Xenoliths from Bunyaruguru volcanic field: Some insights into lithology of East African Rift upper mantle

NASA Astrophysics Data System (ADS)

Muravyeva, N. S.; Senin, V. G.

2018-01-01

The mineral composition of mantle xenoliths from kamafugites of the Bunyaruguru volcanic field has been determined. The major and some trace elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, Cr, Ni, Ba, Sr, La, Ce, Nd, Nb) has been analyzed in olivine, clinopyroxene, phlogopite, Cr-spinel, titanomagnetite, perovskite and carbonates of xenoliths and their host lavas. Bunyaruguru is one of three (Katwe-Kikorongo, Fort Portal and Bunyaruguru) volcanic fields included in the Toro-Ankole province located on the North end of the West Branch of the East African Rift. The xenoliths from three craters within the Bunyaruguru volcanic field revealed the different character of metasomatic alteration, reflecting the heterogeneity of the mantle on the kilometer scale. The most unusual finding was composite glimmerite-wehrlite xenolith from the crater Kazimiro, which contains the fresh primary high-Mg olivine with inclusions of Cr-spinel that had not been previously identified in this area. The different composition of phenocryst and xenolith minerals indicates that the studied xenoliths are not cumulus of enclosing magma, but the composition of xenoliths characterizes the lithology of the upper mantle of the area. The carbonate melt inclusions in olivine Fo90 demonstrate the existence of primary carbonatitic magmas in Bunyaruguru upper mantle. The results of texture and chemical investigation of the xenolith minerals indicate the time sequence of metasomatic alteration of Bunyaruguru upper mantle: MARID metasomatism at the first stage followed by carbonate metasomatism. The abundances of REE in perovskites from kamafugite are 2-4 times higher than similar values for xenolith. Therefore the kamafugite magma was been generated from a more enriched mantle source than the source of the xenoliths. The evaluation of P-T conditions formation of clinopyroxene xenolith revealed the range of pressure 20-65 kbar and the temperatures range 830-1040 °C. The pressure of clinopyroxene phenocryst crystallization differs from pressure of formation the xenoliths clinopyroxene: it may be higher or lower of it. The results of our investigation have shown that olivine can play a noticeable role in the lithology of the upper mantle Bunyaruguru volcanic field.

Upper mantle fluids evolution, diamond formation, and mantle metasomatism

NASA Astrophysics Data System (ADS)

Huang, F.; Sverjensky, D. A.

2017-12-01

During mantle metasomatism, fluid-rock interactions in the mantle modify wall-rock compositions. Previous studies usually either investigated mineral compositions in xenoliths and xenocrysts brought up by magmas, or examined fluid compositions preserved in fluid inclusions in diamonds. However, a key study of Panda diamonds analysed both mineral and fluid inclusions in the diamonds [1] which we used to develop a quantitative characterization of mantle metasomatic processes. In the present study, we used an extended Deep Earth Water model [2] to simulate fluid-rock interactions at upper mantle conditions, and examine the fluids and mineral assemblages together simultaneously. Three types of end-member fluids in the Panda diamond fluid inclusions include saline, rich in Na+K+Cl; silicic, rich in Si+Al; and carbonatitic, rich in Ca+Mg+Fe [1, 3]. We used the carbonatitic end-member to represent fluid from a subducting slab reacting with an excess of peridotite + some saline fluid in the host environment. During simultaneous fluid mixing and reaction with the host rock, the logfO2 increased by about 1.6 units, and the pH increased by 0.7 units. The final minerals were olivine, garnet and diamond. The Mg# of olivine decreased from 0.92 to 0.85. Garnet precipitated at an early stage, and its Mg# also decreased with reaction progress, in agreement with the solid inclusions in the Panda diamonds. Phlogopite precipitated as an intermediate mineral and then disappeared. The aqueous Ca, Mg, Fe, Si and Al concentrations all increased, while Na, K, and Cl concentrations decreased during the reaction, consistent with trends in the fluid inclusion compositions. Our study demonstrates that fluids coming from subducting slabs could trigger mantle metasomatism, influence the compositions of sub-lithospherc cratonic mantle, precipitate diamonds, and change the oxygen fugacity and pH of the upper mantle fluids. [1] Tomlinson et al. EPSL (2006); [2] Sverjensky, DA et al., GCA (2014), Huang, F, Ph. D. thesis, Johns Hopkins University, (2017); [3] Shirey et al., Rev. Mineral. Geochem. (2013)

Postglacial rebound with a non-Newtonian upper mantle and a Newtonian lower mantle rheology

NASA Technical Reports Server (NTRS)

Gasperini, Paolo; Yuen, David A.; Sabadini, Roberto

1992-01-01

A composite rheology is employed consisting of both linear and nonlinear creep mechanisms which are connected by a 'transition' stress. Background stress due to geodynamical processes is included. For models with a non-Newtonian upper-mantle overlying a Newtonian lower-mantle, the temporal responses of the displacements can reproduce those of Newtonian models. The average effective viscosity profile under the ice-load at the end of deglaciation turns out to be the crucial factor governing mantle relaxation. This can explain why simple Newtonian rheology has been successful in fitting the uplift data over formerly glaciated regions.

Synthetic receiver function profiles through the upper mantle and the transition zone for upwelling scenarios

NASA Astrophysics Data System (ADS)

Nagel, Thorsten; Düsterhöft, Erik; Schiffer, Christian

2017-04-01

We investigate the signature relevant mantle lithologies leave in the receiver function record for different adiabatic thermal gradients down to 800 kilometers depth. The parameter space is chosen to target the visibility of upwelling mantle (a plume). Seismic velocities for depleted mantle, primitive mantle, and three pyroxenites are extracted from thermodynamically calculated phases diagrams, which also provide the adiabatic decompression paths. Results suggest that compositional variations, i.e. the presence or absence of considerable amounts of pyroxenites in primitive mantle should produce a clear footprint while horizontal differences in thermal gradients for similar compositions might be more subtle. Peridotites best record the classic discontinuities at around 410 and 650 kilometers depth, which are associated with the olivin-wadsleyite and ringwoodite-perovskite transitions, respectively. Pyroxenites, instead, show the garnet-perovskite transition below 700 kilometers depth and SiO2-supersaturated compositions like MORB display the coesite-stishovite transition between 300 and 340 kilometers depth. The latter shows the strongest temperature-depth dependency of all significant transitions potentially allowing to infer information about the thermal state if the mantle contains a sufficient fraction of MORB-like compositions. For primitive and depleted mantle compositions, the olivin-wadsleyite transition shows a certain temperature-depth dependency reflected in slightly larger delay times for higher thermal gradients. The lower-upper-mantle discontinuity, however, is predicted to display larger delay times for higher thermal gradients although the associated assemblage transition occurs at shallower depths thus requiring a very careful depth migration if a thermal anomaly should be recognized. This counterintuitive behavior results from the downward replacement of the assemblage wadsleyite+garnet with the assemblage garnet+periclase at high temperatures. This transition causes even lower seismic velocities with greater depth (following an adiabatic gradient), the highly continuous nature of the reaction, however, should produce only a smooth negative conversion. In contrast, a small positive conversion is expected at normal thermal gradients in the same depth range between 500 and 550 kilometers because of the wadsleyite-ringwoodite-transition. Hence, the polarity of the 520 discontinuity also offers a possibility to recognize the thermal state of the upper mantle.

The Isotopic Record From Monogenetic Seamounts: Insights Into Recycling Time Scales In The Upper Mantle

NASA Astrophysics Data System (ADS)

Madrigal Quesada, P.; Gazel, E.

2017-12-01

Monogenetic seamounts related to non-plume intraplate magmatism provide a window into the composition of upper mantle heterogeneities, nevertheless, the origin of these heterogeneities are still not well constrained. Radiogenic isotopes (Sr-Nd-Pb) from present-day ocean island basalts (OIB) produced by this type of magmatism can help establish the source compositions of these chemically and isotopically enriched reservoirs. Here we present evidence that suggests that a highly enriched mantle reservoir can originate from OIB-type subducted material that gets incorporated and stirred throughout the upper mantle. We explore this hypothesis using data from non-plume related OIB volcanism; focusing on isolated monogenetic seamounts with no apparent age progression and interpreted to be related to either plate flexure, shear driven convection and/or edge convection. The isotopic record compiled, added to new results obtained from accreted petit-spot seamounts from Santa Elena Peninsula in Costa Rica, suggest that a highly radiogenic mantle reservoir originated from recycled seamount materials can be formed in a shorter time scale than ancient subducted oceanic crust (>1 Ga), thought to be the forming agent of the HIMU mantle "flavor" found in some of these small-scale seamounts. The implications of these results entail that the recycling of already enriched materials in short time scales and in restricted depths within the Upper Mantle may play an important role in the source of OIBs (plume and non-plume related), as well as, the most enriched suites of EMORBs.

Deep mantle cycling of oceanic crust: evidence from diamonds and their mineral inclusions.

PubMed

Walter, M J; Kohn, S C; Araujo, D; Bulanova, G P; Smith, C B; Gaillou, E; Wang, J; Steele, A; Shirey, S B

2011-10-07

A primary consequence of plate tectonics is that basaltic oceanic crust subducts with lithospheric slabs into the mantle. Seismological studies extend this process to the lower mantle, and geochemical observations indicate return of oceanic crust to the upper mantle in plumes. There has been no direct petrologic evidence, however, of the return of subducted oceanic crustal components from the lower mantle. We analyzed superdeep diamonds from Juina-5 kimberlite, Brazil, which host inclusions with compositions comprising the entire phase assemblage expected to crystallize from basalt under lower-mantle conditions. The inclusion mineralogies require exhumation from the lower to upper mantle. Because the diamond hosts have carbon isotope signatures consistent with surface-derived carbon, we conclude that the deep carbon cycle extends into the lower mantle.

Redox state of recycled crustal lithologies in the convective upper mantle constrained using oceanic basalt CO2-trace element systematics

NASA Astrophysics Data System (ADS)

Eguchi, J.; Dasgupta, R.

2017-12-01

Investigating the redox state of the convective upper mantle remains challenging as there is no way of retrieving samples from this part of the planet. Current views of mantle redox are based on Fe3+/∑Fe of minerals in mantle xenoliths and thermodynamic calculations of fO2 [1]. However, deep xenoliths are only recoverable from continental lithospheric mantle, which may have different fO2s than the convective oceanic upper mantle [1]. To gain insight on the fO2 of the deep parts of the oceanic upper mantle, we probe CO2-trace element systematics of basalts that have been argued to receive contributions from subducted crustal lithologies that typically melt deeper than peridotite. Because CO2 contents of silicate melts at graphite saturation vary with fO2 [2], we suggest CO2-trace element systematics of oceanic basalts which sample deep heterogeneities may provide clues about the fO2 of the convecting mantle containing embedded heterogeneities. We developed a new model to predict CO2 contents in nominally anhydrous silicate melts from graphite- to fluid-saturation over a range of P (0.05- 5 GPa), T (950-1600 °C), and composition (foidite-rhyolite). We use the model to calculate CO2 content as a function of fO2 for partial melts of lithologies that vary in composition from rhyolitic sediment melt to silica-poor basaltic melt of pyroxenites. We then use modeled CO2 contents in mixing calculations with partial melts of depleted mantle to constrain the fO2 required for partial melts of heterogeneities to deliver sufficient CO2 to explain CO2-trace element systematics of natural basalts. As an example, Pitcairn basalts, which show evidence of a subducted crustal component [3] require mixing of 40% of partial melts of a garnet pyroxenite at ΔFMQ -1.75 at 3 GPa. Mixing with a more silicic composition such as partial melts of a MORB-eclogite cannot deliver enough CO2 at graphite saturation, so in this scenario fO2 must be above the EMOG/D buffer at 4 GPa. Results suggest convecting upper mantle may be more oxidized than continental lithospheric mantle, and fO2 profiles of continental lithospheric mantle may not be applicable to convective upper mantle.[1] Frost, D, McCammon, C. 2008. An Rev E & P Sci. (36) p.389-420; [2] Holloway, J, et al. 1992. Eu J. Min. (4) p. 105-114; [3] Woodhead, J, Devey C. 1993. EPSL. (116) p. 81-99.

The evolution of continental roots in numerical thermo-chemical mantle convection models including differentiation by partial melting

NASA Astrophysics Data System (ADS)

de Smet, J. H.; van den Berg, A. P.; Vlaar, N. J.

1999-09-01

Incorporating upper mantle differentiation through decompression melting in a numerical mantle convection model, we demonstrate that a compositionally distinct root consisting of depleted peridotite can grow and remain stable during a long period of secular cooling. Our modeling results show that in a hot convecting mantle partial melting will produce a compositional layering in a relatively short time of about 50 Ma. Due to secular cooling mantle differentiation finally stops before 1 Ga. The resulting continental root remains stable on a billion year time scale due to the combined effects of its intrinsically lower density and temperature-dependent rheology. Two different parameterizations of the melting phase-diagram are used in the models. The results indicate that during the Archaean melting occurred on a significant scale in the deep regions of the upper mantle, at pressures in excess of 15 GPa. The compositional depths of continental roots extend to 400 km depending on the potential temperature and the type of phase-diagram parameterization used in the model. The results reveal a strong correlation between lateral variations of temperature and the thickness of the continental root. This shows that cold regions in cratons are stabilized by a thick depleted root.

Electrical conductivity of (Mg,Fe)SiO3 Perovskite and a Perovskite-dominated assemblage at lower mantle conditions

NASA Technical Reports Server (NTRS)

Li, Xiaoyuan; Jeanloz, Raymond

1987-01-01

Electrical conductivity measurements of Perovskite and a Perovskite-dominated assemblage synthesized from pyroxene and olivine demonstrate that these high-pressure phases are insulating to pressures of 82 GPa and temperatures of 4500 K. Assuming an anhydrous upper mantle composition, the result provides an upper bound of 0.01 S/m for the electrical conductivity of the lower mantle between depths of 700 and 1900 km. This is 2 to 4 orders of magnitude lower than previous estimates of lower-mantle conductivity derived from studies of geomagnetic secular variations.

Isotopic Evidence For Chaotic Imprint In The Upper Mantle Heterogeneity

NASA Astrophysics Data System (ADS)

Armienti, P.; Gasperini, D.

2006-12-01

Heterogeneities of the asthenospheric mantle along mid-ocean ridges have been documented as the ultimate effect of complex processes dominated by temperature, pressure and composition of the shallow mantle, in a convective regime that involves mass transfer from the deep mantle, occasionally disturbed by the occurrence of hot spots (e.g. Graham et al., 2001; Agranier et al., 2005; Debaille et al., 2006). Alternatively, upper mantle heterogeneity is seen as the natural result of basically athermal processes that are intrinsic to plate tectonics, such as delamination and recycling of continental crust and of subducted aseismic ridges (Meibom and Anderson, 2003; Anderson, 2006). Here we discuss whether the theory of chaotic dynamical systems applied to isotopic space series along the Mid-Atlantic Ridge (MAR) and the East Pacific Rise (EPR) can delimit the length-scale of upper mantle heterogeneities, then if the model of marble-cake mantle (Allègre and Turcotte, 1986) is consistent with a fractal distribution of such heterogeneity. The correlations between the isotopic (Sr, Nd, Hf, Pb) composition of MORB were parameterized as a function of the ridge length. We found that the distribution of isotopic heterogenity along both the MAR and EPR is self- similar in the range of 7000-9000 km. Self-similarity is the imprint of chaotic mantle processes. The existence of strange attractors in the distribution of isotopic composition of the asthenosphere sampled at ridge crests reveals recursion of the same mantle process(es), endured over long periods of time, up to a stationary state. The occurrence of the same fractal dimension for both the MAR and EPR implies independency of contingent events, suggesting common mantle processes, on a planetary scale. We envisage the cyclic route of "melting, melt extraction and recycling" as the main mantle process which could be able to induce scale invariance. It should have happened for a significant number of times over the Earth's mantle history before it acquired a chaotic structure, thus calling for ancient mantle events. The dimension of 7000 km might be related to the common size of the mantle region which has been affected by these processes.

Applications of the 190Pt-186Os isotope system to geochemistry and cosmochemistry

USGS Publications Warehouse

Walker, R.J.; Morgan, J.W.; Beary, E.S.; Smoliar, M.I.; Czamanske, G.K.; Horan, M.F.

1997-01-01

Platinum is fractionated from osmium primarily as a consequence of processes involving sulfide and metal crystallization. Consequently, the 190Pt-186Os isotope system (190Pt ??? 186Os + ??) shows promise for dating some types of magmatic sulfide ores and evolved iron meteorites. The first 190Pt-186Os isochrons are presented here for ores from the ca. 251 Ma Noril'sk, Siberia plume, and for group IIAB magmatic iron meteorites. Given the known age of the Noril'sk system, a decay constant for 190Pt is determined to be 1.542 ?? 10-12a-1, with ??1% uncertainty. The isochron generated for the IIAB irons is consistent with this decay constant and the known age of the group. The 186Os/188Os ratios of presumably young, mantle-derived osmiridiums and also the carbonaceous chondrite Allende were measured to high-precision to constrain the composition of the modern upper mantle. These compositions overlap, indicating that the upper mantle is chondritic within the level of resolution now available. Our best estimate for this 186Os/188Os ratio is 0.119834 ?? 2 (2??M). The 190Pt/186Os ratios determined for six enstatite chondrites average 0.001659 ?? 75, which is very similar to published values for carbonaceous chondrites. Using this ratio and the presumed composition of the modern upper mantle and chondrites, a solar system initial 186Os/188Os ratio of 0.119820 is calculated. In comparison to the modern upper mantle composition, the 186Os/188Os ratio of the Noril'sk plume was approximately 0.012% enriched in 186Os. Possible reasons for this heterogeneity include the recycling of Pt-rich crust into the mantle source of the plume and derivation of the osmium from the outer core. Derivation of the osmium from the outer core is our favored model. Copyright ?? 1997 Elsevier Science Ltd.

Osmium Isotopic Evolution of the Mantle Sources of Precambrian Ultramafic Rocks

NASA Astrophysics Data System (ADS)

Gangopadhyay, A.; Walker, R. J.

2006-12-01

The Os isotopic composition of the modern mantle, as recorded collectively by ocean island basalts, mid- oceanic ridge basalts (MORB) and abyssal peridotites, is evidently highly heterogeneous (γ Os(I) ranging from <-10 to >+25). One important question, therefore, is how and when the Earth's mantle developed such large-scale Os isotopic heterogeneities. Previous Os isotopic studies of ancient ultramafic systems, including komatiites and picrites, have shown that the Os isotopic heterogeneity of the terrestrial mantle can be traced as far back as the late-Archean (~ 2.7-2.8 Ga). This observation is based on the initial Os isotopic ratios obtained for the mantle sources of some of the ancient ultramafic rocks determined through analyses of numerous Os-rich whole-rock and/or mineral samples. In some cases, the closed-system behavior of these ancient ultramafic rocks was demonstrated via the generation of isochrons of precise ages, consistent with those obtained from other radiogenic isotopic systems. Thus, a compilation of the published initial ^{187}Os/^{188}Os ratios reported for the mantle sources of komatiitic and picritic rocks is now possible that covers a large range of geologic time spanning from the Mesozoic (ca. 89 Ma Gorgona komatiites) to the Mid-Archean (e.g., ca. 3.3 Ga Commondale komatiites), which provides a comprehensive picture of the Os isotopic evolution of their mantle sources through geologic time. Several Precambrian komatiite/picrite systems are characterized by suprachondritic initial ^{187}Os/^{188}Os ratios (e.g., Belingwe, Kostomuksha, Pechenga). Such long-term enrichments in ^{187}Os of the mantle sources for these rocks may be explained via recycling of old mafic oceanic crust or incorporation of putative suprachondritic outer core materials entrained into their mantle sources. The relative importance of the two processes for some modern mantle-derived systems (e.g., Hawaiian picrites) is an issue of substantial debate. Importantly, however, the high-precision initial Os isotopic compositions of the majority of ultramafic systems show strikingly uniform initial ^{187}Os/^{188}Os ratios, consistent with their derivation from sources that had Os isotopic evolution trajectory very similar to that of carbonaceous chondrites. In addition, the Os isotopic evolution trajectories of the mantle sources for most komatiites show resolvably lower average Re/Os than that estimated for the Primitive Upper Mantle (PUM), yet significantly higher than that obtained in some estimates for the modern convecting upper mantle, as determined via analyses of abyssal peridotites. One possibility is that most of the komatiites sample mantle sources that are unique relative to the sources of abyssal peridotites and MORB. Previous arguments that komatiites originate via large extents of partial melting of relatively deep upper mantle, or even lower mantle materials could, therefore, implicate a source that is different from the convecting upper mantle. If so, this source is remarkably uniform in its long-term Re/Os, and it shows moderate depletion in Re relative to the PUM. Alternatively, if the komatiites are generated within the convective upper mantle through relatively large extents of partial melting, they may provide a better estimate of the Os isotopic composition of the convective upper mantle than that obtained via analyses of MORB, abyssal peridotites and ophiolites.

Magnesium stable isotope composition of Earth's upper mantle

NASA Astrophysics Data System (ADS)

Handler, Monica R.; Baker, Joel A.; Schiller, Martin; Bennett, Vickie C.; Yaxley, Gregory M.

2009-05-01

The mantle is Earth's largest reservoir of Mg containing > 99% of Earth's Mg inventory. However, no consensus exists on the stable Mg isotope composition of the Earth's mantle or how variable it is and, in particular, whether the mantle has the same stable Mg isotope composition as chondrite meteorites. We have determined the Mg isotope composition of olivine from 22 mantle peridotites from eastern Australia, west Antarctica, Jordan, Yemen and southwest Greenland by pseudo-high-resolution MC-ICP-MS on Mg purified to > 99%. The samples include fertile lherzolites, depleted harzburgites and dunites, cryptically metasomatised ('dry') peridotites and modally metasomatised apatite ± amphibole-bearing harzburgites and wehrlites. Olivine from these samples of early Archaean through to Permian lithospheric mantle have δ25Mg DSM-3 = - 0.22 to - 0.08‰. These data indicate the bulk upper mantle as represented by peridotite olivine is homogeneous within current analytical uncertainties (external reproducibility ≤ ± 0.07‰ [2 sd]). We find no systematic δ25Mg variations with location, lithospheric age, peridotite fertility, or degree or nature of mantle metasomatism. Although pyroxene may have slightly heavier δ25Mg than coexisting olivine, any fractionation between mantle pyroxene and olivine is also within current analytical uncertainties with a mean Δ25Mg pyr-ol = +0.06 ± 0.10‰ (2 sd; n = 5). Our average mantle olivine δ25Mg DSM-3 = - 0.14 ± 0.07‰ and δ26Mg DSM-3 = - 0.27 ± 0.14‰ (2 sd) are indistinguishable from the average of data previously reported for terrestrial basalts, confirming that basalts have stable Mg isotope compositions representative of the mantle. Olivine from five pallasite meteorites have δ25Mg DSM-3 = - 0.16 to - 0.11‰ that are identical to terrestrial olivine and indistinguishable from the average δ25Mg previously reported for chondrites. These data provide no evidence for measurable heterogeneity in the stable Mg isotope composition of the source material in the proto-planetary disc from which Earth and chondrite and pallasite parent bodies accreted.

Facilitating atmosphere oxidation through mantle convection

NASA Astrophysics Data System (ADS)

Lee, K. K. M.; Gu, T.; Creasy, N.; Li, M.; McCammon, C. A.; Girard, J.

2017-12-01

Earth's mantle connects the surface with the deep interior through convection, and the evolution of its redox state will affect the distribution of siderophile elements, recycling of refractory isotopes, and the oxidation state of the atmosphere through volcanic outgassing. While the rise of oxygen in the atmosphere, i.e., the Great Oxidation Event (GOE) occurred 2.4 billion years ago (Ga), multiple lines of evidence point to oxygen production in the atmosphere well before 2.4 Ga. In contrast to the fluctuations of atmospheric oxygen, vanadium in Archean mantle lithosphere suggests that the mantle redox state has been constant for 3.5 Ga. Indeed, the connection between the redox state of the deep Earth and the atmosphere is enigmatic as is the effect of redox state on mantle dynamics. Here we show a redox-induced density contrast affects mantle convection and may potentially cause the oxidation of the upper mantle. We compressed two synthetic enstatite chondritic samples with identical bulk compositions but formed under different oxygen fugacities (fO2) to lower mantle pressures and temperatures and find Al2O3 forms its own phase separate from the dominant bridgmanite phase in the more reduced composition, in contrast to a more Al-rich, bridgmanite-dominated assemblage for a more oxidized starting composition. As a result, the reduced material is 1-1.5% denser than the oxidized material. Subsequent experiments on other plausible mantle compositions, which differ only in redox state of the starting glass materials, show similar results: distinct mineral assemblages and density contrasts up to 4%. Our geodynamic simulations suggest that such a density contrast causes a rapid ascent and accumulation of oxidized material in the upper mantle, with descent of the denser reduced material to the core-mantle boundary. The resulting heterogeneous redox conditions in Earth's interior may have contributed to the large low-shear velocity provinces in the lower mantle and the rise of oxygen in Earth's atmosphere.

Genetic relations of oceanic basalts as indicated by lead isotopes

USGS Publications Warehouse

Tatsumoto, M.

1966-01-01

The isotopic compositions of lead and the concentrations of lead, uranium, and thorium in samples of oceanic tholeiite and alkali suites are determined, and the genetic relations of the oceanic basalts are discussed. Lead of the oceanic tholeiites has a varying lead-206 : lead-204 ratio between 17.8 and 18.8, while leads of the alkali basalt suites from Easter Island and Guadalupe Island are very radiogenic with lead-206 : lead-204 ratios between 19.3 and 20.4. It is concluded that (i) the isotopic composition of lead in oceanic tholeiite suggests that the upper mantle source region of the tholeiite was differentiated from an original mantle material more than 1 billion years ago and that the upper mantle is not homogeneous at the present time, (ii) less than 20 million years was required for the crystal differentiation within the alkali suite from Easter Island, (iii) no crustal contamination was involved in the course of differentiation of rocks from Easter Island; however, some crustal contamination may have affected Guadalupe Island rocks, and (iv) alkali basalt may be produced from the tholeiite in the oceanic region by crystal differentiation. Alternatively the difference in the isotopic composition of lead in oceanic basalts may be produced by partial melting at different depths of a differentiated upper mantle.

Orogenic, Ophiolitic, and Abyssal Peridotites

NASA Astrophysics Data System (ADS)

Bodinier, J.-L.; Godard, M.

2003-12-01

"Tectonically emplaced" mantle rocks include subcontinental, suboceanic, and subarc mantle rocks that were tectonically exhumed from the upper mantle and occur:(i) as dispersed ultramafic bodies, a few meters to kilometers in size, in suture zones and mountain belts (i.e., the "alpine," or "orogenic" peridotite massifs - De Roever (1957), Thayer (1960), Den Tex (1969));(ii) as the lower ultramafic section of large (tens of kilometers) ophiolite or island arc complexes, obducted on continental margins (e.g., the Oman Ophiolite and the Kohistan Arc Complex - Coleman (1971), Boudier and Coleman (1981), Burg et al. (1998));(iii) exhumed above the sea level in ocean basins (e.g., Zabargad Island in the Red Sea, St. Paul's islets in the Atlantic and Macquarie Island in the southwestern Pacific - Tilley (1947), Melson et al. (1967), Varne and Rubenach (1972), Bonatti et al. (1981)).The "abyssal peridotites" are samples from the oceanic mantle that were dredged on the ocean floor, or recovered from drill cores (e.g., Bonatti et al., 1974; Prinz et al., 1976; Hamlyn and Bonatti, 1980).Altogether, tectonically emplaced and abyssal mantle rocks provide insights into upper mantle compositions and processes that are complementary to the information conveyed by mantle xenoliths (See Chapter 2.05). They provide coverage to vast regions of the Earth's upper mantle that are sparsely sampled by mantle xenoliths, particularly in the ocean basins and beneath passive continental margins, back-arc basins, and oceanic island arcs.Compared with mantle xenoliths, a disadvantage of some tectonically emplaced mantle rocks for representing mantle compositions is that their original geodynamic setting is not exactly known and their significance is sometimes a subject of speculation. For instance, the provenance of orogenic lherzolite massifs (subcontinental lithosphere versus upwelling asthenosphere) is still debated (Menzies and Dupuy, 1991, and references herein), as is the original setting of ophiolites (mid-ocean ridges versus supra-subduction settings - e.g., Nicolas, 1989). In addition, the mantle structures and mineralogical compositions of tectonically emplaced mantle rocks may be obscured by deformation and metamorphic recrystallization during shallow upwelling, exhumation, and tectonic emplacement. Metamorphic processes range from high-temperature recrystallization in the stability field of plagioclase peridotites ( Rampone et al., 1993) to complete serpentinization (e.g., Burkhard and O'Neill, 1988). Some garnet peridotites record even more complex evolutions. They were first buried to, at least, the stability field of garnet peridotites, and, in some cases to greater than 150 km depths ( Dobrzhinetskaya et al., 1996; Green et al., 1997; Liou, 1999). Then, they were exhumed to the surface, dragged by buoyant crustal rocks ( Brueckner and Medaris, 2000).Alternatively, several peridotite massifs are sufficiently well preserved to allow the observation of structural relationships between mantle lithologies that are larger than the sampling scale of mantle xenoliths. It is possible in these massifs to evaluate the scale of mantle heterogeneities and the relative timing of mantle processes such as vein injection, melt-rock reaction, deformation, etc… Detailed studies of orogenic and ophiolitic peridotites on centimeter- to kilometer-scale provide invaluable insights into melt transfer mechanisms, such as melt flow in lithospheric vein conduits and wall-rock reactions (Bodinier et al., 1990), melt extraction from mantle sources via channeled porous flow ( Kelemen et al., 1995) or propagation of kilometer-scale melting fronts associated with thermalerosion of lithospheric mantle ( Lenoir et al., 2001). In contrast, mantle xenoliths may be used to infer either much smaller- or much larger-scale mantle heterogeneities, such as micro-inclusions in minerals ( Schiano and Clocchiatti, 1994) or lateral variations between lithospheric provinces ( O'Reilly et al., 2001).The abyssal peridotites are generally strongly affected by oceanic hydrothermal alteration. Most often, their whole-rock compositions are strongly modified and cannot be used straightforwardly to assess mantle compositions (e.g., Baker and Beckett, 1999). However, even in the worst cases the samples generally contain fresh, relic minerals (mainly clinopyroxene) that represent the only available direct information on the oceanic upper mantle in large ocean basins, away from hot-spot volcanic centers. In situ trace-element data on clinopyroxenes from abyssal peridotites provide constraints on melting processes at mid-ocean ridges (Johnson et al., 1990).In this chapter, we review the main inferences on upper mantle composition and heterogeneity that may be drawn from geochemical analyses of the major elements, lithophile trace elements, and Nd-Sr isotopes in tectonically emplaced and abyssal mantle rocks. In addition we emphasize important insights into the mechanisms of melt/fluid transfer that can be deduced from detailed studies of these mantle materials.

Three-dimensional shear wave velocity structure in the Atlantic upper mantle

NASA Astrophysics Data System (ADS)

James, Esther Kezia Candace

Oceanic lithosphere constitutes the upper boundary layer of the Earth's convecting mantle. Its structure and evolution provide a vital window on the dynamics of the mantle and important clues to how the motions of Earth's surface plates are coupled to convection in the mantle below. The three-dimensional shear-velocity structure of the upper mantle beneath the Atlantic Ocean is investigated to gain insight into processes that drive formation of oceanic lithosphere. Travel times are measured for approximately 10,000 fundamental-mode Rayleigh waves, in the period range 30-130 seconds, traversing the Atlantic basin. Paths with >30% of their length through continental upper mantle are excluded to maximize sensitivity to the oceanic upper mantle. The lateral distribution of Rayleigh wave phase velocity in the Atlantic upper mantle is explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two-dimensional parameterization is utilized in order to capture perturbations to age-dependent structure. Phase velocity shows a strong dependence on seafloor age, and removing age-dependent velocity from the 2-D maps highlights areas of anomalously low velocity, almost all of which are proximal to locations of hotspot volcanism. Depth-dependent variations in vertically-polarized shear velocity (Vsv) are determined with two sets of 3-D models: a layered model that requires constant VSV in each depth layer, and a splined model that allows VSV to vary continuously with depth. At shallow depths (˜75 km) the seismic structure shows the expected dependence on seafloor age. At greater depths (˜200 km) high-velocity lithosphere is found only beneath the oldest seafloor; velocity variations beneath younger seafloor may result from temperature or compositional variations within the asthenosphere. The age-dependent phase velocities are used to constrain temperature in the mantle and show that, in contrast to previous results for the Pacific, phase velocities for the Atlantic are not consistent with a half-space cooling model but are best explained by a plate-cooling model with thickness of 75 km and mantle temperature of 1400°C. Comparison with data such as basalt chemistry and seafloor elevation helps to separate thermal and compositional effects on shear velocity.

Hydrogen Isotopic Systematics of Nominally Anhydrous Phases in Martian Meteorites

NASA Astrophysics Data System (ADS)

Tucker, Kera

Hydrogen isotope compositions of the martian atmosphere and crustal materials can provide unique insights into the hydrological and geological evolution of Mars. While the present-day deuterium-to-hydrogen ratio (D/H) of the Mars atmosphere is well constrained (~6 times that of terrestrial ocean water), that of its deep silicate interior (specifically, the mantle) is less so. In fact, the hydrogen isotope composition of the primordial martian mantle is of great interest since it has implications for the origin and abundance of water on that planet. Martian meteorites could provide key constraints in this regard, since they crystallized from melts originating from the martian mantle and contain phases that potentially record the evolution of the H 2O content and isotopic composition of the interior of the planet over time. Examined here are the hydrogen isotopic compositions of Nominally Anhydrous Phases (NAPs) in eight martian meteorites (five shergottites and three nakhlites) using Secondary Ion Mass Spectrometry (SIMS). This study presents a total of 113 individual analyses of H2O contents and hydrogen isotopic compositions of NAPs in the shergottites Zagami, Los Angeles, QUE 94201, SaU 005, and Tissint, and the nakhlites Nakhla, Lafayette, and Yamato 000593. The hydrogen isotopic variation between and within meteorites may be due to one or more processes including: interaction with the martian atmosphere, magmatic degassing, subsolidus alteration (including shock), and/or terrestrial contamination. Taking into consideration the effects of these processes, the hydrogen isotope composition of the martian mantle may be similar to that of the Earth. Additionally, this study calculated upper limits on the H2O contents of the shergottite and nakhlite parent melts based on the measured minimum H2O abundances in their maskelynites and pyroxenes, respectively. These calculations, along with some petrogenetic assumptions based on previous studies, were subsequently used to infer the H2O contents of the mantle source reservoirs of the depleted shergottites (200-700 ppm) and the nakhlites (10-100 ppm). This suggests that mantle source of the nakhlites is systematically drier than that of the depleted shergottites, and the upper mantle of Mars may have preserved significant heterogeneity in its H2O content. Additionally, this range of H2O contents is not dissimilar to the range observed for the Earth's upper mantle.

Worldwide occurrence of silica-rich melts in sub-continental and sub-oceanic mantle minerals

NASA Astrophysics Data System (ADS)

Schiano, P.; Clocchiatti, R.

1994-04-01

ROCK samples derived from the Earth's upper mantle commonly show indirect evidence for chemical modification. Such modification, or 'metasomatism', can be recognized by the precipitation of exotic minerals such as phlogopite, amphibole or apatite1, and by the overprinting of the bulk compositions of the mantle rocks by a chemical signature involving the enrichment of potassium and other 'incompatible' elements2. Here we study the composition of the metasomatic agents more directly by examining melt and fluid inclusions trapped in mantle minerals. These inclusions are secondary, forming trails along healed fracture planes. A systematic study of the chemical compositions and entrapment temperatures and pressures of inclusions from 14 ultramaflc peridotites from both continental and oceanic intraplate regions shows that volatile- and silica-rich metasomatic melts are present throughout the litho-sphere. Their compositions, which differ dramatically from those of erupted, mantle-derived magmas, are more akin to continental than to oceanic crust.

Episodic large-scale overturn of two-layer mantles in terrestrial planets

NASA Astrophysics Data System (ADS)

Herrick, D. L.; Parmentier, E. M.

1994-01-01

It is usually assumed that the upper and lower mantles of a chemically stratified planet are arranged so that the upper mantle is chemically less dense and that these layers convect separately. Possible buoyant overturn of the two mantle layers has not previously been considered. Such overturn would initially occur when thermal expansion of a chemically denser lower mantle more than offsets the compositional density difference between the layers, reversing the relative sense of buoyancy. Once overturn has occurred, the chemically denser, but thermally less dense upper mantle cools more efficiently than the lower mantle and loses its relative thermal buoyancy. If mixing is slow, this leads to repeated overturns that result in thermal histories that differ radically from those obtained without this large-scale overturning. Thermal evolution calculations, for a two-layer mantle over a wide range of parameter space, show that large-scale overturn occurs cyclically with a well-defined period. This period depends most strongly on the viscosity of the lower mantle, to which it is approximately proportional. Geologically interesting overturn periods on the order of 107 to 109 years result for lower mantle viscosities of 1022 to 1024 Pa s for the Earth and Venus, and 1021 to 1023 Pa s for Mars. The mantles of Mercury and the Moon are too thin to permit two-layer convection, and therefore the model is not appropriate for them. Overturn cannot occur on Earth or Venus if the compositional density difference between the layers exceeds about 4%, or on Mars if it exceeds about 2%. Large-scale mantle overturn could have significant tectonic consequences such as the initiation of a new plate tectonic cycle on the Earth or a major resurfacing event on Mars or Venus. Such episodic events in the evolution of a planet are not easily explained by whole mantle thermal convection.

Episodic large-scale overturn of two-layer mantles in terrestrial planets

NASA Technical Reports Server (NTRS)

Herrick, David L.; Parmentier, E. M.

1994-01-01

It is usually assumed that the upper and lower mantles of a chemically stratified planet are arranged so that the upper mantle is chemically less dense and that these layers convect separately. Possible buoyant overturn of the two mantle layers has not previously been considered. Such overturn would initially occur when thermal expansion of a chemically denser lower mantle more than offsets the compositional density difference between the layers, reversing the relative sense of buoyancy. Once overturn has occurred, the chemically denser, but thermally less dense upper mantle cools more efficiently than the lower mantle and loses its relative thermal buoyancy. If mixing is slow, this leads to repeated overturns that result in thermal histories that differ radically from those obtained without this large-scale overturning. Thermal evolution calculations, for a two-layer mantle over a wide range of parameter space, show that large-scale overturn occurs cyclically with a well-defined period. This period depends most strongly on the viscosity of the lower mantle, to which it is approximately proportional. Geologically interesting overturn periods on the order of 10(exp 7) to 10(exp 9) years result for lower mantle viscosities of 10(exp 22) to 10(exp 24) Pa s for the Earth and Venus, and 10(exp 21) to 10(exp 23) Pa s for Mars. The mantles of Mercury and the Moon are too thin to permit two-layer convection, and therefore the model is not appropriate for them. Overturn cannot occur on Earth or Venus if the compositional density difference between the layers exceeds about 4%, or on Mars if it exceeds about 2%. Large-scale mantle overturn could have significant tectonic consequences such as the initiation of a new plate tectonic cycle on the Earth or a major resurfacing event on Mars or Venus. Such episodic events in the evolution of a planet are not easily explained by whole mantle thermal convection.

Mantle flow tectonics - The influence of a ductile lower crust and implications for the formation of topographic uplands on Venus

NASA Technical Reports Server (NTRS)

Bindschadler, Duane L.; Parmentier, E. Marc

1990-01-01

The crust and mantle of Venus can be represented by a model of a layered structure stratified in both density and viscosity. This structure consists of a brittle-elastic upper crustal layer; a ductile weaker crustal layer; a strong upper mantle layer, about 10 percent denser than the crust; and a weaker substrate, representing the portion of the mantle in which convective flow occurs which is a primary source of large-scale topographic and tectonic features. This paper examines the interactions between these four layers and the mantle flow driven by thermal or compositional variations. Solutions are found for a flow driven by a buoyancy-force distribution within the mantle and by relief at the surface and crust-mantle boundary. It is shown that changes in crustal thickness are driven by vertical normal stresses due to mantle flow and by shear coupling of horizontal mantle flow into the crust.

Isotopic and trace element compositions of upper mantle and lower crustal xenoliths, Cima volcanic field, California: Implications for evolution of the subcontinental lithospheric mantle

USGS Publications Warehouse

Mukasa, S.B.; Wilshire, H.G.

1997-01-01

Ultramafic and mafic xenoliths from the Cima volcanic field, southern California, provide evidence of episodic modification of the upper mantle and underplating of the crust beneath a portion of the southern Basin and Range province. The upper mantle xenoliths include spinel peridotite and anhydrous and hydrous pyroxenite, some cut by igneous-textured pyroxenite-gabbro veins and dikes and some by veins of amphibole ?? plagioclase. Igneous-textured pyroxenites and gabbros like the dike rocks also occur abundantly as isolated xenoliths inferred to represent underplated crust. Mineral and whole rock trace element compositions among and within the different groups of xenoliths are highly variable, reflecting multiple processes that include magma-mantle wall rock reactions, episodic intrusion and it filtration of basaltic melts of varied sources into the mantle wall rock, and fractionation. Nd, Sr, and Pb isotopic compositions mostly of clinopyroxene and plagioclase mineral separates show distinct differences between mantle xenoliths (??Nd = -5.7 to +3.4; 87Sr/86Sr = 0.7051 - 0.7073; 206Pb/204Pb = 19.045 - 19.195) and the igneous-textured xenoliths (??Nd = +7.7 to +11.7; 87Sr/86Sr = 0.7027 - 0.7036 with one carbonate-affected outlier at 0.7054; and 206Pb/204Pb = 18.751 - 19.068), so that they cannot be related. The igneous-textured pyroxenites and gabbros are similar in their isotopic compositions to the host basaltic rocks, which have ??Nd of+5.1 to +9.3; 87Sr/86Sr of 0.7028 - 0.7050, and 206Pb/204Pb of 18.685 - 21.050. The igneous-textured pyroxenites and gabbros are therefore inferred to be related to the host rocks as earlier cogenetic intrusions in the mantle and in the lower crust. Two samples of peridotite, one modally metasomatized by amphibole and the other by plagioclase, have isotopic compositions intermediate between the igneous-textured xenoliths and the mantle rock, suggesting mixing, but also derivation of the metasomatizing magmas from two separate and distinct sources. Sm-Nd two-mineral "isochrons" yield apparent ages for petrographically identical rocks believed to be coeval ranging from -0 to 113 ?? 26 Ma, indicating the unreliability of dating these rocks with this method. Amphibole and plagioclase megacrysts are isotopically like the host basalts and probably originate by mechanical breakup of veins comagmatic with the host basaltic rocks. Unlike other Basin and Range localities, Cima Cr-diopside group isotopic compositions do not overlap with those of the host basalts. Copyright 1997 by the American Geophysical Union.

The lunar interior

NASA Technical Reports Server (NTRS)

Anderson, D. L.; Kovach, R. L.

1972-01-01

The compressional velocities are estimated for materials in the lunar interior and compared with lunar seismic results. The lower crust has velocities appropriate for basalts or anorthosites. The high velocities associated with the uppermost mantle imply high densities and a change in composition to a lighter assemblage at depths of the order of 120 km. Calcium and aluminum are probably important components of the upper mantle and are deficient in the lower mantle. Much of the moon may have accreted from material similar in composition to eucrites. The important mineral of the upper mantle is garnet; possible accessory minerals are kyanite, spinel, and rutile. If the seismic results stand up, the high velocity layer in the moon is more likely to be a high pressure form of anorthosite than eclogite, pyroxenite, or dunite. The thickness of the layer is of the order of 50 km. Cosmic abundances can be maintained if the lower mantle is ferromagnesium silicate with minimal amounts of calcium and aluminum. Achondrites such as eucrites and howardites have more of the required characteristics of the lunar interior than carbonaceous chondrites. A density inversion in the moon is a strong possibility.

Magmatic plumbing system from lower mantle of Hainan plume

NASA Astrophysics Data System (ADS)

Xia, Shaohong; Sun, Jinlong; Xu, Huilong; Huang, Haibo; Cao, Jinghe

2017-04-01

Intraplate volcanism during Late Cenozoic in the Leiqiong area of southernmost South China, with basaltic lava flows covering a total of more than 7000 km2, has been attributed to an underlying Hainan plume. However, detailed features of Hainan plume, such as morphology of magmatic conduits, depth of magmatic pool in the upper mantle and pattern of mantle upwelling, are still enigmatic. Here we present seismic tomographic images of the upper 1100 km of the mantle beneath the southern South China. Our results show a mushroom-like continuous low-velocity anomaly characterized by a columnar tail with diameter of about 200-300 km that tilts downward to lower mantle beneath north of Hainan hotspot and a head that spreads laterally near the mantle transition zone, indicating a magmatic pool in the upper mantle. Further upward, this head is decomposed into small patches, but when encountering the base of the lithosphere, a pancake-like anomaly is shaped again to feed the Hainan volcanism. Our results challenge the classical model of a fixed thermal plume that rises vertically to the surface, and propose the new layering-style pattern of magmatic upwelling of Hainan plume. This work indicates the spatial complexities and differences of global mantle plumes probably due to heterogeneous compositions and changefully thermochemical structures of deep mantle.

Three-dimensional velocity structure of crust and upper mantle in southwestern China and its tectonic implications

USGS Publications Warehouse

Wang, Chun-Yong; Chan, W.W.; Mooney, W.D.

2003-01-01

Using P and S arrival times from 4625 local and regional earthquakes recorded at 174 seismic stations and associated geophysical investigations, this paper presents a three-dimensional crustal and upper mantle velocity structure of southwestern China (21??-34??N, 97??-105??E). Southwestern China lies in the transition zone between the uplifted Tibetan plateau to the west and the Yangtze continental platform to the east. In the upper crust a positive velocity anomaly exists in the Sichuan Basin, whereas a large-scale negative velocity anomaly exists in the western Sichuan Plateau, consistent with the upper crustal structure under the southern Tibetan plateau. The boundary between these two anomaly zones is the Longmen Shan Fault. The negative velocity anomalies at 50-km depth in the Tengchong volcanic area and the Panxi tectonic zone appear to be associated with temperature and composition variations in the upper mantle. The Red River Fault is the boundary between the positive and negative velocity anomalies at 50-km depth. The overall features of the crustal and the upper mantle structures in southwestern China are a low average velocity, large crustal thickness variations, the existence of a high-conductivity layer in the crust or/and upper mantle, and a high heat flow value. All these features are closely related to the collision between the Indian and the Asian plates.

P- T phase relations of silicic, alkaline, aluminous liquids: new results and applications to mantle melting and metasomatism

NASA Astrophysics Data System (ADS)

Draper, David S.; Green, Trevor H.

1999-07-01

We report new experimental results obtained under nominally anhydrous conditions at 1.0-1.5 GPa on a synthetic melt whose composition is typical of extreme-composition xenolith glasses. These results demonstrate that part of this extreme compositional range is in equilibrium with a lherzolitic assemblage (olivine, orthopyroxene, and clinopyroxene on the liquidus), extending our earlier findings [D.S. Draper, T.H. Green P- T phase relations of silicic, alkaline, aluminous mantle-xenolith glasses under anhydrous and C-O-H fluid-saturated conditions, J. Petrol. 38 (1997) 1187-1224] showing saturation with harzburgite minerals (olivine and orthopyroxene on the liquidus). The new results strengthen the view that such liquids can readily coexist with upper mantle rocks. Our results also bear on the current debate regarding the nature of low-degree mantle melts between proponents of the diamond-aggregate technique [who argue for comparatively silica- and alkali-rich low-degree melts; e.g., M.B. Baker, M.M. Hirschmann, M.S. Ghiorso, E.M. Stolper, Compositions of near-solidus peridotite melts from experiments and thermodynamic calculations, Nature 375 (1995) 308-311; M.B. Baker, M.M. Hirschmann, L.E. Wasylenki, E.M. Stolper, M.S. Ghiorso, Quest for low-degree mantle melts, Nature 381 (1996) 286] and those favoring the sandwich technique [who question the value of the diamond-aggregate work and argue that near-solidus melts must be nepheline- and olivine-normative; T.J. Falloon, D.H. Green, H.St.C. O'Neill, C.G. Ballhaus, Quest for low-degree mantle melts, Nature 381 (1996) 285; T.J. Falloon, D.H. Green, H.St.C. O'Neill, W.O. Hibberson, Experimental tests of low degree peridotite partial melt compositions: implications for the nature of anhydrous near-solidus peridotite melts at 1 GPa, Earth Planet. Sci. Lett. 152 (1997) 149-162]. Our results support aspects of both views. The sandwich-technique view is supported, for example, because all our liquids coexisting with mantle minerals are nepheline- and olivine-normative; and our olivine-liquid Fe-Mg exchange KD values fall on a trend similar to that supported by those workers. The diamond-aggregate view is supported, for example, because we find equilibrium between highly silicic, alkaline liquids and mantle minerals, showing the effect of high alkali contents to allow high silica contents at silica activities buffered by magnesian olivine and orthopyroxene at low pressure [M.M. Hirschmann, M.B. Baker, E.M. Stolper, The effect of alkalis on the silica content of mantle-derived melts, Geochim. Cosmochim. Acta 62 (1998) 883-902]. Additionally, the melting trends put forward by the sandwich-technique workers include revised low-degree melt compositions, as reported by Hirschmann et al., and our compositions fall on extensions of these trends. These new analyses also yield an olivine-liquid KD that more closely follows the trend of KD vs. melt alkali contents. The views of both sides of this controversy appear to permit, under certain conditions, the existence of small amounts of melt in the upper mantle with compositions similar to the extreme-composition xenolith glasses that are the focus of our work. On the basis of our new results, we conclude that extreme-composition xenolith glasses can act as agents of cryptic metasomatism in the upper mantle.

Self-gravity, self-consistency, and self-organization in geodynamics and geochemistry

NASA Astrophysics Data System (ADS)

Anderson, Don L.

The results of seismology and geochemistry for mantle structure are widely believed to be discordant, the former favoring whole-mantle convection and the latter favoring layered convection with a boundary near 650 km. However, a different view arises from recognizing effects usually ignored in the construction of these models, including physical plausibility and dimensionality. Self-compression and expansion affect material properties that are important in all aspects of mantle geochemistry and dynamics, including the interpretation of tomographic images. Pressure compresses a solid and changes physical properties that depend on volume and does so in a highly nonlinear way. Intrinsic, anelastic, compositional, and crystal structure effects control seismic velocities; temperature is not the only parameter, even though tomographic images are often treated as temperature maps. Shear velocity is not a good proxy for density, temperature, and composition or for other elastic constants. Scaling concepts are important in mantle dynamics, equations of state, and wherever it is necessary to extend laboratory experiments to the parameter range of the Earth's mantle. Simple volume-scaling relations that permit extrapolation of laboratory experiments, in a thermodynamically self-consistent way, to deep mantle conditions include the quasiharmonic approximation but not the Boussinesq formalisms. Whereas slabs, plates, and the upper thermal boundary layer of the mantle have characteristic thicknesses of hundreds of kilometers and lifetimes on the order of 100 million years, volume-scaling predicts values an order of magnitude higher for deep-mantle thermal boundary layers. This implies that deep-mantle features are sluggish and ancient. Irreversible chemical stratification is consistent with these results; plausible temperature variations in the deep mantle cause density variations that are smaller than the probable density contrasts across chemical interfaces created by accretional differentiation and magmatic processes. Deep-mantle features may be convectively isolated from upper-mantle processes. Plate tectonics and surface geochemical cycles appear to be entirely restricted to the upper ˜1,000 km. The 650-km discontinuity is mainly an isochemical phase change but major-element chemical boundaries may occur at other depths. Recycling laminates the upper mantle and also makes it statistically heterogeneous, in agreement with high-frequency scattering studies. In contrast to standard geochemical models and recent modifications, the deeper layers need not be accessible to surface volcanoes. There is no conflict between geophysical and geochemical data, but a physical basis for standard geochemical and geodynamic mantle models, including the two-layer and whole-mantle versions, and qualitative tomographic interpretations has been lacking.

An experimental study of Fe-Ni exchange between sulfide melt and olivine at upper mantle conditions: implications for mantle sulfide compositions and phase equilibria

NASA Astrophysics Data System (ADS)

Zhang, Zhou; von der Handt, Anette; Hirschmann, Marc M.

2018-03-01

The behavior of nickel in the Earth's mantle is controlled by sulfide melt-olivine reaction. Prior to this study, experiments were carried out at low pressures with narrow range of Ni/Fe in sulfide melt. As the mantle becomes more reduced with depth, experiments at comparable conditions provide an assessment of the effect of pressure at low-oxygen fugacity conditions. In this study, we constrain the Fe-Ni composition of molten sulfide in the Earth's upper mantle via sulfide melt-olivine reaction experiments at 2 GPa, 1200 and 1400 °C, with sulfide melt X_{{{Ni}}}^{{{Sulfide}}}={{Ni}}/{{Ni+{Fe}}} (atomic ratio) ranging from 0 to 0.94. To verify the approach to equilibrium and to explore the effect of {f_{{{O}2}}} on Fe-Ni exchange between phases, four different suites of experiments were conducted, varying in their experimental geometry and initial composition. Effects of Ni secondary fluorescence on olivine analyses were corrected using the PENELOPE algorithm (Baró et al., Nucl Instrum Methods Phys Res B 100:31-46, 1995), "zero time" experiments, and measurements before and after dissolution of surrounding sulfides. Oxygen fugacities in the experiments, estimated from the measured O contents of sulfide melts and from the compositions of coexisting olivines, were 3.0 ± 1.0 log units more reduced than the fayalite-magnetite-quartz (FMQ) buffer (suite 1, 2 and 3), and FMQ - 1 or more oxidized (suite 4). For the reduced (suites 1-3) experiments, Fe-Ni distribution coefficients K_{{D}}{}={(X_{{{Ni}}}^{{{sulfide}}}/X_{{{Fe}}}^{{{sulfide}}})}/{(X_{{{Ni}}^{{{olivine}}}/X_{{{Fe}}}^{{{olivine}}})}} are small, averaging 10.0 ± 5.7, with little variation as a function of total Ni content. More oxidized experiments (suite 4) give larger values of K D (21.1-25.2). Compared to previous determinations at 100 kPa, values of K D from this study are chiefly lower, in large part owing to the more reduced conditions of the experiments. The observed difference does not seem attributable to differences in temperature and pressure between experimental studies. It may be related in part to the effects of metal/sulfur ratio in sulfide melt. Application of these results to the composition of molten sulfide in peridotite indicates that compositions are intermediate in composition (X_{{{Ni}}}^{{{sulfide}}} 0.4-0.6) in the shallow mantle at 50 km, becomes more Ni rich with depth as the O content of the melt diminishes, reaching a maximum (0.6-0.7) at depths near 80-120 km, and then becomes more Fe rich in the deeper mantle where conditions are more reduced, approaching (X_{{{Ni}}}^{{{sulfide}}} 0.28) > 140 km depth. Because Ni-rich sulfide in the shallow upper mantle melts at lower temperature than more Fe-rich compositions, mantle sulfide is likely molten in much of the deep continental lithosphere, including regions of diamond formation.

The interaction of plume heads with compositional discontinuities in the Earth's mantle

NASA Technical Reports Server (NTRS)

Manga, Michael; Stone, Howard A.; O'Connell, Richard J.

1993-01-01

The effects of compositional discontinuities of density and viscosity in the Earth's mantle on the ascent of mantle plume heads is studied using a boundary integral numerical technique. Three specific problems are considered: (1) a plume head rising away from a deformable interface, (2) a plume head passing through an interface, and (3) a plume head approaching the surface of the Earth. For the case of a plume attached to a free-surface, the calculated time-dependent plume shapesare compared with experimental results. Two principle modes of plume head deformation are observed: plume head elingation or the formation of a cavity inside the plume head. The inferred structure of mantle plumes, namely, a large plume head with a long tail, is characteristic of plumes attached to their source region, and also of buoyant material moving away from an interface and of buoyant material moving through an interface from a high- to low-viscosity region. As a rising plume head approaches the upper mantle, most of the lower mantle will quickly drain from the gap between the plume head and the upper mantle if the plume head enters the upper mantle. If the plume head moves from a high- to low-viscosity region, the plume head becomes significantly elongated and, for the viscosity contrasts thought to exist in the Earth, could extend from the 670 km discontinuity to the surface. Plume heads that are extended owing to a viscosity decrease in the upper mantle have a cylindrical geometry. The dynamic surface topography induced by plume heads is bell-shaped when the top of the plume head is at depths greater than about 0.1 plume head radii. As the plume head approaches the surface and spreads, the dynamic topography becomes plateau-shaped. The largest stresses are produced in the early stages of plume spreading when the plume head is still nearly spherical, and the surface expression of these stresses is likely to be dominated by radial extension. As the plume spreads, compressional stresses on the surface are produced beyond the edges of the plume; consequently, extensional features will be produced above the plume head and may be surrounded by a ring of compressional features.

Seismic structure and lithospheric rheology from deep crustal xenoliths, central Montana, USA

NASA Astrophysics Data System (ADS)

Mahan, K. H.; Schulte-Pelkum, V.; Blackburn, T. J.; Bowring, S. A.; Dudas, F. O.

2012-10-01

Improved resolution of lower crustal structure, composition, and physical properties enhances our understanding and ability to model tectonic processes. The cratonic core of Montana and Wyoming, USA, contains some of the most enigmatic lower crust known in North America, with a high seismic velocity layer contributing to as much as half of the crustal column. Petrological and physical property data for xenoliths in Eocene volcanic rocks from central Montana provide new insight into the nature of the lower crust in this region. Inherent heterogeneity in xenoliths derived from depths below ˜30 km support a composite origin for the deep layer. Possible intralayer velocity steps may complicate the seismic definition of the crust/mantle boundary and interpretations of crustal thickness, particularly when metasomatized upper mantle is considered. Mafic mineral-dominant crustal xenoliths and published descriptions of mica-bearing peridotite and pyroxenite xenoliths suggest a strong lower crust overlying a potentially weaker upper mantle.

Basin Excavation, Lower Crust, Composition, and Bulk Moon Mass balance in Light of a Thin Crust

NASA Technical Reports Server (NTRS)

Jolliff, B. L.; Korotev, R. L.; Ziegler, R. A.

2013-01-01

New lunar gravity results from GRAIL have been interpreted to reflect an overall thin and low-density lunar crust. Accordingly, crustal thickness has been modeled as ranging from 0 to 60 km, with thinnest crust at the locations of Crisium and Moscoviense basins and thickest crust in the central farside highlands. The thin crust has cosmochemical significance, namely in terms of implications for the Moon s bulk composition, especially refractory lithophile elements that are strongly concentrated in the crust. Wieczorek et al. concluded that the bulk Moon need not be enriched compared to Earth in refractory lithophile elements such as Al. Less Al in the crust means less Al has been extracted from the mantle, permitting relatively low bulk lunar mantle Al contents and low pre- and post-crust-extraction values for the mantle (or the upper mantle if only the upper mantle underwent LMO melting). Simple mass-balance calculations using the method of [4] suggests that the same conclusion might hold for Th and the entire suite of refractory lithophile elements that are incompatible in olivine and pyroxene, including the KREEP elements, that are likewise concentrated in the crust.

Bases of the Mantle-Carbonatite Conception of Diamond Genesis

NASA Astrophysics Data System (ADS)

Litvin, Yuriy; Spivak, Anna; Kuzyura, Anastasia

2016-04-01

In the mantle-carbonatite conception of diamond genesis, the results of physic-chemical experiments are coordinated with the data of analytic mineralogy of primary inclusions in natural diamonds. Generalization of the solutions of principal genetic problems constitutes the bases of the conception. The solutions are following: (1) it is grounded that diamond-parental melts of the upper mantle have peridotite/eclogite - carbonatite - carbon compositions, of the transition zone - (wadsleite↔ringwoodite) - majorite - stishovite - carbonatite - carbon compositions, and of the lower mantle - periclase/wustite - bridgmanite - Ca-perovskite -stishovite - carbonatite - carbon compositions; (2) a construction of generalized diagrams for the diamond-parental media, which reveal changeable compositions of the growth melts of diamonds and associated phases, their genetic relations to the mantle substance, and classification connections of the primary inclusions in natural diamonds; (3) experimental equilibrium phase diagrams of syngenesis of diamonds and primary inclusions, which characterize the nucleation and growth conditions of diamonds and a capture of paragenetic and xenogenetic minerals by the growing diamonds; (4) a determination of the phase diagrams of diamonds and inclusions syngenesis under the regime of fractional crystallization, which discover the regularities of ultrabasic-basic evolution and paragenesis transitions in the diamond-forming systems of the upper and lower mantle. The evidence of the physic-chemically united mode of diamond genesis at the mantle depths with different mineralogy is obtained. References. Litvin Yu.A. (2007). High-pressure mineralogy of diamond genesis. In: Advances in High-Pressure Mineralogy (edited by Eiji Ohtani), Geological Society of America Special paper 421, 83-103. Litvin Yu.A. (2012). Experimental study of physic-chemical conditions of natural diamond formation on an example of the eclogite-carbonatite-sulphide-diamond system. Geology of Ore Deposits. 54(6), 523-539. Litvin Yu.A., Spivak A.V., Solopova N.A., Dubrovinsky L.S. (2014). On origin of lower-mantle diamonds and their primary inclusions. Physics of the Earth and Planetary Interiors, 228 (The Liebermann Volume), 176-185. http://dx.doi.org/10.1016/j/pepi/2013.12.007

High-Resolution P'P' Precursor Imaging of Nazca-South America Plate Boundary Zones and Inferences for Transition Zone Temperature and Composition

NASA Astrophysics Data System (ADS)

Gu, Y. J.; Schultz, R.

2013-12-01

Knowledge of upper mantle transition zone stratification and composition is highly dependent on our ability to efficiently extract and properly interpret small seismic arrivals. A promising high-frequency seismic phase group particularly suitable for a global analysis is P'P' precursors, which are capable of resolving mantle structures at vertical and lateral resolution of approximately 5 and 200 km, respectively, owing to their shallow incidence angle and small, quasi-symmetric Fresnel zones. This study presents a simultaneous analysis of SS and P'P' precursors based on deconvolution, Radon transform and depth migration. Our multi-resolution survey of the mantle near Nazca-South America subduction zone reveals both olivine and garnet related transitions at depth below 400 km. We attribute a depressed 660 to thermal variations, whereas compositional variations atop the upper-mantle transition zone are needed to explain the diminished or highly complex reflected/scattered signals from the 410 km discontinuity. We also observe prominent P'P' reflections within the transition zone, especially near the plate boundary zone where anomalously high reflection amplitudes result from a sharp (~10 km thick) mineral phase change resonant with the dominant frequency of the P'P' precursors. Near the base of the upper mantle, the migration of SS precursors shows no evidence of split reflections near the 660-km discontinuity, but potential majorite-ilmenite (590-640 km) and ilmenite-perovskite transitions (740-750 km) are identified based on similarly processed high-frequency P'P' precursors. At nominal mantle temperatures these two phase changes may be seismically indistinguishable, but colder mantle conditions from the descending Nazca plate, the presence of water and variable Fe contents may cause sufficient separation for a reliable analysis. In addition, our preliminary results provide compelling evidence for multiple shallow lower-mantle reflections (at ~800 km) along the elongated plate boundary zones of South America. Slab stagnation at the base of the transition zone could play a key role, though a proper interpretation of this finding would likely entail compositional (rather than strictly thermal) variations in the vicinity of the descending oceanic crust and lithosphere. Overall, the resolution and sensitivity differences between low/intermediate- S and high-frequency P wave reflections are key considerations toward reconciling seismic and mineralogical models of transition zone structure, both at the study location and worldwide.

Study on 3-D velocity structure of crust and upper mantle in Sichuan-yunnan region, China

USGS Publications Warehouse

Wang, C.; Mooney, W.D.; Wang, X.; Wu, J.; Lou, H.; Wang, F.

2002-01-01

Based on the first arrival P and S data of 4 625 regional earthquakes recorded at 174 stations dispersed in the Yunnan and Sichuan Provinces, the 3-D velocity structure of crust and upper mantle in the region is determined, incorporating with previous deep geophysical data. In the upper crust, a positive anomaly velocity zone exists in the Sichuan basin, whereas a negative anomaly velocity zone exists in the western Sichuan plateau. The boundary between the positive and negative anomaly zones is the Longmenshan fault zone. The images of lower crust and upper mantle in the Longmenshan fault, Xianshuihe fault, Honghe fault and others appear the characteristic of tectonic boundary, indicating that the faults litely penetrate the Moho discontinuity. The negative velocity anomalies at the depth of 50 km in the Tengchong volcanic area and the Panxi tectonic zone appear to be associated with the temperature and composition variations in the upper mantle. The overall features of the crustal and the upper mantle structures in the Sichuan-Yunnan region are the lower average velocity in both crust and uppermost mantle, the large crustal thickness variations, and the existence of high conductivity layer in the crust or/and upper mantle, and higher geothermal value. All these features are closely related to the collision between the Indian and the Asian plates. The crustal velocity in the Sichuan-Yunnan rhombic block generally shows normal.value or positive anomaly, while the negative anomaly exists in the area along the large strike-slip faults as the block boundary. It is conducive to the crustal block side-pressing out along the faults. In the major seismic zones, the seismicity is relative to the negative anomaly velocity. Most strong earthquakes occurred in the upper-mid crust with positive anomaly or normal velocity, where the negative anomaly zone generally exists below.

Noble gas systematics of the Skaergaard intrusion

NASA Astrophysics Data System (ADS)

Horton, F.; Farley, K. A.; Taylor, H. P.

2017-12-01

The noble gas isotopic compositions of olivines from the Skaergaard layered mafic intrusion in Greenland reveal that magmas readily exchange noble gases with their environment after emplacement. Although Skaergaard magmas are thought to have derived from the upper mantle, all of the olivine separates we analyzed have 3He/4He ratios less than that of the upper mantle ( 8 Ra, where Ra = 3He/4He of the atmosphere, 1.39 x 10-6). This suggests that crustal and/or atmospheric noble gases have contaminated all Skaergaard magmas to some extent. We obtained the highest 3He/4He ratios ( 2 Ra) from olivines found in the lowermost exposed layers of the intrusion away from the margins. Excess radiogenic 4He (indicated by Ra<1) along the margin of the intrusion indicates that noble gases from the Archean host-rock were incorporated into the cooling magma chamber, probably via magmatic assimilation. Noble gases in olivines from the upper portions of the intrusion have atmospheric isotopic compositions, but higher relative helium abundances than the atmosphere. We suggest that post-crystallization hydrothermal circulation introduced atmosphere-derived noble gases into uppermost layers of the intrusion. Such high temperature exchanges of volatiles between plutons and their immediate surroundings may help explain why so few mantle-derived rocks retain mantle-like noble gas signatures.

Reconciling laboratory and observational models of mantle rheology in geodynamic modelling

NASA Astrophysics Data System (ADS)

King, Scott D.

2016-10-01

Experimental and geophysical observations constraining mantle rheology are reviewed with an emphasis on their impact on mantle geodynamic modelling. For olivine, the most studied and best-constrained mantle mineral, the tradeoffs associated with the uncertainties in the activation energy, activation volume, grain-size and water content allow the construction of upper mantle rheology models ranging from nearly uniform with depth to linearly increasing from the base of the lithosphere to the top of the transition zone. Radial rheology models derived from geophysical observations allow for either a weak upper mantle or a weak transition zone. Experimental constraints show that wadsleyite and ringwoodite are stronger than olivine at the top of the transition zone; however the uncertainty in the concentration of water in the transition zone precludes ruling out a weak transition zone. Both observational and experimental constraints allow for strong or weak slabs and the most promising constraints on slab rheology may come from comparing inferred slab geometry from seismic tomography with systematic studies of slab morphology from dynamic models. Experimental constraints on perovskite and ferropericlase strength are consistent with general feature of rheology models derived from geophysical observations and suggest that the increase in viscosity through the top of the upper mantle could be due to the increase in the strength of ferropericlase from 20-65 GPa. The decrease in viscosity in the bottom half of the lower mantle could be the result of approaching the melting temperature of perovskite. Both lines of research are consistent with a high-viscosity lithosphere, a low viscosity either in the upper mantle or transition zone, and high viscosity in the lower mantle, increasing through the upper half of the lower mantle and decreasing in the bottom half of the lower mantle, with a low viscosity above the core. Significant regions of the mantle, including high-stress regions of the lower mantle, may be in the dislocation creep (power-law) regime. Due to our limited knowledge of mantle grain size, the best hope to resolve the question of whether a region is in diffusion creep (Newtonian rheology) or dislocation or grain-boundary creep (power-law rheology), may be the presence of absence of seismic anisotropy, because there is no mechanism to rotate crystals in diffusion creep which would be necessary to develop anisotropy from lattice preferred orientation. While non-intuitive, the presence or absence of a weak region in the upper mantle has a profound effect on lower mantle flow. With an asthenosphere, the lower mantle organizes into a long-wavelength plan form with one or two (degree 1 or degree 2) large downwellings and updrafts, which may contain a cluster of plumes. The boundary between the long-wavelength lower mantle flow and upper region flow may be deeper, likely 800-1200 km, than the usually assumed base of the transition zone. There are competing hypotheses as to whether this change in flow pattern is caused by a change in rheology, composition, or phase.

Constraining the Composition of the Subcontinental Lithospheric Mantle Beneath the East African Rift: FTIR Analysis of Water in Spinel Peridotite Mantle Xenoliths

NASA Technical Reports Server (NTRS)

Erickson, Stephanie Gwen; Nelson, Wendy R.; Peslier, Anne H.; Snow, Jonathan E.

2014-01-01

The East African Rift System was initiated by the impingement of the Afar mantle plume on the base of the non-cratonic continental lithosphere (assembled during the Pan-African Orogeny), producing over 300,000 kmof continental flood basalts approx.30 Ma ago. The contribution of the subcontinental lithospheric mantle (SCLM) to this voluminous period of volcanism is implied based on basaltic geochemical and isotopic data. However, the role of percolating melts on the SCLM composition is less clear. Metasomatism is capable of hybridizing or overprinting the geochemical signature of the SCLM. In addition, models suggest that adding fluids to lithospheric mantle affects its stability. We investigated the nature of the SCLM using Fourier transform infrared spectrometry (FTIR) to measure water content in mantle xenoliths entrained in young (1 Ma) basaltic lavas from the Ethiopian volcanic province. The mantle xenoliths consist dominantly of spinel lherzolites and are composed of nominally anhydrous minerals, which can contain trace water as H in mineral defects. Eleven mantle xenoliths come from the Injibara-Gojam region and two from the Mega-Sidamo region. Water abundances of olivines in six samples are 1-5ppm H2O while the rest are below the limit of detection (<0.5 ppm H2O); orthopyroxene and clinopyroxene contain 80-238 and 111-340 ppm wt H2O, respectively. Two xenoliths have higher water contents - a websterite (470 ppm) and dunite (229 ppm), consistent with involvement of ascending melts. The low water content of the upper SCLM beneath Ethiopia is as dry as the oceanic mantle except for small domains represented by percolating melts. Consequently, rifting of the East African lithosphere may not have been facilitated by a hydrated upper mantle.

Extreme isotopic variations in the upper mantle: evidence from Ronda

NASA Astrophysics Data System (ADS)

Reisberg, Laurie; Zindler, Alan

1986-12-01

The Ronda Ultramafic Complex in southern Spain represents a piece of the Earth's mantle which has been tectonically emplaced into the crust. Nd and Sr isotopic analyses are presented for leached, hand-picked Cr-diopside separates prepared from 15 rock and 18 river sediment samples from Ronda. These results demonstrate that within this small, contiguous body there exists the entire range of Nd isotopic compositions, and much of the range of Sr compositions, found in rocks derived from the sub-oceanic mantle. The sediment cpx samples show that the average isotopic composition of the massif becomes progressively less "depleted" moving from SW to NE along the long axis of the massif. The rock cpx samples document 143Nd/ 144Nd variations from 0.5129 to 0.5126 and 87Sr/ 86Sr variations from 0.7031 to 0.7039 within a uniform outcrop less than 10 m in extent. Thus, extreme isotopic fluctuations exist over a wide range of wavelengths. Sr and Nd isotopes are generally inversely correlated, forming a trend on a Nd-Sr diagram that sharply crosscuts that of the "mantle array". Many of the 143Nd/ 144Nd values, and all of the Sm/Nd values, from one section of the massif are lower than that SCV015SCV0 of the bulk earth, implying that this region existed, or was influenced by a component which existed, in a LREE-enriched environment for a significant period of time. Among the sediment cpxs there is a positive correlation between 143Nd/ 144Nd and 147Sm/ 144Nd. The rock cpx separates display considerably more scatter. A simple, single-stage differentiation event starting with a uniform mantle source cannot explain these results. At least one episode of mixing with a LREE-enriched component is required. If these results from Ronda are typical of the upper mantle, basalts with different isotopic compositions need not derive from spatially separated mantle sources.

The Cedars ultramafic mass, Sonoma County, California

USGS Publications Warehouse

Blake, M. Clark; Bailey, Edgar H.; Wentworth, Carl M.

2012-01-01

The Cedars ultramafic mass is a mantle fragment that consists of partially serpentinized spinel harzburgite and dunite. Compositional layering and a chromite lineation define a penetrative metamorphic foliation that almost certainly formed in the upper mantle. Although detailed petrofabric and mineral chemistry are presently lacking, it seems reasonable that the Cedars peridotite represents a slice of mantle tectonite that once formed the base of the Coast Range ophiolite, and not an abyssal peridotite tectonically emplaced within the Franciscan accretionary prism.

Primordial domains in the depleted upper mantle identified by noble gases in MORBs

NASA Astrophysics Data System (ADS)

Tucker, J.; Mukhopadhyay, S.; Langmuir, C. H.; Hamelin, C.; Fuentes, J.

2017-12-01

The distribution of noble gas isotopic compositions in the mantle provides important constraints on the large-scale mantle evolution, as noble gases can trace the interaction between degassed, or processed, mantle domains and undegassed, or primitive, mantle domains. Data from the radiogenic He, Ne, Ar and Xe isotopic systems have shown that plume-related lavas sample relatively undegassed mantle domains, and the recent identification of isotopic anomalies in the short-lived I-Xe and Hf-W isotopic systems in plume-related lavas further suggests that these domains may be ancient, dating back to Earth's accretion. However, little is known about the potential variability of the heavy noble gas systems and the distribution of undegassed domains in the ambient upper mantle not influenced by plumes. Here, we present new high-precision He, Ne, Ar, and Xe isotopic data for a series of MORBs from a depleted section of the subtropical north Mid-Atlantic Ridge, distant from any known plume influence. Some samples have extremely low (unradiogenic) 4He/3He, 21Ne/22Ne, 40Ar/36Ar, and 129Xe/130Xe ratios, including some of the lowest values ever determined for MORBs. Such unradiogenic compositions are reminiscent of OIBs and plume-influenced E-MORBs, suggesting the presence of a relatively undegassed or primitive reservoir in the source of these depleted MORBs. The He, Ne, and Ar isotopic systems are sensitive to the long-term degassing history, suggesting that this domain in the MORB source is ancient. The 129Xe/130Xe ratio is sensitive to degassing only during the first 100 Ma of Earth history, suggesting that some of the isotopic character of these samples has been preserved since Earth's accretion. Together, these observations suggest that primordial or undegassed material is not only sampled in plumes-related lavas, but also normal, depleted MORBs. Along with data from E-MORBs in the southern EPR (Kurz et al., 2005), southern MAR (Sarda et al., 2000), and equatorial MAR (Tucker et al., 2012), our new data suggest that primordial material may be present throughout the MORB source. Such material could either have been stored for a long term in the upper mantle, or recently mixed into the upper mantle from a deeper reservoir.

A 1.5 Ma record of plume-ridge interaction at the Western Galápagos Spreading Center (91°40‧-92°00‧W)

NASA Astrophysics Data System (ADS)

Herbrich, Antje; Hauff, Folkmar; Hoernle, Kaj; Werner, Reinhard; Garbe-Schönberg, Dieter; White, Scott

2016-07-01

Shallow (elevated) portions of mid-ocean ridges with enriched geochemical compositions near hotspots document the interaction of hot, geochemically-enriched plume mantle with shallow depleted upper mantle. Whereas the spatial variations in geochemical composition of ocean crust along the ridge axis in areas where plume-ridge interaction is taking place have been studied globally, only restricted information exists concerning temporal variations in geochemistry of ocean crust formed through plume-ridge interaction. Here we present a detailed geochemical study of 0-1.5 Ma ocean crust sampled from the Western Galápagos Spreading Center (WGSC) axis to 50 km north of the axis, an area that is presently experiencing a high influx of mantle material from the Galápagos hotspot. The tholeiitic to basaltic andesitic fresh glass and few bulk rock samples have incompatible element abundances and Sr-Nd-Pb isotopic compositions intermediate between depleted normal mid-ocean-ridge basalt (N-MORB) from >95.5°W along the WGSC and enriched lavas from the Galápagos Archipelago, displaying enriched (E-)MORB type compositions. Only limited and no systematic geochemical variations are observed with distance from the ridge axis for <1.0 Ma old WGSC crust, whereas 1.0-1.5 Ma old crust trends to more enriched isotopic compositions in 87Sr/86Sr, 143Nd/144Nd, 207Pb/204Pb and 208Pb/204Pb isotope ratios. On isotope correlation diagrams, the data set displays correlations between depleted MORB and two enriched components. Neither the geographically referenced geochemical domains of the Galápagos Archipelago nor the end members used for principal component analysis can successfully describe the observed mixing relations. Notably an off-axis volcanic cone at site DR63 has the appropriate composition to serve as the enriched component for the younger WGSC and could represent a portion of the northern part of the Galápagos plume not sampled south of the WGSC. Similar compositions to samples from volcanic cone DR63 have been found in the northern part of the 11-14 Ma Galápagos hotspot track offshore Costa Rica, indicating that this composition is derived from the northern portion of the Galápagos plume. The older WGSC requires involvement of an enriched mantle two (EMII) type source, not recognized thus far in the Galápagos system, and is interpreted to reflect entrained material either from small-scale heterogeneities within the upper mantle or from the mantle transition zone. Overall the source material for the 0-1.5 Ma WGSC ocean crust appears to represent mixing of depleted upper mantle with Northern Galápagos Plume material of relatively uniform composition in relatively constant proportions.

Whole-mantle convection with tectonic plates preserves long-term global patterns of upper mantle geochemistry.

PubMed

Barry, T L; Davies, J H; Wolstencroft, M; Millar, I L; Zhao, Z; Jian, P; Safonova, I; Price, M

2017-05-12

The evolution of the planetary interior during plate tectonics is controlled by slow convection within the mantle. Global-scale geochemical differences across the upper mantle are known, but how they are preserved during convection has not been adequately explained. We demonstrate that the geographic patterns of chemical variations around the Earth's mantle endure as a direct result of whole-mantle convection within largely isolated cells defined by subducting plates. New 3D spherical numerical models embedded with the latest geological paleo-tectonic reconstructions and ground-truthed with new Hf-Nd isotope data, suggest that uppermost mantle at one location (e.g. under Indian Ocean) circulates down to the core-mantle boundary (CMB), but returns within ≥100 Myrs via large-scale convection to its approximate starting location. Modelled tracers pool at the CMB but do not disperse ubiquitously around it. Similarly, mantle beneath the Pacific does not spread to surrounding regions of the planet. The models fit global patterns of isotope data and may explain features such as the DUPAL anomaly and long-standing differences between Indian and Pacific Ocean crust. Indeed, the geochemical data suggests this mode of convection could have influenced the evolution of mantle composition since 550 Ma and potentially since the onset of plate tectonics.

Sound velocities of olivine at high pressures and temperatures and the composition of Earth's upper mantle

NASA Astrophysics Data System (ADS)

Zhang, Jin S.; Bass, Jay D.

2016-09-01

We present the elastic properties of San Carlos olivine up to P = 12.8(8) GPa and T = 1300(200) K using Brillouin spectroscopy with CO2 laser heating. A comparison of our results with the global seismic model AK135 yields average olivine content near 410 km depth of about 37% and 43% in a dry and wet (1.9 wt % H2O) upper mantle, respectively. These olivine contents are far less than in the pyrolite model. However, comparisons of our results with regional seismic models lead to very different conclusions. High olivine contents of up to 87% are implied by seismic models of the western U.S. and eastern Pacific regions. In contrast, we infer less than 35% olivine under the central Pacific. Strong variations of olivine content and upper mantle lithologies near the 410 km discontinuity are suggested by regional seismic models.

Density structure of the lithosphere in the southwestern United States and its tectonic significance

USGS Publications Warehouse

Kaban, M.K.; Mooney, W.D.

2001-01-01

We calculate a density model of the lithosphere of the southwestern United States through an integrated analysis of gravity, seismic refraction, drill hole, and geological data. Deviations from the average upper mantle density are as much as ?? 3%. A comparison with tomographic images of seismic velocities indicates that a substantial part (>50%) of these density variations is due to changes in composition rather than temperature. Pronounced mass deficits are found in the upper mantle under the Basin and Range Province and the northern part of the California Coast Ranges and adjacent ocean. The density structure of the northern and central/southern Sierra Nevada is remarkably different. The central/southern part is anomalous and is characterized by a relatively light crust underlain by a higher-density upper mantle that may be associated with a cold, stalled subducted plate. High densities are also determined within the uppermost mantle beneath the central Transverse Ranges and adjoining continental slope. The average density of the crystalline crust under the Great Valley and western Sierra Nevada is estimated to be up to 200 kg m~3 higher than the regional average, consistent with tectonic models for the obduction of oceanic crust and uppermost mantle in this region.

Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth's upper mantle

NASA Astrophysics Data System (ADS)

Wang, Ze-Zhou; Liu, Sheng-Ao; Liu, Jingao; Huang, Jian; Xiao, Yan; Chu, Zhu-Yin; Zhao, Xin-Miao; Tang, Limei

2017-02-01

The zinc (Zn) stable isotope system has great potential for tracing planetary formation and differentiation processes due to its chalcophile, lithophile and moderately volatile character. As an initial approach, the terrestrial mantle, and by inference, the bulk silicate Earth (BSE), have previously been suggested to have an average δ66Zn value of ∼+0.28‰ (relative to JMC 3-0749L) primarily based on oceanic basalts. Nevertheless, data for mantle peridotites are relatively scarce and it remains unclear whether Zn isotopes are fractionated during mantle melting. To address this issue, we report high-precision (±0.04‰; 2SD) Zn isotope data for well-characterized peridotites (n = 47) from cratonic and orogenic settings, as well as their mineral separates. Basalts including mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) were also measured to avoid inter-laboratory bias. The MORB analyzed have homogeneous δ66Zn values of +0.28 ± 0.03‰ (here and throughout the text, errors are given as 2SD), similar to those of OIB obtained in this study and in the literature (+0.31 ± 0.09‰). Excluding the metasomatized peridotites that exhibit a wide δ66Zn range of -0.44‰ to +0.42‰, the non-metasomatized peridotites have relatively uniform δ66Zn value of +0.18 ± 0.06‰, which is lighter than both MORB and OIB. This difference suggests a small but detectable Zn isotope fractionation (∼0.1‰) during mantle partial melting. The magnitude of inter-mineral fractionation between olivine and pyroxene is, on average, close to zero, but spinels are always isotopically heavier than coexisting olivines (Δ66ZnSpl-Ol = +0.12 ± 0.07‰) due to the stiffer Zn-O bonds in spinel than silicate minerals (Ol, Opx and Cpx). Zinc concentrations in spinels are 11-88 times higher than those in silicate minerals, and our modelling suggests that spinel consumption during mantle melting plays a key role in generating high Zn concentrations and heavy Zn isotopic compositions of MORB. Therefore, preferential melting of spinel in the peridotites may account for the Zn isotopic difference between spinel peridotites and basalts. By contrast, the absence of Zn isotope fractionation between silicate minerals suggests that Zn isotopes are not significantly fractionated during partial melting of spinel-free garnet-facies mantle. If the studied non-metasomatized peridotites represent the refractory upper mantle, mass balance calculation shows that the depleted MORB mantle (DMM) has a δ66Zn value of +0.20 ± 0.05‰ (2SD), which is lighter than the primitive upper mantle (PUM) estimated in previous studies (+0.28 ± 0.05‰, 2SD, Chen et al., 2013b; +0.30 ± 0.07‰, 2SD, Doucet et al., 2016). This indicates that the Earth's upper mantle has a heterogeneous Zn isotopic composition vertically, which is probably due to shallow mantle melting processes.

Sr, Nd and Pb Isotope Geochemistry of Near-ridge Seamounts in Eastern Pacific: Implications for Upper Mantle Composition and EPR Magmatic Segmentation

NASA Astrophysics Data System (ADS)

Castillo, P. R.; White, W. M.; Batiza, R.

2005-12-01

Near-ridge seamount lavas tend to reflect the true composition of the upper mantle source of MORB because these are generated by relatively smaller degrees of melting of smaller volumes of the mantle compared to nearby axial lavas; they also by-pass the axial chamber mixing and fractionation processes that are responsible for the relatively more uniform chemical and isotopic composition of normal-MORB. New Sr, Nd and Pb isotope data combined with published data for lavas from near-ridge seamounts on either side of the EPR segment between the 11o45' OSC and Orozco Transform at 15o00' show latitudinal isotopic variation very similar to that shown by the rise axial lavas (Castillo et al., G3 1, 1999). Seamount and axial lavas at both ends of the rise segment have on average slightly higher and more limited range of 143Nd/144Nd, but slightly lower 206Pb/204Pb and 87Sr/86Sr ratios than lavas at the center of the segment. Some of the seamounts are located on ~8 Ma rise flank crust although most of the seamount lavas are fairly young (e.g., lavas from Seamount 6 on ~3 Ma crust are only 3 to 900 kyr - Graham et al., Nature 326, 1987). Thus near-ridge seamount isotope data provide the first documentation for a large-scale (~350 km long x ~720 km wide), systematic compositional variation of the upper mantle source of EPR MORB. Such a scale of variation is larger and longer than the size and <1 myr life span of the majority of non-transform offsets, which are supposed to be responsible for the along-axis compositional variations of EPR MORB according to the "bottoms up" model of magmatic segmentation.

Noble gas composition of Indian carbonatites (Amba Dongar, Siriwasan): Implications on mantle source compositions and late-stage hydrothermal processes

NASA Astrophysics Data System (ADS)

Hopp, Jens; Viladkar, Shrinivas G.

2018-06-01

Within a stepwise crushing study we determined the noble gas composition of several calcite separates, one aegirine and one pyrochlore-aegirine separate of the carbonatite ring dyke complex of Amba Dongar and carbonatite sill complex of Siriwasan, India. Both carbonatites are related to the waning stages of volcanic activity of the Deccan Igneous Province ca. 65 Ma ago. Major observations are a clear radiogenic 4He* and nucleogenic 21Ne* imprint related to in situ production from U and Th in mineral impurities, most likely minute apatite grains, or late incorporation of crustal fluids. However, in first crushing steps of most calcites from Amba Dongar a well-resolvable mantle neon signal is observed, with lowest air-corrected mantle 21Ne/22Ne-compositions equivalent to the Réunion hotspot mantle source. In case of the aegirine separate from Siriwasan we found a neon composition similar to the Loihi hotspot mantle source. This transition from a mantle plume signal in first crushing step to a more nucleogenic signature with progressive crushing indicates the presence of an external (crustal) or in situ nucleogenic component unrelated and superposed to the initial mantle neon component whose composition is best approximated by results of first crushing step(s). This contradicts previous models of a lithospheric mantle source of the carbonatitic magmas from Amba Dongar containing recycled crustal components which base on nucleogenic neon compositions. Instead, the mantle source of both investigated carbonatite complexes is related to a primitive mantle plume source that we tentatively ascribe to the postulated Deccan mantle plume. If, as is commonly suggested, the present location of the Deccan mantle plume source is below Réunion Island, the currently observed more nucleogenic neon isotopic composition of the Réunion hotspot might be obliterated by significant upper mantle contributions. In addition, compared with other carbonatite complexes worldwide a rather significant contribution of atmospheric noble gases is observed. This is documented in cut-off 20Ne/22Ne-ratios of ca. 10.2 (Amba Dongar) and 10.45 (Siriwasan) and cut-off 40Ar/36Ar-ratios of about 1500. This atmospheric component had been added at shallow levels during the emplacement process or later during hydrothermal alteration. However, understanding the late-stage interaction between atmospheric gases and magmatic mantle fluids still requires further investigation.

Diamond and moissanite in ophiolitic mantle rocks and podiform chromitites: A deep carbon source?

NASA Astrophysics Data System (ADS)

Yang, J.; Xu, X.; Wiedenbeck, M.; Trumbull, R. B.; Robinson, P. T.

2010-12-01

Diamonds are known from a variety of occurreces, mainly from mantle-derived kimberlites, meteorite impact craters, and continental deep subduction and collision zones. Recently, an unusual mineral group was discovered in the Luobusa ophiolitic chromitites from the Yarlung Zangbu suture, Tibet, which probably originated from a depth of over 300 km in the mantle. Minerals of deep origin include coesite apparently pseudomorphing stishovite, and diamond as individual grains or inclusions in OsIr alloy. To determine if such UHP and unusual minerals occur elsewhere, we collected about 1.5 t of chromitite from two orebodies in an ultramafic body in the Polar Urals. Thus far, more than 60 different mineral species have been separated from these ores. The most exciting discovery is the common occurrence of diamond, a typical UHP mineral in the Luobusa chromitites. These minerals are very similar in composition and structure to those reported from the Luobusa chromitites. So far diamond and/or moissanite have been discovered from many different ophiolitic ultramafic rocks, including in-situ grains in polished chromitite fragments. These discoveries demonstrate that the Luobusa ophiolite is not a unique diamond-bearing massif. Secondary ion mass spectrometric (SIMS) analysis shows that the ophiolite-hosted diamond has a distinctive 13C-depleted isotopic composition (δ13C from -18 to -28‰, n=70), compatible to the ophiolite-hosted moissanite (δ13C from -18 to -35‰, n=36), both are much lighter than the main carbon reservoir in the upper mantle (δ13C near -5‰). The compiled data from moissanite from kimberlites and other mantle settings share the characteristic of strongly 13C-depleted isotopic composition. This suggests that diamond and moissanite originates from a separate carbon reservoir in the mantle or that its formation involved strong isotopic fractionation. Subduction of biogenic carbonaceous material could potentially satisfy both the unusual isotopic and redox constraints on diamond and moissanite formation, but this material would need to stay chemically isolated from the upper mantle until it reached the high-T stability field of diamond and moissanite. The origin of diamond and moissanite in the mantle is still unsolved, but all evidence from the upper mantle indicates that they cannot have formed there, except under special and local redox conditions. We suggest, alternatively, that diamond and moissanite may have formed in the lower mantle, where the existence of 13C-depleted carbon is strongly suspected.

Coupling geodynamic with thermodynamic modelling for reconstructions of magmatic systems

NASA Astrophysics Data System (ADS)

Rummel, Lisa; Kaus, Boris J. P.; White, Richard

2016-04-01

Coupling geodynamic with petrological models is fundamental for understanding magmatic systems from the melting source in the mantle to the point of magma crystallisation in the upper crust. Most geodynamic codes use very simplified petrological models consisting of a single, fixed, chemistry. Here, we develop a method to better track the petrological evolution of the source rock and corresponding volcanic and plutonic rocks by combining a geodynamic code with a thermodynamic model for magma generation and evolution. For the geodynamic modelling a finite element code (MVEP2) solves the conservation of mass, momentum and energy equations. The thermodynamic modelling of phase equilibria in magmatic systems is performed with pMELTS for mantle-like bulk compositions. The thermodynamic dependent properties calculated by pMELTS are density, melt fraction and the composition of the liquid and solid phase in the chemical system: SiO2-TiO2-Al2O3-Fe2O3-Cr2O3-FeO-MgO-CaO-Na2O-K2O-P2O5-H2O. In order to take into account the chemical depletion of the source rock with increasing melt extraction events, calculation of phase diagrams is performed in two steps: 1) With an initial rock composition density, melt fraction as well as liquid and solid composition are computed over the full upper mantle P-T range. 2) Once the residual rock composition (equivalent to the solid composition after melt extraction) is significantly different from the initial rock composition and the melt fraction is lower than a critical value, the residual composition is used for next calculations with pMELTS. The implementation of several melt extraction events take the change in chemistry into account until the solidus is shifted to such high temperatures that the rock cannot be molten anymore under upper mantle conditions. An advantage of this approach is that we can track the change of melt chemistry with time, which can be compared with natural constraints. In the thermo-mechanical code the thermodynamic dependent properties from pre-computed phase diagrams are carried by each particle using marker-in-cell method . Thus the physical and chemical properties can change locally as a function of previous melt extraction events, pressure and temperature conditions. After each melt extraction event, the residual rock composition is compared with the bulk composition of previous computed phase diagrams, so that the used phase diagram is replaced by the phase diagram with the closest bulk chemistry. In the thermo-mechanical code, the melt is extracted directly to the surface as volcanites and within the crust as plutonites. The density of the crust and new generated crust is calculated with the thermodynamic modelling tool Perple_X. We have investigated the influence of several input parameters on the magma composition to compare it with real rock samples from Eifel (West-Germany). In order to take the very inhomogeneous chemistry of European mantle into account, we include not only primitive mantle but also metasomatised mantle fragments in the melting source of a plume (Eifel plume).

Xenolith constraints on seismic velocities in the upper mantle beneath southern Africa

NASA Astrophysics Data System (ADS)

James, D. E.; Boyd, F. R.; Schutt, D.; Bell, D. R.; Carlson, R. W.

2004-01-01

We impose geologic constraints on seismic three-dimensional (3-D) images of the upper mantle beneath southern Africa by calculating seismic velocities and rock densities from approximately 120 geothermobarometrically calibrated mantle xenoliths from the Archean Kaapvaal craton and adjacent Proterozoic mobile belts. Velocity and density estimates are based on the elastic and thermal moduli of constituent minerals under equilibrium P-T conditions at the mantle source. The largest sources of error in the velocity estimates derive from inaccurate thermo-barometry and, to a lesser extent, from uncertainties in the elastic constants of the constituent minerals. Results are consistent with tomographic evidence that cratonic mantle is higher in velocity by 0.5-1.5% and lower in density by about 1% relative to off-craton Proterozoic samples at comparable depths. Seismic velocity variations between cratonic and noncratonic xenoliths are controlled dominantly by differences in calculated temperatures, with compositional effects secondary. Different temperature profiles between cratonic and noncratonic regions have a relatively minor influence on density, where composition remains the dominant control. Low-T cratonic xenoliths exhibit a positive velocity-depth curve, rising from about 8.13 km/s at uppermost mantle depths to about 8.25 km/s at 180-km depth. S velocities decrease slightly over the same depth interval, from about 4.7 km/s in the uppermost mantle to 4.65 km/s at 180-km depth. P and S velocities for high-T lherzolites are highly scattered, ranging from highs close to those of the low-T xenoliths to lows of 8.05 km/s and 4.5 km/s at depths in excess of 200 km. These low velocities, while not asthenospheric, are inconsistent with seismic tomographic images that indicate high velocity root material extending to depths of at least 250 km. One plausible explanation is that high temperatures determined for the high-T xenoliths are a nonequilibrium consequence of relatively recent thermal perturbation and compositional modification associated with emplacement of kimberlitic fluids into the deep tectospheric root. Seismic velocities and densities for cratonic xenoliths differ significantly from those predicted for both primitive mantle peridotite and mantle eclogite. A model primitive mantle under cratonic P-T conditions exhibits velocities about 1% lower for P and about 1.5% lower for S, a consequence of a more fertile composition and different modal composition. Primitive mantle is also about 2% more dense at 150-km depth than low-T garnet lherzolite at cratonic P-T conditions. Similar calculations based on an oceanic geotherm are consistent with the isopycnic hypothesis of comparable density columns beneath oceanic and cratonic regions. Calculations for a hypothetical "cratonic" eclogite (50:50 garnet/omphacite) with an assumed cratonic geotherm produce extremely high VP and VS (8.68 km/s and 4.84 km/s, respectively, at 150 km depth) as well as high density (˜3.54 gm/cc). The very high velocity of eclogite should render it seismically conspicuous in the cratonic mantle if present as large volume blocks or slabs. We discuss how the seismic velocity data we have compiled in this paper from both xenoliths and generic petrologic models of the upper mantle differ from commonly used standard earth models IASPEI and PREM.

An internally consistent pressure calibration of geobarometers applicable to the Earth’s upper mantle using in situ XRD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Beyer, Christopher; Rosenthal, Anja; Myhill, Robert

We have performed an experimental cross calibration of a suite of mineral equilibria within mantle rock bulk compositions that are commonly used in geobarometry to determine the equilibration depths of upper mantle assemblages. Multiple barometers were compared simultaneously in experimental runs, where the pressure was determined using in-situ measurements of the unit cell volumes of MgO, NaCl, Re and h-BN between 3.6 and 10.4 GPa, and 1250 and 1500 °C. The experiments were performed in a large volume press (LVPs) in combination with synchrotron X-ray diffraction. Noble metal capsules drilled with multiple sample chambers were loaded with a range ofmore » bulk compositions representative of peridotite, eclogite and pyroxenite lithologies. By this approach, we simultaneously calibrated the geobarometers applicable to different mantle lithologies under identical and well determined pressure and temperature conditions. We identified discrepancies between the calculated and experimental pressures for which we propose simple linear or constant correction factors to some of the previously published barometric equations. As a result, we establish internally-consistent cross-calibrations for a number of garnet-orthopyroxene, garnet-clinopyroxene, Ca-Tschermaks-in-clinopyroxene and majorite geobarometers.« less

Geothermal Heat Flux and Upper Mantle Viscosity across West Antarctica: Insights from the UKANET and POLENET Seismic Networks

NASA Astrophysics Data System (ADS)

O'Donnell, J. P.; Dunham, C.; Stuart, G. W.; Brisbourne, A.; Nield, G. A.; Whitehouse, P. L.; Hooper, A. J.; Nyblade, A.; Wiens, D.; Aster, R. C.; Anandakrishnan, S.; Huerta, A. D.; Wilson, T. J.; Winberry, J. P.

2017-12-01

Quantifying the geothermal heat flux at the base of ice sheets is necessary to understand their dynamics and evolution. The heat flux is a composite function of concentration of upper crustal radiogenic elements and flow of heat from the mantle into the crust. Radiogenic element concentration varies with tectonothermal age, while heat flow across the crust-mantle boundary depends on crustal and lithospheric thicknesses. Meanwhile, accurately monitoring current ice mass loss via satellite gravimetry or altimetry hinges on knowing the upper mantle viscosity structure needed to account for the superimposed glacial isostatic adjustment (GIA) signal in the satellite data. In early 2016 the UK Antarctic Network (UKANET) of 10 broadband seismometers was deployed for two years across the southern Antarctic Peninsula and Ellsworth Land. Using UKANET data in conjunction with seismic records from our partner US Polar Earth Observing Network (POLENET) and the Antarctic Seismographic Argentinian Italian Network (ASAIN), we have developed a 3D shear wave velocity model of the West Antarctic crust and uppermost mantle based on Rayleigh and Love wave phase velocity dispersion curves extracted from ambient noise cross-correlograms. We combine seismic receiver functions with the shear wave model to help constrain the depth to the crust-mantle boundary across West Antarctica and delineate tectonic domains. The shear wave model is subsequently converted to temperature using a database of densities and elastic properties of minerals common in crustal and mantle rocks, while the various tectonic domains are assigned upper crustal radiogenic element concentrations based on their inferred tectonothermal ages. We combine this information to map the basal geothermal heat flux variation across West Antarctica. Mantle viscosity depends on factors including temperature, grain size, the hydrogen content of olivine and the presence of melt. Using published mantle xenolith and magnetotelluric data to constrain grain size and hydrogen content, respectively, we use the temperature model to estimate the regional upper mantle viscosity structure. The viscosity information will be incorporated in a 3D GIA model that will better constrain estimates of current ice loss from the West Antarctic Ice Sheet.

Geophysical and petrological modelling of the structure and composition of the crust and upper mantle in complex geodynamic settings: The Tyrrhenian Sea and surroundings

NASA Astrophysics Data System (ADS)

Panza, G. F.; Peccerillo, A.; Aoudia, A.; Farina, B.

2007-01-01

Information on the physical and chemical properties of the lithosphere-asthenosphere system (LAS) can be obtained by geophysical investigation and by studies of petrology-geochemistry of magmatic rocks and entrained xenoliths. Integration of petrological and geophysical studies is particularly useful in geodynamically complex areas characterised by abundant and compositionally variable young magmatism, such as in the Tyrrhenian Sea and surroundings. A thin crust, less than 10 km, overlying a soft mantle (where partial melting can reach about 10%) is observed for Magnaghi, Vavilov and Marsili, which belong to the Central Tyrrhenian Sea backarc volcanism where subalkaline rocks dominate. Similar characteristics are seen for the uppermost crust of Ischia. A crust about 20 km thick is observed for the majority of the continental volcanoes, including Amiata-Vulsini, Roccamonfina, Phlegraean Fields-Vesuvius, Vulture, Stromboli, Vulcano-Lipari, Etna and Ustica. A thicker crust is present at Albani - about 25 km - and at Cimino-Vico-Sabatini — about 30 km. The structure of the upper mantle, in contrast, shows striking differences among various volcanic provinces. Volcanoes of the Roman region (Vulsini-Sabatini-Alban Hills) sit over an upper mantle characterised by Vs mostly ranging from about 4.2 to 4.4 km/s. At the Alban Hills, however, slightly lower Vs values of about 4.1 km/s are detected between 60 and 120 km of depth. This parallels the similar and rather homogeneous compositional features of the Roman volcanoes, whereas the lower Vs values detected at the Alban Hills may reflect the occurrence of small amounts of melts within the mantle, in agreement with the younger age of this volcano. The axial zone of the Apennines, where ultrapotassic kamafugitic volcanoes are present, has a mantle structure with high-velocity lid ( Vs ˜ 4.5 km/s) occurring at the base of a 40-km-thick crust. Beneath the Campanian volcanoes of Vesuvius and Phlegraean Fields, the mantle structure shows a rigid body dipping westward, a feature that continues southward, up to the eastern Aeolian arc. In contrast, at Ischia the upper mantle contains a shallow low-velocity layer ( Vs = 3.5-4.0 km/s) just beneath a thin but complex crust. The western Aeolian arc and Ustica sit over an upper mantle with Vs ˜ 4.2-4.4 km/s, although a rigid layer ( Vs = 4.55 km/s) from about 80 to 150 km occurs beneath the western Aeolian arc. In Sardinia, no significant differences in the LAS structure are detected from north to south. The petrological-geochemical signatures of Italian volcanoes show strong variations that allow us to distinguish several magmatic provinces. These often coincide with mantle sectors identified by Vs tomography. For instance, the Roman volcanoes show remarkable similar petrological and geochemical characteristics, mirroring similar structure of the LAS. The structure and geochemical-isotopic composition of the upper mantle change significantly when we move to the Stromboli-Campanian volcanoes. The geochemical signatures of Ischia and Procida volcanoes are similar to other Campanian centres, but Sr-Pb isotopic ratios are lower marking a transition to the backarc mantle of the Central Tyrrhenian Sea. The structural variations from Stromboli to the central (Vulcano and Lipari) and western Aeolian arc are accompanied by strong variations of geochemical signatures, such as a decrease of Sr-isotope ratios and an increase of Nd-, Pb-isotope and LILE/HFSE ratios. The dominance of mafic subalkaline magmatism in the Tyrrhenian Sea basin denotes large degrees of partial melting, well in agreement with the soft characteristics of the uppermost mantle in this area. In contrast, striking isotopic differences of Plio-Quaternary volcanic rocks from southern to northern Sardinia does not find a match in the LAS geophysical characteristics. The combination of petrological and geophysical constraints allows us to propose a 3D schematic geodynamic model of the Tyrrhenian basin and bordering volcanic areas, including the subduction of the Ionian-Adria lithosphere in the southern Tyrrhenian Sea, and to place constraints on the geodynamic evolution of the whole region.

Iron-titanium oxyhydroxides which transport water into the deep upper mantle and mantle transition zone

NASA Astrophysics Data System (ADS)

Matsukage, K. N.; Nishihara, Y.

2015-12-01

We experimentally discovered a new hydrous phase in the system FeOOH-TiO2 at pressures of 10-16 GPa and temperatures of 1000-1600°C which corresponds to conditions of the deep upper mantle and the Earth's mantle transition zone. Seven different compositions in the FeOOH-TiO2 system having molar ratios of x = Ti/(Fe + Ti) = 0, 0.125, 0.25, 0.375, 0.5, 0.75 that were prepared by mixing reagent grade a-FeOOH (goethite) and TiO2 (anatase) powders were used as starting materials. High-pressure and high-temperature experiments were carried out using Kawai-type multi-anvil apparatus (Orange-1000 at Ehime University and SPI-1000 at Tokyo Institute of Technology). In this system, we identified two stable iron-titanium oxyhydroxide phases whose estimated composition is expressed by (FeH)1 - xTixO2 . One is the Fe-rich solid solution (x < 0.23) with e-FeOOH type crystal structure (e-phase, orthorhombic, P21nm) that was described by the previous studies (e.g., Suzuki 2010), and the other is the more Ti-rich solid solution (x > 0.35) with a-PbO2 type structure (a-phase, orthorhombic, Pbcn). The a-phase is stable up to 1500ºC for a composition of x = 0.5 and at least to 1600ºC for x = 0.75. Our result means that this phase is stable at average mantle temperature in the Earth's mantle transition zone. The Iron-titanium-rich hydrous phases was possible to stable in basalt + H2O system (e.g., Hashimoto and Matsukage 2013). Therefore our findings suggest that water transport in the Earth's deep interior is probably much more efficient than had been previously thought.

Magnesium Isotopic Compositions of Continental Basalts From Various Tectonic Settings

NASA Astrophysics Data System (ADS)

Yang, W.; Li, S.; Tian, H.; Ke, S.

2016-12-01

Recycled sedimentary carbonate through subduction is the main light Mg isotopic reservoir in Earth's deep interior, thus Mg isotopic variation of mantle-derived melts provides a fresh perspective on investigating deep carbon cycling. Here we investigate Mg isotopic compositions of continental basalts from various tectonic settings: (1) The Cenozoic basalts from eastern China, coinciding with the stagnant Pacific slab in the mantle transition zone revealed by seismic tomography; (2) The Cenozoic basalts from Tengchong area, southwestern China, which comprises a crucial part of the collision zone between the Indian and Eurasian plates; (3) The Permian basalts from Emeishan large igneous province, related to a mantle plume. The Cenozoic basalts from both eastern China and Tengchong area exhibit light Mg isotopic compositions (δ26Mg = -0.60 to -0.30‰ and -0.51 to -0.33‰), suggesting recycled sedimentary carbonates in their mantle sources. This is supported by their low Fe/Mn, high CaO/Al2O3, low Hf/Hf* and low Ti/Ti* ratios, which are typical features of carbonated peridotite-derived melt. The Tengchong basalts also show high 87Sr/86Sr, high radiogenic Pb and upper crustal-like trace element pattern, indicating contribution of recycled continental crustal materials. By contrast, all Emeishan basalts display a mantle-like Mg isotopic composition, with δ26Mg ranging from -0.35 to -0.19‰. Since the Emeishan basalts derived from a mantle plume, their mantle-like Mg isotopic composition may indicate limited sedimentary carbonated recycled into the lower mantle. This is consistent with a recent experimental study which concluded that direct recycling of carbon into the lower mantle may have been highly restricted throughout most of the Earth's history.

Markov Chain Monte Carlo Inversion of Mantle Temperature and Composition, with Application to Iceland

NASA Astrophysics Data System (ADS)

Brown, Eric; Petersen, Kenni; Lesher, Charles

2017-04-01

Basalts are formed by adiabatic decompression melting of the asthenosphere, and thus provide records of the thermal, chemical and dynamical state of the upper mantle. However, uniquely constraining the importance of these factors through the lens of melting is challenging given the inevitability that primary basalts are the product of variable mixing of melts derived from distinct lithologies having different melting behaviors (e.g. peridotite vs. pyroxenite). Forward mantle melting models, such as REEBOX PRO [1], are useful tools in this regard, because they can account for differences in melting behavior and melt pooling processes, and provide estimates of bulk crust composition and volume that can be compared with geochemical and geophysical constraints, respectively. Nevertheless, these models require critical assumptions regarding mantle temperature, and lithologic abundance(s)/composition(s), all of which are poorly constrained. To provide better constraints on these parameters and their uncertainties, we have coupled a Markov Chain Monte Carlo (MCMC) sampling technique with the REEBOX PRO melting model. The MCMC method systematically samples distributions of key REEBOX PRO input parameters (mantle potential temperature, and initial abundances and compositions of the source lithologies) based on a likelihood function that describes the 'fit' of the model outputs (bulk crust composition and volume and end-member peridotite and pyroxenite melts) relative to geochemical and geophysical constraints and their associated uncertainties. As a case study, we have tested and applied the model to magmatism along Reykjanes Peninsula in Iceland, where pyroxenite has been inferred to be present in the mantle source. This locale is ideal because there exist sufficient geochemical and geophysical data to estimate bulk crust compositions and volumes, as well as the range of near-parental melts derived from the mantle. We find that for the case of passive upwelling, the models that best fit the geochemical and geophysical observables require elevated mantle potential temperatures ( 120 °C above ambient mantle), and 5% pyroxenite. The modeled peridotite source has a trace element composition similar to depleted MORB mantle, whereas the trace element composition of the pyroxenite is similar to enriched mid-ocean ridge basalt. These results highlight the promise of this method for efficiently exploring the range of mantle temperatures, lithologic abundances, and mantle source compositions that are most consistent with available observational constraints in individual volcanic systems. 1 Brown and Lesher (2016), G-cubed, 17, 3929-3968

Geochemistry of ultramafic xenoliths from Kapfenstein, Austria: evidence for a variety of upper mantle processes

NASA Astrophysics Data System (ADS)

Kurat, G.; Palme, H.; Spettel, B.; Baddenhausen, Hildegard; Hofmeister, H.; Palme, Christl; Wänke, H.

1980-01-01

Major, minor, and trace element contents have been determined in seven ultramafic xenoliths, the host basanite, and some mineral separates from xenoliths from Kapfenstein, Austria. Most of the xenoliths represent residues after extraction of different amounts of basaltic liquid. Within the sequence Iherzolite to harzburgite contents of Al, Ca, Ti, Na, Sc, V, Cr and the HREE decrease systematically with increasing Mg/Fe and decreasing Yb/Sc. Although all samples are depleted in highly incompatible elements, the less depleted end of our suite very closely approaches the chondritic Yb/Sc ratio and consequently the primitive upper mantle composition. Chromium behaved as a non-refractory element. Consequently it should have higher abundances in basalts than observed, suggesting that most basalts experienced Cr fractionation by chromite separation during ascent. Several processes have been active in addition to partial melting within the upper mantle beneath Kapfenstein: (1) a hornblendite has been identified as wet alkali-basaltic mobilisate; (2) an amphibole Iherzolite is the product of alkali-basalt metasomatism of a common depleted Iherzolite; (3) two amphibole Iherzolites contain evidence for rather pure water metasomatism of normal depleted Iherzolites; (4) a garnet-spinel websterite was a tholeiitic liquid trapped within the upper mantle and which suffered a subsequent partial melting event (partial remobilization of a mobilisate). (5) Abundances of highly incompatible elements are generally very irregular, indicating contamination of upper mantle rocks by percolating liquids (in the mantle). Weathering is an important source of contamination: e.g. U mobilization by percolating groundwater. Contamination of the xenoliths by the host basanite liquid can only amount to approximately 5.5 × 10 -4 parts. Distributions of minor and trace elements between different minerals apparently reflect equilibrium and vary with equilibration temperature.

Recycling Seamounts: Implications for Mantle Source Heterogeneities

NASA Astrophysics Data System (ADS)

Madrigal, P.; Gazel, E.

2016-12-01

Isolated seamounts formed away from plate boundaries and/or known hotspot tracks are widely distributed in the Earth's oceanic plates. Despite their pervasiveness, the origin and composition of the magmatic sources that create these seamounts are still unknown. Moreover, as the seamount provinces travel along with the oceanic plate towards subduction trenches these volcanic edifices become subducted materials that are later recycled into the mantle. Using radiogenic isotopes (Sr-Nd-Pb) from present-day non-plume ocean island basalts (OIB) sampled by drilling and dredging as well as by normal processes of accretion to subduction margins, we modeled the isotopic evolution of these enriched reservoirs to assess their role as discrete components contributing to upper mantle heterogeneity. Our evidence suggests that a highly enriched mantle reservoir can originate from OIB-type subducted material that gets incorporated and stirred throughout the upper mantle in a shorter time period ( 200 Ma-500 Ma) than other highly enriched components like ancient subducted oceanic crust (>1 Ga), thought to be the forming agent of the HIMU mantle reservoir endmember. Enriched signatures from intraplate volcanism can be described by mixing of a depleted component like DMM and an enriched reservoir like non-plume related seamounts. Our data suggests that the isotopic evolution in time of a seamount-province type of reservoir can acquire sufficiently enriched compositions to resemble some of the most enriched magmas on Earth. This "fast-forming" (between 200 and 500 Ma) enriched reservoir could also explain some of the enriched signatures commonly present in intraplate and EMORB magmas unrelated to deep mantle plume upwellings.

Calcium Isotopic Compositions of Forearc Sediments from DSDP Site 144

NASA Astrophysics Data System (ADS)

Zhang, Z.; Zhu, H.; Nan, X.; Li, X.; Huang, F.

2016-12-01

It is important to investigate calcium isotopic compositions of reservoirs of the Earth for better application of Ca isotopes into studies of a variety of geochemical problems. Because Ca isotopic compositions for igneous rocks and carbonates are increasingly reported, this maybe bring new requirements on carefully understanding the isotopic compositions of subducted marine sediments. Marine sediments mainly contains carbonates and clays, controlling the compositions of slab-derived materials which are added to the mantle wedge. Obviously, it could have different elemental and calcium isotopic compositions with marine carbonate. Thus, it could also put biases on calcium isotopic signatures of basalts resulted from recycling oceanic carbonate into the mantle. Here, we report calcium isotopic compositions of 17 sediment samples from Deep Sea Drilling Project (DSDP) site 144 (09°27.23' N, 54°20.52' W) which is located about 400 km north of Surinam on the northern flank of the Demerara Rise with a water depth of 2957 meters. These samples have CaO contents ranging from 14.56 wt.% to 41.46 wt.% with an average of 29.61 ± 18.21 (2SD), δ44/40Ca ranges from 0.19 to 0.58 (relative to SRM915a) with an average of 0.40 ± 0.22 (2SD). These carbonate-rich sediments can be used to represent an endmember with high CaO content and low δ44/40Ca, which could modify chemical composition of the upper mantle and subduction zone lavas if they are recycled to the convective mantle during subduction. The positive linear correlation between CaO and δ44/40Ca in the sediments cannot be explained by a simple mixing between marine carbonate and clay. Instead, δ44/40Ca of these samples roughly increase from the Upper Cretaceous to the Early Oligocene, which might reflect the evolution of calcium isotopic compositions of seawater through time.

