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Sample records for low-degree mantle melts

  1. Ascent Dynamics of Low Degree Mantle Partial Melts, Constrained from CO2 Solubility Experiments.

    NASA Astrophysics Data System (ADS)

    Moussallam, Y.; Morizet, Y.; Massuyeau, M.; Gaillard, F.

    2014-12-01

    Low degree partial melting of carbonated mantle peridotite generates strongly silica-undersaturated melts containing substantial amount of carbon dioxide (several tens of wt%). Kimberlite melts are one of these volatile-rich mantle product and are believed to ascent through the upper mantle and crust at great speed (~5 to 50 ms-1). The role of volatiles in propelling this ascent has remained poorly quantified due to experimental difficulties in quenching such compositions to a glass. In this study, we used a range of melt compositions in the Si-C-Al-Ca-Mg-Fe-Na-K-O system addressing the chemical complexity needed to closely mimic kimberlitic to carbonatitic characteristics. These melts can, furthermore, be quenched fast enough to produce a glass and be used to determine the CO2 solubility as a function of composition and pressure. Our results suggest that the solubility of CO2 decreases steadily with increasing amount of network forming cations from ~30 wt% CO2 at 12 wt% SiO2 down to ~3 wt% CO2 at 40 wt% SiO2 and that pressure has limited effect on the solubility of CO2 up until very shallow depth (~ last 3 km). This peculiar pressure-solubility relation in kimberlite melt can explain the highly explosive nature of kimberlite magma and characteristic geo-morphological features of their root zone. We present a general CO2 solubility model based on thermodynamic formalism covering a large range of melt composition from 11 to 53 wt% SiO2 spanning the transition from carbonatitic to basaltic melts at pressures up to 1500 MPa.

  2. Evidence from meimechites and other low-degree mantle melts for redox controls on mantle-crust fractionation of platinum-group elements

    PubMed Central

    Mungall, James E.; Hanley, Jacob J.; Arndt, Nicholas T.; Debecdelievre, Anne

    2006-01-01

    Understanding of the geochemistry of the chalcophile elements [i.e., Os, Ir, Ru, Pt, Pd (platinum-group elements), and Au, Cu, Ni] has been informed for at least 20 years by the common assumption that when crust-forming partial melts are extracted from the upper mantle, sulfide liquid in the restite sequesters chalcophile elements until the extent of partial melting exceeds ≈25% and all of the sulfide has been dissolved in silicate melt [Hamlyn, P. R. & Keays, R. R. (1985) Geochim. Cosmochim. Acta 49, 1797–1811]. Here we document very high, unfractionated, chalcophile element concentrations in small-degree partial melts from the mantle that cannot be reconciled with the canonical residual sulfide assumption. We show that the observed high, unfractionated platinum-group element concentrations in small-degree partial melts can be attained if the melting takes place at moderately high oxygen fugacity, which will reduce the amount of sulfide due to the formation of sulfate and will also destabilize residual monosulfide solid solution by driving sulfide melts into the spinel-liquid divariant field. Magmas formed at high oxygen fugacity by small degrees of mantle melting can be important agents for the transfer of chalcophile elements from the upper mantle to the crust and may be progenitors of significant ore deposits of Pt, Pd, and Au. PMID:16908861

  3. Oceanic slab melting and mantle metasomatism.

    PubMed

    Scaillet, B; Prouteau, G

    2001-01-01

    Modern plate tectonic brings down oceanic crust along subduction zones where it either dehydrates or melts. Those hydrous fluids or melts migrate into the overlying mantle wedge trigerring its melting which produces arc magmas and thus additional continental crust. Nowadays, melting seems to be restricted to cases of young (< 50 Ma) subducted plates. Slab melts are silicic and strongly sodic (trondhjemitic). They are produced at low temperatures (< 1000 degrees C) and under water excess conditions. Their interaction with mantle peridotite produces hydrous metasomatic phases such as amphibole and phlogopite that can be more or less sodium rich. Upon interaction the slab melt becomes less silicic (dacitic to andesitic), and Mg, Ni and Cr richer. Virtually all exposed slab melts display geochemical evidence of ingestion of mantle material. Modern slab melts are thus unlike Archean Trondhjemite-Tonalite-Granodiorite rocks (TTG), which suggests that both types of magmas were generated via different petrogenetic pathways which may imply an Archean tectonic model of crust production different from that of the present-day, subduction-related, one.

  4. Continuous eclogite melting and variable refertilisation in upwelling heterogeneous mantle.

    PubMed

    Rosenthal, Anja; Yaxley, Gregory M; Green, David H; Hermann, Joerg; Kovács, István; Spandler, Carl

    2014-08-18

    Large-scale tectonic processes introduce a range of crustal lithologies into the Earth's mantle. These lithologies have been implicated as sources of compositional heterogeneity in mantle-derived magmas. The model being explored here assumes the presence of widely dispersed fragments of residual eclogite (derived from recycled oceanic crust), stretched and stirred by convection in the mantle. Here we show with an experimental study that these residual eclogites continuously melt during upwelling of such heterogeneous mantle and we characterize the melting reactions and compositional changes in the residue minerals. The chemical exchange between these partial melts and more refractory peridotite leads to a variably metasomatised mantle. Re-melting of these metasomatised peridotite lithologies at given pressures and temperatures results in diverse melt compositions, which may contribute to the observed heterogeneity of oceanic basalt suites. We also show that heterogeneous upwelling mantle is subject to diverse local freezing, hybridization and carbonate-carbon-silicate redox reactions along a mantle adiabat.

  5. Continuous eclogite melting and variable refertilisation in upwelling heterogeneous mantle

    PubMed Central

    Rosenthal, Anja; Yaxley, Gregory M.; Green, David H.; Hermann, Joerg; Kovács, István; Spandler, Carl

    2014-01-01

    Large-scale tectonic processes introduce a range of crustal lithologies into the Earth's mantle. These lithologies have been implicated as sources of compositional heterogeneity in mantle-derived magmas. The model being explored here assumes the presence of widely dispersed fragments of residual eclogite (derived from recycled oceanic crust), stretched and stirred by convection in the mantle. Here we show with an experimental study that these residual eclogites continuously melt during upwelling of such heterogeneous mantle and we characterize the melting reactions and compositional changes in the residue minerals. The chemical exchange between these partial melts and more refractory peridotite leads to a variably metasomatised mantle. Re-melting of these metasomatised peridotite lithologies at given pressures and temperatures results in diverse melt compositions, which may contribute to the observed heterogeneity of oceanic basalt suites. We also show that heterogeneous upwelling mantle is subject to diverse local freezing, hybridization and carbonate-carbon-silicate redox reactions along a mantle adiabat. PMID:25130275

  6. Low-Degree Partial Melting Experiments of CR and H Chondrite Compositions: Implications for Asteroidal Magmatism Recorded in GRA 06128 and GRA 06129 T

    NASA Technical Reports Server (NTRS)

    Usui, T.; Jones, John H.; Mittlefehldt, D. W.

    2010-01-01

    Studies of differentiated meteorites have revealed a diversity of differentiation processes on their parental asteroids; these differentiation mechanisms range from whole-scale melting to partial melting without the core formation [e.g., 1]. Recently discovered paired achondrites GRA 06128 and GRA 06129 (hereafter referred to as GRA) represent unique asteroidal magmatic processes. These meteorites are characterized by high abundances of sodic plagioclase and alkali-rich whole-rock compositions, implying that they could originate from a low-degree partial melt from a volatile-rich oxidized asteroid [e.g., 2, 3, 4]. These conditions are consistent with the high abundances of highly siderophile elements, suggesting that their parent asteroid did not segregate a metallic core [2]. In this study, we test the hypothesis that low-degree partial melts of chondritic precursors under oxidizing conditions can explain the whole-rock and mineral chemistry of GRA based on melting experiments of synthesized CR- and H-chondrite compositions.

  7. Melting of subducted basalt at the core-mantle boundary.

    PubMed

    Andrault, Denis; Pesce, Giacomo; Bouhifd, Mohamed Ali; Bolfan-Casanova, Nathalie; Hénot, Jean-Marc; Mezouar, Mohamed

    2014-05-23

    The geological materials in Earth's lowermost mantle control the characteristics and interpretation of seismic ultra-low velocity zones at the base of the core-mantle boundary. Partial melting of the bulk lower mantle is often advocated as the cause, but this does not explain the nonubiquitous character of these regional seismic features. We explored the melting properties of mid-oceanic ridge basalt (MORB), which can reach the lowermost mantle after subduction of oceanic crust. At a pressure representative of the core-mantle boundary (135 gigapascals), the onset of melting occurs at ~3800 kelvin, which is ~350 kelvin below the mantle solidus. The SiO2-rich liquid generated either remains trapped in the MORB material or solidifies after reacting with the surrounding MgO-rich mantle, remixing subducted MORB with the lowermost mantle.

  8. Abyssal Peridotites and Mantle Melting Beneath Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Dick, H. J.; Snow, J. E.; Hellebrand, E.; Shimizu, N.

    2005-12-01

    Studies of abyssal peridotite from ultraslow and slow spreading ridges show significant regional variability; with a strong correlation between the compositions of peridotite averaged by locality and spatially associated MORB reflecting higher degrees of mantle melting near mantle hot spots. Local variability of peridotite compositions, however, is often large, and may equal the regional variability along ocean ridges. The latter is attributed to local melting and melt transport processes such as melt channelization or late-stage melt impregnation in the lithosphere. The observed regional correlation appears only when many samples are averaged to eliminate local and outcrop scale variability. Almost all the peridotites used in these correlations are from transforms, and therefore represent similar thermal and mantle melting histories. Thus, regional differences in mantle composition are preserved. Until recently, little data were available for peridotites away from transforms representing the central mantle environment beneath magmatic segments. This is key, as geophysical and geologic evidence suggest focused melt flow beneath slow spreading ridges. If so, beneath individual magmatic segments there should be a corresponding mantle melting cell in which melt is focused from a broad melting region to a melt transport zone at its mid-point that feeds an overlying crustal magmatic center. High melt fluxes in the transport zone would produce very depleted peridotites stripped of pyroxene by melt-rock reaction during magma ascent. Studies of peridotites far from transforms at ultraslow Gakkel and SW Indian Ridges indicate this is the case: with near-Cpx free intergranular harzburgite and dunite locally abundant in contrast to transform peridotites. Recent mapping of the plutonic foundation of an ancient 35-km long slow spreading ridge segment at the Kane Core Complex also found a narrow 10-km wide zone of focused melt flow through the mantle marked by abundant dunite

  9. Experimental derivation of nepheline syenite and phonolite liquids by partial melting of upper mantle peridotites

    NASA Astrophysics Data System (ADS)

    Laporte, Didier; Lambart, Sarah; Schiano, Pierre; Ottolini, Luisa

    2014-10-01

    Piston-cylinder experiments were performed to characterize the composition of liquids formed at very low degrees of melting of two fertile lherzolite compositions with 430 ppm and 910 ppm K2O at 1 and 1.3 GPa. We used the microdike technique (Laporte et al., 2004) to extract the liquid phase from the partially molten peridotite, allowing us to analyze liquid compositions at degrees of melting F down to 0.9%. At 1.3 GPa, the liquid is in equilibrium with olivine + orthopyroxene + clinopyroxene + spinel in all the experiments; at 1 GPa, plagioclase is present in addition to these four mineral phases up to about 5% of melting (T≈1240 °C). Important variations of liquid compositions are observed with decreasing temperature, including strong increases in SiO2, Na2O, K2O, and Al2O3 concentrations, and decreases in MgO, FeO, and CaO concentrations. The most extreme liquid compositions are phonolites with 57% SiO2, 20-22% Al2O3, Na2O + K2O up to 14%, and concentrations of MgO, FeO, and CaO as low as 2-3%. Reversal experiments confirm that low-degree melts of a fertile lherzolite have phonolitic compositions, and pMELTS calculations show that the amount of phonolite liquid generated at 1.2 GPa increases from 0.3% in a source with 100 ppm K2O to 3% in a source with 2000 ppm K2O. The enrichment in silica and alkalis with decreasing melt fraction is coupled with an increase of the degree of melt polymerization, which has important consequences for the partitioning of minor and trace elements. Thus Ti4+ in our experiments and, by analogy with Ti4+, other highly charged cations, and rare earth elements become less incompatible near the peridotite solidus. Our study brings a strong support to the hypothesis that phonolitic lavas or their plutonic equivalents (nepheline syenites) may be produced directly by partial melting of upper mantle rock-types at moderate pressures (1-1.5 GPa), especially where large domains of the subcontinental lithospheric mantle has been enriched in

  10. The Fate of Sulfur during Mantle Melting - Implications for Sulfur Concentration of OIB versus MORB Sources

    NASA Astrophysics Data System (ADS)

    Ding, S.; Dasgupta, R.

    2015-12-01

    Sulfur is one of the key volatiles that are less affected by degassing and may capture deeper magmatic processes and mantle source characteristics. S content of OIBs is controlled by S solubility of partial melts at sulfide saturation which depends on P, T, fO2 and melt composition (e.g., [1]), S content of the mantle, and fractional crystallization of primary basalts. In this study, we coupled mantle melting and basalt differentiation models with sulfur solubility models and compared our results with the S abundance data of OIBs. The goal was to compare and contrast the efficiency of S extraction from MORB and OIB sources by partial melting and put constraints on the S content of the mantle source. S contents in basalts from Loihi, Samoa, and Galapagos islands range from 800 to 2500 ppm with MgO from 11 to 4 wt.%. Modeling of S solubility change during isobaric fractional crystallization for basalts from these three islands with MELTS and an SCSS parameterization [1] showed that SCSS are distinctly higher than S concentration in the basalts at MgO>8wt.% (e.g., [2-3]). Assuming that there could be ≤20% S6+/SS present in the OIBs derived from a potentially more oxidized source [4], sulfide-undersaturated primitive melts with 700-900 ppm S are required to match the S concentration in the primitive OIBs. Derivation of sulfide undersaturated low-degree melts (Loihi: F=2-4%; [5]; Samoa: F=1-6%; [6]) from a mantle source with potential temperature (TP) of 1420 °C requires that the mantle source S content is ≤50 ppm. For Galapagos spreading center with F~10% [7], ≤100 ppm S is required in the mantle source. While low F constrains low S abundance in OIB mantle, more sulfide could be present in the MORB source (eg. 146±35 ppm S, [8]) owing to higher average extent of melting. The estimated S content for OIB sources above should be the lower bounds since TP in OIB mantle could be hotter and increasing TP increases SCSS significantly because of increased FeO* in the

  11. Mantle Partial Melting Beneath Gakkel Ridge Reflected in the Petrography of Spinel Lherzolites

    NASA Astrophysics Data System (ADS)

    Snow, J. E.; Dick, H.; Buechl, A.; Michael, P.; Hellebrand, E.; Ship Sc Parties HEALY 102-POLARSTERN 59,; Ship Sc Parties HEALY 102-POLARSTERN 59,; Ship Sc Parties HEALY 102-POLARSTERN 59,

    2001-12-01

    One of the main aims of the AMORE expedition to Gakkel Ridge was to investigate the nature of mantle residues of low-degree partial melting. Previous results from a single sample of highly serpentinized Gakkel peridotite were unable to conclusively resolve many of the issues of mantle melting and mantle veining involved (1). We have made a preliminary examination of 46 thin sections and hundreds of hand samples of mantle peridotites made on board PFS POLARSTERN and HEALY in the course of the expedition. Most of these peridotites are altered 60-90%, like most abyssal peridotites. Some however are stunningly fresh, containing no detectable serpentine in thin section. The distribution of mantle rock types is similar to that from other mid-ocean ridges. Dunites are present but rare, in contrast to the SW Indian Ridge oblique spreading center at 12° E, as are plagioclase peridotites, in contrast to their abundance at Molloy Ridge further south on the arctic ridge system. There are two differences between this sample set and those commonly observed on mid-ocean ridges that are of particular note. First is the relative abundance of clinopyroxene. The mean clinopyroxene content and size observed in thin section are both qualitatively greater than is commonly observed in abyssal peridotites. Second, the spinels are more nearly euhedral, more abundant and commonly very pale in color. The pale color is well known to be a sign of low Cr content (and thus high activity of Al) in the residual system. All of these observations suggest a low degree of partial melting in the Gakkel Ridge mantle, in accordance with theoretical predictions. What has not been observed to date in even the largest and freshest samples is any evidence of significant mantle veining. It may be that mantle veins have sufficiently low solidi that they melt out completely without a trace even at the lowest degrees of partial melting. The petrographic evidence however suggests that there never was significant

  12. The importance of the melting process for quantifying mantle heterogeneity

    NASA Astrophysics Data System (ADS)

    Lambart, S.

    2015-12-01

    A variety of data requires that the mantle source for basaltic magmatism is heterogeneous. Thanks to numerous experimental studies, parameterizations are available to model the melting behavior of peridotite and pyroxenite compositions that are thought to be present in the mantle (e.g., 1, 2). Based on these parameterizations, numerous studies have attempted to estimate the proportion of pyroxenites in magmatic sources. However, while almost all melting experiments correspond to a batch melting process, it is likely that oceanic basalts are formed by near fractional melting rather than batch melting (e.g., 3). Due to the limited extent of melting of peridotites under upper mantle conditions, their magmatic productivity and melt compositions are similar for batch and fractional melting (e.g., 4). In contrast, pyroxenites undergo much higher meting degrees during decompression of a heterogeneous, peridotite-rich mantle source. Using pMELTS, I investigated the effect of near-fractional melting of pyroxenite. Results suggest that the nature of the melting process for pyroxenites can significantly affect (1) the melt productivity of pyroxenites and thus their potential contribution in basalt genesis, (2) the major element composition of melts and thus their interaction with the surrounding peridodite, and (3) the concentration of minor elements such as Ni and consequently the estimation of pyroxenite proportion in magma-source (e.g., 5). In particular, calculations imply that the proportion of solid pyroxenite in the magma source is likely to be underestimated using "batch melting" rather than "fractional melting" parameterization. An increase in the pyroxenite proportion may affect the buoyancy of the mixture in the upper mantle and have important geodynamical implications. 1-Katz et al., 2003, GGG 4; 2-Lambart et al., 2013, Lithos 160-161; 3- Hirose & Kawamura, 1994, Geophy. Res. Let 21; 4-Johnson et al., 1990, J. Geophy. Res. 95; 5-Sobolev et al., 2007, Science 316

  13. Experimental Partitioning of Chalcophile Elements between Mantle Silicate Minerals and Basaltic Melt at High Pressures and Temperatures - Implications for Sulfur Geochemistry of Mantle and Crust

    NASA Astrophysics Data System (ADS)

    Dasgupta, R.; Jego, S.; Ding, S.; Li, Y.; Lee, C. T.

    2015-12-01

    The behavior of chalcophile elements during mantle melting, melt extraction, and basalt differentiation is critical for formation of ore deposits and geochemical model and evolution of crust-mantle system. While chalcophile elements are strongly partitioned into sulfides, their behavior with different extent of melting, in particular, in the absence of sulfides, can only be modeled with complete knowledge of the partitioning behavior of these elements between dominant mantle minerals and basaltic melt with or without dissolved sulfide (S2-). However, experimental data on mineral-melt partitioning are lacking for many chalcophile elements. Crystallization experiments were conducted at 3 GPa and 1450-1600 °C using a piston cylinder and synthetic silicate melt compositions similar to low-degree partial melt of peridotite. Starting silicate mixes doped with 100-300 ppm of each of various chalcophile elements were loaded into Pt/graphite double capsules. To test the effect of dissolved sulfur in silicate melt on mineral-melt partitioning of chalcophile elements, experiments were conducted on both sulfur-free and sulfur-bearing (1100-1400 ppm S in melt) systems. Experimental phases were analyzed by EPMA (for major elements and S) and LA-ICP-MS (for trace elements). All experiments produced an assemblage of cpx + melt ± garnet ± olivine ± spinel and yielded new partition coefficients (D) for Sn, Zn, Mo, Sb, Bi, Pb, and Se for cpx/melt, olivine/melt, and garnet/melt pairs. Derived Ds (mineral/basalt) reveal little effect of S2- in the melt on mineral-melt partition coefficients of the measured chalcophile elements, with Ds for Zn, Mo, Bi, Pb decreasing by less than a factor of 2 from S-free to S-bearing melt systems or remaining similar, within error, between S-free and S-bearing melt systems. By combining our data with existing partitioning data between sulfide phases and silicate melt we model the fractionation of these elements during mantle melting and basalt

  14. Dihedral angle of carbonatite melts in mantle residue near the upper mantle and transition zone

    NASA Astrophysics Data System (ADS)

    Ghosh, S. K.; Rohrbach, A.; Schmidt, M. W.

    2015-12-01

    Carbonate melts are thought to be ideal metasomatic agents in the deep upper mantle (Green & Wallace, 1988) and these melts are low in viscosities (10-1-10-3 Pa·s) compared to primitive basalt (101-102 Pa·s), furthermore the ability to form an interconnected grain-edge melt network at low melt fractions (< 1%) make carbonate melts extremely mobile. They are molten at relatively low temperatures and have solidus temperatures hundreds of degrees lower than silicate melts at >3 GPa (Dasgupta et al. 2006, Ghosh et al., 2009), dissolve a number of geochemically incompatible elements much better than silicate melts (Blundy and Dalton, 2000). Previous studies of carbonate melt dihedral angles in olivine-dominated matrices yielded 25-30oat 1-3 GPa, relatively independent of melt composition (Watson et al., 1990) and temperature (Hunter and McKenzie, 1989). Dihedral angles of carbonate melts in contact with deep mantle silicate phases (e.g. garnet, wadsleyite, and ringwoodite) which constitute more than 70 % of the deep upper mantle and transition zone have not been studied yet. We have performed multi-anvil experiments on carbonate-bearing peridotites with 5.0 wt% CO2 from 13.5 to 20 GPa 1550 oC to investigate the dihedral angle of magnesio-carbonatite melts in equilibrium with garnet, olivine (and its high-pressure polymorphs), and clinoenstatite. The dihedral angle of carbonate melts in the deep upper mantle and transition zone is ~30° for majorite garnet and olivine (and its polymorphs) dominated matrices. It does not change with increasing pressure in the range 13.5-20 GPa. Our results suggest that very low melt fractions of carbonatite melt forming in the deep upper mantle and transition zone are interconnected at melt fractions less than 0.01. Consistent with geophysical observations, this could possibly explain low velocity regions in the deep mantle and transition zone.

  15. Impact-Induced Melting of the Martian Mantle

    NASA Astrophysics Data System (ADS)

    Ghods, A.; Arkani-Hamed, J.

    2008-12-01

    A large impact not only creates a giant basin on a planet but also results in considerable melting in the mantle, especially if the impact occurs in the early history of the planet. There are generally 4 stages of melting caused by a large impact: 1) Melting of a major part of the impactor and target material beneath the impact site, due to release of the kinematic energy of the impactor that is largely converted to heat energy; 2) Melting in the upper mantle due to immediate depressurization caused by excavation of crustal material from the impact site. This stage of melting is simultaneous with the excavation process and it usually occurs in the upper mantle because the pre-impact temperature is usually close to, or at, the melting temperature, and the sudden depressurization allows melting to occur; 3) Melting in the upper mantle due to its upwelling in order to achieve isostatic compensation, during which rocks from deeper parts of the upper mantle move to low- pressure upper parts. This process involves appreciable displacement of the mantle material when the resulting excavation volume is very large, and it may take up to a few thousand years to accomplish; 4) Melting in the entire mantle by convection circulations that develop in response to the temperature perturbations in the upper mantle caused by the second and third stages of melting. Depending on the size of the impactor and the pre-impact temperature condition of the mantle, this secondary convection may take a much longer time to develop and it usually results in enormous amount of melt and extensive volcanism, which are by far more important than those associated with the first three stages of melting. We study the secondary convection induced in the Martian mantle by large impacts that created giant basins such as Utopia, Acidalia, Ares, Deadalia, Hellas, Isidis, and Argyre, as well as the giant Borealis impact that likely created the major part of the northern low lands. We investigate two

  16. Helium isotopic evidence for episodic mantle melting and crustal growth.

    PubMed

    Parman, S W

    2007-04-19

    The timing of formation of the Earth's continental crust is the subject of a long-standing debate, with models ranging from early formation with little subsequent growth, to pulsed growth, to steadily increasing growth. But most models do agree that the continental crust was extracted from the mantle by partial melting. If so, such crustal extraction should have left a chemical fingerprint in the isotopic composition of the mantle. The subduction of oceanic crust and subsequent convective mixing, however, seems to have largely erased this record in most mantle isotopic systems (for example, strontium, neodymium and lead). In contrast, helium is not recycled into the mantle because it is volatile and degasses from erupted oceanic basalts. Therefore helium isotopes may potentially preserve a clearer record of mantle depletion than recycled isotopes. Here I show that the spectrum of 4He/3He ratios in ocean island basalts appears to preserve the mantle's depletion history, correlating closely with the ages of proposed continental growth pulses. The correlation independently predicts both the dominant 4He/3He peak found in modern mid-ocean-ridge basalts, as well as estimates of the initial 4He/3He ratio of the Earth. The correspondence between the ages of mantle depletion events and pulses of crustal production implies that the formation of the continental crust was indeed episodic and punctuated by large, potentially global, melting events. The proposed helium isotopic evolution model does not require a primitive, undegassed mantle reservoir, and therefore is consistent with whole mantle convection.

  17. Hot, shallow mantle melting under the Cascades volcanic arc

    USGS Publications Warehouse

    Elkins Tanton, Linda T.; Grove, Timothy L.; Donnelly-Nolan, Julie

    2001-01-01

    Melting occurs at progressively greater depths and higher temperatures from west to east across the Cascades volcanic arc in northern California, as demonstrated by compositional variations observed in high-alumina olivine tholeiites. The lavas studied erupted from seven vents defining a 75-km-long, east-west transect across the arc, from near Mount Shasta to east of Medicine Lake volcano. The increase in melting depth across the arc parallels modeled isotherms in the mantle wedge and does not parallel the inferred dip of the slab. The depth of mantle melting at which the high-alumina olivine tholeiites were created is ∼36 km at the western end of the transect and 66 km at the eastern end. The very high temperatures of dry melting so close to the crust indicate a transitory condition of the mantle.

  18. Hot, shallow mantle melting under the Cascades volcanic arc

    NASA Astrophysics Data System (ADS)

    Elkins Tanton, Linda T.; Grove, Timothy L.; Donnelly-Nolan, Julie

    2001-07-01

    Melting occurs at progressively greater depths and higher temperatures from west to east across the Cascades volcanic arc in northern California, as demonstrated by compositional variations observed in high-alumina olivine tholeiites. The lavas studied erupted from seven vents defining a 75-km-long, east-west transect across the arc, from near Mount Shasta to east of Medicine Lake volcano. The increase in melting depth across the arc parallels modeled isotherms in the mantle wedge and does not parallel the inferred dip of the slab. The depth of mantle melting at which the high-alumina olivine tholeiites were created is ˜36 km at the western end of the transect and 66 km at the eastern end. The very high temperatures of dry melting so close to the crust indicate a transitory condition of the mantle.

  19. Zn isotopic heterogeneity in the mantle: A melting control?

    NASA Astrophysics Data System (ADS)

    Doucet, Luc S.; Mattielli, Nadine; Ionov, Dmitri A.; Debouge, Wendy; Golovin, Alexander V.

    2016-10-01

    We present new Zn elemental and isotope data on seventeen fertile and refractory mantle peridotite xenoliths. Eleven fertile peridotites are garnet and spinel lherzolites from Vitim and Tariat (Siberia and Mongolia) and represent some of the most pristine fertile peridotites available. Six refractory peridotites are spinel harzburgites from the Udachnaya kimberlite (Siberian craton) that are nearly pristine residues of high-degree polybaric melting at high pressure (7-4 GPa). Geochemical data suggest that Zn isotopic compositions in the peridotites have not been affected by post-melting processes such as metasomatism, contamination by the host-magmas or alteration. The fertile peridotites have uniform Zn concentrations (59 ± 2 ppm) and Zn isotopic compositions with δ66Zn (relative to JMC-Lyon-03-0749l) = +0.30 ± 0.03‰ consistent with the Bulk Silicate Earth estimates of δ66Zn = +0.28 ± 0.05‰ (Chen et al., 2013). The refractory peridotites have Zn concentrations ranging from 30 to 48 ppm and δ66Zn from + 0.10 ± 0.01 ‰ to + 0.18 ± 0.01 ‰ with an average of + 0.14 ± 0.03 ‰. Our data suggest that the lithospheric mantle has a heterogeneous Zn isotopic composition. Modeling of Zn isotope partitioning during partial melting of fertile mantle suggests that high degrees of melt extraction (>30%) may significantly fractionate Zn isotopes (up to 0.16‰) and that during mantle melting, Zn concentrations and isotopic compositions are mainly controlled by the stability of clinopyroxene and garnet within the melting residue. Because the stability of clinopyroxene and garnet is mainly pressure dependent we suggest that both the depth and the degrees of melt extraction may control Zn isotope fractionation during mantle melting.

  20. Geophysical constraints on partial melt in the upper mantle

    SciTech Connect

    Shankland, T.J.; O'Connell, R.J.; Waff, H.S.

    1981-08-01

    This paper adresses the conditions under which partial melt can exist in the mantle in order to be observed as a geophysical 'anomaly'. Typical observed anomalies are high electrical conductivity of the order of 0.1 S/m or greater, velocity decreases of 7--10%, seismic Q values less than 100, and a frequency band for seismic effects in the region mear 1 Hz. Existing theories of electrical conduction in partial melts and of frequency-dependent seismic properties together with recent measurements of melt electrical conductivity, viscosity, and partial melt texture can be used to establish requirements for melt to be observed by geophysical methods. From electrical anomalies, mainly sensitive to melt volume and its interconnection, one can require a minimum melt fraction of several percent at temperatures close to the solidus (1150/sup 0/--1300/sup 0/C). However, seismic models demand only a small volume in very flattened shapes (aspect ratio approx. =0.001, melt fraction approx.0.1%). Further, if melt configuration permits seismic dissipation in bulk, that is, there exist flattened voids intersecting more or less equant voids, then it is possible to infer melt fractions for elastic anomalies that are consistent with the several percent required for electrical anomalies. Observed equilibrium textures of partly melted peridotite together with inferred melt-solid surface energies suggest that melt on a grain size scale in a gravitational field segregates into a strongly anisotropic pattern. Thus if partial melt causes mantle geophysical anomalies, it should exist in a variety of void shapes and probably of sizes. While the association of electrical and elastic anomalies with indications of reduced density, volcanism, and high heat flow makes the hypothesis of partial melting an attractive explanation, the minimum physical requirement is for existence of relatively high temperature.

  1. Isotopic constraints of mantle derived carbonatitic melts from Calatrava, Spain

    NASA Astrophysics Data System (ADS)

    Humphreys, E. R.; Bailey, K.; Hawkesworth, C. J.; Wall, F.; Avanzinelli, R.

    2010-12-01

    Carbonatite volcanism is typically associated both spatially and temporally with alkaline, ultramafic volcanism (Woolley & Church, 2005). Recent discoveries in Calatrava, Spain illustrate the activity of carbonatite in the source melts of leucitite volcanism. Melilitite pyroclastic lapilli tuffs also show a clear association with volcanic carbonate. Carbonatitic activity has been shown to initiate at depths greater than 100km (Humphreys et al., 2010) despite a maximum estimate of lithospheric thickness of 80km. The presence of aragonite and abundant mantle xenoliths in many deposits are clear indication of the rapid emplacement rates of such magmas. Carbonatitic activity in the source of the leucitite melts is indicated by carbonate inclusions within olivine xenocrysts and the presence of occasional carbonatite lenses. The composition of lead and strontium isotope ratios in the bulk rock, and spatially resolved analysis of carbonate from the groundmass and from inclusions demonstrates a genetic affinity between the inclusions and the related bulk rock composition. Lead and strontium isotopic analysis suggest that such melts do not represent the composition of convecting asthenospheric mantle. 87Sr/86Sr (0.7055-0.7068) values are higher than those of MORB and most OIB. Lead isotope ratios show a trend displaced to higher 207Pb/204Pb relative to MORB and OIB. Carbonate inclusions have less radiogenic lead values than the more radiogenic bulk rocks. Our data indicate that carbonatitic activity in the mantle is intrinsic in the generation of the leucitite lava. However, petrographic and isotopic evidence suggest a complex melt history. Olivine xenocrysts are not in equilibrium with the host leucitite, despite inclusions within olivine showing an isotopic affinity to the bulk rock. We suggest that in this example, alkaline magmatism was induced by the presence of CO2 in the mantle source. Isotopic evidence shows that the mantle producing such melts was not

  2. Evolved Rocks in Ocean Islands Formed by Melting of Metasomatized Mantle

    NASA Astrophysics Data System (ADS)

    Ashwal, L. D.; Torsvik, T. H.; Horvath, P.; Harris, C.; Webb, S. J.; Werner, S. C.; Corfu, F.

    2015-12-01

    Evolved rocks like trachyte occur as minor components of many plume-related basaltic ocean islands (e.g. Hawaii, Gran Canaria, Azores, Réunion), and are typically interpreted as products of extreme fractional crystallization from broadly basaltic magmas. Trachytes from Mauritius (Indian Ocean) suggest otherwise. Here, 6.8 Ma nepheline-bearing trachytes (SiO2 ~63%, Na2O + K2O ~12%) are enriched in all incompatible elements except Ba, Sr and Eu, which show prominent negative anomalies. Initial eNd values cluster at 4.03 ± 0.15 (n = 13), near the lower end of the range for Mauritian basalts (eNd = 3.70 - 5.75), but initial Sr is highly variable (ISr = 0.70408 - 0.71034) suggesting secondary deuteric alteration. Fractional crystallization models starting with a basaltic parent fail, because when plagioclase joins olivine in the crystallizing assemblage, residual liquids become depleted in Al2O3, produce no nepheline, and do not approach trachytic compositions. Mauritian basalts and trachytes do not fall near the ends of known miscibility gaps, eliminating liquid immiscibility processes. Partial melting of extant gabbroic bodies, either from the oceanic crust or from Réunion plume-related magmas should yield quartz-saturated melts different from the critically undersaturated Mauritian trachytes. A remaining possibility is that the trachytes represent direct, small-degree partial melts of fertile, perhaps metasomatized mantle. This is supported by the presence of trachytic glasses in many mantle xenoliths, and experimental results show that low-degree trachytic melts can be produced from mantle peridotites even under anhydrous conditions. If some feldspar is left behind as a residual phase, this would account for the negative Ba, Sr and Eu anomalies observed in Mauritian trachytes. Two trachyte samples that are less depleted in these elements contain xenocrysts of anorthoclase, Al-rich cpx and Cl-rich kaersutite that are out of equilibrium with host trachyte magmas

  3. Melting the hydrous, subarc mantle: the origin of primitive andesites

    NASA Astrophysics Data System (ADS)

    Mitchell, Alexandra L.; Grove, Timothy L.

    2015-08-01

    This experimental study is the first comprehensive investigation of the melting behavior of an olivine + orthopyroxene ± spinel—bearing fertile mantle (FM) composition as a function of variable pressure and water content. The fertile composition was enriched with a metasomatic slab component of ≤0.5 % alkalis and investigated from 1135 to 1470 °C at 1.0-2.0 GPa. A depleted lherzolite with 0.4 % alkali addition was also studied from 1225 to 1240 °C at 1.2 GPa. Melts of both compositions were water-undersaturated: fertile lherzolite melts contained 0-6.4 wt% H2O, and depleted lherzolite melts contained ~2.5 wt% H2O. H2O contents of experimental glasses are measured using electron microprobe, secondary ion mass spectrometry, and synchrotron-source reflection Fourier transform infrared spectroscopy, a novel technique for analyzing H2O in petrologic experiments. Using this new dataset in conjunction with results from previous hydrous experimental studies, a thermobarometer and a hygrometer-thermometer are presented to determine the conditions under which primitive lavas were last in equilibration with the mantle. These predictive models are functions of H2O content and pressure, respectively. A predictive melting model is also presented that calculates melt compositions in equilibrium with an olivine + orthopyroxene ± spinel residual assemblage (harzburgite). This model quantitatively predicts the following influences of H2O on mantle lherzolite melting: (1) As melting pressure increases, melt compositions become more olivine-normative, (2) as melting extent increases, melt compositions become depleted in the normative plagioclase component, and (3) as melt H2O content increases, melts become more quartz-normative. Natural high-Mg# [molar Mg/(Mg + Fe2+)], high-MgO basaltic andesite and andesite lavas—or primitive andesites (PAs)—contain high SiO2 contents at mantle-equilibrated Mg#s. Their compositional characteristics cannot be readily explained by melting

  4. The fate of sulfide during decompression melting of peridotite - implications for sulfur inventory of the MORB-source depleted upper mantle

    NASA Astrophysics Data System (ADS)

    Ding, Shuo; Dasgupta, Rajdeep

    2017-02-01

    Magmatism at mid ocean ridges is one of the main pathways of S outflux from deep Earth to the surface reservoirs and is a critical step in the global sulfur cycle, yet our understanding of the behavior of sulfide during decompression melting of the upper mantle is incomplete. In order to constrain the sulfur budget of the mantle and reconcile the sulfur and chalcophile element budget of mantle partial melts parental to primitive mid-ocean ridge basalts (MORBs), here we developed a model to describe the behavior of sulfide and Cu during decompression melting by combining the pMELTS thermodynamic model and empirical sulfur contents at sulfide concentration (SCSS) models, taking into account the effect of the presence of Ni and Cu in sulfides on SCSS of mantle-derived melts. Calculation of SCSS along melting adiabat at mantle potential temperature of 1380 °C with variable initial S content in the mantle indicates that the complete consumption or partial survival of sulfide in the melting residue depends on initial S content and degree of melting. Primitive MORBs (Mg# > 60) with S and Cu mostly concentrated in 800-1000 ppm and 80-120 ppm are likely mixture of sulfide undersaturated high degree melts and sulfide saturated low degree melts derived from depleted peridotite containing 100-200 ppm S. Model calculations to capture the effects of variable mantle potential temperatures (1280-1420 °C) indicate that for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle owing to the effect of temperature in enhancing sulfide solubility in silicate melt, and higher mantle temperature stabilizing partial melt with higher FeO* and lower SiO2 and Al2O3, all of which generally enhance sulfide solubility. However, sulfide can still be exhausted by ∼ 10- 15% melting with bulk S of 100-150 ppm in the mantle when TP is as low as 1300 °C. We also show that although variation of DCuperidotite/ melt and initial Cu in the

  5. Chalcophile element partitioning between sulfide phases and hydrous mantle melt: Applications to mantle melting and the formation of ore deposits

    NASA Astrophysics Data System (ADS)

    Li, Yuan

    2014-11-01

    Understanding the geochemical behavior of chalcophile elements in magmatic processes is hindered by the limited partition coefficients between sulfide phases and silicate melt, in particular at conditions relevant to partial melting of the hydrated, metasomatized upper mantle. In this study, the partitioning of elements Co, Ni, Cu, Zn, As, Mo, Ag, and Pb between sulfide liquid, monosulfide solid solution (MSS), and hydrous mantle melt has been investigated at 1200 °C/1.5 GPa and oxygen fugacity ranging from FMQ-2 to FMQ+1 in a piston-cylinder apparatus. The determined partition coefficients between sulfide liquid and hydrous mantle melt are: 750-1500 for Cu; 600-1200 for Ni; 35-42 for Co; 35-53 for Pb; and 1-2 for Zn, As, and Mo. The partition coefficients between MSS and hydrous mantle melt are: 380-500 for Cu; 520-750 for Ni; ∼50 for Co; <0.5 for Zn; 0.3-6 for Pb; 0.1-2 for As; 1-2 for Mo; and >34 for Ag. The variation of the data is primarily due to differences in oxygen fugacity. These partitioning data in conjunction with previous data are applied to partial melting of the upper mantle and the formation of magmatic-hydrothermal Cu-Au deposits and magmatic sulfide deposits. I show that the metasomatized arc mantle may no longer contain sulfide after >10-14% melt extraction but is still capable of producing the Cu concentrations in the primitive arc basalts, and that the comparable Cu concentrations in primitive arc basalts and in MORB do not necessarily imply similar oxidation states in their source regions. Previous models proposed for producing Cu- and/or Au-rich magmas have been reassessed, with the conclusions summarized as follows. (1) Partial melting of the oxidized (fO2 > FMQ), metasomatized arc mantle with sulfide exhaustion at degrees >10-14% may not generate Cu-rich, primitive arc basalts. (2) Partial melting of sulfide-bearing cumulates in the root of thickened lower continental crust or lithospheric mantle does not typically generate Cu- and

  6. A benchmark initiative on mantle convection with melting and melt segregation

    NASA Astrophysics Data System (ADS)

    Schmeling, Harro; Dannberg, Juliane; Dohmen, Janik; Kalousova, Klara; Maurice, Maxim; Noack, Lena; Plesa, Ana; Soucek, Ondrej; Spiegelman, Marc; Thieulot, Cedric; Tosi, Nicola; Wallner, Herbert

    2016-04-01

    In recent years a number of mantle convection models have been developed which include partial melting within the asthenosphere, estimation of melt volumes, as well as melt extraction with and without redistribution at the surface or within the lithosphere. All these approaches use various simplifying modelling assumptions whose effects on the dynamics of convection including the feedback on melting have not been explored in sufficient detail. To better assess the significance of such assumptions and to provide test cases for the modelling community we carry out a benchmark comparison. The reference model is taken from the mantle convection benchmark, cases 1a to 1c (Blankenbach et al., 1989), assuming a square box with free slip boundary conditions, the Boussinesq approximation, constant viscosity and Rayleigh numbers of 104 to 10^6. Melting is modelled using a simplified binary solid solution with linearly depth dependent solidus and liquidus temperatures, as well as a solidus temperature depending linearly on depletion. Starting from a plume free initial temperature condition (to avoid melting at the onset time) five cases are investigated: Case 1 includes melting, but without thermal or dynamic feedback on the convection flow. This case provides a total melt generation rate (qm) in a steady state. Case 2 is identical to case 1 except that latent heat is switched on. Case 3 includes batch melting, melt buoyancy (melt Rayleigh number Rm) and depletion buoyancy, but no melt percolation. Output quantities are the Nusselt number (Nu), root mean square velocity (vrms), the maximum and the total melt volume and qm approaching a statistical steady state. Case 4 includes two-phase flow, i.e. melt percolation, assuming a constant shear and bulk viscosity of the matrix and various melt retention numbers (Rt). These cases are carried out using the Compaction Boussinseq Approximation (Schmeling, 2000) or the full compaction formulation. For cases 1 - 3 very good agreement

  7. Experimental evidence supports mantle partial melting in the asthenosphere.

    PubMed

    Chantel, Julien; Manthilake, Geeth; Andrault, Denis; Novella, Davide; Yu, Tony; Wang, Yanbin

    2016-05-01

    The low-velocity zone (LVZ) is a persistent seismic feature in a broad range of geological contexts. It coincides in depth with the asthenosphere, a mantle region of lowered viscosity that may be essential to enabling plate motions. The LVZ has been proposed to originate from either partial melting or a change in the rheological properties of solid mantle minerals. The two scenarios imply drastically distinct physical and geochemical states, leading to fundamentally different conclusions on the dynamics of plate tectonics. We report in situ ultrasonic velocity measurements on a series of partially molten samples, composed of mixtures of olivine plus 0.1 to 4.0 volume % of basalt, under conditions relevant to the LVZ. Our measurements provide direct compressional (V P) and shear (V S) wave velocities and constrain attenuation as a function of melt fraction. Mantle partial melting appears to be a viable origin for the LVZ, for melt fractions as low as ~0.2%. In contrast, the presence of volatile elements appears necessary to explaining the extremely high V P/V S values observed in some local areas. The presence of melt in LVZ could play a major role in the dynamics of plate tectonics, favoring the decoupling of the plate relative to the asthenosphere.

  8. Experimental evidence supports mantle partial melting in the asthenosphere

    PubMed Central

    Chantel, Julien; Manthilake, Geeth; Andrault, Denis; Novella, Davide; Yu, Tony; Wang, Yanbin

    2016-01-01

    The low-velocity zone (LVZ) is a persistent seismic feature in a broad range of geological contexts. It coincides in depth with the asthenosphere, a mantle region of lowered viscosity that may be essential to enabling plate motions. The LVZ has been proposed to originate from either partial melting or a change in the rheological properties of solid mantle minerals. The two scenarios imply drastically distinct physical and geochemical states, leading to fundamentally different conclusions on the dynamics of plate tectonics. We report in situ ultrasonic velocity measurements on a series of partially molten samples, composed of mixtures of olivine plus 0.1 to 4.0 volume % of basalt, under conditions relevant to the LVZ. Our measurements provide direct compressional (VP) and shear (VS) wave velocities and constrain attenuation as a function of melt fraction. Mantle partial melting appears to be a viable origin for the LVZ, for melt fractions as low as ~0.2%. In contrast, the presence of volatile elements appears necessary to explaining the extremely high VP/VS values observed in some local areas. The presence of melt in LVZ could play a major role in the dynamics of plate tectonics, favoring the decoupling of the plate relative to the asthenosphere. PMID:27386548

  9. Kinetics of melt migration in upper mantle-type rocks

    NASA Astrophysics Data System (ADS)

    Riley, G. N.; Kohlstedt, D. L.

    1991-08-01

    Experiments have been performed to determine the permeability of an aggregate of olivine plus a silicate melt, as well as the viscosity of the matrix. Melt migration couples were formed between discs composed of olivine with about 12% of a synthetic potassium-aluminum silicate glass and discs of polycrystalline San Carlos olivine. Four melt infiltration experiments were carried out at temperatures between 1050 and 1255°C at 300 MPa in a gas-medium apparatus; each couple was held at the experimental conditions for 2 h. At temperature, capillary forces cause the molten glass to infiltrate into the dunite along triple junctions because the dihedral angle is less than 60°. In order to analyze the resulting melt migration profiles, the coupled differential equations governing melt migration via "porous flow" driven by capillary forces and resisted by compaction/dilation of the matrix were solved numerically. The effects of dihedral angle, melt fraction exponent and amount of the melt in the source region on the spatial and temporal evolution of the melt distribution were investigated with numerical simulations for the initial and boundary conditions imposed by the experimental geometry. The permeability of the aggregate and the viscosity of the matrix were determined by comparison of the melt migration profiles obtained from the experiments with those generated from simulation. The permeability of the partially molten aggregate increased approximately linearly with increasing melt fraction and, at 1255°C, the permeability of the rock and viscosity of the olivine are about 8 × 10 -16 m 2 and 8 × 10 9 Pa s, respectively, for a grain size of 4.2 μm and a melt fraction of 0.145. These results predict a relatively high permeability at low melt fractions, indicating that only very small amounts of melt (˜ 0.1%) could be maintained in a dunitic mantle.

  10. Melting of the Primitive Mercurian Mantle, Insights into the Origin of Its Surface Composition

    NASA Technical Reports Server (NTRS)

    Boujibar, A.; Righter, K.; Rapp, J. F.; Ross, D. K.; Pando, K. M.; Danielson, L. R.; Fontaine, E.

    2016-01-01

    Recent findings of the MESSENGER mission on Mercury have brought new evidence for its reducing nature, widespread volcanism and surface compositional heteregeneity. MESSENGER also provided major elemental ratios of its surface that can be used to infer large-scale differentiation processes and the thermal history of the planet. Mercury is known as being very reduced, with very low Fe-content and high S and alkali contents on its surface. Its bulk composition is therefore likely close to EH enstatite chondrites. In order to elucidate the origin of the chemical diversity of Mercury's surface, we determined the melting properties of EH enstatite chondrites, at pressures between 1 bar and 3 GPa and oxygen fugacity of IW-3 to IW-5, using piston-cylinder experiments, combined with a previous study on EH4 melting at 1 bar. We found that the presence of Ca-rich sulfide melts induces significant decrease of Ca-content in silicate melts at low pressure and low degree of melting (F). Also at pressures lower than 3 GPa, the SiO2-content decreases with F, while it increases at 3 GPa. This is likely due to the chemical composition of the bulk silicate which has a (Mg+Fe+Ca)/Si ratio very close to 1 and to the change from incongruent to congruent melting of enstatite. We then tested whether the various chemical compositions of Mercury's surface can result from mixing between two melting products of EH chondrites. We found that the majority of the geochemical provinces of Mercury's surface can be explained by mixing of two melts, with the exception of the High-Al plains that require an Al-rich source. Our findings indicate that Mercury's surface could have been produced by polybaric melting of a relatively primitive mantle.

  11. A benchmark initiative on mantle convection with melting and melt segregation

    NASA Astrophysics Data System (ADS)

    Schmeling, Harro; Dohmen, Janik; Wallner, Herbert; Noack, Lena; Tosi, Nicola; Plesa, Ana-Catalina; Maurice, Maxime

    2015-04-01

    In recent years a number of mantle convection models have been developed which include partial melting within the asthenosphere, estimation of melt volumes, as well as melt extraction with and without redistribution at the surface or within the lithosphere. All these approaches use various simplifying modelling assumptions whose effects on the dynamics of convection including the feedback on melting have not been explored in sufficient detail. To better assess the significance of such assumptions and to provide test cases for the modelling community we initiate a benchmark comparison. In the initial phase of this endeavor we focus on the usefulness of the definitions of the test cases keeping the physics as sound as possible. The reference model is taken from the mantle convection benchmark, case 1b (Blanckenbach et al., 1989), assuming a square box with free slip boundary conditions, the Boussinesq approximation, constant viscosity and a Rayleigh number of 1e5. Melting is modelled assuming a simplified binary solid solution with linearly depth dependent solidus and liquidus temperatures, as well as a solidus temperature depending linearly on depletion. Starting from a plume free initial temperature condition (to avoid melting at the onset time) three cases are investigated: Case 1 includes melting, but without thermal or dynamic feedback on the convection flow. This case provides a total melt generation rate (qm) in a steady state. Case 2 includes batch melting, melt buoyancy (melt Rayleigh number Rm), depletion buoyancy and latent heat, but no melt percolation. Output quantities are the Nusselt number (Nu), root mean square velocity (vrms) and qm approaching a statistical steady state. Case 3 includes two-phase flow, i.e. melt percolation, assuming a constant shear and bulk viscosity of the matrix and various melt retention numbers (Rt). These cases should be carried out using the Compaction Boussinseq Approximation (Schmeling, 2000) or the full compaction

  12. Reactive melt migration in mantle rocks: an experimental study

    NASA Astrophysics Data System (ADS)

    Pec, M.; Kohlstedt, D. L.; Zimmerman, M. E.; Holtzman, B. K.

    2013-12-01

    Several physical and chemical driving forces are important during the segregation and focusing of melt from a partially molten mantle toward a mid-oceanic spreading center. We performed a series of experiments to investigate the coupling between stress-driven and reaction-driven melt segregation. A core of melt-rich source material containing olivine, chromite and 20 vol% alkali basalt was placed within a ring of melt-poor sink containing olivine, enstatite, chromite and 4 vol% of alkali basalt. The gradient in silica activity at the source-sink interface provides a chemical driving force for melt migration. These source-sink couples were deformed in torsion in a gas-medium deformation apparatus at 1200°C, 300 MPa confining pressure and shear strain rates of ~10-4 s-1 to different finite shear strains (γ = 0 - 5). Microstructural observations of tangential sections show that at low strains (γ ≤ 2) melt aligns along grain boundaries in the sink at ~45° antithetic to the shear plane (parallel to σ1). At higher strains (γ > 3.5), melt-rich bands develop at 0 to 20° antithetic to the shear direction with a mean spacing between bands of 100 to 250 μm. The melt-rich bands contain 20 - 30% melt in the form of individual melt pockets (dequ ≈ 2.5 μm) aligned 10 - 45° with respect to the applied shear. Typically, a band contains 2 to 10 euhedral to rounded, small, mostly olivine grains (dequ ≈ 3 μm) almost entirely surrounded by melt. Bands terminate at melt-depleted lenses (< 1 vol% melt) in a zone 2 - 4 grains wide containing ~2 vol% melt in small (≤1 μm) pockets. Melt-rich zones do not develop in the sink at the source-sink interface after annealing a sample hydrostatically for >9 h. However, once melt bands are well developed by stress-driven melt segregation (γ ≥ 3.5) dissolution of enstatite reaches ~200 μm into the sink from the source-sink interface. For the pressure-temperature conditions and timescales of our experiments, the observations

  13. 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

  14. Deep mantle melting-solidifying and produced heterogeneities

    NASA Astrophysics Data System (ADS)

    Fomin, Ilya; Tackley, Paul

    2015-04-01

    Model for solid-liquid equilibrium and substance properties in lower mantle conditions is important to understand the early stages of evolution of terrestrial planets, such as core formation and magma ocean crystallization. This model is also necessary to prove theories on some modern seismic features of the Earth (e.g. ultra-low velocity zones) and petrological observations (e.g. lower mantle mineral assemblage inclusions in diamonds). Numerous experimental and numerical studies of the lower mantle phases provide sufficient amount of data to build up a thermodynamic model, which can be used in geophysical fluid dynamics research. Molecular Dynamics modeling provides data on thermodynamic properties of solids and liquids (density, heat capacity, thermal expansion, latent heat of melting etc.). Absence of minor components (iron, alkali etc.) makes it to overestimate melting temperatures significantly (up to 20-30%), so experimental data are also very important. Our model is based on MD data by [de Koker et al., 2013] with evaluation of all important parameters according to classical thermodynamic equations. Melting temperatures (especially at eutectic points) are corrected along Clausius-Clapeyron slopes to agree with modern experimental data ([Andrault et al., 2011], [Andrault et al., 2014], [Fiquet et al., 2010], [Hirose et al., 1999], [Mosenfelder et al., 2007], [Nomura et al., 2014], [Ozawa et al., 2011], [Zerr et al., 1998]). KD value for iron reported by [Andrault et al., 2012] was used. Proposed model was implemented into StagYY software (e.g. [Tackley, 2008]). It is a finite-volume discretization code for advection of solid and liquid in a planetary scale. A principal new feature of the used code modification is that we use separated variables for chemical compounds: SiO2, FeO, MgO and other (list can be extended). So it is possible to trace mantle heterogeneities produced by melting and solidifying events. Calculations predict appearing and disappearing

  15. Lower Mantle melting model and it's geodynamical applications

    NASA Astrophysics Data System (ADS)

    Fomin, I.; Tackley, P. J.

    2014-12-01

    Model of solid-liquid equilibrium laws and substances properties in lower mantle conditions is important to understand the early stages of evolution of terrestrial planets, such as core formation and magma ocean crystallization. This model is also necessary to prove theories on some modern seismic features of the Earth (e.g. ultra-low velocity zones) and petrological observations (e.g. lower mantle mineral assemblage inclusions in diamonds). Numerous experimental and numerical studies of the lower mantle phases provide sufficient amount of data to build up a thermodynamic model, which can be used in geophysical fluid dynamics research. Experimental studies are the direct source of soliduses values, but other thermodynamic parameters stay unclear. Molecular Dynamics modeling provides data on thermodynamic properties of solids and liquids (density, heat capacity, latent heat of melting etc.). But absence of minor components (iron, alkali etc.) and some numerical issues (e.g. [Belonoshko, 2001]) make it to overestimate melting temperatures significantly (up to 20-30%). Our approach is to develop a model based on MD data by [de Koker et al., 2013] with evaluation of all important parameters according to classical thermodynamic equations. But melting temperatures (especially at eutectic points) are corrected along Clausius-Clapeyron slopes to agree with modern experimental data ([Andrault et al., 2011], [Andrault et al., 2014], [Fiquet et al., 2010], [Hirose et al., 1999], [Mosenfelder et al., 2007], [Nomura et al., 2014],[Ozawa et al., 2011], [Zerr et al., 1998]). Notable effect on melt and solid densities has iron partitioning, so KD value reported by [Andrault et al., 2012] was used. Proposed model was implemented into StagYY software (e.g. [Tackley, 2008]). It is a finite-volume discretization code for advection of solid and liquid in a planetary scale. CMB temperature was set to be 4000-4400 K. Calculations predict appearing and disappearing batches containing up

  16. Consequences and Resolution of Lunar Lower Mantle Partial Melt

    NASA Astrophysics Data System (ADS)

    Fuqua, H.; Bremner, P. M.; Diamond, M. R.; Garapic, G.; Lock, S. J.; Mallik, A.; Nishikawa, Y.; Panovska, S.; Shahar, A.; Lognonne, P. H.; Panero, W. R.; Faul, U.; Panning, M. P.; Jimenez-Perez, H.; Schmerr, N. C.; Williams, Q. C.

    2014-12-01

    Existence of a partially molten layer at depth has been proposed to explain the lack of observed farside deep moonquakes, the observation of reflected phases from deep moonquakes, and the dissipation of tidal energy within the lunar interior. However, subsequent models explore the possibility that dissipation due to elevated temperatures alone can explain the observed dissipation factor (Q) and tidal love numbers. We have explored the hypothesis that high titanium melt compositions associated with lunar mantle overturn may sink to the base of the mantle, locally or regionally. We have performed forward calculations varying composition and thickness of layers to evaluate if a partially molten layer at the base of the mantle is well constrained by the observational data. Self-consistent physical parameters are calculated for each compositional model that are then compared against the observed data to determine a subset of permissible models. The data constraints considered by this study include bulk density, moment of inertia, real and imaginary parts of the Love numbers, seismic travel times, and electrical conductivity. Dynamic calculations using ASPECT have also been considered to determine the implications of early lunar mantle convection for the survivability of the partially molten layer. Finally, and as a perspective for a future NASA New Frontiers Geophysical Network, we present 1D synthetic seismograms calculated for each proposed structure of the Moon to investigate the future seismological resolution of these deep lunar structure features. This work was initiated at the CIDER 2014 program.

  17. Deformation-driven melt segregation and organization in the mantle

    NASA Astrophysics Data System (ADS)

    Holtzman, B. K.; Kohlstedt, D. L.; Zimmerman, M. E.; Daines, M. J.; Phipps Morgan, J.

    2003-04-01

    The strongest constraint on the dynamics of melt extraction is the observation that most transport occurs in chemically isolated channels. Replacive dunites clearly form by reactive porous flow and may constitute the major melt pathways in the mantle beneath spreading centers, as inferred from the Oman ophiolite. However, deformation and flow of the upwelling mantle beneath ridges and arcs may also be very important in both the initiation of melt segregation and in the organization of channels. With shear and torsional deformation experiments of partially molten rocks, we have demonstrated that deformation alone can drive melt to segregate. To apply this phenomenon to natural settings, we must understand 1) the dynamics of deformation-driven self-organization at the experimental scale and 2) the relations necessary for extrapolating to natural length and time scales. 1) Dynamics: The deformation-driven segregation process involves a positive feedback due to melt fraction-dependent weakening of the crystalline matrix, as discussed by Stevenson in 1989, coupled to (and counteracted by) the concentration of strain into melt-rich bands. The observed band orientation corresponds to the angle at which the pressure difference between melt in the band- and non-band regions is minimized. 2) Scaling: In experiments, band spacing scales with compaction length; this simplest of scaling relations predicts band spacings on the order of channelized-flow features observed in ophiolites (i.e., dunites), suggesting that deformation-driven and reaction-driven segregation processes may interact closely. Deformation can drive the channels to self-organize into connected (anastomosing) networks of melt-rich shear zones. These networks will act as transport pathways and will significantly reduce the effective viscosity of the partially molten regions. While trying to detect a melt-rich network with shear wave splitting is an important test, we ask further if there are unique and

  18. Melting and reactive flow of a volatilized mantle beneath mid-ocean ridges: theory and numerical models

    NASA Astrophysics Data System (ADS)

    Keller, Tobias; Katz, Richard F.

    2015-04-01

    Laboratory experiments indicate that even small concentrations volatiles (H2O or CO2) in the upper mantle significantly affect the silicate melting behavior [HK96,DH06]. The presence of volatiles stabilizes volatile-rich melt at high pressure, thus vastly increasing the volume of the upper mantle expected to be partially molten [H10,DH10]. These small-degree melts have important consequences for chemical differentiation and could affect the dynamics of mantle flow. We have developed theory and numerical implementation to simulate thermo-chemically coupled magma/mantle dynamics in terms of a two-phase (rock+melt), three component (dunite+MORB+volatilized MORB) physical model. The fluid dynamics is based on McKenzie's equations [McK84], while the thermo-chemical formulation of the system is represented by a novel disequilibrium multi-component melting model based on thermo-dynamic theory [RBS11]. This physical model is implemented as a parallel, two-dimensional, finite-volume code that leverages tools from the PETSc toolkit. Application of this simulation code to a mid-ocean ridge system suggests that the methodology captures the leading-order features of both hydrated and carbonated mantle melting, including deep, low-degree, volatile-rich melt formation. Melt segregation leads to continuous dynamic thermo-chemical dis-equilibration, while phenomenological reaction rates are applied to continually move the system towards re-equilibration. The simulations will be used first to characterize volatile extraction from the MOR system assuming a chemically homogeneous mantle. Subsequently, simulations will be extended to investigate the consequences of heterogeneity in lithology [KW12] and volatile content. These studies will advance our understanding of the role of volatiles in the dynamic and chemical evolution of the upper mantle. Moreover, they will help to gauge the significance of the coupling between the deep carbon cycle and the ocean/atmosphere system. REFERENCES

  19. Experiments on melt-rock reaction in the shallow mantle wedge

    NASA Astrophysics Data System (ADS)

    Mitchell, Alexandra L.; Grove, Timothy L.

    2016-12-01

    This experimental study simulates the interaction of hotter, deeper hydrous mantle melts with shallower, cooler depleted mantle, a process that is expected to occur in the upper part of the mantle wedge. Hydrous reaction experiments ( 6 wt% H2O in the melt) were conducted on three different ratios of a 1.6 GPa mantle melt and an overlying 1.2 GPa harzburgite from 1060 to 1260 °C. Reaction coefficients were calculated for each experiment to determine the effect of temperature and starting bulk composition on final melt compositions and crystallizing assemblages. The experiments used to construct the melt-wall rock model closely approached equilibrium and experienced <5% Fe loss or gain. Experiments that experienced higher extents of Fe loss were used to critically evaluate the practice of "correcting" for Fe loss by adding iron. At low ratios of melt/mantle (20:80 and 5:95), the crystallizing assemblages are dunites, harzburgites, and lherzolites (as a function of temperature). When the ratio of deeper melt to overlying mantle is 70:30, the crystallizing assemblage is a wehrlite. This shows that wehrlites, which are observed in ophiolites and mantle xenoliths, can be formed by large amounts of deeper melt fluxing though the mantle wedge during ascent. In all cases, orthopyroxene dissolves in the melt, and olivine crystallizes along with pyroxenes and spinel. The amount of reaction between deeper melts and overlying mantle, simulated here by the three starting compositions, imposes a strong influence on final melt compositions, particularly in terms of depletion. At the lowest melt/mantle ratios, the resulting melt is an extremely depleted Al-poor, high-Si andesite. As the fraction of melt to mantle increases, final melts resemble primitive basaltic andesites found in arcs globally. An important element ratio in mantle lherzolite composition, the Ca/Al ratio, can be significantly elevated through shallow mantle melt-wall rock reaction. Wall rock temperature is a key

  20. Global-scale modelling of melting and isotopic evolution of Earth's mantle: melting modules for TERRA

    NASA Astrophysics Data System (ADS)

    van Heck, Hein J.; Davies, J. Huw; Elliott, Tim; Porcelli, Don

    2016-04-01

    Many outstanding problems in solid-Earth science relate to the geodynamical explanation of geochemical observations. Currently, extensive geochemical databases of surface observations exist, but satisfying explanations of underlying mantle processes are lacking. One way to address these problems is through numerical modelling of mantle convection while tracking chemical information throughout the convective mantle. We have implemented a new way to track both bulk compositions and concentrations of trace elements in a finite-element mantle convection code. Our approach is to track bulk compositions and trace element abundances via particles. One value on each particle represents bulk composition and can be interpreted as the basalt component. In our model, chemical fractionation of bulk composition and trace elements happens at self-consistent, evolving melting zones. Melting is defined via a composition-dependent solidus, such that the amount of melt generated depends on pressure, temperature and bulk composition of each particle. A novel aspect is that we do not move particles that undergo melting; instead we transfer the chemical information carried by the particle to other particles. Molten material is instantaneously transported to the surface layer, thereby increasing the basalt component carried by the particles close to the surface and decreasing the basalt component in the residue. The model is set to explore a number of radiogenic isotopic systems, but as an example here the trace elements we choose to follow are the Pb isotopes and their radioactive parents. For these calculations we will show (1) the evolution of the distribution of bulk compositions over time, showing the buildup of oceanic crust (via melting-induced chemical separation in bulk composition), i.e. a basalt-rich layer at the surface, and the transportation of these chemical heterogeneities through the deep mantle; (2) the amount of melt generated over time; (3) the evolution of the

  1. Silicate melt metasomatism in the lithospheric mantle beneath SW Poland

    NASA Astrophysics Data System (ADS)

    Puziewicz, Jacek; Matusiak-Małek, Magdalena; Ntaflos, Theodoros; Grégoire, Michel; Kukuła, Anna

    2014-05-01

    The xenoliths of peridotites representing the subcontinental lithospheric mantle (SCLM) beneath SW Poland and adjacent parts of Germany occur in the Cenozoic alkaline volcanic rocks. Our study is based on detailed characterization of xenoliths occurring in 7 locations (Steinberg in Upper Lusatia, Księginki, Pilchowice, Krzeniów, Wilcza Góra, Winna Góra and Lutynia in Lower Silesia). One of the two major lithologies occurring in the xenoliths, which we call the "B" lithology, comprises peridotites (typically harzburgites) with olivine containing from 90.5 to 84.0 mole % of forsterite. The harzburgites contain no clinopyroxene or are poor in that mineral (eg. in Krzeniów the group "B" harzburgites contain < 1 vol. % of the mineral). They exhibit significant variation in orthopyroxene contents, which varies from 25 to 10 vol. %. Some of the xenoliths are more impoverished in orthopyroxene and have dunitic compositions. The ortho- and clinopyroxene exhibit mg# similar to that of olivine, and typically are low aluminous (Al < 0.10 atom pfu in ortho-, and < 0.20 atom pfu in clinopyroxene). The exception are xenoliths from Księginki, which contain pyroxenes characterised by negative correlation between mg# and Al. The REE patterns of both ortho- and clinopyroxene in the group "B" peridotites suggest equilibration with silicate melt. The rocks of "B" lithology were formed due to alkaline silicate melt percolation in the depleted peridotitic protolith. The basaltic melts formed at high pressure are usually undersaturated in both ortho- and clinopyroxene at lower pressures (Kelemen et al. 1992). Because of cooling and dissolution of ortho- and clinopyroxene the melts change their composition and become saturated in one or both of those phases. Experimental results (e.g. Tursack & Liang 2012 and references therein) show that the same refers to alkaline basaltic silicate melts and that its reactive percolation in the peridotitic host leads to decrease of Mg

  2. 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

  3. Formation of harzburgite by pervasive melt/rock reaction in the upper mantle

    USGS Publications Warehouse

    Kelemen, P.B.; Dick, H.J.B.; Quick, J.E.

    1992-01-01

    Many mantle peridotite samples are too rich in SiO2 (in the form of orthopyroxene) and have ratios of light to heavy rare earth elements that are too high to be consistent with an origin as the residuum of partial melting of the primitive mantle. Trace element studies of melt/rock reaction zones in the Trinity peridotite provide evidence for reaction of the mantle lithosphere with ascending melts, which dissolved calcium-pyroxene and precipitated orthopyroxene as magma mass decreased. This process can account for the observed major and trace element compositions of lithospheric mantle samples, and may accordingly be prevalent in the upper mantle.

  4. Mantle Sources, Mantle Melting and the Genesis of the Central East Greenland Plateau Lavas

    NASA Astrophysics Data System (ADS)

    Brown, E. L.; Barfod, G. H.; Lesher, C. E.

    2006-12-01

    The Central East Greenland (CEG) plateau lavas (56-54 Ma) contain a very complete geochemical record of the opening of the North Atlantic basin in response to the breakup of Pangaea. This record provides an unique opportunity for identifying the mantle source compositions and melting processes involved in the genesis of the North Atlantic Igneous Province (NAIP). The plateau lavas consist of three compositional suites: the volumetrically dominant high-Ti suite (TiO2 ca. 1.67 - 4 wt. %) (HTS) and the minor low-Ti (TiO2 < 1.96 wt. %) and very high-Ti (TiO2 ca. 4 - 6 wt. %) suites (LTS and VHTS, respectively). We present detailed Hf-Nd-Sr data and trace element data for VHTS and LTS lavas closely associated within the lava succession. These uncontaminated lava suites represent the extreme compositional ranges of the plateau lavas and show limited variability in ɛHf (9.58 - 10.96 [VHTS] and 14.39 - 14.68 [LTS]) and a somewhat broader variation in ɛNd (5.42 - 6.73 [VHTS] and 8.29 - 9.68 [LTS]). The LTS and VHTS source compositions bracket the chemical range observed for the HTS lavas. Drawing from the model of [1], we propose that the mantle sources for the VHTS and LTS were intimately associated within the mantle melting regime beneath CEG and were present throughout the generation of the plateau lavas. Correlations between trace element and isotopic data can be accounted for by a forward melting model involving a heterogeneous source containing fusible eclogite and refractory peridotite. These findings are in contrast to the model of [2] proposing that temporal sampling of three distinct and isolated mantle domains within a zoned plume is the dominant control on plateau lava chemistry. Our study highlights the importance of combining isotope and trace element data in understanding melt production in the NAIP and elsewhere. (1) Tegner et al., 1998, Nature, v 395, p 591-594; (2) Barker et al., 2006, Geology, v 34, p 481-484

  5. Mantle melts, metasomatism and diamond formation: Insights from melt inclusions in xenoliths from Diavik, Slave Craton

    NASA Astrophysics Data System (ADS)

    Araújo, D. P.; Griffin, W. L.; O'Reilly, S. Y.

    2009-11-01

    Abundant carbonatitic to ultramafic melt inclusions 0.2-2.5mm in diameter occur in the Cr-diopside of megacrystalline lherzolite xenoliths from the A154 kimberlite of the Diavik mine, Lac de Gras area. The melts range from carbonatitic (50-97% carbonate) to Ca-Mg-silicic (10-50% carbonate) to Mg-silicic (< 10% dispersed calcite) compositions, and are connected by veinlets of similar material, or by fractures bordered by spongy Cr-diopside. Phenocrysts and quench crystals of calcite, olivine and mica are set in carbonatitic to Mg-silicic matrices, and irregular volumes of carbonatite and Mg-silicate melt appear to have unmixed from one another within single inclusions. Calculated bulk compositions of the more silicic melts are similar in major- and trace elements to kimberlites from the Slave province. The Cr-diopside adjacent to melt inclusions is enriched in LREE, Ba, alkali elements, HFSE, Th and U. Calculated compositions of the metasomatising fluids are strongly enriched in these elements relative to the trapped melts, and are similar to fluids trapped in the opaque coats found on many Diavik diamonds. The microstructures, the metasomatic effects and the genetic relationship to diamond formation suggest that the melt inclusions formed when kimberlite-like melts penetrated the lherzolites along fractures deep in the lithospheric mantle. The melts began to differentiate into carbonatitic and ultramafic end-members, were trapped as globular inclusions during recrystallisation and necking-down prior to entrainment of the xenoliths in the kimberlite, and were quenched during ascent. The evolution of saline, water- and carbonate-rich fluids from melts such as these may play an important role in diamond genesis.

  6. Constraints on Mantle Plume Melting Conditions in the Martian Mantle Based on Improved Melting Phase Relationships of Olivine-Phyric Shergottite Yamato 980459

    NASA Technical Reports Server (NTRS)

    Kiefer, Walter S.; Rapp, Jennifer F.; Usui, Tomohiro; Draper, David S.; Filiberto, Justin

    2016-01-01

    Martian meteorite Yamato 980459 (hereafter Y98) is an olivine-phyric shergottite that has been interpreted as closely approximating a martian mantle melt [1-4], making it an important constraint on adiabatic decompression melting models. It has long been recognized that low pressure melting of the Y98 composition occurs at extremely high temperatures relative to martian basalts (1430 degC at 1 bar), which caused great difficulties in a previous attempt to explain Y98 magma generation via a mantle plume model [2]. However, previous studies of the phase diagram were limited to pressures of 2 GPa and less [2, 5], whereas decompression melting in the present-day martian mantle occurs at pressures of 3-7 GPa, with the shallow boundary of the melt production zone occurring just below the base of the thermal lithosphere [6]. Recent experimental work has now extended our knowledge of the Y98 melting phase relationships to 8 GPa. In light of this improved petrological knowledge, we are therefore reassessing the constraints that Y98 imposes on melting conditions in martian mantle plumes. Two recently discovered olivine- phyric shergottites, Northwest Africa (NWA) 5789 and NWA 6234, may also be primary melts from the martian mantle [7, 8]. However, these latter meteorites have not been the subject of detailed experimental petrology studies, so we focus here on Y98.

  7. Redox freezing and melting in the Earth's deep mantle resulting from carbon-iron redox coupling.

    PubMed

    Rohrbach, Arno; Schmidt, Max W

    2011-04-14

    Very low seismic velocity anomalies in the Earth's mantle may reflect small amounts of melt present in the peridotite matrix, and the onset of melting in the Earth's upper mantle is likely to be triggered by the presence of small amounts of carbonate. Such carbonates stem from subducted oceanic lithosphere in part buried to depths below the 660-kilometre discontinuity and remixed into the mantle. Here we demonstrate that carbonate-induced melting may occur in deeply subducted lithosphere at near-adiabatic temperatures in the Earth's transition zone and lower mantle. We show experimentally that these carbonatite melts are unstable when infiltrating ambient mantle and are reduced to immobile diamond when recycled at depths greater than ∼250 kilometres, where mantle redox conditions are determined by the presence of an (Fe,Ni) metal phase. This 'redox freezing' process leads to diamond-enriched mantle domains in which the Fe(0), resulting from Fe(2+) disproportionation in perovskites and garnet, is consumed but the Fe(3+) preserved. When such carbon-enriched mantle heterogeneities become part of the upwelling mantle, diamond will inevitably react with the Fe(3+) leading to true carbonatite redox melting at ∼660 and ∼250 kilometres depth to form deep-seated melts in the Earth's mantle.

  8. Melting curve of the deep mantle applied to properties of early magma ocean and actual core-mantle boundary

    NASA Astrophysics Data System (ADS)

    Andrault, Denis; Lo Nigro, Giacomo; Bolfan-Casanova, Nathalie; Bouhifd, Mohamed A.; Garbarino, Gaston; Mezouar, Mohamed

    2010-05-01

    Our planet experienced partial melting early in its history as a consequence of energy release due to accretion. Partial mantle melting could still happen today in the lowermost mantle. Occurrence of melting is primordial for the chemical segregation between the different Earth's reservoirs and for the dynamics of the whole planet. Melting of iron-alloys is relatively easy to achieve, but the silicated mantle happens to be more refractory. We investigated experimentally melting properties of two starting material, forsterite and chondritic-mantle, at pressures ranging from 25 to 140 GPa, using laser-heated diamond anvil cell coupled with synchrotron radiation. We show that partial melting in the lowermost mantle, as suggested by seismology on the basis of the ultra-low velocity zones (ULVZ), requires temperatures above 4200 K at the core-mantle boundary. At low pressures, our curve plots significantly lower than previous reports. Compared to recent estimates of mantle geotherm, while this temperature remains possible if the Earth's core is very hot, it is more likely that ULVZs correspond to high concentration of incompatible elements driven down to the D"-layer by subducting slabs or extracted out from the outer core. When our chondritic melting curve is coupled with recent isentropic temperature profiles for a magma ocean, we obtain a correlation between magma ocean depth and the potential temperature (Tp) at its surface; an ocean depth of 1000 km (equivalent to ~40 GPa) corresponds to Tp=2000 K, which happens to be significantly hotter than the estimated surface temperature of a sustained magma ocean. It emphasizes the importance of a lid at the magma ocean surface at an epoch as early as that of core-mantle segregation.

