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

NASA Astrophysics Data System (ADS)

Stuart, Finlay; Rogers, Nick; Davies, Marc

2016-04-01

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

Identifying mantle lithosphere inheritance in controlling intraplate orogenesis

NASA Astrophysics Data System (ADS)

Heron, Philip J.; Pysklywec, Russell N.; Stephenson, Randell

2016-09-01

Crustal inheritance is often considered important in the tectonic evolution of the Wilson Cycle. However, the role of the mantle lithosphere is usually overlooked due to its difficulty to image and uncertainty in rheological makeup. Recently, increased resolution in lithosphere imaging has shown potential scarring in continental mantle lithosphere to be ubiquitous. In our study, we analyze intraplate deformation driven by mantle lithosphere heterogeneities from ancient Wilson Cycle processes and compare this to crustal inheritance deformation. We present 2-D numerical experiments of continental convergence to generate intraplate deformation, exploring the limits of continental rheology to understand the dominant lithosphere layer across a broad range of geological settings. By implementing a "jelly sandwich" rheology, common in stable continental lithosphere, we find that during compression the strength of the mantle lithosphere is integral in generating deformation from a structural anomaly. We posit that if the continental mantle is the strongest layer within the lithosphere, then such inheritance may have important implications for the Wilson Cycle. Furthermore, our models show that deformation driven by mantle lithosphere scarring can produce tectonic patterns related to intraplate orogenesis originating from crustal sources, highlighting the need for a more formal discussion of the role of the mantle lithosphere in plate tectonics.

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

NASA Astrophysics Data System (ADS)

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

2017-12-01

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

Eight good reasons why the uppermost mantle could be magnetic

NASA Astrophysics Data System (ADS)

Ferre, E. C.; Friedman, S. A.; Martin Hernandez, F.; Till, J. L.; Ionov, D. A.; Conder, J. A.

2012-12-01

The launch of Magsat in 1979 prompted a broad magnetic investigation of mantle xenoliths (Wasilewski et al., 1979). The study concluded that no magnetic remanence existed in the uppermost mantle and that even if present, such sources would be at temperatures too high to contribute to long wavelength magnetic anomalies (LWMA). However, new collections of unaltered mantle xenoliths from four different tectonic settings, along with updated views on the sources of LWMA and modern petrologic constraints on fO2 in the mantle indicate that the uppermost mantle could, in certain cases, contain ferromagnetic minerals. 1. The analysis of some LWMA over areas such as, for example, Bangui in the Central African Craton, the Cascadia subduction zone and serpentinized oceanic lithosphere suggest magnetic sources in the uppermost mantle. 2. The most common ferromagnetic phase in the uppermost mantle is pure magnetite, which has a pressure-corrected Curie temperature at 10 kbars of 600C instead of the generally used value of 580C. Assuming 30 km-thick continental crust, and crustal and mantle geotherms of 15C/km and 5C/km, respectively, the 600C Curie temperature implies the existence of a 30 km-thick layer of mantle rocks, whose remanent and induced magnetizations could contribute to LWMA. The thickness of this layer decreases to about 15 km for a 35 km-thick crust. 3. The uppermost mantle is cooler than 600C in some tectonic settings, including Archean and Proterozoic shields (>350C), subduction zones (>300C) and old oceanic basins (>250C). 4. Recently investigated sets of unaltered mantle xenoliths contain pure SD and PSD magnetite inclusions exsolved in olivine and pyroxene. The fact that these magnetite grains are not associated with any alteration phases, such as serpentine, and exhibit a subhedral shape, demonstrates that they formed in equilibrium with the host silicate. 5. The ascent of mantle xenoliths in volcanic conduits through cratons and subduction zones occurs in less than a day. Numerical models of Fe diffusion in silicates suggest that it is unlikely for exsolved magnetite grains to reach greater than superparamagnetic sizes within this time frame. 6. Demagnetization of natural remanent magnetization (NRM) of unaltered mantle xenoliths unambiguously indicates only a single component. The demagnetization of NRM spectra resembles that of laboratory-imparted anhysteretic remanent magnetizations, suggesting that the NRM is of thermal origin, and most likely acquired upon cooling at the Earth's surface. Yet mantle peridotites had to be magnetized before extraction from the mantle source. 7. Modern experimental data suggest that the wüstite-magnetite oxygen buffer and the fayalite-magnetite-quartz oxygen buffer extend several tens of km at depth within the uppermost mantle. Modern petrologic models also indicate that fO2 in the uppermost mantle varies significantly with tectonic setting. 8. The magnetic properties of mantle xenoliths vary consistently across island arc, craton, hot spot and mantle plume regions. The intensity of their NRMs appear to be influenced by their tectonic setting, in accordance with petrologic models. In conclusion, the model of a uniformaly non-magnetic mantle no longer agrees with multiple lines of evidence and should be revisited, especially because the most strongly magnetic xenoliths originate from cold geotherm settings.

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.

Variable styles of rifting expressed in crustal structure across three rift segments of the Gulf of California

NASA Astrophysics Data System (ADS)

Lizarralde, D. D.; Axen, G. J.; Brown, H. E.; Fletcher, J. M.; Fernandez, A. G.; Harding, A. J.; Holbrook, W. S.; Kent, G. M.; Paramo, P.; Sutherland, F. H.; Umhoefer, P. J.

2007-05-01

We present a summary of results from a crustal-scale seismic experiment conducted in the southern Gulf of California. This experiment, the PESCADOR experiment, imaged crustal structure across three rift segments, the Alarcon, Guaymas, and San José del Cabo to Puerto Vallarta (Cabo-PV) segments, using seismic refraction/wide-angle reflection data acquired with airgun sources and recorded by closely spaced (10-15 km) ocean-bottom seismometers (OBSs). The imaged crustal structure reveals a surprisingly large variation in rifting style and magmatism between these segments: the Alarcon segment is a wide rift with apparently little syn-rift magmatism; the Guaymas segment is a narrow, magmatically robust rift; and the Cabo-PV segment is a narrow, magmatically "normal" rift. Our explanation for the observed variability is non-traditional in that we do not invoke mantle temperature, the factor commonly invoked to explain end-member volcanic and non-volcanic rifted margins, as the source of the considerable, though non-end-member variability we observe. Instead, we invoke mantle depletion related to pre-rift arc volcanism to account for observed wide, magma-poor rifting and mantle fertility and possibly the influence of sediments to account for robust rift and post-rift magmatism. These factors may commonly vary over small lateral spatial scales in regions that have transitioned from convergent to extensional tectonics, as is the case for the Gulf of California and many other rifts. Our hypothesis suggests that substantial lateral variability may exist within the uppermost mantle beneath the Gulf of California today, and it is hoped that ongoing efforts to image upper mantle structure here will provide tests for this hypothesis.

Recycling Seamounts: Implications for Mantle Source Heterogeneities

NASA Astrophysics Data System (ADS)

Madrigal, P.; Gazel, E.

2016-12-01

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

Re-Os isotope and platinum group elements of a FOcal ZOne mantle source, Louisville Seamounts Chain, Pacific ocean

NASA Astrophysics Data System (ADS)

Tejada, Maria Luisa G.; Hanyu, Takeshi; Ishikawa, Akira; Senda, Ryoko; Suzuki, Katsuhiko; Fitton, Godfrey; Williams, Rebecca

2015-02-01

The Louisville Seamount Chain (LSC) is, besides the Hawaiian-Emperor Chain, one of the longest-lived hotspot traces. We report here the first Re-Os isotope and platinum group element (PGE) data for Canopus, Rigil, and Burton Guyots along the chain, which were drilled during IODP Expedition 330. The LSC basalts possess (187Os/188Os)i = 0.1245-0.1314 that are remarkably homogeneous and do not vary with age. A Re-Os isochron age of 64.9 ± 3.2 Ma was obtained for Burton seamount (the youngest of the three seamounts drilled), consistent with 40Ar-39Ar data. Isochron-derived initial 187Os/188Os ratio of 0.1272 ± 0.0008, together with data for olivines (0.1271-0.1275), are within the estimated primitive mantle values. This (187Os/188Os)i range is similar to those of Rarotonga (0.124-0.139) and Samoan shield (0.1276-0.1313) basalts and lower than those of Cook-Austral (0.136-0.155) and Hawaiian shield (0.1283-0.1578) basalts, suggesting little or no recycled component in the LSC mantle source. The PGE data of LSC basalts are distinct from those of oceanic lower crust. Variation in PGE patterns can be largely explained by different low degrees of melting under sulfide-saturated conditions of the same relatively fertile mantle source, consistent with their primitive mantle-like Os and primordial Ne isotope signatures. The PGE patterns and the low 187Os/188Os composition of LSC basalts contrast with those of Ontong Java Plateau (OJP) tholeiites. We conclude that the Re-Os isotope and PGE composition of LSC basalts reflect a relatively pure deep-sourced common mantle sampled by some ocean island basalts but is not discernible in the composition of OJP tholeiites.

Petrologic evolution of Miocene-Pliocene mafic volcanism in the Kangal and Gürün basins (Sivas-Malatya), central east Anatolia: Evidence for Miocene anorogenic magmas contaminated by continental crust

NASA Astrophysics Data System (ADS)

Kocaarslan, Ayça; Ersoy, E. Yalçın

2018-06-01

This study discusses the geochemical features of the Early-Middle Miocene and Pliocene basaltic (SiO2 = 46-52; MgO = 6-10 wt%) to andesitic (SiO2 = 59; MgO = 4 wt%) rocks exposed in the Gürün and Kangal basins (Sivas, eastern part of central Anatolia), respectively. The basaltic rocks are characterized by alkaline to tholeiitic affinities, while the more evolved andesitic samples show calc-alkaline affinity. Trace element variations reveal that they can be evaluated in three sub-groups, each represented by different contents of trace elements for given Nb contents. Primary magmas of each groups were likely produced by different degrees of partial melting ( 1-2, 2-3, 7-10% respectively) from a common mantle source, subsequently underwent different degrees of fractionation and crustal contamination. Derivation from a common mantle source of the primitive magmas of each group is supported by similar Sr, Nd and Pb isotopic ratios. Increasing degrees of partial melting seem to be responsible for the alkaline to tholeiitic variation among the basaltic samples, while higher degrees of crustal contamination (AFC) resulted in calc-alkaline affinity of the more evolved samples. Most primitive Pliocene samples show intra-plate (anorogenic) geochemical features, while the more evolved Miocene calc-alkaline samples resemble geochemically subduction-related (orogenic) magmatic rocks. However, on the basis of detailed geochemical models, we propose that the calc-alkaline affinity among the Miocene samples can also be gained by crustal contamination of their primary magmas which were also anorogenic in character. If this is true, overall, the Miocene and Pliocene basaltic to andesitic rocks in the Gürün and Kangal basins appear to may have formed by variable degrees of partial melting of a common anorogenic mantle that had not been subject to subduction-related metasomatism. This is an alternative approach to the general view assuming the Early-Middle Miocene magmatic activity in the region was derived from subduction-modified mantle sources in response to subduction of the Arabian Plate under the Anatolian Plate. This hypothesis further implies that either delamination of the sub-continental lithosphere or slab break-off processes beneath the central to eastern Anatolia might took place well before the Miocene, thus allowing upwelling unaltered mantle to provide the source of the Miocene to Pliocene volcanic rocks.

Heterogeneous seismic anisotropy in the transition zone and uppermost lower mantle: evidence from South America, Izu-Bonin and Japan

NASA Astrophysics Data System (ADS)

Lynner, Colton; Long, Maureen D.

2015-06-01

Measurements of seismic anisotropy are commonly used to constrain deformation in the upper mantle. Observations of anisotropy at mid-mantle depths are, however, relatively sparse. In this study we probe the anisotropic structure of the mid-mantle (transition zone and uppermost lower mantle) beneath the Japan, Izu-Bonin, and South America subduction systems. We present source-side shear wave splitting measurements for direct teleseismic S phases from earthquakes deeper than 300 km that have been corrected for the effects of upper mantle anisotropy beneath the receiver. In each region, we observe consistent splitting with delay times as large as 1 s, indicating the presence of anisotropy at mid-mantle depths. Clear splitting of phases originating from depths as great as ˜600 km argues for a contribution from anisotropy in the uppermost lower mantle as well as the transition zone. Beneath Japan, fast splitting directions are perpendicular or oblique to the slab strike and do not appear to depend on the propagation direction of the waves. Beneath South America and Izu-Bonin, splitting directions vary from trench-parallel to trench-perpendicular and have an azimuthal dependence, indicating lateral heterogeneity. Our results provide evidence for the presence of laterally variable anisotropy and are indicative of variable deformation and dynamics at mid-mantle depths in the vicinity of subducting slabs.

Magnetic properties of the upper mantle beneath the continental United States

NASA Astrophysics Data System (ADS)

Friedman, S. A.; Ferre, E. C.; Demory, F.; Rochette, P.; Martin Hernandez, F.; Conder, J. A.

2012-12-01

The interpretation of long wavelength satellite magnetic data (Magsat, Oersted, CHAMP, SWARM) requires an understanding of magnetic mineralogy in the lithospheric mantle and reliable models of induced and remanent magnetic sources in the lithospheric mantle and the crust. Blakely et al. (2005) proposed the hypothesis of a magnetic lithospheric mantle in subduction zones. This prompted us to reexamine magnetic sources in the lithospheric mantle in different tectonic settings where unaltered mantle xenolith have been reported since the 1990s. Xenoliths from the upper mantle beneath the continental United States show different magnetic properties depending on the tectonic setting in which they equilibrated. Three localities in the South Central United States (San Carlos, AZ; Kilbourne Hole, NM; Knippa, TX) produced lherzolite and harzburgite xenoliths, while the Bearpaw Mountains in Montana (subduction zone) produced dunite and phlogopite-rich dunite xenoliths. Paleomagnetic data on these samples shows the lack of secondary alteration which is commonly caused by post-eruption serpentinization and the lack of basalt contamination. The main magnetic carrier is pure magnetite. The ascent of mantle xenoliths to the surface of the Earth generally takes only a few hours. Numerical modelling shows that nucleation of magnetite during ascent would form superparamagnetic grains and therefore cannot explain the observed magnetic grain sizes. This implies that the ferromagnetic phases present in the studied samples formed at mantle depth. The samples from the South Central United States exhibit a small range in low-field magnetic susceptibility (+/- 0.00003 [SI]), and Natural Remanent Magnetization (NRM) between 0.001 - 0.100 A/m. To the contrary samples from the Bearpaw Mountains exhibit a wider range of low-field susceptibilities (0.00001 to 0.0015 [SI]) and NRM (0.01 and 9.00 A/m). These samples have been serpentinized in-situ by metasomatic fluids related to the Farallon plate (Facer et al., 2009). Hence, the magnetic properties of the lithospheric mantle beneath the continental United States differ significantly depending on tectonic setting. The combination of the low geotherm observed in the Bearpaw Mountains with the stronger induced and remanent magnetization of mantle rocks in this area may produce a detectable LWMA.

Osmium Isotopic Evolution of the Mantle Sources of Precambrian Ultramafic Rocks

NASA Astrophysics Data System (ADS)

Gangopadhyay, A.; Walker, R. J.

2006-12-01

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

An analytic model of axisymmetric mantle plume due to thermal and chemical diffusion

NASA Technical Reports Server (NTRS)

Liu, Mian; Chase, Clement G.

1990-01-01

An analytic model of axisymmetric mantle plumes driven by either thermal diffusion or combined diffusion of both heat and chemical species from a point source is presented. The governing equations are solved numerically in cylindrical coordinates for a Newtonian fluid with constant viscosity. Instead of starting from an assumed plume source, constraints on the source parameters, such as the depth of the source regions and the total heat input from the plume sources, are deduced using the geophysical characteristics of mantle plumes inferred from modelling of hotspot swells. The Hawaiian hotspot and the Bermuda hotspot are used as examples. Narrow mantle plumes are expected for likely mantle viscosities. The temperature anomaly and the size of thermal plumes underneath the lithosphere can be sensitive indicators of plume depth. The Hawaiian plume is likely to originate at a much greater depth than the Bermuda plume. One suggestive result puts the Hawaiian plume source at a depth near the core-mantle boundary and the source of the Bermuda plume in the upper mantle, close to the 700 km discontinuity. The total thermal energy input by the source region to the Hawaiian plume is about 5 x 10(10) watts. The corresponding diameter of the source region is about 100 to 150 km. Chemical diffusion from the same source does not affect the thermal structure of the plume.

The Complex History of Alarcon Rise Mid-Ocean Ridge Rhyolite Revealed through Mineral Chemistry

NASA Astrophysics Data System (ADS)

Dreyer, B. M.; Portner, R. A.; Clague, D. A.; Daczko, N. R.; Castillo, P.; Bindeman, I. N.

2014-12-01

A suite of basalts to rhyolites recovered from the Alarcon Rise, the northern extension of the intermediate spreading-rate East Pacific Rise, provides an unparalleled test of established mechanisms for high-Si lava formation at ridges. Rhyolites are ≤35% phyric and poorly vesicular. Mafic xenoclasts are common, and plagioclase is the dominant phase. Olivine and clinopyroxene are also common, and orthopyroxene, FeTi-oxides, zircon, and rare pyrite blebs are present. Major and trace element glass data are consistent with MELTS models of fractional crystallization from a parental melt, but a diverse mineral population records added complexity. Olivine and plagioclase compositions are broadly consistent with models, with the exception of more variable Fo52-77 and An87-28 in a basaltic andesitic composition where pigeonite is predicted to replace olivine in the crystallizing assemblage between ~1085-1015°C; pigeonites analyzed in an andesite have lower Ca and Fe than predicted. Clinopyroxene variability generally increases with host melt SiO2, from Mg# 86-84 in basalts to Mg# 80-21 in rhyolites, and zoning is common with higher-MgO anhedral cores mantled by lower-MgO euhedral rims. Cooler magmas aided the preservation of disequilibrium and are supported by ~715-835°C Ti-in-zircon and ilmenite-magnetite thermometry in rhyolites. Despite a well-predicted liquid line of decent, multiple signals of chemical disequilibrium in intermediate to silicic melts support mixing of magmatic batches and/or assimilation of partially hydrous crust. Assimilation is permissible given δ18O values that are lower than expected solely from fractional crystallization (i.e., <6.3‰ at 77% SiO2), but assimilation extent is limited on the basis of δD ~82±8 and Pacific MORB-like 87Sr/86Sr. Zircon Hf-isotopes and trace element patterns support a juvenile oceanic crustal source. Whereas depleted Pacific MORB mantle source reservoir is supported by whole rock Sr-Nd isotopes, slight enrichments in zircon 176Hf/177Hf and whole rock 207,206Pb/204Pb may indicate an enriched MORB mantle component. In conclusion, mid-ocean rhyolite at Alarcon formed from a variety of petrogenetic processes including magma-mixing, assimilation, and crystallization following partial melting of slightly heterogeneous mantle source(s).

Refertilization of the Subcontinental Lithospheric Mantle and its link to the formation of Metallogenic Provinces

NASA Astrophysics Data System (ADS)

Tassara, C. S.; González-Jiménez, J. M.; Reich, M.; Morata, D.; Barra, F.; Gregoire, M.; Saunders, J. E.; Cannatelli, C.

2017-12-01

Refertilisation of the subcontinental lithospheric mantle is a key process controlling the noble metal budget of the mantle, and recent views point to anomalously enriched mantle sources as a critical factor in the formation of noble metal (e.g., Au) provinces at a lithospheric scale. Here we test this hypothesis by studying peridotite xenoliths from the mantle beneath the Deseado Massif auriferous province in southern Patagonia, Argentina. Extensive Neogene back-arc plateau magmatism composed of alkaline basalts ( 3.5 Ma) has brought to the surface deep-seated mantle xenoliths from beneath the crust that host the Au mineralization. In the studied xenolith samples we found gold particles enclosed within primary olivine and pyroxene, and embedded in a highly alkaline interstitial glass or sulphides. Detailed inspection of the sulphide hosts using FESEM reveals abundant native Au nanoparticles, which are consistent with the high Au (up to 6 ppm) obtained by LA-ICP-MS analysis of these sulphides. It is relevant to note that these sulphides also contain significant amounts of Ag (up to 163 ppm). Different generations of sulphides were identified on the basis of their chondrite-normalized PGE patterns, and they can be systematically associated with different events of melt depletion and metasomatism in the mantle. Noticeably, Cu-Pd-Pt-Au rich sulfides are associated with clinopyroxene showing typical carbonatite markers (i.e., large LREE/HREE, Zr and Hf negative anomalies) and accessory minerals such as carbonates and apatite. Still, clinopyroxene commonly has high Ti contents suggesting that a "basaltic" component was also present during the metasomatism. These results suggest that overprinting of events of melt depletion and metasomatism lead to the formation of several generations of sulfides. We propose that the Cu-Pd-Pt-Au rich sulfides may be associated with carbonated silicate melts in the mantle. Our results point to 1) a link between an enriched source of gold (and silver) in the mantle and the formation of the Deseado Massif auriferous province; and 2) carbonated silicate melt metasomatism as an important factor in the PPGE + Au refertilisation of the mantle.

Plume dynamics beneath the African plate inferred from the geochemistry of the Tertiary basalts of southern Ethiopia

NASA Astrophysics Data System (ADS)

George, R. M.; Rogers, N. W.

2002-09-01

Southern Ethiopian flood basalts erupted in two episodes: the pre-rift Amaro and Gamo transitional tholeiites (45-35 million years) followed by the syn-extensional Getra-Kele alkali basalts (19-11 million years). These two volcanic episodes are distinct in both trace element and isotope ratios (Zr/Nb ratios in Amaro/Gamo lavas fall between 7 and 14, and 3-4.7 in the Getra-Kele lavas whereas 206Pb/204Pb ratios fall between 18-19 and 18.9-20, respectively). The distinctive chemistries of the two eruptive phases record the tapping of two distinct source regions: a mantle plume source for the Amaro/Gamo phase and an enriched continental mantle lithosphere source for the Getra-Kele phase. Isotope and trace element variations within the Amaro/Gamo lavas reflect polybaric fractional crystallisation initiated at high pressures accompanied by limited crustal contamination. We show that clinopyroxene removal at high (0.5 GPa) crustal pressures provides an explanation for the common occurrence of transitional tholeiites in Ethiopia relative to other, typically tholeiitic flood basalt provinces. The mantle plume signature inferred from the most primitive Amaro basalts is isotopically distinct from that contributing to melt generation in central Ethiopian and Afar. This, combined with Early Tertiary plate reconstructions and similarities with Kenyan basalts farther south, lends credence to derivation of these melts from the Kenyan plume rather than the Afar mantle plume. The break in magmatism between 35 and 19 Ma is consistent with the northward movement away from the Kenya plume predicted from plate tectonic reconstructions. In this model the Getra-Kele magmatism is a response to heating of carbonatitically metasomatised lithosphere by the Afar mantle plume beneath southern Ethiopia at this time.

Plate Tectonic Cycling and Whole Mantle Convection Modulate Earth's 3He/22Ne Ratio

NASA Astrophysics Data System (ADS)

Dygert, N. J.; Jackson, C.; Hesse, M. A.; Tremblay, M. M.; Shuster, D. L.; Gu, J.

2016-12-01

3He and 22Ne are not produced in the mantle or fractionated by partial melting, and neither isotope is recycled back into the mantle by subduction of oceanic basalt or sediment. Thus, it is a surprise that large 3He/22Ne variations exist within the mantle and that the mantle has a net elevated 3He/22Ne ratio compared to volatile-rich planetary precursor materials. Depleted subcontinental lithospheric mantle and mid-ocean ridge basalt (MORB) mantle have distinctly higher 3He/22Ne compared to ocean island basalt (OIB) sources ( 4-12.5 vs. 2.5-4.5, respectively) [1,2]. The low 3He/22Ne of OIBs approaches chondritic ( 1) and solar nebula values ( 1.5). The high 3He/22Ne of the MORB mantle is not similar to solar sources or any known family of meteorites, requiring a mechanism for fractionating He from Ne in the mantle and suggesting isolation of distinct mantle reservoirs throughout geologic time. We model the formation of a MORB source with elevated and variable 3He/22Ne though diffusive exchange between dunite channel-hosted basaltic liquids and harzburgite wallrock beneath mid-ocean ridges. Over timescales relevant to mantle upwelling beneath spreading centers, He may diffuse tens to hundreds of meters into wallrock while Ne is relatively immobile, producing a regassed, depleted mantle lithosphere with elevated 3He/22Ne. Subduction of high 3He/22Ne mantle would generate a MORB source with high 3He/22Ne. Regassed, high 3He/22Ne mantle lithosphere has He concentrations 2-3 orders of magnitude lower than undegassed mantle. To preserve the large volumes of high 3He/22Ne mantle required by the MORB source, mixing between subducted and undegassed mantle reservoirs must have been limited throughout geologic time. Using the new 3He/22Ne constraints, we ran a model similar to [3] to quantify mantle mixing timescales, finding they are on the order of Gyr assuming physically reasonable seafloor spreading rates, and that Earth's convecting mantle has lost >99% of its primordial volatile elements. Most significantly, mantle convection is not and cannot have been layered for most of geologic time. [1] Graham (2002), RiMG 74, 247-317. [2] Jalowitzki et al. (2016), EPSL 450, 263-273. [3] Gonnermann & Mukhopadhyay (2009), Nature, 560-563.

Sub-Moho Reflectors, Mantle Faults and Lithospheric Rheology

NASA Astrophysics Data System (ADS)

Brown, L. D.

2013-12-01

One of the most unexpected and dramatic observations from the early years of deep reflection profiling of the continents using multichannel CMP techniques was the existing of prominent reflections from the upper mantle. The first of these, the Flannan thrust/fault/feature, was traced by marine profiling of the continental margin offshore Britain by the BIRPS program, which soon found them to be but one of several clear sub-crustal discontinuities in that area. Subsequently, similar mantle reflectors have been observed in many areas around the world, most commonly beneath Precambrian cratonic areas. Many, but not all, of these mantle reflections appear to arise from near the overlying Moho or within the lower crust before dipping well into the mantle. Others occur as subhorizontal events at various depths with the mantle, with one suite seeming to cluster at a depth of about 75 km. The dipping events have been variously interpreted as mantle roots of crustal normal faults or the deep extension of crustal thrust faults. The most common interpretation, however, is that these dipping events are the relicts of ancient subduction zones, the stumps of now detached Benioff zones long since reclaimed by the deeper mantle. In addition to the BIRPS reflectors, the best known examples include those beneath Fennoscandia in northern Europe, the Abitibi-Grenville of eastern Canada, and the Slave Province of northwestern Canada (e.g. on the SNORCLE profile). The most recently reported example is from beneath the Sichuan Basin of central China. The preservation of these coherent, and relatively delicate appearing, features beneath older continental crust and presumably within equally old (of not older) mantle lithosphere, has profound implications for the history and rheology of the lithosphere in these areas. If they represent, as widely believe, some form of faulting with the lithosphere, they provide corollary constraints on the nature of faulting in both the lower crust and upper mantle. The SNORCLE mantle reflectors, which can be traced deep within the early Precambrian (?) mantle by both surface (controlled source) reflection profiles and passive (receiver function) images most clearly illustrates the rheological implications of such feature. The SNORCLE events appear to root upwards into the lower crust and extend to depths approaching 200 km into the mantle. This would seem to require the preservation of undeformed mantle lithosphere for almost 2.5 billion years in this area. This preservation is clearly inconsistent with the interpretation of nearby shallower mantle interfaces as marking the modern lithosphere-asthenosphere boundary. In summary, dipping mantle reflections imply preservation of substantial thicknesses of mantle lithosphere for very long periods of time, and localization of mantle deformation during the formation of these structures along relatively narrow, discrete interfaces rather than across broad zones of diffuse deformation. .

The peculiar geochemical signatures of São Miguel (Azores) lavas: Metasomatised or recycled mantle sources?

NASA Astrophysics Data System (ADS)

Beier, Christoph; Stracke, Andreas; Haase, Karsten M.

2007-07-01

The island of São Miguel, Azores consists of four large volcanic systems that exhibit a large systematic intra-island Sr-Nd-Pb-Hf isotope and trace element variability. The westernmost Sete Cidades volcano has moderately enriched Sr-Nd-Pb-Hf isotope ratios. In contrast, lavas from the easternmost Nordeste volcano have unusually high Sr and Pb and low Nd and Hf isotope ratios suggesting a long-term evolution with high Rb/Sr, U/Pb, Th/Pb, Th/U and low Sm/Nd and Lu/Hf parent-daughter ratios. They have trace element concentrations similar to those of the HIMU islands, with the exception of notably higher alkali element (Cs, Rb, K, Ba) and Th concentrations. The time-integrated parent-daughter element evolution of both the Sete Cidades and Nordeste source matches the incompatibility sequence commonly observed during mantle melting and consequently suggests that the mantle source enrichment is caused by a basaltic melt, either as a metasomatic agent or as recycled oceanic crust. Our calculations show that a metasomatic model involving a small degree basaltic melt is able to explain the isotopic enrichment but, invariably, produces far too enriched trace element signatures. We therefore favour a simple recycling model. The trace element and isotopic signatures of the Sete Cidades lavas are consistent with the presence of ancient recycled oceanic crust that has experienced some Pb loss during sub-arc alteration. The coherent correlation of the parent-daughter ratios (e.g. Rb/Sr, Th/U, U/Pb) and incompatible element ratios (e.g. Nb/Zr, Ba/Rb, La/Nb) with the isotope ratios in lavas from the entire island suggest that the Sete Cidades and Nordeste source share a similar genetic origin. The more enriched trace element and isotopic variations of Nordeste can be reproduced by recycled oceanic crust in the Nordeste source that contains small amounts of evolved lavas (˜ 1-2%), possibly from a subducted seamount. The rare occurrence of enriched source signatures comparable to Nordeste may be taken as circumstantial evidence that stirring processes in the Earth's mantle are not able to homogenise material within the size of seamounts over timescales of mantle recycling.

Fe-based redox state of mantle eclogites: Inherited from oceanic protoliths, modified during subduction or overprinted during metasomatism?

NASA Astrophysics Data System (ADS)

Aulbach, S.; Woodland, A. B.; Vasilyev, P.; Viljoen, F.

2016-12-01

Kimberlite-borne mantle eclogite xenoliths of Archaean and Palaeoproterozoic age are commonly interpreted as representing former oceanic crust. As such, they may retain a memory of the redox state of the convecting mantle source that gave rise to their magmatic protoliths and which controls the speciation of volatiles in planetary interiors. Mantle eclogite suites commonly include both cumulate and variably evolved extrusive varieties [1], which may be characterised by initial differences in Fe3+/Fetotal. However, in the warmer ancient mantle, they were also subject to modification due to partial melt loss upon subduction (if a plate tectonic regime existed) and, after capture in the cratonic mantle lithosphere, may be overprinted by interaction with metasomatic melts and fluids. Data are as yet sparse, but new Fe-based oxybarometry shows mantle eclogites to have highly variable fO2 (FMQ-3 to FMQ), whereby low fO2 relative to modern MORB may relate to subduction of more reducing Archaean oceanic crust or loss of ferric Fe during partial melt loss [2,3]. Indeed, using V/Sc as a redox proxy, it was recently shown that Archaean mantle eclogites are more reduced than modern MORB (ΔFMQ-1.3 vs. ΔFMQ -0.4), leading to a shallower depth of redox melting [4]. Although higher Fe contents of eclogites compared to peridotites may translate into greater robustness during metasomatism after emplacement into the cratonic lithosphere, it is possible that this is at least in part responsible for their highly variable Fe-based fO2. In order to help further constrain the redox state of mantle eclogites and unravel the effect of primary and secondary processes, we are currently measuring Fe3+/Fetotal by Mössbauer in garnet from two compositionally well-characterised mantle eclogite suites (Kaapvaal craton and West African craton), with the aim to use recently calibrated oxybarometers [2,3] to calculate fO2. The results will bear on the speciation and hence mobility of carbon during a variety of mantle processes ranging from partial melting of the convecting mantle to metamorphic reactions upon subduction and metasomatic interactions. [1] Aulbach and Jacob (in press) Lithos; [2] Stagno et al. (2015) Contrib Mineral Petrol 42: 207-219; [3] Vasilyev (2016) PhD Thesis, Australian Nat Univ; [4] Aulbach and Stagno (in press) Geology

Late Mesozoic-Cenozoic intraplate magmatism in Central Asia and its relation with mantle diapirism: Evidence from the South Khangai volcanic region, Mongolia

NASA Astrophysics Data System (ADS)

Yarmolyuk, Vladimir V.; Kudryashova, Ekaterina A.; Kozlovsky, Alexander M.; Lebedev, Vladimir A.; Savatenkov, Valery M.

2015-11-01

The South Khangai volcanic region (SKVR) comprises fields of Late Mesozoic-Cenozoic volcanic rocks scattered over southern and central Mongolia. Evolution of the region from the Late Jurassic to the Late Cenozoic includes 13 successive igneous episodes that are more or less evenly distributed in time. Major patterns in the distribution of different-aged volcanic complexes were controlled by a systematic temporal migration of volcanic centers over the region. The total length of their trajectory exceeds 1600 km. Principle characteristics of local magmatism are determined. The composition of igneous rocks varies from basanites to rhyolites (predominantly, high-K rocks), with geochemistry close to that of OIB. The rock composition, however, underwent transformations in the Mesozoic-Cenozoic. Rejuvenation of mafic rocks is accompanied by decrease in the contents of HREE and increase of Nb and Ta. According to isotope data, the SKVR magmatic melts were derived from three isotope sources that differed in the Sr, Nd, and Pb isotopic compositions and successively alternated in time. In the Early Cretaceous, the predominant source composition was controlled by interaction of the EMII- and PREMA-type mantle materials. The PREMA-type mantle material dominated quantitatively in the Late Cretaceous and initial Early Cenozoic. From the latest Early Cenozoic to Late Cenozoic, the magma source also contained the EMI-type material along with the PREMA-type. The structural fabric, rock composition, major evolutionary pattern, and inner structure of SKVR generally comply with the criteria used to distinguish the mantle plume-related regions. Analogous features can be seen in other regions of recent volcanism in Central Asia (South Baikal, Udokan, Vitim, and Tok Stanovik). The structural autonomy of these regions suggests that distribution of the Late Mesozoic-Cenozoic volcanism in Central Asia was controlled by a group of relatively small hot finger-type mantle plumes associated with the common hot mantle field of Central Asia.

Mantle and crustal contributions to continental flood volcanism

USGS Publications Warehouse

Arndt, N.T.; Czamanske, G.K.; Wooden, J.L.; Fedorenko, V.A.

1993-01-01

Arndt, N.T., Czamanske, G.K., Wooden, J.L. and Fedorenko, V.A., 1993. Mantle and crustal contributions to continental flood volcanism. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near the Earth's Surface. Tectonophysics, 223: 39-52. Most continental flood basalts are enriched in incompatible elements and have high initial 87Sr/86Sr ratios and low ??{lunate}Nd values. Many are depleted in Nb and Ta. The commonly-held view that these characteristics are inherited directly from a source in metasomatized lithospheric mantle is inconsistent with the following arguments: (1) thermomechanical modelling demonstrates that flood basalt magmas come mainly from an asthenospheric or plume source, with minimal direct melting of the continental lithospheric mantle. The low water contents of most flood basalts argue against proposals that hydrous lithosphere was the source. (2) Lithospheric mantle normally has low concentrations of incompatible elements, and chondrite-normalized Nb and Ta contents similar to those of other incompatible elements. Such material cannot be the unmodified source of Nb-Ta-depleted basalts such as those from the Karoo, Ferrar, or Columbia River provinces. We suggest there are two main controls on the compositions of continental flood basalts. The first is lithospheric thickness, which strongly influences the depth and degree of mantle melting of a plume or asthenospheric source, and thus has an important influence on the composition of primary magmas. All liquids formed by partial melting of peridotite at sub-lithosphere depths are highly magnesian (20-25 wt.% MgO) but have variable trace-element contents. Where the lithosphere is thick, the source melts at high pressure, garnet is present, the degree of melting is low, and trace-element concentrations are high. This type of magma evolves to produce the high-Ti type of continental flood basalt. Where the lithosphere is thinner, the source ascends to shallower levels, the degree of melting is greater, garnet may be exhausted, and the magmas have lower trace-element contents; these magmas yield low-Ti basalts. The second control is processing of magmas in chambers that were periodically replenished and tapped, while continuously fractionating and assimilating their wall rocks. The uniform compositions of basalts that evolve in such chambers are far removed from those of their picritic parental magmas. Major elements in continental flood basalts reflect control by olivine, pyroxene, and plagioclase crystallization, and this assemblage places the magma chambers at crustal depth. We believe that trace-element and isotopic compositions are also buffered, and that the erupted basalts represent steady-state liquids tapped from these magma chambers. These processes impose a crustal signature on the magmas, as expressed most strongly in the concentrations of incompatible elements (e.g., Nb-Ta anomalies) and their isotopic characteristics. ?? 1993.

Simultaneous Quantification of Temperature, Pyroxenite Abundance, and Upwelling Rates in the Iceland Mantle Source

NASA Astrophysics Data System (ADS)

Brown, E.; Lesher, C. E.

2014-12-01

The compositions and volumes of basalts erupted at the earth's surface are a function of mantle temperature, mantle composition, and the rate at which the mantle upwells through the melting zone. Thus, basaltic magmatism has long been used to probe the thermal and physiochemical state of the earth's mantle. Great insight has been gained into the mantle beneath the global spreading ridge system, where the mantle source is assumed to be homogeneous peridotite that upwells passively [1]. However, it is now recognized that many basalt source regions are lithologically heterogeneous (i.e. containing recycled lithospheric material ranging from harzburgite to pyroxenite) and upwell at rates in excess of those governed by plate separation. To account for these complexities, we have developed a forward melting model for lithologically heterogeneous mantle that incorporates thermodynamically and experimentally constrained melting functions for a range of peridotite and pyroxenite lithologies. The model is unique because it quantifies mantle upwelling rates based on the net buoyancy of the source, thus providing a means for linking basalt compositions/volumes to mantle flow while accounting for source heterogeneity. We apply the model to investigate the mantle properties governing magmatism along different rift segments in Iceland, where lithologic heterogeneity and variable upwelling rates have been inferred through geochemical means [2,3]. Using constraints from seismically determined crustal thicknesses and recent estimates of the proportion of pyroxenite-derived melt contributing to Icelandic basalt compositions [4,5], we show that mantle sources beneath Iceland have excess potential temperatures >85 °C, contain <7% pyroxenite, and maximum upwelling rates ~14 times the passive rate. Our modeling highlights the dominant role of elevated mantle temperature and enhanced upwelling for high productivity magmatism in Iceland, and a subordinate role for mantle heterogeneity, which is required to account for much of the observed chemical and isotopic diversity. [1] Langmuir et al, 1992, AGU Geophys. Mono. Ser. 71 [2] Chauvel & Hemond, 2000, G-cubed, v 1 [3] Kokfelt et al, 2003, EPSL, v 214 [4] Sobolev et al, 2007, Science, v 316 [5] Shorttle et al, 2014, EPSL, v 395

186Os- 187Os systematics of Gorgona Island komatiites: implications for early growth of the inner core

NASA Astrophysics Data System (ADS)

Brandon, Alan D.; Walker, Richard J.; Puchtel, Igor S.; Becker, Harry; Humayun, Munir; Revillon, Sidonie

2003-02-01

The presence of coupled enrichments in 186Os/ 188Os and 187Os/ 188Os in some mantle-derived materials reflects long-term elevation of Pt/Os and Re/Os relative to the primitive upper mantle. New Os data for the 89 Ma Gorgona Island, Colombia komatiites indicate that these lavas are also variably enriched in 186Os and 187Os, with 186Os/ 188Os ranging between 0.1198397±22 and 0.1198470±38, and with γOs correspondingly ranging from +0.15 to +4.4. These data define a linear trend that converges with the previously reported linear trend generated from data for modern Hawaiian picritic lavas and a sample from the ca. 251 Ma Siberian plume, to a common component with a 186Os/ 188Os of approximately 0.119870 and γOs of +17.5. The convergence of these data to this Os isotopic composition may imply a single ubiquitous source in the Earth's interior that mixes with a variety of different mantle compositions distinguished by variations in γOs. The 187Os- and 186Os-enriched component may have been generated via early crystallization of the solid inner core and consequent increases in Pt/Os and Re/Os in the liquid outer core, with time leading to suprachondritic 186Os/ 188Os and γOs in the outer core. The presence of Os from the outer core in certain portions of the mantle would require a mechanism that could transfer Os from the outer core to the lower mantle, and thence to the surface. If this is the process that generated the isotopic enrichments in the mantle sources of these plume-derived systems, then the current understanding of solid metal-liquid metal partitioning of Pt, Re and Os requires that crystallization of the inner core began prior to 3.5 Ga. Thus, the Os isotopic data reported here provide a new source of data to better constrain the timing of inner core formation, complementing magnetic field paleo-intensity measurements as data sources that constrain models based on secular cooling of the Earth.

The role of water in the petrogenesis of Marina trough magmas

NASA Astrophysics Data System (ADS)

Stolper, Edward; Newman, Sally

1994-02-01

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

Eocene-Miocene igneous activity in Provence (SE France): 40Ar/39Ar data, geochemical-petrological constraints and geodynamic implications

NASA Astrophysics Data System (ADS)

Lustrino, Michele; Fedele, Lorenzo; Agostini, Samuele; Di Vincenzo, Gianfranco; Morra, Vincenzo

2017-09-01

Provence (SE France) was affected by two main phases of sporadic igneous activity during the Cenozoic. New 40Ar/39Ar laser step-heating data constrain the beginning of the oldest phase to late Eocene (40.82 ± 0.73 Ma), with activity present until early Miocene ( 20 Ma). The products are mainly andesites, microdiorites, dacites and basaltic andesites mostly emplaced in the Agay-Estérel area. Major- and trace-element constraints, together with Srsbnd Ndsbnd Pb isotopic ratios suggest derivation from a sub-continental lithosphere mantle source variably modified by subduction-related metasomatic processes. The compositions of these rocks overlap those of nearly coeval (emplaced 38-15 Ma) late Eocene-middle Miocene magmatism of Sardinia. The genesis of dacitic rocks cannot be accounted for by simple fractional crystallization alone, and may require interaction of evolved melts with lower crustal lithologies. The youngest phase of igneous activity comprises basaltic volcanic rocks with mildly sodic alkaline affinity emplaced in the Toulon area 10 Myr after the end of the previous subduction-related phase. These rocks show geochemical and isotopic characteristics akin to magmas emplaced in intraplate tectonic settings, indicating a sub-lithospheric HiMu + EM-II mantle source for the magmas, melting approximately in the spinel/garnet-lherzolite transition zone. New 40Ar/39Ar laser step-heating ages place the beginning of the volcanic activity in the late Miocene-Pliocene (5.57 ± 0.09 Ma). The emplacement of "anorogenic" igneous rocks a few Myr after rocks of orogenic character is a common feature in the Cenozoic districts of the Central-Western Mediterranean area. The origin of such "anorogenic" rocks can be explained with the activation of different mantle sources not directly modified by subduction-related metasomatic processes, possibly located in the sub-lithospheric mantle, and thus unrelated to the shallower lithospheric mantle source of the "orogenic" magmatism.

Origin and Role of Recycled Crust in Flood Basalt Magmatism: Case Study of the Central East Greenland Rifted Margin

NASA Astrophysics Data System (ADS)

Brown, E.; Lesher, C. E.

2015-12-01

Continental flood basalts (CFB) are extreme manifestations of mantle melting derived from chemically/isotopically heterogeneous mantle. Much of this heterogeneity comes from lithospheric material recycled into the convecting mantle by a range of mechanisms (e.g. subduction, delamination). The abundance and petrogenetic origins of these lithologies thus provide important constraints on the geodynamical origins of CFB magmatism, and the timescales of lithospheric recycling in the mantle. Basalt geochemistry has long been used to constrain the compositions and mean ages of recycled lithologies in the mantle. Typically, this work assumes the isotopic compositions of the basalts are the same as their mantle source(s). However, because basalts are mixtures of melts derived from different sources (having different fusibilities) generated over ranges of P and T, their isotopic compositions only indirectly represent the isotopic compositions of their mantle sources[1]. Thus, relating basalts compositions to mantle source compositions requires information about the melting process itself. To investigate the nature of lithologic source heterogeneity while accounting for the effects of melting during CFB magmatism, we utilize the REEBOX PRO forward melting model[2], which simulates adiabatic decompression melting in lithologically heterogeneous mantle. We apply the model to constrain the origins and abundance of mantle heterogeneity associated with Paleogene flood basalts erupted during the rift-to-drift transition of Pangea breakup along the Central East Greenland rifted margin of the North Atlantic igneous province. We show that these basalts were derived by melting of a hot, lithologically heterogeneous source containing depleted, subduction-modified lithospheric mantle, and <10% recycled oceanic crust. The Paleozoic mean age we calculate for this recycled crust is consistent with an origin in the region's prior subduction history, and with estimates for the mean age of recycled crust in the modern Iceland plume[3]. These results suggest that this lithospheric material was not recycled into the lower mantle before becoming entrained in the Iceland plume. [1] Rudge et al. (2013). GCA, 114, p112-143; [2] Brown & Lesher (2014). Nat. Geo., 7, p820-824; [3] Thirlwall et al. (2004). GCA, 68, p361-386

Petrology and geochemistry of the Tasse mantle xenoliths of the Canadian Cordillera: A record of Archean to Quaternary mantle growth, metasomatism, removal, and melting

NASA Astrophysics Data System (ADS)

Polat, Ali; Frei, Robert; Longstaffe, Fred J.; Thorkelson, Derek J.; Friedman, Eyal

2018-07-01

Mantle xenoliths hosted by the Quaternary Tasse alkaline basalts in the Canadian Cordillera, southeastern British Columbia, are mostly spinel lherzolite originating from subcontinental lithospheric mantle. The xenoliths contain abundant feldspar veins, melt pockets and spongy clinopyroxene, recording extensive alkaline metasomatism and partial melting. Feldspar occurs as veins and interstitial crystal in melt pockets. Melt pockets occur mainly at triple junctions, along grain boundaries, and consist mainly of olivine, cpx, opx and spinel surrounded by interstitial feldspar. The Nd, Sr and Pb isotopic compositions of the xenoliths indicate that their sources are characterized by variable mixtures of depleted MORB mantle and EM1 and EM2 mantle components. Large variations in εNd values (-8.2 to +9.6) and Nd depleted mantle model ages (TDM = 66 to 3380 Ma) are consistent with multiple sources and melt extraction events, and long-term (>3300 Ma) isolation of some source regions from the convecting mantle. Samples with Archean and Paleoproterozoic Nd model ages are interpreted as either have been derived from relict Laurentian mantle pieces beneath the Cordillera or have been eroded from the root of the Laurentian craton to the east and transported to the base of the Cordilleran lithosphere by edge-driven convection currents. The oxygen isotope compositions of the xenoliths (average δ18O = +5.1 ± 0.5‰) are similar to those of depleted mantle. The average δ18O values of olivine (+5.0 ± 0.2‰), opx (+5.9 ± 0.6‰), cpx (+6.0 ± 0.6‰) and spinel (+4.5 ± 0.2‰) are similar to mantle values. Large fractionations for olivine-opx, olivine-cpx and opx-cpx pairs, however, reflect disequilibrium stemming from metasomatism and partial melting. Whole-rock trace element, Nd, Sr, Pb and O isotope compositions of the xenoliths and host alkaline basalts indicate different mantle sources for these two suites of rocks. The xenoliths were derived from shallow lithospheric sources, whereas the alkaline basalts originated from a deeper asthenospheric mantle source.

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

NASA Astrophysics Data System (ADS)

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

2017-10-01

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

The Nature and Origin of the ~1.88 Ga Circum-Superior Large Igneous Province

NASA Astrophysics Data System (ADS)

Minifie, M.; Kerr, A. C.; Ernst, R. E.

2009-12-01

The Circum-Superior Large Igneous Province (LIP) is composed of a discontinuous belt of magmatic rocks, predominantly mafic-ultramafic in composition, circumscribing the cratonic margins of the Superior Province in the Canadian Shield for >3000 km. In addition to the cratonic margin magmatism, magmatic rocks of the same age are found in the interior of the craton in the form of mafic-ultramafic dykes and also carbonatite complexes along the Kapuskasing Structural Zone. Recent U-Pb geochronological studies have shown a tight age grouping for these magmatic rocks between 1885 and 1864 Ma. Previous studies have treated the various segments of the Circum-Superior LIP individually and models on the origin of the magmatism include seafloor spreading, back-arc basin rifting, foredeep basin flexure, volcanic arc activity, transtension in pull-apart basins, and mantle plume activity. This study is the first to create a cohesive geochemical and Sr-Nd-Pb-Hf-Os isotopic database for the whole of the Circum-Superior LIP and to assess its petrogenesis as a single entity. The geochemical and isotopic evidence strongly favour a mantle plume origin for the Circum-Superior LIP magmatism. A common trace element signature, very much like that of the Ontong Java oceanic plateau, is persistent throughout most of this LIP. Most samples possess Zr/Y and Nb/Y ratios almost identical to Ontong Java and other oceanic plateau lavas. Utilisation of the PRIMELT2 software of Herzberg & Asimow (2008) shows that the parental magmas of the Circum-Superior LIP were derived from ~30-35% pooled fractional melting of a source composition similar to that of primitive mantle with 1% continental crust extracted from it at mantle potential temperatures ranging from 1515 to 1610° C. Basalts from islands in Hudson Bay possess slightly enriched trace element profiles with small positive Nb anomalies and highlight a degree of heterogeneity within the plume source. The Circum-Superior LIP magmatic rocks possess similar isotopic compositions which further support the notion of a common mantle source for the whole LIP. The isotopic composition of this source is distinct to that of N-MORB which precludes the role of ambient upper mantle in the petrogenesis of the Circum-Superior magmatism suggested by previous studies. Ni-Cu-PGE sulphide deposits are associated with some regions of the Circum-Superior LIP. Subtle differences in the geochemistry of the volcanic rocks in areas which are fertile with respect to Ni-Cu-PGE deposits and areas which are barren may have implications for ore prospecting in other LIPs around the world. Herzberg, C. & Asimow, P.D. 2008. Petrology of some oceanic island basalts: PRIMELT2.XLS software for primary magma calculation. Geochemistry Geophysics Geosystems 9, doi: 10.1029/2008GC002057.

Mantle Sulfur Cycle: A Case for Non-Steady State ?

NASA Astrophysics Data System (ADS)

Cartigny, Pierre; Labidi, Jabrane

2016-04-01

Data published over the last 5 years show that the early inference that mantle is isotopically homogeneous is no more valid. Instead, new generation data on lavas range over a significant 34S/32S variability of up to 5‰ with δ 34S values often correlated to Sr- and Nd-isotope compositions. This new set of data also reveals the Earth's mantle to have a sub-chondritic 34S/32S ratio, by about ˜ 1‰. We will present at the conference our published and unpublished data on samples characterizing the different mantle components (i.e. EM1, EM2, HIMU and LOMU). All illustrate 34S-enrichments compared to MORB with Δ 33S and Δ 36S values indistinguishable from CDT or chondrites at the 0.03‰ level. These data are consistent with the recycling of subducted components carrying sulfur with Δ 33S and Δ 36S-values close to zero. Archean rocks commonly display Δ 33S and Δ 36S values deviating from zero by 1 to 10 ‰. The lack of variations for Δ 33S and Δ 36S values in present day lava argue against the sampling of any subducted protolith of Archean age in their mantle source. Instead, our data are consistent with the occurrence of Proterozoic subducted sulfur in the source of the EM1, EM2, LOMU and HIMU endmember at the St-Helena island. This is in agreement with the age of those components early derived through the use of the Pb isotope systematic. Currently, the negative δ 34S-values of the depleted mantle seem to be associated with mostly positive values of enriched components. This would be inconsistent with the concept a steady state of sulfur. Assuming that the overall observations of recycled sulfur are not biased, the origin of such a non-steady state remains unclear. It could be related to the relatively compatible behavior of sulfur during partial melting, as the residue of present-day melting can be shown to always contain significant amounts of sulfide (50{%} of what is observed in a fertile source). This typical behavior likely prevents an efficient extraction of mantle S over time, hence inhibiting quantitative mixing between surface and mantle S. This also allows the preservation of any primitive signature of the deep sulfur cycle to be potentially recorded.

Redox Heterogenity in MORB

NASA Astrophysics Data System (ADS)

Cottrell, E.; Kelley, K. A.

2012-12-01

Mantle oxygen fugacity (fO2) has a first-order effect on the petrogenesis of mantle-derived melts and the speciation of mantle fluids. Current debate centers on the spatial uniformity of upper mantle fO2 and its constancy through geologic time. We use iron K-edge X-ray absorption near-edge structure (μXANES) spectroscopy to provide Fe3+ /ΣFe ratios of submarine mantle-derived basalts from mid-ocean ridges (MORB) as a proxy for fO2. A global survey of primitive (>8.75 wt% MgO) MORB glasses at spreading centers, unaffected by plumes, reveals a decrease in Fe3+ /ΣFe ratio of 12% relative with indices of mantle enrichment such as 87/86Sr, 208/204Pb, Ba/La, and Rb/Sr ratios. The strong negative correlation between upper mantle fO2 and enrichment recorded by MORB glasses contrasts with the positive relationship hinted at by abyssal peridotite oxybarometry (e.g. Ballhaus, CMP, 1993) and the general prediction of a positive correlation born of the expectation that Fe3+ can be treated as more incompatible than Fe2+ during mantle melting. These data unequivocally link upper mantle oxidation state to mantle source enrichment. EMORB generation is commonly attributed to subduction-related processes. That EMORB is more reduced than NMORB implies that deeply subducted and recycled lithologies, such as anoxic sediment, may be more reduced than ambient mantle. Negative correlations between traditional tracers of recycled sediment (e.g. +Nb anomaly, high 87/86Sr, high LILE/LREE) and redox support this hypothesis. Preservation of redox signatures on plate-recycling timescales of hundreds of millions to billions of years would require the mantle to be very poorly buffered. Alternatively, MORB Fe3+ /ΣFe ratios may be generated in situ beneath ridges as a function of variable carbon content. The shallow MORB source is too oxidized to stabilize graphite (Cottrell and Kelley, EPSL, 2011) and carbon exists as oxides. Decreasing fO2 with increasing depth eventually stabilizes reduced carbon species (diamond, carbides, alloys), however, and aCO2 may buffer mantle assemblages. Upon ascent, reduced carbon in upwelling mantle must oxidize, reducing Fe in the process such that more carbon-rich mantle would arrive at the surface with a lower Fe3+ /ΣFe ratio. We cannot directly correlate Fe3+ /ΣFe ratios with CO2 concentrations because submarine basalts have variably degassed CO2; however, the unequivocally carbon-rich sample 2πD43 (popping rock) does record a low Fe3+ /ΣFe ratio. CO2 variations on the order of 80 ppm in the mantle source would explain the range of MORB/EMORB Fe3+ /ΣFe ratios we observe, indicating a possible range of carbon concentrations in subduction-related lithologies. The relationships between MORB Fe3+ /ΣFe ratios, trace elements, and isotopes are consistent with modeled mixtures of depleted melts and low-degree carbonatitic melts of ancient subducted igneous crust plus 5-15% sediment (Stracke et al., G3, 2001) using the near-solidus carbonatitic partition coefficients of Dasgupta et al., Chem Geol, (2009). It may be that low degree carbonatitic melts even act through geologic time to scavenge and fractionate trace elements, creating enriched high-carbon reservoirs. Low Fe3+ /ΣFe ratios, and even EMORB itself, may therefore herald greater carbon concentrations, and the influence of low-degree carbonatitic melts, in Earth's mantle.

Deep mantle: Enriched carbon source detected

NASA Astrophysics Data System (ADS)

Barry, Peter H.

2017-09-01

Estimates of carbon in the deep mantle vary by more than an order of magnitude. Coupled volcanic CO2 emission data and magma supply rates reveal a carbon-rich mantle plume source region beneath Hawai'i with 40% more carbon than previous estimates.

Chondritic Xenon in the Earth's mantle: new constrains on a mantle plume below central Europe

NASA Astrophysics Data System (ADS)

Caracausi, Antonio; Avice, Guillaume; Bernard, Peter; Furi, Evelin; Marty, Bernard

2016-04-01

Due to their inertness, their low abundances, and the presence of several different radiochronometers in their isotope systematics, the noble gases are excellent tracers of mantle dynamics, heterogeneity and differentiation with respect to the atmosphere. Xenon deserves particular attention because its isotope systematic can be related to specific processes during terrestrial accretion (e.g., Marty, 1989; Mukhopadhyay, 2012). The origin of heavy noble gases in the Earth's mantle is still debated, and might not be solar (Holland et al., 2009). Mantle-derived CO2-rich gases are particularly powerful resources for investigating mantle-derived noble gases as large quantities of these elements are available and permit high precision isotope analysis. Here, we report high precision xenon isotopic measurements in gases from a CO2 well in the Eifel volcanic region (Germany), where volcanic activity occurred between 700 ka and 11 ka years ago. Our Xe isotope data (normalized to 130Xe) show deviations at all masses compared to the Xe isotope composition of the modern atmosphere. The improved analytical precision of the present study, and the nature of the sample, constrains the primordial Xe end-member as being "chondritic", and not solar, in the Eifel mantle source. This is consistent with an asteroidal origin for the volatile elements in Earth's mantle and it implies that volatiles in the atmosphere and in the mantle originated from distinct cosmochemical sources. Despite a significant fraction of recycled atmospheric xenon in the mantle, primordial Xe signatures still survive in the mantle. This is also a demonstration of a primordial component in a plume reservoir. Our data also show that the reservoir below the Eifel region contains heavy-radiogenic/fissiogenic xenon isotopes, whose ratios are typical of plume-derived reservoirs. The fissiogenic Pu-Xe contribution is 2.26±0.28 %, the UXe contribution is negligible, the remainder being atmospheric plus primordial. Our data support the notion that the fraction of plutonium-derived Xe in plume sources (oceanic as well as continental) is higher than in the MORB source reservoir. Hence, the MORB - type reservoirs appear to be well distinguished and more degassed than the plume sources (oceanic as well as continental) supporting the heterogeneity of Earth's mantle. Finally this study highlights that xenon isotopes in the Eifel gas have preserved a chemical signature that is characteristic of other mantle plume sources. This is very intriguing because the presence of a mantle plume in this sector of Central Europe was already inferred from geophysical and geochemical studies(Buikin et al., 2005; Goes et al., 1999). Notably, tomographic images show a low-velocity structure down to 2000 km depth, representing deep mantle upwelling under central Europe, that may feed smaller upper-mantle plumes (Eifel volcanic district-Germany). References Buikin A., Trieloff M., HoppJ., Althaus T., Korochantseva E., Schwarz W.H. &Altherr R., (2005), Noble gas isotopessuggestdeepmantleplume source of late Cenozoicmaficalkalinevolcanism in Europe, Earth Planet. Sci. Lett. 230, 143-162. Goes S., Spakman W. &BijwaardH., (1999), A lowermantle source for centraleuropeanvolcanism, Science, 286, 1928-1931.G. Holland, M. Cassidy, C.J. Ballentine, Meteorite Kr in the Earth's mantle suggests a late accretionary source for the atmosphere, Science, 326, 1522-1525, (2009). Marty, B. Neon and xenon isotopes in MORB: implications for the Earth-atmosphere evolution. Earth Planet. Sci. Lett. 94, 45-56 (1989). Mukhopadhyay S., Early differentiation and volatile accretion recorded in deep-mantle neon and xenon Nature, 486, 101-106, (2013).

Iron speciation and redox state of mantle eclogites: Implications for ancient volatile cycles during mantle melting and oceanic crust subduction

NASA Astrophysics Data System (ADS)

Aulbach, Sonja; Woodand, Alan; Vasilyev, Prokopiy; Viljoen, Fanus

2017-04-01

Kimberlite-borne mantle eclogite xenoliths of Archaean and Palaeoproterozoic age are commonly interpreted as representing former oceanic crust. As such, they may retain a memory of the redox state of the ancient convecting mantle sources that gave rise to their magmatic protoliths and which controls the speciation of volatiles in planetary interiors. Mantle eclogite suites commonly include both cumulate and variably evolved extrusive varieties [1], which may be characterised by initial differences in Fe3+/Fetotal. Recent Fe-based oxybarometry shows mantle eclogites to have fO2 relative to the fayalite-magnetite-quartz buffer (ΔFMQ) of -3 to 0, whereby low fO2 relative to modern MORB may relate to subduction of more reducing Archaean oceanic crust or loss of ferric Fe during partial melt loss [2]. Indeed, using V/Sc as a redox proxy, it was recently shown that Archaean mantle eclogites are more reduced than modern MORB (ΔFMQ-1.3 vs. ΔFMQ -0.4) [3]. However, in the warmer ancient mantle, they were also subject to modification due to partial melt loss upon recycling and, after capture in the cratonic mantle lithosphere, may be overprinted by interaction with metasomatic melts and fluids. In order to help further constrain the redox state of mantle eclogites and unravel the effect of primary and secondary processes, we measured Fe3+/Fetotal by Mössbauer in garnet from mantle eclogites from the Lace kimberlite (Kaapvaal craton), comprising samples with melt- and cumulate-like oceanic crustal protoliths as well as metasomatised samples. Fe3+/ΣFe in garnet shows a strong negative correlation with jadeite content and bulk-rock Li and Cu abundances, suggesting increased partitioning of Fe3+ into jadeite in the presence of monovalent cations with which it can form coupled substitutions. Broad negative correlation with whole-rock Al2O3/TiO2 and positive correlation with ΣREE are interpreted as incompatible behaviour of Fe3+ during olivine-plagioclase accumulation (exclusion of TiO2 and REE). NMORB-normalised Nd/Yb, as a proxy of partial melt loss from subducting oceanic crust (<1) and metasomatism by typically LREE-enriched liquids (>1), shows no relationship with Fe3+/ΣFe. ΔFMQ, calculated using recently calibrated oxybarometers [2,4], broadly decreases with increasing pressure, which is ascribed to increasing garnet modes in metabasalts into which Fe3+ can be sequestered, similar to peridotite. The very low Fe3+/ΣFe, like V/Sc, appears to be a relatively robust indicator of low-pressure igneous processes and, potentially, the redox state of the ambient convecting mantle source to the protoliths of mantle eclogites. In contrast, Fe-based fO2 predominantly reflects pressure and bulk composition, and controls the speciation and mobility of volatiles in mafic heterogeneities during subduction and after emplacement in the cratonic mantle. The highly reduced nature of Archaean oceanic crust combined with further reduction upon pressure increase suggests that refractory graphite/diamond will be the stable carbon species. This may have prevented significant carbon output in Archaean subduction zones. [1] Aulbach and Jacob (2016) Lithos 262: 586-605; [2] Stagno et al. (2015) Contrib Mineral Petrol 42: 207-219; [3] Aulbach and Stagno (2016) Geology 44: 751-754; [4] Vasilyev (2016) PhD Thesis, Australian Nat Univ

Petroleum formation during serpentinization: the evidence of trace elements

NASA Astrophysics Data System (ADS)

Szatmari, P.; Fonseca, T. C.; Miekeley, N. F.

2002-05-01

An organic source of petroleum formation is well attested by many biomarkers. This need not, however, exclude contribution from inorganic sources. During serpentinization, in the absence of free oxygen, oxidation of bivalent Fe to magnetite breaks up the water molecule, generating hydrogen and creating one of the most reducing environments near the Earth's surface (Janecky & Seyfried, 1986). Szatmari (1989) proposed that some petroleum forms at plate boundaries by Fischer-Tropsch-type synthesis over serpentinizing peridotites and suggested that Ni, an element rare in the continental crust but important in both petroleum and the mantle, may be indicative of such a source. Recently, Holm and Charlou (2001) observed hydrocarbon formation by Fischer-Tropsch-type synthesis over serpentinizing peridotites of the Mid-Atlantic Ridge. To test whether the relative amounts of other trace elements in petroleum are in agreement with a serpentinizing source, we analyzed by internally coupled plasma-mass spectroscopy (ICP-MS) 22 trace elements in 68 oils sampled in seven sedimentary basins throughout Brazil. We found that trace elements in the oils correlate well with mantle peridotites and reflects the process of hydrothermal serpentinization during continental breakup. Four groups may be distinguished. In serpentinites, trace elements of the first group, Ti, Cr, Mn, and Fe, are largely retained in low-solubility magnetite and other spinels formed during serpentinization or inherited from the original peridotites. In the oils, when normalized to mantle peridotites, these elements are at relatively low levels, about 10,000 times less than their abundances in mantle peridotites, reflecting their low availability from stable minerals. In contrast, trace elements of the second group, which includes V, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Ba, La, Ce, and Nd, pass during serpentinization mostly into serpentine minerals or solution. In the oils, when normalized to mantle peridotites, these elements are at higher levels than those of the first group, about 300 times less than their abundances in mantle peridotites, reflecting their higher availability during serpentinization. Within both groups, trace metal ratios and A/(A+B) type proportionalities in the oils are close to mantle peridotites. V behaves somewhat differently: in lacustrine sequences V contents in the oils are low and the ratios of V to other elements of the second group are mantle-like, whereas in marine sequences V and its ratios to other trace elements rise by orders of magnitude. Trace elements commonly enriched in formation fluids and hydrothermal brines (Rb, Sr, Ba, Cu, Zn), when normalized to mantle peridotites, are enriched in the oils by about 0.5 order of magnitude relative to other elements of the second group. The third group of elements includes S, Mo, and As. These elements occur in the oils at abundances similar to sea water and are, when normalized to mantle peridotites and Ni, enriched in the oils by several orders of magnitude, indicating sea water reacting with peridotites during sepentinization as their possible source. Finally trace elements of the fourth group, such as Pb and Ag, are enriched in the oils by several orders of magnitude relative to both mantle peridotites and sea water and were presumably mobilized from shales by hydrothermal fluids. References:Holm, N.G. and Charlou, J.L., 2001, EPSL 191, 1-8. Janecky, D.R. and Seyfried, W.E., 1986, Geochim. Cosmochim. Acta 50, 1357-1378. Szatmari, P., 1989, AAPG Bull. 73, 989-998.

The temperature of primary melts and mantle sources of komatiites, OIBs, MORBs and LIPs

NASA Astrophysics Data System (ADS)

Sobolev, Alexander

2015-04-01

There is general agreement that the convecting mantle, although mostly peridotitic in composition, is compositionally and thermally heterogeneous on different spatial scales. The amount, sizes, temperatures and compositions of these heterogeneities significantly affect mantle dynamics because they may diverge greatly from dominant peridotites in their density and fusibility. Differences in potential temperature and composition of mantle domains affect magma production and cannot be easily distinguished from each other. This has led to radically different interpretations of the melting anomalies that produce ocean-island basalts, large igneous provinces and komatiites: most scientists believe that they originate as hot, deep-sourced mantle plumes; but a small though influential group (e.g. Anderson 2005, Foulger, 2010) propose that they derive from high proportions of easily fusible recycled or delaminated crust, or in the case of komatiites contain large amount of H2O (e.g. Grove & Parman, 2004). The way to resolve this ambiguity is an independent estimation of temperature and composition of mantle sources of various types of magma. In this paper I report application of newly developed olivine-spinel-melt geothermometers based on partition of Al, Cr, Sc and Y for different primitive lavas from mid-ocean ridges, ocean-island basalts, large igneous provinces and komatiites. The results suggest significant variations of crystallization temperature for the same Fo of high magnesium olivines of different types of mantle-derived magmas: from the lowest (down to 1220 degree C) for MORB to the highest (up to over 1500 degree C) for komatiites and Siberian meimechites. These results match predictions from Fe-Mg olivine-melt equilibrium and confirm the relatively low temperature of the mantle source of MORB and higher temperatures in the mantle plumes that produce the OIB of Iceland, Hawaii, Gorgona, Archean komatiites and several LIPs (e.g Siberian and NAMP). The established liquidus temperatures and compositions of primary melts allow estimating potential temperatures and compositions of their mantle sources. The results strongly confirm mantle plume theory and presence of variable amounts of recycled crustal material in the mantle sources. This study has been founded by Russian Science Foundation grant 14-17-00491.

The source location of mantle plumes from 3D spherical models of mantle convection

NASA Astrophysics Data System (ADS)

Li, Mingming; Zhong, Shijie

2017-11-01

Mantle plumes are thought to originate from thermal boundary layers such as Earth's core-mantle boundary (CMB), and may cause intraplate volcanism such as large igneous provinces (LIPs) on the Earth's surface. Previous studies showed that the original eruption sites of deep-sourced LIPs for the last 200 Myrs occur mostly above the margins of the seismically-observed large low shear velocity provinces (LLSVPs) in the lowermost mantle. However, the mechanism that leads to the distribution of the LIPs is not clear. The location of the LIPs is largely determined by the source location of mantle plumes, but the question is under what conditions mantle plumes form outside, at the edges, or above the middle of LLSVPs. Here, we perform 3D geodynamic calculations and theoretical analyses to study the plume source location in the lowermost mantle. We find that a factor of five decrease of thermal expansivity and a factor of two increase of thermal diffusivity from the surface to the CMB, which are consistent with mineral physics studies, significantly reduce the number of mantle plumes forming far outside of thermochemical piles (i.e., LLSVPs). An increase of mantle viscosity in the lowermost mantle also reduces number of plumes far outside of piles. In addition, we find that strong plumes preferentially form at/near the edges of piles and are generally hotter than that forming on top of piles, which may explain the observations that most LIPs occur above LLSVP margins. However, some plumes originated at pile edges can later appear above the middle of piles due to lateral movement of the plumes and piles and morphologic changes of the piles. ∼65-70% strong plumes are found within 10 degrees from pile edges in our models. Although plate motion exerts significant controls over the large-scale mantle convection in the lower mantle, mantle plume formation at the CMB remains largely controlled by thermal boundary layer instability which makes it difficult to predict geographic locations of most mantle plumes. However, all our models show consistently strong plumes originating from the lowermost mantle beneath Iceland, supporting a deep mantle plume origin of the Iceland volcanism.

Seychelles alkaline suite records the culmination of Deccan Traps continental flood volcanism

NASA Astrophysics Data System (ADS)

Owen-Smith, T. M.; Ashwal, L. D.; Torsvik, T. H.; Ganerød, M.; Nebel, O.; Webb, S. J.; Werner, S. C.

2013-12-01

Silhouette and North Islands in the Seychelles represent an alkaline plutonic-volcanic complex, dated at 63 to 63.5 Ma by U-Pb zircon and 40Ar/39Ar methods. This magmatism coincides with the final stages of the cataclysmic Deccan Traps continental flood volcanism in India (67 to 63 Ma), and thus a causal link has been suggested. Recent reconstructions have placed the Seychelles islands adjacent to the Laxmi Ridge and at the western margin of the Réunion mantle plume at the time of formation of the complex. Here we present geochemical evidence in support of the notion that the Seychelles alkaline magmatism was initiated by the peripheral activity of the Réunion mantle plume and is thus part of the Deccan magmatic event. Positive εNd (0.59 to 3.76) and εHf (0.82 to 6.79) and initial Sr of 0.703507 to 0.705643 at 65 Ma indicate derivation of the Seychelles alkaline magmas from a Réunion-like mantle source with an additional minor enriched component, suggesting entrainment of sub-continental lithospheric mantle. The similarity in trace element composition between the Seychelles suite and Deccan alkaline felsic and mafic rocks provides additional evidence for a common mantle source for the Seychelles and Deccan magmatism. Furthermore, we demonstrate the role of fractional crystallisation in the evolution of the alkaline suite. Modelling using major elements suggests that fractional crystallisation and varying degrees of accumulation of olivine, plagioclase, ilmenite, clinopyroxene, alkali feldspar and apatite can describe the spectrum of rock types, from gabbro, through syenite, to granite.

A Partial Late Veneer for the Source of 3.8 Ga Isua Rocks: Evidence from Highly Siderophile Elements and 182W

NASA Astrophysics Data System (ADS)

Dale, C.; Kruijer, T.; Burton, K. W.; Kleine, T.; Moorbath, S.

2015-12-01

Highly siderophile elements (HSE) were strongly sequestered into metallic planetary cores, leaving silicate mantles almost devoid of HSE. Late accretion partially replenished HSE in planetary mantles soon after core formation had ceased [1], which for Earth probably postdated the giant Moon-forming impact. Ancient isolated domains in Earth's mantle - such as the source of 3.8 Ga Isua basalts - might represent mantle isolated from late accreted material, as suggested based on their small 182W excesses compared to Earth's present-day mantle [2]. However, such 182W excesses may also represent signatures of early differentiation in Earth's mantle, which have been preserved through the giant impact [3]. To assess the origin of 182W anomalies and the 182W composition of the pre-late veneer mantle, we determined HSE abundances and 182W compositions of a suite of mafic to ultramafic rocks from Isua. Our data show that the Isua source mantle had HSE abundances at ~60% of the present-day mantle, inconsistent with isolation from the late veneer. For the same samples we obtained a 13±4 ppm 182W excess over the modern terrestrial mantle, in excellent agreement with previous data [2]. Using a range of possible late veneer compositions and taking into account the recently revised 182W value for the Moon [4], we calculate that the Isua mantle source, containing 60% late veneer, would have a 182W value of 9±4 ppm, in very good agreement with the measured value for Isua. The combined HSE-W data, therefore, are consistent with only partial addition of the late veneer to the Isua mantle source, and with the interpretation that the 27±4 ppm 182W excess of the Moon represents the 182W composition of the pre-late veneer Earth's mantle [4]. [1] Dale et al. (2012) Science 336, 72. [2] Willbold et al. (2011) Nature 477, 195. [3] Touboul et al. (2012) Science 335, 1065-1069. [4] Kruijer et al. (2015) Nature 7548, 534

The Upper Mantle Flow Field around South-Africa as Reflected by Isotopic Provinciality

NASA Astrophysics Data System (ADS)

Meyzen, C.; Blichert-Toft, J.; Ludden, J.; Humler, E.; Mevel, C.; Albarede, F.

2006-12-01

Isotopic studies of MORB have established the existence of broad isotopic provinces within the underlying asthenosphere, such as in the Indian Ocean (DUPAL). How these features relate to mantle circulation is, however, still unknown. The steepness of the transition between such isotopic provinces will define the geometry of the velocity field in the upper mantle. In this respect, the transition between the Indian and South Atlantic provinces, two domains that are isotopically contrasted, should be readily identifiable over this long ridge segment. Here, we present Hf isotope data for 60 samples dredged along the SWIR between 35° and 69°E. The new Hf isotope data show that the Indian asthenosphere does not spill directly into the South Atlantic upper mantle: the general decreasing southward gradient observed for ^{176}Hf/^{177}Hf down the mid- Atlantic Ridge, and also for Sr isotopes and model Th/U ratios (derived from Pb isotopes), is overprinted by material with radiogenic Sr, unradiogenic Hf and high Th/U. The Indian domain grades into the South Atlantic around Bouvet, while the South Atlantic collides with the Atlantic province around Tristan. We interpret these features to represent fronts between three adjacent isotopic provinces similar to what has been suggested for the Australian-Antarctic Discordance. The common DUPAL signature of MORB and OIB from the Indian province and the geochemistry of Gulf of Aden MORB and the Afar plume suggest that the source of this distinctive mantle component is deep and lies to the north of the province. This is also what the three-dimensional flow field computed by Behn et al. (2004) from shear-wave splitting shows with a major lower mantle upwelling radiating at the base of the asthenosphere under the Afar plume. Lower mantle gushing out from this source flows southward unimpeded along the Indian ridges, whereas it only reaches the South Atlantic ridge after first having been deflected under the deep roots of the South African Archean cratons. Erosion of these roots by the asthenospheric drift confers a distinct continental signature on the source of South Atlantic MORB. This pattern is also consistent with the observation that the lowest He isotope values occur, on average, along the South Atlantic ridge. To some extent, the dynamics of the North Atlantic upper mantle mirrors the Indian situation: the flow field of Behn et al. (2004) shows that the North Atlantic asthenosphere also fills up through deep mantle upwellings, which is consistent with the Dupal-like isotopic signature of the Arctic ridges. M.D. Behn, C.P. Conrad and P.G. Silver (2004), Detection of upper mantle flow associated with the African Superplume, Earth. Planet. Sci. Lett., 224, 259-274.

Sr-Nd-Pb Isotope Geochemistry of Melange Formation: Implications for Identification of Fluid Sources in the Mantle Wedge and the Arc

NASA Astrophysics Data System (ADS)

Bebout, G. E.; King, R. L.; Moriguti, T.; Nakamura, E.

2004-12-01

Paramount to our ability to decipher the behavior of fluids and melts within the mantle wedge and the overall subduction system are the chemical compositions of rocks adjacent to the slab-mantle interface. Profound metamorphic and metasomatic alteration of pre-subduction lithologies to form melange along the slab-mantle interface may yield rock types inheriting mixed chemical compositions of diverse pre-subduction lithologies. Early work on melange geochemistry indicates competitive effects between mechanical mixing, metasomatism by fluids or melts, and mineral stabilities imposed by the resulting bulk composition. We have explored the Sr-Nd-Pb isotope geochemistry of low- to high-grade melange zones in the Catalina Schist, CA, to address this crucial missing component in studies of subduction-zone mass flux. The Catalina Schist contains lawsonite-albite (LA), lawsonite-blueschist (LB), and amphibolite (AM) facies melange zones, all with mineralogy dominated by talc, chlorite, and Na-Ca amphiboles, with additional minerals such as micas, rutile, zircon, and apatite stabilized based on bulk sample chemistry. Major element compositions vary, from strongly ultramafic in the AM melange, to more crustal-like compositions (i.e., more reminiscent of basaltic to sedimentary protoliths) for LA and LB melange. However, initial Sr and Nd isotope ratios for all grades of melange are essentially indistinguishable, displaying a wide variation from 87Sr/86Sr=0.703-0.709 and ɛ Nd= +15 to -15. Covariations are generally negative, similar to that of the mantle array, but with some samples extending to higher Sr ratios at constant ɛ Nd that probably reflect inheritance of seawater Sr. No clear mixing relationships between 87Sr/86Sr and 1/Sr exist, suggesting either localized buffering of Sr isotope ratios or that mixing relations are obscured by secondary devolatilization. However, a clear mixing trend for Nd indicates two end-members, one a high-concentration, positive ɛ Nd source (AOC?), the other with low-concentration and negative ɛ Nd (devolatilized sediments?). Likewise, initial Pb isotope ratios for all grades of melange form a single array independent of rock type or inferred protolith. Melange matrix of the Catalina Schist preserves initial 206Pb/204Pb of 18.95-19.59, 207Pb/204Pb of 15.61-15.68, and 208Pb/204Pb of 37.85-39.05. Such elevated Pb ratios are typical of subducting oceanic sediments, but not of MORB-like oceanic crust or peridotites of the depleted mantle. The similarity of these initial ratios suggests pervasive alteration of Pb isotope signatures within diverse rock types by fluids during subduction. As Pb concentrations decline from LA/LB to AM melange, this suggests devolatilization of Pb from the ultramafic AM melange will transfer crustal-like Pb isotope ratios. Sr-Nd-Pb isotope systematics for arc volcanic rocks are commonly used as indicators of fluid sources from the subducting slab to the arc magma source region. Our results suggest such an assumption is extremely dangerous, as hybridization processes common to melange zones are more likely to occur along the slab-mantle interface than is preservation of a pre-subduction section. Such metamorphic mediation and buffering of "slab" compositions is essentially unknown, yet our data support an interpretation where these processes impart a fundamental control on the chemistry of fluids passed to the mantle wedge.

Mantle source volumes and the origin of the mid-Tertiary ignimbrite flare-up in the southern Rocky Mountains, western U.S.

NASA Astrophysics Data System (ADS)

Farmer, G. Lang; Bailley, Treasure; Elkins-Tanton, Linda T.

2008-04-01

Voluminous intermediate to silicic composition volcanic rocks were generated throughout the southern Rocky Mountains, western U.S., during the mid-Tertiary "ignimbrite flare-up", principally at the San Juan and Mogollon-Datil volcanic fields. At both volcanic centers, radiogenic isotope data have been interpreted as evidence that 50% or more of the volcanic rocks (by mass) were derived from mantle-derived, mafic parental magmas, but no consensus exists as to whether melting was largely of lithospheric or sub-lithospheric mantle. Recent xenolith studies, however, have revealed that thick (> 100 km), fertile, and hydrated continental lithosphere was present beneath at least portions of the southern Rocky Mountains during the mid-Tertiary. The presence of such thick mantle lithosphere, combined with an apparent lack of syn-magmatic extension, leaves conductive heating of lithospheric mantle as a plausible method of generating the mafic magmas that fueled the ignimbrite flare-up in this inland region. To further assess this possibility, we estimated the minimum volume of mantle needed to generate the mafic magmas parental to the preserved mid-Tertiary igneous rocks. Conservative estimates of the mantle source volumes that supplied the Mogollon-Datil and San Juan volcanic fields are ˜ 2 M km 3 and ˜ 7 M km 3, respectively. These volumes could have comprised only lithospheric mantle if at least the lower ˜ 20 km of the mantle lithosphere beneath the entire southern Rocky Mountains region underwent partial melting during the mid-Tertiary and if the resulting mafic magmas were drawn laterally for distances of up to ˜ 300 km into each center. Such widespread melting of lithospheric mantle requires that the lithospheric mantle have been uniformly fertile and primed for melting in the mid-Tertiary, a possibility if the lithospheric mantle had experienced widespread hydration and refrigeration during early Tertiary low angle subduction. Exposure of the mantle lithosphere to hot, upwelling sub-lithospheric mantle during mid-Tertiary slab roll back could have then triggered the mantle melting. While a plausible source for mid-Tertiary basaltic magmas in the southern Rocky Mountains, lithospheric mantle could not have been the sole source for mafic magmas generated to the south in that portion of the ignimbrite flare-up now preserved in the Sierra Madre Occidental of northern Mexico. The large mantle source volumes (> 45 M km 3) required to fuel the voluminous silicic ignimbrites deposited in this region (> 400 K km 3) are too large to have been accommodated within the lithospheric mantle alone, implying that melting in sub-lithospheric mantle must have played a significant role in generating this mid-Tertiary magmatic event.

Multiple subduction imprints in the mantle below Italy detected in a single lava flow

NASA Astrophysics Data System (ADS)

Nikogosian, Igor; Ersoy, Özlem; Whitehouse, Martin; Mason, Paul R. D.; de Hoog, Jan C. M.; Wortel, Rinus; van Bergen, Manfred J.

2016-09-01

Post-collisional magmatism reflects the regional subduction history prior to collision but the link between the two is complex and often poorly understood. The collision of continents along a convergent plate boundary commonly marks the onset of a variety of transitional geodynamic processes. Typical responses include delamination of subducting lithosphere, crustal thickening in the overriding plate, slab detachment and asthenospheric upwelling, or the complete termination of convergence. A prominent example is the Western-Central Mediterranean, where the ongoing slow convergence of Africa and Europe (Eurasia) has been accommodated by a variety of spreading and subduction systems that dispersed remnants of subducted lithosphere into the mantle, creating a compositionally wide spectrum of magmatism. Using lead isotope compositions of a set of melt inclusions in magmatic olivine crystals we detect exceptional heterogeneity in the mantle domain below Central Italy, which we attribute to the presence of continental material, introduced initially by Alpine and subsequently by Apennine subduction. We show that superimposed subduction imprints of a mantle source can be tapped during a melting episode millions of years later, and are recorded in a single lava flow.

Early Precambrian mantle derived rocks in the southern Prince Charles Mountains, East Antarctica: age and isotopic constraints

USGS Publications Warehouse

Mikhalsky, E.V.; Henjes-Kunst, F.; Roland, N.W.

2007-01-01

Mafic and ultramafic rocks occurring as lenses, boudins, and tectonic slabs within metamorphic units in the southern Mawson Escarpment display mantle characteristics of either a highly enriched, or highly depleted nature. Fractionation of these mantle rocks from their sources may be as old as Eoarchaean (ca 3850 Ma) while their tectonic emplacement probably occurred prior to 2550 Ma (U-Pb SHRIMP data). These results provide for the first time evidence for Archaean suturing within East Antarctica. Similar upper mantle sources are likely present in the northern Mawson Escarpment. A younger age limit of these rocks is 2200 Ma, as indicated by presumably metamorphic zircon ages while their magmatic age may be constrained by single zircon dates at 2450-2250 Ma. The area of the northern Mawson Escarpment is most likely of ensimatic origin and includes mafic rocks which were derived from distinct mantle source(s) during Palaeoproterozoic time.

Feldspar palaeo-isochrons from early Archaean TTGs: Pb-isotope evidence for a high U/Pb terrestrial Hadean crust

NASA Astrophysics Data System (ADS)

Kamber, B. S.; Whitehouse, M. J.; Moorbath, S.; Collerson, K. D.

2001-12-01

Feldspar lead-isotope data for 22 early Archaean (3.80-3.82 Ga) tonalitic gneisses from an area south of the Isua greenstone belt (IGB),West Greenland, define a steep linear trend in common Pb-isotope space with an apparent age of 4480+/-77 Ma. Feldspars from interleaved amphibolites yield a similar array corresponding to a date of 4455+/-540 Ma. These regression lines are palaeo-isochrons that formed during feldspar-whole rock Pb-isotope homogenisation a long time (1.8 Ga) after rock formation but confirm the extreme antiquity (3.81 Ga) of the gneissic protoliths [1; this study]. Unlike their whole-rock counterparts, feldspar palaeo-isochrons are immune to rotational effects caused by the vagaries of U/Pb fractionation. Hence, comparison of their intercept with mantle Pb-isotope evolution models yields meaningful information regarding the source history of the magmatic precursors. The locus of intersection between the palaeo-isochrons and terrestrial mantle Pb-isotope evolution lines shows that the gneissic precursors of these 3.81 Ga gneisses were derived from a source with a substantially higher time-integrated U/Pb ratio than the mantle. Similar requirements for a high U/Pb source have been found for IGB BIF [2], IGB carbonate [3], and particularly IGB galenas [4]. Significantly, a single high U/Pb source that separated from the MORB-source mantle at ca. 4.3 Ga with a 238U/204Pb of ca. 10.5 provides a good fit to all these observations. In contrast to many previous models based on Nd and Hf-isotope evidence we propose that this reservoir was not a mantle source but the Hadean basaltic crust which, in the absence of an operating subduction process, encased the early Earth. Differentiation of the early high U/Pb basaltic crust could have occurred in response to gravitational sinking of cold mantle material or meteorite impact, and produced zircon-bearing magmatic rocks. The subchondritic Hf-isotope ratios of ca. 3.8 Ga zircons support this model [5] provided that the redetermined 176Lu decay constant of Scherer et al. [6] is correct. Our model of a stable basaltic Hadean shell for the pre-plate tectonic era explicitly refutes operation of processes such as sediment recycling or melting of hydrated material in subduction zones as far back as 4.4 Ga (as recently suggested by [7]; and [8]). Instead, we propose that initiation of terrestrial subduction occurred at ca. 3.75 Ga, at which stage most of the Hadean basaltic shell (and its differentiation products) was recycled into the mantle, because of the lack of a stabilising mantle lithosphere. We further argue that >3.75 Ga terrestrial rocks and minerals were not preserved by chance, but because of creation of a lithospheric mantle keel concommitant with intrusion of voluminous granitoids immediately after establishment of global subduction. In other words, the only portions of >3.75 Ga crust (basaltic and otherwise) that survived were those that were involved in voluminous arc magmatism along the earliest subduction zones. [1] Nutman A.P. et al. (1999). Contr. Min. Pet. 137, 364. [2] Moorbath S. et al. (1973). Nature 245, 138. [3] Kamber B. S. et al.. (2001). Geol. Soc. London, Spec. Publ. 190, 177. [4] Frei R. & Rosing M. T. (in press). Chem. Geol. [5] Amelin Y. et al. (2000). GCA 64, 4205. [6] Scherer E. et al (2001) Science 293, 683. [7] Wilde S. A. et al.(2001). Nature 409, 175. [8] Mojzsis S. J. (2001). Nature 409, 178.