Sr isotopic composition of Afar volcanics and its implication for mantle evolution

NASA Astrophysics Data System (ADS)

Barberi, F.; Civetta, L.; Varet, J.

1980-10-01

Investigations of Rb-Sr systematics of basalts from the Afar depression (Ethiopia) indicate the presence of a heterogeneous mantle source region. The Sr isotopic compositions of the basalts from the Afar axial and transverse ranges identify source regions which are enriched in LIL elements and radiogenic Sr (axial ranges) and others which are relatively depleted (transverse ranges). Sr isotopic composition of basalts from the Red Sea, Gulf of Aden and Gulf of Tadjoura, which range from 0.70300 to 0.70340 are also reported and compared with the more radiogenic Afar region, which is characterized by 87Sr/ 86Sr ranging from 0.70328 to 0.70410. Available geochemical and isotopic data suggest that a relation exists between magma composition and the advancement of the rifting process through progressive lithosphere attenuation leading to continental break-up. However, the petrogenetic process is not simple and probably implies a vertically zoned mantle beneath the Afar region. Sr isotopic evidence suggests that the vertically zoned mantle is more radiogenic and enriched in LIL elements in its upper part.

The role of thermal effect on mantle seismic anomalies from observations of GIA

NASA Astrophysics Data System (ADS)

Wu, P.; Wang, H. S.; Steffen, H.

2012-04-01

Recent advance in seismic tomography reveals the structure inside the mantle. An outstanding issue is the role of thermal versus non-thermal (e.g. compositional, partial melting) contribution to seismic velocity anomalies. Here we use observations of Glacial Isostatic Adjustment (GIA), e.g. global relative sea level data, GRACE observations (with recent hydrology contributions removed) and GPS crustal uplift rates in combination with 3D GIA models to address this issue. Both ICE-4G and ICE-5G models are tested, but ICE-4G gives much better overall fit to these observations. Also, several 1-D background viscosity profiles, with different viscosity contrast at 670 km depth have also been tested and the one that gives consistent results is model RF3 which has a moderate viscosity increase across 670 km. Lateral mantle viscosity variation is inferred from Ekstrom & Dziewonski's S20A seismic tomography model using a scaling law that includes both the effect of anharmonicity and anelasticity. Thermal contribution to seismic tomography appears as the beta factor in the scaling law. The values of beta in the upper mantle, shallow part of the lower mantle and the deep part of the lower mantle are allowed to be different and the solution space of the beta values is searched to find the best combination that gives the best fit to the GIA observations simultaneously. The result of our best model (RF3 with lateral heterogeneity) shows that thermal effect increases from about 65% in the upper mantle to 80% in the shallow part of the lower mantle and to about 100% in the deep lower mantle above the D" layer. This is consistent with temperature excess in the lower mantle from high core heating. However, the uncertainty increases from < 1% in the upper mantle to 20% in the shallow lower mantle and is not very well constrained in the deep lower mantle.

Symplectite in spinel lherzolite xenoliths from the Little Hungarian Plain, Western Hungary: A key for understanding the complex history of the upper mantle of the Pannonian Basin

NASA Astrophysics Data System (ADS)

Falus, György; Szabó, Csaba; Kovács, István; Zajacz, Zoltán; Halter, Werner

2007-03-01

Two spinel lherzolite xenoliths from Hungary that contain pyroxene-spinel symplectites have been studied using EPMA, Laser ablation ICP-MS and universal stage. Based on their geochemical and structural characteristics, the xenoliths represent two different domains of the shallow subcontinental lithospheric mantle beneath the Pannonian Basin. The occurrence of symplectites is attributed to the former presence and subsequent breakdown of garnets due to significant pressure decrease related to lithospheric thinning. This implies that both mantle domains were once part of the garnet lherzolitic upper mantle and had a similar history during the major extension that formed the Pannonian Basin. Garnet breakdown resulted in distinct geochemical characteristics in the adjacent clinopyroxene crystals in both xenoliths. This is manifested by enrichment in HREE, Y, Zr and Hf towards the clinopyroxene porphyroclast rims and also in the neoblasts with respect to porphyroclast core compositions. This geochemical feature, together with the development and preservation of the texturally very sensitive symplectites, enables us to determine the relative timing of mantle processes. Our results indicate that garnets had been metastable in the spinel lherzolite environment and their breakdown to pyroxene and spinel is one of the latest processes that took place within the upper mantle before the xenoliths were brought to the surface.

Low-3He/4He sublithospheric mantle source for the most magnesian magmas of the Karoo large igneous province

NASA Astrophysics Data System (ADS)

Heinonen, Jussi S.; Kurz, Mark D.

2015-09-01

The massive outpourings of Karoo and Ferrar continental flood basalts (CFBs) ∼180 Ma ago mark the initial Jurassic rifting stages of the Gondwana supercontinent. The origin and sources of these eruptions have been debated for decades, largely due to difficulties in defining their parental melt and mantle source characteristics. Recent findings of Fe- and Mg-rich dikes (depleted ferropicrite suite) from Vestfjella, western Dronning Maud Land, Antarctica, have shed light on the composition of the deep sub-Gondwanan mantle: these magmas have been connected to upper mantle sources presently sampled by the Southwest Indian Ocean mid-ocean ridge basalts (SWIR MORBs) or to high 3He/4He plume-entrained non-chondritic primitive mantle sources formed early in Earth's history. In an attempt to determine their He isotopic composition and relative contributions from magmatic, cosmogenic, and radiogenic He sources, we performed in-vacuo stepwise crushing and melting analyses of olivine mineral separates, some of which were abraded to remove the outer layer of the grains. The best estimate for the mantle isotopic composition is given by a sample with the highest amount of He released (>50%) during the first crushing step of an abraded coarse fraction. It has a 3He/4He of 7.03 ± 0.23 (2σ) times the atmospheric ratio (Ra), which is indistinguishable from those measured from SWIR MORBs (6.3-7.3 Ra; source 3He/4He ∼6.4-7.6 Ra at 180 Ma) and notably lower than in the most primitive lavas from the North Atlantic Igneous Province (up to 50 Ra), considered to represent the epitome magmas from non-chondritic primitive mantle sources. Previously published trace element and isotopic (Sr, Nd, and Pb) compositions do not suggest a direct genetic link to any modern hotspot of Indian or southern Atlantic Oceans. Although influence of a mantle plume cannot be ruled out, the high magma temperatures and SWIR MORB-like geochemistry of the suite are best explained by supercontinent insulation of a precursory Indian Ocean upper mantle source. Such a model is also supported by the majority of the recent studies on the structure, geochronology, and petrology of the Karoo CFBs.

Cratonic roots under North America are shifted by basal drag: new evidence from gravity and geodynamic modeling

NASA Astrophysics Data System (ADS)

Kaban, M. K.; Petrunin, A.; Mooney, W. D.

2013-12-01

The impact of basal drag on the long-lived cratonic roots has been debated since the discovering of plate tectonics. Previously, evidence for a shifted mantle structure under North America was postulated from a comparison of the surface expression of the Great Meteor hotspot track versus its location at 200 km depth as inferred from seismic tomography (Eaton and Frederiksen, 2007). We present new results that are based on the integrative modeling of gravity and seismic data. The starting point is the residual gravity anomaly and residual topography, which are computed by removing of the crustal effect and of the effect of temperature variations in the upper mantle from the observed fields (Mooney and Kaban, 2010). After the temperature correction both residual fields chiefly reflect compositional density heterogeneity of the upper mantle. The residual gravity and topography are jointly inverted to determine the 3D density structure of the upper mantle. The inversion technique accounts for the fact that although these parameters are controlled by the same factors, the effect depends on depth and wavelength. Therefore, we can resolve the vertical distribution of density more reliable than by interpreting only one parameter. We found a strong negative anomaly under the North American craton, as expected for a depleted mantle. However, starting from a depth of about 200 km the depleted root is shifted west-southwest. The maximal shift reaches about 1000 km at a depth of 300 km. The direction agrees with the North American plate movement and with the anisotropy pattern in the upper mantle (e.g. Bokelmann, 2002). The results of the gravity modeling are confirmed by geodynamic modeling. The mantle flow is estimated from the density and temperature distribution derived from seismic tomography models. A 3D viscosity model is supplemented with weak boundaries based on an integrated model of plate boundary deformations. The calculated plate velocities are in a good agreement with the GPS-based models. We found a vertical gradient of the horizontal mantle flow velocity under the North American craton that relates to shear stresses deforming the cratonic root. The lateral velocity within the lowermost part of the lithosphere is about 2 mm/y faster than the overlying plate velocity. If we extrapolate this value to the past, the observed shift of the cratonic root could be achieved in about 500 Ma. Bokelmann GHR, (2002) Convection-driven motion of the North American craton: Evidence from P-wave anisotropy, Geoph. J. Int., 148, 278-287. Eaton DW and Frederiksen A, (2007) Seismic evidence for convection-driven motion of the North American plate, Nature 446, 428-431. Mooney WD, Kaban, MK., (2010). The North American Upper Mantle: Density, Composition, and Evolution, J. Geophys. Res., 115, B12424.

Mantle melting and melt refertilization beneath the Southwest Indian Ridge: Mineral composition of abyssal peridotites

NASA Astrophysics Data System (ADS)

Chen, Ling; Zhu, Jihao; Chu, Fengyou; Dong, Yan-hui; Liu, Jiqiang; Li, Zhenggang; Zhu, Zhimin; Tang, Limei

2017-04-01

As one of the slowest spreading ridges of the global ocean ridge system, the Southwest Indian Ridge (SWIR) is characterized by discontinued magmatism. The 53°E segment between the Gallieni fracture zone (FZ) (52°20'E) and the Gazelle FZ (53°30'E) is a typical amagmatic segment (crustal thickness <2km) (Zhou and Dick, 2013) that opens a window to the mantle thus provides a chance to detect the mantle composition directly. We examine the mineral compositions of 17 peridotite samples from the 53°E amagmatic segment. The results show that the peridotites can be divided into two groups. The Group 1 peridotites are characterized by clinopyroxenes having LREE depleted patterns that is typical for the abyssal peridotite, thus are thought to be the residue of the mantle melting. The Group 2 peridotites show the lowest HREE content within the SWIR peridotites but are anomaly enriched in LREE, with flat or U-type REE patterns, thus cannot be the pure residue of mantle melting. Mineral compositions of the Group 2 peridotites are more depleted than that of peridotites sampled near the Bouvet hot spot (Johnson et al., 1990), implying that the depleted mantle beneath the 53°E segment may be the residue of ancient melting event. This hypothesis is supported by the the low Ol/Opx ratios, coarse grain sizes (>1cm) Opx, and Mg-rich mineral compositions akin to harzburgite xenoliths that sample old continental lithospheric mantle (Kelemen et al., 1998). Melt refertilization model shows that Group 2 peridotites were affected by an enriched low-degree partial melt from the garnet stability field. These results indicate that depleted mantle which experiences ancient melting event are more sensitive to melt refertilization, thus may reduce the melt flux, leading to extremely thin crust at 53°E segment. This research was granted by the National Basic Research Programme of China (973 programme) (grant No. 2013CB429705) and the Fundamental Research Funds of Second Institute of Oceanography, State Oceanic Administration (JG1603, SZ1507). References: Johnson K T M, Dick H J B, Shimizu N. Melting in the oceanic upper mantle: An ion microprobe study of diopsides in abyssal peridotites[J]. Journal of Geophysical Research, 1990, 95(B3):2661-2678. Kelemen P B, Hart S R, Bernstein S. Silica enrichment in the continental upper mantle via melt/rock reaction[J]. Earth & Planetary Science Letters, 1998, 164(1-2):387-406. Zhou H, Dick H J. Thin crust as evidence for depleted mantle supporting the Marion Rise.[J]. Nature, 2013, 494(7436):195-200.

Relationship between the upper mantle high velocity seismic lid and the continental lithosphere

NASA Astrophysics Data System (ADS)

Priestley, Keith; Tilmann, Frederik

2009-04-01

The lithosphere-asthenosphere boundary corresponds to the base of the "rigid" plates - the depth at which heat transport changes from advection in the convecting deeper upper mantle to conduction in the shallow upper mantle. Although this boundary is a fundamental feature of the Earth, mapping it has been difficult because it does not correspond to a sharp change in temperature or composition. Various definitions of the lithosphere and asthenosphere are based on the analysis of different types of geophysical and geological observations. The depth to the lithosphere-asthenosphere boundary determined from these different observations often shows little agreement when they are applied to the same region because the geophysical and geological observations (i.e., seismic velocity, strain rate, electrical resistivity, chemical depletion, etc.) are proxies for the change in rheological properties rather than a direct measure of the rheological properties. In this paper, we focus on the seismic mapping of the upper mantle high velocity lid and low velocity zone and its relationship to the lithosphere and asthenosphere. We have two goals: (a) to examine the differences in how teleseismic body-wave travel-time tomography and surface-wave tomography image upper mantle seismic structure; and (b) to summarise how upper mantle seismic velocity structure can be related to the structure of the lithosphere and asthenosphere. Surface-wave tomography provides reasonably good depth resolution, especially when higher modes are included in the analysis, but lateral resolution is limited by the horizontal wavelength of the long-period surface waves used to constrain upper mantle velocity structure. Teleseismic body-wave tomography has poor depth resolution in the upper mantle, particularly when no strong lateral contrasts are present. If station terms are used, features with large lateral extent and gradual boundaries are attenuated in the tomographic image. Body-wave models are not useful in mapping the thickness of the high velocity upper mantle lid because this type of analysis often determines wave speed perturbations from an unknown horizontal average and not absolute velocities. Thus, any feature which extends laterally across the whole region beneath a seismic network becomes invisible in the teleseismic body-wave tomographic image. We compare surface-wave and body-wave tomographic results using southern Africa as an example. Surface-wave tomographic images for southern Africa show a strong, high velocity upper mantle lid confined to depths shallower than ~ 200 km, whereas body-wave tomographic images show weak high velocity in the upper mantle extending to depths of ~ 300 km or more. However, synthetic tests show that these results are not contradictory. The absolute seismic velocity structure of the upper mantle provided by surface wave analysis can be used to map the thermal lithosphere. Priestley and McKenzie (Priestley, K., McKenzie, D., 2006. The thermal structure of the lithosphere from shear wave velocities. Earth and Planetary Science Letters 244, 285-301.) derive an empirical relationship between shear wave velocity and temperature. This relationship is used to obtain temperature profiles from the surface-wave tomographic models of the continental mantle. The base of the lithosphere is shown by a change in the gradient of the temperature profiles indicative of the depth where the mode of heat transport changes from conduction to advection. Comparisons of the geotherms determined from the conversion of surface-wave wave speeds to temperatures with upper mantle nodule-derived geotherms demonstrate that estimates of lithospheric thickness from Vs and from the nodule mineralogy agree to within about 25 km. The lithospheric thickness map for Africa derived from the surface-wave tomographic results shows that thick lithosphere underlies most of the Archean crust in Africa. The distribution of diamondiferous kimberlites provides an independent estimate of where thick lithosphere exists. Diamondiferous kimberlites generally occur where the lower part of the thermal lithosphere as indicated by seismology is in the diamond stability field.

Nucleogenic production of Ne isotopes in Earth's crust and upper mantle induced by alpha particles from the decay of U and Th

NASA Astrophysics Data System (ADS)

Leya, Ingo; Wieler, Rainer

1999-07-01

The production of nucleogenic Ne in terrestrial crust and upper mantle by alpha particles from the decay of U and Th was calculated. The calculations are based on stopping powers for the chemical compounds and thin-target cross sections. This approach is more rigorous than earlier studies using thick-target yields for pure elements, since our results are independent of limiting assumptions about stopping-power ratios. Alpha induced reactions account for >99% of the Ne production in the crust and for most of the 20,21Ne in the upper mantle. On the other hand, our 22Ne value for the upper mantle is a lower limit because the reaction 25Mg(n,α)22Ne is significant in mantle material. Production rates calculated here for hypothetical crustal and upper mantle material with average major element composition and homogeneously distributed F, U, and Th are up to 100 times higher than data presented by Kyser and Rison [1982] but agree within error limits with the results by Yatsevich and Honda [1997]. Production of nucleogenic Ne in "mean" crust and mantle is also given as a function of the weight fractions of O and F. The alpha dose is calculated by radiogenic 4He as well as by the more retentive fissiogenic 136Xe. U and Th is concentrated in certain accessory minerals. Since the ranges of alpha particles from the three decay chains are comparable to mineral dimensions, most nucleogenic Ne is produced in U- and Th-rich minerals. Therefore nucleogenic Ne production in such accessories was also calculated. The calculated correlation between nucleogenic 21Ne and radiogenic 4He agrees well with experimental data for Earth's crust and accessories. Also, the calculated 22Ne/4He ratios as function of the F concentration and the dependence of 21Ne/22Ne from O/F for zircon and apatite agree with measurements.

Constraining the Composition of the Subcontinental Lithospheric Mantle Beneath the East African Rift: FTIR Analysis of Water in Spinel Peridotite Mantle Xenoliths

NASA Astrophysics Data System (ADS)

Erickson, S. G.; Nelson, W. R.; Peslier, A. H.; Snow, J. E.

2014-12-01

The East African Rift System was initiated by the impingement of the Afar mantle plume on the base of the non-cratonic continental lithosphere (assembled during the Pan-African Orogeny), producing over 300,000 km3 [1] of continental flood basalts ~30 Ma ago. The contribution of the subcontinental lithospheric mantle (SCLM) to this voluminous period of volcanism is implied based on basaltic geochemical and isotopic data. However, the role of percolating melts on the SCLM composition is less clear. Metasomatism is capable of hybridizing or overprinting the geochemical signature of the SCLM. In addition, models suggest that adding fluids to lithospheric mantle affects its stability [e.g. 2, 3]. We investigated the nature of the SCLM using Fourier transform infrared spectrometry (FTIR) to measure water content in mantle xenoliths entrained in young (1 Ma) basaltic lavas from the Ethiopian volcanic province. The mantle xenoliths consist dominantly of spinel lherzolites and are composed of nominally anhydrous minerals, which can contain trace water as H in mineral defects. Eleven mantle xenoliths come from the Injibara-Gojam region and two from the Mega-Sidamo region. Water abundances of olivines in six samples are 1-5ppm H2O while the rest are below the limit of detection (<0.5 ppm H2O); orthopyroxene and clinopyroxene contain 80-238 and 111-340 ppm wt H2O, respectively. Two xenoliths have higher water contents - a websterite (470 ppm) and dunite (229 ppm), consistent with involvement of ascending melts. The low water content of the upper SCLM beneath Ethiopia is as dry as the oceanic mantle [2] except for small domains represented by percolating melts. Consequently, rifting of the East African lithosphere may not have been facilitated by a hydrated upper mantle. [1] Hoffman et al., 1997 Nature 389, 838-841. [2] Peslier et al., 2010 Nature 467, 78-81. [3] Lee et al., 2011 AREPS 39, 59-90.

Geophysical and petrological modeling of the lower crust and uppermost mantle in the Variscan and Proterozoic surroundings of the Trans-European Suture Zone in Central Europe

NASA Astrophysics Data System (ADS)

Puziewicz, Jacek; Polkowski, Marcin; Grad, Marek

2017-04-01

High-quality seismic data on the lower crust and uppermost lithospheric mantle in the Central European part of the Trans European Suture Zone, together with thermal and gravimetric data, enables the quantitative modeling of the rocks occurring in those parts of the lithosphere, including their mineral compositions and the chemical composition of individual minerals. The P3 seismic profile is located at the SW margin of the East European Craton. The lower crust is dominated by gabbronoritic intrusions (plagioclase An45Ab55, clinopyroxene Di80Hed20, orthopyroxene En74Fs26), and the uppermost mantle is harzburgitic (olivine and orthopyroxene Mg# 0.91). The lower crust and upper mantle of the P1 seismic profile belong to the Trans European Suture Zone, albeit the upper crust is of Variscan affinity. The P1 lower crust has gabbronoritic composition which is layered from plagioclase-rich compositions on the top to the orthopyroxene-rich ones at the bottom (plagioclase An45Ab55, clinopyroxene Di80Hed20, orthopyroxene En85Fs15), and is lithologically different Proterozoic and Variscan surroundings. The 100 × 200 km eclogite slice (garnet Alm48Gr25Py27, clinopyroxene Di51Hed10Jd39), with a thickness of 5-10 km, occurs in the uppermost mantle sampled by the P1 profile. The Niedźwiedź Massif is located at the NE margin of the Bohemian Massif, which shows an exposed Variscan basement. The lower crust beneath the Niedźwiedź Massif consists of gabbroic rock of variable proportions of plagioclase (An45Ab55) and clinopyroxene (Di80Hed20), whereas the uppermost mantle is supposedly spinel harzburgite (olivine, ortho- and clinopyroxene Mg# 0.90). Our models show that the lowermost crust and uppermost mantle of the East European Craton do not continue to the SW into the Trans European Suture Zone in its Central European section in Poland.

Joint geophysical and petrological models for the lithosphere structure of the Antarctic Peninsula continental margin

NASA Astrophysics Data System (ADS)

Yegorova, Tamara; Bakhmutov, Vladimir; Janik, Tomasz; Grad, Marek

2011-01-01

The Antarctic Peninsula (AP) is a composite magmatic arc terrane formed at the Pacific margin of Gondwana. Through the late Mesozoic and Cenozoic subduction has stopped progressively from southwest to northeast as a result of a series of ridge trench collisions. Subduction may be active today in the northern part of the AP adjacent to the South Shetland Islands. The subduction system is confined by the Shackleton and Hero fracture zones. The magmatic arc of the AP continental margin is marked by high-amplitude gravity and magnetic anomaly belts reaching highest amplitudes in the region of the South Shetland Islands and trench. The sources for these anomalies are highly magnetic and dense batholiths of mafic bulk composition, which were intruded in the Cretaceous, due to partial melting of upper-mantle and lower-crustal rocks. 2-D gravity and magnetic models provide new insights into crustal and upper-mantle structure of the active and passive margin segments of the northern AP. Our models incorporate seismic refraction constraints and physical property data. This enables us to better constrain both Moho geometry and petrological interpretations in the crust and upper mantle. Model along the DSS-12 profile crosses the AP margin near the Anvers Island and shows typical features of a passive continental margin. The second model along the DSS-17 profile extends from the Drake Passage through the South Shetland Trench/Islands system and Bransfield Strait to the AP and indicates an active continental margin linked to slow subduction and on-going continental rifting in the backarc region. Continental rifting beneath the Bransfield Strait is associated with an upward of hot upper mantle rocks and with extensive magmatic underplating.

Controls on melting at spreading ridges from correlated abyssal peridotite - mid-ocean ridge basalt compositions

NASA Astrophysics Data System (ADS)

Regelous, Marcel; Weinzierl, Christoph G.; Haase, Karsten M.

2016-09-01

Variations in the volume and major element composition of basalt erupted along the global mid-ocean ridge system have been attributed to differences in mantle potential temperature, mantle composition, or plate spreading rate and lithosphere thickness. Abyssal peridotites, the residues of mantle melting beneath mid-ocean ridges, provide additional information on the melting process, which could be used to test these hypotheses. We compiled a global database of abyssal peridotite compositions averaged over the same ridge segments defined by Gale et al. (2013). In addition, we calculated the distance of each ridge segment to the nearest hotspots. We show that Cr# in spinel in abyssal peridotites is negatively correlated with Na90 in basalts from the same ridge segments on a global scale. Ridge segments that erupt basalts apparently produced by larger degrees of mantle melting are thus underlain by peridotites from which large amounts of melt have been extracted. We find that near-ridge hotspots have a more widespread influence on mid-ocean ridge basalt (MORB) composition and ridge depth than previously thought. However, when these hotspot-influenced ridge segments are excluded, the remaining segments show clear relationships between MORB composition, peridotite composition, and ridge depth with spreading rate. Very slow-spreading ridges (<20 mm/yr) are deeper, erupt basalts with higher Na90, Al90, K90/Ti90, and lower Fe90, Ca90/Al90, and expose peridotites with lower Cr# than intermediate and fast-spreading ridges. We show that away from hotspots, the spreading-rate dependence of the maximum degree of mantle melting inferred from Cr# in peridotites (FM) and the bulk degree of melting inferred from Na90 in basalts (FB) from the same ridge segments is unlikely to be due to variations in mantle composition. Nor can the effects of dynamic mantle upwelling or incomplete melt extraction at low spreading rates satisfactorily explain the observed compositions of abyssal peridotites and MORB from very slow-spreading ridges. Instead, the distinctive compositions of abyssal peridotites and MORB from very slow-spreading ridges could result from the presence of a thick lithospheric lid, leading to a lower average degree of melting, and a higher contribution to melting from more fertile mantle lithologies. Alternatively, spreading rate influences the thermal structure of the upper mantle such that the mantle beneath very slow-spreading ridges is cooler.

Isotopic and trace element constraints on the petrogenesis of lavas from the Mount Adams volcanic field, Washington

USGS Publications Warehouse

Jicha, B.R.; Hart, G.L.; Johnson, C.M.; Hildreth, Wes; Beard, B.L.; Shirey, S.B.; Valley, J.W.

2009-01-01

Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P)n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. ??18O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust. ?? Springer-Verlag 2008.

New Numerical Approaches for Modeling Thermochemical Convection in a Compositionally Stratified Fluid

NASA Astrophysics Data System (ADS)

Puckett, E. G.; Turcotte, D. L.; He, Y.; Lokavarapu, H. V.; Robey, J.; Kellogg, L. H.

2017-12-01

Geochemical observations of mantle-derived rocks favor a nearly homogeneous upper mantle, the source of mid-ocean ridge basalts (MORB), and heterogeneous lower mantle regions.Plumes that generate ocean island basalts are thought to sample the lower mantle regions and exhibit more heterogeneity than MORB.These regions have been associated with lower mantle structures known as large low shear velocity provinces below Africa and the South Pacific.The isolation of these regions is attributed to compositional differences and density stratification that, consequently, have been the subject of computational and laboratory modeling designed to determine the parameter regime in which layering is stable and understanding how layering evolves.Mathematical models of persistent compositional interfaces in the Earth's mantle may be inherently unstable, at least in some regions of the parameter space relevant to the mantle.Computing approximations to solutions of such problems presents severe challenges, even to state-of-the-art numerical methods.Some numerical algorithms for modeling the interface between distinct compositions smear the interface at the boundary between compositions, such as methods that add numerical diffusion or `artificial viscosity' in order to stabilize the algorithm. We present two new algorithms for maintaining high-resolution and sharp computational boundaries in computations of these types of problems: a discontinuous Galerkin method with a bound preserving limiter and a Volume-of-Fluid interface tracking algorithm.We compare these new methods with two approaches widely used for modeling the advection of two distinct thermally driven compositional fields in mantle convection computations: a high-order accurate finite element advection algorithm with entropy viscosity and a particle method.We compare the performance of these four algorithms on three problems, including computing an approximation to the solution of an initially compositionally stratified fluid at Ra = 105 with buoyancy numbers {B} that vary from no stratification at B = 0 to stratified flow at large B.

Origin of olivine at Copernicus

NASA Technical Reports Server (NTRS)

Pieters, C. M.; Wilhelms, D. E.

1985-01-01

The central peaks of Copernicus are among the few lunar areas where near-infrared telescopic reflectance spectra indicate extensive exposures of olivine. Other parts of Copernicus crater and ejecta, which were derived from highland units in the upper parts of the target site, contain only low-Ca pyroxene as a mafic mineral. The exposure of compositionally distinct layers including the presence of extensive olivine may result from penetration to an anomalously deep layer of the crust or to the lunar mantle. It is suggested that the Procellarum basin and the younger, superposed Insularum basin have provided access to these normally deep-seated crustal or mantle materials by thinning the upper crustal material early in lunar history. The occurrences of olivine in portions of the compositionally heterogeneous Aristarchus Region, in a related geologic setting, may be due to the same sequence of early events.

Enrichments of the mantle sources beneath the Southern Volcanic Zone (Andes) by fluids and melts derived from abraded upper continental crust

NASA Astrophysics Data System (ADS)

Holm, Paul Martin; Søager, Nina; Dyhr, Charlotte Thorup; Nielsen, Mia Rohde

2014-05-01

Mafic basaltic-andesitic volcanic rocks from the Andean Southern Volcanic Zone (SVZ) exhibit a northward increase in crustal components in primitive arc magmas from the Central through the Transitional and Northern SVZ segments. New elemental and Sr-Nd-high-precision Pb isotope data from the Quaternary arc volcanic centres of Maipo (NSVZ) and Infernillo and Laguna del Maule (TSVZ) are argued to reflect mainly their mantle source and its melting. For the C-T-NSVZ, we identify two types of source enrichment: one, represented by Antuco in CSVZ, but also present northward along the arc, was dominated by fluids which enriched a pre-metasomatic South Atlantic depleted MORB mantle type asthenosphere. The second enrichment was by melts having the characteristics of upper continental crust (UCC), distinctly different from Chile trench sediments. We suggest that granitic rocks entered the source mantle by means of subduction erosion in response to the northward increasingly strong coupling of the converging plates. Both types of enrichment had the same Pb isotope composition in the TSVZ with no significant component derived from the subducting oceanic crust. Pb-Sr-Nd isotopes indicate a major crustal compositional change at the southern end of the NSVZ. Modelling suggests addition of around 2 % UCC for Infernillo and 5 % for Maipo.

Constraints on Thermochemical Convection of the Mantle from Plume-related Observations

NASA Astrophysics Data System (ADS)

Zhong, S.

2005-05-01

Although geochemical observations have long suggested a layered mantle with more enriched mantle material in the bottom layer to provide a significant amount of heat to the top layer, the nature of such a layering remains unclear. An important observation that has been used to argue against the conventional layered mantle model (i.e., the layering at the 670 km depth) was the plume heat flux [Davies, 1999]. Plume heat flux is estimated as ~ 3.5 TW, or 10% of the surface heat flux [Davies, 1988; Sleep, 1990]. In this study, we demonstrate with 3-D spherical models of mantle convection with depth- and temperature-dependent viscosity that observed plume heat flux, plume excess temperature (<350°C), and upper mantle temperature (~ 1300°C) can pose important constraints on the layered mantle convection. We show that for a purely thermal convection model (i.e., a whole mantle convection), the observations of plume heat flux, plume excess temperature, and upper mantle temperature can be simultaneously explained only when internal heating rate is about 65%. For smaller internal heating rate, plume heat flux and plume excess temperature would be too large, and upper mantle temperature would be too small, compared with the observed. This suggests that for a whole mantle convection the CMB heat flux needs to be > 10 TW. For a core with no significant heat producing elements, such large CMB heat flux may lead to too rapid cooling of the core or a too young inner core. A layered mantle convection may help reduce the CMB heat flux. For layered convection models, we found that the top layer needs to be ~70% internally heated to explain the upper mantle temperature and plume-related observations, and this required internal heating ratio is insensitive to the layer thickness for the bottom layer (we used ~600 km and 1100 km thicknesses). This result suggests that heat generation rate for the bottom layer cannot be significantly larger (< a factor of 2) than that for the top layer. thus challenging the conventional geochemical inference for an significantly enriched bottom layer. However, this is more consistent with recent estimate of the MORB source composition that increases heat producing element concentration by a factor of three compared with the previously proposed.

The role and conditions of second-stage mantle melting in the generation of low-Ti tholeiites and boninites: the case of the Manihiki Plateau and the Troodos ophiolite

NASA Astrophysics Data System (ADS)

Golowin, Roman; Portnyagin, Maxim; Hoernle, Kaj; Sobolev, Alexander; Kuzmin, Dimitry; Werner, Reinhard

2017-12-01

High-Mg, low-Ti volcanic rocks from the Manihiki Plateau in the Western Pacific share many geochemical characteristics with subduction-related boninites such as high-Ca boninites from the Troodos ophiolite on Cyprus, which are believed to originate by hydrous re-melting of previously depleted mantle. In this paper we compare the Manihiki rocks and Troodos boninites using a new dataset on the major and trace element composition of whole rocks and glasses from these locations, and new high-precision, electron microprobe analyses of olivine and Cr-spinel in these rocks. Our results show that both low-Ti Manihiki rocks and Troodos boninites could originate by re-melting of a previously depleted lherzolite mantle source (20-25% of total melting with 8-10% melting during the first stage), as indicated by strong depletion of magmas in more to less incompatible elements (Sm/Yb < 0.8, Zr/Y < 2, Ti/V < 12) and high-Cr-spinel compositions (Cr# > 0.5). In comparison with Troodos boninites, the low-Ti Manihiki magmas had distinctively lower H2O contents (< 0.2 vs. > 2 wt% in boninites), 100 °C higher liquidus temperatures at a given olivine Fo-number, lower fO2 (ΔQFM < + 0.2 vs. ΔQFM > + 0.2) and originated from deeper and hotter mantle (1.4-1.7 GPa, 1440 °C vs. 0.8-1.0 GPa, 1300 °C for Troodos boninites). The data provide new evidence that re-melting of residual upper mantle is not only restricted to subduction zones, where it occurs under hydrous conditions, but can also take place due to interaction of previously depleted upper mantle with mantle plumes from the deep and hotter Earth interior.

Modification of the Western Gondwana craton by plume-lithosphere interaction

NASA Astrophysics Data System (ADS)

Hu, Jiashun; Liu, Lijun; Faccenda, Manuele; Zhou, Quan; Fischer, Karen M.; Marshak, Stephen; Lundstrom, Craig

2018-03-01

The longevity of cratons is generally attributed to persistence of neutrally-to-positively buoyant and mechanically strong lithosphere that shields the cratonic crust from underlying mantle dynamics. Here we show that large portions of the cratonic lithosphere in South America and Africa, however, experienced significant modification during and since the Mesozoic era, as demonstrated by widespread Cretaceous uplift and volcanism, present-day high topography, thin crust, and the presence of seismically fast but neutrally buoyant upper-mantle anomalies. We suggest that these observations reflect a permanent increase in lithospheric buoyancy due to plume-triggered delamination of deep lithospheric roots during the Late Cretaceous and early Cenozoic periods. Lithosphere in these regions has been thermally reestablished since then, as confirmed by its present-day low heat flow, high seismic velocities and realigned seismic anisotropy. We conclude that the original lowermost cratonic lithosphere is compositionally denser than the asthenospheric mantle and can be removed when perturbed by underlying mantle upwelling. Therefore, it is the buoyancy of the upper lithosphere that perpetuates stabilization of cratons.

Les xénolites ultramafiques du volcanisme alcalin quaternaire d'Oranie (Tell, Algérie occidentale), témoins d'une lithosphère cisaillée et enrichieUltramafic xenoliths from Quaternary alkali volcanism from Oranie (Tell, western Algeria): witnesses of a sheared and enriched lithosphere

NASA Astrophysics Data System (ADS)

Zerka, Mohamed; Cottin, Jean-Yves; Grégoire, Michel; Lorand, Jean-Pierre; Megartsi, M'Hamed; Midoun, Mohamed

Numerous ultramafic xenoliths occur within the Aı̈n-Temouchent volcanic complex (Northwestern Oranie, Algeria). Most of them are type I mantle tectonites (lherzolites and harzburgites) and composite xenoliths (harzburgite/clinopyroxenite) are rare. Only a few samples of spinel lherzolites display relatively fertile compositions when the major part of type I xenoliths have refractory major element compositions but enriched LREE contents showing that they have been affected by mantle metasomatism. The composite xenoliths are witnesses of reactions of alkaline magmas with the upper mantle. An asthenospheric rising, in relation with the large strike slip fault affecting the North African plate margin at Trias time is proposed as a possible geodynamical setting. To cite this article: M. Zerka et al., C. R. Geoscience 334 (2002) 387-394.

Magmatic infiltration and melting in the lower crust and upper mantle beneath the Cima volcanic field, California

USGS Publications Warehouse

Wilshire, H.G.; McGuire, A.V.

1996-01-01

Xenoliths of lower crustal and upper mantle rocks from the Cima volcanic field (CVF) commonly contain glass pockets, veins, and planar trains of glass and/or fluid inclusions in primary minerals. Glass pockets occupy spaces formerly occupied by primary minerals of the host rocks, but there is a general lack of correspondence between the composition of the glass and that of the replaced primary minerals. The melting is considered to have been induced by infiltration of basaltic magma and differentiates of basaltic magma from complex conduits formed by hydraulic fracturing of the mantle and crustal rocks, and to have occurred during the episode of CVF magmatism between ???7.5 Ma and present. Variable compositions of quenched melts resulted from mixing of introduced melts and products of melting of primary minerals, reaction with primary minerals, partial crystallization, and fractionation resulting from melt and volatile expulsion upon entrainment of the xenoliths. High silica melts (> ??? 60% SiO2) may result by mixing introduced melts with siliceous melts produced by reaction of orthopyroxene. Other quenched melt compositions range from those comparable to the host basalts to those with intermediate Si compositions and elevated Al, alkalis, Ti, P, and S; groundmass compositions of CVF basalts are consistent with infiltration of fractionates of those basalts, but near-solidus melting may also contribute to formation of glass with intermediate silica contents with infiltration only of volatile constituents.

Seismic evidence for widespread serpentinized forearc upper mantle along the Cascadia margin

USGS Publications Warehouse

Brocher, T.M.; Parsons, T.; Trehu, A.M.; Snelson, C.M.; Fisher, M.A.

2003-01-01

Petrologic models suggest that dehydration and metamorphism of subducting slabs release water that serpentinizes the overlying forearc mantle. To test these models, we use the results of controlled-source seismic surveys and earthquake tomography to map the upper mantle along the Cascadia margin forearc. We find anomalously low upper-mantle velocities and/or weak wide-angle reflections from the top of the upper mantle in a narrow region along the margin, compatible with recent teleseismic studies and indicative of a serpentinized upper mantle. The existence of a hydrated forearc upper-mantle wedge in Cascadia has important geological and geophysical implications. For example, shearing within the upper mantle, inferred from seismic reflectivity and consistent with its serpentinite rheology, may occur during aseismic slow slip events on the megathrust. In addition, progressive dehydration of the hydrated mantle wedge south of the Mendocino triple junction may enhance the effects of a slap gap during the evolution of the California margin.

Seismic Evidence of Imprints of Malani and Deccan Volcanism in Northwestern India

NASA Astrophysics Data System (ADS)

Mohan, G.; Mangalampally, R. K.; Ahmad, F.

2017-12-01

The evolution of the Neoproterozoic (750 Ma) Malani igneous province(MIP), the site of the largest felsic magmatism in India is debatable with theories supporting extensional tectonics, mantle plume or subduction processes. The MIP that lies to the west of the Proterozoic Aravalli mountain range and east of the Late Mesozoic-Teritary Barmer-Sanchor rift systems, hosts acidic volcanics in an area of 0.5 million sq.km in northwestern India. In this study, the crustal and upper mantle structure beneath the MIP is investigated through a deployment of 12 broadband seismographs in phases, at 18 locations during a period of five years from 2011-2016. The P wave receiver function(RF) analysis was carried out to image the crust and the 410 km and 660 km mantle transition zone discontinuities. About 1500 teleseismic waveforms with signal to noise ratios > 2.5 are utilized. The RFs at most stations are marked by strong conversions from the base of the sediments and the Moho. The crustal thickness estimated through the Neighbourhood algorithm approach, ranges from 35 to 42km. The crustal Poisson's ratio ranges from 0.26 - 0.29. The crustal thickness and Poisson's ratio are observed to increase from west to east viz., from the rift zone to the mountain belt. A significant finding is the presence of a 5-10km thick mid-crustal low velocity zone with a reduced shear velocity of 3.0-3.2km/s. The Ps conversions from the 410km and 660km mantle discontinuities are delayed by about 1sec with respect to the timings predicted by the IASP91 standard earth model. The observed delays are attributed to the reduction in velocity due to compositional/thermal perturbations in the uppermost upper mantle above the 410km discontinuity. The presence of alkaline complexes in MIP which are of pre-Deccan age (68 Ma) led us to surmise that the low velocity anomalies observed in the upper mantle might be linked to the mantle source associated with the 65 Ma Deccan volcanism which erupted further south of MIP. It is likely that the mantle source may have overprinted or obliterated the mantle signatures of the Neoproterozoic tectonic event. However, the intracrustal low velocities overlying an underplated crust in MIP are interpreted to be the compositional imprints of the felsic magma associated with the bimodel Malani volcanism.

The Hadean upper mantle conundrum: evidence for source depletion and enrichment from Sm-Nd, Re-Os, and Pb isotopic compositions in 3.71 Gy boninite-like metabasalts from the Isua Supracrustal Belt, Greenland

NASA Astrophysics Data System (ADS)

Frei, Robert; Polat, Ali; Meibom, Anders

2004-04-01

Here we present Sm-Nd, Re-Os, and Pb isotopic data of carefully screened, least altered samples of boninite-like metabasalts from the Isua Supracrustal Belt (ISB, W Greenland)that characterize their mantle source at the time of their formation. The principal observations of this study are that by 3.7-3.8 Ga melt source regions existed in the upper mantle with complicated enrichment/depletion histories. Sm-Nd isotopic data define a correlation line with a slope corresponding to an age of 3.69 ± 0.18 Gy and an initial εNd value of +2.0 ± 4.7. This Sm-Nd age is consistent with indirect (but more precise) U-Pb geochronological estimates for their formation between 3.69-3.71 Ga. Relying on the maximum formation age of 3.71 Gy defined by the external age constraints, we calculate an average εNd [T = 3.71 Ga] value of +2.2 ± 0.9 (n = 18, 1σ) for these samples, which is indicative of a strongly depleted mantle source. This is consistent with the high Os concentrations, falling in the range between 1.9-3.4 ppb, which is similar to the estimated Os concentration for the primitive upper mantle. Re-Os isotopic data (excluding three outliers) yield an isochron defining an age of 3.76 ± 0.09 Gy (with an initial γOs value of 3.9 ± 1.2), within error consistent with the Sm-Nd age and the indirect U-Pb age estimates. An average initial γOs [T = 3.71 Ga] value of + 4.4 ± 1.2 (n = 8; 2σ) is indicative of enrichment of their source region during, or prior to, its melting. Thus, this study provides the first observation of an early Archean upper mantle domain with a distinctly radiogenic Os isotopic signature. This requires a mixing component characterized by time-integrated suprachondritic Re/Os evolution and a Os concentration high enough to strongly affect the Os budget of the mantle source; modern sediments, recycled basaltic crust, or the outer core do not constitute suitable candidates. At this point, the nature of the mantle or crustal component responsible for the radiogenic Os isotopic signature is not known. Compared with the Sm-Nd and Re-Os isotope systems, the Pb isotope systematics show evidence for substantial perturbation by postformational hydrothermal-metasomatic alteration processes accompanying an early Archean metamorphic event at 3510 ± 65 Ma and indicate that the U-Th-Pb system was partially opened to Pb-loss on a whole rock scale. Single stage mantle evolution models fail to provide a solution to the Pb isotopic data, which requires that a high-μ component was mixed with the depleted mantle component before or during the extrusion of the basalts. Relatively high 207Pb/204Pb ratios (compared to contemporaneous mantle), support the hypothesis that erosion products of the ancient terrestrial protocrust existed for several hundred My before recycling into the mantle before ∼3.7 Ga. Our results are broadly consistent with models favoring a time-integrated Hadean history of mantle depletion and with the existence of an early Hadean protocrust, the complement to the Hadean depleted mantle, which after establishment of subduction-like processes was, at least locally, recycled into the upper mantle before 3.7 Ga. Thus, already in the Hadean, the upper mantle seems to be characterized by geochemical heterogeneity on a range of length scales; one property that is shared with the modern upper mantle. However, a simple two component mixing scenario between depleted mantle and an enriched-, crustal component with a modern analogue can not account for the complicated and contradictory geochemical properties of this particular Hadean upper mantle source.

Barium isotope composition of altered oceanic crust from the IODP Site 1256 at the East Pacific Rise

NASA Astrophysics Data System (ADS)

Nan, X.; Yu, H.; Gao, Y.

2017-12-01

To understand the behavior of Ba isotopes in the oceanic crust during seawater alteration, we analyzed Ba isotopes for altered oceanic crust (AOC) from the IODP Site 1256 at the East Pacific Rise (EPR). The samples include 33 basalts, 5 gabbros, and 1 gabbronorite. This drill profile has four sections from top to bottom, including the volcanic section, transition zone, sheeted dyke complex, and plutonic complex. They display various degrees of alteration with obviously variable temperatures and water/rock ratios (Gao et al., 2012). The volcanic section is slightly to moderately altered by seawater at 100 to 250°; the transition zone is a mixing zone between upwelling hydrothermal fluids and downwelling seawater; and the sheeted dyke complex and plutonic complex are highly altered by hydrothermal fluids (˜250°). Ba isotopes were analyzed on a Neptune Plus MC-ICP-MS at the University of Science and Technology of China. The long-term precision of δ137/134Ba is better than 0.04‰ (2SD). The δ137/134Ba of the volcanic section and the top of the transition zone range between -0.01 and 0.30‰, higher than the δ137/134Ba of fresh MORB and upper mantle (0.020 ± 0.021‰, 2SE, Huang et al., 2015). Similarly, the δ137/134Ba of the sheeted dyke complex ranges from 0.05 to 0.28‰. The plutonic section has δ137/134Ba from -0.17 to -0.05‰, which is lower than the upper mantle, with one exception that has δ137/134Ba of 0.19‰. No correlation exists between Ba contents and δ137/134Ba. The weighted average δ137/134Ba of the AOC samples is 0.13±0.04‰ (2SE), significantly higher than that of the upper mantle. In all, our AOC data reveal obvious Ba isotopic fractionation, reflecting alteration of the AOC by hydrothermal fluids and seawater. The obvious difference of Ba isotope composition between the AOC and the upper mantle further indicates that recycling of the AOC could result in Ba isotope heterogeneity of the mantle. References: Gao Y, Vils F, Cooper K M, et al. (2012) Downhole variation of lithium and oxygen isotopic compositions of oceanic crust at East Pacific Rise, ODP Site 1256. Geochemistry, Geophysics, Geosystems,13(10). Huang F., Nan X., Hu M., Huang S. and Huang J. (2015) Barium isotope compositions of igneous rocks. Goldschm. Abstr.2015, 1331.

Lithospheric evolution of the Northern Arabian Shield: Chemical and isotopic evidence from basalts, xenoliths and granites

NASA Technical Reports Server (NTRS)

Stein, M.

1988-01-01

The evolution of the upper-mantle and the lower-crust (the conteinental lithosphere), is the area of Israel and Sinai was studied, using the chemical composition and the Nd-Sr isotopic systematics from mantle and crustal nodules, their host basalts, and granites. The magmatism and the metasomatism making the lithosphere are related to uprise of mantle diapirs in the uppermost mantle of the area. These diapirs heated the base of the lithosphere, eroded, and replaced it with new hot material. It caused a domal uplift of the lithosphere (and the crust). The doming resulted in tensional stresses that in turn might develop transport channels for the basalt.

Redox-influenced seismic properties of upper-mantle olivine

NASA Astrophysics Data System (ADS)

Cline, C. J., II; Faul, U. H.; David, E. C.; Berry, A. J.; Jackson, I.

2018-03-01

Lateral variations of seismic wave speeds and attenuation (dissipation of strain energy) in the Earth’s upper mantle have the potential to map key characteristics such as temperature, major-element composition, melt fraction and water content. The inversion of these data into meaningful representations of physical properties requires a robust understanding of the micromechanical processes that affect the propagation of seismic waves. Structurally bound water (hydroxyl) is believed to affect seismic properties but this has yet to be experimentally quantified. Here we present a comprehensive low-frequency forced-oscillation assessment of the seismic properties of olivine as a function of water content within the under-saturated regime that is relevant to the Earth’s interior. Our results demonstrate that wave speeds and attenuation are in fact strikingly insensitive to water content. Rather, the redox conditions imposed by the choice of metal sleeving, and the associated defect chemistry, appear to have a substantial influence on the seismic properties. These findings suggest that elevated water contents are not responsible for low-velocity or high-attenuation structures in the upper mantle. Instead, the high attenuation observed in hydrous and oxidized regions of the upper mantle (such as above subduction zones) may reflect the prevailing oxygen fugacity. In addition, these data provide no support for the hypothesis whereby a sharp lithosphere–asthenosphere boundary is explained by enhanced grain boundary sliding in the presence of water.

New numerical approaches for modeling thermochemical convection in a compositionally stratified fluid

NASA Astrophysics Data System (ADS)

Puckett, Elbridge Gerry; Turcotte, Donald L.; He, Ying; Lokavarapu, Harsha; Robey, Jonathan M.; Kellogg, Louise H.

2018-03-01

Geochemical observations of mantle-derived rocks favor a nearly homogeneous upper mantle, the source of mid-ocean ridge basalts (MORB), and heterogeneous lower mantle regions. Plumes that generate ocean island basalts are thought to sample the lower mantle regions and exhibit more heterogeneity than MORB. These regions have been associated with lower mantle structures known as large low shear velocity provinces (LLSVPS) below Africa and the South Pacific. The isolation of these regions is attributed to compositional differences and density stratification that, consequently, have been the subject of computational and laboratory modeling designed to determine the parameter regime in which layering is stable and understanding how layering evolves. Mathematical models of persistent compositional interfaces in the Earth's mantle may be inherently unstable, at least in some regions of the parameter space relevant to the mantle. Computing approximations to solutions of such problems presents severe challenges, even to state-of-the-art numerical methods. Some numerical algorithms for modeling the interface between distinct compositions smear the interface at the boundary between compositions, such as methods that add numerical diffusion or 'artificial viscosity' in order to stabilize the algorithm. We present two new algorithms for maintaining high-resolution and sharp computational boundaries in computations of these types of problems: a discontinuous Galerkin method with a bound preserving limiter and a Volume-of-Fluid interface tracking algorithm. We compare these new methods with two approaches widely used for modeling the advection of two distinct thermally driven compositional fields in mantle convection computations: a high-order accurate finite element advection algorithm with entropy viscosity and a particle method that carries a scalar quantity representing the location of each compositional field. All four algorithms are implemented in the open source finite element code ASPECT, which we use to compute the velocity, pressure, and temperature associated with the underlying flow field. We compare the performance of these four algorithms on three problems, including computing an approximation to the solution of an initially compositionally stratified fluid at Ra =105 with buoyancy numbers B that vary from no stratification at B = 0 to stratified flow at large B .

Lunar Magma Ocean Crystallization: Constraints from Fractional Crystallization Experiments

NASA Technical Reports Server (NTRS)

Rapp, J. F.; Draper, D. S.

2015-01-01

The currently accepted paradigm of lunar formation is that of accretion from the ejecta of a giant impact, followed by crystallization of a global scale magma ocean. This model accounts for the formation of the anorthosite highlands crust, which is globally distributed and old, and the formation of the younger mare basalts which are derived from a source region that has experienced plagioclase extraction. Several attempts at modelling the crystallization of such a lunar magma ocean (LMO) have been made, but our ever-increasing knowledge of the lunar samples and surface have raised as many questions as these models have answered. Geodynamic models of lunar accretion suggest that shortly following accretion the bulk of the lunar mass was hot, likely at least above the solidus]. Models of LMO crystallization that assume a deep magma ocean are therefore geodynamically favorable, but they have been difficult to reconcile with a thick plagioclase-rich crust. A refractory element enriched bulk composition, a shallow magma ocean, or a combination of the two have been suggested as a way to produce enough plagioclase to account for the assumed thickness of the crust. Recently however, geophysical data from the GRAIL mission have indicated that the lunar anorthositic crust is not as thick as was initially estimated, which allows for both a deeper magma ocean and a bulk composition more similar to the terrestrial upper mantle. We report on experimental simulations of the fractional crystallization of a deep (approximately 100km) LMO with a terrestrial upper mantle-like (LPUM) bulk composition. Our experimental results will help to define the composition of the lunar crust and mantle cumulates, and allow us to consider important questions such as source regions of the mare basalts and Mg-suite, the role of mantle overturn after magma ocean crystallization and the nature of KREEP

Melt extraction and mantle source at a Southwest Indian Ridge Dragon Bone amagmatic segment on the Marion Rise

NASA Astrophysics Data System (ADS)

Gao, Changgui; Dick, Henry J. B.; Liu, Yang; Zhou, Huaiyang

2016-03-01

This paper works on the trace and major element compositions of spatially associated basalts and peridotites from the Dragon Bone amagmatic ridge segment at the eastern flank of the Marion Platform on the ultraslow spreading Southwest Indian Ridge. The rare earth element compositions of basalts do not match the pre-alteration Dragon Bone peridotite compositions, but can be modeled by about 5 to 10% non-modal batch equilibrium melting from a DMM source. The Dragon Bone peridotites are clinopyroxene-poor harzburgite with average spinel Cr# 27.7. The spinel Cr# indicates a moderate degree of melting. However, CaO and Al2O3 of the peridotites are lower than other abyssal peridotites at the same Mg# and extent of melting. This requires a pyroxene-poor initial mantle source composition compared to either hypothetical primitive upper mantle or depleted MORB mantle sources. We suggest a hydrous melting of the initial Dragon Bone mantle source, as wet melting depletes pyroxene faster than dry. According to the rare earth element patterns, the Dragon Bone peridotites are divided into two groups. Heavy REE in Group 1 are extremely fractionated from middle REE, which can be modeled by 7% fractional melting in the garnet stability field and another 12.5 to 13.5% in the spinel stability field from depleted and primitive upper mantle sources, respectively. Heavy REE in Group 2 are slightly fractionated from middle REE, which can be modeled by 15 to 20% fractional melting in the spinel stability field from a depleted mantle source. Both groups show similar melting degree to other abyssal peridotites. If all the melt extraction occurred at the middle oceanic ridge where the peridotites were dredged, a normal 6 km thick oceanic crust is expected at the Dragon Bone segment. However, the Dragon Bone peridotites are exposed in an amagmatic ridge segment where only scattered pillow basalts lie on a partially serpentinized mantle pavement. Thus their depletion requires an earlier melting occurred at other place. Considering the hydrous melting of the initial Dragon Bone mantle source, we suggest the earlier melting event occurred in an arc terrain, prior to or during the closure of the Mozambique Ocean in the Neproterozoic, and the subsequent assembly of Gondwana. Then, the Al2O3 depleted and thus buoyant peridotites became the MORB source for Southwest Indian Ridge and formed the Marion Rise during the Gondwana breakup.

New Estimates of Rhenium in the Crust: Implications for Mantle Re-Os Budgets

NASA Astrophysics Data System (ADS)

Bennett, V. C.; Sun, W.

2002-12-01

The 187Re-187Os isotopic system has provided a new probe of mantle chemical structure with, for example, now numerous studies balancing estimates of the Os isotopic compositions of the upper modern mantle with sizes and ages of proposed conjugate reservoirs stored within the deep mantle. This style of modeling is dependent upon estimates of the parent Re in the various reservoirs including total crust, upper mantle, MORB and ocean island basalts. New laser ICP-MS in situ and ID whole rock results from OIB, arc and back-arc basalts suggest Re concentrations in oceanic and crustal domains may have been greatly underestimated. For example Hawaiian OIBs show a clear distinction between subaerial and submarine erupted samples with the latter having Re much closer to the higher MORB estimates (1) than to previous OIB estimates. This difference has been attributed to Re volatility and loss during syn- and post-eruption degassing of subaerial samples. Recent work has produced similar results for submarine arc samples using both dredged glasses and melt inclusions in olivines from primitive basalts. Both have much higher average Re (ca. 1.5 and 3.4 ppb; 2,3) than literature values for arcs (ca. 0.30ppb) determined largely from sub-aerial samples, or for average crust estimated from loess (0.2 ppb; 4). If the undegassed arc samples are representative, then the total crust may have more than 5 times the Re previously estimated. Re lost during arc eruptions may ultimately be concentrated in anoxic seafloor sediments. Prior under-estimates may be linked to the extremely heterogeneous concentration (> 5 orders of magnitude) of the chalcophile, redox sensitive Re in crustal environments. If the residence time of high Re in the crust is long (>1 Ga) then, 1) much smaller reservoirs of stored Re in the deep mantle are required to balance Re depletions in the upper mantle, and 2) significant portions of the upper mantle are likely Re depleted. Alternatively Re may be rapidly recycled in oceanic sediments (short residence time) resulting in a smaller affect on Re-Os budgets, but creating areas of extreme Re heterogeneity in the upper mantle. Refs: 1. Bennett, Norman and Garcia, EPSL 2000. 2. Sun et al. (in press, Chemical Geology) 3. Sun et al. (submitted). 4. Peucker-Ehrenbrink and Jahn, G3, 2001.

Numerical Mantle Convection Models of Crustal Formation in an Oceanic Environment in the Early Earth

NASA Astrophysics Data System (ADS)

van Thienen, P.; van den Berg, A. P.; Vlaar, N. J.

2001-12-01

The generation of basaltic crust in the early Earth by partial melting of mantle rocks, subject to investigation in this study, is thought to be a first step in the creation of proto-continents (consisting largely of felsic material), since partial melting of basaltic material was probably an important source for these more evolved rocks. In the early Archean the earth's upper mantle may have been hotter than today by as much as several hundred degrees centigrade. As a consequence, partial melting in shallow convective upwellings would have produced a layering of basaltic crust and underlying depleted (lherzolitic-harzburgitic) mantle peridotite which is much thicker than found under modern day oceanic ridges. When a basaltic crustal layer becomes sufficiently thick, a phase transition to eclogite may occur in the lower parts, which would cause delamination of this dense crustal layer and recycling of dense eclogite into the upper mantle. This recycling mechanism may have contributed significantly to the early cooling of the earth during the Archean (Vlaar et al., 1994). The delamination mechanism which limits the build-up of a thick basaltic crustal layer is switched off after sufficient cooling of the upper mantle has taken place. We present results of numerical modelling experiments of mantle convection including pressure release partial melting. The model includes a simple approximate melt segregation mechanism and basalt to eclogite phase transition, to account for the dynamic accumulation and recycling of the crust in an upper mantle subject to secular cooling. Finite element methods are used to solve for the viscous flow field and the temperature field, and lagrangian particle tracers are used to represent the evolving composition due to partial melting and accumulation of the basaltic crust. We find that this mechanism creates a basaltic crust of several tens of kilometers thickness in several hundreds of million years. This is accompanied by a cooling of some hundred degrees centigrade. Vlaar, N.J., P.E. van Keken and A.P. van den Berg (1994), Cooling of the Earth in the Archaean: consequences of pressure-release melting in a hotter mantle, Earth and Planetary Science Letters, vol 121, pp. 1-18

Teleseismic array analysis of upper mantle compressional velocity structure. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Walck, M. C.

1984-01-01

Relative array analysis of upper mantle lateral velocity variations in southern California, analysis techniques for dense data profiles, the P-wave upper mantle structure beneath an active spreading center: the Gulf of California, and the upper mantle under the Cascade ranges: a comparison with the Gulf of California are presented.

Joint inversion of satellite-detected tidal and magnetospheric signals constrains electrical conductivity and water content of the upper mantle and transition zone.

PubMed

Grayver, A V; Munch, F D; Kuvshinov, A V; Khan, A; Sabaka, T J; Tøffner-Clausen, L

2017-06-28

We present a new global electrical conductivity model of Earth's mantle. The model was derived by using a novel methodology, which is based on inverting satellite magnetic field measurements from different sources simultaneously. Specifically, we estimated responses of magnetospheric origin and ocean tidal magnetic signals from the most recent Swarm and CHAMP data. The challenging task of properly accounting for the ocean effect in the data was addressed through full three-dimensional solution of Maxwell's equations. We show that simultaneous inversion of magnetospheric and tidal magnetic signals results in a model with much improved resolution. Comparison with laboratory-based conductivity profiles shows that obtained models are compatible with a pyrolytic composition and a water content of 0.01 wt% and 0.1 wt% in the upper mantle and transition zone, respectively.

A geophysical perspective on mantle water content and melting: Inverting electromagnetic sounding data using laboratory-based electrical conductivity profiles

NASA Astrophysics Data System (ADS)

Khan, A.; Shankland, T. J.

2012-02-01

This paper applies electromagnetic sounding methods for Earth's mantle to constrain its thermal state, chemical composition, and "water" content. We consider long-period inductive response functions in the form of C-responses from four stations distributed across the Earth (Europe, North America, Asia and Australia) covering a period range from 3.9 to 95.2 days and sensitivity to ~ 1200 km depth. We invert C-responses directly for thermo-chemical state using a self-consistent thermodynamic method that computes phase equilibria as functions of pressure, temperature, and composition (in the Na2O-CaO-FeO-MgO-Al2O3-SiO2 model system). Computed mineral modes are combined with recent laboratory-based electrical conductivity models from independent experimental research groups (Yoshino (2010) and Karato (2011)) to compute bulk conductivity structure beneath each of the four stations from which C-responses are estimated. To reliably allocate water between the various mineral phases we include laboratory-measured water partition coefficients for major upper mantle and transition zone minerals. This scheme is interfaced with a sampling-based algorithm to solve the resulting non-linear inverse problem. This approach has two advantages: (1) It anchors temperatures, composition, electrical conductivities, and discontinuities that are in laboratory-based forward models, and (2) At the same time it permits the use of geophysical inverse methods to optimize conductivity profiles to match geophysical data. The results show lateral variations in upper mantle temperatures beneath the four stations that appear to persist throughout the upper mantle and parts of the transition zone. Calculated mantle temperatures at 410 and 660 km depth lie in the range 1250-1650 °C and 1500-1750 °C, respectively, and generally agree with the experimentally-determined temperatures at which the measured phase reactions olivine → β-spinel and γ-spinel → ferropericlase + perovskite occur. The retrieved conductivity structures beneath the various stations tend to follow trends observed for temperature with the strongest lateral variations in the uppermost mantle; for depths > 300 km conductivities appear to depend less on the particular conductivity database. Conductivities at 410 km and at 660 km depth are found to agree overall with purely geophysically-derived global and semi-global one-dimensional conductivity models. Both electrical conductivity databases point to < 0.01 wt.% H2O in the upper mantle. For transition zone minerals results from the laboratory database of Yoshino (2010) suggest that a much higher water content (up to 2 wt.% H2O) is required than in the other database (Karato, 2011), which favors a relatively "dry" transition zone (< 0.01 wt.% H2O). Incorporating laboratory measurements of hydrous silicate melting relations and available conductivity data allows us to consider the possibility of hydration melting and a high-conductivity melt layer above the 410-km discontinuity. The latter appears to be 1) regionally localized and 2) principally a feature from the Yoshino (2010) database. Further, there is evidence of lateral heterogeneity: The mantle beneath southwestern North America and central China appears "wetter" than that beneath central Europe or Australia.

The He isotope composition of the earliest picrites erupted by the Ethiopia plume, implications for mantle plume source

NASA Astrophysics Data System (ADS)

Stuart, Finlay; Rogers, Nick; Davies, Marc

2016-04-01

The earliest basalts erupted by mantle plumes are Mg-rich, and typically derived from mantle with higher potential temperature than those derived from the convecting upper mantle at mid-ocean ridges and ocean islands. The chemistry and isotopic composition of picrites from CFB provide constraints on the composition of deep Earth and thus the origin and differentiation history. We report new He-Sr-Nd-Pb isotopic composition of the picrites from the Ethiopian flood basalt province from the Dilb (Chinese Road) section. They are characterized by high Fe and Ti contents for MgO = 10-22 wt. % implying that the parent magma was derived from a high temperature low melt fraction, most probably from the Afar plume head. The picrite 3He/4He does not exceed 21 Ra, and there is a negative correlation with MgO, the highest 3He/4He corresponding to MgO = 15.4 wt. %. Age-corrected 87Sr/86Sr (0.70392-0.70408) and 143Nd/144Nd (0.512912-0.512987) display little variation and are distinct from MORB and OIB. Age-corrected Pb isotopes display a significant range (e.g. 206Pb/204Pb = 18.70-19.04) and plot above the NHRL. These values contrast with estimates of the modern Afar mantle plume which has lower 3He/4He and Sr, Nd and Pb isotope ratios that are more comparable with typical OIB. These results imply either interaction between melts derived from the Afar mantle plume and a lithospheric component, or that the original Afar mantle plume had a rather unique radiogenic isotope composition. Regardless of the details of the origins of this unusual signal, our observations place a minimum 3He/4He value of 21 Ra for the Afar mantle plume, significantly greater than the present day value of 16 Ra, implying a significant reduction over 30 Myr. In addition the Afar source was less degassed than convecting mantle but more degassed than mantle sampled by the proto-Iceland plume (3He/4He ~50 Ra). This suggests that the largest mantle plumes are not sourced in a single deep mantle domain with a common depletion history and that they do not mix with shallower mantle reservoirs to the same extent.

Investigation of upper mantle seismic discontinuities beneath the Indian Ocean using array seismology methods

NASA Astrophysics Data System (ADS)

van Driel, J.; Reiss, A. S.; Thomas, C.

2016-12-01

The topography of upper mantle seismic discontinuities can be used to constrain regional variations in composition and temperature of the Earths mantle. The 410 km discontinuity is caused by the solid-solid phase transition from olivine to wadsleyite. Due to its positive Clapeyron slope, the discontinuity is depressed in hot regimes. The phase transition from ringwoodite to bridgemanite and magnesiowüstite in contrast has a negative Clapeyron slope and therefore is elevated when hot material is present. Cold material is expected to yield an opposing topographic signature, culminating in an elevated 410 km and a depressed 660 km discontinuity. As part of the RHUM-RUM project (Réunion Hotspot and Upper Mantle - Réunions Unterer Mantel) we extract relevant geophysical parameters, by investigating the properties of upper mantle seismic discontinuities beneath the Indian Ocean. The topography of the 410 and 660 km discontinuities, which define the upper and lower bounds of the mantle transition zone, have been mapped using PP and SS underside reflections. This study has utilised over 8500 events with Mw ≥ 5.8, distributed over the entire Indian Ocean. Our robust data set yields a dense coverage of points, which are defined by consistently crossing ray paths. Array seismology methods, such as vespagrams and slowness-backazimuth analysis, are used to enhance the signal-to-noise-ratio and detect and identify weak precursor signals. The differential travel times are corrected for crustal features and converted into depth values of the discontinuities by comparing the measured travel times with theoretical ones derived from ray tracing through the 1D reference Earth model ak135. A `travel-time' stacking method has also been applied for 4° radius bins around each of the bounce points. The addition of a secondary method derives greater stability of our results and allows an enhanced error analysis procedure. In order to better constrain the mineralogical processes taking place within the mantle transition zone, amplitude ratios, polarities and velocity gradients have also been investigated.

Geochemical evidence for airborne dust additions to soils in Channel Islands National Park, California

USGS Publications Warehouse

Muhs, D.R.; Budahn, J.R.; Johnson, D.L.; Reheis, M.; Beann, J.; Skipp, G.; Fisher, E.; Jones, J.A.

2008-01-01

There is an increasing awareness that dust plays important roles in climate change, biogeochemical cycles, nutrient supply to ecosystems, and soil formation. In Channel Islands National Park, California, soils are clay-rich Vertisols or Alfisols and Mollisols with vertic properties. The soils are overlain by silt-rich mantles that contrast sharply with the underlying clay-rich horizons. Silt mantles contain minerals that are rare or absent in the volcanic rocks that dominate these islands. Immobile trace elements (Sc-Th-La and Ta-Nd-Cr) and rare-earth elements show that the basalt and andesite on the islands have a composition intermediate between upper-continental crust and oceanic crust. In contrast, the silt fractions and, to a lesser extent, clay fractions of the silt mantle have compositions closer to average upper-continental crust and very similar to Mojave Desert dust. Island shelves, exposed during the last glacial period, could have provided a source of eolian sediment for the silt mantles, but this is not supported by mineralogical data. We hypothesize that a more likely source for the silt-rich mantles is airborne dust from mainland California and Baja California, either from the Mojave Desert or from the continental shelf during glacial low stands of sea. Although average winds are from the northwest in coastal California, easterly winds occur numerous times of the year when "Santa Ana" conditions prevail, caused by a high-pressure cell centered over the Great Basin. The eolian silt mantles constitute an important medium of plant growth and provide evidence that abundant eolian silt and clay may be delivered to the eastern Pacific Ocean from inland desert sources. ?? 2007 Geological Society of America.

Heterogeneous hydrogen distribution in orthopyroxene from veined mantle peridotite (San Carlos, Arizona): Impact of melt-rock interactions

NASA Astrophysics Data System (ADS)

Denis, Carole M. M.; Demouchy, Sylvie; Alard, Olivier

2018-03-01

Experimental studies have shown that hydrogen embedded as a trace element in mantle mineral structures affects the physical properties of mantle minerals and rocks. Nevertheless, hydrogen concentrations in mantle minerals are much lower than predicted by hydrogen solubilities obtained experimentally at high pressure and temperature. Here, we report textural analyses and major and trace element concentrations (including hydrogen) in upper mantle minerals from a spinel-bearing composite xenolith (dunite and pyroxenite) transported by silica-undersaturated mafic alkaline lavas from the San Carlos volcanic field (Arizona, USA). Our results suggest that the composite xenolith results from the percolation-reaction of a basaltic liquid within dunite channels, and is equilibrated with respect to trace elements. Equilibrium temperatures range between 1011 and 1023 °C. Hydrogen concentrations (expressed in ppm H2O by weight) obtained from unpolarized and polarized Fourier transform infrared spectroscopy are low, with average values <2 ppm H2O, 24 ppm H2O, and 53 ppm H2O for olivine, orthopyroxene, and clinopyroxene, respectively; hydrogen concentrations in olivine are below the detection limit. These low hydrogen concentrations are consistent with depletion by high melt-rock ratio interactions. Clinopyroxene hydrogen concentrations are homogeneous, whereas polarized infrared profile measurements across single-crystals of orthopyroxene reveal hydrogen-depleted rims, which are interpreted as the result of dehydration by ionic diffusion, possibly triggered by melt-rock interactions. We conclude that pyroxenes, like olivine, are unreliable hydrogen proxies, and that the remaining hydrogen concentrations observed in peridotites might only represent the 'tip of the iceberg' of the water stored in the Earth's upper mantle.

3D Integrated geophysical-petrological modelling of the Iranian lithosphere

NASA Astrophysics Data System (ADS)

Mousavi, Naeim; Ardestani, Vahid E.; Ebbing, Jörg; Fullea, Javier

2016-04-01

The present-day Iranian Plateau is the result of complex tectonic processes associated with the Arabia-Eurasia Plate convergence at a lithospheric scale. In spite of previous mostly 2D geophysical studies, fundamental questions regarding the deep lithospheric and sub-lithospheric structure beneath Iran remain open. A robust 3D model of the thermochemical lithospheric structure in Iran is an important step toward a better understanding of the geological history and tectonic events in the area. Here, we apply a combined geophysical-petrological methodology (LitMod3D) to investigate the present-day thermal and compositional structure in the crust and upper mantle beneath the Arabia-Eurasia collision zone using a comprehensive variety of constraining data: elevation, surface heat flow, gravity potential fields, satellite gravity gradients, xenoliths and seismic tomography. Different mantle compositions were tested in our model based on local xenolith samples and global data base averages for different tectonothermal ages. A uniform mantle composition fails to explain the observed gravity field, gravity gradients and surface topography. A tectonically regionalized lithospheric mantle compositional model is able to explain all data sets including seismic tomography models. Our preliminary thermochemical lithospheric study constrains the depth to Moho discontinuity and intra crustal geometries including depth to sediments. We also determine the depth to Curie isotherm which is known as the base of magnetized crustal/uppermost mantle bodies. Discrepancies with respect to previous studies include mantle composition and the geometry of Moho and Lithosphere-Asthenosphere Boundary (LAB). Synthetic seismic Vs and Vp velocities match existing seismic tomography models in the area. In this study, depleted mantle compositions are modelled beneath cold and thick lithosphere in Arabian and Turan platforms. A more fertile mantle composition is found in collision zones. Based on our 3D thermochemical model we propose a new scenario to interpret the geodynamical history of area. In this context the present-day central Iran block would be as remain of the older and larger Iranian block present before the onset of Turan platform subduction beneath the Iranian Plateau. Further analysis of sub-lithospheric density anomalies (e.g., subducted slabs) is required to fully understand the geodynamics of the area.

a View of the Marble-Cake Mantle from the Southeast Indian Ridge

NASA Astrophysics Data System (ADS)

Hanan, B. B.; Graham, D. W.; Hemond, C.; Blichert-Toft, J.; Albarede, F.

2014-12-01

Along the Southeast Indian Ridge, variations in axial depth, crustal thickness, hydrothermal venting [1], basaltic major elements and U-series disequilibria [2] all indicate a west-to-east decrease in magma supply and mantle temperature from the Amsterdam-St. Paul hotspot to the Australian-Antarctic Discordance. Paired Hf-Pb isotopes in closely spaced glasses (5-10 km) from 81-100°E define two populations revealing compositional streaks in the upper mantle [3]. The number density of the streaks follows a Poisson distribution with a characteristic thickness of ~20 km. K/Ti and Na8 do not correlate with Pb or Hf isotopes, and both isotopic domains encompass N- and E-MORB types indicating the variations represent mantle source heterogeneities. 3He/4He varies from 7.5 - 10.2 RA, more than half the range in global MORB away from hotspot influence [4]. No systematic relationship exists between 3He/4He and Pb or Hf isotopes. A general negative correlation between K/Ti and Fe8 for the SEIR resembles that for MORBs globally, with higher K/Ti associated with lower 3He/4He. Collectively the observations suggest the presence of lithologically heterogeneous mantle. Lower 3He/4He derives from a source containing a few percent pyroxenite or ecologite, while 3He/4He > 9 RA arises from peridotite. Mantle convection has folded together distinct composite reservoirs of heterogeneous mantle, and stretched them into streaks that remain discernible units. The mantle 'unit' giving rise to each MORB sample represents a 'mixture of mixtures' with a multi-stage mixing history. Spectral analysis of the length scales of Hf, Pb and He isotopic variability allows a visual representation of this upper mantle 'texture'. The dominant length scales reflect large (1000, 500 km) and regional scale (100-150 km) structures in mantle flow, and sampling of heterogeneities during partial melting (20-30 km). 1-Baker et al., doi:10.1002/2014GC005344; 2-Russo et al., doi:10.1016/j.epsl.2008.11.016; 3-Hanan et al., doi:10.1016/j.epsl.2013.05.028; 4-Graham et al., doi:10.1002/2014GC005264

Thermal anomalies and magmatism due to lithospheric doubling and shifting

NASA Astrophysics Data System (ADS)

Vlaar, N. J.

1983-11-01

We present some thermal and magmatic consequences of the processes of lithospheric doubling and lithospheric shifting. Lithospheric doubling concerns the obduction of a cold continental or old oceanic lithospheric plate over a young and hot oceanic lithosphere/upper mantle system, including an oceanic ridge. Lithospheric shifting concerns the translation and rotation of a lithospheric plate relative to the upper mantle. In both cases the resulting thermal state of the upper mantle below the obducting or shifting lithosphere may be perturbed relative to a "normal" continental or oceanic geothermal situation. The perturbed geothermal state gives rise to a density inversion at depth and thus induces a vertical gravitational instability which favours magmatism. We speculate about the magmatic consequences of this situation and infer that in the case of lithospheric doubling our model may account for the petrology and geochemistry of the resulting magma. The original layering and composition of the overridden young oceanic lithosphere may strongly influence magmatic processes. We dwell shortly on the genesis of kimberlites within the framework of our lithospheric doubling model and on magmatism in general. Lithospheric recycling is inherent to the mechanism of lithospheric doubling.

Can the composition and structure of the lower ocean crust and upper mantle be known without deep ocean drilling?

NASA Astrophysics Data System (ADS)

Dick, H.; Natland, J.

2003-04-01

No. With few exceptions, lower ocean crust sampled by dredge or submersible in tectonic windows such as Atlantis Bank in the Indian Ocean or the MARK area on the Mid-Atlantic Ridge are not representative of the ocean crust. They represent tectonic mixing of rocks from the mantle and crust on large faults that also localize late magmatic intrusion. Where this can be sorted out, the in-situ crustal sections may generally represent a sub-horizontal cross-section through the lower crust and mantle and not a vertical one. The gabbroic rocks exposed represent largely high-level intrusions, highly hybridized by late melt flow along deep faults, or highly evolved gabbro at the distal ends of larger intrusions emplaced into the mantle near transforms. Oceanic gabbros have average compositions that lie outside the range of primary MORB compositions, and rarely are equivalent to spatially associated MORB either as a parent to, or as a residue of their crystallization. Oceanic gabbros sampled from these complexes generally are very coarse-grained, and are unlike those seen in nearly all ophiolites and layered intrusions. In addition, there are few exposures of gabbro and lower ocean crust and mantle in Pacific tectonic windows, though there the possibility of more representative sections is greater due to their exposure in propagating rifts. Limited samples of the mantle from near the midpoints of ocean ridge segments at slow-spreading rifts are from anomalous crustal environments such as ultra-slow spreading ridges or failed rifts. These include abundant dunites, as opposed to samples from fracture zones, which contain only about 1% dunite. While this indicates focused mantle flow towards the midpoint of a ridge, it also shows that fracture zone peridotites are not fully representative of the oceanic upper mantle. Major classes of rocks common in ophiolites, such as fine to medium grained layered primitive olivine gabbros, troctolites, wherlites and dunites, sheeted dikes, and epidosites are rarely or even not exposed. Models of lower ocean crust stratigraphy drawn from deep sea sampling, certainly from slow spreading ridges, do not match those for major intact ophiolites. Thus the ophiolite hypothesis remains unconfirmed for the lower ocean crust and shallow mantle, and it is nearly impossible to accurately identify the ocean ridge environment of any one ophiolite. The one deep drill hole that exists in lower ocean crust, 1.5 km Hole 735B, has a bulk composition too fractionated to mass balance MORB back to a primary mantle melt composition. Thus, a large mass of primitive cumulates is missing and could be situated in the crust below the base of the hole or in the underlying mantle. This is an unresolved question that is critical to understanding the evolution of the most common magma on earth: MORB. Since lower ocean crust and mantle represent a major portion of the crust and the exchange of mass, heat and volatiles from the earth's interior to its exterior this leaves a major hole in our understanding of the global geochemical and tectonic cycle which can only be filled by deep drilling.

Stability of ferrous-iron-rich bridgmanite under reducing midmantle conditions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shim, Sang-Heon; Grocholski, Brent; Ye, Yu

2017-06-05

Our current understanding of the electronic state of iron in lower-mantle minerals leads to a 8 considerable disagreement in bulk sound speed with seismic measurements if the lower mantle 9 has the same composition as the upper mantle (pyrolite). In the modelling studies, the content 10 and oxidation state of Fe in the minerals have been assumed to be constant throughout the lower 11 mantle. Here, we report high pressure experimental results in which Fe becomes dominantly 1 Fe2+ 12 in bridgmanite synthesized at 40–70GPa and 2,000K, while it is in mixed oxidation state (Fe3+/ P Fe = 60%) inmore » the samples synthesized below and above the pressure range. Little Fe3+ 13 14 in bridgmanite combined with the strong partitioning of Fe2+ into ferropericlase will alter the Fe 15 content for these minerals at 1,100–1,700 km depths. Our calculations show that the change in 16 iron content harmonizes the bulk sound speed of pyrolite with the seismic values in this region. 17 Our experiments support no significant changes in bulk composition for most of the mantle, 18 while possible changes in physical properties and processes (such as viscosity and mantle flow 19 patterns) in the mid mantle.« less

Numerical Mantle Convection Models With a Flexible Thermodynamic Interface

NASA Astrophysics Data System (ADS)

van den Berg, A. P.; Jacobs, M. H.; de Jong, B. H.

2001-12-01

Accurate material properties are needed for deep mantle (P,T) conditions in order to predict the longterm behavior of convection planetary mantles. Also the interpretation of seismological observations concerning the deep mantle in terms of mantle flow models calls for a consistent thermodynamical description of the basic physical parameters. We have interfaced a compressible convection code using the anelastic liquid approach based on finite element methods, to a database containing a full thermodynamic description of mantle silicates (Ita and King, J. Geophys. Res., 99, 15,939-15,940, 1994). The model is based on high resolution (P,T) tables of the relevant thermodynamic properties containing typically 50 million (P,T) table gridpoints to obtain resolution in (P,T) space of 1 K and an equivalent of 1 km. The resulting model is completely flexible such that numerical mantle convection experiments can be performed for any mantle composition for which the thermodynamic database is available. We present results of experiments for 2D cartesian models using a data base for magnesium-iron silicate in a pyrolitic composition (Stixrude and Bukowinski, Geoph.Monogr.Ser., 74, 131-142, 1993) and a recent thermodynamical model for magnesium silicate for the complete mantle (P,T) range, (Jacobs and Oonk, Phys. Chem. Mineral, 269, inpress 2001). Preliminary results of bulksound velocity distribution derived in a consistent way from the convection results and the thermodynamic database show a `realistic' mantle profile with bulkvelocity variations decreasing from several percent in the upper mantle to less than a percent in the deep lower mantle.

Genesis of zircon megacrysts in Cenozoic alkali basalts and the heterogeneity of subcontinental lithospheric mantle, eastern China

NASA Astrophysics Data System (ADS)

Yu, Yao; Xu, Xisheng; Chen, Xiaoming

2010-09-01

Zircon megacrysts are found in alluvial deposits associated with Cenozoic basalts from Changle in Shandong Province, Mingxi in Fujian Province and Penglai in Hainan Province within the coastal area of eastern China. They are colourless, transparent to light brown-maroon, and some of them are up to 16 mm long. U-Pb ages of zircon megacrysts from Changle, Mingxi and Penglai are 19.2 ± 0.7 Ma, 1.2 ± 0.1 Ma and 4.1 ± 0.2 Ma respectively, slightly older than the eruption ages of their corresponding host rocks (16.05-18.87 Ma, 0.9-2.2 Ma, 3 Ma). ɛHf(t) values of zircon megacrysts are 9.02 ± 0.49, 6.83 ± 0.47, 4.46 ± 0.48 for Changle, Mingxi and Penglai, respectively, which indicates their mantle origin. We suggest that the zircon megacrysts originated from metasomatised lithospheric mantle and were later brought up quickly by the host basaltic magma. The euhedral forms, uniform internal structure and chemical homogeneity within a single grain suggest crystallization under stable conditions. Pronounced positive Ce anomalies and negligible Eu anomalies suggest oxidizing conditions and little or no fractional crystallization of plagioclase. The differences in Hf-isotope compositions among the zircon megacrysts from different localities are consistent with the Sr-Nd-Pb isotopic compositions of their respective host basalts. This indicates that the host basalts acquired their isotopic signatures from the lithospheric mantle from which the zircon megacrysts derived. These data document the lateral compositional heterogeneity in the upper mantle beneath eastern China. Like mantle xenoliths, zircon megacrysts also have the potential to fingerprint the composition and evolution of the subcontinental lithospheric mantle.

Petrology and trace element geochemistry of the Honolulu volcanics, Oahu: implications for the oceanic mantle below Hawaii.

USGS Publications Warehouse

Clague, D.A.; Frey, F.A.

1982-01-01

These volcanic rocks are the products of small-volume, late-stage vents along rifts cutting the older massive Koolan tholeiitic shield on Oahu. Most of the lavas and tuffs have the geochemical features expected of near-primary magmas derived from a peridotite source with olivine Fo87-89, e.g. 100 Mg/(Mg + Fe2+) > 65, Ni > 250 p.p.m. and the presence of ultramafic mantle xenoliths at 18 of the 37 vents. Thus the geochemistry of the alkali olivine basalt, basanite, nephelinite and nepheline melilitite lavas and tuffs of these Honolulu volcanic rocks has been used to deduce the composition of their mantle source and the conditions under which they were generated by partial melting in the mantle. New major- and trace-element analyses for 31 samples are tabulated and indicate derivation by partial melting of a garnet (<10%) lherzolite source which was isotopically homogeneous and compositionally uniform for most major and trace elements, though apparently heterogeneous in TiO2, Zr, Hf, Nb and Ta (due perhaps to the low inferred degrees of melting which failed to exhaust the source in minor residual phases). In comparison with estimates of a primordial mantle composition and the mantle source of MORB, the garnet peridotite source of these Honolulu volcanics was increasingly enriched in the sequence heavy REE, Y, Tb, Ti, Sm, Zr and Hf, for which a multi-stage history is required. This composition differs from the source of the previously erupted tholeiitic shield, nor is it represented in the upper-mantle xenoliths in the lavas and tuff of the unit.-R.A.H.

Origin of mantle peridotite: Constraints from melting experiments to 16.5 GPa

NASA Astrophysics Data System (ADS)

Herzberg, Claude; Gasparik, Tibor; Sawamoto, Hiroshi

1990-09-01

Experimental data are reported for the melting of komatiite, peridotite, and chondrite compositions in the pressure range 5-16.5 GPa. All experiments were run using the multiple-anvil apparatus facilities at Nagoya and Stony Brook. Equilibrium between coexisting crystals and liquid is demonstrated to occur in less than 3 min in the 2100°C range. The anhydrous solidus in CaO-MgO-Al2O3-SiO2 has been calibrated and is shown to be about 100° higher than that for naturally occurring peridotite (KLB1). All melting curves have positive dT/dP. The effect of pressure is to expand the crystallization field of garnet at the expense of all other phases, resulting in a change in the liquidus phase from olivine to garnet at high pressures. The melting of rocks which contain the four crystalline phases olivine, orthopyroxene, clinopyroxene, and garnet is restricted to enstatite-rich compositions such as chondrite. For these it is demonstrated that melting is peritectic, rather than eutectic, and takes the form L+Opx = Ol+Cpx+Gt. Partial melting yields liquids with the following properties: 5 GPa for komatiite; and 10-15 GPa for liquid peridotite with about 40% MgO, but one that is unlike mantle peridotite in that it is distinctly enriched in silica. These results provide a test and refutation of the model that upper mantle peridotite originated by direct initial melting of a chondritic mantle (Herzberg and O'Hara, 1985). Unlike chondrite, partial melting of peridotite does not usually involve orthopyroxene. Instead, it occurs by the generation of ultrabasic liquids along a cotectic involving L+Ol+Cpx+Gt. Although the thermal and compositional characteristics of this cotectic have not been fully calibrated, it is very likely that it will degenerate into a thermal minimum (L+Ol+Cpx+Gt), compositionally similar to komatiite at 5 GPa and mantle peridotite at 10-15 GPa. Peridotite liquids that occupy a thermal minimum can be derived from those formed from the melting of chondrite by removal of orthopyroxene, followed by fractional crystallization of olivine, clinopyroxene, and garnet. The possibility exists that the thermal minimum is compositionally identical to mantle peridotite in the 10-15 GPa range. If this can be confirmed by experiment, the upper mantle can be understood as having originated by the fractional crystallization of peridotite liquids in a large-scale differentiation event, consistent with magma ocean models for an early Earth.

A kinematic model for the late Cenozoic development of southern California crust and upper mantle

NASA Technical Reports Server (NTRS)

Humphreys, Eugene D.; Hager, Bradford H.

1990-01-01

A model is developed for the young and ongoing kinematic deformation of the southern California crust and upper mantle. The kinematic model qualitatively explains both the overall seismic structure of the upper mantle and much of the known geological history of the late Cenozoic as consequences of ongoing convection beneath southern California. In this model, the high-velocity upper-mantle anomaly of the Transverse ranges is created through the convergence and sinking of the entire thickness of subcrustal lihtosphere, and the low-velocity upper-mantle anomaly beneath the Salton Trough region is attributed to high temperatures and 1-4 percent partial melt related to adiabatic decompression during mantle upwelling.

Nickel isotopic composition of the mantle

NASA Astrophysics Data System (ADS)

Gall, Louise; Williams, Helen M.; Halliday, Alex N.; Kerr, Andrew C.

2017-02-01

This paper presents a detailed high-precision study of Ni isotope variations in mantle peridotites and their minerals, komatiites as well as chondritic and iron meteorites. Ultramafic rocks display a relatively large range in δ60 Ni (permil deviation in 60 Ni /58 Ni relative to the NIST SRM 986 Ni isotope standard) for this environment, from 0.15 ± 0.07‰ to 0.36 ± 0.08‰, with olivine-rich rocks such as dunite and olivine cumulates showing lighter isotope compositions than komatiite, lherzolite and pyroxenite samples. The data for the mineral separates shed light on the origin of these variations. Olivine and orthopyroxene display light δ60 Ni whereas clinopyroxene and garnet are isotopically heavy. This indicates that peridotite whole-rock δ60 Ni may be controlled by variations in modal mineralogy, with the prediction that mantle melts will display variable δ60 Ni values due to variations in residual mantle and cumulate mineralogy. Based on fertile peridotite xenoliths and Phanerozoic komatiite samples it is concluded that the upper mantle has a relatively homogeneous Ni isotope composition, with the best estimate of δ60Nimantle being 0.23 ± 0.06‰ (2 s.d.). Given that >99% of the Ni in the silicate Earth is located in the mantle, this also defines the Ni isotope composition of the Bulk Silicate Earth (BSE). This value is nearly identical to the results obtained for a suite of chondrites and iron meteorites (mean δ60 Ni 0.26 ± 0.12‰ and 0.29 ± 0.10‰, respectively) showing that the BSE is chondritic with respect to its Ni isotope composition, with little to no Ni mass-dependent isotope fractionation resulting from core formation.

Upper mantle and crustal structure of southwestern Scandinavia: Results of the TopoScandiaDeep project

NASA Astrophysics Data System (ADS)

Köhler, A.; Balling, N.; Ebbing, J.; England, R.; Frassetto, A.; Gradmann, S.; Jacobsen, B. H.; Kvarven, T.; Maupin, V.; Medhus, A. Bondo; Mjelde, R.; Ritter, J.; Schweizer, J.; Stratford, W.; Thybo, H.; Wawerzinek, B.; Weidle, C.

2012-04-01

The origin of the Scandinavian mountains, located far away from any presently active plate margin, is still not well understood. In particular, it is not clear if the mountains are sustained isostatically either by crustal thickening or by light upper mantle material. Within the TopoScandiaDeep project (a collaborative research project within the ESF TOPO-EUROPE programme), we therefore analyse recently collected passive seismological and active seismic data in the southern Scandes and surrounding regions. We infer crustal and upper mantle (velocity) structures and relate them to results of gravity and temperature-composition modelling. The Moho under the high topography of southern Norway appears from controlled source seismic refraction and Receiver Functions as relatively shallow (<= 45 km) compared to the deeper conversion (>55 km) imaged beneath the low topography in Sweden and elsewhere in the Baltic Shield area outside Norway. The Receiver Function modeling as well as the active seismic results suggest that the differences in the observed Moho response may represent the transition between tectonically reworked Moho under southern Norway and an intact, cratonic crust-mantle boundary beneath the Baltic Shield. Furthermore, the 410km-discontinuity and the LAB is imaged, the latter one suggesting a lithospheric thickening in NE direction. Upper mantle P-wave and S-wave velocities in southern Sweden and southern Norway east of the Oslo Graben are correspondingly relatively high while lower velocities are observed in the southwestern part of Norway and northern Denmark. The lateral velocity gradient, interpreted as the southwestern boundary of thick Baltic Shield lithosphere, is remarkably sharp. Differences in upper mantle velocities are found at depths of 100-400 km and amount to ± 2-3%. S-to-P wave conversions, interpreted to originate from the lithosphere-asthenosphere boundary, are preliminary estimated to 90-120 km depth. Inversion of Rayleigh and Love surface wave phase velocity dispersion curves from observations of ambient noise and earthquakes yield another independent model of the crust and upper mantle structure below southern Norway. Inverted crustal velocities and Moho depths are consistent with the results of seismic refraction and receiver functions. Additionally, indications for radial crustal anisotropy of up to about 3% are found. The inferred upper mantle S-wave velocities show that the lithosphere under southern Norway has characteristics usually found under continental platforms and changes towards a cratonic-like velocity structure in the East, in agreement with the body wave tomography. All in all, these separate investigations give a very consistent and stable picture of the crust and upper mantle configuration. Integrated geophysical modeling of the results shows that a lateral transition from thinner, warmer lithosphere under southern Norway towards thicker, colder lithosphere under Sweden results in a density distribution that significantly adds to the isostatic support of Norway's high topography.

Density of alkali carbonate melts in the upper mantle and implications for the mobility of carbon at depth

NASA Astrophysics Data System (ADS)

Ritter, X.; Sanchez-Valle, C.; Laumonier, M.; King, A.; Guignot, N.; Gaillard, F.; Sifre, D.; Perrillat, J. P.

2017-12-01

The occurrence of carbonate-rich mantle rocks and diamonds in kimberlite rocks provide evidence for the presence of CO2 in the mantle. Carbon is recycled into the mantle via subduction and released through volcanic outgassing. An important fraction is retained at depth where partial melting of subducted lithologies produce alkali-rich carbonates along the CaCO3-MgCO3-K2CO3 join that infiltrate the mantle wedge [1]. Although volumetrically minor, these melts act as effective metasomatic agents that are related to source regions for diamond-bearing kimberlites [2]. The mobility of carbon at depth is controlled by the physical properties of carbonate liquids that remain largely unknown [3,4]. Here we report in-situ density measurements of alkaline carbonates at crustal and upper mantle conditions using synchrotron X-ray absorption in a Paris-Edinburgh press at beamline Psiché (Synchrotron Soleil). Experiments were conducted in several compositions along the CaCO3-K2CO3 and MgCO3-K2CO3 join up to 1400 K and 3 GPa. The starting materials included a mixture of synthetic K2CO3 and natural calcite and K2Mg(CO3)2 glasses synthesized at 0.15 GPa and 1098 K in an internally heated pressure vessel. The samples were cold pressurized and heated until the molten stage was confirmed by X-ray diffraction. The results were fitted to derive the first robust model for the density of alkali carbonates that mimic liquids from the incipient melting of subducted lithologies at crustal and upper mantle conditions. We combine the results of the present study with available data on the viscosity of carbonate liquids and molecular dynamic predictions to discuss the mobility and migration rates of carbonate liquids in the upper mantle.[1] Litasov et al. 2012 Geology 41, 79-82. [2] Grassi and Schmidt 2011, Contrib Min Petr 162, 169-191. [3] Dobson et al. 1996, EPSL 143, 207-215. [4] Kono et al. 2014 Nature Communications 5:5091.

Development of diapiric structures in the upper mantle due to phase transitions

NASA Technical Reports Server (NTRS)

Liu, M.; Yuen, D. A.; Zhao, W.; Honda, S.

1991-01-01

Solid-state phase transition in time-dependent mantle convection can induce diapiric flows in the upper mantle. When a deep mantle plume rises toward phase boundaries in the upper mantle, the changes in the local thermal buoyancy, local heat capacity, and latent heat associated with the phase change at a depth of 670 kilometers tend to pinch off the plume head from the feeding stem and form a diapir. This mechanism may explain episodic hot spot volcanism. The nature of the multiple phase boundaries at the boundary between the upper and lower mantle may control the fate of deep mantle plumes, allowing hot plumes to go through and retarding the tepid ones.

Mantle redox evolution and the oxidation state of the Archean atmosphere

NASA Technical Reports Server (NTRS)

Kasting, J. F.; Eggler, D. H.; Raeburn, S. P.

1993-01-01

Current models predict that the early atmosphere consisted mostly of CO2, N2, and H2O, along with traces of H2 and CO. Such models are based on the assumption that the redox state of the upper mantle has not changed, so that volcanic gas composition has remained approximately constant with time. We argue here that this assumption is probably incorrect: the upper mantle was originally more reduced than today, although not as reduced as the metal arrest level, and has become progressively more oxidized as a consequence of the release of reduced volcanic gases and the subduction of hydrated, oxidized seafloor. Data on the redox state of sulfide and chromite inclusions in diamonds imply that the process of mantle oxidation was slow, so that reduced conditions could have prevailed for as much as half of the earth's history. To be sure, other oxybarometers of ancient rocks give different results, so the question of when the mantle redox state has changed remains unresolved. Mantle redox evolution is intimately linked to the oxidation state of the primitive atmosphere: A reduced Archean atmosphere would have had a high hydrogen escape rate and should correspond to a changing mantle redox state; an oxidized Archean atmosphere should be associated with a constant mantle redox state. The converses of these statements are also true. Finally, our theory of mantle redox evolution may explain why the Archean atmosphere remained oxygen-deficient until approximately 2.0 billion years ago (Ga) despite a probable early origin for photosynthesis.

Upper Mantle Anisotropy Under Fast Spreading Mid-ocean Ridges: 2-D Whole Mantle Convection Model With Subduction

NASA Astrophysics Data System (ADS)

Lee, C.; Zhou, Y.; King, S. D.

2008-12-01

Analyses of seismic anisotropy caused by spatial alignments of anisotropic minerals (e.g., olivine) have been widely used to infer mantle flow directions in the upper mantle. Deep seismic anisotropy beneath fast spreading mid-ocean ridges (e.g., East Pacific Rise) has been recently observed at depths of 200-300 km and even down to the transition zone, with polarization changes in radial anisotropy from VSH < VSV (shallow) to VSH < VSV (deep). We investigate the origin of the observed deep seismic anisotropy and polarization changes beneath the EPR in 2-D Cartesian numerical models using both kinematically (prescribed velocity) and dynamically (negative buoyancy) driven ridge spreading. Because subduction is thought to be an important controlling factor in the style of ridge spreading and mantle convection, we consider a subduction zone developing at the prescribed weak zone. A whole mantle domain expressed by a one by four box (2890 by 11560 km) is used to minimize the boundary effects on the subducting slab. For the upper mantle rheology, we consider composite viscosity of diffusion and dislocation creep for dry olivine to evaluate the effects of lateral variation of mantle viscosity and the rheological changes from dislocation to diffusion creep under the mid-ocean ridge. For the lower mantle rheology, we use diffusion creep for dry olivine by increasing grain size to match relevant lower mantle viscosity. We also consider the 660 km phase transition with density and viscosity jump as well as Clapeyron slope. Anisotropy is evaluated using finite-strain ellipses based on the assumption that a-axes of olivine crystals are parallel to the major axes of the finite-strain ellipses. Our preliminary results show 1) in general, the development of VSH < VSV anisotropy is confined only in a narrow region under the ridge axis at depths of 200- 300 km; 2) strong VSH > VSV anisotropy can be found in the 'asthenosphere' beneath the entire spreading oceanic lithosphere; and 3) the dominate creep mechanism changes from dislocation creep to diffusion creep at depths of 300-400 km; indicating a more isotropic lower upper mantle. We conclude that our geodynamical modeling in a passive ridge spreading system does not produce the deep seismic anisotropy recently observed beneath the EPR. However, we do not consider partial melting, dynamic recrystallization and anisotropic viscosity which would change seismic interpretation and mantle flow, and thus further study is required.

Petrological processes in mantle plume heads: Evidence from study of mantle xenoliths in the late Cenozoic alkali Fe-Ti basalts in Western Syria

NASA Astrophysics Data System (ADS)

Sharkov, Evgenii

2015-04-01

It is consensus now that within-plate magmatism is considered with ascending of mantle plumes and adiabatic melting of their head. At the same time composition of the plumes' matter and conditions of its adiabatic melting are unclear yet. The major source of objective information about it can be mantle xenoliths in alkali basalts and basanites which represent fragments of material of the plume heads above magma-generation zone. They are not represent material in melting zone, however, carry important information about material of modern mantle plumes, its phase composition and components, involved in melting. Populations of mantle xenoliths in basalts are characterized by surprising sameness in the world and represented by two major types: (1) dominated rocks of ``green'' series, and (2) more rare rocks of ``black'' series, which formed veins in the ``green'' series matrix. It can evidence about common composition of plume material in global scale. In other words, the both series of xenoliths represent two types of material of thermochemical mantle plumes, ascended from core-mantle boundary (Maruyama, 1994; Dobretsov et al., 2001). The same types of xenoliths are found in basalts and basanites of Western Syria (Sharkov et al., 1996). Rocks of ``green'' series are represented by Sp peridotites with cataclastic and protogranular structures and vary in composition from dominated spinel lherzolites to spinel harzburgites and rare spinel pyroxenites (websterites). It is probably evidence about incomplete homogenizing of the plume head matter, where material, underwent by partial melting, adjoins with more fertile material. Such heterogeneity was survived due to quick cooling of upper rim of the plume head in contact with relatively cold lithosphere. Essential role among xenoliths of the ``black'' series play Al-Ti-augite and water-bearing phases like hornblende (kaersutute) and Ti-phlogopite. Rocks of this series are represented by wehrlite, clinopyroxenite, amphibole clinopyroxenite, hornbledite, etc. as well as megacrysts of Al-Ti-augite, kaersutite, ilmenite, sanidine, etc. Numerous vesicles often occurred in megacrysts, especially in kaersurtite. Sp peridotites of the matrix are sharply different on their geochemical features from the ``black series'' rocks (in this case, megacrysts of kaersutite) which are the most close to composition of xenoliths-bearing alkali basalts. From this follows that geochemistry of plume-related basalts was determined by mantle fluids which occurred in magma-generation zone. Very likely, that these fluids, enriched in Fe, Ti, LREE, alkalis, and incompatible elements, initially were parts of intergranular material of original mantle plume material and were released due to its decompression. Because their high mobility, the fluids percolated upwards and accumulated in the upper part of the mantle plume head, where promoted its melting by lowering of solidus of the matter. Excess of the fluids gathered beneath the cooled upper rim and penetrated in its rocks which led to appearance of centers of secondary melting (melt-pockets). Very likely, that these secondary melts formed rocks of the ``black series'' (Ismail et al., 2008;Ryabchkov et al., 2011; Ma et al., 2014). According to geobarometric estimations, Sp peridotite xenoliths from Syria derived from depths 24-42 km (0.8-1.4 GPa) under temperatures 896-980oC; formation of melt-pockets, enriched in volatiles, occurred at the depths 21-27 km (0.7-0.9 GPa) under 826-981oC (Sharkov et al., 1996; Ismail et al., 2008; Ma et al., 2014). From this follows that plumeheads reached depths approximately 21-30 km which is in agree with practically absence of lower-crustal xenoliths in the populations. One of the problems of plume-related magmatism is coexisting of alkali and tholeiitic basalts, which origin often considered with different PT conditions. However, these basalt not rarely interlayered, especially at low and middle levels of LIPs or in single volcanoes (Hawaii, Etna, etc.) which is not in a good agreement with such idea. We suggest that the situation can be more likely explained by nonuniform impregnation of peridotite matrix with fluid components which composition and/or quantity can play essential role in composition of smeltings. It is especially important because even small differences in their ñomposition near to plane of SiO2 saturation in ``basalt tetrahedron'' (Yoder and Tilley, 1962) lead to appearance of Ne-normative or Ne-free melts at practically similar PT conditions. Thus, judging on composition of the mantle xenoliths in basalts of all occurrences in the world, quite possible that Sp peridotites (mainly lherzolites) together with intergranular geochemical-enriched fluid components represent the matter of the modern thermochemical mantle plumes. Origin of two major types of the plume-related magmas, probably, considered with fluid regime in the plume head.

The role of mantle-hybridization and crustal contamination in the petrogenesis of lithospheric mantle-derived alkaline rocks: constraints from Os and Hf isotopes

NASA Astrophysics Data System (ADS)

Mayer, B.; Jung, S.; Brauns, M.; Münker, C.

2018-06-01

The Rhön area as part of the Central European Volcanic Province (CEVP) hosts an unusual suite of Tertiary 24-Ma old hornblende-bearing alkaline basalts that provide insights into melting and fractionation processes within the lithospheric mantle. These chemically primitive to slightly evolved and isotopically (Sr, Nd, Pb) depleted basalts have slightly lower Hf isotopic compositions than respective other CEVP basalts and Os isotope compositions more radiogenic than commonly observed for continental intraplate alkaline basalts. These highly radiogenic initial 187Os/188Os ratios (0.268-0.892) together with their respective Sr-Nd-Pb isotopic compositions are unlikely to result from crustal contamination alone, although a lack of Os data for lower crustal rocks from the area and limited data for CEVP basalts or mantle xenoliths preclude a detailed evaluation. Similarly, melting of the same metasomatized subcontinental lithospheric mantle as inferred for other CEVP basalts alone is also unlikely, based on only moderately radiogenic Os isotope compositions obtained for upper mantle xenoliths from elsewhere in the province. Another explanation for the combined Nd, Sr and Os isotope data is that the lavas gained their highly radiogenic Os isotope composition through a mantle "hybridization", metasomatism process. This model involves a mafic lithospheric component, such as an intrusion of a sublithospheric primary alkaline melt or a melt derived from subducted oceanic material, sometime in the past into the lithospheric mantle where it metasomatized the ambient mantle. Later at 24 Ma, thermal perturbations during rifting forced the isotopically evolved parts of the mantle together with the peridotitic ambient mantle to melt. This yielded a package of melts with highly correlated Re/Os ratios and radiogenic Os isotope compositions. Subsequent movement through the crust may have further altered the Os isotope composition although this effect is probably minor for the majority of the samples based on radiogenic Nd and unradiogenic Sr isotope composition of the lavas. If the radiogenic Os isotope composition can be explained by a mantle-hybridization and metasomatism model, the isotopic compositions of the hornblende basalts can be satisfied by ca. 5-25% addition of the mafic lithospheric component to an asthenospheric alkaline magma. Although a lack of isotope data for all required endmembers make this model somewhat speculative, the results show that the Re-Os isotope system in continental basalts is able to distinguish between crustal contamination and derivation of continental alkaline lavas from isotopically evolved peridotitic lithosphere that was contaminated by mafic material in the past and later remelted during rifting. The Hf isotopic compositions are slightly less radiogenic than in other alkaline basalts from the province and indicate the derivation of the lavas from low Lu-Hf parts of the lithospheric mantle. The new Os and Hf isotope data constrain a new light of the nature of such metasomatizing agents, at least for these particular rocks, which represent within the particular volcanic complex the first product of the volcanism.

Airborne dust transport to the eastern Pacific Ocean off southern California: Evidence from San Clemente Island

USGS Publications Warehouse

Muhs, D.R.; Budahn, J.; Reheis, M.; Beann, J.; Skipp, G.; Fisher, E.

2007-01-01

Islands are natural dust traps, and San Clemente Island, California, is a good example. Soils on marine terraces cut into Miocene andesite on this island are clay-rich Vertisols or Alfisols with vertic properties. These soils are overlain by silt-rich mantles, 5-20 cm thick, that contrast sharply with the underlying clay-rich subsoils. The silt mantles have a mineralogy that is distinct from the island bedrock. Silt mantles are rich in quartz, which is rare in the island andesite. The clay fraction of the silt mantles is dominated by mica, also absent from local andesite, and contrasts with the subsoils, dominated by smectite. Ternary plots of immobile trace elements (Sc-Th-La and Ta-Nd-Cr) show that the island andesite has a composition intermediate between average upper continental crust and average oceanic crust. In contrast, the silt and, to a lesser extent, clay fractions of the silt mantles have compositions closer to average upper continental crust. The silt mantles have particle size distributions similar to loess and Mojave Desert dust, but are coarser than long-range-transported Asian dust. We infer from these observations that the silt mantles are derived from airborne dust from the North American mainland, probably river valleys in the coastal mountains of southern California and/or the Mojave Desert. Although average winds are from the northwest in coastal California, easterly winds occur numerous times of the year when "Santa Ana" conditions prevail, caused by a high-pressure cell centered over the Great Basin. Examination of satellite imagery shows that easterly Santa Ana winds carry abundant dust to the eastern Pacific Ocean and the California Channel Islands. Airborne dust from mainland North America may be an important component of the offshore sediment budget in the easternmost Pacific Ocean, a finding of potential biogeochemical and climatic significance.

Compositionally Controlled Volatile Content of Nominally Volatile-Free Minerals in the Continental Upper Mantle of Southern Gondwana (Patagonia & W. Antarctica)

NASA Astrophysics Data System (ADS)

Rooks, E. E.; Gibson, S. A.; Leat, P. T.; Petrone, C. M.

2015-12-01

H2O and F contents affect many physical and chemical properties of the upper mantle, including melting temperature and viscosity. These elements are hosted by hydrous and F-rich phases, and by modally abundant, nominally-anhydrous/halogen-free mantle minerals, which can potentially accommodate the entire volatile budget of the upper mantle. We present high-precision SIMS analyses of H2O, and F in mantle xenoliths hosted by recently-erupted (5-10 Ka) alkali basalts from south Patagonia (Pali Aike) and older (c. 25 Ma) alkali basalts from localities along the Antarctic Peninsula. Samples are well characterised peridotites and pyroxenites, from a range of depths in the off-craton lithospheric mantle. Minerals are relatively dry: H2O contents of olivine span 0-49 ppm, orthopyroxene 150-235 ppm and clinopyroxene 100-395 ppm, with highest concentrations found in spinel-garnet lherzolites from Pali Aike. These H2O concentrations fall within the global measured range for off-craton mantle minerals. H2O and F are correlated, and the relative compatibility of F in mantle phases is clinopyroxene>orthopyroxene>olivine. However, elevated F concentrations of 100-210 ppm are found in pyroxenites from two Antarctic localities. This elevated F content is not correlated with high H2O, suggesting that these rocks interacted with a F-rich melt. In clinopyroxenes, F concentration is correlated with Ti, and the ratio of M1Ti to M1Al + M1Cr, suggesting a charge balanced substitution. Consistency between samples (excepting high-F pyroxenites) suggests a constant F-budget, and that concentrations in clinopyroxenes are controlled by mineral chemistry. In orthopyroxene, F correlates with CaO, but no other major or minor elements. Large variability of H2O concentrations within samples is attributed to diffusive loss during ascent. Cl is negligible in all samples, indicating little or no influence of slab fluids from this long-lived subduction zone.

Geochemical Evidence for a Terrestrial Magma Ocean

NASA Technical Reports Server (NTRS)

Agee, Carl B.

1999-01-01

The aftermath of phase separation and crystal-liquid fractionation in a magma ocean should leave a planet geochemically differentiated. Subsequent convective and other mixing processes may operate over time to obscure geochemical evidence of magma ocean differentiation. On the other hand, core formation is probably the most permanent, irreversible part of planetary differentiation. Hence the geochemical traces of core separation should be the most distinct remnants left behind in the mantle and crust, In the case of the Earth, core formation apparently coincided with a magma ocean that extended to a depth of approximately 1000 km. Evidence for this is found in high pressure element partitioning behavior of Ni and Co between liquid silicate and liquid iron alloy, and with the Ni-Co ratio and the abundance of Ni and Co in the Earth's upper mantle. A terrestrial magma ocean with a depth of 1000 km will solidify from the bottom up and first crystallize in the perovskite stability field. The largest effect of perovskite fractionation on major element distribution is to decrease the Si-Mg ratio in the silicate liquid and increase the Si-Mg ratio in the crystalline cumulate. Therefore, if a magma ocean with perovskite fractionation existed, then one could expect to observe an upper mantle with a lower than chondritic Si-Mg ratio. This is indeed observed in modern upper mantle peridotites. Although more experimental work is needed to fully understand the high-pressure behavior of trace element partitioning, it is likely that Hf is more compatible than Lu in perovskite-silicate liquid pairs. Thus, perovskite fractionation produces a molten mantle with a higher than chondritic Lu-Hf ratio. Arndt and Blichert-Toft measured Hf isotope compositions of Barberton komatiites that seem to require a source region with a long-lived, high Lu-Hf ratio. It is plausible that that these Barberton komatiites were generated within the majorite stability field by remelting a perovskite-depleted part of the upper mantle transition zone.

Petrochemical and petrophysical characterization of the lower crust and the Moho beneath the West African Craton, based on Xenoliths from Kimberlites

NASA Technical Reports Server (NTRS)

Haggerty, Stephen E.; Toft, Paul B.

1988-01-01

Additional evidence to the composition of the lower crust and uppermost mantle was presented in the form of xenolith data. Xenoliths from the 2.7-Ga West African Craton indicate that the Moho beneath this shield is a chemically and physically gradational boundary, with intercalations of garnet granulite and garnet eclogite. Inclusions in diamonds indicate a depleted upper mantle source, and zenolith barometry and thermometry data suggest a high mantle geotherm with a kink near the Moho. Metallic iron in the xenoliths indicates that the uppermost mantle has a significant magnetization, and that the depth to the Curie isotherm, which is usually considered to be at or above the Moho, may be deeper than the Moho.

Composition of the earth's upper mantle. II - Volatile trace elements in ultramafic xenoliths

NASA Technical Reports Server (NTRS)

Morgan, J. W.; Wandless, G. A.; Petrie, R. K.; Irving, A. J.

1980-01-01

Radiochemical neutron activation analysis was used to determine the nine volatile elements Ag, Bi, Cd, In, Sb, Se, Te, Tl, and Zn in 19 ultramafic rocks, consisting mainly of spinel and garnet lherzolites. A sheared garnet lherzolite, PHN 1611, may approximate undepleted mantle material and tends to have a higher volatile element content than the depleted mantle material represented by spinel lherzolites. Comparisons of continental basalts with PHN 1611 and of oceanic ridge basalts with spinel lherzolites show similar basalt: source material partition factors for eight of the nine volatile elements, Sb being the exception. The strong depletion of Te and Se in the mantle, relative to lithophile elements of similar volatility, suggests that 97% of the earth's S, Se and Te may be in the outer core.

Two-component mantle melting-mixing model for the generation of mid-ocean ridge basalts: Implications for the volatile content of the Pacific upper mantle

NASA Astrophysics Data System (ADS)

Shimizu, Kei; Saal, Alberto E.; Myers, Corinne E.; Nagle, Ashley N.; Hauri, Erik H.; Forsyth, Donald W.; Kamenetsky, Vadim S.; Niu, Yaoling

2016-03-01

We report major, trace, and volatile element (CO2, H2O, F, Cl, S) contents and Sr, Nd, and Pb isotopes of mid-ocean ridge basalt (MORB) glasses from the Northern East Pacific Rise (NEPR) off-axis seamounts, the Quebrada-Discovery-GoFar (QDG) transform fault system, and the Macquarie Island. The incompatible trace element (ITE) contents of the samples range from highly depleted (DMORB, Th/La ⩽ 0.035) to enriched (EMORB, Th/La ⩾ 0.07), and the isotopic composition spans the entire range observed in EPR MORB. Our data suggest that at the time of melt generation, the source that generated the EMORB was essentially peridotitic, and that the composition of NMORB might not represent melting of a single upper mantle source (DMM), but rather mixing of melts from a two-component mantle (depleted and enriched DMM or D-DMM and E-DMM, respectively). After filtering the volatile element data for secondary processes (degassing, sulfide saturation, assimilation of seawater-derived component, and fractional crystallization), we use the volatiles to ITE ratios of our samples and a two-component mantle melting-mixing model to estimate the volatile content of the D-DMM (CO2 = 22 ppm, H2O = 59 ppm, F = 8 ppm, Cl = 0.4 ppm, and S = 100 ppm) and the E-DMM (CO2 = 990 ppm, H2O = 660 ppm, F = 31 ppm, Cl = 22 ppm, and S = 165 ppm). Our two-component mantle melting-mixing model reproduces the kernel density estimates (KDE) of Th/La and 143Nd/144Nd ratios for our samples and for EPR axial MORB compiled from the literature. This model suggests that: (1) 78% of the Pacific upper mantle is highly depleted (D-DMM) while 22% is enriched (E-DMM) in volatile and refractory ITE, (2) the melts produced during variable degrees of melting of the E-DMM controls most of the MORB geochemical variation, and (3) a fraction (∼65% to 80%) of the low degree EMORB melts (produced by ∼1.3% melting) may escape melt aggregation by freezing at the base of the oceanic lithosphere, significantly enriching it in volatile and trace element contents. Our results are consistent with previously proposed geodynamical processes acting at mid-ocean ridges and with the generation of the E-DMM. Our observations indicate that the D-DMM and E-DMM have (1) a relatively constant CO2/Cl ratio of ∼57 ± 8, and (2) volatile and ITE element abundance patterns that can be related by a simple melting event, supporting the hypothesis that the E-DMM is a recycled oceanic lithosphere mantle metasomatized by low degree melts. Our calculation and model give rise to a Pacific upper mantle with volatile content of CO2 = 235 ppm, H2O = 191 ppm, F = 13 ppm, Cl = 5 ppm, and S = 114 ppm.