  9. Density of hydrous silicate melt at the conditions of Earth's deep upper mantle.

    PubMed

    Matsukage, Kyoko N; Jing, Zhicheng; Karato, Shun-ichiro

    2005-11-24

    The chemical evolution of the Earth and the terrestrial planets is largely controlled by the density of silicate melts. If melt density is higher than that of the surrounding solid, incompatible elements dissolved in the melt will be sequestered in the deep mantle. Previous studies on dry (water-free) melts showed that the density of silicate melts can be higher than that of surrounding solids under deep mantle conditions. However, melts formed under deep mantle conditions are also likely to contain some water, which will reduce the melt density. Here we present data constraining the density of hydrous silicate melt at the conditions of approximately 410 km depth. We show that the water in the silicate melt is more compressible than the other components, and therefore the effect of water in reducing melt density is markedly diminished under high-pressure conditions. Our study indicates that there is a range of conditions under which a (hydrous) melt could be trapped at the 410-km boundary and hence incompatible elements could be sequestered in the deep mantle, although these conditions are sensitive to melt composition as well as the composition of the surrounding mantle.

  10. Spin crossover and iron-rich dense partial melt in pyrolitic lower mantle

    NASA Astrophysics Data System (ADS)

    Hirose, K.; Tateno, S.

    2012-12-01

    Spin crossover of iron may occur not only in solids but also in melts in the lower mantle. The resulting change in Fe partitioning strongly affects the buoyancy of partial melts near the base of the mantle. Nomura et al. [2011 Nature] measured the Fe partitioning in (Mg0.89Fe0.11)2 SiO4 bulk composition over the entire mantle pressure range, demonstrating that Fe-Mg distribution coefficient KD = ([FePv]/[MgPv]) / ([Femelt]/[Mgmelt]) between (Mg,Fe)SiO3 perovskite and melt dropped from ~0.25 to <0.1 around 76 GPa, resulting in strong Fe-enrichment in melts and thereby dense partial melts in the mid-lower mantle. In contrast, the most recent experiments by Andrault et al. [2012 Nature] found much higher KD and less Fe-enrichment in partial melts formed in primitive mantle composition, suggesting that melt is not dense in the lowermost mantle. Here we extend our measurements in pyrolitic natural mantle (KLB-1 peridotite) bulk composition. The distribution coefficient KD (total Fe/Mg) was determined at 40-180 GPa by a combination of laser-heated diamond-anvil cell experiments and chemical analyses of recovered samples using field-emission-type electron microprobe (FE-EPMA). Our results demonstrate that KD between perovskite and melt is about 0.3 up to 58 GPa, consistent with earlier multi-anvil data. It then dropped to ~0.1 above 68 GPa, indicating strong Fe-enrichment in partial melts. These results are in excellent agreement with those of Nomura et al. [2011], indicating that Fe-rich partial melts are more dense than solids below 1600-km depth in the lower mantle. The observed Fe-enrichment in partial melt above 68 GPa can be explained by a spin crossover of iron in silicate melt, as discussed previously in Nomura et al. [2011].

  11. Microtomography of partially molten rocks : three-dimensional melt distribution in mantle peridotite.

    SciTech Connect

    Zhu, W.; Gaetani, G.; Fusseis, F.; Montesi, L.; De Carlo, F.

    2011-04-01

    The permeability of the upper mantle controls melt segregation beneath spreading centers. Reconciling contradictory geochemical and geophysical observations at ocean ridges requires a better understanding of transport properties in partially molten rocks. Using x-ray synchrotron microtomography, we obtained three-dimensional data on melt distribution for mantle peridotite with various melt fractions. At melt fractions as low as 0.02, triple junctions along grain edges dominated the melt network; there was no evidence of an abrupt change in the fundamental character of melt extraction as melt fraction increased to 0.2. The porosity of the partially molten region beneath ocean ridges is therefore controlled by a balance between viscous compaction and melting rate, not by a change in melt topology.

  12. Microtomography of partially molten rocks: three-dimensional melt distribution in mantle peridotite.

    PubMed

    Zhu, Wenlu; Gaetani, Glenn A; Fusseis, Florian; Montési, Laurent G J; De Carlo, Francesco

    2011-04-01

    The permeability of the upper mantle controls melt segregation beneath spreading centers. Reconciling contradictory geochemical and geophysical observations at ocean ridges requires a better understanding of transport properties in partially molten rocks. Using x-ray synchrotron microtomography, we obtained three-dimensional data on melt distribution for mantle peridotite with various melt fractions. At melt fractions as low as 0.02, triple junctions along grain edges dominated the melt network; there was no evidence of an abrupt change in the fundamental character of melt extraction as melt fraction increased to 0.2. The porosity of the partially molten region beneath ocean ridges is therefore controlled by a balance between viscous compaction and melting rate, not by a change in melt topology.

  13. Elemental and Sr-Nd-Pb isotope geochemistry of the Cenozoic basalts in Southeast China: Insights into their mantle sources and melting processes

    NASA Astrophysics Data System (ADS)

    Sun, Pu; Niu, Yaoling; Guo, Pengyuan; Ye, Lei; Liu, Jinju; Feng, Yuexing

    2017-02-01

    We analyzed whole-rock major and trace elements and Sr-Nd-Pb isotopes of the Cenozoic basalts in Southeast China to investigate their mantle source characteristics and melting process. These basalts are spatially associated with three extensional fault systems parallel to the coast line. After correction for the effect of olivine microlites on bulk-rock compositions and the effect of crystal fractionation, we obtained primitive melt compositions for these samples. These primitive melts show increasing SiO2, Al2O3 but decreasing FeO, MgO, TiO2, P2O5, CaO and CaO/Al2O3 from the interior to the coast. Such spatial variations of major element abundances and ratios are consistent with a combined effect of fertile source compositional variation and increasing extent and decreasing pressure of decompression melting from beneath the thick lithosphere in the interior to beneath the thin lithosphere in the coast. These basalts are characterized by incompatible element enrichment but varying extent of isotopic depletion. This element-isotope decoupling is most consistent with recent mantle source enrichment by means of low-degree melt metasomatism that elevated incompatible element abundances without yet having adequate time for isotopic ingrowth in the mantle source regions. Furthermore, Sr and Nd isotope ratios show significant correlations with Nb/Th, Nb/La, Sr/Sr* and Eu/Eu*, which substantiates the presence of recycled upper continental crustal material in the mantle sources of these basalts. Pb isotope ratios also exhibit spatial variation, increasing from the interior to the coastal area. The significant correlations of major element abundances with Pb isotope ratios indicate that the Pb isotope variations also result from varied extent and pressure of decompression melting. We conclude that the elevated Pb isotope ratios from the interior to coast are consistent with increasing extent of decompression melting of the incompatible element depleted mantle matrix, which

  14. Melt-rock reaction in the asthenospheric mantle: Perspectives from laboratory dissolution experiments (Invited)

    NASA Astrophysics Data System (ADS)

    Liang, Y.; Wang, C.; Saper, L.; Dygert, N. J.; Xu, W.

    2013-12-01

    The primary motivation for melt-rock interaction in the mantle is chemical disequilibrium that arises when melt generated in the deep mantle percolates through the overlying mantle. This is a continuous process involving both thermal and chemical exchange between the melt and the solid matrix. It occurs in all major active tectonic regimes within the upper mantle and is responsible for a range of petrologic and geochemical observations. Factors that are important in controlling the kinetics of melt-rock interaction include temperature, pressure, and melt composition. The present study focuses on the effect of reacting melting composition, especially water and silica content, on reaction kinetics. In terms of melt composition, we can broadly divide melt-rock interaction into three groups: (1) reaction between peridotite and silica-undersaturated (anhydrous) melt; (2) reaction between peridotite and silica-saturated (anhydrous) melt; and (3) reaction between peridotite and hydrous melt. (1) is important to melt migration beneath mid-ocean ridges, (2) is relevant to magma genesis involving pyroxenite, and (3) is important to melt generation in the mantle wedge and back-arc basin. Reaction between peridotites and silica-undersaturated melts such as alkali basalt and MORB at moderate to low pressures can produce a melt-bearing dunite-harzburgite or dunite-harzburgite-lherzolite sequence. Reaction between peridotites and siliceous melts derived from melting of eclogite or garnet pyroxenite do not form dunite at moderate to high pressures (1-3.5 GPa and 1200-1550°C). Instead, they produce melt-bearing harzburgite at moderate pressure and opx-rich harzburgite or orthopyroxenite at high pressure. The opx is produced at the expense of olivine and the siliceous melt, which may hinder further reaction because of porosity reduction in these reactions. The rate of peridotite dissolution is considerably lower when the peridotite is subsolidus, and a high-porosity harzburgite or

  15. The Asthenosphere Melting Regimes Alteration due to Changing Conditions of Upper Mantle

    NASA Astrophysics Data System (ADS)

    Perepechko, Y. V.; Sharapov, V. N.; Sorokin, K., Jr.

    2014-12-01

    Analyzed in the article are different asthenosphere magma generation regimes above the upper mantle hot spots as thermodynamic and geometric parameters of the upper mantle and the conditions on its boundaries vary. The two-layer mantle model is applied to consider the formation of decompression melting areas. The thickness of metasomatically altered lithospheric mantle is determined by the mantle substance rheology and the location of the upper boundary of asthenosphere. We also take into consideration the principal solid state phase transitions by using the mantle substance state equation. The sizes and distribution of hot spots as well as their maximal temperature were defined by the thermodynamic conditions of the perovskite transition existence. The numerical analysis results demonstrate the manifestation of three main mantle dynamics modes; the conditions necessary to form the partial melting zones are not reached; some melting areas with the 30 to 65 Ma existence time do occur; the melting areas that are formed exist permanently. The permanently existing asthenosphere zones are marked by quasiperiodical variation in thickness and the degree of melting. The typical temperatures of a hot spot sharing these modes are the 1740°С and 2020°С correspondingly. The originally presupposed heating degree and the temperature ratio of the upper mantle do influence the decompression melting degree substantially and - to a lesser extent - they influence the size of melting zones. The primary evolution of the second mode is described by the development of a complex system of asthenosphere zones that lead to the occurrence of additional convectional cells dividing the partial melting zone. The variation in the rheological properties of the mantle substance also contributes to the manifestation of the complex structure of asthenosphere zone. The work was made with support of the Russian Foundation for Basic Research grant #12-05-00625.

  16. Structure and density of basaltic melts at mantle conditions from first-principles simulations

    PubMed Central

    Bajgain, Suraj; Ghosh, Dipta B.; Karki, Bijaya B.

    2015-01-01

    The origin and stability of deep-mantle melts, and the magmatic processes at different times of Earth's history are controlled by the physical properties of constituent silicate liquids. Here we report density functional theory-based simulations of model basalt, hydrous model basalt and near-MORB to assess the effects of iron and water on the melt structure and density, respectively. Our results suggest that as pressure increases, all types of coordination between major cations and anions strongly increase, and the water speciation changes from isolated species to extended forms. These structural changes are responsible for rapid initial melt densification on compression thereby making these basaltic melts possibly buoyantly stable at one or more depths. Our finding that the melt-water system is ideal (nearly zero volume of mixing) and miscible (negative enthalpy of mixing) over most of the mantle conditions strengthens the idea of potential water enrichment of deep-mantle melts and early magma ocean. PMID:26450568

  17. The amount of recycled crust in sources of mantle-derived melts.

    PubMed

    Sobolev, Alexander V; Hofmann, Albrecht W; Kuzmin, Dmitry V; Yaxley, Gregory M; Arndt, Nicholas T; Chung, Sun-Lin; Danyushevsky, Leonid V; Elliott, Tim; Frey, Frederick A; Garcia, Michael O; Gurenko, Andrey A; Kamenetsky, Vadim S; Kerr, Andrew C; Krivolutskaya, Nadezhda A; Matvienkov, Vladimir V; Nikogosian, Igor K; Rocholl, Alexander; Sigurdsson, Ingvar A; Sushchevskaya, Nadezhda M; Teklay, Mengist

    2007-04-20

    Plate tectonic processes introduce basaltic crust (as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts (10 to 30%), ocean island and continental basalts (many >60%), and komatiites (20 to 30%). These results imply involvement of 2 to 20% (up to 28%) of recycled crust in mantle melting.

  18. Supercritical Clinopyroxene in Upper Mantle Peridotites and their Bearing on the Composition of Mantle Melts

    NASA Astrophysics Data System (ADS)

    Muntener, O.; Pilet, S.; Ulianov, A.; Vonlanthen, P.

    2011-12-01

    demonstrate that the (100), (010) and (001) crystallographic axes of both pyroxenes are parallel, albeit the single crystals display distortion of up to 12%. The EBSD data thus support the idea that exsolution preceeded ductile deformation of the pyroxenes. The recombined composition suggests that the primary clinopyroxene was supercritical. Exsolution from supercritical pyroxenes during subsolidus cooling might produce cpx-bearing harzburgites that were originally cpx-free. If these findings are applicable to ridge systems they have important consequences with respect to compositional aspects of mantle melts. In particular, these findings question the simple approach of applying cpx-liquid partition coefficients to calculate equilibrium melts, since the composition of the subsolidus clinopyroxene is drastically different from the primary supercritical pyroxene. 1. Bertka and Holloway 1993 JGR 98, 19755-19766. 2. Longhi and Bertka, 1996, Am Min 81, 685-695. 3. Muntener and Manatschal, 2006, EPSL 252, 437-452.

  19. Melting Temperature and Partial Melt Chemistry of H2O-Saturated Mantle Peridotite to 11 Gigapascals

    PubMed

    Kawamoto; Holloway

    1997-04-11

    The H2O-saturated solidus of a model mantle composition (Kilborne Hole peridotite nodule, KLB-1) was determined to be just above 1000°C from 5 to 11 gigapascals. Given reasonable H2O abundances in Earth's mantle, an H2O-rich fluid could exist only in a region defined by the wet solidus and thermal stability limits of hydrous minerals, at depths between 90 and 330 kilometers. The experimental partial melts monotonously became more mafic with increasing pressure from andesitic composition at 1 gigapascal to more mafic than the starting peridotite at 10 gigapascals. Because the chemistry of the experimental partial melts is similar to that of kimberlites, it is suggested that kimberlites may be derived by low-temperature melting of an H2O-rich mantle at depths of 150 to 300 kilometers.

  20. Carbon-dioxide-rich silicate melt in the Earth's upper mantle.

    PubMed

    Dasgupta, Rajdeep; Mallik, Ananya; Tsuno, Kyusei; Withers, Anthony C; Hirth, Greg; Hirschmann, Marc M

    2013-01-10

    The onset of melting in the Earth's upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. Although carbonatitic melt could be stable 250 km or less beneath mid-oceanic ridges, owing to the small fraction (∼0.03 wt%) its effects on the mantle properties are unclear. Geophysical measurements, however, suggest that melts of greater volume may be present at ∼200 km (refs 3-5) but large melt fractions are thought to be restricted to shallower depths. Here we present experiments on carbonated peridotites over 2-5 GPa that constrain the location and the slope of the onset of silicate melting in the mantle. We find that the pressure-temperature slope of carbonated silicate melting is steeper than the solidus of volatile-free peridotite and that silicate melting of dry peridotite + CO(2) beneath ridges commences at ∼180 km. Accounting for the effect of 50-200 p.p.m. H(2)O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with ∼100 p.p.m. CO(2) becomes as deep as ∼220-300 km. We suggest that, on a global scale, carbonated silicate melt generation at a redox front ∼250-200 km deep, with destabilization of metal and majorite in the upwelling mantle, explains the oceanic low-velocity zone and the electrical conductivity structure of the mantle. In locally oxidized domains, deeper carbonated silicate melt may contribute to the seismic X-discontinuity. Furthermore, our results, along with the electrical conductivity of molten carbonated peridotite and that of the oceanic upper mantle, suggest that mantle at depth is CO(2)-rich but H(2)O-poor. Finally, carbonated silicate melts restrict the stability of carbonatite in the Earth's deep upper mantle, and the inventory of carbon, H(2)O and other highly incompatible elements at ridges becomes controlled by the flux of the former.

  1. Melting of Bridgmanite to 135 Gpa: Toward a Coherent Model for the Melting Behavior in the Lower Mantle

    NASA Astrophysics Data System (ADS)

    Andrault, D.; Pesce, G.; Mezouar, N.

    2015-12-01

    Our knowledge on the melting behavior in the deep mantle remains based on a limited number of experimental and theoretical works. Today, thanks to (i) availability of very brilliant X-ray synchrotron sources and (ii) improved control of the P-T conditions in the laser-heated diamond anvil cell (LH-DAC), the experimental results should reach some agreement about the melting diagrams. However, it is not the case and major controversies remain. For example, liquidi of peridotitic (1) and chondritic-type (2) mantles are reported with a temperature difference of ~1000 K at a pressure of ~90 GPa (corresponding to ~2000 km depth), which cannot be explained by the relatively small compositional difference between these two materials. To bring new insights about the melting properties of the deep mantle, our strategy has been to study the melting curve of the end-member liquidus phase, the (Mg,Fe)(Al,Si)O3 bridgmanite (Bg), before applying a basic thermodynamical approach to the mineralogical system made of Bg, CaSiO3-perovskite and (Mg,Fe)O ferropericlase. Our approach cannot be as formal as currently done for melting in the shallow mantle, due to lack of major thermodynamical parameters. Still, our analysis yields original information, such as the degree of partial melting as a function of P, T and fraction of Bg in the geological material. The Mg/Si ratio in melts can also be addressed. Concerning the controversy between LH-DAC experiments, it can be solved taking into account migration in the temperature gradient of the pseudo-eutectic melt, when the sample starts to melt. This effect is expected to occur more extensively in absence of an insulating material between the sample and the diamond anvils. It yields an overestimation of the liquidus temperature for a given chemical composition, due to loss of the most fusing elements. References:1. Fiquet et al. (2010) Melting of Peridotite to 140 Gigapascals. Science 329, 1516-1518. 2. Andrault et al. (2011) Melting curve of

  2. Extreme incompatibility of helium during mantle melting: Evidence from undegassed mid-ocean ridge basalts

    NASA Astrophysics Data System (ADS)

    Graham, David W.; Michael, Peter J.; Shea, Thomas

    2016-11-01

    We report total helium concentrations (vesicles + glass) for a suite of thirteen ultradepleted mid-ocean ridge basalts (UD-MORBs) that were previously studied for volatile contents (CO2, H2O) plus major and trace elements. The selected basalts are undersaturated in CO2 + H2O at their depths of eruption and represent rare cases of undegassed MORBs. Sample localities from the Atlantic (2), Indian (1) and Pacific (7) Oceans collectively show excellent linear correlations (r2 = 0.75- 0.92) between the concentrations of helium and the highly incompatible elements C, K, Rb, Ba, Nb, Th and U. Three basalts from Gakkel Ridge in the Arctic were also studied but show anomalous behavior marked by excess lithophile trace element abundances. In the Atlantic-Pacific-Indian suite, incompatible element concentrations vary by factors of 3-4.3, while helium concentration varies by a factor of 13. The strong correlations between the concentrations of helium and incompatible elements are explained by helium behavior as the most incompatible element during mantle melting. Partial melting of an ultradepleted mantle source, formed as a residue of earlier melt extraction, accounts for the observed concentrations. The earlier melting event involved removal of a small degree melt (∼1%) at low but non-zero porosity (0.01-0.5%), leading to a small amount of melt retention that strongly leveraged the incompatible element budget of the ultradepleted mantle source. Equilibrium melting models that produce the range of trace element and helium concentrations from this source require a bulk solid/melt distribution coefficient for helium that is lower than that for other incompatible elements by about a factor of ten. Alternatively, the bulk solid/melt distribution coefficient for helium could be similar to or even larger than that for other incompatible elements, but the much larger diffusivity of helium in peridotite leads to its more effective incompatibility and efficient extraction from a

  3. Dynamics of melt and water circulation in the mantle transition zone

    NASA Astrophysics Data System (ADS)

    Bercovici, David

    2010-05-01

    The presence of melt above the mantle transition zone has been predicted by several groups, and its formation has been attributed - according to the 'water filter model" (Bercovici & Karato 2003) - with causing whole mantle convection to appear geochemically layered. In recent years, various seismological studies (e.g., most recently Jasbinsek and Dueker, 2007) have collectively inferred an extensive low velocity region at 410km depth, suggestive of the predicted melt zone. The leading mechanism proposed for generating this melt zone is by dehydration melting, which is supported by modest transition-zone water concentrations inferred by electromagnetic sounding (Huang, Xu, Karato, 2005). In this mechanism, warm upwelling 'damp" transition-zone material (wadsleyite) crosses the 410km boundary, and arrives above the solidus water limit in the upper-mantle (olivine) partial melt stability field. The fate of the subsequently produced melt is important for inferring the structure, observability and stability of this melt region. The most recent models of a wet melt layer spreading along the 410km boundary and reacting with a background mantle flow predict that the layer will be several 10s of kilometers thick, and that the melt's material will be entrained into the lower mantle well before it reaches any slabs (Leahy & Bercovici, 2010). At these pressures the melt is possibly more dense than the solid, although the density cross-over point is not likely to be far above the 410km boundary. However, unless the density cross-over actually intersects the melt zone, the melt is stable to any Rayleigh-Taylor instability (Youngs & Bercovici, 2009). Finally, continued re-hydration of the transition zone is required to supply the melt layer in the presence of background mantle flow. Slabs foundering and flowing horizontally across the transition zone provide one of the best means for transporting water across the transition zone. Slabs at the bottom of the transition zone

  4. Melting in the Earth's deep upper mantle caused by carbon dioxide.

    PubMed

    Dasgupta, Rajdeep; Hirschmann, Marc M

    2006-03-30

    The onset of partial melting beneath mid-ocean ridges governs the cycling of highly incompatible elements from the mantle to the crust, the flux of key volatiles (such as CO2, He and Ar) and the rheological properties of the upper mantle. Geophysical observations indicate that melting beneath ridges begins at depths approaching 300 km, but the cause of this melting has remained unclear. Here we determine the solidus of carbonated peridotite from 3 to 10 GPa and demonstrate that melting beneath ridges may occur at depths up to 330 km, producing 0.03-0.3% carbonatite liquid. We argue that these melts promote recrystallization and realignment of the mineral matrix, which may explain the geophysical observations. Extraction of incipient carbonatite melts from deep within the oceanic mantle produces an abundant source of metasomatic fluids and a vast mantle residue depleted in highly incompatible elements and fractionated in key parent-daughter elements. We infer that carbon, helium, argon and highly incompatible heat-producing elements (such as uranium, thorium and potassium) are efficiently scavenged from depths of approximately 200-330 km in the upper mantle.

  5. Melt Generation in Heterogeneous Mantle Sources: A Three-Legged Stool Approach

    NASA Astrophysics Data System (ADS)

    Brown, E. L.; Lesher, C. E.

    2009-05-01

    The compositions and volumes of basalts generated by adiabatic decompression melting are primarily a function of three factors: mantle potential temperature, the style of mantle upwelling, and source composition. Attempts to use basalts to infer the relative importance of these three factors in specific localities are made difficult because even for homogeneous mantle sources, basalts are aggregates of melts generated over a range of pressures and temperatures within the melting regime. When source heterogeneity and differences in the melting behavior of source lithologies are accounted for, the complexity of relating basalts to the conditions of melt generation increases substantially. Advances in our understanding of mid - ocean ridge basalt petrogenesis have demonstrated the utility of creating geochemical models for melt generation that are constrained by experimental petrology [e.g. 1]. To better relate basalt compositions to the melting processes within a heterogeneous mantle source, we have developed a forward polybaric melting model that simulates the melting of a source comprised of pyroxenite and peridotite. The model uses thermodynamically - derived polybaric melting functions based on parameterizations of pyroxenite and peridotite melting [2, 3]. The model takes into account mantle potential temperature, style of mantle upwelling and variable amounts of pyroxenite, and outputs the isotopic and trace element compositions and volumes of pooled melts using the residual mantle column method [4]. We propagate uncertainties in model input parameters to assess robustness and compare our results with previous models [5-7]. We apply our model to ocean island and large igneous province environments to constrain potential temperature, upwelling rate and abundance of pyroxenite in the mantle source from observed basalt compositions and volumes. [1] Longhi 2002, G-cubed, doi:10.1029/2001/GC000204; [2] Katz et al. 2003, G-cubed, doi:10.1029/2002GC000433; [3

  6. REEBOX PRO: A forward model simulating melting of thermally and lithologically variable upwelling mantle

    NASA Astrophysics Data System (ADS)

    Brown, Eric L.; Lesher, Charles E.

    2016-10-01

    The compositions and volumes of basalts erupted in divergent margin environments provide a record of the thermal, chemical, and dynamical state of their mantle source regions. To relate basalt compositions and volumes to the underlying thermochemical and dynamical state of their mantle source regions, we have developed REEBOX PRO, a compiled stand-alone application that simulates adiabatic decompression melting of passively or actively upwelling mantle containing up to five distinct lithologies. The model calculates melt compositions using thermodynamic and experimental constraints on the melting behaviors and mineral-melt partitioning behavior of homogeneous and lithologically heterogeneous mantle sources containing anhydrous peridotite, hydrous peridotite, harzburgite, and/or silica-saturated/-undersaturated pyroxenite. Key model outputs include the mean composition and crustal thickness for the bulk basaltic crust, calculated for passive and active upwelling scenarios. Here, we present the mathematical formulations underlying the model and benchmark it against existing hydrous melting models and models for mid-ocean ridge basalt formation. We show that the hydrous and anhydrous peridotite melting models incorporated in REEBOX PRO capture the essential differences in basalt composition and volume demonstrated by previous models, and constrain the ambient mantle beneath the global spreading ridge system to be between 1319 and 1366°C, depending on the relative fertility and/or water content of the mid-ocean ridge mantle source. We also show how model outputs may be manipulated outside of the modeling program to calculate nontraditional melt mixing scenarios. These examples highlight the flexibility of REEBOX PRO for simulating melt generation within a range of geodynamical contexts.

  7. 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

  8. Rates of Melt Migration Following Deglaciation-Induced Mantle Melting Revealed by Studies of Icelandic Table Mountains

    NASA Astrophysics Data System (ADS)

    Eason, D. E.; Sinton, J. M.; Ito, G.; Gronvold, K.; Kurz, M. D.

    2010-12-01

    Deglaciation leads to enhanced melt production under Iceland due to mantle decompression during glacial unloading. Models of isostatic rebound following ice sheet removal predict the greatest melt perturbations at shallow depths, with corresponding changes in the chemical composition of erupted material. Some table mountains (tuyas, or stapi) in Iceland are thought to have erupted through a thinning ice sheet during ice retreat following the last glacial maximum (~15-11 ka). New observations and geochemical analyses of table mountains in the 170 km-long Western Volcanic Zone (WVZ) of Iceland constrain spatial and temporal variations in volcanic production and composition associated with the last deglaciation period. We present new major and trace element compositional data for 19 table mountains in the WVZ, and use a combination of lava surface morphology, passage zone heights, and 3He exposure age dating to construct an eruption chronology for WVZ subglacial units. Late-glacial units, whose surfaces are relatively unmodified and show no evidence of subsequent glaciation, have low incompatible element concentrations (e.g., ~0.05 wt. % K2O at 8.0 wt. % MgO), low FeO* (~10.0 wt. % at 8.0 wt. % MgO), and high SiO2 and CaO concentrations (~49.0 wt. % and 14.0 wt. % at 8.0 wt. % MgO, respectively), while older table mountains whose surfaces that have been modified by glacial processes after formation have chemical compositions more comparable to average WVZ post-glacial units. Decreased levels of highly incompatible to moderately incompatible elements and elevated SiO2 and CaO concentrations are also found in fini-glacial eruption units, which experienced minimal amounts of ice interaction during formation. These geochemical characteristics are consistent with enhanced melting in the upper mantle, as predicted by models of deglaciation-induced mantle decompression. Detailed geologic and geochemical studies of post-glacial units along the WVZ show the eruptive response

  9. A Two-Porosity Double-Lithology Model for Partial Melting, Melt Transport and Melt-Rock Reaction in the Mantle

    NASA Astrophysics Data System (ADS)

    Liang, Y.; Parmentier, M. E.

    2005-12-01

    Several lines of evidence suggest that the melting and melt extraction region of the mantle is heterogeneous consisting of interconnected networks of high porosity dunite channels in a low porosity harzburgite or lherzolite matrix. To better understand the dynamical processes of melting, melt migration, and melt-rock reaction in such a heterogeneous mantle, a two-porosity double lithology model has been developed. Here the region of interest is treated as two overlapping continua occupied by the low porosity matrix and high porosity channel system. Conservation equations for the matrix and channel continuum are coupled through interaction terms that take into account the mass, momentum and heat exchange between the two continua. Exchange terms for the mass conservation equations, for example, include reactive dissolution of the matrix, and diffusive and advective mixings between the melt in the channel and that in the matrix. The matrix dissolution rate is proportional to the extent of undersaturation of pyroxene with respect to the melt in the dunite channel. The diffusive exchange rate is proportional to the porosity of the matrix and the concentration difference between the melts in the channel and the matrix. The advective mixing rate is proportional to the permeability of the matrix and the pressure difference between the melts in the channel and the matrix. Both diffusive and advective mixing rates are inversely proportional to the square of the characteristic channel width (Xc). Essential features of the two-porosity double-lithology model have been investigated using simplified 1-D mass conservation equations. Key parameters include: element partition coefficients, porosity, channel volume fraction, rates of matrix dissolution, diffusive and advective mixing to the rate of matrix melting, and matrix-channel mass flux capacity ratio. The last parameter is a measure of the amount of material transport through the channel vs. that through the matrix. In

  10. Melting of the Earth's lithospheric mantle inferred from protactinium-thorium-uranium isotopic data

    PubMed

    Asmerom; Cheng; Thomas; Hirschmann; Edwards

    2000-07-20

    The processes responsible for the generation of partial melt in the Earth's lithospheric mantle and the movement of this melt to the Earth's surface remain enigmatic, owing to the perceived difficulties in generating large-degree partial melts at depth and in transporting small-degree melts through a static lithosphere. Here we present a method of placing constraints on melting in the lithospheric mantle using 231Pa-235U data obtained from continental basalts in the southwestern United States and Mexico. Combined with 230Th-238U data, the 231Pa-235U data allow us to constrain the source mineralogy and thus the depth of melting of these basalts. Our analysis indicates that it is possible to transport small melt fractions--of the order of 0.1%--through the lithosphere, as might result from the coalescence of melt by compaction owing to melting-induced deformation. The large observed 231Pa excesses require that the timescale of melt generation and transport within the lithosphere is small compared to the half-life of 231Pa (approximately 32.7 kyr). The 231Pa-230Th data also constrain the thorium and uranium distribution coefficients for clinopyroxene in the source regions of these basalts to be within 2% of one another, indicating that in this setting 230Th excesses are not expected during melting at depths shallower than 85 km.

  11. 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

  12. Melt transport and compositional heterogeneities of oceanic mantle: evidence from ophiolites

    NASA Astrophysics Data System (ADS)

    Batanova, Valentina; Savelieva, Galina

    2010-05-01

    Numerous studies of ophiolitic and abyssal peridotites published in last two decades convincingly demonstrate that compositional heterogeneities observed in mantle rocks have been largely produced by two main processes: partial melting and melt migration (e.g. [1]). While the effects and degrees of partial melting are more or less easy to decipher by use of petrologic indicators, the compositional changes of mantle peridotites resulting from the magma migration are highly variable and depend on various factors, among which the more important are the mechanisms of melt transport and composition of migrating magmas. Thermo-mechanical modeling suggests that porous flow of melt is the dominant mode of melt migration in the mantle (e.g. [2]). On the other hand, it is widely accepted that melt extraction from the mantle beneath mid-ocean ridges occurs as a focused flow via chemically isolated channels. In their pioneering works P. Kelemen and co-authors have shown, that mantle dunites mark such highly permeable channels and were formed by complete dissolution of pyroxene in peridotite during reactive melt flow (e.g. [3]). It is assumed that focusing of diffuse porous melt flow into the channel flow may occur as a result of reactive infiltration instability or/and under the influence of stress (e.g. [3, 4]). Focused magma ascent does not rule out a diffuse porous flow of small amount of melts in shallow mantle, resulted in refertilization of mantle peridotites [5]. For the spreading in supra-subduction zone (SSZ) settings (fore-arc, immature island arc or back-arc setting), where many of ophiolites were formed [6] one should consider that melt transport processes should have specific features due to thermal structures of SSZ and influence of fluid and/or melt flux derived from the subducted slab and induced the melting in the wedge. We illustrate the current state of issues discussed above by the examples of the mantle section of Voykar ophiolite, Polar Urals, where

  13. Experimental determination of melt interconnectivity and electrical conductivity in the upper mantle

    NASA Astrophysics Data System (ADS)

    Laumonier, Mickael; Farla, Robert; Frost, Daniel J.; Katsura, Tomoo; Marquardt, Katharina; Bouvier, Anne-Sophie; Baumgartner, Lukas P.

    2017-04-01

    The presence of a small fraction of basaltic melt is a potential explanation for mantle electrical conductivity anomalies detected near the top of the oceanic asthenosphere. The interpretation of magnetotelluric profiles in terms of the nature and proportion of melt, however, relies on mathematical models that have not been experimentally tested at realistically low melt fractions (<0.01). In order to address this, we have performed in situ electrical conductivity measurements on partially molten olivine aggregates. The obtained data suggest that the bulk conductivity follows the conventional Archie's law with the melt fraction exponents of 0.75 and 1.37 at melt fractions greater and smaller than 0.5 vol.% respectively at 1350 °C. Our results imply multiple conducting phases in melt-bearing olivine aggregate and a connectedness threshold at ∼0.5 vol.% of melt. The model predicts that the conductive oceanic upper asthenosphere contains 0.5 to 1 vol.% of melt, which is consistent with the durable presence of melt at depths over millions years while the oceanic plates spread apart at the mid-ocean ridge. Beneath ridges a minimum permeability may allow mid-ocean ridge basalts to rise out of the mantle, where our model indicates that melt is present in proportions of up to 4 vol.%.

  14. Os Isotopic Composition and Highly Siderophile Elements: Tracers of Mantle Melting and Melt Percolation Processes (Voykar Complex, Polar Ural Ophiolites)

    NASA Astrophysics Data System (ADS)

    Batanova, V.; Bruegmann, G.; Savelieva, G.

    2006-12-01

    The Voykar complex is located in the Northern part of the Uralian ophiolite belt and represents Early Devonian lithosphere formed in a suprasubduction environment (e. g. (1)). It is unique in that the mantle peridotites are very well preserved and virtually free of serpentine, and it provides excellent exposures. The mantle section of Voykar complex is composed of harzburgite which is intruded by numerous dunite and pyroxenite channels and contains chromitite ore deposits. The distribution of highly siderophile elements (HSE) in harzburgite and dunite channels can be explained by melt-peridotite reaction processes. The harzburgites (Cr/(Cr+Al) of cr-spinel = 0.3-0.4) show two types of HSE patterns. One is depleted in Pd, Pt and Re relatively to Os, Ir, Ru with (Pt/Ir)N=0.3, the second pattern is flat with (Pt/Ir)N=0.9. The first type of harzburgite has lower ratios of 187Os/188Os (0.1149) compared to the second (0.1236). Dunite and associated orthopyroxenite are significantly enriched in 187Os (187Os/188Os=0.1279 - 0.1327), hence shifted towards lava compositions. Their HSE patterns also show the influence of mantle melts, because they are enriched in Pd ((Pt/Ir)N=5-6). The clinopyroxenite has the highest ratio of 187Os/188Os up to 32.38 and shows even stronger depletion of Os, Ir, Ru relative to Pt, Pd, Re ((Pt/Ir)N=20). They could represent crystallization products of percolating melts. Chromitites have variable 187Os/188Os=0.1244-0.1352, and their HSE concentrations overlap with the range observed in chromitites world-wide. Osmium isotope data show evidence for at least two significant events in the magmatic history of the Voykar ophiolite mantle section: an ancient (2.1-1.9 Ga) melting event formed the depleted spinel harzburgite; a younger melt percolation event (0.6-0.5 Ga) led to the formation of pyroxenite veins, dunite channels and chromitites. The age of melt percolation events based on Os isotopic data coincides with the U-Pb age of zircons found in

  15. Volatile Systematics of the Icelandic Mantle from Olivine-Hosted Melt Inclusions

    NASA Astrophysics Data System (ADS)

    Miller, W. G. R.; Maclennan, J.; Thordarson, T.

    2014-12-01

    The behaviour of volatiles within a volcanic system can tell us about melt storage depths, melt evolution, and degassing processes. It is therefore important to be able to quantify the amount of each volatile species entering the system. Olivine-hosted melt inclusions can provide compositions of the primitive mantle melt entering volcanic systems. In some cases they have been unaffected by processes such as melt mixing, fractional crystallisation and degassing; so chemical variations are source signatures. With the knowledge that volatiles behave as very incompatible elements, and with measured volatile/trace-element ratios, we can estimate volatile concentrations within the Icelandic mantle source. Abundances of CO2, H2O, Cl, F, S and trace elements have been measured by SIMS for over 100 olivine-hosted (Fo87.8-88.6) melt inclusions from Kistufell, a monogenic subglacial eruption known to have elevated 3He/4He (15.5 R/Ra)[1] and situated above the inferred locus of the Icelandic mantle plume. H2O concentrations in the melt inclusions are near uniform and similar to that of the carrier glass (0.10-0.15 wt%), likely due to diffusive equilibration through the olivine host at low pressure. CO2 shows more variation, ranging from 1200 to 0ppm, probably from a combination of melt heterogeneity and degassing. The majority of melt inclusions are incompatible trace element-depleted (La/Yb = 1.3), indicating that they were probably generated by high fraction melting of a depleted source in the shallow mantle. The CO2/Nb ratio of this source mantle is ~308, very similar to that of Borgarhraun, ~314[2], which has a lower 3He/4He ratio (12.2 R/Ra)[3]. This CO2/Nb ratio, along with H2O/Ce ~200 and F/Nd ~18, are consistent with other measured melt inclusion suites from Iceland, suggesting limited volatile/trace-element ratio variation within the Icelandic depleted melt source. By analysing more eruptions across Iceland, we hope to identify any spatial variations in volatile

  16. Phase behavior and reactive transport of partial melt in heterogeneous mantle model

    NASA Astrophysics Data System (ADS)

    Jordan, J.; Hesse, M. A.

    2013-12-01

    The reactive transport of partial melt is the key process that leads to the chemical and physical differentiation of terrestrial planets and smaller celestial bodies. The essential role of the lithological heterogeneities during partial melting of the mantle is increasingly recognized. How far can enriched melts propagate while interacting with the ambient mantle? Can the melt flow emanating from a fertile heterogeneity be localized through a reactive infiltration feedback in a model without exogenous factors or contrived initial conditions? A full understanding of the role of heterogeneities requires reactive melt transport models that account for the phase behavior of major elements. Previous work on reactive transport in the mantle focuses on trace element partitioning; we present the first nonlinear chromatographic analysis of reactive melt transport in systems with binary solid solution. Our analysis shows that reactive melt transport in systems with binary solid solution leads to the formation of two separate reaction fronts: a slow melting/freezing front along which enthalpy change is dominant and a fast dissolution/precipitation front along which compositional changes are dominated by an ion-exchange process over enthalpy change. An intermediate state forms between these two fronts with a bulk-rock composition and enthalpy that are not necessarily bounded by the bulk-rock composition and enthalpy of either the enriched heterogeneity or the depleted ambient mantle. The formation of this intermediate state makes it difficult to anticipate the porosity changes and hence the stability of reaction fronts. Therefore, we develop a graphical representation for the solution that allows identification of the intermediate state by inspection, for all possible bulk-rock compositions and enthalpies of the heterogeneity and the ambient mantle. We apply the analysis to the partial melting of an enriched heterogeneity. This leads to the formation of moving precipitation

  17. Detection of melting atop the mantle transition zone

    NASA Astrophysics Data System (ADS)

    Hier-Majumder, S.; Keel, E. B.; Courtier, A. M.

    2012-12-01

    We explore the combined effect of thermal, chemical, and melting anomalies on shear wave velocities above the transition zone. While thermal and chemical heterogeneities influence the seismic velocities at subsolidus temperatures to some extent, the velocity structures are greatly modified in the presence of partial melting. We interpret two sets of data; ScS reflectivity beneath Coral Sea in the Southwest Pacific and receiver function analysis of P-to-S conversions beneath the Hawaiian islands. Both datasets indicate the presence of a Low Velocity Layer (LVL) at an average depth of 350 km. Temperature and subsolidus composition fail to fully explain the observed impedance contrast atop the LVL. Beneath the Coral Sea, an average melt volume fraction of 1% is necessary to explain the seismic observations, while beneath Hawaiia range of melt fractions describe the seismic signature. Melt fraction appears to increase from Northwest to Southeast along the island chain and trades off with dihedral angle.Map of calculated melt volume fraction at the Low Velocity layer (LVL) located approximately 350 km below the surface of Hawaii. The melt fraction is calculated for variations in the subsolidus basaltic component (X) and reference potential temperature (T).

  18. Seismic evidence for silicate melt atop the 410-km mantle discontinuity

    USGS Publications Warehouse

    Revenaugh, Justin; Sipkin, S.A.

    1994-01-01

    LABORATORY results demonstrating that basic to ultrabasic melts become denser than olivine-rich mantle at pressures above 6 GPa (refs 1-3) have important implications for basalt petrogenesis, mantle differentiation and the storage of volatiles deep in the Earth. A density cross-over between melt and solid in the extensively molten Archaean mantle has been inferred from komatiitic volcanism and major-element mass balances, but present-day evidence of dense melt below the seismic low-velocity zone is lacking. Here we present mantle shear-wave impedance profiles obtained from multiple-ScS reverberation mapping for corridors connecting western Pacific subduction zone earthquakes with digital seismograph stations in eastern China, imaging a ~5.8% impedance decrease roughly 330 km beneath the Sea of Japan, Yellow Sea and easternmost Asia. We propose that this represents the upper surface of a layer of negatively buoyant melt lying on top of the olivine ??? ??- phase transition (the 410-km seismic discontinuity). Volatile-rich fluids expelled from the partial melt zone as it freezes may migrate upwards, acting as metasomatic agents and perhaps as the deep 'proto-source' of kimberlites. The remaining, dense, crystalline fraction would then concentrate above 410 km, producing a garnet-rich layer that may flush into the transition zone.

  19. Seismic evidence for silicate melt atop the 410-km mantle discontinuity

    NASA Astrophysics Data System (ADS)

    Revenaugh, J.; Sipkin, S. A.

    1994-06-01

    LABORATORY results demonstrating that basic to ultrabasic melts become denser than olivine-rich mantle at pressures above 6 GPa (refs 1-3) have important implications for basalt petrogenesis, mantle differentiation and the storage of volatiles deep in the Earth. A density cross-over between melt and solid in the extensively molten Archaean mantle has been inferred from komatiitic volcanism4-6 and major-element mass balances7, but present-day evidence of dense melt below the seismic low-velocity zone is lacking. Here we present mantle shear-wave impedance profiles obtained from multiple-ScS reverberation mapping for corridors connecting western Pacific subduction zone earthquakes with digital seismograph stations in eastern China, imaging a ~5.8% impedance decrease roughly 330 km beneath the Sea of Japan, Yellow Sea and easternmost Asia. We propose that this represents the upper surface of a layer of negatively buoyant melt lying on top of the olivine-->β-phase transition (the 410-km seismic discontinuity). Volatile-rich fluids expelled from the partial melt zone as it freezes may migrate upwards, acting as metasomatic agents8,9 and perhaps as the deep 'proto-souree' of kimberlites10,11. The remaining, dense, crystalline fraction would then concentrate above 410 km, producing a garnet-rich layer that may flush into the transition zone.

  20. Density of Hydrous Ultramafic Silicate Melt under the Earth's Deep Upper Mantle Conditions

    NASA Astrophysics Data System (ADS)

    Jing, Z.; Matsukage, K. N.; Karato, S.

    2005-12-01

    Density of silicate melts is a critical material property in our understanding of geochemical evolution of the Earth. Previous studies (e.g., Agee & Walker 1993; Suzuki et al., 1995) showed that the density of dry silicate melts can be higher than that of surrounding solids under deep upper mantle conditions. However, melts formed under such conditions likely contain some water (Bercovici & Karato, 2003), which will reduce the melt density. In this study, we performed sink/float experiments between 10 and 14GPa and at 2173K to determine the density of hydrous ultramafic silicate melts, using a Kawai-type multianvil apparatus. We choose a target melt composition based on the experimental study by Litasov & Ohtani (2002). With this chemical composition, olivine reacts with the melt above the liquidus, so we used diamond as the density marker. However, diamond is much denser than a melt with a typical mantle like Fe/Mg ratio. Therefore in this study we determined the density of melts with high Fe contents, and from the relation between Fe content and melt density, we inferred the melt density with Earth-like Fe/Mg. Four Fe-rich compositions with 5wt% water and different iron content were chosen as starting materials. Density crossovers between melts and diamond were observed for all compositions. The densities of four melts at 14GPa and 2173K were calculated using the Birch-Murnaghan equation of state. The pressure derivative of isothermal bulk modulus (Kt') of the melts was estimated to be around 4. The density of mantle melt with mantle value of content and 5wt% water at 14GPa, 2173K was extrapolated to be ~3.42±0.4g/cm3. We compared our density results for hydrous melts with previous results on dry melts and found that water is more compressible than other components in melt. The estimated partial molar volume of water at 14GPa and 2173K is ~8±2cm3/mol, which is significantly lower than the value at low pressures. The conditions under which the density crossover

  1. Volatile-rich Melts in the Earth's Upper Mantle (AGU Kuno Medal)

    NASA Astrophysics Data System (ADS)

    Dasgupta, Rajdeep

    2013-04-01

    The onset of silicate magma generation in the Earth's upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. Although carbonatitic fluid with variable water content could be stable ≤250 km beneath mid oceanic ridges [1-3], owing to the small fraction (<< 1 wt.%), its effects on the mantle properties are unclear. Geophysical measurements, however, suggest that melts of greater volume may be present down to ~200 km [4-6] but large melt fractions is thought to be restricted to shallower depths. In this Kuno Award lecture, I will discuss the recent advancements on our understanding of deeper silicate melt generation induced by CO2-H2O volatiles and the relative stability of silicate versus carbonatitic melt in various tectonic settings. I will present recent experiments on carbonated peridotites that constrain the location and the slope of the onset of silicate melting in the mantle [7]. The new finding is that the pressure-temperature slope of carbonated silicate melting is steeper than the solidus of volatile-free peridotite and as a consequence the silicate melting of dry peridotite+CO2 beneath ridges commences at ~180 km. Accounting for the effect of 50-200 ppm of mantle H2O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with ~100 ppm CO2 becomes as deep as ~220-300 km [7]. This melting generates a kimberlitic magma with ~25 wt.% dissolved CO2 and 1-5 wt.% dissolved H2O. Based on the recent constraints of oxygen fugacity of the mantle in the garnet peridotite field [2, 3], we suggest that on a global scale, carbonated silicate melt generation at ~250-180 km deep redox solidus, with destabilization of metal and majorite in the upwelling mantle, explains oceanic low-velocity zone and electrical conductivity structure of the mantle. In locally oxidized domains (i.e., higher than average Fe3+/Fetotal), deeper carbonated

  2. 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

  3. Mapping mantle-melting anomalies in Baja California: a combined helium-seismology approach

    NASA Astrophysics Data System (ADS)

    Negrete-Aranda, R.; Spelz, R. M.; Hilton, D. R.; Tellez, M.; González-Yahimovich, O.

    2015-12-01

    In active tectonic settings, the presence of helium in aqueous fluids with 3He/4He ratios greater than in-situ production values (~0.05 RA where RA = air He or 1.4 x 10-6) indicates the contribution of mantle-derived volatiles to the total volatile inventory. This is an indicative of the presence of mantle-derived melts, which act to transfer volatiles from the solid Earth towards the surface. Thus, He has the potential to map regions of the underlying mantle which are undergoing partial melting - a phenomenon which should also be evident in the seismic record. Reports of high 3He/4He in hot springs in Baja California (BC) has prompted us to initiate a survey of the region to assess relationship(s) between He isotopes and geophysical images of the underlying mantle. Previous studies report 3He/4He ratios of 0.54 RA for submarine hot springs (Punta Banda 108oC; Vidal, 1982) and 1.3 RA for spring waters (81oC) at Bahia Concepcion (Forrest et al.,2005). Our new survey of hot springs in northern BC has revealed that all 6 localities sampled to date, show the presence of mantle He with the highest ratio being 1.74RA (21% mantle-derived) at Puertecitos on the Gulf coast. He ratios are generally lower on the Pacific coast with the minimum mantle He contribution being 5% at Sierra Juárez (0.11RA). Thus, preliminary trends are of a west-to-east increase in the mantle He signal across the peninsula. He results presented in this study correlate well with high resolution Rayleigh wave tomography images by Forsythe et al. (2007). Shear velocity variations in the BC crust and upper mantle have been interpreted as low velocity anomalies associated with dynamic upwelling and active melt production. More extensive sampling throughout BC coupled with analysis of other geochemical indicators of mantle degassing (e.g. CO2) will allow more detailed characterization of the extent and distribution of mantle melts in the region, facilitating assessment of the region's geothermal

  4. Nickel and helium evidence for melt above the core-mantle boundary.

    PubMed

    Herzberg, Claude; Asimow, Paul D; Ionov, Dmitri A; Vidito, Chris; Jackson, Matthew G; Geist, Dennis

    2013-01-17

    High (3)He/(4)He ratios in some basalts have generally been interpreted as originating in an incompletely degassed lower-mantle source. This helium source may have been isolated at the core-mantle boundary region since Earth's accretion. Alternatively, it may have taken part in whole-mantle convection and crust production over the age of the Earth; if so, it is now either a primitive refugium at the core-mantle boundary or is distributed throughout the lower mantle. Here we constrain the problem using lavas from Baffin Island, West Greenland, the Ontong Java Plateau, Isla Gorgona and Fernandina (Galapagos). Olivine phenocryst compositions show that these lavas originated from a peridotite source that was about 20 per cent higher in nickel content than in the modern mid-ocean-ridge basalt source. Where data are available, these lavas also have high (3)He/(4)He. We propose that a less-degassed nickel-rich source formed by core-mantle interaction during the crystallization of a melt-rich layer or basal magma ocean, and that this source continues to be sampled by mantle plumes. The spatial distribution of this source may be constrained by nickel partitioning experiments at the pressures of the core-mantle boundary.

  5. Martian mantle primary melts - An experimental study of iron-rich garnet lherzolite minimum melt composition

    NASA Technical Reports Server (NTRS)

    Bertka, Constance M.; Holloway, John R.

    1988-01-01

    The minimum melt composition in equilibrium with an iron-rich garnet lherzolite assemblage is ascertained from a study of the liquidus relations of iron-rich basaltic compositions at 23 kb. The experimentally determined primary melt composition and its calculated sodium content reveal that Martian garnet lherzolite minimum melts are picritic alkali olivine basalts. Martian primary melts are found to be more picritic than terrestrial garnet lherzolite primary melts.

  6. Isotopic equilibrium between mantle peridotite and melt: Evidence from the Corsica ophiolite

    NASA Astrophysics Data System (ADS)

    Rampone, Elisabetta; Hofmann, Albrecht W.; Raczek, Ingrid

    2009-11-01

    A widely used assumption of mantle geochemistry and the theory of partial melting at oceanic settings is the existence of isotopic equilibrium between mantle source and melt. Yet, recent diffusion studies and isotopic investigations of ophiolites, abyssal peridotites and associated MORBs have cast doubts on this assumption, by providing evidence for isotopic disequilibrium between residual peridotites and MORBs. Here we present Sr and Sm-Nd isotope data on mantle peridotites and gabbroic intrusions from the Mt. Maggiore (Alpine Corsica, France) Tethyan ophiolite, which document Nd isotopic homogeneity, implying isotopic equilibrium, on a 1-kilometer scale. The peridotites record multi-stage melt-rock interaction and melt intrusion occurring at different lithospheric depths. Samples studied are residual cpx-poor spinel lherzolites, reactive spinel harzburgites, impregnated plagioclase peridotites and related gabbronoritic veinlets, later gabbroic dykes. Strontium isotopes in peridotites and gabbros are highly variable, due to interaction with sea-water derived fluids, and cannot be used to test melt-residue isotopic equilibrium. In contrast, Nd isotopes are unaffected by sea-water alteration. Peridotites display present-day high 147Sm/ 144Nd (0.49-0.59) and 143Nd/ 144Nd (0.513367-0.513551) ratios, with no appreciable differences between residual and reactive spinel peridotites, and between spinel and plagioclase peridotites. Gabbroic dykes have present-day Nd isotopic compositions typical of MORB ( 143Nd/ 144Nd = 0.513122-0.513138). Internal (plag-whole rock-cpx) Sm-Nd isochrons for olivine gabbro dykes and a gabbronoritic veinlet yield Jurassic ages (162 ± 10 and 159 ± 15 Ma in ol-gabbros, 155 ± 6 Ma in gabbronorite), and initial ɛNd = 8.9-9.7 indicative of a MORB-type source. Sm-Nd isotopic compositions of peridotites conform to the linear array defined by the gabbroic rocks, and yield initial (160 Ma) ɛNd values of 7.6-8.9, again consistent with a MORB

  7. Fate of MgSiO3 melts at core–mantle boundary conditions

    PubMed Central

    Petitgirard, Sylvain; Malfait, Wim J.; Sinmyo, Ryosuke; Kupenko, Ilya; Hennet, Louis; Harries, Dennis; Dane, Thomas; Burghammer, Manfred; Rubie, Dave C.

    2015-01-01

    One key for understanding the stratification in the deep mantle lies in the determination of the density and structure of matter at high pressures, as well as the density contrast between solid and liquid silicate phases. Indeed, the density contrast is the main control on the entrainment or settlement of matter and is of fundamental importance for understanding the past and present dynamic behavior of the deepest part of the Earth’s mantle. Here, we adapted the X-ray absorption method to the small dimensions of the diamond anvil cell, enabling density measurements of amorphous materials to unprecedented conditions of pressure. Our density data for MgSiO3 glass up to 127 GPa are considerably higher than those previously derived from Brillouin spectroscopy but validate recent ab initio molecular dynamics simulations. A fourth-order Birch–Murnaghan equation of state reproduces our experimental data over the entire pressure regime of the mantle. At the core–mantle boundary (CMB) pressure, the density of MgSiO3 glass is 5.48 ± 0.18 g/cm3, which is only 1.6% lower than that of MgSiO3 bridgmanite at 5.57 g/cm3, i.e., they are the same within the uncertainty. Taking into account the partitioning of iron into the melt, we conclude that melts are denser than the surrounding solid phases in the lowermost mantle and that melts will be trapped above the CMB. PMID:26578761

  8. Trace-element fractionation in Hadean mantle generated by melt segregation from a magma ocean.

    PubMed

    Caro, Guillaume; Bourdon, Bernard; Wood, Bernard J; Corgne, Alexandre

    2005-07-14

    Calculations of the energetics of terrestrial accretion indicate that the Earth was extensively molten in its early history. Examination of early Archaean rocks from West Greenland (3.6-3.8 Gyr old) using short-lived 146Sm-142Nd chronometry indicates that an episode of mantle differentiation took place close to the end of accretion (4.46 +/- 0.11 Gyr ago). This has produced a chemically depleted mantle with an Sm/Nd ratio higher than the chondritic value. In contrast, application of 176Lu-176Hf systematics to 3.6-3.8-Gyr-old zircons from West Greenland indicates derivation from a mantle source with a chondritic Lu/Hf ratio. Although an early Sm/Nd fractionation could be explained by basaltic crust formation, magma ocean crystallization or formation of continental crust, the absence of coeval Lu/Hf fractionation is in sharp contrast with the well-known covariant behaviour of Sm/Nd and Lu/Hf ratios in crustal formation processes. Here we show using mineral-melt partitioning data for high-pressure mantle minerals that the observed Nd and Hf signatures could have been produced by segregation of melt from a crystallizing magma ocean at upper-mantle pressures early in Earth's history. This residual melt would have risen buoyantly and ultimately formed the earliest terrestrial protocrust.

  9. Dynamics and melting of a heterogeneous mantle: Importance to geographic variations in hotspot lava composition

    NASA Astrophysics Data System (ADS)

    Bianco, Todd Anthony

    Geochemical variations in hotspot lava compositions commonly reveal geographical and temporal trends over hundreds of kilometers and millions of years. These trends provide clues regarding the character of mantle heterogeneity and the dynamics feeding hotspot magmatism. This work examines an alternative end-member scenario of mantle structure in the plume hypothesis, in which heterogeneity exists in the form of small veins uniformly distributed throughout the mantle matrix. A numerical model couples equations of 3D mantle convection and melting in order to simulate realistic mantle processes. Mantle components with relatively enriched incompatible-element compositions are assumed to be more fusible than relatively depleted components. Melting is assumed to be fractional, and magma pools at the surface assuming it mixes perfectly. Resulting geographic trends at the surface are described. The overall conclusion of the study is that large-scale (102 km) geographic variations arise from the dynamics of plume-lithosphere interaction. The pattern of compositional trends at the surface is controlled by the thickness of the lithosphere, the reference viscosity of the mantle, and the difference between the depths at which components begin melting. These physical parameters control the compositional pattern because they influence the size, position, extent of melting, and melting rate of the different mantle components. In simulations of intraplate hotspots, the average composition of magma erupted at a volcano changes as it grows such that the influence of less refractory components is greatest in the early stages of volcanism, and the influence of more refractory components increases with time. These predictions are compared to observed variations in Nd and Pb isotope ratios at Hawaii, and observations of Pb isotope ratios at Samoa, and Reunion. In simulations of plumes rising beneath a mid-ocean ridge, the contribution from a less refractory component tends to increase

  10. Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle

    SciTech Connect

    Berry, A.J.; Danyushevsky, L.; O'Neill, H.C.; Newville, M.; Sutton, S.R.

    2008-10-16

    Komatiites are volcanic rocks mainly of Archaean age that formed by unusually high degrees of melting of mantle peridotite. Their origin is controversial and has been attributed to either anhydrous melting of anomalously hot mantle or hydrous melting at temperatures only modestly greater than those found today. Here we determine the original Fe{sup 3+}/{Sigma}Fe ratio of 2.7-Gyr-old komatiitic magma from Belingwe, Zimbabwe, preserved as melt inclusions in olivine, to be 0.10 {+-} 0.02, using iron K-edge X-ray absorption near-edge structure spectroscopy. This value is consistent with near-anhydrous melting of a source with a similar oxidation state to the source of present-day mid-ocean-ridge basalt. Furthermore, this low Fe{sup 3+}/{Sigma}Fe value, together with a water content of only 0.2--0.3 wt%, excludes the possibility that the trapped melt contained significantly more water that was subsequently lost from the inclusions by reduction to H{sub 2} and diffusion. Loss of only 1.5 wt% water by this mechanism would have resulted in complete oxidation of iron (that is, the Fe{sup 3+}/{Sigma}Fe ratio would be {approx}1). There is also no petrographic evidence for the loss of molecular water. Our results support the identification of the Belingwe komatiite as a product of high mantle temperatures ({approx}1,700 C), rather than melting under hydrous conditions (3--5-wt% water), confirming the existence of anomalously hot mantle in the Archaean era.

  11. Pervasive upper mantle melting beneath the western US

    NASA Astrophysics Data System (ADS)

    Hier-Majumder, Saswata; Tauzin, Benoit

    2017-04-01

    We report from converted seismic waves, a pervasive seismically anomalous layer above the transition zone beneath the western US. The layer, characterized by an average shear wave speed reduction of 1.6%, spans over an area of ∼ 1.8 ×106 km2 with thicknesses varying between 25 and 70 km. The location of the layer correlates with the present location of a segment of the Farallon plate. This spatial correlation and the sharp seismic signal atop of the layer indicate that the layer is caused by compositional heterogeneity. Analysis of the seismic signature reveals that the compositional heterogeneity can be ascribed to a small volume of partial melt (0.5 ± 0.2 vol% on average). This article presents the first high resolution map of the melt present within the layer. Despite spatial variations in temperature, the calculated melt volume fraction correlates strongly with the amplitude of P-S conversion throughout the region. Comparing the values of temperature calculated from the seismic signal with available petrological constraints, we infer that melting in the layer is caused by release of volatiles from the subducted Farallon slab. This partially molten zone beneath the western US can sequester at least 1.2 ×1017 kg of volatiles, and can act as a large regional reservoir of volatile species such as H or C.

  12. Experimental melting of phlogopite-bearing mantle at 1 GPa: Implications for potassic magmatism

    NASA Astrophysics Data System (ADS)

    Condamine, Pierre; Médard, Etienne

    2014-07-01

    We have experimentally investigated the fluid-absent melting of a phlogopite peridotite at 1.0 GPa (1000-1300 °C) to understand the source of K2O- and SiO2-rich magmas that occur in continental, post-collisional and island arc settings. Using a new extraction technique specially developed for hydrous conditions combined with iterative sandwich experiments, we have determined the composition of low- to high-degree melts (Φ=1.4 to 24.2 wt.%) of metasomatized lherzolite and harzburgite sources. Due to small amounts of adsorbed water in the starting material, amphibole crystallized at the lowest investigated temperatures. Amphibole breaks down at 1050-1075 °C, while phlogopite-breakdown occurs at 1150-1200 °C. This last temperature is higher than the previously determined in a mantle assemblage, due to the presence of stabilizing F and Ti. Phlogopite-lherzolite melts incongruently according to the continuous reaction: 0.49 phlogopite + 0.56 orthopyroxene + 0.47 clinopyroxene + 0.05 spinel = 0.58 olivine + 1.00 melt. In the phlogopite-harzburgite, the reaction is: 0.70 phlogopite + 1.24 orthopyroxene + 0.05 spinel = 0.99 olivine + 1.00 melt. The K2O content of water-undersaturated melts in equilibrium with residual phlogopite is buffered, depending on the source fertility: from ∼3.9 wt.% in lherzolite to ∼6.7 wt.% in harzburgite. Primary melts are silica-saturated and evolve from trachyte to basaltic andesite (63.5-52.1 wt.% SiO2) with increasing temperature. Calculations indicate that such silica-rich melts can readily be extracted from their mantle source, due to their low viscosity. Our results confirm that potassic, silica-rich magmas described worldwide in post-collisional settings are generated by melting of a metasomatized phlogopite-bearing mantle in the spinel stability field.

  13. 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

  14. Cenozoic magmatism in the South China Basin: Decompression melting and implications of an enriched mantle source

    SciTech Connect

    Flower, M.F.J.; Kan Tu; Ming Zhang ); Guanghong Xie )

    1990-06-01

    A widespread eposide of interplate volcanism followed the cessation of seafloor spreading in the South China Basin (SCB), affecting the South China Sea, and fringing areas of southern China and Indochina. Geochemical data for basalts from South China Sea islands and seamounts, Hainan Island, and Taiwan define an enriched (Dupal-like) mantle domain yielding oceanic island basalt (OIB) suites with {Delta}7/4Pb = 2-13, {Delta}8/4Pb = 45-73, {sup 87}Sr/{sup 86}Sr > {approximately}0.70325, Th/Ta > 2, and Th/Ba > 0.02. Opening of the SCB resulted from disaggregation of the South China block in response to the Indo-Eurasian collision, a process involving at least one seafloor spreading episode, terminated by collision of microcontinents with the Philippines and Borneo. The lack of precursive flood basalt suggests that active mantle upwelling was not involved and that melting was a passive effect of lithosphere stretching. However, while mantle decompression at ambient stretching factors ({approximately}1.7-2.5) appears to permit melting on the observed scale, the enriched source may preclude such a simple mantle dynamic. Three alternatives are considered: (1) passive melting of a mature metasomatised boundary layer, (2) active melting of thermally eroded subcontinental lithosphere (deep enrichment) or metasomatised boundary layer (shallow enrichment), and (3) relict diapirs of pre-SCB and/or Java trench subduction slabs (intermediate/deep enrichment). These models are evaluated in terms of chemical and isotopic mass balances associated with the generation and movement of small melt fractions in depleted, nondepleted, and enriched mantle.

  15. Geochemistry of mafic Paleocene volcanic rocks in the Valle del Cura region: Implications for the petrogenesis of primary mantle-derived melts over the Pampean flat-slab

    NASA Astrophysics Data System (ADS)

    Litvak, Vanesa D.; Poma, Stella

    2010-04-01

    Mafic volcanism of Paleocene age was recently reported in the Valle del Cura region and the El Indio Belt in the aphanitic and very homogenous well-preserved lavas flows of the Río Frío Basalts unit. These are high-K basalts, with high Fe 2O 3 and TiO 2 contents that imply an alkaline tendency and show typical intraplate-type patterns on a MORB normalized trace elements plot. Sr and Nd isotopic ratios evidence a mantle affinity. The chemistry indicates that these rocks are high temperature melts that result from a low degree of melting of an enriched portion of lithospheric mantle, with no contamination from crustal derived components. The alkaline back-arc Las Máquinas Basalts of Lower Miocene age are derived from more primitive magmas closer to the original source. Mantle composition was relatively constant from Paleocene to Lower Miocene in the studied latitudes over the Pampean flat-slab. Both mafic units share the isotopic trend of pre-Miocene mafic lavas from the Central Andes that were not affected by crustal contamination. Post-Miocene mafic lavas show a strong influence from crust-related processes.

  16. Morphology of melt-rich channels formed during reaction infiltration experiments on partially molten mantle rocks

    NASA Astrophysics Data System (ADS)

    Pec, Matej; Holtzman, Benjamin; Zimmerman, Mark; Kohlstedt, David

    2016-04-01

    Geochemical, geophysical and geological observations suggest that melt extraction from the partially molten mantle occurs by some sort of channelized flow. Melt-solid reactions can lead to melt channelization due to a positive feedback between melt flow and reaction. If a melt-solid reaction increases local permeability, subsequent flow is increased as well and promotes further reaction. This process can lead to the development of high-permeability channels which emerge from background flow. In nature, anastomozing tabular dunite bodies within peridotitic massifs are thought to represent fossilized channels that formed by reactive flow. The conditions under which such channels can emerge are treated by the reaction infiltration instability (RII) theory (e.g. Szymczak and Ladd 2014). In this contribution, we report the results of a series of Darcy type experiments designed to study the development of channels due to RII in mantle lithologies (Pec et al. 2015). We sandwiched a partially molten rock between a melt source and a porous sink and annealed it at high-pressures (P = 300 MPa) and high-temperatures (T = 1200° or 1250° C) under a controlled pressure gradient (∇P = 0-100 MPa/mm) for up to 5 hours. The partially molten rock is formed by 50:50 mixtures of San Carlos olivine (Ol, Fo ˜ 88) and clinopyroxene (Cpx) with either 4, 10 or 20 vol% of alkali basalt added. The source and sink are disks of alkali basalt and porous alumina, respectively. During the experiments, silica undersaturated melt from the melt source dissolves Cpx and precipitates an iron rich Ol (Fo ˜ 82) thereby forming a Cpx-free reaction layer at the melt source - partially molten rock interface. The melt fraction in the reaction layer increases significantly (40% melt) compared to the protolith, confirming that the reaction increases the permeability of the partially molten rock. In experiments annealed under a low pressure gradient (and hence slow melt flow velocity) the reaction layer is

  17. Oceanic mantle rocks reveal evidence for an ancient, 1.2-1.3 Ga global melting event

    NASA Astrophysics Data System (ADS)

    Dijkstra, A. H.; Sergeev, D.; McTaminey, L.; Dale, C. W.; Meisel, T. C.

    2011-12-01

    It is now increasingly being recognized that many oceanic peridotites are refertilized harzburgites, and that the refertilization often masks an extremely refractory character of the original mantle rock 'protolith'. Oceanic peridotites are, when the effects of melt refertilization are undone, often too refractory to be simple mantle melting residues after the extraction of mid-ocean ridge basalts at a spreading center. Rhenium-osmium isotope analysis is a powerful method to look through the effects of refertilization and to obtain constraints on the age of the melting that produced the refractory mantle protolith. Rhenium-depletion model ages of such anomalously refractory oceanic mantle rocks - found as abyssal peridotites or as mantle xenoliths on ocean islands - are typically >1 Ga, i.e., much older than the ridge system at which they were emplaced. In my contribution I will show results from two case studies of refertilized anciently depleted mantle rocks (Macquarie Island 'abyssal' peridotites and Lanzarote mantle xenoliths). Interestingly, very refractory oceanic mantle rocks from sites all around the world show recurring evidence for a Mesoproterozoic (~1.2-1.3 Ga) melting event [1]. Therefore, oceanic mantle rocks seem to preserve evidence for ancient melting events of global significance. Alternatively, such mantle rocks may be samples of rafts of ancient continental lithospheric mantle. Laser-ablation osmium isotope 'dating' of large populations of individual osmium-bearing alloys from mantle rocks is the key to better constrain the nature and significance of these ancient depletion events. Osmium-bearing alloys form when mantle rocks are melted to high-degrees. We have now extracted over >250 detrital osmium alloys from placer gold occurrences in the river Rhine. These alloys are derived from outcrops of ophiolitic mantle rocks in the Alps, which include blocks of mantle rocks emplaced within the Tethys Ocean, and ultramafic lenses of unknown

  18. Deep mixing of mantle melts beneath continental flood basalt provinces: Constraints from olivine-hosted melt inclusions in primitive magmas

    NASA Astrophysics Data System (ADS)

    Jennings, Eleanor S.; Gibson, Sally A.; Maclennan, John; Heinonen, Jussi S.

    2017-01-01

    We present major and trace element compositions of 154 re-homogenised olivine-hosted melt inclusions found in primitive rocks (picrites and ferropicrites) from the Mesozoic Paraná-Etendeka and Karoo Continental Flood Basalt (CFB) provinces. The major element compositions of the melt inclusions, especially their Fe/Mg ratios, are variable and erratic, and attributed to the re-homogenisation process during sample preparation. In contrast, the trace element compositions of both the picrite and ferropicrite olivine-hosted melt inclusions are remarkably uniform and closely reflect those of the host whole-rocks, except in a small subset affected by hydrothermal alteration. The Paraná-Etendeka picrites and ferropicrites are petrogenetically related to the more evolved and voluminous flood basalts, and so we propose that compositional homogeneity at the melt inclusion scale implies that the CFB parental mantle melts were well mixed prior to extensive crystallisation. The incompatible trace element homogeneity of olivine-hosted melt inclusions in Paraná-Etendeka and Karoo primitive magmatic rocks has also been identified in other CFB provinces and contrasts with findings from studies of basalts from mid-ocean ridges (e.g. Iceland and FAMOUS on the Mid Atlantic Ridge), where heterogeneity of incompatible trace elements in olivine-hosted melt inclusions is more pronounced. We suggest that the low variability in incompatible trace element contents of olivine-hosted melt inclusions in near-primitive CFB rocks, and also ocean island basalts associated with moderately thick lithosphere (e.g. Hawaii, Galápagos, Samoa), may reflect mixing along their longer transport pathways during ascent and/or a temperature contrast between the liquidus and the liquid when it arrives in the crust. These thermal paths promote mixing of mantle melts prior to their entrapment by growing olivine crystals in crustal magma chambers. Olivine-hosted melt inclusions of ferropicrites from the Paran

  19. Effects of water, depth and temperature on partial melting of mantle-wedge fluxed by hydrous sediment-melt in subduction zones

    NASA Astrophysics Data System (ADS)

    Mallik, Ananya; Dasgupta, Rajdeep; Tsuno, Kyusei; Nelson, Jared

    2016-12-01

    This study investigates the partial melting of variable bulk H2O-bearing parcels of mantle-wedge hybridized by partial melt derived from subducted metapelites, at pressure-temperature (P-T) conditions applicable to the hotter core of the mantle beneath volcanic arcs. Experiments are performed on mixtures of 25% sediment-melt and 75% fertile peridotite, from 1200 to 1300 °C, at 2 and 3 GPa, with bulk H2O concentrations of 4 and 6 wt.%. Combining the results from these experiments with previous experiments containing 2 wt.% bulk H2O (Mallik et al., 2015), it is observed that all melt compositions, except those produced in the lowest bulk H2O experiments at 3 GPa, are saturated with olivine and orthopyroxene. Also, higher bulk H2O concentration increases melt fraction at the same P-T condition, and causes exhaustion of garnet, phlogopite and clinopyroxene at lower temperatures, for a given pressure. The activity coefficient of silica (ϒSiO2) for olivine-orthopyroxene saturated melt compositions (where the activity of silica, aSiO2 , is buffered by the reaction olivine + SiO2 = orthopyroxene) from this study and from mantle melting studies in the literature are calculated. In melt compositions generated at 2 GPa or shallower, with increasing H2O concentration, ϒSiO2 increases from <1 to ∼1, indicating a transition from non-ideal mixing as OH- in the melt (ϒSiO2 <1) to ideal mixing as molecular H2O (ϒSiO2 ∼1). At pressures >2 GPa, ϒSiO2 >1 at higher H2O concentrations in the melt, indicate requirement of excess energy to incorporate molecular H2O in the silicate melt structure, along with a preference for bridging species and polyhedral edge decorations. With vapor saturation in the presence of melt, ϒSiO2 decreases indicating approach towards ideal mixing of H2O in silicate melt. For similar H2O concentrations in the melt, ϒSiO2 for olivine-orthopyroxene saturated melts at 3 GPa is higher than melts at 2 GPa or shallower. This results in melts generated at

  20. Melting Processes and Mantle Heterogeneity Recorded by Individual Phases from Mid-Ocean Ridge Basalts

    NASA Astrophysics Data System (ADS)

    Burton, K. W.; Parkinson, I. J.

    2014-12-01

    Isotope and elemental studies of mantle rocks and oceanic basalts demonstrate that Earth's mantle is heterogeneous, comprising distinct components that have experienced isolated long-term evolution, on both global and local scales. In principle, such heterogeneity will control the onset of melting and at least some of the chemical variation seen in Mid-Ocean Ridge Basalts (MORB) (e.g. [1]). But, the high degrees of melting that generate MORB, together with magma mixing and assimilation, have the effect of homogenising the compositions of lavas erupted at the surface, concealing the true extent of the variability in the mantle source. This study presents high-precision double-spike Pb isotope data for the consituent phases of MORB from a single ridge segment from the FAMOUS region (36°50'N) on the Mid-Atlantic ridge. Separated phases from individual basalts show a remarkable variation in Pb isotope composition, greater than that seen for all samples previously analysed from this ridge segment, and encompassing >70% of the variation seeen globally in MORB. These variations cannot be explained by assimilation of seawater altered oceanic crust or by contamination from the Azores, both of which carry a radiogenic Pb isotope siganture. Rather they indicate mixing between an early extremely unradiogenic melt, from which plagioclase, clinopyroxene and sulphide crystallised, sourced by material showing long-term depletion of U, and a later more radiogenic melt that produced the final glass host. Elemental and isotope data suggest that the source of this early melt was ancient, enriched, with a crust-like chemical signature, producing a relatively volatile-rich melt. This study confirms that signficant information may be preserved in the early crystallising minerals at slow spreading ridges, either phenocryst phases or the melt inclusions that they host (e.g. [2]). Overall, these results suggest that there is a simple relationship between the scale and nature of mantle

  1. Melt-Crystal Density Crossover in the Earth: its Importance in Mantle Dynamics

    NASA Astrophysics Data System (ADS)

    Ohtani, E.