Rhenium-osmium isotopes and highly siderophile elements in ultramafic rocks from the Eoarchean Saglek Block, northern Labrador, Canada: implications for Archean mantle evolution

NASA Astrophysics Data System (ADS)

Ishikawa, Akira; Suzuki, Katsuhiko; Collerson, Kenneth D.; Liu, Jingao; Pearson, D. Graham; Komiya, Tsuyoshi

2017-11-01

We determined highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) concentrations and 187Os/188Os ratios for ultramafic rocks distributed over the Eoarchean gneiss complex of the Saglek-Hebron area in northern Labrador, Canada in order to constrain to what extent variations in HSE abundances are recorded in Early Archean mantle that have well-resolved 182W isotope anomalies relative to the present-day mantle (∼+11 ppm: Liu et al., 2016). The samples analysed here have been previously classified into two suites: mantle-derived peridotites occurring as tectonically-emplaced slivers of lithospheric mantle, and metakomatiites comprising mostly pyroxenitic layers in supracrustal units dominated by amphibolites. Although previous Sm-Nd and Pb-Pb isotope studies provided whole-rock isochrons indicative of ∼3.8 Ga protolith formation for both suites, our whole-rock Re-Os isotope data on a similar set of samples yield considerably younger errorchrons with ages of 3612 ± 130 Ma (MSWD = 40) and 3096 ± 170 Ma (MSWD = 10.2) for the metakomatiite and lithospheric mantle suites, respectively. The respective initial 187Os/188Os = 0.10200 ± 18 for metakomatiites and 0.1041 ± 18 for lithospheric mantle rocks are within the range of chondrites. Re-depletion Os model ages for unradiogenic samples from the two suites are consistent with the respective Re-Os errorchrons (metakomatiite TRD = 3.4-3.6 Ga; lithospheric mantle TRD = 2.8-3.3 Ga). These observations suggest that the two ultramafic suites are not coeval. However, the estimated mantle sources for the two ultramafics suites are similar in terms of their broadly chondritic evolution of 187Os/188Os and their relative HSE patterns. In detail, both mantle sources show a small excess of Ru/Ir similar to that in modern primitive mantle, but a ∼20% deficit in absolute HSE abundances relative to that in modern primitive mantle (metakomatiite 74 ± 18% of PUM; lithospheric mantle 82 ± 10% of PUM), consistent with the ∼3.8 Ga Isua mantle source and Neoarchean komatiite sources around the world (∼70-86% of PUM). This demonstrates that the lower HSE abundances are not unique to the sources of komatiites, but rather might be a ubiquitous feature of Archean convecting mantle. This tentatively suggests that chondritic late accretion components boosted the convecting mantle HSE inventory after core separation in the Hadean, and that the Eoarchean to Neoarchean convecting mantle was depleted in its HSE content relative to that of today. Further investigation of Archean mantle-derived rocks is required to explore this hypothesis.

Noble gas composition of subcontinental lithospheric mantle: An extensively degassed reservoir beneath Southern Patagonia

NASA Astrophysics Data System (ADS)

Jalowitzki, Tiago; Sumino, Hirochika; Conceição, Rommulo V.; Orihashi, Yuji; Nagao, Keisuke; Bertotto, Gustavo W.; Balbinot, Eduardo; Schilling, Manuel E.; Gervasoni, Fernanda

2016-09-01

Patagonia, in the Southern Andes, is one of the few locations where interactions between the oceanic and continental lithosphere can be studied due to subduction of an active spreading ridge beneath the continent. In order to characterize the noble gas composition of Patagonian subcontinental lithospheric mantle (SCLM), we present the first noble gas data alongside new lithophile (Sr-Nd-Pb) isotopic data for mantle xenoliths from Pali-Aike Volcanic Field and Gobernador Gregores, Southern Patagonia. Based on noble gas isotopic compositions, Pali-Aike mantle xenoliths represent intrinsic SCLM with higher (U + Th + K)/(3He, 22Ne, 36Ar) ratios than the mid-ocean ridge basalt (MORB) source. This reservoir shows slightly radiogenic helium (3He/4He = 6.84-6.90 RA), coupled with a strongly nucleogenic neon signature (mantle source 21Ne/22Ne = 0.085-0.094). The 40Ar/36Ar ratios vary from a near-atmospheric ratio of 510 up to 17700, with mantle source 40Ar/36Ar between 31100-6800+9400 and 54000-9600+14200. In addition, the 3He/22Ne ratios for the local SCLM endmember, at 12.03 ± 0.15 to 13.66 ± 0.37, are higher than depleted MORBs, at 3He/22Ne = 8.31-9.75. Although asthenospheric mantle upwelling through the Patagonian slab window would result in a MORB-like metasomatism after collision of the South Chile Ridge with the Chile trench ca. 14 Ma, this mantle reservoir could have remained unhomogenized after rapid passage and northward migration of the Chile Triple Junction. The mantle endmember xenon isotopic ratios of Pali-Aike mantle xenoliths, which is first defined for any SCLM-derived samples, show values indistinguishable from the MORB source (129Xe/132Xe =1.0833-0.0053+0.0216 and 136Xe/132Xe =0.3761-0.0034+0.0246). The noble gas component observed in Gobernador Gregores mantle xenoliths is characterized by isotopic compositions in the MORB range in terms of helium (3He/4He = 7.17-7.37 RA), but with slightly nucleogenic neon (mantle source 21Ne/22Ne = 0.065-0.079). We suggest that this MORB-like metasomatism was capable of overprinting the noble gas composition of Gobernador Gregores due to recent metasomatism of the SCLM because of asthenospheric mantle upwelling in response to opening of the Patagonian slab window. The 40Ar/36Ar ratios vary from a near-atmospheric ratio of 380 up to 6560, with mantle source 40Ar/36Ar between 8100-700+1400 and 17700-3100+4400. The lower 40Ar/36Ar ratio of the Gobernador Gregores mantle source, compared with that of Pali-Aike, attests that the Patagonia SCLM was affected significantly by atmospheric contamination associated with the recycled oceanic lithosphere.

Highly siderophile element and 182W evidence for a partial late veneer in the source of 3.8 Ga rocks from Isua, Greenland

NASA Astrophysics Data System (ADS)

Dale, Christopher W.; Kruijer, Thomas S.; Burton, Kevin W.

2017-01-01

The higher-than-expected concentrations of highly siderophile elements (HSE) in Earth's mantle most likely indicate that Earth received a small amount of late accreted mass after core formation had ceased, known as the 'late veneer'. Small 182W excesses in the Moon and in some Archaean rocks - such as the source of 3.8 billion-year-old Isua magmatics - also appear consistent with the late veneer hypothesis, with a lower proportion received. However, 182W anomalies can also relate to other processes, including early mantle differentiation. To better assess the origin of these W isotope anomalies - and specifically whether they relate to the late veneer - we have determined the HSE abundances and 182W compositions of a suite of mafic to ultramafic rocks from Isua, from which we estimate HSE abundances in the source mantle and ultimately constrain the 182W composition of the pre-late veneer mantle. Our data suggest that the Isua source mantle had HSE abundances at around 50-65% of the present-day mantle, consistent with partial, but not complete, isolation from the late veneer. These data also indicate that at least part of the late veneer had been added and mixed into the mantle at the time the Isua source formed, prior to 3.8 Ga. For the same Isua samples we obtained a 13 ± 4 ppm182W excess, compared to the modern terrestrial mantle, in excellent agreement with previous data. Using combined 182W and HSE data we show that the Moon, Isua, and the present-day bulk silicate Earth (BSE) produce a well-defined co-variation between 182W composition and the mass fraction of late-accreted mass, as inferred from HSE abundances. This co-variation is consistent with the calculated effects of various late accretion compositions on the HSE and 182W signatures of Earth's mantle. The empirical relationship, therefore, implies that the Moon, Isua source and BSE received increasing proportions of late-accreted mass, supporting the idea of disproportional late accretion to the Earth and Moon, and consistent with the interpretation that the lunar 182W value of 27 ± 4 ppm represents the composition of Earth's mantle before the late veneer was added. In this case, the Isua source can represent ambient mantle after the giant moon-forming impact, into which only a part of Earth's full late veneer was mixed, rather than an isotopically distinct mantle domain produced by early differentiation, which would probably require survival through the giant Moon-forming impact.

Early differentiation and volatile accretion recorded in deep-mantle neon and xenon.

PubMed

Mukhopadhyay, Sujoy

2012-06-06

The isotopes (129)Xe, produced from the radioactive decay of extinct (129)I, and (136)Xe, produced from extinct (244)Pu and extant (238)U, have provided important constraints on early mantle outgassing and volatile loss from Earth. The low ratios of radiogenic to non-radiogenic xenon ((129)Xe/(130)Xe) in ocean island basalts (OIBs) compared with mid-ocean-ridge basalts (MORBs) have been used as evidence for the existence of a relatively undegassed primitive deep-mantle reservoir. However, the low (129)Xe/(130)Xe ratios in OIBs have also been attributed to mixing between subducted atmospheric Xe and MORB Xe, which obviates the need for a less degassed deep-mantle reservoir. Here I present new noble gas (He, Ne, Ar, Xe) measurements from an Icelandic OIB that reveal differences in elemental abundances and (20)Ne/(22)Ne ratios between the Iceland mantle plume and the MORB source. These observations show that the lower (129)Xe/(130)Xe ratios in OIBs are due to a lower I/Xe ratio in the OIB mantle source and cannot be explained solely by mixing atmospheric Xe with MORB-type Xe. Because (129)I became extinct about 100 million years after the formation of the Solar System, OIB and MORB mantle sources must have differentiated by 4.45 billion years ago and subsequent mixing must have been limited. The Iceland plume source also has a higher proportion of Pu- to U-derived fission Xe, requiring the plume source to be less degassed than MORBs, a conclusion that is independent of noble gas concentrations and the partitioning behaviour of the noble gases with respect to their radiogenic parents. Overall, these results show that Earth's mantle accreted volatiles from at least two separate sources and that neither the Moon-forming impact nor 4.45 billion years of mantle convection has erased the signature of Earth's heterogeneous accretion and early differentiation.

Subduction-modified oceanic crust in the sources of continental picrite dikes from the Karoo LIP?

NASA Astrophysics Data System (ADS)

Heinonen, J. S.; Carlson, R. W.; Riley, T. R.; Luttinen, A. V.; Horan, M. F.

2013-12-01

The Ahlmannryggen mountain range in East Antarctica hosts unusual LILE-depleted, but Fe- and Ti-enriched ultramafic dikes (Group 3) that belong to the Jurassic (~180 Ma) Karoo continental flood basalt (CFB) province. Their high initial ɛNd (+5 to +9) indicates their origin within the sublithospheric mantle beneath the Gondwana supercontinent. Using the new Pb and Os isotopic data and previously published geochemical and mineral chemical data, we try to constrain their mantle sources. The dikes that lack evidence of crustal contamination exhibit very radiogenic ɛNd (+8.6 to +9.0), relatively radiogenic 206Pb/204Pb (18.2-18.4) and 87Sr/86Sr (0.7035-0.7037), and unradiogenic 187Os/188Os (0.124-0.125) at 180 Ma. These isotopic compositions are unlike those typical of MORBs, excluding depleted mantle as the sole source contributor. The Pb isotopic composition of the dikes plots close to the 4.43 Ga geochron and hence is compatible with derivation from an early-depleted reservoir (EDR), recently suggested to be a major source component in CFBs. However, the high ɛNd of the dikes exceeds the ɛNd estimated for EDR (+4.9 to +8.5 at 180 Ma) and the relative Nb, Fe, and Ti enrichment (pyroxenite fingerprint) of the dikes is not readily ascribed to EDR source. Based on our isotopic and trace element modeling, we regard that the mantle source of the picrite dikes contained seawater-altered and subduction-modified MORB with a recycling age of 0.8 Ga. Such a source component would explain the unusual combination of elevated initial 87Sr/86Sr, ɛNd, and 206Pb/204Pb, relative depletion in fluid-mobile LILE, U, Th, Pb, and LREE, and relative enrichment in Nb, Fe, Ti, and other HFSE. Behavior of Re and Os in subduction environments is not well constrained, but loss of Re from recycled MORB, as observed in some subduction-associated eclogites and blueschists, and predominant contribution of Os from depleted peridotite matrix could have produced the observed low 187Os/188Os. Pyroxenite sources also are consistent with mineral chemical data (e.g., high-Ni olivine) for the picrite dikes. Such peculiar sources were likely not a predominant component in Karoo magmatism in general. Nevertheless, less subduction-modified or more enriched (e.g., additional sediment component) recycled crustal signatures would be difficult to distinguish from the 'lithospheric signatures' of many common CFBs. In addition to depleted mantle or EDR components that have been identified in the high-Mg dikes of the adjacent Vestfjella mountain range, a variety of recycled source components could thus be hiding in the geochemical jungle of the Karoo (and other) CFBs.

The ratios of carbon and non-radiogenic helium and argon isotopes in the mantle and crustal rocks

NASA Technical Reports Server (NTRS)

Lokhov, K.; Levsky, L.

1994-01-01

The studies of the relations of carbon and primary isotopes of noble gases were carried out on the natural gases and on the mantle rocks from the mantle M-type sources, which represent the degassed mantle reservoir (MORB's). These works has the aim of estimation of the values of the C/3He ratios in the deep mantle fluids to determine the flux of the mantle CO2 on the basis of known flux of primary mantle 3He. It was found, that in the natural gases the values of the C/3He ratios fall into the range from 1 times E plus 6 to 1 times E plus 15, and in the fluids of MORB's are constant near 2 times E plus 9. We have studied the mantle rocks from the relatively undergassed mantle P minus type sources: continental; Baikal Rift (Siberia), Mongolia, Catalonia (Spain), Pannonia Depression (central Europe) and ocean; Spietzbergen isl., Hawaii isl., Canarian isl. It ws found, that in mantle xenolites and the host alkaline basalts from the continental rifts and ocean islands, the values of the C/3He ratios fall into the range from E plus 11 to E plus 15 (and this result needed to be explained; the higher carbon to helium ratios is relatively undergassed mantle reservoir compared with the degassed one, requires whether hilly compatibility of helium compared with carbon, whether additional flux of 3He to the degassed mantle reservoir). From the other hand it was found that in the mantle rocks from the sources of P minus and M minus types, continental carbonatites, the values of the C/36Ar ratios are constant in the range from E plus 9 to E plus 10, the close values have the MORB's also.

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

NASA Astrophysics Data System (ADS)

Heinonen, Jussi S.; Kurz, Mark D.

2015-09-01

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

Mantle transition zone-derived EM1 component beneath NE China: Geochemical evidence from Cenozoic potassic basalts

NASA Astrophysics Data System (ADS)

Wang, Xiao-Jun; Chen, Li-Hui; Hofmann, Albrecht W.; Mao, Fu-Gen; Liu, Jian-Qiang; Zhong, Yuan; Xie, Lie-Wen; Yang, Yue-Heng

2017-05-01

The isotopic characteristics of the sub-oceanic mantle are well established, but in continental regions these properties are usually obscured, and therefore controversial, because of the potential effects of crustal contamination together with lithospheric mantle metasomatism and melting. The so-called EM1 (Enriched Mantle-1) signature, characterized by low 206Pb/204Pb and 143Nd/144Nd ratios, is particularly problematic in this respect because EM1-type OIB sources are commonly attributed to recycled crust and/or lithospheric mantle. In this paper we show that a suite of Cenozoic potassic basalts from NE China displays many previously unrecognized correlations between chemical and isotopic parameters, which tightly constrain the isotopic characteristics of an extreme EM1-type mantle source located in the asthenosphere. Its radiogenic isotopes are similar to, but even more extreme than, those of the oceanic endmember composition represented by the Pitcairn hotspot, namely 206Pb/204Pb ≤ 16.5, 143Nd/144Nd ≤ 0.5123 (or εNd ≤ - 6.4), 176Hf/177Hf ≤ 0.2825 (or εHf ≤ - 10.1). These characteristics require a source of recycled crustal material of Precambrian age (∼2.2 Ga). An important new constraint is the Mg isotopic composition of δ26 Mg (≤ - 0.6 ‰), which is lower than normal mantle (δ26 Mg = - 0.25 ± 0.07 ‰) and lower crustal values (δ26 Mg = - 0.29 ± 0.15 ‰), but consistent with sedimentary carbonate (δ26 Mg = - 5.57 ‰ to - 0.38 ‰). The endmember EM1 source produced high-SiO2 melts with low MgO, CaO/Al2O3 and δ26 Mg values, exceptionally high K/U ≅ 50,000, Ba/Th ≅ 400, low U/Pb ≅ 0.06, and positive Zr and Hf anomalies. The chemical and isotopic parameters of this potassic basalt suite form binary mixing arrays, one end point of which is the inferred asthenospheric EM1 reservoir, whereas the other is a more ordinary, depleted mantle component, which is also sampled by local lithospheric mantle xenoliths. These binary arrays include well-developed correlations between Sr, Nd, Hf, Pb and Mg isotopes, negative correlations of 206Pb/204Pb with K2 O, K/U, Hf/Hf*, positive correlations of δ26 Mg with MgO, and 143Nd/144Nd with Fe2OT3 and U/Pb. We propose that the EM1 reservoir contains recycled ancient carbonate-bearing sediments, subducted into the mantle transition zone, where K, Rb, Ba and Pb are sequestered by K-hollandite as suggested by Murphy et al. (2002) for the Gaussberg lamproites. Loss of small amounts of carbonate melt extracted Th, U and some of the LREE, while retaining K, Rb, Ba, Pb, Zr and Hf in the residue, thereby generating the observed trace element anomalies. In Cenozoic time, this deep EM1 reservoir ascended into the shallow asthenosphere and underwent low-degree partial melting, at pressures below the stability field of K-hollandite, thereby releasing K, Rb and Ba into the melt. The partial melts ascended through subcontinental lithosphere and were progressively modified by interaction with the lithospheric mantle, thus accounting for the linear chemical and isotopic trends noted above. This interaction imposed a progressively more depleted character on the erupted melt, both in terms of isotopic composition and trace element enrichment.

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

USGS Publications Warehouse

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

1982-01-01

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

Key new pieces of the HIMU puzzle from olivines and diamond inclusions.

PubMed

Weiss, Yaakov; Class, Cornelia; Goldstein, Steven L; Hanyu, Takeshi

2016-09-29

Mantle melting, which leads to the formation of oceanic and continental crust, together with crust recycling through plate tectonics, are the primary processes that drive the chemical differentiation of the silicate Earth. The present-day mantle, as sampled by oceanic basalts, shows large chemical and isotopic variability bounded by a few end-member compositions. Among these, the HIMU end-member (having a high U/Pb ratio, μ) has been generally considered to represent subducted/recycled basaltic oceanic crust. However, this concept has been challenged by recent studies of the mantle source of HIMU magmas. For example, analyses of olivine phenocrysts in HIMU lavas indicate derivation from the partial melting of peridotite, rather than from the pyroxenitic remnants of recycled oceanic basalt. Here we report data that elucidate the source of these lavas: high-precision trace-element analyses of olivine phenocrysts point to peridotite that has been metasomatized by carbonatite fluids. Moreover, similarities in the trace-element patterns of carbonatitic melt inclusions in diamonds and HIMU lavas indicate that the metasomatism occurred in the subcontinental lithospheric mantle, fused to the base of the continental crust and isolated from mantle convection. Taking into account evidence from sulfur isotope data for Archean to early Proterozoic surface material in the deep HIMU mantle source, a multi-stage evolution is revealed for the HIMU end-member, spanning more than half of Earth's history. Before entrainment in the convecting mantle, storage in a boundary layer, upwelling as a mantle plume and partial melting to become ocean island basalt, the HIMU source formed as Archean-early Proterozoic subduction-related carbonatite-metasomatized subcontinental lithospheric mantle.

The Mantle Isotopic Array: A Tale of Two FOZOs

NASA Astrophysics Data System (ADS)

Apen, F. E.; Mukhopadhyay, S.; Williams, C. D.

2017-12-01

Oceanic basalts display isotopic arrays that suggest mixing between a depleted component, several enriched components, and a primitive component. The topology of the arrays provides information on mantle mixing, the distribution of heterogeneities, and information on mantle structure. Here we use a global compilation of mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) He-Sr-Nd-Pb isotopic data to further analyze the topology of these arrays. Previous work indicated that OIB isotopic arrays converge to a common component [1-3] referred to as the focus zone, or FOZO. Our analyses suggest that while all OIBs do point to a common component with unradiogenic 4He/3He ratios relative to MORBs, this component has to be quite variable in its He, Sr, Nd and Pb isotopic compositions. FOZO cannot be a pure component but must represent a heterogeneous mixture of primitive and recycled material. Our analyses of the MORB and OIB isotopic compositions also indicate that while MORBs and OIBs sample the same components, the topology of their mixing arrays are quite distinct. Different MOR segments show quasi-linear isotopic arrays that all converge to a common component. This component is distinctive from the OIB FOZO being more depleted and more restrictive in its He, Sr, Nd and Pb composition. We suggest two common but distinguishable components are present in the mantle arrays: one common to MORBs and the other to OIBs, and we refer to them as MORB-FOZO and OIB-FOZO, respectively. We interpret the two FOZOs to represent the average composition of small-scale heterogeneities that make up the background matrix in the sources of MORBs and OIBs. The depleted and enriched components that are sampled in MORBs and OIBs reflect relatively large-scale heterogeneities distributed within the matrix, material that have yet to be deformed into the smaller length scales of the matrix material. Differences between the two FOZO compositions reflects the inclusion of a component with primitive He in OIBs along with differences in mixing timescales and mantle processing rates for MORBs and OIBs. The two distinct FOZO compositions must also indicate limited direct mixing between the two over Earth's 4.5 Gyr history. References: [1] Hart et al., Science 1992; [2] Farley et al., EPSL 1992; [3] Hanan and Graham, Science 1996.

Long-Lived Mantle Plumes Sample Multiple Deep Mantle Geochemical Domains: The Example of the Hawaiian-Emperor Chain

NASA Astrophysics Data System (ADS)

Harrison, L.; Weis, D.

2017-12-01

Oceanic island basalts provide the opportunity for the geochemist to study the deep mantle source removed from continental sources of contamination and, for long-lived systems, the evolution of mantle sources with time. In the case of the Hawaiian-Emperor (HE) chain, formation by a long-lived (>81 Myr), deeply-sourced mantle plume allows for insight into plume dynamics and deep mantle geochemistry. The geochemical record of the entire chain is now complete with analysis of Pb-Hf-Nd-Sr isotopes and elemental compositions of the Northwest Hawaiian Ridge (NWHR), which consists of 51 volcanoes spanning 42 Ma between the bend in the chain and the Hawaiian Islands. This segment of the chain previously represented a significant data gap where Hawaiian plume geochemistry changed markedly, along with magmatic flux: only Kea compositions have been observed on Emperor seamounts (>50 Ma), whereas the Hawaiian Islands (<6 Ma) present both Kea and Loa compositions. A database of 700 Hawaiian Island shield basalts Pb-Hf-Nd-Sr isotopic compositions were compiled to construct a logistical regression model of Loa or Kea affinity that sorts data into a dichotomous category and provides insight into the relationship between independent variables. We use this model to predict whether newly analyzed NWHR samples are Loa or Kea composition based on their Pb-Sr-Nd-Hf isotopic compositions. The logistical regression model is significantly better at prediciting Loa or Kea affinity than the constant only model (χ2=263.3, df=4, p<0.0001), with Pb and Sr isotopes providing the most predicitive power. Daikakuji, West Nihoa, Nihoa, and Mokumanamana erupt Loa-type lavas, suggesting that the Loa source is sampled ephemerally during the NWHR and increases in presence and volume towards the younger section of the NWHR (younger than Midway 20-25 Ma). These results complete the picture of Hawaiian mantle plume geochemistry and geodynamics for 81 Myr, and show that the Hawaiian mantle plume has transitioned from a dominately Kea source during the Emperor seamounts and older NWHR to an increasingly enriched Loa source from the mid NWHR to Hawaiian Islands. We propose this is due to Hawaiian mantle plume drift through different lower mantle geohemical domains.

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.

Seismic evidence for a cold serpentinized mantle wedge beneath Mount St Helens

PubMed Central

Hansen, S. M.; Schmandt, B.; Levander, A.; Kiser, E.; Vidale, J. E.; Abers, G. A.; Creager, K. C.

2016-01-01

Mount St Helens is the most active volcano within the Cascade arc; however, its location is unusual because it lies 50 km west of the main axis of arc volcanism. Subduction zone thermal models indicate that the down-going slab is decoupled from the overriding mantle wedge beneath the forearc, resulting in a cold mantle wedge that is unlikely to generate melt. Consequently, the forearc location of Mount St Helens raises questions regarding the extent of the cold mantle wedge and the source region of melts that are responsible for volcanism. Here using, high-resolution active-source seismic data, we show that Mount St Helens sits atop a sharp lateral boundary in Moho reflectivity. Weak-to-absent PmP reflections to the west are attributed to serpentinite in the mantle-wedge, which requires a cold hydrated mantle wedge beneath Mount St Helens (<∼700 °C). These results suggest that the melt source region lies east towards Mount Adams. PMID:27802263

Chondritic xenon in the Earth’s mantle

NASA Astrophysics Data System (ADS)

Caracausi, Antonio; Avice, Guillaume; Burnard, Peter G.; Füri, Evelyn; Marty, Bernard

2016-05-01

Noble gas isotopes are powerful tracers of the origins of planetary volatiles, and the accretion and evolution of the Earth. The compositions of magmatic gases provide insights into the evolution of the Earth’s mantle and atmosphere. Despite recent analytical progress in the study of planetary materials and mantle-derived gases, the possible dual origin of the planetary gases in the mantle and the atmosphere remains unconstrained. Evidence relating to the relationship between the volatiles within our planet and the potential cosmochemical end-members is scarce. Here we show, using high-precision analysis of magmatic gas from the Eifel volcanic area (in Germany), that the light xenon isotopes identify a chondritic primordial component that differs from the precursor of atmospheric xenon. This is consistent with an asteroidal origin for the volatiles in the Earth’s mantle, and indicates that the volatiles in the atmosphere and mantle originated from distinct cosmochemical sources. Furthermore, our data are consistent with the origin of Eifel magmatism being a deep mantle plume. The corresponding mantle source has been isolated from the convective mantle since about 4.45 billion years ago, in agreement with models that predict the early isolation of mantle domains. Xenon isotope systematics support a clear distinction between mid-ocean-ridge and continental or oceanic plume sources, with chemical heterogeneities dating back to the Earth’s accretion. The deep reservoir now sampled by the Eifel gas had a lower volatile/refractory (iodine/plutonium) composition than the shallower mantle sampled by mid-ocean-ridge volcanism, highlighting the increasing contribution of volatile-rich material during the first tens of millions of years of terrestrial accretion.

Chondritic xenon in the Earth's mantle.

PubMed

Caracausi, Antonio; Avice, Guillaume; Burnard, Peter G; Füri, Evelyn; Marty, Bernard

2016-05-05

Noble gas isotopes are powerful tracers of the origins of planetary volatiles, and the accretion and evolution of the Earth. The compositions of magmatic gases provide insights into the evolution of the Earth's mantle and atmosphere. Despite recent analytical progress in the study of planetary materials and mantle-derived gases, the possible dual origin of the planetary gases in the mantle and the atmosphere remains unconstrained. Evidence relating to the relationship between the volatiles within our planet and the potential cosmochemical end-members is scarce. Here we show, using high-precision analysis of magmatic gas from the Eifel volcanic area (in Germany), that the light xenon isotopes identify a chondritic primordial component that differs from the precursor of atmospheric xenon. This is consistent with an asteroidal origin for the volatiles in the Earth's mantle, and indicates that the volatiles in the atmosphere and mantle originated from distinct cosmochemical sources. Furthermore, our data are consistent with the origin of Eifel magmatism being a deep mantle plume. The corresponding mantle source has been isolated from the convective mantle since about 4.45 billion years ago, in agreement with models that predict the early isolation of mantle domains. Xenon isotope systematics support a clear distinction between mid-ocean-ridge and continental or oceanic plume sources, with chemical heterogeneities dating back to the Earth's accretion. The deep reservoir now sampled by the Eifel gas had a lower volatile/refractory (iodine/plutonium) composition than the shallower mantle sampled by mid-ocean-ridge volcanism, highlighting the increasing contribution of volatile-rich material during the first tens of millions of years of terrestrial accretion.

Origin and evolution of primitive melts from the Debunscha Maar, Cameroon: Consequences for mantle source heterogeneity within the Cameroon Volcanic Line

NASA Astrophysics Data System (ADS)

Ngwa, Caroline N.; Hansteen, Thor H.; Devey, Colin W.; van der Zwan, Froukje M.; Suh, Cheo E.

2017-09-01

Debunscha Maar is a monogenetic volcano forming part of the Mt. Cameroon volcanic field, located within the Cameroon Volcanic Line (CVL). Partly glassy cauliflower bombs have primitive basanite-picrobasalt compositions and contain abundant normally and reversely zoned olivine (Fo 77-87) and clinopyroxene phenocrysts. Naturally quenched melt inclusions in the most primitive olivine phenocrysts show compositions which, when corrected for post-entrapment modification, cover a wide range from basanite to alkali basalt (MgO 6.9-11.7 wt%), and are generally more primitive than the matrix glasses (MgO 5.0-5.5 wt%) and only partly fall on a common liquid line of descent with the bulk rock samples and matrix glasses. Melt inclusion trace element compositions lie on two distinct geochemical trends: one (towards high Ba/Nb) is thought to represent the effect of various proportions of anhydrous lherzolite and amphibole-bearing peridotite in the source, while the other (for example, high La/Y) reflects variable degrees of partial melting. Comparatively low fractionation-corrected CaO in the melt inclusions with the highest La/Y suggests minor involvement of a pyroxenite source component that is only visible at low degrees of melting. Most of the samples show elevated Gd/Yb, indicating up to 8% garnet in the source. The range of major and trace elements represented by the melt inclusions covers the complete geochemical range given by basalts from different volcanoes of the Cameroon volcanic line, indicating that geochemical signatures that were previously thought to be volcano-specific in fact are probably present under all volcanoes. Clinopyroxene-melt barometry strongly indicates repeated mixing of compositionally diverse melts within the upper mantle at 830 ± 170 MPa prior to eruption. Mantle potential temperatures estimated for the primitive melt inclusions suggest that the thermal influence of a mantle plume is not required to explain the magma petrogenesis.

Mantle heterogeneity in the source region of mid-ocean ridge basalts along the northern Central Indian Ridge (8°S-17°S)

NASA Astrophysics Data System (ADS)

Kim, Jonguk; Pak, Sang-Joon; Moon, Jai-Woon; Lee, Sang-Mook; Oh, Jihye; Stuart, Finlay M.

2017-04-01

The northern Central Indian Ridge (CIR) between 8°S and 17°S is composed of seven segments whose spreading rates increase southward from ˜35 to ˜40 mm/yr. During expeditions of R/V Onnuri to study hydrothermal activity on the northern CIR in 2009-2011, high-resolution multibeam mapping was conducted and ridge axis basalts were dredged. The major and trace element and Sr-Nd-Pb-He isotopic compositions of basaltic glasses dredged from the spreading axis require three mantle sources: depleted mantle and two distinct enriched mantle sources. The southern segments have Sr, Nd, and Pb that are a mix of depleted mantle and an enriched component as recorded in southern CIR MORB. This enrichment is indistinguishable from Rèunion plume mantle, except for He isotopes. This suggests that the southern segments have incorporated a contribution of the fossil Rèunion plume mantle, as the CIR migrated over hot-spot-modified mantle. The low 3He/4He (7.5-9.2 RA) of this enriched component may result from radiogenic 4He ingrowth in the fossil Rèunion mantle component. Basalts from the northern segments have high 206Pb/204Pb (18.53-19.15) and low 87Sr/86Sr (0.70286-0.70296) that are distinct from the Rèunion plume but consistent with derivation from mantle with FOZO signature, albeit with 3He/4He (9.2-11.8 RA) that are higher than typical. The FOZO-like enriched mantle cannot be attributed to the track of a nearby mantle plume. Instead, this enrichment may have resulted from recycling oceanic crust, possibly accompanied by small plume activity.

Mantle heterogeneity and crustal recycling in Archean granite-greenstone belts - Evidence from Nd isotopes and trace elements in the Rainy Lake area, Superior Province, Ontario, Canada

NASA Technical Reports Server (NTRS)

Shirey, Steven B.; Hanson, Gilbert N.

1986-01-01

Crustal evolution in the Rainy Lake area, Ontario is studied in terms of geochemical characteristics. The Nd isotope data are examined for heterogeneity of the Archean mantle, and the Sm/Nd depletion of the mantle is analyzed. The Nd isotope systematics of individual rock suites is investigated in order to understand the difference between crust and mantle sources; the precursors and petrogenetic processes are discussed. The correlation between SiO2 content and Nd values is considered. Rapid recycling of crustal components, which were previously derived from depleted mantle sources, is suggested based on the similarity of the initial Nd isotopic composition for both mantle-derived and crustally-derived rocks.

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

NASA Astrophysics Data System (ADS)

Frei, Robert; Polat, Ali; Meibom, Anders

2004-04-01

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

Sources and mobility of carbonate melts beneath cratons, with implications for deep carbon cycling, metasomatism and rift initiation

NASA Astrophysics Data System (ADS)

Tappe, Sebastian; Romer, Rolf L.; Stracke, Andreas; Steenfelt, Agnete; Smart, Katie A.; Muehlenbachs, Karlis; Torsvik, Trond H.

2017-05-01

Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their co-existence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr-Nd-Hf-Pb-Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative ΔεHf coupled with > + 5 ‰ δ7Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05-3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (-3.6 to - 2.4 ‰ δ13C for carbonatites versus -5.7 to - 3.6 ‰ δ13C for kimberlites) require open-system fractionation at magmatic temperatures. Given that the oxidation state of Earth's mantle at >150 km depth is too reduced to enable larger volumes of 'pure' carbonate melt to migrate, it is reasonable to speculate that percolating near-solidus melts of carbonated peridotite must be silicate-dominated with only dilute carbonate contents, similar to the Tikiusaaq kimberlite compositions (e.g., 16-33 wt.% SiO2). This concept is supported by our findings from the North Atlantic craton where kimberlite and other deeply derived carbonated silicate melts, such as aillikites, exsolve their carbonate components within the shallow lithosphere en route to the Earth's surface, thereby producing carbonatite magmas. The relative abundances of trace elements of such highly differentiated 'cratonic carbonatites' have only little in common with those of metasomatic agents that act on the deeper lithosphere. Consequently, carbonatite trace element systematics should only be used with caution when constraining carbon mobility and metasomatism at mantle depths. Regardless of the exact nature of carbonate-bearing melts within the mantle lithosphere, they play an important role in enrichment processes, thereby decreasing the stability of buoyant cratons and promoting rift initiation - as exemplified by the Mesozoic-Cenozoic breakup of the North Atlantic craton.

Origin of geochemical mantle components: Role of spreading ridges and thermal evolution of mantle

NASA Astrophysics Data System (ADS)

Kimura, Jun-Ichi; Gill, James B.; van Keken, Peter E.; Kawabata, Hiroshi; Skora, Susanne

2017-02-01

We explore the element redistribution at mid-ocean ridges (MOR) using a numerical model to evaluate the role of decompression melting of the mantle in Earth's geochemical cycle, with focus on the formation of the depleted mantle component. Our model uses a trace element mass balance based on an internally consistent thermodynamic-petrologic computation to explain the composition of MOR basalt (MORB) and residual peridotite. Model results for MORB-like basalts from 3.5 to 0 Ga indicate a high mantle potential temperature (Tp) of 1650-1500°C during 3.5-1.5 Ga before decreasing gradually to ˜1300°C today. The source mantle composition changed from primitive (PM) to depleted as Tp decreased, but this source mantle is variable with an early depleted reservoir (EDR) mantle periodically present. We examine a two-stage Sr-Nd-Hf-Pb isotopic evolution of mantle residues from melting of PM or EDR at MORs. At high-Tp (3.5-1.5 Ga), the MOR process formed extremely depleted DMM. This coincided with formation of the majority of the continental crust, the subcontinental lithospheric mantle, and the enriched mantle components formed at subduction zones and now found in OIB. During cooler mantle conditions (1.5-0 Ga), the MOR process formed most of the modern ocean basin DMM. Changes in the mode of mantle convection from vigorous deep mantle recharge before ˜1.5 Ga to less vigorous afterward is suggested to explain the thermochemical mantle evolution.

Lead Isotopes in Olivine-Phyric Shergottite Tissint: Implications for the Geochemical Evolution of the Shergottite Source Mantle

NASA Technical Reports Server (NTRS)

Moriwaki, R.; Usui, T.; Simon, J. I.; Jones, J. H.; Yokoyama, T.

2015-01-01

Geochemically-depleted shergottites are basaltic rocks derived from a martian mantle source reservoir. Geochemical evolution of the martian mantle has been investigated mainly based on the Rb-Sr, Sm-Nd, and Lu-Hf isotope systematics of the shergottites [1]. Although potentially informative, U-Th- Pb isotope systematics have been limited because of difficulties in interpreting the analyses of depleted meteorite samples that are more susceptible to the effects of near-surface processes and terrestrial contamination. This study conducts a 5-step sequential acid leaching experiment of the first witnessed fall of the geochemically-depleted olivinephyric shergottite Tissint to minimize the effect of low temperature distrubence. Trace element analyses of the Tissint acid residue (mostly pyroxene) indicate that Pb isotope compositions of the residue do not contain either a martian surface or terrestrial component, but represent the Tissint magma source [2]. The residue has relatively unradiogenic initial Pb isotopic compositions (e.g., 206Pb/204Pb = 10.8136) that fall within the Pb isotope space of other geochemically-depleted shergottites. An initial µ-value (238U/204Pb = 1.5) of Tissint at the time of crystallization (472 Ma [3]) is similar to a time-integrated mu- value (1.72 at 472 Ma) of the Tissint source mantle calculated based on the two-stage mantle evolution model [1]. On the other hand, the other geochemically-depleted shergottites (e.g., QUE 94201 [4]) have initial µ-values of their parental magmas distinctly lower than those of their modeled source mantle. These results suggest that only Tissint potentially reflects the geochemical signature of the shergottite mantle source that originated from cumulates of the martian magma ocean

Calcio-carbonatite melts and metasomatism in the mantle beneath Mt. Vulture (Southern Italy)

NASA Astrophysics Data System (ADS)

Rosatelli, Gianluigi; Wall, Frances; Stoppa, Francesco

2007-12-01

At Mt. Vulture volcano (Basilicata, Italy) calcite globules (5-150 μm) are hosted by silicate glass pools or veins cross-cutting amphibole-bearing, or more common spinel-bearing mantle xenoliths and xenocrysts. The carbonate globules are rounded or elongated and are composed of a mosaic of 2-20 μm crystals, with varying optical orientation. These features are consistent with formation from a quenched calciocarbonatite melt. Where in contact with carbonate amphibole has reacted to form fassaitic pyroxene. Some of these globules contain liquid/gaseous CO 2 bubbles and sulphide inclusions, and are pierced by quench microphenocrysts of silicate phases. The carbonate composition varies from calcite to Mg-calcite (3.8-5.0 wt.% MgO) both within the carbonate globules and from globule to globule. Trace element contents of the carbonate, determined by LAICPMS, are similar to those of carbonatites worldwide including ΣREE up to 123 ppm. The Sr-Nd isotope ratios of the xenolith carbonate are similar to the extrusive carbonatite and silicate rocks of Mt. Vulture testifying to derivation from the same mantle source. Formation of immiscibile silicate-carbonatite liquids within mantle xenoliths occurred via disequilibrium immiscibility during their exhumation.

Formation and evolution of a metasomatized lithospheric root at the motionless Antarctic plate: the case of East Island, Crozet Archipelago (Indian Ocean)

NASA Astrophysics Data System (ADS)

Meyzen, Christine; Marzoli, Andrea; Bellieni, Giuliano; Levresse, Gilles

2016-04-01

Sitting atop the nearly stagnant Antarctic plate (ca. 6.46 mm/yr), the Crozet archipelago midway between Madagascar and Antarctica constitutes a region of unusually shallow (1543-1756 m below sea level) and thickened oceanic crust (10-16.5 km), high geoid height, and deep low-velocity zone, which may reflect the surface expression of a mantle plume. Here, we present new major and trace element data for Quaternary sub-aerial alkali basalts from East Island, the easterly and oldest island (ca. 9 Ma) of the Crozet archipelago. Crystallization at uppermost mantle depth and phenocryst accumulation have strongly affected their parental magma compositions. Their trace element patterns show a large negative K anomaly relative to Ta-La, moderate depletions in Rb and Ba with respect to Th-U, and heavy rare earth element (HREE) depletions relative to light REE. These characteristics allow limits to be placed upon the composition and mineralogy of their mantle source. The average trace element spectrum of East Island basalts can be matched by melting of about 2 % of a garnet-phlogopite-bearing peridotite source. The stability field of phlogopite restricts melting depth to lithospheric levels. The modelled source composition requires a multistage evolution, where the mantle has been depleted by melt extraction before having been metasomatized by alkali-rich plume melts. The depleted mantle component may be sourced by residual mantle plume remnants stagnated at the melting locus due to a weak lateral flow velocity inside the melting regime, whose accumulation progressively edifies a depleted lithospheric root above the plume core. Low-degree alkali-rich melts are likely derived from the plume source. Such a mantle source evolution may be general to both terrestrial and extraterrestrial environments where the lateral component velocity of the mantle flow field is extremely slow.

Petrological characterization of the seismic low-velocity anomaly beneath the Eifel volcanic field (West Germany) using major and trace element compositions of olivine macrocrysts

NASA Astrophysics Data System (ADS)

Dejan, Prelevic; Dieter, Mertz; Regina, Mertz-Kraus; Stephan, Buhre

2014-05-01

The Eifel volcanic field is part of the Central European Cenozoic Magmatic Province and was periodically active from the mid-Cretaceous until the latest Pleistocene. Two contrasting models are used to explain sources and magma generation mechanisms of the Pleistocene Eifel volcanism: i) decompressional partial melting at the base of the subcontinental lithosphere as a consequence of extension caused by lithospheric flexuring from emplacement of Alpine nappes (Wilson & Downes, 1991); ii) plume-type thermal upwelling in the asthenosphere on the basis of seismic tomography indicating a low-velocity anomaly beneath the Eifel probably caused by temperatures higher than the normal asthenosphere adiabat (e.g., Ritter et al. 2001). We present high-precision electron microprobe data for major and minor elements as well as laser ablation ICP-MS data for trace elements of olivine from the Eifel in order to put new constraints on the origin of Pleistocene Eifel volcanism. Being an early liquidus phase in the crystallization of basaltic melts, olivine composition may be used to characterize the composition of primary mantle melts and their source region in terms of major and trace elements. Moreover, it is useful for T estimation providing a snapshot of the liquid equilibria at early magmatic stage. In addition, important petrological parameters can be constrained, like the extent of prior melt extraction of their mantle source, the presence of different geochemical components in the source, olivine residence times etc. Olivine macrocrysts occur in most of the Eifel Mg-rich lavas, forming up to 10 vol% of the rocks. We studied olivines from 10 representative lava flows of basanitic composition. Based on compositional and textural differences, three genetic groups are recognized: i) volumetrically dominant igneous olivines or phenocrysts (melt related); they are equilibrated with their host melt showing normal zonation (core-rim Fo89-80) and NiO contents up to 0.3 wt%, whereas Cr2O3 and CaO are around 0.18 wt% and 0.20 wt%, respectively; ii) mantle xenocrysts are typically mantled by olivine of phenocrystal composition, with the plateau-like core compositions typically with Fo91.5 and NiO contents around 0.4 wt%; a number of features supports their mantle origin, namely CaO contents lower than 0.1 wt%, homogeneous compositions within the grain (typical for mantle olivine, resulting from long equilibration times), anhedral shapes showing deformation features such as kink bands etc; iii) a genetic group also demonstrating xenocrystic features (e.g., compositional disequilibration with the host melt, the mantling by olivine of phenocrystal composition); however, it differs from the mantle olivine by having higher CaO (> 0.3 wt%), slightly lower Mg# (up to 90), and considerably lower NiO contents (< 0.1 wt%); we interpret these grains to originate from wherlitic assemblages within the lithospheric mantle. Our preliminary estimation of the olivine-liquid equilibria using compositions of the phenocrysts indicates temperatures not considerably higher than 1300 oC. The trace element composition of olivine phenocrysts and two types of xenocrysts show several important characteristics. Relative to mantle xenocrystal olivine that is depleted in the most trace elements, phenocrysts are considerably enriched in Li and Zn, and depleted in Ti. Low NiO xenocrysts have high Ti with slightly elevated Li concentration. There is a certain overlap between the phenocrysts from Eifel lavas and those from orogenic Mediterranean volcanics, indicating compositional similarities in their mantle sources that may imply the presence of common metasomatizing agent(s). Wilson, M. & Downes, H. (1991). Journal of Petrology 32, 811-849. Ritter, J. R. R., Jordan, M., Christensen, U. R. & Achauer, U. (2001). Earth and Planetary Science Letters 186, 7-14.

The planet beyond the plume hypothesis

NASA Astrophysics Data System (ADS)

Smith, Alan D.; Lewis, Charles

1999-12-01

Acceptance of the theory of plate tectonics was accompanied by the rise of the mantle plume/hotspot concept which has come to dominate geodynamics from its use both as an explanation for the origin of intraplate volcanism and as a reference frame for plate motions. However, even with a large degree of flexibility permitted in plume composition, temperature, size, and depth of origin, adoption of any limited number of hotspots means the plume model cannot account for all occurrences of the type of volcanism it was devised to explain. While scientific protocol would normally demand that an alternative explanation be sought, there have been few challenges to "plume theory" on account of a series of intricate controls set up by the plume model which makes plumes seem to be an essential feature of the Earth. The hotspot frame acts not only as a reference but also controls plate tectonics. Accommodating plumes relegates mantle convection to a weak, sluggish effect such that basal drag appears as a minor, resisting force, with plates having to move themselves by boundary forces and continents having to be rifted by plumes. Correspondingly, the geochemical evolution of the mantle is controlled by the requirement to isolate subducted crust into plume sources which limits potential buffers on the composition of the MORB-source to plume- or lower mantle material. Crustal growth and Precambrian tectonics are controlled by interpretations of greenstone belts as oceanic plateaus generated by plumes. Challenges to any aspect of the plume model are thus liable to be dismissed unless a counter explanation is offered across the geodynamic spectrum influenced by "plume theory". Nonetheless, an alternative synthesis can be made based on longstanding petrological evidence for derivation of intraplate volcanism from volatile-bearing sources (wetspots) in conjunction with concepts dismissed for being incompatible or superfluous to "plume theory". In the alternative Earth, the sources for intraplate volcanism evolve from the source residues of arc volcanism located along sutures in the continental mantle. Continental rifting and the lateral distribution of intraplate sources in the asthenosphere are controlled by Earth rotation. Shear induced on the base of the asthenosphere from the mesosphere as the Earth rotates is transmitted to the lithosphere as basal drag. Attenuation of the drag due to the low viscosity of the asthenosphere, in conjunction with plate motions from boundary forces, results in a rotation differential of up to 5 cm yr -1 between the lithosphere and mesosphere manifest as westward plate lag/eastward mantle flow. Continental rifting results from basal drag supplemented by local convection induced by lithospheric architecture. Large continental igneous provinces are generated by convective melting, with passive margin volcanic sequences following the axis of rifting and flood basalts overlying the intersection of sutures in the continental mantle. As rifting progresses, the convection cells expand, cycling continental mantle from sutures perpendicular to the rift axis to generate intraplate tracks in the ocean basin. Continental mantle not melted on rifting, or delaminated on continental collision, becomes displaced to the east of the continent by differential rotation, which also sets up a means for tapping the material to give fixed melting anomalies. When plates move counter to the Earth's rotation, as in the example of the Pacific plate, asthenospheric flow is characterised by a counterflow regime with a zero velocity layer at depths within the stability field for volatile-bearing minerals. Intraplate volcanism results when melts are tapped from this stationary layer along lithospheric stress trajectories induced by stressing of the plate from variations in the subduction geometry around the margins of the plate. Plate boundary forces acting in the same direction as Earth rotation, as for the Nazca plate, produce fast plate velocities but not counterflow, though convergent margin geometry may still induce propagating fractures which set up melting anomalies. Lateral migration of asthenospheric domains allows the sources of Pacific intraplate volcanism to be traced back to continental mantle eroded during the breakup of Gondwana and the amalgamation of Asia in the Paleozoic. Intraplate volcanism in the South Pacific therefore has a common Gondwanan origin to intraplate volcanism in the South Atlantic and Indian Oceans, hence the DUPAL anomaly is entirely of shallow origin. Such domains constitute a second order geochemical heterogeneity superimposed on a streaky/marble-cake structure arising from remixing of subducted crust with the convecting mantle. During the Proterozoic and Phanerozoic, remixing of slabs has buffered the evolution of the depleted mantle to a rate of 2.2 ɛNd units Ga -1, with fractionation of Lu from Hf in the sediment component imparting the large range in 176Hf/ 177Hf relative to 143Nd/ 144Nd observed in MORB. Only the high ɛNd values of some Archean komatiites are compatible with derivation from unbuffered mantle. The existence of a very depleted reservoir is attributed to stabilisation of a large early continental crust through either obduction tectonics or slab melting regimes which reduced the efficiency of crustal recycling back into the mantle. Generation of komatiite is therefore a consequence of mantle composition, and is permitted in ocean ridge environments and/or under hydrous melting conditions. Correspondingly, as intraplate volcanism depends on survival of volatile-bearing sources, its appearance in the Middle Proterozoic corresponds to the time in the Earth's thermal evolution at which minerals such as phlogopite and amphibole could survive in off-ridge environments in the shallow asthenosphere. The geodynamic evolution of the Earth was thus determined at convergent margins, not by plumes and hotspots, with the decline in thermal regime causing both a reduction in size of crust and continental mantle roots, the latter becoming a source for intraplate volcanism while the crust was incorporated into the convecting mantle.

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

NASA Astrophysics Data System (ADS)

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

2016-03-01

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

Zircon Hf-O isotopic constraints on the origin of Late Mesozoic felsic volcanic rocks from the Great Xing'an Range, NE China

NASA Astrophysics Data System (ADS)

Gong, Mingyue; Tian, Wei; Fu, Bin; Wang, Shuangyue; Dong, Jinlong

2018-05-01

The voluminous Late Mesozoic magmatism was related to extensive re-melting of juvenile materials that were added to the Central East Asia continent in Phanerozoic time. The most favoured magma generation mechanism of Late Mesozoic magmas is partial melting of underplated lower crust that had radiogenic Hf-Nd isotopic characteristics, but this mechanism faces difficulties when interpreting other isotopic data. The tectonic environment controlling the generation of the Late Mesozoic felsic magmas is also in dispute. In this study, we obtained new U-Pb ages, and geochemical and isotopic data of representative Jurassic (154.4 ± 1.5 Ma) and Cretaceous (140.2 ± 1.5 Ma) felsic volcanic samples. The Jurassic sample has inherited zircon cores of Permian age, with depleted mantle-like εHf(t) of +7.4 - +8.5, which is in contrast with those of the magmatic zircons (εHf(t) = +2.4 ± 0.7). Whereas the inherited cores and the magmatic zircons have identical mantle-like δ18O composition ranges (4.25-5.29‰ and 4.69-5.54‰, respectively). These Hf-O isotopic characteristics suggest a mixed source of enriched mantle materials rather than ancient crustal components and a depleted mantle source represented by the inherited Permian zircon core. This mechanism is manifested by the eruption of Jurassic alkaline basalts originated from an enriched mantle source. The Cretaceous sample has high εHf(t) of +7.0 - +10.5, suggesting re-melting of a mafic magma derived from a depleted mantle-source. However, the sub-mantle zircon δ18O values (3.70-4.58‰) suggest the depleted mantle-derived mafic source rocks had experienced high temperature hydrothermal alteration at upper crustal level. Therefore, the Cretaceous felsic magma, if not all, could be generated by re-melting of down-dropped supracrustal volcanic rocks that experienced high temperature oxygen isotope alteration. The two processes, enriched mantle-contribution and supracrustal juvenile material re-melting, are new generation mechanisms of the Late Mesozoic magmas from Central East Asia. Rift settings may have controlled these processes throughout crustal and mantle levels.

Trace element and Sr-Nd-Pb isotope geochemistry of Rungwe Volcanic Province, Tanzania: Implications for a superplume source for East Africa Rift magmatism

NASA Astrophysics Data System (ADS)

Castillo, Paterno; Hilton, David; Halldórsson, Sæmundur

2014-09-01

The recently discovered high, plume-like 3He/4He ratios at Rungwe Volcanic Province (RVP) in southern Tanzania, similar to those at the Main Ethiopian Rift in Ethiopia, strongly suggest that magmatism associated with continental rifting along the entire East African Rift System (EARS) has a deep mantle contribution (Hilton et al., 2011). New trace element and Sr-Nd-Pb isotopic data for high 3He/4He lavas and tephras from RVP can be explained by binary mixing relationships involving Early Proterozoic (+/- Archaean) lithospheric mantle, present beneath the southern EARS, and a volatile-rich carbonatitic plume with a limited range of compositions and best represented by recent Nyiragongo lavas from the Virunga Volcanic Province also in the Western Rift. Other lavas from the Western Rift and from the southern Kenya Rift can also be explained through mixing between the same endmember components. In contrast, lavas from the northern Kenya and Main Ethiopian rifts can be explained through variable mixing between the same mantle plume material and the Middle to Late Proterozoic lithospheric mantle, present beneath the northern EARS. Thus, we propose that the bulk of EARS magmatism is sourced from mixing among three endmember sources: Early Proterozoic (+/- Archaean) lithospheric mantle, Middle to Late Proterozoic lithospheric mantle and a volatile-rich carbonatitic plume with a limited range of compositions. We propose further that the African Superplume, a large, seismically anomalous feature originating in the lower mantle beneath southern Africa, influences magmatism throughout eastern Africa with magmatism at RVP and Main Ethiopian Rift representing two different heads of a single mantle plume source. This is consistent with a single mantle plume origin of the coupled He-Ne isotopic signatures of mantle-derived xenoliths and/or lavas from all segments of the EARS (Halldorsson et al., 2014).

Geochemical and petrological evidence of the subduction of delaminated Adriatic continental lithosphere in the genesis of the Neogene-Quaternary magmatism of central Italy

NASA Astrophysics Data System (ADS)

Serri, G.; Innocenti, F.; Manetti, P.

1993-07-01

Serri, G., Innocenti, F. and Manetti, P., 1993. Geochemical and petrological evidence of the subduction of delaminated Adriatic continental lithosphere in the genesis of the Neogene-Quaternary magmatism of central Italy. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near The Earth's Surface. Tectonophysics, 223: 117-147. The Neogene-Quaternary magmatism of the northern Apenninic arc took place in four phases separated in space and time which become progressively younger from west to east: Phase I, 14 Ma; Phase II, 7.3-6.0 Ma; Phase III, 5.1-2.2 Ma; Phase IV, 1.3-0.1 Ma. This magmatism is the result of the activation of three physically separate sources: (1) the Adriatic continental crust, extracted from the mantle in the late Proterozoic; (2) a strongly refractory, recently K-enriched harzburgitic mantle located in the mechanical boundary layer (MBL) of the lithosphere; and (3) a recently metasomatized, cpx-rich mantle, compositionally variable from Iherzolite to wehrlite-clinopyroxenite, interpreted as an ephemerally K-enriched asthenosphere. The Adriatic continental crust is the dominant source of the acid plutonic and volcanic rocks of the Tuscan region. The acid magmatism is mostly found inside an ellipsoidal area (about 150 × 300 km) centred on Giglio Island, here defined as the Tuscan Crustal Dome. Within this area, mantle-derived magmas unaffected by important crustal contamination processes and mixing with crustal anatectic melts have so far not been found. Pure crustal magmas are rare but are represented, for example by some of the San Vincenzo and Roccastrada rhyolites. Virtually all the Tuscan acid centres show evidence of mixing with potassic mantle-derived magmas. Major and trace elements, as well as {87Sr }/{86Sr } and {143Nd }/{144Nd } data, on primitive rocks (Mg# > 65) reveal two groups of mantle-derived magmas. These define two distinct mantle enrichment trends, both essentially due to the additions of K-rich components which metasomatized separate, compositionally diverse, upper mantle sectors. In both cases the most remarkable mineralogical effect of these enrichment processes is the production of variable amount of phlogopite through reaction between fluids and/or melts with the mantle. The rocks of group I (ol-hy and Q-normative, lamproites, ultrapotassic high-Mg latites, ultrapotassic shoshonites and shoshonites: saturated trend) are considered to be derived by partial melting at low pressure (< 50 km) of strongly (lamproites) to moderately depleted phlogopite harzburgitic sources produced by reaction of residual peridotites with a K-Si-rich, Ca-Sr-poor melt with high ratios of {87Sr }/{86Sr (> 0.717) }, Ce/Sr (> 0.3) and {K 2O }/{Na 2O (> 6-7) }, and low ratios of {143Nd }/{144Nd ( 0.5121-0.5120) } and Ba/La (< 20) ratios; it is proposed that this component was formed by partial melting of subducted carbonate-free material of the upper crustal reservoir (e.g., non-restitic felsic granulites). This material is very common in the central Mediterranean region either as granitoid plutons/terrigenous sediments or as metasedimentary, non-restitic lower crust. The primitive rocks of group II are critically undersaturated, mostly leucitites, tephritic leucitites, leucite basanites, melilitites (undersaturated trend). Experimental petrology suggests that these rocks were formed by partial melting of a variably enriched phlogopite, clinopyroxene-rich mantle at higher pressure than group I primitive magmas. Trace-element modelling indicates that three components were involved in the genesis of group II mantle source: (a) a typical MORB-OIB-like mantle; (b) a component with very high Sr, Ca and Sr/Ce values and very low silica and sodium content, probably carried by a carbonatite melt somehow related to subducted marine carbonates; and (c) a recently added K-rich, Ca-Sr-poor crustal component, relatively well constrained to high {87Sr }/{86Sr (> 0.712) } and {K 2O }/{Na 2O (> 8-9) } values, and low {143Nd }/{144Nd (< 0.51205) }, Ba/La (< 20) and Ce/Sr (> 0.10) ratios. These constraints do not allow to exclude a complete identity between the K-rich components which metasomatized the mantle sources of the saturated and undersaturated trend magmas. The geochemical and isotopic features of the components that metasomatized the mantle sources of the northern Apenninic arc magmatism can be explained by a geodynamic process which causes a large amount of crustal materials to be incorporated within the upper mantle. We propose that the delamination and subduction of the Adriatic continental lithosphere related to the still ongoing northern Apennine continental collision provide a viable mechanism to explain the genesis and eastward discontinuous migration of the magmatism in central Italy. The subduction of delaminated lithospheric mantle with lower crustal slivers would have exposed uppermost mantle (Adriatic MBL) and crustal units previously imbricated in the Apennine chain to the heating advected by the upwelling of a recently and ephemerally K-enriched asthenospheric mantle wedge and by the underplating of magmas derived from it. We consider that the diapiric uprising of a hot, crustally contaminated asthenosphere occurs in the wake left above the sinking of the Adriatic delaminated/subducting continental lithosphere. The delamination/subduction process of the Adriatic lithosphere has probably started in the Early-Middle Miocene, but earlier than 15-14 Ma ago, as indicated by the age and petrologic characteristics of the first magmatic episode (Sisco lamproite) of the northern Apennine orogenesis.

Growth of continental crust: Clues from Nd isotopes and Nb-Th relationships in mantle-derived magmas

NASA Astrophysics Data System (ADS)

Arndt, N. T.; Chauvel, C.; Jochum, K.-P.; Gruau, G.; Hofmann, A. W.

Isotope and trace element geochemistry of Precambrian mantle derived rocks and implications for the formation of the continental crust is discussed. Epsilon Nd values of Archean komatiites are variable, but range up to at least +5, suggesting that the Archean mantle was heterogeneous and, in part, very depleted as far back as 3.4 to 3.5 Ga. This may be taken as evidence for separation of continental crust very early in Earth history. If these komatiite sources were allowed to evolve in a closed system, they would produce modern day reservoirs with much higher epsilon Nd values than is observed. This implies recycling of some sort of enriched material, perhaps subducted sediments, although other possibilities exist. Archean volcanics show lower Nb/Th than modern volcanics, suggesting a more primitive mantle source than that observed nowadays. However, Cretaceous komatiites from Gorgona island have similar Nb/Th to Archean volcanics, indicating either the Archean mantle source was indeed more primitive, or Archean magmas were derived from a deep ocean island source like that proposed for Gorgona.

Growth of continental crust: Clues from Nd isotopes and Nb-Th relationships in mantle-derived magmas

NASA Technical Reports Server (NTRS)

Arndt, N. T.; Chauvel, C.; Jochum, K.-P.; Gruau, G.; Hofmann, A. W.

1988-01-01

Isotope and trace element geochemistry of Precambrian mantle derived rocks and implications for the formation of the continental crust is discussed. Epsilon Nd values of Archean komatiites are variable, but range up to at least +5, suggesting that the Archean mantle was heterogeneous and, in part, very depleted as far back as 3.4 to 3.5 Ga. This may be taken as evidence for separation of continental crust very early in Earth history. If these komatiite sources were allowed to evolve in a closed system, they would produce modern day reservoirs with much higher epsilon Nd values than is observed. This implies recycling of some sort of enriched material, perhaps subducted sediments, although other possibilities exist. Archean volcanics show lower Nb/Th than modern volcanics, suggesting a more primitive mantle source than that observed nowadays. However, Cretaceous komatiites from Gorgona island have similar Nb/Th to Archean volcanics, indicating either the Archean mantle source was indeed more primitive, or Archean magmas were derived from a deep ocean island source like that proposed for Gorgona.

Evidence for a Heterogeneous Distribution of Water in the Martian Interior

NASA Technical Reports Server (NTRS)

McCubbin, Francis; Boyce, Jeremy W.; Srinvasan, Poorna; Santos, Alison R.; Elardo, Stephen M.; Filiberto, Justin; Steele, Andrew; Shearer, Charles K.

2016-01-01

The abundance and distribution of H2O within the terrestrial planets, as well as its timing of delivery, is a topic of vital importance for understanding the chemical and physical evolution of planets and their potential for hosting habitable environments. Analysis of planetary materials from Mars, the Moon, and the eucrite parent body (i.e., asteroid 4Vesta) have confirmed the presence of H2O within their interiors. Moreover, H and N isotopic data from these planetary materials suggests H2O was delivered to the inner solar system very early from a common source, similar in composition to the carbonaceous chondrites. Despite the ubiquity of H2O in the inner Solar System, the only destination with any prospects for past or present habitable environments at this time, outside of the Earth, is Mars. Although the presence of H2O within the martian interior has been confirmed, very little is known regarding its abundance and distribution within the martian interior and how the martian water inventory has changed over time. By combining new analyses of martian apatites within a large number of martian meteorite types with previously published volatile data and recently determined mineral-melt partition coefficients for apatite, we report new insights into the abundance and distribution of volatiles in the martian crust and mantle. Using the subset of samples that did not exhibit crustal contamination, we determined that the enriched shergottite mantle source has 36-73 ppm H2O and the depleted shergottite mantle source has 14-23 ppm H2O. This result is consistent with other observed geochemical differences between enriched and depleted shergottites and supports the idea that there are at least two geochemically distinct reservoirs in the martian mantle. We also estimated the H2O content of the martian crust using the revised mantle H2O abundances and known crust-mantle distributions of incompatible lithophile elements. We determined that the bulk martian crust has approximately 1400 ppm H2O, which is likely distributed toward the martian surface. This crustal water abundance would equate to a global equivalent layer (GEL) of water at a depth of-229 m, which can account for at least some of the surface features on Mars attributed to flowing water and may be sufficient to support the past presence of a shallow sea on Mars' surface.

Source Distributions of Substorm Ions Observed in the Near-Earth Magnetotail

NASA Technical Reports Server (NTRS)

Ashour-Abdalla, M.; El-Alaoui, M.; Peroomian, V.; Walker, R. J.; Raeder, J.; Frank, L. A.; Paterson, W. R.

1999-01-01

This study employs Geotail plasma observations and numerical modeling to determine sources of the ions observed in the near-Earth magnetotail near midnight during a substorm. The growth phase has the low-latitude boundary layer as its most important source of ions at Geotail, but during the expansion phase the plasma mantle is dominant. The mantle distribution shows evidence of two distinct entry mechanisms: entry through a high latitude reconnection region resulting in an accelerated component, and entry through open field lines traditionally identified with the mantle source. The two entry mechanisms are separated in time, with the high-latitude reconnection region disappearing prior to substorm onset.

Re-Os isotopic systematics of primitive lavas from the Lassen region of the Cascade arc, California

USGS Publications Warehouse

Borg, L.E.; Brandon, A.D.; Clynne, M.A.; Walker, R.J.

2000-01-01

Rhenium-osmium isotopic systematics of primitive calc-alkaline lavas from the Lassen region appear to be controlled by mantle wedge processes. Lavas with a large proportion of slab component have relatively low Re and Os abundances, and have radiogenic Os and mid ocean ridge basalt-like Sr and Pb isotopic compositions. Lavas with a small proportion of slab component have higher Re and Os elemental abundances and display mantle-like Os, Sr, Nd, and Pb isotopic compositions. Assimilation with fractional crystallization can only generate the Re-Os systematics of the Lassen lavas from a common parent if the distribution coefficient for Re in sulfide is ~40-1100 times higher than most published estimates and if most incompatible element abundances decrease during differentiation. High Re/Os ratios in mid ocean ridge basalts makes subducted oceanic crust a potential source of radiogenic Os in volcanic arcs. The slab beneath the southernmost Cascades is estimated to have 187Os/188Os ratios as high as 1.4. Mixing between a slab component and mantle wedge peridotite can generate the Os isotopic systematics of the Lassen lavas provided the slab component has a Sr/Os ratio of ~7.5X105 and Os abundances that are 100-600 times higher than mid ocean ridge basalts. For this model to be correct, Os must be readily mobilized and concentrated in the slab component, perhaps as a result of high water and HCl fugacities in this subduction environment. Another possible mechanism to account for the correlation between the magnitude of the subduction geochemical signature and Os isotopic composition involves increasing the stability of an Os-bearing phase in mantle wedge peridotites as a result of fluxing with the slab component. Melting of such a source could yield low Os magmas that are more susceptible to crustal contamination, and hence have more radiogenic Os isotopic compositions, than magmas derived from sources with a smaller contribution from the slab. Thus, the addition of the slab component to the mantle wedge appears to result in either the direct or indirect addition of radiogenic Os to arc magmas. (C) 2000 Elsevier Science B.V. All rights reserved.

Blind source deconvolution for deep Earth seismology

NASA Astrophysics Data System (ADS)

Stefan, W.; Renaut, R.; Garnero, E. J.; Lay, T.

2007-12-01

We present an approach to automatically estimate an empirical source characterization of deep earthquakes recorded teleseismically and subsequently remove the source from the recordings by applying regularized deconvolution. A principle goal in this work is to effectively deblur the seismograms, resulting in more impulsive and narrower pulses, permitting better constraints in high resolution waveform analyses. Our method consists of two stages: (1) we first estimate the empirical source by automatically registering traces to their 1st principal component with a weighting scheme based on their deviation from this shape, we then use this shape as an estimation of the earthquake source. (2) We compare different deconvolution techniques to remove the source characteristic from the trace. In particular Total Variation (TV) regularized deconvolution is used which utilizes the fact that most natural signals have an underlying spareness in an appropriate basis, in this case, impulsive onsets of seismic arrivals. We show several examples of deep focus Fiji-Tonga region earthquakes for the phases S and ScS, comparing source responses for the separate phases. TV deconvolution is compared to the water level deconvolution, Tikenov deconvolution, and L1 norm deconvolution, for both data and synthetics. This approach significantly improves our ability to study subtle waveform features that are commonly masked by either noise or the earthquake source. Eliminating source complexities improves our ability to resolve deep mantle triplications, waveform complexities associated with possible double crossings of the post-perovskite phase transition, as well as increasing stability in waveform analyses used for deep mantle anisotropy measurements.

Olivine and melt inclusion chemical constraints on the source of intracontinental basalts from the eastern North China Craton: Discrimination of contributions from the subducted Pacific slab

NASA Astrophysics Data System (ADS)

Li, Hong-Yan; Xu, Yi-Gang; Ryan, Jeffrey G.; Huang, Xiao-Long; Ren, Zhong-Yuan; Guo, Hua; Ning, Zhen-Guo

2016-04-01

Contributions from fluid and melt inputs from the subducting Pacific slab to the chemical makeup of intraplate basalts erupted on the eastern Eurasian continent have long been suggested but have not thus far been geochemically constrained. To attempt to address this question, we have investigated Cenozoic basaltic rocks from the western Shandong and Bohai Bay Basin, eastern North China Craton (NCC), which preserve coherent relationships among the chemistries of their melt inclusions, their hosting olivines and their bulk rock compositions. Three groups of samples are distinguished: (1) high-Si and (2) moderate-Si basalts (tholeiites, alkali basalts and basanites) which were erupted at ∼23-20 Ma, and (3) low-Si basalts (nephelinites) which were erupted at <9 Ma. The high-Si basalts have lower alkalies, CaO and FeOT contents, lower trace element concentrations, lower La/Yb, Sm/Yb and Ce/Pb but higher Ba/Th ratios, and lower εNd and εHf values than the low-Si basalts. The olivines in the high-Si basalts have higher Ni and lower Mn and Ca at a given Fo value than those crystallizing from peridotite melts, and their corresponding melt inclusions have lower CaO contents than peridotite melts, suggesting a garnet pyroxenitic source. The magmatic olivines from low-Si basalts have lower Ni but higher Mn at a given Fo value than that of the high-Si basalts, suggesting more olivine in its source. The olivine-hosted melt inclusions of the low-Si basalts have major elemental signatures different from melts of normal peridotitic or garnet pyroxenitic mantle sources, pointing to their derivation from a carbonated mantle source consisting of peridotite and garnet pyroxenite. We propose a model involving the differential melting of a subduction-modified mantle source to account for the generation of these three suites of basalts. Asthenospheric mantle beneath the eastern NCC, which entrains garnet pyroxenite with an EM1 isotopic signature, was metasomatized by carbonatitic melts from carbonated eclogite derived from subducted Pacific slab materials present in the deeper mantle. High degree melting of garnet pyroxenites from a shallower mantle source produced the early (∼23-20 Ma) higher-Si basalts. Mixing of these materials with deeper-sourced melts of carbonated mantle source produced the moderate-Si basalts. A thicker lithosphere after 9 Ma precluded melting of shallower garnet pyroxenites, so melts of the deeper carbonated mantle source are responsible for the low-Si basalts.

Petrogenesis of postcollisional magmatism at Scheelite Dome, Yukon, Canada: Evidence for a lithospheric mantle source for magmas associated with intrusion-related gold systems

USGS Publications Warehouse

Mair, John L.; Farmer, G. Lang; Groves, David I.; Hart, Craig J.R.; Goldfarb, Richard J.

2011-01-01

The type examples for the class of deposits termed intrusion-related gold systems occur in the Tombstone-Tungsten belt of Alaska and Yukon, on the eastern side of the Tintina gold province. In this part of the northern Cordillera, extensive mid-Cretaceous postcollisional plutonism took place following the accretion of exotic terranes to the continental margin. The most cratonward of the resulting plutonic belts comprises small isolated intrusive centers, with compositionally diverse, dominantly potassic rocks, as exemplified at Scheelite Dome, located in central Yukon. Similar to other spatially and temporally related intrusive centers, the Scheelite Dome intrusions are genetically associated with intrusion-related gold deposits. Intrusions have exceptional variability, ranging from volumetrically dominant clinopyroxene-bearing monzogranites, to calc-alkaline minettes and spessartites, with an intervening range of intermediate to felsic stocks and dikes, including leucominettes, quartz monzonites, quartz monzodiorites, and granodiorites. All rock types are potassic, are strongly enriched in LILEs and LREEs, and feature high LILE/HFSE ratios. Clinopyroxene is common to all rock types and ranges from salite in felsic rocks to high Mg augite and Cr-rich diopside in lamprophyres. Less common, calcic amphibole ranges from actinolitic hornblende to pargasite. The rocks have strongly radiogenic Sr (initial 87Sr/86Sr from 0.711-0.714) and Pb isotope ratios (206Pb/204Pb from 19.2-19.7), and negative initial εNd values (-8.06 to -11.26). Whole-rock major and trace element, radiogenic isotope, and mineralogical data suggest that the felsic to intermediate rocks were derived from mafic potassic magmas sourced from the lithospheric mantle via fractional crystallization and minor assimilation of metasedimentary crust. Mainly unmodified minettes and spessartites represent the most primitive and final phases emplaced. Metasomatic enrichments in the underlying lithospheric mantle are attributes of the ancient North American cratonic margin that appear to be essential prerequisites to this style of postcollisional magmatism and associated gold-rich fluid exsolution. This type of magmatic hydrothermal activity occurs in a very specific tectonic setting that typically sets intrusion-related gold deposits apart from orogenic gold deposits, which are synorogenic in timing and have no consistent direct relationship to such diverse and contemporaneous lithospheric mantle-derived magmas, although they too are commonly sited adjacent to lithospheric boundaries.

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

USGS Publications Warehouse

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

1994-01-01

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

Lead Isotope Compositions of Acid Residues from Olivine-Phyric Shergottite Tissint: Implications for Heterogeneous Shergottite Source Reservoirs

NASA Technical Reports Server (NTRS)

Moriwaki, R.; Usui, T.; Yokoyama, T.; Simon, J. I.; Jones, J. H.

2015-01-01

Geochemical studies of shergottites suggest that their parental magmas reflect mixtures between at least two distinct geochemical source reservoirs, producing correlations between radiogenic isotope compositions and trace element abundances. These correlations have been interpreted as indicating the presence of a reduced, incompatible element- depleted reservoir and an oxidized, incompatible- element-enriched reservoir. The former is clearly a depleted mantle source, but there is ongoing debate regarding the origin of the enriched reservoir. Two contrasting models have been proposed regarding the location and mixing process of the two geochemical source reservoirs: (1) assimilation of oxidized crust by mantle derived, reduced magmas, or (2) mixing of two distinct mantle reservoirs during melting. The former requires the ancient Martian crust to be the enriched source (crustal assimilation), whereas the latter requires isolation of a long-lived enriched mantle domain that probably originated from residual melts formed during solidification of a magma ocean (heterogeneous mantle model). This study conducts Pb isotope and trace element concentration analyses of sequential acid-leaching fractions (leachates and the final residues) from the geochemically depleted olivine-phyric shergottite Tissint. The results suggest that the Tissint magma is not isotopically uniform and sampled at least two geochemical source reservoirs, implying that either crustal assimilation or magma mixing would have played a role in the Tissint petrogenesis.

Silica-enriched mantle sources of subalkaline picrite-boninite-andesite island arc magmas

NASA Astrophysics Data System (ADS)

Bénard, A.; Arculus, R. J.; Nebel, O.; Ionov, D. A.; McAlpine, S. R. B.

2017-02-01

Primary arc melts may form through fluxed or adiabatic decompression melting in the mantle wedge, or via a combination of both processes. Major limitations to our understanding of the formation of primary arc melts stem from the fact that most arc lavas are aggregated blends of individual magma batches, further modified by differentiation processes in the sub-arc mantle lithosphere and overlying crust. Primary melt generation is thus masked by these types of second-stage processes. Magma-hosted peridotites sampled as xenoliths in subduction zone magmas are possible remnants of sub-arc mantle and magma generation processes, but are rarely sampled in active arcs. Published studies have emphasised the predominantly harzburgitic lithologies with particularly high modal orthopyroxene in these xenoliths; the former characteristic reflects the refractory nature of these materials consequent to extensive melt depletion of a lherzolitic protolith whereas the latter feature requires additional explanation. Here we present major and minor element data for pristine, mantle-derived, lava-hosted spinel-bearing harzburgite and dunite xenoliths and associated primitive melts from the active Kamchatka and Bismarck arcs. We show that these peridotite suites, and other mantle xenoliths sampled in circum-Pacific arcs, are a distinctive peridotite type not found in other tectonic settings, and are melting residues from hydrous melting of silica-enriched mantle sources. We explore the ability of experimental studies allied with mantle melting parameterisations (pMELTS, Petrolog3) to reproduce the compositions of these arc peridotites, and present a protolith ('hybrid mantle wedge') composition that satisfies the available constraints. The composition of peridotite xenoliths recovered from erupted arc magmas plausibly requires their formation initially via interaction of slab-derived components with refractory mantle prior to or during the formation of primary arc melts. The liquid compositions extracted from these hybrid sources are higher in normative quartz and hypersthene (i.e., they have a more silica-saturated character) in comparison with basalts derived from prior melt-depleted asthenospheric mantle beneath ridges. These primary arc melts range from silica-rich picrite to boninite and high-Mg basaltic andesite along a residual spinel harzburgite cotectic. Silica enrichment in the mantle sources of arc-related, subalkaline picrite-boninite-andesite suites coupled with the amount of water and depth of melting, are important for the formation of medium-Fe ('calc-alkaline') andesite-dacite-rhyolite suites, key lithologies forming the continental crust.

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

NASA Astrophysics Data System (ADS)

Brown, Eric; Petersen, Kenni; Lesher, Charles

2017-04-01

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

High Resolution Global Electrical Conductivity Variations in the Earth's Mantle

NASA Astrophysics Data System (ADS)

Kelbert, A.; Sun, J.; Egbert, G. D.

2013-12-01

Electrical conductivity of the Earth's mantle is a valuable constraint on the water content and melting processes. In Kelbert et al. (2009), we obtained the first global inverse model of electrical conductivity in the mantle capable of providing constraints on the lateral variations in mantle water content. However, in doing so we had to compromise on the problem complexity by using the historically very primitive ionospheric and magnetospheric source assumptions. In particular, possible model contamination by the auroral current systems had greatly restricted our use of available data. We have now addressed this problem by inverting for the external sources along with the electrical conductivity variations. In this study, we still focus primarily on long period data that are dominated by quasi-zonal source fields. The improved understanding of the ionospheric sources allows us to invert the magnetic fields directly, without a correction for the source and/or the use of transfer functions. It allows us to extend the period range of available data to 1.2 days - 102 days, achieving better sensitivity to the upper mantle and transition zone structures. Finally, once the source effects in the data are accounted for, a much larger subset of observatories may be used in the electrical conductivity inversion. Here, we use full magnetic fields at 207 geomagnetic observatories, which include mid-latitude, equatorial and high latitude data. Observatory hourly means from the years 1958-2010 are employed. The improved quality and spatial distribution of the data set, as well as the high resolution modeling and inversion using degree and order 40 spherical harmonics mapped to a 2x2 degree lateral grid, all contribute to the much improved resolution of our models, representing a conceptual step forward in global electromagnetic sounding. We present a fully three-dimensional, global electrical conductivity model of the Earth's mantle as inferred from ground geomagnetic observatory data, and use additional constraints to interpret these results in terms of mantle processes and compositional variations.

Trace element and isotopic composition of apatite in carbonatites from the Blue River area (British Columbia, Canada) and mineralogy of associated silicate rocks

NASA Astrophysics Data System (ADS)

Mitchell, Roger; Chudy, Thomas; McFarlane, Christopher R. M.; Wu, Fu-Yuan

2017-08-01

Apatites from the Verity, Fir, Gum, Howard Creek and Felix carbonatites of the Blue River (British Columbia, Canada) area have been investigated with respect to their paragenesis, cathodoluminescence, trace element and Sr-Nd isotopic composition. Although all of the Blue River carbonatites were emplaced as sills prior to amphibolite grade metamorphism and have undergone deformation, in many instances magmatic textures and mineralogy are retained. Attempts to constrain the U-Pb age of the carbonatites by SIMS, TIMS and LA-ICP-MS studies of zircon and titanite were inconclusive as all samples investigated have experienced significant Pb loss during metamorphism. The carbonatites are associated with undersaturated calcite-titanite amphibole nepheline syenite only at Howard Creek although most contain clasts of disaggregated phoscorite-like rocks. Apatite from each intrusion is characterized by distinct, but wide ranges, in trace element composition. The Sr and Nd isotopic compositions define an array on a 87Sr/86Sr vs²Nd diagram at 350 Ma indicating derivation from depleted sub-lithospheric mantle. This array could reflect mixing of Sr and Nd derived from HIMU and EM1 mantle sources, and implies that depleted mantle underlies the Canadian Cordillera. Although individual occurrences of carbonatites in the Blue River region are mineralogically and geochemically similar they are not identical and thus cannot be considered as rocks formed from a single batch of parental magma at the same stage of magmatic evolution. However, a common origin is highly probable. The variations in the trace element content and isotopic composition of apatite from each occurrence suggest that each carbonatite represents a combination of derivation of the parental magma(s) from mineralogically and isotopically heterogeneous depleted mantle sources coupled with different stages of limited differentiation and mixing of these magmas. We do not consider these carbonatites as primary direct partial melts of the sub-lithospheric mantle which have ascended from the asthenosphere without modification of their composition.

Plume magmatism and crustal growth at 2.9 to 3.0 Ga in the Steep Rock and Lumby Lake area, Western Superior Province

NASA Astrophysics Data System (ADS)

Tomlinson, K. Y.; Hughes, D. J.; Thurston, P. C.; Hall, R. P.

1999-01-01

The greenstone belts of the western Superior Province are predominantly 2.78 to 2.69 Ga and provide evidence of oceanic and arc volcanism during the accretionary phase of development of the Superior Province. There is also scattered evidence of Meso-Archean crust (predominantly 2.9 to 3.0 Ga) within the western Superior Province. The Meso-Archean greenstone belts commonly contain platformal sediments and unconformably overlie granitoid basement. The platformal sediments occur associated with komatiitic and tholeiitic volcanic rocks that suggest a history of magmatism associated with rifting during the Meso-Archean. The central Wabigoon Subprovince is a key area of Meso-Archean crust and in its southern portion comprises the Steep Rock, Finlayson and Lumby Lake greenstone belts. The Steep Rock greenstone belt unconformably overlies 3 Ga continental basement and contains platformal sediments succeeded by komatiitic and tholeiitic volcanic rocks. The Lumby Lake greenstone belt contains thick sequences of mafic volcanics, a number of komatiite horizons, and thin platformal sedimentary units. The two belts are joined by the predominantly mafic volcanic Finlayson greenstone belt. The volcanics throughout these three greenstone belts may be correlated to some extent and a range of basaltic and komatiite types is present. Al-undepleted komatiites present in the Lumby Lake greenstone belt have an Al 2O 3/TiO 2 ratio ranging from 14 to 27 and (Gd/Yb) N from 0.7 to 1.3. These are divided into basaltic komatiites with generally unfractionated mantle-normalised multi-element profiles, and spinifex-textured high-Mg basalts with slightly light REE enriched multi-element profiles and small negative Nb and Ta anomalies. The unfractionated basaltic komatiites represent high degree partial melts of the upper mantle whereas the spinifex-textured high-Mg basalts represent evolutionary products of the komatiite liquids following olivine and chromite fractionation and crustal contamination. Al-depleted komatiites are present in both the Lumby Lake and Steep Rock belts and have Al 2O 3/TiO 2 ratio ranges from 2.5 to 5. These display strong enrichment in the light REE and Nb and strong depletion in the heavy REE and Y ((Gd/Yb) N=2-4). They represent a deep mantle plume source generated from a high degree of partial melting in the majorite garnet stability field. The basaltic flows in all three greenstone belts are predominantly slightly light REE depleted and represent a slightly depleted upper mantle source. Basalts spatially associated with the unfractionated basaltic komatiites and the slightly light REE enriched spinifex-textured high-Mg basalts are also slightly enriched in light REE and have negative Nb and Ta anomalies. These basalts represent evolved products of the primitive basaltic komatiites and enriched spinifex-textured high-Mg basalts after further crustal contamination and olivine and clinopyroxene fractionation. The geochemical stratigraphy in the Lumby Lake belt is consistent with an ascending mantle plume model. The light REE depleted basalts were derived from upper mantle melted by an ascending mantle plume. These are overlain by the unfractionated basaltic komatiites and their evolutionary products which represent hotter plume head material derived from a mixture of plume mantle and entrained depleted upper mantle. In turn, these are overlain by strongly light REE and HFSE enriched komatiites that represent a deep plume source that has not been mixed with depleted mantle and are, therefore, likely to have been derived from a plume core or tail. Volcanism was protracted in these three greenstone belts lasting ca. 70 Ma and combined stratigraphic evidence from the Lumby Lake and Steep Rock belts suggests that more than one plume may have ascended and tapped the same mantle sources, over time, within the area. Plume magmatism and rifting of continental platforms thus appears to have been an important feature of crustal development in the Meso-Archean.

Lower-mantle plume beneath the Yellowstone hotspot revealed by core waves

NASA Astrophysics Data System (ADS)

Nelson, Peter L.; Grand, Stephen P.

2018-04-01

The Yellowstone hotspot, located in North America, is an intraplate source of magmatism the cause of which is hotly debated. Some argue that a deep mantle plume sourced at the base of the mantle supplies the heat beneath Yellowstone, whereas others claim shallower subduction or lithospheric-related processes can explain the anomalous magmatism. Here we present a shear wave tomography model for the deep mantle beneath the western United States that was made using the travel times of core waves recorded by the dense USArray seismic network. The model reveals a single narrow, cylindrically shaped slow anomaly, approximately 350 km in diameter that we interpret as a whole-mantle plume. The anomaly is tilted to the northeast and extends from the core-mantle boundary to the surficial position of the Yellowstone hotspot. The structure gradually decreases in strength from the deepest mantle towards the surface and if it is purely a thermal anomaly this implies an initial excess temperature of 650 to 850 °C. Our results strongly support a deep origin for the Yellowstone hotspot, and also provide evidence for the existence of thin thermal mantle plumes that are currently beyond the resolution of global tomography models.

Uranium-lead isotope systematics of Mars inferred from the basaltic shergottite QUE 94201

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

Gaffney, A M; Borg, L E; Connelly, J N

2006-12-22

Uranium-lead ratios (commonly represented as {sup 238}U/{sup 204}Pb = {mu}) calculated for the sources of martian basalts preserve a record of petrogenetic processes that operated during early planetary differentiation and formation of martian geochemical reservoirs. To better define the range of {mu} values represented by the source regions of martian basalts, we completed U-Pb elemental and isotopic analyses on whole rock, mineral and leachate fractions from the martian meteorite Queen Alexandra Range 94201 (QUE 94201). The whole rock and silicate mineral fractions have unradiogenic Pb isotopic compositions that define a narrow range ({sup 206}Pb/{sup 204}Pb = 11.16-11.61). In contrast, themore » Pb isotopic compositions of weak HCl leachates are more variable and radiogenic. The intersection of the QUE 94201 data array with terrestrial Pb in {sup 206}Pb/{sup 204}Pb-{sup 207}Pb/{sup 204}Pb-{sup 208}Pb/{sup 204}Pb compositional space is consistent with varying amounts of terrestrial contamination in these fractions. We calculate that only 1-7% contamination is present in the purified silicate mineral and whole rock fractions, whereas the HCl leachates contain up to 86% terrestrial contamination. Despite the contamination, we are able to use the U-Pb data to determine the initial {sup 206}Pb/{sup 204}Pb of QUE 94201 (11.086 {+-} 0.008) and calculate the {mu} value of the QUE 94201 mantle source to be 1.823 {+-} 0.008. This is the lowest {mu} value calculated for any martian basalt source, and, when compared to the highest values determined for martian basalt sources, indicates that {mu} values in martian source reservoirs vary by at least 100%. The range of source {mu} values further indicates that the {mu} value of bulk silicate Mars is approximately three. The amount of variation in the {mu} values of the mantle sources ({mu} {approx} 2-4) is greater than can be explained by igneous processes involving silicate phases alone. We suggest the possibility that a small amount of sulfide crystallization may generate large extents of U-Pb fractionation during formation of the mantle sources of martian basalts.« less

The Central Atlantic Magmatic Province (CAMP)

NASA Astrophysics Data System (ADS)

Marzoli, A.; Callegaro, S.; Davies, J.; Chiaradia, M.; Reisberg, L. C.; Merle, R.; Jourdan, F.; Bertrand, H.; Youbi, N.