Causes and extent of subduction-related highly siderophile element processing in oceanic mantle

NASA Astrophysics Data System (ADS)

O Driscoll, B.; Walker, R. J.; Day, J. M.; Daly, J. S.; Ash, R. D.

2013-12-01

Oceanic mantle samples that are accessible for study (e.g., abyssal and ophiolite peridotites) are commonly viewed as having undergone melt extraction at mid-ocean ridges (MOR). However, many ophiolite peridotites have been subjected to comparatively higher degrees of partial melting in supra-subduction zone (SSZ) environments too[1]. The ~497 Ma Leka Ophiolite, Norway, offers an ideal location for assessing the extent to which SSZ melting processes overprint the residual signatures of prior melt extraction. It comprises ~15 km[2] of well-exposed mantle and lower crustal peridotites that exhibit relatively limited serpentinisation. Extensive lithological heterogeneity is evident within the harzburgitic host rock, in the form of lenses and sheets of dunite, pyroxenite and chromitite. These have been interpreted as representing successive generations of SSZ-related channelised upper mantle melt migration and melt-rock interaction[2]. The integrated application of highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances and 187Os/188Os measurements of oceanic mantle peridotite has proved valuable in assessing the timing of mantle melting processes occurring within the upper mantle, as well as the scales of upper mantle heterogeneity[3,4]. At 497 Ma, the Os isotopic compositions of Leka harzburgites averaged ~2% more radiogenic than the projected average for abyssal peridotites[4] at that time. Several of the harzburgites are characterised by low initial 187Os/188Os (as low as 0.1202), interpreted as reflecting Proterozoic melt depletion, a common characteristic of melt-depleted peridotites comprising most ophiolites. Dunites, pyroxenites and chromitites show considerably more variable initial 187Os/188Os and HSE abundances; some pyroxenites have extreme Pt abundances (to 1-2 ppm), supra-chondritic Pt/Os and 187Os/188Os, yet some of the dunites are also characterised by 187Os/188Os well within the range of the harzburgites. A number of dunites have relatively radiogenic initial 187Os/188Os (up to 0.1385), suggesting that they either formed from selectively more radiogenic melt or that their development predates that of the ophiolite by ~500 Ma. Assuming that the significant lithological heterogeneity observed in the Leka upper mantle section was generated during SSZ melting, it appears that consequent modification of the HSE compositions and 187Os/188Os was restricted to pyroxenites and some, but perhaps not all, of the dunites. Mineral scale observations, coupled with a comparison of the Leka data and those from the ~492 Ma Shetland Ophiolite (Scotland)[3], suggest that sulphide/arsenide mobilisation during channelised melt-rock interactions is the trigger for fractionation of the HSE and modification of 187Os/188Os in ophiolite dunites. [1] Dilek and Furnes (2011) GSA Bulletin 123(3/4), 387-411 [2] Maaløe (2005) Mineralogy and Petrology 85, 163-204 [3] O'Driscoll et al. (2012) EPSL 333-334, 226-237 [4] Liu et al. (2009) EPSL 283, 122-132

Multiple episodes of partial melting, depletion, metasomatism and enrichment processes recorded in the heterogeneous upper mantle sequence of the Neotethyan Eldivan ophiolite, Turkey

NASA Astrophysics Data System (ADS)

Uysal, Ibrahim; Ersoy, E. Yalçın; Dilek, Yildirim; Kapsiotis, Argyrios; Sarıfakıoğlu, Ender

2016-03-01

The Eldivan ophiolite along the Izmir-Ankara-Erzincan suture zone in north-central Anatolia represents a remnant of the Neotethyan oceanic lithosphere. Its upper mantle peridotites include three lithologically and compositionally distinct units: clinopyroxene (cpx)-harzburgite and lherzolite (Group-1), depleted harzburgite (Group-2), and dunite (Group-3). Relics of primary olivine and pyroxene occur in the less refractory harzburgites, and fresh chromian spinel (Cr-spinel) is ubiquitous in all peridotites. The Eldivan peridotites reflect a petrogenetic history evolving from relatively fertile (lherzolite and cpx-harzburgite) toward more depleted (dunite) compositions through time, as indicated by (i) a progressive decrease in the modal cpx distribution, (ii) a progressive increase in the Cr#s [Cr / (Cr + Al)] of Cr-spinel (0.15-0.78), and (iii) an increased depletion in the whole-rock abundances of some magmaphile major oxides (Al2O3, CaO, SiO2 and TiO2) and incompatible trace elements (Zn, Sc, V and Y). The primitive mantle-normalized REE patterns of the Group-1 and some of the Group-2 peridotites display LREE depletions. Higher YbN and lower SmN/YbN ratios of these rocks are compatible with their formation after relatively low degrees (9-25%) of open-system dynamic melting (OSDM) of a Depleted Mid-ocean ridge Mantle (DMM) source, which was then fluxed with small volumes of oceanic mantle-derived melt [fluxing ratio (β): 0.7-1.2%]. Accessory Cr-spinel compositions (Cr# = 015-0.53) of these rocks are consistent with their origin as residual peridotites beneath a mid-ocean ridge axis. Part of the Group-2 harzburgites exhibit lower YbN and higher SmN/YbN ratios, LREE-enriched REE patterns, and higher Cr-spinel Cr#s ranging between 0.54 and 0.61. Trace element compositions of these peridotites can be modeled by approximately 15% OSDM of a previously 17% depleted DMM, which was then fluxed (β: 0.4%) with subduction-influenced melt. The Group-3 dunite samples contain Cr-spinel with elevated Cr#s (0.73-0.78) and low-TiO2 contents (< 0.13 wt.%), implying higher degrees of melting (21-24%) of an already depleted DMM that was triggered by infiltration of low-Ti boninite melt with fluxing rates of 0.4-4.0%. The existence of interstitial, idiomorphic Cr-spinel (high Cr# and low Ti) in the Group-3 dunites is consistent with this interpretation. The occurrence of both MOR- and SSZ-type peridotites in the Eldivan ophiolite suggests that its heterogeneous upper mantle was produced as a result of different partial melting and melt-rock reaction processes in different tectonic settings within the Neotethyan realm.

Shear wave reflectivity imaging of the Nazca-South America subduction zone: Stagnant slab in the mantle transition zone?

NASA Astrophysics Data System (ADS)

Contenti, Sean; Gu, Yu Jeffrey; Ökeler, Ahmet; Sacchi, Mauricio D.

2012-01-01

In this study we utilize over 5000 SS waveforms to investigate the high-resolution mantle reflectivity structure down to 1200 km beneath the South American convergent margin. Our results indicate that the dynamics of the Nazca subduction are more complex than previously suggested. The 410- and 660-km seismic discontinuities beneath the Pacific Ocean and Amazonian Shield exhibit limited lateral depth variations, but their depths vary substantially in the vicinity of the subducting Nazca plate. The reflection amplitude of the 410-km discontinuity is greatly diminished in a ˜1300-km wide region in the back-arc of the subducting plate, which is likely associated with a compositional heterogeneity on top of the upper mantle transition zone. The underlying 660-km discontinuity is strongly depressed, showing localized depth and amplitude variations both within and to the east of the Wadati-Benioff zone. The width of this anomalous zone (˜1000 km) far exceeds that of the high-velocity slab structure and suggesting significant slab deformation within the transition zone. The shape of the 660-km discontinuity and the presence of lower mantle reflectivity imply both stagnation and penetration are possible as the descending Nazca slab impinges upon the base of the upper mantle.

Compositionally heterogeneous podiform chromitite in the Shetland Ophiolite Complex (Scotland): Implications for chromitite petrogenesis and late-stage alteration in the upper mantle portion of a supra-subduction zone ophiolite

NASA Astrophysics Data System (ADS)

Derbyshire, E. J.; O'Driscoll, B.; Lenaz, D.; Gertisser, R.; Kronz, A.

2013-03-01

The mantle sequence of the ~ 492 Ma Shetland Ophiolite Complex (SOC; Scotland) contains abundant compositionally heterogeneous podiform chromitite bodies enclosed in elongate dunite lenses in the vicinity of the petrological Moho. Chromitite petrogenesis and late-stage alteration events recorded in these seams are examined here using petrography, mineral chemistry and crystal structural data. The resistant nature of Cr-spinel to serpentinisation and other late-stage alteration means that primary igneous compositions are preserved in unaltered crystal cores. Chromitite mineralogy and texture from five sampled localities at The Viels, Hagdale, Harold's Grave, Nikka Vord and Cliff reveal significant inter-pod chemical heterogeneity. The Cr-spinel mineral chemistry is consistent with supra-subduction zone melt extraction from the SOC peridotites. The occurrence of chromitite seams in the centres of the dunite lenses combined with variable Cr-spinel compositions at different chromitite seam localities supports a model of chromitite formation from spatially (and temporally?) fluctuating amounts of melt-rock interaction through channelised and/or porous melt flow. Pervasive serpentinisation of the SOC has led to the almost complete replacement of the primary (mantle) silicate mineral assemblages with serpentine (lizardite with minor chrysotile and antigorite). Magmatic sulphide (e.g., pentlandite) in dunite and chromitite is locally converted to reduced Ni-sulphide varieties (e.g., heazlewoodite and millerite). A post-serpentinisation (prograde) oxidisation event is recorded in the extensively altered Cliff chromitite seams in the west of the studied area, where chromitite Cr-spinel is extensively altered to ferritchromit. The ferritchromit may comprise > 50% of the volume of the Cliff Cr-spinels and contain appreciable quantities of 1-2 μm inclusions of sperrylite (PtAs2) and Ni-arsenide, signifying the coeval formation of these minerals with ferritchromit at temperatures of up to ~ 500 °C. The SOC chromitite Cr-spinels thus not only preserve key insights into the complex melting processes occurring in the upper mantle wedge but can also be utilised to construct a comprehensive alteration history of the lower mantle portions of such supra-subduction zone ophiolites.

Density, temperature, and composition of the North American lithosphere—New insights from a joint analysis of seismic, gravity, and mineral physics data: 2. Thermal and compositional model of the upper mantle

NASA Astrophysics Data System (ADS)

Tesauro, Magdala; Kaban, Mikhail K.; Mooney, Walter D.; Cloetingh, Sierd A. P. L.

2014-12-01

Temperature and compositional variations of the North American (NA) lithospheric mantle are estimated using a new inversion technique introduced in Part 1, which allows us to jointly interpret seismic tomography and gravity data, taking into account depletion of the lithospheric mantle beneath the cratonic regions. The technique is tested using two tomography models (NA07 and SL2013sv) and different lithospheric density models. The first density model (Model I) reproduces the typical compositionally stratified lithospheric mantle, which is consistent with xenolith samples from the central Slave craton, while the second one (Model II) is based on the direct inversion of the residual gravity and residual topography. The results obtained, both in terms of temperature and composition, are more strongly influenced by the input models derived from seismic tomography, rather than by the choice of lithospheric density Model I versus Model II. The final temperatures estimated in the Archean lithospheric root are up to 150°C higher than in the initial thermal models obtained using a laterally and vertically uniform "fertile" compositional model and are in agreement with temperatures derived from xenolith data. Therefore, the effect of the compositional variations cannot be neglected when temperatures of the cratonic lithospheric mantle are estimated. Strong negative compositional density anomalies (<-0.03 g/cm3), corresponding to Mg # (100 × Mg/(Mg + Fe)) >92, characterize the lithospheric mantle of the northwestern part of the Superior craton and the central part of the Slave and Churchill craton, according to both tomographic models. The largest discrepancies between the results based on different tomography models are observed in the Proterozoic regions, such as the Trans Hudson Orogen (THO), Rocky Mountains, and Colorado Plateau, which appear weakly depleted (>-0.025 g/cm3 corresponding to Mg # ˜91) when model NA07 is used, or locally characterized by high-density bodies when model SL2013sv is used. The former results are in agreement with those based on the interpretation of xenolith data. The high-density bodies might be interpreted as fragments of subducted slabs or of the advection of the lithospheric mantle induced from the eastward-directed flat slab subduction. The selection of a seismic tomography model plays a significant role when estimating lithospheric density, temperature, and compositional heterogeneity. The consideration of the results of more than one model gives a more complete picture of the possible compositional variations within the NA lithospheric mantle.

Tectonic and magmatic processes of the post-spreading ridge in the Southwest Sub-basin, South China Sea

NASA Astrophysics Data System (ADS)

Li, J.; Zhang, J.; Ruan, A.; Niu, X.; Ding, W.

2016-12-01

We report here a 3D ocean bottom seismometer experiment on the fossil spreading ridge in the Southwest Sub-basin of the South China Sea. An extreme asymmetric crustal structure across the axis is revealed and caused by lower crust thinning and upper mantle uplifting located on NW side of the ridge. Such crustal extension proposed a low-angle oceanic detachment fault throughout the whole crust on the last or post spreading stages. A low-velocity (7.6-7.9 km/s) on the uplifting upper mantle is possibly induced by both mantle serpentinization and/or decompression melting through the detachment fault. Velocity models also demonstrate that a post-spreading volcano erupted on the axis is mainly formed by an extrusive process with an extrusive/intrusive ratio of 1.92. Very low velocity of upper crust (3.1-4.8 km/s) of the volcano is attributed to the composition of volcaniclastic rocks and high-porosity basalts, which have been observed in the borehole and dredged samples on the seamounts nearby. KEY WORDS post-spreading ridge; wide-angle seismic refraction; crustal structure; South China Sea; Southwest Sub-basin

Azimuthally Anisotropic Global Adjoint Tomography

NASA Astrophysics Data System (ADS)

Bozdag, E.; Orsvuran, R.; Lefebvre, M. P.; Lei, W.; Peter, D. B.; Ruan, Y.; Smith, J. A.; Komatitsch, D.; Tromp, J.

2017-12-01

Earth's upper mantle shows significant evidence of anisotropy as a result of its composition and deformation. After the first-generation global adjoint tomography model, GLAD-M15 (Bozdag et al. 2016), which has transverse isotropy confined to upper mantle, we continue our iterations including surface-wave azimuthal anisotropy with an emphasis on the upper mantle. We are focusing on four elastic parameters that surface waves are known to be most sensitive to, namely, vertically and horizontally polarized shear waves and the density-normalised anisotropic parameters Gc' & Gs'. As part of the current anisotropic inversions, which will lead to our "second-generation" global adjoint tomography model, we have started exploring new misfits based on a double-difference approach (Yuan et al. 2016). We define our misfit function in terms of double-difference multitaper measurements, where each waveform is normalized by its number of pairs in the period ranges 45-100 s & 90-250 s. New measurements result in better balanced gradients while extracting more information underneath clusters of stations, such as USArray. Our initial results reveals multi-scale anisotorpic signals depending on ray (kernel) coverage close to continental-scale resolution in areas with dense coverage, consistent with previous studies.

Large-scale compositional heterogeneity in the Earth's mantle

NASA Astrophysics Data System (ADS)

Ballmer, M.

2017-12-01

Seismic imaging of subducted Farallon and Tethys lithosphere in the lower mantle has been taken as evidence for whole-mantle convection, and efficient mantle mixing. However, cosmochemical constraints point to a lower-mantle composition that has a lower Mg/Si compared to upper-mantle pyrolite. Moreover, geochemical signatures of magmatic rocks indicate the long-term persistence of primordial reservoirs somewhere in the mantle. In this presentation, I establish geodynamic mechanisms for sustaining large-scale (primordial) heterogeneity in the Earth's mantle using numerical models. Mantle flow is controlled by rock density and viscosity. Variations in intrinsic rock density, such as due to heterogeneity in basalt or iron content, can induce layering or partial layering in the mantle. Layering can be sustained in the presence of persistent whole mantle convection due to active "unmixing" of heterogeneity in low-viscosity domains, e.g. in the transition zone or near the core-mantle boundary [1]. On the other hand, lateral variations in intrinsic rock viscosity, such as due to heterogeneity in Mg/Si, can strongly affect the mixing timescales of the mantle. In the extreme case, intrinsically strong rocks may remain unmixed through the age of the Earth, and persist as large-scale domains in the mid-mantle due to focusing of deformation along weak conveyor belts [2]. That large-scale lateral heterogeneity and/or layering can persist in the presence of whole-mantle convection can explain the stagnation of some slabs, as well as the deflection of some plumes, in the mid-mantle. These findings indeed motivate new seismic studies for rigorous testing of model predictions. [1] Ballmer, M. D., N. C. Schmerr, T. Nakagawa, and J. Ritsema (2015), Science Advances, doi:10.1126/sciadv.1500815. [2] Ballmer, M. D., C. Houser, J. W. Hernlund, R. Wentzcovitch, and K. Hirose (2017), Nature Geoscience, doi:10.1038/ngeo2898.

A Global Upper-Mantle Tomographic Model of Shear Attenuation

NASA Astrophysics Data System (ADS)

Karaoglu, H.; Romanowicz, B. A.

2016-12-01

Mapping anelastic 3D structure within the earth's mantle is key to understanding present day mantle dynamics, as it provides complementary constraints to those obtained from elastic structure, with the potential to distinguish between thermal and compositional heterogeneity. For this, we need to measure seismic wave amplitudes, which are sensitive to both elastic (through focusing and scattering) and anelastic structure. The elastic effects are less pronounced at long periods, so previous global upper-mantle attenuation models are based on teleseismic surface wave data, sometimes including overtones. In these studies, elastic effects are considered either indirectly, by eliminating data strongly contaminated by them (e.g. Romanowicz, 1995; Gung and Romanowicz, 2004), or by correcting for elastic focusing effects using an approximate linear approach (Dalton et al., 2008). Additionally, in these studies, the elastic structure is held fixed when inverting for intrinsic attenuation . The importance of (1) having a good starting elastic model, (2) accurate modeling of the seismic wavefield and (3) joint inversion for elastic and anelastic structure, becomes more evident as the targeted resolution level increases. Also, velocity dispersion effects due to anelasticity need to be taken into account. Here, we employ a hybrid full waveform inversion method, inverting jointly for global elastic and anelastic upper mantle structure, starting from the latest global 3D shear velocity model built by our group (French and Romanowicz, 2014), using the spectral element method for the forward waveform modeling (Capdeville et al., 2003), and normal-mode perturbation theory (NACT - Li and Romanowicz, 1995) for kernel computations. We present a 3D upper-mantle anelastic model built by using three component fundamental and overtone surface waveforms down to 60 s as well as long period body waveforms down to 30 s. We also include source and site effects to first order as frequency independent scalar factors. The robustness of the inversion method is assessed through synthetic and resolution tests. We discuss salient features of the resulting anelastic model and in particular the well-resolved strong correlation with tectonics observed in the first 200 km of the mantle.

Seismic evidence for a tilted mantle plume and north-south mantle flow beneath Iceland

USGS Publications Warehouse

Shen, Y.; Solomon, S.C.; Bjarnason, I. Th; Nolet, G.; Morgan, W.J.; Allen, R.M.; Vogfjord, K.; Jakobsdottir, S.; Stefansson, R.; Julian, B.R.; Foulger, G.R.

2002-01-01

Shear waves converted from compressional waves at mantle discontinuities near 410- and 660-km depth recorded by two broadband seismic experiments in Iceland reveal that the center of an area of anomalously thin mantle transition zone lies at least 100 km south of the upper-mantle low-velocity anomaly imaged tomographically beneath the hotspot. This offset is evidence for a tilted plume conduit in the upper mantle, the result of either northward flow of the Icelandic asthenosphere or southward flow of the upper part of the lower mantle in a no-net-rotation reference frame. ?? 2002 Elsevier Science B.V. All rights reserved.

Mantle structure beneath Africa and Arabia from adaptively parameterized P-wave tomography: Implications for the origin of Cenozoic Afro-Arabian tectonism

NASA Astrophysics Data System (ADS)

Hansen, Samantha E.; Nyblade, Andrew A.; Benoit, Margaret H.

2012-02-01

While the Cenozoic Afro-Arabian Rift System (AARS) has been the focus of numerous studies, it has long been questioned if low-velocity anomalies in the upper mantle beneath eastern Africa and western Arabia are connected, forming one large anomaly, and if any parts of the anomalous upper mantle structure extend into the lower mantle. To address these questions, we have developed a new image of P-wave velocity variations in the Afro-Arabian mantle using an adaptively parameterized tomography approach and an expanded dataset containing travel-times from earthquakes recorded on many new temporary and permanent seismic networks. Our model shows a laterally continuous, low-velocity region in the upper mantle beneath all of eastern Africa and western Arabia, extending to depths of ~ 500-700 km, as well as a lower mantle anomaly beneath southern Africa that rises from the core-mantle boundary to at least ~ 1100 km depth and possibly connects to the upper mantle anomaly across the transition zone. Geodynamic models which invoke one or more discrete plumes to explain the origin of the AARS are difficult to reconcile with the lateral and depth extent of the upper mantle low-velocity region, as are non-plume models invoking small-scale convection passively induced by lithospheric extension or by edge-flow around thick cratonic lithosphere. Instead, the low-velocity anomaly beneath the AARS can be explained by the African superplume model, where the anomalous upper mantle structure is a continuation of a large, thermo-chemical upwelling in the lower mantle beneath southern Africa. These findings provide further support for a geodynamic connection between processes in Earth's lower mantle and continental break-up within the AARS.

Shear wave velocity, seismic attenuation, and thermal structure of the continental upper mantle

USGS Publications Warehouse

Artemieva, I.M.; Billien, M.; Leveque, J.-J.; Mooney, W.D.

2004-01-01

Seismic velocity and attenuation anomalies in the mantle are commonly interpreted in terms of temperature variations on the basis of laboratory studies of elastic and anelastic properties of rocks. In order to evaluate the relative contributions of thermal and non-thermal effects on anomalies of attenuation of seismic shear waves, QS-1, and seismic velocity, VS, we compare global maps of the thermal structure of the continental upper mantle with global QS-1 and Vs maps as determined from Rayleigh waves at periods between 40 and 150 S. We limit the comparison to three continental mantle depths (50, 100 and 150 km), where model resolution is relatively high. The available data set does not indicate that, at a global scale, seismic anomalies in the upper mantle are controlled solely by temperature variations. Continental maps have correlation coefficients of <0.56 between VS and T and of <0.47 between QS and T at any depth. Such low correlation coefficients can partially be attributed to modelling arrefacts; however, they also suggest that not all of the VS and QS anomalies in the continental upper mantle can be explained by T variations. Global maps show that, by the sign of the anomaly, VS and QS usually inversely correlate with lithospheric temperatures: most cratonic regions show high VS and QS and low T, while most active regions have seismic and thermal anomalies of the opposite sign. The strongest inverse correlation is found at a depth of 100 km, where the attenuation model is best resolved. Significantly, at this depth, the contours of near-zero QS anomalies approximately correspond to the 1000 ??C isotherm, in agreement with laboratory measurements that show a pronounced increase in seismic attenuation in upper mantle rocks at 1000-1100 ??C. East-west profiles of VS, QS and T where continental data coverage is best (50??N latitude for North America and 60??N latitude for Eurasia) further demonstrate that temperature plays a dominant, but non-unique, role in determining the value of lithospheric VS and QS. At 100 km depth, where the resolution of seismic models is the highest, we compare observed seismic VS and QS with theoretical VST and QST values, respectively, that are calculated solely from temperature anomalies and constrained by experimental data on temperature dependencies of velocity and attenuation. This comparison shows that temperature variations alone are sufficient to explain seismic VS and QS in ca 50 per cent of continental regions. We hypothesize that compositional anomalies resulting from Fe depletion can explain the misfit between seismic and theoretical VS in cratonic lithosphere. In regions of active tectonics, temperature effects alone cannot explain seismic VS and QS in the lithosphere. It is likely that partial melts and/or fluids may affect seismic parameters in these regions. This study demonstrates that lithospheric temperature plays the dominant role in controlling VS and QS anomalies, but other physical parameters, such as compositional variations, fluids, partial melting and scattering, may also play a significant role in determining VS and QS variations in the continental mantle. ?? 2004 RAS.

Experimental constraints on the fate of subducted upper continental crust beyond the "depth of no return"

NASA Astrophysics Data System (ADS)

Zhang, Y.; Wu, Y.; WANG, C.; Jin, Z.

2015-12-01

Large-scale oceanic/continental subduction introduces a range of crustal materials into the Earth's mantle. These subducted material will be gravitationally trapped in the deep mantle when they have been transported to a depth of greater than ~250-300 km ("depth of no return"). However, little is known about the fate of these trapped continental material. Here, we conduct experimental study on a natural continental rock which compositionally similar to the average upper continental crust (UCC) over a pressure and temperature range of 9-16 GPa and 1300-1800 oC to constraint the fate of these trapped continental materials. The experimental results demonstrate that subducted UCC produces ~20-30 wt% K-rich melt (>55 wt% SiO2) in the upper mantle (9-13 GPa). The melting residue is mainly composed of coesite/stishovite + clinopyroxene + kyanite. In contrast, partial melting of subducted UCC in the MTZ produces ~10 wt% K-rich melt (<50 wt% SiO2), together with stishovite, clinopyroxene, K-Hollandite, garnet and CAS-phase as the residue phases. The melting residue phases achieve densities greater than the surrounding mantle, which provides a driving force for descending across the 410 km seismic discontinuity into the MTZ. However, this density relationship is reversed at the base of MTZ, leaving the descended residues being accumulated above the 660 km seismic discontinuity and may contribute to the stagnated "second continent". On the other hand, the melt is ~0.3-0.7 g/cm3 less dense than the surrounding mantle and provides a buoyancy force for the ascending of melt to shallow depth. The ascending melt preserves a significant portion of the bulk-rock REEs and LILEs. Thus, chemical reaction between the melt and the surrounding mantle would leads to a variably metasomatised mantle. Re-melting of the metasomatised mantle may contribute to the origin of the "enriched mantle sources" (EM-sources). Therefore, through subduction, stagnation, partial melting and melt segregation of continental crust may create EM-sources and"second continent" at shallow depth and the base of the MTZ respectively, which may contribute to the observed geochemical/geophysical heterogeneity in Earth's interior.

Modelling the isotopic evolution of the Earth.

PubMed

Paul, Debajyoti; White, William M; Turcotte, Donald L

2002-11-15

We present a flexible multi-reservoir (primitive lower mantle, depleted upper mantle, upper continental crust, lower continental crust and atmosphere) forward-transport model of the Earth, incorporating the Sm-Nd, Rb-Sr, U-Th-Pb-He and K-Ar isotope-decay systematics. Mathematically, the model consists of a series of differential equations, describing the changing abundance of each nuclide in each reservoir, which are solved repeatedly over the history of the Earth. Fluxes between reservoirs are keyed to heat production and further constrained by estimates of present-day fluxes (e.g. subduction, plume flux) and current sizes of reservoirs. Elemental transport is tied to these fluxes through 'enrichment factors', which allow for fractionation between species. A principal goal of the model is to reproduce the Pb-isotope systematics of the depleted upper mantle, which has not been done in earlier models. At present, the depleted upper mantle has low (238)U/(204)Pb (mu) and (232)Th/(238)U (kappa) ratios, but Pb-isotope ratios reflect high time-integrated values of these ratios. These features are reproduced in the model and are a consequence of preferential subduction of U and of radiogenic Pb from the upper continental crust into the depleted upper mantle. At the same time, the model reproduces the observed Sr-, Nd-, Ar- and He-isotope ratios of the atmosphere, continental crust and mantle. We show that both steady-state and time-variant concentrations of incompatible-element concentrations and ratios in the continental crust and upper mantle are possible. Indeed, in some cases, incompatible-element concentrations and ratios increase with time in the depleted mantle. Hence, assumptions of a progressively depleting or steady-state upper mantle are not justified. A ubiquitous feature of this model, as well as other evolutionary models, is early rapid depletion of the upper mantle in highly incompatible elements; hence, a near-chondritic Th/U ratio in the upper mantle throughout the Archean is unlikely. The model also suggests that the optimal value of the bulk silicate Earth's K/U ratio is close to 10000; lower values suggested recently seem unlikely.

Early formation and long-term stability of continents resulting from decompression melting in a convecting mantle

NASA Astrophysics Data System (ADS)

De Smet, J.; Van den Berg, A. P.; Vlaar, N. J.

2000-07-01

The origin of stable old continental cratonic roots is still debated. We present numerical modelling results which show rapid initial formation during the Archaean of continental roots of ca. 200 km thick. These results have been obtained from an upper mantle thermal convection model including differentiation by pressure release partial melting of mantle peridotite. The upper mantle model includes time-dependent radiogenic heat production and thermal coupling with a heat reservoir representing the Earth's lower mantle and core. This allows for model experiments including secular cooling on a time-scale comparable to the age of the Earth. The model results show an initial phase of rapid continental root growth of ca. 0.1 billion year, followed by a more gradual increase of continental volume by addition of depleted material produced through hot diapiric, convective upwellings which penetrate the continental root from below. Within ca. 0.6 Ga after the start of the experiment, secular cooling of the mantle brings the average geotherm below the peridotite solidus thereby switching off further continental growth. At this time the thickness of the continental root has grown to ca. 200 km. After 1 Ga of secular cooling small scale thermal instabilities develop at the bottom of the continental root causing continental delamination without breaking up the large scale layering. This delaminated material remixes with the deeper layers. Two more periods, each with a duration of ca. 0.5 Ga and separated by quiescent periods were observed when melting and continental growth was reactivated. Melting ends at 3 Ga. Thereafter secular cooling proceeds and the compositionally buoyant continental root is stabilized further through the increase in mechanical strength induced by the increase of the temperature dependent mantle viscosity. Fluctuating convective velocity amplitudes decrease to below 10 mma -1 and the volume average temperature of the sub-continental convecting mantle has decreased ca. 340 K after 4 Ga. Surface heatflow values decrease from 120 to 40 mW m -2 during the 4 Ga model evolution. The surface heatflow contribution from an almost constant secular cooling rate was estimated to be 6 mW m -2, in line with recent observational evidence. The modelling results show that the combined effects of compositional buoyancy and strong temperature dependent rheology result in continents which overall remain stable for a duration longer than the age of the Earth. Tracer particles have been used for studying the patterns of mantle differentiation in greater detail. The observed ( p, T, F, t)-paths are consistent with proposed stratification and thermo-mechanical history of the depleted continental root, which have been inferred from mantle xenoliths and other upper mantle samples. In addition, the particle tracers have been used to derive the thermal age of the modelled continental root, defined by a hypothetical closing temperature.

Evolving Mantle Sources in Postcollisional Early Permian-Triassic Magmatic Rocks in the Heart of Tianshan Orogen (Western China)

NASA Astrophysics Data System (ADS)

Tang, Gong-Jian; Cawood, Peter A.; Wyman, Derek A.; Wang, Qiang; Zhao, Zhen-Hua

2017-11-01

Magmatism postdating the initiation of continental collision provides insight into the late stage evolution of orogenic belts including the composition of the contemporaneous underlying subcontinental mantle. The Awulale Mountains, in the heart of the Tianshan Orogen, display three types of postcollisional mafic magmatic rocks. (1) A medium to high K calc-alkaline mafic volcanic suite (˜280 Ma), which display low La/Yb ratios (2.2-11.8) and a wide range of ɛNd(t) values from +1.9 to +7.4. This suite of rocks was derived from melting of depleted metasomatized asthenospheric mantle followed by upper crustal contamination. (2) Mafic shoshonitic basalts (˜272 Ma), characterized by high La/Yb ratios (14.4-20.5) and more enriched isotope compositions (ɛNd(t) = +0.2 - +0.8). These rocks are considered to have been generated by melting of lithospheric mantle enriched by melts from the Tarim continental crust that was subducted beneath the Tianshan during final collisional suturing. (3) Mafic dikes (˜240 Ma), with geochemical and isotope compositions similiar to the ˜280 Ma basaltic rocks. This succession of postcollision mafic rock types suggests there were two stages of magma generation involving the sampling of different mantle sources. The first stage, which occurred in the early Permian, involved a shift from depleted asthenospheric sources to enriched lithospheric mantle. It was most likely triggered by the subduction of Tarim continental crust and thickening of the Tianshan lithospheric mantle. During the second stage, in the middle Triassic, there was a reversion to more asthenospheric sources, related to postcollision lithospheric thinning.

Dynamics of metasomatic transformation of lithospheric mantle rocks under Siberian Craton

NASA Astrophysics Data System (ADS)

Sharapov, Victor; Perepechko, Yury; Tomilenko, Anatoly; Chudnenko, Konstantin; Sorokin, Konstantin

2014-05-01

Numerical problem for one- and two-velocity hydrodynamics of heat and mass transfer in permeable zones over 'asthenospheric lenses' (with estimates for dynamics of non-isothermal metasomatosis of mantle rocks, using the approximation of flow reactor scheme) was formulated and solved based on the study of inclusion contents in minerals of metamorphic rocks of the lithosphere mantle and earth crust, estimates of thermodynamic conditions of inclusions appearance, and the results of experimental modeling of influence of hot reduced gases on rocks and minerals of xenoliths in mantle rocks under the cratons of Siberian Platform (SP): 1) the supply of fluid flows of any composition from upper mantle magma sources results in formation of zonal metasomatic columns in ultrabasic lithosphere mantle in permeable zones of deep faults; 2) when major element or petrogenetic components are supplied from magma source, depleted ultrabasic rocks of the lithosphere mantle are transformed into substrates which can be regarded as deep analogs of crust rodingites; 3) other fluid compositions cause deep calcinations and noticeable salination of metasomated substrate, or garnetization (eclogitization) of primary ultrabasic matrix develops; 4) above these zones the zone of basification appears; it is changed by the area of pyroxenitization, amphibolization, and biotitization; 5) modeling of thermo and mass exchange for two-velocity hydrodynamic problem showed that hydraulic approximation increases velocities of heat front during convective heating and decreases pressure in fluid along the flow. It was shown that grospydites, regarded earlier as eclogites, in permeable areas of lithosphere mantle, are typical zones draining upper mantle magma sources of metasomatic columns. As a result of the convective melting the polybaric magmatic sources may appear. Thus the formation of the (kimberlites?) melilitites or carbonatites is possible at the base of the lithospheric plates. It is shown that the physico - chemical conditions of the carbonation of the depleted mantle peridotites refer to the narrow interval of the possible fluid compositions. The bulk fluid content near 4 weight % with the SiO2 CaO 0.5 - 0.1 molar volumes the 1) the Si/Ca molar ratio is < 1; 2) in the C-H-O system the molar ration should be 1/2/3 - 2/1/2; 3) the pO2 variations should be -8 < lg pO2 < -11; 4) in the fluid the CO2 content is twice higher than H2O and Cl essentially prevail under F. In the system with smaller fraction of the fluid phase less increased by the major element rock components the carbonation is more intensive when the Ca content decrease. The fusions of the basic magmas are possible within the wehrlitization zones. The work is supported by RFBR grant 12-05-00625.

Li Isotope Studies of Olivine in Mantle Xenoliths by SIMS

NASA Technical Reports Server (NTRS)

Bell, D. R.; Hervig, R. L.; Buseck, P. R.

2005-01-01

Variations in the ratio of the stable isotopes of Li are a potentially powerful tracer of processes in planetary and nebular environments [1]. Large differences in the 7Li/6Li ratio between the terrestrial upper mantle and various crustal materials make Li isotope composition a potentially powerful tracer of crustal recycling processes on Earth [2]. Recent SIMS studies of terrestrial mantle and Martian meteorite samples report intra-mineral Li isotope zoning [3-5]. Substantial Li isotope heterogeneity also exists within and between the components of chondritic meteorites [6,7]. Experimental studies of Li diffusion suggest the potential for rapid isotope exchange at elevated temperatures [8]. Large variations in 7Li, exceeding the range of unaltered basalts, occur in terrestrial mantle-derived xenoliths from individual localities [9]. The origins of these variations are not fully understood.

Mantle metasomatism above subduction zones: Trace-element and radiogenic isotope characteristics of peridotite xenoliths from Batan Island (Philippines)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vidal, Ph.; Dupuy, C.; Maury, R.

1989-12-01

Trace-element abundances and radiogenic isotope ratios have been determined for a suite of upper mantle-derived xenoliths collected from Pliocene-Quaternary andesitic lavas on Batan Island, northernmost Philippines. The xenoliths exhibit mineralogical changes and large ion lithophile enrichment indicative of metasomatism involving H{sub 2}O-rich fluids. Strontium and neodymium isotopes in the xenoliths are not totally consistent with those in host lavas, but a common signature is indicated by the fact that all samples plot below the mantle array. The flux of fluids in the mantle wedge probably occurred over a long period of time. The flux induced large but variable changes inmore » mineral and trace and isotopic compositions, and ultimately resulted in the melting of the peridotites and production of island-arc lavas.« less

Length-scales of Slab-induced Asthenospheric Deformation from Geodynamic Modeling, Mantle Deformation Fabric, and Synthetic Shear Wave Splitting

NASA Astrophysics Data System (ADS)

Jadamec, M. A.; MacDougall, J.; Fischer, K. M.

2017-12-01

The viscosity structure of the Earth's interior is critically important, because it places a first order constraint on plate motion and mantle flow rates. Geodynamic models using a composite viscosity based on experimentally derived flow laws for olivine aggregates show that lateral viscosity variations emerge in the upper mantle due to the subduction dynamics. However, the length-scale of this transition is still not well understood. Two-dimensional numerical models of subduction are presented that investigate the effect of initial slab dip, maximum yield stress (slab strength), and viscosity formulation (Newtonian versus composite) on the emergent lateral viscosity variations in the upper-mantle and magnitude of slab-driven mantle flow velocity. Significant viscosity reductions occur in regions of large flow velocity gradients due to the weakening effect of the dislocation creep deformation mechanism. The dynamic reductions in asthenospheric viscosity (less than 1018 Pa s) occur within approximately 500 km from driving force of the slab, with peak flow velocities occurring in models with a lower yield stress (weaker slab) and higher stress exponent. This leads to a sharper definition of the rheological base of the lithosphere and implies lateral variability in tractions along the base of the lithosphere. As the dislocation creep mechanism also leads to mantle deformation fabric, we then examine the spatial variation in the LPO development in the asthenosphere and calculate synthetic shear wave splitting. The models show that olivine LPO fabric in the asthenosphere generally increases in alignment strength with increased proximity to the slab, but can be transient and spatially variable on small length scales. The vertical flow fields surrounding the slab tip can produce shear-wave splitting variations with back-azimuth that deviate from the predictions of uniform trench-normal anisotropy, a result that bears on the interpretation of complexity in shear-wave splitting observed in real subduction zones.

Chemical stratification of cratonic lithosphere: constraints from the Northern Slave craton, Canada

NASA Astrophysics Data System (ADS)

Kopylova, Maya G.; Russell, James K.

2000-08-01

We describe the mineralogical and chemical composition of the Northern Slave mantle as deduced from xenoliths of peridotite within the Jericho kimberlite, Northwest Territories. Our data set includes modal, major, trace and rare earth element compositions of bulk samples of spinel peridotite, low-T and high-T garnet peridotite and minor pyroxenite. Compared to primitive upper mantle, Jericho peridotite shows depletion in the major elements and enrichment in incompatible elements (except for HREE). The Slave mantle is also uniquely stratified. Older, depleted spinel peridotite extends to a depth of 80-100 km and is underlain by garnet peridotite which shows a gradual decrease in Mg# with depth to 200 km. The youngest layer of fertile garnet peridotite, enriched in clinopyroxene and garnet, is underlain by a pyroxenite-rich horizon at the base of the petrological lithosphere. The Northern Slave is further distinguished from the Kaapvaal and Siberian upper mantle by a marked vertical stratification in Mg#, lower abundances of orthopyroxene and higher abundances of clinopyroxene. In addition, a deeper layer of garnet peridotite below Jericho shows less depletion than low-T peridotite from other cratons. The Northern Slave peridotite results from a series of chemical events that include: (i) high-degree melting of pyrolite at P>3 Gpa for low-T peridotite and lower pressure melting for high-T peridotite, (ii) enrichment of low-T spinel peridotite in orthopyroxene, and (iii) pervasive metasomatic enrichment in alkali and LREE's by kimberlite-related fluids. The chemical stratification described for two of the three lithospheric domains of the Slave craton makes this craton an exception among cratons with commonly unstratified lithospheres. The gradual increase in fertility with depth below the Slave craton is related to age stratification and may have formed by incremental downward growth of mantle lithosphere with time, and/or later re-fertilization of deeper mantle horizons.

A Xenolith Perspective on the Composition and Age of the Northern Tanzanian Lithosphere (Invited)

NASA Astrophysics Data System (ADS)

Rudnick, R. L.; Aulbach, S.; Bellucci, J. J.; Blondes, M. S.; Chesley, J.; Lee, C.; Mansur, A. T.; Manya, S.; McDonough, W. F.

2009-12-01

Study of deep crustal and upper mantle xenoliths from rift volcanoes throughout northern Tanzania provides insights into the architecture of the Tanzanian lithosphere, as well as the interaction of this lithosphere with rift magmas. Like the upper crust, the lower crust and mantle lithosphere of the Tanzanian Craton (TC) and Mozambique Belt (MB) are Archean, but the lower crust of the MB has been thermally reactivated during the pan-African Orogeny, whereas that of the craton has not. In addition, both mantle sections have experienced interaction and heating associated with rift magmas. Cratonic lithospheric mantle is compositionally stratified, with highly refractory but strongly LREE-enriched peridotite comprising the bulk of the section (40-130 km depth), underlain by more fertile and deformed peridotites (130-150 km depth), which are also LREE-enriched. Lithospheric mantle of the MB is highly variable in thickness, ranging from a maximum of ˜150 km at Lashaine to <50 km within the Rift axis near the Kenyan border. Like the cratonic lithosphere, this mantle is also refractory and yields Archean Os model ages throughout. Mantle lithospheres of both the TC and MB have interacted with rift magmas, including carbonatites (at Olmani) and alkali basalts (s.l.), which, in some cases, precipitated veins containing phlogopite or amphibole. Late Pleistocene zircons in one of these veins testify to the youth of this interaction. Rift basalt precipitates that formed in the mantle (pyroxenites and glimmerites) and have, thus, never interacted with continental crust, have radiogenic Os isotopic compositions (γOs = +9), providing strong evidence for a plume source of the rift magmas. Sr and Nd isotopes in cpx from peridotites are highly variable: in some they are completely overprinted by rift magmas, whereas others contain Archean components. Granulite-facies xenoliths throughout northern Tanzania are generally mafic (including anorthositic compositions), with a few intermediate compositions; no granulite-facies metapelites have been found. Marbles, schists, quartzites and amphibolites from the MB likely derive from middle-crustal depths. All zircon U-Pb ages are Archean (≥ 2.6 Ga) and many of the samples fall along a 2.6 Ga Sm-Nd reference line. U-Pb thermochronology largely records slow cooling in the MB following the Pan-African Orogeny and is consistent with a present-day conductive geotherm of 47 mW/m2 in a crust with very low heat production (see Blondes et al., this meeting). Despite the fact that ɛNd varies from -4 to -32 in the lower crustal xenoliths, 87Sr/86Sr is much less variable and the data fall along a near-vertical trend in a Sr vs. Nd isotope plot, reflecting ancient Rb depletion relative to Sr. Similarly, the unradiogenic Pb in granulite feldspars from both TC and MB is consistent with ancient U depletion. Collectively, such distinctive radiogenic isotope characteristics can serve as a diagnostic signature of crustal assimilation in rift magmas from northern Tanzania.

Tracing crustal contamination along the Java segment of the Sunda Arc, Indonesia

NASA Astrophysics Data System (ADS)

Jolis, E. M.; Troll, V.; Deegan, F.; Blythe, L.; Harris, C.; Freda, C.; Hilton, D.; Chadwick, J.; Van Helden, M.

2012-04-01

Arc magmas typically display chemical and petrographic characteristics indicative of crustal input. Crustal contamination can take place either in the mantle source region or as magma traverses the upper crust (e.g. [1]). While source contamination is generally considered the dominant process (e.g. [2]), late-stage crustal contamination has been recognised at volcanic arcs too (e.g. [3]). In light of this, we aim to test the extent of upper crustal versus source contamination along the Java segment of the Sunda arc, which, due its variable upper crustal structure, is an exemplary natural laboratory. We present a detailed geochemical study of 7 volcanoes along a traverse from Anak-Krakatau in the Sunda strait through Java and Bali, to characterise the impact of the overlying crust on arc magma composition. Using rock and mineral elemental geochemistry, radiogenic (Sr, Nd and Pb) and, stable (O) isotopes, we show a correlation between upper crustal composition and the degree of upper crustal contamination. We find an increase in 87Sr/86Sr and δ18O values, and a decrease in 143Nd/144Nd values from Krakatau towards Merapi, indicating substantial crustal input from the thick continental basement present. Volcanoes to the east of Merapi and the Progo-Muria fault transition zone, where the upper crust is thinner, in turn, show considerably less crustal input in their isotopic signatures, indicating a stronger influence of the mantle source. Our new data represent a systematic and high-resolution arc-wide sampling effort that allows us to distinguish the effects of the upper crust on the compositional spectrum of individual volcanic systems along the Sunda arc. [1] Davidson, J.P, Hora, J.M, Garrison, J.M & Dungan, M.A 2005. Crustal Forensics in Arc Magmas. J. Geotherm. Res. 140, 157-170; [2] Debaille, V., Doucelance, R., Weis, D., & Schiano, P. 2005. Geochim. Cosmochim. Acta, 70,723-741; [3] Gasparon, M., Hilton, D.R., & Varne, R. 1994. Earth Planet. Sci. Lett., 126, 15-22.

Probing the atomic structure of basaltic melts generated by partial melting of upper mantle peridotite (KLB-1): Insights from high-resolution solid-state NMR study

NASA Astrophysics Data System (ADS)

Park, S. Y.; Lee, S. K.

2015-12-01

Probing the structural disorder in multi-component silicate glasses and melts with varying composition is essential to reveal the change of macroscopic properties in natural silicate melts. While a number of NMR studies for the structure of multi-component silicate glasses and melts including basaltic and andesitic glasses have been reported (e.g., Park and Lee, Geochim. Cosmochim. Acta, 2012, 80, 125; Park and Lee, Geochim. Cosmochim. Acta, 2014, 26, 42), many challenges still remain. The composition of multi-component basaltic melts vary with temperature, pressure, and melt fraction (Kushiro, Annu. Rev. Earth Planet. Sci., 2001, 71, 107). Especially, the eutectic point (the composition of first melt) of nepheline-forsterite-quartz (the simplest model of basaltic melts) moves with pressure from silica-saturated to highly undersaturated and alkaline melts. The composition of basaltic melts generated by partial melting of upper mantle peridotite (KLB-1, the xenolith from Kilbourne Hole) also vary with pressure. In this study we report experimental results for the effects of composition on the atomic structure of Na2O-MgO-Al2O3-SiO2 (NMAS) glasses in nepheline (NaAlSiO4)-forsterite (Mg2SiO4)-quartz (SiO2) eutectic composition and basaltic glasses generated by partial melting of upper mantle peridotite (KLB-1) using high-resolution multi-nuclear solid-state NMR. The Al-27 3QMAS (triple quantum magic angle spinning) NMR spectra of NMAS glasses in nepheline-forsterite-quartz eutectic composition show only [4]Al. The Al-27 3QMAS NMR spectra of KLB-1 basaltic glasses show mostly [4]Al and a non-negligible fraction of [5]Al. The fraction of [5]Al, the degree of configurational disorder, increases from 0 at XMgO [MgO/(MgO+Al2O3)]=0.55 to ~3% at XMgO=0.79 in KLB-1 basaltic glasses while only [4]Al are observed in nepheline-forsterite-quartz eutectic composition. The current experimental results provide that the fraction of [5]Al abruptly increases by the effect of composition as well as pressure in natural silicate melts. The changes of the fraction of highly coordinated Al in multi-component silicate glasses and melts with composition can provide insight into the changes of macroscopic properties (e.g., entropy, viscosity, and diffusivity) with varying composition of melt.

Analysis of converted S-waves and gravity anomaly along the Aegir Ridge: implications for crustal lithology

NASA Astrophysics Data System (ADS)

Rai, A. K.; Breivik, A. J.; Mjelde, R.; Hanan, B. B.; Ito, G.; Sayit, K.; Howell, S.; Vogt, P. R.; Pedersen, R.

2012-12-01

The Aegir Ridge is an extinct spreading ridge in North-East Atlantic ocean. A thinner than normal crust around the Aegir Ridge appears as a hole in the extensively magmatic surroundings. Its proximity to the Iceland hot-spot makes it particularly important for understanding the changing dynamics of hotspot-ridge interaction. An integrated seismic and dredging experiment was conduced during the summer of 2010 with the primary aim to understand the nature of magmatism along the ridge shortly before cessation of seafloor spreading through variations of sub-seafloor lithological properties. Here, we present results of analysis of converted shear-waves recorded on OBS-sesimic data, and ship-gravity data. The shear-wave study enables us to quantify the variation of Vp/Vs in the sediments, crust and the upper-most mantle. We also inverted the gravity data to determine the sub-seafloor density distribution. The P- to S- converted shear-waves were identified on 20 OBSs along a profile with a total length of 550 km parallel to the ridge-axis. The sedimentary section on top of the crystalline crust is well illuminated in the streamer data. The forward modelling of the OBS data reveals that the Vp/Vs ratio in sediments are as high as 4.8, decreasing rapidly to a value of 3.00, primarily due to compaction of sediments with depth. Identification of sufficient PnS and PSn phases enable us to model the crustal and upper-most mantle Vp/Vs. The upper crystalline crust requires a Vp/Vs value of 1.99 and 1.89 for the southern and the northern profiles respectively, to fit the observations. The lower crust and upper-most part of the mantle have a Vp/Vs of ~1.82 and 1.795 respectively. Slightly lower Vp and moderate increase in Vp/Vs in parts of the crust and upper mantle presumably indicate presence of faulting, fracturing in the crust and moderate degree of serpentinization of the upper mantle. A sub-seafloor density model is derived by non-linear inversion of the gravity anomaly. The distribution of sediments appear to control the short-wavelength features of the gravity data, whereas density variations are required in the upper mantle to optimally fit the overall gravity anomaly. Our results suggest certain degree of temperature and/or compositional heterogeneities towards the southern ends of Aegir Ridge, near the Iceland-Faroes Ridge.

The role of the metasomatized oceanic lithosphere on the composition of mid-ocean ridge basalts from the East Pacific Rise

NASA Astrophysics Data System (ADS)

Shimizu, K.; Saal, A. E.

2016-12-01

In the present study, we evaluate the effect of melting of a metasomatized oceanic lithosphere on the chemical composition of MORB using the East Pacific Rise (EPR) mid-ocean ridge basalts (MORB) from the Quebrada-Discovery-GoFar (QDG) transform fault system, Northern EPR seamounts, and Macquarie Island [1-3]. EMORB from the QDG have trace element and volatile-refractory element ratios different from those measured in NEPR seamounts and Macquarie EMORB. The unique chemical composition of the QDG EMORB might indicate contribution from the oceanic lithosphere during the formation of intra-transform spreading centers due to clockwise rotation in Pacific-Nazca plate relative motion. In addition, the compositions of some of the Petit-spot lavas recently erupted along lithospheric fractures in the Pacific Plate in response to its flexure near the Japan Trench [4] have geochemical signatures that might suggest melts derived from a metasomatized oceanic mantle lithosphere. We evaluate this hypothesis using a geochemical model assuming a two-component asthenospheric mantle (DDMM and EDMM) and formation of hydrous cumulates in the oceanic mantle lithosphere by crystallization of low degree melts of the EDMM [3, 5]. The model suggests that melting of the hydrous cumulates can reproduce the composition of EMORB from QDG transform fault and some of the Petit-spot lavas. The process of melting the metasomatized oceanic lithosphere may significantly affect the chemical composition of MORB, and the common assumption for the purely asthenosphere origin of MORB could lead to inaccurate estimates of the Earth's upper mantle composition. We also show that similar process might affect not only oceanic, but also off-craton sub continental mantle lithosphere. References: [1] Niu et al., 2002 EPSL 199. [2] Kamenetsky et al., 2002 J Petrol 43. [3] Shimizu et al., 2016 GCA 176. [4] Hirano et al., 2006 Science 313. [5] Pilet et al., 2011 J Petrol 52.

Effects of mantle rheologies on viscous heating induced by Glacial Isostatic Adjustment

NASA Astrophysics Data System (ADS)

Huang, PingPing; Wu, Patrick; van der Wal, Wouter

2018-04-01

It has been argued that viscous dissipation from mantle flow in response to surface loading during glacial cycles can result in short-term heating and thus trigger transient volcanism or changes in mantle properties, which may in turn affect mantle dynamics. Furthermore, heating near the Earth's surface can also affect the stability of ice sheets. We have studied the magnitude and spatial-temporal distribution of viscous heating induced in the mantle by the realistic ice model ICE-6G and gravitationally consistent ocean loads. Three types of mantle rheologies, including linear, non-linear and composite rheologies are considered to see if non-linear creep can induce larger viscous heating than linear rheology. We used the Coupled-Laplace-Finite-Element model of Glacial Isostatic Adjustment (GIA) to compute the strain, stress and shear heating during a glacial cycle. We also investigated the upper bound of temperature change and surface heat flux change due to viscous heating. We found that maximum viscous heating occurs near the end of deglaciation near the edge of the ice sheet with amplitude as high as 120 times larger than that of the chondritic radioactive heating. The maximum heat flux due to viscous heating can reach 30 mW m-2, but the area with large heat flux is small and the timescale of heating is short. As a result, the upper bound of temperature change due to viscous heating is small. Even if 30 glacial cycles are included, the largest temperature change can be of the order of 0.3 °C. Thus, viscous heating induced by GIA cannot induce volcanism and cannot significantly affect mantle material properties, mantle dynamics nor ice-sheet stability.

On Lateral Viscosity Contrast in the Mantle and the Rheology of Low-Frequency Geodynamics

NASA Technical Reports Server (NTRS)

Ivins, Erik R.; Sammis, Charles G.

1995-01-01

Mantle-wide heterogeneity is largely controlled by deeply penetrating thermal convective currents. These thermal currents are likely to produce significant lateral variation in rheology, and this can profoundly influence overall material behaviour. How thermally related lateral viscosity variations impact models of glacio-isostatic and tidal deformation is largely unknown. An important step towards model improvement is to quantify, or bound, the actual viscosity variations that characterize the mantle. Simple scaling of viscosity to shear-wave velocity fluctuations yields map-views of long- wavelength viscosity variation. These give a general quantitative description and aid in estimating the depth dependence of rheological heterogeneity throughout the mantle. The upper mantle is probably characterized by two to four orders of magnitude variation (peak-to-peak). Discrepant time-scales for rebounding Holocene shorelines of Hudson Bay and southern Iceland are consistent with this characterization. Results are given in terms of a local average viscosity ratio, (Delta)eta(bar)(sub i), of volumetric concentration, phi(sub i). For the upper mantle deeper than 340 km the following reasonable limits are estimated for (delta)eta(bar) approx. equal 10(exp -2): 0.01 less than or equal to phi less than or equal to 0.15. A spectrum of ratios (Delta)eta(bar)(sub i) less than 0.1 at concentration level eta(sub i) approx. equal 10(exp -6) - 10(exp -1) in the lower mantle implies a spectrum of shorter time-scale deformational response modes for second-degree spherical harmonic deformations of the Earth. Although highly uncertain, this spectrum of spatial variation allows a purely Maxwellian viscoelastic rheology simultaneously to explain all solid tidal dispersion phenomena and long-term rebound-related mantle viscosity. Composite theory of multiphase viscoelastic media is used to demonstrate this effect.

Partitioning of H2O on high pressure phase transformation of olivine

NASA Astrophysics Data System (ADS)

Inoue, T.; Wada, T.; Sasaki, R.; Irifune, T.; Yurimoto, H.

2003-12-01

Water is the most abundant volatile component on the Earth's surface, and it has been supplied to the Earth's interiors by subducted slab. Water influences the physical properties and melting temperature of minerals. Olivine is the most abundant mineral in the mantle, and it is clarified that the high-pressure polymorphs of olivine, wadsleyite and ringwoodite, can contain 3wt% of H2O in their crystal structures (e.g. Inoue et al., 1995, 1998). However, the partitioning of H2O among these minerals has not been clarified yet except for olivine-wadsleyite transformation (Chen et al., 2003). We have determined the partitioning of H2O between wadsleyite and ringwoodite and between ringwoodite and perovskite, and clarified the distribution of H2O among upper mantle, mantle transition zone and lower mantle. High-pressure experiments were conducted by MA-8 type (Kawai-type) high-pressure apparatus in Ehime University, and the chemical compositions were determined by EPMA. The water contents of minerals were measured by SIMS in Tokyo Institute of Technology. We succeeded to synthesize large (approximately 50 μ m) coexisting crystals of wadsleyite and ringwoodite, and of ringwoodite and perovskite, and we could clarify the partitioning of H2O between those coexisting minerals. The partition coefficients between wadsleyite and ringwoodite and between ringwoodite and perovskite were about 2 and about 10 or more, respectively. We (Chen et al., 2003) have already determined that the partition coefficients between wadsleyite and olivine is about 5, so the partitioning among upper mantle, 410-520km and 520-660km of mantle transition zone, and lower mantle are 4:20:10:1. Thus the mantle transition zone should be a strong water reservoir in the Earth's interiors.

New Hafnium Isotope and Trace Element Constraints on the Role of a Plume in Genesis of the Eastern Snake River Plain Basalts, Idaho

NASA Astrophysics Data System (ADS)

Taylor, R. D.; Reid, M. R.; Blichert-Toft, J.

2009-12-01

Bimodal volcanism associated with the eastern Snake River Plain (ESRP)-Yellowstone Plateau province has persisted since approximately 16 Ma. A time-transgressive track of rhyolitic eruptions which young progressively to the east and parallel the motion of the North American plate are overlain by younger basalts with no age progression. Interpretations for the origin of these basalts range from a thermo-chemical mantle plume to incipient melting of the shallow upper mantle, and remain controversial. The enigmatic ESRP basalts are characterized by high 3He/4He, diagnostic of a plume source, but also by lithophile radiogenic isotope signatures that are more enriched than expected for plume-derived OIBs. These features could possibly be caused by isotopic decoupling associated with shallow melting of a hybridized upper mantle, or derivation from an atypical mantle plume, or both by way of mixing. New Hf isotope and trace element data further constrain potential sources for the ESRP basalts. Their Hf isotopic signatures (ɛHf = +0.1 to -5.8) are moderately enriched and consistently fall above or in the upper part of the field of OIBs, with similar Nd isotope signatures (ɛNd = -2.0 to -5.8), indicating a source with high time-integrated Lu/Hf compared with Sm/Nd. The isotopic compositions of the basalts lie between those of Archean SCML and a more depleted end-member source, suggestive of contributions from at least two sources. The grouping of isotopic characteristics is compact compared to other regional volcanism, implying that the hybridization process is highly reproducible within the ESRP. Minor localized differences in isotopic composition may signify local variations in the relative proportions of the end-members. Trace element patterns also support genesis of the ESRP basalts from an enriched source. Our data detect evidence of deeper contributions derived from the garnet-stability field, and a greater affinity of the trace element signatures to plume sources than to sources in the mantle lithosphere. The Hf isotope and trace element characteristics of the ESRP basalts thus support a model of derivation from a deep mantle plume with additional melt contributions and isotopic overprinting from SCML.

Mantle Mineral/Silicate Melt Partitioning

NASA Astrophysics Data System (ADS)

McFarlane, E. A.; Drake, M. J.

1992-07-01

Introduction: The partitioning of elements among mantle phases and silicate melts is of interest in unraveling the early thermal history of the Earth. It has been proposed that the elevated Mg/Si ratio of the upper mantle of the Earth is a consequence of the flotation of olivine into the upper mantle (Agee and Walker, 1988). Agee and Walker (1988) have generated a model via mass balance by assuming average mineral compositions to generate upper mantle peridotite. This model determines that upper mantle peridotite could result from the addition of 32.7% olivine and 0.9% majorite garnet into the upper mantle, and subtraction of 27.6% perovskite from the upper mantle (Agee and Walker, 1988). The present contribution uses experimental data to examine the consequences of such multiple phase fractionations enabling an independent evaluation of the above mentioned model. Here we use Mg-perovskite/melt partition coefficients from both a synthetic and a natural system (KLB-1) obtained from this laboratory. Also used are partition coefficient values for majorite garnet/melt, beta spinel/melt and olivine/melt partitioning (McFarlane et al., 1991b; McFarlane et al., 1992). Multiple phase fractionations are examined using the equilibrium crystallization equation and partition coefficient values. The mineral proportions determined by Agee and Walker (1988) are converted into weight fractions and used to compute a bulk partition coefficient value. Discussion: There has been a significant debate concerning whether measured values of trace element partition coefficients permit large-scale fractionation of liquidus phases from an early terrestrial magma ocean (Kato et al., 1988a,b; Walker and Agee, 1989; Drake, 1989; Drake et al., 1991; McFarlane et al., 1990, 1991). It should be noted that it is unclear which, if any, numerical values of partition coefficients are appropriate for examining this question, and certainly the assumptions for the current model must be more fully examined. However, our preliminary calculations do not appear to be consistent with large scale fractionation of phases in the proportions postulated from an early ocean, because approximately chondritic ratios and abundances of refractory lithophile elements inferred for the primitive upper mantle of the Earth would not be preserved. References: Agee, C.B. and Walker, D. (1988) Earth. Planet. Sci. Lett. 90, 144-156. Drake, M.J. (1989) Z. Naturforsch., 44a, 883-890. Drake, M.J. et al. (1991) Magma Oceans Workshop. Drake, M.J. et al. (1989) Geochim. Cosmochim. Acta, 53, 2101-2111. Kato, T. et al. (1988a) Earth. Planet. Sci. Lett. 89, 123-145. Kato, T. et al. (1988b) Earth. Planet. Sci. Lett. 90, 65-68. McFarlane, E.A. et al. (1990) Lunar and Planetary Science 21, 759-760. McFarlane, E.A. et al. (199la) Magma Oceans Workshop. McFarlane, E.A. et al. (199lb) Lunar and Planetary Science 22, 875-876. McFarlane, E.A. et al. (1992) Lunar and Planetary Science 23, 883-884. Walker, D. and Agee, C.B. (1989) Earth. Planet. Sci. Lett. 96, 49-60.

Estimation of Water Within the Lithospheric Mantle of Central Tibet from Petrological-Geophysical Investigations

NASA Astrophysics Data System (ADS)

Vozar, J.; Fullea, J.; Jones, A. G.

2013-12-01

Investigations of the lithosphere and sub-lithospheric upper mantle by integrated petrological-geophysical modeling of magnetotelluric (MT) and seismic surface-wave data, which are differently sensitive to temperature and composition, allows us to reduce the uncertainties associated with modeling these two data sets independently, as commonly undertaken. We use selected INDEPTH MT data, which have appropriate dimensionality and large penetration depths, across central Tibet for 1D modeling. Our deep resistivity models from the data can be classified into two different and distinct groups: (i) the Lhasa Terrane and (ii) the Qiangtang Terrane. For the Lhasa Terrane group, the models show the existence of upper mantle conductive layer localized at depths of 200 km, whereas for the Qiangtang Terrane, this conductive layer is shallower at depths of 120 km. We perform the integrated geophysical-petrological modeling of the MT and surface-wave data using the software package LitMod. The program facilitates definition of realistic temperature and pressure distributions within the upper mantle for given thermal structure and oxide chemistry in the CFMAS system. This allows us to define a bulk geoelectric and seismic model of the upper mantle based on laboratory and xenolith data for the most relevant mantle minerals, and to compute synthetic geophysical observables. Our results suggest an 80-120 km-thick, dry lithosphere in the central part of the Qiangtang Terrane. In contrast, in the central Lhasa Terrane the predicted MT responses are too resistive for a dry lithosphere regardless its thickness; according to seismic and topography data the expected lithospheric thickness is about 200 km. The presence of small amounts of water significantly decreases the electrical resistivity of mantle rocks and is required to fit the MT responses. We test the hypothesis of small amounts of water (ppm scale) in the nominally anhydrous minerals of the lithospheric mantle. Such a small amount of water dramatically affects the resistivity but has no influence on the seismic velocities (and therefore, the calculated surface wave's dispersion curves are unaffected too). Three different proton conduction models for olivine conductivity (1 - Wang et al., 2006; 2 - Yoshino et al., 2009; 3 -Jones et al., 2012) and two water partition coefficients are tested. The presence of water in lithospheric mantle is decreased from 170 km to the LAB depth at 200 km. If we move this water-presentbottom boundary to shallower depth, the lithospheric mantle becomes too resistive. Our results favour a moderately wet (<0.01 wt%) mantle above the underthrusted Indian lithosphere, probably as a result of the dehydration processes. The presence of percolating water-rich fluids has the additional effect of lowering the solidus, and therefore facilitating partial melting in the warm lower crust of Lhasa.

Recycling of subducted crustal components into carbonatite melts revealed by boron isotopes

NASA Astrophysics Data System (ADS)

Hulett, Samuel R. W.; Simonetti, Antonio; Rasbury, E. Troy; Hemming, N. Gary

2016-12-01

The global boron geochemical cycle is closely linked to recycling of geologic material via subduction processes that have occurred over billions of years of Earth’s history. The origin of carbonatites, unique melts derived from carbon-rich and carbonate-rich regions of the upper mantle, has been linked to a variety of mantle-related processes, including subduction and plume-lithosphere interaction. Here we present boron isotope (δ11B) compositions for carbonatites from locations worldwide that span a wide range of emplacement ages (between ~40 and ~2,600 Ma). Hence, they provide insight into the temporal evolution of their mantle sources for ~2.6 billion years of Earth’s history. Boron isotope values are highly variable and range between -8.6‰ and +5.5‰, with all of the young (<300 Ma) carbonatites characterized by more positive δ11B values (>-4.0‰), whereas most of the older carbonatite samples record lower B isotope values. Given the δ11B value for asthenospheric mantle of -7 +/- 1‰, the B isotope compositions for young carbonatites require the involvement of an enriched (crustal) component. Recycled crustal components may be sampled by carbonatite melts associated with mantle plume activity coincident with major tectonic events, and linked to past episodes of significant subduction associated with supercontinent formation.

Rheological properties of the lower crust and upper mantle beneath Baja California: a microstructural study of xenoliths from San Quintin

NASA Astrophysics Data System (ADS)

Van der Werf, Thomas F.; Chatzaras, Vasileios; Tikoff, Basil; Drury, Martyn R.

2016-04-01

Baja California is an active transtensional rift zone, which links the San Andreas Fault with the East Pacific Rise. The erupted basalts of the Holocene San Quintin volcanic field contain xenoliths, which sample the lower crust and upper mantle beneath Baja California. The aim of this research is to gain insight in the rheology of the lower crust and the upper mantle by investigating the xenolith microstructure. Microstructural observations have been used to determine the dominant deformation mechanisms. Differential stresses were estimated from recrystallized grain size piezometry of plagioclase and clinopyroxene for the lower crust and olivine for the upper mantle. The degree of deformation can be inferred from macroscopic foliations and the deformation microstructures. Preliminary results show that both the lower crust and the upper mantle have been affected by multiple stages of deformation and recrystallization. In addition the dominant deformation mechanism in both the lower crust and the upper mantle is dislocation creep based on the existence of strong crystallographic preferred orientations. The differential stress estimates for the lower crust are 10-29 MPa using plagioclase piezometry and 12-35 MPa using clinopyroxene piezometry. For the upper mantle, differential stress estimates are 10-20 MPa. These results indicate that the strength of the lower crust and the upper mantle are very similar. Our data do not fit with the general models of lithospheric strength and may have important implications for the rheological structure of the lithosphere in transtensional plate margins and for geodynamic models of the region.

Integrating EarthScope Data to Constrain the Long-Term Effects of Tectonism on Continental Lithosphere

NASA Astrophysics Data System (ADS)

Porter, R. C.; van der Lee, S.

2017-12-01

One of the most significant products of the EarthScope experiment has been the development of new seismic tomography models that take advantage of the consistent station design, regular 70-km station spacing, and wide aperture of the EarthScope Transportable Array (TA) network. These models have led to the discovery and interpretation of additional compositional, thermal, and density anomalies throughout the continental US, especially within tectonically stable regions. The goal of this work is use data from the EarthScope experiment to better elucidate the temporal relationship between tectonic activity and seismic velocities. To accomplish this, we compile several upper-mantle seismic velocity models from the Incorporated Research Institute for Seismology (IRIS) Earth Model Collaboration (EMC) and compare these to a tectonic age model we compiled using geochemical ages from the Interdisciplinary Earth Data Alliance: EarthChem Database. Results from this work confirms quantitatively that the time elapsed since the most recent tectonic event is a dominant influence on seismic velocities within the upper mantle across North America. To further understand this relationship, we apply mineral-physics models for peridotite to estimate upper-mantle temperatures for the continental US from tomographically imaged shear velocities. This work shows that the relationship between the estimated temperatures and the time elapsed since the most recent tectonic event is broadly consistent with plate cooling models, yet shows intriguing scatter. Ultimately, this work constrains the long-term thermal evolution of continental mantle lithosphere.

Cooling of the Earth in the Archaean: Consequences of pressure-release melting in a hotter mantle

NASA Astrophysics Data System (ADS)

Vlaar, N. J.; van Keken, P. E.; van den Berg, A. P.

1994-01-01

A model is presented to describe the cooling of the Earth in the Archaean. At the higher Archaean mantle temperatures pressure-release melting starts deeper and generates a thicker basaltic or komatiitic crust and depleted harzburgite layer compared with the present-day situation. Intrinsic compositional stability and lack of mechanical coherency renders the mechanism of plate tectonics ineffective. It is proposed that the Archaean continents stabilised early on top of a compositionally stratified root. In the Archaean oceanic lithosphere, hydrated upper crust can founder and recycle through its high-pressure phase eclogite. Eclogite remelting and new pressure-release melting generates new crustal material. Migration of magma and latent heat release by solidification at the surface provides an efficient mechanism to cool the mantle by several hundreds of degrees during the Archaean. This can satisfactorily explain the occurrence of high extrusion temperature komatiites and lower extrusion temperature basalts in greenstone belts as being derived from the same source by different mechanisms.

Partitioning of copper between olivine, orthopyroxene, clinopyroxene, spinel, garnet and silicate melts at upper mantle conditions

NASA Astrophysics Data System (ADS)

Liu, Xingcheng; Xiong, Xiaolin; Audétat, Andreas; Li, Yuan; Song, Maoshuang; Li, Li; Sun, Weidong; Ding, Xing

2014-01-01

Previously published Cu partition coefficients (DCu) between silicate minerals and melts cover a wide range and have resulted in large uncertainties in model calculations of Cu behavior during mantle melting. In order to obtain true DCumineral/melt values, this study used Pt95Cu05 alloy capsules as the source of Cu to experimentally determine the DCu between olivine (ol), orthopyroxene (opx), clinopyroxene (cpx), spinel (spl), garnet (grt) and hydrous silicate melts at upper mantle conditions. Three synthetic silicate compositions, a Komatiite, a MORB and a Di70An30, were used to produce these minerals and melts. The experiments were conducted in piston cylinder presses at 1.0-3.5 GPa, 1150-1300 °C and oxygen fugacities (fO2) of from ∼2 log units below to ∼5 log units above fayalite-magnetite-quartz (FMQ). The compositions of minerals and quenched melts in the run products were measured with EMP and LA-ICP-MS. Attainment of equilibrium is verified by reproducible DCu values obtained at similar experimental conditions but different durations. The results show that DCu for ol/, opx/, spl/ and possibly cpx/melt increase with increasing fO2 when fO2 > FMQ + 1.2, while DCu for cpx/ and spl/melt also increase with increasing Na2O in cpx and Fe2O3 in spinel, respectively. In the investigated P-T-fO2 conditions, the DCumineral/melt values are 0.04-0.14 for ol, 0.04-0.09 for opx, 0.02-0.23 for cpx, 0.19-0.77 for spl and 0.03-0.05 for grt. These results confirm that Cu is highly incompatible (DCu < ∼0.2) in all the silicate minerals and oxides of the upper mantle with the exception of the high-Fe spinel, in which Cu is moderately incompatible (DCu = 0.4-0.8) and thus Cu will be enriched in the derived melts during mantle partial melting and magmatic differentiation if sulfide is absent. These experimental DCu values are used to assess the controls on Cu behavior during mantle melting. The model results suggest that MORBs and most arc basalts must form by sulfide-present melting at relatively reduced conditions, while high Cu (>70 ppm) arc basalts may form at oxidized, sulfide-absent conditions, which is consistent with the possibility of some high fO2 regions present in the arc mantle.

Upper Mantle Responses to India-Eurasia Collision in Indochina, Malaysia, and the South China Sea

NASA Astrophysics Data System (ADS)

Hongsresawat, S.; Russo, R. M.

2016-12-01

We present new shear wave splitting and splitting intensity measurements from SK(K)S phases recorded at seismic stations of the Malaysian National Seismic Network. These results, in conjunction with results from Tibet and Yunnan provide a basis for testing the degree to which Indochina and South China Sea upper mantle fabrics are responses to India-Eurasia collision. Upper mantle fabrics derived from shear wave splitting measurements in Yunnan and eastern Tibet parallel geodetic surface motions north of 26°N, requiring transmission of tractions from upper mantle depths to surface, or consistent deformation boundary conditions throughout the upper 200 km of crust and mantle. Shear wave splitting fast trends and surface velocities diverge in eastern Yunnan and south of 26°N, indicating development of an asthenospheric layer that decouples crust and upper mantle, or corner flow above the subducted Indo-Burma slab. E-W fast shear wave splitting trends southwest of 26°N/104°E indicate strong gradients in any asthenospheric infiltration. Possible upper mantle flow regimes beneath Indochina include development of olivine b-axis anisotropic symmetry due to high strain and hydrous conditions in the syntaxis/Indo-Burma mantle wedge (i.e., southward flow), development of strong upper mantle corner flow in the Indo-Burma wedge with olivine a-axis anisotropic symmetry (i.e., westward flow), and simple asthenospheric flow due to eastward motion of Sundaland shearing underlying asthenosphere. Further south, shear-wave splitting delay times at Malaysian stations vary from 0.5 seconds on the Malay Peninsula to over 2 seconds at stations on Borneo. Splitting fast trends at Borneo stations and Singapore trend NE-SW, but in northern Peninsular Malaysia, the splitting fast polarization direction is NW-SE, parallel to the trend of the Peninsula. Thus, there is a sharp transition from low delay time and NW-SE fast polarization to high delay times and fast polarization directions that parallel the strike of the now-inoperative spreading center in the South China Sea. This transition appears to occur in the central portion of Peninsular Malaysia and may mark the boundary between Tethyan upper mantle extruded from the India-Asia collision zone and supra-subduction upper mantle of the Indonesian arc.

Lithospheric models of the North American continent

NASA Astrophysics Data System (ADS)

Tesauro, Magdala; Kaban, Mikhail; Mooney, Walter; Cloetingh, Sierd

2015-04-01

We constructed NACr14, a 3D model of the North American (NA) crust, based on the most recent seismic data from the USGS database. In comparison with the global crustal model CRUST 1.0, NACr14 is more heterogeneous, showing a larger spatial variability of the thickness and average velocities of the crustal layers. Velocities of the lower crust vary in a larger range than those of the other layers, while the thickness of all the three layers is on average between 11 and 13 km. The largest velocities of the crystalline crust (>6.6 km/s) reflect the presence of a 7.x layer (>7.0 km/s) in the lowermost part of the crust. Using NACr2014, a regional (NA07) and a global (SL201sv) tomography model, and gravity data, we apply an iterative technique, which jointly interprets seismic tomography and gravity data, to estimate temperature and compositional variations in the NA upper mantle. The results obtained demonstrate that temperature of the cratonic mantle is up to 150°C higher than when using a uniform compositional model. The differences between the two tomography models influence the results more strongly than possible changes of the depth distribution of compositional variations. Strong negative compositional density anomalies, corresponding to Mg # >92, characterize the upper mantle of the northwestern part of the Superior craton and the central part of the Slave and Churchill craton. The Proterozoic upper mantle of the western and more deformed part of the NA cratons, appears weakly depleted (Mg# ~91) when NA07 is used, in agreement with the results based on the interpretation of xenolith data. When we use SL2013sv, the same areas are locally characterized by high density bodies, which might be interpreted as the effect due to fragments of subducted slabs, as those close to the suture of the Appalachians and Grenville province. We used the two thermal models to estimate the integrated strength and the effective elastic thickness (Te) of the lithosphere. In the peripheral parts of the cratons, as the Proterozoic Canadian Platform and Grenville, the integrated strength for model NA07 is ten times larger than in model SL2013sv, due to a model-dependent temperature difference of >200˚C in the uppermost mantle. In both models, Proterozoic regions reactivated by Meso-Cenozoic tectonics (e.g., Rocky Mountains and the Mississippi Embayment) show a weak lithosphere due to the absence of the mechanically strong part of the mantle lithospheric layer. Intraplate earthquakes are distributed along the edges of the cratons, characterized by a weak lithosphere or pronounced variations in integrated lithospheric strength and Te. In addition, the sum of the seismic moments shows that most of the energy is released by the weak lithosphere. These results suggest that the edges of the cratons are more prone to accumulation of tectonic stress and subsequent release by earthquakes, in comparison with the stable cratonic regions which resist deformation.

Deep seismic structure and tectonics of northern Alaska: Crustal-scale duplexing with deformation extending into the upper mantle

USGS Publications Warehouse

Fuis, G.S.; Murphy, J.M.; Lutter, W.J.; Moore, Thomas E.; Bird, K.J.; Christensen, N.I.

1997-01-01

Seismic reflection and refraction and laboratory velocity data collected along a transect of northern Alaska (including the east edge of the Koyukuk basin, the Brooks Range, and the North Slope) yield a composite picture of the crustal and upper mantle structure of this Mesozoic and Cenozoic compressional orogen. The following observations are made: (1) Northern Alaska is underlain by nested tectonic wedges, most with northward vergence (i.e., with their tips pointed north). (2) High reflectivity throughout the crust above a basal decollement, which deepens southward from about 10 km depth beneath the northern front of the Brooks Range to about 30 km depth beneath the southern Brooks Range, is interpreted as structural complexity due to the presence of these tectonic wedges, or duplexes. (3) Low reflectivity throughout the crust below the decollement is interpreted as minimal deformation, which appears to involve chiefly bending of a relatively rigid plate consisting of the parautochthonous North Slope crust and a 10- to 15-km-thick section of mantle material. (4) This plate is interpreted as a southward verging tectonic wedge, with its tip in the lower crust or at the Moho beneath the southern Brooks Range. In this interpretation the middle and upper crust, or all of the crust, is detached in the southern Brooks Range by the tectonic wedge, or indentor: as a result, crust is uplifted and deformed above the wedge, and mantle is depressed and underthrust beneath this wedge. (5) Underthrusting has juxtaposed mantle of two different origins (and seismic velocities), giving rise to a prominent sub-Moho reflector. Copyright 1997 by the American Geophysical Union.

Seismic characteristics of central Brazil crust and upper mantle: A deep seismic refraction study

USGS Publications Warehouse

Soares, J.E.; Berrocal, J.; Fuck, R.A.; Mooney, W.D.; Ventura, D.B.R.

2006-01-01

A two-dimensional model of the Brazilian central crust and upper mantle was obtained from the traveltime interpretation of deep seismic refraction data from the Porangatu and Cavalcante lines, each approximately 300 km long. When the lines were deployed, they overlapped by 50 km, forming an E-W transect approximately 530 km long across the Tocantins Province and western Sa??o Francisco Craton. The Tocantins Province formed during the Neoproterozoic when the Sa??o Francisco, the Paranapanema, and the Amazon cratons collided, following the subduction of the former Goia??s ocean basin. Average crustal VP and VP/VS ratios, Moho topography, and lateral discontinuities within crustal layers suggest that the crust beneath central Brazil can be associated with major geological domains recognized at the surface. The Moho is an irregular interface, between 36 and 44 km deep, that shows evidences of first-order tectonic structures. The 8.05 and 8.23 km s-1 P wave velocities identify the upper mantle beneath the Porangatu and Cavalcante lines, respectively. The observed seismic features allow for the identification of (1) the crust has largely felsic composition in the studied region, (2) the absence of the mafic-ultramafic root beneath the Goia??s magmatic arc, and (3) block tectonics in the foreland fold-and-thrust belt of the northern Brasi??lia Belt during the Neoproterozoic. Seismic data also suggested that the Bouguer gravimetric discontinuities are mainly compensated by differences in mass distribution within the lithospheric mantle. Finally, the Goia??s-Tocantins seismic belt can be interpreted as a natural seismic alignment related to the Neoproterozoic mantle domain. Copyright 2006 by the American Geophysical Union.

Experimental Deformation of Polyphase Aggregates at Pressures and Temperatures of the Upper Mantle

NASA Astrophysics Data System (ADS)

Bejina, F.; Bystricky, M.; Ingrin, J.

2011-12-01

Modelling the solid-state flow of the upper mantle requires a thorough understanding of its rheology and therefore necessitates to perform deformation experiments on mantle rocks (or analogues) at very high pressures and temperatures. Minerals other than olivine constitute up to 40 vol% of upper mantle rocks and may have a significant effect on the rheological behavior of these rocks. Nevertheless, most experimental studies to date have focused on the deformation properties of olivine single crystals or monomineralic olivine aggregates. In this study, and as a first step before focusing on more realistic mantle-like compositions, we have performed deformation experiments on polymineralic model aggregates of forsterite and MgO at upper mantle pressures and temperatures. Commercial powders of Mg2SiO4 and MgO were mixed and ground in WC grinders and dried in a one-atmosphere furnace at 1000°C. Powders with different volume proportions of the two phases were sintered by spark plasma sintering (SPS) at 1300-1400°C and 100 MPa for a few minutes, resulting in dense pellets 8 mm in diameter and 3-4 mm in length. Microstructural analysis by SEM reveals equilibrated microstructures with forsterite and MgO grain sizes of a few microns. Deformation experiments on samples 1.2 mm in diameter and ~1.2mm in length were performed at 3-8 GPa and 1000-1300°C in a D-DIA apparatus coupled with synchrotron X-ray radiation. The technique permits in situ measurement of macroscopic strain rates as well as stress levels sustained by different subpopulations of grains of each phase. Typically, two specimens, respectively a monomineralic and a polymineralic aggregate, were deformed concurrently in order to minimize the relative uncertainties in temperature and pressure and to facilitate the comparison of their rheological properties. The samples were deformed to total strains of 15-25%. As expected, the harder phase, forsterite, sustains much higher stress levels than MgO, in agreement with numerical models for two-phase flow. Results on stress and strain partitioning in polyphase aggregates and on the effect of the presence of a weak phase on the rheology of forsterite-rich aggregates will be discussed.

From the Exoplanetary Bestiary to the Exoplanetary Zoo

NASA Astrophysics Data System (ADS)

Unterborn, C. T.; Panero, W. R.; Stixrude, L. P.; Kellogg, L. H.; Lithgow-Bertelloni, C. R.; Diamond, M. R.

2014-12-01

While much attention has been focused on the exoplanetary "bestiary" of super-Earths, lava worlds, and diamond planets, habitable planets are more likely to be found in a more similar exoplanetary "zoo." Many planet-hosting stars are similar in composition to the Sun, with moderate variations in metal abundances. Even for those stars with O and Fe abundances similar to the Sun, many have 100% variations in the refractory, rock-forming elements such as Si, Mg, Al and Ca. For an Earth sized planet, this variation creates planets with drastically different mantle mineral assemblages and variable melting, elastic, and viscous properties, leading to variable dynamical behavior. This dynamical behavior dictates the dominant mode of heat extraction, be it through a conducting rigid lid or via plate tectonics. Without tectonics, there is no mechanism known with which to create a deep water and carbon cycle, thus creating a long-lived habitable surface. We present the results of integrated modeling in which we consider the effects of variations in bulk mantle composition on Earth-mass planets. To explore the variations expected in this planetary zoo, we present condensation sequence calculations for stars of varying refractory element abundances. These calculations constrain the potential refractory mineral reservoir from which Earth-mass terrestrial planets will form. As planets of this size inevitably will convect, the thermal structure of the mantle is controlled by surface melting temperature and the first crust can be estimated from decompression melting of a convecting mantle. The thermodynamic code HeFESTo determines the mineralogy and resulting thermoelastic properties of both the mantle and potential foundering of crustal material. Finally, with parameterized convection modeling and 2- and 3-D convection modeling, we determine terrestrial mantle's convective regime as a function of bulk composition. We therefore consider a planet's potential for Earth-like plate tectonics by applying compositional perturbations from the Earth. Aspects affecting this potential include the location of the basalt-eclogite transition in the upper mantle and the density contrast, and thus negative buoyancy, between the foundering crust and mantle. Portions of this work were initiated at the CIDER 2014 program.

Seismological evidence of the Hales discontinuity in northeast India

NASA Astrophysics Data System (ADS)

Anand, Aakash; Bora, Dipok K.; Borah, Kajaljyoti; Madhab Borgohain, Jayanta

2018-04-01

The crust and upper mantle shear wave velocity structure beneath the northeast India is estimated by joint inversion of Rayleigh wave group velocity and receiver function, calculated from teleseismic earthquakes data recorded at nine broadband seismic stations. The Assam valley and the Shillong-Mikir plateau are the two important tectonic blocks in the northeast India, which are surrounded by the Himalayan collision zone in the north, Indo-Burma subduction zone in the east and by the Bengal basin in the south. The joint inversion followed by forward modeling reveal crustal thicknesses of 30-34 km beneath the Shillong plateau, 36 km beneath the Mikir hills and 38-40 km beneath the Assam valley with an average shear wave velocity (Vs) of 3.4-3.5 km/s. The estimated low upper mantle shear wave velocity (Vsn) 4.2-4.3 km/s may be due to the rock composition or grain size or increased temperature and partial melt (<1%) in the upper mantle, or an effect of all. Also, we report for the first time, the existence of the Hales discontinuity at depths 56-74 km with Vs ∼4.4-4.6 km/s. Variable depth of the Hales discontinuity may be explained by the geotherm and/or addition of Cr3+ and Fe2+ in the spinel-garnet system.

Upper mantle anisotropic attenuation of the Sierra Nevada and surroundings

NASA Astrophysics Data System (ADS)

Bernardino, M. J.; Jones, C. H.; Monsalve, G.

2016-12-01

We investigate the contribution of anelasticity in the generation of seismic velocity variations within the upper mantle of the Sierra Nevada and surrounding regions through teleseismic shear-wave attenuation. Given that anelastic effects are most sensitive to temperature and hydration and less to composition and small degrees of partial melt, we aim constrain the thermal structure beneath this region and identify locations where elevated upper mantle temperatures dominate. We also investigate the dependence of shear-wave attenuation on direction by accounting for seismic anisotropy in our measurements. S-wave t* values are determined from teleseismic S- and SKS- phases recorded on permanent and temporary deployments within the California region with particular focus on the Sierra Nevada Earthscope Project (SNEP) and the Sierran Paradox Experiment (SPE) stations. S-waveforms are rotated into the Sierran SFast, N75°E, and SSlow, N15°W, components. Following the method of Stachnik et al., (2004), S-wave spectra for each event are jointly inverted for a single seismic moment, M0k, and corner frequency, fck, for each event, and separate t* for each ray path. The resulting t*Fast and t*Slow measurements are then inverted for three-dimensional variations in (1/QFast) and (1/QSlow). Results are compared with previous magnetotelluric, surface heat flow, and body-wave velocity inversion studies.

Os-186 and Os-187 Enrichments and High-He-3/He-4 sources in the Earth's Mantle: Evidence from Icelandic Picrites

NASA Technical Reports Server (NTRS)

Brandon, Alan D.; Graham, David W.; Waight, Tod; Gautason, Bjarni

2007-01-01

Picrites from the neovolcanic zones in Iceland display a range in Os-187/Os-188O from 0.1297 to 0.1381 ((gamma)Os = 0.0 to 6.5) and uniform Os-186/Os-188 of 0.1198375+/-32 (2 (sigma)). The value for Os-186/Os-188 is within uncertainty of the present-day value for the primitive upper mantle of 0.1198398+/-16. These Os isotope systematics are best explained by ancient recycled crust or melt enrichment in the mantle source region. If so, then the coupled enrichments displayed in Os-186/Os-188 and Os-187/Os-188 from lavas of other plume systems must result from an independent process, the most viable candidate at present remains core-mantle interaction. While some plumes with high He-3/He-4, such as Hawaii, appear to have been subjected to detectable addition of Os (and possibly He) from the outer core, others such as Iceland do not. A positive correlation between Os-187/Os-188 and He-3/He-4 from 9.6 to 19 RA in Iceland picrites is best modeled as mixtures of 500 Ma or older ancient recycled crust mixed with primitive mantle, creating a hybrid source region that subsequently mixes with the convecting MORB mantle during ascent and melting. This multistage mechanism to explain these isotope systematics is consistent with ancient recycled crust juxtaposed with more primitive, relatively He-rich mantle, in convective isolation from the upper mantle, most likely in the lowermost mantle. This is inconsistent with models that propose random mixing between heterogeneities in the convecting upper mantle as a mechanism to explain the observed isotopic variation in oceanic lavas or models that produce a high He-3/He-4 signature in melt depleted and strongly outgassed, He-poor mantle. Instead these systematics require a deep mantle source to explain the 3He/4He signature in Iceland lavas. The He-3/He-4 of lavas derived from the Iceland plume changed over time, from a maximum of 50 RA at 60 Ma, to approximately 25-27 RA at present. The changes are coupled with distinct compositional gaps between the different aged lavas when H-3/He-4 is plotted versus various geochemical parameters such as Nd-143/Nd-144 and La/Sm. These relationships can be interpreted as an increase in the proportion of ancient recycled crust in the upwelling plume over this time period.

Mantle dynamics and generation of a geochemical mantle boundary along the East Pacific Rise - Pacific/Antarctic ridge

NASA Astrophysics Data System (ADS)

Zhang, Guo-Liang; Chen, Li-Hui; Li, Shi-Zhen

2013-12-01

A large-scale mantle compositional discontinuity was identified along the East Pacific Rise (EPR) and the Pacific-Antarctic Ridge (PAR) with an inferred transition located at the EPR 23°S-32°S. Because of the EPR-Easter hotspot interactions in this area, the nature of this geochemical discontinuity remains unclear. IODP Sites U1367 and U1368 drilled into the ocean crust that was accreted at ∼33.5 Ma and ∼13.5 Ma, respectively, between 28°S and 30°S on the EPR. We use lavas from Sites U1367 and U1368 to track this mantle discontinuity away from the EPR. The mantle sources for basalts at Sites U1367 and U1368 represent, respectively, northern and southern Pacific mantle sub-domains in terms of Sr-Nd-Pb-Hf isotopes. The significant isotopic differences between the two IODP sites are consistent with addition of ancient subduction-processed ocean crust to the south Pacific mantle sub-domain. Our modeling result shows that a trace element pattern similar to that of U1368 E-MORB can be formed by melting a subduction-processed typical N-MORB. The trace element and isotope compositions for Site U1368 MORBs can be formed by mixing a HIMU mantle end-member with Site U1367 MORBs. Comparison of our data with those from the EPR-PAR shows a geochemical mantle boundary near the Easter microplate that separates the Pacific upper mantle into northern and southern sub-domains. On the basis of reconstruction of initial locations of the ocean crust at the two sites, we find that the mantle boundary has moved northward to the Easter microplate since before 33.5 Ma. A model, in which along-axis asthenospheric flow to where asthenosphere consumption is strongest, explains the movement of the apparent mantle boundary.

Saudi Arabian seismic-refraction profile: A traveltime interpretation of crustal and upper mantle structure

USGS Publications Warehouse

Mooney, W.D.; Gettings, M.E.; Blank, H.R.; Healy, J.H.

1985-01-01

The crustal and upper mantle compressional-wave velocity structure across the southwestern Arabian Shield has been investigated by a 1000-km-long seismic refraction profile. The profile begins in Mesozoic cover rocks near Riyadh on the Arabian Platform, trends southwesterly across three major Precambrian tectonic provinces, traverses Cenozoic rocks of the coastal plain near Jizan, and terminates at the outer edge of the Farasan Bank in the southern Red Sea. More than 500 surveyed recording sites were occupied, and six shot points were used, including one in the Red Sea. Two-dimensional ray-tracing techniques, used to analyze amplitude-normalized record sections indicate that the Arabian Shield is composed, to first order, of two layers, each about 20 km thick, with average velocities of about 6.3 km/s and 7.0 km/s, respectively. West of the Shield-Red Sea margin, the crust thins to a total thickness of less than 20 km, beyond which the Red Sea shelf and coastal plain are interpreted to be underlain by oceanic crust. A major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition. Elsewhere along the profile, several high-velocity anomalies in the upper crust correlate with mapped gneiss domes, the most prominent of which is the Khamis Mushayt gneiss. Based on their velocities, these domes may constitute areas where lower crustal rocks have been raised some 20 km. Two intracrustal reflectors in the center of the Shield at 13 km depth probably represent the tops of mafic intrusives. The Mohorovic??ic?? discontinuity beneath the Shield varies from a depth of 43 km and mantle velocity of 8.2 km/s in the northeast to a depth of 38 km and mantle velocity of 8.0 km/s depth in the southwest near the Shield-Red Sea transition. Two velocity discontinuities occur in the upper mantle, at 59 and 70 km depth. The crustal and upper mantle velocity structure of the Arabian Shield is interpreted as revealing a complex crust derived from the suturing of island arcs in the Precarnbrian. The Shield is currently flanked by the active spreading boundary in the Red Sea. ?? 1985.

Re-Os isotopic evidence for an enriched-mantle source for the Noril'sk-type, ore-bearing intrusions, Siberia

USGS Publications Warehouse

Walker, R.J.; Morgan, J.W.; Horan, M.F.; Czamanske, G.K.; Krogstad, E.J.; Fedorenko, V.A.; Kunilov, V.E.

1994-01-01

Magmatic Cu-Ni sulfide ores and spatially associated ultramafic and mafic rocks from the Noril'sk I, Talnakh, and Kharaelakh intrusions are examined for Re-Os isotopic systematics. Neodymium and lead isotopic data also are reported for the ultramafic and mafic rocks. The Re-Os data for most samples indicate closed-system behavior since the ca. 250 Ma igneous crystallization age of the intrusions. There are small but significant differences in the initial osmium isotopic compositions of samples from the three intrusions. Ores from the Noril'sk I intrusion have ??Os values that vary from +0.4 to +8.8, but average +5.8. Ores from the Talnakh intrusion have ??Os values that range from +6.7 to +8.2, averaging +7.7. Ores from the Kharaelakh intrusion have ??Os values that range from +7.8 to +12.9, with an average value of +10.4. The osmium isotopic compositions of the ore samples from the Main Kharaelakh orebody exhibit minimal overlap with those for the Noril'sk I and Talnakh intrusions, indicating that these Kharaelakh ores were derived from a more radiogenic source of osmium than the other ores. Combined osmium and lead data for major orebodies in the three intrusions plot in three distinct fields, indicating derivation of osmium and lead from at least three isotopically distinct sources. Some of the variation in lead isotopic compositions may be the result of minor lower-crustal contamination. However, in contrast to most other isotopic and trace element data, Os-Pb variations are generally inconsistent with significant crustal contamination or interaction with the subcontinental lithosphere. Thus, the osmium and lead isotopic compositions of these intrusions probably reflect quite closely the compositions of their mantle source, and suggest that these two isotope systems were insensitive to lithospheric interaction. Ultramafic and mafic rocks have osmium and lead isotopic compositions that range only slightly beyond the compositions of the ores. These rocks also have relatively uniform ??{lunate}Nd values that range only from -0.8 to + 1.1. This limited variation in neodymium isotopic composition may reflect the characteristics of the mantle sources of the rocks, or it may indicate that somehow similar proportions of crust contaminated the parental melts. The osmium, lead, and neodymium isotopic data for these rocks most closely resemble the mantle sources of certain ocean island basalts (OIB), such as some Hawaiian basalts. Hence, these data are consistent with derivation of primary melts from a mantle source similar to that of some types of hotspot activity. The long-term Re/Os enrichment of this and similar mantle sources, relative to chondritic upper mantle, may reflect 1. (1) incorporation of recycled oceanic crust into the source more than 1 Ga ago, 2. (2) derivation from a mantle plume that originated at the outer core-lower mantle interface, or 3. (3) persistence of primordial stratification of rhenium and osmium in the mantle. ?? 1994.

Scales of Heterogeneities in the Continental Crust and Upper Mantle

NASA Astrophysics Data System (ADS)

Tittgemeyer, M.; Wenzel, F.; Ryberg, T.; Fuchs, K.

1999-09-01

A seismological characterization of crust and upper mantle can refer to large-scale averages of seismic velocities or to fluctuations of elastic parameters. Large is understood here relative to the wavelength used to probe the earth.¶In this paper we try to characterize crust and upper mantle by the fluctuations in media properties rather than by their average velocities. As such it becomes evident that different scales of heterogeneities prevail in different layers of crust and mantle. Although we cannot provide final models and an explanation of why these different scales exist, we believe that scales of inhomogeneities carry significant information regarding the tectonic processes that have affected the lower crust, the lithospheric and the sublithospheric upper mantle.¶We focus on four different types of small-scale inhomogeneities (1) the characteristics of the lower crust, (2) velocity fluctuations in the uppermost mantle, (3) scattering in the lowermost lithosphere and on (4) heterogeneities in the mantle transition zone.

Chromium isotope heterogeneity in the mantle

NASA Astrophysics Data System (ADS)

Xia, Jiuxing; Qin, Liping; Shen, Ji; Carlson, Richard W.; Ionov, Dmitri A.; Mock, Timothy D.

2017-04-01

To better constrain the Cr isotopic composition of the silicate Earth and to investigate potential Cr isotopic fractionation during high temperature geological processes, we analyzed the Cr isotopic composition of different types of mantle xenoliths from diverse geologic settings: fertile to refractory off-craton spinel and garnet peridotites, pyroxenite veins, metasomatised spinel lherzolites and associated basalts from central Mongolia, spinel lherzolites and harzburgites from North China, as well as cratonic spinel and garnet peridotites from Siberia and southern Africa. The δ53CrNIST 979 values of the peridotites range from - 0.51 ± 0.04 ‰ (2SD) to + 0.75 ± 0.05 ‰ (2SD). The results show a slight negative correlation between δ53Cr and Al2O3 and CaO contents for most mantle peridotites, which may imply Cr isotopic fractionation during partial melting of mantle peridotites. However, highly variable Cr isotopic compositions measured in Mongolian peridotites cannot be caused by partial melting alone. Instead, the wide range in Cr isotopic composition of these samples most likely reflects kinetic fractionation during melt percolation. Chemical diffusion during melt percolation resulted in light Cr isotopes preferably entering into the melt. Two spinel websterite veins from Mongolia have extremely light δ53Cr values of - 1.36 ± 0.04 ‰ and - 0.77 ± 0.06 ‰, respectively, which are the most negative Cr isotopic compositions yet reported for mantle-derived rocks. These two websterite veins may represent crystallization products from the isotopically light melt that may also metasomatize some peridotites in the area. The δ53Cr values of highly altered garnet peridotites from southern Africa vary from - 0.35 ± 0.04 ‰ (2SD) to + 0.12 ± 0.04 ‰ (2SD) and increase with increasing LOI (Loss on Ignition), reflecting a shift of δ53Cr to more positive values by secondary alteration. The Cr isotopic composition of the pristine, fertile upper mantle is estimated as δ53Cr = - 0.14 ± 0.12 ‰, after corrections for the effects of partial melting and metasomatism. This value is in line with that estimated for the BSE (- 0.12 ± 0.10 ‰) previously.

Shear Wave Velocities in the Pampean Flat Slab Region from Rayleigh Wave Tomography: Implications for Crustal Composition and Upper Mantle Hydration

NASA Astrophysics Data System (ADS)

Porter, R. C.; Gilbert, H. J.; Zandt, G.; Beck, S. L.; Warren, L. M.; Calkins, J. A.; Alvarado, P. M.; Anderson, M. L.

2011-12-01

The Pampean flat slab region, located in Chile and western Argentina between 29° and 34° S, is characterized by the subducting Nazca plate assuming a sub-horizontal geometry for ~300 km laterally before resuming a more "normal" angle of subduction. The onset of flat slab subduction is associated with the cessation of regional arc related volcanism and the migration of deformation inboard from the high Andes into the thin-skinned Precordillera and thick-skinned Sierras Pampeanas. Developing a better understanding of this region's geology is of particular importance, as it is an ideal area to study flat slab subduction and serves as a modern analogue to Laramide flat slab subduction in the western US. To study the crustal and mantle structure in the region, we combine ambient noise tomography and ballistic surface wave tomography to produce a regional 3D shear wave velocity model that encompasses flat slab subduction in the north and normal subduction geometry in the south, allowing for a comparison of the two. Results from this work show that shear velocities within the upper crust are largely determined by composition, with sedimentary basins and areas with active volcanism exhibiting slower velocities than basement cored uplifts and other bedrock exposures. Though surface waves are not particularly sensitive to the depth of sharp velocity contrasts, we observe an eastward increase in shear velocity at depth that correlates with an eastward decrease in crustal thickness. In both the slab and overlying mantle, we observe significant variations in shear wave velocity. North of 32° S, where flat slab subduction is occurring, the Nazca plate contains low-velocity zones (LVZs) beneath the high Andes and Precordillera that are not present in the east beneath the Sierras Pampeanas. An opposite transition is observed in the overlying mantle, which changes from fast in the west to slow in the east. Both of these observations are consistent with an initially hydrated slab dehydrating and releasing water into the overlying mantle. Within this region we also observe a LVZ immediately above the slab as the subduction angle steepens. This zone potentially represents asthenosphere or hydrated lithospheric mantle. South of 32° S, where subduction is occurring at a more normal angle, the slab is visible as a high-velocity body with a low-velocity mantle wedge present beneath the arc and back arc. The variations in slab and upper mantle shear velocities are consistent with a hydrated flat slab and the presence of a LVZ above the flat slab as it steepens suggests that water is being transported to a significant depth or that an asthenospheric wedge is present between the slab and cratonic lithosphere.

Crustal structure of Yunnan province, People's Republic of China, from seismic refraction profiles

USGS Publications Warehouse

Kan, R.-J.; Hu, H.-X.; Zeng, R.-S.; Mooney, W.D.; McEvilly, T.V.

1986-01-01

Seismic refraction, profiles in Yunnan Province, southwestern China, define the crustal structure in an area of active tectonics, on the southern end of the Himalaya-Burma arc. The crustal thickness ranges from 38 to 46 kilometers, and the relatively low mean crustal velocity indicates a crustal composition compatible with normal continental crust and consisting mainly of meta-sedimentary and silicic intrusive rocks, with little mafic or ultramafic component. This composition suggests a crustal evolution involving sedimentary processes on the flank of the Yangtze platform rather than the accretion of oceanic island arcs, as has been proposed. An anomalously low upper-mantle velocity observed on one profile, but not on another at right angles to it may indicate active tectonic processes in the mantle or seismic anisotropy.

Crustal Structure of Yunnan Province, People's Republic of China, from Seismic Refraction Profiles.

PubMed

Kan, R J; Hu, H X; Zeng, R S; Mooney, W D; McEvilly, T V

1986-10-24

Seismic refraction, profiles in Yunnan Province, southwestern China, define the crustal structure in an area of active tectonics on the southern end of the Himalaya-Burma arc. The crustal thickness ranges from 38 to 46 kilometers, and the relatively low mean crustal velocity indicates a crustal composition compatible with normal continental crust and consisting mainly of meta-sedimentary and silicic intrusive rocks, with little mafic or ultramafic component. This composition suggests a crustal evolution involving sedimentary processes on the flank of the Yangtze platform rather than the accretion of oceanic island arcs, as has been proposed. An anomalously low upper-mantle velocity observed on one profile but not on another at right angles to it may indicate active tectonic processes in the mantle or seismic anisotropy.

Plate-Tectonic Circulation is Driven by Cooling From the Top and is Closed Within the Upper Mantle

NASA Astrophysics Data System (ADS)

Hamilton, W. B.

2001-12-01

Subduction drives plate tectonics and is due to cooling from the top: circulation is self-organized, and likely is closed above the discontinuity near 660 km. The contrary consensus that plate tectonics is driven by bottom heating and involves the entire mantle combines misunderstood kinematics with flawed concepts of through-the-mantle plumes and subduction. Plume conjecture came from the Emperor-Hawaii progression, the 45 Ma inflection in which was assumed to mark a 60-degree change in direction of that part of the Pacific plate over a fixed plume. Smooth spreading patterns around the east and south margin of the Pacific plate, and paleomagnetic data, disprove such a change. Speculations that plumes move, jump, etc. do not revive falsified conjecture. Geochemical distinctions between enriched island and depleted ridge basalts (which overlap) are expected products of normal upper-mantle processes, not plumes. MORB traverses solidus-T asthenosphere, whereas OIB zone-refines through subsolidus lithosphere and crust, crystallizing refractories to retain T of diminishing melt while assimilating and retaining fusibles. Tomographic inference of deep-mantle subduction is presented misleadingly and may reflect methodological and sampling artifacts (downward smearing, and concentration of recorded body waves in bundles within broad anomalies otherwise poorly sampled). Planetological and other data require hot Earth accretion, and thorough early fractionation, from material much more refractory than primitive meteorites, and are incompatible with the little-fractionated lower mantle postulated to permit whole-mantle circulation. The profound seismic discontinuity near 660 km is a thermodynamic and physical barrier to easy mass transfer in either direction. Refractory lower mantle convects slowly, perhaps in layers, and loses primarily original heat, whereas upper mantle churns rapidly, and the 660 decoupling boundary must have evolved into a compositional barrier also. Plate motions are driven by subduction, the passive falling away of oceanic lithosphere which is negatively buoyant because of top-down cooling. Slabs have top and bottom rolling hinges and sink subvertically (inclinations of slabs mark their positions, not trajectories) into the transition zone, where they are laid down on, and depress, the 660-km discontinuity. Rollback of upper hinges into subducting plates is required by plate behavior at all scales. That fronts of overriding plates advance at rollback velocity is required by common preservation atop their thin leading edges of little-deformed fore-arc basins. Convergence velocity also commonly equals rollback but is faster in some arcs. Steeply-sinking inclined slabs push sublithospheric upper mantle forward into the shrinking ocean from which they came, forcing seafloor spreading therein, and pull overriding plates behind them. Continental plates pass over sunken slabs like tanks above their basal treads, and material from, and displaced rearward by, sunken slabs is cycled into pull-apart oceans opening behind the continents, thus transferring mantle from shrinking to enlarging oceans. Hot mantle displaced above slabs enables backarc spreading. Spreading ridges, in both shrinking and enlarging oceans, are passive byproducts of subduction, and migrate because it is more energy efficient to process new asthenosphere than to get partial melt from increasingly distant sources. A plate-motion framework wherein hinges roll back, ridges migrate, Antarctica is approximately fixed, and intraplate deformation is integrated may approximate an absolute reference to sluggish lower mantle, whereas the hotspot frame is invalid, and the no-net-rotation frame minimizes trench and ridge motions.

Peeling linear inversion of upper mantle velocity structure with receiver functions

NASA Astrophysics Data System (ADS)

Shen, Xuzhang; Zhou, Huilan

2012-02-01

A peeling linear inversion method is presented to study the upper mantle (from Moho to 800 km depth) velocity structures with receiver functions. The influences of the crustal and upper mantle velocity ratio error on the inversion results are analyzed, and three valid measures are taken for its reduction. This method is tested with the IASP91 and the PREM models, and the upper mantle structures beneath the stations GTA, LZH, and AXX in northwestern China are then inverted. The results indicate that this inversion method is feasible to quantify upper mantle discontinuities, besides the discontinuities between 3 h M ( h M denotes the depth of Moho) and 5 h M due to the interference of multiples from Moho. Smoothing is used to overcome possible false discontinuities from the multiples and ensure the stability of the inversion results, but the detailed information on the depth range between 3 h M and 5 h M is sacrificed.

Carbon isotope composition of CO2-rich inclusions in cumulate-forming mantle minerals from Stromboli volcano (Italy)

NASA Astrophysics Data System (ADS)

Gennaro, Mimma Emanuela; Grassa, Fausto; Martelli, Mauro; Renzulli, Alberto; Rizzo, Andrea Luca

2017-10-01

We report on measurements of concentration and carbon isotope composition (δ13CCO2) of CO2 trapped in fluid inclusions of olivine and clinopyroxene crystals separated from San Bartolo ultramafic cumulate Xenoliths (SBX) formed at mantle depth (i.e., beneath a shallow Moho supposed to be at 14.8 km). These cumulates, erupted about 2 ka ago at Stromboli volcano (Italy), have been already investigated by Martelli et al. (2014) mainly for Sr-Nd isotopes and for their noble gases geochemistry. The concentration of CO2 varies of one order of magnitude from 3.8·10- 8 mol g- 1 to 4.8·10- 7 mol g- 1, with δ13C values between - 2.8‰ and - 1.5‰ vs V-PDB. These values overlap the range of measurements performed in the crater gases emitted at Stromboli (- 2.5‰ < δ13CCO2 < - 1.0‰). Since SBX formed from relatively primitive mantle-derived basic magmas, we argue that the isotope composition displayed by fluid inclusions and surface gases can be considered representative of the magma volatile imprinting released by partial melting of the mantle source beneath Stromboli (- 2.8‰ < δ13C < - 1.0‰). In addition, the δ13C signature of CO2 is not significantly modified by fractionation due to magmatic degassing or intracrustal contamination processes owing to magma ascent and residence within the volcano plumbing system. Such δ13C values are higher than those commonly reported for MORB-like upper mantle (- 8 ÷ - 4‰) and likely reflect the source contamination of the local mantle wedge by CO2 coming from the decarbonation of the sediments carried by the subducting Ionian slab with a contribution of organic carbon up to 7%.

Differences in the Upper Mantle Structure between 'Hot' and 'Cold' Areas in North America based on USArray Seismic Data along California - Virginia Profile

NASA Astrophysics Data System (ADS)

Dec, M.; Sroda, P.; Tesauro, M.; Kaban, M. K.; Perchuc, E.

2013-12-01

Nowadays, United States is an area extensively studied by seismic research due to the fact that the EarthScope USArray project provides an unique opportunity to verify previous seismological models and improve our understanding of the upper mantle structure. The data from this experiment are fundamental to study the upper mantle structure because they allow us to present much more detailed analysis. In this study we use the data recorded by the Transportable Array of the USArray and data from the ISC bulletin. We refer also to data from longitudinal Early Rise project while analysing New Madrid Seismic Zone. We use the travel time data from the earthquakes recorded at a distance up to 3500 km in order to image the upper mantle down to about 600 km depth. We present P- and S-wave velocity models for the tectonically stable central part of US and for the active western part. The 1D models are constructed based on the forward modelling of traveltimes from the events located along the California - Virginia profile, for e.g. in California, Colorado or Virginia. This provides a possibility to update the previous MP-1 model (Malinowski et al., 2010). The models were corrected for the crustal effect using the crustal model of Tesauro et al. (2013). All the models have been verified by synthetic seismograms calculated using the reflectivity method. The models show significant differences in the first-arrivals observed at the 800-1800 km epicentral distance range. In the Western, tectonically active region, the 300-km discontinuity is observed. It is interpreted based on the refracted phases with the apparent velocity of 8.9-9.0 km/s and clearly observed reflections. In this area, a low-velocity zone at the bottom of the upper mantle significantly deepens the 410-km discontinuity. The stable North American Craton is characterized by blurred arrivals from the 300-km discontinuity. These 1D models of the upper mantle structure in North America served as a starting point for calculation of a 2D model along the profile using forward and inversion approach. We distinguish three parts in our profile: western - tectonically active, central cratonic - stable one and eastern - tectonically active. The New Madrid Seismic Zone is characterized by an anomalous structure in the lower lithosphere at the offset ~2500km. Very interesting part of the studied area is the marginal part of North American Craton, which separates two tectonically different areas. The seismic P- and S-wave velocity models were inverted for temperature using different mantle composition and anelasticity models. The modelling results are in agreement with those obtained for the strength and the elastic thickness of the lithosphere.

Processes accompanying of mantle plume emplacement into continental lithosphere: Evidence from NW Arabian plate, Western Syria

NASA Astrophysics Data System (ADS)

Sharkov, E. V.

2015-12-01

Lower crustal xenoliths occurred in the Middle Cretaceous lamprophyre diatremes in Jabel Ansaria (Western Syria) (Sharkov et al., 1992). They are represented mainly garnet granulites and eclogite-like rocks, which underwent by deformations and retrograde metamorphism, and younger fresh pegmatoid garnet-kaersutite-clinopyroxene (Al-Ti augite) rocks; mantle peridotites are absent in these populations. According to mineralogical geothermobarometers, forming of garnet-granulite suite rocks occurred under pressure 13.5-15.4 kbar (depths 45-54 kn) and temperature 965-1115oC. At the same time, among populations of mantle xenoliths in the Late Cenozoic platobasalts of the region, quite the contrary, lower crustal xenoliths are absent, however, predominated spinel lherzolites (fragments of upper cooled rim of a plume head), derived from the close depths (30-40 km: Sharkov, Bogatikov, 2015). From this follows that ancient continental crust was existed here even in the Middle Cretaceous, but in the Late Cenozoic was removed by extended mantle plume head; at that upper sialic crust was not involved in geomechanic processes, because Precambrian metamorphic rocks survived as a basement for Cambrian to Cenozoic sedimentary cover of Arabian platform. In other words, though cardinal rebuilding of deep-seated structure of the region occurred in the Late Cenozoic but it did not affect on the upper shell of the ancient lithosphere. Because composition of mantle xenolithis in basalts is practically similar worldwide, we suggest that deep-seated processes are analogous also. As emplacement of the mantle plume heads accompanied by powerful basaltic magmatism, very likely that range of lower (mafic) continental crust existence is very convenient for extension of plume heads and their adiabatic melting. If such level, because of whatever reasons, was not reached, melting was limited but appeared excess of volatile matters which led to forming of lamprophyre or even kimberlite.