    2005-12-01

    Density of magma is important for chemical fractionation of the Earth interior. Since magma is extremely compressible compared to crystals, a density crossover between magma and crystals is expected to occur in the deep mantle. Several regions of the magma-crystal density crossover are expected to occur in the mantle. The density crossover between peridotite magma and equilibrium olivine was observed at a pressure around 13 GPa [1]. Thus, in the early earth neutral buoyancy of olivine occurs in the primordial magma ocean and effective separation of olivine did not occur at the depth. This could produce an olivine rich upper mantle in the magma ocean stage. Density crossover in the deep upper mantle also provides a maximum depth for generation of komatiites, i.e., although komatiites can be generated by melting at various depths in the upper mantle, they can not ascend at the depths greater than the base of the upper mantle, the region where melt-crystal density crossover occurs in the early Earth. The density of hydrous magma is also important in the present Earth. We determined the density of hydrous magma by using sink-float method, and estimated the partial molar volume of H2O in magmas at high pressures [2]. The result implies that a density crossover exists between the mantle and hydrous magmas containing H2O up to about 5.2 wt percent, and the magmas could be gravitationally stable at the base of the upper mantle. Low V and low Q regions are reported at the base of the upper mantle by seismic studies [3, 4], implying that the region is likely to be caused by accumulation of gravitationally stable hydrous magma. The base of the lower mantle is also a possible region where a dense magma can accumulate [5]. The ultralow velocity zone has been explained by existence of a dense magma [6, 7]. Ohtani [8] and Ohtani and Maeda [9] speculated a possible existence of dense magmas at the base of the lower mantle by extrapolation of the magma density into the CMB

  2. Numerical models of mantle lithosphere weakening, erosion and delamination induced by melt extraction and emplacement

    NASA Astrophysics Data System (ADS)

    Wallner, Herbert; Schmeling, Harro

    2016-09-01

    Continental rifting caused by extension and heating from below affects the lithosphere or cratons in various ways. Volcanism and melt intrusions often occur along with thinning, weakening and even breaking lithosphere. Although mechanical necking models of the lithosphere are often applied, the aspects of melting and the implications due to melt transport and emplacement at shallower depths are not well understood. A two-phase flow approach employing melt extraction and shallow emplacement associated with thermal weakening is developed and compared with observations. The results of this comparison indicate the importance of partial melts and an asthenospheric magma source for increasing the rising rate of the lithosphere-asthenosphere boundary during extension. Thermo-mechanical physics of visco-plastic flow is approximated using the Finite Difference method with Eulerian formulation in 2D. The conservation of mass, momentum and energy equations are solved for a multi-component (crust-mantle) and two-phase (melt-matrix) system. Rheology is temperature- and stress-dependent. In consideration of depletion and enrichment melting and solidification are controlled by a simplified linear binary solid solution model. Melt is extracted and emplaced in predefined depth regions (emplacement zones) in the lithospheric mantle and crust. The Compaction Boussinesq Approximation was applied; its validity was tested against the Full Compaction formulation and found fully satisfactory for the case of sublithospheric melting models. A simple model guided by the geodynamic situation of the Rwenzori region typically results in updoming asthenosphere with melt-assisted erosion of the lithosphere's base. Even with a conservative approach for a temperature anomaly melting alone doubles the lithospheric erosion rate in comparison with a model without melting. With melt extraction and intrusion lithospheric erosion and upwelling of the lithosphere-asthenosphere boundary speeds up by a

  3. Mantle flow and melt migration beneath oceanic ridges: Models derived from observations in ophiolites

    NASA Astrophysics Data System (ADS)

    Ceuleneer, Georges; Rabinowicz, Michel

    The tectonic and volcanic features of the Earth are largely conditioned by processes taking place in the upper mantle. However our present knowledge of the mantle is essentially derived from indirect evidence. As pointed out by Oliver [1991], the promising technologies of very deep drilling and of high resolution geophysical imagery which should improve our views of the Earth's interior are still to be invented. In the meantime, to progress in the study of geological stuffs, widespread and of easy access at the Earth surface, and in their interpretation in terms of mantle processes, is not a vain task. For example, the Earth's magmatic budget is largely dominated by oceanic ridge and hotspot volcanism [e.g. Crisp, 1984]. The basaltic melts erupted there are likely produced by pressure-release induced partial melting of a peridotitic source. Consequently, their composition and distribution are expected to be sensitive to the thermal and flow structure of the mantle. Potentially, basalts constitute a widespread and abundant pool of information about mantle convection, and one of the goals pursued by petrological studies is to decipher this message [e.g. Klein and Langmuir, 1987; McKenzie and Bickle, 1988; Watson and McKenzie, 1991; Albarede, 1992, Langmuir et al., this volume; Grove et al., this volume].

  4. Origin and Constraints on Ilmenite-rich Partial Melt in the Lunar Lower Mantle

    NASA Astrophysics Data System (ADS)

    Mallik, A.; Fuqua, H.; Bremner, P. M.; Panovska, S.; Diamond, M. R.; Lock, S. J.; Nishikawa, Y.; Jiménez-Pérez, H.; Shahar, A.; Panero, W. R.; Lognonne, P. H.; Faul, U.

    2015-12-01

    Existence of a partially molten layer at the lunar core-mantle boundary has been proposed to explain the lack of observed far-side deep moonquakes, the observation of reflected seismic phases from deep moonquakes, and the dissipation of tidal energy within the lunar interior [1,2]. However, subsequent models explored the possibility that dissipation due to elevated temperatures alone can explain the observed dissipation factor (Q) and tidal love numbers [3]. Using thermo-chemical and dynamic modeling (including models of the early lunar mantle convection), we explore the hypothesis that an ilmenite-rich layer forms below crustal anorthosite during lunar magma ocean crystallization and may sink to the base of the mantle to create a partial melt layer at the lunar core-mantle boundary. Self-consistent physical parameters (including gravity, pressure, density, VP and Vs) are forward calculated for a well-mixed mantle with uniform bulk composition versus a mantle with preserved mineralogical stratigraphy from lunar magma ocean crystallization. These parameters are compared against observed mass, moment of inertia, real and imaginary parts of the Love numbers, and seismic travel times to further limit the acceptable models for the Moon. We have performed a multi-step grid search with over twenty thousand forward calculations varying thicknesses of chemically/mineralogically distinct layers within the Moon to evaluate if a partially molten layer at the base of the lunar mantle is well-constrained by the observed data. Furthermore, dynamic mantle modeling was employed on the best-fit model versions to determine the survivability of a partially molten layer at the core-mantle boundary. This work was originally initiated at the CIDER 2014 program. [1] Weber et al. (2011). Science 331(6015), 309-12. [2] Khan et al. (2014). JGR 119. [3] Nimmo et al. (2012). JGR 117, 1-11.

  5. Origin of primitive andesites by melt-rock reaction in the sub-arc mantle (Invited)

    NASA Astrophysics Data System (ADS)

    Rapp, R. P.

    2009-12-01

    The genetic relationship between primitive granitoids, including high-Mg andesites (HMAs) and bajaites, and primary granitoids, or "pristine" adakites, has been vigorously debated since Defant and Drummond (1991; henceforth D&D) first applied the term "adakite" to refer to Cenozoic arc magmas (andesites and dacites) "associated with young subducting lithosphere", with low Y and Yb, low high-field strength elements (HFSEs), high Sr, and high Sr/Y and (La/Yb)N ratios "relative to island arc andesite-dacite-rhyolite". These characteristics were attributed to an origin for adakites by partial melting of basaltic crust within the subducting slab (hence "slab melts"). That such a process can produce melts with the characteristics described by D&D has since been largely confirmed by dehydration melting experiments on hydrous metabasalt at ~1-4 GPa. Attention was also drawn to the geochemical similarities between "adakites" and large-ion lithophile element (LILE)-enriched, high-field strength element (HFSE) depleted magnesian andesites (HMAs) from Adak Island in the western Aleutians, first described by Kay (1978), implying a genetic relationship between primary granitoid (adakites) formed by partial melting of basaltic ocean crust in the subducting slab, transformed to garnet-amphibolite or eclogite, and primitive magnesian andesites (HMAs) with high Mg-numbers (Mg# = molar Mg/(Mg+Fe)x100) and high concentrations of Ni and Cr. What then is the true origin of these enigmatic arc magmas, with both crustal and mantle, derivative and primitive, geochemical signatures? Kay (1978) suggested a "hybrid" model, in which "hydrous melting of eclogite (slab melting) results in a small volume of dacitic melt ("pristine adakite"), which rises into the hotter overlying peridotite wedge and equilibrates with olivine and orthopyroxene, reacting with olivine until it becomes andesitic". In this paper, I will discuss the results of melt-rock reaction experiments modelling this peridotite

  6. 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

  7. Plume-subduction interaction in southern Central America: Mantle upwelling and slab melting

    NASA Astrophysics Data System (ADS)

    Gazel, Esteban; Hoernle, Kaj; Carr, Michael J.; Herzberg, Claude; Saginor, Ian; den Bogaard, Paul van; Hauff, Folkmar; Feigenson, Mark; Swisher, Carl

    2011-01-01

    The volcanic front in southern Central America is well known for its Galapagos OIB-like geochemical signature. A comprehensive set of geochemical, isotopic and geochronological data collected on volumetrically minor alkaline basalts and adakites were used to better constrain the mantle and subduction magma components and to test the different models that explain this OIB signature in an arc setting. We report a migration of back-arc alkaline volcanism towards the northwest, consistent with arc-parallel mantle flow models, and a migration towards the southeast in the adakites possibly tracking the eastward movement of the triple junction where the Panama Fracture Zone intersects the Middle America Trench. The adakites major and trace element compositions are consistent with magmas produced by melting a mantle-wedge source metasomatized by slab derived melts. The alkaline magmas are restricted to areas that have no seismic evidence of a subducting slab. The geochemical signature of the alkaline magmas is mostly controlled by upwelling asthenosphere with minor contributions from subduction components. Mantle potential temperatures calculated from the alkaline basalt primary magmas increased from close to ambient mantle (~ 1380-1410 °C) in the Pliocene to ~ 1450 °C in the younger units. The calculated initial melting pressures for these primary magmas are in the garnet stability field (3.0-2.7 GPa). The average final melting pressures range between 2.7 and 2.5 GPa, which is interpreted as the lithosphere-asthenosphere boundary at ~ 85-90 km. We provide a geotectonic model that integrates the diverse observations presented here. The slab detached after the collision of the Galapagos tracks with the arc (~ 10-8 Ma). The detachment allowed hotter asthenosphere to flow into the mantle wedge. This influx of hotter asthenosphere explains the increase in mantle potential temperatures, the northwest migration in the back-arc alkaline lavas that tracks the passage of the

  8. Reading the record of mantle melt mixing in Icelandic phenocryst compositions

    NASA Astrophysics Data System (ADS)

    Maclennan, J.; Winpenny, B.

    2011-12-01

    Igneous petrology provides important constraints on magmatic processes under spreading ridges. In the particular case of Iceland, petrological observations can be used in concert with seismic and geodetic observations to understand the transport and storage of melt in the lower oceanic crust. Previous study of olivine-hosted melt inclusions in Icelandic basalts has demonstrated that concurrent mixing and crystallisation of mantle melts occurs in magma chambers in the lower crust. In order to further test this model, and to better constrain the depths of crystallisation, a detailed study of compositional variation within clinopyroxene phenocrysts from the early Holocene Borgarhraun flow in the Theistareykir volcanic system of northern Iceland was undertaken. A suite of microanalytical techniques was used to determine major and trace element compositional variations in the clinopyroxenes, which were found to be variable both in their Mg# (85-92) and in their incompatible trace element contents. The depth of crystallisation of Borgarhraun melts was estimated using an experimental parameterisation of clinopyroxene-liquid equilibrium. A source of uncertainty associated with such barometry arises from the erroneous assumption of chemical equilibrium between crystal-melt compositional pairs. In a system where mixing of melts occurs during crystallisation, early-formed phenocrysts are carried to the surface by liquids which they are not in equilibrium with. In some studies, the Mg-Fe partitioning between crystal and melt as been used as a criterion for equilibrium. However, primitive Icelandic melts show a wide range in composition at fixed Mg/Fe. Therefore, in order to better identify plausible equilibrium liquids for the Borgarhraun clinopyroxenes we therefore searched a large dataset of Icelandic lava compositions, and used both Mg-Fe exchange and the partitioning of trace elements as a check on candidate liquids. This procedure produced a relatively well

  9. 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.

  10. Mixing of mantle melts recorded in Icelandic phenocrysts: The significance of clinopyroxene stability in depleted compositions

    NASA Astrophysics Data System (ADS)

    Winpenny, B.; Maclennan, J.

    2009-12-01

    Mixing of chemically heterogeneous mantle melts in the lower oceanic crust has been invoked to explain the large ranges of incompatible trace element ratios observed in olivine-hosted melt inclusions in primitive Icelandic basalts, and the decrease in this variability with degree of crystal fractionation. We show that before mixing is completed, these melts, sourced from different regions of the melting column, can influence the crystallization and chemistry of phenocryst phases hosted in the erupted basalt. Ion microprobe and LA-ICPMS trace element analyses were performed on high Mg# (85-92) clinopyroxene and high An content (80-90) plagioclase from the primitive Borgarhraun flow, N. Iceland. Major and trace element compositions were used to search for cpx-melt pairs close to equilibrium and thus suitable for thermobarometry. Thermobarometry results indicate that clinopyroxene crystallized at ~9(±2) kbar, close to the Moho. The forsterite content of olivine in Mg-Fe equilibrium with the clinopyroxene and the trace element content of the equilibrium melt were estimated using crystal-crystal and crystal-melt partition coefficients. This conversion allows the compositions of clinopyroxene point analyses to be compared with those of melt inclusions and their host crystals. Both converted clinopyroxene compositions and olivine-hosted melt inclusion data show a wide range in incompatible trace element ratios close to Mg#~90, requiring mixing of mantle melts during crystal fractionation. However, the trace element enriched part of the range observed in the olivine-hosted melt inclusions is absent from clinopyroxene compositions. While the range in La/Yb of 92 olivine-hosted melt inclusions is 0.09-3.23, that for 167 converted clinopyroxene compositions is 0.11-1.29. Phase relations can explain this observation: deep-sourced, enriched melts have a long olivine-only crystallization path and so cannot form high Mg# clinopyroxene prior to mixing with more depleted melts

  11. Highly refractory peridotites in Songshugou, Qinling orogen: Insights into partial melting and melt/fluid-rock reactions in forearc mantle

    NASA Astrophysics Data System (ADS)

    Cao, Yi; Song, Shuguang; Su, Li; Jung, Haemyeong; Niu, Yaoling

    2016-05-01

    The Songshugou ultramafic massif is located in the eastern segment of the Qinling orogenic belt, central China. It is a large spinel peridotite body dominated by coarse-grained, porphyroclastic, and fine-grained dunite with minor harzburgite, olivine clinopyroxenite, and banded/podiform chromitite. The compositions of the bulk-rock dunite and harzburgite, and the constituent olivine and spinel, together with the textures and chemical characteristics of multiphase mineral inclusions, point to the highly refractory nature of these rocks with complex histories of high-temperature boninite melt generation and boninitic melt-rock reaction, probably in a young, warm, and volatile-rich forearc lithospheric mantle setting. Additionally, a subsequent low-temperature fluid-rock reaction is also recorded by TiO2-rich spinel with Ti solubility/mobility enhanced by chloride- or fluoride-rich subduction-zone fluids as advocated by Rapp et al. (2010). The olivine clinopyroxenite, on the other hand, was likely crystallized from a residual boninitic melt that had reacted with harzburgitic residues. The ubiquitous occurrences of hydrous minerals, such as anthophyllite, tremolite, Cr-chlorite, and serpentine (stable at lower P-T crustal conditions) in the matrix, suggest that further low-temperature fluid-rock reaction (or retrograde metamorphism) has affected the original volatile-poor peridotites either in a mature and cool subduction zone, or in a continental crust during their exhumation into the Qinling collisional orogeny at early Paleozoic era, or both. The prolonged and intense ductile/brittle deformation can decrease the mineral grain size through dynamic recrystallization and fracturing, and thus aid the fluid-rock reaction or retrograde metamorphism and mineral chemical re-equilibration processes. Therefore, the Songshugou peridotites present a good example for understanding the petrogenesis and evolution of the mantle wedge, with the emphasis on the complex partial

  12. Evidence of Melt percolation and Mantle Wedge Deformation in the Marum Ophiolite (Papua New Guinea)

    NASA Astrophysics Data System (ADS)

    Kaczmarek, M. A.; Jonda, L.; Davies, H. L.

    2014-12-01

    The Marum ophiolite in Papua New Guinea described an ultra-depleted mantle made by dunite and harzburgite showing compositions of supra-subduction zone peridotite. This depleted mantle has been fertilised by diffuse crystallisation of a low proportion of clinopyroxene in the dunite and formation of ol-websterite and ol-clinopyroxenite at cm scale cross-cutting the foliation and the primary pyroxenite layering. This percolating melt shows silica-rich magnesian affinities (boninite-like) related to supra-subduction zone in a fore-arc environment. The peridotite has also been percolated by a melt with more tholeiite affinities precipitating plagioclase-rich wehrlite and thin gabbroic veins. Crystallographic preferred orientations of minerals show that clinopyroxene and orthopyroxene in the harzburgite and orthopyroxenite veins crystallized under the same deformation constraints as the depleted dunite. However, for low melt proportion, such as the clinopyroxenes in the dunite, their crystallization can be governed by epitaxial growth. Epitaxial growth is also possible in the ol-websterite where an area with low melt fraction (5% of clinopyroxenes), clinopyroxene and orthopyroxene <001> axes are parallel to olivine <100> axes, while in an area with higher melt fraction (28% of clinopyroxenes) clinopyroxenes record their own orientation with <001> axes at nearly 90˚ to the olivine <100> axes suggesting a reorientation of the constraints. The same relationship has been observed within the plagioclase-rich wehrlite suggesting the same deformation conditions during percolation of boninitic affinities melt and a more tholeiitic affinities melt. The major inferred slip system in olivine is (001)[100] E-type slip with the possible activation of both (001)[100] E-type and (010)[100] A-type slip systems, which are activated at high-temperature low-stress conditions. Olivine E-type slip is predicted to operate within the mantle wedge in a back-arc position however the Marum

  13. Experimental halogen partitioning between earth upper mantle minerals and silicate melt

    NASA Astrophysics Data System (ADS)

    Joachim, Bastian; Pawley, Alison; Lyon, Ian; Henkel, Torsten; Burgess, Ray; Ballentine, Christopher J.

    2013-04-01

    Owing to their incompatibility, halogens have similar geochemical properties to noble gases in many systems and may therefore be used as key tracers of volatile transport processes in the earth. Halogen fractionation may occur during partial melting of the upper mantle, fractional crystallization or partitioning between immiscible fluids. Experimental determination of the halogen partitioning behaviour is the basis for the investigation of the concentration and distribution of halogens in the earth's mantle. High P-T partition experiments were performed in a piston cylinder apparatus using a model primitive mantle composition proposed by Jagoutz et al. (1979) simplified to the four components CaO, MgO, A2lO3 and SiO2 (CMAS) according to the procedure of O'Hara (1968). Defined small amounts of halogens (0.2 wt%) were added as CaF2, CaCl2 and CaBr2. All experiments were first heated up to 1720° C and then cooled slowly to the target temperature to guarantee growth of large homogeneous crystals, following the method of Beyer et al. (2011). Pressures range between 1.0 GPa and 2.5 GPa and final experimental temperatures between 1500° C and 1600° C, thus representing partial melting conditions of the earth upper mantle. Back-scattered electron images of polished samples show euhedral, almost rectangular forsterite grains or a mixture of euhedral forsterite and pyroxene grains with a side length of up to 150 μm, which are embedded in a MORB-like melt. Electron microprobe analysis reveals a homogeneous major element composition of the forsterite and pyroxene single crystals as well as of the melt. Halogen mapping, measured via Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS), shows no concentration gradients within the minerals or within the melt. These observations suggest that the experiments were performed at equilibrium conditions. The fact that we were able to produce large pyroxene and forsterite crystals at equilibrium conditions in a halogen doped

  14. Seismic evidence of effects of water on melt transport in the Lau back-arc mantle.

    PubMed

    Wei, S Shawn; Wiens, Douglas A; Zha, Yang; Plank, Terry; Webb, Spahr C; Blackman, Donna K; Dunn, Robert A; Conder, James A

    2015-02-19

    Processes of melt generation and transport beneath back-arc spreading centres are controlled by two endmember mechanisms: decompression melting similar to that at mid-ocean ridges and flux melting resembling that beneath arcs. The Lau Basin, with an abundance of spreading ridges at different distances from the subduction zone, provides an opportunity to distinguish the effects of these two different melting processes on magma production and crust formation. Here we present constraints on the three-dimensional distribution of partial melt inferred from seismic velocities obtained from Rayleigh wave tomography using land and ocean-bottom seismographs. Low seismic velocities beneath the Central Lau Spreading Centre and the northern Eastern Lau Spreading Centre extend deeper and westwards into the back-arc, suggesting that these spreading centres are fed by melting along upwelling zones from the west, and helping to explain geochemical differences with the Valu Fa Ridge to the south, which has no distinct deep low-seismic-velocity anomalies. A region of low S-wave velocity, interpreted as resulting from high melt content, is imaged in the mantle wedge beneath the Central Lau Spreading Centre and the northeastern Lau Basin, even where no active spreading centre currently exists. This low-seismic-velocity anomaly becomes weaker with distance southward along the Eastern Lau Spreading Centre and the Valu Fa Ridge, in contrast to the inferred increase in magmatic productivity. We propose that the anomaly variations result from changes in the efficiency of melt extraction, with the decrease in melt to the south correlating with increased fractional melting and higher water content in the magma. Water released from the slab may greatly reduce the melt viscosity or increase grain size, or both, thereby facilitating melt transport.

  15. Geochemical Systematics of Hotspots and Mid-Ocean Ridges Arising from Melting of a Non-Layered Heterogeneous Mantle

    NASA Astrophysics Data System (ADS)

    Ito, G.; Mahoney, J. J.

    2003-12-01

    Many fundamental geochemical differences between ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs) are often explained by a chemically layered mantle, with lower mantle material delivered to the surface by mantle plumes forming ocean islands, and a compositionally distinct upper mantle feeding mid-ocean ridges. A dilemma arises from geophysical evidence for whole mantle convection, which is predicted to efficiently stir the mantle and prevent any long-lasting, global chemical layering. We present models of decompression melting in which mantle heterogeneities are present as veins or small blobs, equally numerous in plume mantle as they are in the ambient mantle. Three different components are considered. Enriched mantle (EM) is highly concentrated in the most incompatible trace elements, has isotopic characteristics reflecting long-term enrichment, and begins melting deepest. Pyroxenite (PX) is relatively depleted in the most incompatible trace elements, has Pb isotope compositions reflecting a high U/Pb ratio, and begins melting at intermediate depths. Depleted mantle (DM), the most abundant (90%) component, is depleted in the most incompatible elements, has corresponding isotope signatures, and begins melting shallowest. Models predict the deeper melting, EM and PX components to be preferentially extracted at intraplate settings where thick lithosphere limits melting to large depths and low extents. In contrast, DM is more heavily sampled at mid-ocean ridges where thinner lithosphere allows for shallower and more extensive melting. Besides lithospheric thickness, differences in mantle flow also contribute to compositional contrasts between OIBs and MORBs. Plume-driven upwelling is most rapid at depth, decreases to the base of the lithosphere, and therefore enhances the extraction of EM and PX. In contrast, seafloor spreading at mid-ocean ridges allows for more uniform upwelling with depth and more even sampling of all mantle components, including DM

  16. Melting the lithosphere: Metasomes as a source for mantle-derived magmas

    NASA Astrophysics Data System (ADS)

    Rooney, Tyrone O.; Nelson, Wendy R.; Ayalew, Dereje; Hanan, Barry; Yirgu, Gezahegn; Kappelman, John

    2017-03-01

    Peridotite constitutes most of the Earth's upper mantle, and it is therefore unsurprising that most mantle-derived magmas exhibit evidence of past equilibrium with an olivine-dominated source. Although there is mounting evidence for the role of pyroxenite in magma generation within upwelling mantle plumes, a less documented non-peridotite source of melts are metasomatic veins (metasomes) within the lithospheric mantle. Here we present major and trace element analyses of 66 lavas erupted from a small Miocene shield volcano located within the Ethiopian flood basalt province. Erupted lavas are intercalated with lahars and pyroclastic horizons that are overlain by a later stage of activity manifested in small cinder cones and flows. The lavas form two distinctive petrographic and geochemical groups: (A) an olivine-phyric, low Ti group (1.7-2.7 wt.% TiO2; 4.0-13.6 wt.% MgO), which geochemically resembles most of the basalts in the region. These low Ti lavas are the only geochemical units identified in the later cinder cones and associated lava flows; (B) a clinopyroxene-phyric high Ti group (3.1-6.5 wt.% TiO2; 2.8-9.2 wt.% MgO), which resembles the Oligocene HT-2 flood basalts. This unit is found intercalated with low Ti lavas within the Miocene shield. In comparison to the low Ti group, the high Ti lavas exhibit a profound depletion in Ni, Cr, Al, and Si, and significant enrichment in Ca, Fe, V, and the most incompatible trace elements. A characteristic negative K anomaly in primitive-mantle normalized diagrams, and Na2O > K2O, suggests a source rich in amphibole, devoid of olivine, and perhaps containing some carbonate and magnetite. While melt generation during rift development in Ethiopia is strongly correlated with the thermo-chemical anomalies associated with the African Superplume, thermobaric destabilization and melting of mantle metasomes may also contribute to lithospheric thinning. In regions impacted by mantle plumes, such melts may be critical to weakening

  17. Fractionation of Mantle-Derived Melts in the Annieopsquotch Ophiolite, Newfoundland.

    NASA Astrophysics Data System (ADS)

    Lissenberg, C.; Bédard, J. H.; van Staal, C. R.

    2004-12-01

    The Annieopsquotch ophiolite exposes a tectonically bounded section through c. 5.5 km tholeiitic gabbros, sheeted dykes and pillow basalts. The gabbro zone is divided into three parts. The lower 500 m comprises massive cumulate gabbros with enclaves (<50 m) of partly-reacted and digested layered troctolite/leucotroctolite. These are interpreted as relics of the substrate into which the gabbro-sheeted dyke-basalt sequence was emplaced. Overlying this is 1500 m of cumulate olivine gabbros and gabbros which form sills c. 30 m thick that are oriented parallel to the ophiolite pseudostratigraphy. Finer grain sizes at contacts and inward-growing crescumulates indicate cooling from both top and bottom. Gabbros in the sill complex are characterized by cumulate textures with minor intercumulus amphibole and oxides, and rarely show shape-preferred orientations. The upper 500 m of the gabbro zone is dominated by massive gabbros with more abundant interstitial Fe-Ti-oxides, and diabasic pods that grade up into sheeted dykes, suggesting it represents a level of frozen melt. Incompatible element contents of cumulate gabbros in the sill complex generally increase upwards, and modeling indicates that the cumulate sills crystallized from melts with compositions similar to those of the overlying sheeted dykes and basalts. Trapped melt fractions are estimated to be c. 20%, consistent with the absence of compaction structures in these gabbros. Models indicate that the parental magmas of the gabbros, as well as lavas and dykes, can be produced by an average of c. 40-45% fractionation of mantle-derived melts. Both field- and geochemical data thus suggests the Annieopsquotch lower crust records repeated in-situ intrusion and fractionation during upward migration of mantle-derived melts towards the surface, with localized ponding in an axial melt lens at the base of the dyke complex. The similarity in composition and degree of fractionation between the lower and upper crust suggest that

  18. Permeability of asthenospheric mantle and melt extraction rates at mid-ocean ridges.

    PubMed

    Connolly, James A D; Schmidt, Max W; Solferino, Giulio; Bagdassarov, Nikolai

    2009-11-12

    Magmatic production on Earth is dominated by asthenospheric melts of basaltic composition that have mostly erupted at mid-ocean ridges. The timescale for segregation and transport of these melts, which are ultimately responsible for formation of the Earth's crust, is critically dependent on the permeability of the partly molten asthenospheric mantle, yet this permeability is known mainly from semi-empirical and analogue models. Here we use a high-pressure, high-temperature centrifuge, at accelerations of 400g-700g, to measure the rate of basalt melt flow in olivine aggregates with porosities of 5-12 per cent. The resulting permeabilities are consistent with a microscopic model in which melt is completely connected, and are one to two orders of magnitude larger than predicted by current parameterizations. Extrapolation of the measurements to conditions characteristic of asthenosphere below mid-ocean ridges yields proportionally higher transport speeds. Application of these results in a model of porous-media channelling instabilities yields melt transport times of approximately 1-2.5 kyr across the entire asthenosphere, which is sufficient to preserve the observed (230)Th excess of mid-ocean-ridge basalts and the mantle signatures of even shorter-lived isotopes such as (226)Ra (refs 5,11-14).

  19. Unravelling the effects of primary melt depletion and metasomatism in the lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Harvey, J.; Gannoun, A.; Alard, O.; Burton, K. W.; Rogers, N.; Schiano, P.

    2003-04-01

    Osmium behaves as a highly compatible element during mantle melting and is preferentially retained in the residue, whereas Re is moderately incompatible and enters the melt. The Re-Os isotope system thus offers unique information on the timing and nature of melt depletion in the mantle, that is often considered to be immune from secondary metasomatic processes. For the system to be perturbed either Re or "exotic" Os must be added during some later event. However, the Os budget in mantle rocks is almost entirely controlled by sulphide, and thus the integrity of Os is fundamentally dependent upon the behaviour of sulphide. In this study we have reexamined the petrology and geochemistry of a classic suite of spinel lherzolite xenoliths from the Massif Central, France. These samples preserve a range of fertility (0.85 - 3.61 CaO wt %) attributed to variable melt extraction. Equilibration temperatures of 900^oC-1100^oC, and minimum equilibration pressures of 1.3Gpa (from geobarometry and fluid inclusions) indicate that these xenoliths last equilibrated under upper mantle conditions. Whole rock Re-Os data yields a range of 187Os/188Os ratios from 0.115 - 0.125 which correlate well with indices of fertility, consistent these rocks having been simply affected by melt depletion. However, A detailed study of secondary melt inclusions within the silicate phases has revealed that not only are the xenoliths contaminated by exotic, small percentage highly silicic melts but also that sulphide blebs are found in co-genetic relationships with CO_2 rich, silica rich and composite melt inclusions. That, at least some, of the Os in these rocks is hosted in metasomatic sulphide is consistent with in-situ Os isotope data from Mont Briancon (1). This shows that within a single rock only sulphides trapped in olivine preserve unradiogenic compositions, whereas interstitial, metasomatic sulphides are more radiogenic. Preliminary Re-Os data for olivine also yields unradiogenic values

  20. Stability of carbonated basaltic melt at the base of the Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Ghosh, S.; Litasov, K.; Ohtani, E.; Suzuki, A.

    2006-12-01

    Seismological observations of low velocity zones (LVZ) at the top of the 410-km discontinuity reveal possible existence of dense melt at this boundary (e.g. Reveanugh and Sipkin, 1994). Density measurements of anhydrous basaltic melts indicate that it is denser than surrounding mantle near 410-km depth (Ohtani and Maeda, 2001). However, melting temperature of peridotite is much higher than about 1400°C, estimated at 410-km depth. It has been shown recently that hydrous basaltic melt containing up to 2 wt.% H2O is denser than peridotite atop 410-km and therefore can be accumulated at the base of the upper mantle (Sakamaki et al., 2006). CO2 is another major volatile component in the mantle and it could be also important for explanation of LVZ near 410 km. In the present study, we have measured the density of carbonated basaltic melt at high pressures and high temperatures and discussed its possible stability at the base of the upper mantle. The density of the melt was determined using sink/float technique. The starting material was synthetic MORB glass. 5 and 10 wt.% CO2 was added to the glass as CaCO3 and Na2CO3, adjusting to proportions of related oxides. Experiments were carried out at 16-22 GPa and 2200-2300°C using a multianvil apparatus at Tohoku University, Japan. We observed neutral buoyancy of diamond density marker in MORB + 5 wt.% CO2 at 18 GPa and 2300°C, whereas, diamond was completely dissolved in the carbonated MORB melt containing 10 wt.% CO2 in 0.5-1 minute experiments. Based on the buoyancy test, the density of the carbonated basaltic melt, containing 5 wt.% CO2, is 3.56 g/cm3 at 18 GPa and 2300°C using an equation of state of diamond. To calculate the bulk modulus we assume that the pressure derivative of the isothermal bulk modulus is the same as that of the dry MORB melt, dKT/dP=5.0 and zero-pressure partial molar volume of CO2 is 32 cm3/mol (based on low-pressure experiments on carbonated basaltic melts and carbonatites, e.g. Dobson et al

  1. Shock-Induced Transformation and Melting of Lower Mantle Minerals: Implications for Earth Evolution

    NASA Astrophysics Data System (ADS)

    Ahrens, T. J.; Mosenfelder, J. L.; Asimow, P. D.

    2008-12-01

    Shock wave techniques can define high-pressure melting relations for deep Earth minerals by several methods. Pressure-volume-energy equations of state and spectral radiation shock temperature measurements are sensitive to the conditions where Hugoniots of lower mantle mineral compositions cross phase boundaries including both polymorphic phase transitions and partial to complete melting. Method 1 uses the velocity of isentropic rarefaction waves to observe the loss of shear modulus upon melting. Method 2 use radiative temperature along the Hugoniot to seek a deflected in temperature upon intersection of Hugoniot and melting curve. Method 3 applies in the case when the liquid phase is denser than the coexisting solid phase(s). In the absence of any other known polymorphic phase change a sudden density increase is attributed to melting. For SiO2, all three of these methods define the shock pressure and temperature where the Hugoniot of fused silica passes from stishovite to partial melt (73 GPa, 4600 K) and where the Hugoniot of crystal quartz passes from the CaCl2 structure to partial melt (116 GPa, 4900 K). In the case of Mg2SiO4, the forsterite Hugoniot passes from periclase+perovskite phases to melt before 152 GPa and 4300 K, whereas initial wadsleyite material follows a colder path, transforming from periclase+post-perovskite to melt before 151 GPa and 4160 K. Recently, we extended the range of the MgSiO3 glass Hugoniot and demonstrated that this glass transforms into the perovskite structure from 80 to 100 GPa. Above 100 GPa and extending to over 160 GPa, the shock state is molten. Since shock states derived from crystal enstatite are also molten above 160 GPa, precise determination of the high pressure Grüneisen parameter,γ , can be obtained from the finite difference V[dP/dE]V. As previously seen in Mg2SiO4 liquid, γ for molten MgSiO3 increases markedly with compression, going from 0.5 to 1.6 over the 0 to 135 GPa range. This property gives rise to larger

  2. Origin of primitive arc basaltic andesites by shallow, hydrous mantle melting

    NASA Astrophysics Data System (ADS)

    Andrews, A.; Grove, T. L.

    2012-12-01

    Primitive, high-magnesium basaltic andesites and andesites (51-58 wt. % SiO2, 9-11 wt. % MgO) provide a unique window into hydrous melting processes in the shallow mantle wedge. While it is thought that melting the hydrous mantle at shallow depths produces these primitive melts in equilibrium with a harzburgitic (olivine + orthopyroxene) residue (Wood and Turner, 2009), the exact conditions under which these melts form remain elusive. We have conducted a suite of experiments on a synthetic mix of primitive upper mantle composition (Hart and Zindler, 1986) at varying temperatures, pressures, and water contents in pursuit of a systematic understanding of the conditions that allow for the generation of these unique magmas. The data obtained thus far show a strong correlation between decreasing temperature and increasing silica content. Using this relationship, as well as constraints from high-magnesium basaltic andesites produced at 1.2 GPa, 1275-1363°C, and 1-2 wt. % H2O, additional isobaric experiments are underway at lower temperatures and higher water contents to determine whether the correlation continues and more silica-rich compositions are produced. For each experiment in our data set, the activity coefficient of SiO2 in the melt (γ_SiO2^melt) shows a positive deviation from ideality. As the water content is increased, ln γ_SiO2^melt moves farther from ideal conditions, suggesting that only a non-ideal solution model can adequately explain water's effect on the silica content of the melt produced in these experiments. Using this new experimental data set, we are able to explore the conditions that produce primitive, high magnesium basaltic andesites and andesites as well as increase our understanding of the thermodynamics behind these conditions and resulting melts. At this point, a primary melting origin seems to require a combination of very shallow depths (P<1.2 GPa) and very high H2O contents (>6 wt. % H2O).

  3. Ancient melt depletion overprinted by young carbonatitic metasomatism in the New Zealand lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Scott, J. M.; Hodgkinson, A.; Palin, J. M.; Waight, T. E.; Van der Meer, Q. H. A.; Cooper, A. F.

    2014-01-01

    Spinel facies dunite, harzburgite, lherzolite and wehrlite mantle xenoliths from a cluster of Miocene volcanoes in southern New Zealand preserve evidence of the complex evolution of the underlying continental mantle lithosphere. Spinel Cr# records melt extraction with some values indicative of near complete removal of clinopyroxene. LREE-enriched, low Ti/Eu and low Al2O3 clinopyroxene and rare F-, LREE-rich apatite indicates subsequent interaction between peridotite and a metasomatising carbonatitic melt. The clearest metasomatic signature occurs in the formerly highly depleted samples because there was little or no pre-existing clinopyroxene to dilute the carbonatite signature. For the same reason, the isotopic character of the metasomatising agent is best observed in the formerly highly depleted peridotites (87Sr/86Sr = 0.7028-0.7031; 143Nd/144Nd = 0.5129; 206Pb/204Pb = 20.2-20.3). These isotope ratios are very close to, but slightly less radiogenic than, the HIMU end-member mantle reservoir. Nd isotope data imply carbonatite metasomatism occurred within the last several hundred million years, with ubiquitous pyroxene core-to-rim Al diffusion zoning indicating that it must pre-date cooling of the lithospheric mantle following Late Cretaceous-Eocene rifting of Zealandia from Gondwana. Metasomatism was significantly younger than ancient Re-depletion ages of ~2 Ga and shows that decoupling of peridotite isotope systems has occurred.

  4. Formation of an interconnected network of iron melt at Earth’s lower mantle conditions

    SciTech Connect

    Shi, Crystal Y.; Zhang, Li; Yang, Wenge; Liu, Yijin; Wang, Junyue; Meng, Yue; Andrews, Joy C.; Mao, Wendy L.

    2013-10-06

    Core formation represents the most significant differentiation event in Earth’s history. Our planet’s present layered structure with a metallic core and an overlying mantle implies that there must be a mechanism to separate iron alloy from silicates in the initially accreted material. At upper mantle conditions, percolation has been ruled out as an efficient mechanism because of the tendency of molten iron to form isolated pockets at these pressures and temperatures. Here we present experimental evidence of a liquid iron alloy forming an interconnected melt network within a silicate perovskite matrix under pressure and temperature conditions of the Earth’s lower mantle. Using nanoscale synchrotron X-ray computed tomography, we image a marked transition in the shape of the iron-rich melt in three-dimensional reconstructions of samples prepared at varying pressures and temperatures using a laser-heated diamond-anvil cell. We find that, as the pressure increases from 25 to 64GPa, the iron distribution changes from isolated pockets to an interconnected network. Our results indicate that percolation could be a viable mechanism of core formation at Earth’s lower mantle conditions.

  5. Spin crossover and iron-rich silicate melt in the Earth's deep mantle (Invited)

    NASA Astrophysics Data System (ADS)

    Hirose, K.; Nomura, R.; Ozawa, H.; Tateno, S.; Hernlund, J. W.

    2010-12-01

    The volume difference between a silicate solid and its melt diminishes at high pressure, and the possibility that a melt sufficiently enriched in iron might then become more dense than solids at the pressures in the interior of the Earth and other terrestrial bodies has long been a source of considerable speculation. The occurrence of such dense silicate melts in the Earth's lowermost mantle would carry important consequences for its physical and chemical evolution and could provide a unifying model for explaining a variety of observed features in the core-mantle boundary (CMB) region [e.g., Labrosse et al., 2007 Nature]. Recent theoretical calculations [Stixrude et al., 2009 EPSL] combined with estimates of Fe partitioning between (Mg,Fe)SiO3 perovskite and melt at shallower mantle conditions suggest that melt is more dense than solids at pressures in the Earth's deepest mantle, consistent with analysis of shockwave experiments. Here we extend measurements of Fe partitioning in (Mg0.89Fe0.11)2 SiO4 bulk composition over the entire mantle pressure range, by a combination of laser-heated diamond-anvil cell experiments and chemical analyses of recovered samples using field-emission-type electron microprobe (FE-EPMA). The results demonstrate that the Fe-Mg distribution coefficient KD = ([FePv]/[MgPv]) / ([Femelt]/[Mgmelt]) between perovskite and melt is about 0.25 up to 75 GPa, consistent with earlier data found at 25 GPa in Al-free or -depleted peridotite materials using multi-anvil apparatus. On the other hand, the KD suddenly dropped to 0.07±0.02 at 76 GPa, resulting in strong Fe-enrichment in melts. It was almost constant at 0.06-0.08 at higher pressures to 159 GPa. The value did not change practically across the perovskite to post-perovskite phase transition. Additional x-ray emission spectroscopy measurements on (Mg0.95Fe0.05)SiO3 glass indicate the loss of spin around 60-70 GPa, suggesting that the observed change in Fe partitioning could be explained by a

  6. Mantle seismic structure beneath the MELT region of the east pacific rise from P and S wave tomography

    PubMed

    Toomey; Wilcock; Solomon; Hammond; Orcutt

    1998-05-22

    Relative travel time delays of teleseismic P and S waves, recorded during the Mantle Electromagnetic and Tomography (MELT) Experiment, have been inverted tomographically for upper-mantle structure beneath the southern East Pacific Rise. A broad zone of low seismic velocities extends beneath the rise to depths of about 200 kilometers and is centered to the west of the spreading center. The magnitudes of the P and S wave anomalies require the presence of retained mantle melt; the melt fraction near the rise exceeds the fraction 300 kilometers off axis by as little as 1%. Seismic anisotropy, induced by mantle flow, is evident in the P wave delays at near-vertical incidence and is consistent with a half-width of mantle upwelling of about 100 km.

  7. Effect of Mantle Wedge Hybridization by Sediment Melt on Geochemistry of Arc Magma and Arc Mantle Source - Insights from Laboratory Experiments at High Pressures and Temperatures

    NASA Astrophysics Data System (ADS)

    Mallik, A.; Dasgupta, R.; Tsuno, K.; Nelson, J. M.

    2015-12-01

    Generation of arc magmas involves metasomatism of the mantle wedge by slab-derived H2O-rich fluids and/or melts and subsequent melting of the modified source. The chemistry of arc magmas and the residual mantle wedge are not only regulated by the chemistry of the slab input, but also by the phase relations of metasomatism or hybridization process in the wedge. The sediment-derived silica-rich fluids and hydrous partial melts create orthopyroxene-rich zones in the mantle wedge, due to reaction of mantle olivine with silica in the fluid/melt [1,2]. Geochemical evidence for such a reaction comes from pyroxenitic lithologies coexisting with peridotite in supra-subduction zones. In this study, we have simulated the partial melting of a parcel of mantle wedge modified by bulk addition of sediment-derived melt with variable H2O contents to investigate the major and trace element chemistry of the magmas and the residues formed by this process. Experiments at 2-3 GPa and 1150-1300 °C were conducted on mixtures of 25% sediment-derived melt and 75% lherzolite, with bulk H2O contents varying from 2 to 6 wt.%. Partial reactive crystallization of the rhyolitic slab-derived melt and partial melting of the mixed source produced a range of melt compositions from ultra-K basanites to basaltic andesites, in equilibrium with an orthopyroxene ± phlogopite ± clinopyroxene ± garnet bearing residue, depending on P and bulk H2O content. Model calculations using partition coefficients (from literature) of trace elements between experimental minerals and silicate melt suggest that the geochemical signatures of the slab-derived melt, such as low Ce/Pb and depletion in Nb and Ta (characteristic slab signatures) are not erased from the resulting melt owing to reactive crystallization. The residual mineral assemblage is also found to be similar to the supra-subduction zone lithologies, such as those found in Dabie Shan (China) and Sanbagawa Belt (Japan). In this presentation, we will also

  8. The Influence of Water on Mantle Melting and Crystallization in Back-arc Basin Systems

    NASA Astrophysics Data System (ADS)

    Lytle, M. L.; Kelley, K. A.; Hauri, E. H.

    2009-12-01

    Water is the central component that distinguishes back-arc spreading ridges from normal mid-ocean ridges. Specifically, the addition of water from the subducted plate to back-arc basin magma sources strongly influences mantle melting and magmatic crystallization processes. Here, we present new SIMS measurements of magmatic volatiles (H2O, CO2, S, Cl, F) and new LA-ICP-MS trace element data in back-arc basin basalt (BABB) glasses, coupled with previously published major, volatile, and trace element data, from the Mariana trough, Manus, Woodlark, Lau, and North Fiji back-arc basins in the western Pacific, and use these data to relate melt composition to crystallization and melting processes. Glasses record magmatic liquid lines of descent (LLD), which reflect fractional crystallization of olivine, plagioclase and clinopyroxene and are consistent with the modal phenocryst abundances of the rocks. LLD’s also vary with magmatic H2O concentration, and using a multi-element approach (e.g., MgO vs. Al2O3, CaO, CaO/Al2O3, and Eu anomaly), the data show clear delays in the onset of plagioclase fractionation related to higher magmatic H2O content, although not enough to cause clinopyroxene to saturate before plagioclase. The MgO concentration at the onset of plagioclase fractionation shows a linear decrease of ~1.0 wt.% MgO at the point of plag-in with each wt.% increase in melt H2O content. These well-constrained LLD’s permit more accurate corrections of basalt compositions to primary melts, achieved by tracing melt compositions along empirically-determined fractionation slopes to the point of plag-in appropriate for their H2O contents before adding equilibrium olivine until melts are in equilibrium with Fo90. We use the calculated primary melts to constrain pressures and temperatures of melt equilibration using new models of melt thermobarometry (Lee et al., 2009), and apply both an inverse melting model using TiO2 contents of the primary melts to determine melt

  9. Nanoscale 3D distribution of low melt and fluid fractions in mantle rocks

    NASA Astrophysics Data System (ADS)

    Gardes, Emmanuel; Morales, Luiz; Heinrich, Wilhelm; Sifre, David; Hashim, Leila; Gaillard, Fabrice; Katharina, Marquardt

    2016-04-01

    The presence of melts or fluids in the intergranular medium of rocks strongly influences their bulk physico-chemical properties (e.g. mass transport and chemical reactivity, electrical conductivity, seismic wave velocity, etc). Actually, the effects can be so large that only small melt or fluid fractions must sometimes be involved for explaining mantle geophysical discontinuities and anomalies. The investigation of the distribution of such small fractions in the intergranular medium of mantle rocks is therefore crucial for relating them to bulk and large scale properties. However, it involves submicrometric structures which are hardly characterizable using conventional techniques. Here we present how the FIB-SEM-STEM microscope can be used to produce 3D imaging at unequalled resolution. We show that low melt and fluid fractions can form films as thin as 20 nm at olivine grain boundaries, and that they can modify the physico-chemical properties of mantle rocks by orders of magnitude. The fine relationships between films at grain boundaries, tubules at triple junctions and pockets at grain corners can be explored, and appear to be complex and to differ from usual visions.

  10. Melting relations of hydrous and dry mantle compositions and the genesis of komatiites

    NASA Astrophysics Data System (ADS)

    Asahara, Y.; Ohtani, E.; Suzuki, A.

    The hydrous phase relations of primitive upper mantle model compositions in the CaO-MgO-Al2O3-SiO2-H2O system with 1, 2 and 5 wt.% H2O have been investigated with a multianvil high pressure apparatus at 6.5 GPa and temperatures from 1550°C to 2050°C. Liquidus temperatures decrease by about 100°C and the temperature interval to produce about 60% partial melting reduces to less than 50°C in the 1-2 wt.% H2O-bearing systems. At an H2O content of about 5 wt.%, garnet is the first dissolved phase and the stability field of orthopyroxene expands. Aluminum undepleted komatiite magmas can be formed by melting at 200 km depth in a hydrous mantle at significantly lower temperatures than under dry conditions. Thus, komatiite magmas cannot constrain temperatures in the Archean mantle. In addition, cratonic peridotites may have formed as residues of partial melting under various H2O contents.

  11. Water Content of Earth's Continental Mantle Is Controlled by the Circulation of Fluids or Melts

    NASA Technical Reports Server (NTRS)

    Peslier, Anne; Woodland, Alan B.; Bell, David R.; Lazarov, Marina; Lapen, Thomas J.

    2014-01-01

    A key mission of the ARES Directorate at JSC is to constrain models of the formation and geological history of terrestrial planets. Water is a crucial parameter to be measured with the aim to determine its amount and distribution in the interior of Earth, Mars, and the Moon. Most of that "water" is not liquid water per se, but rather hydrogen dissolved as a trace element in the minerals of the rocks at depth. Even so, the middle layer of differentiated planets, the mantle, occupies such a large volume and mass of each planet that when it is added at the planetary scale, oceans worth of water could be stored in its interior. The mantle is where magmas originate. Moreover, on Earth, the mantle is where the boundary between tectonic plates and the underlying asthenosphere is located. Even if mantle rocks in Earth typically contain less than 200 ppm H2O, such small quantities have tremendous influence on how easily they melt (i.e., the more water there is, the more magma is produced) and deform (the more water there is, the less viscous they are). These two properties alone emphasize that to understand the distribution of volcanism and the mechanism of plate tectonics, the water content of the mantle must be determined - Earth being a template to which all other terrestrial planets can be compared.

  12. Redox Interactions between Iron and Carbon in Planetary Mantles: Implications for Degassing and Melting Processes

    NASA Technical Reports Server (NTRS)

    Martin, A.; Righter, K.

    2009-01-01

    Carbon stability in planetary mantles has been studied by numerous authors because it is thought to be the source of C-bearing atmospheres and of C-rich lavas observed at the planetary surface. In the Earth, carbonaceous peridotites and eclogites compositions have been experimentally studied at mantle conditions [1] [2] [3]. [4] showed that the fO2 variations observed in martian meteorites can be explained by polybaric graphite-CO-CO2 equilibria in the Martian mantle. Based on thermodynamic calculations [4] and [5] inferred that the stable form of carbon in the source regions of the Martian basalts should be graphite (and/or diamond), and equilibrium with melts would be a source of CO2 for the martian atmosphere. Considering the high content of iron in the Martian mantle (approx.18.0 wt% FeO; [6]), compared to Earth s mantle (8.0 wt% FeO; [7]) Fe/C redox interactions should be studied in more detail.

  13. How hot is red? Thermal structure of the melting mantle from seismic tomography and thermobarometry

    NASA Astrophysics Data System (ADS)

    Plank, T.; Forsyth, D. W.; Bendersky, C.; Ferguson, D. J.; Gazel, E.; Lee, C.

    2012-12-01

    Seismic tomography is providing ever sharper views of the upper mantle, but the ability to interpret these images continues to be limited by the confounding effects of temperature, water and melt content on S wave velocities (Vs). Globally low Vs is found in many active rift zones (e.g., Basin and Range, East African Rift, mid ocean ridges), and yet it is still not clear if velocities reflect variations in mantle temperature, water concentrations, or melt retention in the mantle. Here we combine Vs results from recent tomographic models with melt thermobarometry to isolate the temperature effect on Vs. We focus on the Western US where we have combined surface and body wave tomography using data from EarthScope's Transportable Array to yield excellent models of Vs to depths of 300 km or more. The widespread basaltic volcanism that has occurred over the past 1 Ma across the region provides samples of the melting mantle. We employ a revised version of the Lee et al. (2009) thermobarometer to estimate pressures and temperatures of last equilibration in the mantle from the Si and Mg compositions of basalts. Results are highly dependent on oxygen fugacity and water content, which we determine with new measurements of water and vanadium in melt inclusions and their olivine hosts. Temperatures and pressures range from ~ 1200°C and 1 GPa (40 km) beneath Big Pine volcanic field to ~ 1450°C and 3.5 GPa (115 km) beneath Lunar Crater volcanic field. Most of the equilibration depths are near the base of the seismically determined lithosphere-asthenosphere boundary, although some are deeper, in the low velocity zone, and some are shallower, in the fast seismic lid. Vs in the source regions varies from ~ 4.4 km/s to 3.9 km/s. There is a good negative correlation of Vs with temperature of equilibration, based on nine volcanic fields across the Basin and Range. Other rifting regions around the world, including back-arc basins, the East Pacific Rise and the East Africa Rift, also

  14. Porous Flow and Diffusion of Water in the Mantle Wedge: Melting and Hydration Patterns

    NASA Astrophysics Data System (ADS)

    Conder, J. A.

    2005-12-01

    It is widely accepted that melting at volcanic arcs is primarily triggered by fluxing the mantle wedge from the dehydrating subducting slab. However, there is less concensus regarding how water moves into and within the mantle wedge. There are at least four possible mechanisms for water migration in the wedge: buoyant porous flow, diffusion through mineral crystals, advection of hydrated minerals, and compositionally buoyant diapers. The latter two mechanisms require at least one of the first two to occur to get water from the slab into the wedge before they can function. Using geodynamic models of mantle flow in a simplified subduction setting, we explore the implications of diffusion and porous flow of water in the wedge, particularly as they would affect the time for recycling water through the subduction factory and the predicted pattern of basalt hydration across the arc. The slab is assumed to dehydrate in a continuous fashion as the solubility of water in subducted oceanic crust decreases with temperature and pressure and the water then enters the wedge via one of the two transport mechanisms. Diffusion is controlled by temperature and by which minerals are present. Although olivine dominates the mantle mineral fraction, pyroxenes may control the diffusion of water in the wedge as the diffusivity of pyroxene is one or more orders of magnitude greater than olivine. Even assuming the faster diffusion rate of orthopyroxene in the models, diffusion can only be an important transport mechanism when subduction rates are slower than ~3 cm/yr. Flux melting occurs in the wedge above where the slab is ~100-160 km deep with the maximum above where the slab is ~120 km deep. Models including porous flow can result in melting at higher subduction rates provided the permeability of the mantle is greater than 10-17 m2. The true magnitude of the permeability likely varies with the corresponding porosity created by the free phase. With porous flow, melting occurs 20-30 km

  15. Petrology-based Modeling of Mantle Melt Electrical Conductivity and Joint-Interpretation of Electromagnetic and Seismic Results

    NASA Astrophysics Data System (ADS)

    Pommier, A.; Garnero, E. J.

    2013-12-01

    The presence of melt in the Earth's interior depends on the thermal state, bulk chemistry and dynamics. Therefore, the investigation of the physical and chemical properties of melt is a probe of the planet's structure, dynamics, and potentially evolution. We present a petrology-based model of the electrical conductivity of fertile and depleted peridotites during partial melting. Seismic and magnetotelluric (MT) studies provide geophysical datasets sensitive to the presence of melt, but they do not necessarily agree on melt fraction estimates. A possible explanation regards the assumptions made about melt chemistry as part of MT data interpretation. Melt fraction estimates from electrical anomalies usually assume a basaltic melt phase, whereas petrological studies suggest that the first liquids produced have a different chemistry, and thus a different conductivity. Our model is based on the existing laboratory database of electrical conductivity of silicate melts and crystals, and allows calculation of the electrical conductivity of peridotite partial melting (valid at pressures up to 2 GPa) that accounts for the dependency of melt composition on the extent of melting. Partial melting of both depleted and fertile peridotites is considered. Chemical compositions and phase proportions of melt and crystals are determined using the MELTS algorithm [1]. The melt composition dependence on electrical conductivity is expressed using the optical basicity parameter. Our results show that melts produced by low-degree peridotite melting (< 14 vol. %) are up to 5 times more conductive than basaltic liquids. Such conductive melts significantly affect bulk rock conductivity, particularly for a fertile peridotite. Our findings demonstrate that the electrical conductivity of low-degree partial melting of a peridotite can be significantly underestimated if the liquid phase is assumed to be basaltic, resulting in overestimated melt fractions from bulk conductivity values. Application

  16. Slab and sediment melting during subduction initiation: granitoid dykes from the mantle section of the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Rollinson, Hugh

    2015-09-01

    New geochemical data are presented for a suite of tonalites, granodiorites, trondhjemites and granites intrusive into depleted mantle harzburgites of the Oman-UAE ophiolite. A detailed field, petrological and geochemical examination suggests that these `mantle granitoids' are the product of three processes: (a) the mixing of melts derived from both mafic and metasedimentary sources, (b) interaction with the mantle harzburgite host and (c) the fractional crystallisation of plagioclase, hornblende ± accessory phases. Geochemical data are used to characterise the identity of the protolith(s) by first screening the data for those samples which have experienced fractional crystallisation during emplacement. The resultant `reduced' data set has moderately fractionated REE, with small negative Eu anomalies and fractionated primitive mantle-normalised trace element patterns with high concentrations of fluid mobile elements and lower concentrations of HFS elements and with positive peaks for Rb and Pb and negative troughs for Ba, Nb, Sr and Ti. The character of the protolith was quantified using a melting model based upon a MORB-type basalt similar in composition to the Oman Geotimes lavas and a model using the MUQ (MUd from Queensland) global sediment composition (Kamber et al. Geochim Cosmochim Acta 69:1041-1058, 2005) both with an amphibolite/granulite facies mineralogy. The two compositions bracket the mantle granitoid data set with partial melts of the MORB source yielding trace element compositions lower than the granitoids, whereas melts of the MUQ source yield melts with compositions higher than the granitoids. Mixing of the calculated melt compositions indicates that the measured granitoid compositions represent between 10 and 30 % mixing of a metasedimentary melt into the melt of a mafic source. Current petrological, structural and geochronological data suggest a model for the origin of the Oman ophiolite in which it is formed by spreading above a subduction zone

  17. Modeling composition of partial melts in mantle upwellings through Earth history: an example of a 2D poster

    NASA Astrophysics Data System (ADS)

    Sparks, D. W.; Cheadle, M.

    2004-12-01

    The composition of magmas created by partial melting of the mantle depend on the interplay of several processes: the mantle phase diagram, the physics of magma migration through the mantle and crust, the patterns of solid-state mantle and fluid circulation and heat transfer, to name a few. This modeling study attempts a self-consistent combination of these physical and chemical processes, to predict the composition of magma created in upwelling mantle over a very broad range of mantle conditions, with particular emphasis on the deep past in a hot Archean mantle. We utilize 1) high P-T melting experiments to constrain the composition of melts formed at different depths in the mantle, 2) thermal and compositional solid-state convection models to constrain the temperature and melting rate and the three-dimensional distribution of these melts, and 3) simplified models of magma migration to predict the accumulation and mixing of these magmas, for comparison with mantle-derived primitive melts over time. An explanation of this study requires a description of details from a number of varied disciplines (Archean geology, trace element geochemistry, experimental petrology, solid-state convection, magma migration). While most interested poster readers will want to know the details of one or two aspects of the calculations, few will want to wade through them all. This goal of this poster design is to present the outline of the story in way that can be scanned quickly at a distance, but with several independent offshoots containing explanation of parts of the story that can be either read or skipped, and yet another level containing details for the experts on a particular topic.

  18. The role of pyroxenite in basalt genesis: Melt-PX, a melting parameterization for mantle pyroxenites between 0.9 and 5 GPa

    NASA Astrophysics Data System (ADS)

    Lambart, Sarah; Baker, Michael B.; Stolper, Edward M.

    2016-08-01

    Geochemical and isotopic data suggest that the source regions of oceanic basalts may contain pyroxenite in addition to peridotite. In order to incorporate the wide range of compositions and melting behaviors of pyroxenites into mantle melting models, we have developed a new parameterization, Melt-PX, which predicts near-solidus temperatures and extents of melting as a function of temperature and pressure for mantle pyroxenites. We used 183 high-pressure experiments (25 compositions; 0.9-5 GPa; 1150-1675°C) to constrain a model of melt fraction versus temperature from 5% melting up to the disappearance of clinopyroxene for pyroxenites as a function of pressure, temperature, and bulk composition. When applied to the global set of experimental data, our model reproduces the experimental F values with a standard error of estimate of 13% absolute; temperatures at which the pyroxenite is 5% molten are reproduced with a standard error of estimate of 30°C over a temperature range of ~500°C and a pressure range of ~4 GPa. In conjunction with parameterizations of peridotite melting, Melt-PX can be used to model the partial melting of multilithologic mantle sources—including the effects of varying the composition and the modal proportion of pyroxenite in such source regions. Examples of such applications include calculations of isentropic decompression melting of a mixed peridotite + pyroxenite mantle; these show that although the potential temperature of the upwelling mantle plays an important role in defining the extent of magma production, the composition and mass fraction of the pyroxenite also exert strong controls.

  19. Noble Gas Partitioning Behaviour During Mantle Melting: A Possible Explanation for 'The He Paradox'?

    NASA Astrophysics Data System (ADS)

    Brooker, R. A.; Heber, V.; Kelley, S. P.; Wood, B. J.

    2003-12-01

    New UVLAMP measurements of experimental noble gas crystal/melt partitioning values (including He) suggest reasonably incompatible behaviour for both olivine and cpx and no significant fractionation of noble gases relative to one another. This is consistent with models of noble gas incorporation at crystal lattice sites in both crystals (1). However the determined D values of approximately 8 x10-4 for cpx and 5 x10-3 for olivine suggest a small but significant amount of noble gas might be retained in the mantle after melting. It is also apparent that He is three orders of magnitude less incompatible than U and Th in olivine. As opx is predicted to show similar characteristic to olivine, melting to produce a highly depleted harzbugitic (low-cpx) mantle would involve the preferential removal of U+Th relative to He. This in turn would allow a relatively undisturbed primordial/radiogenic 3He/4He ratio to be retained in association with low He abundance. Thus, recycling of previously depleted mantle into the source region of 'hot spots' provides one possible explanation for the paradox of high 3/4 He ratios previously thought to indicate an undegassed, primordial lower mantle reservoir, with low He abundance indicating a degassed source (2). Preliminary UVLAMP depth profiles for noble gas diffusion in mantle minerals confirm that although sub-solidus diffusive removal of He relative to other noble gases from a gas-rich mantle plum is theoretically possible, the short distances involved are unlikely to produce an effect that can be sustained though a hot spot melting event. The slow diffusion rates and lack of fractionation of noble gases in our partitioning experiments suggests that low He/Ar (and Ne/Ar) ratios observed at hot spots are most likely to be features inherited from the source, or subsequently imposed by some shallow level process. In our partitioning experiments, it proved surprisingly difficult to grow olivine crystals that are free of bubbles, even from

  20. Evidence of unadulterated mantle-depth, granitic melt inclusions: kumdykolite and kokchetavite crystallized from melt in Bohemian Massif granulites.

    NASA Astrophysics Data System (ADS)

    O´Brien, Patrick J.; Ferrero, Silvio; Ziemann, Martin A.; Walczak, Katarzyna; Wunder, Bernd; Hecht, Lutz; Wälle, Markus

    2016-04-01

    Partial melting under near-UHP conditions of metagranitoids (now HP felsic granulites) at mantle depth in the Orlica-Śnieżnik Dome (Bohemian Massif, Poland) is recorded in small volumes of hydrous melt trapped as primary melt inclusions (MI) in peritectic garnets. When free of cracks connecting the inclusion with the leucocratic matrix, these "nanogranites" (≤ 50μm inclusion diameter) contain a unique assemblage including kumdykolite, kokchetavite and cristobalite - polymorphs of albite, K-feldspar and quartz, respectively. These usually metastable phases crystallized from the melt (glass?) during rapid exhumation (cm/a) at high T but the crack-free state strongly suggests over-pressuring of the inclusion with respect to the pressure-time path followed by the matrix. Reports of both kumdykolite and kokchetavite have been mainly from natural rocks equilibrated in the diamond stability field. The precise calculation of the PT path of the MI on cooling and the comparison with previous studies suggests, however, that pressure is not influential to their formation, ruling out the possible interpretation of kumdykolite and kokchetavite as indicators of ultra-high pressure conditions. Experimental re-homogenization of these crack-free nanogranites was achieved using a piston cylinder apparatus at 2.7 GPa and 875°C. These conditions are consistent with the results of geothermobarometric calculations on the host rock, suggesting that no H2O loss occurred during exhumation as this would have caused a shift of the inclusion melting T toward higher values. Coupled with the absence of H2O-loss microstructural evidence, e.g. decrepitation cracks and/or vesciculation in re-homogenized nanogranites, this evidence suggests that the nanogranites still preserve the original H2O content of the melt. Both experimental and microstructural evidence support the hypothesis that the presence of these polymorphs should be regarded as direct mineralogical criterion to identify former

  1. Petrogenesis of Near-Ridge Seamounts: AN Investigation of Mantle Source Heterogeneity and Melting Processes

    NASA Astrophysics Data System (ADS)

    Baxter, N. L.; Perfit, M. R.; Lundstrom, C.; Clague, D. A.

    2010-12-01

    Near-ridge (NR) seamounts offer an important opportunity to study lavas that have similar sources to ridge basalts but have been less affected by fractionation and homogenization that takes place at adjacent spreading ridge axes. By studying lavas erupted at these off-axis sites, we have the potential to better understand source heterogeneity and melting and transport processes that can be applied to the ridge system as a whole. One purpose of our study is to investigate the role of dunite conduits in the formation of near-ridge seamount chains. We believe that near-ridge seamounts could form due to focusing of melts in dunite channels located slightly off-axis and that such conduits may be important in the formation and transport of melt both on- and off-axis (Lundstrom et al., 2000). New trace element and isotopic analyses of glasses from Rogue, Hacksaw, and T461 seamounts near the Juan de Fuca Ridge (JdFR), the Lamont Seamounts adjacent to the East Pacific Rise (EPR) ~ 10°N, and the Vance Seamounts next to the JdFR ~45°N provide a better understanding of the petrogenesis of NR seamounts. Our data indicate that lavas from these seamounts have diverse incompatible trace element compositions that range from highly depleted to slightly enriched in comparison to associated ridge basalts. Vance A lavas (the oldest in the Vance chain) have the most enriched signatures and lavas from Rogue seamount on the JdFR plate have the most depleted signatures. Sr-Nd-Pb isotopic ratios indicate that NR seamount lava compositions vary within the chains as well as within individual seamounts, and that there is some mixing between heterogeneous, small-scale mantle sources. Using the program PRIMELT2.XLS (Herzberg and Asimow, 2008), we calculated mantle potential temperatures (Tp) for some of the most primitive basalts erupted at these seamounts. Our data indicate that NR seamount lavas have Tp values that are only slightly higher than that of average ambient mantle. Variations in

  2. Experimental evidence for high noble gas solubilities in silicate melts under mantle pressures

    NASA Astrophysics Data System (ADS)

    Schmidt, Burkhard C.; Keppler, Hans

    2002-02-01

    distribution. A geochemical consequence of our results is that noble gases remain incompatible elements at pressure conditions covering most of the upper mantle. Therefore partial melting remains an efficient process in extracting noble gases and other volatiles from the Earth's mantle.

  3. Phase relations and melting of carbonated peridotite between 10 and 20 GPa: a proxy for alkali- and CO2-rich silicate melts in the deep mantle

    NASA Astrophysics Data System (ADS)

    Ghosh, Sujoy; Litasov, Konstantin; Ohtani, Eiji

    2014-02-01

    We determined the melting phase relations, melt compositions, and melting reactions of carbonated peridotite on two carbonate-bearing peridotite compositions (ACP: alkali-rich peridotite + 5.0 wt % CO2 and PERC: fertile peridotite + 2.5 wt % CO2) at 10-20 GPa and 1,500-2,100 °C and constrain isopleths of the CO2 contents in the silicate melts in the deep mantle. At 10-20 GPa, near-solidus (ACP: 1,400-1,630 °C) carbonatitic melts with < 10 wt % SiO2 and > 40 wt % CO2 gradually change to carbonated silicate melts with > 25 wt % SiO2 and < 25 wt % CO2 between 1,480 and 1,670 °C in the presence of residual majorite garnet, olivine/wadsleyite, and clinoenstatite/clinopyroxene. With increasing degrees of melting, the melt composition changes to an alkali- and CO2-rich silicate melt (Mg# = 83.7-91.6; ~ 26-36 wt % MgO; ~ 24-43 wt % SiO2; ~ 4-13 wt % CaO; ~ 0.6-3.1 wt % Na2O; and ~ 0.5-3.2 wt % K2O; ~ 6.4-38.4 wt % CO2). The temperature of the first appearance of CO2-rich silicate melt at 10-20 GPa is ~ 440-470 °C lower than the solidus of volatile-free peridotite. Garnet + wadsleyite + clinoenstatite + carbonatitic melt controls initial carbonated silicate melting at a pressure < 15 GPa, whereas garnet + wadsleyite/ringwoodite + carbonatitic melt dominates at pressure > 15 GPa. Similar to hydrous peridotite, majorite garnet is a liquidus phase in carbonated peridotites (ACP and PERC) at 10-20 GPa. The liquidus is likely to be at ~ 2,050 °C or higher at pressures of the present study, which gives a melting interval of more than 670 °C in carbonated peridotite systems. Alkali-rich carbonated silicate melts may thus be produced through partial melting of carbonated peridotite to 20 GPa at near mantle adiabat or even at plume temperature. These alkali- and CO2-rich silicate melts can percolate upward and may react with volatile-rich materials accumulate at the top of transition zone near 410-km depth. If these refertilized domains migrate upward and convect out of the

  4. Mantle metasomatism by melts of HIMU piclogite components: new insights from Fe-lherzolite xenoliths (Calatrava Volcanic District, Central Spain)

    NASA Astrophysics Data System (ADS)

    Bianchini, Gianluca; Beccaluva, Luigi; Bonadiman, Costanza; Nowell, Geoff M.; Pearson, D. Graham; Siena, Franca; Wilson, Marjorie

    2010-05-01

    Mantle xenoliths from the Calatrava Volcanic District (CLV), central Spain, are characterized by a wide compositional range, which includes lherzolites (prevalent) as well as minor amounts of wehrlite, ol-websterite and rare dunites. They generally have bulk-rock Mg#s <89, lower than any primordial mantle estimates. Intra-suite variations in modal proportions are inconsistent with those predicted by melting models irrespective of the starting composition; mineral and bulk-rock variation diagrams show inconsistencies between the CLV compositions (anomalously enriched in Fe-Ti) and those predicted from partial melting of primordial mantle material. Processes other than pure melt extraction are confirmed by the whole-rock REE budget, typically characterized by LREE enrichments, with LaN/YbN (up to 6.7), probably related to pervasive metasomatism. CLV mantle clinopyroxenes (cpx) generally display fractionated REE patterns with upward convex shapes, characterized by low HREE (Tm-Lu) concentrations (typically <6 x chondrite) and enrichments in Middle/Light REE (NdN/YbN up to 7, LaN/YbN up to 5). These "enriched" cpx compositions either result from re-equilibration of primary mantle cpx with an incoming melt, or represent cpx crystallization directly from the metasomatic agent. The latter was plausibly generated at greater depths in the presence of residual garnet (from peridotite or eclogite starting materials). Separated cpx have homogeneous 87Sr/86Sr compositions between 0.7031 and 0.7032; 143Nd/144Nd ranges from 0.51288 to 0.51295 and 176Hf/177Hf is in the range 0.28302-0.28265. Unlike mantle xenoliths and alpine-type peridotites from other Iberian occurrences, which range in composition from the Depleted Mantle (DM) to the Enriched Mantle (EM), the CLV mantle cpxs approach the composition of the HIMU mantle end-member, the genesis of which is generally interpreted as the result of long-term recycling of oceanic basalts/gabbros (or their eclogitic equivalent) via

  5. CO2 in the mantle: Melting and solid-solid phase boundaries

    NASA Astrophysics Data System (ADS)

    Teweldeberhan, A. M.; Boates, B.; Bonev, S. A.

    2013-07-01

    The high temperature phase boundaries of CO2 in the proximity of the Earth's adiabat are determined using first-principles molecular dynamics simulations based on density functional theory. The melting curve, predicted here up to 71 GPa, and the molecular to polymeric solid phase transition are computed through a phase coexistence approach from free energy calculations. The resulting CO2 phase IV-phase V-liquid triple point is at 31.8 GPa and 1636 K, in excellent agreement with the available experimental data. The Earth's geotherm crosses into the non-molecular phase V near 40 GPa and 2160 K, indicating that free deposits of carbon dioxide in the lower mantle would exist as a polymeric solid. We have also examined the thermodynamic stability of phase V and find no indication of transformations into a dissociated diamond and oxygen phase at mantle conditions.

  6. 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

  7. Evidence of alkali rich melt reactions with mantle peridotite : Natural observations and experimental analogues

    NASA Astrophysics Data System (ADS)

    Grant, T. B.; Milke, R.; Wunder, B.

    2012-04-01

    The Heldburg Phonolite, (Thuringia, Germany) is peculiar in its nature due to its absence of a Eu anomaly, and hence lack of feldspar fractionation, as well as the presence of spinel lherzolite xenocrysts. These observations suggest a higher than normal (mantle) pressure of origin, and its potential as a metasomatic agent at depth is explored in this work. Disequilibrium between the phonolite and its entrained upper mantle xenocrysts resulted in the development of secondary reaction rim assemblages of; (1) phlogopite + minor diopside around olivine, (2) pargasitic amphibole, phlogopite and minor diopside around orthopyroxene. We document both the natural rims and the attempts to reproduce them under experimental conditions, in order to elucidate the likely origin of the phonolite and its efficacy for metasomatising the upper mantle. Platinum capsules were loaded with mixtures of crushed mineral separates, (of pure synthetic forsterite, San Carlos olivine, synthetic enstatite or a natural enstatite from Kilosa, Tanzania) with a synthetic Fe-free phonolite melt in a 16:84% weight ratio, respectively. Experiments were run in a piston cylinder apparatus with CaF2 as the pressure medium. In addition to varying PT conditions, a wide range of water contents were tested (0-14wt%). It was found that pressures of 10-14 kbar, and temperatures of 900-1000°C, satisfy the conditions at which the reactions can form, thus, it is likely that the phonolite existed at upper mantle conditions. Water must be present to stabilize the desired hydrous phases, with >6wt% required at 900°C and 10 kbar. The destabilization of feldspar is also essential to the process, hence higher water contents are needed at the lowest PT conditions compared to 4-5 wt. % H2O at greater PT. The formation of amphibole around enstatite appears to be affected by sluggish reaction kinetics and the orientation of the host pyroxene, sometimes leading to diopside single rims. Furthermore we note some of the

  8. Consequences of Melt-Preferred Orientation for Magmatic Segregation in Deforming Mantle Rock

    NASA Astrophysics Data System (ADS)

    Katz, R. F.; Taylor-West, J.; Allwright, J.; Takei, Y.; Qi, C.; Kohlstedt, D. L.