2017-12-01

Basaltic lava flows, dykes, sills, and layered intrusion of the CAMP (Central Atlantic magmatic province) crop out in Europe, Africa, North and South America over > 10 million square km, making this one of Earth's largest igneous provinces. CAMP is characterized by 100-400 m thick preserved lava piles and by huge shallow intrusions (e.g., > 1.5 million cubic km sills). Magmatism occurred mainly between 201.6 and 201.1 Ma (according to U-Pb and Ar/Ar ages) during the end-Triassic extinction event and a few Ma before break-up of Pangea. Pulsed emplacement seems consistent with high-precision geochronology, but needs further confirmation. All over the province, basalts with quite similar composition reflect a common mantle source. These basalts have low Ti contents (TiO2 ca. 1.0-1.3 wt.%), moderately enriched Sr-Nd-Pb isotopic compositions close to the EM-II mantle end-member, and 187Os/188Os close to 0.130. We attribute these characteristics to a dominant shallow asthenospheric mantle source that was enriched by subduction-related components. Assimilation of crustal rocks generally played a minor role and rarely exceed 5-10%. Instead, assimilation of the sub-continental lithospheric mantle (SCLM) was instead recognized in the high-Ti basalts (TiO2> 2.0 wt.%) that were emplaced in a restricted area around the Man and Amazonian cratons (Sierra Leone, Liberia, Brazil, Guyana). The SCLM-like signature of these basalts suggests assimilation of metasomatically enriched parts of the SCLM. Also early basalts emplaced north of the West African craton (Morocco, Mali) are contaminated by enriched SCLM components even if to a lesser degree, while later basalts from the same African regions have low 187Os/188Os (ca. 0.120) and probably tapped a more depleted cratonic SCLM. Calculated mantle potential temperatures are low (ca. 1450 °C) and geochemical data do not support a significant contribution from mantle-plume material. The only available He isotopic data are just slightly higher than those of MORB. This argues against a substantial contribution from mantle-plume material. The only basalts trending to isotopic compositions similar to those of present-day Atlantic island basalts are quite limited in volume and restricted to a small area of Morocco.

Preliminary Report on U-Th-Pb Isotope Systematics of the Olivine-Phyric Shergottite Tissint

NASA Technical Reports Server (NTRS)

Moriwaki, R.; Usui, T.; Yokoyama, T.; Simon, J. I.; Jones, J. H.

2014-01-01

Geochemical studies of shergottites suggest that their parental magmas reflect mixtures between at least two distinct geochemical source reservoirs, producing correlations between radiogenic isotope compositions, and trace element abundances.. These correlations have been interpreted as indicating the presence of a reduced, incompatible-element- depleted reservoir and an oxidized, incompatible-element-rich reservoir. The former is clearly a depleted mantle source, but there has been a long debate regarding the origin of the enriched reservoir. Two contrasting models have been proposed regarding the location and mixing process of the two geochemical source reservoirs: (1) assimilation of oxidized crust by mantle derived, reduced magmas, or (2) mixing of two distinct mantle reservoirs during melting. The former clearly requires the ancient martian crust to be the enriched source (crustal assimilation), whereas the latter requires a long-lived enriched mantle domain that probably originated from residual melts formed during solidification of a magma ocean (heterogeneous mantle model). This study conducts Pb isotope and U-Th-Pb concentration analyses of the olivine-phyric shergottite Tissint because U-Th-Pb isotope systematics have been intensively used as a powerful radiogenic tracer to characterize old crust/sediment components in mantle- derived, terrestrial oceanic island basalts. The U-Th-Pb analyses are applied to sequential acid leaching fractions obtained from Tissint whole-rock powder in order to search for Pb isotopic source components in Tissint magma. Here we report preliminary results of the U-Th-Pb analyses of acid leachates and a residue, and propose the possibility that Tissint would have experienced minor assimilation of old martian crust.

Upper mantle diapers, lower crustal magmatic underplating, and lithospheric dismemberment of the Great Basin and Colorado Plateau regions, Nevada and Utah; implications from deep MT resistivity surveying

NASA Astrophysics Data System (ADS)

Wannamaker, P. E.; Doerner, W. M.; Hasterok, D. P.

2005-12-01

In the rifted Basin and Range province of the southwestern U.S., a common faulting model for extensional basins based e.g. on reflection seismology data shows dominant displacement along master faults roughly coincident with the main topographic scarp. On the other hand, complementary data such as drilling, earthquake focal mechanisms, volcanic occurrences, and trace indicators such as helium isotopes suggest that there are alternative geometries of crustal scale faulting and material transport from the deep crust and upper mantle in this province. Recent magnetotelluric (MT) profiling results reveal families of structures commonly dominated by high-angle conductors interpreted to reflect crustal scale fault zones. Based mainly on cross cutting relationships, these faults appear to be late Cenozoic in age and are of low resistivity due to fluids or alteration (including possible graphitization). In the Ruby Mtns area of north-central Nevada, high angle faults along the margins of the core complex connect from near surface to a regional lower crustal conductor interpreted to contain high-temperature fluids and perhaps melts. Such faults may exemplify the high angle normal faults upon which the major earthquakes of the Great Basin appear to nucleate. A larger-scale transect centered on Dixie Valley shows major conductive crustal-scale structures connecting to conductive lower crust below Dixie Valley, the Black Rock desert in NW Nevada, and in east-central Nevada in the Monitor-Diamond Valley area. In the Great Basin-Colorado Plateau transition of Utah, the main structures revealed are a series of nested low-angle detachment structures underlying the incipient development of several rift grabens. All these major fault zones appear to overlie regions of particularly conductive lower crust interpreted to be caused by recent basaltic underplating. In the GB-CP transition, long period data show two, low-resistivity upper mantle diapirs underlying the concentrated conductive lower crust and nested faults, and these are advanced as melt source regions for the underplating. MT, with its wide frequency bandwidth, allows views of nearly a complete melting and emplacement process, from mantle source region, through lower crustal intrusion, to brittle regime deformational response.

Plume versus plate origin for the Shatsky Rise oceanic plateau (NW Pacific): Insights from Nd, Pb and Hf isotopes

NASA Astrophysics Data System (ADS)

Heydolph, Ken; Murphy, David T.; Geldmacher, Jörg; Romanova, Irina V.; Greene, Andrew; Hoernle, Kaj; Weis, Dominique; Mahoney, John

2014-07-01

Shatsky Rise, an early Cretaceous igneous oceanic plateau in the NW Pacific, comprises characteristics that could be attributed to either formation by shallow, plate tectonic-controlled processes or to an origin by a mantle plume (head). The plateau was drilled during Integrated Ocean Drilling Program (IODP) Expedition 324. Complementary to a recent trace element study (Sano et al., 2012) this work presents Nd, Pb and Hf isotope data of recovered lava samples cored from the three major volcanic edifices of the Shatsky Rise. Whereas lavas from the oldest edifice yield fairly uniform compositions, a wider isotopic spread is found for lavas erupted on the younger parts of the plateau, suggesting that the Shatsky magma source became more heterogeneous with time. At least three isotopically distinct components can be identified in the magma source: 1) a volumetrically and spatially most common, moderately depleted component of similar composition to modern East Pacific Ridge basalt but with low 3He/4He, 2) an isotopically very depleted component which could represent local, early Cretaceous (entrained) depleted upper mantle, and 3) an isotopically enriched component, indicating the presence of (recycled) continental material in the magma source. The majority of analyzed Shatsky lavas, however, possess Nd-Hf-Pb isotope compositions consistent with a derivation from an early depleted, non-chondritic reservoir. By comparing these results with petrological and trace element data of mafic volcanic rock samples from all three massifs (Tamu, Ori, Shirshov), we discuss the origin of Shatsky Rise magmatism and evaluate the possible involvement of a mantle plume (head).

Transient rheology of the upper mantle beneath central Alaska inferred from the crustal velocity field following the 2002 Denali earthquake

USGS Publications Warehouse

Pollitz, F.F.

2005-01-01

The M7.9 2002 Denali earthquake, Alaska, is one of the largest strike-slip earthquakes ever recorded. The postseismic GPS velocity field around the 300-km-long rupture is characterized by very rapid horizontal velocity up to ???300 mm/yr for the first 0.1 years and slower but still elevated horizontal velocity up to ???100 mm/yr for the succeeding 1.5 years. I find that the spatial and temporal pattern of the displacement field may be explained by a transient mantle rheology. Representing the regional upper mantle as a Burghers body, I infer steady state and transient viscosities of ??1 = 2.8 ?? 1018 Pa s and ??2 = 1.0 ?? 1017 Pa s, respectively, corresponding to material relaxation times of 1.3 and 0.05 years. The lower crustal viscosity is poorly constrained by the considered horizontal velocity field, and the quoted mantle viscosities assume a steady state lower crust viscosity that is 7??1. Systematic bias in predicted versus observed velocity vectors with respect to a fixed North America during the first 3-6 months following the earthquake is reduced when all velocity vectors are referred to a fixed site. This suggests that the post-Denali GPS time series for the first 1.63 years are shaped by a combination of a common mode noise source during the first 3-6 months plus viscoelastic relaxation controlled by a transient mantle rheology.

Bromine partitioning between olivine and melt at OIB source conditions: Indication for volatile recycling

NASA Astrophysics Data System (ADS)

Joachim, Bastian; Ruzié, Lorraine; Burgess, Ray; Pawley, Alison; Clay, Patricia L.; Ballentine, Christopher J.

2016-04-01

Halogens play a key role in our understanding of volatile transport processes in the Earth's mantle. Their moderate (fluorine) to highly (iodine) incompatible and volatile behavior implies that their distribution is influenced by partial melting, fractionation and degassing processes as well as fluid mobilities. The heavy halogens, particularly bromine and iodine, are far more depleted in the Earth's mantle than expected from their condensation temperature (Palme and O'Neill 2014), so that their very low abundances in basalts and peridotites (ppb-range) make it analytically challenging to investigate their concentrations in Earth's mantle reservoirs and their behavior during transport processes (Pyle and Mather, 2009). We used a new experimental technique, which combines the irradiation technique (Johnson et al. 2000), laser ablation and conventional mass spectrometry. This enables us to present the first experimentally derived bromine partition coefficient between olivine and melt. Partitioning experiments were performed at 1500° C and 2.3 GPa, a P-T condition that is representative for partial melting processes in the OIB source region (Davis et al. 2011). The bromine partition coefficient between olivine and silicate melt at this condition has been determined to DBrol/melt = 4.37•10-4± 1.96•10-4. Results show that bromine is significantly more incompatible than chlorine (˜1.5 orders of magnitude) and fluorine (˜2 orders of magnitude) due to its larger ionic radius. We have used our bromine partitioning data to estimate minimum bromine abundances in EM1 and EM2 source regions. We used minimum bromine bulk rock concentrations determined in an EM1 (Pitcairn: 1066 ppb) and EM2 (Society: 2063 ppb) basalt (Kendrick et al. 2012), together with an estimated minimum melt fraction of 0.01 in OIB source regions (Dasgupta et al. 2007). The almost perfect bromine incompatibility results in minimum bromine abundances in EM1 and EM2 OIB source regions of 11 ppb and 20 ppb, respectively. The effect on the partitioning behaviour of other minerals such as pyroxene, mantle inhomogeneity, incongruent melting, a potential effect of iron, temperature, pressure or the presence of fluids, would be to shift the estimated bromine mantle source concentration to higher but not to lower values. Comparing our minimum bromine OIB source region estimate with the estimated primitive mantle bromine abundance (3.6 ppb; Lyubetskaya and Korenaga, 2007) implies that the OIB source mantle is enriched in bromine relative to the primitive mantle by at least a factor of 3 in EM1 source regions and a factor of 5.5 in EM2 source regions. One explanation is that bromine may be efficiently recycled into the OIB source mantle region through recycling of subducted oceanic crust. Dasgupta R, Hirschmann MM, Humayun, ND (2007) J. Petrol. 48, pp. 2093-2124. Davis FA, Hirschmann MM, Humayun M (2011) Earth Planet. Sci. Lett. 308, pp. 380-390. Johnson L, Burgess R, Turner G, Milledge JH, Harris JW (2000) Geochim. Cosmochim. Acta 64, pp. 717-732. Kendrick MA, Woodhead JD, Kamenetsky VS (2012) Geol. 32, pp. 441-444. Lyubetskaya T, Korenaga J (2007) J. Geophys. Res.-Sol. Earth 112, B03211. Palme H, O'Neill HStC (2014). Cosmochemical Estimates of Mantle Composition. Treat. Geochem. 2nd edition, 3, pp. 1-39. Pyle DM, Mather TA (2009) Chem. Geol. 263, pp. 110-121.

Contemporaneous eruption of calc-alkaline and alkaline lavas in a continental arc (Eastern Mexican Volcanic Belt): chemically heterogeneous but isotopically homogeneous source

NASA Astrophysics Data System (ADS)

Carrasco-Núñez, Gerardo; Righter, Kevin; Chesley, John; Siebert, Lee; Aranda-Gómez, José Jorge

2005-11-01

Nearly contemporaneous eruption of alkaline and calc-alkaline lavas occurred about 900 years BP from El Volcancillo paired vent, located behind the volcanic front in the Mexican Volcanic Belt (MVB). Emission of hawaiite (Toxtlacuaya) was immediately followed by calc-alkaline basalt (Río Naolinco). Hawaiites contain olivine microphenocrysts (Fo67-72), plagioclase (An56-60) phenocrysts, have 4-5 wt% MgO and 49.6-50.9 wt% SiO2. In contrast, calc-alkaline lavas contain plagioclase (An64-72) and olivine phenocrysts (Fo81-84) with spinel inclusions, and have 8-9 wt% MgO and 48.4-49.4 wt% SiO2. The most primitive lavas in the region (Río Naolinco and Cerro Colorado) are not as primitive as parental melts in other arcs, and could represent either (a) variable degrees of melting of a subduction modified, garnet-bearing depleted mantle source, followed by AFC process, or (b) melting of two distinct mantle sources followed by AFC processes. These two hypotheses are evaluated using REE, HFSE, and Sr, Os and Pb isotopic data. The Toxtlacuaya flow and the Y & I lavas can be generated by combined fractional crystallization and assimilation of gabbroic granulite, starting with a parental liquid similar to the Cerro Colorado basalt. Although calc-alkaline and alkaline magmas commonly occur together in other areas of the MVB, evidence for subduction component in El Volcancillo magmas is minimal and limited to <1%, which is a unique feature in this region further from the trench. El Volcancillo lavas were produced from two different magma batches: we surmise that the injection of calc-alkaline magma into an alkaline magma chamber triggered the eruption of hawaiites. Our results suggest that the subalkaline and hawaiitic lavas were formed by different degrees of partial melting of a similar, largely depleted mantle source, followed by later AFC processes. This model is unusual for arcs, where such diversity is usually explained by melting of heterogeneous (enriched and depleted) and subduction-modified mantle.

Platinum-group element abundance and distribution in chromite deposits of the Acoje Block, Zambales Ophiolite Complex, Philippines

USGS Publications Warehouse

Bacuta, G.C.; Kay, R.W.; Gibbs, A.K.; Lipin, B.R.

1990-01-01

Platinum-group elements (PGE) occur in ore-grade concentration in some of the chromite deposits related to the ultramafic section of the Acoje Block of the Zambales Ophiolite Complex. The deposits are of three types: Type 1 - associated with cumulate peridotites at the base of the crust; Type 2 - in dunite pods from the top 1 km of mantle harzburgite; and Type 3 - like Type 2, but in deeper levels of the harzburgite. Most of the deposites have chromite compositions that are high in Cr with Cr/(Cr + Al) (expressed as chromium index, Cr#) > 0.6; high-Al (Cr# Pd, thought to be characteristic of PGE-barren deposits) and positive slope (Ir < Pd, characteristic of PGE-rich deposits). Iridium, Ru and Os commonly occur as micron-size laurite (sulfide) inclusions in unfractured chromite. Laurite and native Os are also found as inclusions in interstitial sulfides. Platinum and Pd occur as alloy inclusions (and possibly as solid solution) in interstitial Ni-Cu sulfides and as tellurobismuthides in serpentine and altered sulfides. Variability of PGE distribution may be explained by alteration, crystal fractionation or partial melting processes. Alteration and metamorphism were ruled out, because PGE contents do not correlate with degree of serpentinization or the abundance and type (hydroxyl versus non-hydroxyl) of silicate inclusions in chromite. Preliminary Os isotopic data do not support crustal contamination as a source of the PGEs in the Acoje deposits. The anomalous PGE concentrations in Type 1 high-Cr chromite deposits are attributed to two stages of enrichment: an early enrichment of their mantle source from previous melting events and a later stage of sulfide segregation accompanying chromite crystallization. High-Al chromite deposits which crystallized from basalts derived from relatively low degrees of melting owe their low PGE content to partitioning of PGEs in sulfides and alloys that remain in the mantle. High-Cr deposits crystallized from melts that were previously enriched with PGEs during early melting events of their mantle source; Pt and Pd ore concentrations (ppm levels) are attained by segregation of magmatic sulfides. The Acoje deposits indicate that ophiolites are a potential economic source of the PGEs. ?? 1990.

Geochemical Constraints on Core-Mantle Interaction from Fe/Mn Ratios

NASA Astrophysics Data System (ADS)

Humayun, M.; Qin, L.

2003-12-01

The greater density of liquid iron alloy, and its immiscibility with silicate, maintains the physical separation of the core from the mantle. There are no a priori reasons, however, why the Earth's mantle should be chemically isolated from the core. Osmium isotopic variations in mantle plumes have been interpreted in terms of interaction between outer core and the source regions of deep mantle plumes. If chemical transport occurs across the core-mantle boundary its mechanism remains to be established. The Os isotope evidence has also been interpreted as the signatures of subducted Mn-sediments, which are known to have relatively high Pt/Os. In the mantle, Fe occurs mainly as the divalent ferrous ion, and Mn occurs solely as a divalent ion, and both behave in a geochemically coherent manner because of similarity in ionic charge and radius. Thus, the Fe/Mn ratio is a planetary constant insensitive to processes of mantle differentiation by partial melting. Two processes may perturb the ambient mantle Fe/Mn of 60: a) the subduction of Mn-sediments should decrease the Fe/Mn ratio in plume sources, while b) chemical transport from the outer core may increase the Fe/Mn ratio. The differentiation of the liquid outer core to form the solid inner core may increase abundances of the light element constituents (FeS, FeO, etc.) to the point of exsolution from the core at the CMB. The exact rate of this process is determined by the rate of inner core growth. Two end-member models include 1) inner core formation mainly prior to 3.5 Ga with heat release dominated by radioactive sources, or 2) inner core formation occurring mainly in the last 1.5 Ga with heat release dominated by latent heat. This latter model would imply large fluxes of Fe into the sources of modern mantle plumes. Existing Fe/Mn data for Gorgona and Hawaiian samples place limits on both these processes. We describe a new procedure for the precise determination of the Fe/Mn ratio in magmatic rocks by ICP-MS. This high-resolution study of the Fe/Mn of mantle-derived samples offers a new set of chemical constraints on the rates of inner core differentiation and the viability of Os isotope interpretations.

Super-reducing conditions in ancient and modern volcanic systems: sources and behaviour of carbon-rich fluids in the lithospheric mantle

NASA Astrophysics Data System (ADS)

Griffin, William L.; Huang, Jin-Xiang; Thomassot, Emilie; Gain, Sarah E. M.; Toledo, Vered; O'Reilly, Suzanne Y.

2018-05-01

Oxygen fugacity (ƒO2) is a key parameter of Earth's mantle, because it controls the speciation of the fluids migrating at depth; a major question is whether the sublithospheric mantle is metal-saturated, keeping ƒO2 near the Iron-Wustite (IW) buffer reaction. Cretaceous basaltic pyroclastic rocks on Mt. Carmel, Israel erupted in an intraplate environment with a thin, hot lithosphere. They contain abundant aggregates of hopper-shaped crystals of Ti-rich corundum, which have trapped melts with phenocryst assemblages (Ti2O3, SiC, TiC, silicides, native V) requiring extremely low ƒO2. These assemblages are interpreted to reflect interaction between basaltic melts and mantle-derived fluids dominated by CH4 + H2. Similar highly reduced assemblages are found associated with volcanism in a range of tectonic situations including subduction zones, major continental collisions, intraplate settings, craton margins and the cratons sampled by kimberlites. This distribution, and the worldwide similarity of δ13C in mantle-derived SiC and associated diamonds, suggest a widespread process, involving similar sources and independent of tectonic setting. We suggest that the common factor is the ascent of abiotic (CH4 + H2) fluids from the sublithospheric mantle; this would imply that much of the mantle is metal-saturated, consistent with observations of metallic inclusions in sublithospheric diamonds (e.g. Smith et al. 2016). Such fluids, perhaps carried in rapidly ascending deep-seated magmas, could penetrate high up into a depleted cratonic root, establishing the observed trend of decreasing ƒO2 with depth (e.g. Yaxley et al. in Lithos 140:142-151, 2012). However, repeated metasomatism (associated with the intrusion of silicate melts) will raise the FeO content near the base of the craton over time, developing a carapace of oxidizing material that would prevent the rise of CH4-rich fluids into higher levels of the subcontinental lithospheric mantle (SCLM). Oxidation of these fluids would release CO2 and H2O to drive metasomatism and low-degree melting both in the carapace and higher in the SCLM. This model can explain the genesis of cratonic diamonds from both reduced and oxidized fluids, the existence of SiC as inclusions in diamonds, and the abundance of SiC in some kimberlites. It should encourage further study of the fine fractions of heavy-mineral concentrates from all types of explosive volcanism.

Petro-tectonic Analysis of the Plagiogranite Intrusions in the Khor Fakkan Block of the Semail Ophiolites

NASA Astrophysics Data System (ADS)

Kokkalas, S.; Joun, H.; Tombros, S.

2017-12-01

Plagiogranite intrusions are common in the Khor Fakkan block of the Semail ophiolite, where the mantle sequence is predominant. Several models have been proposed for the source of these leucocratic intrusions, but their genesis is still under debate. The examined plagiogranites are characterized by 68 wt. % SiO2 and display volcanic-arc granite affinity. They have crystallize at temperatures that range from 550° to 720o C and pressures ranging from 5.0 to 6.5 Kbars. The parental plagiogranite melts, based on the relations of the δ18Omelt or δ18OH2O versus eSr suggest mixing of subducted crust with overlying upper mantle. The relatively wide range of the 87Rb/86Sr ratios, at almost constant 87Sr/86Sr, implies that partial melting and mixing was followed by fractional crystallization. The isotopic ages from the examined plagiogranites range between 94.9-98.5 Ma, predating the sole metamorphism. Based on our source contribution calculations, 96% of the igneous and 4% of sedimentary end-member components are involved in formation of plagiogranitic melts. The igneous end-member derived from partial melting of 3 % upper mantle and 97% recycled oceanic crust. We propose that the mafic melts were initially produced by the off-axis melting of recycled oceanic slab under a compressional regime a supra-subduction zone (SSZ) setting. The mafic melts were modified due to mixing with small amount of melts from the upper mantle by influx of slab-derived fluids. Then these melts underwent extended fractional crystallization with crystallization of An-enriched plagioclase and emplaced on the Moho level to form Dadnah plagiogranites in the Khor Fakkan block.

Comparing the composition of the earliest basalts erupted by the Iceland and Afar mantle plumes.

NASA Astrophysics Data System (ADS)

Stuart, Finlay M.

2013-04-01

The first basalts erupted by mantle plumes are typically generated by mantle melting at temperatures 200-300°C higher than average ambient mantle. This is consistent with the derivation of from a thermal boundary layer at the core-mantle boundary. Mantle plume temperatures decrease with time, likely as large plume heads give way to thin plume conduits. Consequently the early, hot plume basalts are a window into the deep mantle. At it's simplest they provide a test of whether the discrete plume source regions are primordial mantle that have been isolated since soon after Earth accretion, or have substantial contributions from subducted slabs. Here I present new isotopic and trace element determinations of the earliest picritic basalts from the ~30 Ma Afar plume in Ethiopia. They will be compared with similar material from the ~60 Ma proto-Iceland plume (PIP) in an effort to test prevailing models regarding the source of mantle plumes. The extremely primordial nature of the helium in the PIP picrites (3He/4He ~ 50 Ra) contrasts with much lower values of the Ethiopian flood basalt province (~21 Ra). The Iceland plume 3He/4He has decreased (linearly) with time, mirroring the secular cooling of the Iceland mantle plume identified by decreasing MgO and FeO in primary melts. In 60 million years the Iceland plume 3He/4He is still higher than the maximum Afar plume value. The Sr-Nd-Pb isotopic composition of the high 3He/4He Ethiopian flood basalt province picrites are remarkably homogenous (e.g. 87Sr/86Sr = 0.70396-0.70412; 206Pb/204Pb = 18.82-19.01). In comparison the PIP picrites have ranges that span nearly the global range of E-MORB and N-MORB. The Afar and proto-Iceland mantle plumes are clearly not initiated in a single deep mantle domain with the same depletion/enrichment and degassing histories, and the same scale of heterogeneity. This implies that there is more than one plume source region/mechanism that is capable of generating comparable volumes of basalt melt at Earth surface.

Xenon isotopic composition of the Mid Ocean Ridge Basalt (MORB) source

NASA Astrophysics Data System (ADS)

Peto, M. K.; Mukhopadhyay, S.

2012-12-01

Although convection models do not preclude preservation of smaller mantle regions with more pristine composition throughout Earth's history, it has been widely assumed that the moon forming giant impact likely homogenizes the whole mantle following a magma ocean that extended all the way to the bottom of the mantle. Recent findings of tungsten and xenon heterogeneities in the mantle [1,2,3,4], however, imply that i) the moon forming giant impact may not have homogenized the whole mantle and ii) plate tectonics was inefficient in erasing early formed compositional differences, particularly for the xenon isotopes. Therefore, the xenon isotope composition in the present day mantle still preserves a memory of early Earth processes. However, determination of the xenon isotopic composition of the mantle source is still scarce, since the mantle composition is overprinted by post-eruptive atmospheric contamination in basalts erupted at ocean islands and mid ocean ridges. The xenon composition of the depleted upper mantle has been defined by the gas rich sample, 2πD43 (also known as "popping rock"), from the North Atlantic (13° 469`N). However, the composition of a single sample is not likely to define the composition of the upper mantle, especially since popping rock has an "enriched" trace element composition. We will present Ne, Ar and Xe isotope data on MORB glass samples with "normal" helium isotope composition (8±1 Ra) from the Southeast Indian Ridge, the South Atlantic Ridge, the Sojourn Ridge, the Juan de Fuca, the East Pacific Rise, and the Gakkel Ridge. Following the approach of [1], we correct for syn- and post-eruptive atmosphere contamination, and determine the variation of Ar and Xe isotope composition of the "normal" MORB source. We investigate the effect of atmospheric recycling in the variation of MORB mantle 40Ar/36Ar and 129Xe/130Xe ratios, and attempt to constrain the average upper mantle argon and xenon isotopic compositions. [1] Mukhopadhyay, Nature 2012; [2] Tucker et al., EPSL (in review); [3] Moreira et al., Nature 1998 [4] Touboul et al., Science 2012.

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 were depleted in Ni relative to melts of pyrolitic peridotite; suggesting they were derived from garnet-free peridotite sources. Neodymium isotopic evidence indicates that the source of alkaline ferropicrites was metasomatically enriched shortly before magma generation (⩽3.0 Ga), but the subalkaline ferropicrites do not show evidence of precursor metasomatism. The metasomatic enrichment of the alkaline ferropicrite sources may have been accompanied by conversion of Fe-rich peridotite to secondary garnet-pyroxenite. Melting experiments on ;pyrolitic; compositions and consideration of the dependence of the density of silicate liquids on pressure and temperature, suggest that ferropicrites cannot originate by melting of normal terrestrial mantle (Mg-number = 0.88-0.92) at high pressures and temperatures. The geochemical similarity between the subalkaline ferropicrites and the shergottite-nakhlite-chassigny (SNC) and howardite-eucrite-diogenite (HED) differentiated meteorites suggests, however, that the Fe-rich mantle may originate from the infall of Fe-rich chondritic meteorites. The occurrence of ca. 2.7 Ga Fe-rich rocks on at least six cratons that are commonly coeval with the more ubiquitous komatiites and Mg-tholeiites is consistent with the existence of heterogeneous Fe-rich ;plums; throughout the Neoarchean mantle. The paucity of ferropicrites in the post-2.7 Ga geological record suggests that majority of these Fe-rich plums have been melted out during the global Neoarchean melting of the mantle.

Chlorine isotope evidence for crustal recycling into the Earth's mantle

NASA Astrophysics Data System (ADS)

John, Timm; Layne, Graham D.; Haase, Karsten M.; Barnes, Jaime D.

2010-09-01

Subduction of oceanic lithosphere is a key feature of terrestrial plate tectonics. However, the effect of this recycled crustal material on mantle composition is debated. Ocean island basalts (OIB) provide direct insights into the composition of Earth's mantle. The distinct composition of the HIMU (high 238U/ 204Pb)- and EM (enriched mantle)-type OIB mantle sources may be due to either recycling of oceanic crust and sediment into the mantle or metasomatic processes within the mantle. Chlorine derived from seawater or crustal fluids potentially provides a tracer for recycled material. Previously reported δ 37Cl values for mid-ocean ridge basalts (MORB) range from ca. - 3.0 to near 0‰. In contrast to MORB, we find a larger variation in OIB glasses representing HIMU- and EM-type mantle sources based on replicate SIMS analyses with δ 37Cl values ranging from - 1.6 to + 1.1‰ for HIMU-type and - 0.4 to + 2.9‰ for EM-type lavas. These δ 37Cl values correlate positively with 87Sr/ 86Sr ratios for both the HIMU- and EM-type samples. The negative δ 37Cl values of some HIMU-type lavas overlap with those of altered oceanic lithosphere, which is assumed to be present in the HIMU source. The EM lavas have high 87Sr/ 86Sr and primarily positive δ 37Cl values. We hypothesize that subducting sediments may have developed high δ 37Cl values by expelling 37Cl-depleted pore fluids, thus accounting for the positive δ 37Cl values recorded in the EM-type lavas.

K-Rich Basaltic Sources beneath Ultraslow Spreading Central Lena Trough in the Arctic Ocean

NASA Astrophysics Data System (ADS)

Ling, X.; Snow, J. E.; Li, Y.

2016-12-01

Magma sources fundamentally influence accretion processes at ultraslow spreading ridges. Potassium enriched Mid-Ocean Ridge Basalt (K-MORB) was dredged from the central Lena Trough (CLT) in the Arctic Ocean (Nauret et al., 2011). Its geochemical signatures indicate a heterogeneous mantle source with probable garnet present under low pressure. To explore the basaltic mantle sources beneath the study area, multiple models are carried out predicting melting sources and melting P-T conditions in this study. P-T conditions are estimated by the experimental derived thermobarometer from Hoang and Flower (1998). Batch melting model and major element model (AlphaMELTs) are used to calculate the heterogeneous mantle sources. The modeling suggests phlogopite is the dominant H2O-K bearing mineral in the magma source. 5% partial melting of phlogopite and amphibole mixing with depleted mantle (DM) melt is consistent with the incompatible element pattern of CLT basalt. P-T estimation shows 1198-1212oC/4-7kbar as the possible melting condition for CLT basalt. Whereas the chemical composition of north Lena Trough (NLT) basalt is similar to N-MORB, and the P-T estimation corresponds to 1300oC normal mantle adiabat. The CLT basalt bulk composition is of mixture of 40% of the K-MORB endmember and an N-MORB-like endmember similar to NLT basalt. Therefore the binary mixing of the two endmembers exists in the CLT region. This kind of mixing infers to the tectonic evolution of the region, which is simultaneous to the Arctic Ocean opening.

Tungsten isotope evidence that mantle plumes contain no contribution from the Earth's core

NASA Astrophysics Data System (ADS)

Scherstén, Anders; Elliott, Tim; Hawkesworth, Chris; Norman, Marc

2004-01-01

Osmium isotope ratios provide important constraints on the sources of ocean-island basalts, but two very different models have been put forward to explain such data. One model interprets 187Os-enrichments in terms of a component of recycled oceanic crust within the source material. The other model infers that interaction of the mantle with the Earth's outer core produces the isotope anomalies and, as a result of coupled 186Os-187Os anomalies, put time constraints on inner-core formation. Like osmium, tungsten is a siderophile (`iron-loving') element that preferentially partitioned into the Earth's core during core formation but is also `incompatible' during mantle melting (it preferentially enters the melt phase), which makes it further depleted in the mantle. Tungsten should therefore be a sensitive tracer of core contributions in the source of mantle melts. Here we present high-precision tungsten isotope data from the same set of Hawaiian rocks used to establish the previously interpreted 186Os-187Os anomalies and on selected South African rocks, which have also been proposed to contain a core contribution. None of the samples that we have analysed have a negative tungsten isotope value, as predicted from the core-contribution model. This rules out a simple core-mantle mixing scenario and suggests that the radiogenic osmium in ocean-island basalts can better be explained by the source of such basalts containing a component of recycled crust.

Geochemical and isotopic investigation of the Laiwu-Zibo carbonatites from western Shandong Province, China, and implications for their petrogenesis and enriched mantle source

NASA Astrophysics Data System (ADS)

Ying, Jifeng; Zhou, Xinhua; Zhang, Hongfu

2004-08-01

Major and trace element and Nd-Sr isotope data of the Mesozoic Laiwu-Zibo carbonatites (LZCs) from western Shandong Province, China, provide clues to the petrogenesis and the nature of their mantle source. The Laiwu-Zibo carbonatites can be petrologically classified as calcio-, magnesio- and ferro-carbonatites. All these carbonatites show a similarity in geochemistry. On the one hand, they are extremely enriched in Ba, Sr and LREE and markedly low in K, Rb and Ti, which are similar to those global carbonatites, on the other hand, they have extremely high initial 87Sr/ 86Sr (0.7095-0.7106) and very low ɛNd (-18.2 to -14.3), a character completely different from those global carbonatites. The small variations in Sr and Nd isotopic ratios suggest that crustal contamination can not modify the primary isotopic compositions of LZC magmas and those values are representatives of their mantle source. The Nd-Sr isotopic compositions of LZCs and their similarity to those of Mesozoic Fangcheng basalts imply that they derived from an enriched lithospheric mantle. The formation of such enriched lithospheric mantle is connected with the major collision between the North China Craton (NCC) and the Yangtze Craton. Crustal materials from the Yangtze Craton were subducted beneath the NCC and melts derived from the subducted crust of the Yangtze Craton produced an enriched Mesozoic mantle, which is the source for the LZCs and Fangcheng basalts. The absence of alkaline silicate rocks, which are usually associated with carbonatites suggest that the LZCs originated from the mantle by directly partial melting.

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

NASA Astrophysics Data System (ADS)

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

2017-11-01

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

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

NASA Astrophysics Data System (ADS)

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

1980-01-01

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

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

NASA Technical Reports Server (NTRS)

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

2007-01-01

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

Three-dimensional crust and mantle structure of Kilauea Volcano, Hawaii

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

Ellsworth, W.L.; Koyanagi, R.Y.

1977-11-10

Teleseismic P wave arrival times recorded by a dense network of seismograph stations located on Kilauea volcano, Hawaii, are inverted to determine lateral variation in crust and upper mantle structure to a depth of 70 km. The crustal structure is dominated by relatively high velocities within the central summit complex and along the two radial rift zones compared with the nonrift flank of the volcano. Both the mean crustal velocity contrast between summit and nonrift flank and the distribution of velocities agree well with results from crustal refraction studies. Comparison of the velocity structure with Bouguer gravity anomalies over themore » volcano through a simple physical model also gives excellent agreement. Mantle structure appears to be more homogeneous than crustal structure. The root mean square velocity variation for the mantle averages only 1.5%, whereas variation within the crust exceeds 4%. The summit of Kilauea is underlain by normal velocity (8.1 km/s) material within the uppermost mantle (12--25 km), suggesting that large magma storage reservoirs are not present at this level and that the passageways from deeper sources must be quite narrow. No evidence is found for substantial volumes of partially molten rock (5%) within the mantle to depths of at least 40 km. Below about 30 km, low-velocity zones (1--2%) underlie the summits of Kilauea and nearby Mauna Loa and extend south of Kilauea into a broad offshore zone. Correlation of volcanic tremor source locations and persistent zones of mantle earthquakes with low-velocity mantle between 27.5- and 42.5-km depth suggests that a laterally extensive conduit system feeds magma to the volcanic summits from sources either at comparable depth or deeper within the mantle. The center of contemporary magmatic production and/or upwelling from deeper in the mantle appears to extend well to the south of the active volcanic summits, suggesting that the Hawaiian Island chain is actively extending to the southeast.« less

Evolution of continental crust and mantle heterogeneity: Evidence from Hf isotopes

USGS Publications Warehouse

Jonathan, Patchett P.; Kouvo, O.; Hedge, C.E.; Tatsumoto, M.

1982-01-01

We present initial 176Hf/177 Hf ratios for many samples of continental crust 3.7-0.3 Gy old. Results are based chiefly on zircons (1% Hf) and whole rocks: zircons are shown to be reliable carriers of essentially the initial Hf itself when properly chosen on the basis of U-Pb studies. Pre-3.0 Gy gneisses were apparently derived from an unfractionated mantle, but both depleted and undepleted mantle are evident as magma sources from 2.9 Gy to present. This mantle was sampled mainly from major crustal growth episodes 2.8, 1.8 and 0.7 Gy ago, all of which show gross heterogeneity of 176Hf/177Hf in magma sources from ??Hf=0 to +14, or about 60% of the variability of the present mantle. The approximate ??Hf=2??Nd relationship in ancient and modern igneous rocks shows that 176Lu/177Hf fractionates in general twice as much as 147Sm/144Nd in mantle melting processes. This allows an estimation of the relative value of the unknown bulk solid/liquid distribution coefficient for Hf. DLu/DHf=??? 2.3 holds for most mantle source regions. For garnet to be an important residual mantle phase, it must hold Hf strongly in order to preserve Hf-Nd isotopic relationships. The ancient Hf initials are consistent with only a small proportion of recycled older cratons in new continental crust, and with quasi-continuous, episodic growth of the continental crust with time. However, recycling of crust less than 150 My old cannot realistically be detected using Hf initials. The mantle shows clearly the general positive ??Hf resulting from a residual geochemical state at least back to 2.9 Gy ago, and seems to have repeatedly possessed a similar degree of heterogeneity, rather than a continuously-developing depletion. This is consistent with a complex dynamic disequilibrium model for the creation, maintenance and destruction of heterogeneity in the mantle. ?? 1981 Springer-Verlag.

The Fine Geochemical Structure of the Hawaiian Mantle Plume: Relation to the Earth's Lowermost Mantle

NASA Astrophysics Data System (ADS)

Weis, D.; Harrison, L.

2017-12-01

The Hawaiian mantle plume has been active for >80 Ma with the highest magmatic flux, also distinctly increasing with time. The identification of two clear geochemical trends (Loa-Kea) among Hawaiian volcanoes in all isotope systems has implications for the dynamics and internal structure of the plume conduit and source in the deep mantle. A compilation of modern isotopic data on Hawaiian shield volcanoes and from the Northwest Hawaiian Ridge (NWHR), focusing specifically on high-precision Pb isotopes integrated with Sr, Nd and Hf isotopes, indicates the presence of source differences for Loa- and Kea-trend volcanoes that are maintained throughout the 1 Ma activity of each volcano. These differences extend back in time on all the Hawaiian Islands ( 5 Ma), and as far back as 47 Ma on the NWHR. In all isotope systems, the Loa-trend basalts are more heterogeneous by a factor of 1.5 than the Kea-trend basalts. The Hawaiian mantle plume overlies the boundary between ambient Pacific lower mantle on the Kea side and the Pacific LLSVP on the Loa side. Geochemical differences between Kea and Loa trends reflect preferential sampling of these two distinct sources of deep mantle material, with additional contribution of ULVZ material sporadically on the Loa side. Plume movement up the gently sloping edge of the LLSVP resulted in entrainment of greater amounts of LLSVP-enriched material over time, and explains why the Hawaiian mantle plume dramatically strengthens over time, contrary to plume models. Similar indications of preferential sampling at the edges of the African LLSVP are found in Kerguelen and Tristan da Cunha basalts in the Indian and Atlantic oceans, respectively. The anomalous low-velocity zones at the core-mantle boundary store geochemical heterogeneities that are enriched in recycled material (EM-I type) with different compositions under the Pacific and under Africa, and that are sampled by strong mantle plumes such as Hawaii and Kerguelen.

Source characteristics and tectonic setting of mafic-ultramafic intrusions in North Xinjiang, NW China: Insights from the petrology and geochemistry of the Lubei mafic-ultramafic intrusion

NASA Astrophysics Data System (ADS)

Chen, Bao-Yun; Yu, Jin-Jie; Liu, Shuai-Jie

2018-05-01

The newly discovered Lubei sulfide-bearing mafic-ultramafic intrusion forms the western extension of the Huangshan-Jin'erquan mafic-ultramafic intrusion belt in East Tianshan, NW China. The Lubei intrusion comprises hornblende peridotite, lherzolite, and harzburgite in its southern portion, gabbro in its middle portion, and hornblende gabbro in its northern portion. Intrusive relationships indicate that three magma pulses were involved in the formation of the intrusion, and that they were likely evolved from a common primitive magma. Estimated compositions of the Lubei primitive magma are similar to those of island arc calc-alkaline basalt except for the low Na2O and CaO contents of the Lubei primitive magma. This paper reports on the mineral compositions, whole-rock major and trace element contents, and Rb-Sr and Sm-Nd isotopic compositions of the Lubei intrusion, and a zircon LA-MC-ICP-MS U-Pb age for hornblende gabbro. The Lubei intrusion is characterized by enrichment in large-ion lithophile elements, depletion in high-field-strength elements, and marked negative Nb and Ta anomalies, with enrichment in chondrite-normalized light rare earth elements. It exhibits low (87Sr/86Sr)i ratios of 0.70333-0.70636 and low (143Nd/144Nd)i ratios of 0.51214-0.51260, with positive εNd values of +4.01 to +6.33. LA-ICP-MS U-Pb zircon ages yielded a weighted-mean age of 287.9 ± 1.6 Ma for the Lubei intrusion. Contemporaneous mafic-ultramafic intrusions in different tectonic domains in North Xinjiang show similar geological and geochemical signatures to the Lubei intrusion, suggesting a source region of metasomatized mantle previously modified by hydrous fluids from the slab subducted beneath the North Xinjiang region in the early Permian. Metasomatism of the mantle was dominated by hydrous fluids and was related to subduction of the Paleo-Asian oceanic lithosphere during the Paleozoic. Sr-Nd-Pb isotopic compositions suggest that the mantle source was a mixture of depleted mid-ocean-ridge-basalt mantle and enriched-mantle I components. The Permian mafic-ultramafic intrusions in North Xinjiang were formed from tholeiitic basaltic magmas derived from decompression partial melting of the metasomatized mantle in a post-collision extensional tectonic setting. The tholeiitic basaltic magmas are equivalent to the voluminous underplated basaltic magmas that formed during vertical crustal growth of the Central Asian Orogenic Belt in the later Paleozoic.

Were komatiites wet?

NASA Astrophysics Data System (ADS)

Arndt, N.; Ginibre, C.; Chauvel, C.; Albarède, F.; Cheadle, M.; Herzberg, C.; Jenner, G.; Lahaye, Y.

1998-08-01

The main arguments used to support the concept that komatiites form by melting of hydrous mantle are as follows: (1) Water reduces liquidus temperatures from extreme values to lower, more “normal” temperatures. (2) Some komatiites are pyroclastic and some contain vesicles, features that have been attributed to magmatic volatiles. (3) It is claimed from experimental studies of peridotite melting that the chemical composition of komatiite requires the presence of water, as does their characteristic spinifex textures. Counterarguments are the following: (1) Loss of volatiles as hydrous komatiite approaches the surface should produce degassing textures and structures, which, though not unknown, are rare in komatiites. Degassing should produce a highly supercooled liquid that partially crystallizes to porphyritic magma; komatiites commonly erupt as phenocryst-poor, highly magnesian lavas. (2) Chemical and isotopic compositions of most komatiites indicate that their mantle source became depleted in incompatible elements soon before magma formation. Such depletion removes water, leaving a dry source. (3) The experimental data are at best ambiguous; neither the chemical composition of komatiites, nor the crystallization of spinifex, requires the presence of water. We conclude that although some rare komatiites may be hydrous, most are dry.

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

NASA Astrophysics Data System (ADS)

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

2017-02-01

The subduction of oceanic slabs is widely accepted to be a main reason for chemical heterogeneities in the mantle. However, determining the contributions of slabs in areas that have experienced multiple subduction events is often difficult due to possible overlapping imprints. Understanding the temporal and spatial variations of source components for widespread intraplate small volume basalts in eastern China may be a basis for investigating the influence of the subducted Pacific slab, which has long been postulated but never confirmed. For this purpose, we investigated the Chaihe-aershan volcanic field (including more than 35 small-volume Quaternary basaltic volcanoes) in NE China and measured the oxygen isotopes and water content of clinopyroxene (cpx) phenocrysts using secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR), respectively. The water content of magma was then estimated based on the partition coefficient of H2O between cpx and the basaltic melt. The δ18O of cpx phenocrysts (4.28‰ to 8.57‰) and H2O content of magmas (0.19 wt.%-2.70 wt.%) show large variations, reflecting the compositional heterogeneity of the mantle source. The δ18O values and H2O content within individual samples also display considerable variation, suggesting the mixing of magmas and that the magma mixing occurred shortly before the eruption. The relation between the δ18O values of cpx phenocrysts and the H2O/Ce ratio, Ba/Th ratio and Eu anomaly of whole rocks demonstrates the contributions of three components to the mantle source (hydrothermally altered upper oceanic crust and marine sediments, altered lower gabbroic oceanic crust, and ambient mantle). The proportions of these three components have varied widely over time (∼1.37 Ma to ∼0.25 Ma). The Pacific slab is constantly subducted under eastern Asia and continuously transports recycled materials to the deep mantle. The temporal heterogeneity of the source components may be caused by ongoing Pacific slab subduction. Combined with other basalt localities in eastern China (Shuangliao basalts, Taihang basalts and Shangdong basalts), the contributions of recycled oceanic components in their mantle source are heterogeneous. This spatial heterogeneity of mantle sources may be induced by variable alterations and dehydration during the recycling process of the Pacific slab. Our results show that the source components of Cenozoic intraplate small-volume basalts in eastern China are temporally and spatially heterogeneous, which is likely induced by the ongoing subduction of the Pacific slab. This demonstrates that integrating the temporal variations in geochemical characteristics and tectonic history of a study region can identify the subducted oceanic plate that induced enriched components in the mantle source of intraplate basalts.

Petrogenesis of metaultramafic rocks from the Quadrilátero Ferrífero and adjacent terrains, Minas Gerais, Brazil: Two events of ultramafic magmatism?

NASA Astrophysics Data System (ADS)

da Fonseca, Gabriela Magalhães; Jordt-Evangelista, Hanna; Queiroga, Gláucia Nascimento

2018-03-01

In the worldwide known Quadrilátero Ferrífero and the adjacent terrains, southeastern Brazil, many serpentinite and soapstone quarries, and some rare bodies of metaultramafic rocks that partially preserve minerals or textures from the original igneous protolith can be found. It is not known if the protoliths and the ages of the metaultramafic rocks found in the Quadrilátero Ferrífero (and its oriental basement) and Mineiro Belt regions are the same or if they represent distinct magmatic episodes. The petrogenetic investigation, specially concerning the REE contents, aimed to gather informations about the type of magmatism and the mantle source in order to compare the metaultramafic rocks of both regions. The interpretation of the data concerning petrography, mineral chemistry and geochemistry shows that the metaultramafic rocks are similar to komatiitic peridotites, with MgO contents > 22 wt % and TiO2 < 0.9 wt %. The plot of the REE for the lithotypes found in the Quadrilátero Ferrífero shows decrease in LREE possibly reflecting the depletion of the mantle source. On the other hand the samples from the Mineiro Belt are enriched in LREE suggesting a mantle source enriched in these elements. This enrichment may have been caused by mantle metassomatism that occurred during accretion of the Paleoproterozoic magmatic arc that generated the Mineiro belt. In this paper, we therefore suggest two periods of ultramafic magmatism. The first one found in the Archean basement of the Quadrilátero Ferrífero, with a depleted mantle source. The second occurred in the Paleoproterozoic basement of the Mineiro belt, having a metassomatized mantle as source.

The Paradox of a Wet (High H2O) and Dry (Low H2O/Ce) Mantle: High Water Concentrations in Mantle Garnet Pyroxenites from Hawaii

NASA Technical Reports Server (NTRS)

Peslier, Anne H.; Bizimis, Michael

2013-01-01

Water dissolved as trace amounts in anhydrous minerals has a large influence on the melting behavior and physical properties of the mantle. The water concentration of the oceanic mantle is inferred from the analyses of Mid-Ocean Ridge Basalt (MORB) and Oceanic Island Basalt (OIB). but there is little data from actual mantle samples. Moreover, enriched mineralogies (pyroxenites, eclogites) are thought as important sources of heterogeneity in the mantle, but their water concentrations and their effect on the water budget and cycling in the mantle are virtually unknown. Here, we analyzed by FTIR water in garnet clinopyroxenite xenoliths from Salt Lake Crater, Oahu, Hawaii. These pyroxenites are high-pressure (>20kb) crystal fractionates from alkalic melts. The clinopyroxenes (cpx) have 260 to 576 ppm wt H2O, with the least differentiated samples (Mg#>0.8) in the 400-500 ppm range. Orthopyroxene (opx) contain 117-265 ppm H2O, about half of that of cpx, consistent with other natural sample studies, but lower than cpx/opx equilibrium from experimental data. The pyroxenite cpx and opx H2O concentrations are at the high-end of on-and off-craton peridotite xenolith concentrations and those of Hawaiian spinel peridotites. In contrast, garnet has extremely low water contents (<5ppm H2O). There is no correlation between H2O in cpx and lithophile element concentrations. Phlogopite is present in some samples, and its modal abundance shows a positive correlation in Mg# with cpx, implying equilibrium. However, there is no correlation between H2O concentrations and or the presence of phlogopite. These data imply that cpx and opx may be at water saturation, far lower than experimental data suggest. Reconstructed bulk rock pyroxenite H2O ranges from 200-460 ppm (average 331 +/- 75 ppm), 2 to 8 times higher than H2O estimates for the MORB source (50-200 ppm), but in the range of E-MORB, OIB and the source of rejuvenated Hawaiian magmas. The average bulk rock pyroxenite H2O/Ce is 69 +/-35, lower than estimates of the MORB source (approx 150) or FOZO, C (200-250) mantle component, but consistent with "dry" EM sources (<100). These data suggest that a metasomatized, refertilized oceanic lithosphere that contains pyroxenitic veins (e.g. the lower part of an oceanic plate, where ascending melts can become trapped and crystallize), will have both higher water concentrations and low H2O/Ce, and may contribute to EM-type OIB sources, like that of Samoa basalts. Therefore, a low H2O/Ce mantle source may not necessarily be "dry".

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

NASA Astrophysics Data System (ADS)

Hamilton, W. B.

2001-12-01

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

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

NASA Astrophysics Data System (ADS)

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

2004-01-01

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

Implications of Nb/U, Th/U and Sm/Nd in plume magmas for the relationship between continental and oceanic crust formation and the development of the depleted mantle

NASA Astrophysics Data System (ADS)

Campbell, Ian H.

2002-05-01

The Nb/U and Th/U of the primitive mantle are 34 and 4.04 respectively, which compare with 9.7 and 3.96 for the continental crust. Extraction of continental crust from the mantle therefore has a profound influence on its Nb/U but little influence on its Th/U. Conversely, extraction of midocean ridge-type basalts lowers the Th/U of the mantle residue but has little influence on its Nb/U. As a consequence, variations in Th/U and Nb/U with Sm/Nd can be used to evaluate the relative importance of continental and basaltic crust extraction in the formation of the depleted (Sm/Nd enriched) mantle reservoir. This study evaluates Nb/U, Th/U, and Sm/Nd variations in suites of komatiites, picrites, and their associated basalts, of various ages, to determine whether basalt and/or continental crust have been extracted from their source region. Emphasis is placed on komatiites and picrites because they formed at high degrees of partial melting and are expected to have Nb/U, Th/U, and Sm/Nd that are essentially the same as the mantle that melted to produce them. The results show that all of the studied suites, with the exception of the Barberton, have had both continental crust and basaltic crust extracted from their mantle source region. The high Sm/Nd of the Gorgona and Munro komatiites require the elevated ratios seen in these suites to be due primarily to extraction of basaltic crust from their source regions, whereas basaltic and continental crust extraction are of subequal importance in the source regions of the Yilgarn and Belingwe komatiites. The Sm/Nd of modern midocean ridge basalts lies above the crustal extraction curve on a plot of Sm/Nd against Nb/U, which requires the upper mantle to have had both basaltic and continental crust extracted from it. It is suggested that the extraction of the basaltic reservoir from the mantle occurs at midocean ridges and that the basaltic crust, together with its complementary depleted mantle residue, is subducted to the core-mantle boundary. When the two components reach thermal equilibrium with their surroundings, the lighter depleted component separates from the denser basaltic component. Both are eventually returned to the upper mantle, but the lighter depleted component has a shorter residence time in the lower mantle than the denser basaltic component. If the difference in the recycling times for the basaltic and depleted components is ˜1.0 to 1.5 Ga, a basaltic reservoir is created in the lower mantle, equivalent to the amount of basalt that is subducted in 1.0 to 1.5 Ga, and that reservoir is isolated from the upper mantle. It is this reservoir that is responsible for the Sm/Nd ratio of the upper mantle lying above the trend predicted by extraction of continental crust on the plot of Sm/Nd against Nb/U.

Melt segregation from partially molten source regions - The importance of melt density and source region size

NASA Technical Reports Server (NTRS)

Stolper, E.; Hager, B. H.; Walker, D.; Hays, J. F.

1981-01-01

An investigation is conducted regarding the changes expected in the density contrast between basic melts and peridotites with increasing pressure using the limited data available on the compressibilities of silicate melts and data on the densities of mantle minerals. It is concluded that since compressibilities of silicate melts are about an order of magnitude greater than those of mantle minerals, the density contrast between basic melts and mantle minerals must diminish significantly with increasing pressure. An earlier analysis regarding the migration of liquid in partially molten source regions conducted by Walker et al. (1978) is extended, giving particular attention to the influence of the diminished density contrast between melt and residual crystals with increasing source region depth and to the influence of source region size. This analysis leads to several generalizations concerning the factors influencing the depths at which magmas will segregate from their source regions and the degrees of partial melting that can be achieved in these source regions before melt segregation occurs.

A probabilistic approach towards understanding how planet composition affects plate tectonics - through time and space.

NASA Astrophysics Data System (ADS)

Stamenkovic, V.

2017-12-01

We focus on the connections between plate tectonics and planet composition — by studying how plate yielding is affected by surface and mantle water, and by variable amounts of Fe, SiC, or radiogenic heat sources within the planet interior. We especially explore whether we can make any robust conclusions if we account for variable initial conditions, current uncertainties in model parameters and the pressure dependence of the viscosity, as well as uncertainties on how a variable composition affects mantle rheology, melting temperatures, and thermal conductivities. We use a 1D thermal evolution model to explore with more than 200,000 simulations the robustness of our results and use our previous results from 3D calculations to help determine the most likely scenario within the uncertainties we still face today. The results that are robust in spite of all uncertainties are that iron-rich mantle rock seems to reduce the efficiency of plate yielding occurring on silicate planets like the Earth if those planets formed along or above mantle solidus and that carbon planets do not seem to be ideal candidates for plate tectonics because of slower creep rates and generally higher thermal conductivities for SiC. All other conclusions depend on not yet sufficiently constrained parameters. For the most likely case based on our current understanding, we find that, within our range of varied planet conditions (1-10 Earth masses), planets with the greatest efficiency of plate yielding are silicate rocky planets of 1 Earth mass with large metallic cores (average density 5500-7000 kg m-3) with minimal mantle concentrations of iron (as little as 0% is preferred) and radiogenic isotopes at formation (up to 10 times less than Earth's initial abundance; less heat sources do not mean no heat sources). Based on current planet formation scenarios and observations of stellar abundances across the Galaxy as well as models of the evolution of the interstellar medium, such planets are suggested to be statistically more common around young stars in the outer disk of the Milky Way. Rocky super-Earths, undifferentiated planets, and still hypothetical carbon planets have the lowest plate yielding efficiencies found in our study. This work aids exoplanet characterization and helps explore the fundamental drivers of plate tectonics.

Oxygen Fugacity Variation From Mantle Transition Zone To Ocean Ridges Recorded By In Situ Diamond-Bearing Peridotite Of Indus Ophiolite

NASA Astrophysics Data System (ADS)

Das, S.; Basu, A. R.

2017-12-01

Our recently discovered transition zone ( 410 - 660 Km) -derived peridotites in the Indus Ophiolite, Ladakh Himalaya [1] provide a unique opportunity to study changes in oxygen fugacity from shallow mantle beneath ocean ridges to mantle transition zone. We found in situ diamond, graphite pseudomorphs after diamond crystals, hydrocarbon (C - H) and hydrogen (H2) fluid inclusions in ultra-high pressure (UHP) peridotites that occur in the mantle - section of the Indus ophiolite and sourced from the mantle transition zone [2]. Diamond occurs as octahedral inclusion in orthoenstatite of one of these peridotites. The graphite pseudomorphs after diamond crystals and primary hydrocarbon (C-H), and hydrogen (H2) fluids are included in olivine of this rock. Hydrocarbon fluids are also present as inclusions in high pressure clinoenstatite (> 8 GPa). The association of primary hydrocarbon and hydrogen fluid inclusions in the UHP peridotites suggest that their source-environment was highly reduced at the base of the upper mantle. We suggest that during mantle upwelling beneath Neo Tethyan spreading center, the hydrocarbon fluid was oxidized and precipitated diamond. The smaller diamonds converted to graphite at shallower depth due to size, high temperature and elevated oxygen fugacity. This process explains how deep mantle upwelling can oxidize reduced fluid carried from the transition zone to produce H2O - CO2. The H2O - CO2 fluids induce deep melting in the source of the mid oceanic ridge basalts (MORB) that create the oceanic crust. References: [1] Das S, Mukherjee B K, Basu A R, Sen K, Geol Soc London, Sp 412, 271 - 286; 2015. [2] Das S, Basu A R, Mukherjee B K, Geology 45 (8), 755 - 758; 2017.

Water Content in the SW USA Mantle Lithosphere: FTIR Analysis of Dish Hill and Kilbourne Hole Pyroxenites

NASA Technical Reports Server (NTRS)

Gibler, Robert; Peslier, Anne H.; Schaffer, Lillian Aurora; Brandon, Alan D.

2014-01-01

Kilbourne Hole (NM, USA) and Dish Hill (CA, USA) mantle xenoliths sample continental mantle in two different tectonic settings. Kilbourne Hole (KH) is located in the Rio Grande rift. Dish Hill (DH) is located in the southern Mojave province, an area potentially affected by subduction of the Farallon plate beneath North America. FTIR analyses were obtained on well characterized pyroxenite, dunite and wehrlite xenoliths, thought to represent crystallized melts at mantle depths. PUM normalized REE patterns of the KH bulk-rocks are slightly LREE enriched and consistent with those of liquids generated by < 5% melting of a spinel peridotite source. Clinopyroxenes contain from 272 to 313 ppm weight H2O similar to the lower limit of KH peridotite clinopyroxenes (250-530 ppm H2O). This is unexpected as crystallized melts like pyroxenites should concentrate water more than residual mantle-like peridotites, given that H is incompatible. PUM normalized bulk REE of the DH pyroxenites are characterized by flat to LREE depleted REE profiles consistent with > 6% melting of a spinel peridotite source. Pyroxenite pyroxenes have no detectable water but one DH wehrlite, which bulk-rock is LREE enriched, has 4 ppm H2O in orthopyroxene and <1ppm in clinopyroxene. The DH pyroxenites may thus come from a dry mantle source, potentially unaffected by the subduction of the Farallon plate. These water-poor melts either originated from shallow oceanic lithosphere overlaying the Farallon slab or from continental mantle formed > 2 Ga. The Farallon subduction appears to have enriched in water the southwestern United States lithospheric mantle further east than DH, beneath the Colorado plateau.

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

NASA Astrophysics Data System (ADS)

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

2012-12-01

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

Mafic dikes at Kahel Tabelbala (Daoura, Ougarta Range, south-western Algeria): New insights into the petrology, geochemistry and mantle source characteristics

NASA Astrophysics Data System (ADS)

Mekkaoui, Abderrahmane; Remaci-Bénaouda, Nacéra; Graïne-Tazerout, Khadidja

2017-09-01

New petrological, geochemical and Sr-Nd isotopic data of the Late Triassic and Early Jurassic Kahel Tabelbala (KT) mafic dikes (south-western Algeria) offer a unique opportunity to examine the nature of their mantle sources and their geodynamic significance. An alkaline potassic Group 1 of basaltic dikes displaying relatively high MgO, TiO2, Cr and Ni, La/YbN ∼ 15, coupled with low 87Sr/86Sri ∼ 0.7037 and relatively high ɛNd(t) ∼ +3, indicates minor olivine and clinopyroxene fractionation and the existence of a depleted mantle OIB source. Their parental magma was generated from partial melting in the garnet-lherzolite stability field. A tholeiitic Group 2 of doleritic dikes displaying low MgO, Cr and Ni contents, La/YbN ∼ 5, positive Ba, Sr and Pb anomalies, the absence of a negative Nb anomaly coupled with moderate 87Sr/86Sri ∼ 0.7044 and low ɛNd(t) ∼ 0 (BSE-like), indicates a contamination of a mantle-derived magma that experienced crystal fractionation of plagioclase and clinopyroxene. This second group, similar to the low-Ti tholeiitic basalts of the Central Atlantic Magmatic Province (CAMP), was derived from partial melting in the peridotite source within the spinel stability field. Lower Mesozoic continental rifting could have been initiated by a heterogeneous mantle plume that supplied source components beneath Daoura, in the Ougarta Range.

Major and trace element, and Sr isotope compositions of clinopyroxene phenocrysts in mafic dykes on Jiaodong Peninsula, southeastern North China Craton: Insights into magma mixing and source metasomatism

NASA Astrophysics Data System (ADS)

Liang, Yayun; Deng, Jun; Liu, Xuefei; Wang, Qingfei; Qin, Cheng; Li, Yan; Yang, Yi; Zhou, Mian; Jiang, Jieyan

2018-03-01

Early Cretaceous mafic dyke swarms are widely developed on Jiaodong Peninsula in the southeastern part of the North China Craton (NCC), but their petrogenesis remains enigmatic. We have examined the in-situ major element, trace element and Sr isotope compositions of the clinopyroxene phenocrysts in these dykes in order to evaluate the extent of magma mixing and source metasomatism. Depending on the type of mineral zoning, the clinopyroxene phenocrysts in our samples can be classified into two groups: Group I (reverse zoning) and Group II (no zoning). Based on core compositions, the Group I phenocrysts with obvious reverse zoning can be divided into two subgroups: Groups IA and IB. The cores of Group IA clinopyroxenes have low values of Mg#, low Al2O3 contents, high Na2O contents, and high 87Sr/86Sr ratios, and they were probably derived from newly accreted lower crust that formed through the underplating of basaltic magma. In contrast, the cores of Group IB clinopyroxenes have lower Mg# values and lower contents of Al2O3, ΣREE (total rare earth elements), and incompatible elements, but they have similar 87Sr/86Sr ratios; these cores crystallised from crust-derived andesitic-dacitic magma. Group IA and IB clinopyroxene phenocryst rims (Group I rims) all have similar compositions with higher values of Mg# and higher Al2O3, Cr and Ni contents than the cores. The rims have high 87Sr/86Sr ratios, are enriched in LREEs (light rare earth elements) and LILEs (large ion lithophile elements), and are depleted in HFSEs (high field strength elements); these characteristics indicate that all the high-Mg rims were derived from a similar magma, possibly a relatively primitive magma derived from lithospheric mantle. We suggest, therefore, that the reversely-zoned clinopyroxene phenocrysts (Group I) in the Jiaodong mafic dykes provide evidence of magma mixing between a magma derived from lithospheric mantle and crust-derived andesitic-dacitic melt alongside with the newly accreted lower crust. The Group II clinopyroxene phenocrysts, which lack zoning, display major and trace element compositions and 87Sr/86Sr ratios that are similar to those of the Group I rims, which indicates that all the high-Mg clinopyroxenes were derived from a common source in the lithospheric mantle. These high-Mg clinopyroxenes exhibit high 87Sr/86Sr ratios, high Sr contents and remarkable depletions in HFSEs, reflecting metasomatism of the mantle source by aqueous fluids derived by dehydration of the subducting slab and its marine sediments. The metasomatism of the source reveals that the lithospheric mantle beneath Jiaodong Peninsula was metasomatised by fluids from the subducting Paleo-Pacific slab. Progressive thinning of the lithosphere mantle under the NCC was induced by continuous thermo-mechanical erosion, promoting the partial melting of lithospheric mantle and generating the mafic dykes at Jiaodong. Table A2 Analytical results for the trace element standards used during LA-ICP-MS analyses of clinopyroxene phenocrysts. Table A3 Analytical results for the Sr isotope standards used during MC-ICP-MS analyses of clinopyroxene phenocrysts. Table A4 Major element contents (wt%) of clinopyroxene phenocrysts from the mafic dykes on Jiaodong Peninsula. Table A5 Representative Sr isotopic compositions of clinopyroxene phenocrysts from the mafic dykes on Jiaodong Peninsula. Table A6 Geochemistry of the mafic dykes on Jiaodong Peninsula. Table A7 Partition coefficients (KD) and end-member components used for REE modeling.

Archean crust-mantle geochemical differentiation

NASA Astrophysics Data System (ADS)

Tilton, G. R.

Isotope measurements on carbonatite complexes and komatiites can provide information on the geochemical character and geochemical evolution of the mantle, including the sub-continental mantle. Measurements on young samples establish the validity of the method. These are based on Sr, Nd and Pb data from the Tertiary-Mesozoic Gorgona komatiite and Sr and Pb data from the Cretaceous Oka carbonatite complex. In both cases the data describe a LIL element-depleted source similar to that observed presently in MORB. Carbonatite data have been used to study the mantle beneath the Superior Province of the Canadian Shield one billion years (1 AE) ago. The framework for this investigation was established by Bell et al., who showed that large areas of the province appear to be underlain by LIL element-depleted mantle (Sr-85/Sr-86=0.7028) at 1 AE ago. Additionally Bell et al. found four complexes to have higher initial Sr ratios (Sr-87/Sr-86=0.7038), which they correlated with less depleted (bulk earth?) mantle sources, or possibly crustal contamination. Pb isotope relationships in four of the complexes have been studied by Bell et al.

Archean crust-mantle geochemical differentiation

NASA Technical Reports Server (NTRS)

Tilton, G. R.

1983-01-01

Isotope measurements on carbonatite complexes and komatiites can provide information on the geochemical character and geochemical evolution of the mantle, including the sub-continental mantle. Measurements on young samples establish the validity of the method. These are based on Sr, Nd and Pb data from the Tertiary-Mesozoic Gorgona komatiite and Sr and Pb data from the Cretaceous Oka carbonatite complex. In both cases the data describe a LIL element-depleted source similar to that observed presently in MORB. Carbonatite data have been used to study the mantle beneath the Superior Province of the Canadian Shield one billion years (1 AE) ago. The framework for this investigation was established by Bell et al., who showed that large areas of the province appear to be underlain by LIL element-depleted mantle (Sr-85/Sr-86=0.7028) at 1 AE ago. Additionally Bell et al. found four complexes to have higher initial Sr ratios (Sr-87/Sr-86=0.7038), which they correlated with less depleted (bulk earth?) mantle sources, or possibly crustal contamination. Pb isotope relationships in four of the complexes have been studied by Bell et al.

Midcontinent rift volcanism in the Lake Superior region: Sr, Nd, and Pb isotopic evidence for a mantle plume origin

USGS Publications Warehouse

Nicholson, S.W.; Shirey, S.B.

1990-01-01

Between 1091 and 1098 Ma, most of a 15- to 20-km thickness of dominantly tholeiitic basalt erupted in the Midcontinent Rift System of the Lake Superior region, North America. The Portage Lake Volcanics in Michigan, which are the younget MRS flood basalts, fall into distinctly high- and low-TiO2 types having different liquid lines of descent. Incompatible trace elements in both types of tholeiites are enriched compared to depleted or primitive mantle and both basalt types are isotopically indistinguishable. The isotopic enrichment of the MRS source compared to depleted mantle is striking and must have occurred at least 700 m.y. before 1100 Ma. There are two likely sources for such magmatism: subcontinental lithospheric mantle enriched during the early Proterozoic or enriched mantle derived from an upwelling plume. Decompression melting of an upwelling enriched mantle plume in a region of lithosphere thinned by extension could have successfully generated the enormous volume (850 ?? 103 km3) of relatively homogeneous magma in a restricted time interval. -from Authors

Scattering - a probe to Earth's small scale structure

NASA Astrophysics Data System (ADS)

Rost, S.; Earle, P.

2009-05-01

Much of the short-period teleseismic wavefield shows strong evidence for scattered waves in extended codas trailing the main arrivals predicted by ray theory. This energy mainly originates from high-frequency body waves interacting with fine-scale volumetric heterogeneities in the Earth. Studies of this energy revealed much of what we know about Earth's structure at scale lengths around 10 km throughout the Earth from crust to core. From these data we can gain important information about the mineral-physical and geochemical constitution of the Earth that is inaccessible to many other seismic imaging techniques. Previous studies used scattered energy related to PKP, PKiKP, and Pdiff to identify and map the small-scale structure of the mantle and core. We will present observations related to the core phases PKKP and P'P' to study fine-scale mantle heterogeneities. These phases are maximum travel-time phases with respect to perturbations at their reflection points. This allows observation of the scattered energy as precursors to the main phase avoiding common problems with traditional coda phases which arrive after the main pulse. The precursory arrival of the scattered energy allows the separation between deep Earth and crustal contributions to the scattered wavefield for certain source-receiver configurations. Using the information from these scattered phases we identify regions of the mantle that shows increased scattering potential likely linked to larger scale mantle structure identified in seismic tomography and geodynamical models.

Geochemistry of the Seamounts at the Southeast Chatham Rise, New Zealand

NASA Astrophysics Data System (ADS)

Jolis, E. M.; Hoernle, K.; Hauff, F.; Garbe-Schönberg, D.; Werner, R.; Gohl, K.

2017-12-01

The submarine Chatham Rise, east Zealandia, is a key location of the early continental breakup of the eastern Gondwana (< 100 Ma; [1]). It has been suggested that a mantle plume beneath Zealandia and West Antarctica existed and that a slab window formed as a consequence of the collision of the Hikurangi oceanic plateau with the Chatham Rise, allowing deeper mantle material to upwell and hence cause the rifting. However, the exact processes that have led to this rifting and the sequence of reorganization in the upper mantle in course of and after the breakup of Zealandia from West Antarctica are still unclear. We present new major and trace element and Sr-Nd and high-precision Pb isotope data from submarine samples recovered during the R/V Sonne research expedition SO246 at the southeast Chatham Rise, covering the Chatham Rise Terrace and adjacent areas of the margin and the abyssal plain. The samples include alkali and tholeiitic basalts and minor basanite and trachybasalt, all of which have a composition between ocean island basalt (OIB) and mid-ocean-ridge basalt (MORB). Trace element ratios (e.g., Th/Yb, Nb/Yb) indicate that all but one seamount were derived from enriched sources at a low degree of melting, while one of the seamounts close to the abyssal plain was derived from a depleted mantle source at a high degree of melting. Sr-Nd-Pb isotope variations further support contribution of at least three distinct mantle source components, including a HIMU (high time-integrated U/Pb)-type sources, an enriched mantle (EM)-type sources, and a depleted mantle (N-MORB)-type source. These observations appear to be consistent with previous published data and models proposed by [2] and [3]. These sources will be placed in a chronological framework by incorporating further geochemical data and 40Ar-39Ar ages, providing us better insights into the sequence of events and magmatic processes that occurred at this region. References:[1] Davy et al. (2008), Hikurangi Plateau: Crustal structure, rifted formation, and Gondwana subduction history, G3, 9, Q07004. [2] Hoernle et al. (2006), Cenozoic intraplate volcanism on New Zealand: Upwelling induced by lithospheric removal, EPSL, 248, 350-367. [3] Timm et al. (2010), Temporal and geochemical evolution of the Cenozoic intraplate volcanism of Zealandia, Earth-Sci. Rev., 98, 38-64.

Subduction-zone magnetic anomalies and implications for hydrated forearc mantle

USGS Publications Warehouse

Blakely, R.J.; Brocher, T.M.; Wells, R.E.

2005-01-01

Continental mantle in subduction zones is hydrated by release of water from the underlying oceanic plate. Magnetite is a significant byproduct of mantle hydration, and forearc mantle, cooled by subduction, should contribute to long-wavelength magnetic anomalies above subduction zones. We test this hypothesis with a quantitative model of the Cascadia convergent margin, based on gravity and aeromagnetic anomalies and constrained by seismic velocities, and find that hydrated mantle explains an important disparity in potential-field anomalies of Cascadia. A comparison with aeromagnetic data, thermal models, and earthquakes of Cascadia, Japan, and southern Alaska suggests that magnetic mantle may be common in forearc settings and thus magnetic anomalies may be useful in mapping hydrated mantle in convergent margins worldwide. ?? 2005 Geological Society of America.

Olivine inclusions in Siberian diamonds and mantle xenoliths: Contrasting water and trace-element contents

NASA Astrophysics Data System (ADS)

Jean, M. M.; Taylor, L. A.; Howarth, G. H.; Peslier, A. H.; Fedele, L.; Bodnar, R. J.; Guan, Y.; Doucet, L. S.; Ionov, D. A.; Logvinova, A. M.; Golovin, A. V.; Sobolev, N. V.

2016-11-01

A subject of continuing debate is how the Earth's lithospheric portion of the upper mantle has remained the thickest (> 200 km) and oldest (> 3 Gy) beneath cratons and is yet surrounded by a vigorously convecting asthenosphere. It is generally admitted that water is a key parameter in the strength and longevity of cratonic roots, because olivine, the main phase of the lithospheric mantle, becomes stronger if its water content decreases. Expanding upon the work presented in Novella et al. (2015) and Taylor et al. (2016), we report new water contents for additional olivine inclusions in diamonds together with the trace-element composition for all olivine inclusions, as well as for mantle xenoliths from various kimberlite pipes located on the Siberian craton. The olivine diamond inclusions from this study have systematically low-water contents (< 50 ppmw H2O), moderate to high forsterite (e.g., Fo91-94) contents and low Ni, Co, and Zn ppm contents (e.g., < 2848, < 108, and < 47 ppm, respectively). In contrast, olivines from Siberian craton mantle xenoliths have a wide range of water contents (6-323 ppmw H2O) and extend to lower-Fo (91-92), Ni, Co, and Zn-rich compositions, compared to the diamond inclusions. Depleted incompatible trace-element concentrations in olivine (0.1-0.001 × Primitive Mantle) advance our hypothesis for the protogenetic origins for the majority of Siberian diamond inclusions. These observations are consistent with the peridotite xenoliths as representing a part of the cratonic lithosphere that has experienced melt re-fertilization, which has also transported water. The olivine diamond inclusions, on the other hand, preserve "micro-samples" of an initial, dry cratonic lithosphere, mostly resulting from melting events. These inclusions are likely sourced from the initial cratonic mantle lithosphere, which thereby, resisted delamination over time, due to its buoyancy and strength, imparted from melt and water depletion, respectively. And thus, our data provides a major argument that the kimberlite-hosted mantle xenoliths may be more metasomatized than common rocks at the base of the Siberian and other cratonic roots away from kimberlite fields.

The oxygen isotope composition of Karoo and Etendeka picrites: High δ18O mantle or crustal contamination?

NASA Astrophysics Data System (ADS)

Harris, Chris; le Roux, Petrus; Cochrane, Ryan; Martin, Laure; Duncan, Andrew R.; Marsh, Julian S.; le Roex, Anton P.; Class, Cornelia

2015-07-01

Oxygen isotope compositions of Karoo and Etendeka large igneous province (LIP) picrites and picrite basalts are presented to constrain the effects of crustal contamination versus mantle source variation. Olivine and orthopyroxene phenocrysts from lavas and dykes (Mg# 64-80) from the Tuli and Mwenezi (Nuanetsi) regions of the ca 180 Ma Karoo LIP have δ18O values that range from 6.0 to 6.7 ‰. They appear to have crystallized from magmas having δ18O values about 1-1.5 ‰ higher than expected in an entirely mantle-derived magma. Olivines from picrite and picrite basalt dykes from the ca 135 Ma Etendeka LIP of Namibia and Karoo-age picrite dykes from Dronning Maud Land, Antarctica, do not have such elevated δ18O values. A range of δ18O values from 4.9 to 6.0 ‰, and good correlations between δ18O value and Sr, Nd and Pb isotope ratios for the Etendeka picrites are consistent with previously proposed models of crustal contamination. Explanations for the high δ18O values in Tuli/Mwenezi picrites are limited to (1) alteration, (2) crustal contamination, and (3) derivation from mantle with an abnormally high δ18O. Previously, a variety of models that range from crustal contamination to derivation from the `enriched' mantle lithosphere have been suggested to explain high concentrations of incompatible elements such as K, and average ɛNd and ɛSr values of -8 and +16 in Mwenezi (Nuanetsi) picrites. However, the primitive character of the magmas (Mg# 73), combined with the lack of correlation between δ18O values and radiogenic isotopic compositions, MgO content, or Mg# is inconsistent with crustal contamination. Thus, an 18O-enriched mantle source having high incompatible trace element concentration and enriched radiogenic isotope composition is indicated. High δ18O values are accompanied by negative Nb and Ta anomalies, consistent with the involvement of the mantle lithosphere, whereas the high δ18O themselves are consistent with an eclogitic source. Magma δ18O values about 1 ‰ higher than expected for mantle-derived magma are also a feature of the Bushveld mafic and ultramafic magmas, and the possibility exists that a long-lived 18O-enriched mantle source has existed beneath southern Africa. A mixed eclogite peridotite source could have developed by emplacement of oceanic lithosphere into the cratonic keel during Archaean subduction.

Petrologic Modeling of Magmatic Evolution in The Elysium Volcanic Province

NASA Astrophysics Data System (ADS)

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

2017-12-01

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

Three-Dimensional Numerical Simulation on Passively Excited Flows by Distributed Local Hot Sources Settled at the D" Layer Below Hotspots and/or Large-Scale Cool Masses at Subduction Zones Within the Static Layered Mantle

NASA Astrophysics Data System (ADS)

Eguchi, T.; Matsubara, K.; Ishida, M.

2001-12-01

To unveil dynamic process associated with three-dimensional unsteady mantle convection, we carried out numerical simulation on passively exerted flows by simplified local hot sources just above the CMB and large-scale cool masses beneath smoothed subduction zones. During the study, we used our individual code developed with the finite difference method. The basic three equations are for the continuity, the motion with the Boussinesq (incompressible) approximation, and the (thermal) energy conservation. The viscosity of our model is sensitive to temperature. To get time integration with high precision, we used the Newton method. In detail, the size and thermal energy of the hot or cool sources are not uniform along the latitude, because we could not select uniform local volumes assigned for the sources within the finite difference grids throughout the mantle. Our results, thus, accompany some latitude dependence. First, we treated the case of the hotspots, neglecting the contribution of the subduction zones. The local hot sources below the currently active hotspots were settled as dynamic driving forces included in the initial condition. Before starting the calculation, we assumed that the mantle was statically layered with zero velocity component. The thermal anomalies inserted instantaneously in the initial condition do excite dynamically passive flows. The type of the initial hot sources was not 'plume' but 'thermal.' The simulation results represent that local upwelling flows which were directly excited over the initial heat sources reached the upper mantle by approximately 30 My during the calculation. Each of the direct upwellings above the hotspots has its own dynamic potential to exert concentric down- and up-welling flows, alternately, at large distances. Simultaneously, the direct upwellings interact mutually within the spherical mantle. As an interesting feature, we numerically observed secondary upwellings somewhere in a wide region covering east Eurasia to the Bering Sea where no hot sources were initially input. It seems that the detailed location of the secondary upwellings depends partly on the numerical parameters such as the radial profile of mantle viscosity especially at the D" layer, etc., because the secondary flows are provoked by dynamic interaction among the distributed direct upwellings just above the CMB. Our results suggest that if we assume not only non-zero time delays during the input of the local hot sources but also parameters related with the difference of their historical surface flux rates, the pattern of the passively excited flows will be different from that obtained with the simultaneously settled hot sources stated above. Second, we simultaneously incorporated simplified thermal anomaly models associated with both the distributed local hotspots and the global subduction zones, as dynamic origins in the initial condition for the static layered mantle. In this case, the simulation result represents that the pattern of secondary radial flows, being different from those in the earlier case, is sensitive to the relative strength between the positive dynamic buoyancy integrated over all of the local hot sources below the hotspots and the total negative buoyancy beneath the subduction zones.

Osmium isotope constraints on ore metal recycling in subduction zones

PubMed

McInnes; McBride; Evans; Lambert; Andrew

1999-10-15

Veined peridotite xenoliths from the mantle beneath the giant Ladolam gold deposit on Lihir Island, Papua New Guinea, are 2 to 800 times more enriched in copper, gold, platinum, and palladium than surrounding depleted arc mantle. Gold ores have osmium isotope compositions similar to those of the underlying subduction-modified mantle peridotite source region, indicating that the primary origin of the metals was the mantle. Because the mantle is relatively depleted in gold, copper, and palladium, tectonic processes that enhance the advective transport and concentration of these fluid soluble metals may be a prerequisite for generating porphyry-epithermal copper-gold deposits.

Forest habitat associations of the golden-mantled ground squirrel: Implications for fuels management

Treesearch

Katharine R. Shick; Dean E. Pearson; Leonard F. Ruggiero

2006-01-01

Golden-mantled ground squirrels are commonly associated with high-elevation habitats near or above upper timberline. This species also occurs in fire-adapted, low-elevation forests that are targeted for forest health restoration (FHR) treatments intended to remove encroaching understory trees and thin overstory trees. Hence, the golden-mantled ground squirrel...

The paradox of a wet (high H2O) and dry (low H2O/Ce) mantle: High water concentrations in mantle garnet pyroxenites from Hawaii

NASA Astrophysics Data System (ADS)

Bizimis, M.; Peslier, A. H.

2013-12-01

Water dissolved as trace amounts in anhydrous minerals has a large influence on the melting behavior and physical properties of the mantle. The water concentration of the oceanic mantle is inferred from the analyses of MORB and OIB [1], but there is little data from actual mantle samples. Moreover, enriched mineralogies (pyroxenites, eclogites) are thought as important sources of heterogeneity in the mantle, but their water concentrations and their effect on the water budget and cycling in the mantle are virtually unknown. We analyzed by FTIR water concentrations in garnet clinopyroxenite xenoliths from Salt Lake Crater, Oahu, Hawaii. These pyroxenites are high-pressure (>20kb) crystal fractionates from alkalic melts. The clinopyroxenes (cpx) have 260 to 576 ppm wt. H2O, with the least differentiated samples (Mg#>0.8) in the 400-500 ppm range. Orthopyroxene (opx) contain 117-265 ppm H2O, about half of that of cpx, consistent with other natural sample studies, but lower than experimental cpx/opx equilibrium data. These pyroxenite cpx and opx water concentrations are at the high-end of on-and off-craton peridotite xenolith concentrations and megacrysts from kimberites [2] and those of Hawaiian spinel peridotites. In contrast, garnet has extremely low water contents (<5ppm H2O). There is no correlation between water in cpx and lithophile element concentrations. Phlogopite is present in some samples, and its modal abundance shows a positive correlation in Mg# with cpx, implying equilibrium. However, there is no correlation between water concentrations and the presence of phlogopite. These data imply that cpx and opx water concentrations may be buffered by phlogopite crystallization. Reconstructed bulk rock pyroxenite water concentrations (not including phlogopite, i.e. minimum) range from 200-460 ppm (average 331× 75 ppm), significantly higher than water estimates for the MORB source (50-200 ppm), but in the range of E-MORB, OIB and the source of rejuvenated Hawaiian magmas [1,3]. The average bulk rock pyroxenite H2O/Ce is 69 × 35, lower than estimates of the MORB source (~150) or FOZO, C (200-250) mantle component, but consistent with 'dry' EM sources (<100) [1]. These data suggest that a metasomatized, refertilized oceanic lithosphere that contains a pyroxenite component (e.g. in the lower part of an oceanic plate, where ascending melts can become trapped and crystallize), will have both higher water concentrations and low H2O/Ce, and may contribute to EM-type OIB sources, like that of Samoan basalts [5]. Therefore, a low H2O/Ce mantle source may not necessarily be 'dry'. [1] Dixon et al., 2002, Nature 420, 385-389. [2] Peslier, 2010 JVGR 197, 239-258. [3] Dixon et al., 1997 JP 38, 911-939. [4] O'Leary et al. 2010 EPSL 297, 111-120. [5] Workman et al., 2006 EPSL 241, 932 - 951.

Sims Analysis of Water Abundance and Hydrogen Isotope in Lunar Highland Plagioclase

NASA Technical Reports Server (NTRS)

Hui, Hejiu; Guan, Yunbin; Chen, Yang; Peslier, Anne H.; Zhang, Youxue; Liu, Yang; Rossman, George R.; Eiler, John M.; Neal, Clive R.

2015-01-01

The detection of indigenous water in mare basaltic glass beads has challenged the view established since the Apollo era of a "dry" Moon. Since this discovery, measurements of water in lunar apatite, olivine-hosted melt inclusions, agglutinates, and nominally anhydrous minerals have confirmed that lunar igneous materials contain water, implying that some parts of lunar mantle may have as much water as Earth's upper mantle. The interpretation of hydrogen (H) isotopes in lunar samples, however, is controversial. The large variation of H isotope ratios in lunar apatite (delta Deuterium = -202 to +1010 per mille) has been taken as evidence that water in the lunar interior comes from the lunar mantle, solar wind protons, and/or comets. The very low deuterium/H ratios in lunar agglutinates indicate that solar wind protons have contributed to their hydrogen content. Conversely, H isotopes in lunar volcanic glass beads and olivine-hosted melt inclusions being similar to those of common terrestrial igneous rocks, suggest a common origin for water in both Earth and Moon. Lunar water could be inherited from carbonaceous chondrites, consistent with the model of late accretion of chondrite-type materials to the Moon as proposed by. One complication about the sources of lunar water, is that geologic processes (e.g., late accretion and magmatic degassing) may have modified the H isotope signatures of lunar materials. Recent FTIR analyses have shown that plagioclases in lunar ferroan anorthosite contain approximately 6 ppm H2O. So far, ferroan anorthosite is the only available lithology that is believed to be a primary product of the lunar magma ocean (LMO). A possible consequence is that the LMO could have contained up to approximately 320 ppm H2O. Here we examine the possible sources of water in the LMO through measurements of water abundances and H isotopes in plagioclase of two ferroan anorthosites and one troctolite from lunar highlands.