A deep crustal fluid channel into the San Andreas Fault system near Parkfield, California

USGS Publications Warehouse

Becken, M.; Ritter, O.; Park, S.K.; Bedrosian, P.A.; Weckmann, U.; Weber, M.

2008-01-01

Magnetotelluric (MT) data from 66 sites along a 45-km-long profile across the San Andreas Fault (SAF) were inverted to obtain the 2-D electrical resistivity structure of the crust near the San Andreas Fault Observatory at Depth (SAFOD). The most intriguing feature of the resistivity model is a steeply dipping upper crustal high-conductivity zone flanking the seismically defined SAF to the NE, that widens into the lower crust and appears to be connected to a broad conductivity anomaly in the upper mantle. Hypothesis tests of the inversion model suggest that upper and lower crustal and upper-mantle anomalies may be interconnected. We speculate that the high conductivities are caused by fluids and may represent a deep-rooted channel for crustal and/or mantle fluid ascent. Based on the chemical analysis of well waters, it was previously suggested that fluids can enter the brittle regime of the SAF system from the lower crust and mantle. At high pressures, these fluids can contribute to fault-weakening at seismogenic depths. These geochemical studies predicted the existence of a deep fluid source and a permeable pathway through the crust. Our resistivity model images a conductive pathway, which penetrates the entire crust, in agreement with the geochemical interpretation. However, the resistivity model also shows that the upper crustal branch of the high-conductivity zone is located NE of the seismically defined SAF, suggesting that the SAF does not itself act as a major fluid pathway. This interpretation is supported by both, the location of the upper crustal high-conductivity zone and recent studies within the SAFOD main hole, which indicate that pore pressures within the core of the SAF zone are not anomalously high, that mantle-derived fluids are minor constituents to the fault-zone fluid composition and that both the volume of mantle fluids and the fluid pressure increase to the NE of the SAF. We further infer from the MT model that the resistive Salinian block basement to the SW of the SAFOD represents an isolated body, being 5-8km wide and reaching to depths >7km, in agreement with aeromagnetic data. This body is separated from a massive block of Salinian crust farther to the SW. The NE terminus of resistive Salinian crust has a spatial relationship with a near-vertical zone of increased seismic reflectivity ???15km SW of the SAF and likely represents a deep-reaching fault zone. ?? 2008 The Authors Journal compilation ?? 2008 RAS.

Origin of the Mackenzie large igneous province and sourcing of flood basalts from layered intrusions

NASA Astrophysics Data System (ADS)

Day, J. M.; Pearson, D.

2013-12-01

The 1.27 Ga Coppermine continental flood basalt (CFB) in northern Canada represents the extrusive manifestation of the Mackenzie large igneous province (LIP) that includes the Mackenzie dyke swarm and the Muskox layered intrusion. New Re-Os isotope and highly siderophile element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundance data are reported together with whole-rock major- and trace-element abundances and Nd isotopes to examine the behaviour of the HSE during magmatic differentiation and to place constraints on the extent of crustal interaction with mantle-derived melts. Mineral-chemical data are also reported for an unusual andesite glass flow (4.9 wt.% MgO) found in proximity to newly recognised picrites (>20 wt.% MgO) in the lowermost stratigraphy of the Coppermine CFB. Compositions of mineral phases in the andesite are similar to equivalent phases found in Muskox Intrusion chromitites and the melt composition is identical to Muskox chromite melt inclusions. Elevated HSE contents (e.g., 3.8 ppb Os) and the mantle-like initial Os isotope composition of this andesitic glass contrast strongly with oxygen isotope and lithophile element evidence for extensive crustal contamination. These signatures implicate an origin for the glass as a magma mingling product formed within the Muskox Intrusion during chromitite genesis. The combination of crust and mantle signatures define roles for both these reservoirs in chromitite genesis, but the HSE appear to be dominantly mantle-sourced. Combined with Nd isotope data that places the feeder for lower Coppermine CFB picrites and basalts within the Muskox Intrusion, this provides the strongest evidence yet for direct processing of some CFB within upper-crustal magma chambers. Modeling of absolute and relative HSE abundances in CFB reveal that HSE concentrations decrease with increasing fractionation for melts with <8×1 wt.% MgO in the Coppermine CFB, with picrites (>13.5wt.% MgO) from CFB having higher Os abundances than ocean island basalt (OIB) equivalents. The differences between CFB and OIB picrite absolute Os abundances may result from higher degrees of partial melting to form CFB but may also reflect incorporation of trace sulphide in CFB picrites from magmas that reached S-saturation in shallow-level magma chambers. Significant inter-element fractionation between (Re+Pt+Pd)/(Os+Ir+Ru) are generated during magmatic differentiation in response to strongly contrasting partitioning of these two groups of elements into sulphides and/or HSE-rich alloys. Furthermore, fractional crystallization has a greater role on absolute and relative HSE abundances than crustal contamination under conditions of CFB petrogenesis due to the dilution effect of continental crust. The Coppermine CFB define a Re-Os isochron with an age of 1263 +16/-20 Ma and initial gamma Os = +2.2×0.8. Combined data for the basaltic and intrusive portions of the Mackenzie LIP indicate a mantle source broadly within the range of the primitive upper mantle. The majority of Archaean komatiites and Phanerozoic CFB also require mantle sources with primitive upper mantle to chondritic Re/Os evolution, with exceptions typically being from analyses of highly-fractionated MgO-poor basalts.

Atlas of the underworld: Slab remnants in the mantle, their sinking history, and a new outlook on lower mantle viscosity

NASA Astrophysics Data System (ADS)

van der Meer, Douwe G.; van Hinsbergen, Douwe J. J.; Spakman, Wim

2018-01-01

Across the entire mantle we interpret 94 positive seismic wave-speed anomalies as subducted lithosphere and associate these slabs with their geological record. We document this as the Atlas of the Underworld, also accessible online at www.atlas-of-the-underworld.org, a compilation comprising subduction systems active in the past 300 Myr. Deeper slabs are correlated to older geological records, assuming no relative horizontal motions between adjacent slabs following break-off, using knowledge of global plate circuits, but without assuming a mantle reference frame. The longest actively subducting slabs identified reach the depth of 2500 km and some slabs have impinged on Large Low Shear Velocity Provinces in the deepest mantle. Anomously fast sinking of some slabs occurs in regions affected by long-term plume rising. We conclude that slab remnants eventually sink from the upper mantle to the core-mantle boundary. The range in subduction-age versus - depth in the lower mantle is largely inherited from the upper mantle history of subduction. We find a significant depth variation in average sinking speed of slabs. At the top of the lower mantle average slab sinking speeds are between 10 and 40 mm/yr, followed by a deceleration to 10-15 mm/yr down to depths around 1600-1700 km. In this interval, in situ time-stationary sinking rates suggest deceleration from 20 to 30 mm/yr to 4-8 mm/yr, increasing to 12-15 mm/yr below 2000 km. This corroborates the existence of a slab deceleration zone but we do not observe long-term (> 60 My) slab stagnation, excluding long-term stagnation due to compositional effects. Conversion of slab sinking profiles to viscosity profiles shows the general trend that mantle viscosity increases in the slab deceleration zone below which viscosity slowly decreases in the deep mantle. This is at variance with most published viscosity profiles that are derived from different observations, but agrees qualitatively with recent viscosity profiles suggested from material experiments.

Birch's Mantle

NASA Astrophysics Data System (ADS)

Anderson, D. L.

2002-12-01

Francis Birch's 1952 paper started the sciences of mineral physics and physics of the Earth's interior. Birch stressed the importance of pressure, compressive strain and volume in mantle physics. Although this may seem to be an obvious lesson many modern paradoxes in the internal constitution of the Earth and mantle dynamics can be traced to a lack of appreciation for the role of compression. The effect of pressure on thermal properties such as expansivity can gravitational stratify the Earth irreversibly during accretion and can keep it chemically stratified. The widespread use of the Boussinesq approximation in mantle geodynamics is the antithesis of Birchian physics. Birch pointed out that eclogite was likely to be an important component of the upper mantle. Plate tectonic recycling and the bouyancy of oceanic crust at midmantle depths gives credence to this suggestion. Although peridotite dominates the upper mantle, variations in eclogite-content may be responsible for melting- or fertility-spots. Birch called attention to the Repetti Discontinuity near 900 km depth as an important geodynamic boundary. This may be the chemical interface between the upper and lower mantles. Recent work in geodynamics and seismology has confirmed the importance of this region of the mantle as a possible barrier. Birch regarded the transition region (TR ; 400 to 1000 km ) as the key to many problems in Earth sciences. The TR contains two major discontinuities ( near 410 and 650 km ) and their depths are a good mantle thermometer which is now being exploited to suggest that much of plate tectonics is confined to the upper mantle ( in Birch's terminology, the mantle above 1000 km depth ). The lower mantle is homogeneous and different from the upper mantle. Density and seismic velocity are very insensitive to temperature there, consistent with tomography. A final key to the operation of the mantle is Birch's suggestion that radioactivities were stripped out of the deeper parts of Earth and placed in the crust and upper mantle. This resolves the lower mantle overheating paradox but the stratified mantle slows down the cooling of the Earth. A completely thermodynamically self-consistent treatment of mantle dynamics, with volume and temperature-dependent parameters has not yet been attempted but the essence of this approach is contained in the 1952 paper, which is must reading for all students of Earth's interior. One implication of this paper is that lower mantle structures should be gigantic and long-lived, a prediction spectacularly confirmed by modern seismic tomography.

Composition of the crust beneath the Kenya rift

USGS Publications Warehouse

Mooney, W.D.; Christensen, N.I.

1994-01-01

We infer the composition of the crust beneath and on the flanks of the Kenya rift based on a comparison of the KRISP-90 crustal velocity structure with laboratory measurements of compressional-wave velocities of rock samples from Kenya. The rock samples studied, which are representative of the major lithologies exposed in Kenya, include volcanic tuffs and flows (primarily basalts and phonolites), and felsic to intermediate composition gneisses. This comparison indicates that the upper crust (5-12 km depth) consists primarily of quartzo-feldspathic gneisses and schists similar to rocks exposed on the flanks of the rift, whereas the middle crust (12-22 km depth) consists of more mafic, hornblende-rich metamorphic rocks, probably intruded by mafic rocks beneath the rift axis. The lower crust on the flanks of the rift may consist of mafic granulite facies rocks. Along the rift axis, the lower crust varies in thickness from 9 km in the southern rift to only 2-3 km in the north, and has a seismic velocity substantially higher than the samples investigated in this study. The lower crust of the rift probably consists of a crust/mantle mix of high-grade metamorphic rocks, mafic intrusives, and an igneous mafic residuum accreted to the base of the crust during differentiation of a melt derived from the upper mantle. ?? 1994.

New Elastic Moduli for Amphiboles and Feldspars: Impact on Interpretations of Seismic Velocities

NASA Astrophysics Data System (ADS)

Brown, J. M.; Angel, R. J.

2016-12-01

Seismic properties (both isotropic and anisotropic) of the crust and upper mantle require re-evaluation in light of improved single crystal properties for feldspars and amphiboles as a function of elemental partitioning. Together these minerals constitute more than half of the crust and are locally important in the lithospheric mantle. Their contribution in understanding seismic structures (both in the crust and mantle) has long been recognized. However, published single crystal elastic moduli, required in predictions of seismic velocities based on mineral properties, have remained inadequate for over 50 years. For example, the contribution of amphiboles to seismic velocities has often been approximated on the basis of the reported moduli for two hornblende crystals of unknown composition. New measurements now accurately characterize the plagioclase feldspars, the potassium feldspars, and the calcium and calcium-sodium amphiboles (including a range of compositions for common hornblende). The new moduli allow successful predictions of rock velocities with and without crystal preferred orientations. In contrast, the older moduli required inappropriate use of the Voigt upper aggregate bound in order to rationalize laboratory measurements. These minerals are also more anisotropic than suggested on the basis of the earlier work where cracks and open cleavage surfaces may have artificially depressed the apparent anisotropy. Both feldspars and amphiboles are nearly as anisotropic as sheet silicates with compressional velocity anisotropy of greater than 50%. The plagioclase feldspars show strong compositional trends with small discontinuities between minor structural transitions. In contrast, potassium substitution for sodium and differences in aluminum ordering have little impact on elastic moduli. In the amphiboles, elastic properties are strongly dependent on total aluminum and iron composition. The bulk modulus is most sensitive to aluminum and the shear modulus is more sensitive to iron. Variations in Poisson's ratio (which depends on the ratio of isotropic compressional and shear wave velocities) associated with compositions within the amphiboles and the feldspars are larger than previously predicted. The extent of modifications to seismic interpretations is evaluated.

The support of long wavelength loads on Venus

NASA Astrophysics Data System (ADS)

Benerdt, W. B.; Saunders, R. S.

1985-04-01

One of the great surprises of the Pioneer Venus mission was the high degree of correlation between topography and gravity found at all wavelengths. This implies a close relationship between topography and lateral subsurface density anomalies, such as those due to passive or dynamic compensation. Sleep-Phillips type compensation model with a variable crustal thickness and a variable upper mantle density was developed. The thin shell theory was used to investigate three end member cases: (1) loading by topographic construction, resulting in a downward deflection of the surface (no mantle support); (2) completely compensated support of a constructional load (no surface deflection); and (3) topography due entirely to upward deflection of the surface supported by a low density upper mantle (no surface load). In general, the models imply relatively thick crust and dense upper mantle for Ishtar Terra and Ovda Regio (western Aphrodite), thinned crust and buoyant upper mantle for Tethus Regio and regions near Sappho and Alpha Regio, and a nearly uniform crust with a buoyant upper mantle for Beta Regio and Atla Regio (eastern Aphrodite).

The Support of Long Wavelength Loads on Venus

NASA Technical Reports Server (NTRS)

Benerdt, W. B.; Saunders, R. S.

1985-01-01

One of the great surprises of the Pioneer Venus mission was the high degree of correlation between topography and gravity found at all wavelengths. This implies a close relationship between topography and lateral subsurface density anomalies, such as those due to passive or dynamic compensation. Sleep-Phillips type compensation model with a variable crustal thickness and a variable upper mantle density was developed. The thin shell theory was used to investigate three end member cases: (1) loading by topographic construction, resulting in a downward deflection of the surface (no mantle support); (2) completely compensated support of a constructional load (no surface deflection); and (3) topography due entirely to upward deflection of the surface supported by a low density upper mantle (no surface load). In general, the models imply relatively thick crust and dense upper mantle for Ishtar Terra and Ovda Regio (western Aphrodite), thinned crust and buoyant upper mantle for Tethus Regio and regions near Sappho and Alpha Regio, and a nearly uniform crust with a buoyant upper mantle for Beta Regio and Atla Regio (eastern Aphrodite).

Interaction of ultra-depleted MORBs with plagioclase: implications for CO2/Ba ratios

NASA Astrophysics Data System (ADS)

Shimizu, K.; Hauri, E.; Saal, A. E.; Perfit, M. R.; Hekinian, R.

2017-12-01

Carbon in Earth's upper mantle can significantly reduce its solidus temperature, which in turn can affect other physical properties through generation of partial melt. Carbon content in the depleted upper mantle can be estimated using ultra-depleted mid-ocean ridge basalt (UD-MORB) glasses and melt inclusions that are undersaturated in CO2. CO2 has been shown to behave as a highly incompatible element during mantle melting both through natural samples and experiments. Given its highly incompatible behavior, CO2/Ba and CO2/Nb ratios in CO2 undersaturated UD-MORBs have been used to estimate the CO2/Ba and CO2/Nb ratios and carbon content in Earth's upper mantle. A potential issue with part of this approach is the effect of melt-plagioclase chemical interaction on the CO2/Ba ratios in UD-MORBs. Plagioclase is ubiquitous in the oceanic crust and is enriched in Ba relative to other phases. Chemical interactions (assimilation and/or diffusion) between MORB melts and plagioclase bearing rocks have been shown to affect the Ba (and Sr and Eu) concentrations in MORBs, implying that such processes may also affect their CO2/Ba ratio. Hence, understanding the effect of chemical interaction between plagioclase and UD-MORBs is important for having better constraints on CO2/Ba ratio and carbon content in Earth's upper mantle. In this study, we report on the compositions of olivine-hosted melt inclusions and glasses from the Siqueiros and Garrett transform faults. A subset of melt inclusions in lavas from both transform faults show potential signatures of chemical interaction with plagioclase such as low CO2/Ba, Nb/Ba, and Nd/Sr. CO2 degassing cannot explain the low CO2/Ba ratio in the samples as they are undersaturated in CO2. To better understand the effect of chemical interaction with plagioclase on the composition of UD-MORBs, we model end-member scenarios, which are (1) assimilation of plagioclase and (2) diffusion of elements from plagioclase into the UD-MORBs. In general, the trends produced by these end-member scenarios bracket those observed in the samples (trends between CO2/Ba, Nb/Ba, and Nd/Sr as well as between Al2O3, FeO, and MgO). Hence, chemical interaction with plagioclase may affect the CO2/Ba ratio in UD-MORBs, and care should be taken to evaluate this effect using Nd/Sr and Nb/Ba ratios.

Understanding the geodynamic setting of São Miguel, Azores: A peculiar bit of mantle in the Central Atlantic

NASA Astrophysics Data System (ADS)

Wilson, M.; Houlie, N.; Khan, A.; Lithgow-Bertelloni, C. R.

2012-12-01

The Azores Plateau and Archipelago in the Central Atlantic Ocean has traditionally been considered as the surface expression of a deep mantle plume or hotspot that has interacted with a mid-ocean ridge. It is geodynamically associated with the triple junction between the North American, African and Eurasian plates. (Yang et al., 2006) used finite frequency seismic tomography to demonstrate the presence of a zone of low P-wave velocities (peak magnitude -1.5%) in the uppermost 200km of the mantle beneath the plateau. The tomographic model is consistent with SW deflection of a mantle plume by regional upper mantle shear flow driven by absolute plate motions. The volcanic island of Sao Miguel is located within the Terceira Rift, believed to represent the boundary between the African and Eurasian plates; magmatic activity has been characterised by abundant basaltic eruptions in the past 30,000 years. The basalts are distinctive within the spectrum of global ocean island basalts for their wide range in isotopic composition, particularly in 87Sr/86Sr. Their Sr-Nd-Pb isotopic compositions show systematic variations from west to east across the island which can be interpreted in terms of melting of a two-component mantle source. The low melting point (enriched) component in the source has been attributed to recycled ancient (~3 Ga) oceanic crust(Elliott et al., 2007). Using the thermo-barometry approach of (Lee et al., 2009) we demonstrate that the pressure and temperature of magma generation below Sao Miguel increase from west (2 GPa, 1425 °C) to east (3.8 GPa, 1575 °C), consistent with partial melting along a mantle geotherm with a potential temperature of ~ 1500 °C. This is consistent with the magnitude of the thermal anomaly beneath the Azores Plateau (ΔT ~ 150-200 °C) inferred on the basis of the seismic tomography study. The site of primary magma generation extends from the base of the local lithosphere (~ 50 km) to ~ 125 km depth. To understand the geodynamic setting of the Sao Miguel magmatism we combine GPS data and mantle convection models with our interpretation of the geochemistry of the basalts. We demonstrate strong south-westerly and downward flow in the asthenospheric mantle above the Transition Zone (410 km seismic discontinuity), consistent with a zone of upper mantle shearing below the base of the lithosphere. The maximum flow velocity is broadly consistent with the depth of magma generation. The advection of the mantle with respect to the oceanic plate "moves" an isotopically distinct mantle source component beneath the active volcanoes of Sao Miguel and carries its previous melting residues to the south-west. We discuss the nature of this mantle source and its contribution to the mantle velocity anomalies determined by seismic tomography. This study opens-up new perspectives for seismic tomography and potentially new connections between the fields of geophysics and geochemistry in oceanic domains.

Structure of the crust and upper mantle in the western United States

USGS Publications Warehouse

Pakiser, L.C.

1963-01-01

Seismic waves generated by underground nuclear and chemical explosions have been recorded in a network of nearly 2,000 stations in the western conterminous United States as a part of the VELA UNIFORM program. The network extends from eastern Colorado to the California coastline and from central Idaho to the border of the United States and Mexico. The speed of compressional waves in the upper-mantle rocks ranges from 7.7 km/sec in the southern part of the Basin and Range province to 8.2 km/sec in the Great Plains province. In general, the speed of compressional waves in the upper-mantle rocks tends to be nearly the same over large areas within individual geologic provinces. Measured crustal thickness ranges from less than 20 km in the Central Valley of California to 50 km in the Great Plains province. Changes in crustal thickness across provincial boundaries are not controlled by regional altitude above sea level unless the properties of the upper mantle are the same across those boundaries. The crust tends to be thick in regions where the speed of compressional waves in the upper-mantle rocks (and presumably the density) is high, and tends to be relatively thin where the speed of compressional waves in the upper-mantle rocks (and density) is lower. With in the Basin and Range province, crustal thickness seems to vary directly with regional altitude above sea level. Evidence that a layer of intermediate compressional-wave speed exists in the lower part of the crust has been accumulated from seismic waves that have traveled least-time paths, as well as secondary arrivals (particularly reflections). On a scale that includes many geologic provinces, isostatic compensation is related largely to variations in the density of the upper- mantle rocks. Within geologic provinces or adjacent provinces, isostatic compensation may be related to variations in the thickness of crustal layers. Regions of thick crust and dense upper mantle have been relatively stable in Cenozoic time. Regions of thinner crust and low-density upper mantle have had a Cenozoic history of intense diastrophism and silicic volcanism.

Lithospheric structure and deformation of the North American continent

NASA Astrophysics Data System (ADS)

Tesauro, Magdala; Kaban, Mikhail; Cloetingh, Sierd; Mooney, Walter

2013-04-01

We estimate the integrated strength and elastic thickness (Te) of the North American lithosphere based on thermal, density and structural (seismic) models of the crust and upper mantle. The temperature distribution in the lithosphere is estimated considering for the first time the effect of composition as a result of the integrative approach based on a joint analysis of seismic and gravity data. We do this via an iterative adjustment of the model. The upper mantle temperatures are initially estimated from the NA07 tomography model of Bedle and Van der Lee (2009) using mineral physics equations. This thermal model, obtained for a uniform composition, is used to estimate the gravity effect and to remove it from the total mantle gravity anomalies, which are controlled by both temperature and compositional variations. Therefore, we can predict compositional variations from the residual gravity anomalies and use them to correct the initial thermal model. The corrected thermal model is employed again in the gravity calculations. The loop is repeated until convergence is reached. The results demonstrate that the lithospheric mantle is characterized by strong compositional heterogeneity, which is consistent with xenolith data. Seismic data from the USGS database allow to define P-wave velocity and thickness of each crustal layer of the North American geological provinces. The use of these seismic data and of the new compositional and thermal models gives us the chance to estimate lateral variation of rheology of the main lithospheric layers and to evaluate coupling-decoupling conditions at the layers' boundaries. In the North American Cordillera the strength is mainly localized in the crust, which is decoupled from the mantle lithosphere. In the cratons the strength is chiefly controlled by the mantle lithosphere and all the layers are generally coupled. These results contribute to the long debates on applicability of the "crème brulée" or "jelly-sandwich" models for the lithosphere structure. Intraplate earthquakes (USGS database) occur mainly in the weak regions, such as the Appalachians, and in the transition zones from low to high strength surrounding the craton. The obtained 3D strength model is used to compute Te of the North American lithosphere. This parameter is derived from the thermo-rheological model using new equations that consider variations of the Young's Modulus in the lithosphere. It shows large variability within the cratons, ranging from 70 km to >100km, while it drops to <30 km in the young Phanerozoic regions. The new crustal model is also used to compute the lateral pressure gradient (LPG) that can initiate horizontal ductile flow in the crust. In general, the crustal flow is directed away from the orogens towards adjacent weaker areas. The results show that the effects of the channel flow superimposed with the regional tectonic forces might result in additional significant horizontal and vertical movements associated with zones of compression or extension.

The mantle beneath the Red Sea margin: xenoliths from western Saudi Arabia

NASA Astrophysics Data System (ADS)

McGuire, Anne Vaughan

1988-07-01

Xenoliths from alkali basalts in western Saudi Arabia provide the opportunity to study the composition and rheology of the mantle beneath the Red Sea rift margins. Characteristics of mantle xenolith suites from each of three localities in western Saudi Arabia can be related to locality position relative to the rift axis, and to crustal thickness and heat flow at each locality. Mantle xenoliths from Harrat al Birk, nearest the rift axis, are dominantly websterites (± spinel, plagioclase, amphibole, olivine), garnet clinopyroxenite, and two-pyroxene gabbro (± olivine); peridotite xenoliths, are rare. Garnet clinopyroxenites contain zoned clinopyroxene with Fe-Al-rich rims and reaction rims on garnet formed by breakdown of garnet to orthopyroxene + clinopyroxene + spinel + plagioclase. Zoning and reaction rims are interpreted as forming under conditions of increasing temperature. Thermobarometry on Harrat al Birk garnet-bearing xenoliths yield high temperatures (1015-1040°C) at about 12 kbar. The abundance of plagioclase-bearing assemblages may be related to a relatively shallow upper mantle which extends up into stability fields for plagioclase-bearing pyroxenite and peridotite. Harrat al Kishb and Harrat Hutaymah are farther from the Red Sea axis, on the flanks of the rift. The mantle xenolith suites of al Kishb and Hutaymah are similar, consisting of abundant spinel peridotite and spinel pyroxenite xenoliths and minor garnet pyroxenites; plagioclase-bearing xenoliths are extremely rare. The Harrat Hutaymah suite includes wehrlite and amphibole-bearing peridotite lithologies not found at al Kishb. Variation of peridotite composition may reflect varying degrees of partial melt extraction. Igneous textures of some pyroxenite xenoliths and structural relationships in composite peridotite/pyroxenite nodules suggest that pyroxenites formed by crystallization of magmas within mantle veins. Abundant pyroxenites and fragments of amphibole veins reflect the activity of magmas and hydrous fluids within the mantle. Thermobarometry of al Kishb and Hutaymah garnet-bearing nodules yield temperatures of 1000-1050 °C at pressures of about 13.5-16.0 kbar. Mineral zoning and exsolution features indicate decreasing temperature conditions in the mantle at the flanks of the Red Sea rift.

Evidence of dehydration in peridotites from Eifel Volcanic Field and estimates of the rate of magma ascent

NASA Astrophysics Data System (ADS)

Denis, Carole M. M.; Demouchy, Sylvie; Shaw, Cliff S. J.

2013-05-01

We report major element compositions and water contents in upper mantle minerals from peridotites transported by silica undersaturated, mafic alkaline lavas from three volcanoes Rockeskyllerkopf, Dreiser Weiher, and Meerfelder Maar of the Eifel Volcanic Field (West Germany). The hydrogen concentrations (expressed in ppm wt. H2O) obtained from unpolarized and polarized Fourier transform infrared (FTIR) spectroscopy give water contents for olivine, enstatite and diopside of ~ 6 ppm wt. H2O, ~ 200 ppm wt. H2O and ~ 285 ppm wt. H2O, respectively. The hydrogen concentration in individual olivine grains is strongly heterogeneous whereas that in pyroxenes is homogeneous. Profiles measured across crystallographically oriented single-crystals of olivine using polarized infrared radiation reveal hydrogen depleted rims that are interpreted to be due to partial dehydration by ionic diffusion during the ascent of the xenolith to the surface. Using experimentally obtained diffusion coefficients for hydrogen in olivine at high temperature and high pressure, we estimate that the duration of the dehydration for the spinel-bearing xenoliths is limited to a few hours yielding rates of magma ascent from 3 ms- 1 to 12 ms- 1. Our study suggests that the water contents of the upper mantle based solely on measurements of mantle-derived olivine, when concentration is not homogeneous, underestimate the true water content of the equilibrated uppermost mantle and that pyroxenes are a better proxy to constrain uppermost mantle water contents.

Early Earth differentiation [rapid communication

NASA Astrophysics Data System (ADS)

Walter, Michael J.; Trønnes, Reidar G.

2004-09-01

The birth and infancy of Earth was a time of profound differentiation involving massive internal reorganization into core, mantle and proto-crust, all within a few hundred million years of solar system formation ( t0). Physical and isotopic evidence indicate that the formation of iron-rich cores generally occurred very early in planetesimals, the building blocks of proto-Earth, within about 3 million years of t0. The final stages of terrestrial planetary accretion involved violent and tremendously energetic giant impacts among core-segregated Mercury- to Mars-sized objects and planetary embryos. As a consequence of impact heating, the early Earth was at times partially or wholly molten, increasing the likelihood for high-pressure and high-temperature equilibration among core- and mantle-forming materials. The Earth's silicate mantle harmoniously possesses abundance levels of the siderophile elements Ni and Co that can be reconciled by equilibration between iron alloy and silicate at conditions comparable to those expected for a deep magma ocean. Solidification of a deep magma ocean possibly involved crystal-melt segregation at high pressures, but subsequent convective stirring of the mantle could have largely erased nascent layering. However, primitive upper mantle rocks apparently have some nonchondritic major and trace element refractory lithophile element ratios that can be plausibly linked to early mantle differentiation of ultra-high-pressure mantle phases. The geochemical effects of crystal fractionation in a deep magma ocean are partly constrained by high-pressure experimentation. Comparison between compositional models for the primitive convecting mantle and bulk silicate Earth generally allows, and possibly favors, 10-15% total fractionation of a deep mantle assemblage comprised predominantly of Mg-perovskite and with minor but geochemically important amounts of Ca-perovskite and ferropericlase. Long-term isolation of such a crystal pile is generally consistent with isotopic constraints for time-integrated Sm/Nd and Lu/Hf ratios in the modern upper mantle and might account for the characteristics of some mantle isotope reservoirs. Although much remains to be learned about the earliest formative period in the Earth's development, a convergence of theoretical, physical, isotopic and geochemical arguments is beginning to yield a self-consistent portrait of the infant Earth.

Inverse problems for torsional modes.

USGS Publications Warehouse

Willis, C.

1984-01-01

Considers a spherically symmetric, non-rotating Earth consisting of an isotropic, perfect elastic material where the density and the S-wave velocity may have one or two discontinuities in the upper mantle. Shows that given the velocity throughout the mantle and the crust and given the density in the lower mantle, then the freqencies of the torsional oscillations of one angular order (one torsional spectrum), determine the density in the upper mantle and in the crust uniquely. If the velocity is known only in the lower mantle, then the frequencies of the torsional oscillations of two angular orders uniquely determine both the density and the velocity in the upper mantle and in the crust. In particular, the position and size of the discontinuities in the density and velocity are uniquely determined by two torsional spectra.-Author

Ultrabasic-basic change over primary inclusions in lower-mantle diamonds: Mineralogical and experimental evidence for crucial role of stishovite paradox

NASA Astrophysics Data System (ADS)

Litvin, Yuriy; Spivak, Anna

2017-04-01

Melting relations of the lower-mantle magmatic system MgO - FeO - CaO - SiO2 are characterized by peritectic reaction of bridgmanite (Mg,Fe)SiO3 and melt with formation of Fe-rich phases of periclase-wustite solid solutions (MgO•FeO)ss and stishovite SiO2. The reaction proceeds also in melts-solutions of lower-mantle diamond-parental system MgO - FeO - CaO - SiO2 - (Mg-Fe-Ca-Na-carbonate) - C. Xenoliths of lower mantle rocks were never found among the deep mantle derived materials. Estimation of lower-mantle mineralogy as ferropericlase+ bridgmanite+ Ca-perovskite association is inferred from high-pressure subsolidus experiments with ultrabasic pyrolite composition (Akaogi, 2007). The paradoxical in situ paragenesis of stishovite and ferropericlase as primary inclusions in lower-mantle diamonds (Kaminsky, 2012) takes its explanation from the bridgmanite peritectic reaction (effect of "stishovite paradox") (Litvin et al., 2014). Based on the data for inclusions, physico-chemical study on syngenesis of diamonds and primary inclusions has experimentally revealed the ferropericlase-bridgmanite-Ca-perovskite-stishovite-magnesiowustite-(Mg-Fe-Ca-Na-carbonate)-carbon compositions of the lower-mantle diamond-forming system .(Litvin et al., 2016). The generalized diagram of diamong-forming media characterizes the variable compositions of growths melts for diamonds and paragenetic phases and their genetic relationships with lower mantle matter, and it is the reason for genetic classifying primary inclusions. Fractional ultrabasic-basic evolution and continuous paragenetic transition from ultrabasic bridgmanite-ferropericlase to basic stishovite-magnesiowustite assemblages in the of lower-mantle diamond-parental melts-solutions are providing by the physico-chemical mechanism of stishovite paradox. References Akaogi M. (2007). Phase transformations of minerals in the transition zone and upper part of the lower mantle. In Advances in High-Pressure Mineralogy (Ohtani E., ed.). Geol. Soc. Am. Spec. Paper 421, 1-13. Kaminsky F.V. (2012). Mineralogy of the lower mantle: a review of "supper-deep" mineral inclusions in diamonds. Earth Sci. Rev. 110, 127-147. Litvin Yu.A., Spivak A.V., Solopova N.A., Dubrovinsky L.S. (2014). On origin of lower-mantle diamonds and their primary inclusions. Phys. Earth Planet. Inter. 228, 176-185. Litvin Yu.A., Spivak A.V., Dubrovinsky L.S. (2016). Magmatic evolution of the material of the Earth's lower mantle: stishovite paradox and origin of superdeep diamonds (experiments at 24-26 GPa). Geochemistry Internat. 54(11, 936-947.)

Silicate garnet studies at high pressures: A view into the Earth's mantle

NASA Astrophysics Data System (ADS)

Conrad, Pamela Gales

Silicate garnets are an abundant component in the Earth's upper mantle and transition zone. Therefore, an understanding of garnet behavior under the pressure and temperature conditions of the mantle is critical to the development of models for mantle mineralogy and dynamics. Work from three projects is presented in this report. Each investigation explores an aspect of silicate garnet behavior under high pressures. Moreover, each investigation was made possible by state-of-the-art methods that have previously been unavailable. Brillouin scattering was used to determine the elastic constants and aggregate elastic moduli of three end-member garnets at high pressures in a diamond anvil cell. These are the first high-pressure measurements of the elastic constants of end-member silicate garnets by direct measurement of acoustic velocities. The results indicate that the pressure dependence of silicate garnet elastic constants varies with composition. Therefore, extrapolation from measurements on mixed composition garnets is not possible. A new method of laser heating minerals in a diamond anvil cell has made possible the determination of the high-pressure and high-temperature stability of almandine garnet. This garnet does not transform to a silicate perovskite phase as does pyrope garnet, but it decomposes to its constituent oxides: FeO, Alsb2Osb3, and SiOsb2. These results disprove an earlier prediction that ferrous iron may expand the stability field of garnet to the lower mantle. The present results demonstrate that this is not the case. The third topic is a presentation of the results of a new technique for studying inclusions in mantle xenoliths with synchrotron X-ray microdiffraction. The results demonstrate the importance of obtaining structural as well as chemical information on inclusions within diamonds and other high-pressure minerals. An unusual phase with garnet composition is investigated and several other phases are identified from a suite of natural diamonds that are thought to have a lower mantle origin.

Magma source evolution beneath the Caribbean oceanic plateau: New insights from elemental and Sr-Nd-Pb-Hf isotopic studies of ODP Leg 165, Site 1001 basalts

NASA Astrophysics Data System (ADS)

Kerr, A. C.; Pearson, G.; Nowell, G.

2008-12-01

Ocean Drilling Project Leg 165 sampled 38m of the basaltic basement of the Caribbean plate at Site 1001 on the Hess Escarpment. The recovered section consists of 12 basaltic flow units which yield a weighted mean Ar-Ar age of 80.9±0.9 Ma (Sinton et al., 2000). The basalts (6.4-8.5 wt.% MgO) are remarkably homogeneous in composition and are more depleted in incompatible trace elements than N-MORB. Markedly, depleted initial radiogenic isotope ratios reveal a long-term history of depletion. Although the Site 1001 basalts are superficially similar to N-MORB, radiogenic isotopes in conjunction with incompatible trace element ratios show that the basalts have more similarity to the depleted basalts and komatiites of Gorgona Island. This chemical composition strongly implies that the Site 1001 basalts are derived from a depleted mantle plume component and not from depleted ambient upper mantle. Therefore the Site 1001 basalts are, both compositionally and tectonically, a constituent part of the Caribbean oceanic plateau. Mantle melt modelling suggests that the Site 1001 lavas have a composition which is consistent with second-stage melting of compositionally heterogeneous mantle plume source material which had already been melted, most likely to form the 90Ma basalts of the plateau. The prolonged residence (>10m.y.) of residual mantle plume source material below the region, confirms computational model predictions and places significant constraints on tectonic models of Caribbean evolution in the late Cretaceous, and the consequent environmental impact of oceanic plateau volcanism. Reference Sinton, C.W., et al., 2000. Geochronology and petrology of the igneous basement at the lower Nicaraguan Rise, Site 1001. Proceedings of the Ocean Drilling Program, Scientific Results. Leg 165. pp. 233-236.

Composition of uppermost mantle beneath the Northern Fennoscandia - numerical modeling and petrological interpretation

NASA Astrophysics Data System (ADS)

Virshylo, Ivan; Kozlovskaya, Elena; Prodaivoda, George; Silvennoinen, Hanna

2013-04-01

Studying of the uppermost mantle beneath the northern Fennoscandia is based on the data of the POLENET/LAPNET passive seismic array. Firstly, arrivals of P-waves of teleseismic events were inverted into P-wave velocity model using non-linear tomography (Silvennoinen et al., in preparation). The second stage was numerical petrological interpretation of referred above velocity model. This study presents estimation of mineralogical composition of the uppermost mantle as a result of numerical modeling. There are many studies concerning calculation of seismic velocities for polymineral media under high pressure and temperature conditions (Afonso, Fernàndez, Ranalli, Griffin, & Connolly, 2008; Fullea et al., 2009; Hacker, 2004; Xu, Lithgow-Bertelloni, Stixrude, & Ritsema, 2008). The elastic properties under high pressure and temperature (PT) conditions were modelled using the expanded Hook's law - Duhamel-Neumann equation, which allows computation of thermoelastic strains. Furthermore, we used a matrix model with multi-component inclusions that has no any restrictions on shape, orientation or concentration of inclusions. Stochastic method of conditional moment with computation scheme of Mori-Tanaka (Prodaivoda, Khoroshun, Nazarenko, & Vyzhva, 2000) is applied instead of traditional Voigt-Reuss-Hill and Hashin-Shtrikman equations. We developed software for both forward and inverse problem calculation. Inverse algorithm uses methods of global non-linear optimization. We prefer a "model-based" approach for ill-posed problem, which means that the problem is solved using geological and geophysical constraints for each parameter of a priori and final models. Additionally, we are checking at least several different hypothesis explaining how it is possible to get the solution with good fit to the observed data. If the a priori model is close to the real medium, the nearest solution would be found by the inversion. Otherwise, the global optimization is searching inside the restricted volume in the multi-dimensional parameter space. In order to constrain concentration of minerals we used equilibrium of mineral associations for selected P-T condition obtained by free Gibbs energy minimization (c.f. Stixrude & Lithgow-Bertelloni, 2005). We also considered the mineralogical composition of upper mantle xenoliths, although the representativeness of xenoliths in Precambrian rocks could be treated with care, if one tries to describe the modern mantle. As a first step, we estimated 1D model of mineralogical composition in the depth range of 35-350 km using the IASP91 reference model (Kennett & Engdahl, 1991). Both the P- and S- wave velocities were used for inversion, in order to improve the reliability of the model. More comprehensive result could be obtained if density distribution is involved. In our study we used the 1D PEMC density model (Dziewonski, Hales & Lapwood, 1975) as it is the most adequate for the continental lithosphere. The 1D modeling showed that the garnet lherzolite model (forsterite, fayalite, enstatite, ferrosilite, diopside, jadeite, pyrope) can be considered as a basic one. The end-members of olivine and orthopyroxene solutions were included with the aim of Fe/Mg ratio estimation. Testing with modified models including hedenbergite, harzburgite spinel, etc. showed that these minerals have no significant influence on bulk elastic properties. Selected set of minerals allows modelling the most species of peridote-pyroxenite associations known from xenoliths investigations (Kukkonen, Kuusisto, Lehtonen, & Peltonen, 2008; Lehtonen, O'Brien, Peltonen, Johanson, & Pakkanen, 2004). However, there exist also a number of evidences for mantle eclogite xenoliths from the region under study and its surrounding (Lehtonen et al., 2004; Peltonen, Kinnunen, & Huhma, 2002). That is why we also made modelling for garnet-clinopyroxene model of eclogite. The volumetric mineral compositions obtained were transformed into weight concentration of rock-forming oxides using stoichiometric formulas. The results indicate significant variation of Fe and Mg oxides concentration in the uppermost mantle. The Mg/Fe ratio could be different from the results of previous studies (Griffin et al., 2003; Svetov & Smolkin, 2003), but it is in agreement with the geophysical models considered in our study. At the same time the SiO2 concentration is close to the chemical composition of xenoliths from the Fennoscandia, including Kola Peninsula and Central Finland (Beard, Downes, Mason, & Vetrin, 2007; Kukkonen et al., 2008; Lehtonen et al., 2004). Brief conclusions from our study could be formulated as follows: 1) Modelling confirms potential significant lateral inhomogeneity of mineral composition of the uppermost mantle of northern Fennoscandian Shield. 2) Lherzolitic composition of the mantle lithosphere generally explains seismic velocities obtained by teleseismic tomography in northern Fennoscandian Shield. It could be used as a primary a priori model for interpretation. But potential presence of eclogites cannot be rejected, at least for some parts of studied area. 3) The future study needs to include more precise evaluation of temperature and density in the upper mantle using gravity and heat flow data. Afonso, J. C., Fernàndez, M., Ranalli, G., Griffin, W. L., & Connolly, J. a. D. (2008). Integrated geophysical-petrological modeling of the lithosphere and sublithospheric upper mantle: Methodology and applications. Geochemistry Geophysics Geosystems, 9(5). doi:10.1029/2007GC001834 Beard, a. D., Downes, H., Mason, P. R. D., & Vetrin, V. R. (2007). Depletion and enrichment processes in the lithospheric mantle beneath the Kola Peninsula (Russia): Evidence from spinel lherzolite and wehrlite xenoliths. Lithos, 94(1-4), 1-24. doi:10.1016/j.lithos.2006.02.002 Dziewonski, A.M., A.L. Hales, & E.R. Lapwood. (1975) Parametrically simple earth models consistent with geophysical data Phys. Earth Plan. Int. 10:12. Fullea, J., Afonso, J. C., Connolly, J. A. D., Fernàndez, M., García-Castellanos, D., & Zeyen, H. (2009). LitMod3D: An interactive 3-D software to model the thermal, compositional, density, seismological, and rheological structure of the lithosphere and sublithospheric upper mantle. Geochemistry Geophysics Geosystems, 10(8), 1-21. doi:10.1029/2009GC002391 Griffin, W. ., O'Reilly, S. ., Abe, N., Aulbach, S., Davies, R. ., Pearson, N. ., Doyle, B. ., et al. (2003). The origin and evolution of Archean lithospheric mantle. Precambrian Research, 127(1-3), 19-41. doi:10.1016/S0301-9268(03)00180-3 Hacker, B. R. (2004). Subduction Factory 3: An Excel worksheet and macro for calculating the densities, seismic wave speeds, and H 2 O contents of minerals and rocks at pressure and temperature. Geochemistry Geophysics Geosystems, 5(1), 1-7. doi:10.1029/2003GC000614 Kennett B.L.N. & Engdahl E.R. (1991) Travel times for global earthquake location and phase association. Geophysical Journal International, 105:429-465. Kukkonen, I., Kuusisto, M., Lehtonen, M., & Peltonen, P. (2008). Delamination of eclogitized lower crust: Control on the crust-mantle boundary in the central Fennoscandian shield. Tectonophysics, 457(3-4), 111-127. doi:10.1016/j.tecto.2008.04.029 Lehtonen, M. L., O'Brien, H. E., Peltonen, P., Johanson, B. S., & Pakkanen, L. K. (2004). Layered mantle at the Karelian Craton margin: P - T of mantle xenocrysts and xenoliths from the Kaavi - Kuopio kimberlites , Finland. Lithos, 77, 593-608. doi:10.1016/j.lithos.2004.04.026 Peltonen, P., Kinnunen, K. A., & Huhma, H. (2002). Petrology of two diamondiferous eclogite xenoliths from the Lahtojoki kimberlite pipe , eastern Finland. Lithos, 63, 151-164. Prodaivoda, G. T., Khoroshun, L. P., Nazarenko, L. V, & Vyzhva, S. A. (2000). Mathematical modeling of the azimuthal anisotropy in thermoelasic properties of the oceanic upper mantle. IzvestiyaPhysics of the Solid Earth, 36(5), 394-405. Stixrude, L., & Lithgow-Bertelloni, C. (2005). Thermodynamics of mantle minerals - I. Physical properties. Geophysical Journal International, 162(2), 610-632. Retrieved from http://discovery.ucl.ac.uk/8891/ Svetov, S. A., & Smolkin, V. F. (2003). Model P - T Conditions of High-Magnesia Magma Generation in the Precambrian of the Fennoscandian Shield. Geochemistry International, 41(8), 799-811. Xu, W., Lithgow-Bertelloni, C., Stixrude, L., & Ritsema, J. (2008). The effect of bulk composition and temperature on mantle seismic structure. Earth and Planetary Science Letters, 275(1-2), 70-79. doi:10.1016/j.epsl.2008.08.012

Os and HSE of the hot upper mantle beneath southern Tibet: Indian mantle affinity?

NASA Astrophysics Data System (ADS)

Zhao, Z.; Dale, C. W.; Pearson, D. G.; Niu, Y.; Zhu, D.; Mo, X.

2011-12-01

The subduction of the Indian plate (including cratonic continental crust and/or upper mantle) beneath southern Tibet is widely accepted from both geological and geophysical studies. Mantle-derived xenoliths from this region provide a means of directly investigating the mantle underlying the southern part of the plateau. Studies of xenoliths hosted in the Sailipu ultrapotassic volcanic rocks, erupted at ~17 Ma, have indicated that the subcontinental mantle of southern Tibetan Plateau is hot and strongly influenced by metasomatism (Zhao et al., 2008a, b; Liu et al., 2011). Here we report comprehensive EPMA and LA-ICP-MS major and trace element data for the Sailipu xenoliths and also whole rock Os isotope and HSE data in order to constrain the depletion history of the mantle and to identify the presence of any potential Indian cratonic mantle. The xenoliths, ranging in size from 0.5cm to 1.5cm in diameter, are mostly peridotites. The calculated temperatures are 1010-1230°C at the given pressures of ~1.6-2.0 GPa (n=47). These P-T conditions are similar to rift-related upper mantle regimes (e.g. Kenya), indicating the influence of regional extension beneath southern Tibet in the Miocene. A series of compositional discriminations for minerals (Cpx, Opx, Ol, and Phl), e.g. Fo<90, suggest that the xenoliths are non-cratonic spinel-peridotite (cratonic peridotite olivine Fo> ~91), with a clear metasomatic signature We obtained Os isotope data and abundances of highly siderophile elements (HSE, including Os, Ir, Ru, Pt, Pd and Re) on a set of six olivine-dominated peridotite samples from Sailipu volcanics, less than 1 cm in dimension. They allow us to further constrain the nature and state of the upper mantle beneath the southern Tibet. Sailipu samples display low total HSE abundances (Os+Ir+Ru+Pt+Pd+Re) ranging from 8.7 to 25 ppb, with nearly constant Os, Ir , and Ru, but rather varied Pt (2-13), Pd (0.4-5.2), and Re (0.01-0.5). Chondrite-normalised Pd/Ir ratios range from 0.2 to 2.4 reflecting significant metasomatism of some samples. The xenoliths exhibit 187Os/188Os ratios of 0.12213-0.12696, corresponding to γOs ranging from -4.2 to -0.4 - much higher than ancient cratonic mantle. Thus, on the basis of mineral chemistry and whole rock Os isotopes, Indian cratonic mantle is absent from our suite of xenoliths. Therefore, assuming the presence of cratonic mantle, it seems likely that the xenoliths do not sample the deep basal section of the lithosphere where cratonic Indian lithosphere is thought to be present under southern Tibet. In which case, testing of the seismic and tectonic models may not be possible without garnet-facies peridotites. More work need to be done to further reveal the mantle compostion and mantle dynamics beneath Tibet. [Financially supported by the National Key Project for Basic Research of China (Project 2011CB403102 and 2009CB421002)]. [1] Zhao Z, et al., 2008a. Acta Petrologica Sinica, 24 (2): 193-202 [2] Zhao Z, et al., 2008b. Geochimica et Cosmochimica Acta, 72, 12 (Supp.): A1095 [3] Liu C-Z, et al., 2011, Geology, in press

Petrologic Modeling of Magmatic Evolution in The Elysium Volcanic Province

NASA Astrophysics Data System (ADS)

Susko, D.; Karunatillake, S.; Hood, D.

2017-12-01

The Elysium Volcanic Province (EVP) on Mars is a massive expanse of land made up of many hundreds of lava flows of various ages1. The variable surface ages within this volcanic province have distinct elemental compositions based on the derived values from the Gamma Ray Spectrometer (GRS) suite2. Without seismic data or ophiolite sequences on Mars, the compositions of lavas on the surface provide some of the only information to study the properties of the interior of the planet. The Amazonian surface age and isolated nature of the EVP in the northern lowlands of Mars make it ideal for analyzing the mantle beneath Elysium during the most recent geologic era on Mars. The MELTS algorithm is one of the most commonly used programs for simulating compositions and mineral phases of basaltic melt crystallization3. It has been used extensively for both terrestrial applications4 and for other planetary bodies3,5. The pMELTS calibration of the algorithm allows for higher pressure (10-30 kbars) regimes, and is more appropriate for modeling melt compositions and equilibrium conditions for a source within the martian mantle. We use the pMELTS program to model how partial melting of the martian mantle could evolve magmas into the surface compositions derived from the GRS instrument, and how the mantle beneath Elysium has changed over time. We attribute changes to lithospheric loading by long term, episodic volcanism within the EVP throughout its history. 1. Vaucher, J. et al. The volcanic history of central Elysium Planitia: Implications for martian magmatism. Icarus 204, 418-442 (2009). 2. Susko, D. et al. A record of igneous evolution in Elysium, a major martian volcanic province. Scientific Reports 7, 43177 (2017). 3. El Maarry, M. R. et al. Gamma-ray constraints on the chemical composition of the martian surface in the Tharsis region: A signature of partial melting of the mantle? Journal of Volcanology and Geothermal Research 185, 116-122 (2009). 4. Ding, S. & Dasgupta, R. The fate of sulfide during decompression melting of peridotite - implications for sulfur inventory of the MORB-source depleted upper mantle. Earth and Planetary Science Letters 459, 183-195 (2017). 5. Sakaia, R., Nagaharaa, H., Ozawaa, K. & Tachibanab, S. Composition of the lunar magma ocean constrained by the conditions for the crust formation. Icarus 229, 45-56 (2014).

Upper mantle structure at Walvis Ridge from Pn tomography

NASA Astrophysics Data System (ADS)

Ryberg, Trond; Braeuer, Benjamin; Weber, Michael

2017-10-01

Passive continental margins offer the unique opportunity to study the processes involved in continental extension and break-up. Within the LISPWAL (LIthospheric Structure of the Namibian continental Passive margin at the intersection with the Walvis Ridge from amphibious seismic investigations) project, combined on- and offshore seismic experiments were designed to characterize the Southern African passive margin at the Walvis Ridge in northern Namibia. In addition to extensive analysis of the crustal structures, we carried out seismic investigations targeting the velocity structure of the upper mantle in the landfall region of the Walvis Ridge with the Namibian coast. Upper mantle Pn travel time tomography from controlled source, amphibious seismic data was used to investigate the sub-Moho upper mantle seismic velocity. We succeeded in imaging upper mantle structures potentially associated with continental break-up and/or the Tristan da Cunha hotspot track. We found mostly coast-parallel sub-Moho velocity anomalies, interpreted as structures which were created during Gondwana break-up.

Pb, Sr, and Nd isotopic compositions of a suite of Late Archean, igneous rocks, eastern Beartooth Mountains: implications for crust-mantle evolution

USGS Publications Warehouse

Wooden, J.L.; Mueller, P.A.

1988-01-01

A series of compositionally diverse, Late Archean rocks (2.74-2.79 Ga old) from the eastern Beartooth Mountains, Montana and Wyoming, U.S.A., have the same initial Pb, Sr, and Nd isotopic ratios. Lead and Sr initial ratios are higher and Nd initial ratios lower than would be expected for rocks derived from model mantle sources and strongly indicate the involvement of an older crustal reservoir in the genesis of these rocks. Crustal contamination during emplacement can be ruled out for a variety of reasons. Instead a model involving subduction of continental detritus and contamination of the overlying mantle as is often proposed for modern subduction environments is preferred. This contaminated mantle would have all the isotopic characteristics of mantle enriched by internal mantle metasomatism but would require no long-term growth or changes in parent to daughter element ratios. This contaminated mantle would make a good source for some of the Cenozoic mafic volcanics of the Columbia River, Snake River Plain, and Yellowstone volcanic fields that are proposed to come from ancient, enriched lithospheric mantle. The isotopic characteristics of the 2.70 Ga old Stillwater Complex are a perfect match for the proposed contaminated mantle which provides an alternative to crustal contamination during emplacement. The Pb isotopic characteristics of the Late Archean rocks of the eastern Beartooth Mountains are similar to those of other Late Archean rocks of the Wyoming Province and suggest that Early Archean, upper crustal rocks were common in this terrane. The isotopic signatures of Late Archean rocks in the Wyoming Province are distinctive from those of other Archean cratons in North America which are dominated by a MORB-like, Archean mantle source (Superior Province) and/or a long-term depleted crustal source (Greenland). ?? 1988.

Radial Anisotropy in the Mantle Transition Zone and Its Implications

NASA Astrophysics Data System (ADS)

Chang, S. J.; Ferreira, A. M.

2016-12-01

Seismic anisotropy is a useful tool to investigate mantle flow, mantle convection, and the presence of melts in mantle, since it provides information on the direction of mantle flow or the orientation of melts by combining it with laboratory results in mineral physics. Although the uppermost and lowermost mantle with strong anisotropy have been well studied, anisotropic properties of the mantle transition zone is still enigmatic. We use a recent global radially anisotropic model, SGLOBE-rani, to examine the patterns of radial anisotropy in the mantle transition zone. Strong faster SV velocity anomalies are found in the upper transition zone beneath subduction zones in the western Pacific, which decrease with depth, thereby nearly isotropic in the lower transition zone. This may imply that the origin for the anisotropy is the lattice-preferred orientation of wadsleyite, the dominant anisotropic mineral in the upper transition zone. The water content in the upper transition zone may be inferred from radial anisotropy because of the report that anisotropic intensity depends on the water content in wadsleyite.

Super-chondritic Sm/Nd ratios in Mars, the Earth and the Moon.

PubMed

Caro, Guillaume; Bourdon, Bernard; Halliday, Alex N; Quitté, Ghylaine

2008-03-20

Small isotopic differences in the atomic abundance of neodymium-142 (142Nd) in silicate rocks represent the time-averaged effect of decay of formerly live samarium-146 (146Sm) and provide constraints on the timescales and mechanisms by which planetary mantles first differentiated. This chronology, however, assumes that the composition of the total planet is identical to that of primitive undifferentiated meteorites called chondrites. The difference in the 142Nd/144Nd ratio between chondrites and terrestrial samples may therefore indicate very early isolation (<30 Myr from the formation of the Solar System) of the upper mantle or a slightly non-chondritic bulk Earth composition. Here we present high-precision 142Nd data for 16 martian meteorites and show that Mars also has a non-chondritic composition. Meteorites belonging to the shergottite subgroup define a planetary isochron yielding an age of differentiation of 40 +/- 18 Myr for the martian mantle. This isochron does not pass through the chondritic reference value (100 x epsilon(142)Nd = -21 +/- 3; 147Sm/144Nd = 0.1966). The Earth, Moon and Mars all seem to have accreted in a portion of the inner Solar System with approximately 5 per cent higher Sm/Nd ratios than material accreted in the asteroid belt. Such chemical heterogeneities may have arisen from sorting of nebular solids or from impact erosion of crustal reservoirs in planetary precursors. The 143Nd composition of the primitive mantle so defined by 142Nd is strikingly similar to the putative endmember component 'FOZO' characterized by high 3He/4He ratios.

Mapping the Earth's thermochemical and anisotropic structure using global surface wave data

NASA Astrophysics Data System (ADS)

Khan, A.; Boschi, L.; Connolly, J. A. D.

2011-01-01

We have inverted global fundamental mode and higher-order Love and Rayleigh wave dispersion data jointly, to find global maps of temperature, composition, and radial seismic anisotropy of the Earth's mantle as well as their uncertainties via a stochastic sampling-based approach. We apply a self-consistent thermodynamic method to systematically compute phase equilibria and physical properties (P and S wave velocity, density) that depend only on composition (in the Na2-CaO-FeO-MgO-Al2O3-SiO2 model system), pressure, and temperature. Our 3-D maps are defined horizontally by 27 different tectonic regions and vertically by a number of layers. We find thermochemical differences between oceans and continents to extend down to ˜250 km depth, with continents and cratons appearing chemically depleted (high magnesium number (Mg #) and Mg/Si ratio) and colder (>100°C) relative to oceans, while young oceanic lithosphere is hotter than its intermediate age and old counterparts. We find what appears to be strong radial S wave anisotropy in the upper mantle down to ˜200 km, while there seems to be little evidence for shear anisotropy at greater depths. At and beneath the transition zone, 3-D heterogeneity is likely uncorrelated with surface tectonics; as a result, our tectonics-based parameterization is tenuous. Despite this weakness, constraints on the gross average thermochemical and anisotropic structure to ˜1300 km depth can be inferred, which appear to indicate that the compositions of the upper (low Mg# and high Mg/Si ratio) and lower mantle (high Mg# and low Mg/Si ratio) might possibly be distinct.

The maximum water storage capacities in nominally anhydrous minerals in the mantle transition zone and lower mantle

NASA Astrophysics Data System (ADS)

Inoue, T.; Yurimoto, H.

2012-12-01

Water is the most important volatile component in the Earth, and affects the physicochemical properties of mantle minerals, e.g. density, elastic property, electrical conductivity, thermal conductivity, rheological property, melting temperature, melt composition, element partitioning, etc. So many high pressure experiments have been conducted so far to determine the effect of water on mantle minerals. To clarify the maximum water storage capacity in nominally anhydrous mantle minerals in the mantle transition zone and lower mantle is an important issue to discuss the possibility of the existence of water reservoir in the Earth mantle. So we have been clarifying the maximum water storage capacity in mantle minerals using MA-8 type (KAWAI-type) high pressure apparatus and SIMS (secondary ion mass spectroscopy). Upper mantle mineral, olivine can contain ~0.9 wt% H2O in the condition just above 410 km discontinuity in maximum (e.g. Chen et al., 2002; Smyth et al., 2006). On the other hand, mantle transition zone mineral, wadsleyite and ringwoodite can contain significant amount (about 2-3 wt.%) of H2O (e.g. Inoue et al., 1995, 1998, 2010; Kawamoto et al., 1996; Ohtani et al., 2000). But the lower mantle mineral, perovskite can not contain significant amount of H2O, less than ~0.1 wt% (e.g. Murakami et al., 2002; Inoue et al., 2010). In addition, garnet and stishovite also can not contain significant amount of H2O (e.g. Katayama et al., 2003; Mookherjee and Karato, 2010; Litasov et al., 2007). On the other hand, the water storage capacities of mantle minerals are supposed to be significantly coupled with Al by a substitution with Mg2+, Si4+ or Mg2+ + Si4+, because Al3+ is the trivalent cation, and H+ is the monovalent cation. To clarify the degree of the substitution, the water contents and the chemical compositions of Al-bearing minerals in the mantle transition zone and the lower mantle were also determined in the Al-bearing systems with H2O. We will introduce the recent results on the maximum water storage capacities in nominally anhydrous minerals in the mantle transition zone and lower mantle from the high pressure experimental point of view.

Isotopic evidence for a large-scale plume-derived mantle domain between the Indian and Pacific mantles beneath the Southern Ocean.

NASA Astrophysics Data System (ADS)

Park, S. H.; Langmuir, C. H.; Scott, S. R.; Sims, K. W. W.; Lin, J.; Kim, S. S.; Blichert-Toft, J.; Choi, H.; Yang, Y. S.; Michael, P. J.

2017-12-01

Earth's upper mantle is characterized by Indian- and Pacific-type domains with distinctive isotope characteristics. The boundary between these two mantle regions has been hypothesized to be located at the Australian-Antarctic-Discordance (AAD), where regions west and east of the AAD are Indian- and Pacific-type, respectively. It was further posited that the Pacific mantle feeds into the Indian mantle as the boundary is moving westward. These scenarios have important implications for the dynamics of mantle convection in the area. In the present model, regions east of the AAD are assumed to be entirely Pacific-type mantle, but our recent recovery of basalts from a 2,000-km sampling gap along the Australian-Antarctic Ridge (AAR), located east of the AAD on the Pacific side, challenges this picture. Here we show that the Hf, Nd, Pb, and Sr isotopic compositions of AAR MORB are distinct from those of Pacific and Indian MORB. Rather, the AAR lavas show mixing relationships with volcanoes from the Hikurangi seamounts, the Balleney and Scott Islands, the West Antarctic Rift System, New Zealand, and east Australia. According to tectonic reconstruction models, these volcanoes are related to super-plume activity that caused Gondwana to break up at 90 Ma. These results imply that a large-scale plume-derived mantle domain exists between the Indian and Pacific mantle domains, and that mantle dynamics along the AAD should be reinterpreted in light of interaction with a super-plume.

A Bayesian method to quantify azimuthal anisotropy model uncertainties: application to global azimuthal anisotropy in the upper mantle and transition zone

NASA Astrophysics Data System (ADS)

Yuan, K.; Beghein, C.

2018-04-01

Seismic anisotropy is a powerful tool to constrain mantle deformation, but its existence in the deep upper mantle and topmost lower mantle is still uncertain. Recent results from higher mode Rayleigh waves have, however, revealed the presence of 1 per cent azimuthal anisotropy between 300 and 800 km depth, and changes in azimuthal anisotropy across the mantle transition zone boundaries. This has important consequences for our understanding of mantle convection patterns and deformation of deep mantle material. Here, we propose a Bayesian method to model depth variations in azimuthal anisotropy and to obtain quantitative uncertainties on the fast seismic direction and anisotropy amplitude from phase velocity dispersion maps. We applied this new method to existing global fundamental and higher mode Rayleigh wave phase velocity maps to assess the likelihood of azimuthal anisotropy in the deep upper mantle and to determine whether previously detected changes in anisotropy at the transition zone boundaries are robustly constrained by those data. Our results confirm that deep upper-mantle azimuthal anisotropy is favoured and well constrained by the higher mode data employed. The fast seismic directions are in agreement with our previously published model. The data favour a model characterized, on average, by changes in azimuthal anisotropy at the top and bottom of the transition zone. However, this change in fast axes is not a global feature as there are regions of the model where the azimuthal anisotropy direction is unlikely to change across depths in the deep upper mantle. We were, however, unable to detect any clear pattern or connection with surface tectonics. Future studies will be needed to further improve the lateral resolution of this type of model at transition zone depths.

Seismic Migration Imaging of the Crust and Upper Mantle Discontinuity Structure beneath Southern Taiwan

NASA Astrophysics Data System (ADS)

Liu, Y.-S.; Kuo, B.-Y.

2009-04-01

Taiwan is located in the convergent plate boundary zone where the Philippine Sea plate has obliquely collided on the Asian continental margin, initiating the arc-continent collision and subsequent mountain-building in Taiwan. Receiver function has been a powerful tool to image seismic velocity discontinuity structure in the crust and upper mantle which can help illuminate the deep dynamic process of active Taiwan orogeny. In this study, we adopt backprojection migration processing of teleseismic receiver functions to investigate the crust and upper mantle discontinuities beneath southern Taiwan, using the data from Southern Taiwan Transect Seismic Array (STTA), broadband stations of Central Weather Bureau (CWB), Broadband Array in Taiwan for Seismology (BATS), and Taiwan Integrated Geodynamics Research (TAIGER). This composite east-west trending linear array has the aperture of about 150 km with the station spacing of ~5-10 km. Superior to the common midpoint (CMP) stack approach, the migration can properly image the dipping, curved, or laterally-varying topography of discontinuous interfaces which very likely exist under the complicated tectonic setting of Taiwan. We first conduct synthetic experiments to test the depth and lateral resolution of migration images based on the WKBJ synthetic waveforms calculated from available source and receiver distributions. We will next construct the 2-D migration image under the array to reveal the topographic variation of the Moho and lithosphere discontinuities beneath southern Taiwan.

Regional magnetic anomalies, crustal strength, and the location of the northern Cordilleran fold-and-thrust belt

USGS Publications Warehouse

Saltus, R.W.; Hudson, T.L.

2007-01-01

The northern Cordilleran fold-and-thrust belt in Canada and Alaska is at the boundary between the broad continental margin mobile belt and the stable North American craton. The fold-and-thrust belt is marked by several significant changes in geometry: cratonward extensions in the central Yukon Territory and northeastern Alaska are separated by marginward re-entrants. These geometric features of the Cordilleran mobile belt are controlled by relations between lithospheric strength and compressional tectonic forces developed along the continental margin. Regional magnetic anomalies indicate deep thermal and compositional characteristics that contribute to variations in crustal strength. Our detailed analysis of one such anomaly, the North Slope deep magnetic high, helps to explain the geometry of the fold-and-thrust front in northern Alaska. This large magnetic anomaly is inferred to reflect voluminous mafic magmatism in an old (Devonian?) extensional domain. The presence of massive amounts of malic material in the lower crust implies geochemical depletion of the underlying upper mantle, which serves to strengthen the lithosphere against thermal erosion by upper mantle convection. We infer that deep-source magnetic highs are an important indicator of strong lower crust and upper mantle. This stronger lithosphere forms buttresses that play an important role in the structural development of the northern Cordilleran fold-and-thrust belt. ?? 2007 The Geological Society of America.

Origin of the Indian Ocean-type isotopic signature in basalts from Philippine Sea plate spreading centers: An assessment of local versus large-scale processes

NASA Astrophysics Data System (ADS)

Hickey-Vargas, Rosemary

1998-09-01

Basalts erupted from spreading centers on the Philippine Sea plate between 50 Ma and the present have the distinctive isotopic characteristics of Indian Ocean mid-ocean ridge basalt (MORB), such as high 208Pb/204Pb and low 143Nd/144Nd for a given 206Pb/204Pb compared with Pacific and Atlantic Ocean MORB. This feature may indicate that the upper mantle of the Philippine Sea plate originated as part of the existing Indian Ocean upper mantle domain, or, alternatively, that local processes duplicated these isotopic characteristics within the sub-Philippine Sea plate upper mantle. Synthesis of new and published isotopic data for Philippine Sea plate basin basalts and island arc volcanic rocks, radiometric ages, and tectonic reconstructions of the plate indicates that local processes, such as contamination of the upper mantle by subducted materials or by western Pacific mantle plumes, did not produce the Indian Ocean-type signature in Philippine Sea plate MORB. It is more likely that the plate originated over a rapidly growing Indian Ocean upper mantle domain that had spread into the area between Australia/New Guinea and southeast Asia before 50 Ma.

Fe-Ni-Cu-C-S phase relations at high pressures and temperatures - The role of sulfur in carbon storage and diamond stability at mid- to deep-upper mantle

NASA Astrophysics Data System (ADS)

Tsuno, Kyusei; Dasgupta, Rajdeep

2015-02-01

Constraining the stable form of carbon in the deep mantle is important because carbon has key influence on mantle processes such as partial melting and element mobility, thereby affecting the efficiency of carbon exchange between the endogenic and exogenic reservoirs. In the reduced, mid- to deep-upper mantle, the chief host of deep carbon is expected to be graphite/diamond but in the presence of Fe-Ni alloy melt in the reduced mantle and owing to high solubility of carbon in such alloy phase, diamond may become unstable. To investigate the nature of stable, C-bearing phases in the reduced, mid- to deep-upper mantle, here we have performed experiments to examine the effect of sulfur on the phase relations of the Ni-rich portion of Fe-Ni ± Cu-C-S system, and carbon solubility in the Fe-Ni solid and Fe-Ni-S liquid alloys at 6-8 GPa and 800-1400 °C using a multianvil press. Low-temperature experiments for six starting mixes (Ni/(Fe + Ni) ∼ 0.61, 8-16 wt.% S) contain C-bearing, solid Fe-Ni alloy + Fe-Ni-C-S alloy melt + metastable graphite, and the solid alloy-out boundary is constrained, at 1150-1200 °C at 6 GPa and 900-1000 °C at 8 GPa for S-poor starting mix, and at 1000-1050 °C at 6 GPa and 900-1000 °C at 8 GPa for the S-rich starting mix. The carbon solubility in the liquid alloy significantly diminishes from 2.1 to 0.8 wt.% with sulfur in the melt increasing from 8 to 24 wt.%, irrespective of temperature. We also observed a slight decrease of carbon solubility in the liquid alloy with increasing pressure when alloy liquid contains >∼18 wt.% S, and with decreasing Ni/(Fe + Ni) ratio from 0.65 to ∼0.53. Based on our results, diamond, coexisting with Ni-rich sulfide liquid alloy is expected to be stable in the reduced, alloy-bearing oceanic mantle with C content as low as 20 to 5 ppm for mantle S varying between 100 and 200 ppm. Deep, reduced root of cratonic mantle, on the other hand, is expected to have C distributed among solid alloy, liquid alloy, and diamond for low-S (≤100 ppm S) domains and between liquid alloy and diamond in high-S (≥150 ppm S) domains. Our findings can explain the observation of Ni-rich sulfide and/or Fe-Ni alloy inclusions in diamond and suggest that diamond stability in the alloy-bearing, reduced mantle does not necessarily require excess C supply from recycled, crustal lithologies. Our prediction of diamond stability in the background, depleted upper mantle, owing to the interaction with mantle sulfides, is also consistent with the carbon isotopic composition of peridotitic diamond (δ13C of - 5 ± 1 ‰), which suggests no significant input from recycled carbon.

Redox state of earth's upper mantle from kimberlitic ilmenites

NASA Technical Reports Server (NTRS)

Haggerty, S. E.; Tompkins, L. A.

1983-01-01

Temperatures and oxygen fugacities are reported on discrete ilmenite nodules in kimberlites from West Africa which demonstrate that the source region in the upper mantle is moderately oxidized, consistent with other nodule suites in kimberlites from southern Africa and the United States. A model is presented for a variety of tectonic settings, proposing that the upper mantle is profiled in redox potential, oxidized in the fertile asthenosphere but reduced in the depleted lithosphere.

Plate tectonics and hotspots: the third dimension.

PubMed

Anderson, D L; Tanimoto, T; Zhang, Y S

1992-06-19

High-resolution seismic tomographic models of the upper mantle provide powerful new constraints on theories of plate tectonics and hotspots. Midocean ridges have extremely low seismic velocities to a depth of 100 kilometers. These low velocities imply partial melting. At greater depths, low-velocity and high-velocity anomalies record, respectively, previous positions of migrating ridges and trenches. Extensional, rifting, and hotspot regions have deep (> 200 kilometers) low-velocity anomalies. The upper mantle is characterized by vast domains of high temperature rather than small regions surrounding hotspots; the asthenosphere is not homogeneous or isothermal. Extensive magmatism requires a combination of hot upper mantle and suitable lithospheric conditions. High-velocity regions of the upper 200 kilometers of the mantle correlate with Archean cratons.

Mantle transition zone input to kimberlite magmatism near a subduction zone: Origin of anomalous Nd-Hf isotope systematics at Lac de Gras, Canada

NASA Astrophysics Data System (ADS)

Tappe, Sebastian; Graham Pearson, D.; Kjarsgaard, Bruce A.; Nowell, Geoff; Dowall, David

2013-06-01

Late Cretaceous-Eocene kimberlites from the Lac de Gras area, central Slave craton, show the most extreme Nd-Hf isotope decoupling observed for kimberlites worldwide. They are characterized by a narrow range of moderately enriched Nd isotope compositions (ɛNd(i)=-0.4 to -3.5) that contrasts strongly with their moderately depleted to highly enriched ɛHf(i) values (+3.9 to -9.9). Although digestion of cratonic mantle material in proto-kimberlite melt can theoretically produce steep arrays in Nd-Hf isotope space, the amount of contaminant required to explain the Lac de Gras data is unrealistic. Instead, it is more plausible that mixing of compositionally discrete melt components within an isotopically variable source region is responsible for the steep Nd-Hf isotope array. As development of strongly negative ΔɛHf requires isotopic aging of a precursor material with Sm/Nd≫Lu/Hf for billion-year timescales, a number of models have been proposed where ancient MORB crust trapped in the mantle transition zone is the ultimate source of the extreme Hf isotope signature. However, we provide a conceptual modification and demonstrate that OIB-type domains within ancient subducted oceanic lithosphere can produce much stronger negative ΔɛHf during long-term isolation. Provided that these OIB-type domains have lower melting points compared with associated MORB crust, they are among the first material to melt within the transition zone during thermal perturbations. The resulting hydrous alkali silicate melts react strongly with depleted peridotite at the top of the transition zone and transfer negative ΔɛHf signatures to less dense materials, which can be more easily entrained within upward flowing mantle. Once these entrained refertilized domains rise above 300 km depth, they may become involved in CO2- and H2O-fluxed redox melting of upper mantle peridotite beneath a thick cratonic lid. We argue that incorporation of ancient transition zone material, which includes ultradeep diamonds, into the convecting upper mantle source region of Lac de Gras kimberlites was due to vigorous mantle return flow. This occurred in direct response to fast and complex subduction along the western margin of North America during the Late Cretaceous.

Deep Mantle Origin for the DUPAL Anomaly?

NASA Astrophysics Data System (ADS)

Ingle, S.; Weis, D.

2002-12-01

Twenty years after the discovery of the Dupal Anomaly, its origin remains a geochemical and geophysical enigma. This anomaly is associated with the Southern Hemisphere oceanic mantle and is recognized by basalts with geochemical characteristics such as low 206Pb/204Pb and high 87Sr/86Sr. Both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) are affected, despite originating from melting at different depths and of different mantle sources. We compile geochemical data for both MORB and OIB from the three major oceans to help constrain the physical distribution and chemical composition of the Dupal Anomaly. There is a clear decrease in 206Pb/204Pb and an increase in 87Sr/86Sr with more southerly latitude for Indian MORB and OIB; these correlations are less obvious in the Atlantic and non-existent in the Pacific. The average* 143Nd/144Nd for Pacific and Atlantic OIB is 0.5129, but is lower for Indian OIB (0.5128). Interestingly, Pacific, Atlantic and Indian OIB all have 176Hf/177Hf averages of 0.2830. Indian MORB also record this phenomenon of low Nd with normal Hf isotopic compositions (Chauvel and Blichert-Toft, EPSL, 2001). Hf isotopes appear, therefore, to be a valid isotopic proxy for measuring the presence and magnitude of the Dupal Anomaly at specific locations. Wen (EPSL, 2001) reported a low-velocity layer at the D'' boundary beneath the Indian Ocean from which the Dupal Anomaly may originate. This hypothesis may be consistent with our compilations demonstrating that the long-lived Dupal Anomaly does not appear to be either mixing efficiently into the upper mantle or spreading to other ocean basins through time. We suggest that the Dupal source could be continually tapped by upwelling Indian Ocean mantle plumes. Plumes would then emplace pockets of Dupal material into the upper mantle and other ascending plumes might further disperse this material into the shallow asthenosphere. This could explain both the presence of the Dupal signature in MORB and OIB and the geochemical similarities between some Indian Ocean mantle plumes, such as Kerguelen, and the Dupal signature. * To avoid sampling biases, data for each ocean island (or group) are averaged and these values are used to calculate the average for each ocean.

Geochemical Relationships between Middle- to Upper-Crustal Exposures of the Alisitos Oceanic Arc (Baja California, Mexico): An Outstanding Field Analog to Active Extensional Oceanic Arcs

NASA Astrophysics Data System (ADS)

Morris, R.; DeBari, S. M.; Busby, C.; Medynski, S.

2016-12-01

The southern volcano-bounded basin of the Rosario segment of the Cretaceous Alisitos oceanic arc provides outstanding 3-D exposures of an extensional arc, where crustal generation processes are recorded in the upper-crustal volcanic units and underlying middle-crustal plutonic rocks. Geochemical linkages between exposed crustal levels provide an analog for extensional arc systems such as the Izu-Bonin-Mariana (IBM) Arc. Upper-crustal units comprise a 3-5 km thick volcanic-volcaniclastic stratigraphy with hypabyssal intrusions. Deep-seated plutonic rocks intrude these units over a transition of <500m, where rafted volcanic blocks and evidence of magma mingling are exposed. Thermobarometry suggests <6 km emplacement depths. Compositional ranges (basalt to rhyolite) and mineral assemblages are similar in both middle-crustal and upper-crustal units, with striking compositional overlap. The most mafic compositions occur in upper-crustal hypabyssal units, and as amphibole cumulates in the plutonic rocks ( 51% SiO2). The most felsic compositions occur in welded ignimbrites and a tonalite pluton ( 71% SiO2). All units are low K with flat REE patterns, and show LILE enrichment and HFSE depletion. Trace element ratios show limited variation throughout the crustal section. Zr/Y and Nb/Y ratios are similar to the Izu active ( 3 Ma to present) zone of extension immediately behind the arc front, suggesting comparable mantle melt % during extension. Th/Zr ratios are more enriched in Alisitos compared to Izu, suggesting greater subducted sediment input. The Alisitos crustal section shows a limited range in ɛNd (5.7-7.1), but a wider range in 87Sr/86Sr (0.7035-0.7055) and 206Pb/204Pb (18.12-19.12); the latter is likely alteration effects. Arc magmas were derived from a subduction-modified MORB mantle source, less depleted than Izu arc front and less enriched than the rear arc, but is a good match with the zone of extension that lies between. Differentiation occurred in a closed system (i.e., fractional crystallization/self-melting with back mixing), producing the entire crustal section in <3 Ma.

Mantle rare gas relative abundances in a steady-state mass transport model

NASA Technical Reports Server (NTRS)

Porcelli, D.; Wasserburg, G. J.

1994-01-01

A model for He and Xe was presented previously which incorporates mass transfer of rare gases from an undegassed lower mantle (P) and the atmosphere into a degassed upper mantle (D). We extend the model to include Ne and Ar. Model constraints on rare gas relative abundances within P are derived. Discussions of terrestrial volatile acquisition have focused on the rare gas abundance pattern of the atmosphere relative to meteoritic components, and the pattern of rare gases still trapped in the Ear,th is important in identifying volatile capture and loss processes operating during Earth formation. The assumptions and principles of the model are discussed in Wasserburg and Porcelli (this volume). For P, the concentrations in P of the decay/nuclear products 4 He, 21 Ne, 40 Ar, and 136 Xe can be calculated from the concentrations of the parent elements U, Th, K, and Pu. The total concentration of the daughter element in P is proportional to the isotopic shifts in P. For Ar, ((40)Ar/(36)Ar)p - ((40)Ar/(36)Ar)o =Delta (exp 40) p= 40 Cp/(exp 36)C where(i)C(sub j) the concentration of isotope i in j. In D, isotope compositions are the result of mixing rare gases from P, decay/nuclear products generated in the upper mantle, and subducted rare gases (for Ar and Xe).

Native iron in the Earth and space

NASA Astrophysics Data System (ADS)

Pechersky, D. M.; Kuzina, D. M.; Markov, G. P.; Tsel'movich, V. A.

2017-09-01

Thermomagnetic and microprobe studies of native iron in the terrestrial upper-mantle hyperbasites (xenoliths in basalts), Siberian traps, and oceanic basalts are carried out. The results are compared to the previous data on native iron in sediments and meteorites. It is established that in terms of the composition and grain size and shape, the particles of native iron in the terrestrial rocks are close to each other and to the extraterrestrial iron particles from sediments and meteorites. This suggests that the sources of the origin of these particles were similar; i.e., the formation conditions in the Earth were close to the conditions in the meteorites' parent bodies. This similarity is likely to be due to the homogeneity of the gas and dust cloud at the early stage of the solar system. The predominance of pure native iron in the sediments can probably be accounted for by the fact that interstellar dust is mostly contributed by the upper-mantle material of the planets, whereas the lower-mantle and core material falls on the Earth mainly in the form of meteorites. A model describing the structure of the planets in the solar system from the standpoint of the distribution of native iron and FeNi alloys is proposed.

Deformation of polyphase aggregates, forsterite+MgO, at pressures and temperatures of the Earth's upper mantle

NASA Astrophysics Data System (ADS)

Bejina, F.; Bystricky, M.; Ingrin, J.

2012-12-01

Modelling the solid-state flow of the upper mantle requires a thorough understanding of its rheology and therefore necessitates to perform deformation experiments on mantle rocks (or analogues) at very high pressures and temperatures. Minerals other than olivine constitute up to 40 vol% of upper mantle rocks and may have a significant effect on the rheological behavior of these rocks. Nevertheless, most experimental studies to date have focused on the deformation properties of olivine single crystals or monomineralic olivine aggregates. In this study, and as a first step before focusing on more realistic mantle-like compositions, we have performed deformation experiments on polymineralic model aggregates of forsterite and MgO, at upper mantle pressures and temperatures. Commercial powders of Mg2SiO4 and MgO were mixed and ground in WC grinders and dried in a one-atmosphere furnace at 1000°C. Powders with different volume proportions of the two phases were sintered by spark plasma sintering (SPS) at temperatures of 1300-1400°C and 100 MPa for a few minutes, resulting in dense pellets 8 mm in diameter and 3-4 mm in length. Microstructural analysis by SEM reveals equilibrated microstructures with forsterite and MgO grain sizes of a few microns. Deformation experiments on samples 1.2 mm in diameter and 1.2 mm in length were performed at 3-8 GPa and 1000-1300°C in a D-DIA apparatus coupled with synchrotron X-ray radiation. The technique permits in situ measurement of macroscopic strain rates as well as stress levels sustained by different subpopulations of grains of each phase. Typically, two specimens, respectively a monomineralic and a polyphase aggregate, were deformed concurrently in order to minimize the relative uncertainties in temperature and pressure and to facilitate the comparison of their rheological properties. The samples were deformed to total strains of 15-25%. As expected, the harder phase, forsterite, sustains much higher stress levels than MgO, in agreement with numerical models for two-phase flow. Results on stress and strain partitioning in polyphase aggregates and on the effect of the presence of a weak phase on the rheology of forsterite-rich aggregates will be discussed.

Formation of hydrocarbons under upper mantle conditions: experimental view

NASA Astrophysics Data System (ADS)

Kolesnikov, Anton; Kutcherov, Vladimir G.

2010-05-01

Main postulates of the theory of abiogenic abyssal origin of petroleum have been developed in the last 50 years in Russia and Ukraine. According to this theory, hydrocarbon compounds were generated in the mantle and migrated through the deep faults into the Earth's crust. There they formed oil and gas deposits in any kinds of rocks and in any kind of their structural positions. Until recently the main obstacle to accept the theory was the lack of reliable and reproducible experimental data confirming the possibility of the synthesis of complex hydrocarbon systems under the mantle conditions. The results received in the last decade by different groups of researchers from Russia, U.S.A. and China have confirmed the possibility of generation of hydrocarbons from inorganic materials, highly distributed in the Earth's mantle, under thermobaric conditions of 70-250 km: 2 - 5 GPa and 1000-1500 K. Experiments made in the CONAC chamber at pressures of 3-5 GPa and temperatures of 1000-1500 K by Kutcherov et al. [1, 2] have demonstrated that the mixtures of hydrocarbons with composition similar to natural hydrocarbon systems have been received as a result of chemical reactions between CaCO3, FeO and H2O. Methane formation from the same compounds was registered after heating up to 600-1500 K at pressures of 4-11 GPa in diamond anvil cells [4, 5, 6]. Influence of oxidation state of carbon donor and cooling rate of the fluid synthesized at high pressure were studied using different types of high pressure equipments. It was shown that composition of the final hydrocarbon mixture depends on these parameters. Experimental investigations of transformation of methane and ethane at 2-5 GPa and 1000-1500 K [3] confirmed thermodynamic stability of heavy hydrocarbons in the upper mantle and showed the possibility of hydrocarbon chain growth even at oxidative environment. For development of the theory of abiogenic abyssal origin of petroleum it is necessary to arrange a set of new experiments to understand the pathways of hydrocarbons formation, influence of pressure, temperature, oxygen fugacity and chemical composition of the environment on the content of the mantle fluid. [1] Kutcherov, V. G., Bendeliani, N. A. Alekseev, V. A. & Kenney, J. F. Synthesis of Hydrocarbons from Minerals at Pressures up to 5 GPa. Doklady Physical Chemistry, 387, 4-6, 328-330 (2002). [2] Kenney, J. F., Kutcherov, V. G., Bendeliani, N. A. & Alekseev, V. A. The Evolution of Multicomponent Systems at High Pressures: VI. The Thermodynamic Stability of the Hydrogen-Carbon System: The Genesis of Hydrocarbons and the Origin of Petroleum. Proc. Natl. Acad. Sci. U.S.A., 99, 10976-10981 (2002). [3] Kolesnikov A., Kutcherov V. G. and Goncharov A. F. Methane-derived hydrocarbons produced under upper-mantle conditions. Nature Geoscience, 2, 566 - 570 (2009). [4] Scott H.P., Hemley R.J, Mao H. et al. Generation of methane in the Earth's mantle: In situ high pressure-temperature measurements of carbonate reduction, Proc. Natl. Acad. Sci. U.S.A., 101 14023-14026 (2004). [5] Chen J.Y., et al. Methane formation from CaCO3 reduction catalyzed by high pressure. Chin. Chem. Lett., 19, 4, 475-478 (2008). [6] Sharma A., Cody G. D., and Hemley R. J. In situ Diamond-anvil cell observations of methanogenesis at high pressures and temperatures. Energy Fuels, 23, 11, 5571-5579 (2009).

Sulfide enrichment at an oceanic crust-mantle transition zone: Kane Megamullion (23°N, MAR)

NASA Astrophysics Data System (ADS)

Ciazela, Jakub; Koepke, Juergen; Dick, Henry J. B.; Botcharnikov, Roman; Muszynski, Andrzej; Lazarov, Marina; Schuth, Stephan; Pieterek, Bartosz; Kuhn, Thomas

2018-06-01

The Kane Megamullion oceanic core complex located along the Mid-Atlantic Ridge (23°30‧N, 45°20‧W) exposes lower crust and upper mantle directly on the ocean floor. We studied chalcophile elements and sulfides in the ultramafic and mafic rocks of the crust-mantle transition and the mantle underneath. We determined mineralogical and elemental composition and the Cu isotope composition of the respective sulfides along with the mineralogical and elemental composition of the respective serpentines. The rocks of the crust-mantle transition zone (i.e., plagioclase harzburgite, peridotite-gabbro contacts, and dunite) overlaid by troctolites are by one order of magnitude enriched in several chalcophile elements with respect to the spinel harzburgites of the mantle beneath. Whereas the range of Cu concentrations in spinel harzburgites is 7-69 ppm, the Cu concentrations are highly elevated in plagioclase harzburgites with a range of 90-209 ppm. The zones of the peridotite-gabbro contacts are even more enriched, exhibiting up to 305 ppm Cu and highly elevated concentrations of As, Zn, Ga, Sb and Tl. High Cu concentrations show pronounced correlation with bulk S concentrations at the crust-mantle transition zone implying an enrichment process in this horizon of the oceanic lithosphere. We interpret this enrichment as related to melt-mantle reaction, which is extensive in crust-mantle transition zones. In spite of the ubiquitous serpentinization of primary rocks, we found magmatic chalcopyrites [CuFeS2] as inclusions in plagioclase as well as associated with pentlandite [(Fe,Ni)9S8] and pyrrhotite [Fe1-xS] in polysulfide grains. These chalcopyrites show a primary magmatic δ65Cu signature ranging from -0.04 to +0.29 ‰. Other chalcopyrites have been dissolved during serpentinization. Due to the low temperature (<300 °C) of circulating fluids chalcophile metals from primary sulfides have not been mobilized and transported away but have been trapped in smaller secondary sulfides and hydroxides. Combined with the Cu deposits documented in the crust-mantle transition zones of various ophiolite complexes, our results indicate that the metal enrichment, increased sulfide modes, and potentially formation of small sulfide deposits could be expected globally along the petrological Moho.

Stress and strain partitioning during high PT deformation of polyphase aggregates

NASA Astrophysics Data System (ADS)

Bejina, F.; Bystricky, M.; Ingrin, J.

2013-12-01

Previous experimental studies on the rheology of the upper mantle have mostly focused on the deformation properties of olivine. Yet minerals other than olivine constitute up to 40 vol% of upper mantle rocks and may have a significant effect on the rheological behavior of these rocks. In this study, and as a first step before focusing on more realistic mantle-like compositions, we have performed deformation experiments on polymineralic model aggregates of forsterite and MgO, at upper mantle pressures and temperatures. Commercial powders of Mg2SiO4 and MgO were mixed and ground in WC grinders and dried in a one-atmosphere furnace at 1000°C. Powders with different volume proportions of the two phases (up to 50 vol% of MgO) were sintered by spark plasma sintering at temperatures of 1300-1400°C and 100 MPa for a few minutes, resulting in dense pellets 8 mm in diameter and 3-4 mm in length. Deformation experiments on samples 1.2 mm in diameter and 1.2 mm in length were performed at 3-8 GPa and 1000-1300°C in a D-DIA apparatus coupled with synchrotron X-ray radiation. The technique permits in situ measurement of macroscopic strain rates as well as stress levels sustained by different subpopulations of grains of each phase. Typically, two specimens were deformed concurrently in order to minimize the relative uncertainties in temperature and pressure and to facilitate the comparison of their rheological properties. The samples were deformed to total strains of 15-25%. The harder phase, forsterite, sustains significantly higher stress levels than MgO, as predicted by numerical models for two-phase flow. Microstructural analysis by SEM reveals equilibrated microstructures in the starting samples, with well-mixed forsterite and MgO layers alternating with forsterite-rich layers. In the deformed samples, this compositional banding is accentuated, with more intense deformation in the well-mixed layers. Results on stress and strain partitioning in polyphase aggregates in the context of existing models of two-phase flow will be discussed.

Enriched and depleted characters of the Amnay Ophiolite upper crustal section and the regionally heterogeneous nature of the South China Sea mantle

NASA Astrophysics Data System (ADS)

Perez, Americus d. C.; Faustino-Eslava, Decibel V.; Yumul, Graciano P.; Dimalanta, Carla B.; Tamayo, Rodolfo A.; Yang, Tsanyao Frank; Zhou, Mei-Fu

2013-03-01

The volcanic section of the Middle Oligocene Amnay Ophiolite in Mindoro, Philippines has previously been shown to be of normalmid-oceanic ridge basalt (NMORB) composition. Here we report for the first time an enriched mantle component that is additionally recorded in this crustal section. New whole rock major and trace element data are presented for nine mafic volcanic rocks from a section of the ophiolite that has not been previously examined. These moderately evolved tholeiitic basalts were found to have resulted from the bulk mixing of ˜10% ocean island basalt components with depleted mantle. Drawing together various geochemical characteristics reported for different rock suites taken as representatives of the South China Sea crust, including the enriched MORB (EMORB) and NMORB of the East Taiwan Ophiolite, the NMORB from previous studies of the Amnay Ophiolite and the younger ocean floor eruptives of the Scarborough Seamount-Reed Bank region, a veined mantle model is proposed for the South China Sea mantle. The NMORB magmatic products are suggested to have been derived from the more depleted portions of the mantle whereas the ocean island basalt (OIB) and EMORB-type materials from the mixing of depleted and veined/enriched mantle regions.

Proxies of oceanic Lithosphere/Asthenosphere Boundary from Global Seismic Anisotropy Tomography

NASA Astrophysics Data System (ADS)

Burgos, Gael; Montagner, Jean-Paul; Beucler, Eric; Trampert, Jeannot; Capdeville, Yann

2013-04-01

Surface waves provide essential information on the knowledge of the upper mantle global structure despite their low lateral resolution. This study, based on surface waves data, presents the development of a new anisotropic tomographic model of the upper mantle, a simplified isotropic model and the consequences of these results for the Lithosphere/Asthenosphere Boundary (LAB). As a first step, a large number of data is collected, these data are merged and regionalized in order to derive maps of phase and group velocity for the fundamental mode of Rayleigh and Love waves and their azimuthal dependence (maps of phase velocity are also obtained for the first six overtones). As a second step, a crustal a posteriori model is developped from the Monte-Carlo inversion of the shorter periods of the dataset, in order to take into account the effect of the shallow layers on the upper mantle. With the crustal model, a first Monte-Carlo inversion for the upper mantle structure is realized in a simplified isotropic parameterization to highlight the influence of the LAB properties on the surface waves data. Still using the crustal model, a first order perturbation theory inversion is performed in a fully anisotropic parameterization to build a 3-D tomographic model of the upper mantle (an extended model until the transition zone is also obtained by using the overtone data). Estimates of the LAB depth are derived from the upper mantle models and compared with the predictions of oceanic lithosphere cooling models. Seismic events are simulated using the Spectral Element Method in order to validate the ability of the anisotropic tomographic model of the upper mantle to re- produce observed seismograms.

Ferropericlase inclusions in ultradeep diamonds from Sao Luiz (Brazil): high Li abundances and diverse Li-isotope and trace element compositions suggest an origin from a subduction mélange

NASA Astrophysics Data System (ADS)

Seitz, Hans-Michael; Brey, Gerhard P.; Harris, Jeffrey W.; Durali-Müller, Soodabeh; Ludwig, Thomas; Höfer, Heidi E.

2018-05-01

The most remarkable feature of the inclusion suite in ultradeep alluvial and kimberlitic diamonds from Sao Luiz (Juina area in Brazil) is the enormous range in Mg# [100xMg/(Mg + Fe)] of the ferropericlases (fper). The Mg-richer ferropericlases are from the boundary to the lower mantle or from the lower mantle itself when they coexist with ringwoodite or Mg- perovskite (bridgmanite). This, however, is not an explanation for the more Fe-rich members and a lowermost mantle or a "D" layer origin has been proposed for them. Such a suggested ultra-deep origin separates the Fe-rich fper-bearing diamonds from the rest of the Sao Luiz ultradeep diamond inclusion suite, which also contains Ca-rich phases. These are now thought to have an origin in the uppermost lower mantle and in the transition zone and to belong either to a peridotitic or mafic (subducted oceanic crust) protolith lithology. We analysed a new set of more Fe-rich ferropericlase inclusions from 10 Sao Luiz ultradeep alluvial diamonds for their Li isotope composition by solution MC-ICP-MS (multi collector inductively coupled plasma mass spectrometry), their major and minor elements by EPMA (electron probe micro-analyser) and their Li-contents by SIMS (secondary ion mass spectrometry), with the aim to understand the origin of the ferropericlase protoliths. Our new data confirm the wide range of ferropericlase Mg# that were reported before and augment the known lack of correlation between major and minor elements. Four pooled ferropericlase inclusions from four diamonds provided sufficient material to determine for the first time their Li isotope composition, which ranges from δ7Li + 9.6 ‰ to -3.9 ‰. This wide Li isotopic range encompasses that of serpentinized ocean floor peridotites including rodingites and ophicarbonates, fresh and altered MORB (mid ocean ridge basalt), seafloor sediments and of eclogites. This large range in Li isotopic composition, up to 5 times higher than `primitive upper mantle' Li-abundances, and an extremely large and incoherent range in Mg# and Cr, Ni, Mn, Na contents in the ferropericlase inclusions suggests that their protoliths were members of the above lithologies. This mélange of altered rocks originally contained a variety of carbonates (calcite, magnesite, dolomite, siderite) and brucite as the secondary products in veins and as patches and Ca-rich members like rodingites and ophicarbonates. Dehydration and redox reactions during or after deep subduction into the transition zone and the upper parts of the lower mantle led to the formation of diamond and ferropericlase inclusions with variable compositions and a predominance of the Ca-rich, high-pressure silicate inclusions. We suggest that the latter originated from peridotites, mafic rocks and sedimentary rocks as redox products between calcite and SiO2.

Early and long-term mantle processing rates derived from xenon isotopes

NASA Astrophysics Data System (ADS)

Mukhopadhyay, S.; Parai, R.; Tucker, J.; Middleton, J. L.; Langmuir, C. H.

2015-12-01

Noble gases, particularly xenon (Xe), in mantle-derived basalts provide a rich portrait of mantle degassing and surface-interior volatile exchange. The combination of extinct and extant radioactive species in the I-Pu-U-Xe systems shed light on the degassing history of the early Earth throughout accretion, as well as the long-term degassing of the Earth's interior in association with plate tectonics. The ubiquitous presence of shallow-level air contamination, however, frequently obscures the mantle Xe signal. In a majority of the samples, shallow air contamination dominates the Xe budget. For example, in the gas-rich popping rock 2ΠD43, 129Xe/130Xe ratios reach 7.7±0.23 in individual step-crushes, but the bulk composition of the sample is close to air (129Xe/130Xe of 6.7). Thus, the extent of variability in mantle source Xe composition is not well-constrained. Here, we present new MORB Xe data and explore constraints placed on mantle processing rates by the Xe data. Ten step-crushes were obtained on a depleted popping glass that was sealed in ultrapure N2 after dredge retrieval from between the Kane-Atlantis Fracture Zone of the Mid Atlantic Ridge in May 2012. 9 steps yielded 129Xe/130Xe of 7.50-7.67 and one yielded 7.3. The bulk 129Xe/130Xe of the sample is 7.6, nearly identical to the estimated mantle source value of 7.7 for the sample. Hence, the sample is virtually free of shallow-level air contamination. Because sealing the sample in N2upon dredge retrieval largely eliminated air contamination, for many samples, contamination must be added after sample retrieval from the ocean bottom. Our new high-precision Xe isotopic measurements in upper mantle-derived samples provide improved constraints on the Xe isotopic composition of the mantle source. We developed a forward model of mantle volatile evolution to identify solutions that satisfy our Xe isotopic data. We find that accretion timescales of ~10±5 Myr are consistent with I-Pu-Xe constraints, and the last giant impact occurred 45-70 Myr after the start of the solar system. After the giant impact stage, the Pu-U-Xe system indicates that degassing of the planet via solid-state mantle convection and plate tectonics continued to liberate volatiles to the atmosphere and has led to between ~5-8 mantle turnovers over the age of the Earth.

Osmium Isotope Compositions of Komatiite Sources Through Time

NASA Astrophysics Data System (ADS)

Walker, R. J.

2001-12-01

Extending Os isotopic measurements to ancient plume sources may help to constrain how and when the well-documented isotopic heterogeneities in modern systems were created. Komatiites and picrites associated with plume-related volcanism are valuable tracers of the Os isotopic composition of plumes because of their typically high Os concentrations and relatively low Re/Os. Re-Os data are now available for a variety of Phanerozoic, Proterozoic and Archean komatiites and picrites. As with modern plumes, the sources of Archean and Proterozoic komatiites exhibit a large range of initial 187Os/188Os ratios. Most komatiites are dominated by sources with chondritic Os isotopic compositions (e.g. Song La; Norseman-Wiluna; Pyke Hill; Alexo), though some (e.g. Gorgona) derive from heterogeneous sources. Of note, however, two ca. 2.7 Ga systems, Kostomuksha (Russia) and Belingwe (Zimbabwe), have initial ratios enriched by 2-3% relative to the contemporary convecting upper mantle. These results suggest that if the 187Os enrichment was due to the incorporation of minor amounts of recycled crust into the mantle source of the rocks, the crust formed very early in Earth history. Thus, the Os results could reflect derivation of melt from hybrid mantle whose composition was modified by the addition of mafic crustal material that would most likely have formed between 4.2 and 4.5 Ga. Alternately, the mantle sources of these komatiites may have derived a portion of their Os from the putative 187Os - and 186Os -enriched outer core. For this hypothesis to be applicable to Archean rocks, an inner core of sufficient mass would have to have crystallized sufficiently early in Earth history to generate an outer core with 187Os enriched by at least 3% relative to the chondritic average. Using the Pt-Re-Os partition coefficients espoused by our earlier work, and assuming linear growth of the inner core started at 4.5 Ga and continued to present, would yield an outer core at 2.7 Ga with a gamma Os value of only +1.2 and a 186Os/188Os enrichment relative to the contemporary upper mantle of only +13 ppm. Greater isotopic enrichments could have been achieved by 2.7 Ga if either the inner core comprised >2.8% of the mass of the core by 2.7 Ga, or if Re and Os solid metal-liquid metal D's for core crystallization were greater that those applied in the initial calculation.

Subducting Slabs: Jellyfishes in the Earth's Mantle

NASA Astrophysics Data System (ADS)

Loiselet, C.; Braun, J.; Husson, L.; Le Carlier de Veslud, C.; Thieulot, C.; Yamato, P.; Grujic, D.

2010-12-01

The constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations.

Subducting slabs: Jellyfishes in the Earth's mantle

NASA Astrophysics Data System (ADS)

Loiselet, Christelle; Braun, Jean; Husson, Laurent; Le Carlier de Veslud, Christian; Thieulot, Cedric; Yamato, Philippe; Grujic, Djordje

2010-08-01

The constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations.

Three Dimensional Viscoelastic Postseismic Deformation of the 2013 Mw8.3 Okhotsk Deep-Focus Earthquake

NASA Astrophysics Data System (ADS)

Hu, Y.; Burgmann, R.; Shestakov, N.; Titkov, N. N.; Serovetnikov, S.; Prytkov, A.; Vasilenko, N. F.; Wang, K.

2016-12-01

The upper mantle rheology at depths within a few hundred kilometers has been well studied through shallow great megathrust earthquakes. However, understanding of the mantle rheology at greater depths, such as in the vicinity of the transition zone, has been limited by the lack of direct or indirect measurements. The largest well-recorded deep earthquake with magnitude Mw 8.3 occurred within the subducting Pacific plate at 600 km depth beneath the Okhotsk Sea on May 24, 2013. Twenty-seven continuous GPS stations in this region recorded coseismic displacements of up to 15 mm in the horizontal direction and up to 20 mm in the vertical direction. Within three years after the earthquake seventeen continuous GPS stations underwent transient westward motion of up to 8 mm/yr and vertical motion of up to 10 mm/yr. The geodetically delineated postseismic crustal deformation thus provides a unique opportunity to study the three dimensional heterogeneity of the mantle rheology and properties of the subducting slab at great depths. We have developed three-dimensional viscoelastic finite element models of the 2013 Okhotsk earthquake to explore these questions. Our initial model includes an elastic lithosphere including the subducting slab, a viscoelastic continental upper mantle and a viscoelastic oceanic upper mantle. We assume that the upper mantle is characterized by a bi-viscous Burgers rheology. For simplicity, we assume that the transient Kelvin viscosity is one order of magnitude lower than that of the steady-state Maxwell viscosity. Our preliminary models indicate that the viscosity of the upper mantle beneath the transition zone has to be at least one order of magnitude lower than that of the upper mantle at shallower depths. A viscoelastic subducting slab at depths >400 km with viscosities of 2-3 orders of magnitude higher than that of the mantle wedge provides a better fit to the observed surface velocities.

Seismic Velocity Gradients Across the Transition Zone

NASA Astrophysics Data System (ADS)

Escalante, C.; Cammarano, F.; de Koker, N.; Piazzoni, A.; Wang, Y.; Marone, F.; Dalton, C.; Romanowicz, B.

2006-12-01

One-D elastic velocity models derived from mineral physics do a notoriously poor job at predicting the velocity gradients in the upper mantle transition zone, as well as some other features of models derived from seismological data. During the 2006 CIDER summer program, we computed Vs and Vp velocity profiles in the upper mantle based on three different mineral physics approaches: two approaches based on the minimization of Gibbs Free Energy (Stixrude and Lithgow-Bertelloni, 2005; Piazzoni et al., 2006) and one obtained by using experimentally determined phase diagrams (Weidner and Wang, 1998). The profiles were compared by assuming a vertical temperature profile and two end-member compositional models, the pyrolite model of Ringwood (1979) and the piclogite model of Anderson and Bass (1984). The predicted seismic profiles, which are significantly different from each other, primarily due to different choices of properties of single minerals and their extrapolation with temperature, are tested against a global dataset of P and S travel times and spheroidal and toroidal normal mode eigenfrequencies. All the models derived using a potential temperature of 1600K predict seismic velocities that are too slow in the upper mantle, suggesting the need to use a colder geotherm. The velocity gradient in the transition zone is somewhat better for piclogite than for pyrolite, possibly indicating the need to increase Ca content. The presence of stagnant slabs in the transition zone is a possible explanation for the need for 1) colder temperature and 2) increased Ca content. Future improvements in seismic profiles obtained from mineral physics will arise from better knowledge of elastic properties of upper mantle constituents and aggregates at high temperature and pressure, a better understanding of differences between thermodynamic models, and possibly the effect of water through and on Q. High resolution seismic constraints on velocity jumps at 400 and 660 km also need to be included. earth.org/2006/workshop.html

Noble gas isotopes and halogens in volatile-rich inclusions in diamonds

NASA Technical Reports Server (NTRS)

Burgess, Raymond; Turner, Grenville

1994-01-01

Application of the (40)Ar-(39)Ar method and noble gas studies to diamonds has increased our understanding of their age relationships to the host kimberlite or lamproite, and of the source and composition of volatile-rich fluids in the upper mantle. The properties of diamond (inert, high mechanical strength and low gas diffusivities) means they are especially useful samples for studying gases trapped deep within the earth (less than 150 km) as they are unlikely to have undergone loss or exchange of entrapped material since formation. Volatile-rich fluids (H2O-CO2) are important agents for metasomatic processes in the upper mantle, and the noble gases and halogens preferentially partition into this phase leading to a strong geochemical coherence between these groups of elements. The abundances of the halogens in the major reservoirs of the Earth shows a marked progression from chlorine, concentrated in the oceans, through to iodine which, through its affinity to organic material, is concentrated mainly in sediments. Abundances in the upper mantle are low. This is particularly true for iodine which is of special interest in view of its potential significance as an indicator of sediment recycling and by way of its link to (129)Xe amomalies in the mantle through the low extinct isotope (129)I. Extensions of the (40)Ar-(39)Ar technique enable measurements of halogens and other elements (K, Ca, Ba, U) by production of noble gas isotopes from these species during neutron irradiation. Samples analyzed in this way include 15 coated stones from an unknown source in Zaire, 3 boarts from the Jwaneng and 1 boart from the Orapa kimberlites, both in Botswana.

Can lower mantle slab-like seismic anomalies be explained by thermal coupling between the upper and lower mantles?

NASA Astrophysics Data System (ADS)

Čížková, Hana; Čadek, Ondřej; van den Berg, Arie P.; Vlaar, Nicolaas J.

Below subduction zones, high resolution seismic tomographic models resolve fast anomalies that often extend into the deep lower mantle. These anomalies are generally interpreted as slabs penetrating through the 660-km seismic discontinuity, evidence in support of whole-mantle convection. However, thermal coupling between two flow systems separated by an impermeable interface might provide an alternative explanation of the tomographic results. We have tested this hypothesis within the context of an axisymmetric model of mantle convection in which an impermeable boundary is imposed at a depth of 660 km. When an increase in viscosity alone is imposed across the impermeable interface, our results demonstrate the dominant role of mechanical coupling between shells, producing lower mantle upwellings (downwellings) below upper mantle downwellings (upwellings). However, we find that the effect of mechanical coupling can be significantly weakened if a narrow low viscosity zone exists beneath the 660-km discontinuity. In such a case, both thermally induced ‘slabs’ in the lower mantle and thermally activated plumes that rise from the upper/lower mantle boundary are observed even though mass transfer between the shells does not exist.

Magma source transition of lunar mare volcanism at 2.3 Ga

NASA Astrophysics Data System (ADS)

Kato, Shinsuke; Morota, Tomokatsu; Yamaguchi, Yasushi; Watanabe, Sei-Ichiro; Otake, Hisashi; Ohtake, Makiko

2017-09-01

Mare basalts provide insights into the composition and thermal history of the lunar mantle. The ages of mare basalts suggest a first peak of magma activity at 3.2-3.8 Ga and a second peak at 2 Ga. In this study, we reassess the correlation between the titanium contents and the eruption ages of mare basalt units using the compositional and chronological data updated by SELENE (Kaguya). Using morphological and geological criteria, we calculated the titanium content of 261 mare units across a representative area of each mare unit. In the Procellarum KREEP Terrane, where the latest eruptions are located, an increase in the mean titanium content is observed during the Eratosthenian period, as reported by previous studies. We found that the increase in the mean titanium content occurred within a relatively short period near approximately 2.3 Ga, suggesting that the magma source of the mare basalts changed at this particular age. Moreover, the high-titanium basaltic eruptions are correlated with a second peak in volcanic activity near 2 Ga. The high-titanium basaltic eruptions occurring during the last volcanic activity period can be explained by the three possible scenarios (1) the ilmenite-bearing cumulate rich layer in the core-mantle boundary formed after the mantle overturn, (2) the basaltic material layers beneath the lunar crust formed through upwelling magmas, and (3) ilmenite-bearing cumulate blocks remained in the upper mantle after the mantle overturn.

Water partitioning in the Earth's mantle

NASA Astrophysics Data System (ADS)

Inoue, Toru; Wada, Tomoyuki; Sasaki, Rumi; Yurimoto, Hisayoshi

2010-11-01

We have conducted H2O partitioning experiments between wadsleyite and ringwoodite and between ringwoodite and perovskite at 1673 K and 1873 K, respectively. These experiments were performed in order to constrain the relative distribution of H2O in the upper mantle, the mantle transition zone, and the lower mantle. We successfully synthesized coexisting mineral assemblages of wadsleyite-ringwoodite and ringwoodite-perovskite that were large enough to measure the H2O contents by secondary ion mass spectrometry (SIMS). Combining our previous H2O partitioning data (Chen et al., 2002) with the present results, the determined water partitioning between olivine, wadsleyite, ringwoodite, and perovskite under H2O-rich fluid saturated conditions are 6:30:15:1, respectively. Because the maximum H2O storage capacity in wadsleyite is ∼3.3 wt% (e.g. Inoue et al., 1995), the possible maximum H2O storage capacity in the olivine high-pressure polymorphs are as follows: ∼0.7 wt% in olivine (upper mantle just above 410 km depth), ∼3.3 wt% in wadsleyite (410-520 km depth), ∼1.7 wt% in ringwoodite (520-660 km depth), and ∼0.1 wt% in perovskite (lower mantle). If we assume ∼0.2 wt% of the H2O content in wadsleyite in the mantle transition zone estimated by recent electrical conductivity measurements (e.g. Dai and Karato, 2009), the estimated H2O contents throughout the mantle are as follows; ∼0.04 wt% in olivine (upper mantle just above 410 km depth), ∼0.2 wt% in wadsleyite (410-520 km depth), ∼0.1 wt% in ringwoodite (520-660 km depth) and ∼0.007 wt% in perovskite (lower mantle). Thus, the mantle transition zone should contain a large water reservoir in the Earth's mantle compared to the upper mantle and the lower mantle.

Acquisition and Early Losses of Rare Gases from the Deep Earth

NASA Technical Reports Server (NTRS)

Porcelli, D.; Cassen, P.; Woolum, D.; Wasserburg, G. J.

1998-01-01

Direct observations show that the deep Earth contains rare gases of solar composition distinct from those in the atmosphere. We examine the implications of mantle rare gas characteristics on acquisition of rare gases from the solar nebula and subsequent losses due to a large impact. Deep mantle rare gas concentrations and isotopic compositions can be obtained from a model of transport and distribution of mantle rare gases. This model assumes the lower mantle closed early, while the upper mantle is open to subduction from the atmosphere and mass transfer from the lower mantle. Constraints are derived that can be incorporated into models for terrestrial volatile acquisition: (1) Calculated lower-mantle Xe-isotopic ratios indicate that the fraction of radiogenic Xe produced by I-129 and Pu-244 during the first about 10(exp 8) yr was lost, a conclusion also drawn for atmospheric Xe. Thus, either the Earth was made from materials that had lost >99% of rare gases about (0.7-2) x 10(exp 8) yr after the solar system formed, or gases were then lost from the fully formed Earth. (2) Concentrations of 3He and 20Ne in the lower mantle were established after these losses. (3) Neon-isotopic data indicates that mantle Ne has solar composition. The model allows for solar Ar/Ne and Xe/Ne in the lower mantle if a dominant fraction of upper mantle Ar and Xe are subduction-derived. If Earth formed in the presence of the solar nebula, it could have been melted by accretional energy and the blanketing effect of a massive, nebula-derived atmosphere. Gases from this atmosphere would have been sequestered within the molten Earth by dissolution at the surface and downward mixing. It was found that too much Ne would be dissolved in the Earth unless the atmosphere began to escape when the Earth was only partially assembled. Here we consider conditions required to initially dissolve sufficient rare gases to account for the present lower mantle concentrations after subsequent losses at 10(exp 8) yr. It is assumed that equilibration of the atmosphere with a thoroughly molten mantle was rapid, so that initial abundances of gases retained in any mantle layer reflected surface conditions when the layer solidified. For subsequent gas loss of 99.5% and typical solubility coefficients, a total pressure of 100 atm was required for an atmosphere of solar composition. Calculations of the pressure at the base of a primordial atmosphere indicate that this value might be exceeded by an order of magnitude or more for an atmosphere supported by accretional energy. Surface temperatures of about 4000 K would have been produced, probably high enough to melt the deep mantle. Initial distributions of retained rare gases would then be determined by the history of surface pressure and temperature during mantle cooling and solidification, i.e., the coupled cooling of Earth and atmosphere. The Earth's thermal state was determined by its surface temperature and the efficiency of convection in the molten mantle, estimated to be sufficient to maintain an adiabatic gradient. Because the melting curve is steeper than the adiabat, solidification of the mantle proceeded outward from the interior. Incorporation of atmospheric gases in the mantle therefore occurred over a range in surface temperature of a few thousand degrees Kelvin. The thermal state of the atmosphere was controlled by total luminosity of the Earth (energy) released by accreting planetesimals and the cooling Earth), nebular temperature and pressure, and atmospheric opacity. The energy released by accretion declined with time as did nebular pressure. Analytical solutions for an idealized (constant opacity radiative atmosphere show that declining energy sources under constant nebular conditions result in slowly diminishing surface temperature but dramatically increasing surface pressure. For such an atmosphere with declining nebular pressure but constant total luminosity, surface pressure decreases gradually with decreasing temperaure. A decline in accretion luminosity might be compensated by energy released as the mantle cools for about 10(exp 5) year, after which luminosity must decline. The total complement of dissolved rare gases will depend on the particular evolutionary path determined by the declining accretional luminosity, the Earth thermal history, removal of the nebula, and opacity variations of the atmosphere. Models for these coupled evolutionary histories for Earth's acquisition of nebular-derived noble gases are in progress. The later losses required at about 10(exp 8) yr (depleting the interior concentrations of the sequestered solar gases by a factor of > 100) were presumably related to the major impact in which the Moon formed.