    2014-12-01

    In partially molten regions of the mantle, deviatoric stresses cause large-scale deformation and mantle flow. The same stresses also lead to preferential wetting of coherently oriented grain boundaries [DK97, T10]. This alignment is called melt-preferred orientation (MPO). Because of the contrast between the physical properties of melt and solid grains, MPO has the potential to introduce anisotropy into the mechanical and transport properties of the liquid/solid aggregate. Here we consider the possible consequences for (and of) anisotropic viscosity and permeability of the partially molten aggregate. The consequences are evaluated in the context of laboratory experiments on partially molten rocks. The controlled experiments involve deformation of an initially uniform mixture of solid olivine and liquid basalt [KZK10]. The resultant patterns of melt segregation include two robust features: (i) melt segregation into bands with high melt fraction oriented at a low angle to the shear plane; and (ii) melt segregation associated with an imposed gradient in shear stress, in experiments where this is present. Although there are other reproducible features of experiments, these are the most robust and provide a challenge to models. A theoretical model for the effect of MPO on mantle viscosity under diffusion creep is available [TH09] and makes predictions that are consistent with laboratory experiments [TK13,KT13,QKKT14,AK14]. We review the mechanics of this model and the predictions for flow in torsional and pipe Poiseuille flow, showing a quantitative comparison with experimental results. Furthermore, it is logical to expect MPO to lead to anisotropy of permeability, and we present a general model of tensorial permeability. We demonstrate the consequences of this anisotropy for simple shear deformation of a partially molten rock. REFERENCES: DK97 = Daines & Kohlstedt (1997), JGR, 10.1029/97JB00393. T10 = Takei (2010), JGR, 10.1029/2009JB006568. KZK10 = King, Zimmerman

  9. Depths and Temperatures of Mantle Melt Extraction in the Southern Cascadia Subduction Zone (Invited)

    NASA Astrophysics Data System (ADS)

    Till, C.; Grove, T. L.; Donnelly-Nolan, J. M.; Carlson, R. W.

    2013-12-01

    Plagioclase and spinel lherzolite thermometry and barometry applied to an extensive suite of <10.5 Ma primitive basaltic lavas (most Mg#>0.70) containing variable H2O contents (<<1 to ~4 wt%) suggests these melts were extracted from the mantle at 40-58 km below Oregon's High Lava Plains, 41-51 km below California's Modoc Plateau, and 37-60 km below the central-southern Cascades volcanic arc. Of the 155 basalt samples investigated, 33 are calc-alkaline basalts (CAB) and the remainder are high alumina olivine tholeiites (HAOT) or mildly alkaline basalts (MAB). All 33 of the CAB are from the subduction-influenced volcanic centers of Lassen, Mt. Shasta, Three Sisters, Medicine Lake, and Newberry in the present-day Cascades arc or rear arc. All of these volcanic centers also erupted HAOT or MAB. Olivine-plagioclase hygrometry for a representative subset of the 20 CAB from Newberry indicates they contained ~4 wt% H2O prior to eruption. Water contents for the remaining CAB were approximated using the H2O-melt composition scaling relationship developed by Ruscitto et al. [2010, EPSL 298(1-2), 153-161] yielding ≤1-3 wt% H2O. The calculated pressures and temperatures of last equilibration with mantle lherzolite for all 33 CAB were adjusted for the effects of H2O following Till et al. [2012, JGR 117(B06206)] and are on average 50×15°C (1s) cooler and 1.65×0.27 km deeper than their calculated temperatures and depths for anhydrous conditions. The minimum depths of melt extraction calculated for all basalts considered (including the CAB) are close to the Moho, as determined by regional geophysical studies. Thus, our results suggest that the geophysical Moho and lithosphere-asthenosphere boundary are located in close proximity to one another (within 5-10 km). The basalts originated at 1185-1383°C and the presence of both wet and dry basalts that were generated at such different temperatures at similar times, depths, and geographic locations in the Cascades arc and rear arc

  10. Initial Melting and wall-rock flux-melting of a wet multi-component mantle and its implications for the formation of MORB

    NASA Astrophysics Data System (ADS)

    Morgan, J. P.; Hasenclever, J.

    2013-12-01

    We explore several simple scenarios for wet melting of a heterogeneous multi- component mantle. In our melting formulation the mantle is viewed as a mixture consisting of a heterogeneously depleted peridotite matrix with embedded veins of fertile peridotite and/or geochemically enriched pyroxenite. These lithological units differ in their mineral composition but are assumed to have diffusively equilibrated both their water/hydrogen content and temperature over the hundreds of millions to billions of years prior to entering a melting region. During the melting process, however, only thermal but not chemical (water) equilibrium is assumed between the lithologies, which is a reasonable assumption for veins with thicknesses on the order of few tens to few hundreds of meters, a thermal diffusivity of 10^-6 m^2/s and a diffusivity of hydrogen of less than 3*10^-9 m^2/s. The thermodynamic formulation of the multi-component melting process, during which all components have to share thermal energy, is based on Phipps Morgan (2001). The wet melting parameterization by Katz et al. (2003) has been included in the thermodynamic formulation by modifying its solidus-depletion-dependence and treating water partitioning during melting as partitioning of a trace element with a D-value like that of Ce. Usually, fractional melting with a small trapped melt fraction is assumed. We will mostly discuss results from 1-D model calculations, which represent the idealized decompression of a multi-component mantle rising underneath a mid-ocean ridge. Melt-migration is assumed to occur as vertical ascent within each column. We have also extended the formulation to examine the effects of rising melts on 'flux-melting' the wall-rock through which they migrate. We are still testing to see if this mechanism can be the reason why ridge melts almost always have major element chemistries in equilibrium with a peridotitic mantle, while the incompatible trace elements in EMORB reflect the influence of

  11. Trace-element modelling of mare basalt parental melts: Implications for a heterogeneous lunar mantle

    NASA Astrophysics Data System (ADS)

    Hallis, L. J.; Anand, M.; Strekopytov, S.

    2014-06-01

    The heterogeneous-source model of mare basalt formation indicates that Lunar Magma Ocean (LMO) overturn produced an uneven mixture of early-formed olivine and pyroxene, and late-formed, ilmenite-rich cumulates, which subsequently partially melted to give rise to mare magmas. These heterogeneous cumulate source regions would not only have been characterised by different mineral modal abundances, but also by different trace element compositions. The aim of this work was to investigate the petrology and geochemistry of a diverse suite of Apollo mare basalts, and utilise trace-element modelling in order to understand their petrogenetic history. Chemical modelling confirms that the mare basalts were produced by relatively small degrees of partial melting (<10%) of the LMO cumulates, and that the dominant melting type (batch vs. fractional) varies among different basalt groups. Similarly, single-source mineralogy cannot be applied to all mare basalt types, confirming that the lunar mantle was heterogeneous at the time of generation of mare magmas. Plagioclase is not required in the source of most mare basalts, with the notable exception of the Apollo 14 high-Al basalts. Addition of more than 1% plagioclase to the source of other basalts produces weaker negative Eu anomalies than those observed in the samples. AFC calculations demonstrate the compositional differences between materials assimilated into the Apollo 14 high-Al and Apollo 11 high-K mare basalt partial melts, highlighting the complexities of mare basalt petrogenesis.

  12. Experiments on flow focusing in soluble porous media, with applications to melt extraction from the mantle

    SciTech Connect

    Kelemen, P.B.; Whitehead, J.A.; Aharonov, E.; Jordahl, K.A.

    1995-01-01

    We demonstrate finite strucutres formed as a consequence of the `reactive infiltration instability` in a series of laboratory and numerical experiments with growth of solution channels parallel to the fluid flow direction. Our experiments demonstrate channel growth in the presence of an initial solution front and without an initial solution front where there is a gradient in the solubility of the solid matrix. In the gradient case, diffuse flow is unstable everywhere, channels can form and grow at any point, and channels may extend over the length scale of the gradient. As a consequence of the gradient results, we suggest that the reactive infiltration instability is important in the Earth`s mantle, where partial melts in the mantle ascend adiabatically. This hypothesis represents an important alternative to mid-oceanic ridge basalts (MORB) extraction in fractures, since fractures may not form in weak, viscously deforming asthenospheric mantle. We also briefly consider the effects of crystallization, rather than dissolution reactions, on the morphology of porous flow via a second set of experiments where fluid becomes supersaturated in a solid phase. This process may produce a series of walled conduits, as in our experiments. Development of a low-porosity cap overlying high porosity conduits may create hydrostatic overpressure sufficient to cause fracture and magma transport to the surface in dikes.

  13. Experimentally melting a Mg 80# Martian mantle at 0.5 to 0.5 GPa: Implications for basalt genesis

    NASA Astrophysics Data System (ADS)

    McCoy, Christopher Lee

    The most widely used and accepted composition for the Martian mantle in experimental petrology is the Dreibus and Wanke (1985) proposed composition based on only eight SNC meteorites. This composition is enriched in iron with respect to the Earth, which follows what we see from samples of Mars. The magnesium number (Mg#=Mg/Mg+Fe) of the Dreibus and Wanke (1985) composition is Mg#75, which is iron rich compared to Earth's Mg# of around 90. However, when experimentally melted as a source for generating Martian basalts, the melt concentrates iron further, higher than the Mars basalt compositions, and requires melting a large percentage of the mantle to reach a composition that is comparable to known Martian basalts. Partial melting experiments of an Mg# 80 mantle composition produced shergottite-like melts with a lower percentage of partial melting than with the Mg#75 compositions. This would be more likely since the Martian mantle would have cooled considerably by the time it would have produced the shergottites, which was only approximately 180 million years ago. The reprised composition is Mg#80 and less iron rich than the DW composition, but more iron-rich than Earth.

  14. Spreading-rate dependence of melt extraction at mid-ocean ridges from mantle seismic refraction data.

    PubMed

    Lizarralde, Daniel; Gaherty, James B; Collins, John A; Hirth, Greg; Kim, Sangmyung D

    2004-12-09

    A variety of observations indicate that mid-ocean ridges produce less crust at spreading rates below 20 mm yr(-1) (refs 1-3), reflecting changes in fundamental ridge processes with decreasing spreading rate. The nature of these changes, however, remains uncertain, with end-member explanations being decreasing shallow melting or incomplete melt extraction, each due to the influence of a thicker thermal lid. Here we present results of a seismic refraction experiment designed to study mid-ocean ridge processes by imaging residual mantle structure. Our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo-spreading rate. Changes in mantle velocity gradient, basement topography and crustal thickness all correlate with this spreading-rate change. These observations can be explained by variations in melt extraction at the ridge, with a gabbroic phase preferentially retained in the mantle at slower spreading rates. The estimated volume of retained melt balances the approximately 1.5-km difference in crustal thickness, suggesting that changes in spreading rate affect melt-extraction processes rather than total melting.

  15. Melting of CaO and CaSiO3 at Deep Mantle Condition Using First Principles Simulations

    NASA Astrophysics Data System (ADS)

    Bajgain, S. K.; Ghosh, D. B.; Karki, B. B.

    2015-12-01

    Accurate prediction of melting temperatures of major mantle minerals at high pressures is important to understand the Hadean Earth as well as to explain the observed seismic anomalies at ultra-low velocity zone (ULVZ). To further investigate the geophysical implications of our recent first principles study of molten CaO and CaSiO­3, we calculated the melting temperatures of the corresponding solid phases by integrating the Clausius-Clapeyron equation. The melting behavior of their high-pressure phases can constrain the lower mantle solidus. Our calculations show melting temperature of 5700 ± 500 kelvins for CaSiO3 and 7800 ± 600 kelvins for CaO at the base of the lower mantle (136 GPa). The bulk sound velocities of CaO and CaSiO3 liquids at the core-mantle boundary are found to be 40 % lower than P-wave seismic velocity and 22 % lower than that of MgSiO3 liquid. With substantial decrease of melting temperature by freezing point depression and iron partitioning, the partial melting of multi-component silicate and its gravitational buoyancy at ULVZ cannot be ruled out.

  16. Origin of the Early Cretaceous continental intraplate volcanism, NW Syria: melting of a metasomatised lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Ma, G. S.; Malpas, J.; Xenophontos, C.; Suzuki, K.; Lo, C.

    2011-12-01

    The Mesozoic evolution of the Neotethys-Eastern Mediterranean between the African-Arabian and Eurasian continents was accompanied by intermittent eruption of alkaline-transitional basalts in Arabia. The causes of the prolonged volcanism remain controversial, whether related to the arrival(s) of mantle plume [1] or prolonged far-field extension of the passive continental margin [2]. In addition, the source(s) of the volcanism is not well constrained, as previous conclusions were drawn before recent understanding of the origin of intraplate magmas - (i) melting of hydrous metasomatic veins within the lithospheric mantle [3] or (ii) melting of an incompatible-element enriched peridotite source ± eclogites in the presence of CO2 [4, 5]. The Mesozoic basalts (ankaramites and transitional basalts) from the Coastal Ranges, NW Syria analysed in this study were dated at 106.3 ± 0.2 Ma and 103.4 ± 0.3 Ma (bulk-rock 40Ar/39Ar ages), representing the last instance of Mesozoic intraplate magmatism in the Levant region. Isotopic and geochemical analysis reveals distinct compositions between the two lava series (ankaramites: ɛNd(t) = 5.1-5.6, 87Sr/87Sr(t) = 0.70293-0.70302, 187Os/188Os(t) = 0.227-0.242; transitional basalts: ɛNd(t) = 4.0-4.6, 87Sr/87Sr(t) = 0.70320-0.70424, 187Os/188Os(t) = 0.392; and lower SiO2, higher TiO2, Nb/U, Nb/Th, Nb/La and Ce/Pb in the ankaramites). Fractional crystallisation and assimilation-fractional crystallisation modelling suggests minor roles for both processes during the evolution of the lavas, despite the generally high Os isotopic ratios. The modelling also precludes derivation of one lava series from the other, suggesting that the isotopic and geochemical distinctions must be inherited from the source. It is interpreted that the chemical characteristics represent a greater component derived from metasomatic amphibole-rich veins in the source region. Both the ankaramites and transitional basalts were generated from this metasomatised

  17. 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

  18. 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.

  19. Three dimensional modeling of mantle melt underneath the Lau Back-Arc spreading center and Tofua Volcanic Arc

    NASA Astrophysics Data System (ADS)

    Tarlow, Scott

    Valu Fa and Eastern Lau's (two regions along Lau's back-arc spreading center) observed axial morphology suggest that Valu Fa is more magmatically robust than Eastern Lau despite Eastern Lau's spreading rate nearly doubling Valu Fa's. Early geochemical [Pearce et al., 1994] and geophysical [Martinez and Taylor, 2002] studies predict a gradational decrease in melting moving north from Valu Fa to Eastern Lau, but more recent geochemical and seismic observations ([Escrig, .et al 2009]; [Dunn and Martinez, 2011]; [Dunn et al., 2011]) show a sharper stepwise decrease in melting as the spreading center's ridge axis sweeps away from the Tofua Volcanic-Arc. As the ridge sweeps away from the volcanic-arc, the influence of the slab hydrated mantle in the melting structure of the ridge decreases. Furthermore, Eastern Lau produces a thinner crust than expected for a robust spreading center. 2-D numerical studies [Harmon and Blackmon, 2010] show a gradational decrease in melting from Valu Fa to Eastern Lau but with no corresponding thinning of Eastern Lau's crust. To understand the melting dynamics underneath Lau's back-arc spreading center and the Tofua Volcanic-Arc implementing the effects of 3-D mantle flow and slab hydration appears to be required. To explain the observed geochemical and seismic observations, three 3-D numerical were performed, using a community developed mantle convection solver (CitcomS). The first model shows that observed geometric and surface kinematic boundary conditions cause a steep gradational increase in relative melting area (anhydrous) moving northward with increasing spreading rate along the ridge axis from Valu Fa to Eastern Lau caused by a northwestern along axis mantle flow. A peak in the relative melting area appears particularly close to Eastern Lau where crust is thinnest. These predictions run in opposition to the observations. The second model shows including a viscosity reduction in the mantle wedge due to slab hydration causes a more

  20. Can LREE Enriched Patterns in Clinopyroxenes in Abyssal Peridotites be Produced by Melting of a Depleted Mantle Source?

    NASA Astrophysics Data System (ADS)

    LIU, B.; Liang, Y.

    2015-12-01

    The enrichment of LREE in clinopyroxenes in abyssal peridotites has often been attributed to shallow level melt refertilization. Here we show an alternative mechanism that involves diffusive fractionation of REE during disequilibrium mantle melting. We present a simple model for trace element fractionation during disequilibrium melting in an upwelling steady-state column. We use linear kinetics to approximate crystal-melt mass exchange rate and obtain analytical solutions for cases of perfect fractional melting and batch melting. A key parameter determining the extent of chemical disequilibrium during partial melting is an element specific dimensionless ratio (ɛ) defined as the melting rate relative to the solid-melt chemical exchange rate for the trace element of interest. In the case of diffusion in mineral limited exchange, ɛ is inversely proportional to diffusivity of the element of interest. Disequilibrium melting is important for the trace element when e is comparable to or greater than the bulk solid-melt partition coefficient for the element (k). The disequilibrium fractional melting model is reduced to the equilibrium perfect fractional melting model when e is much smaller than k. Hence highly incompatible trace elements with slower mobilities in minerals are more susceptible to disequilibrium melting than moderately incompatible elements. Effect of chemical disequilibrium is to hinder the extent of fractionation between residual solid and partial melt, making the residual solid less depleted and the accumulated melt more enriched in incompatible trace element abundances relative the case of equilibrium melting. Application of the disequilibrium fractional melting model to REE abundances in clinopyroxene in abyssal peridotites from the Central Indian Ridge and the Vema Lithospheric Section, Mid-Atlantic Ridge revealed a positive correlation between the disequilibrium parameter ɛ and the degree of melting, which can be explained by an increase in melting

  1. Quantifying melt production and degassing rate at mid-ocean ridges from global mantle convection models with plate motion history

    NASA Astrophysics Data System (ADS)

    Li, Mingming; Black, Benjamin; Zhong, Shijie; Manga, Michael; Rudolph, Maxwell L.; Olson, Peter

    2016-07-01

    The Earth's surface volcanism exerts first-order controls on the composition of the atmosphere and the climate. On Earth, the majority of surface volcanism occurs at mid-ocean ridges. In this study, based on the dependence of melt fraction on temperature, pressure, and composition, we compute melt production and degassing rate at mid-ocean ridges from three-dimensional global mantle convection models with plate motion history as the surface velocity boundary condition. By incorporating melting in global mantle convection models, we connect deep mantle convection to surface volcanism, with deep and shallow mantle processes internally consistent. We compare two methods to compute melt production: a tracer method and an Eulerian method. Our results show that melt production at mid-ocean ridges is mainly controlled by surface plate motion history, and that changes in plate tectonic motion, including plate reorganizations, may lead to significant deviation of melt production from the expected scaling with seafloor production rate. We also find a good correlation between melt production and degassing rate beneath mid-ocean ridges. The calculated global melt production and CO2 degassing rate at mid-ocean ridges varies by as much as a factor of 3 over the past 200 Myr. We show that mid-ocean ridge melt production and degassing rate would be much larger in the Cretaceous, and reached maximum values at ˜150-120 Ma. Our results raise the possibility that warmer climate in the Cretaceous could be due in part to high magmatic productivity and correspondingly high outgassing rates at mid-ocean ridges during that time.

  2. Experimental Constraints on Mantle Heterogeneity and Mantle-Melt Equilibration Depths along the Volcanic Front of the Trans-Mexican Volcanic Belt

    NASA Astrophysics Data System (ADS)

    Weaver, S.; Wallace, P. J.; Johnston, A.

    2012-12-01

    Primitive magmas erupted along the volcanic front in the Trans-Mexican Volcanic Belt (TMVB) span a wide geochemical range, with variations in silica, alkalies, and volatiles, indicating that the subarc mantle wedge is chemically heterogeneous. In this work we present the results of hydrous, near-liquidus piston-cylinder experiments for three chemically distinct primitive magmas that have erupted at the volcanic front along the TMVB. The experiments were aimed to constrain the mineralogy of equilibrium residues and final equilibration pressures and temperatures for these primitive melts over a range of H2O contents (1.5-7 wt%). The results provide an along-arc view of primitive magma equilibration conditions beneath the volcanic front of the TMVB. The experimental starting materials included a medium-K basaltic andesite (JR-28, Jorullo, Central Mexico; Weaver et al., 2011), potassic trachybasalt (JOR-46, La Pilita, central Mexico), and an alkali basalt (AY-509, Ayutla, western Mexico). The residual mineralogy for these three compositions at upper mantle pressures was harzburgite (JR-28) and wehrlite (JOR-46 and AY-509). Experimentally constrained equilibration pressures varied from 1.4 GPa-1.8 GPa, with the lowest pressure observed in the central Mexico lavas (Jorullo and La Pilita) and the highest pressures observed for the Ayutla basalt from western Mexico. Other experimental studies on primitive Mexican lavas have produced similar mantle residues and equilibration pressures; high-Mg basaltic andesite (Pelagatos, Mexico City region; Weber et al., 2011) and primitive absarokite (Mascota, Jalisco, western Mexico; Hesse and Grove, 2003) both equilibrated with harzburgite at 1.3 GPa and 1.6 GPa, respectively. We combine all of the available experimental data with recent geodynamic models of the mantle wedge beneath the TMVB to provide a comprehensive, along-arc perspective of mantle-melt equilibrium beneath the volcanic front. There is significant experimental and

  3. Primitive magmas at five Cascade volcanic fields: Melts from hot, heterogeneous sub-arc mantle

    USGS Publications Warehouse

    Bacon, C.R.; Bruggman, P.E.; Christiansen, R.L.; Clynne, M.A.; Donnelly-Nolan, J. M.; Hildreth, W.

    1997-01-01

    ; and OIB-source-like domains. Lavas with arc and intraplate (OIB) geochemical signatures were erupted close to HAOT, and many lavas are blends of two or more magma types. Pre-eruptive H2O contents of HAOT, coupled with phase-equilibrium studies, suggest that these magmas were relatively dry and last equilibrated in the mantle wedge at temperatures of ???1300??C and depths of ???40 km, virtually at the base of the crust. Arc basalt and basaltic andesite represent greater extents of melting than HAOT, presumably in the same general thermal regime but at somewhat lower mantle separation temperatures, of domains of sub-arc mantle that have been enriched by a hydrous subduction component derived from the young, relatively hot Juan de Fuca plate. The primitive magmas originated by partial melting in response to adiabatic upwelling within the mantle wedge. Tectonic extension in this part of the Cascade arc, one characterized by slow oblique convergence, contributes to mantle upwelling and facilitates eruption of primitive magmas.

  4. Modeling the melting of multicomponent systems: the case of MgSiO3 perovskite under lower mantle conditions.

    PubMed

    Di Paola, Cono; P Brodholt, John

    2016-07-21

    Knowledge of the melting properties of materials, especially at extreme pressure conditions, represents a long-standing scientific challenge. For instance, there is currently considerable uncertainty over the melting temperatures of the high-pressure mantle mineral, bridgmanite (MgSiO3-perovskite), with current estimates of the melting T at the base of the mantle ranging from 4800 K to 8000 K. The difficulty with experimentally measuring high pressure melting temperatures has motivated the use of ab initio methods, however, melting is a complex multi-scale phenomenon and the timescale for melting can be prohibitively long. Here we show that a combination of empirical and ab-initio molecular dynamics calculations can be used to successfully predict the melting point of multicomponent systems, such as MgSiO3 perovskite. We predict the correct low-pressure melting T, and at high-pressure we show that the melting temperature is only 5000 K at 120 GPa, a value lower than nearly all previous estimates. In addition, we believe that this strategy is of general applicability and therefore suitable for any system under physical conditions where simpler models fail.

  5. Modeling the melting of multicomponent systems: the case of MgSiO3 perovskite under lower mantle conditions

    PubMed Central

    Di Paola, Cono; P. Brodholt, John

    2016-01-01

    Knowledge of the melting properties of materials, especially at extreme pressure conditions, represents a long-standing scientific challenge. For instance, there is currently considerable uncertainty over the melting temperatures of the high-pressure mantle mineral, bridgmanite (MgSiO3-perovskite), with current estimates of the melting T at the base of the mantle ranging from 4800 K to 8000 K. The difficulty with experimentally measuring high pressure melting temperatures has motivated the use of ab initio methods, however, melting is a complex multi-scale phenomenon and the timescale for melting can be prohibitively long. Here we show that a combination of empirical and ab-initio molecular dynamics calculations can be used to successfully predict the melting point of multicomponent systems, such as MgSiO3 perovskite. We predict the correct low-pressure melting T, and at high-pressure we show that the melting temperature is only 5000 K at 120 GPa, a value lower than nearly all previous estimates. In addition, we believe that this strategy is of general applicability and therefore suitable for any system under physical conditions where simpler models fail. PMID:27444854

  6. CO2 and potassium in the mantle: carbonaceous pelite melts from the trailing edge of a detached slab hybridizing in the mantle to ultrapotassic kamafugite

    NASA Astrophysics Data System (ADS)

    Schmidt, M. W.

    2007-12-01

    The ultrapotassic magmas from the Intra-Apennine and Roman provinces constitute worldwide endmembers in terms of K2O/Na2O, K2O content and CO2 degassing, and are associated with carbonatites. Group II kimberlites, which are geochemically similar but less extreme, occur on cratons stable since several 100 Ma. This geotectonic situation appears in strong contrast to the subduction setting of central Italy, where plate convergence has slowed down to less than a few mm/a, the slab now tearing off leading hot asthenospheric mantle to flow in between the trailing slab and the crust. A successful recipe for ultrapotassic magmas requires K/Na fractionation at some previous stage. Melts from fluid-absent melting of carbonaceous pelites at >3 GPa are ultrapottasic phonolites (SiO2 ~64 wt%) and have K2O/Na2O up to 9 because of residual cpx with jadeite80. The effect of CO2 is to stabilize residual jadeite, to lower SiO2, and to increase K2O/Na2O ratios (as compared to CO2 free melts with K2O/Na2O of 1-3 and SiO2 = 73-77 wt%). The carbonaceous pelite melts were equilibrated with fertile, refractory but cpx bearing mantle, and wherlite. At sufficient pressures (3.5 GPa) and XCO2 in the volatile component, hybridization of the carbonaceous pelite melts produces highly subsilicic kamafugites, with K2O/Na2O only slightly lowered, and XMg's >0.70 as characteristic of primitive melts. The essential role of CO2 is to reduce the olivine saturation volume and to shift the olivine-cpx-opx cotectic to lower SiO2. The Italian kamafugites are ultracalcic (CaO/Al2O3 = 1.2-1.4), and although carbonaceous pelite melts have little CaO and 20 wt% Al2O3, the assimilation of cpx and production of aluminous opx leads to ultracalcic compositions when equilibrated with refractory peridotite or wherlite. Temperatures necessary for the fluid absent carbonaceous pelite melting are 1050-1150 °C, far above any reasonable subduction geotherm. Hybridization in the mantle requires 1320-1400 °C (at 3

  7. Multstage Melting and Mantle Flow in the Galapagos Plume-Ridge Province

    NASA Astrophysics Data System (ADS)

    Geist, D.

    2010-12-01

    residue from the upper melt zone (depleted in all incompatible elements) overlying residue from the lower melt zone (depleted in volatiles, including helium). The northern Galapagos volcanoes tap either ambient upper mantle or plume that has been depleted by both stages of melting, whereas the GSC is supplied by the deeper return flow, which has only been depleted of its volatile components.

  8. Toward Obtaining the Experimental Constraints on the Role of Water on Melting Under the Lower Mantle Conditions

    NASA Astrophysics Data System (ADS)

    Amulele, G.; Lee, K. K.; Karato, S.

    2012-12-01

    Water and other volatile components (such as carbon dioxide) are known to have important influence on the melting behavior of silicates. The role of these components on the melting under the upper mantle conditions is now reasonably well understood. Recent experimental studies in our lab as well as some of the previous studies do show that water has an important influence on the melting relationship under the lower mantle conditions. The influence of water is not only to reduce the solidus but also to change the composition of the melt to (Mg,Fe)O rich. Quantifying these observations is essential in the understanding of chemical evolution of Earth and other planets. However, there are several challenges in performing these experimental studies. In this presentation, we discuss the issue of quantifying the water effects with special attention to the capability of preserving water content during the high pressure-temperature experiments. The issue of the preservation of water is important firstly because water could escape from a capsule during an experiment, and secondly because the melt is unquenchable in a commonly used processes under the lower mantle conditions. A commonly used practice is to identify the deficit of EPMA measurement from 100% to the water content, but there is no sound basis for this practice. In this presentation, we will show some preliminary results of our new approach to quantify the water content from the high-pressure run products containing melts.

  9. Melt Inclusion Evidence for Subduction-modified Mantle Beneath the Woodlark Spreading Center, Solomon Islands

    NASA Astrophysics Data System (ADS)

    Chadwick, J.; Turner, A.; Collins, E.

    2015-12-01

    The Woodlark Spreading Center (WSC) to the east of Papua New Guinea separates the Indo-Australian plate and Solomon Sea microplate. At its eastern terminus, the WSC is being subducted at the New Britain trench, forming a triple junction near the New Georgia Group arc in the Solomon Islands. Previous studies have shown that lavas recovered from greater than 100 km from the trench on the WSC are N-MORB, but closer to the trench they have arc-like Sr-Nd-Pb isotopic ratios, enrichments in LILE, and depletions in HFSE. In the complex triple junction area of the WSC on the Simbo and Ghizo Ridges, island arc tholeiites to medium-K calc-alkaline andesites and dacites have been recovered, many with trace element and isotopic characteristics that are similar to the true arc lavas in the New Georgia Group on the other side of the trench. We suggest that subduction-modified arc mantle migrates through slab windows created by the subduction of the WSC as the plates continue to diverge after subduction. This transfer of mantle across the plate boundary leads to variable mixing between arc and N-MORB end-members, forming the hybrid to arc-like lavas recovered on the WSC. To test this hypothesis and to characterize the end-member compositions, we have analyzed melt inclusions in olivine, pyroxene, and plagioclase phenocrysts in Simbo and Ghizo Ridge lava samples. Major elements were analyzed using the electron microprobe facility at Fayetteville State University and volatiles were analyzed on the ion probe facility at Woods Hole Oceanographic Institution. The melt inclusions show a wide diversity of magmas from basalts to dacites, and mixing modeling shows that most Woodlark Spreading Center lava compositions are explained by mixing between the most extreme mafic (MORB) and felsic (arc) inclusion compositions.

  10. Experimental petrology of peridotites, including effects of water and carbon on melting in the Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Green, David H.

    2015-02-01

    For over 50 years, the use of high-pressure piston/cylinder apparatus combined with an increasing diversity of microbeam analytical techniques has enabled the study of mantle peridotite compositions and of magmas derived by melting in the upper mantle. The experimental studies have been guided by the petrology and geochemistry of peridotites from diverse settings and by the remarkable range of mantle-derived magma types. Recent experimental study using FTIR spectroscopy to monitor water content of minerals has shown that fertile lherzolite (MORB-source upper mantle) at ~1,000 °C can store ~200 ppm H2O in defect sites in nominally anhydrous minerals (olivine, pyroxenes, garnet and spinel). Water in excess of 200 ppm stabilizes amphibole (pargasite) at P < 3 GPa up to the lherzolite solidus. However, at P > 3 GPa, water in excess of 200 ppm appears as an aqueous vapour phase and this depresses the temperature of the upper mantle solidus. Provided the uppermost mantle (lithosphere) has H2O < 4,000 ppm, the mantle solidus has a distinctive P, T shape. The temperature of the vapour- undersaturated or dehydration solidus is approximately constant at 1,100 °C at pressures up to ~3 GPa and then decreases sharply to ~1,010 °C. The strongly negative d T/d P of the vapour-undersaturated solidus of fertile lherzolite from 2.8 to 3 GPa provides the basis for understanding the lithosphere/asthenosphere boundary. Through upward migration of near-solidus hydrous silicate melt, the asthenosphere becomes geochemically zoned with the `enriched' intraplate basalt source (>500 ppm H2O) overlying the `depleted' MORB source (~200 ppm H2O). From the study of primitive MOR picrites, the modern mantle potential temperature for MORB petrogenesis is ~1,430 °C. The intersection of the 1,430 °C adiabat with the vapour-saturated lherzolite solidus at ~230 km suggests that upwelling beneath mid-ocean ridges begins around this depth. In intraplate volcanism, diapiric upwelling begins from

  11. Melting experiments on anhydrous peridotite KLB-1: Compositions of magmas in the upper mantle and transition zone

    NASA Astrophysics Data System (ADS)

    Herzberg, Claude; Zhang, Jianzhong

    1996-04-01

    Electron microprobe results are reported for liquid and crystalline phases that were synthesized at 5-22.5 GPa in multianvil experiments on anhydrous peridotite KLB-1 [Zhang and Herzberg, 1994]. The results provide information on the partitioning of TiO2, Al2O3, Cr2O3, FeO, MnO, MgO, Na2O, and NiO among liquid and the crystalline phases olivine, modified spinel, garnet, magnesiowustite, and magnesium perovskite. Uncertainties in these partition coefficients stem from quenching problems and from the effects of thermal migration of liquid in a temperature gradient. We have, however, exploited the temperature gradients by determining how the crystalline phase chemistry varies throughout the melting interval from the liquidus to the solidus. This has permitted new constraints to be obtained on the compositions of liquids along the anhydrous peridotite solidus at low melt fractions and at pressures in the 5-18 GPa range. It is demonstrated that the wide range of Al2O3 and CaO/Al2O3 contents in picrites and komatiites can be explained by melt segregation at upper mantle pressures that ranged from 3 to ˜10 GPa. These magmas could have formed by anhydrous melting in plumes with temperatures that were only 100°-200°C higher than ambient mantle below ridges, demonstrating that unusually hot conditions are not required to form komatiites. Primary igneous MgO contents in excess of 26% should be rare, and those that do exist in some komatiites can be explained by advanced melting during adiabatic or superadiabatic ascent, by low Na2O in the source, or by melting in hot plumes from the transition zone and lower mantle. Evidence for deep melting in hot plumes is rather conjectural, but it may be contained in some 2700 Myr komatiites that have high MgO and mantle-like CaO/Al2O3.

  12. Lithological, Chemical and Chronological Constraints on Melt Extraction from the Mantle Section of the ~492 Ma Shetland Ophiolite Complex, Scotland

    NASA Astrophysics Data System (ADS)

    O'Driscoll, B.; Walker, R. J.; Clay, P. L.; Day, J. M.; Ash, R. D.; Daly, J. S.

    2015-12-01

    The mantle sections of ophiolites offer a means of studying the composition and structure of the oceanic mantle. In particular, the relations between different lithologies can be established in the field, permitting an assessment of the relative timing of processes such as melt extraction and melt-rock reaction. The Shetland Ophiolite Complex (SOC) contains a well-preserved mantle section that is dominated by harzburgite (≥70 vol.%), with dominantly chondritic present-day 187Os/188Os compositions1. Melt extraction and melt-rock reaction is evident in the form of dunite and chromitite layers and lenses, with thicknesses ranging from millimetres-to-metres. These lithologies are characteristic of supra-subduction zone processing and are considered to relate to closure of the Iapetus Ocean at ~492 Ma1. However, evidence of much earlier melt extraction has been suggested for some SOC harzburgites, which have relatively unradiogenic 187Os/188Os compositions that yield TRD model ages as old as ~1.4 Ga1. In order to assess the scales at which such compositional heterogeneities are preserved in the mantle, a small (45 m2) area of the SOC mantle section was selected for detailed lithological mapping and sampling. A selection of harzburgites (n=8), dunites (n=6) and pyroxenites (n=2) from this area has been analysed for their Os isotope and highly-siderophile element (HSE) compositions. Six of the harzburgites and four of the dunites have relative HSE abundances and gOs values that are approximately chondritic, with gOs ranging only from -0.6 to +2.7 (n=10). Two dunites have more radiogenic gOs (up to +7.5), that is correlated with enhanced concentrations of accessory base-metal sulphides, suggesting formation via melt percolation and melt-rock reaction. The two remaining harzburgites have less radiogenic gOs (-3.5 and -4), yielding Mesoproterozoic TRD ages. The new data indicate that a comparable range of Os isotope compositions to that previously measured across the

  13. Quantitative Model to Predict Melts on the Ol-Opx Saturation Boundary during Mantle Melting: The Role of H2O

    NASA Astrophysics Data System (ADS)

    Andrews, A. L.; Grove, T. L.

    2014-12-01

    Two quantitative, empirical models are presented that predict mantle melt compositions in equilibrium with olivine (ol) + orthopyroxene (opx) ± spinel (sp) as a function of variable pressure and H2O content. The models consist of multiple linear regressions calibrated using new data from H2O-undersaturated primitive and depleted mantle lherzolite melting experiments as well as experimental literature data. The models investigate the roles of H2O, Pressure, 1-Mg# (1-[XMg/(XMg+XFe)]), NaK# ((Na2O+K2O)/(Na2O+K2O+CaO)), TiO2, and Cr2O3 on mantle melt compositions. Melts are represented by the pseudoternary endmembers Clinopyroxene (Cpx), Olivine (Ol), Plagioclase (Plag), and Quartz (Qz) of Tormey et al. (1987). Model A returns predictive equations for the four endmembers with identical predictor variables, whereas Model B chooses predictor variables for the four compositional endmember equations and temperature independently. We employ the use of Akaike Information Criteria (Akaike, 1974) to determine the best predictor variables from initial variables chosen through thermodynamic reasoning and by previous models. In both Models A and B, the coefficients for H2O show that increasing H2O drives the melt to more Qz normative space, as the Qz component increases by +0.012(3) per 1 wt.% H2O. The other endmember components decrease and are all three times less affected by H2O (Ol: -0.004(2); Cpx: -0.004(2); Plag: -0.004(3)). Consistent with previous models and experimental data, increasing pressure moves melt compositions to more Ol normative space at the expense of the Qz component. The models presented quantitatively determine the influence of H2O, Pressure, 1-Mg#, NaK#, TiO2, and Cr2O3 on mantle melts in equilibrium with ol+opx±sp; the equations presented can be used to predict melts of known mantle source compositions saturated in ol+opx±sp. References Tormey, Grove, & Bryan (1987), doi: 10.1007/BF00375227. Akaike (1974), doi: 10.1109/TAC.1974.1100705.

  14. Partial melting of fertile peridotite fluxed by hydrous rhyolitic melt at 2-3 GPa: implications for mantle wedge hybridization by sediment melt and generation of ultrapotassic magmas in convergent margins

    NASA Astrophysics Data System (ADS)

    Mallik, Ananya; Nelson, Jared; Dasgupta, Rajdeep

    2015-05-01

    We investigated the melting behavior of peridotite fluxed with 25 wt% of H2O-bearing rhyolitic sediment melt (1.8 wt% bulk H2O), by performing experiments from 1100 to 1300 °C at 2 GPa and 1050-1350 °C at 3 GPa. The apparent solidus of our bulk composition lies between 1100 and 1150 °C at both pressures, which is at a higher temperature than the vapor-saturated solidus and close to the pargasite dehydration solidus of peridotite. With increasing temperature, reacted melt fraction increases from 20 to 36 wt% from 1200 to 1300 °C at 2 GPa and 7 to 24 wt% from 1225 to 1350 °C at 3 GPa. Orthopyroxene is present as a residual phase in all the experiments, while olivine is present as a residual phase in all the experiments at 2 GPa only. Amphibole is absent above 1100 °C at both pressures, clinopyroxene disappears above 1200 and 1300 °C at 2 and 3 GPa, respectively, and garnet (only present at 3 GPa) melts out above 1300 °C. Upon reaction with the mantle wedge and subsequent melting of the hybrid rock, subducted sediment-derived rhyolites evolve in composition to a nepheline-normative ultrapotassic leucitite, similar in major element composition to ultrapotassic lavas from active arcs such as Sunda and inactive arcs such as in the Roman Magmatic Province. Fluxing peridotite with H2O versus H2O-bearing sediment melt at similar pressures does not appear to have an effect on isobaric melt productivity, but does have significant effect on melting reactions and resultant melt composition, with influx of sediment melt adding K2O to the system, thereby stabilizing phlogopite, which in turn buffers the reacted melt to ultrapotassic compositions. Previous experimental studies, along with this study, find that phlogopite can be stable near the hotter core of the mantle wedge and, hence, is likely to be subducted to deeper mantle, thereby influencing deeper cycling of volatiles and large ion lithophile elements. Also, because D {Rb/phl/melt} ≫ D {Sr/phl/melt} and D {Nd/phl/melt

  15. Unraveling the geochemistry of melts in exhumed mantle domains in present-day and fossil magma-poor rifted margins

    NASA Astrophysics Data System (ADS)

    Amann, Méderic; Ulrich, Marc; Autin, Julia; Manatschal, Gianreto; Epin, Marie-Eva; Müntener, Othmar; Boiron, Marie-Christine; Sauter, Daniel

    2016-04-01

    The role of magmatic processes occurring during the continental break-up and the onset of steady-state seafloor spreading are still a matter of debate. Beside the tectonic processes like stretching, thinning and exhumation, magmatic processes also play a key role in the evolution and breakup of magma-poor rifted margins. To unravel the impact of such processes, Ocean-Continent-Transitions (OCTs) are of particular interest. OCTs are complex areas where hyper-extended continental crust, exhumed mantle and proto-oceanic crust occur. All these domains have been identified and sampled in both present-day (Iberia/Newfoundland margins) and fossil margins (Platta/Err nappes). In this study, we present preliminary results that enable to characterize the nature of the mantle rocks and the melts found in the OCTs of these paleo- and present-day margins with the aim to investigate how the mantle evolves from initial exhumation to final lithospheric breaks. In OCTs two types of mantle rocks can be observed: (i) a « sub-continental type » free of syn-exhumation melt imprint preserving the early geochemical evolution, and (ii) a « refertilized type » characterized by melt infiltration and mantle-melt interaction. Melts from these domains have different major, trace element and isotopic compositions and can therefore be used to constrain how melt interacts with the mantle and to understand the role of magmatic processes in the break-up. We therefore summarized whole-rock, in-situ and isotopic analysis available in the literature from the Iberia/Newfoundland present-day margin system and completed the existing database with new additional data from the Iberia margin. These new data have been obtained using in-situ technics mainly on clinopyroxenites, serpentinized peridotites and gabbros of ODP drill cores. Around 200 new data have been acquired using the LA-ICPMS technic. Preliminary results show that clinopyroxenes in serpentinized peridodite breccia from ODP site 637A and

  16. Mantle refertilization by melts of crustal-derived garnet pyroxenite: Evidence from the Ronda peridotite massif, southern Spain

    NASA Astrophysics Data System (ADS)

    Marchesi, Claudio; Garrido, Carlos J.; Bosch, Delphine; Bodinier, Jean-Louis; Gervilla, Fernando; Hidas, Károly

    2013-01-01

    Geochemical studies of primitive basalts have documented the presence of crustal-derived garnet pyroxenite in their mantle sources. The processes whereby melts with the signature of garnet pyroxenite are produced in the mantle are, however, poorly understood and somewhat controversial. Here we investigate a natural example of the interaction between melts of garnet pyroxenite derived from recycled plagioclase-rich crust and surrounding mantle in the Ronda peridotite massif. Melting of garnet pyroxenite at ˜1.5 GPa generated spinel websterite residues with MREE/HREE fractionation and preserved the positive Eu anomaly of their garnet pyroxenite precursor in whole-rock and clinopyroxene. Reaction of melts from garnet pyroxenite with depleted surrounding peridotite generated secondary fertile spinel lherzolite. These secondary lherzolites differ from common spinel lherzolite from Ronda and elsewhere by their lower-Mg# in clinopyroxene, orthopyroxene and olivine, lower-Cr# in spinel and higher whole-rock Al2O3, CaO, Sm/Yb and FeO* at a given SiO2. Remarkably, secondary spinel lherzolite shows the geochemical signature of ghost plagioclase in the form of positive Eu and Sr anomalies in whole-rock and clinopyroxene, reflecting the transfer of a low-pressure crustal imprint from recycled pyroxenite to hybridized peridotite. Garnet pyroxenite melting and melt-peridotite interaction, as shown in the Ronda massif, may explain how the signature of subducted or delaminated crust is transferred to the mantle and how a garnet pyroxenite component is introduced into the source region of basalts. The efficiency of these processes in conveying the geochemical imprint of crustal-derived garnet pyroxenite to extruded lavas depends on the reactivity of pyroxenite melt with peridotite and the mantle permeability, which may be controlled by prior refertilization reactions similar to those documented in the Ronda massif. Highly fertile heterogeneities produced by pyroxenite

  17. Subduction of fracture zones controls mantle melting and geochemical signature above slabs.

    PubMed

    Manea, Vlad C; Leeman, William P; Gerya, Taras; Manea, Marina; Zhu, Guizhi

    2014-10-24

    For some volcanic arcs, the geochemistry of volcanic rocks erupting above subducted oceanic fracture zones is consistent with higher than normal fluid inputs to arc magma sources. Here we use enrichment of boron (B/Zr) in volcanic arc lavas as a proxy to evaluate relative along-strike inputs of slab-derived fluids in the Aleutian, Andean, Cascades and Trans-Mexican arcs. Significant B/Zr spikes coincide with subduction of prominent fracture zones in the relatively cool Aleutian and Andean subduction zones where fracture zone subduction locally enhances fluid introduction beneath volcanic arcs. Geodynamic models of subduction have not previously considered how fracture zones may influence the melt and fluid distribution above slabs. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations of subduction, we show that enhanced production of slab-derived fluids and mantle wedge melts concentrate in areas where fracture zones are subducted, resulting in significant along-arc variability in magma source compositions and processes.

  18. Tracking the Martian Mantle Signature in Olivine-Hosted Melt Inclusions of Basaltic Shergottites Yamato 980459 and Tissint

    NASA Technical Reports Server (NTRS)

    Peters, T. J.; Simon, J. I.; Jones, J. H.; Usui, T.; Moriwaki, R.; Economos, R.; Schmitt, A.; McKeegan, K.

    2014-01-01

    The Martian shergottite meteorites are basaltic to lherzolitic igneous rocks that represent a period of relatively young mantle melting and volcanism, approximately 600-150 Ma (e.g. [1,2]). Their isotopic and elemental composition has provided important constraints on the accretion, evolution, structure and bulk composition of Mars. Measurements of the radiogenic isotope and trace element concentrations of the shergottite meteorite suite have identified two end-members; (1) incompatible trace element enriched, with radiogenic Sr and negative epsilon Nd-143, and (2) incompatible traceelement depleted, with non-radiogenic Sr and positive epsilon 143-Nd(e.g. [3-5]). The depleted component represents the shergottite martian mantle. The identity of the enriched component is subject to debate, and has been proposed to be either assimilated ancient martian crust [3] or from enriched domains in the martian mantle that may represent a late-stage magma ocean crystallization residue [4,5]. Olivine-phyric shergottites typically have the highest Mg# of the shergottite group and represent near-primitive melts having experienced minimal fractional crystallization or crystal accumulation [6]. Olivine-hosted melt inclusions (MI) in these shergottites represent the most chemically primitive components available to understand the nature of their source(s), melting processes in the martian mantle, and origin of enriched components. We present trace element compositions of olivine hosted melt inclusions in two depleted olivinephyric shergottites, Yamato 980459 (Y98) and Tissint (Fig. 1), and the mesostasis glass of Y98, using Secondary Ionization Mass Spectrometry (SIMS). We discuss our data in the context of understanding the nature and origin of the depleted martian mantle and the emergence of the enriched component.

  19. Application of Markov Chain Monte Carlo Method to Mantle Melting: An Example from REE Abundances in Abyssal Peridotites

    NASA Astrophysics Data System (ADS)

    LIU, B.; Liang, Y.

    2015-12-01

    Markov chain Monte Carlo (MCMC) simulation is a powerful statistical method in solving inverse problems that arise from a wide range of applications, such as nuclear physics, computational biology, financial engineering, among others. In Earth sciences applications of MCMC are primarily in the field of geophysics [1]. The purpose of this study is to introduce MCMC to geochemical inverse problems related to trace element fractionation during concurrent melting, melt transport and melt-rock reaction in the mantle. MCMC method has several advantages over linearized least squares methods in inverting trace element patterns in basalts and mantle rocks. First, MCMC can handle equations that have no explicit analytical solutions which are required by linearized least squares methods for gradient calculation. Second, MCMC converges to global minimum while linearized least squares methods may be stuck at a local minimum or converge slowly due to nonlinearity. Furthermore, MCMC can provide insight into uncertainties of model parameters with non-normal trade-off. We use MCMC to invert for extent of melting, amount of trapped melt, and extent of chemical disequilibrium between the melt and residual solid from REE data in abyssal peridotites from Central Indian Ridge and Mid-Atlantic Ridge. In the first step, we conduct forward calculation of REE evolution with melting models in a reasonable model space. We then build up a chain of melting models according to Metropolis-Hastings algorithm to represent the probability of specific model. We show that chemical disequilibrium is likely to play an important role in fractionating LREE in residual peridotites. In the future, MCMC will be applied to more realistic but also more complicated melting models in which partition coefficients, diffusion coefficients, as well as melting and melt suction rates vary as functions of temperature, pressure and mineral compositions. [1]. Sambridge & Mosegarrd [2002] Rev. Geophys.

  20. Present Day Hot Spot Melting Inferred from Geodynamics and Thermodynamics Modeling and the Thermal History of the Mantle

    NASA Astrophysics Data System (ADS)

    Tirone, M.; Ganguly, J.

    2011-12-01

    Hot spot melting is the end-product of a complex sequence of processes that most likely starts at the CMB boundary. To provide a complete description and a better constrained understanding of the melting process from the petrological and geophysical point of view, two requirements appear to be essential. 1) accurate knowledge of the physico-chemical and thermodynamic properties of the mantle, and characterization of the thermal and geodynamic conditions for the generation and evolution of a thermal plume. 2) development of a geodynamic numerical procedure that incorporates the properties of the mantle and is capable of simulate the thermal evolution of a plume and the petrological evolution of the melting process. The thermodynamic approach is best suited to accomplish the latter objective. By following these requirements, the model allows us to compute several features that can be independently compared with petrological and geophysical observations which ultimately provide a validation for the whole procedure. The viscosity and the thermal conditions leading to the formation of a thermal plume are retrieved from a separate study of the thermal history of the mantle that is mainly constrained by the requirement for melting in the upper mantle in the past and recent time (session DI19, contribution entitled: A Viscosity Model for the Mantle Based on Diffusion in Minerals and Constrained by the Thermal History and Melting of the Mantle). The main topic of this contribution is the final stage of the plume evolution, that is the the thermal structure and melting in a hot spot setting, focusing in particular on the magmatism associated to the Hawaiian-Emperor seamount chain. The numerical study is a work in progress that reveals several key features. Perhaps the most important aspect that emerge from the modeling work is the formation of periodic instabilities and the thermal erosion of the base of the lithosphere which are mainly induced by the formation and transport

  1. Hydrous metasomatism and melt percolation in the lithospsheric mantle wedge underneath Comallo, Rio Negro Province, Argentina

    NASA Astrophysics Data System (ADS)

    Papadopoulou, Martha; Ntaflos, Theodoros; Bjerg, Ernesto; Gregoire, Michel; Hauzenberger, Christoph

    2015-04-01

    Xenoliths from Comallo, N. Patagonia, are sp-lherzolites, sp-harzburgites, dunites, wehrlites and clinopyroxenites. The rock-forming minerals are olivine, ortho- and clinopyroxene and spinel. Amphibole and phlogopite are present as relicts, suggesting that the region was affected by modal metasomatism. The majority of xenoliths show a dominant well-equilibrated equigranular texture. Small rounded spinels and sulfides enclosed within olivine as well as amphiboles enclosed in clinopyroxenes indicate that these xenoliths are recrystallized. The recrystallized samples show secondary protogranular textures. The amphibole inclusions in clinopyroxenes indicate that the peridotite has experienced a dehydration reaction during the recrystallization process. Amphibole and phlogopite, where present, have been destabilized and show breakdown reactions at the margin, forming secondary ol, cpx and sp. The clinopyroxene REE patterns display a concave-up shape in LREE and MREE whereas the HREE abundances are low. Depending on the presence or not of amphibole and/or phlogopite the cpx REE patterns can be divided into two different groups, both of which show absence of Sr- and weak Zr, Hf and Ti-negative anomalies. These features combined with the REE patterns highlight a cryptic metasomatism due to melt infiltration of alkali basaltic composition. The differences occuring between the two groups may indicate a differentiation at distance from the percolation front. A third group with steep patterns, negative slope and slightly positive Eu anomaly shows a progression from LREE enrichments to depleted HREE. A carbonatitic metasomatism is evidenced by the LREE enrichment as well as a positive Eu-anomaly combined with a negative Ti-anomaly. Calculated equilibrium temperatures at 1.5GPa using the cores of crystals range between 790 and 950°C, whereas the estimated temperatures using rims are ~70°C higher. Such temperatures are relatively low for the lithospheric mantle below Comallo

  2. The Effect of Large Melt Fraction on the Deformation Behavior of Peridotite: Implications for the Rheology of Io' Mantle

    NASA Technical Reports Server (NTRS)

    Scott, T.; Kohlstedt, D. L.

    2004-01-01

    One key constraint needed for refinement of the interior geochemical and geodynamic models of Io is the viscosity of the convecting partially- molten silicate mantle. To date, laboratory studies of partially molten mantle rocks have reached melt fractions up to approx.0.12, a value much smaller than thought to be appropriate for the asthenosphere of Io where the degree of partial melting may be 0.15 0.40 or higher. Therefore, we have performed a series of high temperature, triaxial compressive creep experiments on dry synthetic peridotites in a gas medium apparatus at a confining pressure of 300 MPa and temperatures from 1473 to 1573 K in order to understand the influence of large amounts of melt (0.15 < phi < 0.40) on the rheological behavior of partially molten rocks.

  3. Immiscibility between calciocarbonatitic and silicate melts and related wall rock reactions in the upper mantle: a natural case study from Romanian mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Chalot-Prat, Françoise; Arnold, Michel

    1999-04-01

    This paper presents the textural, mineralogical and chemical study of veinlets cross-cutting peridotite xenoliths from the lithospheric mantle and brought to the surface by alkaline basalts (Persani Mountains, Romania). The veinlets utilized pre-existing zones of weakness in the host rocks or display a random distribution, lining grain boundaries or cross-cutting any mineral, and always forming an interconnected network. They are filled with carbonate patches included in a silicate matrix. Both products are holocrystalline. Carbonate products have alkali-poor calciocarbonatitic to sövitic compositions, while the silicate matrix composition ranges from monzodioritic to monzonitic and alkali feldspar syenitic, depending on the host-sample, i.e., within a rather alkaline silica-saturated series. The mineral phases present in the silicate matrix (F-apatite, armalcolite, chromite, diopside-enstatite series, plagioclase-sanidine series) are usually present in the carbonate zones, where forsterite is also found. Some minerals cross-cut the interface between both types of zones. Only the matrix is different, feldspathic (oligoclase to sanidine) in the former and pure calcite in the latter. Thus, mineralogical and textural relationships between both products are consistent with an origin with equilibrium liquid immiscibility. Mantle minerals cross-cut by veinlets are sometimes resorbed at grain boundaries, and at the contact of the most alkaline silicate and carbonate melts, subhedral diopside/augite formed at the expense of mantle enstatite or olivine. In terms of mineral chemistry, the compositional variations recorded by vein minerals vary along a continuous trend. They generally superpose to those observed from lherzolites to harzburgites, and exhibit the same range of composition as that observed between rims and cores of mantle minerals cross-cut by veinlets. In detail, the Ca-rich pyroxenes of veinlets are Al-poor and Mg-rich; cpx in the carbonate zones are slightly

  4. Kinetics of Peridotite and Pyroxenite-derived Melts Interaction: Implications for the Style and Extent of Melt-rock Reaction in the Mantle

    NASA Astrophysics Data System (ADS)

    Lo Cascio, M.; Liang, Y.

    2006-12-01

    Distinct geochemical and petrologic features of ocean floor basalts and mantle peridotites suggest that the upper mantle is lithologically heterogeneous, consisting predominantly of peridotite and a small amount of eclogite [1]. An important issue of this marble cake mantle, is the nature of the peridotite and pyroxenite interface during mantle melting. It has been suggested that during mantle melting eclogite and peridotite develop a reactive boundary layer composed of a second generation eclogite and a layer of orthopyroxenite [2]. The existence of such a boundary layer has also been used to explain the observation that oceanic basalts are extracted with only limited interaction with the surrounding peridotite [3]. In spite of recent progresses, the kinetics of peridotite and pyroxenite-derived melts reaction is still not well understood. It is likely that there are two regimes of peridotite-pyroxenite melt interaction: a high T/low P regime where both the peridotite and pyroxenite are partially molten; and a low T/high P regime where only pyroxenite is partially molten. In this study we explored the kinetics of such interactions in both regimes by conducting lherzolite dissolution experiments using a pyroxenite-derived melt at 1300°C and 1-2 GPa. Dissolution couples were formed by juxtaposing pre-synthesized rods of a basaltic andesite (54.6% SiO2, Mg# 0.42), whose composition is similar to pyroxenite derived liquid at 1300°C and 2 GPa [1,4], and a lherzolite (ol+opx+cpx) in a Pt and graphite lined Mo capsule. The lherzolite solidus is below 1300°C at 1 GPa [5], but above 1300°C at 2 GPa. Lherzolite hardly dissolves (~35 μm in 6 hours) into the melt at 2 GPa and a thin opx layer (<10 μm thick) decorated with a few garnet crystals is observed at the lherzolite-melt interface. From the concentration profiles of Al2O3 and MgO in the melt, we estimated the effective binary diffusion coefficient at 10^{-12}m2/s. Assuming an average mantle upwelling rate of ~50

  5. Lithosphere and Asthenosphere Properties beneath Oceans and Continents and their Relationship with Domains of Partial Melt Stability in the Mantle

    NASA Astrophysics Data System (ADS)

    Dasgupta, R.

    2014-12-01

    The depth of the lithosphere-asthenosphere boundary (LAB) and the change in properties across the lithosphere, asthenosphere, and LAB in various tectonic settings are captured in a variety of geophysical data, including seismic velocities and electrical conductivity. A sharp drop in shear wave velocity and increase in electrical conductivity can potentially be caused by the appearance of partial melt at or below the LAB but the chemical and dynamic stability of partial melt across lithosphere and at LAB remain debated. Here I apply the recent models of mantle melting in the presence of water and carbon [1, 2] to evaluate the domains of stability of partial melt both beneath continents and oceans. The model allows prediction of the possible presence, the fraction, and composition of partial melt as a function of depth, bulk C and H2O content, and fO2 [3] in various geologic/tectonic settings. The results show that while a hydrous, carbonated melt is stable only beneath LAB and in the asthenospheric mantle beneath oceans, continental mantle can contain a carbonate-rich melt within the lithosphere. For geotherms corresponding to surface heat flux (SHF) of 40-50 mW m-2, which also match P-T estimates beneath cratons based on thermo-barometry of peridotite xenoliths [4], the solidus of fertile peridotite with trace amount of CO2 and H2O is crossed at depths as shallow as 80-120 km [5]. If elevated geotherms of the Proterozoic and Phanerozoic terrains are applied, carbonatitic melt becomes stable somewhat shallower. These depths are similar to those argued for a mid-lithospheric discontinuity (MLD) where a negative velocity gradient has been detected much shallower than the proposed depth of LAB in many places. With a drop in oxygen fugacity with depth, a freezing of carbonatitic melt may be expected at intermediate depths (~150-200 km). At 200-250 km a hydrous, carbonated silicate melt may reappear owing to the interplay of fO2 and freezing point depression effect of CO

  6. 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

  7. 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

  8. Crossover from melting to dissociation of CO[subscript 2] under pressure: Implications for the lower mantle

    SciTech Connect

    Litasov, Konstantin D.; Goncharov, Alexander F.; Hemley, Russell J.

    2011-11-17

    A systematic investigation of the solid-solid phase transitions, melting behavior, and chemical reactivity of CO{sub 2} at pressures of 15-70 GPa and temperatures up to 2500 K has been carried out using in situ Raman spectroscopy in laser-heated diamond anvil cells. We find that molecular CO{sub 2} melts to a molecular fluid up to 33 ({+-} 2) GPa and 1720 K ({+-} 100), where it meets a solid-solid phase line to form a triple point. At higher pressure, non-molecular CO{sub 2} phase V does not melt but instead dissociates to carbon and oxygen with a transition line having a negative PT-slope. A comparison with P-T profiles of the Earth's mantle indicates that polymeric CO{sub 2}-V can be stable near the top of the lower mantle and dissociates at greater depths. Decarbonation reactions of subducted carbonates in the lower mantle would produce diamond and fluid oxygen, which in turn significantly affects redox state, increasing oxygen fugacity by several orders of magnitude. The reaction of free oxygen with lower mantle minerals such as Mg-perovskite can create significant conductivity anomalies.

  9. Geochronology and geochemistry of Eocene-aged volcanic rocks around the Bafra (Samsun, N Turkey) area: Constraints for the interaction of lithospheric mantle and crustal melts

    NASA Astrophysics Data System (ADS)

    Temizel, İrfan; Arslan, Mehmet; Yücel, Cem; Abdioğlu, Emel; Ruffet, Gilles

    2016-08-01

    modelling suggests that the parental magma(s) of the volcanic rocks represent mixtures of melts derived by low-degree (~ 5-10%) partial melting of spinel-lherzolite (40-85%) and garnet-lherzolite (15-60%) mantle sources. Sr-Nd isotopic modelling also suggests that a 25-35% lower crustal component was added in the parental magmas; AFC modelling additionally indicates minor upper crustal contamination during the evolution of the volcanic rocks. In conclusion, integration of the geochemical, petrologic, and isotopic data with regional geology suggests that the analcime-bearing and -free volcanic rocks evolved from parental magma(s) derived from melts of a subcontinental lithospheric mantle and lower crustal sources.

  10. Melting in the FeOsbnd SiO2 system to deep lower-mantle pressures: Implications for subducted Banded Iron Formations

    NASA Astrophysics Data System (ADS)

    Kato, Chie; Hirose, Kei; Nomura, Ryuichi; Ballmer, Maxim D.; Miyake, Akira; Ohishi, Yasuo

    2016-04-01

    Banded iron formations (BIFs), consisting of layers of iron oxide and silica, are far denser than normal mantle material and should have been subducted and sunk into the deep lower mantle. We performed melting experiments on Fe2SiO4 from 26 to 131 GPa in a laser-heated diamond-anvil cell (DAC). The textural and chemical characterization of a sample recovered from the DAC revealed that SiO2 is the liquidus phase for the whole pressure range examined in this study. The chemical compositions of partial melts are very rich in FeO, indicating that the eutectic melt compositions in the FeOsbnd SiO2 binary system are very close to the FeO end-member. The eutectic temperature is estimated to be 3540 ± 150 K at the core-mantle boundary (CMB), which is likely to be lower than the temperature at the top of the core at least in the Archean and Paleoproterozoic eons, suggesting that subducted BIFs underwent partial melting in a thermal boundary layer above the CMB. The FeO-rich melts formed by partial melting of the BIFs were exceedingly dense and therefore migrated downward. We infer that such partial melts have caused iron enrichment in the bottom part of the mantle, which may have contributed to the formation of ultralow velocity zones (ULVZs) observed today. On the other hand, solid residues left after the segregation of the FeO-rich partial melts have been almost pure SiO2, and therefore buoyant in the deep lower mantle to be entrained in mantle upwellings. They have likely been stretched and folded repeatedly by mantle flow, forming SiO2 streaks within the mantle "marble cake". Mantle packages enhanced by SiO2 streaks may be the origin of seismic scatterers in the mid-lower mantle.

  11. High-magnesian andesite from Mount Shasta: A product of magma mixing and contamination, not a primitive mantle melt

    NASA Astrophysics Data System (ADS)

    Streck, Martin J.; Leeman, William P.; Chesley, John

    2007-04-01

    It has been proposed that high-Mg andesites (HMAs) from the Mount Shasta area may represent near-primary mantle melts, carrying signatures of slab melt interaction with the Cascadia mantle wedge. We present strong evidence that their formation involved mixing of dacitic and basaltic magmas and entrainment of ultramafic crystal material, and thus they cannot represent primitive magmas. The rocks contain (1) low-Mg# (65 72) clinopyroxene (cpx) and orthopyroxene (opx) phenocryst cores containing dacitic melt inclusions, and (2) high-Mg# opx and olivine xenocrysts, all of which are rimmed by euhedral overgrowths of cpx or opx similar in Mg# (87) to skeletal olivine phenocrysts. Textural relations indicate that ultramafic xenocrysts reacted with dacitic liquid, after which the contaminated magma mixed with basaltic liquid to produce a hybrid HMA bulk composition. High Mg, Cr, and Ni derive from the latter inputs, whereas high Sr/Y and overall adakite affinity is inherited from the dacite end member, which is arguably crustal in origin. We suggest that open system processes may be more important in the petrogenesis of HMAs than generally recognized, and that their magnesian compositions do not necessarily imply that they are primitive mantle melts.

  12. Hydrous metasomatism and melt percolation in the lithospsheric mantle wedge underneath Comallo, Rio Negro Province, Argentina

    NASA Astrophysics Data System (ADS)

    Papadopoulou, Martha; Ntaflos, Theodoros; Bjerg, Ernesto; Gregoire, Michel

    2014-05-01

    At Comallo, N. Patagonia, xenoliths are depleted sp-lherzolites, sp-harzburgites, wehrlites and clinopyroxenites. The studied samples are fine-grained with a dominant well-equilibrated equigranular texture whereas protogranular and porphyroclastic textures are rare. The rock forming minerals are olivine, ortho- and clinopyroxene and spinel. Relictic amphibole and phlogopite are present as well. The amphibole, where present, has been destabilized and shows breakdown reaction at the margin, forming second generation of ol, cpx and sp. The presence of disseminated amphibole and phlogopite, indicates that the lithospheric mantle underneath Comallo, has experienced limited modal metasomatism. The Fo content in the lherzolites and harzburgites range from 90.8 to 92.0, and in the wherlites from 89.0 to 90.0. Clinopyroxene is diopside with 3.0 wt% Al2O3 in the rim and up to 4 wt% in the core. Spinel is Cr2O3-rich with cr#=0.426. The majority of the clinopyroxene REE patterns show similar shapes such as concave-up LREE and MREE and low HREE abundances. The cpx REE patterns differentiate into two groups depending on the presence or not of amphibole and/or phlogopite. At both groups the absence of Sr- and the weak Zr, Hf and Ti-negative anomalies combined with the REE patterns indicate infiltration of alkali basaltic melt whereas the different REE abundances in both groups may indicate a differentiation at the distance from the percolation front. A third group has steep patterns with negative slope and slightly positive Eu anomaly that shows a progression from LREE enrichments to depleted HREE. In this group the enrichment in LREE as well as the positive Eu-anomaly combined with a negative Ti-anomaly is an evidence for carbonatitic metasomatism. Using the model of batch melting the calculated extraction of basaltic components is variable and ranges between 3 and 20%. Calculated equilibrium temperatures at 1.5GPa pressure, which range between 850 and 920°C, are relative low

  13. Trace Elements in Olivine in Italian Potassic Volcanic Rocks Distinguish Between Mantle Metasomatism by Carbonatitic and Silicate Melts

    NASA Astrophysics Data System (ADS)

    Foley, S. F.; Ammannati, E.; Jacob, D. E.; Avanzinelli, R.; Conticelli, S.

    2014-12-01

    The Italian Peninsula is the site of intense subduction-related potassic magmatism with bimodal character in terms of silica activity: Ca-poor silica-saturated lamproitic rocks and Ca-enriched silica-undersaturated leucitites. Lamproitic magmas formed in the early phases of magmatic activity and were followed by leucititic magmas. The primary magmas are generated in the sub-continental lithospheric mantle at the destructive plate margin, and both series have olivine as the first crystallizing phenocrysts. Trace elements in olivine phenocrysts are important in recognizing metasomatic effects on the mineralogy of the mantle source. Since Ni is the most compatible trace element in olivine, particularly in alkaline melts, modal changes of olivine in the source strongly affect its bulk partition coefficient, and therefore its content in primary melts and in olivine that crystallizes from them.The concentration of other compatible trace elements (e.g. Mn, Co) in olivine phenocrysts also depends on the abundance of olivine in the magma source. Ni contents in olivine of the Italian rocks show a clear bimodal distribution. Olivine from lamproitic samples has systematically higher Fo and Ni contents, whereas olivine from leucititic rocks never exceeds Fo92 and has markedly lower Ni, reaching among the lowest levels ever observed in olivine phenocrysts in primitive melts. The Mn/Fe ratio of olivine is also sensitive to changes of the modal abundance of olivine in the source, 100*Mn/Fe of olivine from lamproitic rocks never exceeds 2, while it is always >1.8 in leucititic rocks, meaning that the leucitite source regions are much richer in olivine. Lithium is generally enriched in the crust and in sediments compared to the lithospheric mantle and to mantle-derived melts,so that Li in olivine above 10 ppm is suggested to indicate recycled sediments. Li contents are up to 35 ppm in leucititic olivines and up to >50 ppm in lamproitic olivines, confirming the recycling of crustal

  14. Melting of phase D in the lower mantle and implications for recycling and storage of H2O in the deep mantle

    NASA Astrophysics Data System (ADS)

    Ghosh, Sujoy; Schmidt, Max W.

    2014-11-01

    We determined the melting phase relations and conditions of the dense hydrous magnesium silicate phase D (nominally MgSi2O4(OH)2) on composition in MgO-SiO2-H2O (MSH), MgO-Al2O3-SiO2-H2O (MASH), and FeO-MgO-Al2O3-SiO2-H2O (FMASH), and on a mixture of phase D + olivine + enstatite (MSH) at 22-32 GPa and 1000-1800 °C. Contrasting to previous studies, we performed H2O-undersaturated experiments. Bulk compositions were synthetic mixtures of brucite + silica or brucite + olivine + enstatite on the silica-rich side of the tie-line perovskite-H2O. At 22-24 GPa, the maximum thermal stability of phase D is between 1350 and 1400 °C in MSH and FMASH, but 1600 °C at 24 GPa in the Fe-free, Al-bearing bulk composition (MASH). Apparently, addition of Al2O3 increases the stability field of phase D by 200 °C, an effect that is counter balanced by addition of FeO. At 32 GPa, the stability of phase D (MSH and FMASH) is between 1350 and 1400 °C. At 22 GPa, phase D melts to a Mg-rich melt coexisting with MgSi-ilmenite + stishovite, whereas at 24-32 GPa melt coexists with perovskite and stishovite. Even melts from bulk compositions in the silica-rich part of the MSH system (molar bulk Mg/Si < 0.5) are magnesian-rich (Mg/Si molar ratio of 2-5) and are distinct from aqueous fluids and hydrous melts at lower pressures. The temperature stabilities determined in this study indicate that slabs that thermally relax when stagnating on top of the 660-km discontinuity or penetrating into the lower mantle will have their last dense hydrous magnesium silicate phase, i.e., phase D, melting and producing a magnesian and hydrous melt that will rise through the transition zone. Such a melt could be responsible for observed low velocity zones, and may be neutrally buoyant at the 410-km discontinuity and will affect the structure and dynamics of the mantle.

  15. Experimental determination of carbon partitioning between upper mantle minerals and silicate melts: initial results and comparison to trace element partitioning (Nb, Rb, Ba, U, Th, K)

    NASA Astrophysics Data System (ADS)

    Rosenthal, A.; Hauri, E. H.; Hirschmann, M. M.; Davis, F. A.; Withers, A. C.; Fogel, M. L.

    2012-12-01

    ). Resulting D's indicate that C is highly incompatible in all major mantle mineral phases, with D's for OL, OPX and CPX of close to 5x10-4, and for GA ~2.2x10-4. D's for H2O (2x10-4 to ~3x10-2) and F (~2.3x10-3 to ~5.8x10-2) are comparable to those found in previous studies. Trace element partition determinations are in progress, but comparison to previous studies indicates that carbon is significantly more incompatible during mantle melting than Nb, U, or Th, and has behavior approximately similar to Ba. We therefore suggest that undegassed C/Ba ratios may be useful indicators of C fluxes and concentrations in basalt source regions where very low degrees of melting might fractionate C/Nb ratios. [1] Saal, A, Hauri, EH, Langmuir, CH, Perfit, M (2002) Nature 419, 451-455. [2] Cartigny, P, Pineau, F, Aubaud, C, Javoy, M (2008) Earth Planet Sci Lett 265, 672-685.

  16. Mantle temperatures, and tests of experimentally calibrated olivine-melt equilibria

    NASA Astrophysics Data System (ADS)

    Putirka, K. D.

    2005-12-01

    precise at both low and high MgO, and hydrous and non-hydrous systems compared to Beattie (1993) and Ford et al. (1983). Herzberg (pers. comm.) has modeled olivine-melt pairs for Hawaii and MOR's, which are in accord with Putirka (2005); these pairs are used to test for the effects of systematic model error on estimates of mantle temperatures. The Beattie (1993) and Ford (1983) models appear to predict too low a value for T (by 100-150 K), given comparisons of lnKd(Mg) v. 1/T(K) for experimental data, and the very low values for Kd(Mg) observed at Hawaii. The new calibrations indicate mantle equilibration temperatures of 1855 K at Hawaii and 1608 K at MOR's, in agreement with calculations by Putirka (2005). Excess temperatures at Hawaii thus likely exceed 200 K, as suggested by dynamic model predictions (Sleep, 1990; Schilling, 1991) for a thermal plume origin for the Hawaiian Islands and other hot spots.

  17. Melting depths and mantle heterogeneity beneath Hawaii and the East Pacific Rise: Constraints from Na/Ti and rare earth element ratios

    SciTech Connect

    Putirka, K.

    1999-02-01

    Mantle melting calculations are presented that place constraints on the mineralogy of the basalt source region and partial melting depths for oceanic basalts. Melting depths are obtained from pressure-sensitive mineral-melt partition coefficients for Na, Ti, Hf, and the rare earth elements (REE). Melting depths are estimated by comparing model aggregate melt compositions to natural basalts from Hawaii and the East Pacific Rise (EPR). Variations in melting depths in a peridotite mantle are sufficient to yield observed differences in Na/Ti, Lu/Hf, and Sm/Yb between Hawaii and the EPR. Initial melting depths of 95{endash}120 km are calculated for EPR basalts, while melting depths of 200{endash}400 km are calculated for Hawaii, indicating a mantle that is 300&hthinsp;{degree}C hotter at Hawaii. Some isotope ratios at Hawaii are correlated with Na/Ti, indicating vertical stratification to isotopic heterogeneity in the mantle; similar comparisons involving EPR lavas support a layered mantle model. Abundances of Na, Ti, and REE indicate that garnet pyroxenite and eclogite are unlikely source components at Hawaii and may be unnecessary at the EPR. The result that some geochemical features of oceanic lavas appear to require only minor variations in mantle mineral proportions (2{percent} or less) may have important implications regarding the efficiency of mantle mixing. Heterogeneity required by isotopic studies might be accompanied by only subtle differences in bulk composition, and material that is recycled at subduction zones might not persist as mineralogically distinct mantle components. {copyright} 1999 American Geophysical Union

  18. Softening of the subcontinental lithospheric mantle by asthenosphere melts and the continental extension/oceanic spreading transition

    NASA Astrophysics Data System (ADS)

    Ranalli, G.; Piccardo, G. B.; Corona-Chávez, P.