Life Cycle of Mantle Plumes: A perspective from the Galapagos Plume (Invited)

NASA Astrophysics Data System (ADS)

Gazel, E.; Herzberg, C. T.

2009-12-01

Hotspots are localized sources of heat and magmatism considered as modern-day evidence of mantle plumes. Some hotspots are related to massive magmatic production that generated Large Igneous Provinces (LIPS), an initial-peak phase of plume activity with a mantle source hotter and more magmatically productive than present-day hotspots. Geological mapping and geochronological studies have shown much lower eruption rates for OIB compared to lavas from Large Igneous Provinces LIPS such as oceanic plateaus and continental flood provinces. Our study is the first quantitative petrological comparison of mantle source temperatures and extent of melting for OIB and LIP sources. The wide range of primary magma compositions and inferred mantle potential temperatures for each LIP and OIB occurrence suggest that this rocks originated form a hotspot, a spatially localized source of heat and magmatism restricted in time. Extensive outcrops of basalt, picrite, and sometimes komatiite with circa 65-95 Ma ages occupy portions of the pacific shore of Central and South America included in the Caribbean Large Igneous Province (CLIP). There is general consensus of a Pacific-origin of CLIP and most studies suggest that it was produced by melting in the Galapagos mantle plume. The Galapagos connection is consistent with isotopic and geochemical similarities with lavas from the present-day Galapagos hotspot. A Galapagos link for rocks in South American oceanic complexes (eg. the island of Gorgona) is more controversial and requires future work. The MgO and FeO contents of lavas from the Galapagos related lavas and their primary magmas have decreased since the Cretaceous. From petrological modeling we infer that these changes reflect a cooling of the Galapagos mantle plume from a potential temperature of 1560-1620 C in the Cretaceous to 1500 C at the present time. These temperatures are higher than 1350 C for ambient mantle associated with oceanic ridges, and provide support for the mantle plume model of the CLIP. The exact form of the secular cooling curve depends on whether the Gorgona komatiites were produced by the Galapagos or another plume. Iceland also exhibits secular cooling, in agreement with previous studies. In general, mantle plumes for LIPS with Paleocene-Permian ages were hotter and melted more extensively than plumes of more modern oceanic islands. This is interpreted to reflect episodic flow from lower mantle domains that are lithologically and geochemically heterogeneous. The majority of lavas from the present-day Galapagos plume formed in a column where melting ended at pressures less than 2 GPa, and this pressure is highly variable. Melting ended at much lower pressures for lavas from the Cocos and Carnegie Ridges, consistent with the channeling of the Galapagos plume to locations of thinner lithosphere. Low pressures of final melting are also inferred for older CLIP lavas, which suggest that the plume head impacted a mid-ocean ridge system.

Sodium Inverse Relationships During Melting in Ultraslow Spreading Regions: Insights from SWIR-Smoothseafloor Peridotites

NASA Astrophysics Data System (ADS)

Cannat, M.; Brunelli, D.; Paquet, M.; Sforna, M. C.; Seyler, M.

2015-12-01

Ultraslow spreading ridges are key regions to unravel mantle processes. Low potential temperatures and reduced melting allow decrypting early melting processes and shad lights on the source short-scale heterogeneities and their interactions with transient melts. Mantle-derived peridotites from the Smoothseafloor region of the eastern Southwest Indian Ridge reveal countertrending Na-Ti relationships. Na apparently behaves as a compatible element during partial melting similarly to light REEs. Heavy REEs, however, follow a normal relationship with the other melting indicators (e.g. Cr#), a behaviour that results in pattern rotation around a pivot element when looking to REE systematic. These relationships can be explained by percolation of relatively enriched, grt-field derived, melts in the spinel-field melting mantle 1. A feature that also explains the inverse Na-Cr# correlation, frequently observed in abyssal mantle rocks. Experimental relationships constraint the grt-field derived melts to be produced by low-melting paragenesis that experience a garnet to spinel phase transition shallower than mantle peridotites for a given temperature. Based on potential mantle temperatures estimated by Cannat et al., 19992, the grt-sp transition can be set at ca. 2.0 and 1.5 GPa for mantle peridotites and Mg pyroxenites respectively with the onset of mantle melting at 1.2 GPa. Mass balance calculations based on the amount of produced melt constrains the pyroxenitic fraction < 10% by mass of the mantle source. The contemporaneous presence of lithologies too depleted with respect to the described process suggests that some portions of the mantle source are inherited from more sustained ancient depletion events not related to present-day processes beneath this ridge portion. PNRA funding : PdR 2013/B1.02 1. Brunelli, D., et al., 2104. Percolation of enriched melts during incremental open-system melting in the spinel field : A REE approach to abyssal peridotites from the Southwest Indian Ridge. Geochim. Cosmochim. Acta 127,190-203. 2. Cannat, M., et al., 1999. Formation of the axial relief at the very slow spreading Southwest Indian Ridge (49° to 69°E). J. Geophys. Res. 104, 22825-22843.

Inferring global upper-mantle shear attenuation structure by waveform tomography using the spectral element method

NASA Astrophysics Data System (ADS)

Karaoǧlu, Haydar; Romanowicz, Barbara

2018-06-01

We present a global upper-mantle shear wave attenuation model that is built through a hybrid full-waveform inversion algorithm applied to long-period waveforms, using the spectral element method for wavefield computations. Our inversion strategy is based on an iterative approach that involves the inversion for successive updates in the attenuation parameter (δ Q^{-1}_μ) and elastic parameters (isotropic velocity VS, and radial anisotropy parameter ξ) through a Gauss-Newton-type optimization scheme that employs envelope- and waveform-type misfit functionals for the two steps, respectively. We also include source and receiver terms in the inversion steps for attenuation structure. We conducted a total of eight iterations (six for attenuation and two for elastic structure), and one inversion for updates to source parameters. The starting model included the elastic part of the relatively high-resolution 3-D whole mantle seismic velocity model, SEMUCB-WM1, which served to account for elastic focusing effects. The data set is a subset of the three-component surface waveform data set, filtered between 400 and 60 s, that contributed to the construction of the whole-mantle tomographic model SEMUCB-WM1. We applied strict selection criteria to this data set for the attenuation iteration steps, and investigated the effect of attenuation crustal structure on the retrieved mantle attenuation structure. While a constant 1-D Qμ model with a constant value of 165 throughout the upper mantle was used as starting model for attenuation inversion, we were able to recover, in depth extent and strength, the high-attenuation zone present in the depth range 80-200 km. The final 3-D model, SEMUCB-UMQ, shows strong correlation with tectonic features down to 200-250 km depth, with low attenuation beneath the cratons, stable parts of continents and regions of old oceanic crust, and high attenuation along mid-ocean ridges and backarcs. Below 250 km, we observe strong attenuation in the southwestern Pacific and eastern Africa, while low attenuation zones fade beneath most of the cratons. The strong negative correlation of Q^{-1}_μ and VS anomalies at shallow upper-mantle depths points to a common dominant origin for the two, likely due to variations in thermal structure. A comparison with two other global upper-mantle attenuation models shows promising consistency. As we updated the elastic 3-D model in alternate iterations, we found that the VS part of the model was stable, while the ξ structure evolution was more pronounced, indicating that it may be important to include 3-D attenuation effects when inverting for ξ, possibly due to the influence of dispersion corrections on this less well-constrained parameter.

A Comparison of Methods for Modeling Geochemical Variability in the Earth's Mantle

NASA Astrophysics Data System (ADS)

Kellogg, J. B.; Tackley, P. J.

2004-12-01

Numerial models of isotopic and chemical heterogeneity of the Earth's mantle fall into three categories, in decreasing order of computational demand. First, several authors have used chemical tracers within a full thermo-chemical convection calculation (e.g., Christensen and Hofmann, 1994, van Keken and Ballentine, 1999; Xie and Tackley, 2004). Second, Kellogg et al. (2002) proposed an extension of the traditional geochemical box model calculations in which numerous subreservoirs were tracked within the bulk depleted mantle reservoir. Third, Allègre and Lewin (1995) described a framework in which the variance in chemical and isotopic ratios were treated as quantities intrinsic to the bulk reservoirs, complete with sources and sinks. Results from these three methods vary, particularly with respect to conclusions drawn about the meaning of the Pb-Pb pseudo-isochron. We revisit these methods in an attempt to arrive at a common understanding. By considering all three we better identify the strengths and weaknesses of each approach and allow each to inform the other. Finally, we present results from a new hybrid model that combines the complexity and regional-scale variability of the thermochemical convection models with the short length-scale sensitivity of the Kellogg et al. approach.

Within-plate Cenozoic Volcanism and Mantle Sources Within The Western-central Mediterranean Area

NASA Astrophysics Data System (ADS)

Beccaluva, L.; Bianchini, G.; Bonadiman, C.; Coltorti, M.; Siena, F.

An integrated study of anorogenic basic magmas and entrained mantle xenoliths rep- resents a promising approach for a comprehension of the magmatogenic events occur- ring within the lithospheric mantle in the western-central Mediterranean area. In this contribution we review the geochemical characteristics of mafic lavas and associated peridotite xenoliths from three anorogenic volcanic districts: Pliocene-Quaternary vol- canism of Sardinia; Pliocene-Quaternary volcanism of the Iblean area (eastern Sicily); Paleocene-Oligocene Veneto Volcanic Province. Investigations have been focused on 1) petrological features of parental magmas, which may contribute to infer the com- positional characteristics of mantle sources and to constrain the modes of partial melt- ing; 2) modelling the depletion events and metasomatic enrichments in mantle xeno- liths of the three volcanic districts, as well as the nature of their causative agents. Petrological features and Sr-Nd-Pb isotopic data, both of lava and xenoliths, indicate that DM+HIMU components distinguish the lithospheric mantle sections of Iblean and Veneto Volcanic Provinces. On the other hand, lavas and xenoliths from Sardinia display a significant different isotopic signature characterised by DM+EM1. Similar geochemical fingerprints, i.e. the significant presence of EM components are gener- ally recorded by mafic lavas and mantle xenoliths from the European Plate, whereas they are not observed in the stable African lithospheric domain.

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

NASA Astrophysics Data System (ADS)

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

2004-12-01

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

Volatiles in the Earth and Moon: Constraints on planetary formation and evolution

NASA Astrophysics Data System (ADS)

Parai, Rita

The volatile inventories of the Earth and Moon reflect unique histories of volatile acquisition and loss in the early Solar System. The terrestrial volatile inventory was established after the giant impact phase of accretion, and the planet subsequently settled into a regime of long-term volatile exchange between the mantle and surface reservoirs in association with plate tectonics. Therefore, volatiles in the Earth and Moon shed light on a diverse array of processes that shaped planetary bodies in the Solar System as they evolved to their present-day states. Here we investigate new constraints on volatile depletion in the early Solar System, early outgassing of the terrestrial mantle, and the long-term evolution of the deep Earth volatile budget. We develop a Monte Carlo model of long-term water exchange between the mantle and surface reservoirs. Previous estimates of the deep Earth return flux of water are up to an order of magnitude too large, and incorporation of recycled slabs on average rehydrates the upper mantle but dehydrates the plume source. We find evidence for heterogeneous recycling of atmospheric argon and xenon into the upper mantle from noble gases in Southwest Indian Ridge basalts. Xenon isotope systematics indicate that xenon budgets of mid-ocean ridge and plume-related mantle sources are dominated by recycled atmospheric xenon, though the two sources have experienced different degrees of degassing. Differences between the mid-ocean ridge and plume sources were initiated within the first 100 million years of Earth history, and the two sources have never subsequently been homogenized. New high-precision xenon isotopic data contribute to an emerging portrait of two mantle reservoirs with distinct histories of outgassing and incorporation of recycled material in association with plate tectonics. Xenon isotopes indicate that the Moon likely formed within ˜70 million years of the start of the Solar System. To further investigate early Solar System chronology, we determined strontium isotopic compositions in a suite of planetary materials. If the Moon is derived from proto-Earth material, then rubidium-strontium systematics in the lunar anorthosite 60025 and Moore County plagioclase indicate that Moon formation occurred within ~62 million years of the start of the Solar System.

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 of the continental lithosphere and the development of rifts.

Tomography & Geochemistry: Precision, Repeatability, Accuracy and Joint Interpretations

NASA Astrophysics Data System (ADS)

Foulger, G. R.; Panza, G. F.; Artemieva, I. M.; Bastow, I. D.; Cammarano, F.; Doglioni, C.; Evans, J. R.; Hamilton, W. B.; Julian, B. R.; Lustrino, M.; Thybo, H.; Yanovskaya, T. B.

2015-12-01

Seismic tomography can reveal the spatial seismic structure of the mantle, but has little ability to constrain composition, phase or temperature. In contrast, petrology and geochemistry can give insights into mantle composition, but have severely limited spatial control on magma sources. For these reasons, results from these three disciplines are often interpreted jointly. Nevertheless, the limitations of each method are often underestimated, and underlying assumptions de-emphasized. Examples of the limitations of seismic tomography include its ability to image in detail the three-dimensional structure of the mantle or to determine with certainty the strengths of anomalies. Despite this, published seismic anomaly strengths are often unjustifiably translated directly into physical parameters. Tomography yields seismological parameters such as wave speed and attenuation, not geological or thermal parameters. Much of the mantle is poorly sampled by seismic waves, and resolution- and error-assessment methods do not express the true uncertainties. These and other problems have become highlighted in recent years as a result of multiple tomography experiments performed by different research groups, in areas of particular interest e.g., Yellowstone. The repeatability of the results is often poorer than the calculated resolutions. The ability of geochemistry and petrology to identify magma sources and locations is typically overestimated. These methods have little ability to determine source depths. Models that assign geochemical signatures to specific layers in the mantle, including the transition zone, the lower mantle, and the core-mantle boundary, are based on speculative models that cannot be verified and for which viable, less-astonishing alternatives are available. Our knowledge is poor of the size, distribution and location of protoliths, and of metasomatism of magma sources, the nature of the partial-melting and melt-extraction process, the mixing of disparate melts, and the re-assimilation of crust and mantle lithosphere by rising melt. Interpretations of seismic tomography, petrologic and geochemical observations, and all three together, are ambiguous, and this needs to be emphasized more in presenting interpretations so that the viability of the models can be assessed more reliably.

Rare gases in Samoan xenoliths

NASA Astrophysics Data System (ADS)

Poreda, R. J.; Farley, K. A.

1992-09-01

The rare gas isotopic compositions of residual harzburgite xenoliths from Savai'i (SAV locality) and an unnamed seamount south of the Samoan chain (PPT locality) provide important constraints on the rare gas evolution of the mantle and atmosphere. Despite heterogeneous trace element compositions, the rare gas characteristics of the xenoliths from each of the two localities are strikingly similar. SAV and PPT xenoliths have 3He/ 4He ratios of11.1 ± 0.5 R A and21.6 ± 1 R A, respectively; this range is comparable to the 3He/ 4He ratios in Samoan lavas and clearly demonstrates that they have trapped gases from a relatively undegassed reservoir. The neon results are not consistent with mixing between MORB and a plume source with an atmospheric signature. Rather, the neon isotopes reflect either a variably degassed mantle (with a relative order of degassing of Loihi < PPT < Reunion < SAV < MORB), or mixing between the Loihi source and MORB. The data supports the conclusions of Honda et al. that the 20Ne/ 22Ne ratio in the mantle more closely resembles the solar ratio than the atmospheric one. 40Ar/ 36Ar ratios in the least contaminated samples range from 4,000 to 12,000 with the highest values in the 22 RA PPT xenoliths. There is no evidence for atmospheric 40Ar/ 36Ar ratios in the mantle source of these samples, which indicates that the lower mantle may have 40Ar/ 36Ar ratios in excess of 5,000. Xenon isotopic anomalies in 129Xe and 136Xe are as high as 6%, or about half of the maximum MORB excess and are consistent with the less degassed nature of the Samoan mantle source. These results contradict previous suggestions that the high 3He/ 4He mantle has a near-atmospheric heavy rare gas isotopic composition.

Yellowstone Hotspot Geodynamics

NASA Astrophysics Data System (ADS)

Smith, R. B.; Farrell, J.; Massin, F.; Chang, W.; Puskas, C. M.; Steinberger, B. M.; Husen, S.

2012-12-01

The Yellowstone hotspot results from the interaction of a mantle plume with the overriding N. America plate producing a ~300-m high topographic swell centered on the Late Quaternary Yellowstone volcanic field. The Yellowstone area is dominated by earthquake swarms including a deadly M7.3 earthquake, extraordinary high heat flow up to ~40,000 mWm-2, and unprecedented episodes of crustal deformation. Seismic tomography and gravity data reveal a crustal magma reservoir, 6 to 15 km deep beneath the Yellowstone caldera but extending laterally ~20 km NE of the caldera and is ~30% larger than previously hypothesized. Kinematically, deformation of Yellowstone is dominated by regional crustal extension at up to ~0.4 cm/yr but with superimposed decadal-scale uplift and subsidence episodes, averaging ~2 cm/yr from 1923. From 2004 to 2009 Yellowstone experienced an accelerated uplift episode of up to 7 cm/yr whose source is modeled as magmatic recharge of a sill at the top of the crustal magma reservoir at 8-10-km depth. New mantle tomography suggest that Yellowstone volcanism is fed by an upper-mantle plume-shaped low velocity body that is composed of melt "blobs", extending from 80 km to 650 km in depth, tilting 60° NW, but then reversing tilt to ~60° SE to a depth of ~1500 km. Moreover, images of upper mantle conductivity from inversion of MT data reveal a high conductivity annulus around the north side of the plume in the upper mantle to resolved depths of ~300 km. On a larger scale, upper mantle flow beneath the western U.S. is characterized by eastward flow beneath Yellowstone at 5 cm/yr that deflects the plume to the west, and is underlain by a deeper zone of westerly return flow in the lower mantle reversing the deflection of the plume body to the SE. Dynamic modeling of the Yellowstone plume including a +15 m geoid anomaly reveals low excess plume temperatures, up to 150°K, consistent with a weak buoyancy flux of ~0.25 Mg/s. Integrated kinematic modeling of GPS, Quaternary fault slip, and seismic data suggest that the gravitational potential of the Yellowstone swell creates a regional extension affecting much of the western U.S. Overall, the Yellowstone hotspot swell is the vertex of tensional stress axes rotation from E-W in the Basin-Range to NE-SW at the Yellowstone Plateau as well as the cause of edge faulting, nucleating the nearby Teton and Centennial faults. We extrapolate the original location of the Yellowstone mantle-source southwestward 800 km to an initial position at 17 million years ago beneath eastern Oregon and Washington suggesting a common origin for the YSRP and Columbia Plateau volcanism. We propose that the original plume head ascended vertically behind the subducting Juan de Fuca plate, but was entrained ~12 Ma ago in a faster mantle flow beneath the continental lithosphere and tilted into its present configuration.

Geochemical constraints on the petrogenesis of the pyroclastic rocks in Abakaliki basin (Lower Benue Rift), Southeastern Nigeria

NASA Astrophysics Data System (ADS)

Chukwu, Anthony; Obiora, Smart C.

2018-05-01

The pyroclastic rocks in the Cretaceous Abakaliki basin occur mostly as oval-shaped bodies, consisting of lithic/lava and vitric fragments. They are commonly characterized by parallel and cross laminations, as well contain xenoliths of shale, mudstone and siltstones from the older Asu River Group of Albian age. The rocks are basic to ultrabasic in composition, comprising altered alkali basalts, altered tuffs, minor lapillistones and agglomerates. The mineral compositions are characterized mainly by laths of calcic plagioclase, pyroxene (altered), altered olivines and opaques. Calcite, zeolite and quartz represent the secondary mineral constituents. Geochemically, two groups of volcaniclastic rocks, are distinguished: alkaline and tholeiitic rocks, both represented by fresh and altered rock samples. The older alkali basalts occur within the core of the Abakaliki anticlinorium while the younger tholeiites occur towards the periphery. Though most of the rocks are moderate to highly altered [Loss on ignition (LOI, 3.43-22.07 wt. %)], the use of immobile trace element such as Nb, Zr, Y, Hf, Ti, Ta and REEs reflect asthenospheric mantle source compositions. The rocks are enriched in incompatible elements and REEs (∑REE = 87.98-281.0 ppm for alkaline and 69.45-287.99 ppm for tholeiites). The ratios of La/Ybn are higher in the alkaline rocks ranging from 7.69 to 31.55 compared to the tholeiitic rocks which range from 4.4 to 16.89 and indicating the presence of garnet-bearing lherzolite in the source mantle. The spidergrams and REEs patterns along with Zr/Nb, Ba/Nb, Rb/Nb ratios suggest that the rocks were generated by a mantle plume from partial melting of mixed enriched mantle sources (HIMU, EMI and EMII) similar to the rocks of the south Atlantic Ocean such as St. Helena (alkaline rocks) and Ascension rocks (tholeiitic rocks). The rocks were formed in a within-plate setting of the intra-continental rift type similar to other igneous rocks in the Benue Rift and are not related to any subduction event as previously suggested.

Is Earth coming out of the recent ice house age in the long-term? - constraints from probable mantle CO2-degassing reconstructions

NASA Astrophysics Data System (ADS)

Hartmann, Jens; Li, Gaojun; West, A. Joshua

2017-04-01

Enhanced partial melting of mantle material probably started when the subduction motor started around 3.2 Ga ago as evidenced by the formation history of the continental crust. Carbon is degassing due partial melting as it is an incompatible element. Therefore, mantle carbon degassing rates would change with time proportionally to the reservoir mantle concentration evolution and the ocean crust production rate, causing a distinct CO2-degassing rate change with time. The evolution of the mantle degassing rate has some implications for the reconstruction of the carbon cycle and therefore climate and Earth surface processes rates, as CO2-degassing rates are used to constrain or to balance the atmosphere-ocean-crust carbon cycle system. It will be shown that compilations of CO2-degassing from relevant geological sources are probably exceeding the established CO2-sink terrestrial weathering, which is often used to constrain long-term mantle degassing rates to close the carbon cycle on geological time scales. In addition, the scenarios for the degassing dynamics from the mantle sources suggest that the mantle is depleting its carbon content since 3 Ga. This has further implications for the long-term CO2-sink weathering. Results will be compared with geochemical proxies for weathering and weathering intensity dynamics, and will be set in context with snow ball Earth events and long-term emplacement dynamics of mafic areas as Large Igneous Provinces. Decreasing mantle degassing rates since about 2 Ga suggest a constraint for the evolution of the carbon cycle and recycling potential of the amount of subducted carbon. If the given scenarios hold further investigation, the contribution of mantle degassing to climate forcing (directly and via recycling) will decrease further.

Isotopic Evidence For Chaotic Imprint In The Upper Mantle Heterogeneity

NASA Astrophysics Data System (ADS)

Armienti, P.; Gasperini, D.

2006-12-01

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

Mantle Sources and Origin of the Four Overlapping Continental LIPs Generated Throughout 2500 m.y. of Kaapvaal Craton History in Southern Africa

NASA Astrophysics Data System (ADS)

Ashwal, L. D.

2017-12-01

The Archean Kaapvaal Craton of southern Africa hosts at least four spatially overlapping Large Igneous Provinces (LIPs), each of which generated substantial volumes ( 1-3 x 106 km3) of mafic magmatic rocks, over short time intervals (5 m.y. or less), between 2.7 and 0.18 Ga: the Ventersdorp Supergroup (2714 Ma, 0.7 x 106 km3), the Bushveld layered intrusion (2056 Ma, 1.5 x 106 km3), the Umkondo Igneous Province (1105 Ma, 2 x 106 km3) and the Karoo LIP (182 Ma, 3 x 106 km3). Therefore, over a time interval of >2500 m.y., a minimum collective volume of 7.2 x 106 km3 of mantle-derived, mafic lavas, sills, dikes and derivative cumulate rocks, was periodically emplaced through, into and/or onto the same cratonic region of Archean lithosphere. This long-term spatial superposition of Kaapvaal LIPs can be used as input to the vigorous debate on the nature of LIP mantle sources, and the possible role of crustal contamination in their petrogenesis. Continental LIP magmas, including all four of the Kaapvaal examples, have commonly been interpreted as products of direct partial melting of sub-continental lithospheric mantle (SCLM) sources, with little or no contribution from upwelling, plume-related, asthenospheric materials that provided the heat for melting. The Kaapvaal SCLM was stabilized at 3 Ga by prior melt extraction events that rendered it chemically depleted, and hence buoyant; it seems unlikely that it might have been capable of generating 1-3 x 106 km3 of basaltic magmas four times during its history. This would require repeated, substantial refertilization to counteract the extensive chemical depletion caused by recurrent extraction of LIP magmas. Chemical enrichment events sufficient to yield such extensive volumes of basaltic magma would necessarily increase bulk SCLM density, compromising its long-term buoyancy and stability. It seems far more likely, therefore, that the Kaapvaal LIPs were generated from sub-lithospheric sources, and that their diverse geochemical and isotopic signatures represent variable assimilation of Archean (dominantly 3.0-3.6 Ga) granitoid crustal contaminants, as many have suggested. These arguments challenge the plausibility of SCLM melting as a viable general process for the origin of other continental LIPs.

Osmium isotopes and mantle convection.

PubMed

Hauri, Erik H

2002-11-15

The decay of (187)Re to (187)Os (with a half-life of 42 billion years) provides a unique isotopic fingerprint for tracing the evolution of crustal materials and mantle residues in the convecting mantle. Ancient subcontinental mantle lithosphere has uniquely low Re/Os and (187)Os/(188)Os ratios due to large-degree melt extraction, recording ancient melt-depletion events as old as 3.2 billion years. Partial melts have Re/Os ratios that are orders of magnitude higher than their sources, and the subduction of oceanic or continental crust introduces into the mantle materials that rapidly accumulate radiogenic (187)Os. Eclogites from the subcontinental lithosphere have extremely high (187)Os/(188)Os ratios, and record ages as old as the oldest peridotites. The data show a near-perfect partitioning of Re/Os and (187)Os/(188)Os ratios between peridotites (low) and eclogites (high). The convecting mantle retains a degree of Os-isotopic heterogeneity similar to the lithospheric mantle, although its amplitude is modulated by convective mixing. Abyssal peridotites from the ocean ridges have low Os isotope ratios, indicating that the upper mantle had undergone episodes of melt depletion prior to the most recent melting events to produce mid-ocean-ridge basalt. The amount of rhenium estimated to be depleted from the upper mantle is 10 times greater than the rhenium budget of the continental crust, requiring a separate reservoir to close the mass balance. A reservoir consisting of 5-10% of the mantle with a rhenium concentration similar to mid-ocean-ridge basalt would balance the rhenium depletion of the upper mantle. This reservoir most likely consists of mafic oceanic crust recycled into the mantle over Earth's history and provides the material that melts at oceanic hotspots to produce ocean-island basalts (OIBs). The ubiquity of high Os isotope ratios in OIB, coupled with other geochemical tracers, indicates that the mantle sources of hotspots contain significant quantities (greater than 10%) of lithologically distinct mafic material which represents ancient oceanic lithosphere cycled through the convecting mantle on a time-scale of 800 million years or more.

Using noble gases and 87Sr/86Sr to constrain heat sources and fluid evolution at the Los Azufres Geothermal Field, Mexico

NASA Astrophysics Data System (ADS)

Wen, T.; Pinti, D. L.; Castro, M. C.; Lopez Hernandez, A.; Hall, C. M.; Shouakar-Stash, O.; Sandoval-Medina, F.

2017-12-01

Geothermal wells and hot springs were sampled for noble gases' volume fraction and isotopic measurements and 87Sr/86Sr in the Los Azufres Geothermal Field (LAGF), Mexico, to understand the evolution of fluid circulation following three decades of exploitation and re-injection of used brines. The LAGF, divided into the Southern Production Zone (SPZ) and the Northern Production Zone (NPZ), is hosted in a Miocene to Pliocene andesitic volcanic complex covered by Quaternary rhyolitic-dacitic units. Air contamination corrected 3He/4He ratios (Rc) normalized to the atmospheric ratio (Ra=1.384 x 10-6), show a median value of 6.58 indicating a dominant mantle helium component. Contributions of crustal helium up to 53% and 18% are observed in NPZ and SPZ, respectively. Observations based on Rc/Ra and 87Sr/86Sr ratios points to the mixing of three magmatic sources supplying mantle helium to the LAGF: (1) a pure mantle He (Rc/Ra = 8) and Sr (87Sr/86Sr = 0.7035) source; (2) a pure mantle helium (Rc/Ra = 8) with some radiogenic Sr (87Sr/86Sr = 0.7049) source possibly resulting from Quaternary rhyolitic volcanism; and (3) a fossil mantle He component (Rc/Ra = 3.8) with some radiogenic Sr (87Sr/86Sr = 0.7038), corresponding possibly to the Miocene andesite reservoir. Intrusions within the last 50 kyrs from sources (1) and (2) are likely responsible for the addition of mantle volatiles and heat to the hydrothermal system of Los Azufres. He and Ar isotopes indicate that heat flow is transported by both convection and conduction. Atmospheric noble gas elemental ratios suggest that geothermal wells located closer to the western re-injection zone are beginning to be dominated by re-injection of used brines (injectate). The area affected by boiling in LAGF has further extended to the north and west since the last noble gas sampling campaign in 2009.

Driving forces: Slab subduction and mantle convection

NASA Technical Reports Server (NTRS)

Hager, Bradford H.

1988-01-01

Mantle convection is the mechanism ultimately responsible for most geological activity at Earth's surface. To zeroth order, the lithosphere is the cold outer thermal boundary layer of the convecting mantle. Subduction of cold dense lithosphere provides tha major source of negative buoyancy driving mantle convection and, hence, surface tectonics. There are, however, importnat differences between plate tectonics and the more familiar convecting systems observed in the laboratory. Most important, the temperature dependence of the effective viscosity of mantle rocks makes the thermal boundary layer mechanically strong, leading to nearly rigid plates. This strength stabilizes the cold boundary layer against small amplitude perturbations and allows it to store substantial gravitational potential energy. Paradoxically, through going faults at subduction zones make the lithosphere there locally weak, allowing rapid convergence, unlike what is observed in laboratory experiments using fluids with temperature dependent viscosities. This bimodal strength distribution of the lithosphere distinguishes plate tectonics from simple convection experiments. In addition, Earth has a buoyant, relatively weak layer (the crust) occupying the upper part of the thermal boundary layer. Phase changes lead to extra sources of heat and bouyancy. These phenomena lead to observed richness of behavior of the plate tectonic style of mantle convection.

A global geochemical model for the evolution of the mantle

NASA Technical Reports Server (NTRS)

Anderson, D. L.

1979-01-01

It is proposed that the upper mantle transition region, 220 to 670 km, is composed of eclogite which has been derived from primitive mantle by about 20 percent partial melting and that this is the source and sink of oceanic crust. The remainder of the upper mantle is garnet peridotite which is the source of continental basalts and hotspot magmas. This region is enriched in incompatible elements by hydrous and CO2 rich metasomatic fluids which have depleted the underlying layers in the L.I.L. elements and L.R.E.E. The volatiles make this a low-velocity, high attenuation, low viscosity region. The eclogite layer is internally heated and its controls the convection pattern in the upper mantle. Plate tectonics is intermittent. The continental thermal anomaly at a depth of 150-220 km triggers kimberlite and carbonatite activity, alkali and flood basalt volcanism, vertical tectonics and continental breakup. Hot spots remain active after the continents leave and build the oceanic islands. Mantle plumes rise from a depth of about 220 km. Midocean ridge basalts rise from the depleted layer below this depth. Material from this layer can also be displaced upwards by subducted oceanic lithosphere to form back-arc basins.

Thermal structure and geodynamics of subduction zones

NASA Astrophysics Data System (ADS)

Wada, Ikuko

The thermal structure of subduction zones depends on the age-controlled thermal state of the subducting slab and mantle wedge flow. Observations indicate that the shallow part of the forearc mantle wedge is stagnant and the slab-mantle interface is weakened. In this dissertation, the role of the interface strength in controlling mantle wedge flow, thermal structure, and a wide range of subduction zone processes is investigated through two-dimensional finite-element modelling and a global synthesis of geological and geophysical observations. The model reveals that the strong temperature-dependence of the mantle strength always results in full slab-mantle decoupling along the weakened part of the interface and hence complete stagnation of the overlying mantle. The interface immediately downdip of the zone of decoupling is fully coupled, and the overlying mantle is driven to flow at a rate compatible with the subduction rate. The sharpness of the transition from decoupling to coupling depends on the rheology assumed and increases with the nonlinearity of the flow system. This bimodal behaviour of the wedge flow gives rise to a strong thermal contrast between the cold stagnant and hot flowing parts of the mantle wedge. The maximum depth of decoupling (MDD) thus dictates the thermal regime of the forearc. Observed surface heat flow patterns and petrologically and geochemically estimated mantle wedge temperatures beneath the volcanic arc require an MDD of 70--80 km in most, if not all, subduction zones regardless of their thermal regime of the slab. The common MDD of 70--80 km explains the observed systematic variations of the petrologic, seismological, and volcanic processes with the thermal state of the slab and thus explains the rich diversity of subduction zones in a unified fashion. Models for warm-slab subduction zones such as Cascadia and Nankai predict shallow dehydration of the slab beneath the cold stagnant part of the mantle wedge, which provides ample fluid for mantle wedge serpentinization in the forearc but little fluid for melt generation beneath the arc. In contrast, models for colder-slab subduction zones such as NE Japan and Kamchatka predict deeper dehydration, which provides greater fluid supply for melt generation beneath the arc and allows deeper occurrence of intraslab earthquakes but less fluid for forearc mantle wedge serpentinization. The common MDD also explains the intriguing uniform configuration of subduction zones, that is, the volcanic arc always tends to be situated where the slab is at about 100 km depth. The sudden onset of mantle wedge flow downdip of the common MDD overshadows the thermal effect of the slab, and the resultant thermal field and slab dehydration control the location of the volcanic arc. The recognition of the fundamental importance of the MDD has important implications to the study of geodynamics and earthquake hazard in subduction zones.

Elemental and Sr-Nd-Pb isotope geochemistry of the Florianópolis Dyke Swarm (Paraná Magmatic Province): crustal contamination and mantle source constraints

NASA Astrophysics Data System (ADS)

Marques, L. S.; De Min, A.; Rocha-Júnior, E. R. V.; Babinski, M.; Bellieni, G.; Figueiredo, A. M. G.

2018-04-01

The Florianópolis Dyke Swarm is located in Santa Catarina Island, comprising also the adjacent continental area, and belongs to the Paraná Magmatic Province (PMP). The dyke outcrops in the island are 0.1-70 m thick and most of them are coast-parallel (NE-SW trending), with subordinate NW-SE trending. The vast majority of the dykes has SiO2 varying from 50 to 55 wt% and relatively high-Ti (TiO2 > 3 wt%) contents and these rocks were divided using the criteria commonly used to distinguish the different magma-types identified in the volcanic rocks from the PMP. The Urubici dykes (Sr > 550 μg/g) are the most abundant and some of them experienced crustal contamination reaching to 10%, as evidenced by low P2O5/K2O (0.30-0.21), high (Rb/Ba)PM (1.0-2.2), and radiogenic Sr and Pb isotope compositions (87Sr/86Sri up to 0.70716 (back to 125 Ma) and 206Pb/204Pbm up to 19.093). The Pitanga (Sr < 550 μg/g) and the basaltic trachyandesite dykes are less abundant and almost all of them were also substantially affected by at least 15% of crustal assimilation, evidenced by high (Rb/Ba)PM (up to 2.6) and Sr (87Sr/86Sri = 0.70737-0.71758) and Pb (206Pb/204Pbm = 18.446-19.441) isotope ratios, as well as low P2O5/K2O values (0.30-0.18). The low-Ti (TiO2 < 2 wt%) dykes are scarce and show a large compositional variability (SiO2: 50.4-64.5 wt%), with similar geochemical characteristics of the low-Ti volcanic rocks (Gramado-Palmas) from southern PMP, although the most primitive dykes show hybrid characteristics of Ribeira and Esmeralda magmas. The presence of granitic xenoliths with border reactions and dykes with diffuse contacts indicate that crustal contamination probably occurred by assimilation from re-melted the host rocks. Considering only the high-Ti Urubici dykes that were not affected by crustal contamination, the Sr, Nd and Pb isotope mixing modelling indicates the participation of a heterogeneous metasomatized (refertilized) subcontinental lithospheric mantle (SCLM). This mantle source was originated by partial melting of a depleted sublithospheric mantle (DMM - Depleted Mantle MORB), which was hybridized by addition of pyroxenite (< 5%) and carbonatite (up to 2%) melts. The isotope mixing modelling also points to a significant participation (up to 50%) of Archean SCLM, not evidenced in the mantle sources of the northern PMP high-Ti Pitanga flows (dominated by Neoproterozoic SCLM).

Mafic mantle sources indicated by the olivine-spinifex basalt-ferropicrite lavas in the accreted Permian oceanic LIP fragments and Miocene low-Ni basalt and adakite lavas in central Japan

NASA Astrophysics Data System (ADS)

Ishiwatari, A.; Ichiyama, Y.; Yamazaki, R.; Katsuragi, T.; Tsuchihashi, H.

2008-12-01

Melting of mafic (eclogitic) rocks in the peridotite mantle diapir may be important to generate a large quantity of magma in a short period of time as required for the LIP basaltic magmatism (e.g. Takahashi et al. 1998; EPSL, 162, 63-). Ferropicritic rocks also occur in some LIPs, and Ichiyama et al. (2006; Lithos, 89, 47-) propose a non-peridotitic, Ti- and Fe-rich eclogitic source (recycled oceanic ferrogabbro?) entrained in the peridotitic LIP mantle plume for the origin of ferropicritic rocks, that occur with olivine-spinifex basalt (Ichiyama et al., 2007; Island Arc, 16, 493-) in a Permian LIP fragment that was captured in the Jurassic Tamba accretionary complex in central Japan. Although Ti-poor ferrokomatiitic magma might form through high- degree melting of a primitive chondritic mantle (25wt% MgO and 25wt% Fe+FeO), Ti- and HFSE-rich ferropicritic and meimechitic magmas can not form in this way. On the other hand, Miocene volcanic rocks distributed along the Japan Sea coast of central Japan also represent a product of large-scale arc magmatism that happened coeval to the spreading of the Japan Sea floor. The chemical and isotopic signatures of the magmas are consistent with the secular change of tectonic setting from continental arc (22- 20 Ma) to island arc (15-11 Ma) (Shuto et al. 2006; Lithos, 86, 1-). Some adakites have already been found from these Miocene volcanic rocks by Shuto"fs group, and mafic rock melting in either subducting slab or lower arc crust has been proposed. We have recently found a wide distribution of low-Ni basalt from Fukui City. The low-Ni basalt contains olivine phenocrysts which are one order of magnitude poorer in Ni (less than 0.02 wt% NiO at Fo87) than those in normal basalt (more than 0.2 wt% NiO at Fo87). The rock is also poor in bulk-rock Ni, rich in K and Ti, and may have formed from an olivine-free pyroxenitic source. Close association of adakite and low-Ni basalt with normal tholeiitic basalt, calc-alkaline andesite-dacite-rhyolite, high-Mg andesite and rare picritic basalt suggests melting of a heterogeneous mantle wedge that was abundantly endorsed with eclogitic and pyroxenitic rocks. Melting pressure greatly differs between the ferropicrite case (5 GPa or more) and the low-Ni basalt-adakite case (2 GPa or less), causing large chemical differences. However, common occurrences of non-peridotite-origin magmas in the LIP and island arc suggest pervasive and voluminous distribution of the mafic materials in the peridotitic mantle and their important role in magma genesis at various tectonic settings.

A mantle plume model for the Equatorial Highlands of Venus

NASA Technical Reports Server (NTRS)

Kiefer, Walter S.; Hager, Bradford H.

1991-01-01

The possibility that the Equatorial Highlands are the surface expressions of hot upwelling mantle plumes is considered via a series of mantle plume models developed using a cylindrical axisymmetric finite element code and depth-dependent Newtonian rheology. The results are scaled by assuming whole mantle convection and that Venus and the earth have similar mantle heat flows. The best model fits are for Beta and Atla. The common feature of the allowed viscosity models is that they lack a pronounced low-viscosity zone in the upper mantle. The shape of Venus's long-wavelength admittance spectrum and the slope of its geoid spectrum are also consistent with the lack of a low-viscosity zone. It is argued that the lack of an asthenosphere on Venus is due to the mantle of Venus being drier than the earth's mantle. Mantle plumes may also have contributed to the formation of some smaller highland swells, such as the Bell and Eistla regions and the Hathor/Innini/Ushas region.

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

USGS Publications Warehouse

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

2003-01-01

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

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

NASA Technical Reports Server (NTRS)

Bindschadler, Duane L.; Parmentier, E. Marc

1990-01-01

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

Re — Os isotopic constraints on the origin of volcanic rocks, Gorgona Island, Colombia: Os isotopic evidence for ancient heterogeneities in the mantle

NASA Astrophysics Data System (ADS)

Walker, R. J.; Echeverria, L. M.; Shirey, S. B.; Horan, M. F.

1991-04-01

The Re — Os isotopic systematics of komatiites and spatially associated basalts from Gorgona Island, Colombia, indicate that they were produced at 155±43 Ma. Subsequent episodes of volcanism produced basalts at 88.1±3.8 Ma and picritic and basaltic lavas at ca. 58 Ma. The age for the ultramafic rocks is important because it coincides with the late-Jurassic, early-Cretaceous disassembly of Pangea, when the North- and South-American plates began to pull apart. Deep-seated mantle upwelling possibly precipitated the break-up of these continental plates and caused a tear in the subducting slab west of Gorgona, providing a rare, late-Phanerozoic conduit for the komatiitic melts. Mantle sources for the komatiites were heterogeneous with respect to Os and Pb isotopic compositions, but had homogeneous Nd isotopic compositions (ɛNd+9±1). Initial 187Os/186Os normalized to carbonaceous chondrites at 155 Ma (γOs) ranged from 0 to +22, and model-initial μ values ranged from 8.17 to 8.39. The excess radiogenic Os, compared with an assumed bulk-mantle evolution similar to carbonaceous chondrites, was likely produced in portions of the mantle with long-term elevated Re concentrations. The Os, Pb and Nd isotopic compositions, together with major-element constraints, suggest that the sources of the komatiites were enriched more than 1 Ga ago by low (<20%) and variable amounts of a basalt or komatiite component. This component was added as either subducted oceanic crust or melt derived from greater depths in the mantle. These results suggest that the Re — Os isotope system may be a highly sensitive indicator of the presence of ancient subducted oceanic crust in mantle-source regions.

Re - Os isotopic constraints on the origin of volcanic rocks, Gorgona Island, Colombia: Os isotopic evidence for ancient heterogeneities in the mantle

USGS Publications Warehouse

Walker, R.J.; Echeverria, L.M.; Shirey, S.B.; Horan, M.F.

1991-01-01

The Re - Os isotopic systematics of komatiites and spatially associated basalts from Gorgona Island, Colombia, indicate that they were produced at 155??43 Ma. Subsequent episodes of volcanism produced basalts at 88.1??3.8 Ma and picritic and basaltic lavas at ca. 58 Ma. The age for the ultramafic rocks is important because it coincides with the late-Jurassic, early-Cretaceous disassembly of Pangea, when the North- and South-American plates began to pull apart. Deep-seated mantle upwelling possibly precipitated the break-up of these continental plates and caused a tear in the subducting slab west of Gorgona, providing a rare, late-Phanerozoic conduit for the komatiitic melts. Mantle sources for the komatiites were heterogeneous with respect to Os and Pb isotopic compositions, but had homogeneous Nd isotopic compositions (??Nd+9??1). Initial 187Os/186Os normalized to carbonaceous chondrites at 155 Ma (??Os) ranged from 0 to +22, and model-initial ?? values ranged from 8.17 to 8.39. The excess radiogenic Os, compared with an assumed bulk-mantle evolution similar to carbonaceous chondrites, was likely produced in portions of the mantle with long-term elevated Re concentrations. The Os, Pb and Nd isotopic compositions, together with major-element constraints, suggest that the sources of the komatiites were enriched more than 1 Ga ago by low (<20%) and variable amounts of a basalt or komatiite component. This component was added as either subducted oceanic crust or melt derived from greater depths in the mantle. These results suggest that the Re - Os isotope system may be a highly sensitive indicator of the presence of ancient subducted oceanic crust in mantle-source regions. ?? 1991 Springer-Verlag.

Volatile elements - water, carbon, nitrogen, noble gases - on Earth

NASA Astrophysics Data System (ADS)

Marty, B.

2017-12-01

Understanding the origin and evolution of life-bearing volatile elements (water, carbon, nitrogen) on Earth is a fruitful and debated area of research. In his pioneering work, W.W. Rubey inferred that the terrestrial atmosphere and the oceans formed from degassing of the mantle through geological periods of time. Early works on noble gas isotopes were consistent with this view and proposed a catastrophic event of mantle degassing early in Earth's history. We now have evidence, mainly from noble gas isotopes, that several cosmochemical sources contributed water and other volatiles at different stages of Earth's accretion. Potential contributors include the protosolar nebula gas that equilibrated with magma oceans, inner solar system bodies now represented by chondrites, and comets. Stable isotope ratios suggest volatiles where primarily sourced by planetary bodies from the inner solar system. However, recent measurements by the European Space Agency Rosetta probe on the coma of Comet 67P/Churyumov-Gerasimenko permit to set quantitative constraints on the cometary contribution to the surface of our planet. The surface and mantle reservoirs volatile elements exchanged volatile elements through time, with rates that are still uncertain. Some mantle regions remained isolated from whole mantle convection within the first tens to hundreds million years after start of solar system formation. These regions, now sampled by some mantle plumes (e.g., Iceland, Eifel) preserved their volatile load, as indicated by extinct and extant radioactivity systems. The abundance of volatile elements in the mantle is still not well known. Different approaches, such as high pressure experimental petrology, noble gas geochemistry, modelling, resulted in somewhat contrasted estimates, varying over one order of magnitude for water. Comparative planetology, that is, the study of volatiles on the Moon, Venus, Mars, Vesta, will shed light on the sources and strengths of these elements in the inner solar system.

Ore deposits in Africa and their relation to the underlying mantle

NASA Technical Reports Server (NTRS)

Liu, H.-S.

1981-01-01

African magmatism is largely related to the tensional stress regimes of the crust which are induced by the hotter upwelling mantle rocks. These mantle rocks may provide emanating forces and thermal energy for the upward movements of primary ore bodies with fluid inclusions in the tensional stress regimes of the crust. In this paper, the Goddard Earth Gravity Model is used to calculate a detailed subcrustal stress system exerted by mantle convection under Africa. The resulting system is found to be correlated with the African metallogenic provinces. Recognition of the full spectrum of ore deposits in Africa that may be associated with the hotter upwelling mantle rocks has provided an independent evidence to support the hypothesis of mantle-derived heat source for ore deposits.

Ground-Water Temperature, Noble Gas, and Carbon Isotope Data from the Espanola Basin, New Mexico

USGS Publications Warehouse

Manning, Andrew H.

2009-01-01

Ground-water samples were collected from 56 locations throughout the Espanola Basin and analyzed for general chemistry (major ions and trace elements), carbon isotopes (delta 13C and 14C activity) in dissolved inorganic carbon, noble gases (He, Ne, Ar, Kr, Xe, and 3He/4He ratio), and tritium. Temperature profiles were measured at six locations in the southeastern part of the basin. Temperature profiles suggest that ground water generally becomes warmer with distance from the mountains and that most ground-water flow occurs at depths 50 years old, consistent with the 14C ages. Terrigenic He (Heterr) concentrations in ground water are high (log Delta Heterr of 2 to 5) throughout much of the basin. High Heterr concentrations are probably caused by in situ production in the Tesuque Formation from locally high concentrations of U-bearing minerals (Northeast zone only), or by upward diffusive/advective transport of crustal- and mantle-sourced He possibly enhanced by basement piercing faults, or by both. The 3He/4He ratio of Heterr (Rterr) is commonly high (Rterr/Ra of 0.3-2.0, where Ra is the 3He/4He ratio in air) suggesting that Espanola Basin ground water commonly contains mantle-sourced He. The 3He/4He ratio of Heterr is generally the highest in the western and southern parts of the basin, closest to the western border fault system and the Quaternary to Miocene volcanics of the Jemez Mountains and Cerros del Rio.

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

NASA Astrophysics Data System (ADS)

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

2009-12-01

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

Helium in the Archaean komatiites revisited: significantly high 3He/4He ratios revealed by fractional crushing gas extraction

NASA Astrophysics Data System (ADS)

Matsumoto, T.; Seta, A.; Matsuda, J.; Chen, Y.; Arai, S.

2001-12-01

In order to provide constraints on 3He/4He ratios in the Archaean mantle source, we have analysed helium isotopic compositions in 2.7Ga old Archaean komatiites from the Abitibi green stone belt, Ontario, Canada. Two spinifex-textured komatiites yielded significantly high 3He/4He ratios of about 30Ra (where Ra denotes the atmospheric 3He/4He ratio) in fractions released by sequential crushing. These results are the first confirmation of the occurrence of high 3He/4He component in Archaean komatiites after the intriguing finding by [Richard et al., Science 273 (1996) 93-95] in komatiites from a nearby locality, Alexo. We also found that the crystal structure of the komatiites was significantly enriched in a radiogenic component (4He) and that the radiogenic 4He in the crystal structure was actually degassed by a crushing gas extraction, indicating that the nominal 3He/4He ratios measured by crushing are lower limits for the 3He/4He ratio of an intrinsic component. By constraining the release behaviour of radiogenic 4He by crushing, we have estimated the initial 3He/4He ratio of an inclusion-trapped component to be 73 (+7.8/-5.5) Ra. A mantle source with such a high 3He/4He ratio at 2.7Ga would, if evolved in a closed-system, have present-day 3He/4He ratio of 46-60Ra, indicating that the komatiites from Munro had been trapped their helium from a mantle reservoir with very high 3He/4He ratio in the context of the present-day value. However, whether or not such a source can be considered as the one that is equivalent to the primitive mantle source (such that sampled at hotspots) is highly model-dependent. If a closed-system evolution model were assumed, helium in the Munro komatiites is not likely to be derived from the MORB-source-like reservoir. However, the notion that the komatiites may be derived from a depleted reservoir in terms of trace elemental and isotopic geochemistry might requires an alternative view for the evolution of 3He/4He ratio in ancient mantle reservoirs, as has been demonstrated by a recent model calculation by [Seta et al., Earth Planet. Sci. Lett. 188 (2001) 211-219] in which the 3He/4He ratios in the MORB mantle source could have been as high as those in the primitive (less-degassed) mantle source in Archaean.

Helium in the Archean komatiites revisited: significantly high 3He/ 4He ratios revealed by fractional crushing gas extraction

NASA Astrophysics Data System (ADS)

Matsumoto, Takuya; Seta, Akihiro; Matsuda, Jun-ichi; Takebe, Masamichi; Chen, Yuelong; Arai, Shoji

2002-03-01

In order to provide constraints on 3He/ 4He ratios in the Archean mantle source, we have analyzed helium isotopic compositions in 2.7 Ga old Archean komatiites from the Abitibi green stone belt, Ontario, Canada. Two spinifex-textured komatiites yielded significantly high 3He/ 4He ratios of about 30 Ra (where Ra denotes the atmospheric 3He/ 4He ratio) in fractions released by sequential crushing. These results are the first confirmation of the occurrence of high 3He/ 4He ratios in Archean komatiites after the intriguing finding by Richard et al. [Science 273 (1996) 93-95] in komatiites from a nearby locality, Alexo. We also found that the crystal structure of the komatiites was significantly enriched in a radiogenic component ( 4He) and that this 4He was actually degassed by crushing gas extraction, indicating that the nominal 3He/ 4He ratios measured by crushing are lower limits for the 3He/ 4He ratio of the intrinsic component. By constraining the release behavior of radiogenic 4He by crushing, we have estimated the initial 3He/ 4He ratio of the inclusion-trapped component to be 73.0 +7.8-5.5 Ra. A mantle source with such a high 3He/ 4He ratio at 2.7 Ga, if evolved in a closed system, would have a present-day 3He/ 4He ratio of 46-60 Ra, indicating that the komatiites from Munro have trapped their helium from a mantle reservoir with a very high 3He/ 4He ratio in the context of the present-day value. However, whether or not such a source can be considered as equivalent to the primitive mantle source (such that sampled at hotspots) is highly model-dependent. If a closed system evolution model is assumed, helium in the Munro komatiites is not likely to be derived from the mid-ocean ridge basalt (MORB) source-like reservoir. However, the notion that the komatiites may be derived from a depleted reservoir in terms of trace elemental and isotopic geochemistry might require an alternative view for the 3He/ 4He evolution in ancient mantle reservoirs, as has been demonstrated by a recent model calculation by Seta et al. [Earth Planet. Sci. Lett. 188 (2001) 211-219] in which the 3He/ 4He ratios in the MORB mantle source could have been as high as those in the primitive (less degassed) mantle source in the Archean.

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

NASA Astrophysics Data System (ADS)

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

2016-04-01

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

On the nature and origin of garnet in highly-refractory Archean lithosphere: implications for continent stabilisation

NASA Astrophysics Data System (ADS)

Gibson, Sally

2014-05-01

The nature and timescales of garnet formation in the Earth's subcontinental lithospheric mantle (SCLM) are important to our understanding of how this rigid outer shell has evolved and stabilised since the Archean. Nevertheless, the widespread occurrence of pyrope garnet in the sub-cratonic mantle remains one of the 'holy grails' of mantle petrology. The paradox is that garnet often occurs in mantle lithologies (dunites and harzburgites) which represent residues of major melting events (up to 40 %) whereas experimental studies on fertile peridotite suggest this phase should be exhausted by <20 % melting. Furthermore, garnets commonly found in mantle peridotite suites have diverse compositions that are typically in equilibrium with high-pressure, small-fraction, mantle melts suggesting they formed as a result of enrichment of the lithospheric mantle following cratonisation. This refertilisation -- which typically involves addition of Fe, incompatible trace elements and volatiles -- affects the lower 30 km of the lithosphere and potentially leads to negative buoyancy and destabilisation. Pyrope garnets found in mantle xenoliths from the eastern margin of the Tanzanian Craton (Lashaine) have diverse compositions and provide major constraints on how the underlying deep (120 to 160 km) mantle stabilised and evolved during the last 3 billion years. The garnets display systematic trends from ultra-depleted to enriched compositions that have not been recognised in peridotite suites from elsewhere (Gibson et al., 2013). Certain harzburgite members of the xenolith suite contain the first reported occurrence of pyrope garnets with rare-earth element (REE) patterns similar to hypothetical garnets proposed by Stachel et al. (2004) to have formed in the Earth's SCLM during the Archean, prior to metasomatism. These rare ultra-depleted low-Cr garnets occur in low temperature (~1050 oC) xenoliths derived from depths of ~120 km and coexist in chemical and textural equilibrium with highly-refractory olivine (Fo95.4) and orthopyroxene (Mg#=96.4). These phases are all more magnesian than generally encountered in global samples of depleted mantle, i.e. harzburgites and diamond inclusion suites. The Tanzanian ultra-depleted garnets form interconnecting networks ('necklaces') around grains of orthopyroxene, which is of key importance to their origin. This close spatial relationship of garnet and orthopyroxene together with the major, trace and REE contents of the ultra-depleted garnets, are consistent with an origin by isochemical exsolution. The significance of ultra-depleted low-Cr garnets has not previously been recognised in global suites of mantle xenoliths or diamond inclusions: they appear to have been overlooked, primarily because of their unusual pre-metasomatic compositions. We believe they are rare because the low concentrations of trace elements make them readily susceptible to geochemical overprinting. This highly-refractory low-density peridotite may be common in the 'shallow' SCLM but is not normally brought to the surface by ascending melts, which tend to metasomatise and preferentially sample their source regions. The modal abundance of garnet formed by isochemical exsolution from orthopyroxene in sub-cratonic mantle is unclear but may prove to be an important consideration in isopycnic models related to the long-term stability of the Earth's continental lithosphere, e.g. Lee et al. (2011). Gibson, S. A., McMahon, S. C., Day, J. A. & Dawson, J. B. (2013). Highly Refractory Lithospheric Mantle beneath the Tanzanian Craton: Evidence from Lashaine Pre-metasomatic Garnet-bearing Peridotites. J. Petrol. doi:10.1093/petrology/egt020 Stachel, T., Aulbach, A., Brey, G.P., Harris, J.W., Leost, I., Tappert, R. & Viljoen, K.S. (2004). The trace element composition of silicate inclusions in diamonds: a review. Lithos 77, 1-19 Lee, C.-T., Luffi, P. & Chin, E. J. (2011). Building and Destroying Continental Mantle. Annu. Rev. Earth Planet. Sci. 39, 59-90

Metasomatic processes in the mantle beneath the Arkhangelsk province, Russia: evidence from garnet in mantle peridotite xenoliths, Grib pipe

NASA Astrophysics Data System (ADS)

Kargin, Alexei; Sazonova, Lyudmila; Nosova, Anna; Kovalchuk, Elena; Minevrina, Elena

2015-04-01

The Arkhangelsk province is located in the northern East European Craton and includes more than 80 bodies of kimberlite, alkaline picrite and other ultramafic and mafic rocks. They erupted through the Archean-Early Proterozoic basement into the Riphean-Paleozoic sedimentary cover. The Grib kimberlite pipe is located in the central part of the Arkhangelsk province in the Verkhotina (Chernoozerskoe) kimberlite field. The age of the Grib kimberlite is 376+-3 Ma (Rb-Sr by phlogopite). The Grib kimberlite pipe is the moderate-Ti kimberlites (TiO2 1-2 wt %) with strongly fractionated REE pattern , (La/Yb)n = 38-87. The Nd isotopic composition of the Grib pipe ranges epsilon Nd from -0.4 to + 1.0 and 87Sr/86Sr(t) from 0.7042 to 0.7069 (Kononova et al., 2006). Geochemical (Jeol JXA-8200 electron microprobe; SIMS; LA-ICP-MS) composition of clinopyroxene and garnet from mantle-derived xenoliths of the Grib kimberlite pipe was studied to provide new insights into metasomatic processes in the mantle beneath the Arkhangelsk province. Based on both major and trace element data, five geochemical groups of peridotitic garnet were distinguished. The partial melting of metasomatic peridotite with crystallization of a garnet-clinopyroxene association, and orthopyroxene assimilation by protokimberlitic melts was simulated and a model of garnet and clinopyroxene metasomatic origin was proposed. The model includes three stages: 1. Mantle peridotite was fertilized by subduction-derived sediment partial melts/fluids at the lithosphere-asthenosphere boundary to yield a CO2-bearing mantle peridotite (source I). 2. The partial melting of the carbonate-bearing mantle source 1 produced carbonatite-like melts (a degree of partial melting was 1,5 %), which could form the carbonatite-kimberlite rocks of the Mela River (Arkhangelsk province, 50 km North-West of Grib kimberlite) and also produce the metasomatic reworking of (carbonate-bearing) mantle peridotite (mantle source II) and form type-1 garnets. 3. The melting of the reworked carbonate-bearing mantle peridotite (mantle source II, degree of partial melting was 1 %) resulted in the generation of proto-kimberlite melts and type-2 garnet. These proto-kimberlite melts interacted with lithospheric mantle orthopyroxene to produce megacryst garnets and melts that formed the Grib kimberlite. This stage was responsible for the formation of the metasomatic equilibrium clinopyroxene -- garnet assemblage (type-3) in lithospheric peridotite and metasomatic transformation of deformed peridotite (type 4 and 5 garnet). This model suggests that peridotitic garnet originated at the first stage in the presence of subduction-generated melts or fluids. Kononova V.A., Nosova A.A., Pervov V.A., Kondrashov I.A. (2006). Compositional variations in kimberlites of the east European platform as a manifestation of sublithospheric geodynamic processes // Doklady Earth Sciences. V. 409. Is. 2. Pp. 952-957.

Barium isotopic compositions of oceanic basalts from São Miguel, Azores Archipelago

NASA Astrophysics Data System (ADS)

Yu, H.; Nan, X.; Huang, F.

2016-12-01

Oceanic island basalts (OIB) provide important information to decipher the processes of mantle convection and crustal material recycling1. OIBs from São Miguel, Azores Archipelago have extreme radiogenic isotope compositions2-3, representing an enriched component in their mantle source. However, the origins of the enriched mantle are still in debate. Previous studies proposed that the enriched component could be subducted terrigenous sediments2,4, delaminated subcontinental lithosphere5-6, recycled oceanic crust with evolved compositions (such as a subducted seamount)7, or enriched (E-MORB type) under-plated basalts which infiltrated the oceanic mantle lithosphere8. In this study, we use Ba isotopes to constrain the origin of enriched component beneath São Miguel because Ba isotopes can be significantly fractionated at the Earth's surface with low temperature environment than in the mantle with high temperature9-10. We analyzed Ba isotopes of 15 basalts from São Miguel. Although these samples have large variations of 87Sr/86Sr (0.703440-0.705996), 206Pb/204Pb (19.319-20.095) and 187Os/188Os (0.127-0.161), they have limited variation of 137Ba/134Ba (-0.003 to +0.048‰). The average 137Ba/134Ba of São Miguel basalts is 0.019±0.033‰ (n=15, 2SD), which is in the range of mantle (0.026±0.090‰, n=32, 2SD)9, indicating there is no surface material in the mantle source of São Miguel. The enriched source of São Miguel could be evolved material from the mantle. 1. Hofmann, 1997, Nature; 2. Hawkesworth et al., 1979, Nature; 3. White et al., 1979, CMP; 4. Turner et al., 1997, CG; 5. Widom et al., 1997, CG; 6. Moreira et al., 1999, EPSL; 7. Beier et al., 2007, EPSL; 8. Elliott et al., 2007, GCA; 9. Huang et al., 2015, Goldschmidt abs 1331; 10. Nan et al., 2016, Goldschmidt abs 2246.

Water content in the SW USA mantle lithosphere: FTIR analysis of Dish Hill and Kilbourne Hole pyroxenites

NASA Astrophysics Data System (ADS)

Gibler, R.; Peslier, A. H.; Schaffer, L. A.; Brandon, A. D.

2014-12-01

Kilbourne Hole (NM, USA) and Dish Hill (CA, USA) mantle xenoliths sample continental mantle in two different tectonic settings. Kilbourne Hole (KH) is located in the Rio Grande rift. Dish Hill (DH) is located in the southern Mojave province, an area potentially affected by subduction of the Farallon plate beneath North America [1]. FTIR analyses were obtained on well characterized pyroxenite, dunite and wehrlite xenoliths, thought to represent crystallized melts at mantle depths. PUM normalized REE patterns of the KH bulk-rocks are slightly LREE enriched and consistent with those of liquids generated by < 5% melting of a spinel peridotite source [2]. Clinopyroxenes contain from 272 to 313 ppm weight H2O similar to the lower limit of KH peridotite clinopyroxenes (250-530 ppm H2O, [3]). This is unexpected as crystallized melts like pyroxenites should concentrate water more than residual mantle-like peridotites, given that H is incompatible. PUM normalized bulk REE of the DH pyroxenites are characterized by flat to LREE depleted REE profiles consistent with > 6% melting of a spinel peridotite source. Pyroxenite pyroxenes have no detectable water but one DH wehrlite, which bulk-rock is LREE enriched, has 4 ppm H2O in orthopyroxene and <1ppm in clinopyroxene. The DH pyroxenites may thus come from a dry mantle source, potentially unaffected by the subduction of the Farallon plate. These water-poor melts either originated from shallow oceanic lithosphere overlaying the Farallon slab [4] or from continental mantle formed > 2 Ga [5]. The Farallon subduction appears to have enriched in water the southwestern United States lithospheric mantle further east than DH, beneath the Colorado plateau [6]. [1] Atwater, 1970 Tectonophysics 31, 145-165. [2] Shaw, 2000 CM 38, 1041-1064. [3] Schaffer et al, 2013 AGU Fall Meeting. [4] Luffi et al, 2009 JGR 114, 1-36. [5] Armytage et al, 2013 GCA 137, 113-133. [6] Li et al, 2008 JGR 113, 1-22.

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

NASA Astrophysics Data System (ADS)

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

2015-12-01

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

Composition of plume-influenced mid-ocean ridge lavas and glasses from the Mid-Atlantic Ridge, East Pacific Rise, Galápagos Spreading Center, and Gulf of Aden

NASA Astrophysics Data System (ADS)

Kelley, Katherine A.; Kingsley, Richard; Schilling, Jean-Guy

2013-01-01

The global mid-ocean ridge system is peppered with localities where mantle plumes impinge on oceanic spreading centers. Here, we present new, high resolution and high precision data for 40 trace elements in 573 samples of variably plume-influenced mid-ocean ridge basalts from the Mid-Atlantic ridge, the Easter Microplate and Salas y Gomez seamounts, the Galápagos spreading center, and the Gulf of Aden, in addition to previously unpublished major element and isotopic data for these regions. Included in the data set are the unconventional trace elements Mo, Cd, Sn, Sb, W, and Tl, which are not commonly reported by most geochemical studies. We show variations in the ratios Mo/Ce, Cd/Dy, Sn/Sm, Sb/Ce, W/U, and Rb/Tl, which are expected not to fractionate significantly during melting or crystallization, as a function of proximity to plume-related features on these ridges. The Cd/Dy and Sn/Sm ratios show little variation with plume proximity, although higher Cd/Dy may signal increases in the role of garnet in the mantle source beneath some plumes. Globally, the Rb/Tl ratio closely approximates the La/SmN ratio, and thus provides a sensitive tracer of enriched mantle domains. The W/U ratio is not elevated at plume centers, but we find significant enrichments in W/U, and to a lesser extent the Mo/Ce and Sb/Ce ratios, at mid-ocean ridges proximal to plumes. Such enrichments may provide evidence of far-field entrainment of lower mantle material that has interacted with the core by deeply-rooted, upwelling mantle plumes.

The early Earth -- A perspective on the Archean

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

Hamilton, W.B.

1993-04-01

Dominant models of Archean tectonics and magmatism involve plate-tectonic mechanisms. Common tenets of geochemistry (e.g., model ages) and petrology visualize a cold-accreted Earth in which primitive mantle gradually fractionated to produce crust during and since Archean time. These popular assumptions appear to be incompatible with cosmologic and planetologic evidence and with Archean geology. All current quantitative and semiquantitative theories agree that the Earth was largely or entirely melted (likely superheated) by giant impacts, including the Mars-size impact which splashed out the Moon, and by separation of the core. The Earth at [approximately]4.5 Ga was a violently convecting anhydrous molten ball.more » Both this history and solar-system position indicate the bulk Earth to be more refractory than chondrite. The outer part of whatever sold shell developed was repeatedly recycled by impacts before 3.9 Ga. Water and CO[sub 2] were added by impactors after the Moon-forming event; the mantle is not a source of primordial volatiles, but rather is a sink that has depleted the hydrosphere. Voluminous liquidus ultramafic lava (komatiite) indicates that much Archean upper mantle was above its solidus. Only komatiitic and basaltic magma entered Archean crust from the mantle. Variably hydrous contamination, secondary melting, and fractionation in the crust produced intermediate and felsic melts. Magmatism was concurrent over vast tracts. Within at least the small sample of Archean crust that has not been recycled into the mantle, heat loss was primarily by voluminous, dispersed magmatism, not, as in the modern Earth, primarily through spreading windows through the crust. Only in Proterozoic time did plate-tectonic mechanisms become prevalent.« less

Theory of Earth

NASA Astrophysics Data System (ADS)

Anderson, D. L.

2014-12-01

Earth is an isolated, cooling planet that obeys the 2nd law. Interior dynamics is driven from the top, by cold sinking slabs. High-resolution broad-band seismology and geodesy has confirmed that mantle flow is characterized by narrow downwellings and ~20 broad slowly rising updrafts. The low-velocity zone (LVZ) consists of a hot melange of sheared peridotite intruded with aligned melt-rich lamellae that are tapped by intraplate volcanoes. The high temperature is a simple consequence of the thermal overshoot common in large bodies of convecting fluids. The transition zone consists of ancient eclogite layers that are displaced upwards by slabs to become broad passive, and cool, ridge feeding updrafts of ambient mantle. The physics that is overlooked in canonical models of mantle dynamics and geochemistry includes; the 2nd law, convective overshoots, subadiabaticity, wave-melt interactions, Archimedes' principle, and kinetics (rapid transitions allow stress-waves to interact with melting and phase changes, creating LVZs; sluggish transitions in cold slabs keep eclogite in the TZ where it warms up by extracting heat from mantle below 650 km, creating the appearance of slab penetration). Canonical chemical geodynamic models are the exact opposite of physics and thermodynamic based models and of the real Earth. A model that results from inverting the assumptions regarding initial and boundary conditions (hot origin, secular cooling, no external power sources, cooling internal boundaries, broad passive upwellings, adiabaticity and whole-mantle convection not imposed, layering and self-organization allowed) results in a thick refractory-yet-fertile surface layer, with ancient xenoliths and cratons at the top and a hot overshoot at the base, and a thin mobile D" layer that is an unlikely plume generation zone. Accounting for the physics that is overlooked, or violated (2nd law), in canonical models, plus modern seismology, undermines the assumptions and conclusions of these models.

The Elephants' Graveyard: Constraints from Mantle Plumes on the Fate of Subducted Slabs and Implications for the Style of Mantle Convection

NASA Astrophysics Data System (ADS)

Lassiter, J. C.

2007-12-01

The style of mantle convection (e.g., layered- vs. whole-mantle convection) is one of the most hotly contested questions in the Geological Sciences. Geochemical arguments for and against mantle layering have largely focused on mass-balance evidence for the existence of "hidden" geochemical reservoirs. However, the size and location of such reservoirs are largely unconstrained, and most geochemical arguments for mantle layering are consistent with a depleted mantle comprising most of the mantle mass and a comparatively small volume of enriched, hidden material either within D" or within seismically anomalous "piles" beneath southern Africa and the South Pacific. The mass flux associated with subduction of oceanic lithosphere is large and plate subduction is an efficient driver of convective mixing in the mantle. Therefore, the depth to which oceanic lithosphere descends into the mantle is effectively the depth of the upper mantle in any layered mantle model. Numerous geochemical studies provide convincing evidence that many mantle plumes contain material which at one point resided close to the Earth's surface (e.g., recycled oceanic crust ± sediments, possibly subduction-modified mantle wedge material). Fluid dynamic models further reveal that only the central cores of mantle plumes are involved in melt generation. The presence of recycled material in the sources of many ocean island basalts therefore cannot be explained by entrainment of this material during plume ascent, but requires that recycled material resides within or immediately above the thermo-chemical boundary layer(s) that generates mantle plumes. More recent Os- isotope studies of mantle xenoliths from OIB settings reveal the presence not only of recycled crust in mantle plumes, but also ancient melt-depleted harzburgite interpreted to represent ancient recycled oceanic lithosphere [1]. Thus, there is increasing evidence that subducted slabs accumulate in the boundary layer(s) that provide the source of mantle plumes, as suggested 25 years ago by Hofmann & White [2]. Determination of the depth of origin of mantle plumes would provide a 1st-order constraint on the depth of plate subduction and the volume of the "upper" mantle. Improved seismic techniques and deployment of OBS arrays may soon allow robust imaging of mantle plumes in the deep mantle, although preliminary results are controversial [3]. Detection of a conclusive geochemical signature of core/mantle interaction would also provide strong evidence for a deep origin of mantle plumes, although there is considerable debate as to what such a signature would entail. In summary, determination of the depth of origin of mantle plumes may provide the key to deciphering the fate of subducted slabs and the overall style of mantle convection. Although this problem remains unresolved after several decades of work, recent developments in both geophysics and geochemistry provide hope for a final resolution within the next 10 years. [1] M Bizimis, M Griselin, JC Lassiter, VJM Salters, G Sen, EPSL 257, 259-293, 2007. [2] AW Hofmann, WM White, EPSL 57, 421-436, 1982. [3] R Montelli, G Nolet, F Dahlens, G Masters, E Engdahl, S-H Hung, Science 303, 338-343, 2004.

Sanukitoids Record the Onset of Widespread Neoarchean Supracrustal Recycling

NASA Astrophysics Data System (ADS)

Bjorkman, K. E.; Kemp, A. I.; Lu, Y. J.; McCuaig, T. C.; Hollings, P. N.

2016-12-01

The sudden appearance of sanukitoid magmatism marks a chemical and isotopic turning point in the late Archean. Petrogenetic models call for mixing between primitive and evolved sources to account for their enrichment in both compatible and incompatible elements. TTG melts and the mantle wedge are the most commonly cited end members, but previous study of oxygen isotopes hinted at a supracrustal contribution. Clarifying the nature of endmembers may illuminate the significance of this shift for crustal growth and geodynamics. Heavy oxygen isotope signatures in zircons from 15 sanukitoid intrusions across 4 terranes in the southwestern Superior Craton of Canada (average δ18Ozrc=6.6‰, extending to 7.4‰) unequivocally fingerprint a supracrustal contribution to the host magmas. This contrasts with the mantle-like oxygen of pre-collisional TTG magmatism. Hafnium isotopes measured in the same zircon domains are less radiogenic than the estimated Superior mantle at 2.7 Ga, with ɛHf ranging from +1.5 to +4.1. Hf-O isotope mixing models require <50% local Archean sediment (δ18OWR=10.6‰, ɛHf 1.5±1) addition to mantle peridotite. Within-sample isotope homogeneity indicates a well-mixed magma during zircon crystallisation. A correlation in ɛHf to local crust implies local sediment input or additional contamination by crustal assimilation. As the terranes are roughly parallel to the Kenoran Orogeny, the local Hf signature is unlikely to be derived from subducted sediments. Rather these data permit (i) extensive mixing of sediment melts with the mantle wedge followed by crustal assimilation, registering the onset of widespread erosion and subduction of sediments, or (ii) assimilation of local supracrustal rocks at depth, and by implication, late crustal overturn. These results are incongruent with the current paradigm for late Archaean magmatism, which links sanukitoid generation to extensive TTG metasomatism of the mantle. Sanukitoid emplacement thus records a critical change in the character of the crust, lithosphere and tectonic regime during the evolution of Archean cratons.

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

NASA Astrophysics Data System (ADS)

Zhang, Yanfei; Wu, Yao; Wang, Chao; Zhu, Lüyun; Jin, Zhenmin

2016-08-01

The subducted continental crust material will be gravitationally trapped in the deep mantle after having been transported to depths of greater than ∼250-300 km (the "depth of no return"). However, little is known about the status of this trapped continental material as well as its contribution to the mantle heterogeneity after achieving thermal equilibrium with the surrounding mantle. Here, we conduct an experimental study over pressure and temperature ranges of 9-16 GPa and 1300-1800 °C to constrain the fate of these trapped upper continental crust (UCC). The experimental results show that partial melting will occur in the subducted UCC along normal mantle geotherm to produce K-rich melt. The residual phases composed of coesite/stishovite + clinopyroxene + kyanite in the upper mantle, and stishovite + clinopyroxene + K-hollandite + garnet + CAS-phase in the mantle transition zone (MTZ), respectively. The residual phases achieve densities greater than the surrounding mantle, which provides a driving force for descent across the 410-km seismic discontinuity into the MTZ. However, this density relationship is reversed at the base of the MTZ, leaving the descended residues to be accumulated above the 660-km seismic discontinuity and may contribute to the "second continent". The melt is ∼0.6-0.7 g/cm3 less dense than the surrounding mantle, which provides a buoyancy force for ascent of melt to shallow depths. The ascending melt, which preserves a significant portion of the bulk-rock rare earth elements (REEs), large ion lithophile elements (LILEs), and high-filed strength elements (HFSEs), may react with the surrounding mantle. Re-melting of the metasomatized mantle may contribute to the origin of the "enriched mantle sources" (EM-sources). Therefore, the deep subducted continental crust may create geochemical/geophysical heterogeneity in Earth's interior through subduction, stagnation, partial melting and melt segregation.

Mineral chemistry and geochemistry of ophiolitic metaultramafics from Um Halham and Fawakhir, Central Eastern Desert, Egypt

NASA Astrophysics Data System (ADS)

Abdel-Karim, Abdel-Aal M.; Ali, Shehata; El-Shafei, Shaimaa A.

2018-03-01

This study is focused on ophiolitic metaultramafics from Um Halham and Fawakhir, Central Eastern Desert of Egypt. The rocks include serpentinized peridotites, serpentinites together with talc- and quartz-carbonates. The primary spinel relict is Al-chromite [Cr# > 60], which is replaced by Cr-magnetite during metamorphism. The high Cr# of Al-chromites resembles supra-subduction zone (SSZ) peridotites and suggests derivation from the deeper portion of the mantle section with boninitic affinity. These mantle rocks equilibrated with boninitic melt have been generated by high melting degrees. The estimated melting degrees ( 19-24%) lie within the range of SSZ peridotites. The high Cr# of spinel and Fo content of olivine together with the narrow compositional range suggest a mantle residual origin. Serpentinized peridotite and serpentinites have low Al2O3/SiO2 ratios (mostly < 0.03) like fore-arc mantle wedge serpentinites and further indicate that their mantle protolith had experienced partial melting before serpentinization process. Moreover, they have very low Nb, Ta, Zr and Hf concentrations along with sub-chondritic Nb/Ta (0.3-16) and Zr/Hf (mostly 1-20) ratios further confirming that their mantle source was depleted by earlier melting extraction event. The high chondrite normalized (La/Sm)N ratios (average 10) reflect input of subduction-related slab melts/fluids into their mantle source.

Cordilleran Longevity, Elevation and Heat Driven by Lithospheric Mantle Removal

NASA Astrophysics Data System (ADS)

Mackay-Hill, A.; Currie, C. A.; Audet, P.; Schaeffer, A. J.

2017-12-01

Cordilleran evolution is controlled by subduction zone back-arc processes that generate and maintain high topography due to elevated uppermost mantle temperatures. In the northern Canadian Cordillera (NCC), the persisting high mean elevation long after subduction has stopped (>50 Ma) requires a sustained source of heat either from small-scale mantle convection or lithospheric mantle removal; however direct structural constraints of these processes are sparse. We image the crust and uppermost mantle beneath the NCC using scattered teleseismic waves recorded on an array of broadband seismograph stations. We resolve two sharp and flat seismic discontinuities: a downward velocity increase at 35 km that we interpret as the Moho; and a deeper discontinuity with opposite velocity contrast at 50 km depth. Based on petrologic estimates, we interpret the deeper interface as the lithosphere-asthenosphere boundary (LAB), which implies an extremely thin ( 15 km) lithospheric mantle. We calculate the temperature at the Moho and the LAB in the range 800-900C and 1200-1300C, respectively. Below the LAB, we find west-dipping features far below the LAB beneath the eastern NCC that we associate with laminar downwelling of Cordilleran lithosphere. Whether these structures are fossilized or active, they suggest that lithospheric mantle removal near the Cordillera-Craton boundary may have provided the source of heat and elevation and therefore played a role in the longevity and stability of the Cordillera.

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

NASA Astrophysics Data System (ADS)

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

2015-12-01

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

Recycling and transport of continental material through the mantle wedge above subduction zones: A Caribbean example

NASA Astrophysics Data System (ADS)

Rojas-Agramonte, Yamirka; Garcia-Casco, Antonio; Kemp, Anthony; Kröner, Alfred; Proenza, Joaquín A.; Lázaro, Concepción; Liu, Dunyi

2016-02-01

Estimates of global growth rates of continental crust critically depend upon knowledge of the rate at which crustal material is delivered back into the mantle at subduction zones and is then returned to the crust as a component of mantle-derived magma. Quantification of crustal recycling by subduction-related magmatism relies on indirect chemical and isotopic tracers and is hindered by the large range of potential melt sources (e.g., subducted oceanic crust and overlying chemical and clastic sediment, sub-arc lithospheric mantle, arc crust), whose composition may not be accurately known. There is also uncertainty about how crustal material is transferred from subducted lithosphere and mixed into the mantle source of arc magmas. We use the resilient mineral zircon to track crustal recycling in mantle-derived rocks of the Caribbean (Greater Antilles) intra-oceanic arc of Cuba, whose inception was triggered after the break-up of Pangea. Despite juvenile Sr and Nd isotope compositions, the supra-subduction zone ophiolitic and volcanic arc rocks of this Cretaceous (∼135-70 Ma) arc contain old zircons (∼200-2525 Ma) attesting to diverse crustal inputs. The Hf-O isotope systematics of these zircons suggest derivation from exposed crustal terranes in northern Central America (e.g. Mexico) and South America. Modeling of the sedimentary component in the most mafic lavas suggests a contribution of no more than 2% for the case of source contamination or less than 4% for sediment assimilation by the magma. We discuss several possibilities for the presence of inherited zircons and conclude that they were transported as detrital grains into the mantle beneath the Caribbean Plate via subduction of oceanic crust. The detrital zircons were subsequently entrained by mafic melts that were rapidly emplaced into the Caribbean volcanic arc crust and supra-subduction mantle. These findings suggest transport of continental detritus, through the mantle wedge above subduction zones, in magmas that otherwise do not show strong evidence for crustal input and imply that crustal recycling rates in some arcs may be higher than hitherto realized.

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

NASA Astrophysics Data System (ADS)

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

2010-12-01

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

Complex Anisotropic Structure of the Mantle Wedge Beneath Kamchatka Volcanoes

NASA Astrophysics Data System (ADS)

Levin, V.; Park, J.; Gordeev, E.; Droznin, D.

2002-12-01

A wedge of mantle material above the subducting lithospheric plate at a convergent margin is among the most dynamic environments of the Earth's interior. Deformation and transport of solid and volatile phases within this region control the fundamental process of elemental exchange between the surficial layers and the interior of the planet. A helpful property in the study of material deformation and transport within the upper mantle is seismic anisotropy, which may reflect both microscopic effects of preferentialy aligned crystals of olivine and orthopyroxene and macroscopic effects of systematic cracks, melt lenses, layering etc. Through the mapping of anisotropic properties within the mantle wedge we can establish patterns of deformation. Volatile content affects olivine alignment, so regions of anomalous volatile content may be evident. Indicators of seismic anisotropy commonly employed in upper mantle studies include shear wave birefringence and mode-conversion between compressional and shear body waves. When combined together, these techniques offer complementary constraints on the location and intensity of anisotropic properties. The eastern coast of southern Kamchatka overlies a vigorous convergent margin where the Pacific plate descends at a rate of almost 80 mm/yr towards the northwest. We extracted seismic anisotropy indicators from two data sets sensitive to the anisotropic properties of the uppermost mantle. Firstly, we evaluated teleseismic receiver functions for a number of sites, and found ample evidence for anisotropicaly-influenced P-to-S mode conversion. Secondly, we measured splitting in S waves of earthquakes with sources within the downgoing slab. The first set of observations provides constraints on the depth ranges where strong changes in anisotropic properties take place. The local splitting data provides constraints on the cumulative strength of anisotropic properties along specific pathways through the mantle wedge and possibly parts of the slab. To explain the vertical stratification of anisotropy implied from receiver functions, and the strong lateral dependence of shear-wave splitting observations, we cannot rely on simple models of mantle wedge behaviour e.g., olivine-crystal alignment through subduction-driven corner flow. Diverse mechanisms can contribute to the observed pattern of anisotropic properties, with volatiles likely being a key influence. For instance, we find evidence in favor of a slow-symmetry-axis anisotropy within the uppermost 10-20 km of the mantle wedge, implying either excessive hydration of the mantle or else a presence of systematically aligned volatile-filled cracks or lenses. Also, shear-wave splitting is weak beneath the Avachinsky-Koryaksky volcanic center, suggesting either vertical flow or the influence of volatiles and/or thermally-enhanced diffusion creep.

Nd-isotopes in selected mantle-derived rocks and minerals and their implications for mantle evolution

USGS Publications Warehouse

Basu, A.R.; Tatsumoto, M.

1980-01-01

The Sm-Nd systematics in a variety of mantle-derived samples including kimberlites, alnoite, carbonatite, pyroxene and amphibole inclusions in alkali basalts and xenolithic eclogites, granulites and a pyroxene megacryst in kimberlites are reported. The additional data on kimberlites strengthen our earlier conclusion that kimberlites are derived from a relatively undifferentiated chondritic mantle source. This conclusion is based on the observation that the e{open}Nd values of most of the kimberlites are near zero. In contrast with the kimberlites, their garnet lherzolite inclusions show both time-averaged Nd enrichment and depletion with respect to Sm. Separated clinopyroxenes in eclogite xenoliths from the Roberts Victor kimberlite pipe show both positive and negative e{open}Nd values suggesting different genetic history. A whole rock lower crustal scapolite granulite xenolith from the Matsoku kimberlite pipe shows a negative e{open}Nd value of -4.2, possibly representative of the base of the crust in Lesotho. It appears that all inclusions, mafic and ultramafic, in kimberlites are unrelated to their kimberlite host. The above data and additional Sm-Nd data on xenoliths in alkali basalts, alpine peridotite and alnoite-carbonatites are used to construct a model for the upper 200 km of the earth's mantle - both oceanic and continental. The essential feature of this model is the increasing degree of fertility of the mantle with depth. The kimberlite's source at depths below 200 km in the subcontinental mantle is the most primitive in this model, and this primitive layer is also extended to the suboceanic mantle. However, it is clear from the Nd-isotopic data in the xenoliths of the continental kimberlites that above 200 km the continental mantle is distinctly different from their suboceanic counterpart. ?? 1980 Springer-Verlag.

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.

Europium and strontium anomalies in the MORB source mantle

NASA Astrophysics Data System (ADS)

Tang, Ming; McDonough, William F.; Ash, Richard D.

2017-01-01

Lower crustal recycling depletes the continental crust of Eu and Sr and returns Eu and Sr enriched materials into the mantle (e.g., Tang et al., 2015, Geology). To test the hypothesis that the MORB source mantle balances the Eu and Sr deficits in the continental crust, we carried out high precision Eu/Eu∗ and Sr/Sr∗ measurement for 72 MORB glasses with MgO >8.5% from the Pacific, Indian, and Atlantic mid-ocean ridges. MORB glasses with MgO ⩾ 9 wt.% have a mean Eu/Eu∗ of 1.025 ± 0.025 (2 σm, n = 46) and Sr/Sr∗ of 1.242 ± 0.093 (2 σm, n = 41) and these ratios are positively correlated. These samples show both positive and negative Eu and Sr anomalies, with no correlations between Eu/Eu∗ vs. MgO or Sr/Sr∗ vs. MgO, suggesting that the anomalies are not produced by plagioclase fractionation at MgO >9 wt.% and, thus, other processes must be responsible for generating the anomalies. We term these MORB samples primitive MORBs, as they record the melt Eu/Eu∗ and Sr/Sr∗ before plagioclase fractionation. Consequently, the mean oceanic crust, including cumulates, has a bulk Eu/Eu∗ of ∼1 and 20% Sr excess. Considering that divalent Sr and Eu(II) diffuse faster than trivalent Pr, Nd, Sm, and Gd, we evaluated this kinetic effect on Sm-Eu-Gd and Pr-Sr-Nd fractionations during spinel peridotite partial melting in the MORB source mantle. Our modeling shows that the correlated Eu and Sr anomalies seen in primitive MORBs may result from disequilibrium mantle melting. Melt fractions produced during early- and late-stage melting may carry positive and negative Eu and Sr anomalies, respectively, that overlap with the ranges documented in primitive MORBs. Because the net effect of disequilibrium melting is to produce partial melts with bulk positive Eu and Sr anomalies, the MORB source mantle must have Eu/Eu∗ < 1.025 ± 0.025 (2 σm) and Sr/Sr∗ < 1.242 ± 0.093 (2 σm). Although we cannot rule out the possibility that recycled lower continental crustal materials, which have positive Eu and Sr anomalies, are partially mixed into the upper mantle (i.e., MORB source region), a significant amount of this crustal component must have been sequestered into the deep mantle, as supported by the negative 206Pb/204Pb-Eu/Eu∗ and 206Pb/204Pb-Sr/Sr∗ correlations in ocean island basalts.