Compositional and isotopic heterogeneities in the Neo-Tethyan upper mantle recorded by coexisting Al-rich and Cr-rich chromitites in the Purang peridotite massif, SW Tibet (China)

NASA Astrophysics Data System (ADS)

Xiong, Fahui; Yang, Jingsui; Xu, Xiangzhen; Kapsiotis, Argyrios; Hao, Xiaolin; Liu, Zhao

2018-06-01

The Purang harzburgite massif in SW Tibet (China) hosts abundant chrome ore deposits. Ores consist of 20 to >95% modal chromian spinel (Cr-spinel) with mylonitic fabric in imbricate shaped pods. The composition of Cr-spinel in these ores ranges from Al-rich [Cr#Sp or Cr/(Cr + Al) × 100 = 47.60-57.56] to Cr-rich (Cr#Sp: 62.55-79.57). Bulk platinum-group element (PGE) contents of chromitites are also highly variable ranging from 17.5 ppb to ∼2.5 ppm. Both metallurgical and refractory chromitites show a general enrichment in the IPGE (Os, Ir and Ru) with respect to the PPGE (Rh, Pt and Pd), resulting mostly in right-sloping primitive mantle (PM)-normalized PGE profiles. The platinum-group mineral (PGM) assemblages of both chromitite types are dominated by heterogeneously distributed, euhedral Os-bearing laurite inclusions in Cr-spinel. The Purang chromitites have quite inhomogeneous 187Os/188Os ratios (0.12289-0.13194) that are within the range of those reported for mantle-hosted chromitites from other peridotite massifs. Geochemical calculations demonstrate that the parental melts of high-Cr chromitites were boninitic, whereas those of high-Al chromitites had an arc-type tholeiitic affinity. Chromite crystallization was most likely stimulated by changes in magma compositions due to melt-peridotite interaction, leading to the establishment of a heterogeneous physicochemical environment during the early crystallization of the PGM. The highly variable PGE contents, inhomogeneous Os-isotopic compositions and varying Cr#Sp ratios of these chromitites imply a polygenetic origin for them from spatially distinct melt inputs. The generally low γOs values (<1) of chromitites indicate that their parental melts originated within different sections of a heterogeneously depleted mantle source region. These melts were most likely produced in the mantle wedge above a downgoing lithospheric slab.

History and evolution of Subduction in the Precambrium

NASA Astrophysics Data System (ADS)

Fischer, R.; Gerya, T.

2013-12-01

Plate tectonics is a global self-organising process driven by negative buoyancy at thermal boundary layers. Phanerozoic plate tectonics with its typical subduction and orogeny is relatively well understood and can be traced back in the geological records of the continents. Interpretations of geological, petrological and geochemical observations from Proterozoic and Archean orogenic belts however (e.g. Brown, 2006), suggest a different tectonic regime in the Precambrian. Due to higher radioactive heat production the Precambrian lithosphere shows lower internal strength and is strongly weakened by percolating melts. The fundamental difference between Precambrian and Phanerozoic subduction is therefore the upper-mantle temperature, which determines the strength of the upper mantle (Brun, 2002) and the further subduction history. 3D petrological-thermomechanical numerical modelling experiments of oceanic subduction at an active plate at different upper-mantle temperatures show these different subduction regimes. For upper-mantle temperatures < 175 K above the present day value a subduction style appears which is close to present day subduction but with more frequent slab break-off. At upper-mantle temperatures 175 - 250 K above present day values steep subduction changes to shallow underplating and buckling. For upper-mantle temperatures > 250 K above the present day value no subduction occurs any more. The whole lithosphere starts to delaminate and drip-off. But the subduction style is not only a function of upper-mantle temperature but also strongly depends on the thickness of the subducting plate. If thinner present day oceanic plates are used in the Precambrian models, no shallow underplating is observed but steep subduction can be found up to an upper-mantle temperature of 200 K above present day values. Increasing oceanic plate thickness introduces a transition from steep to flat subduction at lower temperatures of around 150 K. Thicker oceanic plates in the Precambrium also agree with results from earlier studies, e.g. Abbott (1994). References: Abbott, D., Drury, R., Smith, W.H.F., 1994. Flat to steep transition in subduction style. Geology 22, 937-940. Brown, M., 2006. Duality of thermal regimes is the distinctive characteristic of plate tectonics since the neoarchean. Geology 34, 961-964. Brun, J.P., 2002. Deformation of the continental lithosphere: Insights from brittle-ductile models. Geological Society, London, Special Publications 200, 355-370. Subduction depends strongly on upper-mantle temperature. (a) Modern subduction with present day temperature gradients in upper-mantle and lithosphere. (b) Increase of temperature by 100 K at the lithosphere-asthenosphere boundary (LAB) leads to melting and drip-off of the of the slab-tip. (c) A temperature increase of 200 K leads to buckling of the subducting slab and Rayleigh-Taylor instabilities not only at the slab-tip but the whole LAB. At this stage subduction is no longer possible as the slab melts or breaks before it can be subducted into the mantle.

Compositional Heterogeneity and Spatial Segmentation of Suprasubduction (ssz-type) Ophiolites: Evidence From The Kamchatka Arc

NASA Astrophysics Data System (ADS)

Osipenko, A.; Krylov, K.

In ophiolite complexes from the Eastern Asian accretion belts the spatial heterogeneity of geochemical parameters for different components of an ophiolite sequence is estab- lished: restite mantle-derived peridotites, cumulative layered complex and volcanics. This heterogeneity is displayed as at a regional level (tens - hundred km), and at a level of local structures (hundred i - first tens km). As a rule, distinction is observed on a complex of geochemical parameters (concentration and form of REE spectra, EPG distribution, isotope characteristics, Cr-spinel and pyroxene composition etc.). Revealed at once in several suprasubduction-type ophiolite belts (Kamuikotan, Philip- pines New Guinea etc.), the spatial variations of geochemical parameters have not gradual, and discrete character. For an explanation of the reasons of ophiolite com- positional heterogeneity several mechanisms are offered: (1) tectonical overlapping of various fragments of lithosphere; (2) different specify of deep processes, resulting to compositional heterogeneity of rocks from the same lithosphere level; 3) hetero- geneity of the upper mantle and/or mantle metasomatism; 4) evolution of ophiolites (Shervais, 2001) and/or center of magma generation (mixture of continuous series of melt portions, separated during different stages of progressive mantle source melting (Bazylev et al., 2001)); 5) preservations of relict blocks of low lithosphere and upper mantle from the previous stage in suprasubduction conditions. The authors consider regional geochemical heterogeneity and segmentation of suprasubduction ophiolites (SSZ-type) on an example of peridotites from the Eastern Kamchatka ophiolite belt (EKOB), where sublongitude zones, crossed the basic geological structures of a penin- sula (including EROB) were allocated earlier. For each of zones the complex of geo- chemical attributes, steady is established within the limits of a zone, but distinct from of the characteristics of other zones. Among the factors causing an unequal degree of partial melting of peridotites, a main role play a geothermal regime and composition of fluid phase (first of all, the role of water fluid is great). These parameters, in turn, are supervised by a geodynamic regime of magma generation (such characteristics as speed of subduction and geometry of a subducted plate) and finally determine speed of uplift from the diapir in mantle, depth of the termination of partial melting, amount of 1 extracted melt, form and capacity of the magma chamber etc. The local heterogeneity in SSZ-ophiolites is considered on an example of a complex of the Kamchatka Cape Peninsula - the largest ophiolite complex in EKOB. Isotope, geochemical and miner- alogical study have shown, that a part, prevailing on volume, of this complex consist suprasubduction-type magmatic rocks (restite high-depleted harzburgites and related layered cumulative complex), whereas peridotites of harzburgite-lherzolite series and high-grade metabasites (retrograde eclogites and garnet amphibolites) composition- ally correspond to series of N-MORB and Ò-MORB-type. The presence in ophiolite of the Kamchatka Cape Peninsula alongside with high-depleted harzburgites as well moderately- and low-depleted peridotites of harzburgite-lherzolite series allows to as- sume, that Late Mesozoic suprasubduction ophiolites were formed on peridotitic basis of abyssal type. Thus the transformation of "oceanic" substrate was not complete, that has allowed to be kept relict peridotites of lherzolitic type and high-pressure metamor- phics. Probably it reflects pulsing character of geodynamics of suprasubduction-type ophiolite formation, it is possible is connected with "jumping" of spreading axes in suprasubduction conditions. During followed multistage napping in a northeast direc- tion in the Upper Cretaceous time disintegrated fragments of both mantle complexes were tectonically concurrent. In the report the alternative versions of tectonic models of development are also discussed for the Eastern Kamchatka ophiolites. 2

Continent-Wide Maps of Lg Coda Q Variation and Rayleigh-wave Attenuation Variation for Eurasia

DTIC Science & Technology

2007-01-30

lithosphere and crustal strain lead us to infer that fluids, originating by hydrothermal release from subducting lithosphere or other upper mantle heat...relatively low Qo values in the Arabian Peninsula are produced by fluids that have been released in the upper mantle by hydrothermal processes and have...Advection of plumes in mantle flow: Implications for hotspot motion, mantle viscosity and plume distribution, Geophys. J. Int., 132, 412–434. Talebian, M

Experimental determination of carbon solubility in Fe-Ni-S melts

NASA Astrophysics Data System (ADS)

Zhang, Zhou; Hastings, Patrick; Von der Handt, Anette; Hirschmann, Marc M.

2018-03-01

To investigate the effect of metal/sulfide and Ni/Fe ratio on the C storage capacity of sulfide melts, we determine carbon solubility in Fe-Ni-S melts with various (Fe + Ni)/S and Ni/Fe via graphite-saturated high-pressure experiments from 2-7 GPa and 1200-1600 °C. Consistent with previous results, C solubility is high (4-6 wt.%) in metal-rich sulfide melts and diminishes with increasing S content. Melts with near M/S = 1 (XS > 0.4) have <0.5 wt.% C in equilibrium with graphite. C solubility is diminished modestly with increased Ni/Fe ratio, but the effect is most pronounced for S-poor melts, and becomes negligible in near-monosulfide compositions. Immiscibility between S-rich and C-rich melts is observed in Ni-poor compositions, but above ∼18 wt.% Ni there is complete miscibility. Because mantle sulfide compositions are expected to have high Ni concentrations, sulfide-carbide immiscibility is unlikely in natural mantle melts. An empirical parameterization of C solubility in Ni-Fe-S melts as a function of S and Ni contents allows estimation of the C storage capacity of sulfide in the mantle. Importantly, as the metal/sulfide (M/S) ratio of the melt increases, C storage increases both because C solubility increases and because the mass fraction of melt is enhanced by addition of metal from surrounding silicates. Under comparatively oxidized conditions where melts are near M/S = 1, as prevails at <250 km depth, bulk C storage is <3 ppm. In the deeper, more reduced mantle where M/S increases, up to 200 ppm C in typical mantle with 200 ± 100 ppm S can be stored in Fe-Ni-S melts. Thus, metal-rich sulfide melts are the principal host of carbon in the deep upper mantle and below. Residual carbon is present either as diamond or, if conditions are highly reduced and total C concentrations are low, solid alloy.

Calcium Isotopic Compositions of Normal Mid-Ocean Ridge Basalts From the Southern Juan de Fuca Ridge

NASA Astrophysics Data System (ADS)

Zhu, Hongli; Liu, Fang; Li, Xin; Wang, Guiqin; Zhang, Zhaofeng; Sun, Weidong

2018-02-01

Mantle peridotites show that Ca is isotopically heterogeneous in Earth's mantle, but the mechanism for such heterogeneity remains obscure. To investigate the effect of partial melting on Ca isotopic fractionation and the mechanism for Ca isotopic heterogeneity in the mantle, we report high-precision Ca isotopic compositions of the normal Mid-Ocean Ridge Basalts (N-MORB) from the southern Juan de Fuca Ridge. δ44/40Ca of these N-MORB samples display a small variation ranging from 0.75 ± 0.05 to 0.86 ± 0.03‰ (relative to NIST SRM 915a, a standard reference material produced by the National Institute of Standards and Technology), which are slightly lower than the estimated Upper Mantle value of 1.05 ± 0.04‰ and the Bulk Silicate Earth (BSE) value of 0.94 ± 0.05‰. This phenomenon cannot be explained by fractional crystallization, because olivine and orthopyroxene fractional crystallization has limited influence on δ44/40Ca of N-MORB due to their low CaO contents, while plagioclase fractional crystallization cannot lead to light Ca isotopic compositions of the residue magma. Instead, the lower δ44/40Ca of N-MORB samples compared to their mantle source is most likely caused by partial melting. The offset in δ44/40Ca between N-MORB and BSE indicates that at least 0.1-0.2‰ fractionation would occur during partial melting and light Ca isotopes are preferred to be enriched in magma melt, which is in accordance with the fact that δ44/40Ca of melt-depleted peridotites are higher than fertile peridotites in literature. Therefore, partial melting is an important process that can decrease δ44/40Ca in basalts and induce Ca isotopic heterogeneity in Earth's mantle.

Evolution of the Craters of the Moon Lavas from primitive Snake River Plain basalts: inferences from plagioclase-melt thermobarometers and whole rock compositions

NASA Astrophysics Data System (ADS)

Vaid, N.; Putirka, K.; Kuntz, M.

2005-12-01

The volcanic rocks of the Craters of the Mon Lava field provide an ideal laboratory for testing models of magma transport and evolution. Their compositions, relative ages and volumes are well known, as are the fractionation processes leading to their evolution (Leeman, 1976). The COM is somewhat distinctive in the Snake River Plain (SRP) region, due to its evolved character, and an apparent compositional segregation from associated SRP basalts. Some have suggested that the high Fe liquids of the COM demand an origin separate from that of SRP basalts, possibly involving an Fe-enriched mantle, while others have suggested that the COM lavas may be derived by fractionation at moderate depths (30 km). In either case, there are important implications in regard to mantle composition and the nature and distribution of thermal energy. We use plagioclase-melt pairs and an analysis of whole rock compositions in attempt to test models of COM magmatic evolution. Plagioclase-melt thermobarometers provide rough estimates of crystallization depths, and show that COM and SRP lavas partially crystallized at similar depths of 14 +/- 6 km. However, plagioclase crystallization temperatures for SRP basalts (1400 +/- 25 K; Kings Bowl, Cerro Grande, North and South Robbers) exceed temperatures for COM lavas (1358 +/- 45 K) by 40 K. Our data also show that fractional crystallization (ol + plag) can explain the evolution of surrounding SRP basalt flows, and that the most evolved SRP basalts approach primitive COM lava compositions. The most primitive of COM magmas appear to be characterized by the appearance of apatite + magnetite as fractionating phases. Our results thus confirm the geochemical model of Leeman (1976) and the physical model of Kuntz (1992), with the added insight that SRP basalts are parental to the more evolved COM lavas, through low-pressure fractional crystallization in the upper crust. The principal differences between SRP and COM magmas appear to relate more to the presence or absence of density contrasts in the crust than differences in composition or temperature of mantle source materials. SRP basalts lie near the axis of the SRP where the granitic upper crust may have been obliterated by earlier volcanic episodes. In contrast, COM lavas, whose vents lie off axis, appear to have been trapped within the upper crust for longer periods, sufficient for further differentiation. Finally, SRP rhyolite compositions lie on the same fractionation trend as COM and SRP lavas, at very low values of MgO. We propose that highly evolved lavas throughout the SRP may form by fractional crystallization mechanisms alone, rather than through the partial melting and remobilization of preexisting felsic crustal materials.

Plate and Plume Flux: Constraints for paleomagnetic reference frames and interpretation of deep mantle seismic heterogeneity. (Invited)

NASA Astrophysics Data System (ADS)

Bunge, H.; Schuberth, B. S.; Shephard, G. E.; Müller, D.

2010-12-01

Plate and plume flow are dominant modes of mantle convection, as pointed out by Geoff Davies early on. Driven, respectively, from a cold upper and a hot lower thermal boundary layer these modes are now sufficiently well imaged by seismic tomographers to exploit the thermal boundary layer concept as an effective tool in exploring two long standing geodynamic problems. One relates to the choice of an absolute reference frame in plate tectonic reconstructions. Several absolute reference frames have been proposed over the last decade, including those based on hotspot tracks displaying age progression and assuming either fixity or motion, as well as palaeomagnetically-based reference frames, a subduction reference frame and hybrid versions. Each reference frame implies a particular history of the location of subduction zones through time and thus the evolution of mantle heterogeneity via mixing of subducted slab material in the mantle. Here we compare five alternative absolute plate motion models in terms of their consequences for deep mantle structure. Taking global paleo-plate boundaries and plate velocities back to 140 Ma derived from the new plate tectonic reconstruction software GPlates and assimilating them into vigorous 3-D spherical mantle circulation models, we infer geodynamic mantle heterogeneity and compare it to seismic tomography for each absolute rotation model. We also focus on the challenging problem of interpreting deep mantle seismic heterogeneity in terms of thermal and compositional variations. Using published thermodynamically self-consistent mantle mineralogy models in the pyrolite composition, we find strong plume flux from the CMB, with a high temperature contrast (on the order of 1000 K) across the lower thermal boundary layer is entirely sufficient to explain elastic heterogeneity in the deep mantle for a number of quantitative measures. A high excess temperatures of +1000--1500 K for plumes in the lowermost mantle is particularly important in understanding the strong seismic velocity reduction mapped by tomography in low-velocity bodies of the deep mantle, as this produces significant negative anomalies of shear wave velocity of up to -4%. We note, however, that our results do not account for the curious observation of seismic anti-correlation, which appears difficult to explain in any case. Our results provide important constraints for the integration of plate tectonics and mantle dynamics and their use in forward and inverse geodynamic mantle models.

Experimental constraints on mantle metasomatism caused by silicate and carbonate melts

NASA Astrophysics Data System (ADS)

Gervasoni, Fernanda; Klemme, Stephan; Rohrbach, Arno; Grützner, Tobias; Berndt, Jasper

2017-06-01

Metasomatic processes are responsible for many of the heterogeneities found in the upper mantle. To better understand the metasomatism in the lithospheric mantle and to illustrate the differences between metasomatism caused by hydrous silicate and carbonate-rich melts, we performed various interaction experiments: (1) Reactions between hydrous eclogite-derived melts and peridotite at 2.2-2.5 GPa and 900-1000 °C reproduce the metasomatism in the mantle wedge above subduction zones. (2) Reactions between carbonate-rich melts and peridotite at 2.5 GPa and 1050-1000 °C, and at 6 GPa and 1200-1250 °C simulate metasomatism of carbonatite and ultramafic silicate-carbonate melts in different regions of cratonic lithosphere. Our experimental results show that partial melting of hydrous eclogite produces hydrous Si- and Al-rich melts that react with peridotite and form bi-mineralic assemblages of Al-rich orthopyroxene and Mg-rich amphibole. We also found that carbonate-rich melts with different compositions react with peridotite and form new metasomatic wehrlitic mineral assemblages. Metasomatic reactions caused by Ca-rich carbonatite melt consume the primary peridotite and produce large amounts of metasomatic clinopyroxene; on the other hand, metasomatism caused by ultramafic silicate-carbonate melts produces less clinopyroxene. Furthermore, our experiments show that ultramafic silicate-carbonate melts react strongly with peridotite and cause crystallization of large amounts of metasomatic Fe-Ti oxides. The reactions of metasomatic melts with peridotite also change the melt composition. For instance, if the carbonatite melt is not entirely consumed during the metasomatic reactions, its melt composition may change dramatically, generating an alkali-rich carbonated silicate melt that is similar in composition to type I kimberlites.

Effects of mother lode-type gold mineralization on 187Os/188Os and platinum group element concentrations in peridotite: Alleghany District, California

USGS Publications Warehouse

Walker, R.J.; Böhlke, J.K.; McDonough, W.F.; Li, Ji

2007-01-01

Osmium isotope compositions and concentrations of Re, platinum group elements (PGE), and Au were determined for host peridotites (serpentinites and barzburgites) and hydrothermally altered ultramafic wall rocks associated with Mother Lode-type hydrothermal gold-quartz vein mineralization in the Alleghany district, California. The host peridotites have Os isotope compositions and Re, PGE, and Au abundances typical of the upper mantle at their presumed formation age during the late Proterozoic or early Paleozoic. The hydrothermally altered rocks have highly variable initial Os isotope compositions with ??os, values (% deviation of 187OS/188OS from the chondritic average calculated for the approx. 120 Ma time of mineralization) ranging from -1.4 to -8.3. The lowest Os isotope compositions are consistent with Re depletion of a chondritic source (e.g., the upper mantle) at ca. 1.6 Ga. Most of the altered samples are enriched in Au and have depleted and fractionated abundances of Re and PGE relative to their precursor peridotites. Geoehemical characteristics of the altered samples suggest that Re and some PGE were variably removed from the ultramafic rocks during the mineralization event. In addition to Re, the Pt and Pd abundances of the most intensely altered rocks appear to have been most affected by mineralization. The 187Os-depleted isotopic compositions of some altered rocks are interpreted to be a result of preferential 187Os loss via destruction of Re-rich phases during the event. For these rocks, Os evidently is not a useful tracer of the mineralizing fluids. The results do, however, provide evidence for differential mobility of these elements, and mobility of 187Os relative to the initial bulk Os isotope composition during hydrothermal metasomatic alteration of ultramafic rocks. ?? 2007 Society of Economic Geologists, Inc.

What is the iron isotope composition of the Moon?

NASA Astrophysics Data System (ADS)

Poitrasson, F.; Zambardi, T.; Magna, T.; Neal, C. R.

2016-12-01

It is difficult to estimate the bulk chemical and isotopic composition of the Moon because of severe limitations in our sampling. As a result, there is currently a debate on the bulk Fe isotope composition of the Moon despite the constraints on the lunar accretion modes or differentiation processes it may provide. For this, a proper mass balance estimation of essential planetary reservoirs is required. For instance, the dichotomy in δ57Fe between low- and high-Ti mare basalt varieties as a consequence of differences in degree of fractional crystallization of their respective lunar mantle sources should be rigorously tested. To investigate this, we performed new iron isotope measurements of 33 bulk lunar mare basalts and highland rocks, including KREEP-related materials. The new data show significant Fe isotope differences between high-Ti and low-Ti mare basalts, yielding mean δ57FeIRMM-014=0.277±0.020‰ and δ57FeIRMM-014=0.127±0.020‰, respectively. Assuming that lunar basalts mirror the iron isotope composition of their respective mantle protoliths, the estimated relative proportion of the low-Ti and high-Ti mantle source suggests that the lunar upper mantle should be close to δ57Fe=0.14±0.03‰. At present, it is unclear whether the bulk lunar Fe isotope composition is indistinguishable from that of the Earth (δ57FeIRMM-014=0.10±0.03‰), when estimated solely from mare basalts data, or if it is twice as heavy relative to chondrites, as initially proposed. A large scatter at δ57Fe=0.08±0.19‰ for ferroan anorthosites, Mg-suite rocks and a KREEP basalt imparts more complexities for global isotopic view of the Moon. A better understanding of the cause of Fe isotope heterogeneity among the lunar highland rocks will likely allow to better estimate the bulk Moon composition, and possibly to improve our knowledge about the genesis of the lunar crust itself.

Looking at the roots of the highest mountains: the lithospheric structure of the Himalaya-Tibet and the Zagros orogens. Results from a geophysical-petrological study

NASA Astrophysics Data System (ADS)

Tunini, L.; Jimenez-Munt, I.; Fernandez, M.; Villasenor, A.; Afonso, J. C.; Verges, J.

2013-12-01

The Himalaya-Tibet and Zagros orogens are the two most prominent mountain belts built by continental collision. They are part of a huge belt of Cenozoic age which runs from the Pyrenees to Burma. In its central sector, the collision with the southern margin of the Eurasian plate has resulted not only in the building of mountain ranges over the north-eastern edges of the Arabian and Indian plates but also in widespread deformation 1000-3000 km from the suture zones. Zagros and Himalaya-Tibet orogens share many geodynamic processes but at different rates, amount of convergence and stage of development. The study of their present-day structures provides new insights into their quasi coeval collisional event pointing out differences and similarities in the mountain building processes. We present 2D crust and upper mantle cross-sections down to 400 km depth, along four SW-NE trending profiles. Two profiles cross the Zagros Mountains, running from the Mesopotamian Foreland Basin up to the Alborz and Central Iran. Two other profiles run through the Himalaya-Tibetan orogen: the western transect crosses the western Himalaya, Tarim Basin, Tian Shan Mountains and Junggar Basin; the eastern transect runs from the Indian shield to the Beishan Basin, crossing the eastern Himalaya, Tibetan Plateau, Qaidam Basin and Qilian Mountains. We apply the LitMod-2D code which integrates potential fields (gravity and geoid), isostasy (elevation) and thermal (heat flow and temperature distribution) equations, and mantle petrology. The resulting crust and upper mantle structure is constrained by available data on elevation, Bouguer anomaly, geoid height, surface heat flow and seismic data including P- and S-wave tomography models. Our results show distinct deformation patterns between the crust and the lithospheric mantle beneath the Zagros and Himalaya-Tibetan orogens, indicating a strong strain partitioning in both areas. At crustal level, we found a thickening beneath the Zagros and the Alborz ranges, more pronounced in the southern profile. At sub-crustal level, a lithospheric mantle thinning affects the whole area beneath the Zagros range extending to the north through the zone below the Alborz and the central Iran. In the Himalaya-Tibet region our results show stronger strain partitioning in the horizontal (east-west) direction than in the vertical (depth) direction. At crustal level, the Tibetan Plateau extends more than 1000 km in the eastern profile, whereas it is squeezed between the Himalayan Mountains and the Tarim Basin along the western profile (~600 km). At sub-crustal level, the lithospheric mantle is more homogeneous in thickness and mineral composition along the western profile than the eastern one. Finally, our results on mineral composition show that both collisional regions are characterised by a predominant lherzolitic lithospheric mantle, whereas we observe compositional variations around the suture zones, probably related to subduction and mantle delamination processes.

Upper-mantle origin of the Yellowstone hotspot

USGS Publications Warehouse

Christiansen, R.L.; Foulger, G.R.; Evans, J.R.

2002-01-01

Fundamental features of the geology and tectonic setting of the northeast-propagating Yellowstone hotspot are not explained by a simple deep-mantle plume hypothesis and, within that framework, must be attributed to coincidence or be explained by auxiliary hypotheses. These features include the persistence of basaltic magmatism along the hotspot track, the origin of the hotspot during a regional middle Miocene tectonic reorganization, a similar and coeval zone of northwestward magmatic propagation, the occurrence of both zones of magmatic propagation along a first-order tectonic boundary, and control of the hotspot track by preexisting structures. Seismic imaging provides no evidence for, and several contraindications of, a vertically extensive plume-like structure beneath Yellowstone or a broad trailing plume head beneath the eastern Snake River Plain. The high helium isotope ratios observed at Yellowstone and other hotspots are commonly assumed to arise from the lower mantle, but upper-mantle processes can explain the observations. The available evidence thus renders an upper-mantle origin for the Yellowstone system the preferred model; there is no evidence that the system extends deeper than ???200 km, and some evidence that it does not. A model whereby the Yellowstone system reflects feedback between upper-mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep-mantle plume hypothesis.

Isotopic composition of Mg and Fe in garnet peridotites from the Kaapvaal and Siberian cratons

NASA Astrophysics Data System (ADS)

An, Yajun; Huang, Jin-Xiang; Griffin, W. L.; Liu, Chuanzhou; Huang, Fang

2017-03-01

We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in δ26Mg and δ56Fe of garnet peridotites from these two cratons. δ26Mg of whole rocks varies from -0.243‰ to -0.204‰ with an average of -0.225 ± 0.037‰ (2σ, n = 19), and δ56Fe from -0.038‰ to 0.060‰ with an average of -0.003 ± 0.068‰ (2σ, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show δ26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (⩽±0.05‰ for δ26Mg and δ56Fe, 2σ) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest δ26Mg and δ56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy δ26Mg and much lighter δ56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.

40K-(40)Ar constraints on recycling continental crust into the mantle

PubMed

Coltice; Albarede; Gillet

2000-05-05

Extraction of potassium into magmas and outgassing of argon during melting constrain the relative amounts of potassium in the crust with respect to those of argon in the atmosphere. No more than 30% of the modern mass of the continents was subducted back into the mantle during Earth's history. It is estimated that 50 to 70% of the subducted sediments are reincorporated into the deep continental crust. A consequence of the limited exchange between the continental crust and the upper mantle is that the chemistry of the upper mantle is driven by exchange of material with the deep mantle.

Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth’s upper mantle

PubMed Central

Ohuchi, Tomohiro; Kawazoe, Takaaki; Higo, Yuji; Funakoshi, Ken-ichi; Suzuki, Akio; Kikegawa, Takumi; Irifune, Tetsuo

2015-01-01

Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth’s upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 1019.6 to 1020.7 Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size–sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle. PMID:26601281

Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth's upper mantle.

PubMed

Ohuchi, Tomohiro; Kawazoe, Takaaki; Higo, Yuji; Funakoshi, Ken-Ichi; Suzuki, Akio; Kikegawa, Takumi; Irifune, Tetsuo

2015-10-01

Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth's upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 10(19.6) to 10(20.7) Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size-sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle.

Gravity field over northern Eurasia and variations in the strength of the upper mantle

NASA Technical Reports Server (NTRS)

Kogan, Mikhail G.; Mcnutt, Marcia K.

1993-01-01

The correlation of long-wavelength gravity anomalies in northern Eurasia with seismic velocity anomalies in the upper mantle reverses in sign between western and eastern Eurasia. The difference between western and eastern Eurasia can be explained by the presence of a low-viscosity zone in the uppermost mantle beneath eastern Eurasia that is absent to the west. The location of the lateral change in viscosity corresponds with the geologic boundary between the older shields and platforms of the Baltics, Russia, and Siberia and the younger, geologically active mountain belts of eastern Asia. This relation provides evidence that differences in the strength of the upper mantle control the locus of intracontinental deformation.

Modelling the possible interaction between edge-driven convection and the Canary Islands mantle plume

NASA Astrophysics Data System (ADS)

Negredo, A. M.; Rodríguez-González, J.; Fullea, J.; Van Hunen, J.

2017-12-01

The close location between many hotspots and the edges of cratonic lithosphere has led to the hypothesis that these hotspots could be explained by small-scale mantle convection at the edge of cratons (Edge Driven Convection, EDC). The Canary Volcanic Province hotspot represents a paradigmatic example of this situation due to its close location to the NW edge of the African Craton. Geochemical evidence, prominent low seismic velocity anomalies in the upper and lower mantle, and the rough NE-SW age-progression of volcanic centers consistently point out to a deep-seated mantle plume as the origin of the Canary Volcanic Province. It has been hypothesized that the plume material could be affected by upper mantle convection caused by the thermal contrast between thin oceanic lithosphere and thick (cold) African craton. Deflection of upwelling blobs due to convection currents would be responsible for the broader and more irregular pattern of volcanism in the Canary Province compared to the Madeira Province. In this study we design a model setup inspired on this scenario to investigate the consequences of possible interaction between ascending mantle plumes and EDC. The Finite Element code ASPECT is used to solve convection in a 2D box. The compositional field and melt fraction distribution are also computed. Free slip along all boundaries and constant temperature at top and bottom boundaries are assumed. The initial temperature distribution assumes a small long-wavelength perturbation. The viscosity structure is based on a thick cratonic lithosphere progressively varying to a thin, or initially inexistent, oceanic lithosphere. The effects of assuming different rheologies, as well as steep or gradual changes in lithospheric thickness are tested. Modelling results show that a very thin oceanic lithosphere (< 30 km) is needed to generate partial melting by EDC. In this case partial melting can occur as far as 700 km away from the edge of the craton. The size of EDC cells is relatively small (diameter about 300 km) for lithosphere/asthenosphere viscosity contrasts of 1000. In contrast, models assuming temperature-dependent viscosity and large viscosity variations evolve to large-scale (upper mantle) convection cells, with upwelling of hot material being enhanced by cold downwellings at the edge of cratonic lithosphere.

Mesozoic high-Mg andesites from the Daohugou area, Inner Mongolia: Upper-crustal fractional crystallization of parental melt derived from metasomatized lithospheric mantle wedge

NASA Astrophysics Data System (ADS)

Meng, Fanxue; Gao, Shan; Song, Zhaojun; Niu, Yaoling; Li, Xuping

2018-03-01

Mineral chemistry, major- and trace-element data, zircon U-Pb ages, and Sr-Nd isotopic data are presented for a suite of volcanic rocks from the Daohugou area, Ningcheng City, Inner Mongolia, on the northern margin of the North China Craton. Samples from the suite are of basaltic andesite to rhyolite in composition, with the rocks containing <60 wt% SiO2 have high MgO, Cr, and Ni contents, and classify as high-Mg andesites (HMAs). Zircons from a rhyolite yielded weighted mean 206Pb/238U age of 164 ± 1 Ma, indicating that the Daohugou volcanic suite is coeval with the Tiaojishan Formation of northern Hebei and western Liaoning Province. The HMAs have similar enriched-mantle I (EMI)-type isotopic compositions to each other, with low εNd (t) values, moderate (87Sr/86Sr) i ratios, enrichment in LREEs relative to LILEs, and depletion in HFSEs (e.g., Nb, Ta, Ti), indicating formation through protracted fractional crystallization of a common parental magma. The unusually low CaO contents and CaO/FeO ratios of olivine phenocrysts in the HMAs suggest that the parental melt was subduction-related. The results of Rhyolite-MELTS modelling indicates that HMAs may form through upper-crustal fractional crystallization from arc basalts. Therefore, the Daohugou HMAs were most likely formed through fractional crystallization of a parental melt derived from metasomatized lithospheric mantle at crustal depths. The addition of "water" to the cratonic keel may have played a key role in the destruction of the North China Craton.

Upper mantle velocity structure beneath southern Africa from modeling regional seismic data

NASA Astrophysics Data System (ADS)

Zhao, Ming; Langston, Charles A.; Nyblade, Andrew A.; Owens, Thomas J.

1999-03-01

The upper mantle seismic velocity structure beneath southern Africa is investigated using travel time and waveform data which come from a large mine tremor in South Africa (mb 5.6) recorded by the Tanzania broadband seismic experiment and by several stations in southern Africa. The waveform data show upper mantle triplications for both the 410- and 670-km discontinuities between distances of 2100 and 3000 km. Auxiliary travel time data along similar profiles obtained from other moderate events are also used. P wave travel times are inverted for velocity structure down to ˜800-km depth using the Wiechert-Herglotz technique, and the resulting model is evaluated by perturbing it at three depth intervals and then testing the perturbed model against the travel time and waveform data. The results indicate a typical upper mantle P wave velocity structure for a shield. P wave velocities from the top of the mantle down to 300-km depth are as much as 3% higher than the global average and are slightly slower than the global average between 300- and 420-km depth. Little evidence is found for a pronounced low-velocity zone in the upper mantle. A high-velocity gradient zone is required above the 410-km discontinuity, but both sharp and smooth 410-km discontinuities are permitted by the data. The 670-km discontinuity is characterized by high-velocity gradients over a depth range of ˜80 km around 660-km depth. Limited S wave travel time data suggest fast S wave velocities above ˜150-km depth. These results suggest that the bouyant support for the African superswell does not reside at shallow depths in the upper mantle.

Variations in the mantle transition zone beneath the Ethiopian Rift and Afar

NASA Astrophysics Data System (ADS)

Cornwell, D. G.; Hetenyi, G.; Blanchard, T.; Stuart, G. W.

2010-12-01

We use receiver functions calculated on broadband seismological data across Ethiopia to identify and map 3-D changes in the mantle transition zone (MTZ) thickness beneath the Ethiopian rift, Afar and the uplifted Ethiopian Plateau. The MTZ that divides the upper and lower mantle in the Earth is marked by discontinuities whose position and nature is controlled by local temperature and composition. It is commonly assumed that positive temperature anomalies cause an overall thinning of the MTZ by deepening the mineral phase transition of olivine (α-spinel) to wadsleyite (β-spinel) at around 410 km depth and shallowing the mineral phase transition of ringwoodite (γ-spinel) to magnesiowustite-perovskite at around 660 km depth. Such regions of anomalously hot mantle have been interpreted to extend from the core-mantle boundary (e.g. the African Superplume) to the Earth's surface from global tomographic models. Previous studies in Ethiopia or Afar that invoke receiver functions are mainly restricted to illuminating the MTZ beneath permanent seismological stations and, together with a regional receiver function study, all have found difficulty in imaging the discontinuities. They were unable to provide conclusive evidence for a thinned transition zone and could not constrain lateral changes in MTZ thickness that are required to assess whether the African Superplume intersects the MTZ beneath Ethiopia. We use seismological data from permanent stations as well as from four temporary arrays to compute receiver functions. We perform time-to-depth migration using the common conversion point (CCP) method with a regional velocity model that includes the slow mantle anomalies to estimate the depth-to-discontinuties and produce an MTZ thickness map. The signature of both the 410 and the 660 km discontinuities is clearly identified across ~500x500 km2. The 410 is relatively flat at 444±10 km depth throughout the region. The 660 is more perturbed with steep topographic changes and lies at 685±20 km depth. The mean depth of both interfaces being deeper than the respective nominal depths can be related to the low resolution of the global velocity model. However, the 410 is deepened more than the 660, resulting in a regionally thinned MTZ in the area of study by up to 25 km (equivalent of +150°C anomaly in the MTZ). A locally thickened (+13 km) MTZ is observed beneath part of the rift where the Main Ethiopian Rift opens into Afar. We interpret that elevated temperatures caused by the lower mantle African Superplume interacting with the MTZ in this region explains the thinned MTZ. Furthermore, the very slow upper mantle above the MTZ is a result of heat transfer from lower to upper mantle. This raised the mantle temperature, which facilitated the onset of rifting in Ethiopia.

Toward a coherent model for the melting behavior of the deep Earth's mantle

NASA Astrophysics Data System (ADS)

Andrault, D.; Bolfan-Casanova, N.; Bouhifd, M. A.; Boujibar, A.; Garbarino, G.; Manthilake, G.; Mezouar, M.; Monteux, J.; Parisiades, P.; Pesce, G.

2017-04-01

Knowledge of melting properties is critical to predict the nature and the fate of melts produced in the deep mantle. Early in the Earth's history, melting properties controlled the magma ocean crystallization, which potentially induced chemical segregation in distinct reservoirs. Today, partial melting most probably occurs in the lowermost mantle as well as at mid upper-mantle depths, which control important aspects of mantle dynamics, including some types of volcanism. Unfortunately, despite major experimental and theoretical efforts, major controversies remain about several aspects of mantle melting. For example, the liquidus of the mantle was reported (for peridotitic or chondritic-type composition) with a temperature difference of ∼1000 K at high mantle depths. Also, the Fe partitioning coefficient (DFeBg/melt) between bridgmanite (Bg, the major lower mantle mineral) and a melt was reported between ∼0.1 and ∼0.5, for a mantle depth of ∼2000 km. Until now, these uncertainties had prevented the construction of a coherent picture of the melting behavior of the deep mantle. In this article, we perform a critical review of previous works and develop a coherent, semi-quantitative, model. We first address the melting curve of Bg with the help of original experimental measurements, which yields a constraint on the volume change upon melting (ΔVm). Secondly, we apply a basic thermodynamical approach to discuss the melting behavior of mineralogical assemblages made of fractions of Bg, CaSiO3-perovskite and (Mg,Fe)O-ferropericlase. Our analysis yields quantitative constraints on the SiO2-content in the pseudo-eutectic melt and the degree of partial melting (F) as a function of pressure, temperature and mantle composition; For examples, we find that F could be more than 40% at the solidus temperature, except if the presence of volatile elements induces incipient melting. We then discuss the melt buoyancy in a partial molten lower mantle as a function of pressure, F and DFeBg/melt. In the lower mantle, density inversions (i.e. sinking melts) appear to be restricted to low F values and highest mantle pressures. The coherent melting model has direct geophysical implications: (i) in the early Earth, the magma ocean crystallization could not occur for a core temperature higher than ∼5400 K at the core-mantle boundary (CMB). This temperature corresponds to the melting of pure Bg at 135 GPa. For a mantle composition more realistic than pure Bg, the right CMB temperature for magma ocean crystallization could have been as low as ∼4400 K. (ii) There are converging arguments for the formation of a relatively homogeneous mantle after magma ocean crystallization. In particular, we predict the bulk crystallization of a relatively large mantle fraction, when the temperature becomes lower than the pseudo-eutectic temperature. Some chemical segregation could still be possible as a result of some Bg segregation in the lowermost mantle during the first stage of the magma ocean crystallization, and due to a much later descent of very low F, Fe-enriched, melts toward the CMB. (iii) The descent of such melts could still take place today. There formation should to be related to incipient mantle melting due to the presence of volatile elements. Even though, these melts can only be denser than the mantle (at high mantle depths) if the controversial value of DFeBg/melt is indeed as low as suggested by some experimental studies. This type of melts could contribute to produce ultra-low seismic velocity anomalies in the lowermost mantle.

Formation and tectonic evolution of the Cretaceous Jurassic Muslim Bagh ophiolitic complex, Pakistan: Implications for the composite tectonic setting of ophiolites

NASA Astrophysics Data System (ADS)

Khan, Mehrab; Kerr, Andrew C.; Mahmood, Khalid

2007-10-01

The Muslim Bagh ophiolitic complex Balochistan, Pakistan is comprised of an upper and lower nappe and represents one of a number of ophiolites in this region which mark the boundary between the Indian and Eurasian plates. These ophiolites were obducted onto the Indian continental margin around the Late Cretaceous, prior to the main collision between the Indian and Eurasian plates. The upper nappe contains mantle sequence rocks with numerous isolated gabbro plutons which we show are fed by dolerite dykes. Each pluton has a transitional dunite-rich zone at its base, and new geochemical data suggest a similar mantle source region for both the plutons and dykes. In contrast, the lower nappe consists of pillow basalts, deep-marine sediments and a mélange of ophiolitic rocks. The rocks of the upper nappe have a geochemical signature consistent with formation in an island arc environment whereas the basalts of the lower nappe contain no subduction component and are most likely to have formed at a mid-ocean ridge. The basalts and sediments of the lower nappe have been intruded by oceanic alkaline igneous rocks during the northward drift of the Indian plate. The two nappes of the Muslim Bagh ophiolitic complex are thus distinctively different in terms of their age, lithology and tectonic setting. The recognition of composite ophiolites such as this has an important bearing on the identification and interpretation of ophiolites where the plate tectonic setting is less well resolved.

Upper Mantle Structure Beneath the Whitmore Mountains, West Antarctic Rift System, and Marie Byrd Land from Body-Wave Tomography

NASA Astrophysics Data System (ADS)

Nyblade, A.; Lloyd, A. J.; Anandakrishnan, S.; Wiens, D. A.; Aster, R. C.; Huerta, A. D.; Wilson, T. J.; Shore, P.; Zhao, D.

2011-12-01

As part of the International Polar Year in Antarctica, 37 seismic stations have been installed across West Antarctica as part of the Polar Earth Observing Network (POLENET). 23 stations form a sparse backbone network of which 21 are co-located on rock sites with a network of continuously recording GPS stations. The remaining 14 stations, in conjunction with 2 backbone stations, form a seismic transect extending from the Ellsworth Mountains across the West Antarctic Rift System (WARS) and into Marie Byrd Land. Here we present preliminary P and S wave velocity models of the upper mantle from regional body wave tomography using P and S travel times from teleseismic events recorded by the seismic transect during the first year (2009-2010) of deployment. Preliminary P wave velocity models consisting of ~3,000 ray paths from 266 events indicate that the upper mantle beneath the Whitmore Mountains is seismically faster than the upper mantle beneath Marie Byrd Land and the WARS. Furthermore, we observe two substantial upper mantle low velocity zones located beneath Marie Byrd Land and near the southern boundary of the WARS.

Nitrogen evolution within the Earth's atmosphere-mantle system assessed by recycling in subduction zones

NASA Astrophysics Data System (ADS)

Mallik, Ananya; Li, Yuan; Wiedenbeck, Michael

2018-01-01

Understanding the evolution of nitrogen (N) across Earth's history requires a comprehensive understanding of N's behaviour in the Earth's mantle - a massive reservoir of this volatile element. Investigation of terrestrial N systematics also requires assessment of its evolution in the Earth's atmosphere, especially to constrain the N content of the Archaean atmosphere, which potentially impacted water retention on the post-accretion Earth, potentially causing enough warming of surface temperatures for liquid water to exist. We estimated the proportion of recycled N in the Earth's mantle today, the isotopic composition of the primitive mantle, and the N content of the Archaean atmosphere based on the recycling rates of N in modern-day subduction zones. We have constrained recycling rates in modern-day subduction zones by focusing on the mechanism and efficiency of N transfer from the subducting slab to the sub-arc mantle by both aqueous fluids and slab partial melts. We also address the transfer of N by aqueous fluids as per the model of Li and Keppler (2014). For slab partial melts, we constrained the transfer of N in two ways - firstly, by an experimental study of the solubility limit of N in melt (which provides an upper estimate of N uptake by slab partial melts) and, secondly, by the partitioning of N between the slab and its partial melt. Globally, 45-74% of N introduced into the mantle by subduction enters the deep mantle past the arc magmatism filter, after taking into account the loss of N from the mantle by degassing at mid-ocean ridges, ocean islands and back-arcs. Although the majority of the N in the present-day mantle remains of primordial origin, our results point to a significant, albeit minor proportion of mantle N that is of recycled origin (17 ± 8% or 12 ± 5% of N in the present-day mantle has undergone recycling assuming that modern-style subduction was initiated 4 or 3 billion years ago, respectively). This proportion of recycled N is enough to cause a departure of N isotopic composition of the primitive mantle from today's δ15N of -5‰ to - 6.8 ± 0.9 ‰ or - 6.3 ± 1.2 ‰. Future studies of Earth's parent bodies based on the bulk Earth N isotopic signature should take into account these revised values for the δ15N composition of the primitive mantle. Also, the Archaean atmosphere had a N partial pressure of 1.4-1.6 times higher than today, which may have warmed the Earth's surface above freezing despite a faint young Sun.

Mantle Upwellings Below the Ibero-Maghrebian Region with a Common Deep Source from P Travel-time Tomography

NASA Astrophysics Data System (ADS)

Civiero, C.; Custodio, S.; Silveira, G. M.; Rawlinson, N.; Arroucau, P.

2017-12-01

The processes responsible for the geodynamical evolution of the Ibero-Maghrebian domain are still enigmatic. Several geophysical studies have improved our understanding of the region, but no single model has been accepted yet. This study takes advantage of the dense station networks deployed from France in the north to Canary Islands and Morocco in the south to provide a new high-resolution P-wave velocity model of the structure of the upper-mantle and top of the lower mantle. These images show subvertical small-scale upwellings below Atlas Range, Canary Islands and Central Iberia that seem to cross the transition zone. The results, together with geochemical evidence and a comparison with previous global tomographic models, reveal the ponding or flow of deep-plume material beneath the transition zone, which seems to feed upper-mantle "secondary" pulses. In the upper mantle the plumes, in conjunction with the subduction-related upwellings, allow the hot mantle to rise in the surrounding zones. During its rising, the mantle interacts with horizontal SW slab-driven flow which skirts the Alboran slab and connects with the mantle upwelling below Massif Central through the Valencia Trough rift.

Upper-Mantel Earthquakes in the Australia-Pacific Plate Boundary Zone and the Roots of the Alpine Fault

NASA Astrophysics Data System (ADS)

Boese, C. M.; Warren-Smith, E.; Townend, J.; Stern, T. A.; Lamb, S. H.

2016-12-01

Seismicity in the upper mantle in continental collision zones is relatively rare, but observed around the world. Temporary seismometer deployments have repeatedly detected mantle earthquakes at depths of 40-100 km within the Australia-Pacific plate boundary zone beneath the South Island of New Zealand. Here, the transpressive Alpine Fault constitutes the primary plate boundary structure linking subduction zones of opposite polarity farther north and south. The Southern Alps Microearthquake Borehole Array (SAMBA) has been operating continuously since November 2008 along a 50 km-long section of the central Alpine Fault, where the rate of uplift of the Southern Alps is highest. To date it has detected more than 40 small to moderate-sized mantle events (1≤ML≤3.9). The Central Otago Seismic Array (COSA) has been in operation since late 2012 and detected 15 upper mantle events along the sub-vertical southern Alpine Fault. Various mechanisms have been proposed to explain the occurrence of upper mantle seismicity in the South Island, including intra-continental subduction (Reyners 1987, Geology); high shear-strain gradients due to depressed geotherms and viscous deformation of mantle lithosphere (Kohler and Eberhart-Phillips 2003, BSSA); high strain rates resulting from plate bending (Boese et al. 2013, EPSL), and underthrusting of the Australian plate (Lamb et al. 2015, G3). Focal mechanism analysis reveals a variety of mechanisms for the upper mantle events but predominantly strike-slip and reverse faulting. In this study, we apply spectral analysis to better constrain source parameters for these mantle events. These results are interpreted in conjunction with new information about crustal structure and low-frequency earthquakes near the Moho and in light of existing velocity, attenuation and resistivity models.

Sr, Nd, Pb and Hf Isotopic Compositions of Late Cenozoic Alkali Basalts in South Korea: Evidence for Mixing Between the Two Dominant Asthenospheric Mantle Domains beneath East Asia

NASA Astrophysics Data System (ADS)

Choi, S.; Mukasa, S. B.; Kwon, S.; Andronikov, A. V.

2004-12-01

We determined the Sr, Nd, Pb and Hf isotopic compositions of late Cenozoic basaltic rocks from six lava-field provinces in South Korea, including Baengnyeong Island, Jogokni, Ganseong area, Jeju Island, Ulleung Island and Dog Island, in order to understand the nature of the mantle source. The basalts have OIB-like trace element abundance patterns, and also contain mantle-derived xenoliths. Available isotope data of late Cenozoic basalts from East Asia, along with ours, show that the mantle source has a DMM-EM1 array for northeast China and a DMM-EM2 array for Southeast Asia. We note that the basalts falling on an array between DMM and an intermediate end member between EM1 and EM2, are located between the two large-scale isotopic provinces, i.e., around the eastern part of South Korea. The most intriguing observation on the isotopic correlation diagrams is spatial variation from predominantly EM2 signatures in the basaltic lavas toward increasingly important addition of EM1, starting from Jeju Island to Ulleung and Dog Islands to Ganseong area, and to Baengnyeong Island. This is without any corresponding changes in the basement and the lithospheric mantle beneath the region. These observations suggest that the asthenospheric mantle source is dominant for the Cenozoic intraplate volcanism in East Asia, which is characterized by two distinct, large-scale domains. Previous studies on East Asian Cenozoic volcanic rocks have invoked origins by either plume activity or decompressional melting in a rift environment. On the basis of our new trace element and isotopic compositions which have OIB-like characteristics, we prefer a plume origin for these lavas. However, because tomographic images do not show distinct thermal anomaly that would be interpreted as a plume, we suggest that the magmatism might be the product of small, difficult to image multiple plumes that tapped the shallow part of the asthenosphere (probably the transition zone in the upper mantle).

Surface wave tomography applied to the North American upper mantle

NASA Astrophysics Data System (ADS)

van der Lee, Suzan; Frederiksen, Andrew

Tomographic techniques that invert seismic surface waves for 3-D Earth structure differ in their definitions of data and the forward problem as well as in the parameterization of the tomographic model. However, all such techniques have in common that the tomographic inverse problem involves solving a large and mixed-determined set of linear equations. Consequently these inverse problems have multiple solutions and inherently undefinable accuracy. Smoother and rougher tomographic models are found with rougher (confined to great circle path) and smoother (finite-width) sensitivity kernels, respectively. A powerful, well-tested method of surface wave tomography (Partitioned Waveform Inversion) is based on inverting the waveforms of wave trains comprising regional S and surface waves from at least hundreds of seismograms for 3-D variations in S wave velocity. We apply this method to nearly 1400 seismograms recorded by digital broadband seismic stations in North America. The new 3-D S-velocity model, NA04, is consistent with previous findings that are based on separate, overlapping data sets. The merging of US and Canadian data sets, adding Canadian recordings of Mexican earthquakes, and combining fundamental-mode with higher-mode waveforms provides superior resolution, in particular in the US-Canada border region and the deep upper mantle. NA04 shows that 1) the Atlantic upper mantle is seismically faster than the Pacific upper mantle, 2) the uppermost mantle beneath Precambrian North America could be one and a half times as rigid as the upper mantle beneath Meso- and Cenozoic North America, with the upper mantle beneath Paleozoic North America being intermediate in seismic rigidity, 3) upper-mantle structure varies laterally within these geologic-age domains, and 4) the distribution of high-velocity anomalies in the deep upper mantle aligns with lower mantle images of the subducted Farallon and Kula plates and indicate that trailing fragments of these subducted oceanic plates still reside in the transition zone. The thickness of the high-velocity layer beneath Precambrian North America is estimated to be 250±70 km thick. On a smaller scale NA04 shows 1) high-velocities associated with subduction of the Pacific plate beneath the Aleutian arc, 2) the absence of expected high velocities in the upper mantle beneath the Wyoming craton, 3) a V-shaped dent below 150 km in the high-velocity cratonic lithosphere beneath New England, 4) the cratonic lithosphere beneath Precambrian North America being confined southwest of Baffin Bay, west of the Appalachians, north of the Ouachitas, east of the Rocky Mountains, and south of the Arctic Ocean, 5) the cratonic lithosphere beneath the Canadian shield having higher S-velocities than that beneath Precambrian basement that is covered with Phanerozoic sediments, 6) the lowest S velocities are concentrated beneath the Gulf of California, northern Mexico, and the Basin and Range Province.

Experimental Constraints on the Stability of Clinopyroxene (+) Magnesite in Iron Bearing Planetary Mantles: Implications for Nakhlite Formation

NASA Technical Reports Server (NTRS)

Martin, Audrey M.; Righter, Kevin

2010-01-01

Carbon is present in various forms in the Earth s upper mantle (carbonate- or diamond-bearing mantle xenoliths, carbonatite magmas, CO2 emissions from volcanoes...). Moreover, there is enough carbon in chondritic material to stabilize carbonates into the mantles of Mars or Venus as well as in the Earth. However, the interactions with iron have to be constrained, because Fe is commonly thought to buffer oxygen fugacity into planetary mantles. [1] and [2] show evidences of the stability of clinopyroxene Ca(Mg,Fe)Si2O6 + magnesite (Mg,Fe)CO3 in the Earth s mantle around 6GPa (about 180km). The stability of oxidized forms of carbon (like magnesite) depends on the oxygen fugacity of the system. In the Earth s mantle, the maximum carbon content is 10000 ppm [3]. The fO2 parameter varies vertically as a function of pressure, but also laterally because of geodynamic processes like subduction. Thus, carbonates, graphite, diamond, C-rich gases and melts are all stable forms of carbon in the Earth s mantle. [4] show that the fO2 variations observed in SNC meteorites can be explained by polybaric graphite-CO-CO2 equilibria in the Martian mantle. [5] inferred from thermodynamic calculations that the stable form of carbon in the source regions of the Martian basalts should be graphite (and/or diamond). After [6], a metasomatizing agent like a CO2-rich melt may infiltrate the mantle source of nakhlites. However, according to [7] and [8], the FeO wt% value in the Martian bulk mantle is more than twice that of the Earth s mantle (KLB-1 composition by [9]). As iron and carbon are two elements with various oxidation states, Fe/C interaction mechanisms must be considered.

Petrology and geochemistry of the high-Cr podiform chromitites of the Köycegiz ophiolite, southwest Turkey: implications for the multi-stage evolution of the oceanic upper mantle

NASA Astrophysics Data System (ADS)

Xiong, Fahui; Yang, Jingsui; Dilek, Yildirim; Wang, ChunLian; Hao, Xiaolin; Xu, Xiangzhen; Lian, Dongyang

2018-03-01

Ophiolites exposed across the western Tauride belt in Turkey represent tectonically emplaced fragments of oceanic lithosphere obducted onto the continental margin following the closure of the Neotethys Ocean during the Late Cretaceous. The ultramafic massif of Köycegiz, which is located in the ophiolitic belt of southwestern Turkey, is a major source of metallurgical chromitite ore. The massif comprises a base of tectonized harzburgite with minor dunite overlain by a magmatic sequence of wehrlite, pyroxenite, troctolite and gabbro. Only sparse refractory chromitites occur within the harzburgites; in contrast, the upper and middle sections of the peridotite sequence contain abundant metallurgical chromitites. The peridotites record abundant evidence of mantle metasomatism on various scales, as the Fo values of olivine in harzburgite are 90.1-95.4, whereas those in dunite are 90.1-91.8. The compositions of the melts passing through the peridotites changed gradually from arc tholeiite to boninite due to melt-rock reactions, thus producing more Cr-rich chromitites in the upper part of the body. Most of the chromitites have high Cr numbers (77-78), although systematic changes in the compositions of the olivine and chromian spinel occur from the harzburgites to the dunite envelopes to the chromitites, reflecting melt-rock reactions. The calculated ΔlogfO2 (FMQ) values range from - 2.77 to + 1.03 in the chromitites, - 2.73 to -0.01 in the harzburgites, and - 1.65 to + 0.45 in the dunites. All of the available evidence suggests that the Köycegiz ophiolite formed in a supra-subduction zone (SSZ) mantle wedge. These models indicate that the harzburgites represent the products of first-stage melting and low degrees of melt-rock interaction that occurred in a mid-ocean ridge (MOR) environment. In contrast, the chromitites and dunites represent the products of second-stage melting and related refertilization, which occurred in an SSZ environment.

Cr-pyrope garnets in the lithospheric mantle 2. Compositional populations and their distribution in time and space

NASA Astrophysics Data System (ADS)

Griffin, W. L.; Fisher, N. I.; Friedman, J. H.; O'Reilly, Suzanne Y.; Ryan, C. G.

2002-12-01

Three novel statistical approaches (Cluster Analysis by Regressive Partitioning [CARP], Patient Rule Induction Method [PRIM], and ModeMap) have been used to define compositional populations within a large database (n > 13,000) of Cr-pyrope garnets from the subcontinental lithospheric mantle (SCLM). The variables used are the major oxides and proton-microprobe data for Zn, Ga, Sr, Y, and Zr. Because the rules defining these populations (classes) are expressed in simple compositional variables, they are easily applied to new samples and other databases. The classes defined by the three methods show strong similarities and correlations, suggesting that they are statistically meaningful. The geological significance of the classes has been tested by classifying garnets from 184 mantle-derived peridotite xenoliths and from a smaller database (n > 5400) of garnets analyzed for >20 trace elements by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICPMS). The relative abundances of these classes in the lithospheric mantle vary widely across different tectonic settings, and some classes are absent or very rare in either Archean or Phanerozoic SCLM. Their distribution with depth also varies widely within individual lithospheric sections and between different sections of similar tectonothermal age. These garnet classes therefore are a useful tool for mapping the geology of the SCLM. Archean SCLM sections show high degrees of depletion and varying degrees of metasomatism, and they are commonly strongly layered. Several Proterozoic SCLM sections show a concentration of more depleted material near their base, grading upward into more fertile lherzolites. The distribution of garnet classes reflecting low-T phlogopite-related metasomatism and high-T melt-related metasomatism suggests that many of these Proterozoic SCLM sections consist of strongly metasomatized Archean SCLM. The garnet-facies SCLM beneath Phanerozoic terrains is only mildly depleted relative to Primitive Upper Mantle (PUM) compositions. These data emphasize the secular evolution of SCLM composition defined earlier [Griffin et al., 1998, 1999a] and suggest that at least part of this evolutionary trend reflects reworking and refertilization of SCLM formed in the Archean time.

Asthenospheric kimberlites: Volatile contents and bulk compositions at 7 GPa

NASA Astrophysics Data System (ADS)

Stamm, Natalia; Schmidt, Max W.

2017-09-01

During ascent, kimberlites react with the lithospheric mantle, entrain and assimilate xenolithic material, loose volatiles and suffer from syn- and post-magmatic alteration. Consequently, kimberlite rocks deviate heavily from their primary melt. Experiments at 7 GPa, 1300-1480 °C, 10-30 wt% CO2 and 0.46 wt% H2O on a proposed primitive composition from the Jericho kimberlite show that saturation with a lherzolitic mineral assemblage occurs only at 1300-1350 °C for a carbonatitic melt with <8 wt% SiO2 and >35 wt% CO2. At asthenospheric temperatures of >1400 °C, where the Jericho melt stays kimberlitic, this composition saturates only in low-Ca pyroxene, garnet and partly olivine. We hence forced the primitive Jericho kimberlite into multiple saturation with a lherzolitic assemblage by adding a compound peridotite. Saturation in olivine, low- and high-Ca pyroxene and garnet was obtained at 1400-1650 °C (7 GPa), melts are kimberlitic with 18-29 wt% SiO2 + Al2O3, 22.1-24.6 wt% MgO, 15-27 wt% CO2 and 0.4-7.1 wt% H2O; with a trade-off of H2O vs. CO2 and temperature. Melts in equilibrium with high-Ca pyroxene with typical mantle compositions have ≥2.5 wt% Na2O, much higher than the commonly proposed 0.1-0.2 wt%. The experiments allow for a model of kimberlite origin in the convective upper mantle, which only requires mantle upwelling that causes melting at the depth where elemental carbon (in metal, diamond or carbide) converts to CO2 (at ∼250 km). If primary melts leading to kimberlites contain a few wt% H2O, then adiabatic temperatures of 1400-1500 °C would yield asthenospheric mantle melts that are kimberlitic (>18 wt% SiO2 + Al2O3) but not carbonatitic (<10 wt% SiO2 + Al2O3) in composition, carbonatites only forming 100-200 °C below the adiabat. These kimberlites represent small melt fractions concentrating CO2 and H2O and then acquire part of their chemical signature by assimilation/fractionation during ascent in the subcratonic lithosphere.

Mantle transition zone structure and upper mantle S velocity variations beneath Ethiopia: Evidence for a broad, deep-seated thermal anomaly

NASA Astrophysics Data System (ADS)

Benoit, Margaret H.; Nyblade, Andrew A.; Owens, Thomas J.; Stuart, Graham

2006-11-01

Ethiopia has been subjected to widespread Cenozoic volcanism, rifting, and uplift associated with the Afar hot spot. The hot spot tectonism has been attributed to one or more thermal upwellings in the mantle, for example, starting thermal plumes and superplumes. We investigate the origin of the hot spot by imaging the S wave velocity structure of the upper mantle beneath Ethiopia using travel time tomography and by examining relief on transition zone discontinuities using receiver function stacks. The tomographic images reveal an elongated low-velocity region that is wide (>500 km) and extends deep into the upper mantle (>400 km). The anomaly is aligned with the Afar Depression and Main Ethiopian Rift in the uppermost mantle, but its center shifts westward with depth. The 410 km discontinuity is not well imaged, but the 660 km discontinuity is shallower than normal by ˜20-30 km beneath most of Ethiopia, but it is at a normal depth beneath Djibouti and the northwestern edge of the Ethiopian Plateau. The tomographic results combined with a shallow 660 km discontinuity indicate that upper mantle temperatures are elevated by ˜300 K and that the thermal anomaly is broad (>500 km wide) and extends to depths ≥660 km. The dimensions of the thermal anomaly are not consistent with a starting thermal plume but are consistent with a flux of excess heat coming from the lower mantle. Such a broad thermal upwelling could be part of the African Superplume found in the lower mantle beneath southern Africa.

Un-Earth-like interiors of the Earth-like planets

NASA Astrophysics Data System (ADS)

Shim, S. H. D.; Nisr, C.; Pagano, M.; Chen, H.; Ko, B.; Noble, S.; Leinenweber, K. D.; Young, P.; Desch, S. J.

2015-12-01

A number of exoplanets have been described as "Earth-like" planets (or even exo-earths) based on the mass-radius relations. Yet, significant variations have been documented in elemental abundances of planet-hosting stars, which will result in very different structures and processes in the interiors of rocky exoplanets. Recent data suggest that the Mg/Si ratio can be as small as less than 1 and as large as more than 2, opening the possibilities for the upper mantles to be dominated by pyroxene and olivine, respectively, and the lower mantles to be dominated by bridgmanite and ferropericlase, respectively. The changes in mineralogy will alter key properties, such as discontinuity structures (and therefore scale of mantle mixing), viscosity, and volatiles storage, of the mantle. Partial melting of such mantles would result in different compositions of the crusts, affecting the tectonics. However, the prediction should be made carefully because oxygen fugacity and contents of volatiles can change the mineralogy even for the same bulk composition. In extremely reducing proto-planetary disks, carbides will form instead of oxides and silicates, and become main constituents of planets in the system. Because carbides have high thermal conductivity and low thermal expansivity, internal heat transport of such planets may be dominated by conduction and mantle mixing would be much more limited than that of the Earth. However, the behaviors and properties of carbides need to be understood better at high pressure and high temperature. Some rocky exoplanets may have very thick layers of water and other icy materials. Interactions between ice (or fluid) and rock at extreme conditions would be the key to understand dynamics and habitability of such exoplanets.

Origins of topography in the western U.S.: Mapping crustal and upper mantle density variations using a uniform seismic velocity model

NASA Astrophysics Data System (ADS)

Levandowski, Will; Jones, Craig H.; Shen, Weisen; Ritzwoller, Michael H.; Schulte-Pelkum, Vera

2014-03-01

To investigate the physical basis for support of topography in the western U.S., we construct a subcontinent scale, 3-D density model using 1000 estimated crustal thicknesses and S velocity profiles to 150 km depth at each of 947 seismic stations. Crustal temperature and composition are considered, but we assume that mantle velocity variations are thermal in origin. From these densities, we calculate crustal and mantle topographic contributions. Typical 2σ uncertainty of topography is 500 m, and elevations in 84% of the region are reproduced within error. Remaining deviations from observed elevations are attributed to melt, variations in crustal quartz content, and dynamic topography; compositional variations in the mantle, while plausible, are not necessary to reproduce topography. Support for western U.S. topography is heterogeneous, with each province having a unique combination of mechanisms. Topography due to mantle buoyancy is nearly constant (within 250 m) across the Cordillera; relief there (>2 km) results from variations in crustal chemistry and thickness. Cold mantle provides 1.5 km of ballast to the thick crust of the Great Plains and Wyoming craton. Crustal temperature variations and dynamic pressures have smaller magnitude and/or more localized impacts. Positive gravitational potential energy (GPE) anomalies ( 2 × 1012N/m) calculated from our model promote extension in the northern Basin and Range and near the Sierra Nevada. Negative GPE anomalies (-3 × 1012N/m) along the western North American margin and Yakima fold and thrust belt add compressive stresses. Stresses derived from lithospheric density variations may strongly modulate tectonic stresses in the western U.S. continental interior.

Deep Europe today: Geophysical synthesis of the upper mantle structure and lithospheric processes over 3.5 Ga

USGS Publications Warehouse

Artemieva, I.M.; Thybo, H.; Kaban, M.K.; ,

2006-01-01

We present a summary of geophysical models of the subcrustal lithosphere of Europe. This includes the results from seismic (reflection and refraction profiles, P- and S-wave tomography, mantle anisotropy), gravity, thermal, electromagnetic, elastic and petrological studies of the lithospheric mantle. We discuss major tectonic processes as reflected in the lithospheric structure of Europe, from Precambrian terrane accretion and subduction to Phanerozoic rifting, volcanism, subduction and continent-continent collision. The differences in the lithospheric structure of Precambrian and Phanerozoic Europe, as illustrated by a comparative analysis of different geophysical data, are shown to have both a compositional and a thermal origin. We propose an integrated model of physical properties of the European subcrustal lithosphere, with emphasis on the depth intervals around 150 and 250 km. At these depths, seismic velocity models, constrained by body-and surface-wave continent-scale tomography, are compared with mantle temperatures and mantle gravity anomalies. This comparison provides a framework for discussion of the physical or chemical origin of the major lithospheric anomalies and their relation to large-scale tectonic processes, which have formed the present lithosphere of Europe. ?? The Geological Society of London 2006.

Dehydrogenation of goethite in Earth’s deep lower mantle

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hu, Qingyang; Kim, Duck Young; Liu, Jin

2017-01-31

The cycling of hydrogen influences the structure, composition, and stratification of Earth’s interior. Our recent discovery of pyrite-structured iron peroxide (designated as the P phase) and the formation of the P phase from dehydrogenation of goethite FeO 2H implies the separation of the oxygen and hydrogen cycles in the deep lower mantle beneath 1,800 km. Here we further characterize the residual hydrogen, x, in the P-phase FeO 2Hx. Using a combination of theoretical simulations and high-pressure–temperature experiments, we calibrated the x dependence of molar volume of the P phase. Within the current range of experimental conditions, we observed a compositionalmore » range of P phase of 0.39 < x < 0.81, corresponding to 19–61% dehydrogenation. Increasing temperature and heating time will help release hydrogen and lower x, suggesting that dehydrogenation could be approaching completion at the high-temperature conditions of the lower mantle over extended geological time. Our observations indicate a fundamental change in the mode of hydrogen release from dehydration in the upper mantle to dehydrogenation in the deep lower mantle, thus differentiating the deep hydrogen and hydrous cycles.« less

Self-Organized Mantle Layering After the Magma-Ocean Period

NASA Astrophysics Data System (ADS)

Hansen, U.; Dude, S.

2017-12-01

The thermal history of the Earth, it's chemical differentiation and also the reaction of the interior with the atmosphere is largely determined by convective processes within the Earth's mantle. A simple physical model, resembling the situation, shortly after core formation, consists of a compositionally stable stratified mantle, as resulting from fractional crystallization of the magma ocean. The early mantle is subject to heating from below by the Earth's core and cooling from the top through the atmosphere. Additionally internal heat sources will serve to power the mantle dynamics. Under such circumstances double diffusive convection will eventually lead to self -organized layer formation, even without the preexisting jumps is material properties. We have conducted 2D and 3D numerical experiments in Cartesian and spherical geometry, taking into account mantle realistic values, especially a strong temperature dependent viscosity and a pressure dependent thermal expansivity . The experiments show that in a wide parameter range. distinct convective layers evolve in this scenario. The layering strongly controls the heat loss from the core and decouples the dynamics in the lower mantle from the upper part. With time, individual layers grow on the expense of others and merging of layers does occur. We observe several events of intermittent breakdown of individual layers. Altogether an evolution emerges, characterized by continuous but also spontaneous changes in the mantle structure, ranging from multiple to single layer flow. Such an evolutionary path of mantle convection allows to interpret phenomena ranging from stagnation of slabs at various depth to variations in the chemical signature of mantle upwellings in a new framework.

Geochemical composition, petrography and 40Ar/39Ar age of the Heldburg phonolite: implications on magma mixing and mingling

NASA Astrophysics Data System (ADS)

Abratis, Michael; Viereck, Lothar; Pfänder, Jörg A.; Hentschel, Roland

2015-11-01

Differentiated magmatic rocks such as trachyte and phonolite are volumetrically subordinate to mafic volcanic rocks within the Cenozoic Central European Volcanic Province (exceptions are the East Eifel and the Rhön volcanic fields). Within the volcanic field of the "Heldburg dike swarm" (Heldburger Gangschar), the phonolite of the Burgberg near Heldburg represents the only known occurrence of differentiated magmatic rocks. However, the Heldburg phonolite is famous foremost for containing mantle xenoliths (spinel lherzolite). Former studies proposing a cogenetic relationship between the phonolite and the peridotites concluded that the phonolite magma must have evolved under upper mantle conditions. Herewith, we present petrographic and geochemical evidence for magma mixing and mingling in the Heldburg phonolite melt due to the intrusion of mantle-derived basanitic magma, which is exposed today as dikes at the foot of the Heldburg Burgberg. During this process, the mantle xenoliths were introduced into the phonolite melt as they all contain rims of basanitic magma. Extensive mingling features (e.g., schlieren layers, load casts, flame structures, mafic enclaves) are developed, indicating that the basanite and the zoned phonolitic body were melts at the time of mixing. These petrographic and geochemical indications of two coeval melts of different composition are substantiated by 40Ar/39Ar dating, revealing identical ages of ca. 15 Ma.

Sound velocities of skiagite-iron-majorite solid solution to 56 GPa probed by nuclear inelastic scattering

NASA Astrophysics Data System (ADS)

Vasiukov, D. M.; Ismailova, L.; Kupenko, I.; Cerantola, V.; Sinmyo, R.; Glazyrin, K.; McCammon, C.; Chumakov, A. I.; Dubrovinsky, L.; Dubrovinskaia, N.

2018-05-01

High-pressure experimental data on sound velocities of garnets are used for interpretation of seismological data related to the Earth's upper mantle and the mantle transition zone. We have carried out a Nuclear Inelastic Scattering study of iron-silicate garnet with skiagite (77 mol%)-iron-majorite composition in a diamond anvil cell up to 56 GPa at room temperature. The determined sound velocities are considerably lower than sound velocities of a number of silicate garnet end-members, such as grossular, pyrope, Mg-majorite, andradite, and almandine. The obtained sound velocities have the following pressure dependencies: V p [km/s] = 7.43(9) + 0.039(4) × P [GPa] and V s [km/s] = 3.56(12) + 0.012(6) × P [GPa]. We estimated sound velocities of pure skiagite and khoharite, and conclude that the presence of the iron-majorite component in skiagite strongly decreases V s . We analysed the influence of Fe3+ on sound velocities of garnet solid solution relevant to the mantle transition zone and consider that it may reduce sound velocities up to 1% relative to compositions with only Fe2+ in the cubic site.

Free and forced convection in Earth's upper mantle

NASA Astrophysics Data System (ADS)

Hall, Paul S.

Convective motion within Earth's upper mantle occurs as a combination of two primary modes: (1) buoyant upwelling due to the formation of gravitational instabilities at thermochemical boundary layers, and (2) passive flow associated with the divergence of lithospheric plates at mid-ocean ridges and their re-entry into the mantle at subduction zones. The first mode is driven by variations in density and is therefore classified as 'free' convection. Examples of free convection within the Earth include the diapiric flow of hydrous and/or partially molten mantle at subduction zones and mantle plumes. The second mode, while ultimately driven by density on a global scale, can be treated kinematically on the scale of the upper mantle. This type of flow is designated 'forced' convection. On the scale of individual buoyant upwellings in the upper mantle, the forced convection associated with plate tectonics acts to modify the morphology of the flow associated with free convection. Regions in which such interactions occur are typically associated with transfer of significant quantities of both mass and energy (i.e., heat) between the deep interior and the surface of the Earth and thus afford a window into the dynamics of the Earth's interior. The dynamics and the consequences of the interaction between these two modes of convection is the focus of this dissertation. I have employed both laboratory and numerical modeling techniques to investigate the interaction between free and forced convection in this study. Each of these approaches has its own inherent strengths and weaknesses. These approaches are therefore complementary, and their use in combination is particularly powerful. I have focused on two examples interaction between free and forced convection in the upper mantle in this study. Chapter I considers the interaction between ascending diapirs of hydrous and/or partially molten mantle and flow in the mantle wedge at subduction zones using laboratory models. Chapter II and Chapter III consider the interaction between an ascending mantle plume and the large scale shear flow associated with the divergence of plates at a nearby ridge axis.

Evidence of lower-mantle slab penetration phases in plate motions.

PubMed

Goes, Saskia; Capitanio, Fabio A; Morra, Gabriele

2008-02-21

It is well accepted that subduction of the cold lithosphere is a crucial component of the Earth's plate tectonic style of mantle convection. But whether and how subducting plates penetrate into the lower mantle is the subject of continuing debate, which has substantial implications for the chemical and thermal evolution of the mantle. Here we identify lower-mantle slab penetration events by comparing Cenozoic plate motions at the Earth's main subduction zones with motions predicted by fully dynamic models of the upper-mantle phase of subduction, driven solely by downgoing plate density. Whereas subduction of older, intrinsically denser, lithosphere occurs at rates consistent with the model, younger lithosphere (of ages less than about 60 Myr) often subducts up to two times faster, while trench motions are very low. We conclude that the most likely explanation is that older lithosphere, subducting under significant trench retreat, tends to lie down flat above the transition to the high-viscosity lower mantle, whereas younger lithosphere, which is less able to drive trench retreat and deforms more readily, buckles and thickens. Slab thickening enhances buoyancy (volume times density) and thereby Stokes sinking velocity, thus facilitating fast lower-mantle penetration. Such an interpretation is consistent with seismic images of the distribution of subducted material in upper and lower mantle. Thus we identify a direct expression of time-dependent flow between the upper and lower mantle.

Volcanism on Mars controlled by early oxidation of the upper mantle

NASA Astrophysics Data System (ADS)

Tuff, J.; Wade, J.; Wood, B. J.

2013-06-01

Detailed information about the chemical composition and evolution of Mars has been derived principally from the SNC (shergottite-nakhlite-chassignite) meteorites, which are genetically related igneous rocks of Martian origin. They are chemically and texturally similar to terrestrial basalts and cumulates, except that they have higher concentrations of iron and volatile elements such as phosphorus and chlorine and lower concentrations of nickel and other chalcophile (sulphur-loving) elements. Most Martian meteorites have relatively young crystallization ages (1.4 billion years to 180 million years ago) and are considered to be derived from young, lightly cratered volcanic regions, such as the Tharsis plateau. Surface rocks from the Gusev crater analysed by the Spirit rover are much older (about 3.7 billion years old) and exhibit marked compositional differences from the meteorites. Although also basaltic in composition, the surface rocks are richer in nickel and sulphur and have lower manganese/iron ratios than the meteorites. This has led to doubts that Mars can be described adequately using the `SNC model'. Here we show, however, that the differences between the compositions of meteorites and surface rocks can be explained by differences in the oxygen fugacity during melting of the same sulphur-rich mantle. This ties the sources of Martian meteorites to those of the surface rocks through an early (>3.7 billion years ago) oxidation of the uppermost mantle that had less influence on the deeper regions, which produce the more recent volcanic rocks.

New constraints on subduction inputs and volatile outputs along the Aleutian Arc

NASA Astrophysics Data System (ADS)

Lopez, T. M.; Fischer, T. P.; Plank, T. A.; Rizzo, A. L.; Rasmussen, D. J.; Cottrell, E.; Werner, C. A.; Kern, C.; Ilanko, T.; Buff, L.; Andrys, J.; Kelley, K. A.

2017-12-01

Volatile cycling drives volcanism in subduction zone settings. At arcs, volatiles can originate from the subducted slab, mantle wedge and/or crust. Each region has characteristic isotopic signatures, which can be used to fingerprint volatile provenance. We speculate that differences in subduction parameters, such as convergence angle, plate coupling and subducted sediment fluxes, may lead to differences in volatile provenance, which may in turn influence volcanic eruption style and frequency. Here we combine updated constraints on subduction inputs and volatile outputs to provide new insights into volatile cycling within the Aleutian Arc. The high proportion of organic carbon (80-100% to total carbon) in sediments subducting at the Aleutian trench stands out globally and predicts a light carbon isotopic composition of recycled volcanic fluids. We assess volatile outputs on volcanic timescales and along the arc by combining carbon (C), nitrogen (N) and helium (He) isotopic compositions of volcanic gases and new analyses of He and, where possible, C isotopes in olivine-hosted fluid inclusions. From our preliminary isotopic analyses of volcanic gases, we find a greater proportion of mantle-derived volatiles released from the Western segment of the Aleutian Arc (>40% mantle) compared with other volcanic arcs around the world (<30% mantle), where volatiles are sourced primarily from subducted or upper crustal carbonates. This trend may be due to the oblique convergence and low subducted sediment input in this region. The Aleutian Arc also exhibits among the lightest carbon isotope ratios of arcs worldwide (δ13C = -10 to -15‰), especially in the central part of the arc, where organic-bearing terrigneous sediment fills the trench and the convergence rate is high. New constraints on subduction inputs and outputs presented here will help discriminate between upper crustal and subducted carbon sources, and provide further insights into volatile cycling and subduction processes within the Aleutian Arc.

Finite-frequency wave propagation through outer rise fault zones and seismic measurements of upper mantle hydration

USGS Publications Warehouse

Miller, Nathaniel; Lizarralde, Daniel

2016-01-01

Effects of serpentine-filled fault zones on seismic wave propagation in the upper mantle at the outer rise of subduction zones are evaluated using acoustic wave propagation models. Modeled wave speeds depend on azimuth, with slowest speeds in the fault-normal direction. Propagation is fastest along faults, but, for fault widths on the order of the seismic wavelength, apparent wave speeds in this direction depend on frequency. For the 5–12 Hz Pn arrivals used in tomographic studies, joint-parallel wavefronts are slowed by joints. This delay can account for the slowing seen in tomographic images of the outer rise upper mantle. At the Middle America Trench, confining serpentine to fault zones, as opposed to a uniform distribution, reduces estimates of bulk upper mantle hydration from ~3.5 wt % to as low as 0.33 wt % H2O.

The interpretation of crustal dynamics data in terms of plate motions and regional deformation near plate boundaries

NASA Technical Reports Server (NTRS)

Soloman, Sean C.

1991-01-01

The focus was in two broad areas during the most recent 6-month period: (1) the nature and dynamics of time dependent deformation and stress along major seismic zones; and (2) the nature of long-wavelength oceanic geoid anomalies in terms of lateral variations in upper mantle temperature and composition. The principle findings are described in the accompanying appendices.

Partitioning of V, Mn, Co, Ni, Cu, Zn, As, Mo, Ag, Sn, Sb, W, Au, Pb, and Bi between sulfide phases and hydrous basanite melt at upper mantle conditions

NASA Astrophysics Data System (ADS)

Li, Yuan; Audétat, Andreas

2012-11-01

The partitioning of 15 major to trace metals between monosulfide solid solution (MSS), sulfide liquid (SL) and mafic silicate melt (SM) was determined in piston-cylinder experiments performed at 1175-1300 °C, 1.5-3.0 GPa and oxygen fugacities ranging from 3.1 log units below to 1.0 log units above the quartz-fayalite-magnetite fO2 buffer, which conditions are representative of partial melting in the upper mantle in different tectonic settings. The silicate melt was produced by partial melting of a natural, amphibole-rich mantle source rock, resulting in hydrous (˜5 wt% H2O) basanitic melts similar to low-degree partial melts of metasomatized mantle, whereas the major element composition of the starting sulfide (˜52 wt% Fe; 39 wt% S; 7 wt% Ni; 2 wt% Cu) was similar to the average composition of sulfides in this environment. SL/SM partition coefficients are high (≥100) for Au, Ni, Cu, Ag, Bi, intermediate (1-100) for Co, Pb, Sn, Sb (±As, Mo), and low (≤1) for the remaining elements. MSS/SM partition coefficients are generally lower than SL/SM partition coefficients and are high (≥100) for Ni, Cu, Au, intermediate (1-100) for Co, Ag (±Bi, Mo), and low (≤1) for the remaining elements. Most sulfide-silicate melt partition coefficients vary as a function of fO2, with Mo, Bi, As (±W) varying by a factor >10 over the investigated fO2 range, Sb, Ag, Sn (±V) varying by a factor of 3-10, and Pb, Cu, Ni, Co, Au, Zn, Mn varying by a factor of 3-10. The partitioning data were used to model the behavior of Cu, Au, Ag, and Bi during partial melting of upper mantle and during fractional crystallization of primitive MORB and arc magmas. Sulfide phase relationships and comparison of the modeling results with reported Cu, Au, Ag, and Bi concentrations from MORB and arc magmas suggest that: (i) MSS is the dominant sulfide in the source region of arc magmas, and thus that Au/Cu ratios in the silicate melt and residual sulfides may decrease with increasing degree of partial melting, (ii) both MSS and sulfide liquid are precipitated during fractional crystallization of MORB, and (iii) fractional crystallization of arc magmas is strongly dominated by MSS.

Experimental investigation of flow-induced fabrics in rocks at upper-mantle pressures: Application to understanding mantle dynamics and seismic anisotropy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kohlstedt, David L.

2016-04-26

The goal of this collaborative research effort between W.B. Durham at the Massachusetts Institute of Technology (MIT) and D.L. Kohlstedt and S. Mei at the University of Minnesota (UMN) was to exploit a newly developed technology for high-pressure, high-temperature deformation experimentation, namely, the deformation DIA (D-DIA) to determine the deformation behavior of a number of important upper mantle rock types including olivine, garnet, enstatite, and periclase. Experiments were carried out under both hydrous and anhydrous conditions and at both lithospheric and asthenospheric stress and temperature conditions. The result was a group of flow laws for Earth’s upper mantle that quantitativelymore » describe the viscosity of mantle rocks from shallow depths (the lithosphere) to great depths (the asthenosphere). These flow laws are fundamental for modeling the geodynamic behavior and heat transport from depth to Earth’s surface.« less

Experimental investigation of flow-induced fabrics in rocks at upper-mantle pressures. Application to understanding mantle dynamics and seismic anisotropy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Durham, William B.

2016-05-02

The goal of this collaborative research effort between W.B. Durham at the Massachusetts Institute of Technology (MIT) and D.L. Kohlstedt and S. Mei at the University of Minnesota (UMN) was to exploit a newly developed technology for high-pressure, high-temperature deformation experimentation, namely, the deformation DIA (D-DIA), to determine the deformation behavior of a number of important upper mantle rock types including olivine, garnet, enstatite, and periclase. Experiments were carried out under both hydrous and anhydrous conditions and at both lithospheric and asthenospheric stress and temperature conditions. The result was a group of flow laws for Earth’s upper mantle that quantitativelymore » describe the viscosity of mantle rocks from shallow depths (the lithosphere) to great depths (the asthenosphere). These flow laws are fundamental for modeling the geodynamic behavior and heat transport from depth to Earth’s surface.-« less

Re-Os systematics of komatiites and komatiitic basalts at Dundonald Beach, Ontario, Canada: Evidence for a complex alteration history and implications of a late-Archean chondritic mantle source

NASA Astrophysics Data System (ADS)

Gangopadhyay, A.; Sproule, R. A.; Walker, R. J.; Lesher, C.

2004-12-01

Re-Os concentrations and isotopic compositions have been examined in one komatiite unit and one komatiitic basalt unit at Dundonald Beach, which is part of the spatially-extensive 2.7 Ga Kidd-Munro volcanic assemblage in the Abitibi greenstone belt, Ontario, Canada. The komatiitic rocks in this locality record at least three episodes of alteration of Re-Os elemental and isotope systematics. First, an average of 40% and as much as 75% Re was lost due to shallow degassing during eruption and/or hydrothermal leaching during or immediately after the lava emplacement. Second, the Re-Os isotope systematics of the rocks with 187Re/188Os ratios >1 were reset at ˜2.5 Ga, most likely due to a regional metamorphic event. Finally, there is evidence for relatively recent gain and loss of Re. The variations in Os concentrations in the Dundonald komatiites yield a relative bulk distribution coefficient for Os (DOs solid/liquid) of 2-4, consistent with those obtained for stratigraphically-equivalent komatiites in the nearby Alexo area and in Munro Township. This suggests that Os was moderately compatible during crystal-liquid fractionation of the magma parental to the Kidd-Munro komatiitic rocks. Furthermore, whole-rock samples and chromite separates with low 187Re/188Os ratios (<1) yield a precise chondritic average initial 187Os/188Os ratio of 0.1083 ± 0.0006 (\\gammaOs = 0.0 ± 0.6). The chondritic initial Os isotopic composition of the mantle source for the Dundonald rocks is consistent with that determined for komatiites in the Alexo area and in Munro Township. Our Os isotope results for the Dundonald komatiitic rocks, combined with those in the Alexo and Pyke Hill areas suggest that the mantle source region for the Kidd- Munro volcanic assemblage had evolved along a long-term chondritic Os isotopic trajectory until their eruption at ˜2.7 Ga. The chondritic initial Os isotopic composition of the Kidd-Munro komatiites is indistinguishable from that of the projected contemporaneous convective upper mantle. The uniform chondritic Os isotopic composition of the ˜2.7 Ga mantle source for the Kidd-Munro komatiites contrasts with the typical large-scale Os isotopic heterogeneity in the mantle sources for komatiites from the Gorgona Island, present-day ocean island basalts or arc-related lavas. This suggests a significantly more homogeneous mantle source in the Archean compared to the presentday mantle.

Nano-inclusions in diamond: Evidence of diamond genesis

NASA Astrophysics Data System (ADS)

Wirth, R.

2015-12-01

The use of Focused Ion Beam technology (FIB) for TEM sample preparation introduced approximately 15 years ago revolutionized the application of TEM in Geosciences. For the first time, FIB enabled cutting samples for TEM use from exactly the location we are interested in. Applied to diamond investigation, this technique revealed the presence of nanometre-sized inclusions in diamond that have been simply unknown before. Nanoinclusions in diamond from different location and origin such as diamonds from the Lower and Upper Mantle, metamorphic diamonds (Kazakhstan, Erzgebirge, Bohemia), diamonds from ophiolites (Tibet, Mongolia, Xinjiang, Ural Mountains), diamonds from igneous rocks (Hawaii, Kamchatka) and impact diamonds (Popigai Crater, Siberia) have been investigated during the last 15 years. The major conclusion of all these TEM studies is, that the nanoinclusions, their phases and phase composition together with the micro- and nanostructure evidence the origin of diamond and genesis of diamond. We can discriminate Five different mechanisms of diamond genesis in nature are observed: Diamond crystallized from a high-density fluid (Upper mantle and metamorphic diamond). Diamond crystallized from carbonatitic melt (Lower mantle diamond). Diamond precipitates from a metal alloy melt (Diamond from ophiolites). Diamond crystallized by gas phase condensation or chemical vapour condensation (CVD) (Lavas from Kamchatka, xenoliths in Hawaiian lavas). Direct transformation of graphite into diamond.

Storage and recycling of water and carbon dioxide in the earth

NASA Technical Reports Server (NTRS)

Wood, Bernard J.

1994-01-01

The stabilities and properties of water- and carbon-bearing phases in the earth have been determined from phase equilibrium measurements, combined with new data on the equations of state of water, carbon dioxide, carbonates and hydrates. The data have then been used to predict the fate of calcite and hydrous phases in subducting oceanic lithosphere. From the compositions of MORB's one can estimate concentrations of water and carbon of around 200 ppm and 80 ppm respectively in the upper mantle. Lower mantle estimates are very uncertain, but 1900 ppm water and 2000 ppm C are plausible concentrations. Measurements of the density of supercritical water to 3 GPa demonstrate that this phase is less compressible than anticipated from the equations of state of Haar et al. or Saul and Wagner and is closer to predictions based on molecular dynamics simulations. Conversely, fugacity measurements on carbon dioxide to 7 GPa show that this fluid is more compressible than predicted from the MRK equation of state. The results imply that hydrates are relatively more stable and carbonates less stable at pressures greater than 5 GPa than would be predicted from simple extrapolation of the low pressure data. Nevertheless, carbonates remain extremely refractory phases within both the upper and lower mantle.

Detailed Image of the Subducting Plate and Upper mantle Seismic Discontinuities in the Mariana Subduction Zone

NASA Astrophysics Data System (ADS)

Tibi, R.; Wiens, D. A.; Shiobara, H.; Sugioka, H.; Yuan, X.

2006-12-01

We use P-to-S converted teleseismic phases recorded at island and ocean bottom stations in Mariana to image the subducting plate and the upper mantle seismic discontinuities in the Mariana subduction zone. The land and seafloor stations which operated from June 2003 to May 2004, were deployed within the framework of the MARGINS Subduction Factory experiment of the Mariana system. The crust in the sudducting plate is observed at about 80--90 km depth beneath the islands of Saipan, Tinian and Rota. For most of the island stations, a low velocity layer is imaged in the forearc at depth between about 20 and 60 km, with decreasing depths toward the arc. The nature of this feature is not yet clear. We found evidence for double seismic discontinuities at the base of the transition zone near the Mariana slab. A shallower discontinuity is imaged at depths of ~650--715 km, and a deeper interface lies at ~740-- 770 km depth. The amplitudes of the seismic signals suggest that the shear velocity contrasts across the two features are comparable. These characteristics support the interpretation that the discontinuities are the results of the phase transformations in olivine (ringwoodite to post-spinel) and garnet (ilminite to perovskite), respectively, for the pyrolite model of mantle composition.

Formation and modification of chromitites in the mantle

NASA Astrophysics Data System (ADS)

Arai, Shoji; Miura, Makoto

2016-11-01

Podiform chromitites have long supplied us with unrivaled information on various mantle processes, including the peridotite-magma reaction, deep-seated magmatic evolution, and mantle dynamics. The recent discovery of ultrahigh-pressure (UHP) chromitites not only sheds light on a different aspect of podiform chromitites, but also changes our understanding of the whole picture of podiform chromitite genesis. In addition, new evidence was recently presented for hydrothermal modification/formation chromite/chromitite in the mantle, which is a classical but innovative issue. In this context, we present here an urgently needed comprehensive review of podiform chromitites in the upper mantle. Wall-rock control on podiform chromitite genesis demonstrates that the peridotite-magma reaction at the upper mantle condition is an indispensable process. We may need a large system in the mantle, far larger than the size of outcrops or mining areas, to fulfill the Cr budget requirement for podiform chromitite genesis. The peridotite-magma reaction over a large area may form a melt enriched with Na and other incompatible elements, which mixes with a less evolved magma supplied from the depth to create chromite-oversaturated magma. The incompatible-element-rich magma trapped by the chromite mainly precipitates pargasite and aspidolite (Na analogue of phlogopite), which are stable under upper mantle conditions. Moderately depleted harzburgites, which contain chromite with a moderate Cr# (0.4-0.6) and a small amount of clinopyroxene, are the best reactants for the chromitite-forming reaction, and are the best hosts for podiform chromitites. Arc-type chromitites are dominant in ophiolites, but some are of the mid-ocean ridge type; chromitites may be common beneath the ocean floor, although it has not yet been explored for chromitite. The low-pressure (upper mantle) igneous chromitites were conveyed through mantle convection or subduction down to the mantle transition zone to form ultrahigh-pressure chromitites. Some of these reappear at the shallower mantle, and can coexist with newly formed low-pressure igneous chromitites. High-temperature hydrothermal fluids can dissolve and precipitate chromite, and hydrothermal chromitites (chromitites precipitated from aqueous fluids) are possibly formed within the mantle where the circulation of hydrous fluid is available, e.g., at the mantle wedge.

High-pressure phase relation of KREEP basalts: A clue for finding the lost Hadean crust?

NASA Astrophysics Data System (ADS)

Gréaux, Steeve; Nishi, Masayuki; Tateno, Shigehiko; Kuwayama, Yasuhiro; Hirao, Naohisa; Kawai, Kenji; Maruyama, Shigenori; Irifune, Tetsuo

2018-01-01

The phase relations, mineral chemistry and density of KREEP basalt were investigated at pressures of 12-125 GPa and temperatures up to 2810 K by a combination of large volume multi-anvil press experiments and in situ synchrotron X-ray diffraction measurements in a laser-heated diamond anvil cell. Our results showed that grossular-rich majorite garnet, liebermannite and Al-bearing stishovite are dominant in the upper-to-middle part of the upper mantle while in the lowermost transition zone a dense Ti-rich CaSiO3 perovskite exsoluted from the garnet, which becomes more pyropic with increasing pressure. At lower mantle conditions, these minerals transform into an assemblage of bridgmanite, Ca-perovskite, Al-stishovite, the new aluminium-rich (NAL) phase and the calcium-ferrite type (CF) phase. At pressures higher than 50 GPa, NAL phase completely dissolved into the CF phase, which becomes the main deposit of alkali metals in the lower mantle. The density of KREEP estimated from phase compositions obtained by energy dispersive X-ray spectroscopy (EDS) in scanning (SEM) and transmission (TEM) electron microscopes, was found substantially denser than pyrolite suggesting that the Earth primordial crust likely subducted deep into the Earth's mantle after or slightly before the final solidification of magma ocean at 4.53 Ga. Radiogenic elements U, Th and 40K which were abundant in the final residue of magma ocean were brought down along the subduction of the primordial crust and generate heat by decay after the settlement of the primordial crust on top of the CMB, suggesting the non-homogeneous distribution of radiogenic elements in the Hadean mantle with implications for the thermal history of the Earth.

CO2 content of andesitic melts at graphite-saturated upper mantle conditions with implications for redox state of oceanic basalt source regions and remobilization of reduced carbon from subducted eclogite

NASA Astrophysics Data System (ADS)

Eguchi, James; Dasgupta, Rajdeep

2017-03-01

We have performed experiments to determine the effects of pressure, temperature and oxygen fugacity on the CO2 contents in nominally anhydrous andesitic melts at graphite saturation. The andesite composition was specifically chosen to match a low-degree partial melt composition that is generated from MORB-like eclogite in the convective, oceanic upper mantle. Experiments were performed at 1-3 GPa, 1375-1550 °C, and fO2 of FMQ -3.2 to FMQ -2.3 and the resulting experimental glasses were analyzed for CO2 and H2O contents using FTIR and SIMS. Experimental results were used to develop a thermodynamic model to predict CO2 content of nominally anhydrous andesitic melts at graphite saturation. Fitting of experimental data returned thermodynamic parameters for dissolution of CO2 as molecular CO2: ln( K 0) = -21.79 ± 0.04, Δ V 0 = 32.91 ± 0.65 cm3mol-1, Δ H 0 = 107 ± 21 kJ mol-1, and dissolution of CO2 as CO3 2-: ln (K 0 ) = -21.38 ± 0.08, Δ V 0 = 30.66 ± 1.33 cm3 mol-1, Δ H 0 = 42 ± 37 kJ mol-1, where K 0 is the equilibrium constant at some reference pressure and temperature, Δ V 0 is the volume change of reaction, and Δ H 0 is the enthalpy change of reaction. The thermodynamic model was used along with trace element partition coefficients to calculate the CO2 contents and CO2/Nb ratios resulting from the mixing of a depleted MORB and the partial melt of a graphite-saturated eclogite. Comparison with natural MORB and OIB data suggests that the CO2 contents and CO2/Nb ratios of CO2-enriched oceanic basalts cannot be produced by mixing with partial melts of graphite-saturated eclogite. Instead, they must be produced by melting of a source containing carbonate. This result places a lower bound on the oxygen fugacity for the source region of these CO2-enriched basalts, and suggests that fO2 measurements made on cratonic xenoliths may not be applicable to the convecting upper mantle. CO2-depleted basalts, on the other hand, are consistent with mixing between depleted MORB and partial melts of a graphite-saturated eclogite. Furthermore, calculations suggest that eclogite can remain saturated in graphite in the convecting upper mantle, acting as a reservoir for C.

The role of water in the petrogenesis of Marina trough magmas

NASA Astrophysics Data System (ADS)

Stolper, Edward; Newman, Sally

1994-02-01

Most variations in composition among primitive basalts from the Mariana back-arc trough can be explained by melting mixtures of an N-type mid-ocean ridge basalt (NMORB) mantle source and an H2O rich component, provided the degree of melting is positively and approximately linearly correlated with the proportion of the H2O-rich component in the mixture. We conclude that the degrees of melting by which Mariana trough magmas are generated increase from magmas similar to NMORB, through more H2O-enriched basalts, to 'arc-like' basalts, and that this increase is due to the lowering of the solidus of mantle peridotite that accompanies addition of the H2O-rich component. The H2O-rich component is likely to be ultimately derived from fluid from a subducting slab, but we propose that by the time fluids reach the source regions of Mariana trough basalts, they have interacted with sufficient mantle material that for all but the most incompatible of elements (with respect to fluid-mantle interaction), they are in equilibrium with the mantle. In contrast, fluids added to the source regions of Mariana island-arc magmas have typically interacted with less mantle and thus retain the signature of slab-derived fluids to varying degrees for all but the most compatible elements. Primitive Mariana arc basalts can be generated by melting mixtures of such incompletely exchanged slab-derived fluids and sources similar to NMORB-type mantle sources, but the degrees of melting are typically higher than those of Mariana trough NMORB and the sources have been variably depleted relative to the back-arc sources by previous melt extraction. This depletion may be related to earlier extraction of back-arc basin magmas or may evolve by repeated fluxing of the sources as fluid is continually added to them in the regions of arc magma generation. If fluid with partitioning behavior relative to the solid mantle similar to that deduced for the H2O-rich component involved in the generation of Mariana trough basalts were extracted from primitive mantle, the residual mantle would have many of the minor and trace element characteristics of typical oceanic upper mantle; primitive mantle enriched in such fluid would be a satisfactory source for the continental crust in terms of its trace and minor element chemical composition.

Petrology of exhumed mantle rocks at passive margins: ancient lithosphere and rejuvenation processes

NASA Astrophysics Data System (ADS)

Müntener, Othmar; McCarthy, Anders; Picazo, Suzanne

2014-05-01

Mantle peridotites from ocean-continent transition zones (OCT's) and ultraslow spreading ridges question the commonly held assumption of a simple link between mantle melting and MORB. 'Ancient' and partly refertilized mantle in rifts and ridges illustrates the distribution of the scale of chemical and isotopic upper mantle heterogeneity even on a local scale. Field data and petrology demonstrates that ancient, thermally undisturbed, pyroxenite-veined subcontinental mantle blobs formed parts of the ocean floor next to thinned continental crust. These heterogeneities might comprise an (ancient?) subduction component. Upwelling of partial melts that enter the conductive lithospheric mantle inevitably leads to freezing of the melt and refertilization of the lithosphere and this process might well be at the origin of the difference between magma-poor and volcanic margins. Similar heterogeneity might be created in the oceanic lithosphere, in particular at slow to ultra-slow spreading ridges where the thermal boundary layer (TBM) is thick and may be veined with metasomatic assemblages that might be recycled in subduction zones. In this presentation, we provide a summary of mantle compositions from the European realm to show that inherited mantle signatures from previous orogenies play a key role on the evolution of rift systems and on the chemical diversity of peridotites exposed along passive margins and ultra-slow spreading ridges. Particularly striking is the abundance of plagioclase peridotites in the Alpine ophiolites that are interpreted as recorders of refertilization processes related to thinning and exhumation of mantle lithosphere. Another important result over the last 20 years was the discovery of extremely refractory Nd-isotopic compositions with highly radiogenic 147Sm/144Nd which indicates that partial melting processes and Jurassic magmatism in the Western Thetys are decoupled. Although the isotopic variability might be explained by mantle heterogeneities, an alternative is that these depleted domains represent snapshots of melting processes that are related to Permian and/or even older crust forming processes. The findings of the these refractory mantle rocks over the entire Western Alpine arc and the similarity in model ages of depletion suggests a connection to the Early Permian magmatic activity. Shallow and deep crustal magmatism in the Permian is widespread over Western Europe and the distribution of these mafic rocks are likely to pre-determine the future areas of crustal thinning and exhumation during formation of the Thethyan passive margins.

Abrupt Upper-Plate Tilting Upon Slab-Transition-Zone Collision

NASA Astrophysics Data System (ADS)

Crameri, F.; Lithgow-Bertelloni, C. R.

2017-12-01

During its sinking, the remnant of a surface plate crosses and interacts with multiple boundaries in Earth's interior. The most-prominent dynamic interaction arises at the upper-mantle transition zone where the sinking plate is strongly affected by the higher-viscosity lower mantle. Within our numerical model, we unravel, for the first time, that this very collision of the sinking slab with the transition zone induces a sudden, dramatic downward tilt of the upper plate towards the subduction trench. The slab-transition zone collision sets parts of the higher-viscosity lower mantle in motion. Naturally, this then induces an overall larger return flow cell that, at its onset, tilts the upper plate abruptly by around 0.05 degrees and over around 10 Millions of years. Such a significant and abrupt variation in surface topography should be clearly visible in temporal geologic records of large-scale surface elevation and might explain continental-wide tilting as observed in Australia since the Eocene or North America during the Phanerozoic. Unravelling this crucial mantle-lithosphere interaction was possible thanks to state-of-the-art numerical modelling (powered by StagYY; Tackley 2008, PEPI) and post-processing (powered by StagLab; www.fabiocrameri.ch/software). The new model that is introduced here to study the dynamically self-consistent temporal evolution of subduction features accurate subduction-zone topography, robust single-sided plate sinking, stronger plates close to laboratory values, an upper-mantle phase transition and, crucially, simple continents at a free surface. A novel, fully-automated post-processing includes physical model diagnostics like slab geometry, mantle flow pattern, upper-plate tilt angle and trench location.

Heterogeneous source components of intraplate basalts from NE China induced by the ongoing Pacific slab subduction

NASA Astrophysics Data System (ADS)

Chen, Huan; Xia, Qun-Ke; Ingrin, Jannick; Deloule, Etienne

2016-04-01

In recent few years, the recycled oceanic slab has been increasingly suggested to be the enriched component in the mantle source of widespread intra-plate small-volume basaltic magmatism in eastern China. The recycled oceanic slab is a mixture of sediment, upper oceanic crust and lower gabbro oceanic crust, and will undergo alteration and dehydration during the recycling progress. The influence of these different components on the mantle source needs to be further constrained. The Chaihe-aershan volcanic field in Northeast China is located close to the surface position of the front edge of the subducted Pacific slab and includes more than 35 small-volume Quaternary basaltic volcanoes, which provides an opportunity to study the evolution of mantle source in detail and the small-scale geochemical heterogeneity of the mantle source. We measured the oxygen isotopes and water content of clinopyroxene (cpx) phenocrysts by secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectrometry (FTIR), respectively. The water content of magma was then estimated based on the partition coefficient of H2O between cpx and basaltic melt. The measured δ18O of cpx phenocrysts (4.27 to 8.57) and the calculated H2O content of magmas (0.23-2.70 wt.%) show large variations, reflecting the compositional heterogeneity of the mantle source. The δ18O values within individual samples also display a considerable variation, from 1.28 to 2.31‰ suggesting mixing of magmas or the sustained injection of magmas with different δ18O values during the crystallization. The relationship between the averaged δ18O values of cpx phenocrysts and the H2O/Ce, Ba/Th, Nb/La ratios and Eu anomaly of whole-rocks demonstrates the contribution to three components in the mantle source (hydrothermally altered upper oceanic crust or marine sediments, altered lower gabbroic oceanic crust, ambient mantle). The proportions of these three components varied strongly within a limited period (˜1.27 Ma to ˜0.25 Ma). As only the Pacific slab is constantly subducted to the eastern Asia during that time, we suggested that its ongoing subduction is the only reasonable candidate to result in the compositional heterogeneity and rapid variation of enriched components in such a limited and recent time. Combines with previous studies on other basalt localities of eastern China, these new results confirm that the Pacific slab subduction play a key role in the triggering of the wide spread Cenozoic basaltic volcanism in eastern China.

Origin and Distribution of Water Contents in Continental and Oceanic Lithospheric Mantle

NASA Technical Reports Server (NTRS)

Peslier, Anne H.

2013-01-01

The water content distribution of the upper mantle will be reviewed as based on the peridotite record. The amount of water in cratonic xenoliths appears controlled by metasomatism while that of the oceanic mantle retains in part the signature of melting events. In both cases, the water distribution is heterogeneous both with depth and laterally, depending on localized water re-enrichments next to melt/fluid channels. The consequence of the water distribution on the rheology of the upper mantle and the location of the lithosphere-asthenosphere boundary will also be discussed.

Origin of primary PGM assemblage in сhromitite from a mantle tectonite at Harold's Grave (Shetland Ophiolite Complex, Scotland)

NASA Astrophysics Data System (ADS)

Badanina, Inna Yu.; Malitch, Kreshimir N.; Lord, Richard A.; Meisel, Thomas C.

2013-12-01

In this paper we present textural and mineral chemistry data for a PGM inclusion assemblage and whole-rock platinum-group element (PGE) concentrations of chromitite from Harold's Grave, which occurrs in a dunite pod in a mantle tectonite at Unst in the Shetland Ophiolite Complex (SOC), Scotland. The study utilized a number of analytical techniques, including acid digestion and isotope dilution (ID) ICP-MS, hydroseparation and electron microprobe analysis. The chromitite contains a pronounced enrichment of refractory PGE (IPGE: Os, Ir and Ru) over less refractory PGE (PPGE: Rh, Pt and Pd), typical of mantle hosted `ophiolitic' chromitites. A `primary' magmatic PGM assemblage is represented by euhedrally shaped (up to 60 μm in size) single and composite inclusions in chromite. Polyphase PGM grains are dominated by laurite and osmian iridium, with subordinate laurite + osmian iridium + iridian osmium and rare laurite + Ir-Rh alloy + Rh-rich sulphide (possibly prassoite). The compositional variability of associated laurite and Os-rich alloys at Harold's Grave fit the predicted compositions of experiment W-1200-0.37 of Andrews and Brenan (Can Mineral 40: 1705-1716, 2002) providing unequivocal information on conditions of their genesis, with the upper thermal stability of laurite in equilibrium with Os-rich alloys estimated at 1200-1250 °C and f(S2) of 10-0.39-10-0.07.

Plumes do not Exist: Plate Circulation is Confined to Upper Mantle

NASA Astrophysics Data System (ADS)

Hamilton, W. B.

2002-12-01

Plumes from deep mantle are widely conjectured to define an absolute reference frame, inaugurate rifting, drive plates, and profoundly modify oceans and continents. Mantle properties and composition are assumed to be whatever enables plumes. Nevertheless, purported critical evidence for plume speculation is false, and all data are better interpreted without plumes. Plume fantasies are made ever more complex and ad hoc to evade contradictory data, and have no predictive value because plumes do not exist. All plume conjecture derives from Hawaii and the guess that the Emperor-Hawaii inflection records a 60-degree change in Pacific plate direction at 45 Ma. Paleomagnetic latitudes and smooth Pacific spreading patterns disprove any such change. Rationales for other fixed plumes collapse when tested, and hypotheses of jumping, splitting, and gyrating plumes are specious. Thermal and physical properties of Hawaiian lithosphere falsify plume predictions. Purported tomographic support elsewhere represents artifacts and misleading presentations. Asthenosphere is everywhere near solidus temperature, so melt needs a tensional setting for egress but not local heat. Gradational and inconsistent contrasts between MORB and OIB are as required by depth-varying melt generation and behavior in contrasted settings and do not indicate systematically unlike sources. MORB melts rise, with minimal reaction, through hot asthenosphere, whereas OIB melts react with cool lithosphere, and lose mass, by crystallizing refractories and retaining and assimilating fusibles. The unfractionated lower mantle of plume conjecture is contrary to cosmologic and thermodynamic data, for mantle below 660 km is more refractory than that above. Subduction, due to density inversion by top-down cooling that forms oceanic lithosphere, drives plate tectonics and upper-mantle circulation. It organizes plate motions and lithosphere stress, which controls plate boundaries and volcanic chains. Hinge rollback is the key to kinematics. Arcs advance and collide, fast-spreading Pacific shrinks, etc. A fore-arc basin atop an overriding plate shows that hinge and non-shortening plate front there track together: velocities of rollback and advance are equal. Convergence velocity commonly also equals rollback velocity but often is greater. Slabs sinking broadside push upper mantle back under incoming plates and force rapid Pacific spreading, whereas overriding plates flow forward with retreating hinges. Backarc basins open behind island arcs migrating with hinges. Slabs settle on uncrossable 660-km discontinuity. (Contrary tomographic claims reflect sampling and smearing artifacts, notably due to along-slab raypaths.) Plates advance over sunken slabs and mantle displaced rearward by them, and ridges spread where advancing plates pull away. Ridges migrate over asthenosphere, producing geophysical and bathymetric asymmetry, and tap fresh asthenosphere into which slab material is recycled upward. Sluggish deep-mantle circulation is decoupled from rapid upper-mantle circulation, so plate motions can be referenced to semistable lower mantle. Global plate motions make kinematic sense if Antarctica, almost ringed by departing ridges and varying little in Cenozoic paleomagnetic position, is stationary: hinges roll back, ridges migrate, and directions and velocities of plate rotations accord with subduction, including sliding and crowding of oceanic lithosphere toward free edges, as the dominant drive. (The invalid hotspot and no-net-rotation frames minimize motions of hinges and ridges, and their plate motions lack kinematic sense.) Northern Eurasia also is almost stationary, Africa rotates very slowly counterclockwise toward Aegean and Zagros, Pacific plate races toward surface-exit subduction systems, etc.

Secondary overprinting of S-Se-Te signatures in the Earth's mantle: Implications for the Late Veneer

NASA Astrophysics Data System (ADS)

Koenig, S.; Luguet, A.; Lorand, J.; Pearson, D.

2013-12-01

Sulphur, Selenium and Tellurium are both chalcophile and highly siderophile elements (HSE) with near-chondritic ratios and absolute abundances in the terrestrial mantle that exceed those predicted by core-mantle differentiation[1]. These 'excess' HSE abundances have been attributed to addition of ca. 0.5% of chondrite-like material that hit the Earth in its accretionary stage between 4 to 3.8 billion years ago after core-mantle differentiation (Late Veneer[2]). Therefore, like other HSE, S, Se and Te are considered potential tracers for the composition of the Late Veneer, provided that their bulk silicate Earth abundances are properly constrained. In contrast to ca. 250 ppm S, Se and Te are ultra-trace elements in the terrestrial mantle. Like all HSE, they are furthermore controlled by base metal sulphides (BMS) and micrometric platinum group minerals (PGMs)[3]. This strong control exerted by the host mineralogy and petrology on the S-Se-Te systematics at both the micro-scale and the whole-rock scale makes detailed mineralogical and petrological studies of BMS and PGM a pre-requisite to fully understand and accurately interpret the whole-rock signatures. Here we combine in-situ sulphide data and detailed mineralogical observations with whole-rock S-Se-Te-HSE signatures of both lherzolites and harburgites from different geodynamic settings. We demonstrate that the near-chondritic Se and Te signature of 'fertile' mantle rocks (Se/Te ≈9×5) is not a primitive signature of the Earth's mantle, but rather reflects strong enrichment in metasomatic HSE host phases, which erased previous pristine signatures. Consequently, current attempts to identify a potential Late Veneer composition are seriously flawed because, neither refertilisation/metasomatism nor true melt depletion (e.g. harzburgitic residues) have been taken into account for the Primitive Upper Mantle composition estimate[4]. Our combined whole rock and in-situ sulphide data indicate a refertilisation trend towards sub-chondritic Se/Te ratios (i.e. Se/Te < 2). On the other hand, harzburgites that preserve depletion signatures show suprachondritic Se/Te ratios (< 31). Altogether this shows that metasomatic enrichment of mantle rocks may lead to a systematic bias and hence underestimation of the current Se/Te estimate of the primitive mantle. The metasomatic origin of the reported S, Se and Te ratios in peridotites that reflect the control of metasomatic BMS and PGMs[5;6] furthermore show that not all whole rock signatures in the Earth's mantle that scatter around near-chondritic values are primary and hence challenge the simple conception that these features may readily solve the long-standing conundrum of the Late Veneer composition. Refs: [1] Rose-Weston et al. (2009) GCA 73, 4598-4615; [2] Kimura et al. (1974) GCA 38, 683-701; [3] Lorand and Alard (2010) 67, 4137-4151; [4] Wang and Becker (2013) Nature 499, 328-331; [5] König et al. (2012) GCA 86, 354-366; [6] König et al. (2013, in press), EPSL.