    2007-05-01

    The majority of ophiolitic peridotites in the Alpine-Apennine system show evidence of extensive interaction between subcontinental lithospheric mantle and fractional melts of asthenospheric origin. This interaction led to petrological, structural, and geochemical changes in the lithospheric mantle, and was accompanied by a temperature increase to near-asthenospheric values, resulting in the thermomechanical erosion of the lithosphere. We term the parts of mantle lithosphere thus affected the asthenospherized lithospheric mantle or ALM. The thermal and rheological consequences of thermomechanical erosion are explored by modelling the temperature and rheological properties of the thinned lithosphere as a function of thickness of ALM and time since asthenospherization (i.e., since the beginning of thermal relaxation). Results are given both in terms of rheological profiles (strength envelopes) and total lithospheric strength (TLS) for different lower crustal rheologies. The TLS decreases as a consequence of thermomechanical erosion. This decrease is a non-linear function of the thickness of the ALM. While practically negligible if less than 50% of lithospheric mantle is affected, it becomes significant (up to almost one order of magnitude) if thermomechanical erosion approaches the Moho. The maximum decrease in TLS is achieved within a short time span (˜1-2 Ma) after the end of the heating episode. As a working hypothesis, we propose that thermomechanical erosion of the lithospheric mantle, related to lithosphere/asthenospheric melts interaction, can be an important factor in a geologically rapid decrease in TLS. This softening could lead to whole lithospheric failure and consequently to a transition from continental extension to oceanic spreading.

  19. First Kinetic Reactive-Flow and Melting Calculations for Entropy Budget and Major Elements in Heterogeneous Mantle Lithologies (Invited)

    NASA Astrophysics Data System (ADS)

    Asimow, P. D.

    2009-12-01

    The consequences of source heterogeneity and reactive flow during melt transport in the mantle can be classified by scale. At the smallest spatial and longest temporal scales, we can assume complete equilibrium and use batch melting of homogenized sources or equilibrium porous flow treatments. At large enough spatial scale or short enough temporal scale to prevent any thermal or chemical interaction between heterogeneities or between melt and matrix, we can assume perfectly fractional melting and transport and apply simple melt-mixing calculations. At a somewhat smaller spatial or longer temporal scale, thermal but not chemical interactions are significant and various lithologies and channel/matrix systems must follow common pressure-temperature paths, with energy flows between them. All these cases are tractable to model with current tools, whether we are interested in the energy budget, major elements, trace elements, or isotopes. There remains, however, the very important range of scales where none of these simple theories applies because of partial chemical interaction among lithologies or along the flow path. Such disequilibrium or kinetic cases have only been modeled, in the case of mantle minerals and melts, for trace elements and isotopes, with fixed melting rates instead of complete energy budgets. In order to interpret volumes of magma production and major element basalt and residue compositions that might emerge from a heterogeneous mantle in this last range of scales, we must develop tools that can combine a kinetic formulation with a major element and energy-constrained thermodynamic calculation. The kinetics can be handled either with a chemical kinetic approach with rate constants for various net transfer and exchange reactions, or with a physical diffusion-limited approach. A physical diffusion-limited approach can be built with the following elements. At grain scale, spherical grains of an arbitrary number of solid phases can evolve zoning profiles

  20. Geochemistry of rare high-Nb basalt lavas: Are they derived from a mantle wedge metasomatised by slab melts?

    NASA Astrophysics Data System (ADS)

    Hastie, Alan R.; Mitchell, Simon F.; Kerr, Andrew C.; Minifie, Matthew J.; Millar, Ian L.

    2011-09-01

    Compositionally, high-Nb basalts are similar to HIMU (high U/Pb) ocean island basalts, continental alkaline basalts and alkaline lavas formed above slab windows. Tertiary alkaline basaltic lavas from eastern Jamaica, West Indies, known as the Halberstadt Volcanic Formation have compositions similar to high-Nb basalts (Nb > 20 ppm). The Halberstadt high-Nb basalts are divided into two compositional sub-groups where Group 1 lavas have more enriched incompatible element concentrations relative to Group 2. Both groups are derived from isotopically different spinel peridotite mantle source regions, which both require garnet and amphibole as metasomatic residual phases. The Halberstadt geochemistry demonstrates that the lavas cannot be derived by partial melting of lower crustal ultramafic complexes, metasomatised mantle lithosphere, subducting slabs, continental crust, mantle plume source regions or an upper mantle source region composed of enriched and depleted components. Instead, their composition, particularly the negative Ce anomalies, the high Th/Nb ratios and the similar isotopic ratios to nearby adakite lavas, suggests that the Halberstadt magmas are derived from a compositionally variable spinel peridotite source region(s) metasomatised by slab melts that precipitated garnet, amphibole, apatite and zircon. It is suggested that high-Nb basalts may be classified as a distinct rock type with Nb > 20 ppm, intraplate alkaline basalt compositions, but that are generated in subduction zones by magmatic processes distinct from those that generate other intraplate lavas.

  1. Surface Tension-Driven Melt Flow in the Upper Mantle: An Experimental and Modeling Approach to Studying Capillary Flow of Silicate Melt Through an Olivine Matrix

    NASA Astrophysics Data System (ADS)

    Parsons, R. A.; Nimmo, F.; Hustoft, J. W.; Holtzman, B. K.; Kohlstedt, D. L.

    2006-12-01

    The flow of melt in partially-molten rocks has important implications for the geochemical and geophysical evolution of planetary bodies over a wide range of length scales. Surface tension is usually ignored in favor of differential stresses and buoyancy forces, but may still distribute melt over geologically interesting distances [1], particularly in small bodies such as asteroids. We have investigated experimentally the role of surface tension in the redistribution of melt. Shear deformation of synthetic mantle-type rocks composed of 76 vol% olivine, 20 vol% chromite, and 4 vol% mid-ocean ridge basalt (MORB) at upper mantle temperature and pressure conditions (1523 K, 300 MPa) produces anastomosing networks of melt-enriched (MORB) regions separated by melt-depleted lenses [2]. After the deformation phase of the experiment, each of three samples were statically annealed at 1523 K for 0, 10, or 100 hours to allow some MORB to redistribute back into the melt-depleted olivine plus chromite matrix via surface tension-driven capillary flow.". We modeled melt redistribution resulting from surface tension during the static anneal [3]. Using sample measurements of dihedral angle, and values for MORB and olivine viscosity from the literature (10 Pas [4] and 8x10^{12} Pas [5], [6], respectively), we are able to constrain the sample permeability by matching the model results to the experiments. Permeability is given by κ = d2φn/b. The model uses an exponential melt-dependent viscosity relation of the form ηo*10-α φ where ηo is the dry olivine viscosity, α = 25 [5], and φ is the melt fraction. We find that a permeability on the order of 10-18 m2, corresponding to n = 2 ± 0.2 and b = 7000 ± 2000, gives the best fit to the the experimental anneals. The relatively high value of b is probably due to clogging of melt paths by chromite grains (see Appendix A of Holtzmann et al.). [1] Stevenson D. J. (1986) GRL, 13, 1149-1152. [2] Holtzman B. K. et al. (2003) Science, 301

  2. Ni distribution in MORB-source-mantle pyroxenites: Traces of melt-rock reaction on a cm-scale

    NASA Astrophysics Data System (ADS)

    Sergeev, D.; Dijkstra, A.; Pettke, T.

    2010-12-01

    Introduction The origin of pyroxenites in mantle peridotites is widely discussed as they represent the most important observed lithological inhomogeneity in the upper mantle. We have studied a case of such mantle heterogeneity, consisting of 1-10 cm thick concordant layers of websteritic pyroxenites within residual MORB-source mantle peridotites, in the Jurassic Pindos Ophiolite (N. Greece). Here we report Ni concentrations in minerals in pyroxenites and we compare them with those in the enclosing peridotites We use this data to test whether these pyroxenite layers are cumulate veins, or the product of melt-rock reactions. Results Ni concentrations analysed by Laser ablation ICP-MS on single olivine grains in thick sections show values for the wall-rock peridotites 2580-3480 μg/g, while 2825-3815 μg/g are measured in pyroxenites layers. We consistently observe higher Ni concentrations (10-20%) in pyroxenites compared to peridotites and we observe the same trend in thick sections containing peridotite and pyroxenite parts. The highest Ni concentrations, up to 4145 μg/g, are found in olivine inclusions - older grains surrounded by newly formed orthopyroxene in pyroxenites. Similar elevated Ni concentrations are also characteristic for single orthopyroxene and clinopyroxene grains in pyroxenites compared to peridotites. Intermediate values are typical for transition zones, as there is no strict contact line on the thick section scale between peridotite and pyroxenite. Discussion The data is consistent with a melt-rock reaction origin for the pyroxenites. Melt-rock reaction in the upper mantle between peridotites and SiO2-rich melts would not significantly lower the whole-rock Ni concentration in the newly-formed hybrid rock (pyroxenites). The transformation of olivine to orthopyroxene would strongly concentrate Ni in the remaining olivine and would also enrich other minerals (Sobolev et al. 2007). This interpretation is fully supported by petrographical

  3. Hydrous solidus of CMAS-pyrolite and melting of mantle plumes at the bottom of the upper mantle

    NASA Astrophysics Data System (ADS)

    Litasov, Konstantin; Ohtani, Eiji

    2003-11-01

    We showed in previous experiments that the melting temperature of hydrous pyrolite, at the transition boundary between wadsleyite and olivine, is abruptly reduced by the presence of 2 wt.% H2O. In this paper we determine the apparent solidus for CaO-MgO-Al2O3-SiO2-pyrolite with lower and geologically more reasonable H2O contents (0.5 wt.%). Phase relations and melt compositions have been determined at pressures of 13.5-17.0 GPa and temperatures of 1600 to 2100°C. There was no abrupt decrease of solidus temperature along the phase boundary between olivine and wadsleyite in pyrolite with 0.5 wt.% H2O. However significant gradual decrease of the solidus temperature at pressures below 15-16 GPa still supports previous models for a hydrous origin of some ancient komatiites by dehydration melting of rising wet plumes at pressures of 4-10 GPa.

  4. 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

  5. Partial melting and refertilization of mantle peridotites in the Xigaze ophiolite: constraints from whole-rock and mineral geochemistry

    NASA Astrophysics Data System (ADS)

    Zhang, Chang; Liu, Chuan-Zhou; Wu, Fu-Yuan

    2016-04-01

    Ophiolites along the E-W trending Yarlung-Tsangpo Suture (YTS), which separates the Indian plate from the Eurasian plate, have been regarded as relics of the Neo-Tethyan Ocean. The Xigaze ophiolite in the central YTS has been extensively studied. One of the most intact crust-mantle sequences is preserved in the Luqu (or Beimarang) ophiolite. Mantle peridotites of the Luqu ophiolite are dominated by harzburgites, with 55-65% olivine, 30-40% orthopyroxene, 1-5% clinopyroxene and 1-3% spinel. Minor lherzolites and dunites are also outcropped, and the mode contents of clinopyroxene in lherzolite can be locally up to 10%. This contribution presented whole-rock major element and mineral chemistry including EMPA (Electronic MicroProbe Analysis) and clinopyroxene in situ trace elements. Whole rock Al2O3 (0.23-2.05%) and CaO (0.41-1.7%) contents are very low but show obviously inverse correlation with MgO (39.7-47.0%), indicating that the Luqu peridotites are residues of variable degrees of partial melting. This is supported by the Cr# (=molar Cr/(Cr+Al)) values of spinels which vary from 0.36 to 0.69. Meanwhile, the high Cr# values of spinels and homogenously high Mg# (= molar Mg/(Mg+Fe2+)) values of olivines, clustering at 0.91, indicate high degrees of partial melting. The low REE (rare earth elements) concentrations and chondrite-normalized distribution partterns of clinopyroxenes reflect ultra-depleted natures, with most showing LREE (light REEs) and MREE (medium REEs) depleted patterns and strong fractionations between MREEs and HREE (heavy REEs) ((Sm/Yb)N: 0.021-0.184). Based on the observations and analyses, a model of two-stage melting process was proposed that the primitive mantle underwent 2-8% melting in garnet stability field which was followed by 10-15% melting in spinel stability field. The clinopyroxenes in some peridotites exhibit obvious enrichment of somestrongly incompatible elements (such as sodium and LREE) that reveal later refertilization process for

  6. Carbonatite melt-peridotite interaction at 5.5-7.0 GPa: Implications for metasomatism in lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Sokol, Alexander G.; Kruk, Alexey N.; Chebotarev, Dimity A.; Palyanov, Yury N.

    2016-04-01

    Interaction between carbonatite melt and peridotite is studied experimentally by melting samples of interlayered peridotite-carbonatite-peridotite in graphite containers at 1200-1350 °C and 5.5-7.0 GPa in a split-sphere multianvil apparatus. Starting compositions are lherzolite and harzburgite, as well as carbonatite which may form in the upper part of a slab or in a plume-related source. Most experimental runs were of 150 h duration in order for equilibrium to be achieved. The interaction produced carbonatitic melts with low SiO2 (≤ 7 wt.%) and high alkalis. At 1200 °C, melt-peridotite interaction occurs through Mg-Ca exchange, resulting in elimination of orthopyroxene and crystallization of magnesite and clinopyroxene. At 1350 °C hybridization of the carbonatite and magnesite-bearing peridotite melts occurred with consumption of clinopyroxene and magnesite, and crystallization of orthopyroxene at MgO/CaO ≥ 4.3. The resulting peridotite-saturated melt has Ca# (37-50) depending on primary carbonatite composition. Compositions of silicate phases are similar to those of high-temperature peridotite but are different from megacrysts in kimberlites. CaO and Cr2O3 changes in garnet produced from the melt-harzburgite interaction at 1200 and 1350 °C perfectly match the observed trend in garnet from metasomatized peridotite of the Siberian subcontinental lithospheric mantle. K-rich carbonatite melts equilibrated with peridotite at 5.5-7.0 GPa and 1200-1350 °C correspond to high-Mg inclusions in fibrous diamond. Carbonatite melt is a weak solvent of entrained xenoliths and therefore cannot produce kimberlitic magma if temperatures are ~ 1350 °C on separation from the lithospheric peridotite source and ~ 1000 °C on eruption.

  7. Mesoarchean melting and Neoarchean to Paleoproterozoic metasomatism during the formation of the cratonic mantle keel beneath West Greenland

    NASA Astrophysics Data System (ADS)

    van Acken, D.; Luguet, A.; Pearson, D. G.; Nowell, G. M.; Fonseca, R. O. C.; Nagel, T. J.; Schulz, T.

    2017-04-01

    Highly siderophile element (HSE) concentration and 187Os/188Os isotopic heterogeneity has been observed on various scales in the Earth's mantle. Interaction of residual mantle peridotite with infiltrating melts has been suggested to overprint primary bulk rock HSE signatures originating from partial melting, contributing to the heterogeneity seen in the global peridotite database. Here we present a detailed study of harzburgitic xenolith 474527 from the Kangerlussuaq suite, West Greenland, coupling the Re-Os isotope geochemistry with petrography of both base metal sulfides (BMS) and silicates to assess the impact of overprint induced by melt-rock reaction on the Re-Os isotope system. Garnet harzburgite sample 474527 shows considerable heterogeneity in the composition of its major phases, most notably olivine and Cr-rich garnet, suggesting formation through multiple stages of partial melting and subsequent metasomatic events. The major BMS phases show a fairly homogeneous pentlandite-rich composition typical for BMS formed via metasomatic reaction, whereas the 187Os/188Os compositions determined for 17 of these BMS are extremely heterogeneous ranging between 0.1037 and 0.1981. Analyses by LA-ICP-MS reveal at least two populations of BMS grains characterized by contrasting HSE patterns. One type of pattern is strongly enriched in the more compatible HSE Os, Ir, and Ru over the typically incompatible Pt, Pd, and Re, while the other type shows moderate enrichment of the more incompatible HSE and has overall lower compatible HSE/incompatible HSE composition. The small-scale heterogeneity observed in these BMS highlights the need for caution when utilizing the Re-Os system to date mantle events, as even depleted harzburgite samples such as 474527 are likely to have experienced a complex history of metasomatic overprinting, with uncertain effects on the HSE.

  8. Nano-Diamonds in melt inclusions in ortho- and clinopyropxene from mantle xenoliths, Salt Lake Crater, Hawaii.

    NASA Astrophysics Data System (ADS)

    Wirth, R.; Rocholl, A.

    2002-12-01

    We observed nanocrystalline diamonds in magmatic rocks from Hawaii (Salt Lake Crater). They occur in mantle xenoliths (Ga-pyroxenites) in melt inclusions in ortho- and clinopyroxene. The xenoliths are incorporated in the host lava and have been transported from the Earth's interior to the surface by volcanic eruptions. Consequently, such xenoliths allow an insight into the structure, the chemical composition and the P-T conditions of the Earth's mantle. Salt Lake Crater pyroxenites are interpreted as high-pressure basaltic cumulates trapped and adiabatically cooled within the Hawaiian lithosphere at 1000o - 1150oC and 1.6 - 2.5 GPa (50-80 km). The melt inclusions were investigated by using TEM and AEM. Specimen preparation was performed by focused ion beam technique (FIB) at the GeoForschungsZentrum Potsdam (GFZ). Promising melt inclusions in pyroxene have been selected from thin sections. FIB technique uses oil-free vacuum to avoid contamination of the foil. The resulting TEM foil has the dimensions 20 μm x 10 μm x 100 nm. Coating of the TEM ready foil with carbon was not necessary. Nanocrystalline diamonds are embedded in melt droplets, which are enclosed in pyroxene crystals. The melt inclusions with an average diameter of about 5 æm are always associated with a fluid phase or gas. The matrix of the melt inclusion consists of amorphous material (basaltic glass) containing very small inclusions of e.g. ZnS, Fe-Pd-S, Ag and In-rich minerals, native nanocrystalline iron and copper. Most of the diamonds occur in approximately rectangular shaped aggregates of polycrystalline diamonds, between 20 and 500 nm in size. The grain size of individual diamonds within each aggregate varies from 5 to 50 nm. The diamonds have been identified by X-ray analysis, electron diffraction and by EELS. The carbon K-edge in the EEL spectra allows to discriminate diamond, graphite and amorphous carbon. Some of the diamonds are single crystals; most of them are polycrystalline. Electron

  9. Effect of water on mantle melting and magma differentiation, as modeled using Adiabat_1ph 3.0

    NASA Astrophysics Data System (ADS)

    Antoshechkina, P. M.; Asimow, P. D.; Hauri, E. H.; Luffi, P. I.

    2010-12-01

    MELTS [1] and pMELTS [2] are widely used for modeling hydrous magma differentiation and water-saturated mantle melting, respectively. In pHMELTS [3] the water species can partition into melt, pure vapor, and hydrous or nominally anhydrous minerals so that phase relations for water-undersaturated systems may also be constructed. Adiabat_1ph is a text-based front end to the (pH)MELTS algorithms; in version 3.0 (Antoshechkina and Asimow, this meeting) we have added tools to further explore the effect of water on melting and fractional crystallization. Asimow and coworkers developed two schemes for fractionation-correction of major and trace elements: a forward model that considers the effects of water, pressure, and fO2 on the liquid line of descent (LLD) [3] and a hybrid back- and forward-fractionation model that may be used for individual samples when trends are poorly defined [4]. An extensive melt inclusion dataset for the Mariana Trough [5] shows evidence for simultaneous fractionation and degassing, so we have adapted routines from [4] to cope with hydrous conditions. For H2O < 1.5 wt% the path predicted by the ‘reverse-fractionation’ algorithm passes from cotectic assimilation to olivine accumulation as MgO increases. Higher water contents lend themselves to the ‘amoeba’ routine that, given an initial estimate, varies the parental melt composition until isobaric forward fractionation yields a specified target [4]. The adiabat_1ph version is flexible in the choice of varied oxides and fO2 constraint, and offers optional fitting to the LLD of a whole suite. In test runs, using glass compositions from the 9N region of the East Pacific Rise, we optimize the primary melt composition for fixed initial H2O. A grid search gives the quality of LLD fit as a function of initial H2O and pressure of crystallization. The overall best-fit LLD model and our preferred LLD model (H2O-CO2 vapor saturation pressure and maximum measured H2O) are in excellent agreement

  10. Experimental forward approach to alkali-rich magma generation from the metasomatized mantle: Melting of mantle xenoliths from Tallante (Murcia, Spain)

    NASA Astrophysics Data System (ADS)

    Manjón-Cabeza Córdoba, Antonio; Castro, Antonio; Moreno-Ventas, Ignacio; López-Ruiz, José; María Cebriá, Jose

    2014-05-01

    Alkali-rich volcanism is a wide spread characteristic of the circum Mediterranean terrains. One of the outcrops of this kind of magmas is the "Cabezo Negro" Volcano in Tallante (Murcia, Spain). The "Cabezo Negro" lavas are alkali-rich basalts, with a Na/K ratio higher than 1, that were erupted between 3 and 2 My ago. Previous experiments have shown that the origin of alkali-rich series can be attributed to an Amphibole-rich metasomatized mantle. Continuing this work, we have carried out several forward experiments at the piston cylinder apparatus under different P-T conditions starting from a sample from a metasomatized mantle xenolith hosted in those lavas. The chosen xenoliths are amphibole bearing clinopyroxenites. Although there are some of them that are phlogopite-bearing, we have worked with those bearing only pargasite as a hydrous phase, in order to ease the understanding of the role of the amphibole in the melting process. The experiments were carried out at conditions ranging from 10 to 20 kbar and from 1000 to 1300 °C mostly of them were "dry", but those under higher pressures were also under water saturated conditions. The results show compositions of melts that are very similar to those that can be found in the k-rich magmas around the Alpine Mediterranean orogenes. In particular, silica, the alkalis and the K/Na ratio tend to decrease with temperature and to increase with pressure for the experiments under dry conditions, to sum up, approaching to the amphibole stability-solidus line. For those under water saturated (and higher pressure) conditions, however, a slight increase of those values can occur with increasing temperature. Since in this kind of metasomatized mantle amphibole seems to draw the solidus line, we have have analyzed the REE and other trace-element relations between amphiboles and the obtained melts seeking for the origin of particular REE affinities that can be found in post-orogenic magmas with adakitic or sanukitic signatures.

  11. Chalcophile and Siderophile Element Abundances in Kilbourne Hole Lherzolites: Distinguishing the Signature of Melt Depleted Primitive Mantle from Metasomatic Overprints

    NASA Astrophysics Data System (ADS)

    Harvey, J.; König, S.; Luguet, A.

    2013-12-01

    Selenium, tellurium and the highly siderophile elements in peridotites have the potential to illustrate planetary scale processes that are opaque to lithophile elements. However, the interpretation of chalcophile and siderophile element abundances relies heavily on the selection of representative mantle material and the determination of what processes have affected these elements since melt depletion. Whole rock and in-situ sulfide data demonstrate that chalcophile and HSE systematics of the upper mantle could be significantly modified through sulfide-metasomatism, particularly by C-O-H-S × Cl fluids[1] or sulfide melts[2] i.e., chalcophile and siderophile element abundances result from a complex interplay between sulfide addition and alteration of pre-existing sulfide. Here we present new bulk-rock S-Se-Te-PGE abundances on a suite (n = 17) of lherzolite and harzburgite xenoliths from Kilbourne Hole, USA[3, 4]. Mineral modal abundances, major element contents and LREE/HREE ratios for 10 of these xenoliths are consistent with varying degrees of melt depletion (≤ 20 %) whereas the remainder appear to have been affected by cryptic metasomatism, refertilization, or melt-rock interaction which affected lithophile element abundances [4]. While sulfur, Se and PGE budgets are primarily controlled by sulfides, 50 × 30% of Te in peridotite may be accounted for by Pt-Pd tellurides[5]. Although most Kilbourne Hole peridotite xenoliths have PGE characteristics consistent with varying degrees of melt depletion and somewhat scattered Se/Te ratios, KH96-24 has Pt-Pd-Te abundances consistent with Pt-Pd-telluride precipitation, in addition to petrographic evidence for alteration by secondary processes[4]. S/Se are well correlated within the suite. However, lherzolites that retain a strong melt-depletion signature have distinctly lower abundances of both S and Se (<65 ppm and <31 ppm respectively) compared to peridotites that have had their lithophile element budgets perturbed

  12. Viscosity structure of Earth's mantle inferred from rotational variations due to GIA process and recent melting events

    NASA Astrophysics Data System (ADS)

    Nakada, Masao; Okuno, Jun'ichi; Lambeck, Kurt; Purcell, Anthony

    2015-08-01

    We examine the geodetically derived rotational variations for the rate of change of degree-two harmonics of Earth's geopotential, skew5dot J_2, and true polar wander, combining a recent melting model of glaciers and the Greenland and Antarctic ice sheets taken from the IPCC 2013 Report (AR5) with two representative GIA ice models describing the last deglaciation, ICE5G and the ANU model developed at the Australian National University. Geodetically derived observations of skew4dot J_2 are characterized by temporal changes of -(3.7 ± 0.1) × 10-11 yr-1 for the period 1976-1990 and -(0.3 ± 0.1) × 10-11 yr-1 after ˜2000. The AR5 results make it possible to evaluate the recent melting of the major ice sheets and glaciers for three periods, 1900-1990, 1991-2001 and after 2002. The observed skew4dot J_2 and the component of skew4dot J_2 due to recent melting for different periods indicate a long-term change in skew4dot J_2-attributed to the Earth's response to the last glacial cycle-of -(6.0-6.5) × 10-11 yr-1, significantly different from the values adopted to infer the viscosity structure of the mantle in most previous studies. This is a main conclusion of this study. We next compare this estimate with the values of skew4dot J_2 predicted by GIA ice models to infer the viscosity structure of the mantle, and consequently obtain two permissible solutions for the lower mantle viscosity (ηlm), ˜1022 and (5-10) × 1022 Pa s, for both adopted ice models. These two solutions are largely insensitive to the lithospheric thickness and upper mantle viscosity as indicated by previous studies and relatively insensitive to the viscosity structure of the D″ layer. The ESL contributions from the Antarctic ice sheet since the last glacial maximum (LGM) for ICE5G and ANU are about 20 and 30 m, respectively, but glaciological reconstructions of the Antarctic LGM ice sheet have suggested that its ESL contribution may have been less than ˜10 m. The GIA-induced skew4dot J_2 for GIA

  13. The density, compressibility and seismic velocity of hydrous melts at crustal and upper mantle conditions

    NASA Astrophysics Data System (ADS)

    Ueki, K.; Iwamori, H.

    2015-12-01

    Various processes of subduction zone magmatism, such as upward migration of partial melts and fractional crystallization depend on the density of the hydrous silicate melt. The density and the compressibility of the hydrous melt are key factors for the thermodynamic calculation of phase relation of the hydrous melt, and the geophysical inversion to predict physicochemical conditions of the melting region based on the seismic velocity. This study presents a new model for the calculations of the density of the hydrous silicate melts as a function of T, P, H2O content and melt composition. The Birch-Murnaghan equation is used for the equation of state. We compile the experimentally determined densities of various hydrous melts, and optimize the partial molar volume, compressibility, thermal expansibility and its pressure derivative, and K' of the H2O component in the silicate melt. P-T ranges of the calibration database are 0.48-4.29 GPa and 1033-2073 K. As such, this model covers the P-T ranges of the entire melting region of the subduction zone. Parameter set provided by Lange and Carmichael [1990] is used for the partial molar volume and KT value of the anhydrous silicate melt. K' of anhydrous melt is newly parameterized as a function of SiO2 content. The new model accurately reproduces the experimentally determined density variations of various hydrous melts from basalt to rhyolite. Our result shows that the hydrous melt is more compressive and less dense than the anhydrous melt; with the 5 wt% of H2O in melt, density and KT decrease by ~10% and ~30% from those of the anhydrous melt, respectively. For the application of the model, we calculated the P-wave velocity of the hydrous melt. With the 5 wt% of H2O, P-wave velocity of the silicate melt decreases by >10%. Based on the melt P-wave velocity, we demonstrate the effect of the melt H2O content on the seismic velocity of the partially molten zone of the subduction zone.

  14. Chlorine and fluorine partition coefficients and abundances in sub-arc mantle xenoliths (Kamchatka, Russia): Implications for melt generation and volatile recycling processes in subduction zones

    NASA Astrophysics Data System (ADS)

    Bénard, A.; Koga, K. T.; Shimizu, N.; Kendrick, M. A.; Ionov, D. A.; Nebel, O.; Arculus, R. J.

    2017-02-01

    We report chlorine (Cl) and fluorine (F) abundances in minerals, interstitial glasses, and melt inclusions in 12 andesite-hosted, spinel harzburgite xenoliths and crosscutting pyroxenite veins exhumed from the sub-arc lithospheric mantle beneath Avacha volcano in the Kamchatka Arc (NE Russia). The data are used to calculate equilibrium mineral-melt partition coefficients (D mineral / melt) for Cl and F relevant to subduction-zone processes and unravel the history of volatile depletion and enrichment mechanisms in an arc setting. Chlorine is ∼100 times more incompatible in pyroxenes (DClmineral/melt = 0.005-0.008 [±0.002-0.003]) than F (DFmineral/melt = 0.50-0.57 [±0.21-0.24]), which indicates that partial melting of mantle sources leads to strong depletions in Cl relative to F in the residues. The data set in this study suggests a strong control of melt composition on DCl,Fpyroxene/melt, in particular H2O contents and Al/(Al + Si), which is in line with recent experiments. Fluorine is compatible in Ca-amphibole in the 'wet' sub-arc mantle (DFamphibole/melt = 3.5-3.7 [±1.5]) but not Cl (DClamphibole/melt = 0.03-0.05 [±0.01-0.03]), indicating that amphibole may fractionate F from Cl in the mantle wedge. The inter-mineral partition coefficients for Cl and F in this study are consistent amongst different harzburgite samples, whether they contain glass or not. In particular, disseminated amphibole hosts much of the Cl and F bulk rock budgets of spinel harzburgites (DClamphibole/pyroxene up to 14 and DFamphibole/pyroxene up to 40). Chlorine and fluorine are variably enriched (up to 1500 ppm Cl and 750 ppm F) in the parental arc picrite and boninite melts of primitive pyroxenite veins (and related melt inclusions) crosscutting spinel harzburgites.

  15. Copper systematics during mantle melting and crustal differentiation in arcs: implications for S and Pb budgets of the continental crust

    NASA Astrophysics Data System (ADS)

    Lee, C.; Chin, E. J.; Dasgupta, R.; Luffi, P. I.; Le Roux, V.

    2010-12-01

    During mid-ocean ridge melting, Cu behaves like Sc and is therefore moderately incompatible, as evidenced by the twofold increase in Cu content in MORBs compared to the mantle. However, Cu content in the continental crust is comparable to that of the mantle, implying that during continental crust formation, Cu becomes effectively compatible. Cu is one of the only elements that exhibits Jekyll and Hyde behavior. This switch to being compatible is consistent with the observation that for the majority of arc magmas (as well as MORBs), Cu decreases with increasing SiO2 and decreasing MgO. Using natural samples, we infer new partition coefficients that indicate Cu is incompatible in olivine, pyroxenes, amphiboles, and biotite. The only mineral to exert significant control on Cu partitioning is sulfide. Cu behaves incompatibly during mantle melting because the modal abundance of sulfides relative to silicate minerals is extremely low. The monotonic decrease in Cu in most differentiating arc magmas requires sulfide saturation. In addition, the similar abundances of Cu in many primitive arc magmas compared to MORBs suggests that mantle melting in both environments occurs not only at sulfide saturation but without the need for excess Cu (or S). In a few cases, however, primitive arc magmas begin with high Cu or show increases in Cu with differentiation, which most likely requires unusually high oxygen fugacities in the source or magmatic evolution towards high oxygen fugacity. Such cases may be important for the origin of Cu porphyry deposits, but are generally rare. Because of the close link between Cu and sulfide during magma differentiation, Cu can be used as a proxy for the pre-degassed S content of arc magmas. The S content of continental crust, like many volatile elements, is basically unconstrained, but it can be inferred from Cu, which is much better constrained. Finally, the fact that the continental crust is highly depleted in Cu implies that there is a missing

  16. Experimental investigation of the electrical behavior of olivine during partial melting under pressure and application to the lunar mantle

    NASA Astrophysics Data System (ADS)

    Pommier, A.; Leinenweber, K.; Tasaka, M.

    2015-09-01

    ), and provide a melt conductivity value of 78 (±8) S/m for all Fo77 samples and 45 (±5) S/m for the Fo90 sample. Comparison of our results with electromagnetic sounding data of the deep interior of the Moon supports the hypothesis of the presence of interconnected melt at the base of the lunar mantle. Our results underline that electrical conductivity can be used to investigate in situ melt nucleation and migration in the interior of terrestrial planets.

  17. Eclogite-associated potassic silicate melts and chloride-rich fluids in the mantle: a possible connection

    NASA Astrophysics Data System (ADS)

    Safonov, O.; Butvina, V.

    2009-04-01

    Relics of potassium-rich (4-14 wt. % of K2O and K2O/Na2O > 1.0) melts are a specific features of some partially molten diamondiferous eclogite xenoliths in kimberlites worldwide [1, 2]. In addition, potassic silicic melt inclusions with up to 16 wt. % of K2O are associated with eclogite phases in kimberlitic diamonds (O. Navon, pers. comm.). According to available experimental data, no such potassium contents can be reached by "dry" and hydrous melting of eclogite. These data point to close connection between infiltration of essentially potassic fluids, partial melting and diamond formation in mantle eclogites [2]. Among specific components of these fluids, alkali chlorides, apparently, play an important role. This conclusion follows from assemblages of the melt relics with chlorine-bearing phases in eclogite xenoliths [1], findings of KCl-rich inclusions in diamonds from the xenoliths [3], and concentration of Cl up to 0.5-1.5 wt. % in the melt inclusions in diamonds. In this presentation, we review our experimental data on reactions of KCl melts and KCl-bearing fluids with model and natural eclogite-related minerals and assemblages. Experiments in the model system jadeite(±diopside)-KCl(±H2O) at 4-7 GPa showed that, being immiscible, chloride liquids provoke a strong K-Na exchange with silicates (jadeite). As a result, low-temperature ultrapotassic chlorine-bearing (up to 3 wt. % of Cl) aluminosilicate melts form. These melts is able to produce sanidine, which is characteristic phase in some partially molten eclogites. In addition, in presence of water Si-rich Cl-bearing mica (Al-celadonite-phlogopite) crystallizes in equilibrium with sanidine and/or potassic melt and immiscible chloride liquid. This mica is similar to that observed in some eclogitic diamonds bearing chloride-rich fluid inclusions [4], as well as in diamonds in partially molten eclogites [2]. Interaction of KCl melt with pyrope garnet also produce potassic aluminosilicate melt because of high

  18. 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

    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.

  19. Chemical and Isotopic Heterogeneities in the Deep Earth:Importance of Lower Mantle Carbonate-rich Melts

    NASA Astrophysics Data System (ADS)

    Collerson, K. D.; Williams, Q.; Murphy, D.

    2007-12-01

    Evolution of mantle chemical heterogeneity reflects a spectrum of processes. Nature of reservoirs has been inferred from radiogenic isotope and trace element systematics of mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) [1]. Carbonatites, kimberlites and lamproites [2-4] also sample depleted and enriched reservoirs, however, their origin remains equivocal. Secular decrease in Th/U ratio in MORB mantle (DMM), homogeneity of Th/U inferred from Pb-isotopic data, and systematic variation in Nb/Th and Nb/U ratios in MORBs [5], show that recycled components in DMM are well mixed. Thus isotopically hererogeneous domains in DMM must be transient features and are unlikely to yield HIMU and EM chemistries. Explanations for HIMU and EM OIB chemistries include involvement of: (1) subcontinental lithospheric mantle; (2) subducted oceanic lithosphere; (3) subducted sediment; or (4) an enigmatic lower mantle (LM) "plume component". Elevated 3He/4He in OIBs and kimberlites [6] and excess 129Xe and high 40Ar/39Ar [e.g., 7-8] and solar 20Ne/22Ne [9] in carbonatites indicate that they were derived from a primitive, isolated, and less degassed source than MORB. Primordial compositions show that this reservoir escaped atmospheric contamination by Ar, Xe, and Ne and pollution by 4He-rich material (from recycled 238U) during subduction. This primitive reservoir likely exists below the depth subducted slabs obviously penetrate (ca. 1700 km) e.g., [10]. That kimberlites are deeply sourced is also shown by lower mantle inclusions in diamond, e.g., [11]. Importantly, Gp. 1 and 2 kimberlites are isotopically similar to HIMU and EM-1 OIBs [4]. We interpret Gp 1 kimberlites as mixtures of HIMU and EM sources, while Gp. 2 kimberlites (close to EM-1) are interpreted as melts of a Ca perovskite-rich reservoir, possibly from slabs in the LM. We model melting of LM phases to simulate evolution of EM1 and HIMU 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, 207Pb/204Pb, 206Pb/204Pb and 208Pb/204

  20. Effect of melt composition on basalt and peridotite interaction: laboratory dissolution experiments with applications to mineral compositional variations in mantle xenoliths from the North China Craton

    NASA Astrophysics Data System (ADS)

    Wang, Chunguang; Liang, Yan; Xu, Wenliang; Dygert, Nick

    2013-11-01

    Interaction between basaltic melts and peridotites has played an important role in modifying the lithospheric and asthenospheric mantle during magma genesis in a number of tectonic settings. Compositions of basaltic melts vary considerably and may play an important role in controlling the kinetics of melt-peridotite interaction. To better understand the effect of melt composition on melt-peridotite interaction, we conducted spinel lherzolite dissolution experiments at 2 GPa and 1,425 °C using the dissolution couple method. The reacting melts include a basaltic andesite, a ferro-basalt, and an alkali basalt. Dissolution of lherzolite in the basaltic andesite and the ferro-basalt produced harzburgite-lherzolite sequences with a thin orthopyroxenite layer at the melt-harzburgite interface, whereas dissolution of lherzolite in the alkali basalt produced a dunite-harzburgite-lherzolite sequence. Systematic variations in mineral compositions across the lithological units are observed. These mineral compositional variations are attributed to grain-scale processes that involve dissolution, precipitation, and reprecipitation and depend strongly on reacting melt composition. Comparison of mineral compositional variations across the dissolution couples with those observed in mantle xenoliths from the North China Craton (NCC) helps to assess the spatial and temporal variations in the extent of siliceous melt and peridotite interaction in modifying the lithospheric mantle beneath the NCC. We found that such melt-rock interaction mainly took place in Early Cretaceous, and is responsible for the enrichment of pyroxene in the lithospheric mantle. Spatially, siliceous melt-peridotite interaction took place in the ancient orogens with thickened lower crust.

  1. Density and seismic velocity of hydrous melts under crustal and upper mantle conditions

    NASA Astrophysics Data System (ADS)

    Ueki, Kenta; Iwamori, Hikaru

    2016-05-01

    We present a new model for calculating the density of hydrous silicate melts as a function of P, T, H2O concentration, and melt composition. We optimize VPr,Tr, ∂V/∂T, ∂V/∂P, ∂V2/∂T∂P, and K' of H2O end-member components in hydrous silicate melts, as well as K' of anhydrous silicate melts, using previously reported experimental results. The parameter set for H2O end-member component in silicate melt optimized in this study is internally consistent with the parameter values for the properties of anhydrous silicate melt reported by Lange and Carmichael (1987, 1990). The model calculation developed in this study reproduces the experimentally determined densities of various hydrous melts, and can be used to calculate the relationships between pressures, temperatures, and H2O concentrations of various hydrous melts from ultramafic to felsic compositions at pressures of 0-4.29 GPa. Using the new parameter set, we investigate the effects of H2O content on the seismic velocity of hydrous melts, as well as seismic velocities in partially molten regions of subduction zones. The results show that water content in silicate melt plays a key role in determining seismic velocity structure, and therefore must be taken into account when interpreting seismic tomography.

  2. Tracking the Depleted Mantle Signature in Melt Inclusions and Residual Glass of Basaltic Martian Shergottites using Secondary Ionization Mass Spectrometry

    NASA Technical Reports Server (NTRS)

    Peters, Timothy J.; Simon, Justin I.; Jones, John H.; Usui, Tomohiro; Economos, Rita C.; Schmitt, Axel K.; McKeegan, Kevin D.

    2013-01-01

    Trace element abundances of depleted shergottite magmas recorded by olivine-hosted melt inclusions (MI) and interstitial mesostasis glass were measured using the Cameca ims-1270 ion microprobe. Two meteorites: Tissint, an olivine-­phyric basaltic shergottite which fell over Morocco July 18th 2001; and the Antarctic meteorite Yamato 980459 (Y98), an olivine-phyric basaltic shergottite with abundant glassy mesostasis have been studied. Chondrite-­normalized REE patterns for MI in Tissint and Y98 are characteristically LREE depleted and, within analytical uncertainty, parallel those of their respective whole rock composition; supporting each meteorite to represent a melt composition that has experienced closed-­system crystallization. REE profiles for mesostasis glass in Y98 lie about an order of magnitude higher than those from the MI; with REE profiles for Tissint MI falling in between. Y98 MI have the highest average Sm/Nd and Y/Ce ratios, reflecting their LREE depletion and further supporting Y98 as one of our best samples to probe the depleted shergotitte mantle. In general, Zr/Nb ratios overlap between Y98 and Tissint MI, Ce/Nb ratios overlap between Y98 MI and mesostasis glass, and Sm/Nd ratios overlap between Y98 mesostasis glass and Tissint MI. These features support similar sources for both, but with subtle geochemical differences that may reflect different melting conditions or fractionation paths during ascent from the mantle. Interestingly, the REE patterns for both Y98 bulk and MI analyses display a flattening of the LREE that suggests a crustal contribution to the Y98 parent melt. This observation has important implications for the origins of depleted and enriched shergottites.

  3. Thermal structure and melting conditions in the mantle beneath the Basin and Range province from seismology and petrology

    NASA Astrophysics Data System (ADS)

    Plank, T.; Forsyth, D. W.

    2016-04-01

    To better constrain the temperature structure in the upper mantle, we jointly invert seismic surface wave velocities and basalt thermobarometry. New measurements of the water concentration (1.0-3.5 wt %) and oxygen fugacity (FMQ + 0.5 to + 1.5) of basalts from seven recently active volcanic fields in the Basin and Range province (Cima, Pisgah, Amboy, Big Pine, Black Rock, Snow Canyon, W. Grand Canyon) enable more accurate equilibration pressure (P) and temperature (T) estimates of the mantle melts. We developed a revised thermobarometer that more precisely predicts the results of laboratory experiments on melts equilibrated with olivine and orthopyroxene and accounts for the effects of water and CO2. Applying these methods to basalts from the Basin and Range we find that most equilibrated near the dry solidus in P-T space and at depths in the vicinity of the lithosphere-asthenosphere boundary (LAB) inferred from receiver function analysis and Rayleigh surface wave tomography. The wet basalts should have begun melting well below the dry solidus, so the depths of equilibration probably reflect ponding of rising melts beneath the nominally dry lithosphere. A two-parameter thermal model is sufficient to simultaneously satisfy both the seismological and petrological constraints. In the model, the depth to the dry solidus defines the bottom boundary of the conductive lid, while the potential temperature (Tp) controls the asthenosphere and LAB thermal structure. The optimum estimates of Tp range from <1300 to >1500°C, and depths to the LAB range from ˜55 to 75 km, with uncertainties on the order of ±50°C and ±10 km. In contrast to standard tomographic images or basalt thermobarometry, the output of the joint inversion is a geotherm that can be tested quantitatively against other observations.

  4. Alteration of Mantle Sulfides: the Effects of Oxidation and Melt Infiltration in a Kilbourne Hole Harzburgite Xenolith

    NASA Astrophysics Data System (ADS)

    Barrett, T. J.; Harvey, J.; Warren, J. M.; Klein, F.; Walshaw, R.

    2013-12-01

    Sulfides, while commonly present in volumetrically minor amounts (< 0.1 modal %; e.g.[1]) in the mantle, impart a strong control over many of the highly siderophile and strongly chalcophile elements. The mass balance of some elements, such as Os, are almost completely controlled by heterogeneously distributed sulfide grains[2][3]. Hence, processes that re-distribute sulfides and / or alter their composition can have profound effects on the information preserved within them regarding primary mantle processes. Different generations of interstitial sulfide may partly or completely re-equilibrate with one another or may be exposed to open-system processes that mobilize and / or precipitate sulfides[4]. In mantle xenoliths in particular, supergene weathering at the Earth's surface can oxidize sulfide to soluble sulfate, and its removal affects highly siderophile and strongly chalcophile element abundances [6]. Here we present the initial results from a study of interstitial mantle sulfides (n = 24) recovered from a single harzburgitic xenolith from Kilbourne Hole, NM. Large compositional differences are observed in the sulfides even at the scale of a single xenolith. Mono-sulfide solid solution has exsolved into two Fe-Ni-rich phases, one with a significantly larger Ni content for a given Fe abundance. Occurrences of Cu-rich sulfides are rare, but where present Cu can account for up to 22 weight % of the sulfide. Critically, no fresh, unaltered sulfides were recovered and in all of the sulfides there is evidence for at least two secondary processes. EDS mapping of the sulfides reveals pervasive, but incomplete, oxidation in all of the grains; Raman spectroscopy reveals this oxide to be goethite. In addition, there is also evidence for the interaction of many of the sulfides with a volatile-rich silicate melt. Silicate melt veins cross-cut the original sulfide mineralogy in some areas of the sulfide grain, while leaving other areas virtually untouched. The degree of

  5. Ca. 2.7 Ga ferropicritic magmatism: A record of Fe-rich heterogeneities during Neoarchean global mantle melting

    NASA Astrophysics Data System (ADS)

    Milidragovic, Dejan; Francis, Don

    2016-07-01

    Although terrestrial picritic magmas with FeOTOT ⩾13 wt.% are rare in the geological record, they were relatively common ca. 2.7 Ga during the Neoarchean episode of enhanced global growth of continental crust. Recent evidence that ferropicritic underplating played an important role in the ca. 2.74-2.70 Ga reworking of the Ungava craton provides the impetus for a comparison of ca. 2.7 Ga ferropicrite occurrences in the global Neoarchean magmatic record. In addition to the Fe-rich plutons of the Ungava craton, volumetrically minor ferropicritic flows, pyroclastic deposits, and intrusive rocks form parts of the Neoarchean greenstone belt stratigraphy of the Abitibi, Wawa, Wabigoon and Vermillion domains of the southern and western Superior Province. Neoarchean ferropicritic rocks also occur on five other Archean cratons: West Churchill, Slave, Yilgarn, Kaapvaal, and Karelia; suggesting that ca. 2.7 Ga Fe-rich magmatism was globally widespread. Neoarchean ferropicrites form two distinct groups in terms of their trace element geochemistry. Alkaline ferropicrites have fractionated REE profiles and show no systematic HFSE anomalies, broadly resembling the trace element character of modern-day ocean island basalt (OIB) magmas. Magmas parental to ca. 2.7 Ga alkaline ferropicrites also had high Nb/YPM (>2), low Al2O3/TiO2 (<8) and Sc/Fe (⩽3 × 10-4) ratios, and were enriched in Ni relative to primary pyrolite mantle-derived melts. The high Ni contents of the alkaline ferropicrites coupled with the low Sc/Fe ratios are consistent with derivation from olivine-free garnet-pyroxenite sources. The second ferropicrite group is characterized by decisively non-alkaline primary trace element profiles that range from flat to LREE-depleted, resembling Archean tholeiitic basalts and komatiites. In contrast to the alkaline ferropicrites, the magmas parental to the subalkaline ferropicrites had flat HREE, lower Nb/YPM (<2), higher Al2O3/TiO2 (8-25) and Sc/Fe (⩾4 × 10-4) ratios, and

  6. Melt localization and its relation to deformation in the subcontinental mantle: a case study from layered dunite-harzburgite-lherzolite bodies of the Ronda peridotite massif, Spain

    NASA Astrophysics Data System (ADS)

    Hidas, Karoly; Garrido, Carlos J.; Bodinier, Jean-Louis; Tommasi, Andrea; Booth-Rea, Guillermo; Gervilla, Fernando; Marchesi, Claudio

    2010-05-01

    The processes that take place during the transport of melts through the convecting mantle are the least understood and, therefore, state-of-the art problems among a series of processes of formation and evolution of mantle magmas. It is widely accepted that, dunite channels might be pathways by which mantle melts easily pass through the overlying mantle (e.g. Kelemen et al., 1997). The role of shear strain during the formation of dunite bodies in ophiolites was considered in details by Kelemen & Dick (1995). It was also shown that the stress field can control the melt migration paths marked by dunite bodies occurring oriented regularly relative to the hinge and axial plane of a harzburgite fold (Savelieva et al., 2008). The localization of melt flow and formation of channels under mechanical instability during the formation of dunites is expected to lead to a stronger olivine crystallographic preferred orientation (CPO) in these rocks than in their surroundings. However, accepted models explain formation of dunitic lithology mostly in oceanic environment, but one would face several challenges trying to apply them to the subcontinental lithospheric mantle. The Ronda massif (southern Spain) is the largest (ca. 300km2) of several orogenic peridotite massifs exposed in the Betic and Rif (northern Morocco) mountain belts in the westernmost part of the Alpine orogen that was tectonically emplaced during early Miocene times. One of the most remarkable features of the Ronda massif is the ‘recrystallization front' that represents the transition from the spinel-tectonite to the coarse granular peridotite domain corresponding to a narrow boundary of a partial melting domain caused by thinning and coeval asthenospheric upwelling formed at the expense of former subcontinental lithospheric mantle and associated with melting and kilometer-scale migration of melts by diffuse porous flow through the ‘asthenospherized' domain (Van der Wal & Bodinier, 1996; Lenoir et al., 2001

  7. Evidence for the formation of boninitic melt in the base of the Salahi mantle section, the northern Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Nomoto, Y.; Takazawa, E.

    2013-12-01

    The boninites in the Oman ophiolite occur as lavas and dikes of the Alley volcanic sequence (Ishikawa et al., 2002). Moreover, Yamazaki and Miyashita (2008) reported about boninitic dike swarms in the Fizh crustal section. The boninitic melt generation requires hydrous melting of refractory mantle peridotite under an extremely high temperature and low pressure condition. This condition is generally explained by the addition of slab-derived fluids into a hot young oceanic lithosphere, which previously experienced MORB melt extraction. In this study, we report an ultramafic complex mainly composed of dunite which is in equilibrium with chemical composition of boninites in the southwestern part of the Salahi mantle section in the northern Oman ophiolite. Based on the study by Nomoto and Takazawa (2013) the complex consists mainly of massive dunite associated with minor amounts of harzburgite, pyroxenites and wehrlite. We use spinel Cr# (=Cr/[Cr+Al] atomic ratio) as an indicator of extent of melt extraction in harzburgites. For dunites spinel Cr# varies as a function of extent of reaction and of melt composition (Dick and Bullen, 1984; Arai, 1994; Ozawa, 2008). The spinels in the dunites from the complex have Cr# greater than 0.7 indicating highly refractory signature. The range of spinel Cr# is similar to those of spinels in boninites reported worldwide (Umino, 1986; van der Laan et al., 1992; Sobolev and Danyushevsky, 1994; Ishikawa et al., 2002). The complex might be a section of dunite channel that formed by flux melting of harzburgites as a result of infiltration of a voluminous fluid from the basal thrust. We determined the abundances of rare earth elements (REE) in the peridotite clinopyroxenes (cpxs) by LA-ICP-MS to estimate the compositions of the melts in equilibrium with these clinopyroxenes. The chondrite-normalized patterns for clinopyroxenes in the dunites are characterized by enrichments in light REE (LREE) relative to those of the harzburgite

  8. Clinopyroxene megacrysts from Enmelen melanephelinitic volcanoes (Chukchi Peninsula, Russia): application to composition and evolution of mantle melts

    NASA Astrophysics Data System (ADS)

    Akinin, Vyacheslav V.; Sobolev, Alexander V.; Ntaflos, Theodoros; Richter, Wolfram.

    2005-08-01

    Clinopyroxene megacrysts from young melanephelinitic lavas were divided into Cr-rich and Cr-poor suites. Sr, Nd, and Pb isotopic ratios of leached megacrysts and host lava are indistinguishable from each other and indicate a depleted source. Host lavas do not display chemical evidence for significant fractional crystallization, which is required to explain the compositional range of the megacrysts. This rules out a simple cognate genetic relationship between the two, and strictly defines megacrysts as xenocrysts. The well-defined correlations of trace elements with the Mg-numbers in the megacrysts are interpreted as the result of extensive fractional/equilibrium crystallization of magma over a large temperature range at near isobaric condition in the upper mantle. Trace element variations in megacrysts are consistent with fractional crystallization of clinopyroxene alone for the Cr-rich suite, and clinopyroxene + garnet for the Cr-poor suite from at least two bathes of related melts. Megacrysts parent magma might represent mantle melts, which were never erupted in their initial composition.

  9. Temporal variations in the mantle potential temperatures along the Northwest Hawaiian Ridge using olivine-liquid equilibria: Implications for Hawaiian plume melt flux variations

    NASA Astrophysics Data System (ADS)

    Tree, J. P.; Garcia, M. O.; Putirka, K. D.

    2013-12-01

    The Northwest Hawaiian Ridge extends 2800 km and comprises 47% of the total length of the Hawaiian-Emperor (H-E) Chain. The Ridge contains at least 52 volcanoes whose shape, volume, and distance from neighboring volcanoes vary markedly. The temporal melt flux variation of the H-E has been estimated with various geophysical methods involving fluid dynamics and lithospheric modeling. These models show a dramatic increase (up to 300%) in melt flux over the last 30 Myr. A potential explanation for the increase in melt flux is a temporal increase in the temperature of melting. Olivine thermometry offers the best method for evaluating mantle source temperature variations along the Ridge. An olivine-liquid equilibration temperature estimate can then be recast into a mantle potential temperature after accounting for the heat of fusion during mantle melting and decompression of the mantle as it follows an adiabat to the surface. New whole-rock XRF and olivine analyzes for 18 tholeiitic, three transitional tholeiites, and two picro-basalts from 11 volcanoes spanning the entire length of the Ridge from Middle Bank to Yuryaku (just south of the Bend) were made. These data were used to obtain a temperature estimate of the mantle during shield formation of each volcano. After screening samples for those in chemical equilibrium using a Rhodes diagram and assuming a Kdol-liq(Fe-Mg)=0.345 ×0.03, data from six volcanoes were input into the thermometers of Beattie (1993; Contrib. Mineral. Petr., 118, 103-111) and Putirka et al., (2007; Chemical Geology, 241, 177-206). The results were averaged to estimate mantle potential temperatures. These calculations yield mantle potential temperature estimates that vary positively with volcano volume (e.g., 1460oC at Yuryaku vs 1608-1630oC at Gardner Pinnacles, the largest volume seamount in the Ridge). These results suggest that temperature variations may be playing a significant role in modulating the melt flux of the Ridge. Seven more

  10. Geochemical and Isotopic Evidence for Melting and Erosion of Wyoming Craton Mantle Lithosphere Prior to 48 Ma

    NASA Astrophysics Data System (ADS)

    Duke, G. I.; Carlson, R. W.; Frost, C. D.

    2010-12-01

    Trace-element geochemistry of Cretaceous-Tertiary Great Plains igneous rocks supports isotopic data that reveal a sequence of digestion of lithospheric mantle followed by intrusion of dominantly asthenospheric magmas. Multiple Archean, Proterozoic, and Phanerozoic subduction events beneath the Wyoming craton concentrated Ba and K within the underlying mantle lithosphere, resulting in earliest Cretaceous-Tertiary lithospheric melts with fingerprints of high K, high Ba/Nb and negative epsilon-Nd, but low U, Th, total REE, and less extreme values of LREE/HREE. Youngest (Eocene-Oligocene) magmas were kimberlite and carbonatite, with high U, Th, LREE, extremely high LREE/HREE, and positive epsilon-Nd, but with high-T xenoliths from depths of only 150 km (Carlson et al., 1999). Importantly, in the entire Wyoming craton, the Homestead kimberlite is the only one of K-T age that has transported a diamond—a single micro-diamond discovered. The shallow low-T to high-T xenolith transition, lack of diamonds, and changing magma geochemistry, suggest that a significant portion of the mantle lithosphere beneath the Wyoming Archean craton must have been consumed prior to the ≤48 Ma kimberlite eruptions. In contrast, the earliest phase of Cretaceous magmatism in Arkansas was explosive diamond-containing lamproite (~102 Ma) with a Proterozoic lithospheric isotopic signature (Lambert et al., 1995). In Arkansas, there was no earlier subalkalic magmatism, and no evidence of slow digestion of the mantle lithosphere, although later magmatism trended toward higher positive epsilon-Nd values (i.e. larger asthenospheric component). Removal by melting of a significant portion of the Wyoming mantle lithosphere during late Cretaceous-early Tertiary magmatism, along with heating, may have helped promote lithospheric “relaxation” related to extension further west between 53 Ma and 49 Ma, followed by more facile penetration by asthenospheric magmas, an idea proposed to explain the time

  11. A melt-focusing zone in the lithospheric mantle preserved in the Santa Elena Ophiolite, Costa Rica

    NASA Astrophysics Data System (ADS)

    Madrigal, Pilar; Gazel, Esteban; Denyer, Percy; Smith, Ian; Jicha, Brian; Flores, Kennet E.; Coleman, Drew; Snow, Jonathan

    2015-08-01

    The Santa Elena Ophiolite in Costa Rica is composed of a well-preserved fragment of the lithospheric mantle that formed along a paleo-spreading center. Within its exposed architecture, this ophiolite records a deep section of the melt transport system of a slow/ultra-slow spreading environment, featuring a well-developed melt-focusing system of coalescent diabase dikes that intrude the peridotite in a sub-vertical and sub-parallel arrangement. Here we present an integrated analysis of new structural data, 40Ar/39Ar geochronology, major and trace element geochemistry and radiogenic isotope data from the diabase dikes in order to elucidate the tectonic setting of the Santa Elena Ophiolite. The dikes are basaltic and tholeiitic in composition. Petrological models of fractional crystallization suggest deep pressures of crystallization of > 0.4 GPa for most of the samples, which is in good agreement with similar calculations from slow/ultra-slow spreading ridges and require a relatively hydrated ( 0.5 wt.% H2O) MORB-like source composition. The diabase dikes share geochemical and isotope signatures with both slow/ultra-slow spreading ridges and back-arc basins and indicate mixing of a DMM source and an enriched mantle end-member like EMII. The 40Ar/39Ar geochronology yielded an age of 131 Ma for a previous pegmatitic gabbroic magmatic event that intruded the peridotite when it was hot and plastic and an age of 121 Ma for the diabase intrusions, constraining the cooling from near asthenospheric conditions to lithospheric mantle conditions to 10 Ma. Our findings suggest a complex interplay between oceanic basin and back-arc extension environments during the Santa Elena Ophiolite formation. We propose an alternative hypothesis for the origin of Santa Elena as an obducted fragment of an oceanic core complex (OCC).

  12. Oxygen Fugacity of the Martian Mantle from Pigeonite/Melt Partitioning of Samarium, Europium and Gadolinium

    NASA Technical Reports Server (NTRS)

    Musselwhite, D. S.; Jnes, J. H.; Shearer, C.

    2004-01-01

    This study is part of an ongoing effort to calibrate the pyroxene/melt REE oxybarometer for conditions relevant to the martian meteorites. These efforts have been motivated by reports of redox variations among the shergottites . We have conducted experiments on martian composition pigeonite/melt rare earth element partitioning as a function of fO2.

  13. Low-Velocity Zone of the Earth's Mantle: Incipient Melting Caused by Water.

    PubMed

    Lambert, I B; Wyllie, P J

    1970-08-21

    Experimental phase diagrams for the systems gabbro-water and peridotite-water indicate that, if there is any water in the upper mantle, then traces of hydrous interstitial silicate magma will be produced at depths corresponding to the beginning of the low-velocity zone. This explanation for the zone is more satisfactory than others proposed.

  14. Experimentally-determined carbon isotope fractionation in and between methane-bearing melt and fluid to upper mantle temperatures and pressures

    NASA Astrophysics Data System (ADS)

    Mysen, Bjorn

    2016-07-01

    The behavior of melts and fluids is at the core of understanding formation and evolution of the Earth. To advance our understanding of their role, high-pressure/-temperature experiments were employed to determine melt and fluid structure together with carbon isotope partitioning within and between (CH4 +H2O +H2)-saturated aluminosilicate melts and (CH4 +H2O +H2)-fluids. The samples were characterized with vibrational spectroscopy while at temperatures and pressures from 475° to 850 °C and 92 to 1158 MPa, respectively. The solution equilibrium is 2CH4 + QQn = 2CH3- + H2O + Qn+1 where the superscript, n, in the Qn-notation describes silicate species where n denotes the number of bridging oxygen. The solution equilibrium affects the carbon isotope fractionation factor between melt and fluid, αmelt/fluid. Moreover, it is significantly temperature-dependent. The αmelt/fluid < 1 with temperatures less than about 1050 °C, and is greater than 1 at higher temperature. Methane-bearing melts can exist in the upper mantle at fO2 ≤fO2 (MW) (Mysen et al., 2011). Reduced (Csbnd H)-species in present-day upper mantle magma, therefore, are likely. During melting and crystallization in this environment, the δ13C of melts increases with temperature at a rate of ∼ 0.6 ‰ /°C. From the simple-system data presented here, at T ≤ 1050°C, melt in equilibrium with a peridotite-(CH4 +H2O +H2)-bearing mantle source will be isotopically lighter than fluid. At higher temperatures, melts will be isotopically heavier. Degassing at T ≤ 1050°C will shift δ13C of degassed magma to more positive values, whereas degassing at T ≥ 1050°C, will reduce the δ13C of the degassed magma.

  15. Oxygen Fugacity of the Martian Mantle from Pigeonite/Melt Partitioning of Samarium, Europium and Gadolinium

    NASA Astrophysics Data System (ADS)

    Musselwhite, D. S.; Jones, J. H.; Shearer, C.

    2004-03-01

    We present results from experimental calibration of the pigeonite/melt oxybarometer. Application of these results to martian basalts gives a range of oxygen fugacities similar, but not identical to those determined from Fe-Ti oxides.

  16. Subduction zone mantle enrichment by fluids and Zr-Hf-depleted crustal melts as indicated by backarc basalts of the Southern Volcanic Zone, Argentina

    NASA Astrophysics Data System (ADS)

    Holm, Paul M.; Søager, Nina; Alfastsen, Mads; Bertotto, Gustavo W.

    2016-10-01

    We aim to identify the components metasomatizing the mantle above the subducting Nazca plate under part of the Andean Southern Volcanic Zone (SVZ). We present new major and ICP-MS trace element and Sr, Nd and high-precision Pb isotope analyses of primitive olivine-phyric alkali basalts from the Northern Segment Volcanic Field, part of the Payenia province in the backarc of the Transitional SVZ. One new 40Ar-39Ar age determination confirms the Late Pleistocene age of this most northerly part of the province. All analysed rocks have typical subduction zone type incompatible element enrichment, and the rocks of the Northern Segment, together with the neighbouring Nevado Volcanic Field, have isotopic compositions intermediate between adjacent Transitional SVZ arc rocks and southern Payenia OIB-type basaltic rocks. Modelling the Ba-Th-Sm variation we demonstrate that fluids as well as 1-2% melts of upper continental crust (UCC) enriched their mantle sources, and La-Nb-Sm variations additionally indicate that the pre-metasomatic sources ranged from strongly depleted to undepleted mantle. Low Eu/Eu* and Sr/Nd also show evidence for a UCC component in the source. The contribution of Chile Trench sediments to the magmas seems insignificant. The Zr/Sm and Hf/Sm ratios are relatively low in many of the Northern Segment rocks, ranging down to 17 and 0.45, respectively, which, together with relatively high Th/U, is argued to indicate that the metasomatizing crustal melts were derived by partial melting of subducted UCC that had residual zircon, in contrast to the UCC melts added to Transitional SVZ arc magmas. Mixing between depleted and undepleted mantle, enriched by UCC and fluids, is suggested by Sr, Nd and Pb isotopes of the Northern Segment and Nevado magmas. The metasomatized undepleted mantle south of the Northern Segment is suggested to be part of upwelling OIB-type mantle, whereas the pre-metasomatically depleted mantle also can be found as a component in some arc

  17. The Fate of Sulfur during Decompression Melting of Peridotite and Crystallization of Basalts - Implications for Sulfur Geochemistry of MORB and the Earth's Upper Mantle

    NASA Astrophysics Data System (ADS)

    Ding, S.; Dasgupta, R.

    2014-12-01

    Magmatism in mid-ocean ridges is the main pathway of sulfur (S) from the Earth's mantle to the surficial reservoir. MORB is generally considered sulfide saturated due to the positive correlation between S and FeOT concentration (e.g., [1]). However, most MORBs are differentiated, and both S content and sulfur concentration at sulfide saturation (SCSS) change with P, T, and magma composition (e.g., [2]). Therefore, it remains uncertain, from the MORB chemistry alone, whether mantle melts parental to MORB are sulfide saturated. In this study, we modeled the behavior of S during isentropic partial melting of a fertile peridotite using pMELTS [3] and an SCSS parameterization [4]. Our results show that during decompression melting, at a fixed mantle potential temperature, TP (e.g., 1300 °C), SCSS of aggregate melt first slightly increases then decreases at shallower depth with total variation <200 ppm. However, an increase of TP results in a significant increase of SCSS of primitive melts. Our model shows that at 15% melting (F), sulfide in the residue is exhausted for a mantle with <200 ppm S. The resulted sulfide-undersaturated partial melts contain <1000 ppm S and are 4-6 times enriched in Cu compared to the source. In order to compare our modeled results directly to the differentiated basalts, isobaric crystallization calculation was performed on 5, 10, and 15% aggregate melts. SCSS changes along liquid line of descent with a decrease in T and increase in FeOT. Comparison of S contents between the model results and MORB glasses [5] reveals that many MORBs derive from sulfide undersaturated melts. Further, for a TP of 1300-1350 °C and F of 10-15 wt.%, reproduction of self-consistent S, and Cu budget of many MORB glasses requires that S of their mantle source be ~25-200 ppm. We will discuss the interplay of TP, average F, and the conditions of differentiation to bracket the S geochemistry of MORB and MORB source mantle and develop similar systematics for OIBs and

  18. Aqueous fluids and sedimentary melts as agents for mantle wedge metasomatism, as inferred from peridotite xenoliths at Pinatubo and Iraya volcanoes, Luzon arc, Philippines

    NASA Astrophysics Data System (ADS)

    Yoshikawa, Masako; Tamura, Akihiro; Arai, Shoji; Kawamoto, Tatsuhiko; Payot, Betchaida D.; Rivera, Danikko John; Bariso, Ericson B.; Mirabueno, Ma. Hannah T.; Okuno, Mitsuru; Kobayashi, Tetsuo

    2016-10-01

    Mantle xenoliths entrained in subduction-zone magmas often record metasomatic signature of the mantle wedge. Such xenoliths occur in magmas from Iraya and Pinatubo volcanoes, located at the volcanic front of the Luzon arc in the Philippines. In this study, we present the major element compositions of the main minerals, trace element abundances in pyroxenes and amphiboles, and Nd-Sr isotopic compositions of amphiboles in the peridotite xenoliths from Pinatubo volcano. The data indicate enrichment in fluid-mobile elements, such as Rb, Ba, U, Pb, and Sr, and Nd-Sr isotopic ratios relative to those of mantle. The results are considered in terms of mixing of asthenospheric mantle and subducting oceanic crustal components. The enrichments observed in the Pinatubo mantle xenoliths are much less pronounced than those reported for the Iraya mantle xenoliths. This disparity suggests differences in the metasomatic agents contributing to the two suites; i.e., aqueous fluids infiltrated the mantle wedge beneath the Pinatubo volcano, whereas aqueous fluids and sediment-derived melts infiltrated the mantle wedge beneath the Iraya volcano.

  19. Platinum Group Elements (PGE) geochemistry of komatiites and boninites from Dharwar Craton, India: Implications for mantle melting processes

    NASA Astrophysics Data System (ADS)

    Saha, Abhishek; Manikyamba, C.; Santosh, M.; Ganguly, Sohini; Khelen, Arubam C.; Subramanyam, K. S. V.

    2015-06-01

    , Os and Ru concentrations range from 0.6 to 2.2 ppb, 0.2 to 0.6 ppb and 1.4 to 2.6 ppb respectively in IPGE. The PGE abundances in Bababudan komatiites were controlled by olivine fractionation whereas that in Gadwal boninites were influenced by fractionation of chromite and sulphides. The Al-undepleted Bababudan komatiites are characterized by low CaO/Al2O3, (Gd/Yb)N, (La/Yb)N, with positive Zr, Hf, Ti anomalies and high Cu/Pd, Pd/Ir ratios at low Pd concentrations suggesting the derivation of parent magma by high degrees (>30%) partial melting of mantle under anhydrous conditions at shallow depth with garnet as a residual phase in the mantle restite. The komatiites are geochemically analogous to Al-undepleted Munro type komatiites and their PGE compositions are consistent with Alexo and Gorgona komatiites. The S-undersaturated character of Bababudan komatiites is attributed to decompression and assimilation of lower crustal materials during magma ascent and emplacement. In contrast, the higher Al2O3/TiO2, lower (Gd/Yb)N, for Gadwal boninites in combination with negative Nb, Zr, Hf, Ti anomalies and lower Cu/Pd at relatively higher Pd/Ir and Pd concentrations reflect high degree melting of refractory mantle wedge under hydrous conditions in an intraoceanic subduction zone setting. Higher Pd/Ir ratios and S-undersaturation of these boninites conform to influx of fluids derived by dehydration of subducted slab resulting into high fluid pressure and metasomatism of mantle wedge.

  20. Shear-wave splitting and implications for mantle flow beneath the MELT region of the east pacific rise

    PubMed

    Wolfe; Solomon

    1998-05-22

    Shear-wave splitting across the fast-spreading East Pacific Rise has been measured from records of SKS and SKKS phases on the ocean-bottom seismometers of the Mantle Electromagnetic and Tomography (MELT) Experiment. The direction of fast shear-wave polarization is aligned parallel to the spreading direction. Delay times between fast and slow shear waves are asymmetric across the rise, and off-axis values on the Pacific Plate are twice those on the Nazca Plate. Splitting on the Pacific Plate may reflect anisotropy associated with spreading-induced flow above a depth of about 100 km, as well as a deeper contribution from warm asthenospheric return flow from the Pacific Superswell region.

  1. Petrology of Eocene dikes near Lake Chelan, WA: evidence of mantle and crustal melting during the Challis event

    NASA Astrophysics Data System (ADS)

    Davidson, P.; Tepper, J. H.; Nelson, B. K.

    2015-12-01

    The Eocene was a tectonically and volcanically complex time in the Pacific Northwest, characterized by regional extension, rapid uplift, widespread magmatism, and a high thermal flux. Extension was manifested by core complex formation, normal faulting, and dike emplacement, the latter represented by the 48 Ma Teanaway swarm (TS), 53-46 Ma dikes associated with the Colville Igneous Complex (CIC), the 46.6 Ma Corbaley Canyon swarm (CCS), and a previously unstudied cluster near Lake Chelan (LC) that is the focus of this study. LC dikes generally strike NW, range up to 8m in width, and are compositionally diverse with a general NE-ward increase in SiO2 (47-84 wt. % SiO2) over a distance of ~120 km. All dikes display arc affinities (calc-alkaline, HFSE depletions) but can be divided into two suites: (1) basalt-andesite-dacite-rhyolite (BADR), and (2) adakites (Sr/Y > 40, La/YbN > 10, Sr > 350ppm, Y < 18ppm, Yb < 1.8ppm). Sr and Nd isotopic data (87Sr/86Sri = 0.70360 - 0.70530; ɛNd(t) = +4.45 to -1.92) for the two suites overlap, suggesting both are mixtures of material from depleted mantle and older crustal sources. We suggest LC basalt dikes represent mantle-derived magmas that drove melting of thick arc crust, generating adakite melts from an eclogite lower crust and, at shallower depths, andesite-rhyolite melts from a combination of fractional crystallization and crustal assimilation/melting. LC dikes share chemical similarities with volcanic rocks of the CIC and CCS including calc-alkaline trends and compositions that range from basalt to rhyolite. The TS dikes however, are distinctly different in chemical composition (tholeiitic, dominantly basaltic andesites), Sr-Nd isotopic composition, and orientation (NW-striking). Ongoing U-Pb dating of LC dikes should help to determine the extent to which changes in dike chemistry and orientation are temporal or spatial or both.

  2. Melt inclusion evidence for CO2-rich melts beneath the western branch of the East African Rift: implications for long-term storage of volatiles in the deep lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Hudgins, T. R.; Mukasa, S. B.; Simon, A. C.; Moore, G.; Barifaijo, E.

    2015-05-01

    We present new major element, trace element, and volatile (H2O, CO2, S, F, and Cl) concentrations of olivine-hosted melt inclusions from five high-K, low-silica basanites from the western branch of the East African Rift System and use these data to investigate the generation of H2O- and CO2-rich melts at up to ~150 km depth. Measured H2O and CO2 concentrations reach ~2.5 and ~1 wt%, respectively, representing some of the highest CO2 concentrations measured in a melt inclusion to date. These measurements represent direct evidence of the high CO2 and H2O concentrations required to generate high-K alkaline lavas, and the CO2 that has been previously inferred to be necessary for the low mantle potential temperatures in the area. Ratios of CO2/Nb, CO2/Ba, and CO2/Cl are used to estimate an initial melt CO2 concentration of 5-12 wt%. The measured CO2 concentrations are consistent with CO2 solubilities determined by molecular dynamics calculations and high-pressure experiments for melt generation at 3-6 GPa; the depth of melting suggested by previous studies in the area. These melt inclusions measurements represent direct evidence for the presence of H2O- and CO2-rich melts in the deep upper mantle that have been proposed based on experimental and seismic evidence. Primitive-mantle normalized trace element patterns more closely resemble those found in subduction settings rather than ocean island basalt, and ratios of slab fluid tracers such as Li/Dy and B/Be indicate that the measured volatile abundances may be related to Neoproterozoic subduction during the assembly of Gondwana, implying the storage of volatiles in the mantle by subduction-related metasomatism.

  3. Origin of water and mantle-crust interactions on Mars inferred from hydrogen isotopes and volatile element abundances of olivine-hosted melt inclusions of primitive shergottites

    NASA Astrophysics Data System (ADS)

    Usui, Tomohiro; Alexander, Conel M. O'D.; Wang, Jianhua; Simon, Justin I.; Jones, John H.

    2012-12-01

    Volatile elements have influenced the differentiation and eruptive behavior of Martian magmas and played an important role in the evolution of Martian climate and near-surface environments. However, the abundances of volatiles, and in particular the amount of water in the Martian interior, are disputed. A record of volatile reservoirs is contained in primitive Martian basalts (shergottites). Olivine-hosted melt inclusions from a geochemically depleted shergottite (Yamato 980459, representing a very primitive Martian melt) possess undegassed water with a chondritic and Earth-like D/H ratio (δD≤275‰). Based on volatile measurements in these inclusions, the water content of the depleted shergottite mantle is calculated to be 15-47 ppm, which is consistent with the dry mantle hypothesis. In contrast to D/H in the depleted shergottite, melt from an enriched shergottite (Larkman Nunatak 06319), which either formed by melting of an enriched mantle or by assimilation of crust, exhibits an extreme δD of ˜5000‰, indicative of a surface reservoir (e.g., the Martian atmosphere or crustal hydrosphere). These data provide strong evidence that the Martian mantle had retained the primordial low-δD component until at least the time of shergottite formation, and that young Martian basalts assimilated old Martian crust.