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

PubMed

Parkinson; Hawkesworth; Cohen

1998-09-25

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

Dynamical effects on the core-mantle boundary from depth-dependent thermodynamical properties of the lower mantle

NASA Technical Reports Server (NTRS)

Zhang, Shuxia; Yuen, David A.

1988-01-01

A common assumption in modeling dynamical processes in the lower mantle is that both the thermal expansivity and thermal conductivity are reasonably constant. Recent work from seismic equation of state leads to substantially higher values for the thermal conductivity and much lower thermal expansivity values in the deep mantle. The dynamical consequences of incorporating depth-dependent thermodynamic properties on the thermal-mechanical state of the lower mantle are examined with the spherical-shell mean-field equations. It is found that the thermal structure of the seismically resolved anomalous zone at the base of the mantle is strongly influenced by these variable properties and, in particular, that the convective distortion of the core-mantle boundary (CMB) is reduced with the decreasing thermal expansivity. Such a reduction of the dynamically induced topography from pure thermal convection would suggest that some other dynamical mechanism must be operating at the CMB.

Thermal interaction of the core and the mantle and long-term behavior of the geomagnetic field

NASA Technical Reports Server (NTRS)

Jones, G. M.

1977-01-01

The effects of temperature changes at the earth's core-mantle boundary on the velocity field of the core are analyzed. It is assumed that the geomagnetic field is maintained by thermal convection in the outer core. A model for the thermal interaction of the core and the mantle is presented which is consistent with current views on the presence of heat sources in the core and the properties of the lower mantle. Significant long-term variations in the frequency of geomagnetic reversals may be the result of fluctuating temperatures at the core-mantle boundary, caused by intermittent convection in the lower mantle. The thermal structure of the lower mantle region D double prime, extending from 2700 to 2900 km in depth, constitutes an important test of this hypothesis and offers a means of deciding whether the geomagnetic dynamo is thermally driven.

A distinct source and differentiation history for Kolumbo submarine volcano, Santorini volcanic field, Aegean arc.

PubMed

Klaver, Martijn; Carey, Steven; Nomikou, Paraskevi; Smet, Ingrid; Godelitsas, Athanasios; Vroon, Pieter

2016-08-01

This study reports the first detailed geochemical characterization of Kolumbo submarine volcano in order to investigate the role of source heterogeneity in controlling geochemical variability within the Santorini volcanic field in the central Aegean arc. Kolumbo, situated 15 km to the northeast of Santorini, last erupted in 1650 AD and is thus closely associated with the Santorini volcanic system in space and time. Samples taken by remotely-operated vehicle that were analyzed for major element, trace element and Sr-Nd-Hf-Pb isotope composition include the 1650 AD and underlying K2 rhyolitic, enclave-bearing pumices that are nearly identical in composition (73 wt.% SiO 2 , 4.2 wt.% K 2 O). Lava bodies exposed in the crater and enclaves are basalts to andesites (52-60 wt.% SiO 2 ). Biotite and amphibole are common phenocryst phases, in contrast with the typically anhydrous mineral assemblages of Santorini. The strong geochemical signature of amphibole fractionation and the assimilation of lower crustal basement in the petrogenesis of the Kolumbo magmas indicates that Kolumbo and Santorini underwent different crustal differentiation histories and that their crustal magmatic systems are unrelated. Moreover, the Kolumbo samples are derived from a distinct, more enriched mantle source that is characterized by high Nb/Yb (>3) and low 206 Pb/ 204 Pb (<18.82) that has not been recognized in the Santorini volcanic products. The strong dissimilarity in both petrogenesis and inferred mantle sources between Kolumbo and Santorini suggests that pronounced source variations can be manifested in arc magmas that are closely associated in space and time within a single volcanic field.

A large mantle water source for the northern San Andreas Fault System: A ghost of subduction past

USGS Publications Warehouse

Kirby, Stephen H.; Wang, Kelin; Brocher, Thomas M.

2014-01-01

Recent research indicates that the shallow mantle of the Cascadia subduction margin under near-coastal Pacific Northwest U.S. is cold and partially serpentinized, storing large quantities of water in this wedge-shaped region. Such a wedge probably formed to the south in California during an earlier period of subduction. We show by numerical modeling that after subduction ceased with the creation of the San Andreas Fault System (SAFS), the mantle wedge warmed, slowly releasing its water over a period of more than 25 Ma by serpentine dehydration into the crust above. This deep, long-term water source could facilitate fault slip in San Andreas System at low shear stresses by raising pore pressures in a broad region above the wedge. Moreover, the location and breadth of the water release from this model gives insights into the position and breadth of the SAFS. Such a mantle source of water also likely plays a role in the occurrence of Non-Volcanic Tremor (NVT) that has been reported along the SAFS in central California. This process of water release from mantle depths could also mobilize mantle serpentinite from the wedge above the dehydration front, permitting upward emplacement of serpentinite bodies by faulting or by diapiric ascent. Specimens of serpentinite collected from tectonically emplaced serpentinite blocks along the SAFS show mineralogical and structural evidence of high fluid pressures during ascent from depth. Serpentinite dehydration may also lead to tectonic mobility along other plate boundaries that succeed subduction, such as other continental transforms, collision zones, or along present-day subduction zones where spreading centers are subducting.

Phase equilibria and geochemical constraints on the petrogenesis of high-Ti picrite from the Paleogene East Greenland flood basalt province

NASA Astrophysics Data System (ADS)

Zhang, Yi-Shen; Hou, Tong; Veksler, Ilya V.; Lesher, Charles E.; Namur, Olivier

2018-02-01

Phase equilibrium experiments have been performed on an extremely high-Ti (5.4 wt.% TiO2) picrite from the base of the Paleogene ( 55 Ma) East Greenland Flood Basalt Province. This sample has a high CaO/Al2O3 ratio (1.14), a steep rare-earth elements (REE) profile, is enriched in incompatible trace elements, and is in chemical equilibrium with highly primitive olivine. This all suggests that the picrite is a near-primary melt that did not suffer major chemical evolution during ascent from the mantle source and through the crust. Near-liquidus phase relations were determined over the pressure range of 1 atm, 1 to 1.5 GPa and at temperatures from 1094 to 1400°C. They provide an important constraint on the petrogenesis of these lavas. The high-Ti picritic melt is multi-saturated with olivine (Ol) + orthopyroxene (Opx) at 1 GPa but has only Ol or Opx on the liquidus at lower and higher pressures, respectively. This indicates the primitive melt was last equilibrated with its mantle source at relatively shallow pressure ( 1 GPa). Melting probably started at 2-3 GPa and the picritic melt was produced by 15-30% melting of the mantle source. Such a degree of partial melting requires a mantle with a high potential temperature (1480-1530˚C). The relatively low CaO content and high FeO/MnO ratios of the most primitive East Greenland picrites, the high Ni content of olivine phenocrysts and the presence of low-Ca pyroxene (i.e., pigeonite) at high pressure in our experiments all suggest that the mantle source contained a major component of garnet pyroxenite. Residual garnet in the source could adequately explain the low Al2O3 content (7.92 wt.%) and steep REE patterns of the picrite sample. However, simple melting of a lherzolitic source, even with a major pyroxenite component, cannot explain the formation of magmas with the very high Ti contents observed in some East Greenland basalts. We therefore propose that magmas highly-enriched in Ti were produced by melting of a metasomatized mantle source containing Ti-enriched amphibole and/or phlogopite.

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

NASA Astrophysics Data System (ADS)

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

2014-05-01

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

The role of Late Veneer impacts in the evolution of Venus

NASA Astrophysics Data System (ADS)

Gillmann, C.; Golabek, G.; Tackley, P.; Raymond, S.

2017-09-01

We study how different mechanisms contribute to changes in long term evolution. In particular, the primitive history (the first Gy) of terrestrial planets is heavily influenced by collisions. We investigate how the coupled evolution of Venus' atmosphere and mantle is modified by those impacts. We focus on volatile fluxes: atmospheric escape and mantle degassing. We observe that large impacts are unlikely to erode the atmosphere significantly. They are, on the contrary, an important source of volatiles for the primitive planet. Collisions also generate a lot of melting and rapidly dries the mantle through degassing. Without recycling of volatiles into the mantle (like in plate tectonics regime), the mantle is efficiently depleted.

Mantle ingredients for making the fingerprint of Etna alkaline magmas: implications for shallow partial melting within the complex geodynamic framework of Eastern Sicily

NASA Astrophysics Data System (ADS)

Viccaro, Marco; Zuccarello, Francesco

2017-09-01

Mantle ingredients responsible for the signature of Etnean Na- and K-alkaline magmas and their relationships with short-term geochemical changes of the erupted volcanic rocks have been constrained through a partial melting model that considers major, trace elements and water contents in the produced liquids. Characteristics of the Etnean source for alkaline magmas have been supposed similar to those of the mantle accessible at a regional scale, namely below the Hyblean Plateau. The assumption that the Etnean mantle resembles the one beneath the Hyblean Plateau is justified by the large geochemical affinities of the Etnean hawaiites/K-trachybasalts and the Hyblean hawaiites/alkali basalts for what concerns both trace elements and isotope systematics. We have modeled partial melting of a composite source constituted by two rock types, inferred by lithological and geochemical features of the Hyblean xenoliths: 1) a spinel lherzolite bearing metasomatic, hydrous phases and 2) a garnet pyroxenite in form of veins intruded into the spinel lherzolite. The partial melting modeling has been applied to each rock type and the resulting primary liquids have been then mixed in various proportions. These compositions have been compared with some Etnean alkaline magmas of the post ∼60 ka activity, which were firstly re-equilibrated to mantle conditions through mass balance calculations. Our results put into evidence that concentrations of major and trace elements along with the water obtained from the modeling are remarkably comparable with those of Etnean melts re-equilibrated at primary conditions. Different proportions of the spinel lherzolite with variable modal contents of metasomatic phases and of the garnet pyroxenite can therefore account for the signature of a large spectrum of Etnean alkaline magmas and for their geochemical variability through time, emphasizing the crucial role played by compositional small-scale heterogeneity of the source. These heterogeneities are able to produce magmas with variable compositions and volatile contents, which can then undergo distinct histories of ascent and evolution, leading to the wide range of eruptive styles observed at Mt. Etna volcano. Being partial melting confined in the spinel facies of the mantle, our model implies that the source of Mt. Etna magmas might be rather shallow (<2 GPa; i.e., lesser than ca. 60 km), excluding the presence of deep, plume-like mantle structures responsible for magma generation. Partial melting should occur consequently as a response of mantle decompression within the framework of regional tectonics affecting the Eastern Sicily, which could be triggered by extensional tectonics and/or subduction-induced mantle upwelling.

Interactions between magma and the lithospheric mantle during Cenozoic rifting in Central Europe

NASA Astrophysics Data System (ADS)

Meyer, Romain; Elkins-Tanton, Linda T.

2010-05-01

During the Cenozoic, extensive intraplate volcanic activity occurred throughout Central Europe. Volcanic eruptions extend over France (the Massif Central), central Germany (Eifel, Vogelsberg, Rhön; Heldburg), the Czech Republic (the Eger graben) and SW Poland (Lower Silesia), a region ~1,200 km wide. The origin of this predominantly alkaline intraplate magmatism is often genetically linked to one or several mantle plumes, but there is no convincing evidence for this. We have measured Pb isotope ratios, together with major and trace elements, in a representative set of mafic to felsic igneous rocks from the intra-plate Cenozoic Rhön Mts. and the Heldburg dike swarm in order to gain insight into the melting source and petrogenetic history of these melts. Three different mafic rock types (tholeiitic basalt, alkali basalt, basanite) were distinguished based on petrography and geochemistry within the investigated areas. Except for the lherzolite-bearing phonolite from the Veste Heldburg all other evolved magmas are trachytes. REE geochemistry and calculated partial melting modeling experiments for the three mafic magma types point to different degrees of partial melting in a garnet-bearing mantle source. In addition a new version of the ternary Th-Hf-Ta diagram is presented in this study as a useful petrological tool. This diagram is not only able to define potentially involved melting source end-members (e.g. asthenosphere, sub-continental lithospheric mantle and continental crust) but also interactions between these members are illustrated. An advantage of this diagram compared to partial melting degree sensitive multi-element diagrams is that a ternary diagram is a closed system. An earlier version of this diagram has been recently used to establish the nature and extent of crust mantle melt interaction of volcanic rifted margins magmas (Meyer et al. 2009). The Th-Hf-Ta geochemistry of the investigated magmas is similar to spinel and garnet xenoliths from different continental intra-plate volcanic fields The in the Rhön Mts. and the Heldburg dike swarm tapped mantle source is characterized by an enriched Pb-isotope geology. The highest HIMU component has been measured in the lherzolite-bearing Veste Heldburg phonolite. This higher enriched Pb isotope signature compared to the mafic magmas cannot be explained by crustal contamination. Assimilation fractionation crystallization (AFC) modeling of the Heldburg phonolite allows us to petrogenetically link this melt with HIMU rich shallow mantle amphibole-bearing xenoliths. These new observations suggest that melting started in more depleted mantle segments. And that these melts interacted with more enriched metasomatic overprinted lithospheric mantle domains.

Interactions between magma and the lithospheric mantle during Cenozoic rifting in Central Europe

NASA Astrophysics Data System (ADS)

Meyer, R.; Song, X.; Elkins-Tanton, L. T.

2009-12-01

During the Cenozoic, extensive intraplate volcanic activity occurred throughout Central Europe. Volcanic eruptions extend over France (the Massif Central), central Germany (Eifel, Vogelsberg, Rhön; Heldburg), the Czech Republic (the Eger graben) and SW Poland (Lower Silesia), a region ~1,200 km wide. The origin of this predominantly alkaline intraplate magmatism is often genetically linked to one or several mantle plumes, but there is no convincing evidence for this. We have measured Pb isotope ratios, together with major and trace elements, in a representative set of mafic to felsic igneous rocks from the intra-plate Cenozoic Rhön Mts. and the Heldburg dike swarm in order to gain insight into the melting source and petrogenetic history of these melts. Three different mafic rock types (tholeiitic basalt, alkali basalt, basanite) were distinguished based on petrography and geochemistry within the investigated areas. Except for the lherzolite-bearing phonolite from the Veste Heldburg all other evolved magmas are trachytes. REE geochemistry and calculated partial melting modeling experiments for the three mafic magma types point to different degrees of partial melting in a garnet-bearing mantle source. In addition a new version of the ternary Th-Hf-Ta diagram is presented in this study as a useful petrological tool. This diagram is not only able to define potentially involved melting source end-members (e.g. asthenosphere, sub-continental lithospheric mantle and continental crust) but also interactions between these members are illustrated. An advantage of this diagram compared to partial melting degree sensitive multi-element diagrams is that a ternary diagram is a closed system. An earlier version of this diagram has been recently used to establish the nature and extent of crust mantle melt interaction of volcanic rifted margins magmas (Meyer et al. 2009). The Th-Hf-Ta geochemistry of the investigated magmas is similar to spinel and garnet xenoliths from different continental intra-plate volcanic fields The in the Rhön Mts. and the Heldburg dike swarm tapped mantle source is characterized by an enriched Pb-isotope geology. The highest HIMU component has been measured in the lherzolite-bearing Veste Heldburg phonolite. This higher enriched Pb isotope signature compared to the mafic magmas cannot be explained by crustal contamination. Assimilation fractionation crystallization (AFC) modeling of the Heldburg phonolite allows us to petrogenetically link this melt with HIMU rich shallow mantle amphibole-bearing xenoliths. These new observations suggest that melting started in more depleted mantle segments. And that these melts interacted with more enriched metasomatic overprinted lithospheric mantle domains.

Post-collisional and intraplate Cenozoic volcanism in the rifted Apennines/Adriatic domain

NASA Astrophysics Data System (ADS)

Bianchini, G.; Beccaluva, L.; Siena, F.

2008-02-01

The distinctive tectono-magmatic characteristics of rift volcanism in the Apennines/Adria domains are discussed focussing attention on the nature of mantle sources, stress regimes, and conditions of magma generation. Post-collisional intensive lithospheric rifting and tectonic collapse of the Apennines generate large amounts of Pliocene-Quaternary orogenic magmas which overlie a nearly vertical subducted slab along the peri-Tyrrhenian border. This magmatism includes the Roman Magmatic Province sensu lato (RMP-s.l.) and the Internal Apennines Volcanism (IAV), and consists of high-K calcalkaline, potassic (shoshonitic) and ultrapotassic (leucitites, leucite basanite and minor lamproites and kamafugites) products. Integrated petrological and geochemical studies of these rocks (and associated mantle xenoliths) indicate that most of them could have been generated by a restricted partial melting range ( F ≤ 5-10%) of extremely inhomogeneous phlogopite-veined lithospheric mantle sources, resulting from subduction related K-metasomatic processes. Moreover, the presence of both intermediate anorogenic and subduction related geochemical features in Mt. Vulture magmas support the existence of a slab window beneath the central-southern Apennines, which could have allowed inflow of subduction components to intraplate mantle sources. This slab discontinuity may mark the transition between the already collisioned Adriatic and the still subducting Ionian lithospheric slabs. By contrast, the Paleogene intraplate magmatism of the Adriatic foreland (i.e., the Veneto Province (VVP) and the minor Mt. Queglia and Pietre Nere magmatic bodies) is characterized by small volumes of basic magmas, varying from tholeiitic to strongly Na-alkaline in composition. This magmatism appears to be related to a limited extensional regime typical of the low volcanicity rifts. Petrogenetic modelling of the intraplate Adriatic foreland magmas indicates that their composition is remarkably depth-dependent, with generation of tholeiites to nephelinites/alkaline lamprophyres by decreasing degrees of partial melting ( F = 25 to ≤ 5%) of lherzolite lithospheric sources at progressively increasing depths (ca. 40 to 100 km). Moreover, geochemical features of these anorogenic magmas testify that their mantle sources are remarkable homogeneous, as also confirmed by lack of veining in the VVP mantle xenoliths. This homogeneity suggests that Na-metasomatic agents pervasively affected the overlying Adriatic lithospheric mantle by porous flow mechanisms without causing significant inhomogeneities at a regional scale.

Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone

PubMed Central

Nagaya, Takayoshi; Walker, Andrew M.; Wookey, James; Wallis, Simon R.; Ishii, Kazuhiko; Kendall, J. -Michael

2016-01-01

It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed. PMID:27436676

Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone.

PubMed

Nagaya, Takayoshi; Walker, Andrew M; Wookey, James; Wallis, Simon R; Ishii, Kazuhiko; Kendall, J-Michael

2016-07-20

It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed.

Dynamics of metasomatic transformation of lithospheric mantle rocks under Siberian Craton

NASA Astrophysics Data System (ADS)

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

2014-05-01

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

Geochemistry of Intra-Transform Lavas from the Galápagos Transform Fault

NASA Astrophysics Data System (ADS)

Morrow, T. A.; Mittelstaedt, E. L.; Harpp, K. S.

2013-12-01

The Galápagos plume has profoundly affected the development and evolution of the nearby (<250 km) Galápagos Transform Fault (GTF), a ~100km right-stepping offset in the Galápagos Spreading Center (GSC). The GTF can be divided into two sections that represent different stages of transform evolution: the northern section exhibits fully developed transform fault morphology, whereas the southern section is young, and deformation is more diffuse. Both segments are faulted extensively and include numerous small (<0.5km3) monogenetic volcanic cones, though volcanic activity is more common in the south. To examine the composition of the mantle source and melting conditions responsible for the intra-transform lavas, as well as the influence of the plume on GTF evolution, we present major element, trace element, and radiogenic isotope analysis of samples collected during SON0158, EWI0004, and MV1007 cruises. Radiogenic isotope ratio variations in the Galápagos Archipelago require four distinct mantle reservoirs across the region: PLUME, DM, FLO, and WD. We find that Galápagos Transform lavas are chemically distinct from nearby GSC lavas and neighboring seamounts. They have radiogenic isotopic compositions that lie on a mixing line between DM and PLUME, with little to no contribution from any other mantle reservoirs despite their geographic proximity to WD-influenced lavas erupted along the GSC and at nearby (<50km away) seamounts. Within the transform, lavas from the northern section are more enriched in radiogenic isotopes than lavas sampled in the southern section. Transform lavas are anomalously depleted in incompatible trace elements (ITEs) relative to GSC lavas, suggesting unique melting conditions within the transform. Isotopic variability along the transform axis indicates that mantle sources and/or melting mechanisms vary between the northern and southern sections, which may relate to their distances from the plume or the two-stage development and evolution of the Galápagos Transform Fault. We present a melting model that reproduces GTF lava chemistry from a mixture of two partial melts of PLUME and DM. We assume that the DM source has an ITE composition similar to the depleted upper mantle, melting is purely fractional, and lavas do not fractionate during ascent. Solutions were achieved using a Metropolis algorithm and constrained by observed GTF lava chemistry. Model results predict that GTF lavas are produced by a mixture of a ~3%×1% partial melt of the PLUME source and a ~5%×4% partial melt of the DM source. Our model predicts that a larger proportion of PLUME melts contribute to GTF lavas than DM melts. Absence of the WD component and relatively low concentrations of ITEs may indicate that lavas in the GTF are produced from a source that has already undergone partial melting and is being re-melted beneath the TF. Re-melting may be caused by extension across the GTF, or development of the southern section of the GTF via the ~1Ma ridge jump.

U-Series Disequilibria across the New Southern Ocean Mantle Province, Australian-Antarctic Ridge

NASA Astrophysics Data System (ADS)

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

2017-12-01

Mid-ocean ridge basalts (MORB) provide a unique window into the temporal and spatial scales of mantle evolution. Long-lived radiogenic isotopes in MORB have demonstrated that the mantle contains many different chemical components or "flavors". U-series disequilibria in MORB have further shown that different chemical components/lithologies in the mantle contribute differently to mantle melting processes beneath mid-ocean ridges. Recent Sr, Nd, Hf, and Pb isotopic analyses from newly collected basalts along the Australian-Antarctic Ridge (AAR) have revealed that a large distinct mantle province exists between the Australian-Antarctic Discordance and the Pacific-Antarctic Ridge, extending from West Antarctica and Marie Byrd Land to New Zealand and Eastern Australia (Park et al., submitted). This southern mantle province is located between the Indian-type mantle and the Pacific-type mantle domains. U-series measurements in the Southeast Indian Ridge and East Pacific Rise provinces show distinct signatures suggestive of differences in melting processes and source lithology. To examine whether the AAR mantle province also exhibits different U-series systematics we have measured U-Th-Ra disequilibria data on 38 basalts from the AAR sampled along 500 km of ridge axis from two segments that cross the newly discovered Southern Ocean Mantle province. We compare the data to those from nearby ridge segments show that the AAR possesses unique U-series disequilibria, and are thus undergoing distinct mantle melting dynamics relative to the adjacent Pacific and Indian ridges. (230Th)/(238U) excesses in zero-age basalts (i.e., those with (226Ra)/(230Th) > 1.0) range from 1.3 to 1.7, while (226Ra)/(230Th) ranges from 1.0 to 2.3. (226Ra)/(230Th) and (230Th)/(238U) are negatively correlated, consistent with the model of mixing between deep and shallow melts. The AAR data show higher values of disequilibria compared to the Indian and Pacific Ridges, which can be explained by either lower melting rates and porosities, or a higher gt/cpx ratio in their mantle source. That both long-lived radiogenic isotopes and U-series disequilibria are distinct in these three adjacent mantle provinces suggests that lithological differences are strongly influencing the melting process beneath each of these mid-ocean ridges.

High Sr/Y rocks are not all adakites!

NASA Astrophysics Data System (ADS)

Moyen, Jean-François

2010-05-01

The name of "adakite" is used to describe a far too large group of rocks, whose sole common feature is high Sr/Y and La/Yb ratios. Defining adakites only by this criterion is misleading, as the definition of this group of rocks does include many other criteria, including major elements. In itself, high (or commonly moderate!) Sr/Y ratios can be achieved via different processes: melting of a high Sr/Y (and La/Yb) source; deep melting, with abundant residual garnet; fractional crystallization or AFC; or interactions of felsic melts with the mantle, causing selective enrichment in LREE and Sr over HREE. A database of the compositions of "adakitic" rocks - including "high silica" and "low silica" adakites, "continental" adakites and Archaean adakites—was assembled. Geochemical modeling of the potential processes is used to interpret it, and reveals that (1) the genesis of high-silica adakites requires high pressure evolution (be it by melting or fractionation), in equilibrium with large amounts of garnet; (2) low-silica adakites are explained by garnet-present melting of an adakite-metasomatized mantle, i.e at depths greater than 2.5 GPa; (3) "Continental" adakites is a term encompassing a huge range of rocks, with a corresponding diversity of petrogenetic processes, and most of them are different from both low- and high- silica adakites; in fact in many cases it is a complete misnomer and the rocks studied are high-K calc-alkaline granitoids or even S-type granites; (4) Archaean adakites show a bimodal composition range, with some very high Sr/Y examples (similar to part of the TTG suite) reflecting deep melting (> 2.0 GPa) of a basaltic source with a relatively high Sr/Y, while lower Sr/Y rocks formed by shallower (1.0 GPa) melting of similar sources. Comparison with the Archaean TTG suite highlights the heterogeneity of the TTGs, whose composition spreads the whole combined range of HSA and Archaean adakites, pointing to a diversity of sources and processes contributing to the "TTG suite".

Continental basalts record the crust-mantle interaction in oceanic subduction channel: A geochemical case study from eastern China

NASA Astrophysics Data System (ADS)

Xu, Zheng; Zheng, Yong-Fei

2017-09-01

Continental basalts, erupted in either flood or rift mode, usually show oceanic island basalts (OIB)-like geochemical compositions. Although their depletion in Sr-Nd isotope compositions is normally ascribed to contributions from the asthenospheric mantle, their enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) is generally associated with variable enrichments in the Sr-Nd isotope compositions. This indicates significant contributions from crustal components such as igneous oceanic crust, lower continental crust and seafloor sediment. Nevertheless, these crustal components were not incorporated into the mantle sources of continental basalts in the form of solidus rocks. Instead they were processed into metasomatic agents through low-degree partial melting in order to have the geochemical fractionation of the largest extent to achieve the enrichment of LILE and LREE in the metasomatic agents. Therefore, the mantle sources of continental basalts were generated by metasomatic reaction of the depleted mid-ocean ridge basalts (MORB) mantle with hydrous felsic melts. Nevertheless, mass balance considerations indicate differential contributions from the mantle and crustal components to the basalts. While the depleted MORB mantle predominates the budget of major elements, the crustal components predominate the budget of melt-mobile incompatible trace elements and their pertinent radiogenic isotopes. These considerations are verified by model calculations that are composed of four steps in an ancient oceanic subduction channel: (1) dehydration of the subducting crustal rocks at subarc depths, (2) anataxis of the dehydrated rocks at postarc depths, (3) metasomatic reaction of the depleted MORB mantle peridotite with the felsic melts to generate ultramafic metasomatites in the lower part of the mantle wedge, and (4) partial melting of the metasomatites for basaltic magmatism. The composition of metasomatites is quantitatively dictated by the crustal metasomatism through melt-peridotite reaction at the slab-mantle interface in oceanic subduction channels. Continental basalts of Mesozoic to Cenozoic ages from eastern China are used as a case example to illustrate the above petrogenetic mechanism. Subduction of the paleo-Pacific oceanic slab beneath the eastern edge of Eurasian continent in the Early Mesozoic would have transferred the crustal signatures into the mantle sources of these basalts. This process would be associated with rollback of the subducting slab at that time, whereas the partial melting of metasomatites takes place mainly in the Late Mesozoic to Cenozoic to produce the continental basalts. Therefore, OIB-like continental basalts are also the product of subduction-zone magmatism though they occur in intraplate settings.

Subducted slab-plume interaction traced by magnesium isotopes in the northern margin of the Tarim Large Igneous Province

NASA Astrophysics Data System (ADS)

Cheng, Zhiguo; Zhang, Zhaochong; Xie, Qiuhong; Hou, Tong; Ke, Shan

2018-05-01

Incorporation of subducted slabs may account for the geochemical and isotopic variations of large igneous provinces (LIPs). However, the mechanism and process by which subducted slabs are involved into magmas is still highly debated. Here, we report a set of high resolution Mg isotopes for a suite of alkaline and Fe-rich rocks (including basalts, mafic-ultramafic layered intrusions, diabase dykes and mantle xenoliths in the kimberlitic rocks) from Tarim Large Igneous Province (TLIP). We observed that δ26 Mg values of basalts range from -0.29 to - 0.45 ‰, -0.31 to - 0.42 ‰ for mafic-ultramafic layered intrusions, -0.28 to - 0.31 ‰ for diabase dykes and -0.29 to - 0.44 ‰ for pyroxenite xenoliths from the kimberlitic rocks, typically lighter than the normal mantle source (- 0.25 ‰ ± 0.04, 2 SD). After carefully precluding other possibilities, we propose that the light Mg isotopic compositions and high FeO contents should be ascribed to the involvement of recycled sedimentary carbonate rocks and pyroxenite/eclogite. Moreover, from basalts, through layered intrusions to diabase dykes, (87Sr/86Sr)i values and δ18OV-SMOW declined, whereas ε (Nd) t and δ26 Mg values increased with progressive partial melting of mantle, indicating that components of carbonate rock and pyroxenite/eclogite in the mantle sources were waning over time. In combination with the previous reported Mg isotopes for carbonatite, nephelinite and kimberlitic rocks in TLIP, two distinct mantle domains are recognized for this province: 1) a lithospheric mantle source for basalts and mafic-ultramafic layered intrusions which were modified by calcite/dolomite and eclogite-derived high-Si melts, as evidenced by enriched Sr-Nd-O and light Mg isotopic compositions; 2) a plume source for carbonatite, nephelinite and kimberlitic rocks which were related to magnesite or periclase/perovskite involvement as reflected by depleted Sr-Nd-O and extremely light Mg isotopes. Ultimately, our study suggests that subducted slabs could make important contributions to LIP generation, and establishes a potential linkage between plate tectonics and mantle plume.

Pb-isotopic Features of Primitive Rocks from Hess Deep: Distinguishing between EPR and Cocos-Nazca Mantle Source(s)

NASA Astrophysics Data System (ADS)

Jean, M. M.; Falloon, T.; Gillis, K. M.

2014-12-01

We have acquired high-precision Pb-isotopic signatures of primitive lithologies (basalts/gabbros) recovered from IODP Expedition 345.The Hess Deep Rift, located in the vicinity of the Galapagos triple junction (Cocos, Nazca, and Pacific), is viewed as one the best-studied tectonic windows into fast-spreading crust because a relatively young (<1.5 Ma) cross section of oceanic crust. This allows for (1) characterization of the mantle source(s) at Hess Deep, (2) insight into the extent of isotopic homogeneity or heterogeneity in the area, and (3) constrain the relative contributions from the intruding Cocos-Nazca spreading center. The observed Pb-isotopic variation at Hess Deep covers almost the entire range of EPR MORB (10°N to -5°S). Hess Deep samples range from 208Pb (37.3-38.25), 207Pb (15.47-15.58), 206Pb (17.69-18.91). These compositions suggest that this part of Hess Deep mantle is no more isotopically homogeneous than EPR mantle. Two distinct arrays are also observed: 208Pb-enriched (r2=0.985; n=30) and 208Pb-depleted (r2=0.988; n=6). The 208Pb/204Pb isotopes indicates that the Pb-source for some of the samples at Hess Deep had very low Th/U ratios, whereas other areas around the Galapagos microplate seem to have more "normal" ratios. These trends are less apparent when viewed with 207Pb-isotopes. Instead, the majority of basalts and gabbros follow the NHRL, however, at the depleted-end of this array a negative excursion to more enriched compositions is observed. This negative but linear trend could signify an alteration trend or mixing with an EMI-type mantle source, yet this mixing is not observed with 208Pb. This trend is also observed at Pito Deep, which has similar origins to Hess Deep (Barker et al., 2008; Pollack et al., 2009). The Galapagos region has been considered a testing ground for mixing of HIMU, Enriched Mantle, and Depleted Mantle reservoirs (e.g., Schilling et al., 2002). According to our data, however, an EPR-component must also be considered. We model Hess Deep Pb-isotopes as a 4-component system. EPR-DM-EM comprise a 'local' reservoir, but the majority of samples contain a mixture of modified-HIMU-EM-EPR, a product of incoming plume material entrained within the Galapagos Spreading Center.

Transition Element Abundances in MORB Basalts

NASA Astrophysics Data System (ADS)

Yang, S.; Humayun, M.; Salters, V. J.; Fields, D.; Jefferson, G.; Perfit, M. R.

2012-12-01

The mineralogy of the mantle sources of basalts is an important, but hard to constrain parameter, especially with the basalts as chemical probes of major element mantle composition. Geophysical models imply that the deep mantle may have significant variations in Fe and Si relative to the ambient mantle sampled by MORB. Some petrological models of sub-ridge melting involve both pyroxenite and peridotite, implying that basalts preferentially sample a pyroxenite endmember. The First-Row Transition Elements (FRTE), Ga and Ge are compatible to moderately incompatible during partial melting, and are sensitive to mineralogical variability in the mantle and thus can provide constraints on mantle source mineralogy for MORB. We have analyzed major elements, FRTE, Ga and Ge on 231 basaltic glasses from the Middle Atlantic Ridge (MAR between -23°S to 36.44°N), 30 Mid-Cayman Rise basaltic glasses, 12 glasses from the Siqueiros Fracture Zone (EPR), 9 glasses from the Blanco Trough, Juan de Fuca ridge, and Galapagos Spreading Centers (EPR), and 4 Indian Ocean MORB. Large spots (150 μm) were precisely (±1%) analyzed by a New Wave UP193FX excimer (193 nm) laser ablation system coupled to a high-resolution ICP-MS at the National High Magnetic Field Laboratory using a high ablation rate (50 Hz) to yield blank contributions <1% for all elements, particularly Ge. The data demonstrate that the Ge/Si (6.96 x 10E-6 ± 3%, 1σ) and Fe/Mn (55 ± 2%) ratios for MORB are insensitive to fractional crystallization within the MgO range 6%-10%. MORB have Zn/Fe (9.9 x 10E-4 ± 7%), Ga/Sc (0.37-0.50), Ga/Al (2.2 x 10E-4 ± 11%) ratios, with the variations mostly due to the effects of fractional crystallization. Recent experimental determination of FRTE, Ga and Ge partition coefficients provide a framework within which to interpret these data [1]. Using these new partition coefficients, we have modeled the sensitivity of each element to mineralogical variations in the mantle source. Olivine primarily controls the partitioning of Fe, Zn, Ga and Ge; garnet dominates the Sc abundance; spinel exerts exceptionally strong control over Ga and Zn, and cannot be neglected as a source mineral for these elements. MORB FRTE, Ga and Ge abundances are consistent with partial melting of a spinel peridotite source (<1% garnet) similar to that estimated for DMM, although the abundances of many of these elements need to be better constrained in the model sources. [1] Davis et al. GCA (submitted)

Deformation in the mantle wedge associated with Laramide flat-slab subduction

NASA Astrophysics Data System (ADS)

Behr, W. M.; Smith, D.

2013-12-01

Early Tertiary crustal deformation preserved ~1500 km from the plate boundary in the western U.S. is considered by most to be related to a narrow segment of shallow Farallon-slab subduction, similar to the modern Pampean flat-slab of the central Andes. Evidence that the slab shallowed enough to penetrate several hundred kilometers inboard of the plate boundary includes a) shearing off of lithosphere and underplating of schists derived from the accretionary wedge beneath the volcanic arc; b) a cessation of arc magmatism and eastward sweeping of the magmatic front; and c) mid-Tertiary eruptions as far east as the Four Corners region of serpentinized ultramafic microbreccia (SUM) sourced from very cold, hydrated mantle lithosphere. Included within the SUM diatremes are eclogites interpreted to represent fragments of the slab itself and/or remnants of older rock from the mantle wedge metasomatized and recrystallized to eclogite along the top of the slab. Also included within the SUM diatremes are deformed peridotites that represent pieces of the variably hydrated mantle wedge as well as tectonically eroded and entrained fragments of the plate interface. These include weakly deformed to strongly foliated tectonites, spectacularly sheared mylonites and ultramylonites, and cataclasites, formed at temperatures ranging from 500-650°C. Some of the deformed samples contain hydrous minerals, including antigorite, chlorite, and/or tremolite/pargasite that were formed in-situ prior to or during deformation. We investigate the rheological and seismic properties of the peridotite samples using detailed microstructural and petrological analyses. Initial EBSD data indicate that an antigorite-bearing mylonite exhibits a B-type olivine LPO, whereas an ultramylonite that lacks hydrous minerals exhibits an A-type olivine LPO. This is consistent with experimental data that indicate B-type LPOs form under hydrous conditions; and it suggests that these rocks record a transition from trench-parallel to trench-perpendicular seismic anisotropy, as commonly observed in the mantle wedge above active subduction zones. We also show that the deformation within these sheared peridotites can be used to estimate the magnitude of shear stress along the contact between the Farallon slab and the overlying North American lithosphere. Shear stresses along the plate interface were moderate to high (~40 MPa), allowing a strong degree of interplate coupling, consistent with the stress transfer required to deform the upper plate and produce the basement-cored uplifts characteristic of the Laramide orogeny (e.g. the Rocky Mountains). These results place important natural constraints on flat-slab subduction mechanics. Schematic representation of Laramide flat-slab subduction (modified from Humphreys et al., 2003, Int. Geo. Rev.). The mantle inclusions examined here are sourced from the mantle wedge above the slab and from a serpentinite melange along the slab interface.

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

NASA Astrophysics Data System (ADS)

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

1990-07-01

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

Lateral variation in upper mantle temperature and composition beneath mid-ocean ridges inferred from shear-wave propagation, geoid, and bathymetry. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Sheehan, Anne Francis

1991-01-01

Resolution of both the extent and mechanism of lateral heterogeneity in the upper mantle constraints the nature and scales of mantle convection. Oceanic regions are of particular interest as they are likely to provide the closest glimpse at the patterns of temperature anomalies and convective flow in the upper mantle because of their young age and simple crustal structure relative to continental regions. Lateral variations were determined in the seismic velocity and attenuation structure of the lithosphere and astenosphere beneath the oceans, and these seismological observations were combined with the data and theory of geoid and bathymetry anomalies in order to test and improve current models for seafloor spreading and mantle convection. Variations were determined in mantle properties on a scale of about 1000 km, comparable to the thickness of the upper mantle. Seismic velocity, geoid, and bathymetry anomalies are all sensitive to variations in upper mantle density, and inversions were formulated to combine quantitatively these different data and to search for a common origin. Variations in mantle density can be either of thermal or compositional origin and are related to mantle convection or differentiation.

Mantle Sources Beneath the SW Indian Ridge - Remelting the African Superplume

NASA Astrophysics Data System (ADS)

Dick, H. J. B.; Zhou, H.

2012-04-01

The SW Indian Ridge runs some 7700 km from the Bouvet to the Rodgriguez Triple Junction, crossing over or near two postulated mantle plumes. The latter are associated with large oceanic rises where the ridge axis shoals dramatically in the vicinity of the mantle hotspot. The Marion Rise, extends 3100 km from the Andrew Bain FZ to near the Rodriguez TJ, with an along axis rise of 5600-m to it crest north of Marion Island. The rise has thin crust inferred on the basis of abundant exposures of mantle peridotites along its length. We suggest that this is the result of its sub-axial mantle source, which is a depleted residue originally emplaced by the African Superplume into the asthenosphere beneath southern Africa during the Karoo volcanic event ~185 Ma. Based on shallow mantle anisotropy, plate reconstructions, and hotspot traces, it now forms the mantle substrate for the SW Indian Ridge due to the breakup of Gondwanaland. The Marion Rise is associated with Marion Island, the present location of the Marion Hotspot, some 256 km south of the modern ridge. This plume is a vestigial remnant of the African Superplume now imbedded in and centered on asthenospheric mantle derived from the Karoo event. Based on the numerous large offset fracture zones, which would dam sub-axial asthenospheric flow along the ridge, the low postulated flux of the Marion plume, its off-axis position, and the thin crust along the ridge it is clear that the present day plume does not support the Marion Rise. Instead, this must be supported isostatically by the underlying mantle residue of the Karoo event. The Bouvet Rise is much shorter than the Marion Rise, extending ~664 km from the Conrad FZ on the American-Antarctic Ridge to the Shaka FZ on the SW Indian Ridge. It has ~3000-m of axial relief, peaking at Speiss Smt at Speiss Ridge: the last spreading segment of the SW Indian Ridge adjacent to the Bouvet TJ. Unlike the Marion plume, Bouvet is ridge-centered, and much of its rise is likely supported by sub-axial flow of hot mantle from the present-day plume. It is also clear from the isotopic composition of the Bouvet Plume that while it may also be a manifestation of the underlying seismic anomaly situated above D" that gave rise to the Marion Plume, this source must be compositionally heterogeneous at a very large scale. Secondary mantle heterogeneities are evident beyond those associated with the Marion and Bouvet Plumes. These likely explain the frequently extreme local isotopic variability of MORB along the SW Indian Ridge, and are likely due to entrainment of cratonic lithosphere from beneath Africa into the asthenosphere (e.g.: Meyzen et al., Nature, 2003). This is supported by major element anomalies in peridotites from adjacent to the 750-km offset Andrew Bain FZ, and by anomalously thick crust situated at Atlantis Bank, the site of an abrupt MORB isotopic anomaly, that suggest anomalously fertile mantle sources inconsistent with the regional basalt and peridotite major element compositional gradients attributed to the Superplume.

Sources of Magmatic Volatiles Discharging from Subduction Zone Volcanoes

NASA Astrophysics Data System (ADS)

Fischer, T.

2001-05-01

Subduction zones are locations of extensive element transfer from the Earth's mantle to the atmosphere and hydrosphere. This element transfer is significant because it can, in some fashion, instigate melt production in the mantle wedge. Aqueous fluids are thought to be the major agent of element transfer during the subduction zone process. Volatile discharges from passively degassing subduction zone volcanoes should in principle, provide some information on the ultimate source of magmatic volatiles in terms of the mantle, the crust and the subducting slab. The overall flux of volatiles from degassing volcanoes should be balanced by the amount of volatiles released from the mantle wedge, the slab and the crust. Kudryavy Volcano, Kurile Islands, has been passively degassing at 900C fumarole temperatures for at least 40 years. Extensive gas sampling at this basaltic andesite cone and application of CO2/3He, N2/3He systematics in combination with C and N- isotopes indicates that 80% of the CO2 and approximately 60% of the N 2 are contributed from a sedimentary source. The mantle wedge contribution for both volatiles is, with 12% and 17% less significant. Direct volatile flux measurements from the volcano using the COSPEC technique in combination with direct gas sampling allows for the calculation of the 3He flux from the volcano. Since 3He is mainly released from the astenospheric mantle, the amount of mantle supplying the 3He flux can be determined if initial He concentrations of the mantle melts are known. The non-mantle flux of CO2 and N2 can be calculated in similar fashion. The amount of non-mantle CO2 and N2 discharging from Kudryavy is balanced by the amount of CO2 and N2 subducted below Kudryavy assuming a zone of melting constrained by the average spacing of the volcanoes along the Kurile arc. The volatile budget for Kudryavy is balanced because the volatile flux from the volcano is relatively small (75 t/day (416 Mmol/a) SO2, 360 Mmol/a of non-mantle CO2 and 5.4 Mmol/a of non-mantle N2). Other subduction zone volcanoes are currently degassing a much more substantial amount of volatiles. Popocatepetl, Mexico, has degassed approximately 14 Mt of SO2 to the atmosphere over the past 6 years (Witter et al. 2000). Satsuma-Iwojima, Japan, has degassed for longer than 800 years and is currently releasing 500-1000 tones/day (Kazahaya et al. 2000). At these volcanoes CO2 and N2 discharges from the magma should also be balanced by the supply from slab and crustal sources. The rate of subduction off Mexico and Japan, however, is similar to the rate at the Kuriles. Therefore, large amounts of slab derived volatiles must be, in some fashion, stored in the "subduction factory" to supply the large amounts degassing passively from these volcanoes. Kazahaya et al. (2000) Seventh Field Workshop on Volcanic Gases, IAVCEI. Witter et al (2000) Seventh Field Workshop on Volcanic Gases, IAVCEI.

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

NASA Astrophysics Data System (ADS)

Madrigal Quesada, P.; Gazel, E.

2017-12-01

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

Experimental determination of dissolved CO2 content in nominally anhydrous andesitic melts at graphite/diamond saturation - Remobilization of deeply subducted reduced carbon via partial melts of MORB-like eclogite

NASA Astrophysics Data System (ADS)

Eguchi, J.; Dasgupta, R.

2015-12-01

Experimental phase relations of carbonated lithologies [1] and geochemistry of deep diamonds [2] suggest that deep recycling of carbon has likely been efficient for a significant portion of Earth's history. Both carbonates and organic carbon subduct into the mantle, but with gradual decrease of fO2 with depth [3] most carbon in deep mantle rocks including eclogite could be diamond/graphite [4]. Previous studies investigated the transfer of CO2 from subducted eclogite to the ambient mantle by partial melting in the presence of carbonates, i.e., by generation of carbonate-rich melts [5]. However, the transfer of carbon from subducted eclogite to the mantle can also happen, perhaps more commonly, by extraction of silicate partial melt in the presence of reduced carbon; yet, CO2 solubility in eclogite-derived andesitic melt at graphite/diamond saturation remains unconstrained. CO2content of eclogite melts is also critical as geochemistry of many ocean island basalts suggest the presence of C and eclogite in their source regions [6]. In the present study we determine CO2 concentration in a model andesitic melt [7] at graphite/diamond saturation at conditions relevant for partial melting of eclogite in the convecting upper mantle. Piston cylinder and multi anvil experiments were conducted at 1-6 GPa and 1375-1550 °C using Pt/Gr double capsules. Oxygen fugacity was monitored with Pt-Fe sensors in the starting mix. Completed experiments at 1-3 GPa show that CO2 concentration increases with increasing P, T, and fO2 up to ~0.3 wt%. Results were used to develop empirical and thermodynamic models to predict CO2 concentration in partial melts of graphite saturated eclogite. This allowed us to quantify the extent to which CO2 can mobilize from eclogitic heterogeneities at graphite/diamond saturated conditions. With estimates of eclogite contribution to erupted basaltic lavas, the models developed here allow us to put constraints on the flux of CO2 to mantle source regions coming from subducted crust and investigate the possible role this process may play in the deep carbon cycle. [1] Dasgupta (2013) RiMG. [2] Shirey, et al. (2013) RiMG. [3] Frost & McCammon (2008) Ann Rev Earth Plan Sci. [4] Stagno, et al. (2015) CMP. [5] Kiseeva, et al. (2012) JPet. [6] Mallik & Dasgupta (2014) G3. [7] Spandler, et al. (2008) JPet.

Coupled petrological-geodynamical modeling of a compositionally heterogeneous mantle plume

NASA Astrophysics Data System (ADS)

Rummel, Lisa; Kaus, Boris J. P.; White, Richard W.; Mertz, Dieter F.; Yang, Jianfeng; Baumann, Tobias S.

2018-01-01

Self-consistent geodynamic modeling that includes melting is challenging as the chemistry of the source rocks continuously changes as a result of melt extraction. Here, we describe a new method to study the interaction between physical and chemical processes in an uprising heterogeneous mantle plume by combining a geodynamic code with a thermodynamic modeling approach for magma generation and evolution. We pre-computed hundreds of phase diagrams, each of them for a different chemical system. After melt is extracted, the phase diagram with the closest bulk rock chemistry to the depleted source rock is updated locally. The petrological evolution of rocks is tracked via evolving chemical compositions of source rocks and extracted melts using twelve oxide compositional parameters. As a result, a wide variety of newly generated magmatic rocks can in principle be produced from mantle rocks with different degrees of depletion. The results show that a variable geothermal gradient, the amount of extracted melt and plume excess temperature affect the magma production and chemistry by influencing decompression melting and the depletion of rocks. Decompression melting is facilitated by a shallower lithosphere-asthenosphere boundary and an increase in the amount of extracted magma is induced by a lower critical melt fraction for melt extraction and/or higher plume temperatures. Increasing critical melt fractions activates the extraction of melts triggered by decompression at a later stage and slows down the depletion process from the metasomatized mantle. Melt compositional trends are used to determine melting related processes by focusing on K2O/Na2O ratio as indicator for the rock type that has been molten. Thus, a step-like-profile in K2O/Na2O might be explained by a transition between melting metasomatized and pyrolitic mantle components reproducible through numerical modeling of a heterogeneous asthenospheric mantle source. A potential application of the developed method is shown for the West Eifel volcanic field.

Noble gas data from Goldfield and Tonopah epithermal Au-Ag deposits, ancestral Cascades Arc, USA: Evidence for a primitive mantle volatile source

USGS Publications Warehouse

Manning, Andrew H.; Hofstra, Albert H.

2017-01-01

The He, Ne, and Ar isotopic composition of fluid inclusions in ore and gangue minerals were analyzed to determine the source of volatiles in the high-grade Goldfield and Tonopah epithermal Au-Ag deposits in southwestern Nevada, USA. Ar and Ne are mainly atmospheric, whereas He has only a minor atmospheric component. Corrected 3He/4He ratios (with atmospheric He removed) range widely from 0.05 to 35.8 times the air 3He/4He ratio (RA), with a median of 1.43 RA. Forty-one percent of measured 3He/4He ratios are ≥4 RA, corresponding to ≥50% mantle He assuming a mantle ratio of 8 RA. These results suggest that mafic magmas were part of the magmatic-hydrothermal system underlying Goldfield and Tonopah, and that associated mantle-sourced volatiles may have played a role in ore formation. The three highest corrected 3He/4He ratios of 17.0, 23.7, and 35.8 RAindicate a primitive mantle He source and are the highest yet reported for any epithermal-porphyry system and for the Cascades arc region. Compiled 3He/4He measurements from epithermal-porphyry systems in subduction-related magmatic arcs around the world (n = 209) display a statistically significant correlation between 3He/4He and Au-Ag grade. The correlation suggests that conditions which promote higher fluid inclusion 3He/4He ratios (abundance of mantle volatiles and focused upward volatile transport) have some relation to conditions that promote higher Au-Ag grades (focused flow of metal-bearing fluids and efficient chemical traps). Results of this and previous investigations of He isotopes in epithermal-porphyry systems are consistent with the hypothesis posed in recent studies that mafic magmas serve an important function in the formation of these deposits.

Lunar initial Nd-143/Nd-144 - Differential evolution of the lunar crust and mantle

NASA Technical Reports Server (NTRS)

Lugmair, G. W.; Marti, K.

1978-01-01

The Sm-Nd evolution of Apollo 15 green glass is discussed. The ICE age (intercept with chondritic evolution) of 3.8 + or - 0.4 eons overlaps the range of reported (Ar-39)-(Ar-40) ages and implies a distinct source region for green glass, characterized by very low and unfractionated REE abundances. Evidence is presented that LINd (lunar initial Nd) is compatible with a 'chondritic'-type Nd isotopic evolution as observed in the Juvinas meteorite. This normalization is used to study the Sm-Nd system of various lunar rock types. The results obtained from a limited number of rocks clearly indicate differential Sm-Nd evolution for the lunar crust and mantle. High-Ti basalts returned by the Apollo 11 and 17 missions were derived from distinct source regions. The Nd-143 evolution in KREEP requires a source region which is clearly distinct from any mantle reservoir.

Tungsten Abundances in Hawaiian Picrites: Implications for the Mantle Sources of Hawaiian Volcanoes

NASA Astrophysics Data System (ADS)

Ireland, T. J.; Arevalo, R. D.; Walker, R. J.; McDonough, W. F.

2008-12-01

Tungsten abundances have been measured in a suite of Hawaiian picrites (MgO >13 wt.%) from nine Hawaiian shield volcanoes (Mauna Kea, Mauna Loa, Hualalai, Loihi, Koolau, Kilauea, Kohala, Lanai and Molokai). Tungsten concentrations in the parental melts for these volcanoes have been estimated via the intersection of linear W-MgO trends with the putative MgO content of the parental melt (~16 wt.%). Tungsten behaves as a highly incompatible trace element in mafic to ultramafic systems; thus, given an independent assessment of the degree of partial melting for each volcanic center, the W abundances in their mantle sources can be determined. The mantle sources for Hualalai, Kilauea, Kohala and Loihi have non- uniform estimated W abundances of 11, 13, 16 and 27 ng/g, respectively, giving an average source abundance of 17±5 ng/g. This average source abundance is nearly six times more enriched than Depleted MORB Mantle (DMM: 3.0±2.3 ng/g) and slightly elevated relative to the Bulk Silicate Earth (BSE: 13±10 ng/g). The relatively high abundances of W in the Hawaiian sources relative to the DMM can potentially be explained as a consequence of crustal recycling. For example, incorporation of 30% oceanic crust (30 ng/g W), including 3% sediment (1500 ng/g W), into a DMM source could create the W enrichment observed in the Loihi source, consistent with estimates from earlier models based on other trace elements and isotope systems. The Hualalai source, however, has also been suggested to contain a substantial recycled component, as implied by similarly radiogenic 187Os/188Os, yet this source has the lowest estimated W abundance among the volcanic centers studied. The conflict between these results may: 1) reflect chemical differences among recycled components, 2) indicate a more complex history for Hualalai samples, e.g. involvement of a melt percolation component, or 3) implicate other sources of W.

Redox state of earth's upper mantle from kimberlitic ilmenites

NASA Technical Reports Server (NTRS)

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

1983-01-01

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

HIMU-type Mid-Ocean Ridge Basalts Incorporate a Primitive Component

NASA Astrophysics Data System (ADS)

Tucker, J.; Mukhopadhyay, S.; Schilling, J. E.

2011-12-01

Samples from 5°N to 7°S along the MAR axis span a range of compositions from depleted MORB (La/SmN ~0.5, 206Pb/204Pb ~18) to very enriched MORB (La/SmN ~3, 206Pb/204Pb ~20). The measured 206Pb/204Pb in the enriched samples are among the highest measured MORB values and are thought to represent a HIMU type mantle (high μ where μ is the U/Pb ratio). Therefore, the enriched samples provide a unique opportunity to characterize the heavy noble gas composition of the HIMU mantle. If HIMU mantle is related to recycled crust, then the noble gas measurements can also provide insights into recycling of atmospheric noble gases back into the mantle. Additionally, the depleted equatorial samples provide an opportunity to characterize the Ar and Xe composition of the N-MORB source for comparison to the 14°N E-MORB popping rock. Finally, the large variations in lithophile isotopes over a geographically short distance affords the opportunity to study the nature of coupling between the noble gases and lithophile tracers, and understand the origin of the heterogeneities in the MORB source. Stepwise crushing and rare gas analysis (He, Ne, Ar, Xe) was undertaken for both enriched and depleted samples. Many of the crushing steps yielded 20Ne/22Ne > 12, and good correlations between Ne, Ar, and Xe isotopes allow for mantle source compositions of Ar and Xe to be determined by extrapolating the measured values to a mantle 20Ne/22Ne of 12.5. The highest measured values of Ar and Xe in a depleted N-MORB are comparable to measured values of the E-MORB popping rock (40Ar/36Ar ~28,000, 129Xe/130Xe ~7.7). When extrapolated to a mantle 20Ne/22Ne of 12.5, the depleted MORB sample indicates a 40Ar/36Ar of ~43,000 (higher than popping rock) and 129Xe/130Xe of ~7.8. Enriched MORB samples from this suite, thought to represent the HIMU mantle, have the same He and Ne characteristics as HIMU basalts from the Cook and Austral Islands; more radiogenic He than MORBs is accompanied by less nucleogenic Ne than MORBs. Additionally, the enriched MORB samples also constrain the HIMU mantle 40Ar/36Ar to ~20,000 and 129Xe/130Xe ~7.3-7.5, significantly lower than the depleted MORBs. Like the HIMU basalts from the Cook and Austral Islands, a less degassed reservoir than the MORB source must be invoked to explain the He and Ne systematics in the HIMU-type MORBs. If HIMU represents recycled crust, then it must have entrained or been entrained by a less degassed mantle from the deep interior. This less degassed reservoir would also explain the good correspondence between low 21Ne/22Ne, low 40Ar/36Ar and low 129Xe/130Xe in the HIMU-type samples. While we cannot rule out recycling of atmospheric noble gases to explain the low 40Ar/36Ar and 129Xe/130Xe, involvement of a source less degassed in He and Ne would also be accompanied by a less degassed Ar and Xe isotopic signature. Therefore the simplest explanation of the covariation between the noble gases and lithophile isotopes involves a mixture of a less processed and hence more primitive component, a degassed recycled component, and depleted MORB mantle beneath the equatorial Mid-Atlantic Ridge.

Shield volcanism and lithospheric structure beneath the Tharsis plateau, Mars

NASA Technical Reports Server (NTRS)

Blasius, K. R.; Cutts, J. A.

1976-01-01

The heights of four great shield volcanoes, when interpreted as reflecting the local hydrostatic head on a common source of upwelling magma, provide significant constraints on models of lithospheric structure beneath the Tharsis plateau. If Bouguer gravity anomalies are modeled in terms of a variable thickness crust, and a two-component (crust/mantle) earth-like structure is assumed for the Martian lithosphere, the derived model lithosphere beneath the Tharsis plateau has the following properties: (1) the upper low-density 'crustal' component is thickened beneath the Tharsis plateau; (2) the lower high-density 'mantle' component is thinned beneath the Tharsis plateau; and (3) there is a net gradient on the base of the Martian lithosphere directed downward away from beneath the summit of the Tharsis plateau. A long history of magmatic intrusion is hypothesized to have been the cause of the updoming of the Tharsis plateau and the maintenance of the plateau in a state of only partial compensation.

Hardening mechanisms in olivine single crystal deformed at 1090 °C: an electron tomography study

NASA Astrophysics Data System (ADS)

Mussi, Alexandre; Cordier, Patrick; Demouchy, Sylvie; Hue, Benoit

2017-11-01

The dislocation microstructures in a single crystal of olivine deformed experimentally in uniaxial compression at 1090 °C and under a confining pressure of 300 MPa, have been investigated by transmission electron tomography in order to better understand deformation mechanisms at the microscale relevant for lithospheric mantle deformations. Investigation by electron tomography reveals microstructures, which are more complex than previously described, composed of ? and ? dislocations commonly exhibiting 3D configurations. Numerous mechanisms such as climb, cross-slip, double cross-slip as well as interactions like junction formations and collinear annihilations are the source of this complexity. The diversity observed advocates for microscale deformation of olivine significantly less simple than classic dislocation creep reported in metals or ice close to melting temperature. Deciphering mechanism of hardening in olivine at temperatures where ionic diffusion is slow and is then expected to play very little role is crucial to better understand and thus model deformation at larger scale and at temperatures (900-1100 °C) highly relevant for the lithospheric mantle.

Stretching and smearing of chemical heterogeneity by melting and melt migration beneath mid-ocean ridges

NASA Astrophysics Data System (ADS)

Liu, B.; Liang, Y.

2017-12-01

The size of mantle source heterogeneity is important to the interpretation of isotopic signals observed in residual peridotites and basalts. During concurrent melting and melt migration beneath a mid-ocean ridge, both porosity and melt velocity increase upward, resulting in an upward increase in the effective transport velocity for a trace element. Hence a chemical heterogeneity of finite size will be stretched during its transport in the upwelling mantle. This melt migration induced chemical deformation can be quantified by a simple stretching factor. During equilibrium melting, the isotope signals of Sr, Nd and Hf in a 1 km size enriched mantle will be stretched to 2 6 km at the top of the melting column, depending on the style of melt migration. A finite rate of diffusive exchange between residual minerals and partial melt will result in smearing of chemical heterogeneity during its transport in the upwelling melting column. A Gaussian-shaped enriched source in depleted background mantle would be gradually deformed its transit through the melting column. The width of the enriched signal spreads out between the fronts of melt and solid while its amplitude decreases. This melt migration induced smearing also cause mixing of nearby heterogeneities or absorption of enriched heterogeneity by the ambient mantle. Smaller heterogeneities in the solid is more efficiently mixed or aborted by the background mantle than larger ones. Mixing of heterogeneities in the melt depends on the size in the same sense although the erupted melt is more homogenized due to melt accumulation and magma chamber process. The mapping of chemical heterogeneities observed in residual peridotites and basalts into their source region is therefore highly nonlinear. We will show that the observed variations in Nd and Hf isotopes in the global MORB and abyssal peridotites are consistent with kilometer-scale enriched heterogeneities embedded in depleted MORB mantle.

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.

Seismic anisotropy in the Hellenic subduction zone: Effects of slab segmentation and subslab mantle flow

NASA Astrophysics Data System (ADS)

Evangelidis, C. P.

2017-12-01

The segmentation and differentiation of subducting slabs have considerable effects on mantle convection and tectonics. The Hellenic subduction zone is a complex convergent margin with strong curvature and fast slab rollback. The upper mantle seismic anisotropy in the region is studied focusing at its western and eastern edges in order to explore the effects of possible slab segmentation on mantle flow and fabrics. Complementary to new SKS shear-wave splitting measurements in regions not adequately sampled so far, the source-side splitting technique is applied to constrain the depth of anisotropy and to densify measurements. In the western Hellenic arc, a trench-normal subslab anisotropy is observed near the trench. In the forearc domain, source-side and SKS measurements reveal a trench-parallel pattern. This indicates subslab trench-parallel mantle flow, associated with return flow due to the fast slab rollback. The passage from continental to oceanic subduction in the western Hellenic zone is illustrated by a forearc transitional anisotropy pattern. This indicates subslab mantle flow parallel to a NE-SW smooth ramp that possibly connects the two subducted slabs. A young tear fault initiated at the Kefalonia Transform Fault is likely not entirely developed, as this trench-parallel anisotropy pattern is observed along the entire western Hellenic subduction system, even following this horizontal offset between the two slabs. At the eastern side of the Hellenic subduction zone, subslab source-side anisotropy measurements show a general trench-normal pattern. These are associated with mantle flow through a possible ongoing tearing of the oceanic lithosphere in the area. Although the exact geometry of this slab tear is relatively unknown, SKS trench-parallel measurements imply that the tear has not reached the surface yet. Further exploration of the Hellenic subduction system is necessary; denser seismic networks should be deployed at both its edges in order to achieve a more definite image of the structure and geodynamics of this area.

Silicon isotopes reveal recycled altered oceanic crust in the mantle sources of Ocean Island Basalts

NASA Astrophysics Data System (ADS)

Pringle, Emily A.; Moynier, Frédéric; Savage, Paul S.; Jackson, Matthew G.; Moreira, Manuel; Day, James M. D.

2016-09-01

The study of silicon (Si) isotopes in Ocean Island Basalts (OIB) has the potential to discern between different models for the origins of geochemical heterogeneities in the mantle. Relatively large (∼several per mil per atomic mass unit) Si isotope fractionation occurs in low-temperature environments during biochemical and geochemical precipitation of dissolved Si, where the precipitate is preferentially enriched in the lighter isotopes relative to the dissolved Si. In contrast, only a limited range (∼tenths of a per mil) of Si isotope fractionation has been observed from high-temperature igneous processes. Therefore, Si isotopes may be useful as tracers for the presence of crustal material within OIB mantle source regions that experienced relatively low-temperature surface processes in a manner similar to other stable isotope systems, such as oxygen. Characterizing the isotopic composition of the mantle is also of central importance to the use of the Si isotope system as a basis for comparisons with other planetary bodies (e.g., Moon, Mars, asteroids). Here we present the first comprehensive suite of high-precision Si isotope data obtained by MC-ICP-MS for a diverse suite of OIB. Samples originate from ocean islands in the Pacific, Atlantic, and Indian Ocean basins and include representative end-members for the EM-1, EM-2, and HIMU mantle components. On average, δ30Si values for OIB (-0.32 ± 0.09‰, 2 sd) are in general agreement with previous estimates for the δ30Si value of Bulk Silicate Earth (-0.29 ± 0.07‰, 2 sd; Savage et al., 2014). Nonetheless, some small systematic variations are present; specifically, most HIMU-type (Mangaia; Cape Verde; La Palma, Canary Islands) and Iceland OIB are enriched in the lighter isotopes of Si (δ30Si values lower than MORB), consistent with recycled altered oceanic crust and lithospheric mantle in their mantle sources.

Circumventing shallow air contamination in Mid Ocean Ridge Basalts

NASA Astrophysics Data System (ADS)

Mukhopadhyay, Sujoy; Parai, Rita; Tucker, Jonathan; Middleton, Jennifer; Langmuir, Charles

2016-04-01

Noble gases in mantle-derived basalts provide a rich portrait of mantle degassing and surface-interior volatile exchange. However, the ubiquity of shallow-level air contamination frequently obscures the mantle noble gas signal. In a majority of samples, shallow air contamination dominates the noble gas budget. As a result, reconstructing the variability in heavy noble gas mantle source compositions and inferring the history of deep recycling of atmospheric noble gases is difficult. For example, in the gas-rich popping rock 2ΠD43, 129Xe/130Xe ratios reach 7.7±0.23 in individual step-crushes, but the bulk composition of the sample is close to air (129Xe/130Xe of 6.7). Here, we present results from experiments designed to elucidate the source of shallow air contamination in MORBs. Step-crushes were carried out to measure He, Ne, Ar and Xe isotopic compositions on two aliquots of a depleted popping glass that was dredged from between the Kane and Atlantis Fracture Zones of the Mid-Atlantic Ridge in May 2012. One aliquot was sealed in ultrapure N2 after dredge retrieval, while the other aliquot was left exposed to air for 3.5 years. The bulk 20Ne/22Ne and 129Xe/130Xe ratios measured in the aliquot bottled in ultrapure N2 are 12.3 and 7.6, respectively, and are nearly identical to the estimated mantle source values. On the other hand, step crushes in the aliquot left exposed to air for several years show Ne isotopic compositions that are shifted towards air, with a bulk 20Ne/22Ne of 11.5; the bulk 129Xe/130Xe, however, was close to 7.6. These results indicate that lighter noble gases exchange more efficiently between the bubbles trapped in basalt glass and air, suggesting a diffusive or kinetic mechanism for the incorporation of the shallow air contamination. Importantly, in Ne-Ar or Ar-Xe space, step-crushes from the bottled aliquot display a trend that can be easily fit with a simple two-component hyperbolic mixing between mantle and atmosphere noble gases. Step-crushes in the aliquot left exposed to air display significantly more scatter, which makes it difficult to fit a two-component mixing hyperbola and obtain the mantle source value for this aliquot. In summary, our simple and inexpensive experiment demonstrates that at least in some samples, significant air contamination is added after dredge retrieval from the ocean floor. Bottling samples in ultrapure N2 upon dredge retrieval can largely eliminate this component of shallow-level air contamination. As a result, the number of step crushes required to characterize a sample decreases and estimating the mantle source compositions of the basalts becomes significantly easier, which in turn leads to more refined estimates of mantle degassing and regassing rates.

Drastic shift of lava geochemistry between pre- and post- Japan Sea opening in NE Japan subduction zone: constraints on source composition and slab surface melting processes

NASA Astrophysics Data System (ADS)

Okamura, S.; Inaba, M.; Igarashi, S.; Aizawa, M.; Shinjo, R.

2017-12-01

Isotopic and trace element data imply a temporal change in magma sources and thermal conditions beneath the northern Fossa Magna, NE Japan arc from the Oligocene to the Pleistocene. Less radiogenic 176Hf/177Hf and 143Nd/144Nd, and high Zr/Hf characterize the Oligocene - Early Miocene volcanism in the northern Fossa Magna region. The mantle wedge in the Oligocene - Early Miocene consisted of enriched mantle source. We propose that during the onset of subduction, influx of hot asthenospheric mantle provided sufficient heat to partially melt newly subducting sediment. Geochemical modeling results suggest breakdown of zircon in the slab surface sediments for the Oligocene - Early Miocene lavas in the northern Fossa Magna region. In the Middle Miocene, the injection of hot and depleted asthenospheric material replaced the mantle beneath the northern Fossa Magna region of NE Japan. The Middle Miocene lavas characterized by most radiogenic Hf and Nd isotope ratios, have high Zr/Hf. An appropriate working petrogenetic model is that the Middle Miocene lavas were derived from asthenospheric depleted mantle, slightly (<1%) contaminated by slab melt accompanied by full dissolution of zircon. All the Late Miocene - Pleistocene samples are characterized by distinctly more radiogenic 176Hf/177Hf and 143Nd/144Nd, and are displaced toward lower Zr/Hf, which requires mixing between depleted mantle and a partial melt of subducted metasediment saturated with trace quantity of zircon. The Oligocene - Early Miocene volcanism in the northern Fossa Magna region may represent the early stage of continental margin magmatism associated with a back-arc rift. Here volcanism is dominated by sediment melts. Perhaps asthenospheric injection, triggering Japan Sea opening, allowed higher temperatures and more melting at the slab-mantle interface. The mantle wedge was gradually cooled during the Middle Miocene to the Pleistocene with back-arc opening ending in the Late Miocene. Slab surface temperatures were still high enough for sediments to melt but not too high (< 780 °C) to lose zircon as a residual phase.

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

USGS Publications Warehouse

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

2008-01-01

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

Upper mantle anisotropy from long-period P polarization

NASA Astrophysics Data System (ADS)

Schulte-Pelkum, Vera; Masters, Guy; Shearer, Peter M.

2001-10-01

We introduce a method to infer upper mantle azimuthal anisotropy from the polarization, i.e., the direction of particle motion, of teleseismic long-period P onsets. The horizontal polarization of the initial P particle motion can deviate by >10° from the great circle azimuth from station to source despite a high degree of linearity of motion. Recent global isotropic three-dimensional mantle models predict effects that are an order of magnitude smaller than our observations. Stations within regional distances of each other show consistent azimuthal deviation patterns, while the deviations seem to be independent of source depth and near-source structure. We demonstrate that despite this receiver-side spatial coherence, our polarization data cannot be fit by a large-scale joint inversion for whole mantle structure. However, they can be reproduced by azimuthal anisotropy in the upper mantle and crust. Modeling with an anisotropic reflectivity code provides bounds on the magnitude and depth range of the anisotropy manifested in our data. Our method senses anisotropy within one wavelength (250 km) under the receiver. We compare our inferred fast directions of anisotropy to those obtained from Pn travel times and SKS splitting. The results of the comparison are consistent with azimuthal anisotropy situated in the uppermost mantle, with SKS results deviating from Pn and Ppol in some regions with probable additional deeper anisotropy. Generally, our fast directions are consistent with anisotropic alignment due to lithospheric deformation in tectonically active regions and to absolute plate motion in shield areas. Our data provide valuable additional constraints in regions where discrepancies between results from different methods exist since the effect we observe is local rather than cumulative as in the case of travel time anisotropy and shear wave splitting. Additionally, our measurements allow us to identify stations with incorrectly oriented horizontal components.

Helium isotope data from the Goldfield epithermal system, Nevada: Evidence for volatile input from a primitive mantle source during ore formation

NASA Astrophysics Data System (ADS)

Hofstra, A. H.; Manning, A. H.

2013-12-01

Goldfield is the largest high sulfidation epithermal gold mining district in the United States with over 130 t of gold production and 23 sq. km. of argillic alteration (with alunite, pyrophyllite, or kaolinite). It formed at 20.0×0.5 Ma in an andesite to rhyolite volcanic field in the ancestral Cascades continental magmatic arc. Previous stable isotope studies of quartz, alunite, and sulfide minerals suggest that the gold ores formed in a magmatic vapor plume derived from a subjacent porphyry intrusion, which displaced and mixed with meteoric groundwater at shallow levels. The isotopic compositions of He, Ne, and Ar trapped in fluid inclusions in hydrothermal minerals (Cu-sulfides and sulfosalts, pyrite, quartz) were measured to further constrain volatile source and migration processes. Gases were released by thermal decrepitation at 300°C and analyzed using a high resolution static sector mass spectrometer. The isotopic compositions of Ne and Ar are typical of air-saturated water (ASW), indicating that the samples contain little nucleogenic Ne or radiogenic Ar derived from underlying old crustal sources. In contrast, He/Ne and He/Ar ratios are much greater than ASW, indicating that a component of He was produced in the subsurface. The wide range of He R/Ra values, 0.4 to 20, suggests that He was derived from both crustal and mantle sources. 4He/40Ar* and 4He/21Ne* systematics are characteristic of magma degassing. The highest R/Ra values (15-20) are well above those previously reported for modern volcanic rocks and geothermal fluids in subduction-related arcs. Such R/Ra values indicate a primitive mantle source, perhaps below the subducting slab. We hypothesize that the discharge of metal-laden fluids from the subjacent porphyry intrusion was influenced by the input of hot volatiles from mafic mantle-derived magmas. This scenario implies a magma column that remained open to the flux of volatiles over a considerable depth range, from the mantle to the shallow crust. This exceptional volatile plumbing system may be an important ingredient in the formation of large, high sulfidation gold deposits. The ascent of mantle-sourced volatiles may be related to the coeval transition from transpression to transtension within the western North American plate caused by microplate capture along the San Andreas transform.

Elemental composition of the Martian crust.

PubMed

McSween, Harry Y; Taylor, G Jeffrey; Wyatt, Michael B

2009-05-08

The composition of Mars' crust records the planet's integrated geologic history and provides clues to its differentiation. Spacecraft and meteorite data now provide a global view of the chemistry of the igneous crust that can be used to assess this history. Surface rocks on Mars are dominantly tholeiitic basalts formed by extensive partial melting and are not highly weathered. Siliceous or calc-alkaline rocks produced by melting and/or fractional crystallization of hydrated, recycled mantle sources, and silica-poor rocks produced by limited melting of alkali-rich mantle sources, are uncommon or absent. Spacecraft data suggest that martian meteorites are not representative of older, more voluminous crust and prompt questions about their use in defining diagnostic geochemical characteristics and in constraining mantle compositional models for Mars.

Alkaline lavas from southern Mendoza, Argentina, extend the Patagonian DUPAL mantle field to the north

NASA Astrophysics Data System (ADS)

Soager, N.; Holm, P. M.; Llambias, E.

2010-12-01

The lavas sampled around Río Colorado ~37°S at the border of Mendoza and Neuquén provinces, Argentina, define an OIB-like end-member composition for the Pleistocene and Holocene activity in the Payún Matrú volcanic field. Although positioned in the far back-arc of the Andes, only a few lavas show signs of involvement of slab fluids or crustal contamination such as relatively high LILEs relative to Nb. The very low La/Nb (~0.66) and Zr/Nb (~5) and high U/Pb (0.3-0.4) of the end-member composition clearly distinguish the source from normal MORB mantle, while high Ba/Nb (~10) and K/Nb (370-400) compared to FOZO and HIMU type OIBs suggest an EM type of mantle. Overall, the trace element patterns of the Río Colorado lavas are similar to the central and north Patagonian intraplate basalts and to South Atlantic E-MORB affected by the Discovery plume and the LOMU component (le Roux et al., 2002, EPSL 203). The isotopic composition of the Río Colorado component has a 206Pb/204Pb = 18.4, 207Pb/204Pb = 15.58, 208Pb/204Pb = 38.3, 87Sr/86Sr = 0.70353 and 143Nd/144Nd = 0.51285. This composition overlaps the central and north Patagonian intraplate basalts in Pb-isotopic space but is slightly less enriched in Sr and Nd-isotopes. It is distinctly different from the FOZO like composition of the south Patagonian intraplate basalts and the nearby Juan Fernandéz plume but similar to the South Atlantic N-MORB and MORB from the southern Chile Ridge segment 4 (Sturm et al., 1999, JGR 104) described as DUPAL type. The DUPAL-MORB type isotopic composition and the plume-like trace element patterns of the Río Colorado lavas suggest the presence of a weak plume beneath the area. The eruption of the large Payún Matrú volcano and the gigantic Pleistocene flood basalts also calls for a thermal anomaly to produce these melts during a weakly compressive tectonic regime with no significant addition of slab fluids. This was supported by Burd et al. (2008, Abstr., 7th Int. Sym. And. Geo.) who recognized a plume-like conductive structure beneath Payún Matrú volcano on an electrical resistivity profile across the Payún Matrú volcanic field. The many Argentine and Chile Ridge EM1 basalts form part of the global DUPAL-anomaly (Hart, 1984, Nature 309) which suggests a common mode of formation of the enriched mantle sources; most likely anciently subducted components in the underlying upper mantle, either in a larger reservoir or as dispersed bodies of material. From there they can rise as small plumes or be entrained in a convecting MORB source mantle.

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

NASA Astrophysics Data System (ADS)

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

2017-12-01

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

Mantle helium in ground waters of eastern North America: Time and space constraints on sources

USGS Publications Warehouse

Torgersen, T.; Drenkard, S.; Stute, M.; Schlosser, P.; Shapiro, A.

1995-01-01

Mantle helium in continental environments is generally considered to be the result of active volcanism and/or active extension. The latest episodes of volcanism in northeastern North America are the track of the New England hotspot (95–190 Ma) and the closure of the Iapetus sea (before 300 Ma). Thus, the identification of mantle helium in young ground waters of central New England is counter to the conventional wisdom. On the basis of evaluation of helium evolution in emplaced magmas, we postulate an “aged” mantle source for the excess helium component in ground waters of central New England that is either (1) a local, near-surface–emplaced, gas-rich magma that has retained significant volatiles (e.g., in fluid inclusions) or (2) a deeply emplaced gas-rich magma with high initial 2He/4He (10−5) and helium transport (with dispersion) through the crust over time. This gas-rich initial condition may support the concept of a volatile-enriched mantle wedge and thus explain the increased buoyancy flux of the New England hotspot as it traversed eastern North America, as has been suggested by others.

The Origin and Mantle Dynamics of Quaternary Intraplate Volcanism in Northeast China From Joint Inversion of Surface Wave and Body Wave

NASA Astrophysics Data System (ADS)

Guo, Zhen; Wang, Kai; Yang, Yingjie; Tang, Youcai; John Chen, Y.; Hung, Shu-Huei

2018-03-01

We present a 3-D model of NE China by joint inversion of body and surface waves. The joint inversion significantly improves the resolution at shallow depths compared with body wave tomography alone and provides seismic evidence for the origin of Quaternary volcanism in NE China. Our model reveals that the mantle upwelling beneath the Changbaishan volcano originates from the transition zone and extends up to 60 km, and spreads at the base of the lithosphere with the upwelling head 5 times wider than the raising tail in the lower upper mantle. However, low velocities beneath the Halaha and Abaga volcanoes in the Xingmeng belt are confined to depths shallower than 150 km, suggesting that magmatism in the Xingmeng belt is more likely caused by localized asthenospheric upwelling at shallow depths rather than from the common deep source. A small-scale sublithospheric mantle convection may control the spatial and temporal distribution of Quaternary magmatism in NE China; that is, the upwelling beneath the Changbaishan volcano triggers the downwelling beneath the southern Songliao basin, where the high velocity imaged extends to 300 km. The downwelling may further induce localized upwelling in the surrounding areas, such as the Halaha and Abaga volcanoes. Thanks to the joint constraints from both surface and body waves, we can estimate the dimension of the convection cell. The convection cell is located between 42°N and 45°N, spreads around 500 km in the W-E direction measured from the distance between centers of downwelling and upwelling, and extends to 300 km vertically.

The Universal Cpx Jd-Di barometer for mantle peridotite eclogite and pyroxenites and it using for the mantle petrology

NASA Astrophysics Data System (ADS)

Ashchepkov, Igor

2015-04-01

The Jd-Di exchange in clinopyroxenes used for the calibration of pyroxene barometer (Ashchepkov, 2000;2002; Ashchepkov et al 2010;2011;2012) was transformed to make one universal equation for mantle peridotite eclogites and pyroxenites. The original barometer (Ashchepkov, 2002) calibrated on pressures produced by Opx barometry (McGregor , 1974) was transformed (Ashchepkov et al ., 2004; 2010; 2011) to satisfy the increasing data bases for the mantle xenoliths and experimental values 530 in peridotitic and 650 in elcogitic systems . The obtained difference Pd =Pcpx- Pexp were studied for the dependence on each component and their combination . Instead of the common activities we used the temperature-dependent empirical equations. The three separate equations for the common peridotites, pyroxenites and eclogites (Ashchepkov et al., 2010) were checked and complex To and Al-Na-Fe dependent universal coefficients were received. The KD is determined as follows: KD=Na/AlCr*Mg/Ca The logarithmic dependence between P and KD was transformed to a linear one. Final pressure equations are: AlCr=(Al-0.01) *((T-600)/700)**0.75+Cr*(ToK-100)/1000+(4*Ti-0.0125)/ (T0-801)*650 +0.55*((Fe-0.23) *(T0-900)/10000-K) P=0.26*(5+12*(Al+0.30*Na)KD* ToK**0.75 /(1+Fe+ Fe*(ToK-600)/1000)-ln(1273/ ToK))*40*(7*Na-Al-15*Ti+10*Cr+Mg/4)+7.5*Si-20*( Al*Na*Mg/Ca/(Al-2*Ti+Na-2*Fe/(Fe+Mg))+50*(Na+0.1*Al-2*Ti+0.05*Mg-0.22*Ca-0.7*Na)/Ca). Obtained equation in combination with the (Nimis,Taylor, 2000) thermometer allow to reconstruct position of the magma feeder systems of the alkali basaltic magma withing the mantle diapirs in modern platforms like in Vitim plateau (Ashchepkov et al., 2011) and now was applicated to reconstruct the deep seated magma conduits beneath the mountain collision systems, island arcs ocean plateaus etc. This equation allows to receive the positions of the major groups of eclogites mantle sections and to find out the regularities of their behavior. The Fe rich eclogites commonly trace he boundary between the lower upper part of subcontinental lithospheric mantle (SCLM) at 3 -4 GPa marking pyroxenite eclogites layer. Ca- rich eclogites and especially grospydites in SCLM beneath Precambrian kimberlites occurs near pyroxenite layer but in younger mantle sections they became common in the lower parts marking presence of the subducted sediments. The Mg Cr- less group eclogites commonly diamondiferous and referring to the ancient island arc complexes are also common in the middle part of mantle sections and near 5-6 GPa. The group is often dominated in the young kimberlites and sometimes is highly diamondiferous. Commonly P-Fe# for eclogites in the lower SCLM part show rising Fe# with decreasing pressures which very of then reflect the differentiation of the magmatic systems commonly rather significant. Commonly the Fe#-values for the eclogites show that they can't be simple subucted oceanic basalts but material remelted not only during the low angle "hot"subduction but also under the influence of the kimberlite melts including protokimberlite magmas. The Mg - rich and Fe rich pyroxenites also show the extending in pressures trends which suggest the anatexic melting under the influence of volatiles or under the plum magma hybridization. RBRF grants 05-05-64718, 03-05-64146; 11 -05-00060a; 11-05-91060-PICS. Projects 77-2, 65-03, 02-05 IGM SD RAS and ALROSA Stock Company.

Sensitivities of seismic velocities to temperature, pressure and composition in the lower mantle

NASA Astrophysics Data System (ADS)

Trampert, Jeannot; Vacher, Pierre; Vlaar, Nico

2001-08-01

We calculated temperature, pressure and compositional sensitivities of seismic velocities in the lower mantle using latest mineral physics data. The compositional variable refers to the volume proportion of perovskite in a simplified perovskite-magnesiowüstite mantle assemblage. The novelty of our approach is the exploration of a reasonable range of input parameters which enter the lower mantle extrapolations. This leads to realistic error bars on the sensitivities. Temperature variations can be inferred throughout the lower mantle within a good degree of precision. Contrary to the uppermost mantle, modest compositional changes in the lower mantle can be detected by seismic tomography, with a larger uncertainty though. A likely trade-off between temperature and composition will be largely determined by uncertainties in tomography itself. Given current sources of uncertainties on recent data, anelastic contributions to the temperature sensitivities (calculated using Karato's approach) appear less significant than previously thought. Recent seismological determinations of the ratio of relative S to P velocity heterogeneity can be entirely explain by thermal effects, although isolated spots beneath Africa and the Central Pacific in the lowermost mantle may ask for a compositional origin.

Multi-stage evolution of the lithospheric mantle beneath the westernmost Mediterranean: Geochemical constraints from peridotite xenoliths in the eastern Betic Cordillera (SE Spain)

NASA Astrophysics Data System (ADS)

Marchesi, Claudio; Konc, Zoltán; Garrido, Carlos J.; Bosch, Delphine; Hidas, Károly; Varas-Reus, María Isabel; Acosta-Vigil, Antonio

2017-04-01

Spinel (± plagioclase) peridotite xenoliths from the Tallante and Los Perez volcanic centres in the eastern Betics (SE Spain) range from depleted (clinopyroxene-poor) harzburgites to fertile (clinopyroxene-rich) lherzolites and orthopyroxene-free wehrlites. Significantly, only one harzburgite, which is depleted in heavy rare earth elements (HREE), retains the imprint of ca. 20% ancient melting of an original garnet lherzolite source. In contrast, REE abundances of other harzburgites and lherzolites from the eastern Betics have been increased by melt-rock reaction. The whole-rock and mineral compositions of these mantle rocks are largely controlled by three types of modal metasomatism: 1) common clinopyroxene-orthopyroxene addition and olivine consumption which increased FeOt, SiO2 and Al2O3, and decreased MgO compared to the refractory melting products; 2) subordinate orthopyroxene dissolution and precipitation of clinopyroxene and olivine, which led to higher FeOt and MgO and lower SiO2 than in common (orthopyroxene-rich) lherzolites; and 3) rare orthopyroxene consumption and olivine addition that caused higher FeOt and lower SiO2 compared to the original melting residues. These mineral modal and major element variations have been produced mostly by interactions with relatively FeOt-rich/SiO2-poor melts, likely derived from a peridotite-pyroxenite lithospheric mantle with a highly heterogeneous isotopic composition. Melting of the lithospheric mantle in the western Mediterranean was triggered by upwelling of the asthenosphere induced by back-arc extension in the Late Oligocene-Early Miocene. Trapping of small fractions of exotic melts in whole-rocks - likely the parental magmas of Miocene back-arc dykes that intruded the Betic crust - caused local disequilibrium between the trace element signatures and Pb isotopic compositions of clinopyroxene and whole-rock. Subsequent interaction with SiO2-undersaturated magmas, similar to the parental melts of the Pliocene alkali basalts that host the xenoliths, promoted orthopyroxene consumption and clinopyroxene-olivine enrichment at locations close to magma conduits, and finally generated orthopyroxene-free wehrlites. This event constitutes the last episode of the Cenozoic magmatic evolution of the westernmost Mediterranean which is recorded in the mantle xenoliths from the eastern Betics.

Hess, Harry Hammond (1906-69)

NASA Astrophysics Data System (ADS)

Murdin, P.

2000-11-01

Geophysicist, born in New York City, professor of geology at Princeton, led Project Mohole, the first expedition to drill through the Earth's oceanic crust to the mantle beneath, theorized that spreading of mid-ocean ridges was the source of new mantle-derived continental material. Also a lunar geologist....

On the instability and energy flux of lower hybrid waves in the Venus plasma mantle

NASA Technical Reports Server (NTRS)

Strangeway, R. J.; Crawford, G. K.