Adjoint tomography of crust and upper-mantle structure beneath Continental China

NASA Astrophysics Data System (ADS)

Chen, M.; Niu, F.; Liu, Q.; Tromp, J.

2013-12-01

Four years of regional earthquake recordings from 1,869 seismic stations are used for high-resolution and high-fidelity seismic imaging of the crust and upper-mantle structure beneath Continental China. This unprecedented high-density dataset is comprised of seismograms recorded by the China Earthquake Administration Array (CEArray), NorthEast China Extended SeiSmic Array (NECESSArray), INDEPTH-IV Array, F-net and other global and regional seismic networks, and involves 1,326,384 frequency-dependent phase measurements. Adjoint tomography is applied to this unprecedented dataset, aiming to resolve detailed 3D maps of compressional and shear wavespeeds, and radial anisotropy. Contrary to traditional ray-theory based tomography, adjoint tomography takes into account full 3D wave propagation effects and off-ray-path sensitivity. In our implementation, it utilizes a spectral-element method for precise wave propagation simulations. The tomographic method starts with a 3D initial model that combines smooth radially anisotropic mantle model S362ANI and 3D crustal model Crust2.0. Traveltime and amplitude misfits are minimized iteratively based on a conjugate gradient method, harnessing 3D finite-frequency kernels computed for each updated 3D model. After 17 iterations, our inversion reveals strong correlations of 3D wavespeed heterogeneities in the crust and upper mantle with surface tectonic units, such as the Himalaya Block, the Tibetan Plateau, the Tarim Basin, the Ordos Block, and the South China Block. Narrow slab features emerge from the smooth initial model above the transition zone beneath the Japan, Ryukyu, Philippine, Izu-Bonin, Mariana and Andaman arcs. 3D wavespeed variations appear comparable to or much sharper than in high-frequency P-and S-wave models from previous studies. Moreover our results include new information, such as 3D variations of radial anisotropy and the Vp/Vs ratio, which are expected to shed new light to the composition, thermal state, flow or fabric structure in the crust and upper mantle, as well as the related dynamical processes. We intend to use these seismic images to answer important tectonic questions, namely, 1) what controls the strength of the lithosphere; 2) how does lithosphere deform during the formation of orogens, basins and plateaus; 3) how pervasive is lithospheric delamination or partial removal beneath orogens and plateaus; 3) whether or not (and how) are slab segmentation and penetration into the lower mantle linked to upwellings associated with widespread magmatism in East Asia.

Sound velocity of MgSiO 3 perovskite to Mbar pressure

NASA Astrophysics Data System (ADS)

Murakami, Motohiko; Sinogeikin, Stanislav V.; Hellwig, Holger; Bass, Jay D.; Li, Jie

2007-04-01

Brillouin scattering measurements of the aggregate shear wave velocities in MgSiO 3 perovskite were conducted at high pressure conditions relevant to the Earth's lowermost mantle, approaching 1 Mbar. Infrared laser annealing of samples in a diamond anvil cell allowed us to obtain high quality Brillouin spectra and to drastically extend the upper limit of pressure for Brillouin measurements. We found that the pressure derivative of the shear modulus (d G / d P = G') of MgSiO 3 perovskite is 1.56 ± 0.04, which is distinctly lower than that of previous lower-pressure experiments below 9 GPa. Extrapolation of the high-pressure shear velocities of perovskite to ambient pressure, are in excellent agreement with earlier room pressure Brillouin measurements. The shear modulus, shear velocity and longitudinal velocity at ambient pressure were determined to be G0 = 172.9(15) GPa, VS = 6.49(3) and VP = 10.85(3) km/sec. The mineralogical model that provides a best fit to global seismological 1-D velocity profiles is one that contains 85-90 vol% perovskite in the lower mantle. The results of this study are the first to demonstrate that the elastic wave velocities for a near-adiabatic lower mantle with a bulk composition dominated by magnesium silicate perovskite are consistent with the average lower mantle seismic velocity structure. The large pressure range over which acoustic measurements of MgSiO 3 perovskite performed in this study has thus allowed us to put tighter constraints on compositional models of the Earth's lower mantle.

Seismological Signature of Chemical Differentiation of Earth's Upper Mantle

NASA Astrophysics Data System (ADS)

Matsukage, K. N.; Nishihara, Y.; Karato, S.

2004-12-01

Chemical differentiation from a primitive rock (such as pyrolite) to harzburgite due to partial melting and melt extraction is one of the most important mechanisms that causes the chemical heterogeneity in Earth's upper mantle. In this study, we investigate the seismic signature of chemical differentiation that helps mapping chemical heterogeneity in the upper mantle. The relation between chemical differentiation and its seismological signature is not straightforward because a large number of unknown parameters are involved although the seismological observations provide only a few parameters (e.g., VP, VS, QP). Therefore it is critical to identify a small number of parameters by which the gross trend of chemical evolution can be described. The variation in major element composition in natural samples reflect complicated processes that include not only partial melting but also other complex processes (e.g., metasomatism, influx melting). We investigate the seismic velocities of hypothetical but well-defined simple chemical differentiation processes (e.g., partial melting of various pressure conditions, addition of Si-rich melt or fluid), which cover the chemical variation of the natural mantle peridotites with various tectonic settings (mid ocean ridge, island arc and continent). The seismic velocities of the peridotites were calculated to 13 GPa and 1730 K. We obtained two major conclusions. First is that the variations of seismic velocities of upper mantle peridotites can be interpreted in terms of a few distinct parameters. For one class of peridotites which is formed by simple partial melting (e.g. mid-ocean ridges peridotites), seismic velocities can be described in terms of one parameter, namely Mg# (=Mg/(Mg+Fe) atomic ratio). In contrast, some of the peridotites in the continental (cratonic) environment with high silica content and high Mg# need at least two parameters (such as Mg# and Opx# (the volume fraction of orthopyroxene)) are needed to characterize their seismic velocities. Second is the jump of seismic velocity at 300 km in harzburgite that is caused by orthorhombic (opx) to high-pressure monoclinic phase transition in MgSiO3 pyroxene. If opx-rich harzburgite (the maximum content of opx in continental harzburgite is ˜45 vol%) exists at around 300km, the maximum contrast of jump would be 2.5 % for VS and 0.9 % for VP. This phase transition will correspond to the seismological discontinuity around 300km (X-discontinuity).

Carbon-saturated monosulfide melting in the shallow mantle: solubility and effect on solidus

NASA Astrophysics Data System (ADS)

Zhang, Zhou; Lentsch, Nathan; Hirschmann, Marc M.

2015-12-01

We present high-pressure experiments from 0.8 to 7.95 GPa to determine the effect of carbon on the solidus of mantle monosulfide. The graphite-saturated solidus of monosulfide (Fe0.69Ni0.23Cu0.01S1.00) is described by a Simon and Glatzel (Z Anorg Allg Chem 178:309-316, 1929) equation T (°C) = 969.0[ P (GPa)/5.92 + 1]0.39 (1 ≤ P ≤ 8) and is 80 ± 25 °C below the melting temperature found for carbon-free conditions. A series of comparison experiments using different capsule configurations and preparations document that the observed solidus-lowering is owing to graphite saturation and not an artifact of different capsules or hydrogen contamination. Concentrations of carbon in quenched graphite-saturated monosulfide melt measured by electron microprobe are 0.1-0.3 wt% in monosulfide melt and below the detection limit (<0.2 wt%) in crystalline monosulfide solid solution. Although there is only a small amount of carbon dissolved in monosulfide melts, the substantial effect on monosulfide solidus temperature means that the carbon-saturated monosulfide (Fe0.69Ni0.23Cu0.01S1.00) solidus intersects continental mantle geotherms inferred from diamond inclusion geobarometry at 6-7 GPa ( 200 km), whereas carbon-free monosulfide (Fe0.69Ni0.23Cu0.01S1.00) solidus does not. The composition investigated (Fe0.69Ni0.23Cu0.01S1.00) has a comparatively low metal/sulfur (M/S) ratio and low Ni/(Fe + Ni), but sulfides with higher (M/S) and with greater Ni/(Fe + Ni) should melt at lower temperatures and these should have a broader melt stability field in the diamond formation environment and in the continental lithosphere. Low carbon solubility in monosulfide melt excludes the possibility that diamonds are crystallized from sulfide melt. Although monosulfide melt can store no more than 2 ppm C in a bulk mantle with 225 ppm S, melts with higher M/S could be a primary host of carbon in the deeper part of the upper mantle. For example, the storage capacity of C in sulfide melts in the deep upper mantle ( 400 km) for a depleted mantle domain (MORB source, 120 ± 30 ppm S) is estimated to be 57 ±_{30}^{63} ppm, and so all the C could be in a sulfide melt. In an enriched (OIB source, 225 ± 25 ppm S) mantle domain, the C stored in sulfide melt in the deep upper mantle is estimated to be 86 ±_{44}^{92} ppm, which would amount to about half the available carbon.

Compositional Variation of Terrestrial Mantle Apatites and Implications for the Halogen and Water Budgets of the Terrestrial Mantle

NASA Astrophysics Data System (ADS)

Roden, M.; Patino Douce, A. E.; Chaumba, J. B.; Fleisher, C.; Yogodzinski, G. M.

2011-12-01

Apatite in ultramafic xenoliths from various tectonic enviroments including arc (Kamchatka), plume (Hawaii), and intraplate (Lunar Crater, Nunivak, Colorado Plateau) were analyzed by electron microprobe with the aim of characterizing the Cl and F contents, and from these measured compositions to infer the nature of fluids/melts that the apatites equilibrated with. The impetus for the study derived from the generalization of O'Reilly and Griffin (1) that mantle-derived metasomatic apatites tend to be Cl-rich and mantle-derived igneous apatites tend to be F-rich. Our work largely corroborates their generalization with Cl- and/or H2O-rich compositions characterizing the apatites from Nunivak and Kamchatka while apatites from igneous or Group II xenoliths tend to be Cl-poor and be either nearly pure fluorapatite or a mix of hydroxylapatite and fluorapatite. We attribute the Cl-rich nature of the Kamchatka apatites to formation from Cl-rich fluids generated from subducted lithosphere; however the Nunivak occurrence is far removed from subducted lithosphere and may reflect a deep seated source for Cl as also indicated by brine inclusions in diamonds, Cl-rich apatites in carbonate-bearing xenoliths and a Cl-rich signature in some plumes such as Iceland, Azores and Samoa. One curious aspect of mantle-derived apatite compositions is that xenoliths with evidence of carbonatitic metasomatism commonly have Cl-rich apatites while apatites from carbonatites are invariably Cl-poor - perhaps reflecting loss of Cl in fluids evolved from the carbonatitic magma. Apatites from Group II xenoliths at Hawaii are solid solutions between fluorapatite and hydroxylapatite and show no evidence for deep-seated Cl at Hawaii. Samples of the terrestrial mantle are almost uniformly characterized by mineral assemblages with a single Ca-rich phosphate phase but the mantles of Mars, Vesta and the Moon have two Ca-rich phosphates, apatite and volatile-poor merrillite - apatite compositions existing with merrillite are typically Cl- and F-rich in the case of Mars but F-rich in the case of the Moon and Vesta (2-4). In a single reported example of terrestrial mantle xenoliths containing apatite and and a similar volatile-poor Ca-phosphate, whitlockite, the apatite contained significant Cl and H2O but was F-rich and similar to some lunar apatites. Our thermodynamic analysis of apatite-merrillite equilibria suggests that high phosphorous chemical potentials combined with high halogen and low water fugacities are required for the coexistence of a volatile-poor Ca-phosphate with apatite, and point out the relatively unique and typically water-rich nature of the upper mantle of the Earth compared to other differentiated planetary bodies. References 1. S. O'Reilly & W. Griffin, 2000, Lithos 53: 217. 2. A. Patiño Douce et al., 2011, Chem Geol. in press 3. F. McCubbin et al. 2009, LPSC abs 2246 4. A. Sarafian et al. 2011, Meteor. Soc. Abs 5023

Evaluation of the Lithospheric Contribution to Southern Rio Grande Rift Mafic Melts

NASA Astrophysics Data System (ADS)

Konter, J. G.; Crocker, L.; Anaya, L. M.; Rooney, T. O.

2011-12-01

As continental rifting proceeds, the accommodation of lithospheric thinning by mechanical extension and magmatic intrusion represents an important but poorly constrained tectonic process. Insight into role of the magmatic component may come from the composition of volcanic products, which can record magma-lithosphere interactions. The volcanic activity in continental rift environments is frequently characterized by bimodal associations of mafic and silicic volcanism with heterogenous lithospheric contributions. We present a new integrated data set from several mafic volcanic fields in the Rio Grande Rift, consisting of major and trace element compositions, as well as isotopes. This data set provides insight into asthenospheric melting processes and interactions with the overlying lithosphere. The melting processes and the related extensional volcanism is the result of foundering of the Farallon slab. Large volume silicic eruptions such as those in the Sierra Madre Occidental originate from a large contribution of lithospheric melting, with a subordinate asthenospheric contribution. In contrast, Late Tertiary and Quaternary basaltic volcanic fields in the Rio Grande Rift were likely sourced in the asthenosphere and did not reside in the lithosphere for substantial periods. As a result the region is the ideal natural laboratory to investigate the interaction of asthenospheric melts with the lithosphere. In particular the wide array of volcanic fields contain multiple xenolith localities, such as Kilbourne Hole, providing direct samples of lithosphere and crust. Although previous studies have focused on correlations between amount of extension related to Farallon slab foundering, volcanic compositions, and their mantle sources, we present data that suggest that some compositional signatures may pre-date current tectonic processes. Radiogenic isotope data from several volcanic fields in New Mexico show a converging pattern in Pb isotope compositions, focusing on the unradiogenic Pb isotope composition of lower crustal xenoliths from Kilbourne Hole. The opposite ends of the converging trends are more radiogenic for some volcanic fields than the (lithospheric) mantle xenoliths of the Potrillo, San Carlos and Geranimo volcanic fields. Combined Pb-Sr isotope compositions for these fields are consistent with a trend from lower crustal xenoliths to mantle xenoliths, but show more variability. This variability may be explained by a small upper crustal contribution, in agreement with the Pb isotope systematics. Therefore, a common unradiogenic lower crustal composition likely contributed to the asthenospheric melts, followed by upper crustal contamination. The unradiogenic character of the lower crust implies an ancient event created the required low U/Pb ratios that generated the present-day Pb isotope compositions.

Primordial helium entrained by the hottest mantle plumes

NASA Astrophysics Data System (ADS)

Jackson, M. G.; Konter, J. G.; Becker, T. W.

2017-02-01

Helium isotopes provide an important tool for tracing early-Earth, primordial reservoirs that have survived in the planet’s interior. Volcanic hotspot lavas, like those erupted at Hawaii and Iceland, can host rare, high 3He/4He isotopic ratios (up to 50 times the present atmospheric ratio, Ra) compared to the lower 3He/4He ratios identified in mid-ocean-ridge basalts that form by melting the upper mantle (about 8Ra; ref. 5). A long-standing hypothesis maintains that the high-3He/4He domain resides in the deep mantle, beneath the upper mantle sampled by mid-ocean-ridge basalts, and that buoyantly upwelling plumes from the deep mantle transport high-3He/4He material to the shallow mantle beneath plume-fed hotspots. One problem with this hypothesis is that, while some hotspots have 3He/4He values ranging from low to high, other hotspots exhibit only low 3He/4He ratios. Here we show that, among hotspots suggested to overlie mantle plumes, those with the highest maximum 3He/4He ratios have high hotspot buoyancy fluxes and overlie regions with seismic low-velocity anomalies in the upper mantle, unlike plume-fed hotspots with only low maximum 3He/4He ratios. We interpret the relationships between 3He/4He values, hotspot buoyancy flux, and upper-mantle shear wave velocity to mean that hot plumes—which exhibit seismic low-velocity anomalies at depths of 200 kilometres—are more buoyant and entrain both high-3He/4He and low-3He/4He material. In contrast, cooler, less buoyant plumes do not entrain this high-3He/4He material. This can be explained if the high-3He/4He domain is denser than low-3He/4He mantle components hosted in plumes, and if high-3He/4He material is entrained from the deep mantle only by the hottest, most buoyant plumes. Such a dense, deep-mantle high-3He/4He domain could remain isolated from the convecting mantle, which may help to explain the preservation of early Hadean (>4.5 billion years ago) geochemical anomalies in lavas sampling this reservoir.

Venusian Applications of 3D Convection Modeling

NASA Technical Reports Server (NTRS)

Bonaccorso, Timary Annie

2011-01-01

This study models mantle convection on Venus using the 'cubed sphere' code OEDIPUS, which models one-sixth of the planet in spherical geometry. We are attempting to balance internal heating, bottom mantle viscosity, and temperature difference across Venus' mantle, in order to create a realistic model that matches with current planetary observations. We also have begun to run both lower and upper mantle simulations to determine whether layered (as opposed to whole-mantle) convection might produce more efficient heat transfer, as well as to model coronae formation in the upper mantle. Upper mantle simulations are completed using OEDIPUS' Cartesian counterpart, JOCASTA. This summer's central question has been how to define a mantle plume. Traditionally, we have defined a hot plume the region with temperature at or above 40% of the difference between the maximum and horizontally averaged temperature, and a cold plume as the region with 40% of the difference between the minimum and average temperature. For less viscous cases (1020 Pa?s), the plumes generated by that definition lacked vigor, displaying buoyancies 1/100th of those found in previous, higher viscosity simulations (1021 Pa?s). As the mantle plumes with large buoyancy flux are most likely to produce topographic uplift and volcanism, the low viscosity cases' plumes may not produce observable deformation. In an effort to eliminate the smallest plumes, we experimented with different lower bound parameters and temperature percentages.

Geochemical and NdSr isotopic composition of deep-sea turbidites: Crustal evolution and plate tectonic associations

NASA Astrophysics Data System (ADS)

McLennan, S. M.; Taylor, S. R.; McCulloch, M. T.; Maynard, J. B.

1990-07-01

Petrographic, geochemical, and isotopic data for turbidites from a variety of tectonic settings exhibit considerable variability that is related to tectonic association. Passive margin turbidites (Trailing Edge, Continental Collision) display high framework quartz (Q) content in sands, evolved major element compositions (high Si/Al, K/Na), incompatible element enrichments (high Th/Sc, La/Sc, La/Yb), negative Eu-anomalies and variable Th/U ratios. They have low 143Nd /144Nd and high 87Sr /86Sr ( ɛNd = -26 to -10; 87Sr /86Sr = 0.709 to 0.734 ), indicating a dominance of old upper crustal sources. Active margin settings (Fore Arc, Continental Arc, Back Arc, Strike Slip) commonly exhibit quite different compositions. Th/Sc varies from <0.01 to 1.8, and ɛNd varies from -13.8 to +8.3. Eu-anomalies range from no anomaly ( Eu/Eu ∗ = 1.0 ) to Eu-depletions typical of post-Archean shales ( Eu/Eu ∗ = 0.65 ). Active margin data are explained by mixtures of young arc-derived material, with variable composition and old upper crustal sources. Major element data indicate that passive margin turbidites have experienced more severe weathering histories than those from active settings. Most trace elements are enriched in muds relative to associated sands because of dilution effects from quartz and calcite and concentration of trace elements in clays. Exceptions include Zr, Hf (heavy mineral influence) and Tl (enriched in feldspar) which display enrichments in sands. Active margin sands commonly exhibit higher Eu/Eu ∗ than associated muds, resulting from concentration of plagioclase during sorting. Some associated sands and muds, especially from active settings, have systematic differences in Th/Sc ratios and Nd-isotopic composition, indicating that various provenance components may separate into different grain-size fractions during sedimentary sorting processes. Trace element abundances of modern turbidites, from both active and passive settings, differ from Archean turbidites in several important ways. Modern turbidites have less uniformity, for example, in Th/Sc ratios. On average, modern turbidites have greater depletions in Eu (lower Eu/Eu ∗) than do Archean turbidites, suggesting that the processes of intracrustal differentiation (involving plagioclase fractionation) are of greater importance for crustal evolution at modern continental margins than they were during the Archean. Modern turbidites do not display HREE depletion, a feature commonly seen in Archean data. HREE depletion ( Gd N/Yb N > 2.0 ) in Archean sediments results from incorporation of felsic igneous rocks that were in equilibrium (or their sources were in equilibrium) with garnet sometime in their history. Absence of HREE depletion at modern continental margins suggests that processes of crust formation (or mantle source compositions) may have differed. Differences in trace element abundances for Archean and modern turbidites add support to suggestions that upper continental crust compositions and major processes responsible for continental crust differentiation differed during the Archean. Neodymium model ages, thought to approximate average provenance age, are highly variable ( TDMND = 0-2.6 Ga) in modern turbidites, in contrast with studies that indicate Nd-model ages of lithified Phanerozoic sediment are fairly constant at about 1.5-2.0 Ga. This variability indicates that continental margin sediments incorporate new mantle-derived components, as well as continental crust of widely varying age, during recycling. The apparent dearth of ancient sediments with Nd-model age similar to stratigraphic age supports the suggestion that preservation potential of sediments is related to tectonic setting. Many samples from active settings have isotopic compositions similar to or only slightly evolved from mantle-derived igneous rocks. Subduction of active margin turbidites should be considered in models of crust-mantle recycling. For short-term recycling, such as that postulated for island arc petrogenesis, arc-derived turbidites cannot be easily recognized as a source component because of the lack of time available for isotopic evolution. If turbidites were incorporated into the sources of ocean island volcanics, the isotopic signatures would be considerably more evolved since most models call for long mantle storage times (1.0-2.0 Ga), prior to incorporation. Four provenance components are recognized on the basis of geochemistry and Nd-isotopic composition: (1) Old Upper Continental Crust (old igneous/metamorphic terranes, recycled sediment); (2) Young Undifferentiated Arc (young volcanic/plutonic source that has not experienced plagioclase fractionation); (3) Young Differentiated Arc (young volcanic/plutonic source that has experienced plagioclase fractionation); (4) MORB (minor). Relative proportions of these components are influenced by the plate tectonic association of the provenance and are typically (but not necessarily) reflected in the depositional basin. Provenance of quartzose (mainly passive settings) and non-quartzose (mainly active settings) turbidites can be characterized by bulk composition (e.g., Th/Sc) and Nd-isotopic composition (reflecting age).

Peridotites and basalts reveal broad congruence between two independent records of mantle fO2 despite local redox heterogeneity

NASA Astrophysics Data System (ADS)

Birner, Suzanne K.; Cottrell, Elizabeth; Warren, Jessica M.; Kelley, Katherine A.; Davis, Fred A.

2018-07-01

The oxygen fugacity (fO2) of the oceanic upper mantle has fundamental implications for the production of magmas and evolution of the Earth's interior and exterior. Mid-ocean ridge basalts and peridotites sample the oceanic upper mantle, and retain a record of oxygen fugacity. While fO2 has been calculated for mid-ocean ridge basalts worldwide (>200 locations), ridge peridotites have been comparatively less well studied (33 samples from 11 locations), and never in the same geographic location as basalts. In order to determine whether peridotites and basalts from mid-ocean ridges record congruent information about the fO2 of the Earth's interior, we analyzed 31 basalts and 41 peridotites from the Oblique Segment of the Southwest Indian Ridge. By measuring basalts and peridotites from the same ridge segment, we can compare samples with maximally similar petrogenetic histories. We project the composition and oxygen fugacity of each lithology back to source conditions, and evaluate the effects of factors such as subsolidus diffusion in peridotites and fractional crystallization in basalts. We find that, on average, basalts and peridotites from the Oblique Segment both reflect a source mantle very near the quartz-fayalite-magnetite (QFM) buffer. However, peridotites record a significantly wider range of values (nearly 3 orders of magnitude in fO2), with a single dredge recording a range in fO2 greater than that previously reported for mid-ocean ridge peridotites worldwide. This suggests that mantle fO2 may be heterogeneous on relatively short length scales, and that this heterogeneity may be obscured within aggregated basalt melts. We further suggest that the global peridotite fO2 dataset may not provide a representative sample of average basalt-source mantle. Our study motivates further investigation of the fO2 recorded by ridge peridotites, as peridotites record information about the fO2 of the Earth's interior that cannot be gleaned from analysis of basalts alone.

Constraining Mantle Differentiation Processes with La-Ce and Sm-Nd Isotope Systematics

NASA Astrophysics Data System (ADS)

Willig, M.; Stracke, A.

2016-12-01

Cerium (Ce) and Neodymium (Nd) isotopic ratios in oceanic basalts reflect the time integrated La-Ce and Sm-Nd ratios, and hence the extent of light rare earth element element (LREE) depletion or enrichment of their mantle sources. New high precision Ce-Nd isotope data from several ocean islands define a tight array in ԑCe-ԑNd space with ԑNd = -8.2±0.4 ԑCe + 1.3±0.9 (S.D.), in good agreement with previous data [1, 2]. The slope of the ԑCe-ԑNd array and the overall isotopic range are sensitive indicators of the processes that govern the evolution of the mantle's LREE composition. A Monte Carlo approach is employed to simulate continuous mantle-crust differentiation by partial melting and recycling of crustal materials. Partial melting of mantle peridotites produces variably depleted mantle and oceanic crust, which evolve for different time periods, before the oceanic crust is recycled back into the mantle including small amounts of continental crust (GLOSS [3]). Subsequently, depleted mantle and recycled materials of variable age and composition melt, and the respective melts mix in different proportions. Mixing lines strongly curve towards depleted mantle, and tend to be offset from the data for increasingly older and more depleted mantle. Observed ԑCe-ԑNd in ridge [1] and ocean island basalts and the slope of the ԑCe-ԑNd array therefore define upper limits for the extent and age of LREE depletion preserved in mantle peridotites. Very old average mantle depletion ages (> ca. 1-2 Ga) for the bulk of the mantle are difficult to reconcile with the existing ԑCe-ԑNd data, consistent with the range of Nd-Hf-Os model ages in abyssal peridotites [4-6]. Moreover, unless small amounts of continental crust are included in the recycled material, it is difficult to reproduce the relatively shallow slope of the ԑCe-ԑNd array, consistent with constraints from the ԑNd - ԑHf mantle array [7]. [1] Makishima and Masuda, 1994 Chem. Geol. 118, 1-8. [2] Doucelance et al., 2014 EPSL 407, 175-186. [3] Plank, 2014 ToG, 607-629. [4] Stracke et al., 2011 EPSL 308, 359-368. [5] Mallick et al., 2014 G-cubed 15, 2438-2453. [6] Harvey et al., 2006 EPSL 244, 606-621. [7] Chauvel et al. 2008. Nat. Geosci. 1, 64 - 67.

Silicate melts density, buoyancy relations and the dynamics of magmatic processes in the upper mantle

NASA Astrophysics Data System (ADS)

Sanchez-Valle, Carmen; Malfait, Wim J.

2016-04-01

Although silicate melts comprise only a minor volume fraction of the present day Earth, they play a critical role on the Earth's geochemical and geodynamical evolution. Their physical properties, namely the density, are a key control on many magmatic processes, including magma chamber dynamics and volcanic eruptions, melt extraction from residual rocks during partial melting, as well as crystal settling and melt migration. However, the quantitative modeling of these processes has been long limited by the scarcity of data on the density and compressibility of volatile-bearing silicate melts at relevant pressure and temperature conditions. In the last decade, new experimental designs namely combining large volume presses and synchrotron-based techniques have opened the possibility for determining in situ the density of a wide range of dry and volatile-bearing (H2O and CO2) silicate melt compositions at high pressure-high temperature conditions. In this contribution we will illustrate some of these progresses with focus on recent results on the density of dry and hydrous felsic and intermediate melt compositions (rhyolite, phonolite and andesite melts) at crustal and upper mantle conditions (up to 4 GPa and 2000 K). The new data on felsic-intermediate melts has been combined with in situ data on (ultra)mafic systems and ambient pressure dilatometry and sound velocity data to calibrate a continuous, predictive density model for hydrous and CO2-bearing silicate melts with applications to magmatic processes down to the conditions of the mantle transition zone (up to 2773 K and 22 GPa). The calibration dataset consist of more than 370 density measurements on high-pressure and/or water-and CO2-bearing melts and it is formulated in terms of the partial molar properties of the oxide components. The model predicts the density of volatile-bearing liquids to within 42 kg/m3 in the calibration interval and the model extrapolations up to 3000 K and 100 GPa are in good agreement with results from ab initio calculations. The density model has been applied to examine the mineral-melt buoyancy relations at depth and the implications of these results for the dynamics of magma chambers, crystal settling and the stability and mobility of magmas in the upper mantle will be discussed.

Ancient mantle in a modern arc: osmium isotopes in izu-bonin-mariana forearc peridotites

PubMed

Parkinson; Hawkesworth; Cohen

1998-09-25

Mantle peridotites drilled from the Izu-Bonin-Mariana forearc have unradiogenic 187Os/188Os ratios (0.1193 to 0.1273), which give Proterozoic model ages of 820 to 1230 million years ago. If these peridotites are residues from magmatism during the initiation of subduction 40 to 48 million years ago, then the mantle that melted was much more depleted in incompatible elements than the source of mid-ocean ridge basalts (MORB). This result indicates that osmium isotopes record information about ancient melting events in the convecting upper mantle not recorded by incompatible lithophile isotope tracers. Subduction zones may be a graveyard for ancient depleted mantle material, and portions of the convecting upper mantle may be less radiogenic in osmium isotopes than previously recognized.

Total meltwater volume since the Last Glacial Maximum and viscosity structure of Earth's mantle inferred from relative sea level changes at Barbados and Bonaparte Gulf and GIA-induced J˙2

NASA Astrophysics Data System (ADS)

Nakada, Masao; Okuno, Jun'ichi; Yokoyama, Yusuke

2016-02-01

Inference of globally averaged eustatic sea level (ESL) rise since the Last Glacial Maximum (LGM) highly depends on the interpretation of relative sea level (RSL) observations at Barbados and Bonaparte Gulf, Australia, which are sensitive to the viscosity structure of Earth's mantle. Here we examine the RSL changes at the LGM for Barbados and Bonaparte Gulf ({{RSL}}_{{L}}^{{{Bar}}} and {{RSL}}_{{L}}^{{{Bon}}}), differential RSL for both sites (Δ {{RSL}}_{{L}}^{{{Bar}},{{Bon}}}) and rate of change of degree-two harmonics of Earth's geopotential due to glacial isostatic adjustment (GIA) process (GIA-induced J˙2) to infer the ESL component and viscosity structure of Earth's mantle. Differential RSL, Δ {{RSL}}_{{L}}^{{{Bar}},{{Bon}}} and GIA-induced J˙2 are dominantly sensitive to the lower-mantle viscosity, and nearly insensitive to the upper-mantle rheological structure and GIA ice models with an ESL component of about (120-130) m. The comparison between the predicted and observationally derived Δ {{RSL}}_{{L}}^{{{Bar}},{{Bon}}} indicates the lower-mantle viscosity higher than ˜2 × 1022 Pa s, and the observationally derived GIA-induced J˙2 of -(6.0-6.5) × 10-11 yr-1 indicates two permissible solutions for the lower mantle, ˜1022 and (5-10) × 1022 Pa s. That is, the effective lower-mantle viscosity inferred from these two observational constraints is (5-10) × 1022 Pa s. The LGM RSL changes at both sites, {{RSL}}_{{L}}^{{{Bar}}} and {{RSL}}_{{L}}^{{{Bon}}}, are also sensitive to the ESL component and upper-mantle viscosity as well as the lower-mantle viscosity. The permissible upper-mantle viscosity increases with decreasing ESL component due to the sensitivity of the LGM sea level at Bonaparte Gulf ({{RSL}}_{{L}}^{{{Bon}}}) to the upper-mantle viscosity, and inferred upper-mantle viscosity for adopted lithospheric thicknesses of 65 and 100 km is (1-3) × 1020 Pa s for ESL˜130 m and (4-10) × 1020 Pa s for ESL˜125 m. The former solution of (1-3) × 1020 Pa s is consistent with the inferences from the postglacial differential RSL changes in the Australian region and also inversion study of far-field sea-level data. The inference of the viscosity structure based on these four observational constraints is, however, relatively insensitive to the viscosity structure of D″ layer.

Inference of viscosity jump at 670 km depth and lower mantle viscosity structure from GIA observations

NASA Astrophysics Data System (ADS)

Nakada, Masao; Okuno, Jun'ichi; Irie, Yoshiya

2018-03-01

A viscosity model with an exponential profile described by temperature (T) and pressure (P) distributions and constant activation energy (E_{{{um}}}^{{*}} for the upper mantle and E_{{{lm}}}^* for the lower mantle) and volume (V_{{{um}}}^{{*}} and V_{{{lm}}}^*) is employed in inferring the viscosity structure of the Earth's mantle from observations of glacial isostatic adjustment (GIA). We first construct standard viscosity models with an average upper-mantle viscosity ({\\bar{η }_{{{um}}}}) of 2 × 1020 Pa s, a typical value for the oceanic upper-mantle viscosity, satisfying the observationally derived three GIA-related observables, GIA-induced rate of change of the degree-two zonal harmonic of the geopotential, {\\dot{J}_2}, and differential relative sea level (RSL) changes for the Last Glacial Maximum sea levels at Barbados and Bonaparte Gulf in Australia and for RSL changes at 6 kyr BP for Karumba and Halifax Bay in Australia. Standard viscosity models inferred from three GIA-related observables are characterized by a viscosity of ˜1023 Pa s in the deep mantle for an assumed viscosity at 670 km depth, ηlm(670), of (1 - 50) × 1021 Pa s. Postglacial RSL changes at Southport, Bermuda and Everglades in the intermediate region of the North American ice sheet, largely dependent on its gross melting history, have a crucial potential for inference of a viscosity jump at 670 km depth. The analyses of these RSL changes based on the viscosity models with {\\bar{η }_{{{um}}}} ≥ 2 × 1020 Pa s and lower-mantle viscosity structures for the standard models yield permissible {\\bar{η }_{{{um}}}} and ηlm (670) values, although there is a trade-off between the viscosity and ice history models. Our preferred {\\bar{η }_{{{um}}}} and ηlm (670) values are ˜(7 - 9) × 1020 and ˜1022 Pa s, respectively, and the {\\bar{η }_{{{um}}}} is higher than that for the typical value of oceanic upper mantle, which may reflect a moderate laterally heterogeneous upper-mantle viscosity. The mantle viscosity structure adopted in this study depends on temperature distribution and activation energy and volume, and it is difficult to discuss the impact of each quantity on the inferred lower-mantle viscosity model. We conclude that models of smooth depth variation in the lower-mantle viscosity following η ( z ) ∝ {{ exp}}[ {( {E_{{{lm}}}^* + P( z )V_{{{lm}}}^*} )/{{R}}T( z )} ] with constant E_{{{lm}}}^* and V_{{{lm}}}^* are consistent with the GIA observations.

Mantle structure and tectonic history of SE Asia

NASA Astrophysics Data System (ADS)

Hall, Robert; Spakman, Wim

2015-09-01

Seismic travel-time tomography of the mantle under SE Asia reveals patterns of subduction-related seismic P-wave velocity anomalies that are of great value in helping to understand the region's tectonic development. We discuss tomography and tectonic interpretations of an area centred on Indonesia and including Malaysia, parts of the Philippines, New Guinea and northern Australia. We begin with an explanation of seismic tomography and causes of velocity anomalies in the mantle, and discuss assessment of model quality for tomographic models created from P-wave travel times. We then introduce the global P-wave velocity anomaly model UU-P07 and the tectonic model used in this paper and give an overview of previous interpretations of mantle structure. The slab-related velocity anomalies we identify in the upper and lower mantle based on the UU-P07 model are interpreted in terms of the tectonic model and illustrated with figures and movies. Finally, we discuss where tomographic and tectonic models for SE Asia converge or diverge, and identify the most important conclusions concerning the history of the region. The tomographic images of the mantle record subduction beneath the SE Asian region to depths of approximately 1600 km. In the upper mantle anomalies mainly record subduction during the last 10 to 25 Ma, depending on the region considered. We interpret a vertical slab tear crossing the entire upper mantle north of west Sumatra where there is a strong lateral kink in slab morphology, slab holes between c.200-400 km below East Java and Sumbawa, and offer a new three-slab explanation for subduction in the North Sulawesi region. There is a different structure in the lower mantle compared to the upper mantle and the deep structure changes from west to east. What was imaged in earlier models as a broad and deep anomaly below SE Asia has a clear internal structure and we argue that many features can be identified as older subduction zones. We identify remnants of slabs that detached in the Early Miocene such as the Sula slab, now found in the lower mantle north of Lombok, and the Proto-South China Sea slab now at depths below 700 km curving from northern Borneo to the Philippines. Based on our tectonic model we interpret virtually all features seen in upper mantle and lower mantle to depths of at least 1200 km to be the result of Cenozoic subduction.

Mantle wedge exhumation beneath the Dora-Maira (U)HP dome unravelled by local earthquake tomography (Western Alps)

NASA Astrophysics Data System (ADS)

Solarino, Stefano; Malusà, Marco G.; Eva, Elena; Guillot, Stéphane; Paul, Anne; Schwartz, Stéphane; Zhao, Liang; Aubert, Coralie; Dumont, Thierry; Pondrelli, Silvia; Salimbeni, Simone; Wang, Qingchen; Xu, Xiaobing; Zheng, Tianyu; Zhu, Rixiang

2018-01-01

In continental subduction zones, the behaviour of the mantle wedge during exhumation of (ultra)high-pressure [(U)HP] rocks provides a key to distinguish among competing exhumation mechanisms. However, in spite of the relevant implications for understanding orogenic evolution, a high-resolution image of the mantle wedge beneath the Western Alps is still lacking. In order to fill this gap, we perform a detailed analysis of the velocity structure of the Alpine belt beneath the Dora-Maira (U)HP dome, based on local earthquake tomography independently validated by receiver function analysis. Our results point to a composite structure of the mantle wedge above the subducted European lithosphere. We found that the Dora-Maira (U)HP dome lays directly above partly serpentinized peridotites (Vp 7.5 km/s; Vp/Vs = 1.70-1.72), documented from 10 km depth down to the top of the eclogitized lower crust of the European plate. These serpentinized peridotites, possibly formed by fluid release from the subducting European slab to the Alpine mantle wedge, are juxtaposed against dry mantle peridotites of the Adriatic upper plate along an active fault rooted in the lithospheric mantle. We propose that serpentinized mantle-wedge peridotites were exhumed at shallow crustal levels during late Eocene transtensional tectonics, also triggering the rapid exhumation of (U)HP rocks, and were subsequently indented under the Alpine metamorphic wedge in the early Oligocene. Our findings suggest that mantle-wedge exhumation may represent a major feature of the deep structure of exhumed continental subduction zones. The deep orogenic levels here imaged by seismic tomography may be exposed today in older (U)HP belts, where mantle-wedge serpentinites are commonly associated with coesite-bearing continental metamorphic rocks.

Observations of Quasi-Love Waves in Tibet Indicates Coherent Deformation of the Crust and Upper Mantle

NASA Astrophysics Data System (ADS)

Chen, X.; Park, J. J.

2012-12-01

The high uplift of the Tibet area is caused by the continental collision between the Indian plate and the Eurasian plate. The style of deformation along with the collision is still being debated, particularly whether the deformation is vertically coherent or not, i.e., whether the upper mantle deforms coherently with the crust. In this work, we have used quasi-Love (QL) waves to constrain the anisotropy pattern around the Tibet region. The existence of anisotropy gradients has been identified with the observations of QL waves, which is a converted Rayleigh-wave motion that follows the arrival of the Love wave. Further, the locations of the anisotropy gradients have been pinned with the delay time between the Love wave and the QL wave, which is determined from cross-correlation. Our results show that the frequency content of Tibetan QL wave is centered around 10 mHz, indicating the depth range of anisotropy should be in the asthenosphere. Most of the scatterers of QL wave that we can detect lie outside the Tibet Plateau. Their distribution correlates well with the boundary of the Persia-Tibet- Burma orogeny, which has been identified from surface geologic data. This correlation, between surface geology and upper mantle anisotropy inferred from QL observations at the orogenic boundary, suggests that the crust and upper mantle of the orogeny are deforming coherently. Other scatterers that are off the Persia-Tibet-Burma orogenic boundary mostly cluster in two locations, the Tarim Basin, and the Bangong-Nujiang Suture, where there could exist contrasting anisotropy patterns in the upper mantle. The deformation in the Tibet region is complicated, yet our research suggests a vertically coherent deformation style of the upper mantle in Tibet.

The effects of the theoretical formalism and data selection on mantle models derived from waveform tomography

NASA Astrophysics Data System (ADS)

Mégnin, Charles; Romanowicz, Barbara

1999-08-01

Most global tomographic models to date are derived using a combination of surface wave (or normal-mode) data and body wave traveltime data. The traveltime approach limits the number of phases available for inversion by requiring them to be isolated on the seismogram. This may ultimately result in limiting the resolution of 3-D structure, at least in some depth ranges in the mantle. In a previous study, we successfully derived a degree 12 whole-mantle SH-velocity tomographic model (SAW12D) using exclusively waveform data. In that inversion, a normal-mode formalism suitable for body waveforms, the non-linear asymptotic coupling theory (NACT), was combined with a body wave windowing scheme, referred to as the `individual wavepacket' (IW) technique, which allows one to assign individual weights to different body wave energy packets. We here compare the relative merits of this choice of theoretical formalism and windowing scheme at different depth ranges in the mantle. Choosing as the reference a model obtained using 7500 transverse-component body wave and 8000 surface wave seismograms and the NACT and IW approaches, we discuss the relative performance of the path average approximation (PAVA), a zeroth-order theoretical approximation appropriate for single-mode surface waves, relative to NACT, and compare the IW windowing scheme with a more standard `full window' (FW) approach, in which a single time window is considered from the first body wave arrival to the fundamental-mode surface waves. The combination PAVA/FW is often used in global tomography to supplement the traveltime data. We show that although the quality of the image derived under the PAVA/FW formalism is very similar to that derived under NACT/IW in the first 300 km of the upper mantle, where the resolution is dominated by surface waves, it deteriorates at greater depths. Images of the lower mantle are shown to be strongly sensitive to the theoretical formalism. In contrast, the resolution of structure near the core-mantle boundary depends mostly on the windowing scheme. This is because this resolution is controlled by low-amplitude phases such as S_diff, which are downweighted in the FW scheme. Whilst the image obtained in D'' using the combination NACT/IW is in good agreement with images obtained by other authors using both waveforms and traveltimes, we show that, when using FW, uppermost mantle structure can be mapped into D''. This result is confirmed by synthetic tests performed on a composite of the upper-mantle geodynamic model 3SMAC. We also show, based on synthetic tests, that for structures in the upper mantle with sharp boundaries, differences are observed between NACT and PAVA. Whilst a combination of traveltimes and surface wave data is adequate for resolving relatively smooth features in the mantle, our results show that by potentially increasing the achievable sampling, the waveform approach shows great promise for future high-resolution tomographic modelling of mantle structure, if cast in an appropriate theoretical framework.

Influence of Silicate Melt Composition on Metal/Silicate Partitioning of W, Ge, Ga and Ni

NASA Technical Reports Server (NTRS)

Singletary, S. J.; Domanik, K.; Drake, M. J.

2005-01-01

The depletion of the siderophile elements in the Earth's upper mantle relative to the chondritic meteorites is a geochemical imprint of core segregation. Therefore, metal/silicate partition coefficients (Dm/s) for siderophile elements are essential to investigations of core formation when used in conjunction with the pattern of elemental abundances in the Earth's mantle. The partitioning of siderophile elements is controlled by temperature, pressure, oxygen fugacity, and by the compositions of the metal and silicate phases. Several recent studies have shown the importance of silicate melt composition on the partitioning of siderophile elements between silicate and metallic liquids. It has been demonstrated that many elements display increased solubility in less polymerized (mafic) melts. However, the importance of silicate melt composition was believed to be minor compared to the influence of oxygen fugacity until studies showed that melt composition is an important factor at high pressures and temperatures. It was found that melt composition is also important for partitioning of high valency siderophile elements. Atmospheric experiments were conducted, varying only silicate melt composition, to assess the importance of silicate melt composition for the partitioning of W, Co and Ga and found that the valence of the dissolving species plays an important role in determining the effect of composition on solubility. In this study, we extend the data set to higher pressures and investigate the role of silicate melt composition on the partitioning of the siderophile elements W, Ge, Ga and Ni between metallic and silicate liquid.

Basanite-nephelinite suite from early Kilauea: Carbonated melts of phlogopite-garnet peridotite at Hawaii's leading magmatic edge

USGS Publications Warehouse

Sisson, T.W.; Kimura, Jun-Ichi; Coombs, M.L.

2009-01-01

A basanite-nephelinite glass suite from early submarine Kilauea defines a continuous compositional array marked by increasing concentrations of incompatible components with decreasing SiO2, MgO, and Al2O3. Like peripheral and post-shield strongly alkalic Hawaiian localities (Clague et al. in J Volcanol Geotherm Res 151:279-307, 2006; Dixon et al. in J Pet 38:911-939, 1997), the early Kilauea basanite-nephelinite glasses are interpreted as olivine fractionation products from primary magnesian alkalic liquids. For early Kilauea, these were saturated with a garnet-phlogopite-sulfide peridotite assemblage, with elevated dissolved CO2 contents responsible for the liquids' distinctly low-SiO2 concentrations. Reconstructed primitive liquids for early Kilauea and other Hawaiian strongly alkalic localities are similar to experimental 3 GPa low-degree melts of moderately carbonated garnet lherzolite, and estimated parent magma temperatures of 1,350-1,400??C (olivine-liquid geothermometry) match the ambient upper mantle geotherm shortly beneath the base of the lithosphere. The ???3 GPa source regions were too hot for stable crystalline carbonate and may have consisted of ambient upper mantle peridotite containing interstitial carbonate-silicate or carbonatitic liquid, possibly (Dixon et al. in Geochem Geophys Geosyst 9(9):Q09005, 2008), although not necessarily, from the Hawaiian mantle plume. Carbonate-enriched domains were particularly susceptible to further melting upon modest decompression during upward lithospheric flexure beneath the advancing Hawaiian Arch, or by conductive heating or upward drag by the Hawaiian mantle plume. The early Kilauea basanite-nephelinite suite has a HIMU-influenced isotopic character unlike other Hawaiian magmas (Shimizu et al. in EOS Tran Amer Geophys Union 82(47): abstr V12B-0962, 2001; Shimizu et al. in Geochim Cosmochim Acta 66(15A):710, 2002) but consistent with oceanic carbonatite involvement (Hoernle et al. in Contrib Mineral Petrol 142:520-542, 2002). It may represent the melting products of a fertile domain in the ambient upper mantle impinged upon and perturbed by the sustained plume source that feeds later shield-stage magmatism. ?? US Government 2009.

Basanite-nephelinite suite from early Kilauea: carbonated melts of phlogopite-garnet peridotite at Hawaii's leading magmatic edge

NASA Astrophysics Data System (ADS)

Sisson, T. W.; Kimura, J.-I.; Coombs, M. L.

2009-12-01

A basanite-nephelinite glass suite from early submarine Kilauea defines a continuous compositional array marked by increasing concentrations of incompatible components with decreasing SiO2, MgO, and Al2O3. Like peripheral and post-shield strongly alkalic Hawaiian localities (Clague et al. in J Volcanol Geotherm Res 151:279-307, 2006; Dixon et al. in J Pet 38:911-939, 1997), the early Kilauea basanite-nephelinite glasses are interpreted as olivine fractionation products from primary magnesian alkalic liquids. For early Kilauea, these were saturated with a garnet-phlogopite-sulfide peridotite assemblage, with elevated dissolved CO2 contents responsible for the liquids’ distinctly low-SiO2 concentrations. Reconstructed primitive liquids for early Kilauea and other Hawaiian strongly alkalic localities are similar to experimental 3 GPa low-degree melts of moderately carbonated garnet lherzolite, and estimated parent magma temperatures of 1,350-1,400°C (olivine-liquid geothermometry) match the ambient upper mantle geotherm shortly beneath the base of the lithosphere. The ~3 GPa source regions were too hot for stable crystalline carbonate and may have consisted of ambient upper mantle peridotite containing interstitial carbonate-silicate or carbonatitic liquid, possibly (Dixon et al. in Geochem Geophys Geosyst 9(9):Q09005, 2008), although not necessarily, from the Hawaiian mantle plume. Carbonate-enriched domains were particularly susceptible to further melting upon modest decompression during upward lithospheric flexure beneath the advancing Hawaiian Arch, or by conductive heating or upward drag by the Hawaiian mantle plume. The early Kilauea basanite-nephelinite suite has a HIMU-influenced isotopic character unlike other Hawaiian magmas (Shimizu et al. in EOS Tran Amer Geophys Union 82(47): abstr V12B-0962, 2001; Shimizu et al. in Geochim Cosmochim Acta 66(15A):710, 2002) but consistent with oceanic carbonatite involvement (Hoernle et al. in Contrib Mineral Petrol 142:520-542, 2002). It may represent the melting products of a fertile domain in the ambient upper mantle impinged upon and perturbed by the sustained plume source that feeds later shield-stage magmatism.

The impact of degassing on the oxidation state of basaltic magmas: A case study of Kīlauea volcano

NASA Astrophysics Data System (ADS)

Moussallam, Yves; Edmonds, Marie; Scaillet, Bruno; Peters, Nial; Gennaro, Emanuela; Sides, Issy; Oppenheimer, Clive

2016-09-01

Volcanic emissions link the oxidation state of the Earth's mantle to the composition of the atmosphere. Whether the oxidation state of an ascending magma follows a redox buffer - hence preserving mantle conditions - or deviates as a consequence of degassing remains under debate. Thus, further progress is required before erupted basalts can be used to infer the redox state of the upper mantle or the composition of their co-emitted gases to the atmosphere. Here we present the results of X-ray absorption near-edge structure (XANES) spectroscopy at the iron K-edge carried out for a series of melt inclusions and matrix glasses from ejecta associated with three eruptions of Kīlauea volcano (Hawai'i). We show that the oxidation state of these melts is strongly correlated with their volatile content, particularly in respect of water and sulfur contents. We argue that sulfur degassing has played a major role in the observed reduction of iron in the melt, while the degassing of H2O and CO2 appears to have had a negligible effect on the melt oxidation state under the conditions investigated. Using gas-melt equilibrium degassing models, we relate the oxidation state of the melt to the composition of the gases emitted at Kīlauea. Our measurements and modelling yield a lower constraint on the oxygen fugacity of the mantle source beneath Kīlauea volcano, which we infer to be near the nickel nickel-oxide (NNO) buffer. Our findings should be widely applicable to other basaltic systems and we predict that the oxidation state of the mantle underneath most hotspot volcanoes is more oxidised than that of the associated lavas. We also suggest that whether the oxidation states of a basalt (in particular MORB) reflects that of its source, is primarily determined by the extent of sulfur degassing.

Cenozoic magmatism of north Victoria Land, Antarctica: an experimental study on the mantle source of a primary basanite from the McMurdo Volcanic Group

NASA Astrophysics Data System (ADS)

Armienti, P.; Freda, C.; Misiti, V.; Perinelli, C.

2009-04-01

Volcanoes of the McMurdo Vocanic Group (MMVG) (Antarctica) dot the eastern shoulder of Ross Sea Rift System giving rise to alkaline transitional volcanic suites which in north Victoria Land are emplaced since Early Cenozoic. Geochemical geological, geophysical and geochronological data on Cenozoic volcanic activity in NVL suggest that the region is a site of passive astenospheric rise, rather than affected by a thermally active mantle plume. Furthermore the comparison of geochemical and isotopical data of basic lavas with those provided by mantle xenoliths they carry to the surface, document the compositional heterogeneity of sublithospheric mantle caused by the coupled action of partial melting and metasomatism. In particular the metasomatic episode is probably linked to the amagmatic extensional event that affected the West Antarctic Rift System in the Late Cretaceous. The astenospheric melts generated during this event, moving through the upper mantle, can have crystallized as veins or may have led to the formation of metasomatic minerals such as amphibole or phlogopite. In this scenario the mineralogical and chemical composition of sources responsible for Cenozoic magmatism, amphibole-bearing spinel-peridotite versus pyroxenite in the garnet stability field, it is still a matter of debate. To shed light on this argument a previous experimental study on a basanite of MMVG, representative of primary magma (Orlando et al., 2000) has been integrated with new experimental investigation on the same basanitic composition. The preliminary experiments were conducted to pressures of 1.0 - 2.0GPa in the presence of 0-1% of added water and indicate olivine on the liquidus at 1.0 GPa that is substitute by clinopyroxene at 2.0GPa. The addition of 1% of water induces a decrease of liquidus temperature of about 40°C shifting its value in the T range (1280-1310°C) the same that was inferred by melt inclusions hosted in the olivine phenocrysts of the studied basanite.

Volatile composition of microinclusions in diamonds from the Panda kimberlite, Canada: Implications for chemical and isotopic heterogeneity in the mantle

NASA Astrophysics Data System (ADS)

Burgess, Ray; Cartigny, Pierre; Harrison, Darrell; Hobson, Emily; Harris, Jeff

2009-03-01

In order to better investigate the compositions and the origins of fluids associated with diamond growth, we have carried-out combined noble gas (He and Ar), C and N isotope, K, Ca and halogen (Cl, Br, I) determinations on fragments of individual microinclusion-bearing diamonds from the Panda kimberlite, North West Territories, Canada. The fluid concentrations of halogens and noble gases in Panda diamonds are enriched by several orders of magnitude over typical upper mantle abundances. However, noble gas, C and N isotopic ratios ( 3He/ 4He = 4-6 Ra, 40Ar/ 36Ar = 20,000-30,000, δ 13C = -4.5‰ to -6.9‰ and δ 15N = -1.2‰ to -8.8‰) are within the worldwide range determined for fibrous diamonds and similar to the mid ocean ridge basalt (MORB) source value. The high 36Ar content of the diamonds (>1 × 10 -9 cm 3/g) is at least an order of magnitude higher than any previously reported mantle sample and enables the 36Ar content of the subcontinental lithospheric mantle to be estimated at ˜0.6 × 10 -12 cm 3/g, again similar to estimates for the MORB source. Three fluid types distinguished on the basis of Ca-K-Cl compositions are consistent with carbonatitic, silicic and saline end-members identified in previous studies of diamonds from worldwide sources. These fluid end-members also have distinct halogen ratios (Br/Cl and I/Cl). The role of subducted seawater-derived halogens, originally invoked to explain some of the halogen ratio variations in diamonds, is not considered an essential component in the formation of the fluids. In contrast, it is considered that large halogen fractionation of a primitive mantle ratio occurs during fluid-melt partitioning in forming silicic fluids, and during separation of an immiscible saline fluid.

Gas geochemistry of Sierra Negra volcano, Galapagos hot spot

NASA Astrophysics Data System (ADS)

Taran, Y.; Christenson, B.; Sumino, H.; Kennedy, B.

2010-12-01

We report chemical and isotopic compositions of gases from the Mina Azufral fumarolic field of Sierra Negra volcano, Isabela Island, Galápagos, collected in 2004 and compare our data with the data by Giggenbach (unpublished) collected in 1990 and Goff et al. (2000) collected in 1995. New results include the noble gas elemental and isotope abundances and nitrogen isotope ratios for the discharges. Maximum fumarole temperatures and ratios of major components (C/S/Cl/N) changed very little between 1995 and 2004, but the water fraction varied significantly over this period (39 mol% in 1990; 77% in 1995 and 52% in 2004). Carbon and helium isotopic compositions were stable (-3 to -4‰ and 16-18Ra, respectively), and water isotopic composition showed a notable negative oxygen shift from the local meteoric water value depending on the relative water content and thus controlled by the H2O-CO2 oxygen isotope fractionation. In terms of the noble gas abundances and isotopic ratios, heavy noble gases (Kr and Xe) are mainly of the atmospheric origin. Ne isotopic ratios also show strong meteoric signatures, but fall along the 20Ne/22Ne - 21Ne/22Ne air-deep mantle mixing trend for Fernandina glasses (Kurz et al., 2009). 40Ar/36Ar ratios up to 400 show a notable contribution of radiogenic Ar, and 40Ar*/4He ~ 0.3 ratios are consistent with un-degassed upper mantle values. Despite the high He/Ne ratios in gases collected in 2004, and only trace air contamination attributable to sampling, the nitrogen isotope ratios (~ -1 ‰) show a high fraction of the air-saturated water in the volcanic vapor. The chemical composition of the parent magmatic gas is difficult to characterise due to significant interaction between magmatic and hydrothermal system fluids beneath the Sierra Negra caldera. Never-the-less, some important indicators can be estimated: CO2/3He ≈ 3.5x10^9; N2/He <30; CO2/N2 >500. The last value is much higher than the accepted value of ~ 100 for the upper mantle.

Subduction Related Crustal and Mantle Deformations and Their Implications for Plate Dynamics

NASA Astrophysics Data System (ADS)

Okeler, Ahmet

Ocean-continent convergence and subsequent continental collision are responsible for continental growth, mountain building, and severe tectonic events including volcanic eruptions and earthquake activity. They are also key driving forces behind the extensive thermal and compositional heterogeneities at crustal and mantle depths. Active subduction along the Calabrian Arc in southern Italy and the Hellenic Arc are examples of such collisional tectonics. The first part of this thesis examines the subduction related deformations within the crust beneath the southern Apennines. By modeling regional surface wave recordings of the largest temporary deployment in the southern Apennines, a lower-crustal/upper-mantle low-velocity volume extending down to 50 km beneath the mountain chain is identified. The magnitude (˜ 0.4 km/s slower) and anisotropic nature (˜ 10%) of the anomaly suggest the presence of hot and partially molten emplacement that may extend into the upper-crust towards Mt. Vulture, a once active volcano. Since the Apulian basement units are deformed during the compressional and consequent extensional events, our observations favor the "thick-skin" tectonic growth model for the region. In the deeper mantle, active processes are thermodynamically imprinted on the depth and strength of the phase transitions. This thesis examines more than 15000 SS precursors and provides the present-day reflectivity structure and topography associated with these phase transitions. Through case studies I present ample evidence for both slab penetration into the lower mantle (beneath the Hellenic Arc, Kurile Island and South America) and slab stagnation at the bottom of the Mantle Transition Zone (beneath the Tyrrhenian Sea and eastern China). Key findings include (1) thermal anomalies (˜ 200 K) at the base of the MTZ, which represent the deep source for Cenozoic European Rift Zone, Mount Etna and Mount Cameroon volcanism, (2) significant depressions (by 20-40 km) at the bottom of the Mantle Transition Zone beneath subducting slabs, (3) a strong 520-km reflector near subducting slabs, (4) a weak and elevated (15-25 km) 410-km reflector within active deformation zones, (5) strong lower mantle reflectors (˜ 900 km) while slabs penetrate into the lower mantle, and (6) consistency between the topography of a 300-km reflector and an exothermic phase transformation.

Mantle transition zone structure beneath India and Western China from migration of PP and SS precursors

NASA Astrophysics Data System (ADS)

Lessing, Stephan; Thomas, Christine; Rost, Sebastian; Cobden, Laura; Dobson, David P.

2014-04-01

We investigate the seismic structure of the upper-mantle and mantle transition zone beneath India and Western China using PP and SS underside reflections off seismic discontinuities, which arrive as precursors to the PP and SS arrival. We use high-resolution array seismic techniques to identify precursory energy and to map lateral variations of discontinuity depths. We find deep reflections off the 410 km discontinuity (P410P and S410S) beneath Tibet, Western China and India at depths of 410-440 km and elevated underside reflections of the 410 km discontinuity at 370-390 km depth beneath the Tien Shan region and Eastern Himalayas. These reflections likely correspond to the olivine to wadsleyite phase transition. The 410 km discontinuity appears to deepen in Central and Northern Tibet. We also find reflections off the 660 km discontinuity beneath Northern China at depths between 660 and 700 km (P660P and S660S) which could be attributed to the mineral transformation of ringwoodite to magnesiowuestite and perovskite. These observations could be consistent with the presence of cold material in the middle and lower part of the mantle transition zone in this region. We also find a deeper reflector between 700 and 740 km depth beneath Tibet which cannot be explained by a depressed 660 km discontinuity. This structure could, however, be explained by the segregation of oceanic crust and the formation of a neutrally buoyant garnet-rich layer beneath the mantle transition zone, due to subduction of oceanic crust of the Tethys Ocean. For several combinations of sources and receivers we do not detect arrivals of P660P and S660S although similar combinations of sources and receivers give well-developed P660P and S660S arrivals. Our thermodynamic modelling of seismic structure for a range of compositions and mantle geotherms shows that non-observations of P660P and S660S arrivals could be caused by the dependence of underside reflection coefficients on the incidence angle of the incoming seismic waves. Apart from reflections off the 410 and 660 km discontinuities, we observe intermittent reflectors at 300 and 520 km depth. The discontinuity structure of the study region likely reflects lateral thermal and chemical variations in the upper-mantle and mantle transition zone connected to past and present subduction and mantle convection processes.

Continental crust

USGS Publications Warehouse

Pakiser, L.C.

1964-01-01

The structure of the Earth’s crust (the outer shell of the earth above the M-discontinuity) has been intensively studied in many places by use of geophysical methods. The velocity of seismic compressional waves in the crust and in the upper mantle varies from place to place in the conterminous United States. The average crust is thick in the eastern two-thirds of the United States, in which the crustal and upper-mantle velocities tend to be high. The average crust is thinner in the western one-third of the United States, in which these velocities tend to be low. The concept of eastern and western superprovinces can be used to classify these differences. Crustal and upper-mantle densities probably vary directly with compressional-wave velocity, leading to the conclusion that isostasy is accomplished by the variation in densities of crustal and upper-mantle rocks as well as in crustal thickness, and that there is no single, generally valid isostatic model. The nature of the M-discontinuity is still speculative.

New constraints on the upper mantle structure of the Slave craton from Rayleigh wave inversion

NASA Astrophysics Data System (ADS)

Chen, Chin-Wu; Rondenay, Stéphane; Weeraratne, Dayanthie S.; Snyder, David B.

2007-05-01

Rayleigh wave phase and amplitude data are analyzed to provide new insight into the velocity structure of the upper mantle beneath the Slave craton, in the northwestern Canadian Shield. We invert for phase velocities at periods between 20 s-142 s (with greatest sensitivity at depths of 28-200 km) using crossing ray paths from events recorded by the POLARIS broadband seismic network and the Yellowknife array. Phase velocities obtained for the Slave province are comparable to those from other cratons at shorter periods, but exceed the global average by ~2% at periods above 60 s, suggesting that the Slave craton may be an end member in terms of its high degree of mantle depletion. The one-dimensional inversion of phase velocities yields high upper-mantle S-wave velocities of 4.7 +/- 0.2 km/s that persist to 220 +/- 65 km depth and thus define the cratonic lithosphere. Azimuthal anisotropy is well resolved at all periods with a dominant fast direction of N59°E +/- 20°, suggesting that upper mantle anisotropy beneath the Slave craton is influenced by both lithospheric fabric and sub-lithospheric flow.

Cumulate xenoliths from St. Vincent, Lesser Antilles Island Arc: a window into upper crustal differentiation of mantle-derived basalts

NASA Astrophysics Data System (ADS)

Tollan, P. M. E.; Bindeman, I.; Blundy, J. D.

2012-02-01

In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine-gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4-10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89-5.18‰), plagioclase (5.84-6.28‰), clinopyroxene (5.17-5.47‰) and hornblende (5.48-5.61‰) and hydrogen isotope composition of hornblende (δD = -35.5 to -49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ18OOlivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth within the crust, lowering MgO and Cr2O3 and raising Al2O3 and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of magma.

Sensitivities of seismic velocities to temperature, pressure and composition in the lower mantle

NASA Astrophysics Data System (ADS)

Trampert, Jeannot; Vacher, Pierre; Vlaar, Nico

2001-08-01

We calculated temperature, pressure and compositional sensitivities of seismic velocities in the lower mantle using latest mineral physics data. The compositional variable refers to the volume proportion of perovskite in a simplified perovskite-magnesiowüstite mantle assemblage. The novelty of our approach is the exploration of a reasonable range of input parameters which enter the lower mantle extrapolations. This leads to realistic error bars on the sensitivities. Temperature variations can be inferred throughout the lower mantle within a good degree of precision. Contrary to the uppermost mantle, modest compositional changes in the lower mantle can be detected by seismic tomography, with a larger uncertainty though. A likely trade-off between temperature and composition will be largely determined by uncertainties in tomography itself. Given current sources of uncertainties on recent data, anelastic contributions to the temperature sensitivities (calculated using Karato's approach) appear less significant than previously thought. Recent seismological determinations of the ratio of relative S to P velocity heterogeneity can be entirely explain by thermal effects, although isolated spots beneath Africa and the Central Pacific in the lowermost mantle may ask for a compositional origin.

Of Mantle Plumes, Their Existence, and Their Nature: Insights from Whole Mantle SEM-Based Seismic Waveform Tomography

NASA Astrophysics Data System (ADS)

Romanowicz, B. A.; French, S. W.

2014-12-01

Many questions remain on the detailed morphology of mantle convection patterns. While high resolution P wave studies show a variety of subducted slab behaviors, some stagnating in the transition zone, others penetrating into the lower mantle (e.g. Fukao & Obayashi, 2013), low velocity structures - the upwelling part of flow - are more difficult to resolve at the same scale. Indeed, depth extent and morphology of the low velocity roots of hotspot volcanoes is still debated, along with the existence of "mantle plumes". Using spectral element waveform tomography, we previously constructed a global, radially anisotropic, upper mantle Vs model (SEMum2, French et al., 2013) and have now extended it to the whole mantle by adding shorter period waveform data (SEMUCB-WM1, French & Romanowicz, GJI, in revision). This model shows long wavelength structure in good agreement with other recent global Vs models derived under stronger approximations (Ritsema et al. 2011; Kustowski, et al. 2008), but exhibits better focused, finer scale structure throughout the mantle. SEMUCB-WM1 confirms the presence in all major ocean basins of the quasi-periodic, upper mantle low velocity anomalies, previously seen in SEMum2. At the same time, lower mantle low velocity structure is dominated by a small number (~15 globally) of quasi-vertical anomalies forming discrete "column"" rooted at the base of the mantle. Most columns are positioned near major hotspots, as defined by buoyancy flux, and are wider (~800-1000 km diameter) than expected from the thermal plume model - suggestive of thermo-chemical plumes, which may be stable for long times compared to purely thermal ones. Some columns reach the upper mantle, while others deflect horizontally near 1000 km - the same depth where many slabs appear to stagnate. As they reach the transition zone, the wide columnar structure can be lost, as these "plumes" appear to meander through the upper mantle, perhaps entrained by more vigorous, lower viscosity, convection. Most "plumes" in the Pacific LLSVP region appear as isolated columns rising from the CMB, such as beneath Hawaii (rooted near a known ultra low velocity zone, Cottaar & Romanowicz, 2012). Conversely, the African LLSVP region appears more massive up to mid-mantle depths, with isolated "plumes" at its borders, including that beneath Iceland.

Crustal accretion along the global mid-ocean ridge system based on basaltic glass and olivine-hosted melt inclusion compositions

NASA Astrophysics Data System (ADS)

Wanless, V. D.; Behn, M. D.

2015-12-01

The depth and distribution of crystallization at mid-ocean ridges controls the overall architecture of the oceanic crust, influences hydrothermal circulation, and determines geothermal gradients in the crust and uppermost mantle. Despite this, there is no overall consensus on how crystallization is distributed within the crust/upper mantle or how this varies with spreading rate. Here, we examine crustal accretion at mid-ocean ridges by combining crystallization pressures calculated from major element barometers on mid-ocean ridge basalt (MORB) glasses with vapor-saturation pressures from melt inclusions to produce a detailed map of crystallization depths and distributions along the global ridge system. We calculate pressures of crystallization from >11,500 MORB glasses from the global ridge system using two established major element barometers (1,2). Additionally, we use vapor-saturation pressures from >400 olivine-hosted melt inclusions from five ridges with variable spreading rates to constrain pressures and distributions of crystallization along the global ridge system. We show that (i) crystallization depths from MORB glasses increase and become less focused with decreasing spreading rate, (ii) maximum glass pressures are greater than the maximum melt inclusion pressure, which indicates that the melt inclusions do not record the deepest crystallization at mid-ocean ridges, and (iii) crystallization occurs in the lower crust/upper mantle at all ridges, indicating accretion is distributed throughout the crust at all spreading rates, including those with a steady-state magma lens. Finally, we suggest that the remarkably similar maximum vapor-saturation pressures (~ 3000 bars) in melt inclusion from all spreading rates reflects the CO2 content of the depleted upper mantle feeding the global mid-ocean ridge system. (1) Michael, P. & W. Cornell (1998), Journal of Geophysical Research, 103(B8), 18325-18356; (2) Herzberg, C. (2004), Journal of Petrology, 45(12), 2389.

Lead isotope evolution across the Neoproterozoic boundary between craton and juvenile crust, Bayuda Desert, Sudan

NASA Astrophysics Data System (ADS)

Evuk, David; Lucassen, Friedrich; Franz, Gerhard

2017-11-01

Metaigneous mafic and ultramafic rocks from the juvenile Neoproterozoic Arabian Nubian Shield (ANS) and the Proterozoic, reworked Saharan Metacraton (SMC) have been analysed for major- and trace elements and Sr, Nd, and Pb isotopes. Most of the rocks are amphibolites metamorphosed at amphibolite facies conditions, some with relicts of a granulite facies stage. The other rocks are metapyroxenites, metagabbros, and some ultramafic rocks. Trace element compositions of the metabasaltic (dominantly tholeiitic) rocks resemble the patterns of island arcs and primitive lavas from continental arcs. Variable Sr and Nd isotope ratios indicate depleted mantle dominance for most of the samples. 207Pb/204Pb signatures distinguish between the influence of high 207Pb/204Pb old SMC crust and depleted mantle signatures of the juvenile ANS crust. The Pb isotope signatures for most metabasaltic rocks, metapyroxenites and metagabbros from SMC indicate an autochthonous formation. The interpretation of the new data together with published evidence from mafic xenoliths on SMC and ophiolite from ANS allows an extrapolation of mantle evolution in time. There are two lines of evolution in the regional mantle, one, which incorporates potential upper crust material during Neoproterozoic, and a second one with a depleted mantle signature since pre-Neoproterozoic that still is present in the Red Sea and Gulf of Aden spreading centres.

Partial melting and melt percolation in the mantle: The message from Fe isotopes

NASA Astrophysics Data System (ADS)

Weyer, Stefan; Ionov, Dmitri A.

2007-07-01

High precision Fe isotope measurements have been performed on various mantle peridotites (fertile lherzolites, harzburgites, metasomatised Fe-enriched peridotites) and volcanic rocks (mainly oceanic basalts) from different localities and tectonic settings. The peridotites yield an average δ 56Fe = 0.01‰ and are significantly lighter than the basalts (average δ 56Fe = 0.11‰). Furthermore, the peridotites display a negative correlation of δ 56Fe with Mg# indicating a link between δ 56Fe and degrees of melt extraction. Taken together, these findings imply that Fe isotopes fractionate during partial melting, with heavy isotopes preferentially entering the melt. The slope of depletion trends (δ 56Fe versus Mg#) of the peridotites was used to model Fe isotope fractionation during partial melting, resulting in αmantle-melt ≈ 1.0001-1.0003 or ln αmantle-melt ≈ 0.1-0.3‰. In contrast to most other peridotites investigated in this study, spinel lherzolites and harzburgites from three localities (Horoman, Kamchatka and Lherz) are virtually unaffected by metasomatism. These three sites display a particularly good correlation and define an isotope fractionation factor of ln αmantle-melt ≈ 0.3‰. This modelled value implies Fe isotope fractionation between residual mantle and mantle-derived melts corresponding to Δ56Fe mantle-basalt ≈ 0.2-0.3‰, i.e. significantly higher than the observed difference between averages for all the peridotites and the basalts in this study (corresponding to Δ56Fe mantle-basalt ≈ 0.1‰). Either disequilibrium melting increased the modelled αmantle-melt for these particular sites or the difference between average peridotite and basalt may be reduced by partial re-equilibration between the isotopically heavy basalts and the isotopically light depleted lithospheric mantle during melt ascent. The slope of the weaker δ 56Fe-Mg# trend defined by the combined set of all mantle peridotites from this study is more consistent with the generally observed difference between peridotites and basalts; this slope was used here to estimate the Fe isotope composition of the fertile upper mantle (at Mg# = 0.894, δ 56Fe ≈ 0.02 ± 0.03‰). Besides partial melting, the Fe isotope composition of mantle peridotites can also be significantly modified by metasomatic events, e.g. melt percolation. At two localities (Tok, Siberia and Tariat, Mongolia) δ 56Fe correlates with iron contents of the peridotites, which was increased from about 8% to up to 14.5% FeO by post-melting melt percolation. This process produced a range of Fe isotope compositions in the percolation columns, from extremely light (δ 56Fe = - 0.42‰) to heavy (δ 56Fe = + 0.17‰). We propose reaction with isotopically heavy melts and diffusion (enrichment of light Fe isotopes) as the most likely processes that produced the large isotope variations at these sites. Thus, Fe isotopes might be used as a sensitive tracer to identify such metasomatic processes in the mantle.

Minerals of the earth's deep interior

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schiferl, D.; Zhao, Y.; Shankland, T.J.

1998-11-01

This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The project addresses the major geophysical issue of the nature of the seismic velocity and density discontinuity at 670 km depth (the boundary between upper and lower mantle with temperature about 1,900 K and pressure about 23 GPa). A phase change at this depth would represent a relatively small barrier to mantle convection through the discontinuity, but compositional change would inhibit thermal convection throughout the mantle. To address this problem the authors measured equation of state parameters in mantlemore » minerals as functions of high P-T using single crystal x-ray diffraction with a unique, new diamond-anvil cell (DAC) at simultaneous high temperature and pressure. Single-crystal diffraction improves absolute accuracy in lattice constants over those from powder diffraction by a factor of 5 to 10. The authors have measured equations of state of orthoenstatite MgSiO{sub 3} and hexagonal boron nitride hBN.« less

Linking TERRA and DRex to relate mantle convection and seismic anisotropy

NASA Astrophysics Data System (ADS)

Walker, Andrew; Davies, Huw; Davies, Rhodri; Wookey, James

2015-04-01

Seismic anisotropy caused by flow induced alignment of the olivine crystals in Earth's upper mantle provides a powerful way to test our ideas of mantle convection. We have been working to directly combine computer simulations of mantle dynamics, using fluid mechanics at the continuum scale, with models of rock deformation to capture fabric evolution at the grain scale. By combining models of deformation at these two scales we hope to be able to rigorously test hypothesis linking mantle flow to seismic anisotropy in regions as diverse as subduction zones, the lithosphere-asthenosphere boundary, and the transition zone. We also intend to permit feedback, for example via geometrical softening, from the model of fabric development into the material properties used in the convection simulation. We are building a flexible framework for this approach which we call Theia. Our initial implementation uses the TERRA convection code (Baumgardner, J. Stat. Phys. 39:501-511, 1985; Davies et al. Geosci. Model Dev. 6:1095-1107, 2013) to drive DRex (Kaminski et al. Geophys. J. Int. 158:744-752, 2004), which is used to predict the evolution of crystallographic preferred orientation in the upper mantle. Here we describe our current implementation which makes use of the ability of TERRA to track markers, or particles, through the evolving flow field. These tracers have previously been used to track attributes such as the bulk chemical composition or trace element ratios. Our modification is to use this technology to track a description of the current state of the texture and microstructure (encompassing an orientation distribution function, grain size parameters and dislocation density) such that we can advance models of polycrystalline deformation for many simultaneous DRex instances alongside and in sync with models of mantle convection. We will also describe initial results from our first use of the Theia framework where we are investigating the effect of asthenospheric viscosity on seismic anisotropy beneath the oceans. Key to this work is the ability of TERRA to incorporate plate motion history which acts to correctly locate the predicted anisotropy such that it can be directly compared with the anisotropy measured for the Earth.

Volatiles in the Earth and Moon: Constraints on planetary formation and evolution

NASA Astrophysics Data System (ADS)

Parai, Rita

The volatile inventories of the Earth and Moon reflect unique histories of volatile acquisition and loss in the early Solar System. The terrestrial volatile inventory was established after the giant impact phase of accretion, and the planet subsequently settled into a regime of long-term volatile exchange between the mantle and surface reservoirs in association with plate tectonics. Therefore, volatiles in the Earth and Moon shed light on a diverse array of processes that shaped planetary bodies in the Solar System as they evolved to their present-day states. Here we investigate new constraints on volatile depletion in the early Solar System, early outgassing of the terrestrial mantle, and the long-term evolution of the deep Earth volatile budget. We develop a Monte Carlo model of long-term water exchange between the mantle and surface reservoirs. Previous estimates of the deep Earth return flux of water are up to an order of magnitude too large, and incorporation of recycled slabs on average rehydrates the upper mantle but dehydrates the plume source. We find evidence for heterogeneous recycling of atmospheric argon and xenon into the upper mantle from noble gases in Southwest Indian Ridge basalts. Xenon isotope systematics indicate that xenon budgets of mid-ocean ridge and plume-related mantle sources are dominated by recycled atmospheric xenon, though the two sources have experienced different degrees of degassing. Differences between the mid-ocean ridge and plume sources were initiated within the first 100 million years of Earth history, and the two sources have never subsequently been homogenized. New high-precision xenon isotopic data contribute to an emerging portrait of two mantle reservoirs with distinct histories of outgassing and incorporation of recycled material in association with plate tectonics. Xenon isotopes indicate that the Moon likely formed within ˜70 million years of the start of the Solar System. To further investigate early Solar System chronology, we determined strontium isotopic compositions in a suite of planetary materials. If the Moon is derived from proto-Earth material, then rubidium-strontium systematics in the lunar anorthosite 60025 and Moore County plagioclase indicate that Moon formation occurred within ~62 million years of the start of the Solar System.

Cenozoic volcanism in the Bohemian Massif in the context of P- and S-velocity high-resolution teleseismic tomography of the upper mantle

NASA Astrophysics Data System (ADS)

Plomerová, Jaroslava; Munzarová, Helena; Vecsey, Luděk.; Kissling, Eduard; Achauer, Ulrich; Babuška, Vladislav

2016-08-01

New high-resolution tomographic models of P- and S-wave isotropic-velocity perturbations for the Bohemian upper mantle are estimated from carefully preprocessed travel-time residuals of teleseismic P, PKP and S waves recorded during the BOHEMA passive seismic experiment. The new data resolve anomalies with scale lengths 30-50 km. The models address whether a small mantle plume in the western Bohemian Massif is responsible for this geodynamically active region in central Europe, as expressed in recurrent earthquake swarms. Velocity-perturbations of the P- and S-wave models show similar features, though their resolutions are different. No model resolves a narrow subvertical low-velocity anomaly, which would validate the "baby-plume" concept. The new tomographic inferences complement previous studies of the upper mantle beneath the Bohemian Massif, in a broader context of the European Cenozoic Rift System (ECRIS) and of other Variscan Massifs in Europe. The low-velocity perturbations beneath the Eger Rift, observed in about 200km-broad zone, agree with shear-velocity models from full-waveform inversion, which also did not identify a mantle plume beneath the ECRIS. Boundaries between mantle domains of three tectonic units that comprise the region, determined from studies of seismic anisotropy, represent weak zones in the otherwise rigid continental mantle lithosphere. In the past, such zones could have channeled upwelling of hot mantle material, which on its way could have modified the mantle domain boundaries and locally thinned the lithosphere.

History and Evolution of Precambrian plate tectonics

NASA Astrophysics Data System (ADS)

Fischer, Ria; Gerya, Taras

2014-05-01

Plate tectonics is a global self-organising process driven by negative buoyancy at thermal boundary layers. Phanerozoic plate tectonics with its typical subduction and orogeny is relatively well understood and can be traced back in the geological records of the continents. Interpretations of geological, petrological and geochemical observations from Proterozoic and Archean orogenic belts however (e.g., Brown, 2006), suggest a different tectonic regime in the Precambrian. Due to higher radioactive heat production the Precambrian lithosphere shows lower internal strength and is strongly weakened by percolating melts. The fundamental difference between Precambrian and Phanerozoic tectonics is therefore the upper-mantle temperature, which determines the strength of the upper mantle (Brun, 2002) and the further tectonic history. 3D petrological-thermomechanical numerical modelling experiments of oceanic subduction at an active plate at different upper-mantle temperatures show these different subduction regimes. For upper-mantle temperatures < 175 K above the present day value a subduction style appears which is close to present day subduction but with more frequent slab break-off. At upper-mantle temperatures 175 - 250 K above present day values steep subduction continues but the plates are weakened enough to allow buckling and also lithospheric delamination and drip-offs. For upper-mantle temperatures > 250 K above the present day value no subduction occurs any more. The whole lithosphere is delaminating and due to strong volcanism and formation of a thicker crust subduction is inhibited. This stage of 200-250 K higher upper mantle temperature which corresponds roughly to the early Archean (Abbott, 1994) is marked by strong volcanism due to sublithospheric decompression melting which leads to an equal thickness for both oceanic and continental plates. As a consequence subduction is inhibited, but a compressional setup instead will lead to orogeny between a continental or felsic terrain and an oceanic or mafic terrain as well as internal crustal convection. Small-scale convection with plume shaped cold downwellings also in the upper mantle is of increased importance compared to the large-scale subduction cycle observed for present temperature conditions. It is also observed that lithospheric downwellings may initiate subduction by pulling at and breaking the plate. References: Abbott, D., Drury, R., Smith, W.H.F., 1994. Flat to steep transition in subduction style. Geology 22, 937-940. Brown, M., 2006. Duality of thermal regimes is the distinctive characteristic of plate tectonics since the neoarchean. Geology 34, 961-964. Brun, J.P., 2002. Deformation of the continental lithosphere: Insights from brittle-ductile models. Geological Society, London, Special Publications 200, 355-370.

Re-Os systematics of komatiites and komatiitic basalts at Dundonald Beach, Ontario, Canada: Evidence for a complex alteration history and implications of a late-Archean chondritic mantle source

NASA Astrophysics Data System (ADS)

Gangopadhyay, Amitava; Sproule, Rebecca A.; Walker, Richard J.; Lesher, C. Michael

2005-11-01

Osmium isotopic compositions, and Re and Os concentrations have been examined in one komatiite unit and two komatiitic basalt units at Dundonald Beach, part of the 2.7 Ga Kidd-Munro volcanic assemblage in the Abitibi greenstone belt, Ontario, Canada. The komatiitic rocks in this locality record at least three episodes of alteration of Re-Os elemental and isotope systematics. First, an average of 40% and as much as 75% Re may have been lost due to shallow degassing during eruption and/or hydrothermal leaching during or immediately after emplacement. Second, the Re-Os isotope systematics of whole rock samples with 187Re/ 188Os ratios >1 were reset at ˜2.5 Ga, possibly due to a regional metamorphic event. Third, there is evidence for relatively recent gain and loss of Re in some rocks. Despite the open-system behavior, some aspects of the Re-Os systematics of these rocks can be deciphered. The bulk distribution coefficient for Os (D Ossolid/liquid) for the Dundonald rocks is ˜3 ± 1 and is well within the estimated D values obtained for komatiites from the nearby Alexo area and stratigraphically-equivalent komatiites from Munro Township. This suggests that Os was moderately compatible during crystal-liquid fractionation of the magmas parental to the Kidd-Munro komatiitic rocks. Whole-rock samples and chromite separates with low 187Re/ 188Os ratios (<1) yield a precise chondritic average initial 187Os/ 188Os ratio of 0.1083 ± 0.0006 (γ Os = 0.0 ± 0.6) for their well-constrained ˜2715 Ma crystallization age. The chondritic initial Os isotopic composition of the mantle source for the Dundonald rocks is consistent with that determined for komatiites in the Alexo area and in Munro Township, suggesting that the mantle source region for the Kidd-Munro volcanic assemblage had evolved with a long-term chondritic Re/Os before eruption. The chondritic initial Os isotopic composition of the Kidd-Munro komatiites is indistinguishable from that of the projected contemporaneous convective upper mantle. The uniform chondritic Os isotopic composition of the Kidd-Munro komatiites contrasts with the typical large-scale Os isotopic heterogeneity in the mantle sources for ca. 89 Ma komatiites from the Gorgona Island, arc-related rocks and present-day ocean island basalts. This suggests that the Kidd-Munro komatiites sampled a late-Archean mantle source region that was significantly more homogeneous with respect to Re/Os relative to most modern mantle-derived rocks.

Production and recycling of oceanic crust in the early Earth

NASA Astrophysics Data System (ADS)

van Thienen, P.; van den Berg, A. P.; Vlaar, N. J.

2004-08-01

Because of the strongly different conditions in the mantle of the early Earth regarding temperature and viscosity, present-day geodynamics cannot simply be extrapolated back to the early history of the Earth. We use numerical thermochemical convection models including partial melting and a simple mechanism for melt segregation and oceanic crust production to investigate an alternative suite of dynamics which may have been in operation in the early Earth. Our modelling results show three processes that may have played an important role in the production and recycling of oceanic crust: (1) Small-scale ( x×100 km) convection involving the lower crust and shallow upper mantle. Partial melting and thus crustal production takes place in the upwelling limb and delamination of the eclogitic lower crust in the downwelling limb. (2) Large-scale resurfacing events in which (nearly) the complete crust sinks into the (eventually lower) mantle, thereby forming a stable reservoir enriched in incompatible elements in the deep mantle. New crust is simultaneously formed at the surface from segregating melt. (3) Intrusion of lower mantle diapirs with a high excess temperature (about 250 K) into the upper mantle, causing massive melting and crustal growth. This allows for plumes in the Archean upper mantle with a much higher excess temperature than previously expected from theoretical considerations.

Asteroid 4 Vesta: A Fully Differentiated Dwarf Planet

NASA Technical Reports Server (NTRS)

Mittlefehldt, David

2014-01-01

One conclusion derived from the study of meteorites is that some of them - most irons, stony irons, some achondrites - hail from asteroids that were heated to the point where metallic cores and basaltic crusts were formed. Telescopic observations show that there remains only one large asteroid with a basaltic crust, 4 Vesta; present day mean radius 263 km. The largest clan of achondrites, the howardite, eucrite and diogenite (HED) meteorites, represent the crust of their parent asteroid. Diogenites are cumulate harzburgites and orthopyroxenites from the lower crust whilst eucrites are cumulate gabbros, diabases and basalts from the upper crust. Howardites are impact-engendered breccias of diogenites and eucrites. A strong case can be made that HEDs are derived from Vesta. The NASA Dawn spacecraft orbited Vesta for 14 months returning data allowing geological, mineralogical, compositional and geophysical interpretations of Vesta's surface and structure. Combined with geochemical and petrological observations of HED meteorites, differentiation models for Vesta can be developed. Proto-Vesta probably consisted of primitive chondritic materials. Compositional evidence, primarily from basaltic eucrites, indicates that Vesta was melted to high degree (>=50%) which facilitated homogenization of the silicate phase and separation of immiscible Fe,Ni metal plus Fe sulphide into a core. Geophysical models based on Dawn data support a core of 110 km radius. The silicate melt vigorously convected and initially followed a path of equilibrium crystallization forming a harzburgitic mantle, possibly overlying a dunitic restite. Once the fraction of crystals was sufficient to cause convective lockup, the remaining melt collected between the mantle and the cool thermal boundary layer. This melt undergoes fractional crystallization to form a dominantly orthopyroxenite (diogenite) lower crust. The initial thermal boundary layer of primitive chondritic material is gradually replaced by a mafic crust through impact disruption and foundering. The quenched mafic crust thickens over time through magma extrusion/intrusion. Melt from the residual magma ocean intrudes and penetrates the mafic crust forming cumulate eucrite plutons, and dikes, sills and flows of basaltic eucrite composition. The post-differentiation vestan structure is thus not too dissimilar from that of terrestrial planets: (i) a metallic core; (ii) an ultramafic mantle comprised of a lower dunitic layer (if melting was substantially <100%) and an upper cumulate harzburgitic layer; (iii) a lower crust of harzburgitic and orthopyroxenitic cumulates; and (iv) an upper mafic crust of basalts and diabases (melt compositions) with cumulate gabbro intrusions. Impacts have excavated to the lower crust and delivered howardites, eucrites and diogenites to Earth, but there is yet no evidence demonstrating excavation of the vestan mantlle.

A combined geodynamical-geochemical modelling approach to investigating the Lu-Hf isotopic evolution of the terrestrial mantle and crust

NASA Astrophysics Data System (ADS)

Jones, R.; Van Keken, P. E.; Hauri, E.; Vervoort, J. D.; Ballentine, C. J.

2017-12-01

The chemical and isotopic evolution of the Earth's mantle is largely influenced by the formation of oceanic and continental crust at spreading ridges and through arc volcanism, and the subsequent recycling of this crust back into the mantle via subduction. In this study we use a combined geodynamical-geochemical modelling approach to investigate the Lu-Hf isotopic evolution of the terrestrial mantle and crust. We utilise the geodynamic mantle convection model developed by Brandenburg et al., 2008. This model satisfies the geophysical constraints of oceanic heat flow and average plate velocities, as well as geochemical observations such as 40Ar in the atmosphere. It has also been shown to reproduce the observed geochemical distributions in multiple isotope systems (U-Th-Pb, Rb-Sr, Sm-Nd, and Re-Os) that define the DMM, HIMU and EM1 mantle endmembers. We go on to extend this application to investigate the Lu-Hf isotope system, specifically in combination with Sm-Nd. The model has been updated to include a self-consistent reorganisation of the plates with regions of up-/down-wellings. The model is initiated at 4.55 Ga, assumes continental crust is produced from 4 Ga and that a transition from `dry' to `wet' subduction occurs at 3 Ga. The results of the geodynamic model suggest that the ƐHf composition and evolution of the upper mantle can be generated through the extraction and recycling of oceanic crust, which creates an enriched and radiogenic reservoir at the core-mantle boundary. The formation of continental crust, which is extracted at each time-step from the oceanic crust to imitate subduction zone processes, and the recycling of this continental crust as sediments, plays a lesser role. Depending on the selected partition coefficients DMM, FOZO and HIMU mantle endmember compositions are also produced via the simple extraction and recycling of oceanic crust. The formation of continental crust produces spread in the ƐNd vs. ƐHf array and extends the model values into the HIMU region of the terrestrial array. We go on to use this geodynamic-geochemical model to investigate different models of continental growth, by observing the effects on the coupled crustal-mantle reservoirs. Brandenburg, J.P., Hauri, E.H., van Keken, P.E., Ballentine, C.J., 2008. Earth and Planetary Science Letters 276, 1-13.

High-pressure single-crystal elasticity measurements of Al-Fe-bridgmanite support a Fe3+-rich pyrolitic lower mantle

NASA Astrophysics Data System (ADS)

Marquardt, H.; Kurnosov, A.; Frost, D. J.; Boffa Ballaran, T.; Ziberna, L.

2017-12-01

The chemical composition of the Earth's lower mantle can be constrained by combining seismological observations with mineral physics elasticity measurements. Here, we present single-crystal elasticity data of Al-Fe-bearing bridgmanite. Two crystals of (Mg0.9Fe0.1Si0.9Al0.1)O3 with different crystallographic orientations were cut using a focused ion beam and were loaded in the pressure chamber of a single diamond anvil cell employing helium as pressure medium. Elasticity and density measurements were performed at high-pressures on both samples using a combined Brillouin scattering and X-ray diffraction system at BGI. A fit of all the experimental data collected at different pressures was performed combining the Christoffel equation with the finite strain formalism to derive values of the bulk and shear modus K0 and G0, of their pressure derivatives K0' and G0' as well as of the elastic stiffness coefficients Cij and absolute pressure. Input data for this fit were the experimentally measured acoustic velocities (about 100-150 individual velocities for each pressure point), the crystallographic orientation of the two sample platelets determined by in-situ X-ray measurement, and the unit-cell volume (or density) for every pressure point. Comparison of our results to previous work on MgSiO3 bridgmanite shows that Fe/Al-incorporation reduces the acoustic velocities at room pressure, but a stronger pressure dependence of the shear modulus leads to a shear velocity crossover with MgSiO3bridgmanite at pressures of the lower mantle. We employ our data to model seismic wave velocities in the top portion of the lower mantle assuming a pyrolitic mantle composition. We find good agreement between our mineral physics predictions and the seismic PREM down to at least 1200 km depth, indicating chemical homogeneity of the upper and shallow lower mantle. A high Fe3+/Fe2+ ratio of about 2 in shallow lower mantle bridgmanite is required to match seismic data, implying the presence of metallic iron in an isochemical mantle. Our calculated velocities are in increasingly poor agreement with those of the lower mantle at depths >1200 km, indicating either a change in bridgmanite cation ordering or a decrease in the ferric iron content of the lower mantle.

Tradeoffs in Chemical and Thermal Variations in the Post-perovskite Phase Transition: Mixed Phase Regions in the Deep Lower Mantle?

NASA Astrophysics Data System (ADS)

Giles, G. F.; Spera, F. J.; Yuen, D. A.

2005-12-01

The recent discovery of a phase-transition in Mg-rich perovskite (Pv) to a post-perovskite (pPv) phase at lower mantle depths and its relationship to D", lower mantle heterogeneity and iron content prompted an investigation of the relative importance of lower mantle (LM) compositional and temperature fluctuations in creating topographic undulations on mixed phase regions. Above the transition, Mg-rich Pv makes up ~70 percent by mass of the LM. Using results from experimental phase equilibria, first-principles computations and thermodynamic relations for Fe2+-Mg mixing in silicates, a preliminary thermodynamic model for the perovskite to post-perovskite phase transition in the divariant system MgSiO3-FeSiO3 is developed. Complexities associated with components Fe2O3 and Al2O3 and other phases (Ca-Pv, magnesiowustite) are neglected. The model predicts phase transition pressures are sensitive to the FeSiO3 content of perovskite (~-1.5 GPa per one mole percent FeSiO3). This leads to considerable topography along the top boundary of the mixed phase region. The Clapeyron slope for the Pv to pPv transition at XFeSiO3=0.1 is +11 MPa/K about 20% higher than for pure Mg-Pv. Increasing bulk concentration of iron elevates the mixed (two-phase) layer above the core-mantle boundary (CMB); increasing temperature acts to push the mixed layer deeper into the LM into the D" thermal boundary layer resting upon the (CMB). For various LM geotherms and CMB temperatures, a single mixed layer of thickness ~300 km lies within the bottom 40% of the lower mantle. For low iron contents (XFeSiO3 ~5 mole percent or less), two perched layers are found. This is the divariant analog to the univariant double-crosser. The hotter the mantle, the deeper the mixed phase layer; the more iron-rich the LM, the higher the mixed phase layer. In a hotter Hadean Earth with interior temperatures everywhere 200-500 K warmer pPv is not stable unless the LM bulk composition is Fe-enriched compared to the present upper mantle.

Olivine inclusions in Siberian diamonds and mantle xenoliths: Contrasting water and trace-element contents

NASA Astrophysics Data System (ADS)

Jean, M. M.; Taylor, L. A.; Howarth, G. H.; Peslier, A. H.; Fedele, L.; Bodnar, R. J.; Guan, Y.; Doucet, L. S.; Ionov, D. A.; Logvinova, A. M.; Golovin, A. V.; Sobolev, N. V.

2016-11-01

A subject of continuing debate is how the Earth's lithospheric portion of the upper mantle has remained the thickest (> 200 km) and oldest (> 3 Gy) beneath cratons and is yet surrounded by a vigorously convecting asthenosphere. It is generally admitted that water is a key parameter in the strength and longevity of cratonic roots, because olivine, the main phase of the lithospheric mantle, becomes stronger if its water content decreases. Expanding upon the work presented in Novella et al. (2015) and Taylor et al. (2016), we report new water contents for additional olivine inclusions in diamonds together with the trace-element composition for all olivine inclusions, as well as for mantle xenoliths from various kimberlite pipes located on the Siberian craton. The olivine diamond inclusions from this study have systematically low-water contents (< 50 ppmw H2O), moderate to high forsterite (e.g., Fo91-94) contents and low Ni, Co, and Zn ppm contents (e.g., < 2848, < 108, and < 47 ppm, respectively). In contrast, olivines from Siberian craton mantle xenoliths have a wide range of water contents (6-323 ppmw H2O) and extend to lower-Fo (91-92), Ni, Co, and Zn-rich compositions, compared to the diamond inclusions. Depleted incompatible trace-element concentrations in olivine (0.1-0.001 × Primitive Mantle) advance our hypothesis for the protogenetic origins for the majority of Siberian diamond inclusions. These observations are consistent with the peridotite xenoliths as representing a part of the cratonic lithosphere that has experienced melt re-fertilization, which has also transported water. The olivine diamond inclusions, on the other hand, preserve "micro-samples" of an initial, dry cratonic lithosphere, mostly resulting from melting events. These inclusions are likely sourced from the initial cratonic mantle lithosphere, which thereby, resisted delamination over time, due to its buoyancy and strength, imparted from melt and water depletion, respectively. And thus, our data provides a major argument that the kimberlite-hosted mantle xenoliths may be more metasomatized than common rocks at the base of the Siberian and other cratonic roots away from kimberlite fields.

Seismic anisotropy in the lower crust: The link between rock composition, microstructure, texture and seismic properties.

NASA Astrophysics Data System (ADS)

Czaplinska, Daria; Piazolo, Sandra; Almqvist, Bjarne

2015-04-01

Seismic anisotropy observed in Earth's interior is caused by the presence of aligned anisotropic minerals (crystallographic and shape preferred orientation; CPO and SPO respectively), and fluid and/or melt inclusions related to deformation. Therefore, the variations in seismic anisotropy carry valuable information about the structure of the mantle and crust. For example, anisotropy observed in the upper mantle is mainly attributed to the CPO of olivine, and provides strong evidence for the flow within the upper mantle. Seismic anisotropy in the crust is still poorly constrained, mostly due to the much larger heterogeneity of the crustal rocks in comparison with the more homogenous mantle. Anisotropy in the crust will be affected by the variations in rock composition, microstructure, texture (presence or lack of CPO), brittle structures (e.g. fracture systems) and chemical composition of the minerals. However, once the relationships between those variables and seismic properties of the crustal rocks are established, seismic anisotropy can be used to derive characteristics of rocks otherwise out of reach. Our study focuses on two sets of samples of middle to lower crustal rocks collected in Fiordland (New Zealand) and in Sweden. Samples from Fiordland represent a root of a thick (ca. 80 km) magmatic arc and comprise igneous rocks, which crystallized at high P and T conditions and were subsequently metamorphosed and deformed. Samples from Sweden are derived from a metasedimentary nappe in the Caledonian orogenic belt, which is mostly composed of gneisses, amphibolites and calc-silicates that have experienced different amounts of strain. We use large area EBSD mapping to measure the CPO of the constituent phases and record the geometric relationships of the rock microstructure. Data is then used to calculate the elastic properties of the rock from single-crystal stiffnesses. Here, we utilize the EBSD GUI software (Cook et al., 2013), which offers varied homogenization techniques, including Voigt, Reuss, Hill, geometric mean and self-consistent and Asymptotic Expansion Homogenization (AEH) methods. To test the advantages and disadvantages of the method, results are compared to measured geophysical properties of equivalent rocks. Such comparison, allows refinement of seismic data interpretation for mid to lower crustal rocks. References: Cook, A., Vel., S., Johnson, S.E., Gerbi, C., Song, W.J., 2013. Elastic and Seismic Properties (ESP) Toolbox (beta version); http://umaine.edu/mecheng/faculty-and-staff/senthil-vel/software/ESP_Toolbox/

Deformation, static recrystallization, and reactive melt transport in shallow subcontinental mantle xenoliths (Tok Cenozoic volcanic field, SE Siberia)

NASA Astrophysics Data System (ADS)

Tommasi, Andréa; Vauchez, Alain; Ionov, Dmitri A.

2008-07-01

Partial melting and reactive melt transport may change the composition, microstructures, and physical properties of mantle rocks. Here we explore the relations between deformation and reactive melt transport through detailed microstructural analysis and crystallographic orientation measurements in spinel peridotite xenoliths that sample the shallow lithospheric mantle beneath the southeastern rim of the Siberian craton. These xenoliths have coarse-grained, annealed microstructures and show petrographic and chemical evidence for variable degrees of reaction with silicate melts and fluids, notably Fe-enrichment and crystallization of metasomatic clinopyroxene (cpx). Olivine crystal preferred orientations (CPO) range from strong to weak. [010]-fiber patterns, characterized by a point concentration of [010] normal to the foliation and by dispersion of [100] in the foliation plane with a weak maximum parallel to the lineation, predominate relative to the [100]-fiber patterns usually observed in lithospheric mantle xenoliths and peridotite massifs. Variations in olivine CPO patterns or intensity are not correlated with modal and chemical compositions. This, together with the analysis of microstructures, suggests that reactive melt percolation postdated both deformation and static recrystallization. Preferential crystallization of metasomatic cpx along (010) olivine grain boundaries points to an influence of the preexisting deformation fabrics on melt transport, with higher permeability along the foliation. Similarity between orthopyroxene (opx) and cpx CPO suggests that cpx orientations may be inherited from those of opx during melt-rock reaction. As observed in previous studies, reactive melt transport does not weaken olivine CPO and seismic anisotropy in the upper mantle, except in melt accumulation domains. In contrast, recovery and selective grain growth during static recrystallization may lead to development of [010]-fiber olivine CPO and, if foliations are horizontal, result in apparent isotropy for vertically propagating SKS waves, but strong anisotropy for horizontally propagating surface waves.

Enriched continental flood basalts from depleted mantle melts: modeling the lithospheric contamination of Karoo lavas from Antarctica

NASA Astrophysics Data System (ADS)

Heinonen, Jussi S.; Luttinen, Arto V.; Bohrson, Wendy A.

2016-01-01

Continental flood basalts (CFBs) represent large-scale melting events in the Earth's upper mantle and show considerable geochemical heterogeneity that is typically linked to substantial contribution from underlying continental lithosphere. Large-scale partial melting of the cold subcontinental lithospheric mantle and the large amounts of crustal contamination suggested by traditional binary mixing or assimilation-fractional crystallization models are difficult to reconcile with the thermal and compositional characteristics of continental lithosphere, however. The well-exposed CFBs of Vestfjella, western Dronning Maud Land, Antarctica, belong to the Jurassic Karoo large igneous province and provide a prime locality to quantify mass contributions of lithospheric and sublithospheric sources for two reasons: (1) recently discovered CFB dikes show isotopic characteristics akin to mid-ocean ridge basalts, and thus help to constrain asthenospheric parental melt compositions and (2) the well-exposed basaltic lavas have been divided into four different geochemical magma types that exhibit considerable trace element and radiogenic isotope heterogeneity (e.g., initial ɛ Nd from -16 to +2 at 180 Ma). We simulate the geochemical evolution of Vestfjella CFBs using (1) energy-constrained assimilation-fractional crystallization equations that account for heating and partial melting of crustal wall rock and (2) assimilation-fractional crystallization equations for lithospheric mantle contamination by using highly alkaline continental volcanic rocks (i.e., partial melts of mantle lithosphere) as contaminants. Calculations indicate that the different magma types can be produced by just minor (1-15 wt%) contamination of asthenospheric parental magmas by melts from variable lithospheric reservoirs. Our models imply that the role of continental lithosphere as a CFB source component or contaminant may have been overestimated in many cases. Thus, CFBs may represent major juvenile crustal growth events rather than just recycling of old lithospheric materials.

Geology is the Key to Explain Igneous Activity in the Mediterranean Area

NASA Astrophysics Data System (ADS)

Lustrino, M.

2014-12-01

Igneous activity in tectonically complex areas can be interpreted in many different ways, producing completely different petrogenetic models. Processes such as oceanic and continental subduction, lithospheric delamination, changes in subduction polarity, slab break-off and mantle plumes have all been advocated as causes for changes in plate boundaries and magma production, including rate and temporal distribution, in the circum-Mediterranean area. This region thus provides a natural laboratory to investigate a range of geodynamic and magmatic processes. Although many petrologic and tectonic models have been proposed, a number of highly controversial questions still remain. No consensus has yet been reached about the capacity of plate-tectonic processes to explain the origin and style of the magmatism. Similarly, there is still not consensus on the ability of geochemical and petrological arguments to reveal the geodynamic evolution of the area. The wide range of chemical and mineralogical magma compositions produced within and around the Mediterranean, from carbonatites to strongly silica-undersaturated silico-carbonatites and melilitites to strongly silica-oversaturated rhyolites, complicate models and usually require a large number of unconstrained assumptions. Can the calcalkaline-sodic alkaline transition be related to any common petrogenetic point? Is igneous activity plate-tectonic- (top-down) or deep-mantle-controlled (bottom-up)? Do the rare carbonatites and carbonate-rich igneous rocks derive from the deep mantle or a normal, CO2-bearing upper mantle? Do ultrapotassic compositions require continental subduction? Understanding chemically complex magmas emplaced in tectonically complex areas require open minds, and avoiding dogma and assumptions. Studying the geology and shallow dynamics, not speculating about the deep lower mantle, is the key to understanding the igneous activity.

Origin and evolution of primitive melts from the Debunscha Maar, Cameroon: Consequences for mantle source heterogeneity within the Cameroon Volcanic Line

NASA Astrophysics Data System (ADS)

Ngwa, Caroline N.; Hansteen, Thor H.; Devey, Colin W.; van der Zwan, Froukje M.; Suh, Cheo E.

2017-09-01

Debunscha Maar is a monogenetic volcano forming part of the Mt. Cameroon volcanic field, located within the Cameroon Volcanic Line (CVL). Partly glassy cauliflower bombs have primitive basanite-picrobasalt compositions and contain abundant normally and reversely zoned olivine (Fo 77-87) and clinopyroxene phenocrysts. Naturally quenched melt inclusions in the most primitive olivine phenocrysts show compositions which, when corrected for post-entrapment modification, cover a wide range from basanite to alkali basalt (MgO 6.9-11.7 wt%), and are generally more primitive than the matrix glasses (MgO 5.0-5.5 wt%) and only partly fall on a common liquid line of descent with the bulk rock samples and matrix glasses. Melt inclusion trace element compositions lie on two distinct geochemical trends: one (towards high Ba/Nb) is thought to represent the effect of various proportions of anhydrous lherzolite and amphibole-bearing peridotite in the source, while the other (for example, high La/Y) reflects variable degrees of partial melting. Comparatively low fractionation-corrected CaO in the melt inclusions with the highest La/Y suggests minor involvement of a pyroxenite source component that is only visible at low degrees of melting. Most of the samples show elevated Gd/Yb, indicating up to 8% garnet in the source. The range of major and trace elements represented by the melt inclusions covers the complete geochemical range given by basalts from different volcanoes of the Cameroon volcanic line, indicating that geochemical signatures that were previously thought to be volcano-specific in fact are probably present under all volcanoes. Clinopyroxene-melt barometry strongly indicates repeated mixing of compositionally diverse melts within the upper mantle at 830 ± 170 MPa prior to eruption. Mantle potential temperatures estimated for the primitive melt inclusions suggest that the thermal influence of a mantle plume is not required to explain the magma petrogenesis.

Average Potential Temperature of the Upper Mantle and Excess Temperatures Beneath Regions of Active Upwelling

NASA Astrophysics Data System (ADS)

Putirka, K. D.

2006-05-01

The question as to whether any particular oceanic island is the result of a thermal mantle plume, is a question of whether volcanism is the result of passive upwelling, as at mid-ocean ridges, or active upwelling, driven by thermally buoyant material. When upwelling is passive, mantle temperatures reflect average or ambient upper mantle values. In contrast, sites of thermally driven active upwellings will have elevated (or excess) mantle temperatures, driven by some source of excess heat. Skeptics of the plume hypothesis suggest that the maximum temperatures at ocean islands are similar to maximum temperatures at mid-ocean ridges (Anderson, 2000; Green et al., 2001). Olivine-liquid thermometry, when applied to Hawaii, Iceland, and global MORB, belie this hypothesis. Olivine-liquid equilibria provide the most accurate means of estimating mantle temperatures, which are highly sensitive to the forsterite (Fo) contents of olivines, and the FeO content of coexisting liquids. Their application shows that mantle temperatures in the MORB source region are less than temperatures at both Hawaii and Iceland. The Siqueiros Transform may provide the most precise estimate of TpMORB because high MgO glass compositions there have been affected only by olivine fractionation, so primitive FeOliq is known; olivine thermometry yields TpSiqueiros = 1430 ±59°C. A global database of 22,000 MORB show that most MORB have slightly higher FeOliq than at Siqueiros, which translates to higher calculated mantle potential temperatures. If the values for Fomax (= 91.5) and KD (Fe-Mg)ol-liq (= 0.29) at Siqueiros apply globally, then upper mantle Tp is closer to 1485 ± 59°C. Averaging this global estimate with that recovered at Siqueiros yields TpMORB = 1458 ± 78°C, which is used to calculate plume excess temperatures, Te. The estimate for TpMORB defines the convective mantle geotherm, and is consistent with estimates from sea floor bathymetry and heat flow (Stein and Stein, 1992), and overlap within 1 sigma estimates from phase transitions at the 410 km (Jeanloz and Thompson, 1983) and 670 km (Hirose, 2002) seismic discontinuities. Variations in MORB FeOliq can be used to calculate the variance of TpMORB. FeOliq variations in global MORB show that 95% of the sub-MORB mantle has a T range of 165°C; 68% of MORB fall within temperature variations of ±30°C. In comparison, Te at Hawaii and Iceland are 1706°C and 1646°C respectively, and hence Te> is 248°C at Hawaii and 188°C at Iceland. Tp estimates at Hawaii and Iceland also exceed maximum Tp estimates at MORs (at 95% level) by 171 and 111°C respectively. These Te are in agreement with estimates derived from excess topography and dynamic models of mantle flow and melt generation (e.g., Sleep, 1990, Schilling, 1991, Ito et al., 1999). A clear result is that Hawaii and Iceland are hot relative to MORB. Rayleigh number calculations further show that for these Te, critical depths (i.e., the depths at which Ra > 1000) are < 130 km. Hawaii and Iceland are thus almost assuredly the result of thermally driven, active upwellings, or mantle plumes.

The redox state of the mantle during and just after core formation.

PubMed

Frost, D J; Mann, U; Asahara, Y; Rubie, D C

2008-11-28

Siderophile elements are depleted in the Earth's mantle, relative to chondritic meteorites, as a result of equilibration with core-forming Fe-rich metal. Measurements of metal-silicate partition coefficients show that mantle depletions of slightly siderophile elements (e.g. Cr, V) must have occurred at more reducing conditions than those inferred from the current mantle FeO content. This implies that the oxidation state (i.e. FeO content) of the mantle increased with time as accretion proceeded. The oxygen fugacity of the present-day upper mantle is several orders of magnitude higher than the level imposed by equilibrium with core-forming Fe metal. This results from an increase in the Fe2O3 content of the mantle that probably occurred in the first 1Ga of the Earth's history. Here we explore fractionation mechanisms that could have caused mantle FeO and Fe2O3 contents to increase while the oxidation state of accreting material remained constant (homogeneous accretion). Using measured metal-silicate partition coefficients for O and Si, we have modelled core-mantle equilibration in a magma ocean that became progressively deeper as accretion proceeded. The model indicates that the mantle would have become gradually oxidized as a result of Si entering the core. However, the increase in mantle FeO content and oxygen fugacity is limited by the fact that O also partitions into the core at high temperatures, which lowers the FeO content of the mantle. (Mg,Fe)(Al,Si)O3 perovskite, the dominant lower mantle mineral, has a strong affinity for Fe2O3 even in the presence of metallic Fe. As the upper mantle would have been poor in Fe2O3 during core formation, FeO would have disproportionated to produce Fe2O3 (in perovskite) and Fe metal. Loss of some disproportionated Fe metal to the core would have enriched the remaining mantle in Fe2O3 and, if the entire mantle was then homogenized, the oxygen fugacity of the upper mantle would have been raised to its present-day level.

Experimental study of eclogitization and melting of basic rocks at P = 4 GPa and T = 1200-1400°C

NASA Astrophysics Data System (ADS)

Gorbachev, N. S.; Shapovalov, Yu. B.; Kostyuk, A. V.

2017-06-01

Experimental study of gabbro-norite eclogitization and melting at P = 4 GPa has made it possible to reveal the effective influence of fluid and temperature on the phase relationships. The melt composition varies from andesite-dacite in "dry conditions" to phonolite and carbonate in the presence of a fluid. The Grt-containing melting curve is replaced by the Cpx-containing liquidus as the temperature changes or a fluid is added. Hence, the possible presence of "garnetitite" and "clinopyroxenite" in the upper mantle was proved experimentally. The ultimate pressure of the spinel facies at the depth of the eclogite upper mantle is controlled by the stability of Cht ≤ 4 GPa. The revealed similarity of the spectra of REE-adakite, tonalite-trondhjemite-granodiorite (TTG), and melts formed under the partial melting of eclogitized gabbro-norite does not contradict the existing ideas of the eclogite source of the TTG rocks. Wide variations in the interphase microelement distribution factors D (Grt, Cpx)/L are indicative of effective fractionation of the microelements in the course of eclogite melting and differentiation.