  4. Uranium-series disequilibria of inflated sections of the Juan de Fuca Ridge: Implications for mantle melting

    NASA Astrophysics Data System (ADS)

    Dreyer, B. M.; Gill, J. B.; Ramos, F. C.; Clague, D. A.; Scott, S. R.

    2010-12-01

    diversity in Endeavour basalts, Th/U variance is low, as in Axial basalts. However, there are differences in (230Th)/(232Th). Seventeen samples from Axial Seamount have (230Th)/(232Th) <1.22, the lowest on the JdFR axis. The range at Endeavour is 1.26-1.35, and highest in N-MORB. (230Th)-excesses [(230Th)/(238U) >1] are much higher at Endeavour (20-30%) than Axial Seamount (5-20%) and the rest of the JdFR (8-14%) but more similar to values of the adjacent Gorda Ridge to the south (up to ~25%). If differences in melt column characteristics (i.e., porosity, lithology, potential temperature) are similar along the JdFR, then ingrowth melt models predict slower upwelling of mantle-derived melts (e.g., longer residence time in the melt column) at Endeavour. Alternatively, Endeavour lavas may be generated from a more pyroxenitic (lherzolite) mantle where large degrees of disequilibria can be generated.

  5. Effect of melt/mantle interactions on MORB chemistry at the easternmost Southwest Indian Ridge (61°-67°E)

    NASA Astrophysics Data System (ADS)

    Paquet, M.; Cannat, M.; Brunelli, D.; Hamelin, C.; Humler, E.

    2016-11-01

    The easternmost part of the Southwest Indian Ridge (61°-67°E) is an end-member of the global ridge system in terms of very low magma supply. As such, it is a good laboratory to investigate the effect of melt/mantle interactions on the composition of erupted basalts: for a given volume of erupted basaltic melt, the volume of reacted mantle is potentially greater than at more magmatically robust ridges. We analyzed major, trace element and isotopic compositions in three groups of rocks: plagioclase-bearing ultramafic and gabbroic rocks dredged in nearly amagmatic spreading corridors; basalts from the sparse volcanic cover of these corridors ("ultramafic seafloor basalts"); and basalts dredged from the intervening, more volcanically active domains ("volcanic seafloor basalts"). Ultramafic seafloor basalts have significantly lower CaO and Al2O3 contents at a given MgO than most volcanic seafloor basalts. We propose that both types of basalts are derived from similar parental melts, but that the ultramafic seafloor basalts are more affected by reactions between these parent melts and the mantle rocks in the lithosphere below the ridge. We infer that these reactions occur in the walls of conduits that allow the aggregated melts extracted from the melting mantle to rise through the axial lithosphere and to the eruption sites. The principal effect of these reactions is to enrich the asthenospheric melts in MgO through olivine dissolution. This effect is not expected to be as noticeable, but could still play a role in basalt petrogenesis at more magmatic regions of the global slow-spreading MOR system.

  6. A Coupled Geochemical and Geodynamical Approach for Mantle Melting Beneath Hawaii.

    NASA Astrophysics Data System (ADS)

    Lambart, S.; Kelemen, P. B.

    2014-12-01

    The presence of the Hawaiian plume is manifested by the Hawaiian swell [1] and voluminous eruption of Ni-rich lavas [2] with enriched isotopic compositions [3]. Here we estimate the conditions of melt generation needed to reproduce both features. We used thermodynamic treatment for fractional melting [4] and melting parameterizations for pyroxenites [5] and peridotite [6] to determine pyroxenite contribution in magmas Xpx as functions of potential temperature TP, pyroxenite abundance in the source P, radius of the melting zone R and distance to the plume axis. The final pressure of melting is set to correspond with the base of the lithosphere (3 GPa) at the plume axis and increases with the distance from the axis [7]. The Hawaiian plume axis is thought to be currently between Loihi (L), Kilauea (K) and Mauna Loa (ML), which are 25 km, 32 km and 44 km radially away from the plume axis, respectively [3]. To determine Xpx, we assumed that magmas are accumulated melts produced on a circular sampling zone of 50 km diameter centered beneath each volcano [8]. Preliminary calculations show that for TP = 1525°C, P = 0.07 and R = 55 km, XpxML = 0.59, XpxK = 0.49 and XpxL = 0.45. XpxML and XpxK are similar to values suggested by [2]. Computed liquidus temperatures at 3 GPa are consistent with those of Hawaiian parental melts (1500-1520°C; [9]). XpxL is higher than suggested by [2] (XpxL = 0.09) but their estimate is based on only one glass analysis. Our model is also consistent with isotopic compositions: K and L have similar ɛNd, while ML is more enriched [3]. Finally, we can compute the density deficit using parameterization of [1] and relate it to the volume flux volume flux [10]: we obtain 3.2 km3/Yr, a value similar to the estimations based on the Hawaiian swell model [1]. 1-Ribe & Christensen EPSL 1999; 2-Sobolev et al. Nature 2005; 3-DePaulo et al. GGG 2001; 4-Phipps Morgan GGG 2001; 5-Lambart et al. in prep; 6-Katz et al. GGG 2003; 7-Ito & Mahoney EPSL 2005; 8-De

  7. Two magma bodies beneath the summit of Kīlauea Volcano unveiled by isotopically distinct melt deliveries from the mantle

    NASA Astrophysics Data System (ADS)

    Pietruszka, Aaron J.; Heaton, Daniel E.; Marske, Jared P.; Garcia, Michael O.

    2015-03-01

    The summit magma storage reservoir of Kīlauea Volcano is one of the most important components of the magmatic plumbing system of this frequently active basaltic shield-building volcano. Here we use new high-precision Pb isotopic analyses of Kīlauea summit lavas-from 1959 to the active Halema'uma'u lava lake-to infer the number, size, and interconnectedness of magma bodies within the volcano's summit reservoir. From 1971 to 1982, the 206Pb/204Pb ratios of the lavas define two separate magma mixing trends that correlate with differences in vent location and/or pre-eruptive magma temperature. These relationships, which contrast with a single magma mixing trend for lavas from 1959 to 1968, indicate that Kīlauea summit eruptions since at least 1971 were supplied from two distinct magma bodies. The locations of these magma bodies are inferred to coincide with two major deformation centers identified by geodetic monitoring of the volcano's summit region: (1) the main locus of the summit reservoir ∼2-4 km below the southern rim of Kīlauea Caldera and (2) a shallower magma body <2 km below the eastern rim of Halema'uma'u pit crater. Residence time modeling suggests that the total volume of magma within Kīlauea's summit reservoir during the late 20th century (1959-1982) was exceedingly small (∼0.1-0.5 km3). Voluminous Kīlauea eruptions, such as the ongoing, 32-yr old Pu'u 'Ō'ō rift eruption (>4 km3 of lava erupted), must therefore be sustained by a nearly continuous supply of new melt from the mantle. The model results show that a minimum of four compositionally distinct, mantle-derived magma batches were delivered to the volcano (at least three directly to the summit reservoir) since 1959. These melt inputs correlate with the initiation of energetic (1959 Kīlauea Iki) and/or sustained (1969-1974 Mauna Ulu, 1983-present Pu'u 'Ō'ō and 2008-present Halema'uma'u) eruptions. Thus, Kīlauea's eruptive behavior is partly tied to the delivery of new magma batches

  8. Two magma bodies beneath the summit of Kilauea Volcano unveiled by isotopically distinct melt deliveries from the mantle

    USGS Publications Warehouse

    Pietruszka, Aaron J; Heaton, Daniel E.; Marske, Jared P.; Garcia, Michael O.

    2015-01-01

    The summit magma storage reservoir of Kīlauea Volcano is one of the most important components of the magmatic plumbing system of this frequently active basaltic shield-building volcano. Here we use new high-precision Pb isotopic analyses of Kīlauea summit lavas—from 1959 to the active Halema‘uma‘u lava lake—to infer the number, size, and interconnectedness of magma bodies within the volcano's summit reservoir. From 1971 to 1982, the 206Pb/204Pb ratios of the lavas define two separate magma mixing trends that correlate with differences in vent location and/or pre-eruptive magma temperature. These relationships, which contrast with a single magma mixing trend for lavas from 1959 to 1968, indicate that Kīlauea summit eruptions since at least 1971 were supplied from two distinct magma bodies. The locations of these magma bodies are inferred to coincide with two major deformation centers identified by geodetic monitoring of the volcano's summit region: (1) the main locus of the summit reservoir ∼2–4 km below the southern rim of Kīlauea Caldera and (2) a shallower magma body <2 km below the eastern rim of Halema‘uma‘u pit crater. Residence time modeling suggests that the total volume of magma within Kīlauea's summit reservoir during the late 20th century (1959–1982) was exceedingly small (∼0.1–0.5 km3). Voluminous Kīlauea eruptions, such as the ongoing, 32-yr old Pu‘u ‘Ō‘ō rift eruption (>4 km3 of lava erupted), must therefore be sustained by a nearly continuous supply of new melt from the mantle. The model results show that a minimum of four compositionally distinct, mantle-derived magma batches were delivered to the volcano (at least three directly to the summit reservoir) since 1959. These melt inputs correlate with the initiation of energetic (1959 Kīlauea Iki) and/or sustained (1969–1974 Mauna Ulu, 1983-present Pu‘u ‘Ō‘ō and 2008-present Halema‘uma‘u) eruptions. Thus, Kīlauea's eruptive behavior is partly tied to

  9. Oxo-amphiboles in mantle xenoliths: evidence for H2O-rich melt interacting with the lithospheric mantle of Harrow Peaks (Northern Victoria Land, Antarctica)

    NASA Astrophysics Data System (ADS)

    Gentili, S.; Bonadiman, C.; Biagioni, C.; Comodi, P.; Coltorti, M.; Zucchini, A.; Ottolini, L.

    2015-12-01

    Amphiboles are the most widespread hydrous metasomatic phases in spinel-bearing mantle peridotites from Harrow Peaks (HP), Northern Victoria Land (Antarctica). They occur both in veinlets and disseminated in the peridotite matrix (preferentially associated with clinopyroxene and spinel grains). Four amphibole crystals were investigated by single-crystal X-ray diffraction (SC-XRD), electron microprobe analysis (EMPA), secondary ion mass spectrometry (SIMS) and micro-Mössbauer spectroscopy; these crystal-chemical data allow to constrain upper mantle conditions during growth of these amphiboles and the role of volatile circulation during metasomatic processes in the Antarctic region. The HP amphiboles have low Mg# values (69.3-84.1), high TiO2 (2.74-5.30 wt%) and FeOtot contents (3.40 to 6.90 wt%). The Fe3+/Fetot ratios are significantly high (0.53-0.66). The W-site is mainly occupied by O2- (0.984-1.187 apfu) plus OH (H2O: 0.70-1.01 wt%) and minor F (0.04-0.24 wt%) and Cl (0.03-0.08 wt%). Consequently, HP amphiboles are actually characterized by a significant oxo component. The aH2O values were calculated at 1.5 GPa by dehydration equilibrium equations written as H2O-buffering equilibria among end-member components of amphibole and coexisting peridotite phases. Three out of four HP amphibole-bearing peridotites have values of aH2O ranging from 0.122 to 0.335; whereas one sample has aH2O remarkably higher (0.782) approaching an ideal H2O basalt solubility. The HP fO2 values, determined by the olivine-spinel-orthopyroxene oxygeobarometer (ΔQFM = -1.77 : +0.01), are remarkably different from those calculated on the basis of the amphibole dehydration equilibrium and the application of the dissociation reaction (ΔQFM = -2.60 : +6.8). The high aH2O and the extremely high fO2 values, determined by the oxy-amphibole equilibrium with respect to the redox conditions recorded by the co-existing anhydrous minerals (close to QFM buffer), revealed that: i) the amphibole

  10. Sr-Nd-Pb isotope data for ultramafic xenoliths from Hierro, Canary Islands: Melt infiltration processes in the upper mantle

    NASA Astrophysics Data System (ADS)

    Whitehouse, M. J.; Neumann, E.-R.

    1995-03-01

    recrystallisation and equilibration prior to the Hierro event. Isotopic data presented in this study show that complex interaction with percolating basaltic melts of varying composition was occurring in the upper mantle beneath Hierro prior to and during the volcanic event and was probably related to the generation of earlier Canary Island magmas.

  11. P-rich olivines in a melt vein of a composite mantle xenolith: implications for crystal growth and kinetics

    NASA Astrophysics Data System (ADS)

    Baziotis, Ioannis; Asimow, Paul D.; Ntaflos, Theodoros; Koroneos, Antonios; Perugini, Diego; Stolper, Edward M.

    2014-05-01

    The mineral chemistry of mantle xenoliths, and in particular the presence of phosphorus (P) - a moderately incompatible and slowly diffusing element - may preserve the history of mineral growth and constrain timescales of pre-eruption petrogenetic processes (Boesenberg & Hewins 2010). P-rich zones in olivine may reflect incorporation of P in excess of equilibrium partitioning during rapid growth, in which case zoning patterns primarily record crystal growth rate variations (Milman-Barris et al. 2008; Stolper et al. 2009). We investigated using EMP analyses and X-ray maps a composite amphibole-bearing, mantle xenolith (sample: Ci-1-196) from Cima Volcanic Field (California, USA) that contains second generation P-rich olivines. The xenolith contains multiple lherzolite, websterite, and dunite layers. The host magma (not preserved in our hand-specimen) is thought to be a hawaiite (Wilshire et al. 1988). A thin (average ~200 μm width), dark layer is present along the contact between lherzolite and websterite. Interpreted as a rapidly crystallized melt, this layer consists of olivine + glass + plagioclase + spinel + clinopyroxene + apatite + ilmenite. The layer contains olivines (Fo83-89.3) with 0.03-0.52 wt.% P2O5; the P-rich olivines (P2O5 >0.1 wt.%) are Fo85 to Fo89.3. Apatite inclusions are present near the rim of P-rich olivine (Fo85) and in plagioclase (An54). Glass is widespread (~15 vol.%) in the layer, having variable composition with P2O5 up to 1.2 wt.%. Plagioclase occurs as prismatic, flow-oriented crystals, parallel to the elongation of the layer or intergranular crystals between olivine and/or clinopyroxene. Clinopyroxene formed either as crystallized products within the melt layer or by reaction at the interface between melt and matrix olivine. Spinel occurs as inclusions in the olivine or associated with plagioclase and glass, showing anhedral shape and linear edges; spinel composition varies from chromite to Ti-chromite from core to rim, with an outer

  12. 3D analytical investigation of melting at lower mantle conditions in the laser-heated diamond anvil cel

    NASA Astrophysics Data System (ADS)

    Nabiei, F.; Cantoni, M.; Badro, J.; Dorfman, S. M.; Gaal, R.; Piet, H.; Gillet, P.

    2015-12-01

    The diamond anvil cell is a unique tool to study materials under static pressures up to several hundreds of GPa. It is possible to generate temperatures as high as several thousand degrees in the diamond anvil cell by laser heating. This allows us to achieve deep mantle conditions in the laser-heated diamond anvil cell (LHDAC). The small heated volume is surrounded by thermally conductive diamond anvils results in high temperature gradients which affect phase transformation and chemical distribution in the LH-DAC. Analytical characterization of samples in three dimensions is essential to fully understand phase assemblages and equilibrium in LHDAC. In this study we used San Carlos olivine as a starting material as a simple proxy to deep mantle composition. Three samples were melted at ~3000 K and at ~45 GPa for three different durations ranging from 1 to 6 minutes; two other samples were melted at 30 GPa and 70 GPa. All samples were then sliced by focused ion beam (FIB). From each slice, an electron image and energy dispersive X-ray (EDX) map were acquired by scanning electron microscope (SEM) in the dual beam FIB instrument. These slices were collected on one half of the heated area in each sample, from which we obtained 3D elemental and phase distribution. The other half of the heated area was used to extract a 100 nm thick section for subsequent analysis by analytical transmission electron microscopy (TEM) to obtain diffraction patterns and high resolution EDX maps. 3D reconstruction of SEM EDX results shows at least four differentiated regions in the heated area for all samples. The exact Fe and Mg compositions mentioned below are an example of the sample melted at 45 GPa for 6 minutes. The bulk of the heated are is surrounded by ferropericlase (Mg0.92, Fe0.08)O shell (Fp). Inside this shell we find a thick region of (Mg,Fe)SiO3 perovskite-structured bridgmanite (Brg) coexisting with Fp. In the center lies a Fe-rich core which is surrounded by magnesiow

  13. Oxygen Fugacity of the Martian Mantle from Pigeonite/Melt Partitioning of Samarium, Europium and Gadolinium

    NASA Technical Reports Server (NTRS)

    Musselwhite, S.; Jones, J. H.; Shearer, C.

    2004-01-01

    This study is part of an ongoing effort to calibrate the pyroxene/melt Eu oxybarometer for conditions relevant to the martian meteorites. There is fairly good agreement between a determinations using equilibria between Fe-Ti oxides and the estimates from Eu anomalies in shergottite augites in tenns of which meteorites are more or less oxidized. The Eu calibration was for angrite composition pyroxenes which are rather extreme. However, application of a calibration for martian composition augites 113 does not significantly reduce the discrepancy between the two methods. One possible reason for this discrepancy is that augites are non-liquidus. The use of pigeonite rather than augite as the oxy-barometer phase is considered. We have conducted experiments on martian composition pigeonite/melt REE partitioning as a function of fO2.

  14. Oxygen Fugacity of the Martian Mantle From Pyroxene/Melt Partitioning of REE

    NASA Technical Reports Server (NTRS)

    Musselwhite, D. S.; Jones, J. H.

    2003-01-01

    This study is part of an ongoing effort to calibrate the pyroxene/melt REE oxybarometer for conditions relevant to the martian meteorites. Redox variations have been reported among the shergottites. Wadhwa used the Eu and Gd augite/melt partitioning experiments of McKay, designed for the LEW86010 angrite, to infer a range of fo2 for the shergottites. Others inferred fo2 using equilibria between Fe-Ti oxides. There is fairly good agreement between the Fe-Ti oxide determinations and the estimates from Eu anomalies in terms of which meteorites are more or less oxidized. The Eu anomaly technique and the Fe-Ti oxide technique both essentially show the same trend, with Shergotty and Zagami being the most oxidized and QUE94201 and DaG 476 being the most reduced. Thus, the variation in fo2 appears to be both real and substantive. However, although the redox trends indicated by the two techniques are similar, there is as much as two log unit offset between the results of three researchers. One explanation for this offset is that the Eu calibration used for the shergottites was actually designed for the LEW86010 angrite, a silica-undersaturated basalt whose pyroxene (diopside) compositions are rather extreme. To correct this, experiments have been conducted on the redox relationship of Eu partitioning relative to Sm and Gd for pyroxene/melt compositions more relevant to Martian meteorites. We report here preliminary results for experiments on pigeonite/melt partitioning as a function of fO2.

  15. Silicate diffusion in alkali-carbonatite and hydrous melts at 16.5 and 24 GPa: Implication for the melt transport by dissolution-precipitation in the transition zone and uppermost lower mantle

    NASA Astrophysics Data System (ADS)

    Shatskiy, Anton; Litasov, Konstantin D.; Borzdov, Yuriy M.; Katsura, Tomoo; Yamazaki, Daisuke; Ohtani, Eiji

    2013-12-01

    The diffusivity of dissolved Mg2SiO4 in wadsleyite saturated KMC melt (K2Mg(CO3)2 + 25.7 wt.% MgSiO3) at 16.5 GPa and 1700 °C, MgSiO3 diffusivity in perovskite saturated KMCH (K2Mg(CO3)2 × 2H2O + 31.7 wt.% MgSiO3) and HM (H2O + 75.7 wt.% MgSiO3) melts at 24 GPa and 1500 °C were determined experimentally using a scaled-up version of a Kawai-type multi-anvil apparatus. During a diffusion experiment, silicate saturation was maintained at different levels in the two temperature regions by placing the diffusion cell in the thermal gradient of 20 °C/mm. The diffusivity was computed from the total mass of silicate transported from “hot” to the “cold” region during the course of an experiment. At given conditions silicate diffusivities were estimated to be DKMCMg2SiO4=2×10-9 m/s, DKMCHMgSiO3=4×10-9 m/s, and DHMMgSiO3=5×10-8 m/s. Using obtained diffusivities we estimated possible migration rates of dispersed melt inclusion in the deep mantle by means of dissolution-precipitation considering different driving forces. The rates of melt migration driven by the lateral thermal gradient of 1 °C/km in the mantle plume range from 4 × 10-8 to 8 × 10-7 m/year. This means that during plume ascent time of about 50 Ma, the melt can be moved by 2-40 m. These values clearly demonstrate that the thermal gradient is very weak driving force in terms of melt segregation in the deep mantle. On the other hand, at typical mantle stress of 1 MPa and droplet size of 100 μm the migration rates of the HM, KMCH and KMC melts are estimated to be 22.5, 0.9 and 0.2 m/year, respectively, which are 2-3 orders of magnitude faster than ascent rate of the mantle plume. This implies that all melt droplets on the way of ascending plume would be entrapped by the stressed zone in front of plume and accumulated in the plume head. This mechanism may explain segregation of mantle magmas with the source regions deeper than 150-250 km, such as kimberlites.

  16. Chromian spinels in mafic ultramafic mantle dykes: Evidence for a two-stage melt production during the evolution of the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Python, Marie; Ceuleneer, Georges; Arai, Shoji

    2008-11-01

    This paper describes a comprehensive study of the chromian spinels present in mafic-ultramafic dykes cropping out along the mantle section of the Oman ophiolite. We studied about 1100 samples in thin section and with the electron microprobe. Chromian spinel is almost ubiquitous in primitive dykes (high-Mg# troctolites and pyroxenites) and less common in more differentiated ones (olivine gabbros and gabbronorites). The Cr#, TiO 2 content, and other compositional parameters are well correlated to the nature and composition of the co-genetic silicate assemblage. Chromian spinel composition contributes to establish that the mantle dykes of Oman are more or less evolved cumulates that crystallised from two main types of primary melts: tholeitic melt similar to Mid-Ocean Ridge basalts (MORB; 0.45 < Cr# < 0.63; 0.3 < Mg# < 0.6; TiO 2 up to2 wt.%), and more silicic melts issued from a highly depleted mantle source, similar to boninitic-andesitic melts that preferentially form in subduction zone settings (0.35 < Cr# < 0.80; 0.1 < Mg# < 0.7; TiO 2 up to 0.2 wt.%). The chromian spinel composition presents a higher variability than the associated silicates and allows us to further unravel the petrological evolution and segmentation of the Oman ophiolite. The composition of chromian spinel in mantle dykes and in the spatially related residual harzbugites display well correlated variations at the scale of the Oman ophiolite. This shows that these two lithologies share a common magmatic history, even if, strictly speaking, they cannot be related through direct parent-daughter relationships. The Cr# is on average higher, and the TiO 2 lower in the NW than in the SE, consistent with an increasing influence of "marginal" magmatic processes in the NW, while the southeastern area has petrological characteristics closer to those of an "open" ocean. In this southeastern part, compositional variations of the chromian spinel are correlated to structural characteristics related to the

  17. Geochemical variations in Japan Sea back-arc basin basalts formed by high-temperature adiabatic melting of mantle metasomatized by sediment subduction components

    NASA Astrophysics Data System (ADS)

    Hirahara, Yuka; Kimura, Jun-Ichi; Senda, Ryoko; Miyazaki, Takashi; Kawabata, Hiroshi; Takahashi, Toshiro; Chang, Qing; Vaglarov, Bogdan S.; Sato, Takeshi; Kodaira, Shuichi

    2015-05-01

    The Yamato Basin in the Japan Sea is a back-arc basin characterized by basaltic oceanic crust that is twice as thick as typical oceanic crust. Two types of ocean floor basalts, formed during the opening of the Japan Sea in the Middle Miocene, were recovered from the Yamato Basin during Ocean Drilling Program Legs 127/128. These can be considered as depleted (D-type) and enriched (E-type) basalts based on their incompatible trace element and Sr-Nd-Pb-Hf isotopic compositions. Both types of basalts plot along a common mixing array drawn between depleted mantle and slab sediment represented by a sand-rich turbidite on the Pacific Plate in the NE Japan fore arc. The depleted nature of the D-type basalts suggests that the slab sediment component is nil to minor relative to the dominant mantle component, whereas the enrichment of all incompatible elements in the E-type basalts was likely caused by a large contribution of bulk slab sediment in the source. The results of forward model calculations using adiabatic melting of a hydrous mantle with sediment flux indicate that the melting conditions of the source mantle for the D-type basalts are deeper and hotter than those for the E-type basalts, which appear to have formed under conditions hotter than those of normal mid-oceanic ridge basalts (MORB). These results suggest that the thicker oceanic crust was formed by greater degrees of melting of a hydrous metasomatized mantle source at unusually high mantle potential temperature during the opening of the Japan Sea.

  18. Melting enthalpies of mantle peridotite: calorimetric determinations in the system CaO-MgO-Al 2O 3-SiO 2 and application to magma generation

    NASA Astrophysics Data System (ADS)

    Kojitani, Hiroshi; Akaogi, Masaki

    1997-12-01

    High-temperature drop calorimetry in the temperature range of 1398-1785 K was performed for the samples of mixtures of synthetic anorthite (An), diopside (Di), enstatite (En) and forsterite (Fo) with the same compositions as those of primary melts generated at 1.1, 3 and 4 GPa at most 10° above the solidus of anhydrous mantle peridotite in the CaO-MgO-Al 2O 3-SiO 2 system. From the differences between the heat contents ( H T-H 298) of liquid and that of crystal mixture at the liquidus temperature, melting enthalpies of the samples of 1.1, 3 and 4 GPa-primary melt compositions were determined at 1 atm to be 531 ± 39 J · g -1 at 1583 K, 604 ± 21 J · g -1 at 1703 K, 646 ± 21 J · g -1 at 1753 K, respectively. These heat of fusion values suggest that mixing enthalpy of the melt in the An-Di-En-Fo system is approximately zero within the experimental errors when we use the heat of fusion of Fo by Richet et al. (P. Richet, F. Leclerc, L. Benoist, Melting of forsterite and spinel, with implications for the glass transition of Mg 2SiO 4 liquid, Geophys. Res. Lett. 20 (1993) 1675-1678). The measured enthalpies of melting at 1 atm were converted into those for melting reactions which occur under high pressures by correcting enthalpy changes associated with solid-state mineral reactions. Correcting the effects of pressure, temperature and FeO and Na 2O components on the melting enthalpies at 1 atm, heat of fusion values of a representative mantle peridotite just above the solidus under high pressure were estimated to be 590 J at 1.1 GPa and 1523 K, 692 J at 3 GPa and 1773 K, and 807 J at 4 GPa and 1923 K for melting reactions producing liquid of 1 g, with uncertainties of 50 J. By applying these melting enthalpies to a mantle diapir model which generates present MORBs, a potential mantle temperature of 1533 K has been estimated, assuming an eruption temperature of magma of 1473 K.

  19. Subduction of Fracture Zones control mantle melting and geochemical signature above slabs

    NASA Astrophysics Data System (ADS)

    Constantin Manea, Vlad; Leeman, William; Gerya, Taras; Manea, Marina; Zhu, Guizhi

    2014-05-01

    The geochemistry of arc volcanics proximal to oceanic fracture zones (FZs) is consistent with higher than normal fluid inputs to arc magma sources. Here, enrichment of boron (B/Zr) in volcanic arc lavas is used to evaluate relative along-strike inputs of slab-derived fluids in the Aleutian, Andean, Cascades, and Trans-Mexican arcs. Significant B/Zr spikes coincide with subduction of prominent FZs in the relatively cool Aleutian and Andean subduction zones, but not in the relatively warm Cascadia and Mexican subduction zones, suggesting that FZ subduction locally enhances fluid introduction beneath volcanic arcs, and retention of fluids to sub-arc depths diminishes with subduction zone thermal gradient. Geodynamic treatments of lateral inhomogeneities in subducting plates have not previously considered how FZs may influence the melt and fluid distribution above the slab. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations of subduction, we show that fluids, including melts and water, concentrate in areas where fracture zones are subducted, resulting in along-arc variability in magma source compositions and processes.

  20. Melt-rock interactions, deformation, hydration and seismic properties in the sub-arc lithospheric mantle inferred from xenoliths from seamounts near Lihir, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Soustelle, Vincent; Tommasi, Andréa; Demouchy, Sylvie; Franz, Leander

    2013-11-01

    This study analyses the interactions between deformation and reactive fluid and melt percolation, and their effects on sub-arc mantle seismic properties based on microstructural observations on mantle xenoliths extracted by the Tubaf and Edison seamounts close to the Lihir Island, in the Papua New Guinea archipelago. These xenoliths sample an oceanic lithosphere, which has experienced high-temperature deformation in the presence of fluids or melts. This was followed by metasomatism under static conditions. Syn-kinematic percolation of reactive Si-rich melts or fluids in peridotites has produced pyroxene-enrichment, grain size reduction, and dispersion of olivine crystal preferred orientation (CPO). Fourier transform infrared spectroscopy analyses show that olivine has very low water contents (1-4 wt. ppm H2O), similar to spinel peridotites from other subduction zones. These low values may record both low water solubility in olivine at low pressure and dehydration during transport and exhumation. Water contents in pyroxenes are highly variable and likely result from spatially heterogeneous melt or fluid percolation. Analysis of olivine CPO indicates dominant activation of both (010)[100] and (001)[100] slip systems, which are characteristic of deformation under high temperature, low stress, low pressure and low to moderate hydrous conditions. Fast S-wave polarization and P- and Rayleigh propagation directions are thus parallel to the mantle flow direction. The pyroxene enrichment by melt-rock reactions is accompanied by dispersion of olivine CPO and induces a significant decrease of the maximum S-wave and P-wave anisotropy in the peridotites. The calculated seismic properties also show that the lowest Vp/Vs ratios (< 1.7) mapped in fore-arc mantle may only be explained by taking in consideration the CPO-induced elastic anisotropy of the peridotites.

  1. Upper mantle heterogeneity below the Mid-Atlantic Ridge, 0°-15°N

    NASA Astrophysics Data System (ADS)

    Bonatti, E.; Peyve, A.; Kepezhinskas, P.; Kurentsova, N.; Seyler, M.; Skolotnev, S.; Udintsev, G.

    1992-04-01

    Small-scale variations in composition of mantle-derived peridotites have been investigated in the 0°-15°N portion of the Mid-Atlantic Ridge (MAR), thanks to a relatively close-spaced peridotite sample coverage achieved by combining samples collected by Russian and U.S. expeditions. Areal variations in the composition of mantle-equilibrated minerals olivine, orthopyroxene, clinopyroxene, and spinel have been interpreted as due primarily to regional variations in the initial composition, degree of partial melting, and thermal structure of the upper mantle. Mantle rocks from the eastern part of the Romanche transform frequently contain a trapped fraction of basaltic melt, while undepleted mantle prevails in the western part of the Romanche, suggesting a "cold" upper mantle thermal regime in this region, which prevented significant melting. Immediately to the north, the St. Paul Fracture Zone (FZ) upper mantle shows intermediate degrees of melting, except for St. Peter-Paul Island which exposes metasomatized mantle rocks chemically and isotopically different from other oceanic peridotites. Between St. Paul FZ and 4°N (Strakhov FZ) we have an area of strongly depleted upper mantle. Farther north the Doldrums FZ area (˜8°N) appears to be underlain by moderately depleted upper mantle with some melt entrapment. The Vema FZ (11°N) is underlain by relatively homogenous upper mantle which has undergone a rather low degree of melting. The Mercurius and Marathon transforms (between 12° and 13°N) expose moderately depleted peridotites. Finally, the 15°20' FZ area shows relatively undepleted upper mantle on the northern side of the transform and at sites distant from the MAR axis and strongly depleted mantle south of the transform. The strongly depleted mantle from the 2°-3°N and 14°-15°N regions is associated spatially with light rare earth element enriched mid-ocean ridge basalt showing a "hot spot"-type geochemical signature. The areal association of refractory

  2. X-ray Raman scattering study of MgSiO₃ glass at high pressure: Implication for triclustered MgSiO₃ melt in Earth's mantle

    SciTech Connect

    Lee, Sung Keun; Lin, Jung-Fu; Cai, Yong Q.; Hiraoka, Nozomu; Eng, Peter J.; Okuchi, Takuo; Mao, Ho-kwang; Meng, Yue; Hu, Michael Y.; Chow, Paul; Shu, Jinfu; Li, Baosheng; Fukui, Hiroshi; Lee, Bum Han; Kim, Hyun Na; Yoo, Choong-Shik

    2015-02-09

    Silicate melts at the top of the transition zone and the core-mantle boundary have significant influences on the dynamics and properties of Earth's interior. MgSiO3-rich silicate melts were among the primary components of the magma ocean and thus played essential roles in the chemical differentiation of the early Earth. Diverse macroscopic properties of silicate melts in Earth's interior, such as density, viscosity, and crystal-melt partitioning, depend on their electronic and short-range local structures at high pressures and temperatures. Despite essential roles of silicate melts in many geophysical and geodynamic problems, little is known about their nature under the conditions of Earth's interior, including the densification mechanisms and the atomistic origins of the macroscopic properties at high pressures. Here, we have probed local electronic structures of MgSiO3 glass (as a precursor to Mg-silicate melts), using high-pressure x-ray Raman spectroscopy up to 39 GPa, in which high-pressure oxygen K-edge features suggest the formation of tricluster oxygens (oxygen coordinated with three Si frameworks; [3]O) between 12 and 20 GPa. Our results indicate that the densification in MgSiO3 melt is thus likely to be accompanied with the formation of triculster, in addition to a reduction in nonbridging oxygens. The pressure-induced increase in the fraction of oxygen triclusters >20 GPa would result in enhanced density, viscosity, and crystal-melt partitioning, and reduced element diffusivity in the MgSiO3 melt toward deeper part of the Earth's lower mantle.

  3. Volatiles in melt inclusions and osmium isotopes from Hawaiian lavas: investigating the relationship between CO2 and H2O contents with mantle source lithology

    NASA Astrophysics Data System (ADS)

    Marske, J. P.; Hauri, E. H.; Garcia, M. O.; Pietruszka, A. J.

    2012-12-01

    Variations in radiogenic isotopes (Pb, Sr, Nd, Hf, He, etc.) and magmatic volatiles (CO2 or H2O) in Hawaiian volcanoes reveal a range of important processes that occur from the source to surface (mantle source heterogeneity, extents of melting, magma transportation, mixing, and storage, and eruption style). Hawaiian lavas are thought to originate from partial melting of a heterogeneous plume source containing a mixture of peridotite and ancient recycled oceanic crust (pyroxenite or eclogite). These lavas display a considerable range in volatile abundances (e.g. <1 to >1000 ppm for Kilauea) that is generally attributed to magmatic degassing or mixing during ascent. However, the source region for Hawaiian volcanoes may also be heterogeneous with respect to volatile concentrations. The effect of shallow degassing makes it difficult to determine if there is a relationship between mantle source composition, lithology, and the volatile budget. We will present Os isotopic ratios and volatile contents for over 25 samples from six Hawaiian volcanoes (Koolau, Mauna Kea, Mauna Loa, Hualalai, Kilauea, and Loihi) to determine if pre-eruptive volatiles in shield-stage magmas correlate with the different Hawaiian components (i.e. 'Kea' or 'Loa') and lithologies (i.e. peridotite or pyroxenite) identified by radiogenic isotopes. Rapidly-cooled submarine glasses and tephras will be analyzed, as subaerial lavas cool too slowly to prevent diffusive loss of H2O from melt inclusions. Olivine-hosted melt inclusions or groundmass glasses from each eruption will be analyzed for a range of volatiles (CO2, H2O, S, Cl, F). Additionally, olivine separates from each of these samples will be analyzed for Os isotopes. This study assesses the role of mantle heterogeneity, degree of partial melting, magma transportation, and degassing, in controlling the primary melt volatile concentrations. Although volatile abundances in parental magmas are likely to be modified by variable extents of exsolution

  4. Transition from ultra-enriched to ultra-depleted primary MORB melts in a single volcanic suite (Macquarie Island, SW Pacific): Implications for mantle source, melting process and plumbing system

    NASA Astrophysics Data System (ADS)

    Husen, Anika; Kamenetsky, Vadim S.; Everard, John L.; Kamenetsky, Maya B.

    2016-07-01

    Compositional diversity of basalts forming the oceanic floor is attributed to a variety of factors such as mantle heterogeneities, melting conditions, mixing of individual melt batches, as well as fractionation and assimilation processes during magma ascent and emplacement. In this study the compositional range and origin of mid-ocean ridge basalts (MORB) is approached by petrological, mineralogical and geochemical studies of the Miocene Macquarie Island ophiolite, an uplifted part of the Macquarie Ridge at the boundary between the Australian and Pacific plates. In this study, earlier results on the enriched to ultra-enriched (La/Sm 1.4-7.9), isotopically homogeneous basaltic glasses are complemented by the compositions of olivine-phyric rocks, principal phenocrystic minerals and Cr-spinel hosted melt inclusions. Studied olivine, clinopyroxene and Cr-spinel phenocrysts are among the most primitive known for MORB (85-91 mol% forsterite in olivine, 81-91 Mg# in clinopyroxene, and 66-77 Mg# and 34-60 Cr# in spinel) and represent primary and near-primary compositions of their parental melts. Geochemical characteristics of the liquids parental to clinopyroxene (La/Sm 0.8-6.3) and Cr-spinel (La/Sm 0.4-5) partly overlap with those of the basaltic glasses, but also strongly advocate the role of depleted to ultra-depleted primary melts in the origin of the Macquarie Island porphyritic rocks. The trace element composition of olivine phenocrysts and the systematics of rare-earth elements in glasses, melt inclusions, and clinopyroxene provide evidence for a peridotitic composition of the source mantle. Our data supports the mechanism of fractional "dynamic" melting of a single mantle peridotite producing individual partial melt batches with continuously changing compositions from ultra-enriched towards ultra-depleted. The incipient enriched melt batches, represented by basaltic glasses in this study, may erupt without significant modification, whereas consecutively derived

  5. Shear-induced melt localization and the rheology of the partially molten mantle

    NASA Astrophysics Data System (ADS)

    Katz, R. F.; Spiegelman, M.; Holtzman, B. K.

    2005-12-01

    The emergence of patterns of melt distribution in experiments on partially molten aggregates undergoing simple shear [1,2] provide a rare opportunity to test magma migration theory [3,4,5] by directly comparing experiments and calculations. The fundamental observation is the emergence and persistence to large strains of bands of high porosity and concentrated deformation oriented at about 15-25° to the plane of shear [6]. We have extended the analysis of Spiegelman [7] and report results from new linear analysis and numerical solutions that suggest that band angle in experiments is controlled by a balance between porosity and strain rate-weakening mechanisms. Lower angles are predicted for stronger strain rate-weakening. For the specific model considered here, a power-law stress-dependent rheology, calculations with n~6 are consistent with the observations. These results suggest that partially molten aggregates deforming under shear may have a greater sensitivity to strain rate than previously believed [8]. [1] Zimmerman, M. et al., Geophys. Res. Lett., 26, 1999. [2]Holtzman, B. et al., Geochem. Geophys. Geosyst., 4, 2003. [3]Mc{K}enzie, D., J. Petrol., 25, 1984. [4]Fowler, A., Geophys. Astrophys. Fluid Dyn., 33, 1985. [5]Bercovici, D. et al., J. Geophys. Res.- Solid Earth, 106, 2001. [6]Spiegelman, M., Geochem. Geophys. Geosyst., 4, 2003. [7]Holtzman, B. et al., J. Petrol., 2005. [8]Hirth, G. and D. Kohlstedt, J. Geophys. Res., 100, 1995.

  6. Partitioning of Large-ion Lithophile Elements Between Aqueous Fluids and Melts: Role of Saline Fluids in Sub-arc Mantle

    NASA Astrophysics Data System (ADS)

    Kawamoto, T.; Mibe, K.

    2014-12-01

    Chemical fractionation of slab-derived supercritical fluids can play an important role in elemental transfer from subducting slab to the mantle wedge and arc magmatism [1]. Recent findings of saline fluids from sub-arc mantle peridotite indicate that aqueous fluids in mantle wedge can contain 3.7 wt% NaCl in Ichinomageta, Northeast Japan arc [2] to 5.1 wt% NaCl in Pinatubo, Luzon arc [3]. It is, therefore, important to determine the effect of Cl on the trace element partitioning between aqueous fluids and melts. Synchrotron radiation X-ray fluorescence (XRF) analysis is conducted to know Rb, Sr, and Pb partitioning between aqueous fluids and melts [4]. There is a positive correlation between partition coefficients and pressure, as well as salinity. Two slab-derived components, melt and fluid components, are suggested to explain trace element characteristics of arc-basalts in the Mariana arc [5]. The fluid component is characterized by enrichment of alkali and alkali earth elements. Such features can be explained if the fluid component is a saline fluid, because alkali earth elements and Pb are much less mobile with Cl-free fluids than Cl-rich fluids [4]. We suggest that slab-derived components have compositional features consistent with a saline fluid and a melt, which can be formed through a separation of a slab-derived supercritical fluid [1]. Slab derived supercritical fluids contain Cl, and aqueous fluids inherit much of the Cl and some of the large-ion lithophile elements. [1] Kawamoto et al. 2012, Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism. PNAS, pnas.org/content/109/46/18695 [2] Kumagai et al. Evolution of carbon dioxide bearing saline fluids in the mantle wedge beneath the Northeast Japan arc, CMP [3] Kawamoto et al. 2013, Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab. PNAS, pnas.org/content/110/24/9663 [4] Kawamoto et al. 2014, Large ion

  7. Hydrous melting and partitioning in and above the mantle transition zone: Insights from water-rich MgO-SiO2-H2O experiments

    NASA Astrophysics Data System (ADS)

    Myhill, R.; Frost, D. J.; Novella, D.

    2017-03-01

    Hydrous melting at high pressures affects the physical properties, dynamics and chemical differentiation of the Earth. However, probing the compositions of hydrous melts at the conditions of the deeper mantle such as the transition zone has traditionally been challenging. In this study, we conducted high pressure multianvil experiments at 13 GPa between 1200 and 1900 °C to investigate the liquidus in the system MgO-SiO2-H2O. Water-rich starting compositions were created using platinic acid (H2Pt(OH)6) as a novel water source. As MgO:SiO2 ratios decrease, the T -XH2O liquidus curve develops an increasingly pronounced concave-up topology. The melting point reduction of enstatite and stishovite at low water contents exceeds that predicted by simple ideal models of hydrogen speciation. We discuss the implications of these results with respect to the behaviour of melts in the deep upper mantle and transition zone, and present new models describing the partitioning of water between the olivine polymorphs and associated hydrous melts.

  8. Seismic velocity anisotropy and heterogeneity beneath the Mantle Electromagnetic and Tomography Experiment (MELT) region of the East Pacific Rise from analysis of P and S body waves

    USGS Publications Warehouse

    Hammond, W.C.; Toomey, D.R.

    2003-01-01

    We use teleseismic P and S delay times and shear wave splitting measurements to constrain isotropic and anisotropic heterogeneity in the mantle beneath the southern East Pacific Rise (SEPR). The data comprise 462 P and S delay times and 18 shear wave splitting observations recorded during the Mantle Electromagnetic and Tomography (MELT) Experiment. We estimate the mantle melt content (F) and temperature (T) variation from the isotropic velocity variation. Our results indicate that the maximum variation in F beneath our array is between zero and ???1.2%, and maximum variation in T is between zero and ???100 K. We favor an explanation having partial contributions from both T and F. We approximate the seismic anisotropy of the upper mantle with hexagonal symmetry, consistent with the assumption of two dimensionality of mantle flow. Our new tomographic technique uses a nonlinear inversion of P and slow S polarization delay times to simultaneously solve for coupled VP and VS heterogeneity throughout the model and for the magnitude of anisotropy within discrete domains. The domain dimensions and the dip of the anisotropy are fixed for each inversion but are varied in a grid search, obtaining the misfit of the models to the body wave delay data and to split times of vertically propagating S waves. The data misfit and the isotropic heterogeneity are sensitive to domain dimensions and dip of anisotropy. In a region centered beneath the SEPR the best average dip of the hexagonal symmetry axis is horizontal or dipping shallowly (<30??) west. Given the resolution of our data, a subaxial region characterized by vertically aligned symmetry axes may exist but is limited to be <80 km deep. We infer that the mantle flow beneath the SEPR is consistent with shallow asthenospheric return flow from the direction of the South Pacific superswell.

  9. Comparing the nature of the western and eastern Azores mantle

    NASA Astrophysics Data System (ADS)

    Genske, Felix S.; Beier, Christoph; Stracke, Andreas; Turner, Simon P.; Pearson, Norman J.; Hauff, Folkmar; Schaefer, Bruce F.; Haase, Karsten M.

    2016-01-01

    The Azores islands in the central North-Atlantic originate from a regional melting anomaly, probably created by melting hot, unusually hydrous and geochemically enriched mantle. Here, we present Hf, Pb and Os isotopic data in geochemically well-characterised primitive lavas from the islands Flores and Corvo that are located west of the Mid-Atlantic Ridge (MAR), as well as submarine samples from a subsided island west of Flores and from Deep Sea Drilling Project (DSDP) holes drilled in the western part of the Azores platform and beyond. These are compared to existing data from the Azores islands east of the MAR. The geodynamic origin of the two islands west of the ridge axis and furthest from the inferred plume centre in the central part of the plateau is enigmatic. The new data constrain the source compositions of the Flores and Corvo lavas and show that the western and eastern Azores mantle is isotopically similar, with the exception of an enriched component found exclusively on eastern São Miguel. Trace element ratios involving high field strength elements (HFSE) are distinctly different in the western islands (e.g. twofold higher Nb/Zr) compared to any of the islands east of the MAR. A similar signature is observed in MAR basalts to the south of the Azores platform and inferred to originate from (auto-) metasomatic enrichment of the sub-ridge mantle (Gale et al., 2011, 2013). In a similar fashion, low degree melts from an enriched source component may metasomatise the ambient plume mantle underneath the western Azores islands. Melting such a modified plume mantle can explain the chemical differences between lavas from the western and eastern Azores islands without the need for additional plume components. Recent re-enrichment and intra melting column modification of the upwelling mantle can cause local to regional scale geochemical differences in mantle-derived melts.

  10. The Low-Degree Shape of Mercury

    NASA Astrophysics Data System (ADS)

    Perry, M. E.; Neumann, G. A.; Mazarico, E.; Hauck, S. A., II; Solomon, S. C.; Zuber, M. T.; Smith, D. E.; Phillips, R. J.; Margot, J. L.; Johnson, C. L.; Ernst, C. M.; Oberst, J.

    2015-12-01

    The shape of Mercury, particularly when combined with its geoid, provides clues to the planet's internal structure, thermal evolution, and rotational history. Twenty-five million elevation measurements of the northern hemisphere, acquired by the Mercury Laser Altimeter on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, were combined with 378 occultation measurements of radio-frequency signals from the spacecraft in the planet's southern hemisphere to reveal the low-degree shape of Mercury. We solved for the spherical-harmonic coefficients through degree and order 128 and found that Mercury's mean radius is 2439.36±0.02 km. The offset between the planet's centers of mass and figure is negligible (40±40 m) along the polar axis and modest (140±50 m) in the equatorial plane. Mercury's spherical-harmonic shape spectrum is dominated by degree 2, and the planet's first-order shape is that of a triaxial ellipsoid with semimajor axes a, b, and c. The polar radius, c, is 1.65 km less than (a+b)/2, and the equatorial difference, a-b, is 1.25 km. The long axis is rotated 15° west of Mercury's dynamically defined principal axis. Mercury's geoid is similarly dominated by degree 2 and well described by a triaxial ellipsoid. The degree-2 geoid and shape are highly correlated, but the power spectral density of the geoid at degree 2 is only 1% of its shape counterpart, implying substantial compensation of elevation variations on a global scale and that Mercury is not in hydrostatic equilibrium.

  11. Fluid-present melting of sulfide-bearing ocean-crust: Experimental constraints on the transport of sulfur from subducting slab to mantle wedge

    NASA Astrophysics Data System (ADS)

    Jégo, Sébastien; Dasgupta, Rajdeep

    2013-06-01

    To constrain the sulfur enrichment of arc magma source-regions and the agent of sulfur transport from subducting slab to mantle wedge, here we report experimental measurements of sulfur content at sulfide saturation (SCSS) of slab-derived hydrous partial melts at 2.0 and 3.0 GPa and from 800 to 1050 °C, using Ni-NiO (NNO) and Co-CoO (CCO) external oxygen fugacity (fO2) buffers. A synthetic H2O-saturated MORB with 1 wt.% S (added as pyrite) was used as starting material. All experiments produced pyrrhotite- and fluid-saturated assemblages of silicate glass, clinopyroxene, garnet, quartz, and rutile (plus amphibole at 2 GPa/800 °C and phengite at 3 GPa/850 °C). The silicate partial melt composition evolves from rhyolitic to rhyodacitic compositions with increasing temperature and melting degree in equilibrium with an eclogitic residue, showing substantial decrease in SiO2 and Mg# and increase in FeOT, TiO2 and Na2O. At all temperatures melt sulfur concentrations are very low, with an average of 110 ± 50 ppm S, similar to previous measurements at lower pressures. Melt SCSS appears to be mainly controlled by the melt composition, the activity of water, aH2O and the sulfur fugacity, fS2 (calculated from the composition of pyrrhotite). Mass-balance calculations show that the proportion of bulk sulfur dissolved in the silicate melt is negligible (<0.005 wt.% of the bulk sulfur). In contrast, diminishing proportion of pyrrhotite with increasing temperature suggests that the fluid phase at equilibrium may contain as much as 10-15 wt.% S at ⩾1050 °C, and more than 40 wt.% of the bulk sulfur initially present in the slab may be transferred to the aqueous fluid. Our data also suggest that fluid/melt sulfur partitioning increases with increasing temperature, from ˜300 at 900 °C to ˜1200 at 1050 °C, whereas pressure appears to have less of an effect. With respect to fO2, no real difference of fluid/melt S partitioning, within data uncertainties, between NNO and CCO at

  12. Constraint on a melting regime in upper mantle beneath the central Mariana back-arc spreading center through the geophysical electromagnetic forward modeling

    NASA Astrophysics Data System (ADS)

    Matsuno, T.; Evans, R. L.; Seama, N.

    2011-12-01

    An upper-mantle electrical resistivity structure along a 2D magnetotelluric (MT) transect across the Mariana subduction system shows high resistivities beneath the back-arc spreading system where a melting regime is supposed to exist (Matsuno et al., 2010). This result is contrary to expectations, as interconnected silicate and carbonate melts are electrically conductive (e.g. Roberts and Tyburczy, 1999; Gaillard et al., 2008). Possible reasons for the high resistivity are (1) no more than a small amount of melt that is poorly interconnected is present within the mantle and (2) the slow spreading ridge system results in a 3D melt delivery with limited along-strike dimensions that is difficult to image by the 2D MT transect (Matsuno et al., 2010). Seismic attenuation structures (Pozgay et al., 2009) and a shear velocity structure (Pyle et al., 2010) along the same transect show high attenuation and fast velocity beneath the back-arc spreading center, suggesting the existence of modest amount of melt (< 1%) in a columnar shape with about 75 km width down to about 100 km depth (Pozgay et al., 2009). We have run a series of modeling tests to place constraints on the melt geometry that are consistent with the MT data used in Matsuno et al. (2010). Plausible melting regimes beneath the back-arc spreading ridge are systematically changed into electrical models, assuming an interconnect geometry of silicate melt, and superimposed on the optimal 2D electrical resistivity model of Matsuno et al. (2010). Predicted responses of these new models were compared to the data set. Since the electromagnetic data is more sensitive to resistance (i.e. the product of resistivity and volume) than resistivity itself in a volume, there is a trade-off between the width and length of the acceptable melting regime. For the electrical resistivity structure to be consistent with the ~75 km width melting regime inferred from the seismic attenuation structures (Pozgay et al., 2009) and with the

  13. The limited depth range of a metallic-Fe-bearing layer in the lower mantle and its implications for partial melting

    NASA Astrophysics Data System (ADS)

    Girard, J.; Karato, S. I.

    2014-12-01

    Partial melting in (most of) the lower mantle occurs only by the presence of volatile elements such as hydrogen (and/or carbon). The experimental studies by [Kawamoto, 2004] showed that partial melting is possible even at temperature of 1673 K in the shallow lower mantle if there is sufficient water. However, if metallic Fe is present in the lower mantle as suggested by [Frost et al., 2004]). most of hydrogen will be dissolved in metallic Fe, and thus melting will be prevented, making it difficult to explain a seismic velocity drop in the shallow lower mantle [Schmandt et al., 2014]. In this study we conducted high pressure experiments using the Rotational Drickamer Apparatus (RDA), on bridgmanite (Mg,Fe)SiO3 + (Mg,Fe)O mixture at pressure up to 23-29 GPa and temperature of about 2000-2200K. Using the advantage of the new RDA cell design which provide a larger pressure gradient (~6 GPa across the sample), we report experimental observations showing that metallic Fe is formed only in the low pressure conditions, 24-26.5 GPa (corresponding to the depth range of 660-730 km), leaving the shallow lower mantle minerals "dry". Our results are also consistent with the published results by [Irifune et al., 2010; Sinmyo and Hirose, 2013] where they did not find any metallic Fe above 27 GPa. Therefore we conclude that metallic Fe is present only in the narrow depth range in the lower mantle. In such a case partial melt would be impossible and only occur at depth greater than 730 km. Our results explain why a velocity drop is observed at ~730 km not at 660 km [Schmandt et al., 2014]. The present results also have important implications for other geochemical issues including the behaviors of siderohpile elements during core formation. Frost, D. J., et al., (2004), Nature, 428, 409-412. Irifune, T., et al., (2010), Science, 327, 193-195. Kawamoto, T. (2004), Physics of Earth and Planetary Interiors, 143/144, 387-395. Schmandt, B., et al., (2014), Science, 344(6189), 1265

  14. Frozen melt-rock reaction in a peridotite xenolith from sub-arc mantle recorded by diffusion of trace elements and water in olivine

    NASA Astrophysics Data System (ADS)

    Tollan, P. M. E.; O'Neill, H. St. C.; Hermann, J.; Benedictus, A.; Arculus, R. J.

    2015-07-01

    Inferring the ambient state of the lithospheric mantle from xenoliths may be misleading if the magmatic activity responsible for the exhumation has modified the xenolith. Changes due to melt-xenolith interactions during exhumation are usually identifiable because of the short timescales involved, but changes due to earlier pulses of magma passing through the lithosphere may be more insidious because the longer timescales allow extensive overprinting of the ambient petrography. Here we describe an intermediate stage of melt-rock reaction recorded in a porphyroclastic peridotite xenolith from the upper mantle wedge adjacent to the West Bismarck Island Arc. The texture and chemistry, frozen during rapid exhumation, reflects the progressive change from harzburgite to clinopyroxene-bearing dunite caused by the infiltration of hydrous basaltic melt into the xenolith. The unusually low equilibration temperature of the xenolith (∼650 °C) prior to this event is reflected in very low concentrations of incompatible elements in the olivine, enhancing the diffusion profiles of these elements into the olivine from the interaction with the melt. The Ca concentration profiles correspond to a timescale of approximately three months, which we interpret as the time between the first infiltration of melt into the lithospheric mantle at this locality and the subsequent exhumation of the xenolith by a second pulse of magma. The diffusion profiles of other trace elements (Li, Na, Sc, Ti, V, Cr, Mn, Ni, Cu, Y) and major elements (Fe-Mg) confirm that all these elements diffuse through olivine at rates that differ from each other by less than one order of magnitude. Infrared spectroscopy reveals OH- ("water") contents and incorporation mechanisms typical of other arc mantle peridotites in the olivine cores. Water contents increase towards the crystal rims, similarly to the profiles of other measured trace elements. The increase in water concentration is accompanied by subtle changes in

  15. 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

  16. Evidence for slab melt/mantle reaction: Petrogenesis of Early Cretaceous and Eocene high-Mg andesites from Kitakami Mountains, Japan

    NASA Astrophysics Data System (ADS)

    Tsuchiya, N.; Suzuki, S.; Kimura, J.-I.; Kagami, H.

    2003-04-01

    ) are consisitent with those calculated by assuming Fe-Mg exchange partitioning between olivine and the bulk rock (KD = 0.3, Fe2+/(Fe2+ + Fe3+) = 0.85), suggesting that HMA melts have equilibrated with mantle olivine. However, the olivine phenocrysts show extremely high NiO contents (maximum 0.58 wt%), and are plotted beyond the olivine-spinel mantle array after Takahashi (1986). Since equilibrium crystallization trend for olivine phenocrysts obtained from MELTS calculations (Ghiorso and Sack, 1995) is consistent with the observed chemical compositions of phenocrysts, high-Ni contents in olivine phenocrysts result from high-Ni contents in melt. The HMAs show similar petrochemical characteristics to those of adakite (high La/Yb ratios and Sr contents, low Y and HREE contents) except higher Cr, Ni, and Mg contents. These characteristics of the HMAs in Kitakami resemble those of bajaites (Rogers et al., 1985), which corresponds to transitional adakites (Kepezhinskas et al., 1996) and primitive adakites (Yogodzinski and Kelemen, 1998). The petrochemical features are consistent with the conclusions that the bajaitic magmas can be resulted from reaction of slab melt with overlying mantle peridotite (Rogers and Saunders, 1989; Stern and Kilian, 1996; Yogodzinski and Kelemen, 1998). Modelling calculation using major and trace element chemistry indicates that the HMAs in Kitakami are derived from slab melt/mantle reaction. This model involves bulk mantle assimilation combined with crystallization of orthopyroxene and olivine with large assimilation/fractional crystallization ratio (r = 1.2--1.6; DePaolo 1981), reflecting a relatively hot mantle (Kelemen 1990, 1995). If a melt is reacted with mantle peridotite, Ni content in melts increases as a result of olivine dissolution, because Ni is enriched in olivine relative to orthopyroxene (Kelemen, 1995; 1998). Therefore, particularly Ni-rich nature of olivine phenocrysts in the Iwaizumi HMA may be an evidence for slab melt/mantle

  17. H2O storage capacity of olivine at 5-8 GPa and consequences for dehydration partial melting of the upper mantle

    NASA Astrophysics Data System (ADS)

    Ardia, P.; Hirschmann, M. M.; Withers, A. C.; Tenner, T. J.

    2012-09-01

    The H2O storage capacities of peridotitic minerals place crucial constraints on the onset of hydrous partial melting in the mantle. The storage capacities of minerals in equilibrium with a peridotite mineral assemblage (“peridotite-saturated” minerals) are lower than when the minerals coexist only with fluid because hydrous partial melt is stabilized at a lower activity of H2O. Here, we determine peridotite-saturated olivine H2O storage capacities from 5 to 8 GPa and 1400-1500 °C in layered experiments designed to grow large (∼100-150 μm) olivine crystals in equilibrium with the full hydrous peridotite assemblage (melt+ol+opx+gar+cpx). The peridotite-saturated H2O storage capacity of olivine at 1450 °C rises from 57±26 ppm (by wt.) at 5 GPa to 254±60 ppm at 8 GPa. Combining these with results of a parallel study at 10-13 GPa (Tenner et al., 2011, CMP) yields a linear relation applicable from 5 to 13 GPa for peridotite-saturated H2O storage capacity of olivine at 1450 °C, CH2Oolivine(ppm)=57.6(±16)×P(GPa)-169(±18). Storage capacity diminishes with increasing temperature, but is unaffected by variable total H2O concentration between 0.47 and 1.0 wt%. Both of these are as predicted for the condition in which the water activity in the melt is governed principally by the cryoscopic requirement of melt stability for a given temperature below the dry solidus. Measured olivine storage capacities are in agreement or slightly greater than those predicted by a model that combines data from experimental freezing point depression and olivine/melt partition coefficients of H2O (Hirschmann et al., 2009). Considering the temperature along the mantle geotherm, as well as available constraints on garnet/olivine and pyroxene/olivine partitioning of H2O (DH2Ogar/ol,DH2Opx/ol), we estimate the peridotite H2O storage capacity in the low velocity zone. The CH2O required to initiate melting between 150 and 250 km depth is between 270 and 855 ppm. We conclude that hydrous

  18. Slab melting as a barrier to deep carbon subduction.

    PubMed

    Thomson, Andrew R; Walter, Michael J; Kohn, Simon C; Brooker, Richard A

    2016-01-07

    Interactions between crustal and mantle reservoirs dominate the surface inventory of volatile elements over geological time, moderating atmospheric composition and maintaining a life-supporting planet. While volcanoes expel volatile components into surface reservoirs, subduction of oceanic crust is responsible for replenishment of mantle reservoirs. Many natural, 'superdeep' diamonds originating in the deep upper mantle and transition zone host mineral inclusions, indicating an affinity to subducted oceanic crust. Here we show that the majority of slab geotherms will intersect a deep depression along the melting curve of carbonated oceanic crust at depths of approximately 300 to 700 kilometres, creating a barrier to direct carbonate recycling into the deep mantle. Low-degree partial melts are alkaline carbonatites that are highly reactive with reduced ambient mantle, producing diamond. Many inclusions in superdeep diamonds are best explained by carbonate melt-peridotite reaction. A deep carbon barrier may dominate the recycling of carbon in the mantle and contribute to chemical and isotopic heterogeneity of the mantle reservoir.

  19. Slab melting as a barrier to deep carbon subduction

    NASA Astrophysics Data System (ADS)

    Thomson, Andrew R.; Walter, Michael J.; Kohn, Simon C.; Brooker, Richard A.

    2016-01-01

    Interactions between crustal and mantle reservoirs dominate the surface inventory of volatile elements over geological time, moderating atmospheric composition and maintaining a life-supporting planet. While volcanoes expel volatile components into surface reservoirs, subduction of oceanic crust is responsible for replenishment of mantle reservoirs. Many natural, ‘superdeep’ diamonds originating in the deep upper mantle and transition zone host mineral inclusions, indicating an affinity to subducted oceanic crust. Here we show that the majority of slab geotherms will intersect a deep depression along the melting curve of carbonated oceanic crust at depths of approximately 300 to 700 kilometres, creating a barrier to direct carbonate recycling into the deep mantle. Low-degree partial melts are alkaline carbonatites that are highly reactive with reduced ambient mantle, producing diamond. Many inclusions in superdeep diamonds are best explained by carbonate melt-peridotite reaction. A deep carbon barrier may dominate the recycling of carbon in the mantle and contribute to chemical and isotopic heterogeneity of the mantle reservoir.

  20. Physiognomy and timing of metasomatism in the southern Vourinos ultramafic suite, NW Greece: a chronicle of consecutive episodes of melt extraction and stagnation in the Neotethyan lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Kapsiotis, Argyrios N.

    2016-04-01

    The southern Vourinos massif, located in the Hellenides orogenic belt, forms part of the mantle section of the homonymous Neotethyan ophiolite complex in the NW Greek mainland. The southern domain of the massif is comprised voluminous and strained peridotite outcrops with variable pyroxene and olivine modal abundances, ranging from harzburgite (sensu stricto) to olivine-rich harzburgite and fine- to coarse-grained dunite. These peridotites are intruded by a complex network of undeformed websterite to olivine-rich websterite dykes. The peridotite lithologies are characterized by high Cr# [=Cr/(Cr + Al)] values in Cr-spinel (0.54-0.80), elevated Mg# [=Mg/(Mg + Fe2+)] ratios in olivine (0.91-0.94), poor Al2O3 content in clinopyroxene (up to 1.85 wt%) and very low bulk-rock abundances of Al2O3 (up to 0.66 wt%), CaO (up to 0.84 wt%), V (up to 45 ppm), Sc (up to 11 ppm) and REE, which are suggestive of their strongly depleted nature. They also display a wide range of fO2 values that vary between the fayalite-magnetite-quartz (FMQ-2) and FMQ+1 buffers, signifying their genesis under anoxic to oxidizing conditions. Simple batch and fractional melting models cannot satisfactorily explain their ultradepleted composition, whereas whole-rock Ni/Yb versus Yb systematics can be simulated by up to 27 % closed-system, non-modal, dynamic melting of a primitive mantle source, implying their multifarious origin in a progressively changing, in space and time, geotectonic setting. Chromian spinel chemistry (Cr# vs. TiO2) provides evidence for two consecutive melt-peridotite interaction events pertaining to patent metasomatism. The first incident is related to the release of IAT melts from the deep parts of the southern Vourinos mantle segment, which reacted with harzburgites transforming them into olivine-rich harzburgites and replacive dunites, whereas mixing of different pulses of IAT melts with distinct SiO2 activities generated heterogeneously deformed, cumulitic dunites. The

  1. Distinguishing Mantle Components by Sr and Nd Isotope Analysis of Single Melt Inclusions: Case Study on Italian Potassium-rich Lavas

    NASA Astrophysics Data System (ADS)

    Koornneef, J.; Nikogosian, I.; van Bergen, M.; Smeets, R.; Bouman, C.; Schwieters, J. B.; Davies, G. R.

    2014-12-01

    Melt inclusions record more extreme isotopic variability than their bulk host lavas and offer the potential to better constrain mantle components that contribute to magmatism. Isotopic analyses of melt inclusions are, however, challenging because of their limited size. To date, Sr and Pb isotope data have been obtained for oceanic islands by application of in situ analysis techniques, either by SIMS or LA-(MC)-ICPMS. Comprehensive studies of inclusions from other settings have yet to be performed. Here, we report the first combined Sr and Nd isotope data on individual melt inclusions from Pliocene-Quaternary K-rich lavas from mainland Italy, obtained using wet chemistry techniques and TIMS analysis. We use newly developed 1013 Ohm resistors mounted in the feedback loop of Faraday cup amplifiers. Compared to default 1011 Ohm resistors, use of 1013 Ohm resistors results in a 10-fold improvement of the signal-to-noise ratio and more precise data when analysing small ion beams (< 20 mV). Twenty-one olivine hosted (85 - 92 mol % Fo) melt inclusions from seven volcanic centres along the Italian peninsula were analysed. Inclusions were homogenised and analysed for trace elements prior to isotopic analysis. 87Sr/86Sr ratios in the melt inclusions range from 0.70508 to 0.71543; 143Nd/144Nd ratios from 0.51175 to 0.51268. Significant differences in 87Sr/86Sr and 143Nd/144Nd were found between melt inclusions and host lavas indicating distinct evolution paths for the lava groundmasses and the primitive melts that were trapped in the phenocrysts. The isotopic variability is interpreted to be the result of (1) mixing of melts derived from a heterogeneous mantle modified by subduction-related metasomatism, (2) mingling of distinct batches of olivine-bearing magma within a plumbing system, and (3) local assimilation of crustal material. These preliminary data suggest that combined analysis of individual melt inclusions for trace elements and Sr-Nd isotope ratios is a promising

  2. Infiltration of Refractory Melts into the Sub-Oceanic Mantle: Evidence from Major and Minor Element Compositions of Minerals from the 53° E Amagmatic Segment Abyssal Peridotites at the Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Gao, C.; Dick, H. J.; Zhou, H.; Liu, Y.; Wang, J.

    2014-12-01

    Elevated sodium and titanium in pyroxene and spinel with high TiO2 (> 0.2 wt%) are suggested as the geochemical characteristic for the MORB-like melt infiltration of peridotites. The petrological and geochemical results of melt infiltrating in mantle peridotites are controlled by not only the melt composition but also the melt/rock ratio. Large discordant dunite bodies in the mantle transition zone are the direct observation of large volume melt (high melt/rock ratio) infiltrating by channeled porous flow in the shallow mantle (1). In addition to dunites, melt infiltrating results in a large variety of vein lithologies in mantle, and the occurrence of plagioclases are considered as a petrological signal of melt-reaction at shallow depth (2, 3) with a medium melt/rock ratio. Because the lacking of obviously petrological and geochemical variation of peridotites, melt infiltration of peridotites with a low melt/rock ratio are rarely reported. Peridotites in this study are from the 53° E amagmatic segment at the Southwest Indian Ridge. These peridotites are suggested as highly depleted buoyant mantle drawn up from the asthenosphere beneath southern Africa during the breakup of Gondwanaland (4) and are residues of multi-stage melt extracting in both spinel and garnet field. We present a detailed analysis of mineral compositions by both the EMPA and LA-ICPMS. Mineral phases in 53°E peridotites have mantle major element compositions, although minerals show variations with the crystal size and the location from cores to rims (Fig.1). In conjunction with the profile analysis of large clinopyroxene crystals, our results document the melt infiltration occurred at the ultraslow-spreading environment. At least two kinds of percolation melts are distinguished. They are normally MORB-like melt and ultra-depleted melt. Reference1.P. B. Kelemen, H. J. B. Dick, Journal of Geophysical Research-Solid Earth 100, 423 (Jan, 1995). 2.J. M. Warren, N. Shimizu, Journal of Petrology 51

  3. Melting of a subduction-modified mantle source: A case study from the Archean Marda Volcanic Complex, central Yilgarn Craton, Western Australia

    NASA Astrophysics Data System (ADS)

    Morris, P. A.; Kirkland, C. L.

    2014-03-01

    Subduction processes on early earth are controversial, with some suggestions that tectonics did not operate until the earth cooled to a sufficient point around the Archean-Proterozoic boundary. One way of addressing this issue is to examine well-preserved successions of Archean supracrustal rocks. Here we discuss petrography, whole-rock chemical and isotopic data combined with zircon Hf isotopes from andesites, high-magnesium andesites (HMA), dacites, high-magnesium dacites (HMD), rhyolites and coeval felsic intrusive rocks of the c. 2730 Ma Marda Volcanic Complex (MVC) in the central Yilgarn Craton of Western Australia. We demonstrate that these rocks result from melting of a metasomatized mantle source, followed by fractional crystallization in a crustal magma chamber. Contamination of komatiite by Archean crust, to produce the Marda Volcanic Complex andesites, is not feasible, as most of these crustal sources are too radiogenic to act as viable contaminants. The ɛNd(2730) of MVC andesites can be produced by mixing 10% Narryer semi-pelite with komatiite, consistent with modelling using Hf isotopes, but to achieve the required trace element concentrations, the mixture needs to be melted by about 25%. The most likely scenario is the modification of a mantle wedge above a subducting plate, coeval with partial melting, producing volcanic rocks with subduction signatures and variable Mg, Cr and Ni contents. Subsequent fractionation of cognate phases can account for the chemistry of dacites and rhyolites.

  4. Stochastic melting of the marble cake mantle: evidence from local study of the East Pacific Rise at 12°50'N

    NASA Astrophysics Data System (ADS)

    Prinzhofer, Alain; Lewin, Eric; Allègre, C. J.

    1989-03-01

    Isotopes (Nd, Sr and Pb) and trace elements (REE, Ba, Sr, Rb) have been measured on a set of basaltic glasses from a restricted area (40 × 10 km) at 12°50'N on the East Pacific Rise. The huge variation of incompatible element concentrations (factor 70 for Ba concentrations), and the variable degrees of correlation between element concentrations cannot be explained by usual models of melting and fractional crystallization. A rough correlation between the Ce/Yb ratio and the isotopic ratios favors a "source effect" for the genesis of the glasses. We have developed a model including both partial melting process acting on a heterogeneous mantle source with two components (peridotites and pyroxenites; "marble cake mantle" of Allègre and Turcotte) and fractional crystallization. The purpose of this model is not to obtain values of the four parameters involved (degree of melting in the peridotites, in the pyroxenites, proportion of pyroxenites involved in the melting, degree of fractional crystallization) for each analyzed glass, but to model the whole set of glasses by stochastic genesis and sampling of liquids. We have used the stochastic procedure for the four controlled parameters, currently generating 10, 000 "samples". Our preferred model for this portion of the East Pacific Ridge is obtained with a degree of melting in the peridotites and in the pyroxenites varying uniformly from 6 to 20%, and from 6 to 50% respectively. The degree of mixing between liquids issued from the two sources varies from 0 to 100%, and the degree of fractional crystallization remains small, without noticeable effect on the concentrations, varying from 0 to 6%.

  5. Mantle migration of K-LILE-enriched melts/fluids in supra-subduction settings: evidence from the Finero Complex (Ivrea-Verbano Zone, Southern Alps)

    NASA Astrophysics Data System (ADS)

    Zanetti, A.; Mazzucchelli, M.; Giovanardi, T.; Tiepolo, M.; Vannucci, R.

    2011-12-01

    The Finero Complex is located in the northern sector of the Ivrea-Verbano Zone (Southern Alps). It is placed in contact with the Austro-Alpine terrains of the Sesia-Lanzo Zone by the Insubric line and consists of a dunitic-harzburgitic mantle unit, surrounded by a layered mafic-ultramafic intrusion. Unlike the central and southern sectors of the Ivrea-Verbano Zone, recent geochronological data suggest that metasomatic events of the Finero mantle unit, as well as the emplacement of the layered intrusion, occurred over a time span covering Middle Triassic to Lower Jurassic. A number of different geodynamic scenarios, among which aborted rifting processes and/or mantle plume activity, have been proposed to account for Lower Mesozoic melt-related events. However, the trace element and isotopic evidence points to the occurrence of large amount of crustal component in melts migrating through the mantle unit, which, consistently with regional structural features, has been proposed to be related to the development of roll-back subduction(s) during the early Mesozoic. New field, petrochemical and geochronological work has allowed to place further constraints on the geochemical affinity and evolution of the metasomatic melts/fluids affecting the mantle unit. Particular emphasis has been placed on the reconstruction of different migration stages recorded by decameter-wide dunites. In these bodies, an early migration stage produced dm-wide chromitite layers formed by chromite and orthopyroxene (Opx), with subordinate clinopyroxene (Cpx) and accessory amounts of amphibole (Amph), zircon and zirconolite. Successive migration events produced a lithologic series formed by, in order of crystallization: i) straight, cm-large websterites formed by Cpx-Opx-Amph-Phlogopite (Phl); ii) cm- to dm-wide bands with curved strike, characterized by segregation of Cpx-Phl-Amph-Opx; iii) dm-scale pockets filled by pegmatoidal Amph and Phlog; iv) finally, mm-thick veins with random strike and

  6. 186Os/188Os Isotopic Compositions of Peridotites: Constraints on Melt Depletion and Pt/Os Evolution of the Upper Mantle

    NASA Astrophysics Data System (ADS)

    Chatterjee, R. N.; Lassiter, J. C.

    2014-12-01

    Global correlations between Al2O3 and Pt/Os in mantle peridotites suggest that Pt behaves incompatibly relative to Os during partial melting [c.f., 1]. Because 190Pt decays to 186Os (t1/2 = 468 Ga), correlations between 186Os/188Os and peridotite fertility can be used to constrain the long-term Pt/Os evolution of the depleted mantle and the initial Pt/Os ratio of the primitive upper mantle (PUM). We examined 186Os/188Os in mantle peridotites from continental (Rio Grande Rift/Colorado Plateau) and oceanic (Lena Trough, Hawaiian Islands) settings that span a wide range in fertility (Al2O3 ~0.67-4.42 %) and 187Os/188Os ratios (0.1138-0.1305). The new data define a narrow range in 186Os/188Os (0.1198338 to 0.1198393, 2 SD~24 ppm), placing constraints on long-term Pt/Os variability in the DMM. 186Os/188Os is broadly correlated with indices of melt depletion including spinel Cr#, clinopyroxene Cr#, and clinopyroxene Yb content, consistent with the inferred relative compatibility of Pt and Os during partial melting. Extrapolation of the alumina-186Os/188Os trend to PUM alumina content (~4.5 wt% Al2O3; [2]) suggests a PUM 186Os/188Os of ~0.1198380±15, similar to the 186Os/188Os of H chondrites (~0.1198398±16; [3]). This 186Os/188Os value is consistent with a PUM Pt/Os of 1.8±0.3, similar to Pt/Os values measured in several classes of chondrites (Carbonaceous ~1.9±0.2, Ordinary ~2.0±0.3 and Enstatite ~1.9±0.2; [3]). Whereas ~84% of peridotites worldwide [excluding low-[Os] samples (<1 ppb Os) that may have been compromised by melt-rock reaction and/or weathering and alteration] with measured Pt/Os ratios have Pt/Os between 0.3 and 3.1 (the range permissible from 186Os/188Os variations for melt extraction from PUM at ~1.5 Ga), only ~36% fall between 1.3 and 2.2 (a narrower range consistent with an older ~4.5 Ga melt depletion age). This suggests that much of the observed Pt/Os variability in mantle peridotites is relatively recent. Close agreement between our inferred

  7. Subduction- vs- Intraplate-Type Melt Migration in the Alboran Lithospheric Mantle: Insights From the Tallante Xenoliths (Betic Cordillera, SE Spain)

    NASA Astrophysics Data System (ADS)

    Rampone, E.; Vissers, R. L.; Poggio, M.; Scambelluri, M.; Zanetti, A.