1993-01-01

Waves generated near the lower hybrid resonance frequency by the modified two stream instability have been invoked as a possible source of energy flux into the topside ionosphere of Venus. These waves are observed above the ionopause in a region known as the plasma mantle. The plasma within the mantle appears to be a mixture of magnetosheath and ionospheric plasmas. Since the magnetosheath electrons and ions have temperatures of several tens of eV, any instability analysis of the modified two stream instability requires the inclusion of finite electron and ion temperatures. Finite temperature effects are likely to reduce the growth rate of the instability. Furthermore, the lower hybrid waves are only quasi-electrostatic, and the energy flux of the waves is mainly carried by parallel Poynting flux. The magnetic field in the mantle is draped over the ionopause. Lower hybrid waves therefore cannot transport any significant wave energy to lower altitudes, and so do not act as a source of additional heat to the topside ionosphere.

Age-Orientation Relationships of Northern Hemisphere Martian Gullies and "Pasted-on" Mantling Unit: Implications for Near-Surface Water Migration in Mars' Recent History

NASA Technical Reports Server (NTRS)

Bridges, N. T.; Lackner, C. N.

2005-01-01

The finding of abundant, apparently young, Martian gullies with morphologies indicative of formation by flowing fluid was surprising in that volumes of near-surface liquid water in sufficient quantities to modify the surface geology were not thought possible under current conditions. Original hypotheses on origin of gullies were mostly centered on groundwater seepage and surface runoff and melting of near-surface ground ice. More recently, melting of snow deposited in periods of higher obliquity has been proposed as a possible origin of the gullies. Tied to this hypothesis is the supposition that the "pasted-on" mantling unit observed in association with many gullies is composed of remnant snowpack. The mantling unit has distinct rounded edge on its upper boundary and exhibits features suggestive of flow noted that the uppermost part of the mantle marks where gullies begin, suggesting that the source of water for the gullies was within the mantle. The mantle is found preferentially on cold, pole-facing slopes and, where mantled and non-mantled slopes are found together, gullies are observed incised into the latter. In other cases, the mantling material lacks gullies.

Minerals as mantle fingerprints: Sr-Nd-Pb-Hf in clinopyroxene and He in olivine distinguish an unusual ancient mantle lithosphere beneath the East African Rift System

NASA Astrophysics Data System (ADS)

Nelson, W. R.; Shirey, S. B.; Graham, D. W.

2011-12-01

The East African Rift System is a complex region that holds keys to understanding the fundamental geodynamics of continental break-up. In this region, the volcanic record preserves over 30 Myrs of geochemical variability associated with the interplay between shallow and deep asthenospheric sources, continental lithospheric mantle, and continental crust. One fundamental question that is still subject to debate concerns the relationship between the lithospheric mantle and the voluminous flood basalt province that erupted at ~30 Ma in Ethiopia and Yemen. Whole-rock Re-Os isotopic data demonstrate the high-Ti (HT2) flood basalts (187Os/188Ost = 0.1247-0.1329) and peridotite xenoliths (187Os/188Ost = 0.1235-0.1377) from NW Ethiopia have similar isotopic compositions. However, Sr-Nd-Pb-Hf isotopic signatures from peridotite clinopyroxene grains are different from those of the flood basalts. The peridotite clinopyroxene separates bear isotopic affinities to anciently depleted mantle (87Sr/86Sr = 0.7019-0.7029; ɛNd = 12.6-18.5; ɛHf = 13.8-27.6) - more depleted than the MORB source - rather than to the OIB-like 30 Ma flood basalts (87Sr/86Sr ~ 0.704; ɛNd = 4.7-6.7; ɛHf = 12.1-13.5). Peridotite clinopyroxenes display two groups of 206Pb/204Pb compositions: the higher 206Pb/204Pb group (18.7-19.3) is compositionally similar to the flood basalts (206Pb/204Pb = 18.97-19.02) whereas the lower 206Pb/204Pb group (17.1-17.9) overlaps with depleted mantle. This suggests that the Pb isotope systematics in some of the peridotites have been metasomatically perturbed. Helium isotopes were analyzed by crushing olivine separated from the peridotites and the flood basalts. Olivine in the peridotites has low He concentrations (0.78-4.7 ncc/g) and low 3He/4He (4.6-6.6 RA), demonstrating that they cannot be the petrogenetic precursor to the high 3He/4He (>12 RA) flood basalts. Notably, these peridotites have 3He/4He signatures consistent with a lithospheric mantle source. Therefore, although the flood basalts and lithospheric mantle bear some isotopic similarities, the basalts were not derived from this portion of the lithospheric mantle, nor are the peridotites crystalline cumulates derived from asthenosphere -derived magmas. The isotopic variations in these peridotites demonstrate that the Afro-Arabian lithosphere contains anciently depleted mantle, created during or prior to the late Proterozoic Pan-African orogeny.

Rethinking geochemical feature of the Afar and Kenya mantle plumes and geodynamic implications

NASA Astrophysics Data System (ADS)

Meshesha, Daniel; Shinjo, Ryuichi

2008-09-01

We discuss the spatial and temporal variation in the geochemistry of mantle sources which were sampled by the Eocene to Quaternary mafic magmas in the vicinity of the Afar and Kenya plume upwelling zones, East Africa. Despite the contributions of lithospheric and crustal sources, carefully screened Eocene to Quaternary mafic lavas display wide range of Sr-Nd-Pb isotopic and incompatible trace elemental compositions that can be attributed to significant intraplume heterogeneity. The geochemical variations reflect the involvement of at least four mantle plume components as sources for the northeastern Africa magmatism: (1) isotopically depleted but trace element-enriched component; (2) component characterized by radiogenic Pb isotope signatures (HIMU?); (3) enriched mantle-like component; and (4) high-3He/4He-type (as HT2-type basalts) plume component. The first component disappears in the Miocene-Quaternary magmatism, and the second component is hardly recognized after the eruption of Miocene basalt in southern Ethiopia. Plume-unrelated depleted asthenosphere starts to involve at a nascent stage of seafloor spreading centers in the Red Sea and Gulf of Aden. The other two-plume components have persisted from the late Eocene to present, but their proportions have changed through time and space. We propose a model of multiple impingements of plumelets within the broad upwelling zone connected to the African Superplume in the lower mantle beneath southern Africa. The plumelet contains a matrix of high-3He/4He-type component with blobs, streaks, or ribbons of other components.

Potential Temperatures of Sources of MORB, OIB and LIPs Based on AL Partitioning Between Olivine and Spinel

NASA Astrophysics Data System (ADS)

Sobolev, A. V.; Batanova, V. G.; Krasheninnikov, S.; Borisov, A.; Arndt, N.; Kuzmin, D.; Krivolutskaya, N.; Sushevskaya, N.

2013-12-01

Knowledge of potential temperatures of convecting mantle is required for the understanding the global processes on the Earth [1]. The common way to estimate these is the reconstruction of primary melt compositions and liquidus temperatures based on the Fe-Mg partitioning between olivine and melt. This approach requires knowledge of the compositions of primitive melts in equilibrium with olivine alone as well as composition of olivine equilibrium with primary melts. This information is in most cases unavailable or of questionable quality. Here we report a new approach to obtain crystallization temperatures of primary melts based on the olivine-spinel Al-Cr geothermometer [2]. The advantages of this approach are: (1) low rate of diffusion of Al in the olivine, which promises to preserve high magmatic temperatures and (2) common presence of spinel in assemblage with high-Mg olivine. In order to decipher influence of elevated Ti concentrations in spinel we have run several experiments at high temperatures (1400-1200 degree C), atmospheric pressure and controled oxygen fugacity. We also analysed over two thousand spinel inclusions and high-Mg host olivines from different MORB, OIB, LIP and Archean komatiites on the JXA-8230 EPMA at ISTerre, Grenoble, France. Concentrations of Al, Ti, Na, P, Zn, Cr, Mn, Ca, Co, Ni were determined with a precision of 10 ppm (2 standard errors) using a newly developed protocol [3]. When available, we also analysed matrix glass and glass inclusions in olivine and found that temperature estimations from olivine-spinel (Al-Cr) and olivine-melt (Fe-Mg) [4] equilibrium match within (+/-30 degree C). The results show contrasting crystallization temperatures of Mg-rich olivine of the same Fo content from different types of mantle-derived magmas, from the lowest (down to 1220 degree C) for MORB to the highest (up to 1550 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 convecting 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, Decan). [1] McKenzie & Bickle, 1988, J. Petr. 29, p 625-679. [2] Wan et al, 2008, Am. Min. 93, p1142-1147. [3] Batanova & Sobolev, 2013, Min. Mag.,p 667, DOI :10.1180/minmag2013.077.5.2 [4] Ford et al, 1983, J. Petr. 24, p 256-265.

Geochemical Evolution of the Louisville Seamount Chain

NASA Astrophysics Data System (ADS)

Vanderkluysen, L.; Mahoney, J. J.; Koppers, A. A.; Lonsdale, P. F.

2007-12-01

The Louisville seamount chain is a 4300 km long chain of submarine volcanoes in the southwestern Pacific that is commonly thought to represent a hotspot track. It spans an ~80 Myr age range, comparable to that of the Hawaiian-Emperor chain (Koppers et al., G-cubed, 5 (6), 2004). The few previously dredged igneous samples are dominantly basaltic and alkalic, and have been inferred to represent post-shield volcanism (Hawkins et al., AGU Monograph, 43, 235, 1987). Their isotope and trace element signatures suggest an unusually homogenous mantle source (Cheng et al., AGU Monograph, 43, 283, 1987). Dredging in 2006, during the AMAT02RR cruise of the R.V. Revelle, was carried out in the hope of recovering both shield and post-shield samples and of exploring the geochemical evolution of the chain. Igneous rocks were recovered from 33 stations on 23 seamounts covering some 47 Myr of the chain's history. Our study, focusing on the major and trace element and Sr, Nd and Pb isotopic characteristics of these samples, shows that all are alkalic basalts, basanites and tephrites containing normative nepheline. Variations in major and trace elements appear to be controlled predominantly by variable extents of melting and fractional crystallization, with little influence from mantle source heterogeneity. Indeed, age-corrected isotopic values define only a narrow range, in agreement with long-term source homogeneity relative to the scale of melting; e.g., ɛNd varies from +4.1 to +5.7, 206Pb/204Pb from 19.048 to 19.281, and 87Sr/86Sr from 0.70362 to 0.70398. These values broadly fall within the fields of the proposed "C" or "FOZO" mantle end-members. However, small variations are present, with less radiogenic Nd and Pb isotope ratios at the older, western end of the chain, defining a trend toward a broadly EM2-like composition. Although some workers have postulated that the Louisville hotspot was the source of the ~120 Myr Ontong Java Plateau, our samples are isotopically distinct from any known Ontong Java compositions.

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 rocks. The fluid-borne enrichment seems to have been derived from South Atlantic wedge mantle with no significant transfer of solubles in the slab fluids from the subducting altered Pacific oceanic crust to the wedge. The Northern Segment magmatism is proposed to be related to the steepening of Nazca plate subduction in the Pleistocene after a shallow slab period, where melts of subducted UCC plus slab fluids metasomatized the overlying depleted wedge mantle. During this steepening, the enriched depleted and undepleted mantle mixed or interacted, and yielded the Northern Segment and Nevado magmas.

The record of mantle heterogeneity preserved in Earth's oceanic crust

NASA Astrophysics Data System (ADS)

Burton, K. W.; Parkinson, I. J.; Schiano, P.; Gannoun, A.; Laubier, M.

2017-12-01

Earth's oceanic crust is produced by melting of the upper mantle where it upwells beneath mid-ocean ridges, and provides a geographically widespread elemental and isotopic `sample' of Earth's mantle. The chemistry of mid-ocean ridge basalts (MORB), therefore, holds key information on the compositional diversity of the upper mantle, but the problem remains that mixing and reaction during melt ascent acts to homogenise the chemical variations they acquire. Nearly all isotope and elemental data obtained thus far are for measurements of MORB glass, and this represents the final melt to crystallise, evolving in an open system. However, the crystals that are present are often not in equilibrium with their glass host. Melts trapped in these minerals indicate that they crystallised from primitive magmas that possess diverse compositions compared to the glass. Therefore, these melt inclusions preserve information on the true extent of the mantle that sources MORB, but are rarely amenable to precise isotope measurement. An alternative approach is to measure the isotope composition of the primitive minerals themselves. Our new isotope data indicates that these minerals crystallised from melts with significantly different isotope compositions to their glass host, pointing to a mantle source that has experienced extreme melt depletion. These primitive minerals largely crystallised in the lower oceanic crust, and our preliminary data for lower crustal rocks and minerals shows that they preserve a remarkable range of isotope compositions. Taken together, these results indicate that the upper mantle sampled by MORB is extremely heterogeneous, reflecting depletion and enrichment over much of Earth's geological history.

Understanding the nature of mantle upwelling beneath East-Africa

NASA Astrophysics Data System (ADS)

Civiero, Chiara; Hammond, James; Goes, Saskia; Ahmed, Abdulhakim; Ayele, Atalay; Doubre, Cecile; Goitom, Berhe; Keir, Derek; Kendall, Mike; Leroy, Sylvie; Ogubazghi, Ghebrebrhan; Rumpker, Georg; Stuart, Graham

2014-05-01

The concept of hot upwelling material - otherwise known as mantle plumes - has long been accepted as a possible mechanism to explain hotspots occurring at Earth's surface and it is recognized as a way of removing heat from the deep Earth. Nevertheless, this theory remains controversial since no one has definitively imaged a plume and over the last decades several other potential mechanisms that do not require a deep mantle source have been invoked to explain this phenomenon, for example small-scale convection at rifted margins, meteorite impacts or lithospheric delamination. One of the best locations to study the potential connection between hotspot volcanism at the surface and deep mantle plumes on land is the East African Rift (EAR). We image seismic velocity structure of the mantle below EAR with higher resolution than has been available to date by including seismic data recorded by stations from many regional networks ranging from Saudi Arabia to Tanzania. We use relative travel-time tomography to produce P- velocity models from the surface down into the lower mantle incorporating 9250 ray-paths in our model from 495 events and 402 stations. We add smaller earthquakes (4.5 < mb < 5.5) from poorly sampled regions in order to have a more uniform data coverage. The tomographic results allow us to image structures of ~ 100-km length scales to ~ 1000 km depth beneath the northern East-Africa rift (Ethiopia, Eritrea, Djibouti, Yemen) with good resolution also in the transition zone and uppermost lower mantle. Our observations provide evidence that the shallow mantle slow seismic velocities continue trough the transition zone and into the lower mantle. In particular, the relatively slow velocity anomaly beneath the Afar Depression extends up to depths of at least 1000 km depth while another low-velocity anomaly beneath the Main Ethiopian Rift seems to be present in the upper mantle only. These features in the lower mantle are isolated with a diameter of about 400 km indicating deep multiple sources of upwelling that converge in broader low-velocity bodies along the rift axis at shallow depths. Moreover, our preliminary models show that the low-velocity feature in the transition zone and uppermost lower mantle beneath Afar trends to the northeast beneath the Red Sea and Saudi Arabia as opposed to being linked to the African Superplume towards the southwest.

A distinct source and differentiation history for Kolumbo submarine volcano, Santorini volcanic field, Aegean arc

PubMed Central

Carey, Steven; Nomikou, Paraskevi; Smet, Ingrid; Godelitsas, Athanasios; Vroon, Pieter

2016-01-01

Abstract This study reports the first detailed geochemical characterization of Kolumbo submarine volcano in order to investigate the role of source heterogeneity in controlling geochemical variability within the Santorini volcanic field in the central Aegean arc. Kolumbo, situated 15 km to the northeast of Santorini, last erupted in 1650 AD and is thus closely associated with the Santorini volcanic system in space and time. Samples taken by remotely‐operated vehicle that were analyzed for major element, trace element and Sr‐Nd‐Hf‐Pb isotope composition include the 1650 AD and underlying K2 rhyolitic, enclave‐bearing pumices that are nearly identical in composition (73 wt.% SiO2, 4.2 wt.% K2O). Lava bodies exposed in the crater and enclaves are basalts to andesites (52–60 wt.% SiO2). Biotite and amphibole are common phenocryst phases, in contrast with the typically anhydrous mineral assemblages of Santorini. The strong geochemical signature of amphibole fractionation and the assimilation of lower crustal basement in the petrogenesis of the Kolumbo magmas indicates that Kolumbo and Santorini underwent different crustal differentiation histories and that their crustal magmatic systems are unrelated. Moreover, the Kolumbo samples are derived from a distinct, more enriched mantle source that is characterized by high Nb/Yb (>3) and low 206Pb/204Pb (<18.82) that has not been recognized in the Santorini volcanic products. The strong dissimilarity in both petrogenesis and inferred mantle sources between Kolumbo and Santorini suggests that pronounced source variations can be manifested in arc magmas that are closely associated in space and time within a single volcanic field. PMID:27917071

The role of mantle CO2 in volcanism

USGS Publications Warehouse

Barnes, I.; Evans, William C.; White, L.D.

1988-01-01

Carbon dioxide is the propellant gas in volcanic eruptions and is also found in mantle xenoliths. It is speculated that CO2 occurs as a free gas phase in the mantle because there is no reason to expect CO2 to be so universally associated with volcanic rocks unless the CO2 comes from the same source as the volcanic rocks and their xenoliths. If correct, the presence of a free gas in the mantle would lead to physical instability, with excess gas pressure providing the cause of both buoyancy of volcanic melts and seismicity in volcanic regions. Convection in the mantle and episodic volcanic eruptions are likely necessary consequences. This suggestion has considerable implications for those responsible for providing warnings of impending disasters resulting from volcanic eruptions and earthquakes in volcanic regions. ?? 1988.

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

NASA Astrophysics Data System (ADS)

Gangopadhyay, Amitava; Sproule, Rebecca A.; Walker, Richard J.; Lesher, C. Michael

2005-11-01

Osmium isotopic compositions, and Re and Os concentrations have been examined in one komatiite unit and two komatiitic basalt units at Dundonald Beach, part of the 2.7 Ga Kidd-Munro volcanic assemblage in the Abitibi greenstone belt, Ontario, Canada. The komatiitic rocks in this locality record at least three episodes of alteration of Re-Os elemental and isotope systematics. First, an average of 40% and as much as 75% Re may have been lost due to shallow degassing during eruption and/or hydrothermal leaching during or immediately after emplacement. Second, the Re-Os isotope systematics of whole rock samples with 187Re/ 188Os ratios >1 were reset at ˜2.5 Ga, possibly due to a regional metamorphic event. Third, there is evidence for relatively recent gain and loss of Re in some rocks. Despite the open-system behavior, some aspects of the Re-Os systematics of these rocks can be deciphered. The bulk distribution coefficient for Os (D Ossolid/liquid) for the Dundonald rocks is ˜3 ± 1 and is well within the estimated D values obtained for komatiites from the nearby Alexo area and stratigraphically-equivalent komatiites from Munro Township. This suggests that Os was moderately compatible during crystal-liquid fractionation of the magmas parental to the Kidd-Munro komatiitic rocks. Whole-rock samples and chromite separates with low 187Re/ 188Os ratios (<1) yield a precise chondritic average initial 187Os/ 188Os ratio of 0.1083 ± 0.0006 (γ Os = 0.0 ± 0.6) for their well-constrained ˜2715 Ma crystallization age. The chondritic initial Os isotopic composition of the mantle source for the Dundonald rocks is consistent with that determined for komatiites in the Alexo area and in Munro Township, suggesting that the mantle source region for the Kidd-Munro volcanic assemblage had evolved with a long-term chondritic Re/Os before eruption. The chondritic initial Os isotopic composition of the Kidd-Munro komatiites is indistinguishable from that of the projected contemporaneous convective upper mantle. The uniform chondritic Os isotopic composition of the Kidd-Munro komatiites contrasts with the typical large-scale Os isotopic heterogeneity in the mantle sources for ca. 89 Ma komatiites from the Gorgona Island, arc-related rocks and present-day ocean island basalts. This suggests that the Kidd-Munro komatiites sampled a late-Archean mantle source region that was significantly more homogeneous with respect to Re/Os relative to most modern mantle-derived rocks.

Large volume recycling of oceanic lithosphere over short time scales: geochemical constraints from the Caribbean Large Igneous Province

NASA Astrophysics Data System (ADS)

Hauff, F.; Hoernle, K.; Tilton, G.; Graham, D. W.; Kerr, A. C.

2000-01-01

Oceanic flood basalts are poorly understood, short-term expressions of highly increased heat flux and mass flow within the convecting mantle. The uniqueness of the Caribbean Large Igneous Province (CLIP, 92-74 Ma) with respect to other Cretaceous oceanic plateaus is its extensive sub-aerial exposures, providing an excellent basis to investigate the temporal and compositional relationships within a starting plume head. We present major element, trace element and initial Sr-Nd-Pb isotope composition of 40 extrusive rocks from the Caribbean Plateau, including onland sections in Costa Rica, Colombia and Curaçao as well as DSDP Sites in the Central Caribbean. Even though the lavas were erupted over an area of ˜3×10 6 km 2, the majority have strikingly uniform incompatible element patterns (La/Yb=0.96±0.16, n=64 out of 79 samples, 2σ) and initial Nd-Pb isotopic compositions (e.g. 143Nd/ 144Nd in=0.51291±3, ɛNdi=7.3±0.6, 206Pb/ 204Pb in=18.86±0.12, n=54 out of 66, 2σ). Lavas with endmember compositions have only been sampled at the DSDP Sites, Gorgona Island (Colombia) and the 65-60 Ma accreted Quepos and Osa igneous complexes (Costa Rica) of the subsequent hotspot track. Despite the relatively uniform composition of most lavas, linear correlations exist between isotope ratios and between isotope and highly incompatible trace element ratios. The Sr-Nd-Pb isotope and trace element signatures of the chemically enriched lavas are compatible with derivation from recycled oceanic crust, while the depleted lavas are derived from a highly residual source. This source could represent either oceanic lithospheric mantle left after ocean crust formation or gabbros with interlayered ultramafic cumulates of the lower oceanic crust. High 3He/ 4He in olivines of enriched picrites at Quepos are ˜12 times higher than the atmospheric ratio suggesting that the enriched component may have once resided in the lower mantle. Evaluation of the Sm-Nd and U-Pb isotope systematics on isochron diagrams suggests that the age of separation of enriched and depleted components from the depleted MORB source mantle could have been ≤500 Ma before CLIP formation and interpreted to reflect the recycling time of the CLIP source. Mantle plume heads may provide a mechanism for transporting large volumes of possibly young recycled oceanic lithosphere residing in the lower mantle back into the shallow MORB source mantle.

Rb-Sr and Sm-Nd Isotopic Studies of Lunar Green and Orange Glasses

NASA Technical Reports Server (NTRS)

Shih, C.-Y.; Nyquist, L. E.; Reese, Y.

2012-01-01

Lunar volcanic glassy beads have been considered as quenched basaltic magmas derived directly from deep lunar mantle during fire-fountaining eruptions [1]. Since these sub-mm size glassy melt droplets were cooled in a hot gaseous medium during free flight [2], they have not been subject to mineral fractionations. Thus, they represent primary magmas and are the best samples for the investigation of the lunar mantle. Previously, we presented preliminary Rb- Sr and Sm-Nd isotopic results for green and orange glassy samples from green glass clod 15426,63 and orange soil 74220,44, respectively [3]. Using these isotopic data, initial Sr-87/Sr-86 and Nd ratios for these pristine mare glass sources can be calculated from their respective crystallization ages previously determined by other age-dating techniques. These isotopic data were used to evaluate the mineralogy of the mantle sources. In this report, we analyzed additional glassy samples in order to further characterize isotopic signatures of their source regions. Also, we'll postulate a relationship between these two major mare basalt source mineralogies in the context of lunar magma ocean dynamics.

Partial separation of halogens during the subduction of oceanic crust

NASA Astrophysics Data System (ADS)

Joachim, Bastian; Pawley, Alison; Lyon, Ian; Henkel, Torsten; Clay, Patricia L.; Ruzié, Lorraine; Burgess, Ray; Ballentine, Christopher J.

2014-05-01

Incompatible elements, such as halogens, have the potential to act as key tracers for volatile transport processes in Earth and planetary systems. The determination of halogen abundances and ratios in different mantle reservoirs gives us the ability to better understand volatile input mechanisms into the Earth's mantle through subduction of oceanic crust. Halogen partition coefficients were experimentally determined between forsterite, orthopyroxene and silicate melt at pressures ranging from 1.0 to 2.3 GPa and temperatures ranging from 1500-1600°C, thus representing partial melting conditions of the Earth's mantle. Combining our data with results of recent studies (Beyer et al. 2012; Dalou et al. 2012) shows that halogen partitioning between forsterite and melt increases by factors of about 1000 (fluorine) and 100 (chlorine) between 1300°C and 1600°C and does not show any pressure dependence. Chlorine partitioning between orthopyroxene and melt increases by a factor of about 1500 for a temperature increase of 100°C (anywhere between 1300°C and 1600°C), but decreases by a factor of about 1500 for a pressure increase of 1.0 GPa (anywhere between 1.0 GPa and 2.5 GPa). At similar P-T conditions, a comparable effect is observed for the fluorine partitioning behaviour, which increases by 500-fold for a temperature increase of 100°C and decreases with increasing pressure. Halogen abundances in mid-ocean ridge basalts (MORB; F=3-15, Cl=0.5-14ppm) and ocean island basalts (OIB; F=35-65, Cl=21-55 ppm) source regions were estimated by combining our experimentally determined partition coefficients with natural halogen concentrations in oceanic basalts (e.g. Ruzié et al. 2012). The estimated chlorine OIB source mantle concentration is in almost perfect agreement with primitive mantle estimates (Palme and O'Neill 2003). If we expect an OIB source mantle slightly depleted in incompatible elements, this suggests that at least small amounts of chlorine are recycled deep into the mantle through subduction of oceanic crust, possibly via marine pore fluids (Sumino et al. 2010). The OIB source region is, however, significantly enriched in fluorine relative to the primitive mantle by a factor of 1.4-3.6, which indicates that significantly larger amounts of fluorine are transported deep into the Earth's mantle through subduction. An explanation for the partial separation of chlorine and fluorine during subduction is that the heavy halogens are more likely to escape from the subducting slab in hydrous fluids at an early subduction stage whereas significant amounts of fluorine are likely to remain in the slab, possibly incorporated in the lattice of hydrous amphibole or mica, or in anhydrous high-pressure phases of eclogite. The MORB source mantle is degassed in fluorine (17-88%) and chlorine (22-99%) relative to primitive mantle estimates. Preliminary data suggest that the bromine partitioning behaviour between forsterite and melt is roughly comparable to the behaviour of fluorine and chlorine. If true, this would imply that the Earth's upper mantle is presumably degassed of all halogens despite the more likely escape of heavy halogens from the slab at an early subduction stage, implying that these halogens are at least partly accumulating in the crust after leaving the slab. Beyer C, Klemme S, Wiedenbeck M, Stracke A, Vollmer C (2012) Earth Planet Sci. Lett. 337-338, pp. 1-9. Dalou C, Koga KT, Shimizu N, Boulon J, Devidal JL (2012) Contrib. Mineral. Petrol. 163, pp. 591-609. Palme H, O'Neill HSTC (2003) Treatise Geochem. 2, pp. 1-38. Ruzié L, Burgess R, Hilton DR, Ballentine CJ (2012) AGU Fall Meeting 2012. V31A-2762 (abstr.). Sumino H, Burgess R, Mizukami T, Wallis SR, Holland G, Ballentine CJ (2010) Earth Planet. Sci. Lett. 294, pp. 163-172.

A More Reduced Mantle Source for Enriched Shergottites; Insights from the Olivine-Phyric Shergottite Lar 06319

NASA Technical Reports Server (NTRS)

Peslier, A. H.; Hnatyshin, D.; Herd, C. D. K.; Walton, E. L.; Brandon, A. D.; Lapen, T. J.; Shafer, J.

2010-01-01

A detailed petrographic study of melt inclusions and Cr-Fe-Ti oxides of LAR 06319 leads to two main conclusions: 1) this enriched oxidized olivine- phyric shergottite represents nearly continuous crystallization of a basaltic shergottite melt, 2) the melt became more oxidized during differentiation. The first crystallized mineral assemblages record the oxygen fugacity which is closest to that of the melt s mantle source, and which is lower than generally attributed to the enriched shergottite group.

Early mantle heterogeneities in the Réunion hotspot source inferred from highly siderophile elements in cumulate xenoliths

NASA Astrophysics Data System (ADS)

Peters, Bradley J.; Day, James M. D.; Taylor, Lawrence A.

2016-08-01

Ultramafic cumulate rocks form during intrusive crystallization of high-MgO magmas, incorporating relatively high abundances of compatible elements, including Cr and Ni, and high abundances of the highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re). Here, we utilize a suite of cumulate xenoliths from Piton de la Fournaise, La Réunion (Indian Ocean), to examine the mantle source composition of the Réunion hotspot using HSE abundances and Os isotopes. Dunite and wherlite xenoliths and associated lavas from the Piton de la Fournaise volcanic complex span a range of MgO contents (46 to 7 wt.%), yet exhibit remarkably homogeneous 187Os/188Os (0.1324 ± 0.0014, 2σ), representing the Os-isotopic composition of Réunion hotspot primary melts. A significant fraction of the xenoliths also have primitive upper-mantle (PUM) normalized HSE patterns with elevated Ru and Pd (PUM-normalized Ru/Ir and Pd/Ir of 0.8-6.3 and 0.2-7.2, respectively). These patterns are not artifacts of alteration, fractional crystallization, or partial melting processes, but rather require a primary magma with similar relative enrichments. Some highly olivine-phyric (>40 modal percent olivine) Piton de la Fournaise lavas also preserve these relative Ru and Pd enrichments, while others preserve a pattern that is likely related to sulfur saturation in evolved melts. The estimate of HSE abundances in PUM indicates high Ru/Ir and Pd/Pt values relative to carbonaceous, ordinary and enstatite chondrite meteorite groups. Thus, the existence of cumulate rocks with even more fractionated HSE patterns relative to PUM suggests that the Réunion hotspot samples a yet unrecognized mantle source. The origin of fractionated HSE patterns in Réunion melts may arise from sampling of a mantle source that experienced limited late accretion (<0.2% by mass) compared with PUM (0.5-0.8%), possibly involving impactors that were distinct from present-day chondrites, or limited core-mantle interactions. Given the remarkably homogeneous Os, Pb, and noble-gas isotopic signatures of Réunion, which plot near the convergence point of isotopic data for many hotspots, such a conclusion provides evidence for an early differentiated and subsequently isolated mantle domain that may be partially sampled by some ocean island basalts.

Interferometric Seismic Sources on the Core Mantle Boundary Revealed by Seismic Coda Crosscorrelation

NASA Astrophysics Data System (ADS)

Pham, T. S.; Tkalcic, H.; Sambridge, M.

2017-12-01

The crosscorrelation of earthquake coda can be used to extract seismic body waves which are sensitive to deep Earth interior. The retrieved peaks in crosscorrelation of two seismic records are commonly interpreted as seismic phases that originate at a point source collocated with the first recorder (Huygens-Fresnel principle), reflected upward from prominent underground reflectors and reaching the second recorder. From the time shift of these peaks measured at different interstation distances, new travel time curves can be constructed. This study focuses on a previously unexplained interferometric phase (named temporarily a ghost or "G phase") observed in crosscorrelogram stack sections utilizing seismic coda. In particular, we deploy waveforms recorded by two regional seismic networks, one in Australia and another in Alaska. We show that the G phase cannot be explained by as a reflection. Moreover, we demonstrate that the G phase is explained through the principle of energy partitioning, and specifically, conversions from compressional to shear motions at the core-mantle boundary (CMB). This can be thought of in terms of a continuous distribution of Huygens sources across the CMB that are "activated" in long-range wavefield coda following significant earthquakes. The newly explained phase is renamed to cPS, to indicate a CMB origin and the P to S conversion. This mechanism explains a range of newly observed global interferometric phases that can be used in combination with existing phases to constrain Earth structure.

The Lowest δ7Li Yet Recorded in MORB Glasses: The Connection with Oceanic Core Complex Formation, Refractory Rutile-bearing Eclogitic Mantle Sources and Melt Supply

NASA Astrophysics Data System (ADS)

Casey, J. F.; Gao, Y.; Benavidez, R.; Dragoi, C.

2010-12-01

The region between 12°N and 16°N along the Mid-Atlantic Ridge is known for its prolific development of oceanic core complexes and for a geochemical anomaly centered at ~14°N. We examine the correlation of the geochemical anomaly with a region characterized by low magma supply. Basalt glasses over the geochemical anomaly are unusual in exhibiting E-MORB to T-MORB HIMU-DMM isotopic gradients. The most enriched MORBs exhibit positive Ta and Nb anomalies and negative Th and Pb anomalies that are similar to some OIB basalts. Some more primitive basalts exhibit positive Ti, Sr and Eu anomalies. The center of the geochemical anomaly is characterized by elevated La/Sm ratios that are strongly correlated with Nb/La, Nb/Nb*, Ta/Ta* and Sr, Nd, Pb isotopic anomalies. In addition, we have recently documented a regional anomaly in δ7Li, with the lowest values ever recorded in MORB glasses near the center of the anomaly. We interpret this data to indicate that the mantle source in the 12-16°N region of the Mid-Atlantic Ridge involves subducted slab components including a refractory rutile-bearing eclogitic source that has suffered significant dehydration and a previously depleted mantle source that has undergone an ancient depletion event that results in little melt supply being contributed to the ridge axis. We examine melt supply implications in the context of core complex development and these unusual mantle source characteristics.

Geodynamic Implications of Himu Mantle In The Source of Tertiary Volcanics From The Veneto Region (south Eastern Alps)

NASA Astrophysics Data System (ADS)

Macera, P.; Gasperini, D.; Blichert-Toft; Bosch, D.; del Moro, A.; Dini, G.; Martin, S.; Piromallo, C.

DuringTertiary times extensive mafic volcanism took place in the South-Eastern Alps, along a half-graben structure bounded by the Schio-Vicenza main fault. This mag- matism gave rise to four main volcanic centers: Lessini, Berici, Euganei, and Maros- tica. The dominating rock types are alkali basalts, basanites and transitional basalts, with hawaiites, trachybasalts, tephrites, basaltic andesites, and differentiated rocks be- ing less common. Major and trace element and Sr-Nd-Hf-Pb isotopic data for the most primitive lavas from each volcanic center show the typical features of HIMU hotspot volcanism, variably diluted by a depleted asthenospheric mantle component (87Sr/86Sr48Ma = 0.70314-0.70321; eNd48Ma = +6.4 to +6.5; eHf48Ma = +6.4 to +8.1, 206Pb/204Pb48Ma = 18.786-19.574). Since the HIMU component is consid- ered to be of deep mantle origin, its presence in a tectonic environment dominated by subduction (the Alpine subduction of the European plate below the Adria plate) has significant geodynamic implications. Slab detachment and ensuing rise of deep man- tle material into the lithospheric gap is proposed to be a viable mechanism of hotspot magmatism in a subduction zone setting. Interaction between deep-seated plume ma- terial and shallow depleted asthenospheric mantle may account for the geochemical features of the Veneto volcanics, as well as those of the so-called enriched astheno- spheric reservoir (EAR) component. Ascending counterflow of deep mantle material through the lithospheric gap to the top of the subducting slab further may induce heat- ing of the overriding plate and trigger it to partially melt. Upwelling of the resulting mafic magmas and their subsequent underplating at the mantle-lower crust bound- ary would favor partial melting of the lower crust, thereby giving rise to the bimodal mafic-felsic magmatism that characterizes the whole Periadriatic province. According to this model, the HIMU-like magmatism of the Alpine foreland is therefore closely related to the calc-alkaline magmatism of the Periadriatic Lineament, and caused by the same mechanism of Tertiary Alpine convergence tectonics.

187Os-186Os and He Isotope Systematics of Iceland Picrites

NASA Astrophysics Data System (ADS)

Brandon, A. D.; Brandon, A. D.; Graham, D.; Gautason, B.

2001-12-01

Iceland is one of the longest-lived modern plumes, and seismic imaging supports a model where the roots of this plume are at the base of the lower mantle. Hence, Os isotopic data for lavas from this plume are ideal for further testing the role of core-mantle chemical exchange at the site of plume generation in the lower mantle, and for addressing the origin of Os-He isotopic variation in plumes. Recent work has shown that lavas from some plume systems (Hawaii, Noril'sk-Siberia, Gorgona) show coupled enrichments in 186Os/188Os and 187Os/188Os, not observed in upper mantle materials including abyssal peridotites. Picrites from Hawaii display a positive correlation between 186Os/188Os and He isotopes (R/Ra), where range in 186Os/188Os of 0.119834+/-28 to 0.1198475+/-29 and corresponding R/Ra from +7 to +25. These systematics are consistent with a lower mantle source for the radiogenic 186Os signal in the Hawaiian plume. The coupled Os enrichments in these plumes has been attributed to core-mantle chemical exchange, consistent with generation of the Hawaiian plume at the base of the lower mantle in D". Other potentially viable models await additional scrutiny. New He isotope and high precision 186Os/188Os and 187Os/188Os measurements for Iceland picrites show unique systematics compared to Hawaii. These picrites have 187Os/188Os ranging from 0.1297 to 0.1381 and R/Ra of +9 to +18, with generally higher R/Ra correlating with higher 187Os/188Os. Unlike the Hawaiian picrites from Hualalai and Loihi, which have coupled enrichments in 186Os/188Os and 187Os/188Os, the Iceland picrites show no enrichment 186Os/188Os - 0.1198363+/-28 (2s, n=14). Such Os-He isotopic variations require one end-member source that has high R/Ra, coupled with a long term elevated Re/Os and Pt/Os similar to that of the upper mantle. These systematics are inconsistent with either known upper mantle materials or those purported for ancient recycled slabs and may be a previously unidentified component in the lower mantle.

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

NASA Astrophysics Data System (ADS)

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

2014-12-01

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

Lu-Hf and Sm-Nd evolution in lunar mare basalts

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

Unruh, D.M.; Stille, P.; Patchett, P.J.

1984-02-15

Lu-Hf and Sm-Nd data for mare basalts combined with Rb-Sr and total REE data taken from the literature suggest that the mare basalts were derived by small (< or =10%) degrees of partial melting of cumulate sources, but that the magma ocean from which these sources formed was light REE and Hf-enriched. Calculated source compositions range fromm lherzolite to olivine websterite. Nonmodal melting of small amounts of ilmenite (< or =3%) in the sources seems to be required by the Lu/Hf data. A comparison of the Hf and Nd isotopic characteristics between the mare basalts and terrestrial oceanic basalts revealsmore » that the epsilonHf/epsilonNd ratios of low-Ti mare basalts are much higher than in terrestrial oceanic basalts. The results are qualitatively consistent with the hypothesis that terrestrial basalt sources are partial melt residues whereas mare basalt sources are cumulates. Alternatively, the results may imply that the terrestrial mantle has evolved in two (or more) stages of evolution, and that the net effect was depletion of the mantle during the first approx.1-3 b.y. followed by enrichment during the last 1-2 b.y.; or simply that there is a difference in Lu-Hf crystal-liquid partitioning (relative to Sm-Nd) between the lunar and terrestrial mantles.« less

Source Stacking for Numerical Wavefield Computations - Application to Global Scale Seismic Mantle Tomography

NASA Astrophysics Data System (ADS)

MacLean, L. S.; Romanowicz, B. A.; French, S.

2015-12-01

Seismic wavefield computations using the Spectral Element Method are now regularly used to recover tomographic images of the upper mantle and crust at the local, regional, and global scales (e.g. Fichtner et al., GJI, 2009; Tape et al., Science 2010; Lekic and Romanowicz, GJI, 2011; French and Romanowicz, GJI, 2014). However, the heaviness of the computations remains a challenge, and contributes to limiting the resolution of the produced images. Using source stacking, as suggested by Capdeville et al. (GJI,2005), can considerably speed up the process by reducing the wavefield computations to only one per each set of N sources. This method was demonstrated through synthetic tests on low frequency datasets, and therefore should work for global mantle tomography. However, the large amplitudes of surface waves dominates the stacked seismograms and these cases can no longer be separated by windowing in the time domain. We have developed a processing approach that helps address this issue and demonstrate its usefulness through a series of synthetic tests performed at long periods (T >60 s) on toy upper mantle models. The summed synthetics are computed using the CSEM code (Capdeville et al., 2002). As for the inverse part of the procedure, we use a quasi-Newton method, computing Frechet derivatives and Hessian using normal mode perturbation theory.

Long wavelength magnetic anomalies over continental rifts in cratonic region

NASA Astrophysics Data System (ADS)

Friedman, S. A.; Persaud, P.; Ferre, E. C.; Martín-Hernández, F.; Feinberg, J. M.

2017-12-01

New collections of unaltered mantle xenoliths shed light on potential upper mantle contributions to long wavelength magnetic anomalies (LWMA) in continental rifts in cratonic / shield areas. The new material originates from the East African Rift (Tanzania), the Rio Grande Rift (U.S.A.), the Rhine Rift (Germany), and the West Antarctic Rift (Antarctica). The xenoliths sample the uppermost (<80 km depth) lithospheric mantle in these regions in the spinel-peridotite and plagioclase-peridotite stability fields. The most common lithology by far (95% of samples) is a spinel-lherzolite indicating relatively low oxygen fugacities (FMQ -1). Chrome spinel in these peridotites is non-magnetic (Al + Mg > 0.2 or Fe < 0.3) and primary magnetite (Fe3O4) occurs only in trace amounts, typically yielding low natural remanent magnetizations (NRM < 10-2 A/m). The low Koenigsberger ratios (Qn < 1) of these materials, combined with high geotherms (>60ºC/km) that are characteristic of rifted regions preclude any contribution to LWMA at depths >10 km. Hence, only upper basalts and hypovolcanic mafic sills would constitute potential magnetic sources. In contrast, the margins of these rifted regions consist of refractory cratonic domains, often characterized by oxidized sublithospheric mantle that host significant concentrations of primary magnetite. The higher NRMs of these peridotites (up to 15 A/m, Qn > 2.5) combined with much lower geotherms (as low as 15ºC/km) allows for a 5 to 10 km layer of uppermost mantle to potentially contribute to LWMA. Assuming that Qn values in rift margins are also <1, the new data presented here suggests that relatively young rifts would display a central negative magnetic anomaly surrounded by two broad positive anomalies. The latitude of the rift is predicted to exert a primary control on the magnitude of such anomalies, while the steepness of the magnetic gradient across the rift would primarily reflect thermal equilibration over time.

The Last Word

NASA Astrophysics Data System (ADS)

Anderson, D. L.

2014-12-01

Earth is an isolated, cooling planet, that obeys the 2nd law of thermodynamics. Interior dynamics is driven from the top, by cold sinking slabs. High-resolution broad-band seismology and geodesy have confirmed that mantle flow is characterized by narrow downwellings and ~20 broad slowly rising updrafts. The low-velocity zone (LVZ) consists of a hot melange of sheared peridotite intruded with aligned melt-rich lamellae that are tapped by intraplate volcanoes. The high temperature is a simple consequence of the thermal overshoot common in large bodies of convecting fluids. The transition zone consists of ancient eclogite layers that are displaced upwards by slabs to become broad, passive, cool ridge-feeding updrafts of ambient mantle. The physics that is overlooked in canonical models of mantle dynamics and geochemistry includes; the 2nd law of thermodynamics, convective overshoots, subadiabaticity, wave-melt interactions, Archimedes principle, and kinetics. Rapid transitions allow stress-waves to interact with melting and phase changes, creating LVZs; sluggish transitions in cold slabs keep eclogite in the transition zone where it warms up by extracting heat from mantle below 650 km, creating the appearance of slab penetration. Canonical chemical geodynamic models are the exact opposite of physics- and thermodynamic-based models and of the real Earth. A model that results from inverting the assumptions regarding initial and boundary conditions (hot origin, secular cooling, no external power sources, cooling internal boundaries, broad passive upwellings, adiabaticity and whole-mantle convection not imposed, layering and self-organization allowed) results in a thick refractory-yet-fertile surface layer, with ancient xenoliths and cratons at the top and a hot overshoot at the base. A thin mobile D" layer results, that is an unlikely plume-generation zone. Accounting for the physics that is overlooked or violated (the 2nd law of thermodynamics) in canonical models, plus modern seismology, undermines the assumptions and conclusions of these models.

Trace elements in olivine of ultramafic lamprophyres controlled by phlogopite-rich mineral assemblages in the mantle source

NASA Astrophysics Data System (ADS)

Veter, Marina; Foley, Stephen F.; Mertz-Kraus, Regina; Groschopf, Nora

2017-11-01

Carbonate-rich ultramafic lamprophyres (aillikites) and associated rocks characteristically occur during the early stages of thinning and rifting of cratonic mantle lithosphere, prior to the eruption of melilitites, nephelinites and alkali basalts. It is accepted that they require volatile-rich melting conditions, and the presence of phlogopite and carbonate in the source, but the exact source rock assemblages are debated. Melts similar to carbonate-rich ultramafic lamprophyres (aillikites) have been produced by melting of peridotites in the presence of CO2 and H2O, whereas isotopes and trace elements appear to favor distinct phlogopite-bearing rocks. Olivine macrocrysts in aillikites are usually rounded and abraded, so that it is debated whether they are phenocrysts or mantle xenocrysts. We have analyzed minor and trace element composition in olivines from the type aillikites from Aillik Bay in Labrador, Canada. We characterize five groups of olivines: [1] mantle xenocrysts, [2] the main phenocryst population, and [3] reversely zoned crystals interpreted as phenocrysts from earlier, more fractionated, magma batches, [4] rims on the phenocrysts, which delineate aillikite melt fractionation trends, and [5] rims around the reversely zoned olivines. The main phenocryst population is characterized by mantle-like Ni (averaging 3400 μg g- 1) and Ni/Mg at Mg# of 88-90, overlapping with phenocrysts in ocean island basalts and Mediterranean lamproites. However, they also have low 100 Mn/Fe of 0.9-1.3 and no correlation between Ni and other trace elements (Sc, Co, Li) that would indicate recycled oceanic or continental crust in their sources. The low Mn/Fe without high Ni/Mg, and the high V/Sc (2-5) are inherited from phlogopite in the source that originated by solidification of lamproitic melts at the base of the cratonic lithosphere in a previous stage of igneous activity. The olivine phenocryst compositions are interpreted to result from phlogopite and not high modal pyroxene in the source. The presence of kimberlites and ultramafic lamprophyres of Mesozoic age in Greenland indicates the persistence of a steep edge to the cratonic lithosphere at a time when this had been removed from the western flank in Labrador.

Enrichment of 18O in the mantle sources of the Antarctic portion of the Karoo large igneous province

NASA Astrophysics Data System (ADS)

Heinonen, Jussi S.; Luttinen, Arto V.; Whitehouse, Martin J.

2018-03-01

Karoo continental flood basalt (CFB) province is known for its highly variable trace element and isotopic composition, often attributed to the involvement of continental lithospheric sources. Here, we report oxygen isotopic compositions measured with secondary ion mass spectrometry for hand-picked olivine phenocrysts from 190 to 180 Ma CFBs and intrusive rocks from Vestfjella, western Dronning Maud Land, that form an Antarctic extension of the Karoo province. The Vestfjella lavas exhibit heterogeneous trace element and radiogenic isotope compositions (e.g., ɛ Nd from - 16 to + 2 at 180 Ma) and the involvement of continental lithospheric mantle and/or crust in their petrogenesis has previously been suggested. Importantly, our sample set also includes rare primitive dikes that have been derived from depleted asthenospheric mantle sources ( ɛ Nd up to + 8 at 180 Ma). The majority of the oxygen isotopic compositions of the olivines from these dike rocks (δ18O = 4.4-5.2‰; Fo = 78-92 mol%) are also compatible with such sources. The olivine phenocrysts in the lavas, however, are characterized by notably higher δ18O (6.2-7.5‰; Fo = 70-88 mol%); and one of the dike samples gives intermediate compositions (5.2‒6.1‰, Fo = 83-87 mol%) between the other dikes and the CFBs. The oxygen isotopic compositions do not correlate with radiogenic isotope compositions susceptible to crustal assimilation (Sr, Nd, and Pb) or with geochemical indicators of pyroxene-rich mantle sources. Instead, δ18O correlates positively with enrichments in large-ion lithophile elements (especially K) and 187Os. We suggest that the oxygen isotopic compositions of the Vestfjella CFB olivines primarily record large-scale subduction-related metasomatism of the sub-Gondwanan mantle (base of the lithosphere or deeper) prior to Karoo magmatism. The overall influence of such sources to Karoo magmatism is not known, but, in addition to continental lithosphere, they may be responsible for some of the geochemical heterogeneity observed in the CFBs.

Application of normal mode theory to seismic source and structure problems: Seismic investigations of upper mantle lateral heterogeneity. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Okal, E. A.

1978-01-01

The theory of the normal modes of the earth is investigated and used to build synthetic seismograms in order to solve source and structural problems. A study is made of the physical properties of spheroidal modes leading to a rational classification. Two problems addressed are the observability of deep isotropic seismic sources and the investigation of the physical properties of the earth in the neighborhood of the Core-Mantle boundary, using SH waves diffracted at the core's surface. Data sets of seismic body and surface waves are used in a search for possible deep lateral heterogeneities in the mantle. In both cases, it is found that seismic data do not require structural differences between oceans and continents to extend deeper than 250 km. In general, differences between oceans and continents are found to be on the same order of magnitude as the intrinsic lateral heterogeneity in the oceanic plate brought about by the aging of the oceanic lithosphere.

Using the heterogeneity distribution in Earth's mantle to study structure and flow

NASA Astrophysics Data System (ADS)

Rost, S.; Frost, D. A.; Bentham, H. L.

2016-12-01

The Earth's interior contains heterogeneities on many scale-lengths ranging from continent sized structures such as Large-Low Shear Velocity Provinces (LLSVPs) to grain-sized anomalies resolved using geochemistry. Sources of heterogeneity in Earth's mantle are for example the recycling of crustal material through the subduction process as well as partial melting and compositional variations. The subduction and recycling of oceanic crust throughout Earth's history leads to strong heterogeneities in the mantle that can be detected using seismology and geochemistry. Current models of mantle convection show that the subducted crustal material can be long-lived and is transported passively throughout the mantle by convective flows. Settling and entrainment is dependent on the density structure of the heterogeneity. Imaging heterogeneities throughout the mantle therefore allows imaging mantle flow especially in areas of inhibited flow due to e.g. viscosity changes or changes in composition or dynamics. The short-period seismic wavefield is dominated by scattered seismic energy partly originating from scattering at small-scale heterogeneities in Earth's mantle. Using specific raypath configurations we are able to sample different depths throughout Earth's mantle for the existence and properties of heterogeneities. These scattering probes show distinct variations in energy content with frequency indicating dominant heterogeneity length-scales in the mantle. We detect changes in heterogeneity structure both in lateral and radial directions. The radial heterogeneity structure requires changes in mantle structure at depths of 1000 km and 1800 to 2000 km that could indicate a change in viscosity structure in the mid mantle partly changing the flow of subducted crustal material into the deep mantle. Lateral changes in heterogeneity structure close to the core mantle boundary indicate lateral transport inhibited by the compositional anomalies of the LLSVPs.

SrNdPb isotopic and trace element evidence for crustal contamination of plume-derived flood basalts: Oligocene flood volcanism in western Yemen

NASA Astrophysics Data System (ADS)

Baker, J. A.; Thirlwall, M. F.; Menzies, M. A.

1996-07-01

Oligocene flood basalts from western Yemen have a relatively limited range in initial isotopic composition compared with other continental flood basalts: 87Sr/86Sr = 0.70365-0.70555 ; 143Nd/144Nd = 0.5129-0.51248 ( ɛNd = +6.0 to -2.4) ; 206pb/204Pb = 17.9-19.3 . Most compositions lie outside the isotopic ranges of temporally and spatially appropriate mantle source compositions observed in this area, i.e., Red Sea/Gulf of Aden MORB mantle, the Afar plume, and Pan-African lithospheric mantle Correlations between indices of fractionation, silica, and isotope ratios suggest that crustal contamination has substantially modified the primary isotopic and incompatible trace element characteristics of the flood basalts. However, significant scatter in these correlations was produced by: (a) the heterogeneous isotopic composition of Pan-African crust; (b) the difference in susceptibility of magmas to contamination as a result of variable incompatible trace element contents in primary melts produced by differing degrees of partial melting; (c) the presence or absence of plagioclase as a fractionating phase generating complex contamination trajectories for Sr; (d) sampling over a wide area not representing a single coherent magmatic system; and (e) variation in contamination mechanisms from assimilation associated with fractionation (AFC) to assimilation by hot mafic magmas with little concomitant fractionation. The presence of plagioclase as a fractionating phase in some suites that were undergoing AFC requires assimilation to have taken place within the crust and, coupled with the limited LREE-enrichment accompanying isotopic variations, excludes the possibility that an AFC-type process took place during magma transfer through the lithospheric mantle. Isotopic compositions of some of the inferred crustal assimilants are similar to those postulated by other workers for an enriched lithospheric mantle source of many flood basalts in southwestern Yemen, Ethiopia, and Djibouti. The western Yemen flood basalts contain 0-30% crust which largely swamps their primary lead isotopic signature, but the primary SrNd isotopic signature is close to that of the least contaminated and isotopically most depleted flood basalts. LREE/HFSE and LILE/HFSE ratios also correlate with isotopic data as a result of crustal contamination. However, Nb/La and K/Nb ratios of >1.1 and <150, respectively, in least contaminated samples require an OIB-like source. The pre-contamination isotopic signature is estimated to be: 87Sr/86Sr ˜ 0.7036; 143Nd/144Nd ˜ 0.51292 ; 206Pb/204Pb ˜ 18.4-19.0 . This, coupled with low LILE/HFSE ratios, suggest the source has characteristics akin to the Afar plume. A mantle source isotopically more depleted than Bulk Earth, but not as depleted as MORB, coupled with LILE depletion, also characterises other examples of plume-derived flood volcanism. This mantle reservoir is responsible for the second largest outbursts of volcanism on Earth and has radiogenic isotopic characteristics akin to PREMA mantle, but the incompatible trace element signature of HIMU mantle.

Investigating the Mantle Source of the Lunar Crater Volcanic Field, Nevada: Evidence of a Thermal Plume?

NASA Astrophysics Data System (ADS)

Lee, J. W.; Roden, M.

2016-12-01

The Easy Chair Crater (ECC), located within the Lunar Crater Volcanic Field (LCVF) in central Nevada is particularly interesting because of the unusually high equilibrium temperatures and strain recorded by the mantle-derived xenoliths at LCVF1. In addition, a gravity and elevation anomaly suggests the possibility of an underlying thermal plume in the region2. In order to determine if the rocks at ECC are geochemically similar to rocks from other plume-related regions, we analyzed melt inclusions and olivine phenocrysts collected from basalts near the crater. Chlorine amounts in melt inclusions were normalized to the highly incompatible K to produce a ratio that is insensitive to crystallization within or along the walls of the inclusion3. Because Cl is implicated in lithosphere recycling, the Cl/K ratio can be used to differentiate magmatic source components. Initial results (Fig. 1) indicate that basalts from ECC are geochemically more similar to ocean island basalts than to MORB or arc basalts. Elemental ratios in olivine phenocrysts from basaltic magmas can be used to determine the petrology of the source rock for particular silicate melts. In turn, petrology of mantle sources is thought to correlate with source nature (e.g., plume versus upper mantle)4. Specifically, Ni and Mn amounts were evaluated in order to determine if magma sources were pyroxenite-rich. Preliminary calculations of the wt. fraction of pyroxenite in the source of ECC basalts ranged from 0.13 to 0.68 indicating the possibility of a significant amount of pyroxenite in the magmatic source which would be expected if a plume was present beneath LCVF. References:1Smith, D. (2000) JGR 105: 16769; 2Saltus, R.W. & Thompson, G.A. (1995) Tectonics 14:1235; 3Patiño Douce, A.E. & Roden, M.F. (2006) Geochim Cosmochim Acta 70: 3173; 4Gurenko et al. (2010) Contrib Mineral Petrol 159: 689

Abundant carbon in the mantle beneath Hawai`i

NASA Astrophysics Data System (ADS)

Anderson, Kyle R.; Poland, Michael P.

2017-09-01

Estimates of carbon concentrations in Earth’s mantle vary over more than an order of magnitude, hindering our ability to understand mantle structure and mineralogy, partial melting, and the carbon cycle. CO2 concentrations in mantle-derived magmas supplying hotspot ocean island volcanoes yield our most direct constraints on mantle carbon, but are extensively modified by degassing during ascent. Here we show that undegassed magmatic and mantle carbon concentrations may be estimated in a Bayesian framework using diverse geologic information at an ocean island volcano. Our CO2 concentration estimates do not rely upon complex degassing models, geochemical tracer elements, assumed magma supply rates, or rare undegassed rock samples. Rather, we couple volcanic CO2 emission rates with probabilistic magma supply rates, which are obtained indirectly from magma storage and eruption rates. We estimate that the CO2 content of mantle-derived magma supplying Hawai`i’s active volcanoes is 0.97-0.19+0.25 wt%--roughly 40% higher than previously believed--and is supplied from a mantle source region with a carbon concentration of 263-62+81 ppm. Our results suggest that mantle plumes and ocean island basalts are carbon-rich. Our data also shed light on helium isotope abundances, CO2/Nb ratios, and may imply higher CO2 emission rates from ocean island volcanoes.

Contrasting petrogenesis of spatially related carbonatites from Samalpatti and Sevattur, Tamil Nadu, India

NASA Astrophysics Data System (ADS)

Ackerman, Lukáš; Magna, Tomáš; Rapprich, Vladislav; Upadhyay, Dewashish; Krátký, Ondřej; Čejková, Bohuslava; Erban, Vojtěch; Kochergina, Yulia V.; Hrstka, Tomáš

2017-07-01

Two Neoproterozoic carbonatite suites of spatially related carbonatites and associated silicate alkaline rocks from Sevattur and Samalpatti, south India, have been investigated in terms of petrography, chemistry and radiogenic-stable isotopic compositions in order to provide further constraints on their genesis. The cumulative evidence indicates that the Sevattur suite is derived from an enriched mantle source without significant post-emplacement modifications through crustal contamination and hydrothermal overprint. The stable (C, O) isotopic compositions confirm mantle origin of Sevattur carbonatites with only a modest difference to Paleoproterozoic Hogenakal carbonatite, emplaced in the same tectonic setting. On the contrary, multiple processes have shaped the petrography, chemistry and isotopic systematics of the Samalpatti suite. These include pre-emplacement interaction with the ambient crustal materials with more pronounced signatures of such a process in silicocarbonatites. Calc-silicate marbles present in the Samalpatti area could represent a possible evolved end member due to the inability of common silicate rocks (pyroxenites, granites, diorites) to comply with radiogenic isotopic constraints. In addition, Samalpatti carbonatites show a range of C-O isotopic compositions, and δ13CV-PDB values between + 1.8 and + 4.1‰ found for a sub-suite of Samalpatti carbonatites belong to the highest values ever reported for magmatic carbonates. These heavy C-O isotopic signatures in Samalpatti carbonatites could be indicative of massive hydrothermal interaction with carbonated fluids. Unusual high-Cr silicocarbonatites, discovered at Samalpatti, seek their origin in the reaction of pyroxenites with enriched mantle-derived alkali-CO2-rich melts, as also evidenced by mantle-like O isotopic compositions. Field and petrographic observations as well as isotopic constraints must, however, be combined with the complex chemistry of incompatible trace elements as indicated from their non-uniform systematics in carbonatites and their individual fractions. We emphasise that, beside common carriers of REE like apatite, other phases may be important for incompatible element budgets, such as mckelveyite-(Nd) and kosmochlor, found in these carbonatites. Future targeted studies, including in-situ techniques, could help further constrain temporal and petrologic conditions of formation of Sevattur and Samalpatti carbonatite bodies.

The Solarya Volcano-Plutonic Complex (NW Turkey): Petrography, Petrogenesis and Tectonic Implications

NASA Astrophysics Data System (ADS)

Ünal, Alp; Kamacı, Ömer; Altunkaynak, Şafak

2014-05-01

The post collisional magmatic activity produced several volcano-plutonic complexes in NW Anatolia (Turkey) during the late Oligocene- Middle Miocene. One of the major volcano-plutonic complexes, the Solarya volcano-plutonic complex is remarkable for its coeval and cogenetic plutonic (Solarya pluton), hypabysal and volcanic rocks of Early Miocene (24-21 Ma) age. Solarya pluton is an epizonal pluton which discordantly intruded into metamorphic and nonmetamorphic basement rocks of Triassic age. It is a N-S trending magmatic body covering an area of 220 km2,approximatelly 20 km in length and 10 km in width. Based on the field and petrographic studies, three main rock groups distinguished in Solarya pluton; K-feldspar megacrystalline granodiorite, microgranite-granodiorite and haplogranite. Porphyritic and graphic-granophyric textures are common in these three rock groups. Pluton contains magmatic enclaves and syn-plutonic dykes of dioritic composition. Hypabyssal rocks are represented by porphyritic microdiorite and porphyritic quartz-diorite. They form porphyry plugs, sheet inrusions and dykes around the pluton. Porphyrites have microcrystalline-cryptocrystalline groundmass displaying micrographic and granophyric textures. Petrographically similar to the hypabyssal rocks, volcanic rocks are formed from andesitic and dasitic lavas and pyroclastic rocks. Plutonic, hypabyssal and volcanic rocks of Solarya volcano-plutonic complex show similar major-trace element and Sr-Nd-Pb isotopic compositions, indicating common magmatic evolution and multicomponent melt sources including mantle and crustal components. They are mainly metaluminous, medium to high-K calc alkaline rocks and display enrichment in LILE and depletion in Nb, Ta, P and Ti. They have initial 87Sr/86Sr values of 0.70701- 0.70818 and 143Nd/144Nd values of 0.51241-0.51250. These geochemical characteristics and isotopic signatures are considered to reflect the composition of the magmas derived from a metasomatized lithospheric mantle beneath NW Anatolia and from the overlying crust. Compositional variations in Solarya volcano-plutonic complex are interpreted as a result of AFC. Convective removal or partial delamination of the base of mantle lithosphere and asthenospheric upwelling can be considered as possible mechanisms to provide melting of subcontinental lithospheric mantle metasomatized by earlier subduction, resulting in post collisional magmatic activity in NW Anatolia and the adjacent regions.

Constraints of lithium isotopes on petrogenesis of the Northern Luzon arc in Eastern Taiwan

NASA Astrophysics Data System (ADS)

Hsiao, C. C.; Chu, M. F.; Lai, Y. M.; Lin, T. H.

2017-12-01

Lithium stable isotopes have great potential as a tracer of terrestrial materials in crust-mantle recycling. However, the causes of their variations in arc magmatism remain controversial. The Northern Luzon arc has long been demonstrated incorporation of the sediment melt into its sub-arc mantle. The Li isotopes of volcanic rocks in the Coastal Range, located in Eastern Taiwan, thus are studied to examine the effects of sediment melt on the evolution of Li isotopes in subduction zone and also to constrain the petrogenesis of the northernmost part of Northern Luzon arc. It is worth to note that we had ruled out samples that were significantly influenced by crustal contamination according to the proportion of inherited zircons, trace-elemental and Sr-Nd isotopic geochemistry. Concerning that Li isotopic fractionation is negligible during fractional crystallization and partial melting, the variation of Li/Y and δ7Li in rock samples of this study mainly reflects the geochemistry of magma sources. The overall range of δ7Li is very restricted (δ7Li = +2.9 +5.8) and consistent with that of N-MORB. In addition, ɛNd of the Coastal Range volcanic rocks lowers not only with increasing values of sediment-melt indicators (e.g., Th/Ce, Th/Yb and La/Sm), but also Li/Y (from 0.5 to 1.1 ppm). This suggests the involvement of sediment melt with equivalent δ7Li to and higher Li/Y than those of N-MORB, in magma source of the Coastal Range arc volcanism. In summary, the Li isotopic compositions of the Coastal Range volcanic rocks demonstrate that (1) Li/Y commonly treated as a tracer of fluid in arc magmatism indeed can be significantly affected by the input of sediment melt as well, and (2) sediment melt played a key role in the evolution of Li/Y and lithium isotopes in the mantle wedge, but showed least influence on Li isotopic variation possibly as a result of the similarity between δ7Li of sediments subducted and of the upper mantle.

Kankan diamonds (Guinea) III: δ13C and nitrogen characteristics of deep diamonds

NASA Astrophysics Data System (ADS)

Stachel, T.; Harris, J. W.; Aulbach, S.; Deines, P.

Diamonds from the Kankan area in Guinea formed over a large depth profile beginning within the cratonic mantle lithosphere and extending through the asthenosphere and transition zone into the lower mantle. The carbon isotopic composition, the concentration of nitrogen impurities and the nitrogen aggregation level of diamonds representing this entire depth range have been determined. Peridotitic and eclogitic diamonds of lithospheric origin from Kankan have carbon isotopic compositions (δ13C: peridotitic -5.4 to -2.2‰ eclogitic -19.7 to -0.7‰) and nitrogen characteristics (N: peridotitic 17-648 atomic ppm; eclogitic 0-1,313 atomic ppm; aggregation from IaA to IaB) which are generally typical for diamonds of these two suites worldwide. Geothermobarometry of peridotitic and eclogitic inclusion parageneses (worldwide sources) indicates that both suites formed under very similar conditions within the cratonic lithosphere, which is not consistent with a derivation of diamonds with light carbon isotopic composition from subducted organic matter within subducting oceanic slabs. Diamonds containing majorite garnet inclusions fall to the isotopically heavy side (δ13C: -3.1‰ to +0.9‰) of the worldwide diamond population. Nitrogen contents are low (0-126 atomic ppm) and one of the two nitrogen-bearing diamonds shows such a low level of nitrogen aggregation (30% B-centre) that it cannot have been exposed to ambient temperatures of the transition zone (>=1,400 °C) for more than 0.2 Ma. This suggests rapid upward transport and formation of some Kankan diamonds pene-contemporaneous to Cretaceous kimberlite activity. Similar to these diamonds from the asthenosphere and the transition zone, lower mantle diamonds show a small shift towards isotopic heavy compositions (-6.6 to -0.5‰, mode at -3.5‰). As already observed for other mines, the nitrogen contents of lower mantle diamonds were below detection (using FTIRS). The mutual shift of sublithospheric diamonds towards isotopic heavier compositions suggests a common carbon source, which may have inherited an isotopic heavy composition from a component consisting of subducted carbonates.

Kankan diamonds (Guinea) III: δ13C and nitrogen characteristics of deep diamonds

NASA Astrophysics Data System (ADS)

Stachel, T.; Harris, J. W.; Aulbach, S.; Deines, P.

2001-08-01

Diamonds from the Kankan area in Guinea formed over a large depth profile beginning within the cratonic mantle lithosphere and extending through the asthenosphere and transition zone into the lower mantle. The carbon isotopic composition, the concentration of nitrogen impurities and the nitrogen aggregation level of diamonds representing this entire depth range have been determined. Peridotitic and eclogitic diamonds of lithospheric origin from Kankan have carbon isotopic compositions (δ13C: peridotitic -5.4 to -2.2‰ eclogitic -19.7 to -0.7‰) and nitrogen characteristics (N: peridotitic 17-648 atomic ppm; eclogitic 0-1,313 atomic ppm; aggregation from IaA to IaB) which are generally typical for diamonds of these two suites worldwide. Geothermobarometry of peridotitic and eclogitic inclusion parageneses (worldwide sources) indicates that both suites formed under very similar conditions within the cratonic lithosphere, which is not consistent with a derivation of diamonds with light carbon isotopic composition from subducted organic matter within subducting oceanic slabs. Diamonds containing majorite garnet inclusions fall to the isotopically heavy side (δ13C: -3.1‰ to +0.9‰) of the worldwide diamond population. Nitrogen contents are low (0-126 atomic ppm) and one of the two nitrogen-bearing diamonds shows such a low level of nitrogen aggregation (30% B-centre) that it cannot have been exposed to ambient temperatures of the transition zone (>=1,400 °C) for more than 0.2 Ma. This suggests rapid upward transport and formation of some Kankan diamonds pene-contemporaneous to Cretaceous kimberlite activity. Similar to these diamonds from the asthenosphere and the transition zone, lower mantle diamonds show a small shift towards isotopic heavy compositions (-6.6 to -0.5‰, mode at -3.5‰). As already observed for other mines, the nitrogen contents of lower mantle diamonds were below detection (using FTIRS). The mutual shift of sublithospheric diamonds towards isotopic heavier compositions suggests a common carbon source, which may have inherited an isotopic heavy composition from a component consisting of subducted carbonates.

Evidence for an eolian origin for the silt-enriched soil mantles on the glaciated uplands of eastern Upper Michigan, USA

USGS Publications Warehouse

Schaetzl, R.J.; Loope, W.L.

2008-01-01

We provide textural, geochemical, and mineralogical data on a thin, silty deposit that unconformably mantles glaciated uplands in the eastern Upper Peninsula of Michigan. Previous research on this deposit, which we hypothesize to be loess, is nonexistent. The uplands were islands or narrow peninsulas within one or more glacial lakes. We compare the distribution, likely source and nature of the 20-60??cm thick silty mantle by using the loess formation model of Mason et al. [Mason, J.A., Nater, E.A., Zanner, C.W., Bell, J.C., 1999. A new model of topographic effects on the distribution of loess. Geomorphology 28, 223-236], which focuses on the generation of eolian silt by saltating sand across upwind, barren surfaces. Parabolic dunes, with arms open to the NW, are common on former lake floors upwind of the silt-mantled uplands, attesting to the strength and direction of paleowinds. The abrupt termination of the dunes at the footslopes of the uplands, associated with silt deposition on upland soil surfaces in downwind locations, are both consistent with the model of Mason et al. [Mason, J.A., Nater, E.A., Zanner, C.W., Bell, J.C., 1999. A new model of topographic effects on the distribution of loess. Geomorphology 28, 223-236]. Sediments on former lake floors contain abundant strata of fine/medium sand and silt, and thus are likely sources for the silt and dune sand. The cap, dune and lake sediments are similar along many different geochemical axes, whereas the substrate sediment, i.e., the drift below the cap, is unique. Cap sediments, normally containing roughly 30% silt, are enriched in quartz and depleted in Ti and Zr, relative to dune sediment. The dune sediment, a more residual eolian deposit, is enriched in Ti and Zr, relative to the cap, probably due to its greater abundance of heavy minerals. Therefore, we conclude that the silty cap is loess that was deflated from abandoned lake floors after nearby glacial lakes drained, probably contemporaneously with dune migration across the former lake floors. ?? 2008 Elsevier B.V. All rights reserved.

Numerical study of the origin and stability of chemically distinct reservoirs deep in Earth's mantle

NASA Astrophysics Data System (ADS)

van Thienen, P.; van Summeren, J.; van der Hilst, R. D.; van den Berg, A. P.; Vlaar, N. J.

Seismic tomography is providing mounting evidence for large scale compositional heterogeneity deep in Earth's mantle; also, the diverse geochemical and isotopic signatures observed in oceanic basalts suggest that the mantle is not chemically homogeneous. Isotopic studies on Archean rocks indicate that mantle inhomogeneity may have existed for most of the Earth's history. One important component may be recycled oceanic crust, residing at the base of the mantle. We investigate, by numerical modeling, if such reservoirs may have been formed in the early Earth, before plate tectonics (and subduction) were possible, and how they have survived—and evolved—since then. During Earth's early evolution, thick basaltic crust may have sunk episodically into the mantle in short but vigorous diapiric resurfacing events. These sections of crust may have resided at the base of the mantle for very long times. Entrainment of material from the enriched reservoirs thus produced may account for enriched mantle and high-μ signatures in oceanic basalts, whereas deep subduction events may have shaped and replenished deep mantle reservoirs. Our modeling shows that (1) convective instabilities and resurfacing may have produced deep enriched mantle reservoirs before the era of plate tectonics; (2) such formation is qualitatively consistent with the geochemical record, which shows multiple distinct ocean island basalt sources; and (3) reservoirs thus produced may be stable for billions of years.

Geochemical constraints on possible subduction components in lavas of Mayon and Taal Volcanoes, Southern Luzon, Philippines

USGS Publications Warehouse

Castillo, P.R.; Newhall, C.G.

2004-01-01

Mayon is the most active volcano along the east margin of southern Luzon, Philippines. Petrographic and major element data indicate that Mayon has produced a basaltic to andesitic lava series by fractional crystallization and magma mixing. Trace element data indicate that the parental basalts came from a heterogeneous mantle source. The unmodified composition of the mantle wedge is similar to that beneath the Indian Ocean. To this mantle was added a subduction component consisting of melt from subducted pelagic sediment and aqueous fluid dehydrated from the subducted basaltic crust. Lavas from the highly active Taal Volcano on the west margin of southern Luzon are compositionally more variable than Mayon lavas. Taal lavas also originated from a mantle wedge metasomatized by aqueous fluid dehydrated from the subducted basaltic crust and melt plus fluid derived from the subducted terrigenous sediment. More sediment is involved in the generation of Taal lavas. Lead isotopes argue against crustal contamination. Some heterogeneity of the unmodified mantle wedge and differences in whether the sediment signature is transferred into the lava source through an aqueous fluid or melt phase are needed to explain the regional compositional variation of Philippine arc lavas. ?? Oxford University Press 2004; all rights reserved.

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

NASA Astrophysics Data System (ADS)

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

2009-04-01

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

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

USGS Publications Warehouse

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

2009-01-01

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

Redox Evolution in Magma Oceans Due to Ferric/Ferrous Iron Partitioning

NASA Astrophysics Data System (ADS)

Schaefer, L.; Elkins-Tanton, L. T.; Pahlevan, K.

2017-12-01

A long-standing puzzle in the evolution of the Earth is that while the present day upper mantle has an oxygen fugacity close to the QFM buffer, core formation during accretion would have occurred at much lower oxygen fugacities close to IW. We present a new model based on experimental evidence that normal solidification and differentiation processes in the terrestrial magma ocean may explain both core formation and the current oxygen fugacity of the mantle without resorting to a change in source material or process. A commonly made assumption is that ferric iron (Fe3+) is negligible at such low oxygen fugacities [1]. However, recent work on Fe3+/Fe2+ ratios in molten silicates [2-4] suggests that the Fe3+ content should increase at high pressure for a given oxygen fugacity. While disproportionation was not observed in these experiments, it may nonetheless be occurring in the melt at high pressure [5]. Therefore, there may be non-negligible amounts of Fe3+ formed through metal-silicate equilibration at high pressures within the magma ocean. Homogenization of the mantle and further partitioning of Fe2+/Fe3+ as the magma ocean crystallizes may explain the oxygen fugacity of the Earth's mantle without requiring additional oxidation mechanisms. We present here models using different parameterizations for the Fe2+/Fe3+ thermodynamic relationships in silicate melts to constrain the evolution of the redox state of the magma ocean as it crystallizes. The model begins with metal-silicate partitioning at high pressure to form the core and set the initial Fe3+ abundance. Combined with previous work on oxygen absorption by magma oceans due to escape of H from H2O [6], we show that the upper layers of solidifying magma oceans should be more oxidized than the lower mantle. This model also suggests that large terrestrial planets should have more oxidized mantles than small planets. From a redox perspective, no change in the composition of the Earth's accreting material needs to be invoked to explain mantle oxygen fugacity, although implications for trace elements in the Earth's core and mantle are still being explored. [1] Ringwood, AE (1979) [2] Armstrong, K et al (2016) LPSC, 2580 [3] Armstrong, K et al (2017) ACCRETE workshop [4] Zhang, HL et al (2017) GCA, 204, 83 [5] Frost, DJ et al (2004) Nature, 428, 409 [6] Schaefer, L et al (2016) ApJ, 829, 63

Tectonic plates, D (double prime) thermal structure, and the nature of mantle plumes

NASA Technical Reports Server (NTRS)

Lenardic, A.; Kaula, W. M.

1994-01-01

It is proposed that subducting tectonic plates can affect the nature of thermal mantle plumes by determining the temperature drop across a plume source layer. The temperature drop affects source layer stability and the morphology of plumes emitted from it. Numerical models are presented to demonstrate how introduction of platelike behavior in a convecting temperature dependent medium, driven by a combination of internal and basal heating, can increase the temperature drop across the lower boundary layer. The temperature drop increases dramatically following introduction of platelike behavior due to formation of a cold temperature inversion above the lower boundary layer. This thermal inversion, induced by deposition of upper boundary layer material to the system base, decays in time, but the temperature drop across the lower boundary layer always remains considerably higher than in models lacking platelike behavior. On the basis of model-inferred boundary layer temperature drops and previous studies of plume dynamics, we argue that generally accepted notions as to the nature of mantle plumes on Earth may hinge on the presence of plates. The implication for Mars and Venus, planets apparently lacking plate tectonics, is that mantle plumes of these planets may differ morphologically from those of Earth. A corollary model-based argument is that as a result of slab-induced thermal inversions above the core mantle boundary the lower most mantle may be subadiabatic, on average (in space and time), if major plate reorganization timescales are less than those acquired to diffuse newly deposited slab material.

Submarine Alkalic Lavas Around the Hawaiian Hotspot; Plume and Non-Plume Signatures Determined by Noble Gases

NASA Astrophysics Data System (ADS)

Hanyu, T.; Clague, D. A.; Kaneoka, I.; Dunai, T. J.; Davies, G. R.

2004-12-01

Noble gas isotopic ratios were determined for submarine alkalic volcanic rocks distributed around the Hawaiian islands to constrain the origin of such alkalic volcanism. Samples were collected by dredging or using submersibles from the Kauai Channel between Oahu and Kauai, north of Molokai, northwest of Niihau, Southwest Oahu, South Arch and North Arch volcanic fields. Sites located downstream from the center of the hotspot have 3He/4He ratios close to MORB at about 8 Ra, demonstrating that the magmas erupted at these sites had minimum contribution of volatiles from a mantle plume. In contrast, the South Arch, located upstream of the hotspot on the Hawaiian Arch, has 3He/4He ratios between 17 and 21 Ra, indicating a strong plume influence. Differences in noble gas isotopic characteristics between alkalic volcanism downstream and upstream of the hotspot imply that upstream volcanism contains incipient melts from an upwelling mantle plume, having primitive 3He/4He. In combination with lithophile element isotopic data, we conclude that the most likely source of the upstream magmatism is depleted asthenospheric mantle that has been metasomatised by incipient melt from a mantle plume. After major melt extraction from the mantle plume during production of magmas for the shield stage, the plume material is highly depleted in noble gases and moderately depleted in lithophile elements. Partial melting of the depleted mantle impregnated by melts derived from this volatile depleted plume source may explain the isotopic characteristics of the downstream alkalic magmatism.

Deep permeable fault-controlled helium transport and limited mantle flux in two extensional geothermal systems in the Great Basin, United States

USGS Publications Warehouse

Banerjee, Amlan; Person, Mark; Hofstra, Albert; Sweetkind, Donald S.; Cohen, Denis; Sabin, Andrew; Unruh, Jeff; Zyvoloski, George; Gable, Carl W.; Crossey, Laura; Karlstrom, Karl

2011-01-01

This study assesses the relative importance of deeply circulating meteoric water and direct mantle fluid inputs on near-surface 3He/4He anomalies reported at the Coso and Beowawe geothermal fields of the western United States. The depth of meteoric fluid circulation is a critical factor that controls the temperature, extent of fluid-rock isotope exchange, and mixing with deeply sourced fluids containing mantle volatiles. The influence of mantle fluid flux on the reported helium anomalies appears to be negligible in both systems. This study illustrates the importance of deeply penetrating permeable fault zones (10-12 to 10-15 m2) in focusing groundwater and mantle volatiles with high 3He/4He ratios to shallow crustal levels. These continental geothermal systems are driven by free convection.

Chemical composition of rocks and soils at Taurus-Littrow

NASA Technical Reports Server (NTRS)

Rose, H. J., Jr.; Cuttitta, F.; Berman, S.; Brown, F. W.; Carron, M. K.; Christian, R. P.; Dwornik, E. J.; Greenland, L. P.

1974-01-01

Seventeen soils and seven rock samples were analyzed for major elements, minor elements, and trace elements. Unlike the soils at previous Apollo sites, which showed little difference in composition at each collection area, the soils at Taurus-Littrow vary widely. Three soil types are evident, representative of (1) the light mantle at the South Massif, (2) the dark mantle in the valley, and (3) the surface material at the North Massif. The dark-mantle soils are chemically similar to those at Tranquillitatis. Basalt samples from the dark mantle are chemically similar although they range from fine to coarse grained. It is suggested that they originated from the same source but crystallized at varying depths from the surface.

Plume-related mantle source of super-large rare metal deposits from the Lovozero and Khibina massifs on the Kola Peninsula, Eastern part of Baltic Shield: Sr, Nd and Hf isotope systematics

NASA Astrophysics Data System (ADS)

Kogarko, L. N.; Lahaye, Y.; Brey, G. P.

2010-03-01

The two world’s largest complexes of highly alkaline nepheline syenites and related rare metal loparite and eudialyte deposits, the Khibina and Lovozero massifs, occur in the central part of the Kola Peninsula. We measured for the first time in situ the trace element concentrations and the Sr, Nd and Hf isotope ratios by LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) in loparite, eudialyte an in some other pegmatitic minerals. The results are in aggreement with the whole rock Sr and Nd isotope which suggests the formation of these superlarge rare metal deposits in a magmatic closed system. The initial Hf, Sr, Nd isotope ratios are similar to the isotopic signatures of OIB indicating depleted mantle as a source. This leads to the suggestion that the origin of these gigantic alkaline intrusions is connected to a deep seated mantle source—possibly to a lower mantle plume. The required combination of a depleted mantle and high rare metal enrichment in the source can be explained by the input of incompatible elements by metasomatising melts/fluids into the zones of alkaline magma generation shortly before the partial melting event (to avoid ingrowth of radiogenic isotopes). The minerals belovite and pyrochlore from the pegmatites are abnormally high in 87Sr /86Sr ratios. This may be explained by closed system isotope evolution as a result of a significant increase in Rb/Sr during the evolution of the peralkaline magma.

Continental magnetic anomaly constraints on continental reconstruction

NASA Technical Reports Server (NTRS)

Vonfrese, R. R. B.; Hinze, W. J.; Olivier, R.; Bentley, C. R.

1985-01-01

Crustal magnetic anomalies mapped by the MAGSAT satellite for North and South America, Europe, Africa, India, Australia and Antarctica and adjacent marine areas were adjusted to a common elevation of 400 km and differentially reduced to the radial pole of intensity 60,000 nT. These radially polarized anomalies are normalized for differential inclination, declination and intensity effects of the geomagnetic field, so that in principle they directly reflected the geometric and magnetic polarization attributes of sources which include regional petrologic variations of the crust and upper mantle, and crustal thickness and thermal perturbations. Continental anomalies demonstrate remarkably detailed correlation of regional magnetic sources across rifted margins when plotted on a reconstruction of Pangea. Accordingly, they suggest further fundamental constraints on the geologic evolution of the continents and their reconstructions.

The behaviour of tungsten during mantle melting revisited with implications for planetary differentiation time scales

NASA Astrophysics Data System (ADS)

Babechuk, Michael G.; Kamber, Balz S.; Greig, Alan; Canil, Dante; Kodolányi, János

2010-02-01

Tungsten is a moderately siderophile high-field-strength element that is hydrophile and widely regarded as highly incompatible during mantle melting. In an effort to extend empirical knowledge regarding the behaviour of W during the latter process, we report new high-precision trace element data (W, Th, U, Ba, La, Sm) that represent both terrestrial and planetary reservoirs: MORB (11), abyssal peridotites (8), eucrite basalts (3), and carbonaceous chondrites (8). A full trace element suite is also reported for Cordilleran Permian ophiolite peridotites (12) to better constrain the behaviour of W in the upper mantle. In addition, we report our long-term averages for a number of USGS (BIR-1, BHVO-1, BHVO-2, PCC-1, DTS-1) and GSJ (JA-3, JP-1) standard reference materials, some of which we conclude to be heterogeneous and contaminated with respect to W. The most significant finding of this study is that many of the highly depleted upper mantle peridotites contain far higher W concentrations than expected. In the absence of convincing indications for alteration, re-enrichment or contamination, we propose that the W excess was caused by retention in an Os-Ir alloy phase, whose stability is dependent on fO 2 of the mantle source region. This explanation could help to account for the particularly low W content of N-MORB and implies that the lithophile behaviour of W in basaltic rocks is not an accurate representation of the behaviour in the melt source. These findings then become relevant to the interpretation of W-isotopic data for achondrites, where the fractionation of Hf from W during melting is used to infer the Hf/W of the parent body mantle. This is exemplified by the differentiation chronology of the eucrite parent body (EPB), which has been modeled with a melt source with high Hf/W. By contrast, we explore the alternative scenario with a low mantle Hf/W on the EPB. Using available eucrite literature data, a maximum core segregation age of 1.2 ± 1.2 Myr after the closure of CAIs is calculated with a more prolonged time between core formation and mantle fractionation of ca. 2 Myr. This timeline is consistent with most recent published chronologies of the EPB differentiation based on the 53Mn- 53Cr and 26Al- 26Mg systems.

Tectonic events, continental intraplate volcanism, and mantle plume activity in northern Arabia: Constraints from geochemistry and Ar-Ar dating of Syrian lavas

NASA Astrophysics Data System (ADS)

Krienitz, M.-S.; Haase, K. M.; Mezger, K.; van den Bogaard, P.; Thiemann, V.; Shaikh-Mashail, M. A.

2009-04-01

New 40Ar/39Ar ages combined with chemical and Sr, Nd, and Pb isotope data for volcanic rocks from Syria along with published data of Syrian and Arabian lavas constrain the spatiotemporal evolution of volcanism, melting regime, and magmatic sources contributing to the volcanic activity in northern Arabia. Several volcanic phases occurred in different parts of Syria in the last 20 Ma that partly correlate with different tectonic events like displacements along the Dead Sea Fault system or slab break-off beneath the Bitlis suture zone, although the large volume of magmas and their composition suggest that hot mantle material caused volcanism. Low Ce/Pb (<20), Nb/Th (<10), and Sr, Nd, and Pb isotope variations of Syrian lavas indicate the role of crustal contamination in magma genesis, and contamination of magmas with up to 30% of continental crustal material can explain their 87Sr/86Sr. Fractionation-corrected major element compositions and REE ratios of uncontaminated lavas suggest a pressure-controlled melting regime in western Arabia that varies from shallow and high-degree melt formation in the south to increasingly deeper regions and lower extents of the beginning melting process northward. Temperature estimates of calculated primary, crustally uncontaminated Arabian lavas indicate their formation at elevated mantle temperatures (Texcess ˜ 100-200°C) being characteristic for their generation in a plume mantle region. The Sr, Nd, and Pb isotope systematic of crustally uncontaminated Syrian lavas reveal a sublithospheric and a mantle plume source involvement in their formation, whereas a (hydrous) lithospheric origin of lavas can be excluded on the basis of negative correlations between Ba/La and K/La. The characteristically high 206Pb/204Pb (˜19.5) of the mantle plume source can be explained by material entrainment associated with the Afar mantle plume. The Syrian volcanic rocks are generally younger than lavas from the southern Afro-Arabian region, indicating a northward progression of the commencing volcanism since the arrival of the Afar mantle plume beneath Ethiopia/Djibouti some 30 Ma ago. The distribution of crustally uncontaminated high 206Pb/204Pb lavas in Arabia indicates a spatial influence of the Afar plume of ˜2600 km in northward direction with an estimated flow velocity of plume material on the order of 22 cm/a.

182W and HSE constraints from 2.7 Ga komatiites on the heterogeneous nature of the Archean mantle

NASA Astrophysics Data System (ADS)

Puchtel, Igor S.; Blichert-Toft, Janne; Touboul, Mathieu; Walker, Richard J.