The role of harzburgite layers in the morphology of subducting plates and the behavior of oceanic crustal layers

NASA Astrophysics Data System (ADS)

Yoshida, Masaki

2014-05-01

Previous numerical studies of mantle convection focusing on subduction dynamics have indicated that the viscosity contrast between the subducting plate and the surrounding mantle have a primary effect on the behavior of subducting plates. The seismically observed plate stagnation at the base of the mantle transition zone (MTZ) under the Western Pacific and Eastern Eurasia is considered to mainly result from a viscosity increase at the ringwoodite to perovskite + magnesiowüstite (Rw→Pv+Mw) phase decomposition boundary, i.e., the boundary between the upper and lower mantle. The harzburgite layer, which is sandwiched between basaltic crust and depleted peridotite (lherzolite) layers, is a key component of highly viscous, cold oceanic plates. However, the possible sensitivity of the effective viscosity of harzburgite layers in the morphology of subducting plates that are flattened in the MTZ and/or penetrated in the lower mantle has not been examined systematically in previous three-dimensional (3D) numerical modeling studies that consider the viscosity increase at the boundary between the upper and lower mantle. In this study, in order to investigate the role of harzburgite layers in the morphology of subducting plates and the behavior of oceanic crustal layers, I performed a series of numerical simulations of mantle convection with semi-dynamic plate subduction in 3D regional spherical-shell geometry. The results show that a buckled crustal layer is observed under the "heel" of the stagnant slab that begins to penetrate into the lower mantle, regardless of the magnitude of the viscosity contrast between the harzburgite layer and the underlying mantle, when the factor of viscosity increase at the boundary of the upper and lower mantle is larger than 60-100. As the viscosity contrast between the harzburgite layer and the underlying mantle increases, the curvature of buckling is larger. When the viscosity increase at the boundary of the upper and lower mantle and the viscosity contrast between the harzburgite layer and the underlying mantle are larger, the volumes of crustal and harzburgite materials trapped in the mantle transition zone (MTZ) are also larger, although almost all of the materials penetrate into the lower mantle. These materials are trapped in the MTZ for over tens of millions of years. The bending of crustal layers numerically observed in the present study is consistent with seismological evidence that there is a piece of subducted oceanic crust in the uppermost lower mantle beneath the subducting slab under the Mariana trench [Niu et al., 2003, JGR]. The results of the present study suggest that when the viscosity increase at the boundary of the upper and lower mantle is larger than 60-100, a seismically observed stagnant slab is reproduced. This result is consistent with the previous independent geodynamic studies. For instance, a 2D geodynamic model with lateral viscosity variations suggested that it would need to be substantially greater than 30, say, around 100, to explain the positive geoid anomaly in the subduction zones where the subducting slab reaches the boundary between the upper and lower mantle such as that of the western Pacific [Tosi et al., 2009, GJI]. References: [1] Tajima, F. Yoshida, M. and Ohtani, E., Conjecture with water and rheological control for subducting slab in the mantle transition zone, Geoscience Frontiers, doi:10.1016/j.gsf.2013.12.005, 2014. [2] Yoshida, M. The role of harzburgite layers in the morphology of subducting plates and the behavior of oceanic crustal layers, Geophys. Res. Lett., 40(20), 5387-5392, doi:10.1002/2013GL057578, 2013. [3] Yoshida, M. and Tajima, F., On the possibility of a folded crustal layer stored in the hydrous mantle transition zone, Phys. Earth Planet. Inter., 219, 34-48, doi:10.1016/j.pepi.2013.03.004, 2013.

Upper mantle structure of central and West Antarctica from array analysis of Rayleigh wave phase velocities

NASA Astrophysics Data System (ADS)

Heeszel, David S.; Wiens, Douglas A.; Anandakrishnan, Sridhar; Aster, Richard C.; Dalziel, Ian W. D.; Huerta, Audrey D.; Nyblade, Andrew A.; Wilson, Terry J.; Winberry, J. Paul

2016-03-01

The seismic velocity structure of Antarctica is important, both as a constraint on the tectonic history of the continent and for understanding solid Earth interactions with the ice sheet. We use Rayleigh wave array analysis methods applied to teleseismic data from recent temporary broadband seismograph deployments to image the upper mantle structure of central and West Antarctica. Phase velocity maps are determined using a two-plane wave tomography method and are inverted for shear velocity using a Monte Carlo approach to estimate three-dimensional velocity structure. Results illuminate the structural dichotomy between the East Antarctic Craton and West Antarctica, with West Antarctica showing thinner crust and slower upper mantle velocity. West Antarctica is characterized by a 70-100 km thick lithosphere, underlain by a low-velocity zone to depths of at least 200 km. The slowest anomalies are beneath Ross Island and the Marie Byrd Land dome and are interpreted as upper mantle thermal anomalies possibly due to mantle plumes. The central Transantarctic Mountains are marked by an uppermost mantle slow-velocity anomaly, suggesting that the topography is thermally supported. The presence of thin, higher-velocity lithosphere to depths of about 70 km beneath the West Antarctic Rift System limits estimates of the regionally averaged heat flow to less than 90 mW/m2. The Ellsworth-Whitmore block is underlain by mantle with velocities that are intermediate between those of the West Antarctic Rift System and the East Antarctic Craton. We interpret this province as Precambrian continental lithosphere that has been altered by Phanerozoic tectonic and magmatic activity.

Evidence for Primordial Water in Earths Deep Mantle: D/h Ratios in Baffin Island and Icelandic Picrites

NASA Astrophysics Data System (ADS)

Hallis, L. J.; Huss, G. R.; Nagashima, K.; Taylor, J.; Hilton, D. R.; Mottl, M. J.; Meech, K. J.; Halldorsson, S. A.

2016-12-01

Experimentally based chemical models suggest Jeans escape could have caused an increase in Earth's atmospheric D/H ratio of between a factor of 2 and 9 since the planets formation1. Plate tectonic mixing ensures this change has been incorporated into the mantle. In addition, collisions with hydrogen bearing planetesimals or cometary material after Earth's accretion could have altered the D/H ratio of the planet's surface and upper mantle2. Therefore, to determine Earth's original D/H ratio, a reservoir that has been completely unaffected by these surface and upper mantle changes is required. Most studies suggest that high 3He/4He ratios in some OIBs indicate the existence of relatively undegassed regions in the deep mantle compared to the upper mantle, which retain a greater proportion of their primordial He3-4. Early Tertiary (60-million-year-old) picrites from Baffin Island and west Greenland, which represent volcanic rocks from the proto/early Iceland mantle plume, contain the highest recorded terrestrial 3He/4He ratios3-4. These picrites also have Pb and Nd isotopic ratios consistent with primordial mantle ages (4.45 to 4.55 Ga)5, indicating the persistence of an ancient, isolated reservoir in the mantle. The undegassed and primitive nature6of this reservoir suggests that it could preserve Earth's initial D/H ratio. We measured the D/H ratios of olivine-hosted glassy melt inclusions in Baffin Island and Icelandic picrites to establish whether their deep mantle source region exhibits a different D/H ratio to known upper mantle and surface reservoirs. Baffin Island D/H ratios were found to extend lower than any previously measured mantle values (δD -97 to -218 ‰), suggesting that areas of the deep mantle do preserve a more primitive hydrogen reservoir, hence are unaffected by plate tectonic mixing. Comparing our measured low D/H ratios to those of known extra-terrestrial materials can help determine where Earths water came from. References: [1] Genda and Ikoma, 2008 Icarus 194, 42-52. [2] Abramov, and Mojzsis, (2009) Nature 459, 419-422. [3] Stuart et al. (2003) Nature 424, 57-59. [4] Starkey et al. (2009) Earth Planet. Sci. Lett. 277, 91-100. [5] Jackson et al. (2010) Nature 466, 853-856. [6] Robillard et al. (1992) Contrib. Mineral. Petrol. 112, 230-241.

Lithosphere-asthenosphere interaction beneath Ireland from joint inversion of teleseismic P-wave delay times and GRACE gravity

NASA Astrophysics Data System (ADS)

O'Donnell, J. P.; Daly, E.; Tiberi, C.; Bastow, I. D.; O'Reilly, B. M.; Readman, P. W.; Hauser, F.

2011-03-01

The nature and extent of the regional lithosphere-asthenosphere interaction beneath Ireland and Britain remains unclear. Although it has been established that ancient Caledonian signatures pervade the lithosphere, tertiary structure related to the Iceland plume has been inferred to dominate the asthenosphere. To address this apparent contradiction in the literature, we image the 3-D lithospheric and deeper upper-mantle structure beneath Ireland via non-linear, iterative joint teleseismic-gravity inversion using data from the ISLE (Irish Seismic Lithospheric Experiment), ISUME (Irish Seismic Upper Mantle Experiment) and GRACE (Gravity Recovery and Climate Experiment) experiments. The inversion combines teleseismic relative arrival time residuals with the GRACE long wavelength satellite derived gravity anomaly by assuming a depth-dependent quasilinear velocity-density relationship. We argue that anomalies imaged at lithospheric depths probably reflect compositional contrasts, either due to terrane accretion associated with Iapetus Ocean closure, frozen decompressional melt that was generated by plate stretching during the opening of the north Atlantic Ocean, frozen Iceland plume related magmatic intrusions, or a combination thereof. The continuation of the anomalous structure across the lithosphere-asthenosphere boundary is interpreted as possibly reflecting sub-lithospheric small-scale convection initiated by the lithospheric compositional contrasts. Our hypothesis thus reconciles the disparity which exists between lithospheric and asthenospheric structure beneath this region of the north Atlantic rifted margin.

A global geochemical model for the evolution of the mantle

NASA Technical Reports Server (NTRS)

Anderson, D. L.

1979-01-01

It is proposed that the upper mantle transition region, 220 to 670 km, is composed of eclogite which has been derived from primitive mantle by about 20 percent partial melting and that this is the source and sink of oceanic crust. The remainder of the upper mantle is garnet peridotite which is the source of continental basalts and hotspot magmas. This region is enriched in incompatible elements by hydrous and CO2 rich metasomatic fluids which have depleted the underlying layers in the L.I.L. elements and L.R.E.E. The volatiles make this a low-velocity, high attenuation, low viscosity region. The eclogite layer is internally heated and its controls the convection pattern in the upper mantle. Plate tectonics is intermittent. The continental thermal anomaly at a depth of 150-220 km triggers kimberlite and carbonatite activity, alkali and flood basalt volcanism, vertical tectonics and continental breakup. Hot spots remain active after the continents leave and build the oceanic islands. Mantle plumes rise from a depth of about 220 km. Midocean ridge basalts rise from the depleted layer below this depth. Material from this layer can also be displaced upwards by subducted oceanic lithosphere to form back-arc basins.

Constraints of GRACE on the Ice Model and Mantle Rheology in Glacial Isostatic Adjustment Modeling in North-America

NASA Astrophysics Data System (ADS)

van der Wal, W.; Wu, P.; Sideris, M.; Wang, H.

2009-05-01

GRACE satellite data offer homogeneous coverage of the area covered by the former Laurentide ice sheet. The secular gravity rate estimated from the GRACE data can therefore be used to constrain the ice loading history in Laurentide and, to a lesser extent, the mantle rheology in a GIA model. The objective of this presentation is to find a best fitting global ice model and use it to study how the ice model can be modified to fit a composite rheology, in which creep rates from a linear and non-linear rheology are added. This is useful because all the ice models constructed from GIA assume that mantle rheology is linear, but creep experiments on rocks show that nonlinear rheology may be the dominant mechanism in some parts of the mantle. We use CSR release 4 solutions from August 2002 to October 2008 with continental water storage effects removed by the GLDAS model and filtering with a destriping and Gaussian filter. The GIA model is a radially symmetric incompressible Maxwell Earth, with varying upper and lower mantle viscosity. Gravity rate misfit values are computed for with a range of viscosity values with the ICE-3G, ICE-4G and ICE-5G models. The best fit is shown for models with ICE-3G and ICE-4G, and the ICE-4G model is selected for computations with a so-called composite rheology. For the composite rheology, the Coupled Laplace Finite-Element Method is used to compute the GIA response of a spherical self-gravitating incompressible Maxwell Earth. The pre-stress exponent (A) derived from a uni- axial stress experiment is varied between 3.3 x 10-34/10-35/10-36 Pa-3s-1, the Newtonian viscosity η is varied between 1 and 3 x 1021 Pa-s, and the stress exponent is taken to be 3. Composite rheology in general results in geoid rates that are too small compared to GRACE observations. Therefore, simple modifications of the ICE-4G history are investigated by scaling ice heights or delaying glaciation. It is found that a delay in glaciation is a better way to adjust ice models for composite rheology as it increases geoid rates and improves sea level fit at some sites.

Mantle sources and magma evolution of the Rooiberg lavas, Bushveld Large Igneous Province, South Africa

NASA Astrophysics Data System (ADS)

Günther, T.; Haase, K. M.; Klemd, R.; Teschner, C.

2018-06-01

We report a new whole-rock dataset of major and trace element abundances and 87Sr/86Sr-143Nd/144Nd isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic—andesitic compositions) with > 4 wt% MgO, as well as evolved rocks (andesitic—rhyolitic compositions), characterized by MgO contents of < 4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)N, Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures ( 4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative ɛNdi (- 5.2 to - 9.4) and radiogenic ɛSri (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10-20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events.

The harzburgites-lherzolite cycle: depletion and refertilization processes

NASA Astrophysics Data System (ADS)

Dijkstra, A. H.

2011-12-01

Lherzolites or clinopyroxene-rich harzburgites sampled at the ocean floor are now generally interpreted as refractory harzburgites refertilized by melt-rock reaction or melt impregnation at the spreading center, rather than as relatively undepleted bulk upper mantle. The key evidence for a melt refertilization origin is often textural. Critically, the refertilization can mask the underlying very refractory character: oceanic peridotites prior to melt refertilization at the ridge are often too refractory to be simple mantle residues of bulk upper mantle that was melted at the ridge. This suggests that the upper mantle contains large domains that record prior melting histories. This is supported by ancient rhenium-depletion ages that are common in oceanic peridotites. In this presentation, I will discuss some key examples (e.g., Macquarie Island [1], Pindos, Totalp, Lanzarote) of refertilized oceanic peridotites, which all have recorded previous, ancient depletions. I will show the textural and geochemical evidence for melt refertilization. It has often been assumed that melt refertilization occurs by interaction with mantle melts. However, there is now evidence for melt refertilization through a reaction with eclogite-derived melts, probably at the base of the melting column underneath the ridge system. These eclogitic mantle heterogeneities themselves do not normally survive the melting underneath the spreading center, but their isotopic signature can be recognized in the reacted peridotites. In summary, we have moved away from the idea that oceanic mantle rocks are simple melting residues of homogeneous bulk upper mantle. The picture that emerges is a rich and complex one, suggesting that oceanic mantle rocks record dynamic histories of melting and refertilization. In particular, the melting event in refertilized peridotites can be much older than the age of the ridge system at which they are sampled. Many oceanic peridotites contain evidence for a Mesoproterozoic melting event of perhaps global significance. Regardless of the nature of these melting events, it is now clear that in their complex overprinting history, oceanic peridotites more and more resemble polygenetic metamorphic rocks.

The Oxidation State of Fe in Glasses from the Galapagos Archipelago: Variable Oxygen Fugacity as a Function of Mantle Source

NASA Astrophysics Data System (ADS)

Peterson, M. E.; Kelley, K. A.; Cottrell, E.; Saal, A. E.; Kurz, M. D.

2015-12-01

The oxidation state of the mantle plays an intrinsic role in the magmatic evolution of the Earth. Here we present new μ-XANES measurements of Fe3+/ΣFe ratios (a proxy for ƒO2) in a suite of submarine glasses from the Galapagos Archipelago. Using previously presented major, trace, and volatile elements and isotopic data for 4 groups of glass that come from distinct mantle sources (depleted upper mantle, 2 recycled, and a primitive mantle source) we show that Fe3+/ΣFe ratios vary both with the influence of shallow level processes and with variations in mantle source. Fe3+/ΣFe ratios increase with differentiation (i.e. decreasing MgO), but show a large variation at a given MgO. Progressive degassing of sulfur accompanies decreasing Fe3+/ΣFe ratios, while assimilation of hydrothermally altered crust (as indicated by increasing Sr/Sr*) is shown to increase Fe3+/ΣFe ratios. After taking these processes into account, there is still variability in the Fe3+/ΣFe ratios of the isotopically distinct sample suites studied, yielding a magmatic ƒO2 that ranges from ΔQFM = +0.16 to +0.74 (error < 0.5 log units) and showing that oxidation state varies as a function of mantle source composition in the Galapagos hotspot system. After correcting back to a common MgO content = 8.0 wt%, the trace element depleted group similar to MORB (ITD), and the group similar to Pinta (WD = high Th/La, Δ7/4, Δ8/4 ratios) show Fe3+/ΣFe ratios within the range of MORB (average ITD = 0.162 ± 0.003 and WD = 0.164 ± 0.006). Another trace element enriched group similar to Sierra Negra and Cerro Azul (ITE = enriched Sr and Pb isotopes) shows evidence of mixing between oxidized and reduced sources (ITE oxidized end-member = 0.177). This suggests that mantle sources in the Galapagos that are thought to contain recycled components (i.e., WD and ITE groups) have distinct oxidation states. The high 3He/4He Fernandina samples (HHe group) are shown to be the most oxidized (ave. 0.175 ± 0.006). With C/3He ratios an order of magnitude greater than MORB this suggests that the primitive mantle is a more carbonated and oxidized source than the depleted upper mantle.

Copper isotope fractionation during partial melting and melt percolation in the upper mantle: Evidence from massif peridotites in Ivrea-Verbano Zone, Italian Alps

NASA Astrophysics Data System (ADS)

Huang, Jian; Huang, Fang; Wang, Zaicong; Zhang, Xingchao; Yu, Huimin

2017-08-01

To investigate the behavior of Cu isotopes during partial melting and melt percolation in the mantle, we have analyzed Cu isotopic compositions of a suite of well-characterized Paleozoic peridotites from the Balmuccia and Baldissero massifs in the Ivrea-Verbano Zone (IVZ, Northern Italy). Our results show that fresh lherzolites and harzburgites have a large variation of δ65Cu ranging from -0.133 to 0.379‰, which are negatively correlated with Al2O3 contents as well as incompatible platinum-group (e.g., Pd) and chalcophile element (e.g., Cu, S, Se, and Te) contents. The high δ65Cu can be explained by Cu isotope fractionation during partial melting of a sulfide-bearing peridotite source, with the light isotope (63Cu) preferentially entering the melts. The low δ65Cu can be attributed to precipitation of sulfides enriched in 63Cu during sulfur-saturated melt percolation. Replacive dunites from the Balmuccia massif display high δ65Cu from 0.544 to 0.610‰ with lower Re, Pd, S, Se, and Te contents and lower Pd/Ir ratios relative to lherzolites, which may result from dissolution of sulfides during interactions between S-undersaturated melts and lherzolites at high melt/rock ratios. Thus, our results suggest that partial melting and melt percolation largely account for the Cu isotopic heterogeneity of the upper mantle. The correlation between δ65Cu and Cu contents of the lherzolites and harzburgites was used to model Cu isotope fractionation during partial melting of a sulfide-bearing peridotite, because Cu is predominantly hosted in sulfide. The modelling results indicate an isotope fractionation factor of αmelt-peridotite = 0.99980-0.99965 (i.e., 103lnαmelt-peridotite = -0.20 to -0.35‰). In order to explain the Cu isotopic systematics of komatiites and mid-ocean ridge basalts reported previously, the estimated αmelt-peridotite was used to simulate Cu isotopic variations in melts generated by variable degrees of mantle melting. The results suggest that high degrees (>25%) of partial melting extracts nearly all source Cu and it cannot produce Cu isotope fractionation in komatiites relative to their mantle source, and that sulfide segregation during magma evolution have modified Cu isotopic compositions of mid-ocean ridge basalts.

Seismic structure of the uppermost mantle beneath the Kenya rift

USGS Publications Warehouse

Keller, Gordon R.; Mechie, J.; Braile, L.W.; Mooney, W.D.; Prodehl, C.

1994-01-01

A major goal of the Kenya Rift International Seismic Project (KRISP) 1990 experiment was the determination of deep lithospheric structure. In the refraction/wide-angle reflection part of the KRISP effort, the experiment was designed to obtain arrivals to distances in excess of 400 km. Phases from interfaces within the mantle were recorded from many shotpoints, and by design, the best data were obtained along the axial profile. Reflected arrivals from two thin (< 10 km), high-velocity layers were observed along this profile and a refracted arrival was observed from the upper high-velocity layer. These mantle phases were observed on record sections from four axial profile shotpoints so overlapping and reversed coverage was obtained. Both high-velocity layers are deepest beneath Lake Turkana and become more shallow southward as the apex of the Kenya dome is approached. The first layer has a velocity of 8.05-8.15 km/s, is at a depth of about 45 km beneath Lake Turkana, and is observed at depths of about 40 km to the south before it disappears near the base of the crust. The deeper layer has velocities ranging from 7.7 to 7.8 km/s in the south to about 8.3 km/s in the north, has a similar dip as the upper one, and is found at depths of 60-65 km. Mantle arrivals outside the rift valley appear to correlate with this layer. The large amounts of extrusive volcanics associated with the rift suggest compositional anomalies as an explanation for the observed velocity structure. However, the effects of the large heat anomaly associated with the rift indicate that composition alone cannot explain the high-velocity layers observed. These layers require some anisotropy probably due to the preferred orientation of olivine crystals. The seismic model is consistent with hot mantle material rising beneath the Kenya dome in the southern Kenya rift and north-dipping shearing along the rift axis near the base of the lithosphere beneath the northern Kenya rift. This implies lithosphere thickening towards the north and is consistent with a thermal thinning of the lithosphere from below in the south changing to thinning of the lithosphere due to stretching in the north. ?? 1994.

Ultrasonic Sound Velocity of Diopside Liquid Under High Pressure and High Temperature Conditions

NASA Astrophysics Data System (ADS)

Xu, M.; Jing, Z.; Chantel, J.; Yu, T.; Wang, Y.; Jiang, P.

2017-12-01

The equation of state (EOS) of silicate liquids is of great significance to the understanding of the dynamics and differentiation of the magmatic systems in Earth and other terrestrial planets. Sound velocity of silicate liquids measured at high pressure can provide direct information on the bulk modulus and its pressure derivative and hence tightly constrain the EOS of silicate liquids. In addition, the sound velocity data can be directly compared to seismic observations to infer the presence of melts in the mantle. While the sound velocity for silicate liquids at ambient pressure has been well established, the high-pressure sound velocity data are still lacking due to experimental challenges. In this study, we successfully determined the sound velocities of diopside (CaMgSi2O6) liquid in a multi-anvil apparatus under high pressure-high temperature conditions from 1 to 4 GPa and 1973 to 2473 K by the ultrasonic interferometry in conjunction with synchrotron X-ray techniques. Diopside was chosen to study because it is not only one of the most important phases in the Earth's upper mantle, but also an end-member composition of model basalt. It is thus an ideal simplified melt composition in the upper mantle. Besides, diopside liquid has been studied by ambient-pressure ultrasonic measurements (e.g., Ai and Lange, 2008) and shock-wave experiments at much higher pressure (e.g., Asimow and Ahrens, 2010), which allows comparison with our results over a large pressure range. Our high-pressure results on the sound velocity of Di liquid are consistent with the ambient-pressure data and show an increase of velocity with pressure (from 3039 m/s at 0.1 GPa to 4215 m/s at 3.5 GPa). Fitting to the Murnaghan EOS gives an isentropic bulk modulus (Ks) of 24.8 GPa and its pressure dependence (K'S) of 7.8. These are consistent with the results from shock-wave experiments on Di liquid (Asimow and Ahrens, 2010), indicating that the technique used in this study is capable to accurately determine the sound velocity of silicate liquids at high pressures. We will use these results to better constrain the hard sphere EOS model for silicate liquids (Jing and Karato, 2011), with implications to the stability of melt layers in the deep mantle under gravity and the presence of partial melts in low velocity zones in the mantle.

Isotopic evolution of Mauna Kea volcano: Results from the initial phase of the Hawaii Scientific Drilling Project

USGS Publications Warehouse

Lassiter, J.C.; DePaolo, D.J.; Tatsumoto, M.

1996-01-01

We have examined the Sr, Nd, and Pb isotopic compositions of Mauna Kea lavas recovered by the first drilling phase of the Hawaii Scientific Drilling Project. These lavas, which range in age from ???200 to 400 ka, provide a detailed record of chemical and isotopic changes in basalt composition during the shied/postshield transition and extend our record of Mauna Kea volcanism to a late-shield period roughly equivalent to the last ???100 ka of Mauna Loa activity. Stratigraphic variations in isotopic composition reveal a gradual shift over time toward a more depleted source composition (e.g., higher 143Nd/144Nd, lower 87Sr/86Sr, and lower 3He/4He). This gradual evolution is in sharp contrast with the abrupt appearance of alkalic lavas at ???240 ka recorded by the upper 50 m of Mauna Kea lavas from the core. Intercalated tholeiitic and alkalic lavas from the uppermost Mauna Kea section are isotopically indistinguishable. Combined with major element evidence (e.g., decreasing SiO2 and increasing FeO) that the depth of melt segregation increased during the transition from tholeiitic to alkalic volcanism, the isotopic similarity of tholeiitic and alkalic lavas argues against significant lithosphere involvement during melt generation. Instead, the depleted isotopic signatures found in late shield-stage lavas are best explained by increasing the proportion of melt generated from a depleted upper mantle component entrained and heated by the rising central plume. Direct comparison of Mauna Kea and Mauna Loa lavas erupted at equivalent stages in these volcanoes' life cycles reveals persistent chemical and isotopic differences independent of the temporal evolution of each volcano. The oldest lavas recovered from the drillcore are similar to modern Kilauea lavas, but are distinct from Mauna Loa lavas. Mauna Kea lavas have higher 143Nd/144Nd and 206Pb/204Pb and lower 87Sr/86Sr. Higher concentrations of incompatible trace elements in primary magmas, lower SiO2, and higher FeO also indicate that Mauna Kea lavas formed through smaller degrees of partial melting at greater depth than Mauna Loa lavas. These chemical and isotopic differences are consistently found between volcanoes along the western "Loa" and eastern "Kea" trends and reflect large-scale variations in source composition and melting environment. We propose a simple model of a radially zoned plume centered beneath the Loa trend. Loa trend lavas generated from the hot plume axis reflect high degrees of partial melting from a source containing a mixture of enriched plume-source material and entrained lower mantle. Kea trend lavas, in contrast, are generated from the cooler, peripheral portions of the plume, record lower degrees of partial melting, and tap a source containing a greater proportion of depleted upper mantle.

Heterogeneity in mantle carbon content from CO2-undersaturated basalts

PubMed Central

Le Voyer, M.; Kelley, K.A.; Cottrell, E.; Hauri, E.H.

2017-01-01

The amount of carbon present in Earth's mantle affects the dynamics of melting, volcanic eruption style and the evolution of Earth's atmosphere via planetary outgassing. Mantle carbon concentrations are difficult to quantify because most magmas are strongly degassed upon eruption. Here we report undegassed carbon concentrations from a new set of olivine-hosted melt inclusions from the Mid-Atlantic Ridge. We use the correlations of CO2 with trace elements to define an average carbon abundance for the upper mantle. Our results indicate that the upper mantle carbon content is highly heterogeneous, varying by almost two orders of magnitude globally, with the potential to produce large geographic variations in melt fraction below the volatile-free solidus. Such heterogeneity will manifest as variations in the depths at which melt becomes interconnected and detectable, the CO2 fluxes at mid-ocean ridges, the depth of the lithosphere-asthenosphere boundary, and mantle conductivity. PMID:28082738

Dynamic Linkages Between the Transition Zone & Surface Plate Motions in 2D Models of Subduction

NASA Astrophysics Data System (ADS)

Arredondo, K.; Billen, M. I.

2013-12-01

While slab pull is considered the dominant force controlling plate motion and speed, its magnitude is controlled by slab behavior in the mantle, where tomographic studies show a wide range of possibilities from direct penetration to folding, or stagnation directly above the lower mantle (e.g. Fukao et al., 2009). Geodynamic studies have investigated various parameters, such as plate age and two phase transitions, to recreate observed behavior (e.g. Běhounková and Cízková, 2008). However, past geodynamic models have left out known slab characteristics that may have a large impact on slab behavior and our understanding of subduction processes. Mineral experiments and seismic observations have indicated the existence of additional phase transitions in the mantle transition zone that may produce buoyancy forces large enough to affect the descent of a subducting slab (e.g. Ricard et al., 2005). The current study systematically tests different common assumptions used in geodynamic models: kinematic versus free-slip boundary conditions, the effects of adiabatic heating, viscous dissipation and latent heat, compositional layering and a more complete suite of phase transitions. Final models have a complete energy equation, with eclogite, harzburgite and pyrolite lithosphere compositional layers, and seven composition-dependent phase transitions within the olivine, pyroxene and garnet polymorph minerals. Results show important feedback loops between different assumptions and new behavior from the most complete models. Kinematic models show slab weakening or breaking above the 660 km boundary and between compositional layers. The behavior in dynamic models with a free-moving trench and overriding plate is compared to the more commonly found kinematic models. The new behavior may have important implications for the depth distribution of deep earthquakes within the slab. Though the thermodynamic parameters of certain phase transitions may be uncertain, their presence and feedback to other added processes remain important, which could encourage mineralogical research into multiphase systems. Feedback from the compositionally complex slab to the dynamic trench may improve understanding on the mechanics of slab behavior in the upper and lower mantle and surface behavior of the subducting and overriding plates. Běhounková, M., and H. Cízková, Long-wavelength character of subducted slabs in the lower mantle, Earth and Planetary Science Letters, 275, 43-53, 2008. Fukao, Y., M. Obayashi, T. Nakakuki, and the Deep Slab Project Group, Stagnant slab: A review, Annual Reviews of Earth and Planetary Science, 37, 19-46, 2009. Ricard, Y., E. Mattern, and J. Matas, Synthetic tomographic images of slabs from mineral physics, in Earth's Deep Mantle: Structure, Composition, and Evolution, Geophysical Monograph Series, vol. 160, American Geophysical Union, 2005.

The Earth's core formation: Constraints imposed by partitioning behaviour of Germanium and Copper.

NASA Astrophysics Data System (ADS)

Kegler, P.; Holzheid, A.; Palme, H.

2006-12-01

The abundances of the siderophile (metal-seeking) elements in the present Earth mantle are the result of the core formation in the early Earth. It is commonly assumed that the concentrations of the siderophile elements in the Earth mantle, especially of Ni and Co, can be explained by metal-silicate equilibrium at the base of a deep magma ocean during the core forming event [1,2], assuming values of Ni and Co metal-silicate partition coefficients at the bottom of a magma ocean that are identical to the Ni and Co core-mantle ratios (Ni:~39; pressure, where both partition coefficients have the same value, does exist within the Earth upper mantle [3,4]. In order to better constrain the origin of the siderophile elements in the upper mantle of the Earth we expanded our study and determined high pressure and high temperature metal/silicate partition coefficients of Ge and Cu (Dmet/sil = concentration of, e.g., Ge in metal / concentration of, e.g., Ge in silicate). The experiments were performed with Fe97Ge3 and Fe97Cu3 alloys. The metals were equilibrated with a synthetic basaltic melt (same composition as in [3,4]) within a pressure range from 0.5 to 2.5 GPa and at a temperature of 1450°C. The partition coefficients of both elements decrease within the investigated pressure range and at an oxygen fugacity of 1.4 log units more reduced than the Fe-FeO buffer (Dmet/sil at 0.5 GPa: Ge 5475, Cu 49; Dmet/sil at 2.5 GPa: Ge 4057, Cu 42). Although the pressure range covered by our experiments is narrow and extrapolations to pressures relevant to upper mantle conditions are therefore not very robust, it is still obvious that Dmet/sil of Ge and Cu at conditions at the bottom of a magma ocean would not be equal to their core/mantle ratios (Ge: 26; Cu: 6). These findings, in addition to the earlier results of Ni and Co, question the hypothesis of a simple single stage magma ocean. However, other models like inefficient core formation [5], heterogeneous accretion [6-7], or self oxidation of the Earth mantle with a multiple stage magma ocean [8,9] might be alternative hypotheses to explain the siderophile element abundances in the Earth mantle. [1] Li &Agee 1996 Nature, 381, 686-689. [2] Bouhifd &Jephcoat 2003. EPSL, 209, 245-255. [3] Kegler et al. 2004 LPSC XXXV, 1632. [4] Kegler et al. 2005 LPSC XXXVI, 2030. [5] Jones & Drake 1986 Nature 322, 221- 228. [6] Ringwood 1984 Proc.R.Soc.London, A395, 1-46. [7] Wanke et al. 1984 In: Archean geochemistry (ed. A. Kroener ) pp 1-24. [8] Frost et al. 2004 Nature, 428, 409-412. [9] Wade &Wood 2005 EPSL, 236, 78-95.

Miocene Basaltic Lava Flows and Dikes of the Intervening Area Between Picture Gorge and Steens Basalt of the CRBG, Eastern Oregon

NASA Astrophysics Data System (ADS)

Cahoon, E. B.; Streck, M. J.

2016-12-01

Mid-Miocene basaltic lavas and dikes are exposed in the area between the southern extent of the Picture Gorge Basalt (PGB) and the northern extent of Steens Basalt in a wide corridor of the Malheur National Forest, eastern Oregon. An approximate mid-Miocene age of sampled basaltic units is indicated by stratigraphic relationships to the 16 Ma Dinner Creek Tuff. Lavas provide an opportunity to extend and/or revise distribution areas of either CRBG unit and explore the petrologic transition between them. The PGB and the Steens Basalt largely represent geochemically distinct tholeiitic units of the CRBG; although each unit displays internal complexity. Lavas of PGB are relatively primitive (MgO 5-9 wt.%) while Steens Basalt ranges in MgO from >9 to 3 wt.% but both units are commonly coarsely porphyritic. Conversely, Steens Basalt compositions are on average more enriched in highly incompatible elements (e.g. Rb, Th) and relatively enriched in the lesser incompatible elements (e.g. Y, Yb) compared to the Picture Gorge basalts. These compositional signatures produce inclined and flat patterns on mantle-normalized incompatible trace element plots but with similar troughs and spikes, respectively. New compositional data from our study area indicate basaltic lavas can be assigned as PGB lava flows and dikes, and also to a compositional group chemically distinct between Steens Basalt and PGB. Distribution of lava flows with PGB composition extend this CRBG unit significantly south/southeast closing the exposure gap between PGB and Steens Basalt. We await data that match Steens Basalt compositions but basaltic lavas with petrographic features akin to Steens Basalt have been identified in the study area. Lavas of the transitional unit share characteristics with Upper Steens and Picture Gorge basalt types, but identify a new seemingly unique composition. This composition is slightly more depleted in the lesser incompatible elements (i.e. steeper pattern) on mantle normalized incompatible element diagrams, relatively enriched in Sr, and overall reflects more HFSE depletion than Upper Steens Basalt. Similar compositional patterns have also been observed among lavas of the Strawberry Volcanics located immediately east of our study area.

Effects of Fertile Mantle Compositional Variation and Spreading Rate Variation on the Working of Global Ocean Ridges

NASA Astrophysics Data System (ADS)

Niu, Y.; O'Hara, M. J.

2014-12-01

Mantle temperature variation, plate spreading rate variation and mantle compositional variation have been considered to be the three fundamental variables that govern the working of global ocean ridges [1]. An analysis demonstrates that mantle compositional variation exerts the primary control on ocean ridge processes; it determines (1) variation in both composition and mode of mantle mineralogy, (2) variation of mantle density, (3) variation of ridge axial depth, (4) source-inherited MORB compositional variation, (4) density-controlled variation in the amplitude of mantle upwelling, (5) apparent variation in the extent of melting, and (6) the correlated variation of MORB chemistry with ridge axial depth [2]. The above interpretations are reinforced by the updated MORB database [3]. The new database also confirms spreading rate control on the extent of melting as shown previously [4]. Mantle temperature variation could play a part, but its overstated role [3,5] results from a basic error (1) in treating ridge axial depth variation as evidence of mantle temperature variation by ignoring the intrinsic control of mantle composition, (2) in treating "mantle plume" influenced ridges (e.g., Iceland) as normal ridges of plate spreading origin, and (3) in treating low Vs at greater depths (> 300 km vs. < 200 km beneath ridges) beneath these "mantle plume" influenced ridges as evidence for hot ridge mantle. In order to understand the working of global ocean ridges, we must avoid plume-influenced ridges (e.g., in the vicinity of Iceland) and remove/average out data from such ridges. As a result, the correlations (e.g., between ridge axial depth, mantle low Vs anomaly, and some geochemical parameters) required for the interpretation of mantle temperature control all disappear. There is thus no evidence for large mantle temperature variation away from ridges influenced by "mantle plumes". References: [1] Niu et al., 2001, Earth Planet Sci. Lett., 186, 383-399; [2] Niu & O'Hara, 2008, J. Petrol., 49, 633-664; [3] Gale et al., 2014, J. Petrol, 55, 1051-1082; [4] Niu & Hékinian, 1997, Nature, 385, 326-329; [5] Dalton et al., 2014, Science, 334, 80-83; [6]Niu & Hékinian, 2004, In Oceanic Hotspots, Springer-Verlag, 285-307.

Constraints on the viscosity of the continental crust and mantle from GPS measurements and postseismic deformation models in western Mongolia

USGS Publications Warehouse

Vergnolle, M.; Pollitz, F.; Calais, E.

2003-01-01

We use GPS measurements and models of postseismic deformation caused by seven M6.8 to 8.4 earthquakes that occurred in the past 100 years in Mongolia to assess the viscosity of the lower crust and upper mantle. We find an upper mantle viscosity between 1 ?? 1018 and 4 ?? 1018 Pa s. The presence of such a weak mantle is consistent with results from independent seismological and petrological studies that show an abnormally hot upper mantle beneath Mongolia. The viscosity of the lower crust is less well constrained, but a weak lower crust (3 ?? 1016 to 2 ?? 1017 Pa s) is preferred by the data. Using our best fit upper mantle and lower crust viscosities, we find that the postseismic effects of viscoelastic relaxation on present-day horizontal GPS velocities are small (<2 mm yr-1) but still persist 100 years after the 1905, M8.4, Bolnay earthquake. This study shows that the GPS velocity field in the Baikal-Mongolia area can be modeled as the sum of (1) a rigid translation and rotation of the whole network, (2) a 3-5 mm yr-1 simple shear velocity gradient between the Siberian platform to the north and northern China to the south, and (3) the contribution of postseismic deformation, mostly caused by the 1905 Bolnay-Tsetserleg sequence and by the smaller, but more recent, 1957 Bogd earthquake. Copyright 2003 by the American Geophysical Union.

The failure of 1D seismic model fitting to constrain lower mantle composition

NASA Astrophysics Data System (ADS)

Houser, C. T.; Hernlund, J. W.; Valencia-Cardona, J. J.; Wentzcovitch, R.

2017-12-01

Tests of lower mantle composition models often compare mineral physics data bearing on the elasticity and density of lower mantle phases to the average seismic velocity profile determined by seismology, such a PREM or ak135. We demonstrate why such comparisons between mineralogy and seismology are an inadequate method for definitive discrimination between different scenarios. One issue is that the seismic velocity is more sensitive to temperature than composition for most lower mantle minerals. In practice, this allows one the freedom to choose the geotherm that brings the predicted seismic and density data into agreement with observations. It is commonly assumed that the temperature profile should be adiabatic, however, such a profile presupposes a particular state of the mantle and is only applicable in the absence of layering, buoyancy fluctuations, compositional segregation, and rheological complexities. The mantle temperature should depend on the composition since the latter influences the viscosity of rocks. However, the precise relation between composition, viscosity, and heat transfer would need to be specified, but unfortunately remains highly uncertain. If the mantle contains a mixture of domains with multiple bulk compositions, then the 1D seismic profile comparison is inherently non-unique. Rocks with different bulk composition likely have different isotopic abundances, and can exhibit differing degrees of internal heating and therefore distinct temperatures. Different composition domains can also exhibit variable densities, and tend to congregate at different depths in ways that also affect their thermal evolution and temperature. Therefore, fitting a 1D seismic model alone is an inadequate tool to evaluate lower mantle composition.

Effects of upper mantle heterogeneities on the lithospheric stress field and dynamic topography

NASA Astrophysics Data System (ADS)

Osei Tutu, Anthony; Steinberger, Bernhard; Sobolev, Stephan V.; Rogozhina, Irina; Popov, Anton A.

2018-05-01

The orientation and tectonic regime of the observed crustal/lithospheric stress field contribute to our knowledge of different deformation processes occurring within the Earth's crust and lithosphere. In this study, we analyze the influence of the thermal and density structure of the upper mantle on the lithospheric stress field and topography. We use a 3-D lithosphere-asthenosphere numerical model with power-law rheology, coupled to a spectral mantle flow code at 300 km depth. Our results are validated against the World Stress Map 2016 (WSM2016) and the observation-based residual topography. We derive the upper mantle thermal structure from either a heat flow model combined with a seafloor age model (TM1) or a global S-wave velocity model (TM2). We show that lateral density heterogeneities in the upper 300 km have a limited influence on the modeled horizontal stress field as opposed to the resulting dynamic topography that appears more sensitive to such heterogeneities. The modeled stress field directions, using only the mantle heterogeneities below 300 km, are not perturbed much when the effects of lithosphere and crust above 300 km are added. In contrast, modeled stress magnitudes and dynamic topography are to a greater extent controlled by the upper mantle density structure. After correction for the chemical depletion of continents, the TM2 model leads to a much better fit with the observed residual topography giving a good correlation of 0.51 in continents, but this correction leads to no significant improvement of the fit between the WSM2016 and the resulting lithosphere stresses. In continental regions with abundant heat flow data, TM1 results in relatively small angular misfits. For example, in western Europe the misfit between the modeled and observation-based stress is 18.3°. Our findings emphasize that the relative contributions coming from shallow and deep mantle dynamic forces are quite different for the lithospheric stress field and dynamic topography.

Upper Mantle Dynamics of Bangladesh by Splitting Analyzes of Core Refracted SKS and SKKS Waves

NASA Astrophysics Data System (ADS)

Tiwari, A. K.; Bhushan, K.; Eken, T.; Singh, A.

2017-12-01

New shear wave splitting measurements are obtained from hitherto less studied Bengal Basin using core refracted SKS and SKKS phases. Splitting parameters, time delays (δt) and fast polarization directions (Φ) were estimated through analysis of 64 high-quality waveforms (≥ 2.5 signal to noise ratio) from 29 earthquakes with magnitude ≥5.5 recorded at eight seismic stations deployed over Bangladesh. We found no evidence of splitting which indicates azimuthal isotropy beneath the region. Null measurements can be explained by near vertical axis of anisotropy or by the presence of multiple anisotropic layers with different fast polarization directions, where combined effect results in null. We consider that the presence of partial melts within the upper mantle due to Kerguelen mantle plume activities may be the potential geodynamic cause for observed null measurements. It locally perturbed mantle convection flow beneath the region and reoriented the lattice preferred orientation of the upper mantle mineral mainly olivine as this disabled the core refracted SKS and SKKS phases to scan the anisotropic characteristics of the region, and hence null measurements are obtained.

Tracking silica in Earth's upper mantle using new sound velocity data for coesite to 5.8 GPa and 1073 K: Tracking Silica in Earth's Upper Mantle

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Ting; Liebermann, Robert C.; Zou, Yongtao

The compressional and shear wave velocities for coesite have been measured simultaneously up to 5.8 GPa and 1073 K by ultrasonic interferometry for the first time. The shear wave velocity decreases with pressure along all isotherms. The resulting contrasts between coesite and stishovite reach ~34% and ~45% for P and S wave velocities, respectively, and ~64% and ~75% for their impedance at mantle conditions. The large velocity and impedance contrasts across coesite-stishovite transition imply that to generate the velocity and impedance contrasts observed at the X-discontinuity, only a small amount of silica would be required. The velocity jump dependences onmore » silica, d(lnVP)/d(SiO2) = 0.38 (wt %)-1 and d(lnVS)/d(SiO2) = 0.52 (wt %)-1, are utilized to place constraints on the amount of silica in the upper mantle and provide a geophysical approach to track mantle eclogite materials and ancient subducted oceanic slabs.« less

The African Lithosphere

NASA Astrophysics Data System (ADS)

Priestley, K.; Debayle, E.; McKenzie, D.; Pilidou, S.

2007-12-01

There have been a number of prior, large scale surface wave studies of Africa, the majority of which rely on fundamental mode observations. In this study we use a large data set of multi-mode surface waves recorded over epicentral distances most of which are shorter than 6000 km, to investigate the Sv wave speed heterogeneity of the upper mantle beneath Africa. The inclusion of the higher mode data allow us to build an upper mantle model for the African plate with a horizontal resolution of a few hundred kilometers and a vertical resolution of a few tens of kilometers extending to about 400 km depth. Our tomographic images of the upper mantle beneath Africa displays significant shear velocity features, much of which correlate with surface geology. High velocity mantle persists beneath the West African and Congo cratons to 225-250 km depth, but the high velocity root beneath Kalahari Craton extends to only about 175 km depth. Low velocity upper mantle underlies the Pan- African terranes of Africa with the exception of the Damara mobile belt separating the Congo and Kalahari Cratons. The Damara mobile belt is underlain by a thick high velocity upper mantle lid which is indistinguishable from that beneath the Congo Craton to the north and the Kalahari Craton to the south. Low velocity upper mantle underlie the Hoggar, Tebesti and Darfur volcanic areas of northern Africa, and very low velocities underlie the Afar region to at least 400 km depth. We use the relationship between shear velocity and temperature of Priestley & McKenzie (2006) to derive a model for the African thermal lithosphere. Two types of lithosphere underlie Africa. Thick lithosphere underlies most of western Africa and all of southern Africa; in the latter the extent of the thick lithosphere is significantly different from the distribution of Archean crust mapped at the surface. Thick lithosphere forms one continuous structure beneath the Congo and Kalahari Cratons. Other than the Pan-African Damara mobile belt, the only Pan-African terrane of Africa free of recent (<30 Ma) volcanism, all of the Pan- African is underlain by lithosphere whose thickness is too thin to be resolved by our current surface wave analysis.

Dynamics of upper mantle rocks decompression melting above hot spots under continental plates

NASA Astrophysics Data System (ADS)

Perepechko, Yury; Sorokin, Konstantin; Sharapov, Victor

2014-05-01

Numeric 2D simulation of the decompression melting above the hot spots (HS) was accomplished under the following conditions: initial temperature within crust mantle section was postulated; thickness of the metasomatized lithospheric mantle is determined by the mantle rheology and position of upper asthenosphere boundary; upper and lower boundaries were postulated to be not permeable and the condition for adhesion and the distribution of temperature (1400-2050°C); lateral boundaries imitated infinity of layer. Sizes and distribution of lateral points, their symmetry, and maximum temperature varied between the thermodynamic condition for existences of perovskite - majorite transition and its excess above transition temperature. Problem was solved numerically a cell-vertex finite volume method for thermo hydrodynamic problems. For increasing convergence of iterative process the method of lower relaxation with different value of relaxation parameter for each equation was used. The method of through calculation was used for the increase in the computing rate for the two-layered upper mantle - lithosphere system. Calculated region was selected as 700 x (2100-4900) km. The time step for the study of the asthenosphere dynamics composed 0.15-0.65 Ma. The following factors controlling the sizes and melting degree of the convective upper mantle, are shown: a) the initial temperature distribution along the section of upper mantleb) sizes and the symmetry of HS, c) temperature excess within the HS above the temperature on the upper and lower mantle border TB=1500-2000oC with 5-15% deviation but not exceed 2350oC. It is found, that appearance of decompression melting with HS presence initiate primitive mantle melting at TB > of 1600oC. Initial upper mantle heating influence on asthenolens dimensions with a constant HS size is controlled mainly by decompression melting degree. Thus, with lateral sizes of HS = 400 km the decompression melting appears at TB > 1600oC and HS temperature (THS) > 1900oC asthenolens size ~700 km. When THS = of 2000oC the maximum melting degree of the primitive mantle is near 40%. An increase in the TB > 1900oC the maximum degree of melting could rich 100% with the same size of decompression melting zone (700 km). We examined decompression melting above the HS having LHS = 100 km - 780 km at a TB 1850- 2100oC with the thickness of lithosphere = 100 km.It is shown that asthenolens size (Lln) does not change substantially: Lln=700 km at LHS = of 100 km; Lln= 800 km at LHS = of 780 km. In presence of asymmetry of large HS the region of advection is developed above the HS maximum with the formation of asymmetrical cell. Influence of lithospheric plate thicknesses on appearance and evolution of asthenolens above the HS were investigated for the model stepped profile for the TB ≤ of 1750oS with Lhs = 100km and maximum of THS =2350oC. With an increase of TB the Lln difference beneath lithospheric steps is leveled with retention of a certain difference to melting degrees and time of the melting appearance a top of the HS. RFBR grant 12-05-00625.

Coupled anisotropic and isotropic tomography of the upper mantle beneath northern Fennoscandia - Application of a novel code AniTomo

NASA Astrophysics Data System (ADS)

Munzarova, Helena; Plomerova, Jaroslava; Kissling, Edi; Vecsey, Ludek; Babuska, Vladislav

2017-04-01

Seismological investigations of the continental mantle lithosphere, particularly its anisotropic structure, advance our understanding of plate tectonics and formation of continents. Orientation of the anisotropic fabrics reflects stress fields during the lithosphere origin and its later deformations. To contribute to studies of the large-scale upper-mantle anisotropy, we have developed code AniTomo for regional anisotropic tomography. AniTomo allows a simultaneous inversion of relative travel time residuals of teleseismic P waves for 3D distribution of isotropic-velocity perturbations and anisotropy in the upper mantle. Weak hexagonal anisotropy with symmetry axis oriented generally in 3D is assumed. The code was successfully tested on a large series of synthetic datasets and synthetic structures. In this contribution we present results of the first application of novel code AniTomo to real data, i.e., relative travel-time residuals of teleseismic P waves recorded during passive seismic experiment LAPNET in the northern Fennoscandia between 2007 and 2009. The region of Fennoscandia is a suitable choice for the first application of the new code. This Precambrian region is tectonically stable and has a thick anisotropic mantle lithosphere (Plomerova and Babuska, Lithos 2010) without significant thermal heterogeneities. In the resulting anisotropic model of the upper mantle beneath the northern Fennoscandia, the strongest anisotropy and the largest velocity perturbations concentrate in the mantle lithosphere. We delimit regions of laterally and vertically consistent anisotropy in the mantle-lithospheric part of the model. In general, the identified anisotropic regions correspond to domains detected by joint interpretation of lateral variations of the P- and SKS-wave anisotropic parameters (Plomerova et al., Solid Earth 2011). Particularly, the mantle lithosphere in the western part of the volume studied exhibits a distinct and uniform fabric that is sharply separated from the surrounding regions. The eastern boundary of this region gradually shifts westward with increasing depth in the tomographic model. We connect the retrieved domain-like anisotropic structure of the mantle lithosphere in the northern Fennoscandia with preserved fossil fabrics of the Archean micro-plates, accreted during the Precambrian orogenic processes.

What drives the Tibetan crust to the South East Asia? Role of upper mantle density discontinuities as inferred from the continental geoid anomalies

NASA Astrophysics Data System (ADS)

Rajesh, S.

2012-04-01

The Himalaya-Tibet orogen formed as a result of the northward convergence of India into the Asia over the past 55 Ma had caused the north south crustal shortening and Cenozoic upliftment of the Tibetan plateau, which significantly affected the tectonic and climatic framework of the Asia. Geodetic measurements have also shown eastward crustal extrusion of Tibet, especially along major east-southeast strike slip faults at a slip rate of 15-20 mm a-1 and around 40 mm a-1. Such continental scale deformations have been modeled as block rotation by fault boundary stresses developed due to the India-Eurasia collision. However, the Thin Sheet model explained the crustal deformation mechanism by considering varying gravitational potential energy arise out of varying crustal thickness of the viscous lithosphere. The Channel Flow model, which also suggests extrusion is a boundary fault guided flow along the shallow crustal brittle-ductile regime. Although many models have proposed, but no consensus in these models to explain the dynamics of measured surface geodetic deformation of the Tibetan plateau. But what remains conspicuous is the origin of driving forces that cause the observed Tibetan crustal flow towards the South East Asia. Is the crustal flow originated only because of the differential stresses that developed in the shallow crustal brittle-ductile regime? Or should the stress transfer to the shallow crustal layers as a result of gravitational potential energy gradient driven upper mantle flow also to be accounted. In this work, I examine the role of latter in the light of depth distribution of continental geoid anomalies beneath the Himalaya-Tibet across major upper mantle density discontinuities. These discontinuity surfaces in the upper mantle are susceptible to hold the plastic deformation that may occur as a result of the density gradient driven flow. The distribution of geoid anomalies across these density discontinuities at 220, 410 and 660 km depth in the upper mantle beneath the Himalaya-Tibet has been studied by analyzing the geoid undulation data obtained from various satellite geodetic missions along with the recent and old (EGM2008 and EGM2006) Earth Gravity models. Results show that the net geoid anomaly varies from -65 m to -20 m, which signify a density stratified upper mantle beneath the Himalaya-Tibet and the same has been confirmed from the results of regional seismic tomography studies. The density anomaly distribution beneath Tibet from 163 km depth to its upper mantle thickness of 1063 km show a strong NW-SE elliptically oriented positive geoid anomalies of magnitude around 40 meter. Asymmetric density anomaly gradient have been observed along the Himalayan arc from west to east as well as across the arc from north to south. This caused differential gravitational potential gradient and hence an elliptical flow structure of the Tibetan continental mantle along the resultant NW-SE direction, which is in concurrence with the observed present day direction of the Tibetan crustal flow. Thus the geoid anomalies distributed at various depth ranges show how the gradient in the upper mantle gravitational potential energy, especially across the deformed discontinuity surface, is significant in determining the transfer of deviatoric stresses and providing traction to the flow of crustal layers of the Tibetan Plateau. This suggests the viscous flow model could be a preferable choice, which could better accommodate the dynamics of the upper mantle, in explaining the crustal extrusion processes of the Tibetan Plateau.

Earth's Various Recipes for Making Lherzolites

NASA Astrophysics Data System (ADS)

Becker, H.; van Acken, D.

2007-12-01

Petrological and cosmochemical arguments suggest that the convecting upper mantle overall should have a lherzolitic composition, otherwise, continous production of MORB would not be feasible. The predominance of harzburgites among ocean floor peridotites fits this picture because harzburgites are commonly believed to be the residue of high degrees of partial melting at shallow depths, with fertile components lost during polybaric partial melting. Implicitly, it is commonly assumed that the deeper parts of the asthenosphere and new-formed lithosphere should be residues of low-degree partial melting. This view has been supported by the abundance of lherzolites among mantle xenoliths and orogenic peridotite massifs. But is this model really correct? Data and observations on oceanic and continental peridotites accumulated over recent years hint that reality is more complicated. On the basis of mineral and whole rock compositions, and isotopic data, it has long been suspected that many continental peridotites have undergone some form of pyroxene addition via percolating melts, yet the efficacy of these processes has been uncertain. Novel combination of structural and chemical work by Le Roux et al. (2007) indicates that melt influx may have converted deformed harzburgitic rocks of the Lherz peridotite massif into little-deformed spinel lherzolites. Refertilization by MORB-like sub-lithospheric melts, and marble cake style stretching of pyroxenites have been implicated as major processes that affected the composition of peridotites from the Totalp spinel lherzolite body, a fragment of Jurassic ultra-slow spreading Thetys ocean floor in the Swiss Alps (van Acken et al., 2007). Refertilization by melts has been associated with lherzolites from oceanic fracture zones (e. g., Seyler and Bonatti, 1997) and may be responsible for lherzolites alternating with harzburgitic domains at the Arctic Gakkel ridge (Liu et al. 2007). Evidence for compositional transformation of depleted peridotites into fertile rocks, both in young oceanic and in continental settings brings up questions that need to be addressed in the future: How common are truly residual lherzolites? Are lherzolites suitable to constrain the composition of the primitive mantle? How are fertile components in the asthenosphere distributed? Mantle rocks may have more surprises in stock.

Global structure of mantle isotopic heterogeneity and its implications for mantle differentiation and convection

NASA Astrophysics Data System (ADS)

Iwamori, Hikaru; Albaréde, Francis; Nakamura, Hitomi

2010-11-01

In order to further our understanding of the global geochemical structure and mantle dynamics, a global isotopic data set of oceanic basalts was analyzed by Independent Component Analysis (ICA), a relatively new method of multivariate analysis. The data set consists of 2773 mid-ocean ridge basalts (MORB) and 1515 ocean island basalts (OIB) with five isotopic ratios of Pb, Nd and Sr. The data set spatially covers the major oceans and enables us to compare the results with global geophysical observations. Three independent components (ICs) have been found, two of which are essentially identical to those previously found for basalts from the Atlantic and Indian Oceans. The two ICs (IC1 and IC2) span a compositional plane that accounts for 95.7% of the sample variance, while the third IC (IC3) accounts for 3.7%. Based on the geochemical nature of ICs and a forward model concerning trace elemental and isotopic compositions, the origin of the ICs is discussed. IC1 discriminates OIB from MORB, and may be related to elemental fractionation associated with melting and the subsequent radiogenic in growth with an average recycling time of 0.8 to 2.4 Ga. IC2 tracks the regional provenance of both MORB and OIB and may be related to aqueous fluid-rock interaction and the subsequent radiogenic ingrowth with an average recycling time of 0.3 to 0.9 Ga. IC3 fingerprints upper continental crustal material and its high value appears in limited geographical and tectonic settings. Variations in the melt component (IC1) and in the aqueous fluid component (IC2) inherited in the mantle most likely reflect mid-ocean ridge and subduction zone processes, respectively. Long-term accumulation of dense materials rich in the IC1 melt component at the base of the convective mantle accounts for its longer recycling time with respect to that for less dense materials rich in the aqueous fluid component (IC2). IC2 broadly correlates with the seismic velocity structures of the lowermost mantle and electric conductivity around the mantle transition zones. We propose that IC2 reflects hydrogen distribution within the mantle and that several global domains enriched in hydrogen could exist as vertical sectors extending all the way down to the core-mantle boundary.

Upper mantle seismic velocity structure beneath the Kenya Rift and the Arabian Shield

NASA Astrophysics Data System (ADS)

Park, Yongcheol

Upper mantle structure beneath the Kenya Rift and Arabian Shield has been investigated to advance our understanding of the origin of the Cenozoic hotspot tectonism found there. A new seismic tomographic model of the upper mantle beneath the Kenya Rift has been obtained by inverting teleseismic P-wave travel time residuals. The model shows a 0.5--1.5% low velocity anomaly below the Kenya Rift extending to about 150 km depth. Below ˜150 km depth, the anomaly broadens to the west toward the Tanzania Craton, suggesting a westward dip to the structure. The P- and S-wave velocity structure beneath the Arabian Shield has been investigated using travel-time tomography. Models for the seismic velocity structure of the upper mantle between 150 and 400 depths reveal a low velocity region (˜1.5% in the P model and ˜3% in the S model) trending NW-SE along the western side of the Arabian Shield and broadening to the northeast beneath the MMN volcanic line. The models have limited resolution above 150 km depth everywhere under the Shield, and in the middle part of the Shield the resolution is limited at all depths. Rayleigh wave phase velocity measurements have been inverted to image regions of the upper mantle under the Arabian Shield not well resolved by the body wave tomography. The shear wave velocity model obtained shows upper mantle structure above 200 km depth. A broad low velocity region in the lithospheric mantle (depths of ≤ ˜100 km) across the Shield is observed, and below ˜150 km depth a region of low shear velocity is imaged along the Red Sea coast and MMN volcanic line. A westward dipping low velocity zone beneath the Kenya Rift is consistent with an interpretation by Nyblade et al. [2000] suggesting that a plume head is located under the eastern margin of the Tanzania Craton, or alternatively a superplume rising from the lower mantle from the west and reaching the surface under Kenya [e.g., Debayle et al., 2001; Grand et al., 1997; Ritsema et al., 1999]. For the Arabian Shield, the models are not consistent with a two plume model [Camp and Roobol, 1992] because there is a continuous low velocity zone at depths ≥ 150 km along the western side of the Shield and not separate anomalies. The NW-SE trending low velocity anomaly beneath the western side of the Shield supports the Ebinger and Sleep [1998] model invoking plume flow channeled by thinner lithosphere along the Red Sea coast. The NW-SE low velocity structure beneath the western side of the Shield could also be the northern-most extent of the African Superplume. A low velocity anomaly beneath Ethiopia [Benoit et al., 2006a,b] dips to the west and may extend through the mantle transition zone. The observed low velocities in the upper mantle beneath the Arabian Shield could be caused by hot mantle rock rising beneath Ethiopia and flowing to the north under the Arabian Shield.

African hot spot volcanism: small-scale convection in the upper mantle beneath cratons.

PubMed

King, S D; Ritsema, J

2000-11-10

Numerical models demonstrate that small-scale convection develops in the upper mantle beneath the transition of thick cratonic lithosphere and thin oceanic lithosphere. These models explain the location and geochemical characteristics of intraplate volcanos on the African and South American plates. They also explain the presence of relatively high seismic shear wave velocities (cold downwellings) in the mantle transition zone beneath the western margin of African cratons and the eastern margin of South American cratons. Small-scale, edge-driven convection is an alternative to plumes for explaining intraplate African and South American hot spot volcanism, and small-scale convection is consistent with mantle downwellings beneath the African and South American lithosphere.

Constraints on lateral variations in upper mantle viscosity from Lake Bonneville shorelines

NASA Astrophysics Data System (ADS)

Austermann, Jacqueline; Chen, Christine; Lau, Harriet C. P.

2017-04-01

Lake Bonneville is an extinct pluvial lake that formed and catastrophically drained at the onset of the last deglaciation (˜ 20 - 18ka). With a volume of just over 10 000 km3 this lake was comparable in size to present-day Lake Michigan. During its existence the excess load of water stored in Lake Bonneville depressed the crust and upper mantle. After the drainage of the lake this area rebounded by up to 75 m, which is recorded in the paleoshorelines around the lake periphery and on islands within the lake. The rebound pattern has been used to infer the lithospheric thickness and upper mantle viscosity structure of the area (e.g. Bill et al., 1994). In agreement with the tectonic history of the Basin and Range area, the deformed shorelines point to a thin lithosphere (< 30km) and low upper mantle viscosity (˜ 1019 Pa s). This differs from the upper mantle viscosity inferred from post-glacial data in cratonic regions (e.g., Hudson Bay, Fennoscandia), which is one to two orders of magnitude larger (˜ 5 × 1020 Pa s). Direct constraints on the lateral variability of mantle viscosity are invaluable but in order to utilize such constraints it is important to consider the sensitivity range of different observations before comparing the inferred viscosities. In this study we revisit the earlier inversions of shoreline elevations for mantle and lithospheric structure with an updated dataset of paleoshoreline elevations by Chen and Maloof (2017). We construct depth-dependent sensitivity kernels for the lake rebound and compare them to kernels associated with the rebound from glacial ice sheets over Canada and Scandinavia. This comparison along with the inferred viscosities allows us to evaluate the degree to which lateral viscosity variations are required. We additionally compare our results to estimates of lateral viscosity variations based on perturbations in seismic shear wave speed in the respective areas in order to assess the consistency of our results with independent data. The paleoshorelines of Lake Bonneville have been deflected by not only rebound post-drainage, but also the longer-term subsidence of the Laurentide peripheral bulge. The lake was located on the distal flank of the peripheral bulge of the Laurentide Ice Sheet and after its collapse the peripheral bulge subsided leading to an additional northeast trending tilt in shoreline elevations. We show that the degree of tilt is not only sensitive to shallow mantle structure but has also sensitivity in the upper half of the lower mantle, in contrast to the lake rebound pattern. We independently invert the degree of tilt for mantle viscosity and examine its trade-off with uncertainties in the ice history.

A comparison of satellite systems for gravity field measurements

NASA Technical Reports Server (NTRS)

Argentiero, P. D.; Lowrey, B. E.

1977-01-01

A detailed and accurate earth gravity field model is important to the understanding of the structure and composition of the earth's crust and upper mantle. Various satellite-based techniques for providing more accurate models of the gravity field are analyzed and compared. A high-low configuration satellite-to-satellite tracking mission is recommended for the determination of both the long wavelength and short wavelength portions of the field. Satellite altimetry and satellite gradiometry missions are recommended for determination of the short wavelength portion of the field.

Electrical conductivity of partially-molten olivine aggregate and melt interconnectivity in the oceanic upper mantle

NASA Astrophysics Data System (ADS)

Laumonier, Mickael; Frost, Dan; Farla, Robert; Katsura, Tomoo; Marquardt, Katharina

2016-04-01

A consistent explanation for mantle geophysical anomalies such as the Lithosphere-Astenosphere Boundary (LAB) relies on the existence of little amount of melt trapped in the solid peridotite. Mathematical models have been used to assess the melt fraction possibly lying at mantle depths, but they have not been experimentally checked at low melt fraction (< 2 vol. %). To fill this gap, we performed in situ electrical conductivity (EC) measurement on a partially-molten olivine aggregate (Fo92-olivine from a natural peridotite of Lanzarote, Canary Islands, Spain) containing various amount of basaltic (MORB-like composition) melt (0 to 100%) at upper mantle conditions. We used the MAVO 6-ram press (BGI) combined with a Solartron gain phase analyser to acquire the electrical resistance of the sample at pressure of 1.5 GPa and temperature up to 1400°C. The results show the increase of the electrical conductivity with the temperature following an Arrhenius law, and with the melt fraction, but the effect of pressure between 1.5 and 3.0 GPa was found negligible at a melt fraction of 0.5 vol.%. The conductivity of a partially molten aggregate fits the modified Archie's law from 0.5 to 100 vol.%. At melt fractions of 0.25, 0.15 and 0.0 vol.%, the EC value deviates from the trend previously defined, suggesting that the melt is no longer fully interconnected through the sample, also supported by chemical mapping. Our results extend the previous results obtained on mixed system between 1 and 10% of melt. Since the melt appears fully interconnected down to very low melt fraction (0.5 vol.%), we conclude that (i) only 0.5 to 1 vol.% of melt is enough to explain the LAB EC anomaly, lower than previously determined; and (ii) deformation is not mandatory to enhance electrical conductivity of melt-bearing mantle rocks.

Subduction of hydrated basalt of the oceanic crust: Implications for recycling of water into the upper mantle and continental growth

NASA Technical Reports Server (NTRS)

Rapp, R. P.

1994-01-01

Subduction zones are presently the dominant sites on Earth for recycling and mass transfer between the crust and mantle; they feed hydrated basaltic oceanic crust into the upper mantle, where dehydration reactions release aqueous fluids and/or hydrous melts. The loci for fluid and/or melt generation will be determined by the intersection of dehydration reaction boundaries of primary hydrous minerals within the subducted lithosphere with slab geotherms. For metabasalt of the oceanic crust, amphibole is the dominant hydrous mineral. The dehydration melting solidus, vapor-absent melting phase relationships; and amphibole-out phase boundary for a number of natural metabasalts have been determined experimentally, and the pressure-temperature conditions of each of these appear to be dependent on bulk composition. Whether or not the dehydration of amphibole is a fluid-generating or partial melting reaction depends on a number of factors specific to a given subduction zone, such as age and thickness of the subducting oceanic lithosphere, the rate of convergence, and the maturity of the subduction zone. In general, subduction of young, hot oceanic lithosphere will result in partial melting of metabasalt of the oceanic crust within the garnet stability field; these melts are characteristically high-Al2O3 trondhjemites, tonalites and dacites. The presence of residual garnet during partial melting imparts a distinctive trace element signature (e.g., high La/Yb, high Sr/Y and Cr/Y combined with low Cr and Y contents relative to demonstrably mantle-derived arc magmas). Water in eclogitized, subducted basalt of the oceanic crust is therefore strongly partitioned into melts generated below about 3.5 GPa in 'hot' subduction zones. Although phase equilibria experiments relevant to 'cold' subduction of hydrated natural basalts are underway in a number of high-pressure laboratories, little is known with respect to the stability of more exotic hydrous minerals (e.g., ellenbergite) and the potential for oceanic crust (including metasediments) to transport water deeper into the mantle.

Lithospheric density structure beneath the Tarim basin and surroundings, northwestern China, from the joint inversion of gravity and topography

USGS Publications Warehouse

Deng, Yangfan; Levandowski, William Brower; Kusky, Tim

2017-01-01

Intraplate strain generally focuses in discrete zones, but despite the profound impact of this partitioning on global tectonics, geodynamics, and seismic hazard, the processes by which deformation becomes localized are not well understood. Such heterogeneous intraplate strain is exemplified in central Asia, where the Indo-Eurasian collision has caused widespread deformation while the Tarim block has experienced minimal Cenozoic shortening. The apparent stability of Tarim may arise either because strain is dominantly accommodated by pre-existing faults in the continental suture zones that bound it—essentially discretizing Eurasia into microplates—or because the lithospheric-scale strength (i.e., viscosity) of the Tarim block is greater than its surroundings. Here, we jointly analyze seismic velocity, gravity, topography, and temperature to develop a 3-D density model of the crust and upper mantle in this region. The Tarim crust is characterized by high density, vs, vp, and vp/vs, consistent with a dominantly mafic composition and with the presence of an oceanic plateau beneath Tarim. Low-density but high-velocity mantle lithosphere beneath southern (southwestern) Tarim underlies a suite of Permian plume-related mafic intrusions and A-type granites sourced in previously depleted mantle lithosphere; we posit that this region was further depleted, dehydrated, and strengthened by Permian plume magmatism. The actively deforming western and southern margins of Tarim—the Tien Shan, Kunlun Shan, and Altyn Tagh fault—are underlain by buoyant upper mantle with low velocity; we hypothesize that this material has been hydrated by mantle-derived fluids that have preferentially migrated along Paleozoic continental sutures. Such hydrous material should be weak, and herein strain focuses there because of lithospheric-scale variations in rheology rather than the pre-existence of faults in the brittle crust. Thus this world-class example of strain partitioning arises not simply from the pre-existence of brittle faults but from the thermo-chemical and therefore rheological variations inherited from prior tectonism.

Seismic tomography shows that upwelling beneath Iceland is confined to the upper mantle

USGS Publications Warehouse

Foulger, G.R.; Pritchard, M.J.; Julian, B.R.; Evans, J.R.; Allen, R.M.; Nolet, G.; Morgan, W.J.; Bergsson, B.H.; Erlendsson, P.; Jakobsdottir, S.; Ragnarsson, S.; Stefansson, R.; Vogfjord, K.

2001-01-01

We report the results of the highest-resolution teleseismic tomography study yet performed of the upper mantle beneath Iceland. The experiment used data gathered by the Iceland Hotspot Project, which operated a 35-station network of continuously recording, digital, broad-band seismometers over all of Iceland 1996-1998. The structure of the upper mantle was determined using the ACH damped least-squares method and involved 42 stations, 3159 P-wave, and 1338 S-wave arrival times, including the phases P, pP, sP, PP, SP, PcP, PKIKP, pPKIKP, S, sS, SS, SKS and Sdiff. Artefacts, both perceptual and parametric, were minimized by well-tested smoothing techniques involving layer thinning and offset-and-averaging. Resolution is good beneath most of Iceland from ??? 60 km depth to a maximum of ??? 450 km depth and beneath the Tjornes Fracture Zone and near-shore parts of the Reykjanes ridge. The results reveal a coherent, negative wave-speed anomaly with a diameter of 200-250 km and anomalies in P-wave speed, Vp, as strong as -2.7 per cent and in S-wave speed, Vs, as strong as -4.9 per cent. The anomaly extends from the surface to the limit of good resolution at ??? 450 km depth. In the upper ??? 250 km it is centred beneath the eastern part of the Middle Volcanic Zone, coincident with the centre of the ??? 100 mGal Bouguer gravity low over Iceland, and a lower crustal low-velocity zone identified by receiver functions. This is probably the true centre of the Iceland hotspot. In the upper ??? 200 km, the low-wave-speed body extends along the Reykjanes ridge but is sharply truncated beneath the Tjornes Fracture Zone. This suggests that material may flow unimpeded along the Reykjanes ridge from beneath Iceland but is blocked beneath the Tjornes Fracture Zone. The magnitudes of the Vp, Vs and Vp/Vs anomalies cannot be explained by elevated temperature alone, but favour a model of maximum temperature anomalies <200 K, along with up to ??? 2 per cent of partial melt in the depth range ??? 100-300 km beneath east-central Iceland. The anomalous body is approximately cylindrical in the top 250 km but tabular in shape at greater depth, elongated north-south and generally underlying the spreading plate boundary. Such a morphological change and its relationship to surface rift zones are predicted to occur in convective upwellings driven by basal heating, passive upwelling in response to plate separation and lateral temperature gradients. Although we cannot resolve structure deeper than ??? 450 km, and do not detect a bottom to the anomaly, these models suggest that it extends no deeper than the mantle transition zone. Such models thus suggest a shallow origin for the Iceland hotspot rather than a deep mantle plume, and imply that the hotspot has been located on the spreading ridge in the centre of the north Atlantic for its entire history, and is not fixed relative to other Atlantic hotspots. The results are consistent with recent, regional full-thickness mantle tomography and whole-mantle tomography images that show a strong, low-wave-speed anomaly beneath the Iceland region that is confined to the upper mantle and thus do not require a plume in the lower mantle. Seismic and geochemical observations that are interpreted as indicating a lower mantle, or core-mantle boundary origin for the North Atlantic Igneous Province and the Iceland hotspot should be re-examined to consider whether they are consistent with upper mantle processes.

The subcontinental mantle beneath southern New Zealand, characterised by helium isotopes in intraplate basalts and gas-rich springs

NASA Astrophysics Data System (ADS)

Hoke, L.; Poreda, R.; Reay, A.; Weaver, S. D.

2000-07-01

New helium isotope data measured in Cenozoic intraplate basalts and their mantle xenoliths are compared with present-day mantle helium emission on a regional scale from thermal and nonthermal gas discharges on the South Island of New Zealand and the offshore Chatham Islands. Cenozoic intraplate basaltic volcanism in southern New Zealand has ocean island basalt affinities but is restricted to continental areas and absent from adjacent Pacific oceanic crust. Its distribution is diffuse and widespread, it is of intermittent timing and characterised by low magma volumes. Most of the 3He/ 4He ratios measured in fluid inclusions in mantle xenocrysts and basalt phenocrysts such as olivine, garnet, and amphibole fall within the narrow range of 8.5 ± 1.5 Ra (Ra is the atmospheric 3He/ 4He ratio) with a maximum value of 11.5 Ra. This range is characteristic of the relatively homogeneous and degassed upper MORB-mantle helium reservoir. No helium isotope ratios typical of the lower less degassed mantle (>12 Ra), such as exemplified by the modern hot-spot region of Hawaii (with up to 32 Ra) were measured. Helium isotope ratios of less than 8 Ra are interpreted in terms of dilution of upper mantle helium with a radiogenic component, due to either age of crystallisation or small-scale mantle heterogeneities caused by mixing of crustal material into the upper mantle. The crude correlation between age of samples and helium isotopes with generally lower R/Ra values in mantle xenoliths compared with host rock phenocrysts and the in general depleted Nd and Sr isotope ratios and the light rare earth element enrichment of the basalts supports derivation of melts as small melt fractions from a depleted upper mantle, with posteruptive ingrowth of radiogenic helium as a function of lithospheric age. In comparison, the regional helium isotope survey of thermal and nonthermal gas discharges of the South Island of New Zealand shows that mantle 3He anomalies in general do not show an obvious relationship with either age or proximity to the Cenozoic intraplate volcanic centres or with major faults. In general, areas characterised by mantle 3He emission are interpreted to define those regions beneath which mantle melting and basalt magma addition to the crust are recent. The strongest mantle 3He anomaly (equivalent to >80% mantle helium component) is centred over southern Dunedin, measured in magmatic CO 2-rich mineral water springs issuing from crystalline basement rocks which outcrop at the southern extent of Miocene intraplate basaltic volcanism which ceased 9 Ma ago. This mantle helium anomaly overlaps with an area characterised by elevated surface high heat flow, compatible with a long-lived mantle melt/heat input into the crust. In comparison Banks Peninsula, another Miocene intraplate basaltic centre, is characterised by relatively low surface heat flow and a small mantle helium contribution measured in a nitrogen-rich spring. Here the thermal transient induced by the magmatic event has either dissipated or has not reached the surface. In the former case one might be dealing with storage and mixing of magmatic and crustal gases at shallow crustal levels and in the latter with active to recent mantle-melt degassing at depth. Along the most actively deforming part of the plate boundary zone, the transpressional Alpine Fault and Marlborough fault systems, mantle helium is present in gas-rich springs in all those areas underlain by actively subducting oceanic crust (the Australian plate in the south and Pacific plate in the north), whereas the central part of the Alpine transpressional fault is characterised by pure crustal radiogenic helium. Areas where the mantle helium component is negligible are restricted to the centre part of the South Island, extending along its length from Southland to northern Canterbury and Murchison. These areas are interpreted to delineate the extent of thicker and colder lithosphere compared to all other areas where mantle helium release from partial mantle melts at depth is recent to active being added to the lower lithosphere and/or lower crust. Areas characterised by mantle helium anomalies are equated with areas of thermal mantle anomalies, i.e., localised mantle heterogeneities such as upwelling less dense silicate melts in the upper asthenospheric mantle.

Mantle potential temperature estimates and primary melt compositions of the Low-Ti Emeishan flood basalt

NASA Astrophysics Data System (ADS)

Shellnutt, J. G.; Pham, Thuy T.

2018-05-01

The Late Permian Emeishan large igneous province (ELIP) is considered to be one of the best examples of a mantle plume derived large igneous province. One of the primary observations that favour a mantle plume regime is the presence of ultramafic volcanic rocks. The picrites suggest primary mantle melts erupted and that mantle potential temperatures (TP) of the ELIP were > 200oC above ambient mantle conditions. However, they may represent a mixture of liquid and cumulus olivine and pyroxene rather than primary liquids. Consequently, temperature estimates based on the picrite compositions may not be accurate. Here we calculate mantle potential temperature (TP) estimates and primary liquids compositions using PRIMELT3 for the low-Ti (Ti/Y < 500) Emeishan basalt as they represent definite liquid compositions. The calculated TP yield a range from 1400oC to 1550oC, which is consistent with variability across a mantle plume axis. The primary melt compositions of the basalts are mostly picritic. The results of this study indicate that the Emeishan basalt was produced by a high temperature regime and that a few of the ultramafic volcanic rocks may be indicative of primary liquids.

Osmium isotopes and mantle convection.

PubMed

Hauri, Erik H

2002-11-15

The decay of (187)Re to (187)Os (with a half-life of 42 billion years) provides a unique isotopic fingerprint for tracing the evolution of crustal materials and mantle residues in the convecting mantle. Ancient subcontinental mantle lithosphere has uniquely low Re/Os and (187)Os/(188)Os ratios due to large-degree melt extraction, recording ancient melt-depletion events as old as 3.2 billion years. Partial melts have Re/Os ratios that are orders of magnitude higher than their sources, and the subduction of oceanic or continental crust introduces into the mantle materials that rapidly accumulate radiogenic (187)Os. Eclogites from the subcontinental lithosphere have extremely high (187)Os/(188)Os ratios, and record ages as old as the oldest peridotites. The data show a near-perfect partitioning of Re/Os and (187)Os/(188)Os ratios between peridotites (low) and eclogites (high). The convecting mantle retains a degree of Os-isotopic heterogeneity similar to the lithospheric mantle, although its amplitude is modulated by convective mixing. Abyssal peridotites from the ocean ridges have low Os isotope ratios, indicating that the upper mantle had undergone episodes of melt depletion prior to the most recent melting events to produce mid-ocean-ridge basalt. The amount of rhenium estimated to be depleted from the upper mantle is 10 times greater than the rhenium budget of the continental crust, requiring a separate reservoir to close the mass balance. A reservoir consisting of 5-10% of the mantle with a rhenium concentration similar to mid-ocean-ridge basalt would balance the rhenium depletion of the upper mantle. This reservoir most likely consists of mafic oceanic crust recycled into the mantle over Earth's history and provides the material that melts at oceanic hotspots to produce ocean-island basalts (OIBs). The ubiquity of high Os isotope ratios in OIB, coupled with other geochemical tracers, indicates that the mantle sources of hotspots contain significant quantities (greater than 10%) of lithologically distinct mafic material which represents ancient oceanic lithosphere cycled through the convecting mantle on a time-scale of 800 million years or more.

Origin and mixing timescale of Earth's late veneer

NASA Astrophysics Data System (ADS)

Prescher, C.; Allu Peddinti, D.; Bell, E. A.; Bello, L.; Cernok, A.; Ghosh, N.; Tucker, J.; Wielicki, M. M.; Zahnle, K. J.

2012-12-01

Experimental studies on the partitioning behavior of highly siderophile elements (HSE) between silicate and metallic melts imply that the Earth's mantle should have been highly depleted in these elements by core formation in an early magma ocean. However, present HSE contents of the Earth's mantle are ~3 orders of magnitude higher than that expected by experiments. The apparent over-abundance of HSE has commonly been explained by the addition of meteoritic material in the "late veneer" which describes the exogenous mass addition following the moon forming impact and concluding with the late heavy bombardment at ~3.8-3.9 Ga. The strongest evidence for this theory is that the platinum group element (PGE) contents in today's mantle are present in chondritic relative abundances, as opposed to a fractionated pattern expected with metal-silicate partitioning. Archean komatiites indicate that the PGE content of the Earth's mantle increased from about half their present abundances at 3.5 Ga to their present abundances at 2.9 Ga. This secular increase in PGE content suggests a progressive mixing of the late veneer material into the Earth's mantle. However, this time scale also implies that the whole mantle was relatively well mixed by 2.9 Ga. We use a compilation of existing isotopic and trace element data in order to constrain the origin and composition of the late veneer. We use PGE abundances, W abundances and W isotopic compositions in chondritic meteorites and the primitive upper mantle to compute the amount of mass delivered during the late veneer and find the late veneer mass to be ~0.6 % the mass of the bulk silicate Earth (consistent with earlier estimates). We also use the 187Re-187Os and 190Pt-186Os systems to constrain the composition and timing of delivery of the impacting population. We model the efficiency of mantle mixing in this time frame by using 3-dimensional numerical geodynamical simulations and geochemical constraints. Initial parameters include the amount of mass delivered in the late veneer and the Archean internal heating which is at least 4 times higher than the present values, due to the higher abundance of radioactive elements. Another important parameter is the mechanism of mass addition to the Earth. We test three end-member scenarios: (1) a single very large impactor accounting for the entire mass addition, (2) sprinkling of a large number of small impactors over the whole Earth which then mix into the mantle, or (3) by using a size/frequency distribution estimated from the lunar cratering record and corrected for the difference in gravitational cross section of the Earth and the Moon. This project results from collaborations begun at the CIDER II workshop held at KITP, UCSB, 2012.

Rheological structure of the lithosphere in plate boundary strike-slip fault zones

NASA Astrophysics Data System (ADS)

Chatzaras, Vasileios; Tikoff, Basil; Kruckenberg, Seth C.; Newman, Julie; Titus, Sarah J.; Withers, Anthony C.; Drury, Martyn R.

2016-04-01

How well constrained is the rheological structure of the lithosphere in plate boundary strike-slip fault systems? Further, how do lithospheric layers, with rheologically distinct behaviors, interact within the strike-slip fault zones? To address these questions, we present rheological observations from the mantle sections of two lithospheric-scale, strike-slip fault zones. Xenoliths from ˜40 km depth (970-1100 ° C) beneath the San Andreas fault system (SAF) provide critical constraints on the mechanical stratification of the lithosphere in this continental transform fault. Samples from the Bogota Peninsula shear zone (BPSZ, New Caledonia), which is an exhumed oceanic transform fault, provide insights on lateral variations in mantle strength and viscosity across the fault zone at a depth corresponding to deformation temperatures of ˜900 ° C. Olivine recrystallized grain size piezometry suggests that the shear stress in the SAF upper mantle is 5-9 MPa and in the BPSZ is 4-10 MPa. Thus, the mantle strength in both fault zones is comparable to the crustal strength (˜10 MPa) of seismogenic strike-slip faults in the SAF system. Across the BPSZ, shear stress increases from 4 MPa in the surrounding rocks to 10 MPa in the mylonites, which comprise the core of the shear zone. Further, the BPSZ is characterized by at least one order of magnitude difference in the viscosity between the mylonites (1018 Paṡs) and the surrounding rocks (1019 Paṡs). Mantle viscosity in both the BPSZ mylonites and the SAF (7.0ṡ1018-3.1ṡ1020 Paṡs) is relatively low. To explain our observations from these two strike-slip fault zones, we propose the "lithospheric feedback" model in which the upper crust and lithospheric mantle act together as an integrated system. Mantle flow controls displacement and the upper crust controls the stress magnitude in the system. Our stress data combined with data that are now available for the middle and lower crustal sections of other transcurrent fault systems support the prediction for constant shear strength (˜10 MPa) throughout the lithosphere; the stress magnitude is controlled by the shear strength of the upper crustal faults. Fault rupture in the upper crust induces displacement rate loading of the upper mantle, which in turn, causes strain localization in the mantle shear zone beneath the strike-slip fault. Such forced localization leads to higher stresses and strain rates in the shear zone compared to the surrounding rocks. Low mantle viscosity within the shear zone is critical for facilitating mantle flow, which induces widespread crustal deformation and displacement loading. The lithospheric feedback model suggests that strike-slip fault zones are not mechanically stratified in terms of shear stress, and that it is the time-dependent interaction of the different lithospheric layers - rather than their relative strengths - that governs the rheological behavior of the plate boundary, strike-slip fault zones.

Oxygen fugacity profile of the oceanic upper mantle and the depth of redox melting beneath ridges

NASA Astrophysics Data System (ADS)

Davis, F. A.; Cottrell, E.

2014-12-01

Oxygen fugacity (fO2) of a mantle mineral assemblage, controlled primarily by Fe redox chemistry, sets the depth of the diamond to carbonated melt reaction (DCO3). Near-surface fO2 recorded by primitive MORB glasses and abyssal peridotites anchor the fO2 profile of the mantle at depth. If the fO2-depth relationship of the mantle is known, then the depth of the DCO3 can be predicted. Alternatively, if the DCO3 can be detected geophysically, then its depth can be used to infer physical and chemical characteristics of upwelling mantle. We present an expanded version of a model of the fO2-depth profile of adiabatically upwelling mantle first presented by Stagno et al. (2013), kindly provided by D. Frost. The model uses a chemical mass balance and empirical fits to experimental data to calculate compositions and modes of mantle minerals at specified P, T, and bulk Fe3+/ƩFe. We added P and T dependences to the partitioning of Al and Ca to better simulate the mineralogical changes in peridotite at depth and included majorite component in garnet to increase the depth range of the model. We calculate fO2 from the mineral assemblages using the grt-ol-opx oxybarometer (Stagno et al., 2013). The onset of carbonated melting occurs at the intersection of a Fe3+/ƩFe isopleth with the DCO3. Upwelling mantle is tied to the DCO3 until all native C is oxidized to form carbonated melts by reduction of Fe3+ to Fe2+. The depth of intersection of a parcel of mantle with the DCO3 is a function of bulk Fe3+/ƩFe, potential temperature, and bulk composition. We predict that fertile mantle (PUM) along a 1400 °C adiabat, with 50 ppm bulk C, and Fe3+/ƩFe = 0.05 after C oxidation begins redox melting at a depth of 250 km. The model contextualizes observations of MORB redox chemistry. Because fertile peridotite is richer in Al2O3, the Fe2O3-bearing components of garnet are diluted leading to lower fO2 at a given depth compared to refractory mantle under the same conditions. This may indicate that the negativecorrelation observed between enrichment and fO2 at ridges (Cottrell and Kelley, 2013) is a consequence of the increased fertility of remixing recycled crust into the mantle. Addition of reduced C to the mantle during subduction can also explain this observation. Geophysical detection of the depth of the DCO3 may resolve these hypotheses.

Teaching machines to find mantle composition

NASA Astrophysics Data System (ADS)

Atkins, Suzanne; Tackley, Paul; Trampert, Jeannot; Valentine, Andrew

2017-04-01

The composition of the mantle affects many geodynamical processes by altering factors such as the density, the location of phase changes, and melting temperature. The inferences we make about mantle composition also determine how we interpret the changes in velocity, reflections, attenuation and scattering seen by seismologists. However, the bulk composition of the mantle is very poorly constrained. Inferences are made from meteorite samples, rock samples from the Earth and inferences made from geophysical data. All of these approaches require significant assumptions and the inferences made are subject to large uncertainties. Here we present a new method for inferring mantle composition, based on pattern recognition machine learning, which uses large scale in situ observations of the mantle to make fully probabilistic inferences of composition for convection simulations. Our method has an advantage over other petrological approaches because we use large scale geophysical observations. This means that we average over much greater length scales and do not need to rely on extrapolating from localised samples of the mantle or planetary disk. Another major advantage of our method is that it is fully probabilistic. This allows us to include all of the uncertainties inherent in the inference process, giving us far more information about the reliability of the result than other methods. Finally our method includes the impact of composition on mantle convection. This allows us to make much more precise inferences from geophysical data than other geophysical approaches, which attempt to invert one observation with no consideration of the relationship between convection and composition. We use a sampling based inversion method, using hundreds of convection simulations run using StagYY with self consistent mineral physics properties calculated using the PerpleX package. The observations from these simulations are used to train a neural network to make a probabilistic inference for major element oxide composition of the mantle. We find we can constrain bulk mantle FeO molar percent, FeO/MgO and FeO/SiO2 using observations of the temperature and density structure of the mantle in convection simulations.

Mantle Flow and Dehydration Beneath the Juan de Fuca Plate Revealed by Shear Velocity and Attenuation

NASA Astrophysics Data System (ADS)

Ruan, Y.; Forsyth, D. W.; Bell, S. W.

2017-12-01

At mid-ocean-ridge spreading centers, it is still unclear to what extent the upwelling is purely passive, driven by viscous drag of the separating plates, or dynamically driven by the buoyancy induced by melt retention and depletion of the mantle matrix. The distinct sensitivities of seismic wavespeed and attenuation to temperature, melt porosity, water content and major element composition yield some of the primary constraints on mid-ocean ridge processes and the associated flow pattern, melt distribution, and the interaction of spreading centers with hotspots. Extensive arrays of ocean-bottom seismometers (OBS) with better quality, longer deployment periods, and the application of noise-removal techniques together provided higher quality data in this study than in any previous regional study of velocity and attenuation of the upper mantle beneath a spreading center. Based on the fundamental-mode Rayleigh waves, we imaged shear wave attenuation and velocity models in the vicinity of the Juan de Fuca plate with the best resolution to date of any spreading center. There is strong attenuation centered at depths of 70-80 km, just below the expected dry solidus and somewhat deeper than predicted for a model of passive mantle upwelling beneath the spreading center. The shear velocity structure shows lowest velocities west of the spreading center, particularly near Axial Seamount and high velocities east of the axis extending to a greater depth than predicted by the passive flow model. Together, these observations support a model in which buoyant upwelling west of the spreading center first depletes and dehydrates the mantle above the dry solidus by melt removal and then the associated downwelling carries depleted, melt-free, residual mantle downward beneath the Juan de Fuca plate. This depleted, dehydrated, melt-free layer can explain why the average attenuation is lower than expected and the velocity is higher than expected in the 30 to 70 km depth range. The compositional buoyancy of the depleted mantle may in most places limit downwelling to the vicinity of the spinel peridotite to garnet peridotite transition at a depth of 80 km.

Fractional ultrabasic-basic evolution of upper-mantle magmatism: Evidence from xenoliths in kimberlites, inclusions in diamonds and experiments

NASA Astrophysics Data System (ADS)

Litvin, Yuriy; Kuzyura, Anastasia

2017-04-01

Ultrabasic peridotites and pyroxenites together with basic eclogites are the upper-mantle in situ rocks among xenoliths in kimberlites. Occasionally their diamond-bearing varieties have revealed within the xenoliths. Therewith the compositions of rock-forming minerals demonstrate features characteristic for primary diamond-included minerals of peridotite and eclogite parageneses (the elevated contents of Cr-component in peridotitic garnets and Na-jadeitic component in eclogitic clinopyroxenes). High-pressure experimental study of melting equilibria on the multicomponent peridotie-pyroxenite system olivine Ol - orthopyroxene Opx - clinopyroxene Cpx - garnet Grt showed that Opx disappeared in the peritectic reaction Opx+L→Cpx (Litvin, 1991). As a result, the invariant peritectic equilibrium Ol+Opx+Cpx+Grt+L of the ultrabasic system was found to transform into the univariant cotectic assemblage Ol+Cpx+Grt+L. Further experimental investigation showed that olivine reacts with jadeitic component (Jd) with formation of garnet at higher 4.5 GPa (Gasparik, Litvin, 1997). Study of melting relations in the multicomponent system Ol - Cpx - Jd permits to discover the peritectic point Ol+Omph+Grt+L (where Omph - omphacitic clinopyroxene) at concentration 3-4 wt.% Jd-component in the system. The reactionary loss of Opx and Ol makes it possible to transform the 4-phase garnet lherzolite ultrabasic association into the bimineral eclogite assemblage. The regime of fractional Ol, Cpx and Grt crystallization must be accompanied by increasing content of jadeitic component in residual melts that causes the complete "garnetization of olivine". In the subsequent evolution, the melts would have to fractionate for basic SiO2-saturated compositions responsible for petrogenesis of eclogite varieties marked with accessory corundum Crn, kyanite Ky and coesite Coe. Both the peritectic mechanisms occur in regime of fractional crystallization. The sequence of the upper-mantle fractional ultrabasic-basic magmatic evolution and petrogenesis may be controlled by the following melting relations: from Ol, Opx, L field to cotectic curve Ol, Opx, Cpx, L, peritectic point Ol, Opx, Cpx, Grt, L (loss of Opx), cotectic curve Ol, (Cpx+Jd), Grt, L, peritectic point Ol, (Cpx→Omph), Grt, L (loss of Ol), divariant field Omph,Grt,L, cotectic curve Ky, Omph, Grt, L, eutectic point Ky,Coe,Omph, Grt,L, subsolidus assemblage Ky,Coe,Omph, Grt. The fractional ultrabasic-basic evolution of the upper-mantle silicate-carbonate-carbon melts-solutions, which are responsible for genesis of diamond-and-inclusions associations and diamond-bearing peridotites and eclogites, follows the similar physico-chemical mechanisms (Litvin et al., 2016). This is illustrated by fractional syngenesis diagram for diamonds and associated minerals which construction is based on evidence from high pressure experiments. References Gasparik T., Litvin Yu.A (1997). Stability of Na2Mg2Si2O7 and melting relations on the forsterite - jadeite join at pressures up to 22 GPa. Eur, J. Mineral. 9(2), 311-326. Litvin Yu.A. (1991). Physico-Chemical Study of Melting of Materials from the Deep Earth. Moscow: Nauka. 312 p. Litvin Yu.A., Spivak A.V., Kuzyura A.V. (2016). Fundamentals of the mantle-carbonatite concept of diamond genesis, Geochemistry Internat. 34(10), 839-857.

The ruthenium isotopic composition of the oceanic mantle

NASA Astrophysics Data System (ADS)

Bermingham, K. R.; Walker, R. J.

2017-09-01

The approximately chondritic relative, and comparatively high absolute mantle abundances of the highly siderophile elements (HSE), suggest that their concentrations in the bulk silicate Earth were primarily established during a final ∼0.5 to 1% of ;late accretion; to the mantle, following the cessation of core segregation. Consequently, the isotopic composition of the HSE Ru in the mantle reflects an amalgamation of the isotopic compositions of late accretionary contributions to the silicate portion of the Earth. Among cosmochemical materials, Ru is characterized by considerable mass-independent isotopic variability, making it a powerful genetic tracer of Earth's late accretionary building blocks. To define the Ru isotopic composition of the oceanic mantle, the largest portion of the accessible mantle, we report Ru isotopic data for materials from one Archean and seven Phanerozoic oceanic mantle domains. A sample from a continental lithospheric mantle domain is also examined. All samples have identical Ru isotopic compositions, within analytical uncertainties, indicating that Ru isotopes are well mixed in the oceanic mantle, defining a μ100Ru value of 1.2 ± 7.2 (2SD). The only known meteorites with the same Ru isotopic composition are enstatite chondrites and, when corrected for the effects of cosmic ray exposure, members of the Main Group and sLL subgroup of the IAB iron meteorite complex which have a collective CRE corrected μ100Ru value of 0.9 ± 3.0. This suggests that materials from the region(s) of the solar nebula sampled by these meteorites likely contributed the dominant portion of late accreted materials to Earth's mantle.

P-wave Velocity Structure Across the Mariana Trench and Implications for Hydration

NASA Astrophysics Data System (ADS)

Eimer, M. O.; Wiens, D.; Lizarralde, D.; Cai, C.

2017-12-01

Estimates of the water flux at subduction zones remain uncertain, particularly the amount of water brought into the trench by the subducting plate. Normal faulting related to the bending of the incoming plate has been proposed to provide pathways for water to hydrate the crust and upper mantle. A passive and active source seismic experiment spanning both the incoming plate and forearc was conducted in 2012 in central Mariana to examine the role of hydration at subduction zones. The active-source component of the survey used the R/V M.G. Langsethairgun array and 68 short period sensors, including suspended hydrophones, deployed on 4 transects. This study at the Mariana trench offers a comparison to related studies of incoming plate hydration in Middle America, where differing thermal structures related to plate age predict different stability fields for hydrous minerals. The forearc structure is also of interest, since Mariana is characterized by large serpentine seamounts and may have a serpentinized mantle wedge. The velocity structure will also be important for the relocation of earthquakes in the incoming plate, since the seismicity can offer a constraint for the depth extent of these bending faults. We examine the P-wave velocity structure along a 400-km long wide-angle refraction transect perpendicular to the trench and spanning both the forearc and incoming plate. Preliminary results indicate a velocity reduction in the crust and uppermost mantle at the bending region of the incoming plate, relative to the plate's structure away from the trench. This reduction suggests that outer-rise faults extend into the upper mantle and may have promoted serpentinization of that material. Mantle Pn refraction phases are not observed in the forearc, consistent with the ambient noise tomography results that show upper-mantle velocities similar to that of the lower crust. The lack of contrast between the upper mantle and crustal velocities from the ambient noise has been interpreted to indicate extensive serpentinization of the shallow mantle wedge.

New upper mantle model for North America: no longer a pyrolite composition?

NASA Astrophysics Data System (ADS)

Perchuc, E.; Malinowski, M.

2009-04-01

We compare the traveltimes data for P and S waves from the long range seismic profiles and from the earthquakes recorded to the offset of 3000 km with theoretical traveltimes predicted by standard seismological models: PREM, IASP- 91, AK-135 and especially by seismo-petrological model PREF (Cammarano and Romanowicz - 2007). For our analysis we are used data from north American array also. Our analysis suggests that for several events in the distance range 2000-3000 km, the first-arrivals are characterized by a relatively high velocity of 8.7-8.9 km/s. It is about 2.5% higher than P-wave velocity of the Lehmann phases, observed in the nearest offset and about 3% smaller than velocity below 410 km discontinuity. S waves model suggested significant differences in Vp/Vs ratio. We suggest that this is a new first-order seismological boundary which can be interpreted as a top of the mantle transition zone. Seismological arguments for the existence of such a boundary are as follows: refracted waves with velocity 8.7-8.9 km/s and reflected waves find by Warren at al. (1967) and by Thybo and Perchuc (1997b). Several new publications suggested existence of a low velocity zone above the 410-km discontinuity. We also see this feature in our studies. Important suggestion is existence of 300 km discontinuity below cold areas and it is also difficult to exclude this boundary below "cold" areas however phases from this boundary are in secondary impulses. Depth of this boundary strongly depends on the thermal state of the mantle in particular regions. In conclusion we can say that the mantle transition zone starts much earlier and the lower part of the upper mantle is much faster than predicted by purely pyrolitic mantle model. Several petrological studies suggest influences of fluids (especially H2O) on the character of the 410 km discontinuity and of the transition zone. All the differences in experimental data can be explained by the effect of temperature on the phase transformations within the olivine-wadsleyite system.

The thermal regimes of the upper mantle beneath Precambrian and Phanerozoic structures up to the thermobarometry data of mantle xenoliths

NASA Astrophysics Data System (ADS)

Glebovitsky, V. A.; Nikitina, L. P.; Khiltova, V. Ya.; Ovchinnikov, N. O.

2004-05-01

The thermal state of the upper mantle beneath tectonic structures of various ages and types (Archaean cratons, Early Proterozoic accretionary and collisional orogens, and Phanerozoic structures) is characterized by geotherms and by thermal gradients (TG) derived from data on the P- T conditions of mineral equilibria in garnet and garnet-spinel peridotite xenoliths from kimberlites (East Siberia, Northeastern Europe, India, Central Africa, North America, and Canada) and alkali basalts (Southeastern Siberia, Mongolia, southeastern China, southeastern Australia, Central Africa, South America, and the Solomon and Hawaiian islands). The use of the same garnet-orthopyroxene thermobarometer (Theophrastus Contributions to Advanced Studies in Geology. 3: Capricious Earth: Models and Modelling of Geologic Processes and Objects 2000 44) for all xenoliths allowed us to avoid discrepancies in estimation of the P- T conditions, which may be a result of the mismatch between different thermometers and barometers, and to compare the thermal regimes in the mantle in various regions. Thus, it was established that (1) mantle geotherms and geothermal gradients, obtained from the estimation of P- T equilibrium conditions of deep xenoliths, correspond to the age of crust tectonic structures and respectively to the time of lithosphere stabilization; it can be suggested that the ancient structures of the upper mantle were preserved within continental roots; (2) thermal regimes under continental mantle between the Archaean cratons and Palaeoproterozoic belts are different today; (3) the continental mantle under Neoproterozoic and Phanerozoic belts is characterized by significantly higher values of geothermal gradient compared to the mantle under Early Precambrian structures; (4) lithosphere dynamics seems to change at the boundary between Early and Mezo-Neoproterozoic and Precambrian and Phanerozoic.

Chemical layering in the upper mantle of Mars: Evidence from olivine-hosted melt inclusions in Tissint

NASA Astrophysics Data System (ADS)

Basu Sarbadhikari, A.; Babu, E. V. S. S. K.; Vijaya Kumar, T.

2017-02-01

Melting of Martian mantle, formation, and evolution of primary magma from the depleted mantle were previously modeled from experimental petrology and geochemical studies of Martian meteorites. Based on in situ major and trace element study of a range of olivine-hosted melt inclusions in various stages of crystallization of Tissint, a depleted olivine-phyric shergottite, we further constrain different stages of depletion and enrichment in the depleted mantle source of the shergottite suite. Two types of melt inclusions were petrographically recognized. Type I melt inclusions occur in the megacrystic olivine core (Fo76-70), while type II melt inclusions are hosted by the outer mantle of the olivine (Fo66-55). REE-plot indicates type I melt inclusions, which are unique because they represent the most depleted trace element data from the parent magmas of all the depleted shergottites, are an order of magnitude depleted compared to the type II melt inclusions. The absolute REE content of type II displays parallel trend but somewhat lower value than the Tissint whole-rock. Model calculations indicate two-stage mantle melting events followed by enrichment through mixing with a hypothetical residual melt from solidifying magma ocean. This resulted in 10 times enrichment of incompatible trace elements from parent magma stage to the remaining melt after 45% crystallization, simulating the whole-rock of Tissint. We rule out any assimilation due to crustal recycling into the upper mantle, as proposed by a recent study. Rather, we propose the presence of Al, Ca, Na, P, and REE-rich layer at the shallower upper mantle above the depleted mantle source region during the geologic evolution of Mars.

Crust and Mantle Deformation Revealed from High-Resolution Radially Anisotropic Velocity Models

NASA Astrophysics Data System (ADS)

Li, A.; Dave, R.; Yao, Y.

2017-12-01

Love wave tomography, which can achieve a similar model resolution as Rayleigh wave, so far has limited applications to the USArray data. Recently, we have developed high-resolution Love wave phase velocity maps in the Wyoming craton and Texas using data at the Transportable Array stations. 3-D, radially anisotropic velocity models are obtained by jointly inverting Love and Rayleigh wave phase velocities. A high-velocity anomaly extending to about 200 km depth beneath central Wyoming correlates with negative radial anisotropy (Vsv>Vsh), suggesting that mantle downwelling develops under the cratonic lithosphere. Surprisingly, the significantly low velocity beneath the Yellowstone hotspot, which has been interpreted as partial melting and asthenospheric upwelling, is associated with the largest radial anisotropy (Vsh>Vsv) in the area. This observation does not support mantle upwelling. Instead, it indicates that the upper mantle beneath the hotspot has experienced strong shear deformation probably by the plate motion and large-scale mantle flow. In Texas, positive radial anisotropy in the lower crust extends from the coast to the Ouachita belt, which is characterized by high velocity and negative radial anisotropy. In the upper mantle, large variations of velocity and anisotropy exit under the coastal plain. A common feature in these anisotropic models is that high-velocity anomalies in the upper mantle often correlate with negative anisotropy (Vsv>Vsh) while low-velocity anomalies are associated with positive anisotropy (Vsh>Vsv). The manifestation of mantle downweling as negative radial anisotropy is largely due to the relatively high viscosity of the high-velocity mantle block, which is less affected by the surrounding large-scale horizontal flow. However, mantle upwelling, which is often associated with low-velocity anomalies, presumably low-viscosity mantle blocks, is invisible in radial anisotropy models. Such upwelling may happen too quickly to make last effects or too slow to alter the dominant shear deformation in the asthenosphere.

Experimental constraints on the fate of subducted upper continental crust beyond the "depth of no return"

NASA Astrophysics Data System (ADS)

Zhang, Yanfei; Wu, Yao; Wang, Chao; Zhu, Lüyun; Jin, Zhenmin

2016-08-01

The subducted continental crust material will be gravitationally trapped in the deep mantle after having been transported to depths of greater than ∼250-300 km (the "depth of no return"). However, little is known about the status of this trapped continental material as well as its contribution to the mantle heterogeneity after achieving thermal equilibrium with the surrounding mantle. Here, we conduct an experimental study over pressure and temperature ranges of 9-16 GPa and 1300-1800 °C to constrain the fate of these trapped upper continental crust (UCC). The experimental results show that partial melting will occur in the subducted UCC along normal mantle geotherm to produce K-rich melt. The residual phases composed of coesite/stishovite + clinopyroxene + kyanite in the upper mantle, and stishovite + clinopyroxene + K-hollandite + garnet + CAS-phase in the mantle transition zone (MTZ), respectively. The residual phases achieve densities greater than the surrounding mantle, which provides a driving force for descent across the 410-km seismic discontinuity into the MTZ. However, this density relationship is reversed at the base of the MTZ, leaving the descended residues to be accumulated above the 660-km seismic discontinuity and may contribute to the "second continent". The melt is ∼0.6-0.7 g/cm3 less dense than the surrounding mantle, which provides a buoyancy force for ascent of melt to shallow depths. The ascending melt, which preserves a significant portion of the bulk-rock rare earth elements (REEs), large ion lithophile elements (LILEs), and high-filed strength elements (HFSEs), may react with the surrounding mantle. Re-melting of the metasomatized mantle may contribute to the origin of the "enriched mantle sources" (EM-sources). Therefore, the deep subducted continental crust may create geochemical/geophysical heterogeneity in Earth's interior through subduction, stagnation, partial melting and melt segregation.

Tottori earthquakes and Daisen volcano: Effects of fluids, slab melting and hot mantle upwelling

NASA Astrophysics Data System (ADS)

Zhao, Dapeng; Liu, Xin; Hua, Yuanyuan

2018-03-01

We investigate the 3-D seismic structure of source areas of the 6 October 2000 Western Tottori earthquake (M 7.3) and the 21 October 2016 Central Tottori earthquake (M 6.6) which occurred near the Daisen volcano in SW Japan. The two large events took place in a high-velocity zone in the upper crust, whereas low-velocity (low-V) and high Poisson's ratio (high-σ) anomalies are revealed in the lower crust and upper mantle. Low-frequency micro-earthquakes (M 0.0-2.1) occur in or around the low-V and high-σ zones, which reflect upward migration of magmatic fluids from the upper mantle to the crust under the Daisen volcano. The nucleation of the Tottori earthquakes may be affected by the ascending fluids. The flat subducting Philippine Sea (PHS) slab has a younger lithosphere age and so a higher temperature beneath the Daisen and Tottori area, facilitating the PHS slab melting. It is also possible that a PHS slab window has formed along the extinct Shikoku Basin spreading ridge beneath SW Japan, and mantle materials below the PHS slab may ascend to the shallow area through the slab window. These results suggest that the Daisen adakite magma was affected by the PHS slab melting and upwelling flow in the upper mantle above the subducting Pacific slab.

The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China

NASA Astrophysics Data System (ADS)

Yutao, Shi; Yuan, Gao; Lingxue, Tai; Yuanyuan, Fu

2015-11-01

In order to infer the distribution of local stress and the deep geodynamic process in North China, this study detects seismic anisotropy in the crust and upper mantle beneath the Bohai Sea area. A total of 535 local shear-wave and 721 XKS (including SKS, PKS and SKKS phases) splitting measurements were obtained from stations in permanent regional seismograph networks and a temporary seismic network called ZBnet-E. The dominant fast polarization orientation of local shear-waves in the crust is nearly East-West, suggesting an East-West direction of local maximum compressive stress in the area. Nearly North-South fast orientation was obtained at some stations in the Tan-Lu fault belt and the Zhang-Bo seismic belt. The average fast orientation from XKS splitting analysis is 87.4° measured clockwise from the North. The average time-delays of XKS splitting are range from 0.54 s to 1.92 s, corresponding to a 60-210 km thick layer of anisotropy. The measured results indicate that upper mantle anisotropy beneath Bohai Sea area, even the eastern part of North China, is mainly from asthenospheric mantle flow from the subduction of the Pacific plate. From the complicated anisotropic characteristics in this study, we infer that there might be multiple mechanisms in the crust and upper mantle around the Bohai Sea area that led to the observed anisotropy.

Paleogeothermal record of the Emeishan mantle plume: evidences from borehole Ro data in the Sichuan basin, SW China

NASA Astrophysics Data System (ADS)

Hu, S.

2013-12-01

The Emeishan basalt province located in the southwest of China is widely accepted to be a result of the eruption of a mantle plume at the time of middle-late Permian. If it was a mantle plume, the ambient sedimentary rocks must be heated up during the development of the mantle plume and this thermal effect must be recorded by some geothermometers in the country rocks. The vitrinite reflectance (Ro) data as a maximum paleotemperature recorder from boreholes in Sichuan basin was employed to expose the thermal regime related to the proposed Emeishan mantle plume. The Ro profiles from boreholes which drilled close to the Emeishan basalts shows a ';dog-leg' (break) style at the unconformity between the middle and the upper Permian, and the Ro profiles in the lower subsection (pre-middle Permian) shows a significantly higher slopes (gradients) than those in the upper subsection. In contrast, those Ro profiles from boreholes far away from the center of the basalt province have no break at the uncomformity. Based on the chemical kinetic model of Ro, the paleo-temperature gradients for the upper and the lower subsections in different boreholes, as well as the erosion at the unconformity between the middle and the upper Permian, were reconstructed to reveal the variations of the temperature gradients and erosion thickness with geological time and space. Both the thermal regime and the erosion thickness together with their spatial variation (structure) provide strong geothermal evidence for the existence of the Emeishan mantle plume in the middle-late Permian.

Upper-mantle water stratification inferred from observations of the 2012 Indian Ocean earthquake.

PubMed

Masuti, Sagar; Barbot, Sylvain D; Karato, Shun-Ichiro; Feng, Lujia; Banerjee, Paramesh

2016-10-20

Water, the most abundant volatile in Earth's interior, preserves the young surface of our planet by catalysing mantle convection, lubricating plate tectonics and feeding arc volcanism. Since planetary accretion, water has been exchanged between the hydrosphere and the geosphere, but its depth distribution in the mantle remains elusive. Water drastically reduces the strength of olivine and this effect can be exploited to estimate the water content of olivine from the mechanical response of the asthenosphere to stress perturbations such as the ones following large earthquakes. Here, we exploit the sensitivity to water of the strength of olivine, the weakest and most abundant mineral in the upper mantle, and observations of the exceptionally large (moment magnitude 8.6) 2012 Indian Ocean earthquake to constrain the stratification of water content in the upper mantle. Taking into account a wide range of temperature conditions and the transient creep of olivine, we explain the transient deformation in the aftermath of the earthquake that was recorded by continuous geodetic stations along Sumatra as the result of water- and stress-activated creep of olivine. This implies a minimum water content of about 0.01 per cent by weight-or 1,600 H atoms per million Si atoms-in the asthenosphere (the part of the upper mantle below the lithosphere). The earthquake ruptured conjugate faults down to great depths, compatible with dry olivine in the oceanic lithosphere. We attribute the steep rheological contrast to dehydration across the lithosphere-asthenosphere boundary, presumably by buoyant melt migration to form the oceanic crust.

Northeastern Pacific mantle conductivity profile from long-period magnetotelluric sounding using Hawaii-to-California submarine cable data

NASA Astrophysics Data System (ADS)

Lizarralde, Daniel; Chave, Alan; Hirth, Greg; Schultz, Adam

1995-09-01

We present results of a long-period magnetotelluric (MT) investigation of the electrical structure beneath the eastern North Pacific. The electric field data consist of ˜2 years of continuously recorded voltages across an unpowered, ˜4000-km-long submarine telephone cable (HAW-1) extending from Point Arena, California, to Oahu, Hawaii. The electric field measurements are coherent to some degree with magnetic field measurements from Honolulu Observatory at periods of 0.1 to 45 days. This coherence is enhanced at long periods over that observed with point electric field sensors due to horizontal averaging of the motional electric fields of spatial scale smaller than the cable length, significantly diminishing their effect. Robust, controlled leverage MT response estimates and their jacknife confidence limits are computed for the HAW-1 to Honolulu data. An equivalent scalar MT response obtained from Honolulu magnetic variations data is used to correct the HAW-1 MT response for static shift and to extend the MT response estimate to periods of 100 days. The composite response function satisfies necessary and sufficient conditions for consistency with a one-dimensional conductivity structure and is most sensitive to structure between 150 and 1000 km. Inversion of the MT response reveals a conductive zone (0.05-0.1 S/m) between 150 and 400 km depth and a positive gradient below 500 km; these observations are consistent with previous MT studies in the North Pacific. This upper mantle conductivity is too high to be explained by solid-state conduction in dry olivine using reasonable mantle geotherms. Calculations based on measurements of hydrogen solubility and diffusivity in olivine indicate that H+ dissolved in olivine, possibly combined with a lattice preferred orientation consistent with measured seismic anisotropy, provide sufficient conductivity enhancement to explain the inversion results. The high conductivity may also be explained by the presence of gravitationally stable partial melt. Comparison of the HAW-1 results with long-period MT studies conducted on land reveals differences in upper mantle conductivity between different tectonic regimes. In particular, the upper mantle beneath the Pacific Ocean is considerably more conductive than that beneath the Canadian shield and similar in conductivity to that beneath the Basin and Range.

The Earth's Mantle.

ERIC Educational Resources Information Center

McKenzie, D. P.

1983-01-01

The nature and dynamics of the earth's mantle is discussed. Research indicates that the silicate mantle is heated by the decay of radioactive isotopes and that the heat energizes massive convention currents in the upper 700 kilometers of the ductile rock. These currents and their consequences are considered. (JN)

Challenging the Standard Model: Equation of State of Natural Peridotite at Lower-Mantle Conditions

NASA Astrophysics Data System (ADS)

Jeanloz, R.; Lee, K. K.; Shim, S.