    2008-12-01

    The Alboran Sea region has been affected since the late Oligocene by widespread eruption of tholeiitic to calc-alkaline magmas followed by Late Neogene alkaline basalts. These magmatic episodes are related to Neogene lithospheric extension beneath the Alboran domain, as a consequence of slab roll-back. According to recent models, subduction of oceanic lithosphere caused continental-edge delamination of subcontinental lithosphere, associated with upwelling of plume-type mantle sources. The Alboran lithospheric mantle thus constitutes a unique setting to investigate the effects of subduction- and intraplate-type metasomatism. Here we present a microstructural and geochemical study of mantle xenoliths from the Cabezo Tallante Late Neogene alkaline volcanic center (SE Spain). These xenoliths record multiple episodes of reactive porous melt percolation, and melt entrapment, tracking their progressive extension-related uplift from P > 20 Kb to 7-10 Kb. This is documented by i) crystallization of undeformed olivine replacing pyroxene porphyroclasts, and unstrained opx overgrowing undeformed olivine and pyroxene porphyroclasts, in porphyroclastic spinel peridotites, ii) development of annealed equigranular structure, likely enhanced by heating during melt percolation, iii) crystallization of interstitial (plag±ol±opx) aggregates between mantle minerals in porphyroclastic and equigranular xenoliths. Cpx in equigranular peridotites have smooth trace element spectra characterized by slight LREE depletion; computed equilibrium liquids have a tholeiitic-transitional affinity. Diffuse melt percolation was followed by intrusion of melts with distinct chemical affinity. The first event is documented by the intrusion of cm-sized gabbronoritic veins, showing a fine-grained opx reaction rim against the host peridotite. Similar gabbronoritic lithotypes were previously documented and ascribed to slab-derived melts. A quite remarkable textural feature in these veins is the occurrence

  8. Complex metasomatism of lithospheric mantle by asthenosphere-derived melts: Evidence from peridotite xenoliths in Weichang at the northern margin of the North China Craton

    NASA Astrophysics Data System (ADS)

    Zou, Dongya; Zhang, Hongfu; Hu, Zhaochu; Santosh, M.

    2016-11-01

    The petrology, in situ analyses of major and trace elements as well as Sr isotopic compositions of spinel-facies lherzolite and harzburgite xenoliths from Weichang within the northern margin of the North China Craton (NCC) are reported for the first time in this study to evaluate the nature and evolution of the lithospheric mantle. These peridotite xenoliths display porphyroclastic texture and can be subdivided into two groups. Group I peridotites have slightly higher forsterite contents (Fo) (90.6-91.2) and 87Sr/86Sr ratios (0.7025-0.7043) in the cores than those in the rims (89.8-90.8; 0.7025-0.7038) of olivines and clinopyroxenes, respectively. The clinopyroxenes in these rocks exhibit uniform LREE-depleted patterns. These geochemical features suggest that the Group I peridotites were weakly metasomatized by recent asthenospheric melts. In contrast, Group II peridotites show sieve-texture and clear compositional zoning in minerals. The olivines and clinopyroxenes from these rocks have higher Fo (86.9-91.3) and 87Sr/86Sr ratios (0.7035-0.7049) in the cores than those in the rims (76.9-90.6; 0.7021-0.7046). The clinopyroxenes show three types of REE patterns: LREE-enriched, convex-upward and spoon-shaped. The LREE-enriched clinopyroxenes have the highest (La/Yb)N and lowest Ti/Eu and those with spoon-shaped REE patterns show an increase in LREE, Ba, Th and U contents from the cores to the rims. These features indicate that the Group II peridotites witnessed a high degree of refertilization by recent asthenosphere-derived silicate and carbonatite melts or their mixture. Compared with the data of the Mesozoic NCC lithospheric mantle, the heterogeneous and low 87Sr/86Sr ratios (0.7025-0.7049) in the LREE-depleted clinopyroxenes reveal that the ancient lithospheric mantle could have been modified by asthenospheric melts before the recent metasomatism event. We conclude that the lithospheric mantle beneath Weichang underwent multiple modifications through asthenosphere

  9. Melt-rock reaction in the asthenospheric mantle: Perspectives from high-order accurate numerical simulations in 2D and 3D

    NASA Astrophysics Data System (ADS)

    Tirupathi, S.; Schiemenz, A. R.; Liang, Y.; Parmentier, E.; Hesthaven, J.

    2013-12-01

    The style and mode of melt migration in the mantle are important to the interpretation of basalts erupted on the surface. Both grain-scale diffuse porous flow and channelized melt migration have been proposed. To better understand the mechanisms and consequences of melt migration in a heterogeneous mantle, we have undertaken a numerical study of reactive dissolution in an upwelling and viscously deformable mantle where solubility of pyroxene increases upwards. Our setup is similar to that described in [1], except we use a larger domain size in 2D and 3D and a new numerical method. To enable efficient simulations in 3D through parallel computing, we developed a high-order accurate numerical method for the magma dynamics problem using discontinuous Galerkin methods and constructed the problem using the numerical library deal.II [2]. Linear stability analyses of the reactive dissolution problem reveal three dynamically distinct regimes [3] and the simulations reported in this study were run in the stable regime and the unstable wave regime where small perturbations in porosity grows periodically. The wave regime is more relevant to melt migration beneath the mid-ocean ridges but computationally more challenging. Extending the 2D simulations in the stable regime in [1] to 3D using various combinations of sustained perturbations in porosity at the base of the upwelling column (which may result from a viened mantle), we show the geometry and distribution of dunite channel and high-porosity melt channels are highly correlated with inflow perturbation through superposition. Strong nonlinear interactions among compaction, dissolution, and upwelling give rise to porosity waves and high-porosity melt channels in the wave regime. These compaction-dissolution waves have well organized but time-dependent structures in the lower part of the simulation domain. High-porosity melt channels nucleate along nodal lines of the porosity waves, growing downwards. The wavelength scales

  10. Composition of garnet and clinopyroxene in peridotite xenoliths from the Grib kimberlite pipe, Arkhangelsk diamond province, Russia: Evidence for mantle metasomatism associated with kimberlite melts

    NASA Astrophysics Data System (ADS)

    Kargin, A. V.; Sazonova, L. V.; Nosova, A. A.; Tretyachenko, V. V.

    2016-10-01

    Here we present major and trace element data for garnet and clinopyroxene from mantle-derived peridotite xenoliths of the Grib kimberlite, the Arkhangelsk diamond province, Russia, and provide new insights into the metasomatic processes that occur within the subcontinental lithospheric mantle (SCLM) during the kimberlite generation and ascent. The mantle xenoliths examined in this study are both coarse and sheared garnet peridotites and consist of olivine, orthopyroxene, clinopyroxene, garnet with minor ilmenite, magnetite, and Cr-spinel. Based on garnet and clinopyroxene composition, two groups of peridotite are recognized. One group contains high-Ti, light rare earth elements (LREE) enriched garnets and low-Mg# clinopyroxenes with low (La/Sm)n (C1 chondrite-normalized) values. This mineral assemblage was in equilibrium with a high-temperature carbonate-silicate metasomatic agent, presumably, a protokimberlite melt. Pressure-temperature (P-T) estimates (T = 1220 °C and P = 70 kbar) suggest that this metasomatic event occurred at the base of the SCLM. Another group contains low-Ti garnet with normal to sinusoidal rare earth elements (REE) distribution patterns and high-Mg# clinopyroxenes with wide range of (La/Sm)n values. The geochemical equilibrium between garnet and clinopyroxene coupled with their REE composition indicates that peridotite mantle experienced metasomatic transformation by injection of a low-Ti (after crystallizations of the ilmenite megacrysts) kimberlite melt that subsequently percolated through a refractory mantle column. Peridotites of this group show a wide range of P-T estimates (T = 730-1070 °C and P = 22-44 kbar). It is suggested that evolution of a kimberlite magma from REE-enriched carbonate-bearing to carbonate-rich ultramafic silicate compositions with lower REE occurs during the ascent and interaction with a surrounding lithospheric mantle, and this process leads to metasomatic modification of the SCLM with formation of both high

  11. MORB melting processes beneath the southern Mid-Atlantic Ridge (40-55°S): a role for mantle plume-derived pyroxenite

    NASA Astrophysics Data System (ADS)

    le Roux, P. J.; le Roex, A. P.; Schilling, J.-G.

    2002-07-01

    Major and selected trace element abundances of MORB dredged from the moderately slow spreading southern MAR (40-55°S), in the vicinity of the Shona and Discovery mantle plumes, are used to constrain their melting conditions. All samples plot to high Fe8 relative to the Na8-Fe8 global trend, the high Fe8 of N-MORB in particular being anomalous. Shona-influenced MORB plot at lower Na8 and towards higher Fe8 values along the global trend than associated N-MORB, consistent with deeper initiation of melting. Discovery-influenced MORB extend to higher Na8 and lower Fe8 compositions than associated N-MORB. Anomalous Na8 and Fe8, and Si8, Ca8 and Al8, of most Discovery-influenced MORB are interpreted to reflect increased modal clinopyroxene in their source regions. Calculated Fmean range from 15-17% for N-MORB, 16-19% for Shona-influenced MORB, and 11-18.5% for selected, least-anomalous Discovery(+LOMU)-influenced MORB. Calculated Pinitial are fairly constant for N-MORB ( 18+/-2 kbar), slightly greater for Shona-influenced MORB ( 20+/-3 kbar), whereas for least anomalous Discovery(+LOMU)-influenced MORB calculated Pinitial group around 16+/-2 and 22 kbar. Calculated crustal thicknesses range mostly between 6-9 km, but for Shona- and a single Discovery(+LOMU)-influenced MORB location the range is 8-12 km. Anomalous compositions of some Discovery-influenced MORB are interpreted to reflect variable melting of pyroxenite veins, initially formed as small-degree (2-3%) melts of garnet lherzolite within the upwelling off-axis Discovery mantle plume, and subsequently entrained in the ambient spinel lherzolite beneath the ridge axis. Because of lower solidus temperatures relative to ambient spinel lherzolite, initial melting preferentially consumed the low abundance (2.5-3.5%) pyroxenite veins. Variable mixing between vein- and host mantle-derived melts led to the range of Discovery(+LOMU)-influenced MORB compositions. In the vicinity of transform off-sets, melting is restricted

  12. The Deep Mantle Volatile Cycle Revealed in Superdeep Diamonds and their Mineral Inclusions

    NASA Astrophysics Data System (ADS)

    Walter, Michael; Thomson, Andrew; Frost, Jennifer; Bulanova, Galina; Smith, Chris; Kohn, Simon; Burnham, Antony

    2013-04-01

    Diamonds crystallize in the mantle primarily as a consequence of fluid or melt metasomatism. In doing so they sample the fluid-melt-solid equilibria directly by incorporation of carbon and its isotopic flavours, and by entrapping other phases as they grow. Superdeep diamonds from the transition zone and lower mantle provide evidence for crystallization from melts derived from subducted materials [1, 2]. The presence of deeply subducted volatile components such as carbon and water are important because they lower the solidus of subducted materials. The source of carbon may ultimately be via deposition of biogenic or abiogenic carbon in subducted crust, and water may become available via dehydration of high-pressure hydrous phases in the slab (e.g. superhydrous B, Phase D) [3]. Foundering of slabs around 700 km due to density inversion and thermalization with surrounding mantle leads to the generation of low-degree, volatile-charged melts. Melts from subducted oceanic crust may be carbonated, and diamond crystallization occurs as a consequence of 'redox freezing' when the oxidized slab melts react with reducing mantle rocks [4]. Reaction of slab melts with mantle peridotite may precipitate phases such as Ca-perovskite, Mg-perovskite, majorite and ferropericlase. Here we will survey evidence from the chemistry of superdeep mineral inclusions for a record of this deep mantle reactive transport process, and speculate on the role of deep mantle volatiles. 1. Bulanova, G.P., et al., Contributions to Mineralogy and Petrology, 2010. 160: p. 489-510. 2. Walter, M.J., et al., Nature, 2008. 454: p. 622-U30. 3. Harte, B., Mineralogical Magazine, 2010. 74: p. 189-215. 4. Rohrbach, A. and M.W. Schmidt, Nature, 2011. 472: p. 209-212.

  13. Evidence from Olivine-Hosted Melt Inclusions that the Martian Mantle has a Chondritic D/H Ratio and that Some Young Basalts have Assimilated Old Crust

    NASA Technical Reports Server (NTRS)

    Usui, Tomohiro; Alexander, O'D.; Wang, J.; Simon, J. I.; Jones, J. H.

    2012-01-01

    Magmatic degassing of volatile elements affects the climate and near-surface environment of Mars. Telescopic and meteorite studies have revealed that the Martian atmosphere and near-surface materials have D/H ratios 5-6 times terrestrial values [e.g., 1, 2]. Such high D/H ratios are interpreted to result from the preferential loss of H relative to heavier D from the Martian atmosphere, assuming that the original Martian water inventory had a D/H ratio similar to terrestrial values and to H in primitive meteorites [e.g., 1, 3]. However, the primordial Martian D/H ratio has, until now, not been well constrained. The uncertainty over the Martian primordial D/H ratio has arisen both from the scarcity of primitive Martian meteorites and as a result of contamination by terrestrial and, perhaps, Martian surface waters that obscure the signature of the Martian mantle. This study reports a comprehensive dataset of magmatic volatiles and D/H ratios in Martian primary magmas based on low-contamination, in situ ion microprobe analyses of olivine-hosted melt inclusions from both depleted [Yamato 980459 (Y98)] and enriched [Larkman Nunatak 06319 (LAR06)] Martian basaltic meteorites. Analyses of these primitive melts provide definitive evidence that the Martian mantle has retained a primordial D/H ratio and that young Martian basalts have assimilated old Martian crust.

  14. Protracted, coeval crust and mantle melting during Variscan late-orogenic evolution: U-Pb dating in the eastern French Massif Central

    NASA Astrophysics Data System (ADS)

    Laurent, Oscar; Couzinié, Simon; Zeh, Armin; Vanderhaeghe, Olivier; Moyen, Jean-François; Villaros, Arnaud; Gardien, Véronique; Chelle-Michou, Cyril

    2017-03-01

    The late stages of the Variscan orogeny are characterized by middle to lower crustal melting and intrusion of voluminous granitoids throughout the belt, which makes it akin to "hot" orogens. These processes resulted in the development of large granite-migmatite complexes, the largest of which being the 305-300-Ma-old Velay dome in the eastern French Massif Central (FMC). This area also hosts a wide range of late-Variscan plutonic rocks that can be subdivided into four groups: (i) cordierite-bearing peraluminous granites (CPG); (ii) muscovite-bearing peraluminous granites (MPG); (iii) K-feldspar porphyritic, calc-alkaline granitoids (KCG) and (iv) Mg-K-rich (monzo)diorites and lamprophyres ("vaugnerites"). New results of LA-SF-ICP-MS U-Pb zircon and monazite dating on 33 samples from all groups indicate that both granites and mafic rocks emplaced together over a long period of 40 million years throughout the Carboniferous, as shown by intrusion ages between 337.4 ± 1.0 and 298.9 ± 1.8 Ma for the granitoids, and between 335.7 ± 2.1 and 299.1 ± 1.3 Ma for the vaugnerites. Low zircon saturation temperatures and abundant inherited zircons with predominant late Ediacaran to early Cambrian ages indicate that the CPG and MPG formed through muscovite or biotite dehydration melting of ortho- and paragneisses from the Lower Gneiss Unit. The KCG and vaugnerites contain very few inherited zircons, if any, suggesting higher magma temperatures and consistent with a metasomatized lithospheric mantle source for the vaugnerites. The KCG can be explained by interactions between the CPG/MPG and the vaugnerites, or extensive differentiation of the latter. The new dataset provides clear evidence that the eastern FMC was affected by a long-lived magmatic episode characterized by coeval melting of both crustal and mantle sources. This feature is suggested here to result from a lithospheric-scale thermal anomaly, triggered by the removal of the lithospheric mantle root. The spatial

  15. Protracted, coeval crust and mantle melting during Variscan late-orogenic evolution: U-Pb dating in the eastern French Massif Central

    NASA Astrophysics Data System (ADS)

    Laurent, Oscar; Couzinié, Simon; Zeh, Armin; Vanderhaeghe, Olivier; Moyen, Jean-François; Villaros, Arnaud; Gardien, Véronique; Chelle-Michou, Cyril

    2017-01-01

    The late stages of the Variscan orogeny are characterized by middle to lower crustal melting and intrusion of voluminous granitoids throughout the belt, which makes it akin to "hot" orogens. These processes resulted in the development of large granite-migmatite complexes, the largest of which being the 305-300-Ma-old Velay dome in the eastern French Massif Central (FMC). This area also hosts a wide range of late-Variscan plutonic rocks that can be subdivided into four groups: (i) cordierite-bearing peraluminous granites (CPG); (ii) muscovite-bearing peraluminous granites (MPG); (iii) K-feldspar porphyritic, calc-alkaline granitoids (KCG) and (iv) Mg-K-rich (monzo)diorites and lamprophyres ("vaugnerites"). New results of LA-SF-ICP-MS U-Pb zircon and monazite dating on 33 samples from all groups indicate that both granites and mafic rocks emplaced together over a long period of 40 million years throughout the Carboniferous, as shown by intrusion ages between 337.4 ± 1.0 and 298.9 ± 1.8 Ma for the granitoids, and between 335.7 ± 2.1 and 299.1 ± 1.3 Ma for the vaugnerites. Low zircon saturation temperatures and abundant inherited zircons with predominant late Ediacaran to early Cambrian ages indicate that the CPG and MPG formed through muscovite or biotite dehydration melting of ortho- and paragneisses from the Lower Gneiss Unit. The KCG and vaugnerites contain very few inherited zircons, if any, suggesting higher magma temperatures and consistent with a metasomatized lithospheric mantle source for the vaugnerites. The KCG can be explained by interactions between the CPG/MPG and the vaugnerites, or extensive differentiation of the latter. The new dataset provides clear evidence that the eastern FMC was affected by a long-lived magmatic episode characterized by coeval melting of both crustal and mantle sources. This feature is suggested here to result from a lithospheric-scale thermal anomaly, triggered by the removal of the lithospheric mantle root. The spatial

  16. Asteroids and Archaean crustal evolution: Tests of possible genetic links between major mantle/crust melting events and clustered extraterrestrial bombardments

    NASA Technical Reports Server (NTRS)

    Glikson, A. Y.

    1992-01-01

    Since the oldest intact terrestrial rocks of ca. 4.0 Ga and oldest zircon xenocrysts of ca. 4.3 Ga measured to date overlap with the lunar late heavy bombardment, the early Precambrian record requires close reexamination vis a vis the effects of megaimpacts. The identification of microtektite-bearing horizons containing spinals of chondritic chemistry and Ir anomalies in 3.5-3.4-Ga greenstone belts provides the first direct evidence for large-scale Archaean impacts. The Archaean crustal record contains evidence for several major greenstone-granite-forming episodes where deep upwelling and adiabatic fusion of the mantle was accompanied by contemporaneous crustal anatexis. Isotopic age studies suggest evidence for principal age clusters about 3.5, 3.0, and 2.7 (+/- 0.8) Ga, relics of a ca. 3.8-Ga event, and several less well defined episodes. These peak events were accompanied and followed by protracted thermal fluctuations in intracrustal high-grade metamorphic zones. Interpretations of these events in terms of internal dynamics of the Earth are difficult to reconcile with the thermal behavior of silicate rheologies in a continuously convecting mantle regime. A triggering of these episodes by mantle rebound response to intermittent extraterrestrial asteroid impacts is supported by (1) identification of major Archaean impacts from microtektite and distal ejecta horizons marked by Ir anomalies; (2) geochemical and experimental evidence for mantle upwelling, possibly from levels as deep as the transition zone; and (3) catastrophic adiabatic melting required to generate peridotitic komatites. Episodic differentiation/accretion growth of sial consequent on these events is capable of resolving the volume problem that arises from comparisons between modern continental crust and the estimated sial produced by continuous two-stage mantle melting processes. The volume problem is exacerbated by projected high accretion rates under Archaean geotherms. It is suggested that

  17. Melting relations of multicomponent carbonate MgCO3-FeCO3-CaCO3-Na2CO3 system at 12-26 GPa: application to deeper mantle diamond formation

    NASA Astrophysics Data System (ADS)

    Spivak, Anna; Solopova, Natalia; Dubrovinsky, Leonid; Litvin, Yuriy

    2015-11-01

    Carbonatic components of parental melts of the deeper mantle diamonds are inferred from their primary inclusions of (Mg, Fe, Ca, Na)-carbonate minerals trapped at PT conditions of the Earth's transition zone and lower mantle. PT phase diagrams of MgCO3-FeCO3-CaCO3-Na2CO3 system and its ternary MgCO3-FeCO3-Na2CO3 boundary join were studied at pressures between 12 and 24 GPa and high temperatures. Experimental data point to eutectic solidus phase relations and indicate liquidus boundaries for completely miscible (Mg, Fe, Ca, Na)- and (Mg, Fe, Ca)-carbonate melts. PT fields for partial carbonate melts associated with (Mg, Fe)-, (Ca, Fe, Na)-, and (Na2Ca, Na2Fe)-carbonate solid solution phases are determined. Effective nucleation and mass crystallization of deeper mantle diamonds are realized in multicomponent (Mg, Fe, Ca, Na)-carbonatite-carbon melts at 18 and 26 GPa. The multicomponent carbonate systems were melted at temperatures that are lower than the geothermal ones. This gives an evidence for generation of diamond-parental carbonatite melts and formation of diamonds at the PT conditions of transition zone and lower mantle.

  18. Transformation of mantle to lower crust: melt-rock reaction processes in peridotites from Atlantis Massif, 30°N, Mid-Atlantic ridge

    NASA Astrophysics Data System (ADS)

    von der Handt, A.; Hellebrand, E.

    2010-12-01

    Very olivine-rich plutonic lithologies (> 70% olivine), in the following referred to as olivine-rich troctolites (ORT), have now been identified at several locations dominantly along slow-spreading mid-ocean ridges. Their compositions (high Mg#, Cr-rich cpx) and textures (rounded olivine enclosed by poikilitic plagioclase and less abundant clinopyroxene) are consistent with the conventional definition of a primitive cumulate produced by crystallization of MORB-type parental melts. Particularly well preserved ORT were recovered at Atlantis Massif, 30°N, Mid-Atlantic Ridge, an oceanic core complex drilled during IODP Legs 304/305. In both drill holes at Site U1309, ORT comprise around 50 m of core length that is otherwise dominantly composed of gabbroic lithologies. Several studies on ORT from Atlantis Massif concluded that they instead represent reactively overprinted residual mantle peridotites, transformed in several steps by large melt influx at shallow levels (1;2;3;4). Common to all models is an early stage of dunitization and disaggregation of mantle peridotite that occurs at high melt-rock ratios followed by concomitant precipitation of plagioclase and clinopyroxene in the pore space. We performed a detailed petrologic investigation of mantle peridotites drilled at Atlantis Massif. Mantle peridotites represent a subordinate lithology in IODP drill holes U1309B and D (<0.5%) and occur as screens surrounded by troctolites and olivine gabbros. The unmodified protolith at Atlantis Massif is probably best represented by a highly depleted harzburgite (cpx: Na2O 0.04 wt%, TiO2 0.05 wt%; forsterite 0.91, NiO 0.39 wt%) that occurs in an interval farthest away from any magmatic contacts. All other peridotite samples show replacive mineral textures and compositional variation in agreement with prolonged melt-rock reaction. We interpret these mineral changes as a glimpse into the early stages in the formation of ORT and use our observations to amend existing models of

  19. Numerical modeling of convective erosion and peridotite-melt interaction in big mantle wedge: Implications for the destruction of the North China Craton

    NASA Astrophysics Data System (ADS)

    He, Lijuan

    2014-04-01

    The deep subduction of the Pacific Plate underneath East Asia is thought to have played a key role in the destruction of the North China Craton (NCC). To test this hypothesis, this paper presents a new 2-D model that includes an initial stable equilibrated craton, the formation of a big mantle wedge (BMW), and erosion by vigorous mantle convection. The model shows that subduction alone cannot thin the cold solid craton, but it can form a low-viscosity BMW. The amount of convective erosion is directly proportional to viscosity within the BMW (η0bmw), and the rheological boundary layer thins linearly with decreasing log10(η0bmw), thereby contributing to an increase in heat flow at the lithospheric base. This model also differs from previous modeling in that the increase in heat flow decays linearly with t1/2, meaning that the overall thinning closely follows a natural log relationship over time. Nevertheless, convection alone can only cause a limited thinning due to a minor increase in basal heat flow. The lowering of melting temperature by peridotite-melt interaction can accelerate thinning during the early stages of this convection. The two combined actions can thin the craton significantly over tens of Myr. This modeling, combined with magmatism and heat flow data, indicates that the NCC evolution has involved four distinct stages: modification in the Jurassic by Pacific Plate subduction and BMW formation, destruction during the Early Cretaceous under combined convective erosion and peridotite-melt interaction, extension in the Late Cretaceous, and cooling since the late Cenozoic.

  20. Nd, Sr and Pb isotopic composition of metasomatised xenoliths from the backarc Patagonian Mantle Wedge: Insights into the origin of the uprising melts

    NASA Astrophysics Data System (ADS)

    Zanetti, Alberto; Mazzucchelli, Maurizio; Hemond, Christope; Cipriani, Anna; Bertotto, Gustavo W.; Cingolani, Carlos; Vannucci, Riccardo

    2010-05-01

    Information about the geochemical composition of metasomatic melts migrating through the Patagonian mantle wedge is provided by the ultramafic xenoliths occurrence of Tres Lagos (TL; lat. 49.13°S, long. 71.18°W), Argentina. Such a locality is placed at the eastern border of the Meseta de la Muerte backarc basaltic plateau, where a post-plateau volcanic diatreme contains mantle xenoliths in both pyroclastites and lavas. Its latitude corresponds with the Northern limit of the Austral Volcanic Arc (AVZ), which is separated from the Southern Volcanic Zone (SVZ) by a gap in the arc magmatism ranging between 49° and 46°30' latitude S. The analysed xenoliths have been distinguished into two groups (Group 1 & 2). Group 1 consists of lherzolites and harzburgites, whereas Group 2 is formed by harzburgites. The texture of the Group 1 lherzolites varies from protogranular to granoblastic to porphyroblastic, whereas Group 1 harzburgites have always granoblastic texture. Group 2 harzburgites have granular texture, which may change to porphyroblastic owing to the random concentration of large olivine and orthopyroxene crystals. The clinopyroxenes (Cpx) from Group 1 lherzolites have PM-normalised REE patterns ranging from LREE-depleted (LaN/SmN= 0.24-0.37), to LREE-enriched (LaN/YbN up to 4.08) and spoon-shaped: the latter have minimum at Pr and Pr-Yb concentrations similar to those shown by the LREE-depleted Cpx. The Cpx from Group 1 harzburgites have lower REE concentrations with respect to the lherzolite ones and their REE patterns vary from HREE-enriched, steadily fractionated, (LaN/YbN = 0.21-0.35, Ybn ~ 1-2) to spoon-shaped (LaN/SmN = 2.81; SmN/YbN = 0.89; YbN ~ 3. The Cpx from the Group 2 harzburgites have convex-upward (LaN/SmN = 0.31; SmN/YbN = 1.50) to LREE-enriched (LaN/YbN = 2.94) patterns. The Sr, Nd and Pb isotopic compositions of the Group 1 clinopyroxenes form arrays spanning from DM to the field delimited by the TL basaltic lavas, pointing to EMI end

  1. Anomalous density and elastic properties of basalt at high pressure: Reevaluating of the effect of melt fraction on seismic velocity in the Earth's crust and upper mantle

    NASA Astrophysics Data System (ADS)

    Clark, Alisha N.; Lesher, Charles E.; Jacobsen, Steven D.; Wang, Yanbin

    2016-06-01

    Independent measurements of the volumetric and elastic properties of Columbia River basalt glass were made up to 5.5 GPa by high-pressure X-ray microtomography and GHz-ultrasonic interferometry, respectively. The Columbia River basalt displays P and S wave velocity minima at 4.5 and 5 GPa, respectively, violating Birch's law. These data constrain the pressure dependence of the density and elastic moduli at high pressure, which cannot be modeled through usual equations of state nor determined by stepwise integrating the bulk sound velocity as is common practice. We propose a systematic variation in compression behavior of silicate glasses that is dependent on the degree of polymerization and arises from the flexibility of the aluminosilicate network. This behavior likely persists into the liquid state for basaltic melts resulting in weak pressure dependence for P wave velocities perhaps to depths of the transition zone. Modeling the effect of partial melt on P wave velocity reductions suggests that melt fraction determined by seismic velocity variations may be significantly overestimated in the crust and upper mantle.

  2. Unravelling the effects of melt depletion and secondary infiltration on mantle Re-Os isotopes beneath the French Massif Central

    NASA Astrophysics Data System (ADS)

    Harvey, J.; Gannoun, A.; Burton, K. W.; Schiano, P.; Rogers, N. W.; Alard, O.

    2010-01-01

    Spinel lherzolite xenoliths from Mont Briançon, French Massif Central, retain evidence for multiple episodes of melt depletion and melt/fluid infiltration (metasomatism). Evidence for primary melt depletion is still preserved in the co-variation of bulk-rock major elements (MgO 38.7-46.1 wt.%; CaO 0.9-3.6 wt.%), and many samples yield unradiogenic bulk-rock Os isotope ratios ( 187Os/ 188Os = 0.11541-0.12626). However, many individual xenoliths contain interstitial glasses and melt inclusions that are not in equilibrium with the major primary minerals. Incompatible trace element mass balance calculations demonstrate that metasomatic components comprise a significant proportion of the bulk-rock budget for these elements in some rocks, ranging to as much as 25% of Nd and 40% of Sr Critically, for Re-Os geochronology, melt/fluid infiltration is accompanied by the mobilisation of sulfide. Consequently, bulk-rock isotope measurements, whether using lithophile (e.g. Rb-Sr, Sm-Nd) or siderophile (Re-Os) based isotope systems, may only yield a perturbed and/or homogenised average of these multiple events. Osmium mass balance calculations demonstrate that bulk-rock Os in peridotite is dominated by contributions from two populations of sulfide grain: (i) interstitial, metasomatic sulfide with low [Os] and radiogenic 187Os/ 188Os, and (ii) primary sulfides with high [Os] and unradiogenic 187Os/ 188Os, which have been preserved within host silicate grains and shielded from interaction with transient melts and fluid. The latter can account for >97% of bulk-rock Os and preserve geochronological information of the melt from which they originally precipitated as an immiscible liquid. The Re-depletion model ages of individual primary sulfide grains preserve evidence for melt depletion beneath the Massif Central from at least 1.8 Gyr ago despite the more recent metasomatic event(s).

  3. Constraining late stage melt-peridotite interaction in the lithospheric mantle of southern Ethiopia: evidence from lithium elemental and isotopic compositions

    NASA Astrophysics Data System (ADS)

    Alemayehu, Melesse; Zhang, Hong-Fu; Seitz, Hans-Michael

    2017-02-01

    Lithium (Li) elemental and isotopic compositions for mineral separates of coexisting olivine, orthopyroxene and clinopyroxene of mantle xenoliths from the Quaternary volcanic rocks of southern Ethiopian rift (Dillo and Megado) reveal the influence of late stage melt-peridotite interaction on the early depleted and variably metasomatized lithospheric mantle. Two types of lherzolites are reported (LREE-depleted La/Sm(N) = 0.11-0.37 × Cl and LREE-enriched, La/Sm(N) = 1.88-15.72 × Cl). The depleted lherzolites have variable range in Li concentration (olivine: 2.1-5.4 ppm; opx: 1.1-2.3 ppm; cpx: 1.0-1.8 ppm) and in Li isotopic composition (δ7Li in olivine: -9.4 to 1.5‰; in opx: -4.5 to 3.6‰; in cpx: -17.0 to 4.8‰), indicating strong disequilibrium in Li partitioning and Li isotope fractionation between samples. The enriched lherzolites have limited range in both Li abundances (olivine: 2.7-3.0 ppm; opx: 1.1-3.1 ppm; cpx: 1.1-2.3 ppm) and Li isotopic compositions (δ7Li in olivine: -1.3 to +1.3‰; in opx: -2.0 to +5.0‰; in cpx: -7.5 to +4.8‰), suggest that the earlier metasomatic event which lead to LREE enrichment could also homogenize the Li contents and its isotopes. The enriched harzburgite and clinopyroxenite minerals show limited variation in Li abundances and variable Li isotopic compositions. The Li enrichments of olivine and clinopyroxene correlate neither with the incompatible trace element enrichment nor with the Sr-Nd isotopic compositions of clinopyroxene. These observations indicate that the metasomatic events which are responsible for the LREE enrichment and for the Li addition are distinct, whereby the LREE-enrichment pre-dates the influx of Li. The presence of large Li isotopic disequilibria within and between minerals of depleted and enriched peridotites suggest that the lithospheric mantle beneath the southern Ethiopian rift has experienced recent melt-peridotite interaction. Thus, the Li data set reported in this study offer new

  4. Crustal melting during subduction at mantle depth: anatomy of near-UHP nanogranites (Orlica-Śnieżnik Dome, Bohemian Massif)

    NASA Astrophysics Data System (ADS)

    Ferrero, Silvio; Ziemann, Martin; Walczak, Katarzyna; Wunder, Bernd; O'Brien, Patrick J.; Hecht, Lutz

    2015-04-01

    Small volumes (≤ 50µm) of hydrous melt were trapped as primary inclusions in peritectic garnets during partial melting of metagranitoids from the Orlica-Śnieżnik Dome (Bohemian Massif) at mantle depth [1]. Detailed microstructural/microchemical investigation confirmed the occurrence of a granitic assemblage (biotite+feldspars+quartz) in every investigated inclusion, i.e they are nanogranites [2]. MicroRaman mapping of unexposed inclusions showed the occurrence of residual, H2O-rich glass in interstitial position. Despite the oddity of this finding within a classic regional HP/HT terrain, an incomplete crystallization of the melt inclusions (MI) is consistent with the (relatively) rapid exhumation of the Orlica-Śnieżnik Dome proposed by some authors [e.g. 3]. Moreover glassy and partially crystallized MI have been already reported in lower-P (<1 GPa) migmatites [4]. MicroRaman investigation also showed the possible presence of kumdykolite, a high-temperature polymorph of albite reported in UHP rocks from the Kokchetav Massif as well as the Bohemian massif ([5] and references therein). Experimental re-homogenization of nanogranites was achieved using a piston cylinder apparatus at 2.7 GPa and 875°C under dry conditions, in order to investigate melt composition and H2O content with in situ techniques. The trapped melt is granitic, hydrous (6 wt% H2O) and metaluminous (ASI=1.03), and it is similar to those produced experimentally from crustal lithologies at mantle conditions. Re-homogenization conditions are consistent with the results of geothermobarometric calculations on the host rock, suggesting that no H2O loss occurred during exhumation - this would have caused a shift of the inclusion melting T toward higher values. Coupled with the absence of H2O-loss microstructural evidence, e.g. decrepitation cracks and/or vesciculation [4] in re-homogenized nanogranites, this evidence suggests that the nanogranites still preserves the original H2O content of the

  5. Multiple sulfur isotope composition of oxidized Samoan melts and the implications of a sulfur isotope 'mantle array' in chemical geodynamics

    NASA Astrophysics Data System (ADS)

    Labidi, J.; Cartigny, P.; Jackson, M. G.

    2015-05-01

    To better address how subducted protoliths drive the Earth's mantle sulfur isotope heterogeneity, we report new data for sulfur (S) and copper (Cu) abundances, S speciation and multiple S isotopic compositions (32S, 33S, 34S, 36S) in 15 fresh submarine basaltic glasses from the Samoan archipelago, which defines the enriched-mantle-2 (EM2) endmember. Bulk S abundances vary between 835 and 2279 ppm. About 17 ± 11% of sulfur is oxidized (S6+) but displays no consistent trend with bulk S abundance or any other geochemical tracer. The S isotope composition of both dissolved sulfide and sulfate yield homogeneous Δ33S and Δ36S values, within error of Canyon Diablo Troilite (CDT). In contrast, δ34S values are variable, ranging between +0.11 and +2.79‰ (±0.12‰ 1σ) for reduced sulfur, whereas oxidized sulfur values vary between +4.19 and +9.71‰ (±0.80‰, 1σ). Importantly, δ34S of the reduced S pool correlates with the 87Sr/86Sr ratios of the glasses, in a manner similar to that previously reported for South-Atlantic MORB, extending the trend to δ34S values up to + 2.79 ± 0.04 ‰, the highest value reported for undegassed oceanic basalts. As for EM-1 basalts from the South Atlantic ridge, the linear δ34S-87Sr/86Sr trend requires the EM-2 endmember to be relatively S-rich, and only sediments can account for these isotopic characteristics. While many authors argue that both the EM-1 and EM-2 mantle components record subduction of various protoliths (e.g. upper or lower continental crust, lithospheric mantle versus intra-metasomatized mantle, or others), it is proposed here that they primarily reflect sediment recycling. Their distinct Pb isotope variation can be accounted for by varying the proportion of S-poor recycled oceanic crust in the source of mantle plumes.

  6. Understanding magma formation and mantle conditions in the Lassen segment of the Cascade Arc: Insights from volatile contents of olivine-hosted melt inclusions

    NASA Astrophysics Data System (ADS)

    Walowski, K. J.; Rasmussen, D. J.; Wallace, P. J.; Clynne, M. A.

    2012-12-01

    The Cascade Arc is associated with subduction of some of the youngest and consequently warmest oceanic crust on Earth. In the southernmost portion of the arc, from Mt. Shasta to Lassen Peak, volcanism is a result of the oblique subduction of the deformed Gorda plate, and the magmas produced are some of the most compositionally diverse of the entire arc. This compositional variability along with an abundance of primitive mafic magmas make it an ideal location to investigate the influences of a young subducting slab and mantle heterogeneities on arc volcanism. Previous geochemical studies of cinder cones in the Lassen region have shown that calc-alkaline mafic lavas in the forearc generally show evidence for a greater amount of a subducted slab component (e.g., higher Sr/P and Ba/Nb), whereas lavas in the region behind the axis of the arc show much less of a slab component (Borg et al., 1997, Can. Min.). These data suggest a change in fluid input across the arc and also indicate a change in mantle source composition from MORB-source-like peridotite beneath the forearc to OIB-source-like peridotite in the rear arc. We have analyzed volatile, major and trace elements in olivine-hosted melt inclusions (Fo84-90) from the tephra of six cinder cones that span a range of Sr/P values. The maximum volatile contents for inclusions from each cone (1.2 - 3.6 wt% H2O, ≤1500 wt% CO2, 100-650 ppm Cl, and 900-1800 ppm S) are similar to those found in melt inclusions from the central Oregon segment of the Cascades (Ruscitto et al., 2010, EPSL). The values overlap with data for other arcs worldwide but are lower, on average, than most other arcs, consistent with the young subducted plate age, which likely results in substantial dehydration beneath the forearc region. Key ratios such as H2O/Ce and Cl/Nb correlate positively with Sr/P, indicating coupling between volatile and trace element enrichment related to slab dehydration beneath the arc. Slab surface temperatures have also been

  7. Porosity of the melting zone and variations in the solid mantle upwelling rate beneath Hawaii: Inferences from {sup 238}U-{sup 230}Th-{sup 226}Ra and {sup 235}U-{sup 231}Pa disequilibria

    SciTech Connect

    Sims, K.W.W.; DePaolo, D.J.; Murrell, M.T.; Baldridge, W.S.; Goldstein, S.; Clague, D.; Jull, M.

    1999-12-01

    Measurements of {sup 238}U-{sup 230}Th-{sup 226}Ra and {sup 235}U-{sup 231}Pa disequilibria in a suite of tholeiitic-to-basanitic lavas provide estimates of porosity, solid mantle upwelling rate and melt transport times beneath Hawaii. The observation that ({sup 230}Th/{sup 238}U) {gt} 1 indicates that garnet is required as a residual phase in the magma sources for all of the lavas. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Hawaiian basalts. For chromatographic porous flow, the calculated maximum porosity in the melting zone ranges from 0.3--3% for tholeiites and 0.1--1% for alkali basalts and basanites, and solid mantle upwelling rates range from 40 to 100 cm/yr for tholeiites and from 1 to 3 cm/yr for basanites. For dynamic melting, the escape or threshold porosity is 0.5--2% for tholeiites and 0.1--0.8% for alkali basalts and basanites, and solid mantle upwelling rates range from 10 to 30 cm/yr for tholeiites and from 0.1 to 1 cm/yr for basanites. Assuming a constant melt productivity, calculated total melt fractions range from 15% for the tholeiitic basalts to 3% for alkali basalts and basanites.

  8. The basalt-high magnesium andesite association formed by multi-stage partial melting of a heterogeneous source mantle: Evidence from Hirado-Seto, Northwest Kyushu, Southwest Japan

    NASA Astrophysics Data System (ADS)

    Mashima, Hidehisa

    2009-10-01

    An association of basalts and high magnesium andesites (HMAs), erupted at 7 Ma after the opening of the Sea of Japan, exposed at Hirado-Seto in northwest Kyushu, southwest Japan. The rocks are aphyric and are characterized by enrichments in incompatible trace elements similar to those seen for oceanic island basalts, although the HMAs show a weak negative Nb anomaly. High MgO, Ni and low FeO*/MgO indicate that the Hirado-Seto rocks were originally primitive magmas. They do not show a positive correlation between K 2O/La and SiO 2, or between Pb/La and SiO 2, indicating that hydrous components derived from a subducting slab did not play a significant role in the genesis of the Hirado-Seto basalt-HMA magmas. Alternatively, the normative olivine-quartz-[Jd + CaTs] compositions indicate that the Hirado-Seto basalt-HMA magmas were formed by multi-stage partial melting of the source mantle at pressures ranging from 1 to 0.5 GPa along the 1300 °C mantle adiabat, assuming anhydrous conditions. Basalt magmas separated from the source mantle at 1 GPa. HMA magmas separated at 0.5 GPa. A weak negative anomaly for Nb in HMAs can be explained by precipitation of Ti-P oxides during their ascent under high fO 2 condition. Thinning of the Hirado-Seto lithosphere caused by transtensional strain during the opening of the Sea of Japan would have enabled separation of HMA magmas at unusually low pressures.

  9. Eocene melting of Precambrian lithospheric mantle: Analcime-bearing volcanic rocks from the Challis-Kamloops belt of south central British Columbia

    NASA Astrophysics Data System (ADS)

    Dostal, J.; Breitsprecher, K.; Church, B. N.; Thorkelson, D.; Hamilton, T. S.

    2003-08-01

    Potassic silica-undersaturated mafic volcanic rocks form a minor portion of the predominantly calc-alkaline Eocene Challis-Kamloops volcanic belt, which extends from the northwestern United States into central British Columbia (Canada). Their major occurrence is in the Penticton Group in south central British Columbia, where they reach a thickness of up to 500 m and form the northwestern edge of the Montana alkaline province. These analcime-bearing rocks (˜53-52 Ma old) are typically rhomb porphyries of ternary feldspar (An 28Ab 52Or 20). Additional phenocryst phases include clinopyroxene, analcime, phlogopite and rare olivine. The rocks are characterized by high total alkalis, particularly K 2O (>4.5 wt%) as well as by a distinct enrichment of large-ion lithophile elements versus heavy rare-earth elements and high-field-strength elements. They have unusual isotopic compositions compared to most other rocks of the Challis-Kamloops belt, particularly high negative ɛNd values and elevated but relatively uniform initial 87Sr/ 86Sr ratios (˜0.7065). The potassic silica-undersaturated rocks overlie Precambrian crust and lithosphere and were at least in part derived from ancient metasomatized subcontinental mantle lithosphere, which was modified in a Precambrian subduction setting. The alkaline rocks of the Challis-Kamloops belt are related to a slab-window environment. In particular, they were formed above the southern edge of the Kula plate adjacent to the Kula-Farallon slab window, whereas the Montana alkaline province situated well to the southeast was formed directly above the Kula-Farallon slab window. Upwelling of the hotter asthenospheric mantle may have been the thermal trigger necessary to induce melting of fertile and metasomatized lithospheric mantle.

  10. Theoretical studies of {sup 238}U-{sup 230}Th-{sup 226}Ra and {sup 235}U-{sup 231}Pa disequilibria in young lavas produced by mantle melting

    SciTech Connect

    Zou, H.; Zindler, A.

    2000-05-01

    This paper provides ready-to-use equations to describe variations in uranium-series (U-series) disequilibrium as a function of elemental distribution coefficients, melting porosity, melting rate, and melting time. The effects of these melting parameters on U-series disequilibria are quantitatively evaluated in both an absolute and relative sense. The importance of net elemental fractionation and ingrowth of daughter nuclides are also described and compared in terms of their relative contributions to total U-series disequilibrium. In addition, the authors compare the production of U-series disequilibrium during mantle melting to trace element fractionations produced by melting in a similar context. Trace element fractionations depend externally on the degree to which a source is melted, whereas U-series disequilibrium depends upon both the degree and rate of melting. In contrast to previous models, their approach to modeling U-series disequilibrium during dynamic melting collapses simply to a description of trace element behavior during dynamic melting when the appropriate decay terms are omitted. Their formulation shows that extremely small degrees of melting, sometimes called upon to explain observed extents of U-series disequilibrium, are not always required.

  11. Geochemistry of volcanic glasses from the Louisville Seamount Trail (IODP Expedition 330): Implications for eruption environments and mantle melting

    NASA Astrophysics Data System (ADS)

    Nichols, Alexander R. L.; Beier, Christoph; Brandl, Philipp A.; Buchs, David M.; Krumm, Stefan H.

    2014-05-01

    glasses recovered from four guyots during drilling along the Louisville Seamount Trail, southwest Pacific, have been analyzed for major, trace, and volatile elements (H2O, CO2, S, and Cl), and oxygen isotopes. Compared to other oceanic island settings, they are geochemically homogeneous, providing no evidence of the tholeiitic stage that characterizes Hawai'i. The degrees and depth of partial melting remained constant over 1-3 Ma represented by the drill holes, and along-chain over several million years. The only exception is Hadar Guyot with compositions that suggest small degree preferential melting of an enriched source, possibly because it erupted on the oldest and thickest lithosphere. Incompatible element enriched glass from late-stage volcaniclastics implies lower degrees of melting as the volcanoes moved off the melting anomaly. Volcaniclastic glasses from throughout the igneous basement are degassed suggesting generation during shallow submarine eruptions (<20 mbsl) or as subaerial flows entered the sea. Drill depths may no longer reflect relative age due to postquench downslope movement. Higher volatile contents in late-stage volcaniclastics indicate submarine eruptions at 118-258 mbsl and subsidence of the edifices below sea level by the time they erupted, or generation in flank eruptions. Glass from intrusion margins suggests emplacement ˜100 m below the surface. The required uplift to achieve these paleo-quench depths and the subsequent subsidence to reach their current depths exceeds that expected for normal oceanic lithosphere, consistent with the Louisville melting anomaly being <100°C hotter than normal asthenosphere at 50-70 Ma when the guyots were erupted.

  12. 87Sr/86Sr in spinel peridotites from Borée, Massif Central, France: melt depletion and metasomatism in the sub-continental lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Barnett, Caroline; Harvey, Jason

    2016-04-01

    Radiogenic isotopes and elemental concentrations in peridotite xenoliths may be used to model the timing and degree of partial melting in the upper mantle, but this primary melt depletion signature may be overwritten by subsequent episodes of melt or fluid infiltration. Spinel peridotites from the Maar de Borée, Massif Central, France have mainly poikilitic protogranular textures and clear petrographic evidence of a melt phase apparently unrelated to host basalt infiltration. Bulk rock major and compatible trace element concentrations are consistent with varying degrees of partial melting but incompatible trace element concentrations indicate cryptic metasomatism in some samples. Lithophile trace element mass balance cannot always be reconciled by the inclusion of the chemically characterized melt phase and suggest a contribution from a trace abundance grain boundary phase1. 87Sr/86Sr values for unleached bulk rocks and clinopyroxene mineral separates are higher than those for their leached equivalents, consistent with the removal of a radiogenic grain boundary phase. While unleached bulk rock 87Sr/86Sr is sometimes indistinguishable (within error) from its constituent unleached clinopyroxene, in two samples they show distinct patterns, as do the REE trends in these two xenoliths. BO01-01 bulk-rock is LREE-enriched (La/YbN = 3.6)2, and constituent clinopyroxene shows a similar relative enrichment trend. Bulk-rock 87Sr/86Sr is 0.70342±1 while that of clinopyroxene is lower at 0.70332±2. Clinopyroxene modal abundance is 11%. BO01-03 bulk-rock is only slightly LREE-enriched (La/YbN = 1.2) and both bulk-rock and clinopyroxene show a generally flatter profile. Bulk-rock 87Sr/86Sr is 0.70285±1 while that of clinopyroxene is in this case higher at 0.70296±2. Clinopyroxene modal abundance is also higher at 15%, consistent with a greater contribution by clinopyroxene to the bulk-rock Sr-isotope budget. The results appear to be inconsistent with a simple model of single

  13. Mantle cryptology

    SciTech Connect

    Zindler, A.; Jagoutz, E.

    1988-02-01

    A group of anhydrous peridotites from Peridot Mesa, Arizona, document isotopic and trace element heterogeneity in the source mantle. LREE enrichments in two spinel periodotites may have occurred immediately prior to entrainment through interaction with a melt similar to the hose basanite. Detailed characterization of inclusion-free peridotite phases, and washed and unwahsed whole-rock samples, verifies the presence of a ubiquitous secondary contaminant which derives from interaction of the peridotites with local ground waters and host magma. Once the veil of this contamination is removed, coexisting phases are found to be in isotopic equilibrium. Further, a comparison of washed whole rocks and calculated clean-bulk compositions documents the occurrence of an important intragranular fluid-hosted trace element component. For the very incompatible elements (K, Rb, Cs, and Ba, and probably U, Th, Pb and gaseous components as well) this component dominates the nodule budget for two of the three samples studied in detail. Production of basaltic magmas from fertile but incompatible-element-depleted peridotite requires the action of melting processes such as those recently proposed by McKenzie (1985) and O'Hara (1985). The distinctive feature of these models is that they call on effectively larger source volumes for more incompatible elements. In this context, depletions of incompatible trace elements in MORB source mantle will be more extreme than has heretofore been suspected. This would essentially preclude the long-term total isolation of a MORB source mantle above the 670 km seismic discontinuity.

  14. Experimental petrology of the ultramafic lavas remotely sensed on the surface of Mercury: Constraints on melting and differentiation of Mercury's mantle (Invited)

    NASA Astrophysics Data System (ADS)

    Grove, T. L.; Charlier, B.; Zuber, M. T.; Brown, S. M.

    2013-12-01

    The orbiting MESSENGER spacecraft has measured major element ratios using x-ray fluorescence spectra. These ratios can be used to calculate absolute abundances of the major oxide components in the lavas exposed on the surface of Mercury. The first compositional data reported by Nittler (2011, Science 333, 1847-50) represented large areas of mixed high-reflectance volcanic plains and low-reflectance materials and did not include the northern volcanic plains. These large area measurements reveal high MgO, high SiO2 and low FeO lavas that are most similar to terrestrial volcanic rocks known as basaltic komatiites. Two compositional groups are distinguished by the presence or absence of a clinopyroxene component. Continued measurement of smaller areas on the surface (Weider et al., 2012, JGR 117, E00L05) confirms the existence of two groups. Melting experiments at one atmosphere on the average compositions of each of the two groups were performed to provide constraints on melting and crystallization processes. The two compositional groups cannot be related to each other by any fractional crystallization process at any pressure within the silicate interior of Mercury. This result suggests that the silicate interior of Mercury contains differentiated source regions. One possibility for development of distinctive source regions is early multi-stage differentiation and remelting processes in Mercury's interior. Magma ocean crystallization followed by adiabatic decompression of the differentiated cumulate layers during post magma ocean overturn and/or convection could lead to conditions that would be adequate to cause melting and volcanism that is preserved in the surface lavas. High-pressure phase equilibria on compositional systems similar to the Mercury lavas supports partial melting at pressures > 1 GPa if the Na detected by gamma ray spectroscopy is assumed to be included in the surface lavas. This depth corresponds to equilibration of the melts close to the crust-mantle

  15. Volatile cycling of H2O, CO2, F, and Cl in the HIMU mantle: A new window provided by melt inclusions from oceanic hot spot lavas at Mangaia, Cook Islands

    NASA Astrophysics Data System (ADS)

    Cabral, Rita A.; Jackson, Matthew G.; Koga, Kenneth T.; Rose-Koga, Estelle F.; Hauri, Erik H.; Whitehouse, Martin J.; Price, Allison A.; Day, James M. D.; Shimizu, Nobumichi; Kelley, Katherine A.

    2014-11-01

    hosts the most radiogenic Pb-isotopic compositions observed in ocean island basalts and represents the HIMU (high µ = 238U/204Pb) mantle end-member, thought to result from recycled oceanic crust. Complete geochemical characterization of the HIMU mantle end-member has been inhibited due to a lack of deep submarine glass samples from HIMU localities. We homogenized olivine-hosted melt inclusions separated from Mangaia lavas and the resulting glassy inclusions made possible the first volatile abundances to be obtained from the HIMU mantle end-member. We also report major and trace element abundances and Pb-isotopic ratios on the inclusions, which have HIMU isotopic fingerprints. We evaluate the samples for processes that could modify the volatile and trace element abundances postmantle melting, including diffusive Fe and H2O loss, degassing, and assimilation. H2O/Ce ratios vary from 119 to 245 in the most pristine Mangaia inclusions; excluding an inclusion that shows evidence for assimilation, the primary magmatic H2O/Ce ratios vary up to ˜200, and are consistent with significant dehydration of oceanic crust during subduction and long-term storage in the mantle. CO2 concentrations range up to 2346 ppm CO2 in the inclusions. Relatively high CO2 in the inclusions, combined with previous observations of carbonate blebs in other Mangaia melt inclusions, highlight the importance of CO2 for the generation of the HIMU mantle. F/Nd ratios in the inclusions (30 ± 9; 2σ standard deviation) are higher than the canonical ratio observed in oceanic lavas, and Cl/K ratios (0.079 ± 0.028) fall in the range of pristine mantle (0.02-0.08).

  16. A combined basalt and peridotite perspective on 14 million years of melt generation at the Atlantis Bank segment of the Southwest Indian Ridge: Evidence for temporal changes in mantle dynamics?

    USGS Publications Warehouse

    Coogan, L.A.; Thompson, G.M.; MacLeod, C.J.; Dick, H.J.B.; Edwards, S.J.; Hosford, Scheirer A.; Barry, T.L.

    2004-01-01

    Little is known about temporal variations in melt generation and extraction at midocean ridges largely due to the paucity of sampling along flow lines. Here we present new whole-rock major and trace element data, and mineral and glass major element data, for 71 basaltic samples (lavas and dykes) and 23 peridotites from the same ridge segment (the Atlantis Bank segment of the Southwest Indian Ridge). These samples span an age range of almost 14 My and, in combination with the large amount of published data from this area, allow temporal variations in melting processes to be investigated. Basalts show systematic changes in incompatible trace element ratios with the older samples (from ???8-14 Ma) having more depleted incompatible trace element ratios than the younger ones. There is, however, no corresponding change in peridotite compositions. Peridotites come from the top of the melting column, where the extent of melting is highest, suggesting that the maximum degree of melting did not change over this interval of time. New and published Nd isotopic ratios of basalts, dykes and gabbros from this segment suggest that the average source composition has been approximately constant over this time interval. These data are most readily explained by a model in which the average source composition and temperature have not changed over the last 14 My, but the dynamics of mantle flow (active-to-passive) or melt extraction (less-to-more efficient extraction from the 'wings' of the melting column) has changed significantly. This hypothesised change in mantle dynamics occurs at roughly the same time as a change from a period of detachment faulting to 'normal' crustal accretion. We speculate that active mantle flow may impart sufficient shear stress on the base of the lithosphere to rotate the regional stress field and promote the formation of low angle normal faults. ?? 2004 Elsevier B.V. All rights reserved.

  17. Experience melting through the Earth's lower mantle via LH-DAC experiments on MgO-SiO2 and CaO-MgO-SiO2 systems

    NASA Astrophysics Data System (ADS)

    Baron, Marzena A.; Lord, Oliver T.; Walter, Michael J.; Trønnes, Reidar G.

    2015-04-01

    The large low shear-wave velocity provinces (LLSVPs) and ultra-low velocity zones (ULVZs) of the lowermost mantle [1] are likely characterized by distinct chemical compositions, combined with temperature anomalies. The heterogeneities may have originated by fractional crystallization of the magma ocean during the earliest history of the Earth [2,3] and/or the continued accretion at the CMB of subducted basaltic oceanic crust [4,5]. These structures and their properties control the distribution and magnitude of the heat flow at the CMB and therefore the convective dynamics and evolution of the whole Earth. To determine the properties of these structures and thus interpret the seismic results, a good understanding of the melting phase relations of relevant basaltic and peridotitic compositions are required throughout the mantle pressure range. The melting phase relations of lower mantle materials are only crudely known. Recent experiments on various natural peridotitic and basaltic compositions [6-8] have given wide ranges of solidus and liquidus temperatures at lower mantle pressures. The melting relations for MgO, MgSiO3 and compositions along the MgO-SiO2 join from ab initio theory [e.g. 9,10] is broadly consistent with a thermodynamic model for eutectic melt compositions through the lower mantle based on melting experiments in the MgO-SiO2 system at 16-26 GPa [3]. We have performed a systematic study of the melting phase relations of analogues for peridotitic mantle and subducted basaltic crust in simple binary and ternary systems that capture the major mineralogy of Earth's lower mantle, using the laser-heated diamond anvil cell (LH-DAC) technique at 25-100 GPa. We determined the eutectic melting temperatures involving the following liquidus mineral assemblages: 1. bridgmanite (bm) + periclase (pc) and bm + silica in the system MgO-SiO2 (MS), corresponding to model peridotite and basalt compositions 2. bm + pc + Ca-perovskite (cpv) and bm + silica + cpv in the

  18. Metasomatic Enrichment of Oceanic Lithospheric Mantle Documented by Petit-Spot Xenoliths

    NASA Astrophysics Data System (ADS)

    Pilet, S.; Abe, N.; Rochat, L.; Hirano, N.; Machida, S.; Kaczmarek, M. A.; Muntener, O.

    2015-12-01

    Oceanic lithosphere is generally interpreted as mantle residue after MORB extraction. It has been proposed, however, that metasomatism could take place at the interface between the low-velocity zone and the cooling and thickening oceanic lithosphere or by the percolation of low-degree melts produced in periphery of Mid Ocean Ridges. This later process is observed in slow spreading ridges and ophiolites where shallow oceanic lithospheric mantle could be metasomatized/refertilized during incomplete MORB melt extraction. Nevertheless, direct evidence for metasomatic refertilization of the deep part of the oceanic lithospheric mantle is still missing. Xenoliths and xenocrysts sampled by petit-spot volcanoes interpreted as low-degree melts extracted from the base of the lithosphere in response to plate flexure, provide important new information about the nature and the processes associated with the evolution of oceanic lithospheric mantle. Here, we report, first, the presence of a garnet xenocryst in petit-spot lavas from Japan characterized by low-Cr, low-Ti content and mostly flat MREE-HREE pattern. This garnet is interpreted as formed during subsolidus cooling of pyroxenitic or gabbroic cumulates formed at ~1 GPa during the incomplete melt extraction at the periphery of the Pacific mid-ocean ridge. It is the first time that such processes are documented in fast spreading context. Second, we report petit-spot mantle xenoliths with cpx trace element "signatures" characterized by high U, Th, relative depletion in Nb, Pb, Ti and high but variable LREE/HREE ratio suggesting equilibration depth closed to the Gt/Sp transition zone. Such "signatures" are unknown from oceanic settings and show unexpected similarity to melt-metasomatized gt-peridotites sampled by kimberlites. This similarity suggests that metasomatic processes are not restricted to continental setting, but could correspond to a global mechanism at the lithosphere-asthenosphere boundary. As plate flexure

  19. Timing of Precambrian melt depletion and Phanerozoic refertilization events in the lithospheric mantle of the Wyoming Craton and adjacent Central Plains Orogen

    USGS Publications Warehouse

    Carlson, R.W.; Irving, A.J.; Schulze, D.J.; Hearn, B.C.

    2004-01-01

    Garnet peridotite xenoliths from the Sloan kimberlite (Colorado) are variably depleted in their major magmaphile (Ca, Al) element compositions with whole rock Re-depletion model ages generally consistent with this depletion occurring in the mid-Proterozoic. Unlike many lithospheric peridotites, the Sloan samples are also depleted in incompatible trace elements, as shown by the composition of separated garnet and clinopyroxene. Most of the Sloan peridotites have intermineral Sm-Nd and Lu-Hf isotope systematics consistent with this depletion occurring in the mid-Proterozoic, though the precise age of this event is poorly defined. Thus, when sampled by the Devonian Sloan kimberlite, the compositional characteristics of the lithospheric mantle in this area primarily reflected the initial melt extraction event that presumably is associated with crust formation in the Proterozoic-a relatively simple history that may also explain the cold geotherm measured for the Sloan xenoliths. The Williams and Homestead kimberlites erupted through the Wyoming Craton in the Eocene, near the end of the Laramide Orogeny, the major tectonomagmatic event responsible for the formation of the Rocky Mountains in the late Cretaceous-early Tertiary. Rhenium-depletion model ages for the Homestead peridotites are mostly Archean, consistent with their origin in the Archean lithospheric mantle of the Wyoming Craton. Both the Williams and Homestead peridotites, however, clearly show the consequences of metasomatism by incompatible-element-rich melts. Intermineral isotope systematics in both the Homestead and Williams peridotites are highly disturbed with the Sr and Nd isotopic compositions of the minerals being dominated by the metasomatic component. Some Homestead samples preserve an incompatible element depleted signature in their radiogenic Hf isotopic compositions. Sm-Nd tie lines for garnet and clinopyroxene separates from most Homestead samples provide Mesozoic or younger "ages" suggesting

  20. Inferring Mantle From Basalt Composition

    NASA Astrophysics Data System (ADS)

    Stracke, A.

    2014-12-01

    Isotope ratios in oceanic basalts, first reported by Gast and co-workers 50 years ago, are unique tracers of mantle composition, because they are expected to mirror the composition of their mantle sources. While the latter is certainly true for homogeneous sources, the plethora of studies over the last 50 years have shown that mantle sources are isotopically heterogeneous on different length scales. Isotopic differences exist between basalts from different ocean basins, volcanoes of individual ocean islands, lava flows of a single volcano, and even in μm sized melt inclusions in a single mineral grain. Diffusion, which acts to homogenize isotopic heterogeneity over Gyr timescales, limits the length scale of isotopic heterogeneity in the mantle to anywhere between several mm to 10s of meters. Melting regions, however, are typically several 100 km wide and up to 100 km deep. The scale of melting is thus generally orders of magnitude larger than the scale of isotopic heterogeneity. How partial melts mix during melting, melt transport, and melt storage then inevitably influences how isotopic heterogeneity is conveyed from source to melt. The isotopic composition of oceanic basalts hence provides an integrated signal of isotopically diverse melts. Recent mixing models and observed isotopic differences between source (abyssal peridotites) and melts (MORB) show that the range of isotopic heterogeneity of erupted melts need NOT directly reflect that of their source(s), nor need observed isotopic endmembers in source and melts be congruent. Many geochemical models, however, implicitly assume equivalence of source and melt composition. Especially when attempting to infer spatial patterns of isotopic heterogeneity in the mantle from those observed in erupted melts, or for linking isotopic diversity to geophysical structures in the mantle requires a more profound understanding to what extent erupted melts represent the isotopic composition of their mantle sources.

  1. Compositional variability in mafic arc magmas over short spatial and temporal scales: Evidence for the signature of mantle reactive melt channels

    NASA Astrophysics Data System (ADS)

    Rawson, Harriet; Keller, Tobias; Fontijn, Karen; Pyle, David M.; Mather, Tamsin A.; Smith, Victoria C.; Naranjo, José A.

    2016-12-01

    Understanding arc magma genesis is critical to deciphering the construction of continental crust, understanding the relationship between plutonic and volcanic rocks, and for assessing volcanic hazards. Arc magma genesis is complex. Interpreting the underlying causes of major and trace element diversity in erupted magmas is challenging and often non-unique. To navigate this complexity mafic magma diversity is investigated using sample suites that span short temporal and spatial scales. These constraints allow us to evaluate models of arc magma genesis and their geochemical implications based on physical arguments and recent model results. Young volcanic deposits (≲18 kyr) are analysed from the Southern Volcanic Zone (SVZ), Chile, in particular suites of scoria cones on the flanks of arc stratovolcanoes that have erupted relatively primitive magmas of diverse compositions. Our study is centred on the high-resolution post-glacial tephrochronological record for Mocho-Choshuenco volcano where tight age constraints and a high density of scoria cones provide a spatially well-resolved mafic magma dataset. Two compositional trends emerge from the data. Firstly, magmas from cones on the flanks of the main edifice become more mafic with distance from the central vent. This is attributed to fractional crystallisation processes within the crust, with distal cones sampling less differentiated magmas. Secondly, there is a set of cones with distinct major and trace element compositions that are more primitive but enriched in incompatible elements relative to the central system and other 'normal SVZ' magmas. This distinct signature - termed the 'Kangechi' signature - is observed at three further clusters of cones within the SVZ. This is attributed to greater preservation of the enriched melt signature arising from reactive melt transport within the mantle wedge. Our model has important implications for arc magma genesis in general, and in particular for the spatial and temporal

  2. Geochemistry and petrology of spinel lherzolite xenoliths from Xalapasco de La Joya, San Luis Potosi, Mexico: Partial melting and mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Liang, Yan; Elthon, Don

    1990-09-01

    Spinel Iherzolite xenoliths from Xalapasco de La Joya, San Luis Potosi, Mexico, are divided into two distinct groups according to their major element and trace element characteristics. Group Ia xenoliths are characterized by light rare earth element (LREE) depletion ((La/Lu)N = 0.10-0.77 in clinopyroxene) and linear major and compatible trace element relationships. Group Ib xenoliths are characterized by FeO and Na2O enrichment and higher (La/Lu)N ratios (0.80-4.1 in clinopyroxene) and complex major element relationships. These samples, which have a range of equilibrium temperatures of 910°-1070°C, exhibit protogranular textures and typical orthopyroxene+clinopyroxene+spinel clusters. Modal abundances and chemical compositions of the group Ia xenoliths vary from primitive (15.2% clinopyroxene, 38.5% MgO, 1824 ppm Ni) to moderately depleted (6.4-8.7% clinopyroxene, 43.8-44.1% MgO, 2192 ppm Ni). Systematic variations of major elements and compatible trace elements in the group Ia xenoliths are interpreted to result from various degrees (<25%) of partial melting and melt extraction, followed by subsolidus equilibration and annealing. The extracted melts have a range of compositions similar to picritic basalts. Abundances of moderately incompatible trace elements, Sc and Cr, in the group Ia minerals have been substantially redistributed during subsolidus equilibration. In a few of these xenoliths there appears to be vestiges of incipient metasomatism, but metasomatism has not substantially influenced the group as a whole. Group Ib xenoliths have been substantially influenced by metasomatic processes. The ∑FeO and Na2O contents of the cores of clinopyroxenes in group Ib xenoliths are higher than clinopyroxenes in group Ia samples. The higher La contents and La/Lu ratios in group Ib clinopyroxenes (compared to group Ia), together with this FeO and Na2O enrichment, suggest that equilibration of basanites with residual mantle has been a major process in the evolution

  3. Rapid Ascent of Aphyric Mantle Melts through the Overriding Crust in Subduction Zones: Evidence from Variable Uranium-Series Disequilibria, Amorphous Hydrous Alteration Microtextures in Crystal Rims, and Two-Pyroxene Pseudo-Decompression Paths

    NASA Astrophysics Data System (ADS)

    Zellmer, G. F.; Freymuth, H.; Hsieh, H. H.; Hwang, S. L.; Iizuka, Y.; Miller, C. A.; Rubin, K. H.; Sakamoto, N.; Yurimoto, H.

    2014-12-01

    Volcanic hazard mitigation at subduction zones critically depends on knowledge of magma generation and ascent processes and timescales. Two diametrically opposite scenarios are presently debated: One paradigm is the generation of low-silica (basaltic) melts in the mantle wedge, followed by protracted sub-liquidus magma ascent and evolution through crystal growth and fractionation in crustal reservoirs, which are tapped during volcanic eruptions. In contrast, a diametrically opposite model favours the generation of higher silica melts in the mantle or in a lower crustal hot zone, followed by rapid decompression to the surface under super-liquidus conditions. In the latter case, crystals are picked up during magma ascent, and are in the process of dissolving. We present multiple lines of evidence that point to crystal uptake as the principal processes by which arc melts acquire their crystal cargo: (i) variable 234U-238U disequilibria in mineral separates; (ii) hydrous mineral rims with amorphous alteration textures; and (iii) two-pyroxene pseudo-decompression paths; cf. Zellmer et al. (2014a,b,c), doi: 10.1144/SP385.3 and 10.1144/SP385.9 and 10.1144/SP410.1. These observations point to a scarcity of true phenocrysts in many arc magmas, and thus to decompression of aphyric melts that take up their crystal cargo during ascent. The data imply that many hydrous wedge melts are more silica-rich than basalts and achieve super-liquidus conditions during rapid ascent from great depth.

  4. Melt Focusing Along Permeability Barriers in Various Tectonic Settings

    NASA Astrophysics Data System (ADS)

    Montesi, L. G.; Hebert, L. B.

    2012-12-01

    The lithosphere, cold and rigid, acts as a barrier to the migration of melt from sources in the convecting mantle to the surface. In mid-ocean ridge settings in particular, the contrast between the width of the melt production zone at depths, reaching tens to hundreds of kilometer from the ridge axis, and the zone of crustal accretion, only one or two kilometers wide, points to the presence of an efficient focusing mechanism. The development of a zone impermeable to melt, or permeability barrier, at the base of the thermal boundary layer, and transport of melt in a high porosity channel at the base of this barrier provides a reasonable explanation for this focusing. Applied to various segmented and non-segmented mid-ocean ridges like the ultraslow Southwest Indian Ridge and the ultrafast East Pacific Rise at the Siqueiros transform, this process predicts along-strike variations in crustal thickness that compare favorably with observations. Although the concept of permeability barriers has been discussed mainly in the context of mid-ocean ridges, it may apply to other locations where melting in the upper mantle occurs. Permeability barriers form when ascending melt cools and crystallizes as it enters the thermal boundary layer at the base of the lithosphere. Such a setup is present at subduction zones as melts ascending from the mantle wedge interact with the overriding plate. Convection in the wedge introduces thermal gradients that may focus melt roughly to a point above the transition from a coupled to decoupled slab interface. This location is close to where volcanic arcs are observed. Above mantle plumes, a permeability barrier may develop coincident with the lithosphere-asthenosphere boundary, allowing low-degree melts to stall and form a low-velocity layer that has been observed seismically. To date, the hypothesis of a permeability barrier has been thoroughly tested only in the context of mid-ocean ridges. Whether crystallization would be rapid enough in

  5. Experimental determination of C, F, and H partitioning between mantle minerals and carbonated basalt, CO2/Ba and CO2/Nb systematics of partial melting, and the CO2 contents of basaltic source regions

    NASA Astrophysics Data System (ADS)

    Rosenthal, A.; Hauri, E. H.; Hirschmann, M. M.

    2015-02-01

    To determine partitioning of C between upper mantle silicate minerals and basaltic melts, we executed 26 experiments between 0.8 and 3 GPa and 1250-1500 °C which yielded 37 mineral/glass pairs suitable for C analysis by secondary ion mass spectrometry (SIMS). To enhance detection limits, experiments were conducted with 13C-enriched bulk compositions. Independent measurements of 13C and 12C in coexisting phases produced two C partition coefficients for each mineral pair and allowed assessment of the approach to equilibrium during each experiment. Concentrations of C in olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) and garnet (gt) range from 0.2 to 3.5 ppm, and resulting C partition coefficients for ol/melt, opx/melt, cpx/melt and gt/melt are, respectively, 0.0007 ± 0.0004 (n = 2), 0.0003 ± 0.0002 (n = 45), 0.0005 ± 0.0004 (n = 17) and 0.0001 ± 0.00007 (n = 5). The effective partition coefficient of C during partial melting of peridotite is 0.00055 ± 0.00025, and therefore C is significantly more incompatible than Nb, slightly more compatible than Ba, and, among refractory trace elements, most similar in behavior to U or Th. Experiments also yielded partition coefficients for F and H between minerals and melts. Combining new and previous values of DFmineral/melt yields bulk DFperidotite/melt = 0.011 ± 0.002, which suggests that F behaves similarly to La during partial melting of peridotite. Values of DHpyx/melt correlate with tetrahedral Al along a trend consistent with previously published determinations. Small-degree partial melting of the mantle results in considerable CO2/Nb fractionation, which is likely the cause of high CO2/Nb evident in some Nb-rich oceanic basalts. CO2/Ba is much less easily fractionated, with incompatible-element-enriched partial melts having lower CO2/Ba than less enriched basalts. Comparison of calculated behavior of CO2, Nb, and Ba to systematics of oceanic basalts suggests that depleted (DMM-like) sources have 75 ± 25

  6. Geologic Map and Eruptive History of Veniaminof Volcano Record Aleutian Arc Processing of Mantle-Derived Melts

    NASA Astrophysics Data System (ADS)

    Bacon, C. R.; Sisson, T. W.; Calvert, A. T.; Nye, C. J.