2018-05-01

While the isotopically heterogeneous nature of the terrestrial mantle has long been established, the origin, scale, and longevity of the heterogeneities for different elements and isotopic systems are still debated. Here, we report Nd, Hf, W, and Os isotopic and highly siderophile element (HSE) abundance data for the Boston Creek komatiitic basalt lava flow (BCF) in the 2.7 Ga Abitibi greenstone belt, Canada. This lava flow is characterized by strong depletions in Al and heavy rare earth elements (REE), enrichments in light REE, and initial ε143Nd = +2.5 ± 0.2 and intial ε176Hf = +4.2 ± 0.9 indicative of derivation from a deep mantle source with time-integrated suprachondritic Sm/Nd and Lu/Hf ratios. The data plot on the terrestrial Nd-Hf array suggesting minimal involvement of early magma ocean processes in the fractionation of lithophile trace elements in the mantle source. This conclusion is supported by a mean μ142Nd = -3.8 ± 2.8 that is unresolvable from terrestrial standards. By contrast, the BCF exhibits a positive 182W anomaly (μ182W = +11.7 ± 4.5), yet is characterized by chondritic initial γ187Os = +0.1 ± 0.3 and low inferred source HSE abundances (35 ± 5% of those estimated for the present-day Bulk Silicate Earth, BSE). Collectively, these characteristics are unique among Archean komatiite systems studied so far. The deficit in the HSE, coupled with the chondritic Os isotopic composition, but a positive 182W anomaly, are best explained by derivation of the parental BCF magma from a mantle domain characterized by a predominance of HSE-deficient, differentiated late accreted material. According to the model presented here, the mantle domain that gave rise to the BCF received only ∼35% of the present-day HSE complement in the BSE before becoming isolated from the rest of the convecting mantle until the time of komatiite emplacement at 2.72 Ga. These new data provide strong evidence for a highly heterogeneous Archean mantle in terms of absolute HSE abundances and W isotopic composition, and also indicate slow mixing, on a timescale of at least 1.8 billion years. Additionally, the data are consistent with a stagnant-lid plate tectonic regime in the Hadean and Archean, prior to the onset of modern-style plate tectonics.

Using mineral geochemistry to decipher slab, mantle, and crustal inputs to the generation of high-Mg andesites from Mount Baker and Glacier Peak, northern Cascade arc

NASA Astrophysics Data System (ADS)

Sas, M.; DeBari, S. M.; Clynne, M. A.; Rusk, B. G.

2015-12-01

A fundamental question in geology is whether subducting plates get hot enough to generate melt that contributes to magmatic output in volcanic arcs. Because the subducting plate beneath the Cascade arc is relatively young and hot, slab melt generation is considered possible. To better understand the role of slab melt in north Cascades magmas, this study focused on petrogenesis of high-Mg andesites (HMA) and basaltic andesites (HMBA) from Mt. Baker and Glacier Peak, Washington. HMA have unusually high Mg# relative to their SiO2 contents, as well as elevated La/Yb and Dy/Yb ratios that are interpreted to result from separation of melt from a garnet-bearing residuum. Debate centers on the garnet's origin as it could be present in mineral assemblages from the subducting slab, deep mantle, thick lower crust, or basalt fractionated at high pressure. Whole rock analyses were combined with major, minor, and trace element analyses to understand the origin of these HMA. In the Tarn Plateau (Mt. Baker) flow unit (51.8-54.0 wt.% SiO2, Mg# 68-70) Mg#s correlate positively with high La/Yb in clinopyroxene equilibrium liquids, suggesting an origin similar to that of Aleutian adakites, where slab-derived melts interact with the overlying mantle to become Mg-rich and subsequently mix with mantle-derived basalts. The source for high La/Yb in the Glacier Creek (Mt. Baker) flow unit (58.3-58.7 wt.% SiO2, Mg# 63-64) is more ambiguous. High whole rock Sr/P imply origin from a mantle that was hydrated by an enriched slab component (fluid ± melt). In the Lightning Creek (Glacier Peak) flow unit (54.8-57.9 SiO2, Mg# 69-72) Cr and Mg contents in Cr-spinel and olivine pairs suggest a depleted mantle source, and high whole rock Sr/P indicate hydration-induced mantle melting. Hence Lightning Creek is interpreted have originated from a refractory mantle source that interacted with a hydrous slab component (fluid ± melt). Our results indicate that in addition to slab-derived fluids, slab-derived melts also have an important role in the production of HMA in the north Cascade arc.

Istopically Defined Source Reservoirs of Primitive Magmas in the East African Rift.

NASA Astrophysics Data System (ADS)

Rooney, T. O.; Furman, T.; Hanan, B.

2005-12-01

Extension within the East African Rift is a function of the interaction between plume-driven uplift and far-field stresses associated with plate tectonic processes. Geochemical and isotopic investigation of primitive basalts from the Main Ethiopian Rift (MER) reveals systematic spatial variations in the contributions from distinct and identifiable source reservoirs that, in turn help identify the mechanisms by which along-axis rifting has progressed. The Sr-Nd-Pb isotopic characteristics of MER basalts can be described by a three-component mixing model involving the long-lived Afar plume, a depleted mantle component similar to the source region for Gulf of Aden MORB from east of 48° E and a reservoir that is likely lithospheric (sub-continental mantle lithosphere, magmatic underplate or lower crust). Quaternary basalts in the central MER exhibit a systematic decrease in plume influence southward from 9.5° N to 8° N, i.e., away from the modern surface expression of the Afar plume in Djibouti and Erta 'Ale. The composition of the Afar plume component is comparable to the "C" mantle reservoir. This southward decrease in plume influence is coupled with an increase in the influence of the lithospheric and depleted mantle components. Linear arrays observed within Pb-Pb isotopic space at each eruptive center require distinctive ratio of lithospheric + depleted mantle components mixing with variable amounts of the "C"-like plume component. This isotopic evidence suggests the depleted mantle and lithosphere mixed prior to the generation of the recent magmas. To the south, the Sr-Nd-Pb isotopic compositions of Turkana (Kenya) rift basalts record a mix of a similar "C"-like plume component and a fourth HIMU-like source component. Low 3He/4He values observed in the HIMU-dominated lavas from Turkana contrast with the higher ratios found in basalts associated with the "C"-like Afar plume. Further analysis of "C"-HIMU lavas at Turkana is required to fully constrain the He isotopic signatures. Thus, along-axis patterns in Quaternary EARS magmatism are compatible with two "C"-like plumes with contributions from the upper mantle and chemically distinct lithospheric components. Alternatively, a single "C"-like plume can account for these relationships. In the single plume scenario, the HIMU source component present in the 30 Ma Turkana lavas may represent melting of metasomatised lithosphere, derived from the accretion of island-arc-backarc basins during Pan-African events (e.g. Schilling et al., 1992). The recent plume-dominated activity in Turkana and Afar are separated by a region characterized by waning plume influence and a greater contribution from the depleted mantle. This intermediate zone, which is located in the south-central MER represents the modern site of contact between the northward propagating Kenya / Turkana Rift and the southward propagating Afar Rift zone.

Temporal Variations in the Mantle Source of MORB near the Vema Fracture Zone (Central Atlantic): Nd and Sr Isotopes in Peridotites and Basaltic Glasses

NASA Astrophysics Data System (ADS)

Cipriani, A.; Cipriani, A.; Brunelli, D.; Brueckner, H. K.; Brueckner, H. K.; Bonatti, E.; Bonatti, E.

2001-12-01

Sr-Nd-Pb isotopic ratios of zero age basalts sampled along Mid-Ocean Ridges (MOR) have demonstrated that the mantle is heterogeneous at a regional scale. However, how the mantle evolves through time below a single segment of MOR it is still matter of debate. Peridotites and basaltic glasses were collected along a lithospheric section uplifted and exposed on the southern side of the Vema transform (10o North, Atlantic Ocean) along a seafloor spreading flow line for a stretch of almost 200 km (corresponding to roughly 10 my). This set of samples offers a unique opportunity to detect changes through time of the mantle signature in a segment of Mid Atlantic Ridge, by analyzing radiogenic isotopes in the clinopyroxenes (cpx) from peridotites and glasses from the overlying basalts. Work is in progress; initial Sr and Nd measurements from cpxs within peridotites indicate several things. First, the cpxs display "depleted" mantle signatures. Second, there is a considerable variation of the isotopic ratios along the exposed section (143Nd/144Nd varies from 0.51293 to 0.51345, 87Sr/86Sr varies from 0.70228 to 0.70422) and these variations occur over a short time scale (some occur within an interval of one million year). Next, the Sr and Nd ratios are inversely correlated and fall along the mantle array. Finally, cpx Nd ratios are inversely correlated with the Cr/Al ratio of the spinel and ortopyroxene (opx) from the peridotites while Sr ratios are positively correlated. Thus, the chemically most depleted peridotite with high Cr/Al ratios show the most enriched isotopic signatures, a pattern that has also been observed in alpine-type peridotites and peridotite nodules and that is generally interpreted as metasomatism by enriched fluids affecting depleted peridotite more extensively than less depleted peridotite. This may indicate that the temporal variations in the extent of melting detected by Cr/Al ratio in spinel and opx (Bonatti et al., Variations with age of mantle ultramafic composition near the Vema Fracture Zone, Central Atlantic. EOS, Vol.79, No.45, F919) are related to rapid changes in the degree of depletion of the upwelling mantle sources and that the degree of depletion of these mantle sources is an inherited feature from earlier processes rather than the result of melting at the MOR.

Volcanism on Mercury (dikes, lava flows, pyroclastics): Crust/mantle density contrasts, the evolution of compressive stress and the presence of mantle volatiles

NASA Astrophysics Data System (ADS)

Wilson, L.; Head, J. W., III

2008-09-01

Background. There is great uncertainty about the internal structure of Mercury and the composition of the mantle [e.g., 1, 2]. The high mean density of the body suggests that it may have lost parts of its crust and mantle in a giant impact at some stage after most of its initial accretion was sufficiently complete that at least partial separation of a core had occurred. It is the uncertainty about the timing of the giant impact, and hence the physico-chemical state of proto-Mercury at the time that it occurred, that leads to difficulties in predicting the interior structure and mantle composition. However, it seems reasonable to assume that the Mercury we see today has some combination of a relatively low-density crust and a relatively highdensity mantle; uncertainty remains about the presence and types of volatiles [2]. The second uncertainty is the nature of the surface plains units, specifically, are these lava flows and pyroclastics erupted from the interior, or impact-reworked earlier crust [3-5] (Figs. 1-2)? The detection of candidate pyroclastic deposits [4] has very important implications for mantle volatiles. Furthermore, whatever the surface composition, the presence of planet-wide systems of wrinkle ridges and thrust faults implies that a compressive crustal stress regime became dominant at some stage in the planet's history [3, 6]. If the plains units are indeed lava flows, then the fact that the products of the compressive regime deform many plains units suggests that the development of the compressive stresses may have played a vital role in determining when and if surface eruptions of mantle-derived magmas could occur. This would be analogous to the way in which the change with time from extensional to compressive global stresses in the lithosphere of the Moon influenced the viability of erupting magmas from deep mantle sources [7-9]. Analysis: To investigate the relationship between lithospheric stresses and magma eruption conditions [e.g., 9-11] we have assumed a series of permutations of crustal density, crustal thickness, mantle density, magma density, source depth in mantle of melt generation, and crustal compressive stress, and investigated which permutations will allow the transfer of magma from source to surface. With so many variables it is easiest to illustrate the results by choosing one set of densities and varying the depths and stresses. We begin with crustal density of 2700 kg m-3, a mantle density of 3400 kg m-3 and a melt density of 3000 kg m-3. Table 1 then shows, as a function of the thickness of the crust (Hc), the minimum depth below the surface (Hm) from which mantle melts must be derived if their positive buoyancy in the mantle is to just compensate for their negative buoyancy in the crust and so enable them to reach the surface and erupt. For the values of Hm in Table 1 to be valid, the stress conditions in the crust must be such that a dike can remain open at all depths. However, this may not be possible in the presence of a horizontal compressive stress. The third and fourth columns of the table show the maximum horizontal compressive stress allowed if a dike is to remain open when the compressive stress is either uniform, i.e. the same at all depths in the crust (Su), or variable, specifically decreasing from the value given (Sv) at the surface to zero at the base of the crust. We now increase the crustal density slightly to 2800 kg m-3 but keep the mantle and melt densities the same. The results in Table 2 show, as expected, that the reduced amount of negative buoyancy of magma in the crust means that mantle melt sources need not be quite as deep as before. However, if a pathway is to remain open at all depths, significantly smaller compressive stresses are needed than in the previous case. Implications: This comparison demonstrates the major trend that we find: as the crust becomes denser it is easier, in terms of magma buoyancy alone, to erupt magma from a given depth in the mantle. Given that all intrusions and eruptions emplace magma at some level into the crust, and therefore increase its density with time, this at first sight implies that surface eruptions of magma coming directly from the mantle could have become commoner with time on Mercury. However, the fact that the thermal history of the planet is likely to dictate that crustal compressive stresses increased with time, together with our finding that such an increase progressively suppresses the possibility of maintaining continuously open pathways between the mantle and the surface, suggests that conditions were much more finely balanced. By analogy with the Moon's thermal history [12, 13], compressive stresses at least a factor of two greater than those found here to suppress stable dikes must have been reached about half way through Mercury's lifetime, with even greater compressive stresses being needed to cause the observed thrust faults. Thus deepseated eruptive activity must eventually have ceased on Mercury, with the timing of its cessation being very finely tuned by its density and stress structure. As our knowledge of the surface composition and internal structure of Mercury improves with future exploration by MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) [14] and BepiColombo [15, 16], it will become possible to greatly refine the models presented here. Furthermore, the distribution of suspected pyroclastic deposits [4] can be used to assess mantle volatile content. References: [1] M. T. Zuber et al. (2007) SSR, 131. 105- 132. [2] W. V. Boynton et al. (2007) SSR, 131, 85-104. [3] J. W. Head et al. (2007) SSR, 131, 41-84. [4] M. Robinson and P. Lucey (1997) Science, 275, 197-200. [5] J. W. Head et al. (2008) Science, in press. [6] R. G. Strom et al. (1975) JGR, 80, 2478-2507. [7] S. C. Solomon and J. W. Head (1979) JGR, 84, 1667-1682. [8] S. C. Solomon and J. W. Head (1989) RGSP, 18, 107-141. [9] J. W. Head and L. Wilson (1992) G&CA, 55, 2155-2175. [10] J. W. Head and L. Wilson (2001) Workshop on Mercury: Space Environment, Surface and Interior (LPI), 44-45. [11] L. Wilson and J. W. Head (2008) LPSC 39, #1104. [12] M.A. Wieczorek et al. (2007) New Views of the Moon, MSA-RMG 60, 221-364. [13] C. K. Shearer et al. (2007) New Views of the Moon, MSARMG 60, 365-518. [14] S. C. Solomon et al. (2007) SSR, 131, 3-39. [15] R. Grard et al. (2000) ESA Bull., 103, 11-19. [16] A. Anselmi and G. Scoon (2001) PSS, 49, 1409-1420.

The Moho as a magnetic boundary. [Earth crust-mantle boundary

NASA Technical Reports Server (NTRS)

Wasilewski, P. J.; Thomas, H. H.; Mayhew, M. A.

1979-01-01

Magnetism in the crust and the upper mantle and magnetic results indicating that the seismic Moho is a magnetic boundary are considered. Mantle derived rocks - peridotites from St. Pauls rocks, dunite xenoliths from the Kaupulehu flow, and peridotite, dunite, and eclogite xenoliths from Roberts Victor and San Carlos diatremes - are weakly magnetic with saturation magnetization values from 0.013 emu/gm to less than 0.001 emu/gm which is equivalent to 0.01 to 0.001 wt% Fe304. Literature on the minerals in mantle xenoliths shows that metals and primary Fe304 are absent, and that complex Cr, Mg, Al, and Fe spinels are dominant. These spinels are non-magnetic at mantle temperatures, and the crust/mantle boundary can be specified as a magnetic mineralogy discontinuity. The new magnetic results indicate that the seismic Moho is a magnetic boundary, the source of magnetization is in the crust, and the maximum Curie isotherm depends on magnetic mineralogy and is located at depths which vary with the regional geothermal gradient.

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

NASA Astrophysics Data System (ADS)

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

2004-12-01

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

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

NASA Astrophysics Data System (ADS)

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

1980-10-01

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

Molybdenum mobility and isotopic fractionation during subduction at the Mariana arc

NASA Astrophysics Data System (ADS)

Freymuth, Heye; Vils, Flurin; Willbold, Matthias; Taylor, Rex N.; Elliott, Tim

2015-12-01

The fate of crustal material recycled into the convecting mantle by plate tectonics is important for understanding the chemical and physical evolution of the planet. Marked isotopic variability of Mo at the Earth's surface offers the promise of providing distinctive signatures of such recycled material. However, characterisation of the behaviour of Mo during subduction is needed to assess the potential of Mo isotope ratios as tracers for global geochemical cycles. Here we present Mo isotope data for input and output components of the archetypical Mariana arc: Mariana arc lavas, sediments from ODP Sites 800, 801 and 802 near the Mariana trench and the altered mafic, oceanic crust (AOC), from ODP Site 801, together with samples of the deeper oceanic crust from ODP Site 1256. We also report new high precision Pb isotope data for the Mariana arc lavas and a dataset of Pb isotope ratios from sediments from ODP Sites 800, 801 and 802. The Mariana arc lavas are enriched in Mo compared to elements of similar incompatibility during upper mantle melting, and have distinct, isotopically heavy Mo (high 98Mo/95Mo) relative to the upper mantle, by up to 0.3 parts per thousand. In contrast, the various subducting sediment lithologies dominantly host isotopically light Mo. Coupled Pb and Mo enrichment in the Mariana arc lavas suggests a common source for these elements and we further use Pb isotopes to identify the origin of the isotopically heavy Mo. We infer that an aqueous fluid component with elevated [Mo], [Pb], high 98Mo/95Mo and unradiogenic Pb is derived from the subducting, mafic oceanic crust. Although the top few hundred metres of the subducting, mafic crust have a high 98Mo/95Mo, as a result of seawater alteration, tightly defined Pb isotope arrays of the Mariana arc lavas extrapolate to a fluid component akin to fresh Pacific mid-ocean ridge basalts. This argues against a flux dominantly derived from the highly altered, uppermost mafic crust or indeed from an Indian-like mantle wedge. Thus we infer that the Pb and Mo budgets of the fluid component are dominated by contributions from the deeper, less altered (cooler) portion of the subducting Pacific crust. The high 98Mo/95Mo of this flux is likely caused by isotopic fractionation during dehydration and fluid flow in the slab. As a result, the residual mafic crust becomes isotopically lighter than the upper mantle from which it was derived. Our results suggest that the continental crust produced by arc magmatism should have an isotopically heavy Mo composition compared to the mantle, whilst a contribution of deep recycled oceanic crust to the sources of some ocean island basalts might be evident from an isotopically light Mo signature.

Crustal Anatexis by Upwelling Mantle Melts in the N.Atlantic Igneous Province: the Isle of Rum, NW Scotland.

NASA Astrophysics Data System (ADS)

Hertogen, J.; Meyer, R.; Nicoll, G.; Troll, V. R.; Ellam, R. M.; Emeleus, C. H.

2008-12-01

Crustal anatexis is a common process in the rift-to-drift evolution during continental breakup and the formation of Volcanic Rifted Margins (VRM) systems. 'Early felsic-later mafic' volcanic rock associations on the Continent Ocean Boundary (COB) of the N.Atlantic Ocean have been sampled by ODP drilling on the SE Greenland margin and the the Vøring Plateau (Norwegian Sea). Such associations also occur further inland in the British Paleocene Igneous Province, such as on the Isle of Rum (e.g., Troll et al., Contrib. Min. Petrol., 2004, 147, p.722). Sr and Nd isotope and trace element geochemistry show that the Rum rhyodacites are the products of melting of Lewisian amphibolite gneiss. There are no indications of a melt contribution from Lewisian granulite gneiss. The amphibolite gneiss parent rock had experienced an ancient Cs and Rb loss, possibly during a Caledonian event, which caused 87Sr/86Sr heterogeneity in the crustal source of silicic melts. The dacites and early gabbros of Rum are mixtures of crustal melts and primary mantle melts. Rare Earth Element modelling shows that late stage picritic melts on Rum are close analogues for the parent melts of the Rum Layered Suite, and for the mantle melts that caused crustal anatexis of the Lewisian gneiss. These primary mantle melts have close affinities to MORB whose trace element content varies from slightly depleted to slightly enriched. The 'early felsic-later mafic' volcanic associations from Rum, and from the now drowned seaward dipping wedges on the shelf of SE Greenland and on the Vøring Plateau show geochemical differences that result from variations in the regional crustal composition and the depth at which crustal anatexis took place.

New 40Ar / 39Ar age and geochemical data from seamounts in the Canary and Madeira volcanic provinces: Support for the mantle plume hypothesis

NASA Astrophysics Data System (ADS)

Geldmacher, J.; Hoernle, K.; Bogaard, P. v. d.; Duggen, S.; Werner, R.

2005-08-01

The role of mantle plumes in the formation of intraplate volcanic islands and seamount chains is being increasingly questioned. Particular examples are the abundant and somewhat irregularly distributed island and seamount volcanoes off the coast of northwest Africa. New 40Ar / 39Ar ages and Sr-Nd-Pb isotope geochemistry of volcanic rocks from seamounts northeast of the Madeira Islands (Seine and Unicorn) and northeast of the Canary Islands (Dacia and Anika), however, provide support for the plume hypothesis. The oldest ages of shield stage volcanism from Canary and Madeira volcanic provinces confirm progressions of increasing age to the northeast. Average volcanic age progression of ∼1.2 cm/a is consistent with rotation of the African plate at an angular velocity of ∼0.20° ± 0.05 /Ma around a common Euler pole at approximately 56° N, 45° W computed for the period of 0-35 Ma. A Euler pole at 35° N, 45° W is calculated for the time interval of 35-64 Ma. The isotope geochemistry further confirms that the Madeira and Canary provinces are derived from different sources, consistent with distinct plumes having formed each volcanic group. Conventional hotspot models, however, cannot easily explain the up to 40 m.y. long volcanic history at single volcanic centers, long gaps in volcanic activity, and the irregular distribution of islands and seamounts in the Canary province. A possible explanation could involve interaction of the Canary mantle plume with small-scale upper mantle processes such as edge-driven convection. Juxtaposition of plume and non-plume volcanism could also account for observed inconsistencies of the classical hotspot concept in other volcanic areas.

Transcriptomics Provide Insight Into Mussel (Mytilus galloprovincialis) Mantle Function And Its Role In Biomineralization

NASA Astrophysics Data System (ADS)

Zaghdoudi-Allan, N.; Yarra, T.; Churcher, A.; Felix, R. C.; Cardoso, J.; Clark, M.; Power, D. M.

2016-02-01

With over 90,000 extant species, the Mollusca is one of the most successful and species-rich phyla, comprising 23% of known marine fauna. Common to all molluscs, the mantle is a multi-functional highly muscular tissue that contacts the shell and envelops vital organs. In bivalves, the epithelial cells of the mantle secrete the external shell by a complex network of mechanisms that remain poorly understood. To date, the bulk of the work on Mytilus mantle has focused on two of its features: the mantle edge and the pallial mantle and relatively little is known about the factors regulating its function. We hypothesize that the mantle edge in Mytilus species is heterogeneous in cellular structure and function and use next generation sequencing to mine for receptors involved in biomineralization. The mantle edge of the Mediterranean mussel (Mytilus galloprovincialis) was sectioned into three parts and sequenced using the Illumina platform. The transcriptome sequences generated assembled into 179,879 transcripts with a 34% GC content, congruent with other bivalve asssemblies. The transcriptome was annotated and String analysis (http://www.string-db.org) was used for a preliminary characterisation of biological processes. To test our hypothesis, we compared the transcripts from the 3 mantle segments and the expression levels of putative receptors such as the G -protein coupled receptors (GPCRs) in the sectioned mantle of 6 individuals using qPCR. Candidates were chosen based on their regulatory function and potential involvement in shell formation. Our results show differences in transcript abundance and cellular function amongst the three mantle sections. Combining our transcriptomic study with histological studies of the mantle tissue, we present evidence of both molecular and structural heterogeneity of the mussel mantle and identify several putative regulatory networks.

Re-Os systematics of early proterozoic ferropicrites, Pechenga Complex, northwestern Russia: Evidence for ancient 187Os-enriched plumes

NASA Astrophysics Data System (ADS)

Walker, Richard J.; Morgan, John W.; Hanski, Eero J.; Smolkin, Valery F.

1997-08-01

The Re-Os isotopic systematics of various ferropicritic flows and sills of the Pechenga Complex, Russia, have been examined. During crystallization about 1.98 Ga ago, many of these bodies became highly differentiated. In addition, some of the larger igneous units are associated with major NiCu ore deposits. The melts that produced these rocks have been termed ferropicritic because of their high FeO and MgO contents. They are also enriched in light rare earth elements (LREEs), TiO 2, Zr, and many other incompatible trace elements. Previous studies have concluded that the ferropicrites were most likely derived from an Fe-rich mantle plume that had a complex history of long-term LREE depletion (initial ɛNd = + 1.4), but that also experienced a LREE enrichment event within 200 Ma of the generation of the rocks. Whole rock samples believed to be most representative of primary melt compositions indicate that initial melt concentrations of rhenium and osmium were approximately 1.1 ppb and 0.5 ppb, respectively. The high primary melt concentrations presumably made the osmium contained in the melts relatively immune to the effects of crustal contamination. Nonetheless, all ore-bearing intrusions examined show osmium isotopic evidence for crustal contamination. For example, the initial γOs for some primary magmatic sulfides from the Pilgujärvi intrusion average +46. Other ore-bearing intrusions, such as the Kammikivi sill, appear to have been similarly contaminated by crustal osmium during the injection of magma, with initial yo, values as high as +251. The seemingly high levels of crustal osmium may be attributed to the rapidly diminishing concentrations of osmium in the melts as the larger bodies differentiated, combined with localized in situ assimilation of the metasedimentary rocks that comprise the country rocks. The Re-Os systematics of some whole rock samples of both mineralized and sulfide-poor intrusions were affected by post-magmatic events, especially the greenschist grade metamorphism that impacted the rocks between about 1.7 and 1.8 Ga ago. The metamorphic effects are reflected in the recrystallization of many of the primary sulfides. As a consequence of this open-system behavior in many whole rock samples, the primary igneous Re-Os systematics of these rocks are best examined via analysis of magmatic phases such as chromite, olivine, clinopyroxene, and primary sulfides. Chromite and ilmenite+sulfide separates from two sulfide-poor lava flows, the Lammas and Keskitunturi, have characteristically low 187Re/ 188Os ( < l), and because of the limited age correction, precisely define the initial γOs of these systems to be +6.0±0.7. Because of the identical initial compositions of the two, spatially distinct lava flows, and the fact that these flows were extruded onto only slightly older volcanic rocks, we conclude that the +6.0 value reflects the composition of the mantle source and not minor crustal contamination. Although 187Os-enriched, plume-derived systems are common during the Phanerozoic, this is the earliest known evidence for the existence of long-term, Re-enriched mantle reservoirs. The most commonly invoked model to explain 187Os enrichments in Phanerozoic systems, oceanic crustal recycling, in this instance requires that very large proportions of oceanic crust were recycled into the mantle source and that the event was likely very ancient. Other options, such as core-mantle interaction and a stratified mantle, are also discussed.

Concentration, behavior and storage of H/sub 2/O in the suboceanic upper mantle: implications for mantle metasomatism

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

Michael, P.J.

1988-02-01

Mid-ocean ridge basalt glasses from the Pacific-Nazca Ridge and the northern Juan de Fuca Ridge were analyzed for H/sub 2/O by gas chromatography. Incompatible element enriched (IEE) glasses have higher H/sub 2/O contents than depleted (IED) glasses. H/sub 2/O increases systematically with decreasing Mg/Mg + Fe/sup 2 +/ within each group. Near-primary IED MORBs have an average of about 800 ppm H/sub 2/O, while near-primary IEE MORBs (with chondrite normalized Nb/Zr or La/Sm approx. 2) have about 2100 ppm H/sub 2/O. If these basalts formed by 10-20% partial melting then the IED mantle source had 100-180 ppm H/sub 2/O, whilemore » the IEE source had 250-450 ppm H/sub 2/O. The ratio H/sub 2/O/(Ce + Nd) is fairly constant at 95 +/- 30 for all oceanic basalts from the Pacific. During trace element fractionation in the suboceanic upper mantle, H/sub 2/O behaves more compatibly than K, Rb, Nb, and Cl, but less compatibly than Sm, Zr and Ti. H/sub 2/O is contained mostly in amphibole in the shallow upper mantle. At pressures greater than the amphibole stability limit, it is likely that a significant proportion of H/sub 2/O is contained in a mantle phase which is more refractory than phlogopite at these pressures. The role of H/sub 2/O in mantle enrichment processes is examined by assuming that an enriched component was added. The modeled concentrations of K, Na, Ti and incompatible trace elements in this component are high relative to H/sub 2/O, indicating that suboceanic mantle enrichment is caused by silicate melts such as basanites and not by aqueous fluids.« less

Source of Volatiles in Earth's Deep Mantle from Neon Isotope Systematics in the South Atlantic

NASA Astrophysics Data System (ADS)

Williams, C. D.; Mukhopadhyay, S.

2016-12-01

The noble gases play an important role in understanding Earth's accretion and subsequent evolution. Neon isotopes in particular have the potential to distinguish between distinct sources of Earth's volatiles e.g., acquisition of nebular gas, solar wind implanted materials or chondritic meteorites and their components. The neon isotopic composition of the deep mantle remains subject to debate with the majority of mantle-derived basalts displaying maximum 20Ne/22Ne ratios less than 12.5, similar to values determined for the convective mantle (20Ne/22Ne = 12.49 +/- 0.04; [1]). These values are also much lower than those of solar wind (20Ne/22Ne = 13.8; [2,3]) and estimates of the nebular gas (20Ne/22Ne = 13.4; [4]) but comparable to solar wind implanted meteoritic materials (20Ne/22Ne = 12.5-12.7; [5]). Here we determine the neon isotopic composition of mantle-derived materials from the south Atlantic. These samples display strong linear correlations in 20Ne/22Ne-21Ne/22Ne space with maximum 20Ne/22Ne ratios that are resolvable from and higher than materials derived from the convecting mantle as well as models of solar wind implantation. These results supplement a growing database of mantle materials characterized by 20Ne/22Ne ratios greater than 12.5, challenging the notion that the entire mantle acquired volatiles from solar wind implanted meteoritic materials. In this presentation we will explore alternative origins for these volatiles and provide testable predictions for each scenario. [1] G. Holland, C.J. Ballentine.. Nature 441 (2006), 186-191. [2] A. Gimberg et al. GCA 72 (2008), 626-645. [3] V.S. Heber et al. GCA 73 (2009), 7414-7432. [4] V. S. Heber et al. ApJ 759 (2012), 121. [5] R. Wieler in: D. Porcelli, C.J. Ballentine, R. Wieler (Eds.), Reviews in Mineralogy and Geochemistry 47 (2002), 21-70.

P-wave Velocity Structure Across the Mariana Trench and Implications for Hydration

NASA Astrophysics Data System (ADS)

Eimer, M. O.; Wiens, D.; Lizarralde, D.; Cai, C.

2017-12-01

Estimates of the water flux at subduction zones remain uncertain, particularly the amount of water brought into the trench by the subducting plate. Normal faulting related to the bending of the incoming plate has been proposed to provide pathways for water to hydrate the crust and upper mantle. A passive and active source seismic experiment spanning both the incoming plate and forearc was conducted in 2012 in central Mariana to examine the role of hydration at subduction zones. The active-source component of the survey used the R/V M.G. Langsethairgun array and 68 short period sensors, including suspended hydrophones, deployed on 4 transects. This study at the Mariana trench offers a comparison to related studies of incoming plate hydration in Middle America, where differing thermal structures related to plate age predict different stability fields for hydrous minerals. The forearc structure is also of interest, since Mariana is characterized by large serpentine seamounts and may have a serpentinized mantle wedge. The velocity structure will also be important for the relocation of earthquakes in the incoming plate, since the seismicity can offer a constraint for the depth extent of these bending faults. We examine the P-wave velocity structure along a 400-km long wide-angle refraction transect perpendicular to the trench and spanning both the forearc and incoming plate. Preliminary results indicate a velocity reduction in the crust and uppermost mantle at the bending region of the incoming plate, relative to the plate's structure away from the trench. This reduction suggests that outer-rise faults extend into the upper mantle and may have promoted serpentinization of that material. Mantle Pn refraction phases are not observed in the forearc, consistent with the ambient noise tomography results that show upper-mantle velocities similar to that of the lower crust. The lack of contrast between the upper mantle and crustal velocities from the ambient noise has been interpreted to indicate extensive serpentinization of the shallow mantle wedge.

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

NASA Astrophysics Data System (ADS)

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

2018-07-01

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

Complex interactions between diapirs and 4-D subduction driven mantle wedge circulation.

NASA Astrophysics Data System (ADS)

Sylvia, R. T.; Kincaid, C. R.

2015-12-01

Analogue laboratory experiments generate 4-D flow of mantle wedge fluid and capture the evolution of buoyant mesoscale diapirs. The mantle is modeled with viscous glucose syrup with an Arrhenius type temperature dependent viscosity. To characterize diapir evolution we experiment with a variety of fluids injected from multiple point sources. Diapirs interact with kinematically induced flow fields forced by subducting plate motions replicating a range of styles observed in dynamic subduction models (e.g., rollback, steepening, gaps). Data is collected using high definition timelapse photography and quantified using image velocimetry techniques. While many studies assume direct vertical connections between the volcanic arc and the deeper mantle source region, our experiments demonstrate the difficulty of creating near vertical conduits. Results highlight extreme curvature of diapir rise paths. Trench-normal deflection occurs as diapirs are advected downward away from the trench before ascending into wedge apex directed return flow. Trench parallel deflections up to 75% of trench length are seen in all cases, exacerbated by complex geometry and rollback motion. Interdiapir interaction is also important; upwellings with similar trajectory coalesce and rapidly accelerate. Moreover, we observe a new mode of interaction whereby recycled diapir material is drawn down along the slab surface and then initiates rapid fluid migration updip along the slab-wedge interface. Variability in trajectory and residence time leads to complex petrologic inferences. Material from disparate source regions can surface at the same location, mix in the wedge, or become fully entrained in creeping flow adding heterogeneity to the mantle. Active diapirism or any other vertical fluid flux mechanism employing rheological weakening lowers viscosity in the recycling mantle wedge affecting both solid and fluid flow characteristics. Many interesting and insightful results have been presented based upon 2-D, steady-state thermal and flow regimes. We reiterate the importance of 4-D time evolution in subduction models. Analogue experiments allow added feedbacks and complexity improving intuition and providing insight for further investigation.

Does the "mantle" helium signature provide useful information about lithospheric architecture of Tibet/Himalaya?

NASA Astrophysics Data System (ADS)

Klemperer, S. L.; Liu, T.; Hilton, D. R.; Karlstrom, K. E.; Crossey, L. J.; Zhao, P.

2015-12-01

Measurements of 3He/4He > 0.1*Ra (where Ra = 3He/4He in Earth's atmosphere) in geothermal fluids are conventionally taken to represent derivation from a mantle source. 3He/4He values < 0.1*Ra are taken to represent only radiogenic helium with no modern mantle input (the canonical 3He/4He ratio for the crust is 0.02*Ra). Upward transport rates are hard to constrain, but transit times of 3He through the crust in a CO2-rich carrier fluid may be as short as a few years, so 3He/4He measurements offer a proxy for mantle temperature on geologically short time-scales. In Tibet, enhanced 3He/4He ratios could in principle represent (1) incipient partial melt of Indian lithospheric mantle; (2) of Asian lithospheric mantle; (3) upwelling asthenosphere north of underthrust India or along tears in the subducting Indian plate; and/or (4) high-T prograde metamorphism releasing previously trapped 3He from older, voluminous mafic/ultramafic rocks in the crust. We present data from our recent field campaigns and our compilations from the western and Chinese literature. Any individual observation of 3He/4He > 0.1*Ra may still be argued to result from mantle-derived 3He previously stored in the crust. However, our growing regional database of widely spaced observations of 3He/4He > 0.1*Ra, from the Karakoram Fault in the west to the Sangri-Cona rift and Yalaxiangbo Dome in the east, and from south of the Yarlung-Zangbo suture (YZS) to north of the Banggong-Nujiang suture, makes such special pleading increasingly implausible. The observation of 3He/4He > 0.1*Ra at the YZS and even within the Tethyan Himalaya south of the YZS cannot represent melting of Indian mantle close to the Moho unless existing thermal models are grossly in error. The source of 3He close to the YZS is likely either asthenosphere accessed by faults and shear zones that cut through subducting Indian lithospheric mantle; or incipient melt of Asian lithospheric mantle at the Moho north of the northern edge of underthrust India (the "mantle suture") which must therefore lie close to the YZS. Thus far we have barely tapped the rich potential that helium-isotope data offer for understanding transit of mantle volatiles through some of Earth's thickest (and ductilely flowing) crust.

Geochemical constraints on the spatial distribution of recycled oceanic crust in the mantle source of late Cenozoic basalts, Vietnam

NASA Astrophysics Data System (ADS)

Hoang, Thi Hong Anh; Choi, Sung Hi; Yu, Yongjae; Pham, Trung Hieu; Nguyen, Kim Hoang; Ryu, Jong-Sik

2018-01-01

This study presents a comprehensive analysis of the major and trace element, mineral, and Sr, Nd, Pb and Mg isotopic compositions of late Cenozoic intraplate basaltic rocks from central and southern Vietnam. The Sr, Nd, and Pb isotopic compositions of these basalts define a tight linear array between Indian mid-ocean-ridge basalt (MORB)-like mantle and enriched mantle type 2 (EM2) components. These basaltic rocks contain low concentrations of CaO (6.4-9.7 wt%) and have high Fe/Mn ratios (> 60) and FeO/CaO-3MgO/SiO2 values (> 0.54), similar to partial melts derived from pyroxenite/eclogite sources. This similarity is also supported by the composition of olivine within these samples, which contains low concentration of Ca and high concentrations of Ni, and shows high Fe/Mn ratios. The basaltic rocks have elevated Dy/Yb ratios that fall within the range of melts derived from garnet lherzolite material, although their Yb contents are much higher than those of modeled melts derived from only garnet lherzolite material and instead plot near the modeled composition of eclogite-derived melts. The Vietnamese basaltic rocks have lighter δ26Mg values (- 0.38 ± 0.06‰) than is expected for the normal mantle (- 0.25 ± 0.07‰), and these values decrease with decreasing Hf/Hf* and Ti/Ti* ratios, indicating that these basalts were derived from a source containing carbonate material. On primitive mantle-normalized multi-element variation diagrams, the central Vietnamese basalts are characterized by positive Sr, Eu, and Ba anomalies. These basalts also plot within the pelagic sediment field in Pbsbnd Pb isotopic space. This suggests that the mantle source of the basalts contained both garnet peridotite and recycled oceanic crust. A systematic analysis of variations in geochemical composition in basalts from southern to central Vietnam indicates that the recycled oceanic crust (possibly the paleo-Pacific slab) source material contains varying proportions of gabbro, basalt, and sediment. The basalts from south-central Vietnam (12°N-14°N) may be dominated by the lowest portion of the residual slab that contains rutile-bearing plagioclase-rich gabbroic eclogite, whereas the uppermost portion of the recycled slab, including sediment and basaltic material with small amounts of gabbro, may be a major constituent of the source for the basalts within the central region of Vietnam (14°N-16°N). Finally, the southern region (10°N-12°N) contains basalts sourced mainly from recycled upper oceanic crust that is basalt-rich and contains little or no sediment.

The temporal evolution of back-arc magmas from the Auca Mahuida shield volcano (Payenia Volcanic Province, Argentina)

NASA Astrophysics Data System (ADS)

Pallares, Carlos; Quidelleur, Xavier; Gillot, Pierre-Yves; Kluska, Jean-Michel; Tchilinguirian, Paul; Sarda, Philippe

2016-09-01

In order to better constrain the temporal volcanic activity of the back-arc context in Payenia Volcanic Province (PVP, Argentina), we present new K-Ar dating, petrographic data, major and trace elements from 23 samples collected on the Auca Mahuida shield volcano. Our new data, coupled with published data, show that this volcano was built from about 1.8 to 1.0 Ma during five volcanic phases, and that Auca Mahuida magmas were extracted from, at least, two slightly different OIB-type mantle sources with a low partial melting rate. The first one, containing more garnet, was located deeper in the mantle, while the second contains more spinel and was thus shallower. The high-MgO basalts (or primitive basalts) and the low-MgO basalts (or evolved basalts), produced from the deeper and shallower lherzolite mantle sources, respectively, are found within each volcanic phase, suggesting that both magmatic reservoirs were sampled during the 1 Myr lifetime of the Auca Mahuida volcano. However, a slight increase of the proportion of low-MgO basalts, as well as of magmas sampled from the shallowest source, can be observed through time. Similar overall petrological characteristics found in the Pleistocene-Holocene basaltic rocks from Los Volcanes and Auca Mahuida volcano suggest that they originated from the same magmatic source. Consequently, it can be proposed that the thermal asthenospheric anomaly is probably still present beneath the PVP. Finally, our data further support the hypothesis that the injection of hot asthenosphere with an OIB mantle source signature, which was triggered by the steepening of the Nazca subducting plate, induced the production of a large volume of lavas within the PVP since 2 Ma.

Petrogenesis of a Mesoproterozoic shoshonitic lamprophyre dyke from the Wajrakarur kimberlite field, eastern Dharwar craton, southern India: Geochemical and Sr-Nd isotopic evidence for a modified sub-continental lithospheric mantle source

NASA Astrophysics Data System (ADS)

Pandey, Ashutosh; Chalapathi Rao, N. V.; Chakrabarti, Ramananda; Pandit, Dinesh; Pankaj, Praveer; Kumar, Alok; Sahoo, Samarendra

2017-11-01

Mineralogy and geochemistry of the Udirpikonda lamprophyre, located within the Mesoproterozoic diamondiferous Wajrakarur kimberlite field (WKF), towards the western margin of the Paleo-Mesoproterozoic Cuddapah basin are presented. The lamprophyre is characterised by a panidiomorphic-porphyritic texture imparted by clinopyroxene, olivine and biotite set in a groundmass of feldspar and spinel. Olivine occurs as the microphenocrysts with a composition range of Fo87-78. Clinopyroxenes display reverse as well as oscillatory optical zoning and are diopsidic in nature with a variation in the composition from core (Wo47 En28 Fs20Ac5) to rim (Wo46En41Fs11Ac3). Biotite (Mg# < 0.6) is the only mica present and spinels are titano-magnetites showing ulvospinel- magnetite solid solution. Plagioclase is the dominant feldspar with a variable compositional range of An41-8Ab82-56Or33-3. Based on the mineralogy, the lamprophyre can be classified to be of calc-alkaline variety but its geochemistry display mixed signals of both alkaline and calc-alkaline lamprophyres. K2O/Na2O ranges from 1.49 to 2.79, making it distinctly potassic and highlights its shoshonitic character. Moderate Mg# (60-65), Ni (110-200 ppm) and Cr (110-260 ppm) contents in the bulk-rock indicate substantial fractional crystallization of olivine and clinopyroxene. Fractionated chondrite normalized REE patterns (average (La/Yb)N = 37.56) indicates involvement of an enriched mantle source from within the garnet stability field whereas slightly negative Ta-Nb-Ti and Hf anomalies displayed on the primitive mantle normalized multi-element spider gram highlight involvement of a subducted component in the mantle source. Given the spatial disposition of the studied lamprophyre, the age of the emplacement is considered to be coeval with WKF kimberlites ( 1.1 Ga) and the initial 143Nd/144Nd (0.510065-0.510192) and 87Sr/86Sr (0.705333-0.706223) are strikingly similar to those observed for the Smoky Butte lamproites, Montana, USA. Fluid-related subduction enrichment of the mantle source is apparent from the enriched ratios of La/Nb, Ba/Nb and (Hf/Sm)N, (Ta/La)N < 1. Petrogenetic modelling reveals melt generation from 1 to 2% partial melting of an enriched mantle source that subsequently underwent fractional crystallization. Our study provides geochemical and isotopic evidence for a sub-continental lithospheric mantle (SCLM) modified by subduction and asthenospheric upwelling in the Eastern Dharwar Craton. The partial melting of a resulting heterogeneous Eastern Dharwar Craton SCLM to generate Udiripikonda lamprophyre and Wajrakarur kimberlites has been attributed to the Mesoproterozoic regional lithospheric extension event.

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

NASA Astrophysics Data System (ADS)

Schiano, P.; Clocchiatti, R.

1994-04-01

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

Receiver function imaging of the mantle discontinuties beneath Fennoscandia and northern Europe

NASA Astrophysics Data System (ADS)

Frassetto, Andrew; Thybo, Hans

2010-05-01

Receiver functions from the Mantle Investigations of Norwegian Uplift Structure experiment (MAGNUS) are depth-converted using interval wavespeeds from AK-135 for the 410-km and 660-km discontinuities and combined using common-conversion-point stacking. This preliminary work shows a potentially complex mantle-transition-zone beneath southern Norway, with reduction in the amplitude of the 410-arrival and 20-30 km of shallowing of the 660-arrival beneath the axis of the Oslo Rift. To refine these measurements and place them in a regional context, we incorporate the MAGNUS dataset with permanent stations and previous temporary seismic deployments across Fennoscandia and northern Europe. New constraints on the depth to the lithosphere-asthenosphere boundary and character of the mantle-transition-zone will aid in understanding the causes for potentially recent uplift in the southern Scandes and the region of unusually slow upper mantle resolved beneath the region (Weidle and Maupin, 2008).

Petrology of some oceanic island basalts: PRIMELT2.XLS software for primary magma calculation

NASA Astrophysics Data System (ADS)

Herzberg, C.; Asimow, P. D.

2008-09-01

PRIMELT2.XLS software is introduced for calculating primary magma composition and mantle potential temperature (TP) from an observed lava composition. It is an upgrade over a previous version in that it includes garnet peridotite melting and it detects complexities that can lead to overestimates in TP by >100°C. These are variations in source lithology, source volatile content, source oxidation state, and clinopyroxene fractionation. Nevertheless, application of PRIMELT2.XLS to lavas from a wide range of oceanic islands reveals no evidence that volatile-enrichment and source fertility are sufficient to produce them. All are associated with thermal anomalies, and this appears to be a prerequisite for their formation. For the ocean islands considered in this work, TP maxima are typically ˜1450-1500°C in the Atlantic and 1500-1600°C in the Pacific, substantially greater than ˜1350°C for ambient mantle. Lavas from the Galápagos Islands and Hawaii record in their geochemistry high TP maxima and large ranges in both TP and melt fraction over short horizontal distances, a result that is predicted by the mantle plume model.

Line-source excitation of realistic conformal metasurface cloaks

NASA Astrophysics Data System (ADS)

Padooru, Yashwanth R.; Yakovlev, Alexander B.; Chen, Pai-Yen; Alù, Andrea

2012-11-01

Following our recently introduced analytical tools to model and design conformal mantle cloaks based on metasurfaces [Padooru et al., J. Appl. Phys. 112, 034907 (2012)], we investigate their performance and physical properties when excited by an electric line source placed in their close proximity. We consider metasurfaces formed by 2-D arrays of slotted (meshes and Jerusalem cross slots) and printed (patches and Jerusalem crosses) sub-wavelength elements. The electromagnetic scattering analysis is carried out using a rigorous analytical model, which utilizes the two-sided impedance boundary conditions at the interface of the sub-wavelength elements. It is shown that the homogenized grid-impedance expressions, originally derived for planar arrays of sub-wavelength elements and plane-wave excitation, may be successfully used to model and tailor the surface reactance of cylindrical conformal mantle cloaks illuminated by near-field sources. Our closed-form analytical results are in good agreement with full-wave numerical simulations, up to sub-wavelength distances from the metasurface, confirming that mantle cloaks may be very effective to suppress the scattering of moderately sized objects, independent of the type of excitation and point of observation. We also discuss the dual functionality of these metasurfaces to boost radiation efficiency and directivity from confined near-field sources.

Isotopes as tracers of the sources of the lunar material and processes of lunar origin.

PubMed

Pahlevan, Kaveh

2014-09-13

Ever since the Apollo programme, isotopic abundances have been used as tracers to study lunar formation, in particular to study the sources of the lunar material. In the past decade, increasingly precise isotopic data have been reported that give strong indications that the Moon and the Earth's mantle have a common heritage. To reconcile these observations with the origin of the Moon via the collision of two distinct planetary bodies, it has been proposed (i) that the Earth-Moon system underwent convective mixing into a single isotopic reservoir during the approximately 10(3) year molten disc epoch after the giant impact but before lunar accretion, or (ii) that a high angular momentum impact injected a silicate disc into orbit sourced directly from the mantle of the proto-Earth and the impacting planet in the right proportions to match the isotopic observations. Recently, it has also become recognized that liquid-vapour fractionation in the energetic aftermath of the giant impact is capable of generating measurable mass-dependent isotopic offsets between the silicate Earth and Moon, rendering isotopic measurements sensitive not only to the sources of the lunar material, but also to the processes accompanying lunar origin. Here, we review the isotopic evidence that the silicate-Earth-Moon system represents a single planetary reservoir. We then discuss the development of new isotopic tracers sensitive to processes in the melt-vapour lunar disc and how theoretical calculations of their behaviour and sample observations can constrain scenarios of post-impact evolution in the earliest history of the Earth-Moon system. © 2014 The Author(s) Published by the Royal Society. All rights reserved.

Abundant carbon in the mantle beneath Hawai`i

USGS Publications Warehouse

Anderson, Kyle R.; Poland, Michael

2017-01-01

Estimates of carbon concentrations in Earth’s mantle vary over more than an order of magnitude, hindering our ability to understand mantle structure and mineralogy, partial melting, and the carbon cycle. CO2 concentrations in mantle-derived magmas supplying hotspot ocean island volcanoes yield our most direct constraints on mantle carbon, but are extensively modified by degassing during ascent. Here we show that undegassed magmatic and mantle carbon concentrations may be estimated in a Bayesian framework using diverse geologic information at an ocean island volcano. Our CO2 concentration estimates do not rely upon complex degassing models, geochemical tracer elements, assumed magma supply rates, or rare undegassed rock samples. Rather, we couple volcanic CO2 emission rates with probabilistic magma supply rates, which are obtained indirectly from magma storage and eruption rates. We estimate that the CO2content of mantle-derived magma supplying Hawai‘i’s active volcanoes is 0.97−0.19+0.25 wt%—roughly 40% higher than previously believed—and is supplied from a mantle source region with a carbon concentration of 263−62+81 ppm. Our results suggest that mantle plumes and ocean island basalts are carbon-rich. Our data also shed light on helium isotope abundances, CO2/Nb ratios, and may imply higher CO2 emission rates from ocean island volcanoes.

Tracing subducted crustal materials in the mantle by using magnesium isotopes

NASA Astrophysics Data System (ADS)

Teng, F. Z.

2016-12-01

Recent studies show that some continental basalt, mantle-metasomatised peridotite and cratonic eclogite have heterogeneous Mg isotopic compositions. These isotopically distinct Mg isotopic compositions have been explained by the incorporation of subducted materials in their mantle sources though the detailed mechanisms are still not well understood. In particular, how Mg-poor crustal materials can modify Mg isotopic systematics of Mg-rich mantle is unknown. Subduction zones are the most efficient sites for crust and mantle interactions, hence should be where the most prominent Mg isotopic variation occurs. However, to date, little is known on Mg isotope systematics in the subduction factory. Here I first review and report new Mg isotopic data for arc lava, subarc peridotite and the subducted slab (marine sediment, altered basalt and abyssal peridotite), then use them to constrain the origins of mantle Mg isotopic heterogeneity and lay the foundation for using Mg isotopes as new tools for tracing crust-mantle interactions. The main conclusions are 1) fluid-rock interactions can modify Mg isotopic systematics of abyssal peridotites; 2) island arc lavas have non-MORB Mg isotopic compositions, reflecting distinct surbarc mantle Mg isotopic signature; 3) continental arcs have non-MORB Mg isotopic compositions, likely resulting from crustal contamination and 4) the isotopically heterogeneous continental basalts are mainly produced by mixing of isotopically distinct magmas instead of being partial melting products of metasomatised mantle peridotites.

Early episodes of high-pressure core formation preserved in plume mantle

NASA Astrophysics Data System (ADS)

Jackson, Colin R. M.; Bennett, Neil R.; Du, Zhixue; Cottrell, Elizabeth; Fei, Yingwei

2018-01-01

The decay of short-lived iodine (I) and plutonium (Pu) results in xenon (Xe) isotopic anomalies in the mantle that record Earth’s earliest stages of formation. Xe isotopic anomalies have been linked to degassing during accretion, but degassing alone cannot account for the co-occurrence of Xe and tungsten (W) isotopic heterogeneity in plume-derived basalts and their long-term preservation in the mantle. Here we describe measurements of I partitioning between liquid Fe alloys and liquid silicates at high pressure and temperature and propose that Xe isotopic anomalies found in modern plume rocks (that is, rocks with elevated 3He/4He ratios) result from I/Pu fractionations during early, high-pressure episodes of core formation. Our measurements demonstrate that I becomes progressively more siderophile as pressure increases, so that portions of mantle that experienced high-pressure core formation will have large I/Pu depletions not related to volatility. These portions of mantle could be the source of Xe and W anomalies observed in modern plume-derived basalts. Portions of mantle involved in early high-pressure core formation would also be rich in FeO, and hence denser than ambient mantle. This would aid the long-term preservation of these mantle portions, and potentially points to their modern manifestation within seismically slow, deep mantle reservoirs with high 3He/4He ratios.

Tomographic and Geodynamic Constraints on Convection-Induced Mixing in Earth's Deep Mantle

NASA Astrophysics Data System (ADS)

Hafter, D. P.; Forte, A. M.; Bremner, P. M.; Glisovic, P.

2017-12-01

Seismological studies reveal two large low-shear-velocity provinces (LLSVPs) in the lowermost mantle (e.g., Su et al. 1994; Wang & Wen 2007; He & Wen 2012), which may represent accumulations of subducted slabs at the CMB (Tan & Gurnis 2005; Christensen & Hoffman 1994) or primordial material generated in the early differentiation of Earth (e.g. Li et al. 2014). The longevity or stability of these large-scale heterogeneities in the deep mantle depends on the vigor and spatial distribution of the convective circulation, which is in turn dependent on the distribution of mantle buoyancy and viscosity (e.g. Glisovic & Forte 2015). Here we explore the state of convective mixing in the mantle using the ASPECT convection code (Kronbichler et al. 2012). A series of experiments are conducted to consider the geochemical and dynamical contributions of LLSVPs to deep-mantle upwellings and corresponding plume-sourced volcanism. The principal feature of these experiments is the use of particle tracers to track geochemical changes in the LLSVPs and mantle plumes in addition to identifying those parts of the mantle that may remain unmixed. We employ 3-D mantle density anomalies derived from joint inversions of seismic, geodynamic and mineral physics constraints and geodynamically-constrained viscosity distributions (Glisovic et al. 2015) to ensure that the predicted flow fields yield a good match to key geophysical constraints (e.g. heat flow, global gravity anomalies and plate velocities).

Osmium isotope constraints on Earth's late accretionary history

USGS Publications Warehouse

Morgan, J.W.

1985-01-01

Osmium isotope measurements reported by Alle??gre and Luck 1,2 indicate that terrestrial osmiridiums evolved in a mantle source region in which the osmium/rhenium ratio falls strictly within the range found in chondrites. This suggests that the highly siderophile elements in the Earth's mantle were introduced by a late influx of chondritic material and are not a result of endogenous processes. I have now examined the available data in more detail and conclude that the inferred Os/Re ratio of the Earth's mantle matches the E group and C3 chondrites, but that C1 and probably C2 chondrites were not major components of the material accreted in the late stages of mantle formation. ?? 1985 Nature Publishing Group.

Boron Isotopic Composition of Metasomatized Mantle Xenoliths from the Western Rift, East Africa

NASA Astrophysics Data System (ADS)

Hudgins, T.; Nelson, W. R.

2017-12-01

The Western Branch of the East African Rift System is known to have a thick lithosphere and sparse, alkaline volcanism associated with a metasomatized mantle source. Recent work investigating the relationship between Western Branch metasomatized mantle xenoliths and associated lavas has suggested that these metasomes are a significant factor in the evolution of the rift. Hydrous/carbonated fluids or silicate melts are potent metasomatic agents, however gaining insight into the source of a metasomatic agent proves challenging. Here we investigate the potential metasomatic fluid sources using B isotope analysis of mineral separates from Western Branch xenoliths. Preliminary SIMS analyses of phlogopite from Katwe Kikorongo and Bufumbira have and average B isotopic composition of -28.2‰ ± 5.1 and -16.4‰ ± 3.6, respectively. These values are are dissimilar to MORB (-7.5‰ ± 0.7; Marschall and Monteleone, 2015), primitive mantle (-10‰ ± 2; Chaussidon and Marty, 1995), and bulk continental crust (-9.1‰ ± 2.4; Marschall et al., 2017) and display significant heterogeneity across a relatively short ( 150km) portion of the Western Branch. Though displaying large variability, these B isotopic compositions are indicative of a metasomatic agent with a more negative B isotopic composition than MORB, PM, or BCC. These results are consistent with fluids that released from a subducting slab and may be related to 700 Ma Pan-African subduction.

Evidence from Xenon isotopes for limited mixing between MORB sources and plume sources since 4.45 Ga

NASA Astrophysics Data System (ADS)

Mukhopadhyay, S.

2011-12-01

Xenon isotopes provide unique insights into the sources of volatile material for planet Earth, the degassing of the mantle, and the chemical evolution of the mantle [1-4]. 129Xe is produced from 129I, which has a half-life of 16 Myrs, and 131-136Xe are produced from 244Pu, which has a half-life of 80 Myrs. To a smaller extent, 131-136Xe are also produced from 238U fission. Thus, ratios of Pu-derived to U-derived fission xenon and 129I-derived to Pu-derived fission xenon constrain the rate and degree of outgassing of a mantle reservoir. Here, I report on the Pu-derived to U-derived fission xenon and Pu/I ratio of the Iceland plume. I then compare the plume observations with the gas rich popping rock from the North Mid Atlantic Ridge that samples the upper mantle [4]. Through step crushing of multiple aliquots of a basalt glass from Iceland, 51 high-precision He, Ne, Ar, and Xe isotopic compositions were generated. Combined He, Ne, and Xe measurements provide unequivocal evidence that the Iceland plume has a lower 129Xe/130Xe ratio than MORBs because it evolved with a I/Xe ratio distinct from the MORB source and not because of recycled atmosphere (which has low 129Xe/130Xe) in the plume source. Since 129I became extinct 80 Myrs after solar system formation, limited mixing between plume and MORB source is a stringent requirement since 4.45 Ga. Of the 51 different isotopic analyses, 42 data points were distinct from the atmospheric 129Xe/130Xe composition at two standard deviations. These 42 data points were utilized to calculate the ratio of Pu- to U-derived fission xenon. The starting composition of terrestrial Xe is a matter of debate. However, for reasonable starting compositions of air, non-radiogenic atmosphere, solar wind, and U-Xe [5-7], the Iceland plume ,on average, has approximately a factor of two higher Pu-derived xenon than the MORB source. These data thus, provide unequivocal evidence that the Iceland plume is less degassed than the MORB source and that the differences must have existed early on because Pu becomes extinct after ~ 400 Myrs. Thus, the Xe isotopic data suggests that differences between plume and MORB sources are the result of different mantle processing rates and not related to the preferential recycling of atmospheric gases into the plume source. Furthermore, if the plumes are derived from the large low shear wave velocity (LLSVPs) provinces at the base of the lower mantle [8], then our results require that LLSVPs are not made of solely recycled material. Rather, primitive material must constitute some fraction of the LLSVPs, and LLSVPs are ancient, having persisted through most of Earth's history. [1] Holland and Ballentine, Nature, 2006. [2] Yokochi and Marty, EPSL, 2004. [3] Coltice et al., Chem Geol., 2009. [4] Moriera et al., Science, 1998. [5] Caffee et al., Science, 1998. [6] Kunz et al., Science 1998. [7] Pepin and Porcelli, EPSL, 2006. [8] Torsvik et al., Nature, 2010.

Mantle sources for Central Atlantic Magmatic Province basalts from Hf isotopes

NASA Astrophysics Data System (ADS)

Elkins, L. J.; Marzoli, A.; Bizimis, M.; Meyzen, C. M.; Callegaro, S.; Sorsen, N.; Lassiter, J. C.; Ernesto, M.

2017-12-01

The Central Atlantic Magmatic Province (CAMP) was one of the most voluminous LIP events in Earth history and likely triggered the end-Triassic mass extinction. The tectonic and mantle processes that produced such significant magmatic emplacement are thus of great interest. To further explore the origins of CAMP, we present new 176Hf/177Hf isotope data for a broad geographic sampling of CAMP dikes, sills, and basalt flows. We find that basaltic intrusions from the Carolinas in Eastern North America trend along a shallower slope than the terrestrial array on a diagram of 176Hf/177Hf vs. 143Nd/144Nd. This trend may reflect the presence of variable quantities of sediment-derived material in the mantle source region. This is consistent with previous suggestions that the asthenosphere beneath CAMP has been partially metasomatised by fluids derived from subducted sediments, as well as with isotopic trends observed in other LIP, such as Karoo [Jourdan et al., 2007, Jour. Petrology, doi:10.1093/petrology/egm010]. Distinct from the Carolina trend, we further observe that high-TiO2 basalts from Amazonia exhibit unusually radiogenic 176Hf/177Hf for a given 208Pb/206Pb ratio. The high-TiO­2 basalts, which trend towards EM1-type compositions, may be asthenospheric melts that have experienced the addition of melts from local subcontinental lithospheric mantle (SCLM). Similarly high-TiO2 CAMP rocks from Sierra Leone may likewise have incorporated enriched lithospheric melts of lamproite-like composition in the source region [Callegaro et al., JPet, accepted; GSA Abstract #302853, 2017]. Low-TiO2 basalts from the same region in Brazil and of similar age to the high-TiO2 basalts lack the observed radiogenic 176Hf/177Hf ratios. This suggests that the melt source region beneath Brazil was heterogeneous, containing variable material with relatively radiogenic 176Hf/177Hf ratios, perhaps due to the greater age of subcontinental lithosphere and the presence of garnet. It remains unclear, however, whether the hypothesized SCLM source represents lithospheric domains which are still intact, or if this material reentered the convecting mantle by delamination prior to melting.

Cainozoic Orogenic Magmatism In The Western-central Mediterranean Area: Implications For The Subduction-modified Mantle Sources

NASA Astrophysics Data System (ADS)

Beccaluva, L.; Bianchini, G.; Coltorti, M.; Siena, F.; Verde, M.

In this contribution new REE and Sr-Nd isotopic data carried out on Cainozoic subduction-related volcanic rocks from the western-central Mediterranean are dis- cussed within a general review of the Cainozoic orogenic magmatism of the area. These volcanic events are related to subduction processes which occurred along the Paleo-European margin at least since Eocene and migrated (trough passive sinking and slab roll-back) southeastward up to the present in the peri-Tyrrhenian margin of Italy. Orogenic rocks from Provence (34-20 Ma) are characterised by 87Sr/86Sr be- tween 0.70453 and 0.70579, and 143Nd/144Nd between 0.51292 and 0.51265, which are consistent with mantle sources modified by subduction fluids released by altered oceanic crust. Sr-Nd isotopic composition of orogenic rocks from Sardinia (32-13 Ma), show a more complex picture: some compositions with relatively low 87Sr/86Sr (<0.706) and high 143Nd/144Nd (>0.5125), are compatible with the subduction of pure oceanic crust, while compositions with very high 87Sr/86Sr (up to 0.7113) and low 143Nd/144Nd (down to 0.51219) require additional components of continental crust affinity in the mantle wedge (partial fusion of subducted terrigenous sediments?). As concerns the Aeolian volcanics (< 1.3 Ma), compositions are compatible with man- tle sources solely enriched by fluid components from subducted oceanic crust. How- ever, it is interesting to note that shoshonites from the younger series of Stromboli display distinctly higher 87Sr/86Sr (up to 0.7075) and lower 143Nd/144Nd composi- tion (down to 0.51242), thus requiring once again recycle of continental crust materials in their mantle sources. The influence of such continental crust-derived components appear to be even more important in the mantle sources of the Campania volcanics, where extreme Sr-Nd isotopic compositions are recorded (87Sr/86Sr up to 0.7097; 143Nd/144Nd down to 0.5122).

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 OIB source. References: [1] Le Roux et al. (2006) EPSL, 251, 209-231. [2] Baker and Moritti (2011) Rev. in Mineral. Geochem, 73, 167-213. [3] Ghiorso et al. (2002) Geochem. Geophy. Geosy. 3, 5. [4] Li and Ripley (2009) Econ. Geol. 104, 405-412. [5] Jenner and O'Neill (2012) Geochem. Geophy. Geosy. 13, 1.

Effects of Fertile Mantle Compositional Variation and Spreading Rate Variation on the Working of Global Ocean Ridges

NASA Astrophysics Data System (ADS)

Niu, Y.; O'Hara, M. J.

2014-12-01

Mantle temperature variation, plate spreading rate variation and mantle compositional variation have been considered to be the three fundamental variables that govern the working of global ocean ridges [1]. An analysis demonstrates that mantle compositional variation exerts the primary control on ocean ridge processes; it determines (1) variation in both composition and mode of mantle mineralogy, (2) variation of mantle density, (3) variation of ridge axial depth, (4) source-inherited MORB compositional variation, (4) density-controlled variation in the amplitude of mantle upwelling, (5) apparent variation in the extent of melting, and (6) the correlated variation of MORB chemistry with ridge axial depth [2]. The above interpretations are reinforced by the updated MORB database [3]. The new database also confirms spreading rate control on the extent of melting as shown previously [4]. Mantle temperature variation could play a part, but its overstated role [3,5] results from a basic error (1) in treating ridge axial depth variation as evidence of mantle temperature variation by ignoring the intrinsic control of mantle composition, (2) in treating "mantle plume" influenced ridges (e.g., Iceland) as normal ridges of plate spreading origin, and (3) in treating low Vs at greater depths (> 300 km vs. < 200 km beneath ridges) beneath these "mantle plume" influenced ridges as evidence for hot ridge mantle. In order to understand the working of global ocean ridges, we must avoid plume-influenced ridges (e.g., in the vicinity of Iceland) and remove/average out data from such ridges. As a result, the correlations (e.g., between ridge axial depth, mantle low Vs anomaly, and some geochemical parameters) required for the interpretation of mantle temperature control all disappear. There is thus no evidence for large mantle temperature variation away from ridges influenced by "mantle plumes". References: [1] Niu et al., 2001, Earth Planet Sci. Lett., 186, 383-399; [2] Niu & O'Hara, 2008, J. Petrol., 49, 633-664; [3] Gale et al., 2014, J. Petrol, 55, 1051-1082; [4] Niu & Hékinian, 1997, Nature, 385, 326-329; [5] Dalton et al., 2014, Science, 334, 80-83; [6]Niu & Hékinian, 2004, In Oceanic Hotspots, Springer-Verlag, 285-307.

The isotopic and chemical evolution of Mount St. Helens

USGS Publications Warehouse

Halliday, A.N.; Fallick, A.E.; Dickin, A.P.; Mackenzie, A.B.; Stephens, W.E.; Hildreth, W.

1983-01-01

Isotopic and major and trace element analysis of nine samples of eruptive products spanning the history of the Mt. St. Helens volcano suggest three different episodes; (1) 40,000-2500 years ago: eruptions of dacite with ??{lunate}Nd = +5, ??{lunate}Sr = -10, variable ??18O, 206Pb/204Pb ??? 18.76, Ca/Sr ??? 60, Rb/Ba ??? 0.1, La/Yb ??? 18, (2) 2500-1000 years ago: eruptions of basalt, andesite and dacite with ??{lunate}Nd = +4 to +8, ??{lunate}Sr = -7 to -22, variable ??18O (thought to represent melting of differing mantle-crust reservoirs), 206Pb/204Pb = 18.81-18.87, variable Ca/Sr, Rb/Ba, La/Yb and high Zr, (3) 1000 years ago to present day: eruptions of andesite and dacite with ??{lunate}Nd = +6, ??{lunate}Sr = -13, ??18O ???6???, variable 206Pb/204Pb, Ca/Sr ??? 77, Rb/Ba = 0.1, La/Yb ??? 11. None of the products exhibit Eu anomalies and all are LREE enriched. There is a strong correlation between 87Sr/86Sr and differentiation indices. These data are interpreted in terms of a mantle heat source melting young crust bearing zircon and garnet, but not feldspar, followed by intrusion of this crustal reservoir by mantle-derived magma which caused further crustal melting and contaminated the crustal magma system with mafic components. Since 1000 years ago all the eruptions have been from the same reservoir which has displayed a much more gradual re-equilibration of Pb isotopic compositions than other components suggesting that Pb is being transported via a fluid phase. The Nd and Sr isotopic compositions lie along the mantle array and suggest that the mantle underneath Mt. St. Helens is not as depleted as MORB sources. There is no indication of seawater involvement in the source region. ?? 1983.

Rapid Mantle Source Variations During the Latest Episode of Kilauea's Prolonged Pu'u O'o Eruption, Hawaii

NASA Astrophysics Data System (ADS)

Marske, J. P.; Garcia, M. O.; Pietruszka, A. J.; Norman, M. D.; Rhodes, J. M.

2006-12-01

Nearly 24 years of continuous geochemical monitoring of lavas from the current Pu'u O'o eruption allow us to probe the mantle processes beneath Kilauea Volcano in unparalleled detail. Here we present new measurements Pb, Sr, and Nd isotope ratios and major- and trace-element abundances for lavas from episode 55 (1997-2006), which marks the longest and most voluminous interval of this eruption. Pu'u O'o lavas erupted since 1985 display systematic decreases in their TiO2, K2O, P2O5 and CaO abundances (normalized to 10 wt. % MgO to correct for olivine control) due to changes in the parental magma composition. Incompatible element ratios (e.g., Ba/Nb and La/Y) also show overall temporal decreases. Earlier erupted Pu'u O'o lavas displayed the most significant decrease in incompatible element ratios with near constant SiO2 contents, and a gradual increase in 87Sr/86Sr ratios. However, episode 55 lavas record significant increases in MgO- normalized SiO2 contents and 87Sr/86Sr with nearly constant (e.g. Ba/Nb) or a slightly reversed (e.g., TiO2 and K2O) trends in incompatible element ratios and abundances. There is little variation of 206Pb/204Pb ratios in lavas (18.38-18.43) erupted since 1985. Neither a single mantle source composition nor a change in partial melting conditions alone can explain these observations. Based on the isotopic and chemical variability, we conclude that early Pu'u O'o lavas originated from two distinct mantle source components: (1) a long-term depleted component (with relatively low 87Sr/86Sr ratios) that originated within the deep source of the Hawaiian plume that characterizes the earlier part of the eruption (1985-1992), and (2) a recently depleted component (i.e. a component that was recently depleted by prior melting) with low abundances of incompatible elements became increasingly important from 1992-1997. More recently, Pu'u O'o has tapped greater proportions of a new (3) long-term less depleted component (with higher 87Sr/86Sr ratios than observed from 1985-1992) that originated within the deep source region of the plume. This third component lies within typical Pb, Sr and Nd isotopic space for Kilauea, but represents a new source composition for the Pu'u O'o eruption. The systematic geochemical evolution of Pu'u O'o lavas reflects changes in the proportions of the mantle source components tapped throughout the eruption. The rapid isotope variations (on a time scale of years) in the most recent lavas suggest the mantle source components are heterogeneous on an extremely small scale, relative to the size of Kilauea's melting region.

Crustal structure and evolution of the Pyrenean-Cantabrian belt: A review and new interpretations from recent concepts and data

NASA Astrophysics Data System (ADS)

Teixell, A.; Labaume, P.; Ayarza, P.; Espurt, N.; de Saint Blanquat, M.; Lagabrielle, Y.

2018-01-01

This paper provides a synthesis of current data and interpretations on the crustal structure of the Pyrenean-Cantabrian orogenic belt, and presents new tectonic models for representative transects. The Pyrenean orogeny lasted from Santonian ( 84 Ma) to early Miocene times ( 20 Ma), and consisted of a spatial and temporal succession of oceanic crust/exhumed mantle subduction, rift inversion and continental collision processes at the Iberia-Eurasia plate boundary. A good coverage by active-source (vertical-incidence and wide-angle reflection) and passive-source (receiver functions) seismic studies, coupled with surface data have led to a reasonable knowledge of the present-day crustal architecture of the Pyrenean-Cantabrian belt, although questions remain. Seismic imaging reveals a persistent structure, from the central Pyrenees to the central Cantabrian Mountains, consisting of a wedge of Eurasian lithosphere indented into the thicker Iberian plate, whose lower crust is detached and plunges northwards into the mantle. For the Pyrenees, a new scheme of relationships between the southern upper crustal thrust sheets and the Axial Zone is here proposed. For the Cantabrian belt, the depth reached by the N-dipping Iberian crust and the structure of the margin are also revised. The common occurrence of lherzolite bodies in the northern Pyrenees and the seismic velocity and potential field record of the Bay of Biscay indicate that the precursor of the Pyrenees was a hyperextended and strongly segmented rift system, where narrow domains of exhumed mantle separated the thinned Iberian and Eurasian continental margins since the Albian-Cenomanian. The exhumed mantle in the Pyrenean rift was largely covered by a Mesozoic sedimentary lid that had locally glided along detachments in Triassic evaporites. Continental margin collision in the Pyrenees was preceded by subduction of the exhumed mantle, accompanied by the pop-up thrust expulsion of the off-scraped sedimentary lid above. To the west, oceanic subduction of the Bay of Biscay under the North Iberian margin is supported by an upper plate thrust wedge, gravity and magnetic anomalies, and 3D inclined sub-crustal reflections. However, discrepancies remain for the location of continent-ocean transitions in the Bay of Biscay and for the extent of oceanic subduction. The plate-kinematic evolution during the Mesozoic, which involves issues as the timing and total amount of opening, as well as the role of strike-slip drift, is also under debate, discrepancies arising from first-order interpretations of the adjacent oceanic magnetic anomaly record.

Adjoint tomography of Europe

NASA Astrophysics Data System (ADS)

Zhu, H.; Bozdag, E.; Peter, D. B.; Tromp, J.

2010-12-01

We use spectral-element and adjoint methods to image crustal and upper mantle heterogeneity in Europe. The study area involves the convergent boundaries of the Eurasian, African and Arabian plates and the divergent boundary between the Eurasian and North American plates, making the tectonic structure of this region complex. Our goal is to iteratively fit observed seismograms and improve crustal and upper mantle images by taking advantage of 3D forward and inverse modeling techniques. We use data from 200 earthquakes with magnitudes between 5 and 6 recorded by 262 stations provided by ORFEUS. Crustal model Crust2.0 combined with mantle model S362ANI comprise the initial 3D model. Before the iterative adjoint inversion, we determine earthquake source parameters in the initial 3D model by using 3D Green functions and their Fréchet derivatives with respect to the source parameters (i.e., centroid moment tensor and location). The updated catalog is used in the subsequent structural inversion. Since we concentrate on upper mantle structures which involve anisotropy, transversely isotropic (frequency-dependent) traveltime sensitivity kernels are used in the iterative inversion. Taking advantage of the adjoint method, we use as many measurements as can obtain based on comparisons between observed and synthetic seismograms. FLEXWIN (Maggi et al., 2009) is used to automatically select measurement windows which are analyzed based on a multitaper technique. The bandpass ranges from 15 second to 150 second. Long-period surface waves and short-period body waves are combined in source relocations and structural inversions. A statistical assessments of traveltime anomalies and logarithmic waveform differences is used to characterize the inverted sources and structure.

Veined pyroxenite xenoliths in Ugandan kamafugites: mantle or magma? Using in situ techniques for 87Sr/86Sr-isotopes and trace elements as tools

NASA Astrophysics Data System (ADS)

Link, Klemens; Tommasini, Simone; Braschi, Eleonora; Conticelli, Sandro; Barifaijo, Erasmus; Tiberindwa, John V.; Foley, Stephen F.

2010-05-01

The genesis of pyroxenite nodules in Ugandan kamafugites and their possible genetic relationships is a matter of debate. In earlier studies the pyroxenites were considered either as xenoliths from pervasively metasomatized peridotite mantle (Lloyd, 1981) or as distinct paragenesises occurring as veins within the peridotitic mantle (Harte et al., 1993). In both cases the xenoliths would represent mantle material that was at least partly involved as source material for the kamafugite melts. A third alternative could be that they represent cumulates of the lavas. In any case, the nodules provide important information for understanding the generation of ultrapotassic lavas and for characterizing the rift-related lithosphere mantle as part of the initial continental rift process. Originally the ultrapotassic kamafugites were considered to be single stage partial melts of pervasively metasomatized mantle but new geochemical studies indicate a multistage development (Rosenthal et al., 2009). Nd, Hf and Os isotopes point to mixing between components derived from metasomatically influenced peridotite and mica-pyroxenite. In-situ investigation of the Sr-isotope and trace element compositions of individual minerals in a number of xenoliths allows us to constrain their genesis and relation to the host lavas. The nodules appear to originate by near-liquidus crystallization of melts derived from enriched peridotite within the cratonic lithosphere mantle. They later partially remelted to form one source of the potassium-rich kamafugites. Sr-isotopes from different domains within single mineral grains in the nodules and host lavas are used to trace the nodules' role as a potential source to lavas, and trace element measurements are used to support the conclusions. Rb/Sr- measurements from the biotites to constrain the time between nodule crystallization and eruption of the Quaternary lavas to about 3.3 Ma. This also suggests a significant increase of the geothermal gradient beneath the preceding rift within that time. Structures on microscopic scale indicate at least two different generations of mineral growth clearly related to multiphase magmatic events forming the nodules. Rare composite samples allow a correlation between the older and younger parageneses, demonstrating reaction between the older matrix pyroxenite and the younger, high-Ti melt. The relatively low (~0,13wt%) Cr2O3-contents together with the high LREE concentrations measured in the oldest observed clinopyroxenes (La~12,4 x PRIMA with La/Lu~21) as well as the lack of any other characteristic mineral relicts argue against a pervasively overprinted peridotite mantle. Comparable 87Sr/86Sr- values close to bulk earth values as well as similar 143Nd/144Nd- ratios in the nodules (0,512480-0,5122573) and the lavas (average: 0,512551) support a genetic link between the kamafugites and the nodules as suggested by experiments (Lloyd et al. 1985). Low radiogenic 87Sr/86Sr ratios in Rb-free clinopyroxene and perovskite (0,704459-0,704487) constrain initial values for the source whereas slightly more radiogenic values from cogenetic Rb-bearing biotites (0,704754- 0,704762) are the result of radioactive decay after mineral growth. The majority of the kamafugite 87Sr/86Sr values lie between the two end-members (0,704624- 0,704717). Additionally considering microscale structures showing melting processes we conclude that the nodules represent one source and that the intermediate 87Sr/86Sr values of the lavas reflect the melting of differing proportions of biotite and clinopyroxene in the source region.

The geochemistry of primitive volcanic rocks of the Ankaratra volcanic complex, and source enrichment processes in the genesis of the Cenozoic magmatism in Madagascar

NASA Astrophysics Data System (ADS)

Melluso, L.; Cucciniello, C.; le Roex, A. P.; Morra, V.

2016-07-01

The Ankaratra volcanic complex in central Madagascar consists of lava flows, domes, scoria cones, tuff rings and maars of Cenozoic age that are scattered over 3800 km2. The mafic rocks include olivine-leucite-nephelinites, basanites, alkali basalts and hawaiites, and tholeiitic basalts. Primitive samples have high Mg# (>60), high Cr and Ni concentrations; their mantle-normalized patterns peak at Nb and Ba, have troughs at K, and smoothly decrease towards the least incompatible elements. The Ankaratra mafic rocks show small variation in Sr-Nd-Pb isotopic compositions (e.g., 87Sr/86Sr = 0.70377-0.70446, 143Nd/144Nd = 0.51273-0.51280, 206Pb/204Pb = 18.25-18.87). These isotopic values differ markedly from those of Cenozoic mafic lavas of northern Madagascar and the Comoro archipelago, typical Indian Ocean MORB and oceanic basalt end-members. The patterns of olivine nephelinitic magmas can be obtained through 3-10% partial melting of a mantle source that was enriched by a Ca-rich alkaline melt, and that contained garnet, carbonates and phlogopite. The patterns of tholeiitic basalts can be obtained after 10-12% partial melting of a source enriched with lower amounts of the same alkaline melt, in the spinel- (and possibly amphibole-) facies mantle, hence in volumes where carbonate is not a factor. The significant isotopic change from the northernmost volcanic rocks of Madagascar and those in the central part of the island implicates a distinct source heterogeneity, and ultimately assess the role of the continental lithospheric mantle as source region. The source of at least some volcanic rocks of the still active Comoro archipelago may have suffered the same time-integrated geochemical and isotopic evolution as that of the northern Madagascar volcanic rocks.

Origin of the DUPAL anomaly in mantle xenoliths of Patagonia (Argentina) and geodynamic consequences

NASA Astrophysics Data System (ADS)

Mazzucchelli, Maurizio; Cipriani, Anna; Hémond, Christophe; Zanetti, Alberto; Bertotto, Gustavo Walter; Cingolani, Carlos Alberto

2016-04-01

The sub-continental lithospheric mantle of South America has been known for some time to carry the DUPAL isotope anomaly as seen in volcanics from the Paraná volcanic province. However, this has not allowed discriminating whether the DUPAL anomaly is a primary feature of the mantle source or acquired during the upwelling and emplacement of the primary magmas. We discovered mantle xenoliths from the Tres Lagos location in Patagonia that carry evidence of percolation by metasomatic melts that imparted the DUPAL isotope anomaly signature. We discuss a model that requires four isotope components (LCC, EM2, HIMU and DM) to account for the Sr, Nd and Pb isotope variability of our samples. We propose that upwelling of hot astenosphere during the Miocene could have triggered the melting of the LCC and EM2 components carrying the DUPAL anomaly, previously entrained in the subcontinental mantle by subduction. These ascending melts would have then metasomatised the local SCLM characterised by DMM and HIMU geochemical affinity generating the hybrid DUPAL-bearing mantle sampled by the Tres Lagos xenoliths.

Deep long-period earthquakes west of the volcanic arc in Oregon: evidence of serpentine dehydration in the fore-arc mantle wedge

USGS Publications Warehouse

Vidale, John E.; Schmidt, David A.; Malone, Stephen D.; Hotovec-Ellis, Alicia J.; Moran, Seth C.; Creager, Kenneth C.; Houston, Heidi

2014-01-01

Here we report on deep long-period earthquakes (DLPs) newly observed in four places in western Oregon. The DLPs are noteworthy for their location within the subduction fore arc: 40–80 km west of the volcanic arc, well above the slab, and near the Moho. These “offset DLPs” occur near the top of the inferred stagnant mantle wedge, which is likely to be serpentinized and cold. The lack of fore-arc DLPs elsewhere along the arc suggests that localized heating may be dehydrating the serpentinized mantle wedge at these latitudes and causing DLPs by dehydration embrittlement. Higher heat flow in this region could be introduced by anomalously hot mantle, associated with the western migration of volcanism across the High Lava Plains of eastern Oregon, entrained in the corner flow proximal to the mantle wedge. Alternatively, fluids rising from the subducting slab through the mantle wedge may be the source of offset DLPs. As far as we know, these are among the first DLPs to be observed in the fore arc of a subduction-zone system.

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

NASA Technical Reports Server (NTRS)

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

1988-01-01

Compositionally diverse Late Archean rocks (2.74-2.79 Ga old) from the eastern Beartooth Mountains (Montana and Wyoming) were studied and shown to have the same initial Pb, Sr, and Nd isotopic ratios. Lead and Sr initial ratios are higher and Nd initial values lower than predicted for rocks derived from model mantle sources and strongly indicate the involvement of an older crustal reservoir in the genesis of these rocks. A model involving subduction of continental detritus and contamination of the overlying mantle is suggested.

Probing the structure of the sub-Salinia mantle lithosphere using spinel lherzolite xenoliths from Crystal Knob, Santa Lucia Range, California

NASA Astrophysics Data System (ADS)

Quinn, D. P.; Saleeby, J.; Ducea, M. N.; Luffi, P. I.

2013-12-01

We present the first petrogenetic analysis of a suite of peridotite xenoliths from the Crystal Knob volcanic neck in the Santa Lucia Range, California. The neck was erupted during the Plio-Pleistocene through the Salinia terrane, a fragment of the Late Cretaceous southern Sierra-northwest Mojave supra-subduction core complex that was displaced ~310 km in the late Cenozoic along the dextral San Andreas fault. The marginal tectonic setting makes these xenoliths ideal for testing different models of upper-mantle evolution along the western North American plate boundary. Possible scenarios include the early Cenozoic underplating of Farallon-plate mantle lithosphere nappes (Luffi et al., 2009), Neogene slab window opening (Atwater and Stock, 1998), and the partial subduction and stalling of the Monterey microplate (Pisker et al., 2012). The xenoliths from Crystal Knob are spinel lherzolites, which sample the mantle lithosphere underlying Salinia, and dunite cumulates apparently related to the olivine-basalt host. Initial study is focused on the spinel lherzolites: these display an allotriomorphic granular texture with anisotropy largely absent. However, several samples exhibit a weak shape-preferred orientation in elongate spinels. Within each xenolith, the silicate phases are in Fe-Mg equilibrium; between samples, Mg# [molar Mg/(Mg+Fe)*100] ranges from 87 to 91. Spinels have Cr# [molar Cr/(Cr+Al)*100] ranging from 10 to 27. Clinopyroxene Rb-Sr and Sm-Nd radiogenic isotope data show that the lherzolites are depleted in large-ion lithophile (LIL) elements, with uniform enrichment in 143Nd (ɛNd from +10.3 to +11.0) and depletion in 87Sr (87/86Sr of .702). This data rules out origin in the continental lithosphere, such as that observed in xenoliths from above the relict subduction interface found at at Dish Hill and Cima Dome in the Mojave (Luffi et al., 2009). The Mesozoic mantle wedge, which is sampled by xenoliths from beneath the southern Sierra Nevada batholith (Ducea and Saleeby, 1998), is also ruled out as a source locale. The isotopic data are consistent with oceanic mantle originating from either the Farallon plate (underplated during Paleocene shallow subduction) or the Monterey plate (partially subducted during the Miocene). Ascended asthenosphere, presumably of slab-window origin, is also a possible source. Pyroxene Ca-Mg exchange geothermometry is in progress and will enable thermal modeling and comparisons with contemporary heat flow data. These results, along with trace-element analysis of clinopyroxene crystals, will be used to distinguish between the possible sources of LIL-depleted mantle in the sub-Salinia mantle lithosphere. The full petrogenetic survey of these xenoliths adds a distal constraint to the makeup of the mantle lithosphere beneath the western North American margin.