2002-12-01

High-resolution x-ray diffraction of natural peridotite, before and after (subsolidus) laser heating at pressures as high as 107 GPa, yields results challenging the paradigm that the Earth's mantle is a homogeneously mixed layer having the bulk composition of pyrolite. The starting material for the experiments is representative of fertile upper mantle, and is indistinguishable from Ringwood's pyrolite compositions. It transforms to an assemblage of 76 (2)% (Mg0.88Fe0.06Al0.12Si0.94)O3 orthorhombic perovskite (opv) by volume at zero pressure, 17 (2)% (Mg0.80Fe0.20)O magnesiow\\x81stite (mw) and 7 (1)% CaSiO3 perovskite (cpv), and room-temperature isotherms for each phase within the assemblage are in good agreement with past results on the individual mineral phases. Different measurement techniques yield reproducible results, with the observed scatter being well explained by the (small) compositional variations within the mineral phases of the natural starting material. We find values of the opv/mw Fe/Mg partition coefficient consistent with prior results, 0.20 (0.10) with no evidence of any pressure dependence, and recent work on CaSiO3 perovskite shows that its structure exhibits slight tetragonal distortion at lower-mantle pressures. The thermal equation of state of the high-pressure assemblage, described in terms of the Debye temperature, Gruneisen parameter and its volume dependence, is well determined if past measurements at high pressures and temperatures are reanalysed in terms of internally-consistent calibration standards. In particular, one model for the thermal equation of state of gold that has been used to calibrate several key experiments is faulty and yields biased results. Our re-analysis shows that all experiments point to relatively high values for the thermal expansion of opv (hence of the entire high-pressure assemblage), compatible with earlier rather than more recent analyses. The resulting high-pressure, high-temperature bulk modulus of the high-pressure assemblage is constrained to about 5% at lower-mantle conditions, and is expected to be relatively insensitive to Fe abundance. Minimum temperatures of about 2000 K at 700 km depth rising to about 3000 K at 2500 km depth are required for the bulk modulus of the high-pressure assemblage to match the seismologically observed bulk modulus of the lower mantle. These values of temperature are in good accord with current estimates. The density of the pyrolite-composition high-pressure assemblage is then found to be at least 2 (1)%, (and plausibly 4 (2)%) lower than the seismologically determined density at corresponding depths. The density mismatch is partly attributable to the effect of Al on the volume of opv, as also found by others. Uncertainties in the measurements and analysis appear to be well constrained, and rule out pyrolite as a viable bulk composition for the preponderance of the mantle.

Continental lithosphere of the Arabian Plate: A geologic, petrologic, and geophysical synthesis

NASA Astrophysics Data System (ADS)

Stern, Robert J.; Johnson, Peter

2010-07-01

The Arabian Plate originated ˜ 25 Ma ago by rifting of NE Africa to form the Gulf of Aden and Red Sea. It is one of the smaller and younger of the Earth's lithospheric plates. The upper part of its crust consists of crystalline Precambrian basement, Phanerozoic sedimentary cover as much as 10 km thick, and Cenozoic flood basalt (harrat). The distribution of these rocks and variations in elevation across the Plate cause a pronounced geologic and topographic asymmetry, with extensive basement exposures (the Arabian Shield) and elevations of as much as 3000 m in the west, and a Phanerozoic succession (Arabian Platform) that thickens, and a surface that descends to sea level, eastward between the Shield and the northeastern margin of the Plate. This tilt in the Plate is partly the result of marginal uplift during rifting in the south and west, and loading during collision with, and subduction beneath, the Eurasian Plate in the northeast. But a variety of evidence suggests that the asymmetry also reflects a fundamental crustal and mantle heterogeneity in the Plate that dates from Neoproterozoic time when the crust formed. The bulk of the Plate's upper crystalline crust is Neoproterozoic in age (1000-540 Ma) reflecting, in the west, a 300-million year process of continental crustal growth between ˜ 850 and 550 Ma represented by amalgamated juvenile magmatic arcs, post-amalgamation sedimentary and volcanic basins, and granitoid intrusions that make up as much as 50% of the Shield's surface. Locally, Archean and Paleoproterozoic rocks are structurally intercalated with the juvenile Neoproterozoic rocks in the southern and eastern parts of the Shield. The geologic dataset for the age, composition, and origin of the upper crust of the Plate in the east is smaller than the database for the Shield, and conclusions made about the crust in the east are correspondingly less definitive. In the absence of exposures, furthermore, nothing is known by direct observation about the composition of the crust north of the Shield. Nonetheless, available data indicate a geologic history for eastern Arabian crust different to that in the west. The Neoproterozic crust (˜ 815-785 Ma) is somewhat older than in the bulk of the Arabian Shield, and igneous and metamorphic activity was largely finished by 750 Ma. Thereafter, the eastern part of the Plate became the site of virtually continuous sedimentation from 725 Ma on and into the Phanerozoic. This implies that a relatively strong lithosphere was in place beneath eastern Arabia by 700 Ma in contrast to a lithospheric instability that persisted to ˜ 550 Ma in the west. Lithospheric differentiation is further indicated by the Phanerozoic depositional history with steady subsidence and accumulation of a sedimentary succession 5-14 km thick in the east and a consistent high-stand and thin to no Phanerozoic accumulation over the Shield. Geophysical data likewise indicate east-west lithospheric differentiation. Overall, the crustal thickness of the Plate (depth to the Moho) is ˜ 40 km, but there is a tendency for the crust to thicken eastward by as much as 10% from 35-40 km beneath the Shield to 40-45 km beneath eastern Arabia. The crust also becomes structurally more complex with as many as 5 seismically recognized layers in the east compared to 3 layers in the west. A coincident increase in velocity is noted in the upper-crust layers. Complementary changes are evidenced in some models of the Arabian Plate continental upper mantle, indicating eastward thickening of the lithospheric mantle from ˜ 80 km beneath the Shield to ˜ 120 km beneath the Platform, which corresponds to an overall lithospheric thickening (crust and upper mantle) from ˜ 120 km to ˜ 160 km eastward. The locus of these changes coincides with a prominent magnetic anomaly (Central Arabian Magnetic Anomaly, CAMA) in the extreme eastern part of the Arabian Shield that extends north across the north-central part of the Arabian Plate. The CAMA also coincides with a major structural boundary separating a region of northerly and northwesterly basement trends in the west from a region of northerly and northeasterly trends in the northeastern part of the Plate, and with the transition from high-stand buoyant Shield to subsided Platform. Its coincidence with geophysically indicated changes in the lower crust and mantle structure suggests that a fundamental lithospheric boundary is present in the central part of the Arabian Plate. The ages and isotopic characteristics of xenoliths brought to the surface in Cenozoic basalt eruptions indicate that the lower crust and upper mantle are largely juvenile Neoproterozoic additions, meaning that the lower crust and upper mantle formed about the same time as the upper crust. This implies that the lithospheric boundary in the central part of the Arabian Plate dates from Neoproterozoic time. We conclude that lithospheric differentiation across the Arabian Plate is long lived and has controlled much of the Phanerozoic sedimentary history of the Plate.

Mantle-circulation models with sequential data assimilation: inferring present-day mantle structure from plate-motion histories.

PubMed

Bunge, Hans-Peter; Richards, M A; Baumgardner, J R

2002-11-15

Data assimilation is an approach to studying geodynamic models consistent simultaneously with observables and the governing equations of mantle flow. Such an approach is essential in mantle circulation models, where we seek to constrain an unknown initial condition some time in the past, and thus cannot hope to use first-principles convection calculations to infer the flow history of the mantle. One of the most important observables for mantle-flow history comes from models of Mesozoic and Cenozoic plate motion that provide constraints not only on the surface velocity of the mantle but also on the evolution of internal mantle-buoyancy forces due to subducted oceanic slabs. Here we present five mantle circulation models with an assimilated plate-motion history spanning the past 120 Myr, a time period for which reliable plate-motion reconstructions are available. All models agree well with upper- and mid-mantle heterogeneity imaged by seismic tomography. A simple standard model of whole-mantle convection, including a factor 40 viscosity increase from the upper to the lower mantle and predominantly internal heat generation, reveals downwellings related to Farallon and Tethys subduction. Adding 35% bottom heating from the core has the predictable effect of producing prominent high-temperature anomalies and a strong thermal boundary layer at the base of the mantle. Significantly delaying mantle flow through the transition zone either by modelling the dynamic effects of an endothermic phase reaction or by including a steep, factor 100, viscosity rise from the upper to the lower mantle results in substantial transition-zone heterogeneity, enhanced by the effects of trench migration implicit in the assimilated plate-motion history. An expected result is the failure to account for heterogeneity structure in the deepest mantle below 1500 km, which is influenced by Jurassic plate motions and thus cannot be modelled from sequential assimilation of plate motion histories limited in age to the Cretaceous. This result implies that sequential assimilation of past plate-motion models is ineffective in studying the temporal evolution of core-mantle-boundary heterogeneity, and that a method for extrapolating present-day information backwards in time is required. For short time periods (of the order of perhaps a few tens of Myr) such a method exists in the form of crude 'backward' convection calculations. For longer time periods (of the order of a mantle overturn), a rigorous approach to extrapolating information back in time exists in the form of iterative nonlinear optimization methods that carry assimilated information into the past through the use of an adjoint mantle convection model.

Plume versus plate origin for the Shatsky Rise oceanic plateau (NW Pacific): Insights from Nd, Pb and Hf isotopes

NASA Astrophysics Data System (ADS)

Heydolph, Ken; Murphy, David T.; Geldmacher, Jörg; Romanova, Irina V.; Greene, Andrew; Hoernle, Kaj; Weis, Dominique; Mahoney, John

2014-07-01

Shatsky Rise, an early Cretaceous igneous oceanic plateau in the NW Pacific, comprises characteristics that could be attributed to either formation by shallow, plate tectonic-controlled processes or to an origin by a mantle plume (head). The plateau was drilled during Integrated Ocean Drilling Program (IODP) Expedition 324. Complementary to a recent trace element study (Sano et al., 2012) this work presents Nd, Pb and Hf isotope data of recovered lava samples cored from the three major volcanic edifices of the Shatsky Rise. Whereas lavas from the oldest edifice yield fairly uniform compositions, a wider isotopic spread is found for lavas erupted on the younger parts of the plateau, suggesting that the Shatsky magma source became more heterogeneous with time. At least three isotopically distinct components can be identified in the magma source: 1) a volumetrically and spatially most common, moderately depleted component of similar composition to modern East Pacific Ridge basalt but with low 3He/4He, 2) an isotopically very depleted component which could represent local, early Cretaceous (entrained) depleted upper mantle, and 3) an isotopically enriched component, indicating the presence of (recycled) continental material in the magma source. The majority of analyzed Shatsky lavas, however, possess Nd-Hf-Pb isotope compositions consistent with a derivation from an early depleted, non-chondritic reservoir. By comparing these results with petrological and trace element data of mafic volcanic rock samples from all three massifs (Tamu, Ori, Shirshov), we discuss the origin of Shatsky Rise magmatism and evaluate the possible involvement of a mantle plume (head).

Mantle thermal history during supercontinent assembly and breakup

NASA Astrophysics Data System (ADS)

Rudolph, M. L.; Zhong, S.

2013-12-01

We use mantle convection simulations driven by plate motion boundary conditions to investigate changes in mantle temperature through time. It has been suggested that circum-Pangean subduction prevented convective thermal mixing between sub-continental and sub-oceanic regions. We performed thermo-chemical simulations of mantle convection with velocity boundary conditions based on plate motions for the past 450 Myr using Earth-like Rayleigh number and ~60% internal heating using three different plate motion models for the last 200 Myr [Lithgow-Bertelloni and Richards 1998; Gurnis et al. 2012; Seton et al. 2012; Zhang et al. 2010]. We quantified changes in upper-mantle temperature between 200-1000 km depth beneath continents (defined as the oldest 30% of Earth's surface) and beneath oceans. Sub-continental upper mantle temperature was relatively stable and high between 330 and 220 Ma, coincident with the existence of the supercontinent Pangea. The average sub-continental temperature during this period was, however, only ~10 K greater than during the preceding 100 Myr. In the ~200 Myr since the breakup of Pangea, sub-continental temperatures have decreased only ~15 K in excess of the 0.02 K/Myr secular cooling present in our models. Sub-oceanic upper mantle temperatures did not vary more than 5 K between 400 and 200 Ma and the cooling trend following Pangea breakup is less pronounced. Recent geochemical observations imply rapid upper mantle cooling of O(10^2) K during continental breakup; our models do not produce warming of this magnitude beneath Pangea or cooling of similar magnitude associated with the breakup of Pangea. Our models differ from those that produce strong sub-continental heating in that the circum-Pangean subduction curtain does not completely inhibit mixing between the sub-continental and sub-oceanic regions and we include significant internal heating, which limits the rate of temperature increase. Heat transport in our simulations is controlled to first order by plate motions. Most of the temporal variability in surface heat flow is driven by variations in seafloor spreading rate and the accompanying changes in slab velocities dominate variations in buoyancy flux at all mantle depths. Variations in plume buoyancy flux are small but are correlated with the slab buoyancy flux variations.

Eclogite formation beneath the northern Slave craton constrained by diamond inclusions: Oceanic lithosphere origin without a crustal signature

NASA Astrophysics Data System (ADS)

Smart, Katie A.; Chacko, Thomas; Stachel, Thomas; Tappe, Sebastian; Stern, Richard A.; Ickert, Ryan B.; EIMF

2012-02-01

We report the geochemical and oxygen isotope compositions for eclogitic mineral inclusions in diamonds hosted by high-MgO eclogite xenoliths from the Jericho kimberlite, Canada. These data are used to constrain the nature and evolution of the eclogite protolith. The garnet and clinopyroxene diamond inclusions (DIs) are compositionally different than their host eclogite counterparts. In particular, garnet DIs have much lower Mg-numbers (54 vs. 82) and Cr2O3 contents (0.1 vs. 0.6 wt.%) and higher CaO contents (7.6 vs. 4.3 wt.%) than host eclogite garnet. DI and host eclogite clinopyroxenes are more similar but differences include lower Mg-numbers (78-81 vs. 93) and higher Na2O contents (2.3 vs. 1.8 wt.%) in the DIs. The DIs lack typical shallow oceanic crust signatures such as strong positive Eu and Sr anomalies, and oxygen isotope compositions that deviate significantly from the pristine mantle average. On the contrary, both the Jericho DIs and host eclogite garnets have small negative Eu and Sr anomalies, fractionated HREE patterns ((LuN/GdN) ~ 3-5) and pristine mantle-like δ18O values of 5.2-6.0‰, indicating that shallow, plagioclase-rich oceanic crust protoliths are unlikely. The eclogitic DI trace-element characteristics require that both garnet and plagioclase were present in the protolith, which likely crystallized in the shallow upper mantle. DI-based reconstructed whole-rock eclogite compositions have higher Mg-numbers and lower Al2O3 contents than found in typical basaltic or gabbroic oceanic crust, and are similar to pyroxenitic veins found in orogenic peridotite massifs. Due to the lack of clear oceanic crust signatures and the mantle-like δ18O values of the studied DIs, we propose that the Jericho diamond eclogites originally crystallized as pyroxenite cumulates that formed veins within the oceanic mantle lithosphere. Following partial melt extraction, the eclogite protoliths were subducted into the diamond stability field beneath the evolving Slave craton. Hence, the Jericho DIs and host high-MgO eclogites may represent an example of eclogite formation in an oceanic setting without the diagnostic 'crustal signatures' that are typically observed in cratonic eclogite xenolith suites worldwide.

Geochemical constraints on the spatial distribution of recycled oceanic crust in the mantle source of late Cenozoic basalts, Vietnam

NASA Astrophysics Data System (ADS)

Hoang, Thi Hong Anh; Choi, Sung Hi; Yu, Yongjae; Pham, Trung Hieu; Nguyen, Kim Hoang; Ryu, Jong-Sik

2018-01-01

This study presents a comprehensive analysis of the major and trace element, mineral, and Sr, Nd, Pb and Mg isotopic compositions of late Cenozoic intraplate basaltic rocks from central and southern Vietnam. The Sr, Nd, and Pb isotopic compositions of these basalts define a tight linear array between Indian mid-ocean-ridge basalt (MORB)-like mantle and enriched mantle type 2 (EM2) components. These basaltic rocks contain low concentrations of CaO (6.4-9.7 wt%) and have high Fe/Mn ratios (> 60) and FeO/CaO-3MgO/SiO2 values (> 0.54), similar to partial melts derived from pyroxenite/eclogite sources. This similarity is also supported by the composition of olivine within these samples, which contains low concentration of Ca and high concentrations of Ni, and shows high Fe/Mn ratios. The basaltic rocks have elevated Dy/Yb ratios that fall within the range of melts derived from garnet lherzolite material, although their Yb contents are much higher than those of modeled melts derived from only garnet lherzolite material and instead plot near the modeled composition of eclogite-derived melts. The Vietnamese basaltic rocks have lighter δ26Mg values (- 0.38 ± 0.06‰) than is expected for the normal mantle (- 0.25 ± 0.07‰), and these values decrease with decreasing Hf/Hf* and Ti/Ti* ratios, indicating that these basalts were derived from a source containing carbonate material. On primitive mantle-normalized multi-element variation diagrams, the central Vietnamese basalts are characterized by positive Sr, Eu, and Ba anomalies. These basalts also plot within the pelagic sediment field in Pbsbnd Pb isotopic space. This suggests that the mantle source of the basalts contained both garnet peridotite and recycled oceanic crust. A systematic analysis of variations in geochemical composition in basalts from southern to central Vietnam indicates that the recycled oceanic crust (possibly the paleo-Pacific slab) source material contains varying proportions of gabbro, basalt, and sediment. The basalts from south-central Vietnam (12°N-14°N) may be dominated by the lowest portion of the residual slab that contains rutile-bearing plagioclase-rich gabbroic eclogite, whereas the uppermost portion of the recycled slab, including sediment and basaltic material with small amounts of gabbro, may be a major constituent of the source for the basalts within the central region of Vietnam (14°N-16°N). Finally, the southern region (10°N-12°N) contains basalts sourced mainly from recycled upper oceanic crust that is basalt-rich and contains little or no sediment.

Composition of the Ultra-Low Velocity Zone from Shock Data

NASA Astrophysics Data System (ADS)

Ahrens, T. J.; Asimow, P. D.

2009-12-01

Composition of the Ultra-Low Velocity Zone from Shock Data Thomas J. Ahrens and Paul D. Asimow Recent models of the thermal structure of a putative magma ocean upon accretion of the Earth are derived from construction of isentropes centered at the core-mantle boundary (CMB) pressure and temperature (133 GPa and 4300 K). These models were motivated by the idea that the seismologically mapped ultra-low velocity zones (ULVZ) above the CMB are partially molten remnants of a basal magma ocean [1]. Magma ocean thermal models are derived from the observation of strongly increasing Grüneisen parameter (γ) upon compression of silicate liquids both in ab initio molecular dynamics modeling of MgSiO3 melt [2] and in new shock wave data on MgSiO3 phases reaching CMB conditions. Shock EOS (and limited Hugoniot radiative temperature) data for Mg2SiO4 (initially forsterite and wadsleyite) access perovskite (and post-perovskite) + periclase and melt regimes [3]. MgSiO3 (initially enstatite, perovskite, and glass) EOS and radiative temperature data in the perovskite, post-perovskite, and melt regimes, together with static P-V-T data, define the properties of these phases [4]. With recent Caltech Hugoniot radiative temperature measurements on pre-heated (1923 K) MgO [5], we have experimental constraints on melting temperatures of all major minerals in the MgO-SiO2 binary at lower-most mantle pressures. Recently extended (to 130 GPa) pre-heated (1673 K) Hugoniot data for molten and solid diopside - anorthite aggregate (64 mol % diopside, 36 mol % anorthite) also show the strong increase in γ, over the pressure range of the mantle, previously observed for ultramafic compositions. For long-term gravitational stability, the presumed molten silicate liquid of the ULVZ must be neutrally buoyant, or denser, than the ambient lowermost mantle. Surprisingly, unlike the situation in the upper mantle low-velocity zone, the density of even partially Fe-enriched, Di0.64An0.36 composition, ~5.1 g/cm3 , is much too low to be stable in the ambient, ~5.6 g/cm3, solid mineral assemblage at lower-most mantle conditions. In contrast, a molten magma of MgSiO3 composition, not necessarily requiring significant Fe enrichment, appears to approximately satisfy ULVZ constraints of melting temperature and density. [1] Labrosse, S., et al. (2007), Nature, 450, 866. [2] Stixrude, L., and B. Karki (2005), Science, 310, 297. [3] Mosenfelder, J. L., et al. (2007),, J. Geophys. Res., 112B, 6208. [4] Mosenfelder, J. L., et al. (2009), J. Geophys. Res., 114B,1203. [5] Fat’yanov O. V., et al. (2009), APS SCCM.

Deep Roots of Cratons From Surface-wave Tomography

NASA Astrophysics Data System (ADS)

Cara, M.; Debayle, E.; Lévêque, J. J.

Thanks to the application of multimode waveform inversion techniques to various sets of surface wave seismograms recorded on global networks of broad-band seismome- ters, either permanent (IRIS, Geoscope) or temporary (PASSCAL, INSU), unprece- dented lateral- and depth-resolution can be achieved in upper-mantle surface-wave tomography. With a depth-resolution around 50 km and a lateral resolution around 250 km in the upper mantle, Sv velocity models beneath Australia, South-America, Eurasia and East-Africa show fast velocity anomalies associated with shield generally confined to the uppermost 200 km of the mantle. We show on cross-sections taken across different continents that there is no evidence so far for "thermal and/or com- positional" lithospheric roots extending deeper than 300 km in the continental regions we have investigated. In addition, surface wave azimuthal anisotropy can be used as an indicator of the me- chanical thickness of the lithosphere when a clear change in the pattern of anisotropic directions is observed with depth. The fast moving Australian plate shows the clear- est example of such a change occuring at relatively shallow depths (150 km) within the high seismic velocity lid. This suggests that seismic anisotropy defines a "me- chanical" lithosphere that does not coincide with the "thermal and/or compositional" lithosphere probably imaged by velocity anomalies. However, beneath other slowly moving plates, such a change in pattern is less clear and there is a tendency of seismic anisotropy to disappear at the bottom of the lid.

Relative arrival-time upper-mantle tomography and the elusive background mean

NASA Astrophysics Data System (ADS)

Bastow, Ian D.

2012-08-01

The interpretation of seismic tomographic images of upper-mantle seismic wave speed structure is often a matter of considerable debate because the observations can usually be explained by a range of hypotheses, including variable temperature, composition, anisotropy, and the presence of partial melt. An additional problem, often overlooked in tomographic studies using relative as opposed to absolute arrival-times, is the issue of the resulting velocity model's zero mean. In shield areas, for example, relative arrival-time analysis strips off a background mean velocity structure that is markedly fast compared to the global average. Conversely, in active areas, the background mean is often markedly slow compared to the global average. Appreciation of this issue is vital when interpreting seismic tomographic images: 'high' and 'low' velocity anomalies should not necessarily be interpreted, respectively, as 'fast' and 'slow' compared to 'normal mantle'. This issue has been discussed in the seismological literature in detail over the years, yet subsequent tomography studies have still fallen into the trap of mis-interpreting their velocity models. I highlight here some recent examples of this and provide a simple strategy to address the problem using constraints from a recent global tomographic model, and insights from catalogues of absolute traveltime anomalies. Consultation of such absolute measures of seismic wave speed should be routine during regional tomographic studies, if only for the benefit of the broader Earth Science community, who readily follow the red = hot and slow, blue = cold and fast rule of thumb when interpreting the images for themselves.

Open system models of isotopic evolution in Earth's silicate reservoirs: Implications for crustal growth and mantle heterogeneity

NASA Astrophysics Data System (ADS)

Kumari, Seema; Paul, Debajyoti; Stracke, Andreas

2016-12-01

An open system evolutionary model of the Earth, comprising continental crust (CC), upper and lower mantle (UM, LM), and an additional isolated reservoir (IR) has been developed to study the isotopic evolution of the silicate Earth. The model is solved numerically at 1 Myr time steps over 4.55 Gyr of Earth history to reproduce both the present-day concentrations and isotope ratios of key radioactive decay systems (Rb-Sr, Sm-Nd, and U-Th-Pb) in these terrestrial reservoirs. Various crustal growth scenarios - continuous versus episodic and early versus late crustal growth - and their effect on the evolution of Sr-Nd-Pb isotope systematics in the silicate reservoirs have been evaluated. Modeling results where the present-day UM is ∼60% of the total mantle mass and a lower mantle that is non-primitive reproduce the estimated geochemical composition and isotope ratios in Earth's silicate reservoirs. The isotopic evolution of the silicate Earth is strongly affected by the mode of crustal growth; only an exponential crustal growth pattern with crustal growth since the early Archean satisfactorily explains the chemical and isotopic evolution of the crust-mantle system and accounts for the so-called Pb paradoxes. Assuming that the OIB source is located in the deeper mantle, our model could, however, not reproduce its target ɛNd of +4.6 for the UM, which has been estimated from the average isotope ratios of 32 individual ocean island localities. Hence, either mantle plumes sample the LM in a non-representative way, or the simplified model set-up does not capture the full complexity of Earth's lower mantle (Nd isotope) evolution. Compared to the results obtained for a 4.55 Ga Earth, a model assuming a protracted U-Pb evolution of silicate Earth by ca. 100 Myr reproduces a slightly better fit for the Pb isotope ratios in Earth's silicate reservoirs. One notable feature of successful models is the early depletion of incompatible elements (as well as rapid decrease in Th/U) in the UM within the initial 500 Myr, as a result of early formation of CC, which supports other evidence in favor of the presence of Hadean continental crust. Therefore, a chondritic Th/U ratio (4 ± 0.2) in the UM until 2 Gyr appears rather unlikely. We find that the κ conundrum - the observation that measured Th/U ratios and those deduced from 208Pb-206Pb isotope systematics differ - is a natural outcome of an open system evolution in which preferential recycling of U for the past 2 Gyr has played a dominant role. Overall, our simulations strongly favor exponential crustal growth, starting in the early Hadean, the transient preservation of compositionally distinct mantle reservoirs over billion year time periods, and a generally less incompatible element depleted, but non-primitive composition of the lower mantle.

3D Rheological Modeling of NW Intraplate Europe, Deciphering Spatial Integrated strength patterns, Mechanical Strong Layering and EET

NASA Astrophysics Data System (ADS)

Beekman, F.; Hardebol, N.; Cloetingh, S.; Tesauro, M.

2006-12-01

Better understanding of 3D rheological heterogeneity of the European Lithosphere provide the key to tie the recorded intraplate deformation pattern to stress fields transmitted into plate interior from plate boundary forces. The first order strain patterns result from stresses transmitted through the European lithosphere that is marked by a patchwork of high strength variability from inherited structural and compositional heterogeneities and upper mantle thermal perturbations. As the lithospheric rheology depends primarily on its spatial structure, composition and thermal estate, the 3D strength model for the European lithosphere relies on a 3D compositional model that yields the compositional heterogeneities and an iteratively calculated thermal cube using Fouriers law for heat conduction. The accurate appraisal of spatial strength variability results from proper mapping and integration of the geophysical compositional and thermal input parameters. Therefore, much attention has been paid to a proper description of first order structural and tectonic features that facilitate compilation of the compositional and thermal input models. As such, the 3D strength model reflects the thermo-mechanical structure inherited from the Europeans polyphase deformation history. Major 3D spatial mechanical strength variability has been revealed. The East-European and Fennoscandian Craton to the NE exhibit high strength (30-50 1012 N/m) from low mantle temperatures and surface heatflow of 35-60 mW/m2 while central and western Europe reflect a polyphase Phanerozoic thermo- tectonic history. Here, regions with high rigidity are formed primarily by patches of thermally stabilized Variscan Massifs (e.g. Rhenish, Armorican, Bohemian, and Iberian Massif) with low heatflow and lithospheric thickness values (50-65 mW/m2; 110-150 km) yielding strengths of ~15-25 1012 N/m. In contrast, major axis of weakened lithosphere coincides with Cenozoic Rift System (e.g. Upper and Lower Rhine Grabens, Pannonian Basin and Massif Central) attributed to the presence of tomographically imaged plumes. This study has elucidated the memory of the present-days Europeans lithosphere induced by compositional and thermal heterogeneities. The resulting lateral strength variations has a clear signature of the pst lithospheres polyphase deformation and also entails active tectonics, tectonically induced topography and surface processes.

Bending-related faulting and mantle serpentinization at the Middle America trench.

PubMed

Ranero, C R; Morgan, J Phipps; McIntosh, K; Reichert, C

2003-09-25

The dehydration of subducting oceanic crust and upper mantle has been inferred both to promote the partial melting leading to arc magmatism and to induce intraslab intermediate-depth earthquakes, at depths of 50-300 km. Yet there is still no consensus about how slab hydration occurs or where and how much chemically bound water is stored within the crust and mantle of the incoming plate. Here we document that bending-related faulting of the incoming plate at the Middle America trench creates a pervasive tectonic fabric that cuts across the crust, penetrating deep into the mantle. Faulting is active across the entire ocean trench slope, promoting hydration of the cold crust and upper mantle surrounding these deep active faults. The along-strike length and depth of penetration of these faults are also similar to the dimensions of the rupture area of intermediate-depth earthquakes.

Regional Vp, Vs, Vp/Vs, and Poisson's ratios across earthquake source zones from Memphis, Tennessee, to St. Louis, Missouri

USGS Publications Warehouse

Catchings, R.D.

1999-01-01

Models of P- and S-wave velocity, Vp/Vs ratios, Poisson's ratios, and density for the crust and upper mantle are presented along a 400-km-long profile trending from Memphis, Tennessee, to St. Louis, Missouri. The profile crosses the New Madrid seismic zone and reveals distinct regional variations in the crustal velocity structure north and south of the latitude of New Madrid. In the south near Memphis, the upper few kilometers of the crust are dominated by upper crustal sedimentary basins or graben with P-wave velocities less than 5 km/sec and S-wave velocities of about 2 km/sec. P-wave velocities of the upper and middle crust range from 6.0 to 6.5 km/sec at depths above 25 km, and corresponding S-wave velocities range from 3.5 to 3.7 km/sec. The lower crust consists of a high-velocity layer (Vp = 7.4 km/sec; Vs ~4.2 km/sec) that is up to 20-km thick at the latitude of New Madrid but thins to about 15 km near Memphis. To the north, beneath the western-most Illinois basin, low-velocity (Vp < 5 km/sec; Vs < 2.3 km/sec) sedimentary basins are less than 1-km deep. The average velocities (Vp = 6.0 km/sec; Vs = 3.5 km/sec) of the underlying, near-surface rocks argue against large thickness of unconsolidated noncarbonate sediments within 50 km of the western edge of the Illinois basin. Most of the crust beneath the Illinois basin is modeled as one layer, with velocities up to 6.8 km/sec (Vs = 3.7 km/sec) at 37-km depth. The thick, high-velocity (Vp = 7.4 km/sec; Vs ~4.2 km/sec) lower crustal layer thins from about 20 km near New Madrid to about 6 km beneath the western Illinois basin. Refractions from the Moho and upper mantle occur as first arrivals over distances as a great as 160 km and reveal upper mantle layering to 60 km depth. Upper mantle layers with P-wave velocities of 8.2 km/sec (Vs = 4.5 km/sec) and 8.4 km/sec (Vs = 4.7 km/sec) are modeled at 43 and 60 km depth, respectively. Crustal Vp/Vs ratios range between 1.74 and 1.83, and upper mantle Vp/V s ratios range from 1.78 to 1.84. Poisson's ratios range from about 0.26 to 0.33 in the crust and from about 0.27 to 0.29 in the upper mantle. Modeled average densities range from about 2.55 in the sedimentary basins to 3.43 in the upper mantle. Geophysical characteristics of the crust and upper mantle within the New Madrid seismic zone are consistent with other continental rifts, but the crustal structure of the Illinois basin is not characteristics of most continental rift settings. Seismic and gravity data suggest a buried horst near the middle of Reelfoot rift, beneath which is a vertical zone of seismicity and velocity anomalies. The relative depth of the Reelfoot rift north and south of the Reelfoot graben suggests that the rift and its bounding faults may extend eastward beneath the city of Memphis.

Multiple enrichment of the Carpathian-Pannonian mantle: Pb-Sr-Nd isotope and trace element constraints

NASA Astrophysics Data System (ADS)

Rosenbaum, Jeffrey M.; Wilson, Marjorie; Downes, Hilary

1997-07-01

Pb isotope compositions of acid-leached clinopyroxene and amphibole mineral separates from spinel peridotite mantle xenoliths entrained in Tertiary-Quaternary alkali basalts from the Carpathian-Pannonian Region of eastern Europe provide important constraints on the processes of metasomatic enrichment of the mantle lithosphere in an extensional tectonic setting associated with recent subduction. Principal component analysis of Pb-Sr-Nd isotope and rare earth element compositions of the pyroxenes is used to identify the geochemical characteristics of the original lithospheric mantle protolith and a spectrum of infiltrating metasomatic agents including subduction-related aqueous fluids and silicate melts derived from a subduction-modified mantle wedge which contains a St. Helena-type (HIMU) plume component. The mantle protolith is highly depleted relative to mid-ocean ridge basalt-source mantle with Pb-Nd-Sr isotope compositions consistent with an ancient depletion event. Silicate melt infiltration into the protolith accounts for the primary variance in the Pb-Sr-Nd isotope compositions of the xenoliths and has locally generated metasomatic amphibole. Infiltration of aqueous fluids has introduced radiogenic Pb and Sr without significantly perturbing the rare earth element signature of the protolith. The Pb isotope compositions of the fluid-modified xenoliths suggest that they reacted with aqueous fluids released from a subduction zone which had equilibrated with sediment derived from an ancient basement terrain. We propose a model for mantle lithosphere evolution consistent with available textural and geochemical data for the xenolith population. The Pb-Sr-Nd isotope compositions of both alkaline mafic magmas and rare, subduction-related, calc-alkaline basaltic andesites from the region provide important constraints for the nature of the asthenospheric mantle wedge and confirm the presence of a HIMU plume component. These silicate melts contribute to the metasomatism of the mantle lithosphere rather than being derived therefrom.

The temperature of primary melts and mantle sources of komatiites, OIBs, MORBs and LIPs

NASA Astrophysics Data System (ADS)

Sobolev, Alexander

2015-04-01

There is general agreement that the convecting mantle, although mostly peridotitic in composition, is compositionally and thermally heterogeneous on different spatial scales. The amount, sizes, temperatures and compositions of these heterogeneities significantly affect mantle dynamics because they may diverge greatly from dominant peridotites in their density and fusibility. Differences in potential temperature and composition of mantle domains affect magma production and cannot be easily distinguished from each other. This has led to radically different interpretations of the melting anomalies that produce ocean-island basalts, large igneous provinces and komatiites: most scientists believe that they originate as hot, deep-sourced mantle plumes; but a small though influential group (e.g. Anderson 2005, Foulger, 2010) propose that they derive from high proportions of easily fusible recycled or delaminated crust, or in the case of komatiites contain large amount of H2O (e.g. Grove & Parman, 2004). The way to resolve this ambiguity is an independent estimation of temperature and composition of mantle sources of various types of magma. In this paper I report application of newly developed olivine-spinel-melt geothermometers based on partition of Al, Cr, Sc and Y for different primitive lavas from mid-ocean ridges, ocean-island basalts, large igneous provinces and komatiites. The results suggest significant variations of crystallization temperature for the same Fo of high magnesium olivines of different types of mantle-derived magmas: from the lowest (down to 1220 degree C) for MORB to the highest (up to over 1500 degree C) for komatiites and Siberian meimechites. These results match predictions from Fe-Mg olivine-melt equilibrium and confirm the relatively low temperature of the mantle source of MORB and higher temperatures in the mantle plumes that produce the OIB of Iceland, Hawaii, Gorgona, Archean komatiites and several LIPs (e.g Siberian and NAMP). The established liquidus temperatures and compositions of primary melts allow estimating potential temperatures and compositions of their mantle sources. The results strongly confirm mantle plume theory and presence of variable amounts of recycled crustal material in the mantle sources. This study has been founded by Russian Science Foundation grant 14-17-00491.

The helium flux from the continents and ubiquity of low-3He/4He recycled crust and lithosphere

NASA Astrophysics Data System (ADS)

Day, James M. D.; Barry, Peter H.; Hilton, David R.; Burgess, Ray; Pearson, D. Graham; Taylor, Lawrence A.

2015-03-01

New helium isotope and trace-element abundance data are reported for pyroxenites and eclogites from South Africa, Siberia, and the Beni Bousera Massif, Morocco that are widely interpreted to form from recycled oceanic crustal protoliths. The first He isotope data are also presented for Archaean peridotites from the Kaapvaal (South Africa), Slave (Canada), and Siberian cratons, along with recently emplaced off-craton peridotite xenoliths from Kilbourne Hole, San Carlos (USA) and Vitim (Siberia), to complement existing 3He/4He values obtained for continental and oceanic peridotites. Helium isotope compositions of peridotite xenoliths vary from 7.3 to 9.6 RA in recently (<10 kyr) emplaced xenoliths, to 0.05 RA in olivine from cratonic peridotite xenoliths of the 1179 Ma Premier kimberlite, South Africa. The helium isotope compositions of the peridotites can be explained through progressive sampling of 4He produced from radiogenic decay of U and Th in the mineral lattice in the older emplaced peridotite xenoliths. Ingrowth of 4He is consistent with generally higher 4He concentrations measured in olivine from older emplaced peridotite xenoliths relative to those from younger peridotite xenoliths. Collectively, the new data are consistent with pervasive open-system behaviour of He in peridotite xenoliths from cratons, mobile belts and tectonically-active regions. However, there is probable bias in the estimate of the helium isotope composition of the continental lithospheric mantle (6.1 ± 2.1 RA), since previously published databases were largely derived from peridotite xenoliths from non-cratonic lithosphere, or phenocrysts/xenocrysts obtained within continental intraplate alkaline volcanics that contain a contribution from asthenospheric sources. Using the new He isotope data for cratonic peridotites and assuming that significant portions (>50%) of the Archaean and Proterozoic continental lithospheric mantle are stable and unaffected by melt or fluid infiltration on geological timescales (>0.1 Ga), and that U and Th contents vary between cratonic lithosphere and non-cratonic lithosphere, calculations yield a 3He flux of 0.25-2.2 atoms/s/cm2 for the continental lithospheric mantle. These estimates differ by a factor of ten from non-cratonic lithospheric mantle and are closer to the observed 3He flux from the continents (<1 atoms/s/cm2). Pyroxenites and eclogites from the continental regions are all characterized by 3He/4He (0.03-5.6 RA) less than the depleted upper mantle, and relatively high U and Th contents. Together with oceanic and continental lithospheric peridotites, these materials represent reservoirs with low time-integrated 3He/(U + Th) in the mantle. Pyroxenites and eclogites are also characterized by higher Fe/Mg, more radiogenic Os-Pb isotope compositions, and more variable δ18O values (∼3‰ to 7‰), compared with peridotitic mantle. These xenoliths are widely interpreted to be the metamorphic/metasomatic equivalents of recycled oceanic crustal protoliths. The low-3He/4He values of these reservoirs and their distinctive compositions make them probable end-members to explain the compositions of some low-3He/4He OIB, and provide an explanation for the low-3He/4He measured in most HIMU lavas. Continental lithospheric mantle and recycled oceanic crust protoliths are not reservoirs for high-3He/4He and so alternative, volumetrically significant, He-rich reservoirs, such as less-degassed (lower?) mantle, are required to explain high-3He/4He signatures measured in some intraplate lavas. Recycling of oceanic crust represents a fundamental process for the generation of radiogenic noble gases in the mantle, and can therefore be used effectively as tracers for volatile recycling.

Heterogeneity of the North Atlantic oceanic lithosphere based on integrated analysis of GOCE satellite gravity and geological data

NASA Astrophysics Data System (ADS)

Barantseva, Olga; Artemieva, Irina; Thybo, Hans; Herceg, Matija

2015-04-01

We present the results from modelling the gravity and density structure of the upper mantle for the off-shore area of the North Atlantic region. The crust and upper mantle of the region is expected to be anomalous: Part of the region affected by the Icelandic plume has an anomalously shallow bathymetry, whereas the northern part of the region is characterized by ultraslow spreading. In order to understand the links between deep geodynamical processes that control the spreading rate, on one hand, and their manifestations such as oceanic floor bathymetry and heat flow, on the other hand, we model the gravity and density structure of the upper mantle from satellite gravity data. The calculations are based on interpretation of GOCE gravity satellite data for the North Atlantics. To separate the gravity signal responsible for density anomalies within the crust and upper mantle, we subtract the lower harmonics caused by deep density structure of the Earth (the core and the lower mantle). The gravity effect of the upper mantle is calculated by subtracting the gravity effect of the crust for two crustal models. We use a recent regional seismic model for the crustal structure (Artemieva and Thybo, 2013) based om seismic data together with borehole data for sediments. For comparison, similar results are presented for the global CRUST 1.0 model as well (Laske, 2013). The conversion of seismic velocity data for the crustal structure to crustal density structure is crucial for the final results. We use a combination of Vp-to-density conversion based on published laboratory measurements for the crystalline basement (Ludwig, Nafe, Drake, 1970; Christensen and Mooney, 1995) and for oceanic sediments and oceanic crust based on laboratory measurements for serpentinites and gabbros from the Mid-Atlantic Ridge (Kelemen et al., 2004). Also, to overcome the high degree of uncertainty in Vp-to-density conversion, we account for regional tectonic variations in the Northern Atlantics as constrained by numerous published seismic profiles and potential-field models across the Norwegian off-shore crust (e.g. Breivik et al., 2005, 2007). The results demonstrate the presence of strong gravity and density heterogeneity of the upper mantle in the North Atlantic region. In particular, there is a sharp contrast at the continent-ocean transition, which also allows for recognising mantle gravity anomalies associated with continental fragments and with anomalous oceanic lithosphere.

Subduction Drive of Plate Tectonics

NASA Astrophysics Data System (ADS)

Hamilton, W. B.

2003-12-01

Don Anderson emphasizes that plate tectonics is self-organizing and is driven by subduction, which rights the density inversion generated as oceanic lithosphere forms by cooling of asthenosphere from the top. The following synthesis owes much to many discussions with him. Hinge rollback is the key to kinematics, and, like the rest of actual plate behavior, is incompatible with bottom-up convection drive. Subduction hinges (which are under, not in front of, thin leading parts of arcs and overriding plates) roll back into subducting plates. The Pacific shrinks because bounding hinges roll back into it. Colliding arcs, increasing arc curvatures, back-arc spreading, and advance of small arcs into large plates also require rollback. Forearcs of overriding plates commonly bear basins which preclude shortening of thin plate fronts throughout periods recorded by basin strata (100 Ma for Cretaceous and Paleogene California). This requires subequal rates of advance and rollback, and control of both by subduction. Convergence rate is equal to rates of rollback and advance in many systems but is greater in others. Plate-related circulation probably is closed above 650 km. Despite the popularity of concepts of plumes from, and subduction into, lower mantle, there is no convincing evidence for, and much evidence against, penetration of the 650 in either direction. That barrier not only has a crossing-inhibiting negative Clapeyron slope but also is a compositional boundary between fractionated (not "primitive"), sluggish lower mantle and fertile, mobile upper mantle. Slabs sink more steeply than they dip. Slabs older than about 60 Ma when their subduction began sink to, and lie down on and depress, the 650-km discontinuity, and are overpassed, whereas younger slabs become neutrally buoyant in mid-upper mantle, into which they are mixed as they too are overpassed. Broadside-sinking old slabs push all upper mantle, from base of oceanic lithosphere down to the 650, back under shrinking oceans, forcing rapid Pacific spreading. Slabs suck forward overriding arcs and continental lithosphere, plus most subjacent mantle above the transition zone. Changes in sizes of oceans result primarily from transfer of oceanic lithosphere, so backarcs and expanding oceans spread only slowly. Lithosphere parked in, or displaced from, the transition zone, or mixed into mid-upper mantle, is ultimately recycled, and regional variations in age of that submerged lithosphere may account for some regional contrasts in MORB. Plate motions make no kinematic sense in either the "hotspot" reference frame (HS; the notion of fixed plumes is easily disproved) or the no-net-rotation frame (NNR) In both, for example, many hinges roll forward, impossible with gravity drive. Subduction-drive predictions are fulfilled, and paleomagnetic data are satisfied (as they are not in HS and NNR), in the alternative framework of propulsionless Antarctica fixed relative to sluggish lower mantle. Passive ridges migrate away from Antarctica on all sides, and migration of these and other ridges permits tapping fresh asthenosphere. (HS and NNR tend to fix ridges). Ridge migration and spreading rates accord with subduction drive. All trenches roll back when allowance is made for back-arc spreading and intracontinental deformation. Africa rotates slowly toward subduction systems in the NE, instead of moving rapidly E as in HS and NNR. Stable NW Eurasia is nearly stationary, instead of also moving rapidly, and S and E Eurasian deformation relates to subduction and rollback. The Americas move Pacificward at almost the full spreading rates of passive ridges behind them. Lithosphere has a slow net westward drift. Reference: W.B. Hamilton, An alternative Earth, GSA Today, in press.

V/Sc in olivine as a proxy for magma redox conditions

NASA Astrophysics Data System (ADS)

Locmelis, M.; Arevalo, R. D., Jr.; Puchtel, I. S.; Fiorentini, M. L.

2017-12-01

Although olivine is the most abundant mineral in the upper mantle and a major constituent of most mantle-derived rocks, studies on its trace element chemistry are underrepresented. This is especially the case for komatiites, an ultramafic, olivine-dominated rock type that formed via high degrees of partial melting (up to 50%) of the mantle. Komatiites were mostly emplaced in the Archean and therefore provide a unique perspective on the composition and evolution of the early Earth's mantle. Here, we present the V/Sc compositions of olivines from a global set of Paleo- (3.5-3.3 Ga) and Neo-Archean (2.7 Ga) komatiites analyzed via laser ablation ICP-MS. Vanadium and Sc behave similar during partial melting, but V is redox-sensitive (V2+-V5+) and most compatible in olivine as V2+. Scandium is monovalent (Sc3+) and its compatibility in olivine is not affected by the oxygen fugacity (fO2) of the komatiite lava. Therefore, V/Sc ratios in olivines are potentially indicative of the fO2 of the magma they crystallized from. Our data show that V/Sc ratios measured in Neo-Archean olivines (V/Scmean = 1.0 ± 0.1; 2σm) are significantly lower than in their Paleo-Archean counterparts (V/Scmean = 2.1 ± 0.1; 2σm). Geochemical models show that the elevated V/Sc ratios captured by the Paleo-Archean olivines reflect crystallization from magmas that were 1.6 log units (relative to QFM) more reduced than their Neo-Archean counterparts, and thus contained a higher proportion of V2+. Because assimilation of sedimentary bedrocks can alter the composition of komatiites upon emplacement, it may be argued that the redox states recorded by the olivines do not reflect mantle fO2, but the assimilation of crustal material. However, such an effect is not visible in our data as V/Sc ratios in olivines from localities that show evidence of crustal assimilation do not differ from komatiites that did not assimilate significant amounts of crustal rocks. Rather than a crustal signature, the V/Sc ratios suggest a secular increase of fO2 in the Archean mantle of potentially as much as 1.6 log units (relative to QFM) between 3.5-3.3 Ga and 2.7 Ga. These results are intriguing because an increase of only 0.5 log units in mantle fO2 has previously been identified as a sufficient kick-starter for the 2.4 Ga Great Oxidation Event.

Melting relations in the Fe-rich portion of the system FeFeS at 30 kb pressure

USGS Publications Warehouse

Brett, R.; Bell, P.M.

1969-01-01

The melting relations of FeFeS mixtures covering the composition range from Fe to Fe67S33 have been determined at 30 kb pressure. The phase relations are similar to those at low pressure. The eutectic has a composition of Fe72.9S27.1 and a temperature of 990??C. Solubility of S in Fe at elevated temperatures at 30 kb is of the same order of magnitude as at low pressure. Sulfur may have significantly lowered the melting point of iron in the upper mantle during the period of coalescence of metal prior to core formation in the primitive earth. ?? 1969.

The lithospheric Structure of the Sahara Metacraton From Joint Analysis of Satellite Gravity Gradients and Seismological Data

NASA Astrophysics Data System (ADS)

Sobh, M.; Ebbing, J.; Goetze, H. J.; Abdelsalam, M. G.

2016-12-01

For the Saharan Metacraton in northern Africa only a few geophysical results exists, which can be used to characterize its deep structure. We combine recent seismological models with satellite gravity gradients to build a 3D lithospheric density model of the metacraton and its surrounding regions. Due to the sparse distribution of seismic data, we estimate the Moho boundary by non-linear gravity inversion in spherical coordinates. The model is constrained by some wide angle refraction seismic profiles and receiver function Moho depths. Despite the high topography of the Darfur and Tibisti Cenozoic volcanic provinces, we estimate thin crust which indicates an upper mantle contribution to the isostatic balance. In combination with seismic tomography models, we found that the lithospheric thickness in the western part of the Metacraton is thicker than in the eastern part. This indicates that the western resembles the remnants of the pre-Neoproterozoic Sahara craton (e.g. the Marzuk craton which escaped the metacratonization process). In order to explain the partial loss of the expected cratonic root beneath the Metacraton, we present different petrological-geophysical scenario testing for different upper mantle compositions.

Carbon isotopic composition and origin of SiC from kimberlites of Yakutia, Russia

NASA Astrophysics Data System (ADS)

Mathez, E. A.; Fogel, R. A.; Hutcheon, I. D.; Marshintsev, V. K.

1995-02-01

The stability of moissanite (SiC) has been computed for upper mantle conditions using the internally optimized thermodynamic dataset for the MgSiO compounds of Fei et al. (1990). The computations consider the effects of pressure and temperature on the elastic properties of phases involved in the reactions. The maximum stability of moissanite throughout the upper mantle is typically five to six orders of magnitude lower in oxygen fugacity ( fO2) than the Fe metal-wüstite oxygen buffer at equivalent temperature and pressure, in agreement with previous calculations. Under conditions of SiC stability, silicates will be Fe-free, Fe metal will contain substantial amounts of Si but little C in solution, and Mg-rich sulfides will be stable. Moissanite from the heavy mineral concentrate of the Mir and Aikhal kimberlite pipes, Yakutia, has been studied. Moissanite crystals are gemmy and vary in color from a characteristic blue-green to pale green to nearly colorless to blue-black. Most exhibit crystallographic faces and are in the size range 0.5 to 1 mm in long dimension. Their compositions include small quantities of Fe, which is ubiquitous, Al, Ca, V, Cr, and Mn, all of which may be present in concentrations > 100 ppmwt. Mineral inclusions are present in some crystals. Silicon metal is the most common; inclusions of ferrosilicite (Fe 3Si 7), FeTi silicides, REE silicate, and sinoite (Si 2N 2O) have also been observed. The carbon isotopic compositions of individual moissanite grains have been determined by ion microprobe. The nine analyzed crystals from Aikhal and fourteen from Mir are characterized by a narrow range in δ 13C values of -22 to -29‰; the majority of crystals fall within a more restricted range of -24 to -27‰. Two grains were analyzed for N and found to have a δ 15N of +9.7 ± 4.0 and +5.6 ± 2.0‰. Five mechanisms for the formation of moissanite are considered. Moissanite may be a relict of a reduced, primordial Earth and now present only as a trace phase in an otherwise oxidized mantle. Alternatively, there may be present-day global regions of the Earth that are both highly reduced and characterized by light carbon isotopic compositions. Although these possiblities cannot be disproved, they are not supported by observations. Two other possibilities, namely that moissanite stability extends to more oxidized conditions at pressures of the lower mantle or that it may form metastably, cannot be evaluated with present knowledge. The possibility most consistent with, although not proven by, the isotopic data is that moissanite formed by metamorphism of reduced, carbonaceous sediments during subduction.

Confirmation of a change in the global shear velocity pattern at around 1000 km depth

NASA Astrophysics Data System (ADS)

Durand, S.; Debayle, E.; Ricard, Y.; Zaroli, C.; Lambotte, S.

2017-12-01

In this study, we confirm the existence of a change in the shear velocity spectrum around 1000 km depth based on a new shear velocity tomographic model of the Earth's mantle, SEISGLOB2. This model is based on Rayleigh surface wave phase velocities, self- and cross-coupling structure coefficients of spheroidal normal modes and body wave traveltimes which are, for the first time, combined in a tomographic inversion. SEISGLOB2 is developed up to spherical harmonic degree 40 and in 21 radial spline functions. The spectrum of SEISGLOB2 is the flattest (i.e. richest in 'short' wavelengths corresponding to spherical harmonic degrees greater than 10) around 1000 km depth and this flattening occurs between 670 and 1500 km depth. We also confirm various changes in the continuity of slabs and mantle plumes all around 1000 km depth where we also observed the upper boundary of Large Low Shear Velocity Provinces. The existence of a flatter spectrum, richer in short-wavelength heterogeneities, in a region of the mid-mantle can have great impacts on our understanding of the mantle dynamics and should thus be better understood in the future. Although a viscosity increase, a phase change or a compositional change can all concur to induce this change of pattern, its precise origin is still very uncertain.

Southern hemisphere craton modification by plume-lithosphere interaction

NASA Astrophysics Data System (ADS)

Hu, J.; Liu, L.; Faccenda, M.; Zhou, Q.; Fischer, K. M.; Marshak, S.; Lundstrom, C.

2017-12-01

The longevity of cratons is generally attributed to neutrally-to-positively buoyant and mechanically strong lithosphere that shields the cratonic crust from underlying mantle dynamics. Large portions of the cratonic lithospheres in South America and Africa, however, have experienced significant modification since the Mesozoic, as demonstrated by widespread Cretaceous uplift and volcanism, present-day high topography, thin crust, and the presence of seismically fast but neutrally buoyant upper-mantle anomalies. We show that these observations reflect a permanent increase in lithospheric buoyancy due to plume-triggered lithosphere deformation and deep lithospheric loss during Late Cretaceous to early Tertiary, as further evidenced by positive lithosphere residual topography, negative lithosphere residual gravity and the realignment of seismic anisotropy in the cratonic roots. Lithosphere in these regions has been thermally reestablished since then, as confirmed by its present-day low heat flow and high seismic velocities. We conclude that lowermost cratonic lithospheres is compositionally denser than the asthenospheric mantle and can be episodically removed when perturbed by underlying mantle dynamics, while the shallower buoyant lithosphere helps to stabilize cratonic crust over billions of years. We further propose that zones where lithosphere was lost would take tens of millions of years to recover thermally, but the density of the new thermal root would remain less than that of the intact root.

On gravity from SST, geoid from Seasat, and plate age and fracture zones in the Pacific

NASA Technical Reports Server (NTRS)

Marsh, B. D.; Marsh, J. G.; Williamson, R. G. (Principal Investigator)

1984-01-01

A composite map produced by combining 90 passes of SST data show good agreement with conventional GEM models. The SEASAT altimeter data were deduced and found to agree with both the SST and GEM fields. The maps are dominated (especially in the east) by a pattern of roughly east-west anomalies with a transverse wavelength of about 2000 km. Comparison with regional bathymetric data shows a remarkedly close correlation with plate age. Most anomalies in the east half of the Pacific could be partly caused by regional differences in plate age. The amplitude of these geoid or gravity anomalies caused by age differences should decrease with absolute plate age, and large anomalies (approximately 3 m) over old, smooth sea floor may indicate a further deeper source within or perhaps below the lithosphere. The possible plume size and ascent velocity necessary to supply deep mantle material to the upper mantle without complete thermal equilibration was considered. A plume emanating from a buoyant layer 100 km thick and 10,000 times less viscous than the surrounding mantle should have a diameter of about 400 km and must ascend at about 10 cm/yr to arrive still anomalously hot in the uppermost mantle.

Petrology of Hualalai volcano, Hawaii: Implication for mantle composition

USGS Publications Warehouse

Clague, D.A.; Jackson, E.D.; Wright, T.L.

1980-01-01

Hualalai is one of five volcanoes whose eruptions built the island of Hawaii. The historic 1800-1801 flows and the analyzed prehistoric flows exposed at the surface are alkalic basalts except for a trachyte cone and flow at Puu Waawaa and a trachyte maar deposit near Waha Pele. The 1800-1801 eruption produced two flows: the upper Kaupulehu flow and the lower Huehue flow. The analyzed lavas of the two 1800-1801 flows are geochemically identical with the exception of a few samples from the toe of the Huehue flow that appear to be derived from a separate magmatic batch. The analyzed prehistoric basalts are nearly identical to the 1800-1801 flows but include some lavas that have undergone considerable shallow crystal fractionation. The least fractionated alkalic basalts from Hualalai are in equilibrium with mantle olivine (Fo87) indicating that the Hawaiian mantle source region is not unusually iron-rich. The 1800-1801 and analyzed prehistoric basalts can be generated by about 5-10% partial fusion of a garnet-bearing source relatively enriched in the light-rare-earths. The mantle underlying the Hawaiian Islands is chemically and mineralogically heterogeneous before and after extraction of the magmas that make up the volcanoes. ?? 1980 Intern. Association of Volcanology and Chemistry of the Earth's Interior.

Hydrous orthopyroxene-rich pyroxenite source of the Xinkailing high magnesium andesites, Western Liaoning: Implications for the subduction-modified lithospheric mantle and the destruction mechanism of the North China Craton

NASA Astrophysics Data System (ADS)

Hong, Lu-Bing; Zhang, Yin-Hui; Xu, Yi-Gang; Ren, Zhong-Yuan; Yan, Wen; Ma, Qiang; Ma, Liang; Xie, Wei

2017-06-01

Metasomatism of the lithospheric mantle by subduction-related fluids/melts is recorded in the Early Cretaceous Xinkailing high magnesium andesites (HMAs) from Western Liaoning. Olivine-hosted melt inclusions within the Xinkailing HMAs are alkaline and record a much lower SiO2 content and higher Al2O3 and CaO contents than the sub-alkaline bulk rock compositions. These observed compositional differences between bulk rocks and melt inclusions suggest that a crustally derived, high-SiO2 melt was incorporated in the Xinkailing HMAs within the pre-eruptive magma chamber. The process of this incorporation accounts for the compositional differences between upper (HMAs) and lower (high magnesium basalts) successions of the Yixian Formation. Olivine phenocrysts also record unusually high Ni and Ni/MgO contents with high Fo values. Based on the fact that bulk rocks record low Ni contents, whereas olivine crystals record a steep correlation between Fo and Ni and low CaO and CaO/FeO contents, in addition to the likely considerable depression of the olivine liquidus temperature, we argue that a hydrous (2-6% H2O) orthopyroxene-rich pyroxenite source was formed by the reaction between subducted slab-released SiO2-rich fluids and overlying mantle peridotite. We further propose that during a series of Phanerozoic successive subduction events around the Eastern NCC, a significant amount of water may have been transported to the lithospheric mantle, thus lowering its viscosity and ultimately destabilizing the cratonic lithosphere. Hydrous experiments data (circles filled by yellow color) used to parameterize the equation after screened several data significantly deviates from the line (circles without color). Data source: Gaetani and Grove (1998); Almeev et al. (2007); Médard and Grove (2008); Tenner et al. (2009); Mitchell and Grove (2015).

Two mantle sources, two plumbing systems: Tholeiitic and alkaline magmatism of the Maymecha River basin, Siberian flood volcanic province

USGS Publications Warehouse

Arndt, N.; Chauvel, C.; Czamanske, G.; Fedorenko, V.

1998-01-01

Rocks of two distinctly different magma series are found in a ???4000-m-thick sequence of lavas and tuffs in the Maymecha River basin which is part of the Siberian flood-volcanic province. The tholeiites are typical low-Ti continental flood basalts with remarkably restricted, petrologically evolved compositions. They have basaltic MgO contents, moderate concentrations of incompatible trace elements, moderate fractionation of incompatible from compatible elements, distinct negative Ta(Nb) anomalies, and ??Nd values of 0 to + 2. The primary magmas were derived from a relatively shallow mantle source, and evolved in large crustal magma chambers where they acquired their relatively uniform compositions and became contaminated with continental crust. An alkaline series, in contrast, contains a wide range of rock types, from meymechite and picrite to trachytes, with a wide range of compositions (MgO from 0.7 to 38 wt%, SiO2 from 40 to 69 wt%, Ce from 14 to 320 ppm), high concentrations of incompatible elements and extreme fractionation of incompatible from compatible elements (Al2O3/TiO2 ??? 1; Sm/Yb up to 11). These rocks lack Ta(Nb) anomalies and have a broad range of ??Nd values, from -2 to +5. The parental magmas are believed to have formed by low-degree melting at extreme mantle depths (>200 km). They bypassed the large crustal magma chambers and ascended rapidly to the surface, a consequence, perhaps, of high volatile contents in the primary magmas. The tholeiitic series dominates the lower part of the sequence and the alkaline series the upper part; at the interface, the two types are interlayered. The succession thus provides evidence of a radical change in the site of mantle melting, and the simultaneous operation of two very different crustal plumbing systems, during the evolution of this flood-volcanic province. ?? Springer-Verlag 1998.

Fine-scale structure of the mid-mantle characterised by global stacks of PP precursors

NASA Astrophysics Data System (ADS)

Bentham, H. L. M.; Rost, S.; Thorne, M. S.

2017-08-01

Subduction zones are likely a major source of compositional heterogeneities in the mantle, which may preserve a record of the subduction history and mantle convection processes. The fine-scale structure associated with mantle heterogeneities can be studied using the scattered seismic wavefield that arrives as coda to or as energy preceding many body wave arrivals. In this study we analyse precursors to PP by creating stacks recorded at globally distributed stations. We create stacks aligned on the PP arrival in 5° distance bins (with range 70-120°) from 600 earthquakes recorded at 193 stations stacking a total of 7320 seismic records. As the energy trailing the direct P arrival, the P coda, interferes with the PP precursors, we suppress the P coda by subtracting a best fitting exponential curve to this energy. The resultant stacks show that PP precursors related to scattering from heterogeneities in the mantle are present for all distances. Lateral variations are explored by producing two regional stacks across the Atlantic and Pacific hemispheres, but we find only negligible differences in the precursory signature between these two regions. The similarity of these two regions suggests that well mixed subducted material can survive at upper and mid-mantle depth. To describe the scattered wavefield in the mantle, we compare the global stacks to synthetic seismograms generated using a Monte Carlo phonon scattering technique. We propose a best-fitting layered heterogeneity model, BRT2017, characterised by a three layer mantle with a background heterogeneity strength (ɛ = 0.8%) and a depth-interval of increased heterogeneity strength (ɛ = 1%) between 1000 km and 1800 km. The scalelength of heterogeneity is found to be 8 km throughout the mantle. Since mantle heterogeneity of 8 km scale may be linked to subducted oceanic crust, the detection of increased heterogeneity at mid-mantle depths could be associated with stalled slabs due to increases in viscosity, supporting recent observations of mantle viscosity increases due to the iron spin transition at depths of ∼1000 km.

Paleoproterozoic Collisional Structures in the Hudson Bay Lithosphere Constrained by Multi-Observable Probabilistic Inversion

NASA Astrophysics Data System (ADS)

Darbyshire, F. A.; Afonso, J. C.; Porritt, R. W.

2015-12-01

The Paleozoic Hudson Bay intracratonic basin conceals a Paleoproterozoic Himalayan-scale continental collision, the Trans-Hudson Orogen (THO), which marks an important milestone in the assembly of the Canadian Shield. The geometry of the THO is complex due to the double-indentor geometry of the collision between the Archean Superior and Western Churchill cratons. Seismic observations at regional scale show a thick, seismically fast lithospheric keel beneath the entire region; an intriguing feature of recent models is a 'curtain' of slightly lower wavespeeds trending NE-SW beneath the Bay, which may represent the remnants of more juvenile material trapped between the two Archean continental cores. The seismic models alone, however, cannot constrain the nature of this anomaly. We investigate the thermal and compositional structure of the Hudson Bay lithosphere using a multi-observable probabilistic inversion technique. This joint inversion uses Rayleigh wave phase velocity data from teleseismic earthquakes and ambient noise, geoid anomalies, surface elevation and heat flow to construct a pseudo-3D model of the crust and upper mantle. Initially a wide range of possible mantle compositions is permitted, and tests are carried out to ascertain whether the lithosphere is stratified with depth. Across the entire Hudson Bay region, low temperatures and a high degree of chemical depletion characterise the mantle lithosphere. Temperature anomalies within the lithosphere are modest, as may be expected from a tectonically-stable region. The base of the thermal lithosphere lies at depths of >250 km, reaching to ~300 km depth in the centre of the Bay. Lithospheric stratification, with a more-depleted upper layer, is best able to explain the geophysical data sets and surface observables. Some regions, where intermediate-period phase velocities are high, require stronger mid-lithospheric depletion. In addition, a narrow region of less-depleted material extends NE-SW across the Bay, likely associated with the trace of the THO collision and the entrapment of juvenile material between the highly-depleted Archean cores.

Role of the Yakutat collision and upper mantle dynamics in the present-day tectonics of the North America Northern Cordillera

NASA Astrophysics Data System (ADS)

Mazzotti, S.; Tarayoun, A.; Marechal, A.; Audet, P.

2017-12-01

The Northern Cordillera of North America is a type example of present-day strain distribution across a wide orogeny. Several geodynamic models are proposed to explain this large-scale tectonic activity, with two main end-members: strain transfer from the Yakutat collision zone (orogenic float) and strain transfer from upper mantle convection (lithosphere basal traction). One of the main differences between these is the lithosphere vertical rheology profile: the former requires significant crust - mantle decoupling to allow far field strain transfer, whereas the latter requires a vertically coupled lithosphere. Here we combine recent data across the eastern region of the Northern Cordillera (eastern Alaska, Yukon, western Northwest Territories) to characterize its states of strain rate, stress, and crustal and lithospheric structure, in order to test the role of the Yakutat collision and upper mantle convection in its present-day tectonics. Recent GPS data confirm the radial, east- to northeastward motion of the central Yukon and foreland belt (Mackenzie and Richardson Mountains), albeit at a much lower velocity than previously proposed. This motion is primarily accommodated by E-W to NE-SW shortening, mainly in the foreland belt, and small to near-zero lateral motion on the major Denali and Tintina strike-slip faults. Seismic anisotropy data further suggest that these two major faults, like most of the Yukon Cordillera, have kept their early Cenozoic crustal and upper mantle structures, as shown by the fault-parallel (NW-SE) fast anisotropy orientation. We use these new data, combined with numerical models of strain distribution under various boundary conditions, to provide constraints on the respective role of the Yakutat collision and upper mantle convection in the present-day tectonics. Preliminary results suggest that, whichever the driving mechanism (or combination thereof), the total strain associated with the present-day tectonics must remain small in order to preserve the inherited crustal and mantle fabrics. Such small cumulative strain appears in contradiction with a thin decoupling layer (such as lower crust decoupling in the orogenic float model) and seems more suggestive of distributed shear across a large part of the lithosphere.

Sensitivity analysis of seismic waveforms to upper-mantle discontinuities using the adjoint method

NASA Astrophysics Data System (ADS)

Koroni, Maria; Bozdağ, Ebru; Paulssen, Hanneke; Trampert, Jeannot

2017-09-01

Using spectral-element simulations of wave propagation, we investigated the sensitivity of seismic waveforms, recorded on transverse components, to upper-mantle discontinuities in 1-D and 3-D background models. These sensitivity kernels, or Fréchet derivatives, illustrate the spatial sensitivity to model parameters, of which those for shear wave speed and the surface topography of internal boundaries are discussed in this paper. We focus on the boundaries at 400 and 670 km depth of the mantle transition zone. SS precursors have frequently been used to infer the topography of upper-mantle discontinuities. These seismic phases are underside reflections off these boundaries and are usually analysed in the distance range of 110°-160°. This distance range is chosen to minimize the interference from other waves. We show sensitivity kernels for consecutive time windows at three characteristic epicentral distances within the 110°-160° range. The sensitivity kernels are computed with the adjoint method using synthetic data. From our simulations we can draw three main conclusions: (i) The exact Fréchet derivatives show that in all time windows, and also in those centred on the SS precursors, there is interference from other waves. This explains the difficulty reported in the literature to correct for 3-D shear wave speed perturbations, even if the 3-D structure is perfectly known. (ii) All studies attempting to map the topography of the 400 and 670 km discontinuities to date assume that the traveltimes of SS precursors can be linearly decomposed into a 3-D elastic structure and a topography part. We recently showed that such a linear decomposition is not possible for SS precursors, and the sensitivity kernels presented in this paper explain why. (iii) In agreement with previous work, we show that other parts of the seismograms have greater sensitivity to upper-mantle discontinuities than SS precursors, especially multiply bouncing S waves exploiting the S-wave triplications due to the mantle transition zone. These phases can potentially improve the inference of global topographic variations of the upper-mantle discontinuities in the context of full waveform inversion in a joint inversion for (an)elastic parameters and topography.

Unexpected HIMU-type late-stage volcanism on the Walvis Ridge

NASA Astrophysics Data System (ADS)

Homrighausen, S.; Hoernle, K.; Geldmacher, J.; Wartho, J.-A.; Hauff, F.; Portnyagin, M.; Werner, R.; van den Bogaard, P.; Garbe-Schönberg, D.

2018-06-01

Volcanic activity at many oceanic volcanoes, ridges and plateaus often reawakens after hiatuses of up to several million years. Compared to the earlier magmatic phases, this late-stage (rejuvenated/post-erosional) volcanism is commonly characterized by a distinct geochemical composition. Late-stage volcanism raises two hitherto unanswered questions: Why does volcanism restart after an extended hiatus and what is the origin of this volcanism? Here we present the first 40Ar/39Ar age and comprehensive trace element and Sr-Nd-Pb-Hf isotopic data from seamounts located on and adjacent to the Walvis Ridge in the South Atlantic ocean basin. The Walvis Ridge is the oldest submarine part of the Tristan-Gough hotspot track and is famous as the original type locality for the enriched mantle one (EM I) end member. Consistent with the bathymetric data, the age data indicates that most of these seamounts are 20-40 Myr younger than the underlying or nearby Walvis Ridge basement. The trace element and isotope data reveal a distinct compositional range from the EM I-type basement. The composition of the seamounts extend from the St. Helena HIMU (high time-integrated 238U/204Pb mantle with radiogenic Pb isotope ratios) end member to an enriched (E) Mid-Ocean-Ridge Basalt (MORB) type composition, reflecting a two-component mixing trend on all isotope diagrams. The EMORB end member could have been generated through mixing of Walvis Ridge EM I with normal (N) MORB source mantle, reflecting interaction of Tristan-Gough (EM I-type) plume melts with the upper mantle. The long volcanic quiescence and the HIMU-like geochemical signature of the seamounts are unusual for classical hotspot related late-stage volcanism, indicating that these seamounts are not related to the Tristan-Gough hotspot volcanism. Two volcanic arrays in southwestern Africa (Gibeon-Dicker Willem and Western Cape province) display similar ages to the late-stage Walvis seamounts and also have HIMU-like compositions, suggesting a larger-scale event at ∼77-49 Ma. We propose that the EM I-like mantle plumes rise from the edges of the African Large Low Shear Velocity Province (LLSVP; Tristan-Gough, Discovery and Shona hotspot), whereas the HIMU-dominated intraplate lavas (St. Helena, Gibeon-Dicker Willem and Western Cape province) and the late-stage Walvis seamounts tap material from internal portions of the African LLSVP, suggesting possible lateral and/or vertical chemical zonation of the African LLSVP.

Imaging 3D crustal and upper mantle structures of the Northeast China using local and teleseismic data

NASA Astrophysics Data System (ADS)

Huang, J.

2017-12-01

Northeast China is located in the composite part of Paleo Asia ocean and Pacific ocean Domain, it undergone multi-stage tectonism and has complicated geological structure. In this region, two major geologic and geophysical boundaries are distinct, the NNE-trending North South Gravity Lineament (NSGL) and Tanlu fault. With respect to North China Craton (NCC), Northeast China is more closely adjacent to the subduction zone of Pacific slab. Along the eastern boundary of Northeast China, the subducting Pacific plate approaches depths of 600 km, many deep earthquakes occurred here. This region becomes an ideal place to investigate deep structure related to deep subduction, deep earthquakes as well as intraplate volcanism. In this study, we determined high-resolution three dimensional P- and S-wave velocity models of the crust and upper mantle to 800 km depth by jointly inverting arrival times from local events and relative residuals from teleseismic events. Our results show that main velocity anomalies exhibited block feature and are generally oriented in NE to NNE direction, which is consistent with regional tectonic direction. The NSGL is characterized by a high-velocity (high-V) anomaly belt with a width of approximately 100 km, and the high-V anomaly extents to the bottom of upper mantle or mantle transition zone. The songliao basin, which is located between NSGL and Tanlu fault tectonic boundaries, obvious low-velocity anomaly extends to about depth of 200 km(. Under the Great Xing'an Range on the west side of NSGL, the low velocity extend to the lithosphere. Our results also show that most of deep earthquakes all occurred in deep subduction zone with high-velocity anomaly. Further, we also observed that extensive low velocity exists above deep-earthquakes zones, this result suggests that deep subduction of the Pacific slab maybe affect overlying lithosphere, resulting in the state of molten, semi-molten or high water.This research is supported by the National Science Foundation of China (91114204) and National Key R&D Plan (2017YFC0601406)

S-Wave Velocity Models Under the Saudi Arabian Portable Broadband Deployment: Evidence for Lithospheric Erosion Beneath the Arabian Shield

NASA Astrophysics Data System (ADS)

Julià, J.; Ammon, C. J.; Herrmann, R. B.

2002-12-01

Models of crustal evolution strongly rely on our knowledge on the mineralogical composition of subsurface rocks, as well as pressure and temperature conditions. Direct sampling of subsurface rocks is often not possible, so that constraints have to be placed from indirect estimates of rock properties. Detailed seismic imaging of subsurface rocks has the potential for providing such constraints, and probe the extent at depth of surface geologic observations. In this study, we provide detailed S-wave velocity profiles for the crust and uppermost mantle beneath the Saudi Arabian Portable Broadband Deployment stations. Seismic velocities have been estimated from the joint inversion of receiver functions and fundamental mode group velocities. Receiver functions are sensitive to S-wave velocity contrasts and vertical travel times, and surface-wave dispersion is sensitive to vertical S-wave velocity averages, so that their combination bridge resolution gaps associated with each individual data set. Our resulting models correlate well with surface geology observations in the Arabian Shield and characterize its terranes at depth: the Asir terrane consists of a 10-km thick upper crust of 3.3~km/s overlying a lower crust with shear-wave velocities of 3.7-3.8 km/s; the Afif terrane is made of a 20-km thick upper crust with average velocity of 3.6 km/s and a lower crust with a shear-velocity of about 3.8~km/s; the Nabitah mobile belt has a gradational, 15-km thick upper crust up to 3.6 km/s overlying a gradational lower crust with velocities up to 4.0 km/s. The crust-mantle transition is sharper in terranes of continental affinity and more gradational beneath terranes of oceanic affinity. In the uppermost mantle, our models suggest a thin lid between up to 50-60 km depth overlying a low velocity zone beneath station TAIF, located close to a region of upwelling mantle material. Temperatures in the lid are estimated to be about 1000 C, which are in good agreement with independent xenolith data, and suggest that the lithosphere could be eroded to a thickness as little as 50~km under this station.

Deep and persistent melt layer in the Archaean mantle

NASA Astrophysics Data System (ADS)

Andrault, Denis; Pesce, Giacomo; Manthilake, Geeth; Monteux, Julien; Bolfan-Casanova, Nathalie; Chantel, Julien; Novella, Davide; Guignot, Nicolas; King, Andrew; Itié, Jean-Paul; Hennet, Louis

2018-02-01

The transition from the Archaean to the Proterozoic eon ended a period of great instability at the Earth's surface. The origin of this transition could be a change in the dynamic regime of the Earth's interior. Here we use laboratory experiments to investigate the solidus of samples representative of the Archaean upper mantle. Our two complementary in situ measurements of the melting curve reveal a solidus that is 200-250 K lower than previously reported at depths higher than about 100 km. Such a lower solidus temperature makes partial melting today easier than previously thought, particularly in the presence of volatiles (H2O and CO2). A lower solidus could also account for the early high production of melts such as komatiites. For an Archaean mantle that was 200-300 K hotter than today, significant melting is expected at depths from 100-150 km to more than 400 km. Thus, a persistent layer of melt may have existed in the Archaean upper mantle. This shell of molten material may have progressively disappeared because of secular cooling of the mantle. Crystallization would have increased the upper mantle viscosity and could have enhanced mechanical coupling between the lithosphere and the asthenosphere. Such a change might explain the transition from surface dynamics dominated by a stagnant lid on the early Earth to modern-like plate tectonics with deep slab subduction.

Mantle transition zone, stagnant slab and intraplate volcanism in Northeast Asia

NASA Astrophysics Data System (ADS)

Chen, Chuanxu; Zhao, Dapeng; Tian, You; Wu, Shiguo; Hasegawa, Akira; Lei, Jianshe; Park, Jung-Ho; Kang, Ik-Bum

2017-04-01

3-D P- and S-wave velocity structures of the mantle down to a depth of 800 km beneath NE Asia are investigated using ∼981 000 high-quality arrival-time data of local earthquakes and teleseismic events recorded at 2388 stations of permanent and portable seismic networks deployed in NE China, Japan and South Korea. Our results do not support the existence of a gap (or a hole) in the stagnant slab under the Changbai volcano, which was proposed by a previous study of teleseismic tomography. In this work we conducted joint inversions of both local-earthquake arrival times and teleseismic relative traveltime residuals, leading to a robust tomography of the upper mantle and the mantle transition zone (MTZ) beneath NE Asia. Our joint inversion results reveal clearly the subducting Pacific slab beneath the Japan Islands and the Japan Sea, as well as the stagnant slab in the MTZ beneath the Korean Peninsula and NE China. A big mantle wedge (BMW) has formed in the upper mantle and the upper part of the MTZ above the stagnant slab. Localized low-velocity anomalies are revealed clearly in the crust and the BMW directly beneath the active Changbai and Ulleung volcanoes, indicating that the intraplate volcanism is caused by hot and wet upwelling in the BMW associated with corner flows in the BMW and deep slab dehydration as well.

Alkaline lavas from southern Mendoza, Argentina, extend the Patagonian DUPAL mantle field to the north

NASA Astrophysics Data System (ADS)

Soager, N.; Holm, P. M.; Llambias, E.

2010-12-01

The lavas sampled around Río Colorado ~37°S at the border of Mendoza and Neuquén provinces, Argentina, define an OIB-like end-member composition for the Pleistocene and Holocene activity in the Payún Matrú volcanic field. Although positioned in the far back-arc of the Andes, only a few lavas show signs of involvement of slab fluids or crustal contamination such as relatively high LILEs relative to Nb. The very low La/Nb (~0.66) and Zr/Nb (~5) and high U/Pb (0.3-0.4) of the end-member composition clearly distinguish the source from normal MORB mantle, while high Ba/Nb (~10) and K/Nb (370-400) compared to FOZO and HIMU type OIBs suggest an EM type of mantle. Overall, the trace element patterns of the Río Colorado lavas are similar to the central and north Patagonian intraplate basalts and to South Atlantic E-MORB affected by the Discovery plume and the LOMU component (le Roux et al., 2002, EPSL 203). The isotopic composition of the Río Colorado component has a 206Pb/204Pb = 18.4, 207Pb/204Pb = 15.58, 208Pb/204Pb = 38.3, 87Sr/86Sr = 0.70353 and 143Nd/144Nd = 0.51285. This composition overlaps the central and north Patagonian intraplate basalts in Pb-isotopic space but is slightly less enriched in Sr and Nd-isotopes. It is distinctly different from the FOZO like composition of the south Patagonian intraplate basalts and the nearby Juan Fernandéz plume but similar to the South Atlantic N-MORB and MORB from the southern Chile Ridge segment 4 (Sturm et al., 1999, JGR 104) described as DUPAL type. The DUPAL-MORB type isotopic composition and the plume-like trace element patterns of the Río Colorado lavas suggest the presence of a weak plume beneath the area. The eruption of the large Payún Matrú volcano and the gigantic Pleistocene flood basalts also calls for a thermal anomaly to produce these melts during a weakly compressive tectonic regime with no significant addition of slab fluids. This was supported by Burd et al. (2008, Abstr., 7th Int. Sym. And. Geo.) who recognized a plume-like conductive structure beneath Payún Matrú volcano on an electrical resistivity profile across the Payún Matrú volcanic field. The many Argentine and Chile Ridge EM1 basalts form part of the global DUPAL-anomaly (Hart, 1984, Nature 309) which suggests a common mode of formation of the enriched mantle sources; most likely anciently subducted components in the underlying upper mantle, either in a larger reservoir or as dispersed bodies of material. From there they can rise as small plumes or be entrained in a convecting MORB source mantle.

Evolution of the Archean Mohorovičić discontinuity from a synaccretionary 4.5 Ga protocrust

NASA Astrophysics Data System (ADS)

Hamilton, Warren B.

2013-12-01

This review evaluates and rejects the currently dominant dogmas of geodynamics and geochemistry, which are based on 1950s-1970s assumptions of a slowly differentiating Earth. Evidence is presented for evolution of mantle, crust, and early Moho that began with fractionation of most crustal components, synchronously with planetary accretion, into mafic protocrust by ~ 4.5 Ga. We know little about Hadean crustal geology (> 3.9 Ga) except that felsic rocks were then forming, but analogy with Venus, and dating from the Moon, indicate great shallow disruption by large and small impact structures, including huge fractionated impact-melt constructs, throughout that era. The mantle sample and Archean (< 3.9 Ga) crustal geology integrate well. The shallow mantle was extremely depleted by early removal of thick mafic protocrust, which was the primary source of the tonalite, trondhjemite, and granodiorite (TTG) that dominate preserved Archean crust to its base, and of the thick mafic volcanic rocks erupted on that crust. Lower TTG crust, kept mobile by its high radioactivity and by insulating upper crust, rose diapirically into the upper crust as dense volcanic rocks sagged synformally. The mobile lower crust simultaneously flowed laterally to maintain subhorizontal base and surface, and dragged overlying brittler granite-and-greenstone upper crust. Petrologically required garnet-rich residual protocrust incrementally delaminated, sank through low-density high-mantle magnesian dunite, and progressively re-enriched upper mantle, mostly metasomatically. Archean and earliest Proterozoic craton stabilization and development of final Mohos followed regionally complete early delamination of residual protocrust, variously between ~ 2.9 and 2.2 Ga. Where some protocrust remained, Proterozoic basins, filled thickly by sedimentary and volcanic rocks, developed on Archean crust, beneath which delamination of later residual protocrust continued top-down enrichment of upper mantle. That reenrichment enabled modern-style plate tectonics after ~ 600 Ma, with a transition regime beginning ~ 850 Ma.

Crustal and Upper Mantle Velocity and Q Structures of Mainland China

DTIC Science & Technology

1979-11-01

CLASIFICATION OFTHIS PAGE(117..t- [).(t ntred) with identical source-receiver geometry. The generalized surface wave inversion technique was applied...in the recent past. A particularly unusual crustal and upper mantle structure is found underlying the Tibet Dlateau. AOceSIon For DDC TAB Ubazmnounced...the AIR FORCE OFFICE OF SCIENTIFIC RESEARCH by the GEOPHYSICAL LABORATORY UNIVERSITY OF SOUTHERN CALIFORNIA Contractor: University of Southern

Seismic Structure of the Antarctic Upper Mantle and Transition Zone Unearthed by Full Waveform Adjoint Tomography

NASA Astrophysics Data System (ADS)

Lloyd, A. J.; Wiens, D.; Zhu, H.; Tromp, J.; Nyblade, A.; Anandakrishnan, S.; Aster, R. C.; Huerta, A. D.; Winberry, J. P.; Wilson, T. J.; Dalziel, I. W. D.; Hansen, S. E.; Shore, P.

2017-12-01

The upper mantle and transition zone beneath Antarctica and the surrounding ocean are among the poorest seismically imaged regions of the Earth's interior. Over the last 1.5 decades researchers have deployed several large temporary broadband seismic arrays focusing on major tectonic features in the Antarctic. The broader international community has also facilitated further instrumentation of the continent, often operating stations in additional regions. As of 2016, waveforms are available from almost 300 unique station locations. Using these stations along with 26 southern mid-latitude seismic stations we have imaged the seismic structure of the upper mantle and transition zone using full waveform adjoint techniques. The full waveform adjoint inversion assimilates phase observations from 3-component seismograms containing P, S, Rayleigh, and Love waves, including reflections and overtones, from 270 earthquakes (5.5 ≤ Mw ≤ 7.0) that occurred between 2001-2003 and 2007-2016. We present the major results of the full waveform adjoint inversion following 20 iterations, resulting in a continental-scale seismic model (ANT_20) with regional-scale resolution. Within East Antarctica, ANT_20 reveals internal seismic heterogeneity and differences in lithospheric thickness. For example, fast seismic velocities extending to 200-300 km depth are imaged beneath both Wilkes Land and the Gamburtsev Subglacial Mountains, whereas fast velocities only extend to 100-200 km depth beneath the Lambert Graben and Enderby Land. Furthermore, fast velocities are not found beneath portions of Dronning Maud Land, suggesting old cratonic lithosphere may be absent. Beneath West Antarctica slow upper mantle seismic velocities are imaged extending from the Balleny Island southward along the Transantarctic Mountains front, and broaden beneath the southern and northern portion of the mountain range. In addition, slow upper mantle velocities are imaged beneath the West Antarctic coast extending from Marie Byrd Land to the Antarctic Peninsula. This region of slow velocity only extends to 150-200 km depth beneath the Antarctic Peninsula, while elsewhere it extends to deeper upper mantle depths and possibly into the transition zone as well as offshore, suggesting two different geodynamic processes are at play.

Imaging of Upper-Mantle Upwelling Beneath the Salton Trough, Southern California, by Joint Inversion of Ambient Noise Dispersion Curves and Receiver Functions

NASA Astrophysics Data System (ADS)

Klemperer, S. L.; Barak, S.

2016-12-01

We present a new 2D shear-wave velocity model of the crust and upper-mantle across the Salton Trough, southern California, obtained by jointly inverting our new dataset of receiver functions and our previously published Rayleigh-wave group-velocity model (Barak et al., G-cubed, 2015), obtained from ambient-noise tomography. Our results show an upper-mantle low-velocity zone (LVZ) with Vs ≤4.2 km/s extending from the Elsinore Fault to the Sand Hills Fault, that together bracket the full width of major San Andreas dextral motion since its inception 6 Ma b.p., and underlying the full width of low topography of the Imperial Valley and Salton Trough. The lateral extent of the LVZ is coincident with the lateral extent of an upper-mantle anisotropic region interpreted as a zone of SAF-parallel melt pockets (Barak & Klemperer, Geology, 2016). The shallowest part of the LVZ is 40 km depth, coincident with S-receiver function images. The western part of the LVZ, between the Elsinore and San Jacinto faults (the region of greatest modern dextral slip), appears to continue to significantly greater depth; but a puzzling feature of our preliminary models is that the eastern part of the LVZ, from the San Jacinto Fault to the Sand Hills Fault, appears to be underlain by more-normalvelocity upper mantle (Vs ≥ 4.5 km/s) below 75 km depth. We compare our model to the current SCEC community models CVM-H and CVM-S, and to P-wave velocity models obtained by the active-source Salton Sea Imaging Project (SSIP). The hypothesized lower-crustal low-velocity zone beneath the Salton Trough in our previous model (Barak et al., G-cubed, 2015), there interpreted as a region of partial melt, is not supported by our new modeling. Melt may be largely absent from the lower crust of the Salton trough; but appears required in the upper mantle at depths as shallow as 40 km.

Geochemical Diversity of the Mantle: 50 Years of Acronyms

NASA Astrophysics Data System (ADS)

Hart, S. R.

2014-12-01

50 years ago, Gast, Tilton and Hedge demonstrated that the oceanic mantle is isotopically heterogeneous. 28 years ago, Zindler and Hart formalized the concept of geochemical mantle components, with an attendant, to some, odious, acronym soup. Work on a marriage of mantle geochemistry and dynamics continues unabated. We know unequivocally that the mantle is chemically heterogeneous; we do not know the scale lengths of these heterogeneities. We know unequivocally that these heterogeneities have persisted for eons (Gy); we do not know where they were formed or where they are stored. Through the kind auspices of the Plume Model, we plausibly have access to the whole mantle. The most accessible and well understood mantle reservoir is the upper depleted MORB mantle (DMM). Classically, this mantle was depleted by extraction of oceanic and continental crust from a "chondritic" bulk silicate Earth. In this post-Boyet and Carlson world, the complementary enriched reservoir may instead be hidden in the deepest mantle. In this case, DMM will become an endangered acronym. Hofmann and White (1982) argued that radiogenic Pb mantle (HIMU) is re-cycled ocean crust, and this is a comfortably viable model. It does require some ad hoc chemical manipulations during subduction. Given 2 Gy of aggregate mantle strains, the mafic component in HIMU may be of small length scale (< 50 m), possibly subsumed into the dominant peridotitic lithology. This mantle species is globally widespread. Enriched mantles (EM1 and EM2) almost certainly reflect recycling of enriched continental material. This was splendidly verified by Jackson et al (2007), with 87Sr/86Sr in Samoan EM2 lavas up to 0.721. The lithology and length scale of EM1 and EM2 is unconstrained. EM1 is globally present; EM2 is confined to the SW Pacific hotspots. FOZO is a work in progress; many would like to see it become extinct! The trace element signatures of HIMU and FOZO mantles have been constrained using melting models; in both cases the spidergrams are "enriched" with peaks at Nb-Ta of 2x and 4x bulk silicate earth, respectively, but with quite different shapes. As is typical with OIB, the derived source compositions are incompatible with the isotopic signatures, requiring a fairly recent "enrichment" event (possibly auto-metasomatism).

Constraints on mantle viscosity from convection models with plate motion history

NASA Astrophysics Data System (ADS)

Mao, W.; Zhong, S.

2017-12-01

The Earth's long-wavelength geoid and dynamic topography are mainly controlled by the mantle buoyancy and viscosity structure. Previous dynamical models for the geoid provide constraints on the 1-D mantle viscosity, using mantle buoyancy derived from seismic topography models. However, it is a challenge in these studies on how to convert seismic velocity to density anomalies and mantle buoyancy. Furthermore, these studies provide constraints only on relative viscosity variations but not on absolute magnitude of viscosity. In this study, we formulate time-dependent 3-D spherical mantle convection models with imposed plate motion history and seek constraints on mantle viscosity structure for both its radial relative variations and its absolute magnitude (i.e., Rayleigh number), using the geoid from the convection models. We found that the geoid at intermediate wavelengths of degrees 4-9 is mainly controlled by the subducted slabs in the upper mantle and the upper part of lower mantle that result from subduction from the last 50 Myr or the Cenozoic. To fit the degrees 4-9 geoid, we need viscosity contrast β defined as the ratio of the lower mantle viscosity and the asthenospheric viscosity to be larger than 2000 and Ra to be 1e8 (defined by the Earth's radius). The best fit model leads to 57% variance reduction and 76% correlation between the model and the observations. However, the long-wavelength geoid at degrees 2-3 is controlled by the lower mantle structure which requires much longer time scale to develop, as seen from our modeling. The preferred viscosity structure and Rayleigh number as constrained by the Cenozoic plate motion and the degrees 4-9 geoid no longer provide adequate fit to the geoid in models with the plate motion history for the last 450 Myr. The degrees 4-9 geoid amplitude is smaller for the models with longer plate motion history and a smaller Ra is required to fit the observation. In order to satisfy the relative amplitude between degrees 2-3 and degrees 4-9 geoid, either a gradually increase of viscosity in the upper part of lower mantle or larger thermal expansivity in the lower mantle is needed. We also consider thermo-chemical models to examine the effects of the African and Pacific thermochemical piles (i.e., LLSVPSs) on the geoid and the inferred mantle viscosity and Ra.

Upper mantle Q and thermal structure beneath Tanzania, East Africa from teleseismic P wave spectra

NASA Astrophysics Data System (ADS)

Venkataraman, Anupama; Nyblade, Andrew A.; Ritsema, Jeroen

2004-08-01

We measure P wave spectral amplitude ratios from deep-focus earthquakes recorded at broadband seismic stations of the Tanzania network to estimate regional variation of sublithospheric mantle attenuation beneath the Tanzania craton and the eastern branch of the East African Rift. One-dimensional profiles of QP adequately explain the systematic variation of P wave attenuation in the sublithospheric upper mantle: QP ~ 175 beneath the cratonic lithosphere, while it is ~ 80 beneath the rifted lithosphere. By combining the QP values and a model of P wave velocity perturbations, we estimate that the temperature beneath the rifted lithosphere (100-400 km depth) is 140-280 K higher than ambient mantle temperatures, consistent with the observation that the 410 km discontinuity in this region is depressed by 30-40 km.

Shear velocity structure of central Eurasia from inversion of surface wave velocities

NASA Astrophysics Data System (ADS)

Villaseñor, A.; Ritzwoller, M. H.; Levshin, A. L.; Barmin, M. P.; Engdahl, E. R.; Spakman, W.; Trampert, J.

2001-04-01

We present a shear velocity model of the crust and upper mantle beneath central Eurasia by simultaneous inversion of broadband group and phase velocity maps of fundamental-mode Love and Rayleigh waves. The model is parameterized in terms of velocity depth profiles on a discrete 2°×2° grid. The model is isotropic for the crust and for the upper mantle below 220 km but, to fit simultaneously long period Love and Rayleigh waves, the model is transversely isotropic in the uppermost mantle, from the Moho discontinuity to 220 km depth. We have used newly available a priori models for the crust and sedimentary cover as starting models for the inversion. Therefore, the crustal part of the estimated model shows good correlation with known surface features such as sedimentary basins and mountain ranges. The velocity anomalies in the upper mantle are related to differences between tectonic and stable regions. Old, stable regions such as the East European, Siberian, and Indian cratons are characterized by high upper-mantle shear velocities. Other large high velocity anomalies occur beneath the Persian Gulf and the Tarim block. Slow shear velocity anomalies are related to regions of current extension (Red Sea and Andaman ridges) and are also found beneath the Tibetan and Turkish-Iranian Plateaus, structures originated by continent-continent collision. A large low velocity anomaly beneath western Mongolia corresponds to the location of a hypothesized mantle plume. A clear low velocity zone in vSH between Moho and 220 km exists across most of Eurasia, but is absent for vSV. The character and magnitude of anisotropy in the model is on average similar to PREM, with the most prominent anisotropic region occurring beneath the Tibetan Plateau.

The Evolution of Eastern Himalayan Syntaxis of Tibetan Plateau

NASA Astrophysics Data System (ADS)

Zhang, S.; Wu, T.; Li, M.; Zhang, Y.; Hua, Y.; Zhang, B.

2017-12-01

Indian plate has been colliding with Eurasian plate since 50Ma years ago, resulting in the Tethys extinction, crust shortening and Tibetan plateau uplift. But it is still a debate how the Tibetan Plateau material escaped. This study tries to invert the distributions of dispersion phase velocity and anisotropy in Eastern Himalayan Syntaxis (EHS) based on the seismic data. We focused on the seven sub-blocks around EHS region. Sub-block "EHS" represents EHS corner with high velocity anomalies, significantly compressed in the axle and strike directions. Sub-blocks "LSD", "QTB" and "SP-GZB" are located at its northern areas with compressions also, and connected with low-velocity anomalies in both crustal and upper mantle rocks. Sub-block "ICB" is located at its southern area with low velocity anomaly, and connected with Tengchong volcano. Sub-blocks "SYDB" and "YZB" are located at its eastern areas with high velocity anomalies in both crustal and upper mantle rocks. Our results demonstrated that significant azimuthal anisotropy of crust (t£30s) and upper mantle (30s£t£60s). Crustal anisotropy indicates the orogenic belt matched well with the direction of fast propagation, and upper mantle anisotropy represents the lattic-preferred orientation (LPO) of mantle minerals (e.g. olivine and basalt), indicating the features of subducting Indian plate. Besides, Red River fault is a dextral strike fault, controlling the crustal and mantle migration. There is a narrow zone to be the channel flow of Tibetan crustal materials escaping toward Yunnan area. The evolution of EHS seems constrained by gravity isostatic mechanism. Keywords: Tibetan Plateau; Eastern Himalayan Syntaxis; Red River fault; crustal flow; surface wave; anisotropy

NoMelt Experiment: High-resolution constraints on Pacific upper mantle fabric inferred from radial and azimuthal anisotropy

NASA Astrophysics Data System (ADS)

Russell, J. B.; Gaherty, J. B.; Lin, P. P.; Lizarralde, D.; Collins, J. A.; Hirth, G.; Evans, R. L.

2017-12-01

Observations of seismic anisotropy in the ocean basins are important for constraining deformation and melting processes in the upper mantle. The NoMelt OBS array was deployed on relatively pristine, 70 Ma seafloor in the central Pacific with the aim of constraining upper mantle circulation and the evolution of the lithosphere-asthenosphere system. Surface-waves traversing the array provide a unique opportunity to estimate a comprehensive set of anisotropic parameters. Azimuthal variations in Rayleigh-wave velocity over a period band of 15-180 s suggest strong anisotropic fabric both in the lithosphere and deep in the asthenosphere. High-frequency ambient noise (4-10 s) provides constraints on average VSV and VSH as well as azimuthal variations in both VS and VP in the upper ˜10 km of the mantle. Our best fitting models require radial anisotropy in the uppermost mantle with VSH > VSV by 3 - 7% and as much as 2% radial anisotropy in the crust. Additionally, we find a strong azimuthal dependence for Rayleigh- and Love-wave velocities, with Rayleigh 2θ fast direction parallel to the fossil spreading direction (FSD) and Love 2θ and 4θ fast directions shifted 90º and 45º from the FSD, respectively. These are some of the first direct observations of the Love 2θ and 4θ azimuthal signal, which allows us to directly invert for anisotropic terms G, B, and E in the uppermost Pacific lithosphere, for the first time. Together, these observations of radial and azimuthal anisotropy provide a comprehensive picture of oceanic mantle fabric and are consistent with horizontal alignment of olivine with the a-axis parallel to fossil spreading and having an orthorhombic or hexagonal symmetry.

Forward Modelling of Long-wavelength Magnetic Anomaly Contributions from the Upper Mantle

NASA Astrophysics Data System (ADS)

Idoko, C. M.; Conder, J. A.; Ferre, E. C.; Friedman, S. A.

2016-12-01

Towards the interpretation of the upcoming results from SWARM satellite survey, we develop a MATLAB-based geophysical forward-modeling of magnetic anomalies from tectonic regions with different upper mantle geotherms including subduction zones (Kamchaka island arcs), cratons (Siberian craton), and hotspots (Hawaii hotspots and Massif-central plumes). We constrain the modeling - using magnetic data measured from xenoliths collected across these regions. Over the years, the potency of the upper mantle in contributing to long-wavelength magnetic anomalies has been a topic of debate among geoscientists. However, recent works show that some low geotherm tectonic environments such as forearcs and cratons contain mantle xenoliths which are below the Curie-Temperature of magnetite and could potentially contribute to long-wavelength magnetic anomalies. The modeling pursued here holds the prospect of better understanding the magnetism of the upper mantle, and the resolution of the mismatch between observed long-wavelength anomalies and surface field anomaly upward continued to satellite altitude. The SWARM satellite survey provides a unique opportunity due to its capacity to detect more accurately the depth of magnetic sources. A preliminary model of a hypothetical craton of size 2000km by 1000km by 500km discretized into 32 equal and uniformly distributed prism blocks, using magnetic data from Siberian craton with average natural remanent magnetization value of 0.0829 A/m (randomnly oriented) for a magnetized mantle thickness of 75km, and induced magnetization, varying according to the Curie-Weiss law from surface to 500km depth with an average magnetization of 0.02 A/m, shows that the contributions of the induced and remanent phases of magnetizations- with a total-field anomaly amplitude of 3 nT may impart a measurable signal to the observed long-wavelength magnetic anomalies in low geotherm tectonic environments.

Redox Evolution in Magma Oceans Due to Ferric/Ferrous Iron Partitioning

NASA Astrophysics Data System (ADS)

Schaefer, L.; Elkins-Tanton, L. T.; Pahlevan, K.

2017-12-01

A long-standing puzzle in the evolution of the Earth is that while the present day upper mantle has an oxygen fugacity close to the QFM buffer, core formation during accretion would have occurred at much lower oxygen fugacities close to IW. We present a new model based on experimental evidence that normal solidification and differentiation processes in the terrestrial magma ocean may explain both core formation and the current oxygen fugacity of the mantle without resorting to a change in source material or process. A commonly made assumption is that ferric iron (Fe3+) is negligible at such low oxygen fugacities [1]. However, recent work on Fe3+/Fe2+ ratios in molten silicates [2-4] suggests that the Fe3+ content should increase at high pressure for a given oxygen fugacity. While disproportionation was not observed in these experiments, it may nonetheless be occurring in the melt at high pressure [5]. Therefore, there may be non-negligible amounts of Fe3+ formed through metal-silicate equilibration at high pressures within the magma ocean. Homogenization of the mantle and further partitioning of Fe2+/Fe3+ as the magma ocean crystallizes may explain the oxygen fugacity of the Earth's mantle without requiring additional oxidation mechanisms. We present here models using different parameterizations for the Fe2+/Fe3+ thermodynamic relationships in silicate melts to constrain the evolution of the redox state of the magma ocean as it crystallizes. The model begins with metal-silicate partitioning at high pressure to form the core and set the initial Fe3+ abundance. Combined with previous work on oxygen absorption by magma oceans due to escape of H from H2O [6], we show that the upper layers of solidifying magma oceans should be more oxidized than the lower mantle. This model also suggests that large terrestrial planets should have more oxidized mantles than small planets. From a redox perspective, no change in the composition of the Earth's accreting material needs to be invoked to explain mantle oxygen fugacity, although implications for trace elements in the Earth's core and mantle are still being explored. [1] Ringwood, AE (1979) [2] Armstrong, K et al (2016) LPSC, 2580 [3] Armstrong, K et al (2017) ACCRETE workshop [4] Zhang, HL et al (2017) GCA, 204, 83 [5] Frost, DJ et al (2004) Nature, 428, 409 [6] Schaefer, L et al (2016) ApJ, 829, 63

New Constraints on Upper Mantle Structure Underlying the Diamondiferous Central Slave Craton, Canada, from Teleseismic Body Wave Tomography

NASA Astrophysics Data System (ADS)

Esteve, C.; Schaeffer, A. J.; Audet, P.

2017-12-01

Over the past number of decades, the Slave Craton (Canada) has been extensively studied for its diamondiferous kimberlites. Not only are diamonds a valuable resource, but their kimberlitic host rocks provide an otherwise unique direct source of information on the deep upper mantle (and potentially transition zone). Many of the Canadian Diamond mines are located within the Slave Craton. As a result of the propensity for diamondiferous kimberlites, it is imperative to probe the deep mantle structure beneath the Slave Craton. This work is further motivated by the increase in high-quality broadband seismic data across the Northern Canadian Cordillera over the past decade. To this end we have generated a P and S body wave tomography model of the Slave Craton and its surroundings. Furthermore, tomographic inversion techniques are growing ever more capable of producing high resolution Earth models which capture detailed structure and dynamics across a range of scale lengths. Here, we present preliminary results on the structure of the upper mantle underlying the Slave Craton. These results are generated using data from eight different seismic networks such as the Canadian National Seismic Network (CNSN), Yukon Northwest Seismic Network (YNSN), older Portable Observatories for Lithospheric Analysis and Reseach Investigating Seismicity (POLARIS), Regional Alberta Observatory for Earthquake Studies Network (RV), USArray Transportable Array (TA), older Canadian Northwest Experiment (CANOE), Batholith Broadband (XY) and the Yukon Observatory (YO). This regional model brings new insights about the upper mantle structure beneath the Slave Craton, Canada.

Convection in three dimensions with surface plates - Generation of toroidal flow

NASA Technical Reports Server (NTRS)

Gable, Carl W.; O'Connell, Richard J.; Travis, Bryan J.

1991-01-01

This work presents numerical calculations of mantle convection that incorporate some of the basic observational constraints imposed by plate tectonics. The model is three-dimensional and includes surface plates; it allows plate velocity to change dynamically according to the forces which result from convection. It is shown that plates are an effective means of introducing a toroidal component into the flow field. After initial transients the plate motion is nearly parallel to transform faults and in the direction that tends to minimize the toroidal flow field. The toroidal field decays with depth from its value at the surface; the poloidal field is relatively constant throughout the layer but falls off slightly at the top and bottom boundaries. Layered viscosity increasing with depth causes the toroidal field to decay more rapidly, effectively confining it to the upper, low-viscosity layer. The effect of viscosity layering on the poloidal field is relatively small, which is attributed to its generation by temperature variations distributed throughout the system. The generation of toroidal flow by surface plates would seem to account for the observed nearly equal energy of toroidal and poloidal fields of plate motions on the earth. A low-viscosity region in the upper mantle will cause the toroidal flow to decay significantly before reaching the lower mantle. The resulting concentration of toroidal flow in the upper mantle may result in more thorough mixing there and account for some of the geochemical and isotopic differences proposed to exist between the upper and lower mantles.

Inference of mantle viscosity for depth resolutions of GIA observations

NASA Astrophysics Data System (ADS)

Nakada, Masao; Okuno, Jun'ichi

2016-11-01

Inference of the mantle viscosity from observations for glacial isostatic adjustment (GIA) process has usually been conducted through the analyses based on the simple three-layer viscosity model characterized by lithospheric thickness, upper- and lower-mantle viscosities. Here, we examine the viscosity structures for the simple three-layer viscosity model and also for the two-layer lower-mantle viscosity model defined by viscosities of η670,D (670-D km depth) and ηD,2891 (D-2891 km depth) with D-values of 1191, 1691 and 2191 km. The upper-mantle rheological parameters for the two-layer lower-mantle viscosity model are the same as those for the simple three-layer one. For the simple three-layer viscosity model, rate of change of degree-two zonal harmonics of geopotential due to GIA process (GIA-induced J˙2) of -(6.0-6.5) × 10-11 yr-1 provides two permissible viscosity solutions for the lower mantle, (7-20) × 1021 and (5-9) × 1022 Pa s, and the analyses with observational constraints of the J˙2 and Last Glacial Maximum (LGM) sea levels at Barbados and Bonaparte Gulf indicate (5-9) × 1022 Pa s for the lower mantle. However, the analyses for the J˙2 based on the two-layer lower-mantle viscosity model only require a viscosity layer higher than (5-10) × 1021 Pa s for a depth above the core-mantle boundary (CMB), in which the value of (5-10) × 1021 Pa s corresponds to the solution of (7-20) × 1021 Pa s for the simple three-layer one. Moreover, the analyses with the J˙2 and LGM sea level constraints for the two-layer lower-mantle viscosity model indicate two viscosity solutions: η670,1191 > 3 × 1021 and η1191,2891 ˜ (5-10) × 1022 Pa s, and η670,1691 > 1022 and η1691,2891 ˜ (5-10) × 1022 Pa s. The inferred upper-mantle viscosity for such solutions is (1-4) × 1020 Pa s similar to the estimate for the simple three-layer viscosity model. That is, these analyses require a high viscosity layer of (5-10) × 1022 Pa s at least in the deep mantle, and suggest that the GIA-based lower-mantle viscosity structure should be treated carefully in discussing the mantle dynamics related to the viscosity jump at ˜670 km depth. We also preliminarily put additional constraints on these viscosity solutions by examining typical relative sea level (RSL) changes used to infer the lower-mantle viscosity. The viscosity solution inferred from the far-field RSL changes in the Australian region is consistent with those for the J˙2 and LGM sea levels, and the analyses for RSL changes at Southport and Bermuda in the intermediate region for the North American ice sheets suggest the solution of η670,D > 1022, ηD,2891 ˜ (5-10) × 1022 Pa s (D = 1191 or 1691 km) and upper-mantle viscosity higher than 6 × 1020 Pa s.

How to make a craton

NASA Astrophysics Data System (ADS)

Lee, C.; Chin, E. J.; Erdman, M.; Gaschnig, R. M.; Lederer, G. W.; Savage, P. S.; Zhong, S.; Zincone, S.

2013-12-01

Most Archean cratons are underlain by long-lived 200-300 km thick thermal boundary layers, significantly thicker than oceanic boundary layers, which eventually subduct. The longevity of cratons is perplexing because cold thermal boundary layers should be gravitationally unstable or should thermally erode with time. However, it is agreed that thermal contraction of the cratonic root is compensated by intrinsic compositional buoyancy due to extreme melt depletion. This melt depletion is also thought to have dehydrated the peridotitic residue, strengthening the cratonic mantle, making it resistant to thermo-mechanical erosion. Exactly how cratonic mantle arrives at this chemically buoyant and dehydrated state is unknown. Possible scenarios include formation by melting within a large plume head, accretion of oceanic lithosphere, and accretion of sub-arc mantle. The high degrees of melting would seem to imply formation in hot plume heads, but low Al and heavy rare earth element contents suggest formation in the spinel stability field, implying formation at shallower depths than their current equilibration pressures. We present a new thermobarometer designed to estimate the average melting pressures and temperatures of residual peridotites using whole rock major element compositions. We find that the average melting pressures and temperatures of cratonic peridotites range between 3-4 GPa and 1600 °C. If cratonic peridotites melted via adiabatic decompression, these average pressures represent maximum bounds on the final pressures of melt extraction. Currently, cratonic peridotites derive from 4-7 GPa, implying that the building blocks of peridotites experienced an increase of 1-3 GPa, equivalent to 30-90 km of overburden. Our results thus imply that cratonic mantle most likely formed by tectonic thickening of oceanic or arc lithospheres. But because both arc and oceanic lithospheres might be expected to be wet due to hydrous flux melting and serpentinization, respectively, cratons should be weak. This dilemma can be reconciled by considering the thermal and magmatic evolution of juvenile crust formed in the Archean. Thickening of juvenile crust increases total heat production within the upper part of the nascent lithosphere. With higher heat production in the past, such thickening causes the crust to heat up on timescales of 100 Myr, resulting in a post-orogenic thermal pulse that generates a wave of crustal anatexis and downward heating of the lithospheric mantle, driving off residual water and increasing the kinetics of grain growth, both of which strengthen the lithosphere. Crustal melting will also advectively concentrate radiogenics towards the surface with no observable change in surface heat flow. This upward migration of radiogenics will be followed by cooling of the lower crust and lithospheric mantle, causing further strengthening. With secular cooling of the ambient convecting mantle over much longer timescales, cratons emerge in elevation, leading to erosion of the radiogenically enriched upper crust and leaving behind a continental block with the low surface heat flow characteristic of cratons today. In summary, cratons form by tectonic thickening of cold building blocks, followed by a thermal pulse that further dehydrates and anneals the cratonic mantle. The last step requires sufficient radiogenics to operate, which may explain why cratons formed early in Earth's history.

Seismic anisotropy in the upper mantle beneath the MAGIC array, mid-Atlantic Appalachians: Constraints from SKS splitting and quasi-Love wave propagation

NASA Astrophysics Data System (ADS)

Aragon, J. C.; Long, M. D.; Benoit, M. H.; Servali, A.

2016-12-01

North America's eastern passive continental margin has been modified by several cycles of supercontinent assembly. Its complex surface geology and distinct topography provide evidence of these events, while also raising questions about the extent of deformation in the continental crust, lithosphere, and mantle during past episodes of rifting and mountain building. The Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) is an EarthScope and GeoPRISMS-funded project that involves a collaborative effort among seismologists, geodynamicists, and geomorphologists. One component of the project is a broadband seismic array consisting of 28 instruments in a linear path from coastal Virginia to western Ohio, which operated between October 2013 and October 2016. A key science question addressed by the MAGIC project is the geometry of past lithospheric deformation and present-day mantle flow beneath the Appalachians, which can be probed using observations of seismic anisotropy Here we present observations of SKS splitting and quasi-Love wave arrivals from stations of the MAGIC array, which together constrain seismic anisotropy in the upper mantle. SKS splitting along the array reveals distinct regions of upper mantle anisotropy, with stations in and to the west of the range exhibiting fast directions parallel to the strike of the mountains. In contrast, weak splitting and null SKS arrivals dominate eastern stations in the coastal plain. Documented Love-to-Rayleigh wave scattering for surface waves originating the magnitude 8.3 Illapel, Chile earthquakes in September 2015 provides complementary constraints on anisotropy. These quasi-Love wave arrivals suggest a pronounced change in upper mantle anisotropy at the eastern edge of present-day Appalachian topography. Together, these observations increase our understanding of the extent of lithospheric deformation beneath North America associated with Appalachian orogenesis, as well as the pattern of present-day mantle flow beneath the passive margin.

Interaction of the Cyprus/Tethys Slab With the Mantle Transition Zone Beneath Anatolia

NASA Astrophysics Data System (ADS)

Thompson, D. A.; Rost, S.; Taylor, G.; Cornwell, D. G.

2017-12-01

The geodynamics of the eastern Mediterranean are dominated by northward motion of the Arabian/African continents and subduction of the oldest oceanic crust on the planet along the Aegean and Cyprean trenches. These slabs have previously been imaged using seismic tomography on a continental scale, but detailed information regarding their descent from upper to lower mantle and how they interact with the mantle transition zone have been severely lacking. The Dense Array for North Anatolia (DANA) was a 73 station passive seismic deployment active between 2012-2013 with the primary aim of imaging shallow structure beneath the North Anatolian Fault. However, we exploit the exceptional dataset recorded by DANA to characterise a region where the Cyprus Slab impinges upon the mantle transition zone beneath northern Turkey, providing arguably the most detailed view of a slab as it transits from the upper to lower mantle. We map varying depths and amplitudes of the transition zone seismic discontinuities (`410', `520' and `660') in 3D using over 1500 high quality receiver functions over an area of approximately 200km x 300km. The `410' is observed close to its predicted depth, but the `660' is depressed to >670 km across the entirety of the study region. This is consistent with an accumulation of cold subducted material at the base of the upper mantle, and the presence of a `520' discontinuity in the vicinity of the slab surface also suggests that the slab is present deep within the transition zone. Anomalous low velocity layers above and within the transition zone are constrained and may indicate hydration and ongoing mass/fluid flux between upper and lower mantle in the presence of subduction. The results of the study have implications not only for the regional geodynamics of Anatolia, but also for slab dynamics globally.

Experimental constraints on CO2 and H2O in the Martian mantle and primary magmas

NASA Technical Reports Server (NTRS)

Holloway, John R.; Domanik, Kenneth J.; Cocheo, Peter A.

1993-01-01

We present new data on the stability of hornblende in a Martian mantle composition, on CO2 solubility in iron-rich basaltic magmas, and on the solubility of H2O in an alkalic basaltic magma. These new data are combined with a summary of data from the literature to present a summary of the current state of our estimates of solubilities of H2O and CO2 in probable Martian magmas and the stability of hornblende in a slightly hydrous mantle. The new results suggest that hornblende stability is not sensitive to the Mg/(Mg+Fe) ratio (mg#) of the mantle, that is the results for terrestrial mantle compositions are similar to the more iron-rich Martian composition. Likewise, CO2 solubility in iron-rich tholeiitic basaltic magmas is similar to iron-poor terrestrial compositions. The solubility of H2O has been measured in an alkalic basaltic (basanite) composition for the first time, and it is significantly lower than predicted for models of water solubility in magmas. The lack of mg# dependence observed in hornblende stability and on CO2 solubility that in many cases terrestrial results can be applied to Martian compositions. This conclusion does not apply to other phenomena such as primary magma compositions and major mantle mineral mineralogy.