    2009-12-01

    Mount Veniaminof, one of the largest volcanoes in the Aleutian arc, has a basal diameter of ~40 km, a volume of ~350 km3, an 8-km-diameter ice-filled caldera, and an active intracaldera cone. The geology of this tholeiitic basalt-to-dacite volcano has been mapped at 1:50,000 scale. Over 100 Quaternary volcanic map units are characterized by 600 chemical analyses of rocks and nearly 100 40Ar/39Ar and K-Ar ages. Throughout its history, lava flows from Veniaminof recorded alternately ice/melt-water chilling or ice-free conditions that are consistent with independent paleoclimatic records. Exposures from deep glacial valleys to the caldera rim reveal a long history dominated by basalt and basaltic andesite from ≥260 ka to 150 ka that includes compositions as primitive as 9.4% MgO and 130 ppm Ni at 50% SiO2. Basaltic andesite, common throughout Veniaminof's history, has low compatible-element contents that indicate an origin by fractionation of basaltic magma. Repeated eruption of more differentiated melts from a shallow intrusive complex, represented by granodiorite (crystallized dacitic magma) and cumulate gabbro and diorite xenoliths in pyroclastic deposits, has featured virtually aphyric andesite since 150 ka and dacite (to 69.5% SiO2) beginning ~110 ka. These variably differentiated liquids segregated from crystal mush, possibly by gas-driven filter pressing, and commonly vented but also solidified at depth. A large composite cone was present at least as early as 200 ka. Although asymmetric edifice morphology hints at early sector collapse to the southeast, coeval vents on northwest and southeast flanks and the distribution of extensive lava units indicate that a large cone (again) was present by 120 ka. Flank eruption of a wide variety of Veniaminof magmas was common from plate-convergence-parallel northwest-trending fissures from at least as early as ca. 80 ka. At 56 ka and at 46 ka, voluminous dacite lava erupted on both northwest and southeast flanks. A

  7. Effect of variable CO2 on eclogite-derived andesite and lherzolite reaction at 3 GPa—Implications for mantle source characteristics of alkalic ocean island basalts

    NASA Astrophysics Data System (ADS)

    Mallik, Ananya; Dasgupta, Rajdeep

    2014-04-01

    have performed reaction experiments between 1, 4, and 5 wt % CO2-bearing MORB-eclogite (recycled oceanic crust)-derived low-degree andesitic partial melt and fertile peridotite at 1375°C, 3 GPa for infiltrating melt fractions of 25% and 33% by weight. We observe that the reacted melts are alkalic with degree of alkalinity or Si undersaturation increasing with increasing CO2 content in reacting melt. Consequently, an andesite evolves through basanite to nephelinite owing to greater drawdown of SiO2 from melt and enhanced precipitation of orthopyroxene in residue. We have developed an empirical model to predict reacted melt composition as a function of reacting andesite fraction and source CO2 concentration. Using our model, we have quantified the mutual proportions of equilibrated melt from andesite-peridotite (+ CO2) hybridization and subsequent peridotite (± CO2)-derived melt required to produce the major element composition of various ocean island basalts. Our model can thus be applied to characterize the source of ocean islands from primary alkalic lava composition. Accordingly, we determined that average HIMU source requires 24 wt % of MORB-eclogite-derived melt relative to peridotite containing 2 wt % CO2 and subsequent contribution of 45% of volatile-free peridotite partial melt. We demonstrate that mantle hybridization by eclogite melt-peridotite (± CO2) reaction in the system can produce high MgO (>15 wt %) basaltic melts at mantle potential temperature (TP) of 1350°C. Therefore, currently used thermometers to estimate TP using MgO content of primary alkalic melts need to be revised, with corrections for melt-rock reaction in a heterogeneous mantle as well as presence of CO2.

  8. Temporal distribution of mantle-derived potassic rocks and carbonatites linked to stabilization of mantle lithosphere and redox states during subduction

    NASA Astrophysics Data System (ADS)

    Foley, S. F.

    2014-12-01

    Mantle-derived potassic igneous rocks and carbonatites first appear in the geological record in the late Archean, coinciding with major crust-forming events on most continents. The compositions of potassic rocks require sources including discrete ultramafic rocks with phlogopite and pyroxenes, whereas carbonatites and ultramafic lamprophyres (carbonate-rich potassic rocks) require oxidizing conditions in which carbonate is stable. The presence of these source rocks from this time is probably related to the stabilization of mantle lithosphere. If mantle lithosphere had not been stable for considerable periods of time, then melting would be restricted to peridotite, which is not a viable option for strongly potassic rocks. The phlogopite-rich source-rock assemblages that are necessary precursors for potassic melts could be introduced into the lithosphere by either subduction processes or by multiple stages of low-degree melting. Many modern examples involve subducted sedimentary material, which concentrates potassium by the stabilization of micas in subduction metamorphism. Subduction involves a great variety of redox states, but the bulk effect is the return of oxidized material from the surface into the mantle. However, we cannot apply uniformitarianism unthinkingly, because subduction processes at and before 2.7 Ga may have had different redox states. Before the Great Oxidation Event the distribution and abundances of geological formations such as banded iron formations, red beds, and uraninites indicate that geological reservoirs became gradually oxidized, preventing an earlier increase in atmospheric oxygen. This means that the function of the subduction process to oxidize the upper mantle by the return of oxidized rocks from the surface was much weaker in the early Earth. Early continental mantle lithosphere was, therefore, likely to accumulate carbon in reduced form, which would be more easily remobilized in melts through low-temperature redox melting much

  9. A Melt Inclusion Study in Primitive Olivines from the Adventive Cones of the Piton de la Fournaise Volcano, La Réunion Island : Implications for the Nature of the Réunion Mantle Plume

    NASA Astrophysics Data System (ADS)

    Valer, M.; Schiano, P.; Bachelery, P.

    2015-12-01

    According to Courtillot et al. (2003), the mantle plume that forms the Réunion hot spot originates from the deepest part of the lower mantle. Based on the isotopic compositions of the lavas, this long-lived plume appears relatively homogeneous during the last 65 My (e.g Fisk et al., 1988), and is believed to correspond to an ubiquitous mantle component common to ocean island basalts (e.g Bosch et al., 2008). Here, we give additional information on the nature of the Réunion mantle plume by studying the chemical composition of silicate melt inclusions trapped within early-formed, primitive olivine crystals (Fo>85%) from the adventive cones of the Piton de la Fournaise Volcano. These cones have emitted distinct magmas from the historical lavas. In particular, we focus on very incompatible trace element ratios, which reflect the long-term characteristics of the basalt sources and do not depend on the age of the source. The results indicate that the trapped melts have very primitive compositions (up to 11.93 wt% MgO) relative to the lavas. They also suggest that the magmas found in the adventive cones originate from a common chemical source, corresponding to either (1) a homogeneous mixed source between different mantle components (HIMU, EM 1, EM 2 and DMM), or (2) a near-primitive less-differentiated mantle source. Some very incompatible trace element ratios (e.g Th/La, Nb/La) display values similar to the primitive mantle ones, giving thus further support for hypothesis (2), as also inferred by Vlastélic et al. (2006) and Schiano et al. (2012). If based on Ce/Pb and Nb/U systematics, Hofmann et al. (1986) argued that the sources of all oceanic basalts (MORB and OIB) have undergone continental crust extraction, we propose an intermediate origin for the Réunion plume, between a primitive-like mantle domain and a depleted one, almost not affected by the recycling processes.

  10. A melt inclusion study in primitive olivines from the adventive cones of the Piton de la Fournaise volcano, La Réunion Island : Implications for the nature of the Réunion mantle plume

    NASA Astrophysics Data System (ADS)

    Valer, Marina; Schiano, Pierre; Bachèlery, Patrick

    2016-04-01

    According to Courtillot et al. (2003), the mantle plume that forms the Réunion hot spot originates from the deepest part of the lower mantle. Based on the isotopic compositions of the lavas, this long-lived plume appears relatively homogeneous during the last 65 My (e.g Fisk et al., 1988), and is believed to correspond to an ubiquitous mantle component common to ocean island basalts (e.g Bosch et al., 2008). Here, we give additional information on the nature of the Réunion mantle plume by studying the chemical composition of silicate melt inclusions trapped within early-formed, primitive olivine crystals (Fo>85%) from the adventive cones of the Piton de la Fournaise volcano. These cones have emitted distinct magmas from the historical lavas. In particular, we focus on very incompatible trace element ratios, which reflect the long-term characteristics of the basalt sources and do not depend on the age of the source. The results indicate that the trapped melts have primitive compositions (up to 10.55 wt.% MgO) relative to the lavas. They also suggest that the magmas found in the adventive cones originate from a common chemical source, corresponding to either (1) a homogeneous mixed source between different mantle components (HIMU, EM 1, EM 2 and DMM), or (2) a near-primitive less-differentiated mantle source. Some very incompatible trace element ratios (e.g Th/La, Nb/La) display values similar to the primitive mantle ones, giving thus further support for hypothesis (2), as also inferred by Vlastélic et al. (2006) and Schiano et al. (2012). If based on Ce/Pb and Nb/U systematics, Hofmann et al. (1986) argued that the sources of all oceanic basalts (MORB and OIB) have undergone continental crust extraction, we propose an intermediate origin for the Réunion plume, between a primitive-like mantle domain and a depleted one, almost not affected by the recycling processes.

  11. Effect of chlorine on near-liquidus crystallization of olivine-phyric shergottite NWA 6234 at 1 GPa: Implication for volatile-induced melting of the Martian mantle

    NASA Astrophysics Data System (ADS)

    Farcy, Benjamin J.; Gross, Juliane; Carpenter, Paul; Hicks, Jacob; Filiberto, Justin

    2016-11-01

    Martian magmas are thought to be rich in chlorine compared with their terrestrial counterparts. Here, we experimentally investigate the effect of chlorine on liquidus depression and near-liquidus crystallization of olivine-phyric shergottite NWA 6234 and compare these results with previous experimental results on the effect of chlorine on near-liquidus crystallization of the surface basalts Humphrey and Fastball. Previous experimental results showed that the change in liquidus temperature is dependent on the bulk composition of the basalt. The effect of chlorine on liquidus depression is greater for lower SiO2 and higher Al2O3 magmas than for higher SiO2 and lower Al2O3 magmas. The bulk composition for this study has lower Al2O3 and higher FeO contents than previous work; therefore, we provide additional constraints on the effect of the bulk composition on the influence of chlorine on near-liquidus crystallization. High pressure and temperature crystallization experiments were performed at 1 GPa on a synthetic basalt, of the bulk composition of NWA 6234, with 0-4 wt% Cl added to the sample as AgCl. The results are consistent with previous notions that with increasing wt% Cl in the melt, the crystallization temperature decreases. Importantly, our results have a liquidus depression ∆T (°C) from added chlorine that is consistent with the difference in bulk composition and suggest a dependence on both the bulk Al2O3 and FeO content. Our results suggest that the addition of chlorine to the Martian mantle may lower magma genesis temperatures and potentially aid in the petrogenesis of Martian magmas.

  12. Multi-stage melt-rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence

    NASA Astrophysics Data System (ADS)

    Rampone, Elisabetta; Piccardo, Giovanni B.; Hofmann, Albrecht W.

    2008-10-01

    Spinel and plagioclase peridotites from the Mt.Maggiore (Corsica, France) ophiolitic massif record a composite asthenosphere-lithosphere history of partial melting and subsequent multi-stage melt-rock interaction. Cpx-poor spinel lherzolites are consistent with mantle residues after low-degree fractional melting ( F = 5-10%). Opx + spinel symplectites at the rims of orthopyroxene porphyroclasts indicate post-melting lithospheric cooling ( T = 970-1,100°C); this was followed by formation of olivine embayments within pyroxene porphyroclasts by melt-rock interaction. Enrichment in modal olivine (up to 85 wt%) at constant bulk Mg values, and variable absolute REE contents (at constant LREE/HREE) indicate olivine precipitation and pyroxene dissolution during reactive porous melt flow. This stage occurred at spinel-facies depths, after incorporation of the peridotites in the thermal lithosphere. Plagioclase-enriched peridotites show melt impregnation microtextures, like opx + plag intergrowths replacing exsolved cpx porphyroclasts and interstitial gabbronoritic veinlets. This second melt-rock interaction stage caused systematic chemical changes in clinopyroxene (e.g. Ti, REE, Zr, Y increase), related to the concomitant effects of local melt-rock interaction at decreasing melt mass, and crystallization of small (<3%) trapped melt fractions. LREE depletion in minerals of the gabbronoritic veinlets indicates that the impregnating melts were more depleted than normal MORB. Preserved microtextural evidence of previous melt-rock interaction in the impregnated peridotites suggests that they were progressively uplifted in response to lithosphere extension and thinning. Migrating melts were likely produced by mantle upwelling and melting related to extension; they were modified from olivine-saturated to opx-saturated compositions, and caused different styles of melt-rock interaction (reactive spinel harzburgites, vs. impregnated plagioclase peridotites) depending on the

  13. Experimental, in-situ carbon solution mechanisms and isotope fractionation in and between (C-O-H)-saturated silicate melt and silicate-saturated (C-O-H) fluid to upper mantle temperatures and pressures

    NASA Astrophysics Data System (ADS)

    Mysen, Bjorn

    2017-02-01

    Our understanding of materials transport processes in the Earth relies on characterizing the behavior of fluid and melt in silicate-(C-O-H) systems at high temperature and pressure. Here, Raman spectroscopy was employed to determine structure of and carbon isotope partitioning between melts and fluids in alkali aluminosilicate-C-O-H systems. The experimental data were recorded in-situ while the samples were at equilibrium in a hydrothermal diamond anvil cell at temperatures and pressures to 825 °C and >1300 MPa, respectively. The carbon solution equilibrium in both (C-O-H)-saturated melt and coexisting, silicate-saturated (C-O-H) fluid is 2CO3 + H2O + 2Qn + 1 = 2HCO3 + 2Qn. In the Qn-notation, the superscript, n, is the number of bridging oxygen in silicate structural units. At least one oxygen in CO3 and HCO3 groups likely is shared with silicate tetrahedra. The structural behavior of volatile components described with this equilibrium governs carbon isotope fractionation factors between melt and fluid. For example, the ΔH equals 3.2 ± 0.7 kJ/mol for the bulk 13C/12C exchange equilibrium between fluid and melt. From these experimental data, it is suggested that at deep crustal and upper mantle temperatures and pressures, the δ13C-differences between coexisting silicate-saturated (C-O-H) fluid and (C-O-H)-saturated silicate melts may change by more than 100‰ as a function of temperature in the range of magmatic processes. Absent information on temperature and pressure, the use of carbon isotopes of mantle-derived magma to derive isotopic composition of magma source regions in the Earth's interior, therefore, should be exercised with care.

  14. Relation of slab-derived carbonate melts to kimberlite magma genesis and diamond formation

    NASA Astrophysics Data System (ADS)

    Golubkova, A.; Schmidt, M. W.

    2012-12-01

    Geochemical and experimental studies demonstrated that kimberlites originate from mantle regions, which were both depleted by melt extraction and metasomatized by carbonate (or carbonate-silicate) melts or fluids [Becker+LeRoex, 2006, JPet; Brey et al., 2008, JPet]. The metasomatizing melts are low-degree partial melts formed in carbonated lithologies, which impose their geochemical features on mantle peridotites. Trace-element abundances and isotopic characteristics indicate that some C stems from a recycled crustal component [Deines et al., 1991, GCA] introduced into the mantle during subduction. Subduction-related fluids or melts probably affected lithospheric source region of Group II kimberlites in South Africa [Becker+LeRoex, 2006]. Depletion in high-field strength elements of some lamproites is also associated with collision and subduction-accretion episodes. Consequently, subducted carbonated lithologies constitute one of the sources of carbonate melts metasomatizing the kimberlites source-region mantle and introducing a crustal imprint. Melting of different mantle assemblages with/without volatiles and liquidus studies on natural kimberlites investigated the source mineralogy of kimberlites. As a result, volatiles (in particular CO2) play an important role in kimberlite magma generation; moreover, the source region should be enriched in incompatible elements and alkalis. In our study we investigate the formation of metasomatized mantle portions enriched in CO2 and alkalis, and provide a mechanism, which relates subduction processes to kimberlite magma genesis. For this purpose we simulate metasomatic reactions between slab-derived carbonate melts [Grassi+Schmidt, 2011, JPet] and mantle peridotites (spinel lherzolite and harzburgite) at 8 and 13 GPa employing multi-anvil apparatus. This pressure range corresponds to the depths, at which K-rich carbonate melts may form in subducted carbonated metapelites and is also consistent with depth estimates for

  15. Chemical stratification of the mantle

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.

    1979-01-01

    A possible scenario for the chemical stratification of the earth's mantle is presented. Differentiation of the mantle by either the production of basaltic magmas or partial melting by the upper mantle is proposed to lead to a thick basalt layer, the lower part of which is converted to eclogite as the earth cools. Density estimates indicate that the eclogite formed would not be able to sink to below 670 km. The eclogite layer is thus demonstrated to be trapped as a result of whole-mantle convection and possible irreversible differentiation of the mantle into eclogite and overlying residual peridotite layers.

  16. Reply to Comment on “Steady-state 226Ra/230Th disequilibrium in mantle minerals: implications for melt transport rates in island arcs” by R. George, M. Reagan, S. Turner, J. Gill, B. Bourdon

    NASA Astrophysics Data System (ADS)

    Feineman, M. D.; DePaolo, D. J.

    2004-12-01

    We agree with George et al. that high 226Ra/ 230Th and correlated Ba/Th, 238U/ 230Th, and other elemental and isotopic ratios are the result of fluid addition to the wedge. The Feineman and DePaolo [Earth Planet. Sci. Lett. 215 (2003) 339-355] manuscript addresses more specifically the pathway followed by the fluid en route to the surface. In particular, we address whether the 226Ra/ 230Th data directly yield the total time available for the fluid to migrate from the point of origin in the slab to the melting region in the wedge, and ultimately to the surface with the melt. All of the processes involved in generating arc volcanism would of necessity have to take place very rapidly within the constraints proposed by George et al. The alternative is that the upward fluid movement is hindered by the fluid flow regime and solid mantle flow, allowing more time for reaction with the mantle wedge. There is evidence in the U-series data for more than one time scale, and our model helps to explain how this could come about. The extent to which grainscale 226Ra/ 230Th disequilibrium affects the inferred melt transport time is yet another issue, and this could be achieved either with the small-degree melt mechanism we discussed or by additional effects associated with melt and fluid migration at later stages in the melt generation process. We agree that the final melt transport event is likely to happen quickly. One of the most puzzling features of volcanic arcs is that the volcanic front emerges considerably behind the presumed location of primary water release from the subducting slab (depth to slab is ˜120 km beneath the volcanic front, as opposed to the predicted ˜80 km). This offset between the expected site of fluid release and the location of the volcanic front can be attributed to coupled transport of the fluid—in the form of hydrous minerals, such as amphibole and/or phlogopite in the wedge [J. Geophys. Res. 97 (1992) 2037-2070] or redirection of the melt due to

  17. Wüstite stability in the presence of a CO2-fluid and a carbonate-silicate melt: Implications for the graphite/diamond formation and generation of Fe-rich mantle metasomatic agents

    NASA Astrophysics Data System (ADS)

    Bataleva, Yuliya V.; Palyanov, Yuri N.; Sokol, Alexander G.; Borzdov, Yuri M.; Bayukov, Oleg A.

    2016-02-01

    Experimental simulation of the interaction of wüstite with a CO2-rich fluid and a carbonate-silicate melt was performed using a multianvil high-pressure split-sphere apparatus in the FeO-MgO-CaO-SiO2-Al2O3-CO2 system at a pressure of 6.3 GPa and temperatures in the range of 1150 °C-1650 °C and with run time of 20 h. At relatively low temperatures, decarbonation reactions occur in the system to form iron-rich garnet (Alm75Prp17Grs8), magnesiowüstite (Mg# ≤ 0.13), and CO2-rich fluid. Under these conditions, magnesiowüstite was found to be capable of partial reducing CO2 to C0 that leads to the formation of Fe3+-bearing magnesiowüstite, crystallization of magnetite and metastable graphite, and initial growth of diamond seeds. At T ≥ 1450 °C, an iron-rich carbonate-silicate melt (FeO ~ 56 wt.%, SiO2 ~ 12 wt.%) forms in the system. Interaction between (Fe,Mg)O, SiO2, fluid and melt leads to oxidation of magnesiowüstite and crystallization of fayalite-magnetite spinel solid solution (1450 °C) as well as to complete dissolution of magnesiowüstite in the carbonate-silicate melt (1550 °C-1650 °C). In the presence of both carbonate-silicate melt and CO2-rich fluid, dissolution (oxidation) of diamond and metastable graphite was found to occur. The study results demonstrate that under pressures of the lithospheric mantle in the presence of a CO2-rich fluid, wüstite/magnesiowüstite is stable only at relatively low temperatures when it is in the absolute excess relative to CO2-rich fluid. In this case, the redox reactions, which produce metastable graphite and diamond with concomitant partial oxidation of wüstite to magnetite, occur. Wüstite is unstable under high concentrations of a CO2-rich fluid as well as in the presence of a carbonate-silicate melt: it is either completely oxidized or dissolves in the melt or fluid phase, leading to the formation of Fe2 +- and Fe3 +-enriched carbonate-silicate melts, which are potential metasomatic agents in the

  18. The effect of f[subscript O2] on the partitioning and valence of V and Cr in garnet/melt pairs and the relation to terrestrial mantle V and Cr content

    SciTech Connect

    Righter, K.; Sutton, S.; Danielson, L.; Pando, K.; Schmidt, G.; Yang, H.; Berthet, S.; Newville, M.; Choi, Y.; Downs, R.T.; Malavergne, V.

    2011-09-16

    Chromium and vanadium are stable in multiple valence states in natural systems, and their distribution between garnet and silicate melt is not well understood. Here, the partitioning and valence state of V and Cr in experimental garnet/melt pairs have been studied at 1.8-3.0 GPa, with variable oxygen fugacity between IW-1.66 and the Ru-RuO{sub 2} (IW+9.36) buffer. In addition, the valence state of V and Cr has been measured in several high-pressure (majoritic garnet up to 20 GPa) experimental garnets, some natural megacrystic garnets from the western United States, and a suite of mantle garnets from South Africa. The results show that Cr remains in trivalent in garnet across a wide range of oxygen fugacities. Vanadium, on the other hand, exhibits variable valence state from 2.5 to 3.7 in the garnets and from 3.0 to 4.0 in the glasses. The valence state of V is always greater in the glass than in the garnet. Moreover, the garnet/melt partition coefficient, D(V), is highest when V is trivalent, at the most reduced conditions investigated (IW-1.66 to FMQ). The V{sup 2.5+} measured in high P-T experimental garnets is consistent with the reduced nature of those metal-bearing systems. The low V valence state measured in natural megacrystic garnets is consistent with f{sub O{sub 2}} close to the IW buffer, overlapping the range of f{sub O{sub 2}} measured independently by Fe{sup 2+}/Fe{sup 3+} techniques on similar samples. However, the valence state of V measured in a suite of mantle garnets from South Africa is constant across a 3 log f{sub O{sub 2}} unit range (FMQ-1.8 to FMQ-4.5), suggesting that the valence state of V is controlled by the crystal chemistry of the garnets rather than f{sub O{sub 2}} variations. The compatibility of V and Cr in garnets and other deep mantle silicates indicates that the depletion of these elements in the Earth's primitive upper mantle could be due to partitioning into lower mantle phases as well as into metal.

  19. 187Os/188Os in Spinel Peridotites from Borée, Massif Central, France: Seeing through the Effects of Melt Infiltration in the Sub-continental Lithospheric Mantle

    NASA Astrophysics Data System (ADS)

    Barnett, C. J.; Harvey, J.

    2015-12-01

    The Re-Os isotope system can be used to model the timing of melt extraction in peridotites, although secondary metasomatic processes can obscure primary melt depletion signatures, implying that bulk-rock Os model ages should be treated with caution.1Spinel peridotites from the volcanic Maar de Borée (French Massif Central) have equigranular to protogranular and occasionally poikilitic textures. Their bulk-rock chemistry are consistent with moderate degrees of partial melting, but elevated incompatible trace element ratios (e.g. La/YbN) are indicative of subsequent secondary processes. Petrographic observation reveals no infiltration of host basalt, but melt infiltration unrelated to the host basalt has occurred, most likely within the sub-continental lithospheric mantle prior to entrainment as xenoliths. The peridotites have a mean [Os] concentration of 2.35 ng g-1 and 187Os/188Os values from 0.12081 ± 16 to 0.12639 ± 14 (cf. PUM = 0.1296 ± 00082), with rhenium depletion model ages (TRD) ranging from 0.48 to 1.30 Ga. Silicate melt contains up to 2 orders of magnitude less Os than peridotites3 but the 187Os/188Os of melt infiltrated peridotite can be skewed by the precipitation of immiscible sulfide when an infiltrating melt reaches S-saturation4. The Borée peridotites retain an unradiogenic Os-isotope signature despite silicate melt infiltration; this may be due to primary base metal sulfides enclosed in silicate minerals and therefore protected from interaction with infiltrating melts. TRD of enclosed sulphides should therefore be able to 'see through' any secondary metasomatic events and reveal melt depletion ages significantly older than those obtained from bulk-rock analyses (cf. 4). 1. Rudnick & Walker (2009) Lithos 112S, 1083-1095. 2. Meisel et al. (2001) Geochim Cosmochim Ac 65, 1311-1323. 3. Day, J.M.D. (2013) Chem Geol 341, 50-74. 4. Harvey et al. (2010) Geochim Cosmochim Acta 74, 293-320.

  20. Magnesium isotopic variation of oceanic island basalts generated by partial melting and crustal recycling

    NASA Astrophysics Data System (ADS)

    Zhong, Yuan; Chen, Li-Hui; Wang, Xiao-Jun; Zhang, Guo-Liang; Xie, Lie-Wen; Zeng, Gang

    2017-04-01

    garnet pyroxenite (recycled altered oceanic crust) and garnet peridotite can generate melts with low-δ26Mg signature for low-degree partial melting. Therefore, if the degree of partial melting can be independently constrained for the generation of parental OIB magma, the Mg isotopic compositions of their source can be estimated to investigate the chemical heterogeneity of the deep mantle.

  1. Refertilization of oceanic mantle by old depleted melts beneath a slow spreading ridge: An Os isotope study of the peridotites drilled at ODP Site 1274 (15°20 FZ, Mid-Atlantic Ridge)

    NASA Astrophysics Data System (ADS)

    Alard, O.; Gréau, Y.; Godard, M.; Lorand, J.-P.; O'Reilly, S. Y.

    2009-04-01

    During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge was drilled (Site 1274) in the vicinity of the Fifteen-Twenty Fracture Zone that offsets one of the slowest portions of the spreading ridge. Bulk rock geochemistry indicates that Site 1274 peridotites represent the most depleted peridotites sampled so far at a slow spreading ridge. Their composition can be explained by open system partial melting and incomplete melt extraction; nevertheless, observation of interstitial clinopyroxene (Cpx) and local variations in bulk trace element contents suggests the occurrence of a late melt freezing reaction with melts from different mantle sources (Godard et al, 2008). Recent studies of mantle-derived peridotites have shown that several sulphide populations, characterised by different microstructural occurrences and elemental and isotopic compositions, coexist at the thin section scale. Thus by establishing the Re-Os isotopic systematic of the different sulphide populations together with an in situ trace element characterization of the associated silicates, we can shed some light on the intricacy of melt-extraction and melt-percolation processes beneath mid-ocean ridges. Site 1274 peridotites show several sulphide populations. Sulphides 1 (Sulf-1) are either enclosed in relict Ol1 and Opx1, or form isolated round blebs of sulphide within the serpentine matrix (Ol1). Their mineralogical and microstructural features are mostly characteristic of sulphide residual after melting. Sulphide-2 are partly embayed in Opx1 porphyroclasts and show an abnormal Cu-rich composition more akin to the solidification products of a sulphide partial melt. Finally, a third type of magmatic sulphide (Sulf-3) formed of pentlandite and primary bornite occurs as large (100-500 µm) convoluted patches intimately associated with Cpx2. Microstructural features suggest that the Sulf3-Cpx2(±Spl2) assemblage represents the crystallization product of a Cu-Ni-rich sulphide-bearing melt

  2. Time-Variable Gravity from Satellite Laser-Ranging: The Low-Degree Components and Their Connections with Geophysical/Climatic Changes

    NASA Technical Reports Server (NTRS)

    Chao, Benjamin F.; Cox, Christopher M.

    2004-01-01

    Satellite laser-ranging (SLR) has been observing the tiny variations in Earth s global gravity for over 2 decades. The oblateness of the Earth's gravity field, J2, has been observed to undergo a secular decrease of J2 due mainly to the post-glacial rebound of the mantle. Sometime around 1998 this trend reversed quite suddenly. This reversal persisted until 2001, at which point the atmosphere-corrected time series appears to have reversed yet again towards normal. This anomaly signifies a large interannual change in global mass distribution. A number of possible causes have been considered, with oceanic mass redistribution as the leading candidate although other effects, such as glacial melting and core effects may be contributing. In fact, a strong correlation has been found between the J2 variability and the Pacific decadal oscillation. It is relatively more difficult to solve for corresponding signals in the shorter wavelength harmonics from the existing SLR-derived time variable gravity results, although it appears that geophysical fluid mass transport is being observed. For example, the recovered J3 time series shows remarkable agreement with NCEP-derived estimates of atmospheric gravity variations. Likewise, some of the non-zonal harmonic components have significant interannual signal that appears to be related to mass transport related to climatic effects such as El Nino Southern Oscillation. We will present recent updates on the J2 evolution, as well as a monthly time sequence of low-degree component map of the time-variable gravity complete through degree 4, and examine possible geophysical/climatic causes.

  3. Importance of the Small-Scale Processes Melting, Plate Boundary Formation and Mineralogy on the Large-Scale, Long-Term Thermo-Chemical Evolution of Earth's Mantle-Plate System

    NASA Astrophysics Data System (ADS)

    Tackley, P.

    2015-12-01

    Seismic observations of the deep Earth reveal the presence of two large low shear velocity provinces (LLSVPs) that are typically inferred to be dense chemically-distinct material, as well as discontinuities that are typically linked to the post-perovskite (pPv) phase transition. Several possible origins of chemically-dense material have been proposed, including recycling of mid-ocean ridge basalt (MORB), primordial differentiation events, crystallisation of a basal magma ocean, or some combination of these creating a basal melange (BAM; Tackley 2012 Earth Sci. Rev.). Each of these possibilities would result in a different composition hence different mineralogy. In order to constrain this we have been running calculations of thermo-chemical mantle evolution over 4.5 billion years that include melting-induced differentiation, plate tectonics induced by strongly temperature-dependent viscosity and plastic yielding, core cooling and compressibility with reasonable assumptions about the pressure-dependence of other material properties. Some of our simulations start from a magma ocean state so initial layering is developed self-consistently. Already-published results (Nakagawa et al., 2009 GCubed, 2010 PEPI, 2012 GCubed) already indicate the importance of exact MORB composition on the amount of MORB segregating above the CMB, which in turn influences mantle thermal structure and the evolution of the core and geodynamo. In more recent results we have been additionally including primordial material. We find that melting-induced differentiation has several first-order effects on the dynamics, including (i) making plate tectonics easier (through stresses associated with lateral variations in crustal thickness) and (ii) reducing heat flux through the CMB (due to the build-up of dense material above the CMB); also (iii) tectonic mode (continuous plate tectonics, episodic lid or stagnant lid) also makes a first-order difference to mantle structure and dynamics. This emphasises

  4. 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

  5. Trace element composition of silicate inclusions in sub-lithospheric diamonds from the Juina-5 kimberlite: Evidence for diamond growth from slab melts

    NASA Astrophysics Data System (ADS)

    Thomson, A. R.; Kohn, S. C.; Bulanova, G. P.; Smith, C. B.; Araujo, D.; Walter, M. J.

    2016-11-01

    The trace element compositions of inclusions in sub-lithospheric diamonds from the Juina-5 kimberlite, Brazil, are presented. Literature data for mineral/melt partition coefficients were collated, refitted and employed to interpret inclusion compositions. As part of this process an updated empirical model for predicting the partitioning behaviour of trivalent cations for garnet-melt equilibrium calibrated using data from 73 garnet-melt pairs is presented. High levels of trace element enrichment in inclusions interpreted as former calcium silicate perovskite and majoritic garnet preclude their origin as fragments of an ambient deep mantle assemblage. Inclusions believed to represent former bridgmanite minerals also display a modest degree of enrichment relative to mantle phases. The trace element compositions of 'NAL' and 'CF phase' minerals are also reported. Negative Eu, Ce, and Y/Ho anomalies alongside depletions of Sr, Hf and Zr in many inclusions are suggestive of formation from a low-degree carbonatitic melt of subducted oceanic crust. Observed enrichments in garnet and 'calcium perovskite' inclusions limit depths of melting to less than 600 km, prior to calcium perovskite saturation in subducting assemblages. Less enriched inclusions in sub-lithospheric diamonds from other global localities may represent deeper diamond formation. Modelled source rock compositions that are capable of producing melts in equilibrium with Juina-5 'calcium perovskite' and majorite inclusions are consistent with subducted MORB. Global majorite inclusion compositions suggest a common process is responsible for the formation of many superdeep diamonds, irrespective of geographic locality. Global transition zone inclusion compositions are reproduced by fractional crystallisation from a single parent melt, suggesting that they record the crystallisation sequence and melt evolution during this interaction of slab melts with ambient mantle. All observations are consistent with the

  6. Heat sources for mantle plumes

    NASA Astrophysics Data System (ADS)

    Beier, C.; Rushmer, T.; Turner, S. P.

    2008-06-01

    Melting anomalies in the Earth's upper mantle have often been attributed to the presence of mantle plumes that may originate in the lower mantle, possibly from the core-mantle boundary. Globally, mantle plumes exhibit a large range in buoyancy flux that is proportional to their temperature and volume. Plumes with higher buoyancy fluxes should have higher temperatures and experience higher degrees of partial melting. This excess heat in mantle plumes could reflect either (1) an enrichment of the heat-producing elements (HPE: U, Th, K) in their mantle source leading to an increase of heat production by radioactive decay, (2) material transport from core to mantle (either advective or diffusive), or (3) conductive heat transport across the core-mantle boundary. The advective/diffusive transport of heat may result in a physical contribution of material from the core to the lower mantle. If core material is incorporated into the lower mantle, mantle plumes with a higher buoyancy flux should have higher core tracers, e.g., increased 186Os, 187Os, and Fe concentrations. Geophysical and dynamic modeling indicate that at least Afar, Easter, Hawaii, Louisville, and Samoa may all originate at the core-mantle boundary. These plumes encompass the whole range of known buoyancy fluxes from 0.9 Mg s-1 (Afar) to 8.7 Mg s-1 (Hawaii), providing evidence that the buoyancy flux is largely independent of other geophysical parameters. In an effort to explore whether the heat-producing elements are the cause of excess heat we looked for correlations between fractionation-corrected concentrations of the HPE and buoyancy flux. Our results suggest that there is no correlation between HPE concentrations and buoyancy flux (with and without an additional correction for variable degrees of partial melting). As anticipated, K, Th, and U are positively correlated with each other (e.g., Hawaii, Iceland, and Galapagos have significantly lower concentrations than, e.g., Tristan da Cunha, the Canary

  7. Petrogenesis of nephelinites from the Tarim Large Igneous Province, NW China: Implications for mantle source characteristics and plume-lithosphere interaction

    NASA Astrophysics Data System (ADS)

    Cheng, Zhiguo; Zhang, Zhaochong; Hou, Tong; Santosh, M.; Zhang, Dongyang; Ke, Shan

    2015-04-01

    The nephelinite exposed in the Wajilitage area in the northwestern margin of the Tarim large igneous province (TLIP), Xinjiang, NW China display porphyritic textures with clinopyroxene, nepheline and olivine as the major phenocryst phases, together with minor apatite, sodalite and alkali feldspar. The groundmass typically has cryptocrystalline texture and is composed of crystallites of clinopyroxene, nepheline, Fe-Ti oxides, sodalite, apatite, rutile, biotite, amphibole and alkali feldspar. We report rutile SIMS U-Pb age of 268 ± 30 Ma suggesting that the nephelinite may represent the last phase of the TLIP magmatism, which is also confirmed by the field relation. The nephelinite shows depleted Sr-Nd isotopic compositions with age-corrected 87Sr/86Sr and εNd(t) values of 0.70348-0.70371 and + 3.28 to + 3.88 respectively indicating asthenospheric mantle source. Based on the reconstructed primary melt composition, the depth of magma generation is estimated as 115-140 km and the temperatures of mantle melting as 1540-1575 °C. The hotter than normal asthenospheric mantle temperature suggests the involvement of mantle thermal plume. The Mg isotope values display a limited range of δ26Mg from - 0.35 to - 0.55‰, which are lower than the mantle values (- 0.25‰). The Mg isotopic compositions, combined with the Sr-Nd isotopes and major and trace element data suggest that the Wajilitage nephelinite was most likely generated by low-degree partial melting of the hybridized carbonated peridotite/eclogite source, which we correlate with metasomatism by subducted carbonates within the early-middle Paleozoic convergent regime. A plume-lithosphere model is proposed with slight thinning of the lithosphere and variable depth and degree of melting of the carbonated mantle during the plume-lithosphere interaction. This model also accounts for the variation in lithology of the TLIP.

  8. Deep melting of recycled crust from stagnant slab and genesis of alkaline basalts in eastern China

    NASA Astrophysics Data System (ADS)

    Chen, L.; Hofmann, A. W.; Zeng, G.; Yu, X.

    2013-12-01

    Recycled oceanic crust from the core-mantle boundary has been widely accepted as important components in the sources of many hot spot-associated basalts. However, other than the core-mantle boundary, the mantle transition zone may be the other ';graveyard' for subducted crust, because the subducted slabs are usually stagnant there. To date, whether and how such recycled crust of stagnant slab contributes to the genesis of intraplate basalts is still poorly understood. In eastern China, the subducted Pacific slab is stagnant as a high-velocity anomaly in the mantle transition zone, and Cenozoic alkaline basalts are widely distributed as typical intraplate basalts in continental background, which provide a chance to explore this question. Here we found that alkaline basalts from Shandong, a province just above the eastern front of the stagnant Pacific slab in central eastern China, can be mainly produced by mixing of two endmember components. The two components are represented by two kinds of alkaline basalts which have similar (and moderately depleted) isotopic compositions but complementary (sub-mantle and super-mantle) incompatible element ratios of K/U, Ba/Th, and Ti/Gd. These complementary geochemical signatures are accordant with those of carbonatitic melts and solid residue from recycled young oceanic crust, respectively. This observation supports that recycled crust from the stagnant slab has experienced recent low-degree melting in deep upper mantle, possibly in an adiabatic process induced by a kind of edge flow at the eastern front of the stagnant slab, and feed the shallow sources of alkaline basalts with two kinds of components, carbonatitic liquids and eclogitic residues, respectively.

  9. Wet inside and out? Constraints on water in the Martian mantle and on outgassed water, based on melt inclusions in SNC meteorites

    NASA Technical Reports Server (NTRS)

    Mcsween, H. Y., Jr.; Harvey, R. P.

    1993-01-01

    Constraints on the volatile inventory and outgassing history of Mars are critical to understanding the origin of ancient valley systems and paleoclimates. Planetary accretion models for Mars allow either a volatile-rich or volatile-poor mantle, depending on whether the accreted materials were fully oxidized or whether accretion was homogeneous so that water was lost through reaction with metallic iron. The amount of water that has been outgassed from the interior is likewise a contentious subject, and estimates of globally distributed water based on various geochemical and geological measurements vary from a few meters to more than a thousand meters. New data on SNC meteorites, which are thought to be Martian igneous rocks, provide constraints on both mantle and outgassed water.

  10. Mantle to surface degassing of alkalic magmas at Erebus volcano, Antarctica

    USGS Publications Warehouse

    Oppenheimer, C.; Moretti, R.; Kyle, P.R.; Eschenbacher, A.; Lowenstern, J. B.; Hervig, R.L.; Dunbar, N.W.

    2011-01-01

    Continental intraplate volcanoes, such as Erebus volcano, Antarctica, are associated with extensional tectonics, mantle upwelling and high heat flow. Typically, erupted magmas are alkaline and rich in volatiles (especially CO2), inherited from low degrees of partial melting of mantle sources. We examine the degassing of the magmatic system at Erebus volcano using melt inclusion data and high temporal resolution open-path Fourier transform infrared (FTIR) spectroscopic measurements of gas emissions from the active lava lake. Remarkably different gas signatures are associated with passive and explosive gas emissions, representative of volatile contents and redox conditions that reveal contrasting shallow and deep degassing sources. We show that this unexpected degassing signature provides a unique probe for magma differentiation and transfer of CO2-rich oxidised fluids from the mantle to the surface, and evaluate how these processes operate in time and space. Extensive crystallisation driven by CO2 fluxing is responsible for isobaric fractionation of parental basanite magmas close to their source depth. Magma deeper than 4kbar equilibrates under vapour-buffered conditions. At shallower depths, CO2-rich fluids accumulate and are then released either via convection-driven, open-system gas loss or as closed-system slugs that ascend and result in Strombolian eruptions in the lava lake. The open-system gases have a reduced state (below the QFM buffer) whereas the closed-system gases preserve their deep oxidised signatures (close to the NNO buffer). ?? 2011 Elsevier B.V.

  11. Mantle to surface degassing of alkalic magmas at Erebus volcano, Antarctica

    NASA Astrophysics Data System (ADS)

    Oppenheimer, Clive; Moretti, Roberto; Kyle, Philip R.; Eschenbacher, Al; Lowenstern, Jacob B.; Hervig, Richard L.; Dunbar, Nelia W.

    2011-06-01

    Continental intraplate volcanoes, such as Erebus volcano, Antarctica, are associated with extensional tectonics, mantle upwelling and high heat flow. Typically, erupted magmas are alkaline and rich in volatiles (especially CO 2), inherited from low degrees of partial melting of mantle sources. We examine the degassing of the magmatic system at Erebus volcano using melt inclusion data and high temporal resolution open-path Fourier transform infrared (FTIR) spectroscopic measurements of gas emissions from the active lava lake. Remarkably different gas signatures are associated with passive and explosive gas emissions, representative of volatile contents and redox conditions that reveal contrasting shallow and deep degassing sources. We show that this unexpected degassing signature provides a unique probe for magma differentiation and transfer of CO 2-rich oxidised fluids from the mantle to the surface, and evaluate how these processes operate in time and space. Extensive crystallisation driven by CO 2 fluxing is responsible for isobaric fractionation of parental basanite magmas close to their source depth. Magma deeper than 4 kbar equilibrates under vapour-buffered conditions. At shallower depths, CO 2-rich fluids accumulate and are then released either via convection-driven, open-system gas loss or as closed-system slugs that ascend and result in Strombolian eruptions in the lava lake. The open-system gases have a reduced state (below the QFM buffer) whereas the closed-system gases preserve their deep oxidised signatures (close to the NNO buffer).

  12. Scales of mantle heterogeneity

    NASA Astrophysics Data System (ADS)

    Moore, J. C.; Akber-Knutson, S.; Konter, J.; Kellogg, J.; Hart, S.; Kellogg, L. H.; Romanowicz, B.

    2004-12-01

    A long-standing question in mantle dynamics concerns the scale of heterogeneity in the mantle. Mantle convection tends to both destroy (through stirring) and create (through melt extraction and subduction) heterogeneity in bulk and trace element composition. Over time, these competing processes create variations in geochemical composition along mid-oceanic ridges and among oceanic islands, spanning a range of scales from extremely long wavelength (for example, the DUPAL anomaly) to very small scale (for example, variations amongst melt inclusions). While geochemical data and seismic observations can be used to constrain the length scales of mantle heterogeneity, dynamical mixing calculations can illustrate the processes and timescales involved in stirring and mixing. At the Summer 2004 CIDER workshop on Relating Geochemical and Seismological Heterogeneity in the Earth's Mantle, an interdisciplinary group evaluated scales of heterogeneity in the Earth's mantle using a combined analysis of geochemical data, seismological data and results of numerical models of mixing. We mined the PetDB database for isotopic data from glass and whole rock analyses for the Mid-Atlantic Ridge (MAR) and the East Pacific Rise (EPR), projecting them along the ridge length. We examined Sr isotope variability along the East Pacific rise by looking at the difference in Sr ratio between adjacent samples as a function of distance between the samples. The East Pacific Rise exhibits an overall bowl shape of normal MORB characteristics, with higher values in the higher latitudes (there is, however, an unfortunate gap in sampling, roughly 2000 km long). These background characteristics are punctuated with spikes in values at various locations, some, but not all of which are associated with off-axis volcanism. A Lomb-Scargle periodogram for unevenly spaced data was utilized to construct a power spectrum of the scale lengths of heterogeneity along both ridges. Using the same isotopic systems (Sr, Nd

  13. The molecular structure of melts along the carbonatite-kimberlite-basalt compositional joint: CO2 and polymerisation

    NASA Astrophysics Data System (ADS)

    Moussallam, Yves; Florian, Pierre; Corradini, Dario; Morizet, Yann; Sator, Nicolas; Vuilleumier, Rodolphe; Guillot, Bertrand; Iacono-Marziano, Giada; Schmidt, Burkhard C.; Gaillard, Fabrice

    2016-01-01

    Transitional melts, intermediate in composition between silicate and carbonate melts, form by low degree partial melting of mantle peridotite and might be the most abundant type of melt in the asthenosphere. Their role in the transport of volatile elements and in metasomatic processes at the planetary scale might be significant yet they have remained largely unstudied. Their molecular structure has remained elusive in part because these melts are difficult to quench to glass. Here we use FTIR, Raman, 13C and 29Si NMR spectroscopy together with First Principle Molecular Dynamic (FPMD) simulations to investigate the molecular structure of transitional melts and in particular to assess the effect of CO2 on their structure. We found that carbon in these glasses forms free ionic carbonate groups attracting cations away from their usual 'depolymerising' role in breaking up the covalent silicate network. Solution of CO2 in these melts strongly modifies their structure resulting in a significant polymerisation of the aluminosilicate network with a decrease in NBO/Si of about 0.2 for every 5 mol% CO2 dissolved. This polymerisation effect is expected to influence the physical and transport properties of transitional melts. An increase in viscosity is expected with increasing CO2 content, potentially leading to melt ponding at certain levels in the mantle such as at the lithosphere-asthenosphere boundary. Conversely an ascending and degassing transitional melt such as a kimberlite would become increasingly fluid during ascent hence potentially accelerate. Carbon-rich transitional melts are effectively composed of two sub-networks: a carbonate and a silicate one leading to peculiar physical and transport properties.

  14. Compositional variations and heterogeneity in fertile lithospheric mantle: peridotite xenoliths in basalts from Tariat, Mongolia

    NASA Astrophysics Data System (ADS)

    Rubolini, Diego; Ambrosini, Roberto; Caffi, Mario; Brichetti, Pierandrea; Armiraglio, Stefano; Saino, Nicola

    2007-10-01

    Clinopyroxene-rich, poorly metasomatised spinel lherzolites are rare worldwide but predominate among xenoliths in five Quaternary basaltic eruption centres in Tariat, central Mongolia. High-precision analyses of the most fertile Tariat lherzolites are used to evaluate estimates of primitive mantle compositions; they indicate Mg#PM = 0.890 while lower Mg# in the mantle are likely related to metasomatic enrichments in iron. Within a 10 × 20 km area, and between ~45 and ≥60 km depth, the sampled xenoliths suggest that the Tariat mantle does not show km-scale chemical heterogeneities and mainly consists of residues after low-degree melt extraction at 1 3 GPa. However, accessory (<1%) amphibole and phlogopite are unevenly distributed beneath the eruption centres. Ca abundances in olivine are controlled by temperature whereas Al and Cr abundances also depend on Cr/Al in coexisting spinel. Comparisons of conventional and high-precision analyses obtained for 30 xenoliths show that high-quality data, in particular for whole-rocks and olivines, are essential to constrain the origin of mantle peridotites.

  15. Time-Variable Gravity from Satellite-Laser-Ranging and Doppler Measurements: An Update on the Low-degree components as well as the connections with Geophysical/Climatic Processes

    NASA Technical Reports Server (NTRS)

    Cox, Christopher M.; Chao, Benjamin F.; Au, Andrew Y.; Boy, J.-P.

    2003-01-01

    The oblateness of the Earth's gravity field, 52, has long been observed to undergo a slight decrease due to post-glacial rebound of the mantle. Sometime around 1998 this trend reversed quite suddenly. This reversal persisted until 2001, at which point the atmosphere-corrected time series appears to have reversed yet again. Presently, the time series appears to be returning to the value that would nominally have been reached had the anomaly not occurred. This anomaly signifies a large interannual change in global mass distribution whose J2 effect overshadows that of the post-glacial rebound over such timescales. A number of possible causes have been considered, with oceanic mass redistribution as the leading candidate although other effects, such as glacial melting and core effects may be contributing. The amount by which J2 returns to it's nominal value provides a valuable constraint on the separation of the causes, and will be considered. We will present our latest Satellite Laser Ranging and DORIS Doppler derived time series for J2, and various other low-degree harmonic terms, as well as our investigations into the causes. In addition, we will show the comparison of the J2 results with those derived from CHAMP, as computed at NASA GSFC, and the recently released GRACE gravity model.

  16. Lithospheric Mantle heterogeneities beneath northern Santa Cruz province, Argentina

    NASA Astrophysics Data System (ADS)

    Mundl, Andrea; Ntaflos, Theodoros; Bjerg, Ernesto

    2013-04-01

    interstitial clinopyroxene appears to be of metasomatic origin. The clinopyroxene from cumulate dunites has depleted LREE abundances and low HREE indicating that they have been formed from residual melts. In contrast, clinopyroxene from mantle dunites has enriched LREE (10 x PM) and LILE suggesting that the metasomatic agent was fluid-rich silicate melt. Calculated equilibrium conditions cover a wide range, from 800 to 1100 °C. Considering the crustal thickness in the area being around 35 km, a pressure between 12 and 17 kbar can be assumed as reasonable, indicating that xenoliths were extracted from shallow depths, in the order of 40 to 60 km. Model calculations have shown that the Lithospheric Mantle beneath Don Camilo is fertile and that spinel peridotites experienced low degrees of partial melting (2-8% batch melting in the spinel peridotite field). The metasomatic agent was a fluid rich silicate melt presumably similar to that which affected the xenoliths from Cerro Clark locality, north of Don Camilo. The clinopyroxenes with the highest Sr and lowest Nd isotopic signatures suggest that the metasomatism was an old event apparently not associated to the interaction of the Lithospheric Mantle in southern Patagonia with downgoing Nazca and Antarctic plates.

  17. Redox conditions for mantle plumes

    NASA Astrophysics Data System (ADS)

    Heister, L. E.; Lesher, C. E.

    2005-12-01

    The vanadium to scandium ratio (V/Sc) for basalts from mid-ocean ridge (MOR) and arc environments has been proposed as a proxy for fO2 conditions during partial melting (e.g. [1] and [2]). Contrary to barometric measurements of the fO2 of primitive lavas, the V/Sc ratio of the upper mantle at mid-ocean ridges and arcs is similar, leading previous authors to propose that the upper mantle has uniform redox potential and is well-buffered. We have attempted to broaden the applicability of the V/Sc parameter to plume-influenced localities (both oceanic and continental), where mantle heterogeneities associated with recycled sediments, mafic crust, and metasomatized mantle, whether of shallow or deep origin, exist. We find that primitive basalts from the North Atlantic Igneous Province (NAIP), Hawaii (both the Loa and Kea trends), Deccan, Columbia River, and Siberian Traps show a range of V/Sc ratios that are generally higher (average ~9) than those for MOR (average ~ 6.7) or arc (average ~7) lavas. Based on forward polybaric decompression modeling, we attribute these differences to polybaric melting and melt segregation within the garnet stability field rather than the presence of a more oxidized mantle in plume-influenced settings. Like MORB, the V/Sc ratios for plume-influenced basalts can be accounted for by an oxidation state approximately one log unit below the Ni-NiO buffer (NNO-1). Our analysis suggests that source heterogeneities have little, if any, resolvable influence on mantle redox conditions, although they have significant influence on the trace element and isotopic composition of mantle-derived melts. We suggest that variations in the redox of erupted lavas is largely a function of shallow lithospheric processes rather than intrinsic to the mantle source, regardless of tectonic setting. [1] Li and Lee (2004) EPSL, [2] Lee et al. (2005) J. of Petrology

  18. Mantle metasomatism

    SciTech Connect

    Menzies, M.; Hawkesworth, C.

    1986-01-01

    The concept of metasomatism and its role in the geochemical enrichment and depletion processes in upper mantle rocks remains contentious. This volume makes a comprehensive contribution to the study of metasomatic and enrichment processes: origin and importance in determining trace element and isotopic heterogeneity in the lithospheric mantle. It begins with a theoretical thermodynamic and experimental justification for metasomatism and proceeds to present evidence for this process from the study of mantle xenoliths. Finally the importance of metasomatism in relation to basaltic volcanism is assessed. The contents are as follows: Dynamics of Translithospheric Migration of Metasomatic Fluid and Alkaline Magma. Solubility of Major and Trace Elements in Mantle Metasomatic Fluids: Experimental Constraints. Mineralogic and Geochemical Evidence for Differing Styles of Metasomatism in Spinel Lherzolite Xenoliths: Enriched Mantle Source Regions of Basalts. Characterization of Mantle Metasomatic Fluids in Spinel Lherzolites and Alkali Clinophyroyxenites from the West Eifel and South-West Uganda. Metasomatised Harzburgites in Kimberlite and Alkaline Magmas: Enriched Resites and ''Flushed'' Lherzolites. Metasomatic and Enrichment Phenomena in Garnet-Peridotite Facies Mantle Xenoliths from the Matsoku Kimberlite Pipe Lesotho. Evidence for Mantle Metasomatism in Periodite Nodules from the Kimberley Pipes South Africa. Metasomatic and Enrichment Processes in Lithospheric Peridotites, an Effective of Asthenosphere-Lithosphere Interaction. Isotope Variations in Recent Volcanics: A Trace Element Perspective. Source Regions of Mid-Ocean Ridge Basalts: Evidence for Enrichment Processes. The Mantle Source for the Hawaiian Islands: Constraints from the Lavas and Ultramafic Inclusions.

  19. Not so hot "hot spots" in the oceanic mantle.

    PubMed

    Bonath, E

    1990-10-05

    Excess volcanism and crustal swelling associated with hot spots are generally attributed to thermal plumes upwelling from the mantle. This concept has been tested in the portion of the Mid-Atlantic Ridge between 34 degrees and 45 degrees (Azores hot spot). Peridotite and basalt data indicate that the upper mantle in the hot spot has undergone a high degree of melting relative to the mantle elsewhere in the North Atlantic. However, application of various geothermometers suggests that the temperature of equilibration of peridotites in the mantle was lower, or at least not higher, in the hot spot than elsewhere. The presence of H(2)O-rich metasomatized mantle domains, inferred from peridotite and basalt data, would lower the melting temperature of the hot spot mantle and thereby reconcile its high degree ofmelting with the lack of a mantle temperature anomaly. Thus, some so-called hot spots might be melting anomalies unrelated to abnormally high mantle temperature or thermal plumes.

  20. Mafic and silica-rich glasses in mantle xenoliths from Wau-en-Namus, Libya: Textural and geochemical evidence for peridotite-melt reactions

    NASA Astrophysics Data System (ADS)

    Miller, C.; Zanetti, A.; Thöni, M.; Konzett, J.; Klötzli, U.

    2012-01-01

    Anhydrous spinel peridotite xenoliths in Quaternary nepheline-basanite and melilite- or sodalite-bearing lavas of the Wau-en-Namus volcano in S Libya range from lherzolites to harzburgites recording melt extraction in a shallow setting (≤ 2 GPa). Primary clinopyroxenes have distinct trace element characteristics documenting LILE (large ion lithophile element) depletion or enrichment events predating the formation of glass pockets and veins in the xenoliths. These glasses are aluminous and alkali-rich, range in composition from ultrabasic to silicic (43-67 wt.% SiO 2) and may contain empty vugs and micro-phenocrysts of olivine, clinopyroxene, spinel, plagioclase, sodalite, apatite that are similar in composition to phenocrysts in the host lavas. Reactions of infiltrating melt and xenolith minerals are documented by diffuse Fe-Ca-rich rims of olivine in contact with glass, and by spongy-textured reaction domains caused by incongruent dissolution of primary pyroxenes and spinel. Some glasses have trace element characteristics similar to that of the host Ne-basanite, suggesting they were derived from the same source during entrainment and transport to the surface. Incompatible element enrichment and Sr-Nd isotopic compositions of the analyzed host lava are similar to HIMU (high μ; μ = 238Pb/ 204Pb)-type magmas, but the Pb isotopic composition is less radiogenic compared to other intra-plate Neogene magmatic rocks from N Africa.

  1. Crystal chemical control of clinopyroxene-melt partitioning in the Di-Ab-An system: implications for elemental fractionations in the depleted mantle

    SciTech Connect

    Lundstrom, C. C.; Shaw, H. F.; Ryerson, F. J.; Williams, Q.; Gill, J.

    1998-08-01

    The partitioning of fifteen trace elements (Rb, Sr, Zr, Nb, Ba, La, Ce, Nd, Sm, Gd, Yb, Hf, Ta, Pb and Th) between clinopyroxene and synthetic melt has been studied in two compositions along an isotherm in the diopside-albite-anorthite ternary at 1 bar pressure. The two compositions correspond to ~ Di65An35 and ~ Di55Ab45 and produce clinopyroxenes distinct in chemistry while melt compositions range from 49 wt % SiO2 to 61 wt. % SiO2. The partition coefficients of high field strength elements (HFSE) increase by factors of 2 to 8 in Di-An experiments relative to Di-Ab experiments while other elements show very little change (+/- 20%) between compositions. The change in HFSE partitioning correlates with increases in tetrahedral Al2O3 (IVAl) content of clinopyroxenes in the anorthite-bearing experiments. Changes in DTa/DNb also correlate with IVA1 based on a survey of previously published determinations.

  2. Nature of the lithospheric mantle beneath the Arabian Shield and genesis of Al-spinel micropods: Evidence from the mantle xenoliths of Harrat Kishb, Western Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Ahmed, Ahmed H.; Moghazi, Abdel Kader M.; Moufti, Mohamed R.; Dawood, Yehia H.; Ali, Kamal A.

    2016-01-01

    The Harrat Kishb area of western Saudi Arabia is part of the Cenozoic volcanic fields in the western margin of the Arabian Shield. Numerous fresh ultramafic xenoliths are entrained in the basanite lava of Harrat Kishb, providing an opportunity to study the nature and petrogenetic processes involved in the evolution of the lithospheric mantle beneath the Arabian Shield. Based on the petrological characteristics and mineralogical compositions, the majority of the mantle xenoliths ( 92%) are peridotites (lherzolites and pyroxene-bearing harzburgites); the remaining xenoliths ( 8%) are unusual spinel-rich wehrlites containing black Al-spinel micropods. The two types of mantle xenoliths display magmatic protogranular texture. The peridotite xenoliths have high bulk-rock Mg#, high forsterite (Fo90-Fo92) and NiO (0.24-0.46 wt.%) contents of olivine, high clinopyroxene Mg# (0.91-0.93), variable spinel Cr# (0.10-0.49, atomic ratio), and approximately flat chondrite-normalized REE patterns. These features indicate that the peridotite xenoliths represent residues after variable degrees of melt extraction from fertile mantle. The estimated P (9-16 kbar) and T (877-1227 °C) as well as the oxidation state (∆logfO2 = - 3.38 to - 0.22) under which these peridotite xenoliths originated are consistent with formation conditions similar to most sub-arc abyssal-type peridotites worldwide. The spinel-rich wehrlite xenoliths have an unusual amount ( 30 vol.%) of Al-spinel as peculiar micropods with very minor Cr2O3 content (< 1 wt.%). Olivines of the spinel-rich wehrlites have low-average Fo (Fo81) and NiO (0.18 wt.%) contents, low-average cpx Mg# (0.79), high average cpx Al2O3 content (8.46 wt.%), and very low-average spinel Cr# (0.01). These features characterize early mantle cumulates from a picritic melt fraction produced by low degrees of partial melting of a garnet-bearing mantle source. The relatively high Na2O and Al2O3 contents of cpx suggest that the spinel-rich wehrlites

  3. Melting of peridotite to 140 gigapascals.

    PubMed

    Fiquet, G; Auzende, A L; Siebert, J; Corgne, A; Bureau, H; Ozawa, H; Garbarino, G

    2010-09-17

    Interrogating physical processes that occur within the lowermost mantle is a key to understanding Earth's evolution and present-day inner composition. Among such processes, partial melting has been proposed to explain mantle regions with ultralow seismic velocities near the core-mantle boundary, but experimental validation at the appropriate temperature and pressure regimes remains challenging. Using laser-heated diamond anvil cells, we constructed the solidus curve of a natural fertile peridotite between 36 and 140 gigapascals. Melting at core-mantle boundary pressures occurs at 4180 ± 150 kelvin, which is a value that matches estimated mantle geotherms. Molten regions may therefore exist at the base of the present-day mantle. Melting phase relations and element partitioning data also show that these liquids could host many incompatible elements at the base of the mantle.

  4. The system Na2CO3-CaCO3-MgCO3 at 6 GPa and 900-1250°C and its relation to the partial melting of carbonated mantle

    NASA Astrophysics Data System (ADS)

    Shatskiy, Anton; Litasov, Konstantin D.; Sharygin, Igor S.; Egonin, Ilya A.; Mironov, Aleksandr M.; Palyanov, Yuri N.; Ohtani, Eiji

    2016-01-01

    In order to constrain the Na2CO3-CaCO3-MgCO3 T-X diagram at 6 GPa in addition to the binary and pseudo-binary systems we conducted experiments along the Na2CO3-Ca0.5Mg0.5CO3 join. At 900-1000°C, melting does not occur and isothermal sections are presented by one-, two- and three-phase regions containing Ca-bearing magnesite, aragonite, Na2CO3 (Na2) and Na2(Ca1-0.9Mg0-0.1)3-4(CO3)4-5 (Na2Ca3-4), Na4(Ca1-0.6Mg0-0.4)(CO3)3 (Na4Ca), Na2(Ca0-0.08Mg1-0.92)(CO3)2 (Na2Mg) phases with intermediate compositions. The minimum melting point locates between 1000°C and 1100°C. This point would resemble that of three eutectics: Mgs-Na2Ca3-Na2Mg, Na2Mg-Na2Ca3-Na4Ca or Na2Mg-Na4Ca-Na2, in the compositional interval of [45Na2CO3.55(Ca0.6Mg0.4)CO3]-[60Na2CO3.40Ca0.6Mg0.4CO3]. The liquidus projection has seven primary solidification phase regions for Mgs, Dol, Arg, Na2Ca3, Na4Ca, Na2 and Na2Mg. The results suggest that extraction of Na and Ca from silicate to carbonate components has to decrease minimum melting temperature of carbonated mantle rocks to 1000-1100°C at 6 GPa and yields Na-rich dolomitic melt with a Na# (Na2O/(Na2O + CaO + MgO)) ≥ 28 mol%.

  5. Where is mantle's carbon?

    NASA Astrophysics Data System (ADS)

    Oganov, A. R.; Ono, S.; Ma, Y.

    2008-12-01

    Due to the strongly reducing conditions (the presence of metallic iron was suggested both by experiments [1] and theory [2]), diamond was believed to be the main host of carbon through most of the lower mantle [3]. We showed [4] that cementite Fe3C is another good candidate to be the main host of "reduced" carbon in the mantle, reinforcing an earlier hypothesis [5]. The fate of "oxidised" carbon (in subducted slabs) is of particular importance - if carbonates decompose producing fluid CO2, this would have important implications for the chemistry and rheology of the mantle. Knowledge of crystal structures and phase diagrams of carbonates is crucial here. The high-pressure structures of CaCO3 were predicted [6] and subsequently verified by experiments. For MgCO3, Isshiki et al. [7] found a new phase above 110 GPa, and several attempts were made to solve it [8,9]. Here [4], using an evolutionary algorithm for crystal structure prediction [10], we show that there are two post-magnesite phases at mantle-relevant pressure range, one stable at 82-138 GPa, and the other from 138 GPa to ~160 GPa. Both are based on threefold rings of CO4-tetrahedra and are more favourable than all previously proposed structures. We show that through most of the P-T conditions of the mantle, MgCO3 is the major host of oxidized carbon in the Earth. We predict the possibility of CO2 release at the very bottom of the mantle (in SiO2-rich basaltic part of subducted slabs), which could enhance partial melting of rocks and be related to the geodynamical differences between the Earth and Venus. 1.Frost D.J., Liebske C., Langenhorst F., McCammon C.A., Tronnes R.G., Rubie D.C. (2004). Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle. Nature 428, 409-412. 2.Zhang F., Oganov A.R. (2006). Valence and spin states of iron impurities in mantle-forming silicates. Earth Planet. Sci. Lett. 249, 436-443. 3.Luth R.W. (1999). Carbon and carbonates in the mantle. In: Mantle

  6. Rogue mantle helium and neon.

    PubMed

    Albarède, Francis

    2008-02-15

    The canonical model of helium isotope geochemistry describes the lower mantle as undegassed, but this view conflicts with evidence of recycled material in the source of ocean island basalts. Because mantle helium is efficiently extracted by magmatic activity, it cannot remain in fertile mantle rocks for long periods of time. Here, I suggest that helium with high 3He/4He ratios, as well as neon rich in the solar component, diffused early in Earth's history from low-melting-point primordial material into residual refractory "reservoir" rocks, such as dunites. The difference in 3He/4He ratios of ocean-island and mid-ocean ridge basalts and the preservation of solar neon are ascribed to the reservoir rocks being stretched and tapped to different extents during melting.

  7. Early stages of core segregation recorded by Fe isotopes in an asteroidal mantle

    NASA Astrophysics Data System (ADS)

    Barrat, J. A.; Rouxel, O.; Wang, K.; Moynier, F.; Yamaguchi, A.; Bischoff, A.; Langlade, J.

    2015-06-01

    Ureilite meteorites are achondrites that are debris of the mantle of a now disrupted differentiated asteroid rich in carbon. They provide a unique opportunity to study the differentiation processes of such a body. We analyzed the iron isotopic compositions of 30 samples from the Ureilite Parent Body (UPB) including 29 unbrecciated ureilites and one ureilitic trachyandesite (ALM-A) which is at present the sole large crustal sample of the UPB. The δ56Fe of the whole rocks fall within a restricted range, from 0.01 to 0.11‰, with an average of + 0.056 ± 0.008‰, which is significantly higher than that of chondrites. We show that this difference can be ascribed to the segregation of S-rich metallic melts at low degrees of melting at a temperature close to the Fe-FeS eutectic, and certainly before the onset of the melting of the silicates (< 1100 °C), in agreement with the marked S depletions, and the siderophile element abundances of the ureilites. These results point to an efficient segregation of S-rich metallic melts during the differentiation of small terrestrial bodies.

  8. Stagnation and Storage of Strongly Depleted Melts in Slow-Ultraslow Spreading Oceans: Evidence from the Ligurian Tethys

    NASA Astrophysics Data System (ADS)

    Piccardo, Giovanni; Guarnieri, Luisa; Padovano, Matteo

    2013-04-01

    Our studies of Alpine-Apennine ophiolite massifs (i.e., Lanzo, Voltri, Ligurides, Corsica) show that the Jurassic Ligurian Tethys oceanic basin was a slow-ultraslow spreading basin, characterized by the exposures on the seafloor of mantle peridotites with extreme compositional variability. The large majority of these peridotites are made of depleted spinel harzburgites and plagioclase peridotites. The former are interpreted as reactive peridotites formed by the reactive percolation of under-saturated, strongly trace element depleted asthenospheric melts migrated by porous flow through the mantle lithosphere. The latter are considered as refertilized peridotites formed by peridotite impregnation by percolated silica-saturated, strongly trace element depleted melts. Strongly depleted melts were produced as low-degrees, single melt increments by near fractional melting of the passively upwelling asthenosphere during the rifting stage of the basin. They escaped single melt increment aggregation, migrated isolated through the mantle lithosphere by reactive porous or channeled flow before oceanic opening, and were transformed into silica-saturated derivative liquids that underwent entrapment and stagnation in the shallow mantle lithosphere forming plagioclase-enriched peridotites. Widespread small bodies of strongly depleted gabbro-norites testify for the local coalescence of these derivative liquids. These melts never reached the surface (i.e., the hidden magmatism), since lavas with their composition have never been found in the basin. Subsequently, aggregated MORB melts upwelled within replacive dunite channels (as evidenced by composition of magmatic clinopyroxenes in dunites), intruded at shallow levels as olivine gabbro bodies and extruded as basaltic lavas, to form the crustal rocks of the oceanic lithosphere (i.e., the oceanic magmatism). Km-scale bodies of MORB olivine gabbros were intruded into the plagioclase-enriched peridotites, which were formed in the

  9. Pyroxenites - Melting or Migration?: Evidence from the Balmuccia massif

    NASA Astrophysics Data System (ADS)

    Sossi, Paolo; O'Neill, Hugh

    2014-05-01

    The recognition of pyroxenites in the mantle, combined with their lower solidus temperatures than peridotite, have been proposed as contributors to melting (Pertermann and Hirschmann, 2003; Sobolev et al, 2005; 2007). Geochemical fingerprints of this process invoke an unspecified 'pyroxenite' as the putative source. In reality, mantle pyroxenites are diverse (Downes, 2007), requiring that their mode of origin and compositional variability be addressed. Due to the excellent preservation and exposure of the Balmuccia massif, it has become an archetype for orogenic peridotites, providing information on their composition, field relationships and metamorphic history (Shervais and Mukasa, 1991; Hartmann and Wedepohl, 1993; Rivalenti et al., 1995; Mazzucchelli et al., 2009). The Balmuccia massif consists of fertile lherzolite with subordinate harzburgite and dunite and is riddled with pyroxenite bands, which fall into two suites - Chrome-Diopside (Cr-Di) and Aluminous-Augite (Al-Aug), a pairing present in most massif peridotites. Two-pyroxene thermometry gives temperatures of 850±25°C at 1-1.5 GPa, 500°C lower than asthenospheric mantle at that pressure, meaning they do not preserve their original, high temperature mineralogy. Decimetre-sized Cr-Di bands (≡75% CPX, 25% OPX) occur as initially Ol-free and bound by refractory dunite, but, as the bands are rotated into the plane of foliation, they mechanically incorporate olivine. Al-Aug veins (60% CPX, 25% OPX, 15% Sp) discordantly cut the body, intruding lherzolites which show enrichments in Fe, Al and Ti adjacent to the dykes. Both the Cr-Di suite and the Al-Aug series have indistinguishable Sr-, Nd-isotopic compositions to the host peridotite (Mukasa and Shervais, 1999). The major element compositions of pyroxenes in the Cr-Di bands and those in the surrounding peridotites are identical. Together with isotopic evidence, this suggests a local source, not only chemically but spatially, where a very low degree melt (

  10. Metasomatic processes within the fertile lithospheric Mantle beneath Don Camilo, Santa Cruz, Argentina

    NASA Astrophysics Data System (ADS)

    Ntaflos, Th.; Mundl, A.; Bjerg, E. A.; Tschegg, C.; Kosler, J.

    2009-04-01

    formed from residual melts. In contrast, clinopyroxene from mantle dunites enriched LREE (10 x PM) and LILE suggesting that the metasomatic agent was fluid-rich silicate melt. Calculated equilibrium P-T conditions cover a wide range from 800 to 1100 °C. Considering the crustal thickness in the area being around 35 km, a pressure between 12 and 17 kbar could be assumed as reasonable, indicating that these xenoliths were extracted from shallow depths of 40 to 60 km. Model calculations have shown that the lithospheric Mantle beneath Don Camilo, in Santa Cruz province is fertile and that spinel peridotites experienced low degrees of partial melting (2-8% batch melting in the spinel peridotite field). The metasomatic agent was a fluid-rich silicate melt of alkalibasaltic composition, presumably similar to this, which affected the Cerro Clark xenoliths north of Don Camilo locality. Don Camilo mantle xenoliths, like Tres Lagos, Cerro Redondo and Gobernador Gregores, does not show evidence for interaction of the lithospheric Mantle in southern Patagonia with subduction related components.

  11. Early Eocene clinoenstatite boninite and boninite-series dikes of the ophiolite of New Caledonia; a witness of slab-derived enrichment of the mantle wedge in a nascent volcanic arc

    NASA Astrophysics Data System (ADS)

    Cluzel, Dominique; Ulrich, Marc; Jourdan, Fred; Meffre, Sebastien; Paquette, Jean-Louis; Audet, Marc-Antoine; Secchiari, Arianna; Maurizot, Pierre

    2016-09-01

    Clinoenstatite-bearing boninites (CE-boninite) from the serpentinite sole of the Cenozoic ophiolite of New Caledonia near Nepoui have been dated by the 40Ar/39Ar method, yielding two plateau ages of 47.4 ± 0.9 Ma and 50.4 ± 1.3 Ma. Coarser grained, geochemically similar boninite-series felsic dikes consistently yielded U-Pb zircon ages of ca. 54 Ma. Nepoui CE-boninites display whole rock geochemical features similar to that of Cape Vogel boninites (Papua-New Guinea). They similarly have been generated by low degree hydrous melting of depleted peridotite. High contents in LILE and LREE, and some elemental ratios suggest source enrichment by subduction-derived fluids and melts. However, unlike the Cape Vogel boninite, moderately depleted MORB-like isotopic signatures (εNd50 = 7.9) rule out the role of OIB-like, or E-MORB component that might account for the relatively high LREE and LILE contents measured in the rocks. Nd isotopic ratios and positive anomalies in Zr and Hf are closely similar to that of the slightly older felsic dikes (55-50 Ma) that crosscut the peridotite from the ophiolite in New Caledonia. Most of these magmas have been generated by slab melting during the early stages of intra-oceanic subduction. The Early Eocene subduction started at or near the "oceanic" ridge and involved young and hot lithosphere; therefore, slab-derived melts may have reacted locally with hot depleted peridotites. Finally, water influx into the mantle wedge during the subduction of slightly older (cooler and hydrated) lithosphere initiated a low degree partial melting event in the mantle wedge and generated the CE-boninite magma. Geochemical modeling of hydrous melting of a depleted mantle re-enriched by slab melts suggest that the additional slab melt component was derived from the partial melting of a BABB-like barroisite-bearing eclogite, similar to some elements of the Eocene HP-LT Pouebo terrane. This potential magma source is similar to the BABB-like HT amphibolites

  12. Thermal and chemical convection in planetary mantles

    NASA Technical Reports Server (NTRS)

    Dupeyrat, L.; Sotin, C.; Parmentier, E. M.

    1995-01-01

    Melting of the upper mantle and extraction of melt result in the formation of a less dense depleted mantle. This paper describes series of two-dimensional models that investigate the effects of chemical buoyancy induced by these density variations. A tracer particles method has been set up to follow as closely as possible the chemical state of the mantle and to model the chemical buoyant force at each grid point. Each series of models provides the evolution with time of magma production, crustal thickness, surface heat flux, and thermal and chemical state of the mantle. First, models that do not take into account the displacement of plates at the surface of Earth demonstrate that chemical buoyancy has an important effect on the geometry of convection. Then models include horizontal motion of plates 5000 km wide. Recycling of crust is taken into account. For a sufficiently high plate velocity which depends on the thermal Rayleigh number, the cell's size is strongly coupled with the plate's size. Plate motion forces chemically buoyant material to sink into the mantle. Then the positive chemical buoyancy yields upwelling as depleted mantle reaches the interface between the upper and the lower mantle. This process is very efficient in mixing the depleted and undepleted mantle at the scale of the grid spacing since these zones of upwelling disrupt the large convective flow. At low spreading rates, zones of upwelling develop quickly, melting occurs, and the model predicts intraplate volcanism by melting of subducted crust. At fast spreading rates, depleted mantle also favors the formation of these zones of upwelling, but they are not strong enough to yield partial melting. Their rapid displacement toward the ridge contributes to faster large-scale homogenization.