Objective estimates of mantle 3He in the ocean and implications for constraining the deep ocean circulation

NASA Astrophysics Data System (ADS)

Holzer, Mark; DeVries, Timothy; Bianchi, Daniele; Newton, Robert; Schlosser, Peter; Winckler, Gisela

2017-01-01

Hydrothermal vents along the ocean's tectonic ridge systems inject superheated water and large amounts of dissolved metals that impact the deep ocean circulation and the oceanic cycling of trace metals. The hydrothermal fluid contains dissolved mantle helium that is enriched in 3He relative to the atmosphere, providing an isotopic tracer of the ocean's deep circulation and a marker of hydrothermal sources. This work investigates the potential for the 3He/4He isotope ratio to constrain the ocean's mantle 3He source and to provide constraints on the ocean's deep circulation. We use an ensemble of 11 data-assimilated steady-state ocean circulation models and a mantle helium source based on geographically varying sea-floor spreading rates. The global source distribution is partitioned into 6 regions, and the vertical profile and source amplitude of each region are varied independently to determine the optimal 3He source distribution that minimizes the mismatch between modeled and observed δ3He. In this way, we are able to fit the observed δ3He distribution to within a relative error of ∼15%, with a global 3He source that ranges from 640 to 850 mol yr-1, depending on circulation. The fit captures the vertical and interbasin gradients of the δ3He distribution very well and reproduces its jet-sheared saddle point in the deep equatorial Pacific. This demonstrates that the data-assimilated models have much greater fidelity to the deep ocean circulation than other coarse-resolution ocean models. Nonetheless, the modelled δ3He distributions still display some systematic biases, especially in the deep North Pacific where δ3He is overpredicted by our models, and in the southeastern tropical Pacific, where observed westward-spreading δ3He plumes are not well captured. Sources inferred by the data-assimilated transport with and without isopycnally aligned eddy diffusivity differ widely in the Southern Ocean, in spite of the ability to match the observed distributions of CFCs and radiocarbon for either eddy parameterization.

Searching for Seismic Signatures of a Plume Source at the Base of the Mantle Below the Galapagos Island Hotspot

NASA Astrophysics Data System (ADS)

Vanacore, E.; Niu, F.

2007-12-01

This study analyzes SKS and SKKS waveforms recorded on the BOLIVAR array in Venezuela and the BANJO array in South America from earthquake sources located in Tonga and Alaska regions to characterize the lower mantle beneath the Galapagos Islands. The data analysis applies two independent methods, residual differential SKKS-SKS travel times and anisotropy measurements, to examine the historically unsampled region. The residual differential travel time observations were performed using 21 earthquakes from the Tonga trench with magnitudes greater than 5.5 Mw that were recorded on the Bolivar array. Only data that was deemed to have a high SNR for both the SKS and SKKS phases were retained for analysis. Significant positive values of differential travel time that indicate low velocity along the SKKS raypaths are detected east of ~\\m270° longitude. The anisotropy data set consists of 31 intermediate and deep focus earthquakes from the Tonga and Aleutian trenches recorded on the BOLIVAR and BANJO arrays respectively. The anisotropy fast axis angle and time lag of the two phases are calculated using the 1-layer cross-convolution method of Menke and Levin (2003) with a maximum time lag window of 3 seconds. We retain results with an amplitude normalized squared L2 norm value of 0.6 or less for analysis. Because the raypaths of the SKS and SKKS phases are similar in the upper mantle and sample different regions of the lower mantle, we attribute inconsistencies between the two anisotropy to difference of the mantle structure near the CMB. We define significant difference in the azimuth of the fast axis as any difference between the SKSac and SKKSac measurements greater than 15 degrees. The dataset is dominated by inconsistent fast axis azimuth measurements between the SKSac and SKKSac phases, but does not isolate a single geographic region. Comparison of the splitting time measurements yields that inconsistency between the two phases is more significant, greater than 0.5 s, in the Northeast portion of the sampled region bounded to the south and west at approximately \\m-3°S and \\m267° longitude. While the residual differential travel times and the anisotropy measurements do not conclusively show that there is a mantle plume source at the base of the mantle in this region, the data does indicate there the lower mantle beneath the Galapagos Islands has significant structure meriting further study.

Mesoproterozoic orangeites of Karelia (Kostomuksha-Lentiira): evidence for composition of mantle lithosphere

NASA Astrophysics Data System (ADS)

Kargin, Alexey; Nosova, Anna; Larionova, Yulia; Kononova, Voctoria; Borisovskiy, Sergey; Kovalchuk, Elena; Griboedova, Irina

2014-05-01

The 1.23-1.20 Ga old diamondiferous lamproites and orangeites (kimberlites of II group) of the Kostomuksha-Taloveys and the Lentiira-Kuhmo dyke fields intrude the Archaean crust of the Karelian craton, NE of the East European Platform. Mineral (a trend of compositional evolution of mica, presence of carbonate minerals in basis, composition of olivine) and geochemical (major elements, ratio of trace elements, primitive mantle normalized trace elements patterns) characteristics of these rocks suggest an orangeitic rather than lamproitic or lamprophyric nature. The composition of Phl-Ol orangeites suggests intensive processes of fractional crystallization for their melts. Cpx-Phl-Ol orangeites indicate higher intensity of lithospheric mantle assimilation then other orangeitic types. Phl-Carb orangeites of the Taloveys area and Cpx-Phl-Ol one of the Lentiira area are closest to primary melts. The Ol-Phl-Cpx orangeites of the Lentiira area contain three generations of unaltered olivine that vary in composition and origin: a) xenocryst derived from depleted mantle peridotite; b) orangeitic olivine phenocryst and c) and olivine like early stage crystallization of megacryst assemblage or a product of metasomatic interaction between mantle peridotite and protokimberlitic melt. Orangeites of Kostomuksha-Lentiira have low- and medium-radiogenic value of (87Sr/86Sr)1200 that range from 0.7038 to 0.7067. Phl-Carb orangeites of Taloveys have less radiogenic isotopic composition of Nd (eNd -11 ... -12) then Cpx-Phl-Ol and Phl-Ol orangeites of Kostomuksha (eNd -6.9 ... -9.4). The study of Sm-Nd and Rb-Sr isotopic systems suggests that an ancient metasomatic mantle source took part in origin of orangeites. We propose a two-steps model of origin of their source (Kargin et al., 2014): 1) The metasomatic component of mantle source (like as MARID-type veins) formed during Lapland-Kola and/or Svecofennian orogeny events (2.1-1.8 Ga ago). 2) The intrusion of orangeites is comparable by time and geological setting with back-arc extensions (such as magmatism of CSDG) during the first stage of Sveconorwegian orogeny, initial stage of assembly of the supercontinent Rodinia (1.31-1.14 Ga). These back-arc extensions inspired of asthenosphere-derived melts to metasomatic lithospheric mantle and a generation of orangeitic melts. Kargin A.V., Nosova A.A., Larionova Yu.O. et al. Mesoproterozoic orangeites (kimberlites II) of Western Karelia: mineralogical, geochemical and Nd-Sr isotopic-geochemical characteristics // Petrology. 2014. V.2. in press.

Tracing the HIMU component within Pan-African lithosphere beneath northeast Africa: Evidence from Late Cretaceous Natash alkaline volcanics, Egypt

NASA Astrophysics Data System (ADS)

Abu El-Rus, M. A.; Chazot, G.; Vannucci, R.; Paquette, J.-L.

2018-02-01

A large late Cretaceous ( 90 Ma) volcanic field (the Natash volcanic province) crops out in southeast Egypt at the northwestern boundary of the Arabian-Nubian shield. The lavas are mainly of alkaline affinity and exhibit a continuous compositional range from alkali olivine basalt (AOB) to trachyte and rhyolite. All basaltic lavas in the province record various extents of fractional crystallization of olivine, clinopyroxene, plagioclase and spinel. The basaltic lavas show variations in Sr-Nd-Pb-Hf isotopic ratios [(87Sr/86Sr)i = 0.7030-0.70286; (143Nd/144Nd)i = 0.512653-0.512761; (206Pb/204Pb)i = 19.28-19.94; (177Hf-176Hf)i = 0.28274-0.28285], that correlate markedly with the major and trace element ratios and abundances. Assimilation of crustal material cannot explain these correlations, and we invoke instead melting of a multicomponent mantle source. We infer the existence of High-μ (HIMU), Enriched mantle type-I (EM-I) and Depleted mantle (DM) domains in the melting source, with a predominant contribution from the HIMU-type. We suggests further that the basaltic lavas originate from low degrees of partial melting (F < 5%) at moderate potential temperatures (TP) 1391-1425 °C and pressures of 2.0-2.6 GPa. The melting pressure estimations imply that melting entirely occurred within lithospheric mantle, most likely in the presence of residual amphibole as presence negative K-anomalies in the primitive mantle-normalized patterns of the fractionation-corrected melts. The presence of amphibole within the lithosphere is a strong evidence that the lithospheric mantle underwent metasomatic enrichment prior to melting in Late Cretaceous. This metasomatic event affected on the Pb isotopic compositions of the Natash volcanics by adding Th and U to the melting source. Time-integrated calculations to remove the decoupling between 206Pb and 207Pb isotopes that most probably resulted from the metasomatic event indicate a tentative link between the metasomatism occurring in the Pan-African lithospheric mantle and the formation of juvenile crust during the Pan-African Orogeny. A two stage evolution model is therefore proposed for volcanism in the Natash area: fluxing of the lithosphere by hydrous fluids during Pan-African Orogeny forming a hybrid lithospheric mantle that in Late Cretaceous underwent thermal erosion and melting in response to upwelling asthenosphere, possibly at the onset of the extensional fracturing preceded the doming of the Afro-Arabian Shield.

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 compare the major and trace element characteristics of bulk rock and minerals found in orthopyroxenites from supra-subduction zones with the residua formed in our experiments, to differentiate between melt versus fluid, and sediment- versus basalt-derived flux in the mantle wedge. [1] Mallik et al. (2015) CMP169(5) [2] Sekine & Wyllie (1982) CMP 81(3)

South-to-north pyroxenite-peridotite source variation correlated with an OIB-type to arc-type enrichment of magmas from the Payenia backarc of the Andean Southern Volcanic Zone (SVZ)

NASA Astrophysics Data System (ADS)

Brandt, Frederik Ejvang; Holm, Paul Martin; Søager, Nina

2017-01-01

New high-precision minor element analysis of the most magnesian olivine cores (Fo85-88) in fifteen high-MgO (Mg#66-74) alkali basalts or trachybasalts from the Quaternary backarc volcanic province, Payenia, of the Andean Southern Volcanic Zone in Argentina displays a clear north-to-south decrease in Mn/Feol. This is interpreted as the transition from mainly peridotite-derived melts in the north to mainly pyroxenite-derived melts in the south. The peridotite-pyroxenite source variation correlates with a transition of rock compositions from arc-type to OIB-type trace element signatures, where samples from the central part of the province are intermediate. The southernmost rocks have, e.g., relatively low La/Nb, Th/Nb and Th/La ratios as well as high Nb/U, Ce/Pb, Ba/Th and Eu/Eu* = 1.08. The northern samples are characterized by the opposite and have Eu/Eu* down to 0.86. Several incompatible trace element ratios in the rocks correlate with Mn/Feol and also reflect mixing of two geochemically distinct mantle sources. The peridotite melt end-member carries an arc signature that cannot solely be explained by fluid enrichment since these melts have relatively low Eu/Eu*, Ba/Th and high Th/La ratios, which suggest a component of upper continental crust (UCC) in the metasomatizing agent of the northern mantle. However, the addition to the mantle source of crustal materials or varying oxidation state cannot explain the variation in Mn and Mn/Fe of the melts and olivines along Payenia. Instead, the correlation between Mn/Feol and whole-rock (wr) trace element compositions is evidence of two-component mixing of melts derived from peridotite mantle source enriched by slab fluids and UCC melts and a pyroxenite mantle source with an EM1-type trace element signature. Very low Ca/Fe ratios ( 1.1) in the olivines of the peridotite melt component and lower calculated partition coefficients for Ca in olivine for these samples are suggested to be caused by higher H2O contents in the magmas derived from subduction zone enriched mantle. Well-correlated Mn/Fe ratios in the wr and primitive olivines demonstrate that the Mn/Fewr of these basalts that only fractionated olivine and chromite reflects the Mn/Fe of the primitive melts and can be used as a proxy for the amount of pyroxenite melt in the magmas. Using Mn/Fewr for a large dataset of primitive Payenia rocks, we show that decreasing Mn/Fewr is correlated with decreasing Mn and increasing Zn/Mn as expected for pyroxenite melts.

Geochemistry and petrogenesis of lava flows around Linga, Chhindwara area in the Eastern Deccan Volcanic Province (EDVP), India

NASA Astrophysics Data System (ADS)

Ganguly, Sohini; Ray, Jyotisankar; Koeberl, Christian; Saha, Abhishek; Thöni, Martin; Balaram, V.

2014-09-01

Based on systematic three-tier arrangement of vesicles, entablature and columnar joints, three distinct quartz normative tholeiitic lava flows (I, II and III) were recognized in the area around Linga, in the Eastern Deccan Volcanic Province (EDVP). Each of the flows exhibits intraflow chemical variations marked by high Mg#-low Ti, and low Mg#-high Ti contents. The MgO (4.27-7.74 wt.%), Mg# (23.45-41.89) and Zr (161.5-246.3 ppm) of Linga flows suggest an evolved chemistry marked by fractional crystallization and crustal contamination processes. Positive Rb and Th anomalies, negative Nb anomalies, relative enrichment of LILE-LREE with respect to Nb, Nb/Th:3.71-6.77 indicate crustal contamination of magma by continental materials through magma-crust interaction during melt migration and contributions from sub-continental lithospheric mantle (SCLM). Negative K, Sr and Ti anomalies corroborate an intracontinental, rift-controlled tectonic setting for the genesis and evolution of Linga basalts. Chondrite-normalized REE patterns reflect low HREE abundances and prominent LREE/HREE, MREE/HREE fractionation thereby pointing towards partial melting of garnet peridotite mantle source. Nb, Zr, Y variations suggest 10-15% partial melting of mantle source for the derivation of parent tholeiitic melt that suffered crystal fractionation of phenocrystal phases and subsequent liquid immiscibility. Critical evaluation of Srinitial and Ndinitial (65 Ma) isotopic compositions (87Sr/86Srinitial between 0.705656 and 0.706980 and 143Nd/144Ndinitial between 0.512523 and 0.512598) suggests that these basalts were derived from an enriched mantle (∼EM I-EM II) source. The εSr (21.84-41.27) and εNd (-0.28 to 1.10) isotopic signatures defined by higher εSr and lower εNd fingerprint a plume-related source. Positive and negative values of εNd indicate an isotopically heterogeneous mantle source marked by mixing of depleted (DM) and enriched mantle (EM I-EM II) components at the source region and together with 87Sr/86Srinitial ranging from 0.705656 to 0.706980 suggest two stage contamination of parent magma which is much similar to that of Poladpur, Toranmal, Mhow, Chikaldara flows. Ba/Y versus 87Sr/86Sr and Nb/Y versus Rb/Y variations show an Ambenali-Poladpur contamination trend for the Linga basalts thereby suggesting the role of upper continental granitic crust as the contaminant of these flows through magma-crust interaction during melt migration. The lava flows of Linga are geochemically correlatable with the Poladpur flows of southwestern and Toranmal flows of northern Deccan and show genetic coherence with the basalts of Jabalpur, Seoni, Chakhla-Delakhari of eastern Deccan.

Age and geochemistry of the Newania dolomite carbonatites, India: implications for the source of primary carbonatite magma

NASA Astrophysics Data System (ADS)

Ray, Jyotiranjan S.; Pande, Kanchan; Bhutani, Rajneesh; Shukla, Anil D.; Rai, Vinai K.; Kumar, Alok; Awasthi, Neeraj; Smitha, R. S.; Panda, Dipak K.

2013-12-01

The Newania carbonatite complex of India is one of the few dolomite-dominated carbonatites of the world. Intruding into Archean basement gneisses, the rocks of the complex have undergone limited diversification and are not associated with any alkaline silicate rock. Although the magmatic nature of the complex was generally accepted, its age of emplacement had remained equivocal because of the disturbed nature of radioisotope systems. Many questions about the nature of its mantle source and mode of origin had remained unanswered because of lack of geochemical and isotopic data. Here, we present results of our effort to date the complex using 147Sm-143Nd, 207Pb-206Pb and 40Ar-39Ar dating techniques. We also present mineral chemistry, major and trace element geochemistry and Sr-Nd isotopic ratio data for these carbonatites. Our age data reveal that the complex was emplaced at ~1,473 Ma and parts of it were affected by a thermal event at ~904 Ma. The older 207Pb-206Pb ages reported here (~2.4 Ga) and by one earlier study (~2.3 Ga; Schleicher et al. Chem Geol 140:261-273, 1997) are deemed to be a result of heterogeneous incorporation of crustal Pb during the post-emplacement thermal event. The thermal event had little effect on many magmatic signatures of these rocks, such as its dolomite-magnesite-ankerite-Cr-rich magnetite-magnesio-arfvedsonite-pyrochlore assemblage, mantle like δ13C and δ18O and typical carbonatitic trace element patterns. Newania carbonatites show fractional crystallization trend from high-Mg to high-Fe through high-Ca compositions. The least fractionated dolomite carbonatites of the complex possess very high Mg# (≥80) and have similar major element oxide contents as that of primary carbonatite melts experimentally produced from peridotitic sources. In addition, lower rare earth element (and higher Sr) contents than a typical calcio-carbonatite and mantle like Nb/Ta ratios indicate that the primary magma for the complex was a magnesio-carbonatite melt and that it was derived from a carbonate bearing mantle. The Sr-Nd isotopic data suggest that the primary magma originated from a metasomatized lithospheric mantle. Trace element modelling confirms such an inference and suggests that the source was a phlogopite bearing mantle, located within the garnet stability zone.

The Xenon record of Earth's early differentiaiton

NASA Astrophysics Data System (ADS)

Peto, M. K.; Mukhopadhyay, S.; Kelley, K. A.

2011-12-01

Xenon isotopes in mantle derived rocks provide information on the early differentiation of the silicate mantle of our planet. {131,132 134,136}Xe isotopes are produced by the spontaneous fission of two different elements: the now extinct radionuclide 244Pu, and the long-lived 238U. These two parent nuclides, however, yield rather different proportion of fissiogenic Xenon isotopes. Hence, the proportion of Pu- to U-derived fission xenon is indicative of the degree and rate of outgassing of a mantle reservoir. Recent data obtained from Iceland in our lab confirm that the Xenon isotopic composition of the plume source(s) is characterized by lower 136Xe/130Xe ratios than the MORB source and the Iceland plume is more enriched in the Pu-derived Xenon component. These features are interpreted as reflecting different degrees of outgassing and appear not to be the result of preferential recycling of Xenon to the deep mantle. To further investigate how representative the Icelandic measurements might be of other mantle plumes, we measured noble gases (He, Ne, Ar, Xe) in gas-rich basalt glasses from the Rochambeau Ridge (RR) in the Northern Lau Basin. Recent work suggests the presence of a "Samoan-like" OIB source in the northern Lau Basin and our measurements were performed on samples with plume-like 3He/4He ratios (15-28 RA) [1]. The Xenon isotopic measurements indicate that the maximum measured 136Xe/130Xe ratios in the Rochambeau samples are similar to Iceland. In particular, for one of the gas rich samples we were able to obtain 77 different isotopic measurements through step-crushing. Preliminary investigation of this sample suggests higher Pu- to U-derived fission Xenon than in MORBs. To quantitatively evaluate the degree and rate of outgassing of the plume and MORB reservoirs, particularly during the first few hundred million years of Earth's history, we have modified a geochemical reservoir model that was previously developed to investigate mantle overturn and mixing from He, Ar and lithophile isotopes [2]. We will present the results from this geochemical reservoirs model, which is constrained by our high precision dataset from the Rochambeau Rift (Northern Lau Basin) and Iceland along with the Xenon dataset from popping rock [3]. [1] Lupton et al., GRL, 2009. [2] Gonnermann and Mukhopadhyay, Nature, 2009. [3] Kunz et al., Science, 1998.

Ubiquitous radiogenic Os in Miocene to recent basalts from diverse mantle domains beneath the Colorado Plateau, USA

NASA Astrophysics Data System (ADS)

Schlieder, T.; Reid, M. R.; Widom, E.; Blichert-Toft, J.

2015-12-01

The source of magmatism and mechanisms responsible for the observed geochemical signatures in Miocene to Recent Colorado Plateau (CP) basalts has been a renewed focus of investigation in light of Earthscope results. We report new Os and Nd isotopic data for magnesian basalts (Mg#=62-72) and interpret them in light of previously reported Hf isotope data to help constrain contributions from olivine-poor source lithologies and subduction-derived metasomatism in the genesis of recent CP volcanism. The basalts studied span a large range in Hf isotope compositions and represent melts last equilibrated at a variety of depths beneath the Colorado Plateau and its transition zones. We distinguished at least three mantle domains on the basis of paired Hf-Nd isotope, other isotopic, and geochemical characteristics of CP lavas. Domain 1 likely represents a depleted, variably metasomatized, lithospheric source, with relatively radiogenic ɛHf (+5.2 to +11.8) and highly variable ɛNd (-6.2 to +6.2). Domain 2 could represent either ancient or Farallon subduction-modified mantle and is displaced above the Hf-Nd mantle array (ɛHf=+1.0 to +7.3; ɛNd=-6.1 to -3.5). Domain 3 may be melts of pyroxenite/mica-rich veins or layers within lithospheric mantle and is characterized by unradiogenic Hf and Nd (ɛHf=-12.9 to +0.6; ɛNd=-10.0 to -2.9). The isotopic variability in CP-related lavas can largely be attributed to contributions from these mantle domains. Preliminary Os isotope data show no correlation with proxies for differentiation or crustal contamination. Osmium and Hf isotope compositions are negatively correlated between domains 1 and 2 (187Os/188Os=0.31 to 0.59), whereas the Os isotope ratios in two domain 3 basalts have both lower and higher values (187Os/188Os=0.25 and 0.68). Significantly, Os isotope signatures are highly radiogenic (vs. values of <0.12 for SW US peridotite xenoliths [1]), overlapping and extending the range for inferred melts of pyroxene- and mica-rich veins and/or layers [2]. Thus mafic magmatism associated with the CP appears to ubiquitously tap sources at least locally modified by processes such as recycling of ancient oceanic crust, introduction of terrigenous sediments, or subduction-related metasomatism. [1] Lee et al., Nature, 2001. [2] Carlson and Nowell, G-Cubed, 2001.

Olivine water contents in the continental lithosphere and the longevity of cratons.

PubMed

Peslier, Anne H; Woodland, Alan B; Bell, David R; Lazarov, Marina

2010-09-02

Cratons, the ancient cores of continents, contain the oldest crust and mantle on the Earth (>2 Gyr old). They extend laterally for hundreds of kilometres, and are underlain to depths of 180-250 km by mantle roots that are chemically and physically distinct from the surrounding mantle. Forming the thickest lithosphere on our planet, they act as rigid keels isolated from the flowing asthenosphere; however, it has remained an open question how these large portions of the mantle can stay isolated for so long from mantle convection. Key physical properties thought to contribute to this longevity include chemical buoyancy due to high degrees of melt-depletion and the stiffness imparted by the low temperatures of a conductive thermal gradient. Geodynamic calculations, however, suggest that these characteristics are not sufficient to prevent the lithospheric mantle from being entrained during mantle convection over billions of years. Differences in water content are a potential source of additional viscosity contrast between cratonic roots and ambient mantle owing to the well-established hydrolytic weakening effect in olivine, the most abundant mineral of the upper mantle. However, the water contents of cratonic mantle roots have to date been poorly constrained. Here we show that olivine in peridotite xenoliths from the lithosphere-asthenosphere boundary region of the Kaapvaal craton mantle root are water-poor and provide sufficient viscosity contrast with underlying asthenosphere to satisfy the stability criteria required by geodynamic calculations. Our results provide a solution to a puzzling mystery of plate tectonics, namely why the oldest continents, in contrast to short-lived oceanic plates, have resisted recycling into the interior of our tectonically dynamic planet.

Mantle-cell lymphoma.

PubMed

Barista, I; Romaguera, J E; Cabanillas, F

2001-03-01

During the past decade, mantle-cell lymphoma has been established as a new disease entity. The normal counterparts of the cells forming this malignant lymphoma are found in the mantle zone of the lymph node, a thin layer surrounding the germinal follicles. These cells have small to medium-sized nuclei, are commonly indented or cleaved, and stain positively with CD5, CD20, cyclin D1, and FMC7 antibodies. Because of its morphological appearance and a resemblance to other low-grade lymphomas, many of which grow slowly, this lymphoma was initially thought to be an indolent tumour, but its natural course was not thoroughly investigated until the 1990s, when the BCL1 oncogene was identified as a marker for this disease. Mantle-cell lymphoma is a discrete entity, unrelated to small lymphocytic or small-cleaved-cell lymphomas.

Quantifying Textures of Rapakivi Granites and Mantle Formation Insights

NASA Astrophysics Data System (ADS)

Ashauer, Z.; Currier, R. M.

2017-12-01

Rapakivi texture, the mantling of plagioclase on alkali feldspar, is a common occurrence in granitoids derived from crustal melting. Presented here, are several textural analyses that quantify mantle thickness and the overall distribution of crystal populations. Analyses were performed on outcrops and slabbed samples from the Wolf River Batholith, Wisconsin, USA and the Wiborg Batholith, Finland. Both localities are "classical" rapakivi granites of Proterozoic age associated with incipient rifting of the supercontinent Nuna/Columbia. Mantle thickness analysis reveals a relationship between the characteristic size of the mantle and the size of the core. The thickest mantles tend to be on relatively small cores while relatively large cores display thin mantles. This relationship is consistent with a replacement origin as a result of alkali feldspar dissolution with concomitant reprecipitation of plagioclase, due to disequilibrium between crystal and melt. If this is the case then crystal size distributions should be similar between unmantled and mantled megacrysts. Preliminary results confirm this supposition: rapakivi mantle formation in these classical systems appear to be the result of replacement. These textural analyses immediately call into question the viability of epitaxial growth models. A certain amount of disequilibrium is required to drive the replacement reaction. Two potential mechanisms are 1) mechanical transfer of crystals into a magma of more mafic composition (i.e., magma mixing), and 2) the production of a heterogeneous melt during rapid melting of granitic rock and reaction between unmelted crystals and partial melt. The classical rapakivi granites are associated with prolonged bimodal magmatism, and so there is clear potential to drive either of these mantling mechanisms.

Self-Organized Mantle Layering After the Magma-Ocean Period

NASA Astrophysics Data System (ADS)

Hansen, U.; Dude, S.

2017-12-01

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

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

NASA Technical Reports Server (NTRS)

Haggerty, Stephen E.; Toft, Paul B.

1988-01-01

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

Osmium-187 enrichment in some plumes: Evidence for core-mantle interaction?

USGS Publications Warehouse

Walker, R.J.; Morgan, J.W.; Horan, M.F.

1995-01-01

Calculations with data for asteroidal cores indicate that Earth's outer core may have a rhenium/osmium ratio at least 20 percent greater than that of the chondritic upper mantle, potentially leading to an outer core with an osmium-187/osmium-188 ratio at least 8 percent greater than that of chondrites. Because of the much greater abundance of osmium in the outer core relative to the mantle, even a small addition of metal to a plume ascending from the D??? layer would transfer the enriched isotopic signature to the mixture. Sources of certain plume-derived systems seem to have osmium-187/osmium-188 ratios 5 to 20 percent greater than that for chondrites, consistent with the ascent of a plume from the core-mantle boundary.

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

NASA Technical Reports Server (NTRS)

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

1980-01-01

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

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. Based on the data in this study, the main inferences on the behaviour of Cl and F during melting and metasomatic processes in the sub-arc mantle are as follow: (i) Melting models show that most depleted mantle protoliths of intra-oceanic arc sources can have extremely low Cl/F (0.002-0.007) before being overprinted by subduction-derived components. (ii) Chlorine has a higher percolation distance in the mantle than F. Even for small fluid or melt volumes, Cl and F signatures of partial melting are overprinted by those of pervasive percolation, which increases Cl/F in percolating agents and bulk peridotites during chromatographic interaction and/or amphibole-forming metasomatic reactions. These processes ultimately control the bulk Cl and F compositions of the residual mantle lithosphere beneath arcs, and likely in other tectonic settings. (iii) Fluxed melting models suggest that Cl enrichment in arc picrite and boninite melts in this study, and in many arc melt inclusions reported in the literature, could be related to the infiltration of high Cl/F fluids derived from subducted serpentinite or altered crust in mantle wedge sources. However, these high Cl/F signatures should be re-evaluated with new models in light of the possible overprint of pervasive percolation effects in the mantle. The breakdown of amphibole (and/or mica) in the deep metasomatised mantle at higher pressure and temperature conditions than in the slab may explain, at least in part, the positive correlations between F abundances and Cl/F in primitive arc melt inclusions and slab depth.

On seismic resolution of lateral heterogeneity in the Earth's outermost core

NASA Astrophysics Data System (ADS)

Garnero, Edward J.; Helmberger, Donald V.

1995-03-01

Issues concerning resolution of seismically determined outermost core properties are presented with an example from three earthquakes in the Fiji-Tonga region. Travel time behavior of the commonly used family of S mKS waves, which travel as S in the mantle, P in the core, reflecting m - 1 times at the underside of the core-mantle boundary (CMB), are analyzed over a large distance range (125-165°). Data having wavepaths through an area of known D″ heterogeneity (±2%) exhibit systematic anomalies in S mKS differential times. Two-dimensional wave propagation experiments demonstrate how large-scale lower-mantle velocity perturbations can explain long-wavelength behavior of such anomalous S mKS times, though heterogeneity on smaller scales may be responsible for the observed scatter about these trends. If lower-mantle heterogeneity is not properly accounted for in deriving a core model, misfit of the mantle model maps directly into core structure. The existence of outermost core heterogeneity is difficult to resolve at present, owing to uncertainties in global lower-mantle structure. Resolving a one-dimensional chemically stratified outermost core also remains difficult, owing to the same uncertainties. Inclusion of the slowly accruing broadband data should help in this regard. Restricting study to higher multiples of S mKS ( m = 2, 3, 4) can help reduce the effect of mantle heterogeneity, because of the closeness of the mantle legs of the wavepaths. S mKS waves are ideal in providing additional information on the details of lower-mantle heterogeneity.

The mantle lithosphere and the Wilson Cycle

NASA Astrophysics Data System (ADS)

Heron, Philip; Pysklywec, Russell; Stephenson, Randell

2017-04-01

In the view of the conventional theory of plate tectonics (e.g., the Wilson Cycle), crustal inheritance is often considered important in tectonic evolution. However, the role of the mantle lithosphere is usually overlooked due to its difficulty to image and uncertainty in rheological makeup. Deep seismic imaging has shown potential scarring in continental mantle lithosphere to be ubiquitous. Recent studies have interpreted mantle lithosphere heterogeneities to be pre-existing structures, and as such linked to the Wilson Cycle and inheritance. In our study, we analyze intraplate deformation driven by mantle lithosphere heterogeneities from ancient Wilson Cycle processes and compare this to crustal inheritance deformation. We present 2-D numerical experiments of continental convergence to generate intraplate deformation, exploring the limits of continental rheology to understand the dominant lithosphere layer across a broad range of geological settings. By implementing a "jelly sandwich" rheology, characteristic of stable continental lithosphere, we find that during compression the strength of the mantle lithosphere is integral in controlling deformation from a structural anomaly. We posit that if the continental mantle is the strongest layer within the lithosphere, then such inheritance may have important implications for the Wilson Cycle. Furthermore, our models show that deformation driven by mantle lithosphere scarring can produce tectonic patterns related to intraplate orogenesis originating from crustal sources, highlighting the need for a more formal discussion of the role of the mantle lithosphere in plate tectonics. We outline the difficulty in unravelling the causes of tectonic deformation, alongside discussing the role of deep lithosphere processes in plate tectonics.

The Temperature of the Icelandic Mantle Plume from Aluminium-in-Olivine Thermometry

NASA Astrophysics Data System (ADS)

Matthews, S.; Shorttle, O.; Maclennan, J.

2015-12-01

Temperature is a key control on the physical properties of the mantle, in particular the extent of melting during upwelling. It is not, however, a unique control on many of the parameters used to estimate mantle temperature. For example igneous crustal thickness which has often been used as a first-order proxy for mantle temperature, is also affected by mantle lithology and plume flux. Alternatives to geophysical indicators of mantle temperature are petrological thermometers. However, these record crystallisation temperatures, therefore a series of assumptions about the coupled melt- solid mantle thermal history must be made when calculating back to mantle potential temperature. In this study we investigate how these assumptions may affect mantle temperature estimates and how crystallisation temperatures may offer insights into the melting and melt transport processes, focussing on a new set of crystallisation temperature estimates we have made on primitive Icelandic basalts.We used the aluminium-in-olivine thermometer of Coogan et al. (2014) to estimate crystallisation temperatures of olivine phenocrysts in a suite of samples from the Northern Volcanic Zone (NVZ) of Iceland. The data suggest that within a single volcanic system crystallisation temperature depends strongly on the olivine forsterite content, thus the history of melt evolution, and how the eruption samples this, must be considered when extrapolating to mantle temperature. To assess the influence of the assumptions required to obtain mantle temperature we constructed a simple thermal model incorporating varying proportions of lherzolite, pyroxenite and harzburgite undergoing decompression melting. A trade off between increasing mantle temperature and decreasing pyroxenite (or increasing harzburgite) in the source is observed. Using this dataset and our model, calculations reveal a potential temperature of 1470±130 °C for Iceland, and a temperature excess of 150±40 °C relative to ambient mantle. These estimates are consistent with temperatures estimated using crustal thickness and melt chemistry.

Thermal Structure and Mantle Dynamics of Rocky Exoplanets

NASA Astrophysics Data System (ADS)

Wagner, F. W.; Tosi, N.; Hussmann, H.; Sohl, F.

2011-12-01

The confirmed detections of CoRoT-7b and Kepler-10b reveal that rocky exoplanets exist. Moreover, recent theoretical studies suggest that small planets beyond the Solar System are indeed common and many of them will be discovered by increasingly precise observational surveys in the years ahead. The knowledge about the interior structure and thermal state of exoplanet interiors provides crucial theoretical input not only for classification and characterization of individual planetary bodies, but also to better understand the origin and evolution of the Solar System and the Earth in general. These developments and considerations have motivated us to address several questions concerning thermal structure and interior dynamics of terrestrial exoplanets. In the present study, depth-dependent structural models of solid exoplanet interiors have been constructed in conjunction with a mixing length approach to calculate self-consistently the radial distribution of temperature and heat flux. Furthermore, 2-D convection simulations using the compressible anelastic approximation have been carried through to examine the effect of thermodynamic quantities (e.g., thermal expansivity) on mantle convection pattern within rocky planets more massive than the Earth. In comparison to parameterized convection models, our calculated results predict generally hotter planetary interiors, which are mainly attributed to a viscosity-regulating feedback mechanism involving temperature and pressure. We find that density and thermal conductivity increase with depth by a factor of two to three, however, thermal expansivity decreases by more than an order of magnitude across the mantle for planets as massive as CoRoT-7b or Kepler-10b. The specific heat capacity is observed to stay almost constant over an extended region of the lower mantle. The planform of mantle convection is strongly modified in the presence of depth-dependent thermodynamic quantities with hot upwellings (plumes) rising across the whole mantle and cold downwellings (slabs) disperse in the mid-mantle. This may have a significant effect on thermal evolution, magnetic field generation, and the propensity of plate tectonics on rocky super-Earths. Model calculations also indicate that modest radiogenic heating through the decay of long-lived radioactive elements such as U, Th, and K has a negligible effect on the interior structure of rocky exoplanets. However, the calculated body tide Love numbers strongly scale with planetary mass suggesting that in resonant and sufficiently eccentric orbits the dissipation of tidal energy would substantially affect present thermal state and orbital evolution. Therefore, tidal heating provides a viable present-day heat source for close-in exoplanets such as CoRoT-7b and Kepler-10b.

Mantle-derived trace element variability in olivines and their melt inclusions

NASA Astrophysics Data System (ADS)

Neave, David A.; Shorttle, Oliver; Oeser, Martin; Weyer, Stefan; Kobayashi, Katsura

2018-02-01

Trace element variability in oceanic basalts is commonly used to constrain the physics of mantle melting and the chemistry of Earth's deep interior. However, the geochemical properties of mantle melts are often overprinted by mixing and crystallisation processes during ascent and storage. Studying primitive melt inclusions offers one solution to this problem, but the fidelity of the melt-inclusion archive to bulk magma chemistry has been repeatedly questioned. To provide a novel check of the melt inclusion record, we present new major and trace element analyses from olivine macrocrysts in the products of two geographically proximal, yet compositionally distinct, primitive eruptions from the Reykjanes Peninsula of Iceland. By combining these macrocryst analyses with new and published melt inclusion analyses we demonstrate that olivines have similar patterns of incompatible trace element (ITE) variability to the inclusions they host, capturing chemical systematics on intra- and inter-eruption scales. ITE variability (element concentrations, ratios, variances and variance ratios) in olivines from the ITE-enriched Stapafell eruption is best accounted for by olivine-dominated fractional crystallisation. In contrast, ITE variability in olivines and inclusions from the ITE-depleted Háleyjabunga eruption cannot be explained by crystallisation alone, and must have originated in the mantle. Compatible trace element (CTE) variability is best described by crystallisation processes in both eruptions. Modest correlations between host and inclusion ITE contents in samples from Háleyjabunga suggest that melt inclusions can be faithful archives of melting and magmatic processes. It also indicates that degrees of ITE enrichment can be estimated from olivines directly when melt inclusion and matrix glass records of geochemical variability are poor or absent. Inter-eruption differences in olivine ITE systematics between Stapafell and Háleyjabunga mirror differences in melt inclusion suites, and confirm that the Stapafell eruption was fed by lower degree melts from greater depths within the melting region than the Háleyjabunga eruption. Although olivine macrocrysts from Stapafell are slightly richer in Ni than those from Háleyjabunga, their overall CTE systematics (e.g., Ni/(Mg/Fe), Fe/Mn and Zn/Fe) are inconsistent with being derived from olivine-free pyroxenites. However, the major element systematics of Icelandic basalts require lithological heterogeneity in their mantle source in the form of Fe-rich and hence fusible domains. We thus conclude that enriched heterogeneities in the Icelandic mantle are composed of modally enriched, yet nonetheless olivine-bearing, lithologies and that olivine CTE contents provide an incomplete record of lithological heterogeneity in the mantle. Modally enriched peridotites may therefore play a more important role in oceanic magma genesis than previously inferred.

Experimental Phase Relations of Hydrous, Primitive Melts: Implications for variably depleted mantle melting in arcs and the generation of primitive high-SiO2 melts

NASA Astrophysics Data System (ADS)

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

2010-12-01

There has been considerable experimental and theoretical work on how the introduction of H2O-rich fluids into the mantle wedge affects partial melting in arcs and chemical evolution of mantle melts as they migrate through the mantle. Studies aimed at describing these processes have become largely quantitative, with an emphasis on creating models that suitably predict the production and evolution of melts and describe the thermal state of arcs worldwide. A complete experimental data set that explores the P-T conditions of melt generation and subsequent melt extraction is crucial to the development, calibration, and testing of these models. This work adds to that data set by constraining the P-T-H2O conditions of primary melt extraction from two end-member subduction zones, a continental arc (Mexico) and an intraoceanic arc (Aleutians). We present our data in context with primitive melts found worldwide and with other experimental studies of melts produced from fertile and variably depleted mantle sources. Additionally, we compare our experimental results to melt compositions predicted by empirical and thermodynamic models. We used a piston-cylinder apparatus and employed an inverse approach in our experiments, constraining the permissible mantle residues with which our melts could be in equilibrium. We confirmed our inverse approach with forced saturation experiments at the P-T-H2O conditions of melt-mantle equilibration. Our experimental results show that a primitive, basaltic andesite melt (JR-28) from monogenetic cinder cone Volcan Jorullo (Central Mexico) last equilibrated with a harzburgite mantle residue at 1.2-1.4 GPa and 1150-1175°C with H2O contents in the range of 5.5-7 wt% H2O prior to ascent and eruption. Phase relations of a tholeiitic high-MgO basaltic melt (ID-16) from the Central Aleutians (Okmok) show the conditions of last equilibration with a fertile lherzolite mantle residue at shallower (1.2 GPa) but hotter (1275°C) conditions with approximately 2 wt% H2O. Given the estimated crustal thicknesses of these two regions, our data suggest that both samples equilibrate with mantle minerals just below the Moho. Recent viscosity dependent thermal models that account for slab geometry suggest that JR-28 melts last equilibrate with harzburgite in a cooler region of the mantle wedge. In contrast, ID-16 equilibrated with a fertile source near the hotter core of the mantle wedge. Our results support the hypothesis that lherzolite melting (wet or dry) produces essentially basaltic melts, whereas more Si-rich primitive melts require shallow hydrous melting of harzburgite or reequilibration of basaltic melts with harzburgite in the uppermost part of the wedge.

The geochemical and Sr-Nd-Pb-He isotopic characterization of the mantle source of Rungwe Volcanic Province: comparison with the Afar mantle domain

NASA Astrophysics Data System (ADS)

Castillo, P. R.; Hilton, D. R.; Halldorsson, S. A.; Wang, R.

2012-12-01

The ultimate source of heat and magmatism associated with continental rifting in the East African Rift System (EARS) is generally viewed to be the African Superplume, but there is continuing debate on the surface expression of this large anomalous feature, which originates in the lower mantle. Previous studies have demonstrated an insignificant role for crustal contamination thereby identifying a single mantle plume signature in Quaternary basalts from the Main Ethiopian Rift in the northern EARS. This is designated to be the Afar plume and is characterized by, e.g., 3He/4He >15 RA, 206Pb/204Pb = 19.5 and 87Sr/86Sr = 0.7035 [Rooney et al., J. Pet. 53, 2012]. In contrast, the signature of plume(s) in the southern EARS is less constrained. Rogers et al. [EPSL 176, 2000] proposed a plume in the sub-lithospheric Kenyan mantle with characteristically lower 43Nd/144Nd than the Afar plume whereas Furman [JAES 48, 2007] advocated a high μ [HIMU] plume based primarily on the high 206Pb/204Pb ratios of lavas in all areas within and south of the Turkana Depression: both models assume a 3He/4He lower than the Afar plume. Here we report the trace element and Sr-Nd-Pb isotopic composition of basaltic lavas from the Rungwe Volcanic Province (RVP) in the southern extreme of the Western Rift previously identified as a high 3He/4He locality (~15 RA; [Hilton et al., GRL 38, 2011]). Trace element analyses are within the previously reported range of lava compositions that include a relatively large lithospheric component. More importantly, we identify correlations among incompatible trace element and isotopic ratios (e.g., 3He/4He vs 206Pb/204Pb, Rb/Sr, Nb/Ta; 87Sr/86Sr vs 208Pb/204Pb). Our new results suggest the presence of a distinct, high 3He/4He mantle source beneath RVP that is more radiogenic (e.g., 206Pb/204Pb up to ~19.8; 87Sr/86Sr up to 0.7055) than the Afar mantle plume. There is also very little or no HIMU signature in RPV basalts based on their high Sr and low Nd isotopic ratios.

Geochemistry of southern Pagan Island lavas, Mariana arc: The role of subduction zone processes

USGS Publications Warehouse

Marske, J.P.; Pietruszka, A.J.; Trusdell, F.A.; Garcia, M.O.

2011-01-01

New major and trace element abundances, and Pb, Sr, and Nd isotopic ratios of Quaternary lavas from two adjacent volcanoes (South Pagan and the Central Volcanic Region, or CVR) located on Pagan Island allow us to investigate the mantle source (i.e., slab components) and melting dynamics within the Mariana intra-oceanic arc. Geologic mapping reveals a pre-caldera (780-9.4ka) and post-caldera (<9.4ka) eruptive stage for South Pagan, whereas the eruptive history of the older CVR is poorly constrained. Crystal fractionation and magma mixing were important crustal processes for lavas from both volcanoes. Geochemical and isotopic variations indicate that South Pagan and CVR lavas, and lavas from the northern volcano on the island, Mt. Pagan, originated from compositionally distinct parental magmas due to variations in slab contributions (sediment and aqueous fluid) to the mantle wedge and the extent of mantle partial melting. A mixing model based on Pb and Nd isotopic ratios suggests that the average amount of sediment in the source of CVR (~2.1%) and South Pagan (~1.8%) lavas is slightly higher than Mt. Pagan (~1.4%) lavas. These estimates span the range of sediment-poor Guguan (~1.3%) and sediment-rich Agrigan (~2.0%) lavas for the Mariana arc. Melt modeling demonstrates that the saucer-shaped normalized rare earth element (REE) patterns observed in Pagan lavas can arise from partial melting of a mixed source of depleted mantle and enriched sediment, and do not require amphibole interaction or fractionation to depress the middle REE abundances of the lavas. The modeled degree of mantle partial melting for Agrigan (2-5%), Pagan (3-7%), and Guguan (9-15%) lavas correlates with indicators of fluid addition (e.g., Ba/Th). This relationship suggests that the fluid flux to the mantle wedge is the dominant control on the extent of partial melting beneath Mariana arc volcanoes. A decrease in the amount of fluid addition (lower Ba/Th) and extent of melting (higher Sm/Yb), and an increase in the sediment contribution (higher Th/Nb, La/Sm, and Pb isotopic ratios) from Mt. Pagan to South Pagan could reflect systematic cross-arc or irregular along-arc melting variations. These observations indicate that the length scale of compositional heterogeneity in the mantle wedge beneath Mariana arc volcanoes is small (~10km).

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

NASA Astrophysics Data System (ADS)

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

2013-06-01

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

Sulfur and Metal Fertilization of the Lower Continental Crust

NASA Technical Reports Server (NTRS)

Locmelis, Marek; Fiorentini, Marco L.; Rushmer, Tracy; Arevalo, Ricardo, Jr.; Adam, John; Denyszyn, Steven W.

2015-01-01

Mantle-derived melts and metasomatic fluids are considered to be important in the transport and distribution of trace elements in the subcontinental lithospheric mantle. However, the mechanisms that facilitate sulfur and metal transfer from the upper mantle into the lower continental crust are poorly constrained. This study addresses this knowledge gap by examining a series of sulfide- and hydrous mineral-rich alkaline mafic-ultramafic pipes that intruded the lower continental crust of the Ivrea-Verbano Zone in the Italian Western Alps. The pipes are relatively small (<300 m diameter) and primarily composed of a matrix of subhedral to anhedral amphibole (pargasite), phlogopite and orthopyroxene that enclose sub-centimeter-sized grains of olivine. The 1 to 5 m wide rim portions of the pipes locally contain significant blebby and disseminated Fe-Ni-Cu-PGE sulfide mineralization.Stratigraphic relationships, mineral chemistry, geochemical modeling and phase equilibria suggest that the pipes represent open-ended conduits within a large magmatic plumbing system. The earliest formed pipe rocks were olivine-rich cumulates that reacted with hydrous melts to produce orthopyroxene, amphibole and phlogopite.Sulfides precipitated as immiscible liquid droplets that were retained within a matrix of silicate crystals and scavenged metals from the percolating hydrous melt. New high-precision chemical abrasion TIMS-UPb dating of zircons from one of the pipes indicates that these pipes were emplaced at 249.1+/-0.2 Ma, following partial melting of lithospheric mantle pods that were metasomatized during the Eo-Variscan oceanic to continental subduction (approx. 420-310 Ma). The thermal energy required to generate partial melting of the metasomatized mantle was most likely derived from crustal extension, lithospheric decompression and subsequent asthenospheric rise during the orogenic collapse of the Variscan belt (<300 Ma). Unlike previous models, outcomes from this study suggest a significant temporal gap between the occurrence of mantle metasomatism, subsequent partial melting and emplacement of the pipes.We argue that this multi-stage process is a very effective mechanism to fertilize the commonly dry and refractory lower continental crust in metals and volatiles. During the four-dimensional evolution of the thermo-tectonic architecture of any given terrain, metals and volatiles stored in the lower continental crust may become available as sources for subsequent ore-forming processes, thus enhancing the prospectivity of continental block margins for a wide range of mineral systems.

Mantle shear-wave tomography and the fate of subducted slabs.

PubMed

Grand, Steven P

2002-11-15

A new seismic model of the three-dimensional variation in shear velocity throughout the Earth's mantle is presented. The model is derived entirely from shear bodywave travel times. Multibounce shear waves, core-reflected waves and SKS and SKKS waves that travel through the core are used in the analysis. A unique aspect of the dataset used in this study is the use of bodywaves that turn at shallow depths in the mantle, some of which are triplicated. The new model is compared with other global shear models. Although competing models show significant variations, several large-scale structures are common to most of the models. The high-velocity anomalies are mostly associated with subduction zones. In some regions the anomalies only extend into the shallow lower mantle, whereas in other regions tabular high-velocity structures seem to extend to the deepest mantle. The base of the mantle shows long-wavelength high-velocity zones also associated with subduction zones. The heterogeneity seen in global tomography models is difficult to interpret in terms of mantle flow due to variations in structure from one subduction zone to another. The simplest interpretation of the seismic images is that slabs in general penetrate to the deepest mantle, although the flow is likely to be sporadic. The interruption in slab sinking is likely to be associated with the 660 km discontinuity.

The evolution of mollusc shells.

PubMed

McDougall, Carmel; Degnan, Bernard M

2018-05-01

Molluscan shells are externally fabricated by specialized epithelial cells on the dorsal mantle. Although a conserved set of regulatory genes appears to underlie specification of mantle progenitor cells, the genes that contribute to the formation of the mature shell are incredibly diverse. Recent comparative analyses of mantle transcriptomes and shell proteomes of gastropods and bivalves are consistent with shell diversity being underpinned by a rapidly evolving mantle secretome (suite of genes expressed in the mantle that encode secreted proteins) that is the product of (a) high rates of gene co-option into and loss from the mantle gene regulatory network, and (b) the rapid evolution of coding sequences, particular those encoding repetitive low complexity domains. Outside a few conserved genes, such as carbonic anhydrase, a so-called "biomineralization toolkit" has yet to be discovered. Despite this, a common suite of protein domains, which are often associated with the extracellular matrix and immunity, appear to have been independently and often uniquely co-opted into the mantle secretomes of different species. The evolvability of the mantle secretome provides a molecular explanation for the evolution and diversity of molluscan shells. These genomic processes are likely to underlie the evolution of other animal biominerals, including coral and echinoderm skeletons. This article is categorized under: Comparative Development and Evolution > Regulation of Organ Diversity Comparative Development and Evolution > Evolutionary Novelties. © 2018 Wiley Periodicals, Inc.

The hottest lavas of the Phanerozoic from a reservoir at the core-mantle boundary

NASA Astrophysics Data System (ADS)

Gazel, E.; Trela, J.; Sobolev, A. V.; Bizimis, M.; Jicha, B. R.; Batanova, V. G.

2017-12-01

Petrologic models suggest that modern plume-derived melts generate at high mantle temperatures (>1500 °C) relative to those produced at ambient mid-ocean ridge conditions ( 1350 °C). Earth's mantle has cooled during its history due to heat loss and decrease in radioactive heat production, thus the temperatures of these modern-day basalts are substantially lower than those produced during the Archean (>2.5 Ga), as recorded by komatiites (>1700 °C). Surprisingly, we discovered that the 90 Ma Galapagos-related Tortugal Suite accreted in Costa Rica not only records mantle potential temperatures as high as ancient Archean komatiites ( 1800 °C), but also the highest olivine-spinel crystallization temperatures ever reported ( 1600 °C). These new results from Tortugal (and other anomalously hot Phanerozoic locations) imply that if the mantle is still producing melts as hot as during the Archean, then there must exist reservoirs that preserve Archean temperatures at the base of the lower mantle. These anomalously hot reservoirs could be sustained over time by the steady-state temperature conditions at the core-mantle boundary buffered by the crystallization of Earth's core. Although our results suggest that even modern plumes can produce melts at similar P-T conditions as inferred during the Archean, these occurrences are not common as plumes will likely interact with the cooler ambient mantle.

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

NASA Astrophysics Data System (ADS)

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

2011-12-01

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

Evidence for an abrupt transition in the mantle-derived source to the Long Valley Caldera rhyolites after the climactic eruption: from subduction-modified lithosphere to asthenosphere

NASA Astrophysics Data System (ADS)

Waters, L.; Lange, R. A.

2014-12-01

Shortly after the climactic eruption of ~600 km3 of Bishop Tuff zoned rhyolitic magma, ~100 km3 of crystal-poor Early Rhyolite erupted inside Long Valley Caldera between ~750-650 ka as domes, glassy lavas, and tuffs (Hildreth, 2004). Despite similarities in bulk composition (e.g., 73-75 wt% SiO2; ~100 ppm Sr), there are marked differences between the Late (≥ 790°C) Bishop Tuff and postcaldera Early Rhyolites. Although crystal-poor (<5%), the Early Rhyolites are often saturated with 7-8 mineral phases (plag + opx + ilm + tmte + biotite + apatite + zircon ± pyrrhotite), but without the quartz, sanidine, and cpx additionally found in the more crystal-rich (12-24%) Late Bishop Tuff. Pre-eruptive temperatures, on the basis of two Fe-Ti oxides, range from 720-860°C, and ΔNNO values range from-0.4 to -0.9 (consistent with abundant ilmenite). Thus the Early Rhyolites record fO2 values that are nearly two orders of magnitude lower than those in the Late Bishop Tuff (ΔNNO = +1; Hildreth and Wilson, 2007). Application of the plagioclase-liquid hygrometer to Early Rhyolites gives pre-eruptive water contents ≤ 4.4 wt% H2O. The phenocrysts in Early Rhyolite obsidians often display euhedral and/or diffusion-limited growth textures, suggesting degassing-induced crystallization during rapid ascent. Isotopic data from the literature (e.g., Simon et al., 2014 and references therein) show that Long Valley rhyolites were derived from both crustal and mantle sources. We hypothesize that the drop in fO2 between the Late Bishop Tuff and Early Rhyolites may reflect a transition in their respective mantle source, from subduction-modified lithosphere to asthenosphere. Such a time-progressive transition in the mantle source of erupted basalts is seen throughout the Great Basin, occurring earliest in its central region and more recently toward its western margin (e.g. Cousens et al., 2012). Although the geochemistry of Quaternary basalts erupted around Long Valley indicate a subduction-modified lithosphere source (Cousens, 1996), the Early Rhyolites may be recording the crustal emplacement of basalts from the asthenosphere before any have yet erupted. If so, the Early Rhyolites may be derived from a greater proportion of crustal sources than calculated from isotopic data on the assumption of a lithospheric mantle source.

Osmium Isotope Compositions of Komatiite Sources Through Time

NASA Astrophysics Data System (ADS)

Walker, R. J.

2001-12-01

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

Extreme isotopic variations in the upper mantle: evidence from Ronda

NASA Astrophysics Data System (ADS)

Reisberg, Laurie; Zindler, Alan

1986-12-01

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

Multi-stage mixing in subduction zone: Application to Merapi volcano, Indonesia

NASA Astrophysics Data System (ADS)

Debaille, V.; Doucelance, R.; Weis, D.; Schiano, P.

2003-04-01

Basalts sampling subduction zone volcanism (IAB) often show binary mixing relationship in classical Sr-Nd, Pb-Pb, Sr-Pb isotopic diagrams, generally interpreted as reflecting the involvement of two components in their source. However, several authors have highlighted the presence of minimum three components in such a geodynamical context: mantle wedge, subducted and altered oceanic crust and subducted sediments. The overlying continental crust can also contribute by contamination and assimilation in magma chambers and/or during magma ascent. Here we present a multi-stage model to obtain a two end-member mixing from three components (mantle wedge, altered oceanic crust and sediments). The first stage of the model considers the metasomatism of the mantle wedge by fluids and/or melts released by subducted materials (altered oceanic crust and associated sediments), considering mobility and partition coefficient of trace elements in hydrated fluids and silicate melts. This results in the generation of two distinct end-members, reducing the number of components (mantle wedge, oceanic crust, sediments) from three to two. The second stage of the model concerns the binary mixing of the two end-members thus defined: mantle wedge metasomatized by slab-derived fluids and mantle wedge metasomatized by sediment-derived fluids. This model has been applied on a new isotopic data set (Sr, Nd and Pb, analyzed by TIMS and MC-ICP-MS) of Merapi volcano (Java island, Indonesia). Previous studies have suggested three distinct components in the source of indonesian lavas: mantle wedge, subducted sediments and altered oceanic crust. Moreover, it has been shown that crustal contamination does not significantly affect isotopic ratios of lavas. The multi-stage model proposed here is able to reproduce the binary mixing observed in lavas of Merapi, and a set of numerical values of bulk partition coefficient is given that accounts for the genesis of lavas.

Neogene volcanism associated with back-arc basin tectonics at the northern Fossa Magna, NE Japan

NASA Astrophysics Data System (ADS)

Okamura, S.; Inaba, M.; Shinjo, R.; Adachi, Y.

2016-12-01

New isotopic and trace element data presented here imply a temporal change in magma sources and thermal conditions beneath the northern Fossa Magna of NE Japan from the Miocene to the Pliocene. Rocks from more sediment melt-rich Early Miocene volcanoes have less radiogenic 176Hf/177Hf and 143Nd/144Nd, high Zr/Hf, and little or no Hf anomaly (Hf/Hf*; ˜1.0). The mantle wedge in the Early Miocene consisted of enriched mantle source. We propose that during the onset of subduction, influx of hot asthenospheric mantle provided sufficient heat to partially melt newly subducting sediment. Geochemical modeling results suggest breakdown of zircon in the slab surface sediments for the Early Miocene lavas in the northern Fossa Magna region. In the Middle Miocene, the injection of hot and depleted asthenospheric material replaced the mantle beneath the northern Fossa Magna region of NE Japan. This caused the isotopic signature of the rocks to change from enriched to depleted. The Middle Miocene lavas characterized by most radiogenic Hf and Nd isotope ratios, have high Zr/Hf, low Lu/Hf, and little or no Hf anomaly. An appropriate working petrogenetic model is that the Middle Miocene lavas were derived from asthenospheric depleted mantle, slightly ( < 1%) contaminated by slab melt accompanied by full dissolution of zircon. All the Late Miocene and Pliocene samples are characterized by distinctly more radiogenic 176Hf/177Hf and 143Nd/144Nd, and more negative Hf anomalies (greater Hf/Hf* variability; ˜0.3). The Pliocene samples are displaced toward lower Hf/Hf* and Zr/Hf, and higher Lu/Hf relative to the Middle Miocene samples, which requires mixing between depleted mantle and a partial melt of subducted metasediment saturated with trace quantity of zircon.

Search for seismic discontinuities in the lower mantle

NASA Astrophysics Data System (ADS)

Vinnik, Lev; Kato, Mamoru; Kawakatsu, Hitoshi

2001-09-01

Indications of lower mantle discontinuities have been debated for decades, but still little is known about their properties, and their origins are enigmatic. In our study broad-band recordings of deep events are examined for the presence of signals from the lower-mantle discontinuities with a novel technique. We deconvolve vertical component of the P-wave coda in the period range around 10s by the S waveform and stack many deconvolved traces with moveout time corrections. In synthetic seismograms for an earth model without lower mantle discontinuities, the strongest signal thus detected in the time window of interest is often s`410'P phase (generated as S and reflected as P from the `410km' discontinuity above the source). In actual seismograms there are other phases that can be interpreted as converted from S to P at discontinuities in the lower mantle beneath the seismic source. We summarize the results of processing the seismograms (1) of deep events in Sunda arc at seismograph stations in east Asia, (2) deep Kermadec-Fiji-Tonga events at the J-array and FREESIA networks in Japan and stations in east Asia, and (3) deep events in the northwest Pacific region (Mariana, Izu-Bonin and the Japan arc) recorded at stations in north America. In our data there are indications of discontinuities near 860-880, 1010-1120, 1170-1250 and 1670-1800km depths. The clearest signals are obtained from the discontinuity at a depth of 1200km. We argue that the `900', `1200' and `1700km' discontinuities are global, but laterally variable in both depth and strength. Seismic stratification of the lower mantle may have bearings on the patterns of subduction, as revealed by tomographic models.

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.

Multiple mantle upwellings in the transition zone beneath the northern East-African Rift system from relative P-wave travel-time tomography

NASA Astrophysics Data System (ADS)

Civiero, Chiara; Hammond, James O. S.; Goes, Saskia; Fishwick, Stewart; Ahmed, Abdulhakim; Ayele, Atalay; Doubre, Cecile; Goitom, Berhe; Keir, Derek; Kendall, J.-Michael; Leroy, Sylvie; Ogubazghi, Ghebrebrhan; Rümpker, Georg; Stuart, Graham W.

2015-09-01

Mantle plumes and consequent plate extension have been invoked as the likely cause of East African Rift volcanism. However, the nature of mantle upwelling is debated, with proposed configurations ranging from a single broad plume connected to the large low-shear-velocity province beneath Southern Africa, the so-called African Superplume, to multiple lower-mantle sources along the rift. We present a new P-wave travel-time tomography model below the northern East-African, Red Sea, and Gulf of Aden rifts and surrounding areas. Data are from stations that span an area from Madagascar to Saudi Arabia. The aperture of the integrated data set allows us to image structures of ˜100 km length-scale down to depths of 700-800 km beneath the study region. Our images provide evidence of two clusters of low-velocity structures consisting of features with diameter of 100-200 km that extend through the transition zone, the first beneath Afar and a second just west of the Main Ethiopian Rift, a region with off-rift volcanism. Considering seismic sensitivity to temperature, we interpret these features as upwellings with excess temperatures of 100 ± 50 K. The scale of the upwellings is smaller than expected for lower mantle plume sources. This, together with the change in pattern of the low-velocity anomalies across the base of the transition zone, suggests that ponding or flow of deep-plume material below the transition zone may be spawning these upper mantle upwellings. This article was corrected on 28 SEP 2015. See the end of the full text for details.

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

NASA Astrophysics Data System (ADS)

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

2018-01-01

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

A model for osmium isotopic evolution of metallic solids at the core-mantle boundary

NASA Astrophysics Data System (ADS)

Humayun, Munir

2011-03-01

Some plumes are thought to originate at the core-mantle boundary, but geochemical evidence of core-mantle interaction is limited to Os isotopes in samples from Hawaii, Gorgona (89 Ma), and Kostomuksha (2.7 Ga). The Os isotopes have been explained by physical entrainment of Earth's liquid outer core into mantle plumes. This model has come into conflict with geophysical estimates of the timing of core formation, high-pressure experimental determinations of the solid metal-liquid metal partition coefficients (D), and the absence of expected 182W anomalies. A new model is proposed where metallic liquid from the outer core is partially trapped in a compacting cumulate pile of Fe-rich nonmetallic precipitates (FeO, FeS, Fe3Si, etc.) at the top of the core and undergoes fractional crystallization precipitating solid metal grains, followed by expulsion of the residual metallic liquid back to the outer core. The Os isotopic composition of the solids and liquids in the cumulate pile is modeled as a function of the residual liquid remaining and the emplacement age using 1 bar D values, with variable amounts of oxygen (0-10 wt %) as the light element. The precipitated solids evolve Os isotope compositions that match the trends for Hawaii (at an emplacement age of 3.5-4.5 Ga; 5%-10% oxygen) and Gorgona (emplacement age < 1.5 Ga; 0%-5% oxygen). The Fe-rich matrix of the cumulate pile dilutes the precipitated solid metal decoupling the Fe/Mn ratio from Os and W isotopes. The advantages to using precipitated solid metal as the Os host include a lower platinum group element and Ni content to the mantle source region relative to excess iron, miniscule anomalies in 182W (<0.1 ɛ), and no effects for Pb isotopes, etc. A gradual thermomechanical erosion of the cumulate pile results in incorporation of this material into the base of the mantle, where mantle plumes subsequently entrain it. Fractional crystallization of metallic liquids within the CMB provides a consistent explanation of both Os isotope correlations, Os-W isotope systematics, and Fe/Mn evidence for core-mantle interaction over the entire Hawaiian source.

Modeling Earth's surface topography: decomposition of the static and dynamic components

NASA Astrophysics Data System (ADS)

Guerri, M.; Cammarano, F.; Tackley, P. J.

2017-12-01

Isolating the portion of topography supported by mantle convection, the so-called dynamic topography, would give us precious information on vigor and style of the convection itself. Contrasting results on the estimate of dynamic topography motivate us to analyse the sources of uncertainties affecting its modeling. We obtain models of mantle and crust density, leveraging on seismic and mineral physics constraints. We use the models to compute isostatic topography and residual topography maps. Estimates of dynamic topography and associated synthetic geoid are obtained by instantaneous mantle flow modeling. We test various viscosity profiles and 3D viscosity distributions accounting for inferred lateral variations in temperature. We find that the patterns of residual and dynamic topography are robust, with an average correlation coefficient of 0.74 and 0.71, respectively. The amplitudes are however poorly constrained. For the static component, the considered lithospheric mantle density models result in topographies that differ, on average, 720 m, with peaks reaching 1.7 km. The crustal density models produce variations in isostatic topography averaging 350 m, with peaks of 1 km. For the dynamic component, we obtain peak-to-peak topography amplitude exceeding 3 km for all the tested mantle density and viscosity models. Such values of dynamic topography produce geoid undulations that are not in agreement with observations. Assuming chemical heterogeneities in the lower mantle, in correspondence with the LLSVPs (Large Low Shear wave Velocity Provinces), helps to decrease the amplitudes of dynamic topography and geoid, but reduces the correlation between synthetic and observed geoid. The correlation coefficients between the residual and dynamic topography maps is always less than 0.55. In general, our results indicate that, i) current knowledge of crust density, mantle density and mantle viscosity is still limited, ii) it is important to account for all the various sources of uncertainties when computing static and dynamic topography. In conclusion, a multidisciplinary approach, which involves multiple geophysics observations and constraints from mineral physics, is necessary for obtaining robust density models and, consequently, for properly estimating the dynamic topography.

Transfer Rates of Magma From Planetary Mantles to the Surface.

NASA Astrophysics Data System (ADS)

Wilson, L.; Head, J. W.; Parfitt, E. A.

2008-12-01

We discuss the speed at which magma can be transferred to a planetary surface from the deep interior. Current literature describes a combination of slow percolation of melt in the mantle where convection-driven pressure-release melting is occurring, concentration of melt by source region deformation, initiation and growth of magma-filled brittle fractures (dikes) providing wider pathways for melt movement, additional growth and interconnection of dikes with decreasing depth, rise of magma to storage zones (reservoirs) located at levels of neutral buoyancy at the base of or within the crust, and transfer from the storage zones in dikes to feed eruptions or intrusions. We do not take issue with these mechanisms but think that their relative importance in various circumstances is poorly appreciated. On Earth, preservation of diamonds in kimberlites implies very rapid (hours) transfer of melts from depths of 100-300 km, and there is strong geochemical evidence that magmas at mid-ocean ridges reach shallow depths faster than is possible by percolation alone. On the Moon, the petrology of pyroclasts involved in dark-mantle-forming eruptions implies rapid (again probably hours) magma transfer from depths of up to 400 km. The ureilite meteorites, samples of the mantle of a disrupted asteroid 200 km in diameter, have compositions only consistent with the rapid (months) extraction of mafic melt from the mantle. All of these examples imply that brittle fractures (dikes) can sometimes be initiated at depths where mantle rheology would normally be expected to be plastic rather than elastic, and that melt can be fed into these dikes extremely efficiently. Further evidence for this is provided by the giant radial dike swarms observed on Earth, Mars and Venus. The dikes observed (on Earth) and inferred from the presence of radiating graben systems (Mars) and radiating fracture and graben systems (Venus) are so voluminous that they can only be understood if they are fed from extremely large magma reservoirs, probably located at the base of the crust, that are supplied from the mantle (i.e. buffered) while the dikes are being emplaced, again implying extremely efficient melt extraction from mantle source regions.

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

NASA Astrophysics Data System (ADS)

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

2018-03-01

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

Can basal magma oceans generate magnetic fields?

NASA Astrophysics Data System (ADS)

Stegman, D. R.; Ziegler, L. B.; Davies, C.

2015-12-01

Earth's magnetic field is very old, with recent data now showing the field possibly extended back to 4.1 billion years ago (Tarduno et al., Science, 2015). Yet, based upon our current knowledge there are difficulties in sustained a core dynamo over most of Earth's history. Moreover, recent estimates of thermal and electrical conductivity of liquid iron at core conditions from mineral physics experiments indicate that adiabatic heat flux is approximately 15 TW, nearly 3 times larger than previously thought, exacerbating difficulties for driving a core dynamo by convective core cooling alone throughout Earth history. A long-lived basal magma ocean in the lowermost mantle has been proposed to exist in the early Earth, surviving perhaps into the Archean. While the modern, solid lower mantle is an electromagnetic insulator, electrical conductivities of silicate melts are known to be higher, though as yet they are unconstrained for lowermost mantle conditions. Here we explore the geomagnetic consequences of a basal magma ocean layer for a range of possible electrical conductivities. For the highest electrical conductivities considered, we find a basal magma ocean could be a primary dynamo source region. This would suggest the proposed three magnetic eras observed in paleomagnetic data originate from distinct sources for dynamo generation: from 4.5-2.45 Ga within a basal magma ocean, from 2.25-0.4 Ga within a superadiabatically cooled liquid core, and from 0.4-present within a quasi-adiabatic core that includes a solidifying inner core. We have extended this work by developing a new code, Dynamantle, which is a model with an entropy-based approach, similar to those commonly used in core dynamics models. We present new results using this code to assess the conditions under which basal magma oceans can generate positive ohmic dissipation. This is more generally useful than just considering the early Earth, but also for many silicate exoplanets in which basal magma oceans are even more likely to exist.

Compositional diversity in peridotites as result of a multi-process history: The Pacific-derived Santa Elena ophiolite, northwest Costa Rica

NASA Astrophysics Data System (ADS)

Escuder-Viruete, Javier; Baumgartner, Peter O.; Castillo-Carrión, Mercedes

2015-08-01

The Santa Elena ophiolite (SEO) is an ultramafic nappe of more than 270 km2 overlying a tectonic serpentinite-matrix mélange in northwest Costa Rica. It is mainly composed of Cpx-rich and Cpx-poor harzburgites (~ 2.5 km-thick), with minor lherzolite, dunite and chromitite, as well as intrusive mafic sills and subvertical dikes, which coalesce into an upper Isla Negritos gabbroic sill complex. Minerals and whole-rock features of the Cpx-rich and Cpx-poor harzburgites share features of the abyssal and supra-subduction zone (SSZ) peridotites, respectively. To explain these characteristics two-stages of melting and refertilization processes are required. By means of trace element modeling, the composition of Cpx-rich harzburgites may be reproduced by up to ~ 5-10% melting of a primitive mantle source, and the composition of Cpx-poor harzburgites and dunites by ~ 15-18% melting of an already depleted mantle. Therefore, the Cpx-rich harzburgites can be interpreted as product of first-stage melting and low-degrees of melt-rock interaction in a mid-ocean ridge environment, and the Cpx-poor harzburgites and dunites as the product of second-stage melting and refertilization in a SSZ setting. The mafic sills and the Isla Negrito gabbros are genetically related and can be explained as crystallization from the liquids that were extracted from the lower SSZ mantle levels and emplaced at shallow conditions. The Murciélagos Island basalts are not directly related to the ultramafic and mafic rocks of the SEO. Their E-MORB-like composition is similar to most of the CLIP mafic lavas and suggests a common Caribbean plume-related source. The SEO represents a fragment of Pacific-derived, SSZ oceanic lithosphere emplaced onto the southern North America margin during the late Cretaceous. Because of the predominance of rollback-induced extension during its history, only a limited amount of crustal rocks were formed and preserved in the SEO.

Geochemistry of Woranso-Mille Pliocene basalts from west-central Afar, Ethiopia: Implications for mantle source characteristics and rift evolution

NASA Astrophysics Data System (ADS)

Alene, Mulugeta; Hart, William K.; Saylor, Beverly Z.; Deino, Alan; Mertzman, Stanley; Haile-Selassie, Yohannes; Gibert, Luis B.

2017-06-01

The Woranso-Mille (WORMIL) area in the west-central Afar, Ethiopia, contains several Pliocene basalt flows, tuffs, and fossiliferous volcaniclastic beds. We present whole-rock major- and trace-element data including REE, and Sr-Nd-Pb isotope ratios from these basalts to characterize the geochemistry, constrain petrogenetic processes, and infer mantle sources. Six basalt groups are distinguished stratigraphically and geochemically within the interval from 3.8 to 3 Ma. The elemental and isotopic data show intra- and inter-group variations derived primarily from source heterogeneity and polybaric crystallization ± crustal inputs. The combined Sr-Nd-Pb isotope data indicate the involvement of three main reservoirs: the Afar plume, depleted mantle, and enriched continental lithosphere (mantle ± crust). Trace element patterns and ratios further indicate the basalts were generated from spinel-dominated shallow melting, consistent with significantly thinned Pliocene lithosphere in western Afar. The on-land continuation of the Aden rift into western Afar during the Pliocene is reexamined in the context of the new geochemistry and age constraints of the WORMIL basalts. The new data reinforce previous interpretations that progressive rifting and transformation of the continental lithosphere to oceanic lithosphere allows for increasing asthenospheric inputs through time as the continental lithosphere is thinned. Accepted trace element values for BHVO-2 are those recently recommended by Jochum et al. (2016) rounded to provide the same significant figures as the data. Ternary model after Schilling et al. (1992); Endmembers from Rooney et al. (2012).

Jurassic rifting at the Eurasian Tethys margin: Geochemical and geochronological constraints from granitoids of North Makran, southeastern Iran

NASA Astrophysics Data System (ADS)

Hunziker, Daniela; Burg, Jean-Pierre; Bouilhol, Pierre; von Quadt, Albrecht

2015-03-01

This study focuses on an east-west trending belt of granitic to intermediate intrusions and their volcanic cover in the northern Dur Kan Complex, a continental slice outcropping to the north of the exposed Makran accretionary wedge in southeastern Iran. Field observations, petrographic descriptions, trace element, and isotope analyses combined with U-Pb zircon geochronology are presented to determine the time frame of magmatism and tectonic setting during the formation of these rocks. Results document three magmatic episodes with different melt sources for (1) granites, (2) a diorite-trondhjemite-plagiogranite sequence, and (3) diabases and lavas. Granites, dated at 170-175 Ma, represent crystallized melt with a strong continental isotopic contribution. The diorite-trondhjemite-plagiogranite sequence is 165-153 Ma old and derives from a mantle magma source with minor continental contribution. East-west trending diabase dikes and bodies intruded the granitoids, which were eroded and then covered by Valanginian (140-133 Ma) alkaline lavas and sediments. Alkaline dikes and lavas have a mantle isotopic composition. Temporal correlation with plutonites of the Sanandaj-Sirjan Zone to the northwest defines a narrow, NW-SE striking and nearly 2000 km long belt of Jurassic intrusions. The increasing mantle influence in the magma sources is explained by thinning of continental lithosphere and related mantle upwelling/decompression melting. Accordingly, the formation of the studied igneous rocks is related to the extension of the Iranian continental margin, which ultimately led to the formation of the Tethys-related North Makran Ophiolites.

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.

Nd-Sr-Pb isotopic variations along the Gulf of Aden - Evidence for Afar mantle plume-continental lithosphere interaction

NASA Astrophysics Data System (ADS)

Schilling, Jean-Guy; Kingsley, Richard H.; Hanan, Barry B.; McCully, Brian L.

1992-07-01

The rare-earth-element concentrations and Nd, Sr, and Pb isotopic compositions of the basalts in the Gulf of Aden are described and related to asthenospheric and lithospheric interactions with a thermal toruslike plume. Specific attention is given to the spatial and temporal traits of the mantle sources, and isotopic and geochemical data are used to determine the extent to which basaltic volcanism is derived from a mantle plume, the mantle lithosphere, and upwelling of the depleted atmosphere. The impingement and dispersion of a plume head is confirmed beneath the Afar region, and the geological record shows continental stretching and rifting prior to the impingement in the outskirts of the Horn of Africa. The data suggest that the isotopic variations along the Gulf of Aden/Red Sea/Ethiopia Rift system can be explained by the interaction of a thermal toruslike plume with the depleted asthenosphere and the overlying continental mantle lithosphere.

Upper mantle structure at Walvis Ridge from Pn tomography

NASA Astrophysics Data System (ADS)

Ryberg, Trond; Braeuer, Benjamin; Weber, Michael

2017-10-01

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

Composition, structure and chemistry of interstellar dust

NASA Technical Reports Server (NTRS)

Tielens, Alexander G. G. M.; Allamandola, Louis J.

1986-01-01

The observational constraints on the composition of the interstellar dust are analyzed. The dust in the diffuse interstellar medium consists of a mixture of stardust (amorphous silicates, amorphous carbon, polycyclic aromatic hydrocarbons, and graphite) and interstellar medium dust (organic refractory material). Stardust seems to dominate in the local diffuse interstellar medium. Inside molecular clouds, however, icy grain mantles are also important. The structural differences between crystalline and amorphous materials, which lead to differences in the optical properties, are discussed. The astrophysical consequences are briefly examined. The physical principles of grain surface chemistry are discussed and applied to the formation of molecular hydrogen and icy grain mantles inside dense molecular clouds. Transformation of these icy grain mantles into the organic refractory dust component observed in the diffuse interstellar medium requires ultraviolet sources inside molecular clouds as well as radical diffusion promoted by transient heating of the mantle. The latter process also returns a considerable fraction of the molecules in the grain mantle to the gas phase.

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.

Magnetic resonance imaging with pathological correlation in a case of mantle cell lymphoma of the parotid gland: a case report

PubMed Central

2010-01-01

Introduction Mantle cell lymphoma is a rare non-Hodgkin's lymphoma. It is a subtype of B-cell lymphoma with frequent involvement of the bone marrow and the gastrointestinal tract. Isolated parotid gland involvement seldom occurs. Here we report an unusual case of isolated infiltration of the parotid gland by mantle cell lymphoma. The aim of our study is to correlate magnetic resonance imaging findings with the histological features of the disease. To the best of our knowledge, no similar radiological findings of mantle cell lymphoma have been published before. Case presentation A 72-year-old Caucasian woman presented with a painful left parotid enlargement. She was diagnosed with mantle cell lymphoma involving the left submandibular gland seven years prior to presentation. Her whole body CT scan showed the absence of pathologically enlarged lymph nodes. However, a magnetic resonance imaging showed enlargement of her left parotid gland and an abnormal parenchyma with mixed-type solid and cystic lesions. A biopsy of her left parotid gland and subsequent histological examination confirmed a mantle cell lymphoma (common variant) relapse. Conclusion Although rare, the involvement of parotid gland with mantle cell lymphoma must be considered in the differential diagnosis of parotid tumors. PMID:20350332

Upper-mantle origin of the Yellowstone hotspot

USGS Publications Warehouse

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

2002-01-01

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

Mantle transition zone structure beneath India and Western China from migration of PP and SS precursors

NASA Astrophysics Data System (ADS)

Lessing, Stephan; Thomas, Christine; Rost, Sebastian; Cobden, Laura; Dobson, David P.

2014-04-01

We investigate the seismic structure of the upper-mantle and mantle transition zone beneath India and Western China using PP and SS underside reflections off seismic discontinuities, which arrive as precursors to the PP and SS arrival. We use high-resolution array seismic techniques to identify precursory energy and to map lateral variations of discontinuity depths. We find deep reflections off the 410 km discontinuity (P410P and S410S) beneath Tibet, Western China and India at depths of 410-440 km and elevated underside reflections of the 410 km discontinuity at 370-390 km depth beneath the Tien Shan region and Eastern Himalayas. These reflections likely correspond to the olivine to wadsleyite phase transition. The 410 km discontinuity appears to deepen in Central and Northern Tibet. We also find reflections off the 660 km discontinuity beneath Northern China at depths between 660 and 700 km (P660P and S660S) which could be attributed to the mineral transformation of ringwoodite to magnesiowuestite and perovskite. These observations could be consistent with the presence of cold material in the middle and lower part of the mantle transition zone in this region. We also find a deeper reflector between 700 and 740 km depth beneath Tibet which cannot be explained by a depressed 660 km discontinuity. This structure could, however, be explained by the segregation of oceanic crust and the formation of a neutrally buoyant garnet-rich layer beneath the mantle transition zone, due to subduction of oceanic crust of the Tethys Ocean. For several combinations of sources and receivers we do not detect arrivals of P660P and S660S although similar combinations of sources and receivers give well-developed P660P and S660S arrivals. Our thermodynamic modelling of seismic structure for a range of compositions and mantle geotherms shows that non-observations of P660P and S660S arrivals could be caused by the dependence of underside reflection coefficients on the incidence angle of the incoming seismic waves. Apart from reflections off the 410 and 660 km discontinuities, we observe intermittent reflectors at 300 and 520 km depth. The discontinuity structure of the study region likely reflects lateral thermal and chemical variations in the upper-mantle and mantle transition zone connected to past and present subduction and mantle convection processes.

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

NASA Astrophysics Data System (ADS)

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

2017-12-01

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

Picrite "Intelligence" from the Middle-Late Triassic Stikine arc: Composition of mantle wedge asthenosphere

NASA Astrophysics Data System (ADS)

Milidragovic, D.; Zagorevski, A.; Weis, D.; Joyce, N.; Chapman, J. B.

2018-05-01

Primitive, near-primary arc magmas occur as a volumetrically minor ≤100 m thick unit in the Canadian Cordillera of northwestern British Columbia, Canada. These primitive magmas formed an olivine-phyric, picritic tuff near the base of the Middle-Late Triassic Stuhini Group of the Stikine Terrane (Stikinia). A new 40Ar/39Ar age on hornblende from a cross-cutting basaltic dyke constrains the tuff to be older than 221 ± 2 Ma. An 87Sr/86Sr isochron of texturally-unmodified tuff samples yields 212 ± 25 Ma age, which is interpreted to represent syn-depositional equilibration with sea-water. Parental trace element magma composition of the picritic tuff is strongly depleted in most incompatible trace elements relative to MORB and implies a highly depleted ambient arc mantle. High-precision trace element and Hf-Nd-Pb isotopic analyses indicate an origin by mixing of a melt of depleted ambient asthenosphere with ≤2% of subducted sediment melt. Metasomatic addition of non-conservative incompatible elements through melting of subducted Panthalassa Ocean floor sediments accounts for the arc signature of the Stuhini Group picritic tuff, enrichment of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) and high field strength elements (HFSE), and anomalous enrichment in Pb. The inferred Panthalassan sediments are similar in composition to the Neogene-Quaternary sediments of the modern northern Cascadia Basin. The initial Hf isotopic composition of the picritic tuff closely approximates that of the ambient Middle-Late Triassic asthenosphere beneath Stikinia and is notably less radiogenic than the age-corrected Hf isotopic composition of the Depleted (MORB) Mantle reservoir (DM or DMM). This suggests that the ambient asthenospheric mantle end-member experienced melt depletion (F ≤ 0.05) a short time before picrite petrogenesis. The mantle end-member in the source of the Stuhini Group picritic tuff is isotopically similar to the mantle source of enriched mid-ocean ridge basalts (E-MORB) erupted today at the southern end of the Explorer Ridge in northeastern Pacific Ocean. The isotopic similarity between the Middle-Late Triassic ambient mantle under Stikinia, and mantle presently tapped at the southern Explorer Ridge suggests that enriched domains in the northeastern Pacific mantle are long-lived (≥222 million years).

Crustal and Upper Mantle Velocity and Q Structures of Mainland China

DTIC Science & Technology

1979-11-01

CLASIFICATION OFTHIS PAGE(117..t- [).(t ntred) with identical source-receiver geometry. The generalized surface wave inversion technique was applied...in the recent past. A particularly unusual crustal and upper mantle structure is found underlying the Tibet Dlateau. AOceSIon For DDC TAB Ubazmnounced...the AIR FORCE OFFICE OF SCIENTIFIC RESEARCH by the GEOPHYSICAL LABORATORY UNIVERSITY OF SOUTHERN CALIFORNIA Contractor: University of Southern

Geochemical Overview of the East African Rift System

NASA Astrophysics Data System (ADS)

Furman, T.

2003-12-01

Mafic volcanics of the East African Rift System (EARS) record a protracted history of continental extension that is linked to mantle plume activity. The modern EARS traverses two post-Miocene topographic domes separated by a region of polyphase extension in northern Kenya and southern Ethiopia. Basaltic magmatism commenced ˜45 Ma in this highly extended region, while the onset of plume-related activity took place ˜30 Ma with eruption of flood basalts in central Ethiopia. A spatial and temporal synthesis of EARS volcanic geochemistry shows progressive lithospheric removal (by erosion and melting) as the degree of rifting increases, with basalts in the most highly extended areas recording melting of depleted asthenosphere. Plume contributions are indicated locally in the northern half of the EARS, but are absent from the southern half. The geochemical signatures are compatible with a physical model in which the entire EARS is fed by a discontinuous plume emanating from the core-mantle boundary as the South African Superswell. Quaternary basaltic lavas erupted in the Afar triangle, Red Sea and Gulf of Aden define the geochemical signature attributed to the Afar plume (87Sr/86Sr 0.7034-0.7037, 143Nd/144Nd 0.5129-0.5130; La/Nb 0.6-0.9; Nb/U 40-50). These suites commonly record mixing with ambient upper mantle having less radiogenic isotopes but generally overlapping incompatible trace element abundances. Within the Ethiopian dome both lithospheric and sub-lithoshperic contributions can be documented clearly; lithospheric contributions are manifest in more radiogenic isotope values (87Sr/86Sr up to 0.7050) and distinctive trace element abundances (e.g., La/Nb <2.0, Nb/U > 10). The degree of lithospheric contribution is lowest within the active Main Ethiopian Rift and increases towards the southern margin of the dome. The estimated depth of melting (65-75 km) is consistent with geophysical observations of lithospheric thickness. In regions of prolonged volcanism the lithospheric contributions and estimated melting depths decrease through time, corresponding to a higher degree of rifting. In the Kenyan dome, including the western rift, the degree of extension is low and lithospheric melting is the dominant source for basaltic magmatism. Mafic lavas from these regions have generally lower MgO but higher contents of alkalis, P2O5 and many incompatible trace elements than are observed in the Ethiopian Rift. High values of 87Sr/86Sr, 207Pb/204Pb and Zr/Hf relative to other parts of the EARS indicate melting of metasomatized lithosphere. Melting in this area occurs at depths up to 100+ km, consistent with the thick crustal section observed seismically. Between the topographic domes, basalts from the Turkana region record melting at shallow levels ( ˜35 km) consistent with seismic evidence for nearly complete rifting of the crustal section. The geochemistry of these lavas is dominated by asthenospheric source materials, with only minor lithospheric involvement. Temporal evolution of EARS geochemistry reflects progressive rifting of the thick craton. This change is manifest within lavas that are interpreted as plume-derived, as Tb/Yb values decrease from 30 Ma through the present. The modern thermal anomaly associated with Afar volcanism does not appear to extend below the shallow mantle, but may reflect a large blob of deep mantle material that became stuck to Africa 30 Ma and has contributed to regional volcanism ever since. Relative contributions from this deep mantle source, shallow asthenosphere and lithosphere are controlled by the extent of rifting and cannot be predicted solely on the basis of surface topography.

Magma source transition of lunar mare volcanism at 2.3 Ga

NASA Astrophysics Data System (ADS)

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

2017-09-01

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

Sn-wave velocity structure of the uppermost mantle beneath the Australian continent

NASA Astrophysics Data System (ADS)

Wei, Zhi; Kennett, Brian L. N.; Sun, Weijia

2018-06-01

We have extracted a data set of more than 5000 Sn traveltimes for source-station pairs within continental Australia, with 3-D source relocation using Pn arrivals to improve data consistency. We conduct tomographic inversion for S-wave-speed structure down to 100 km using the Fast Marching Tomography (FMTOMO) method for the whole Australian continent. We obtain a 3-D model with potential resolution of 3.0° × 3.0°. The new S-wave-speed model provides strong constraints on structure in a zone that was previously poorly characterized. The S velocities in the uppermost mantle are rather fast, with patterns of variation generally corresponding to those for Pn. We find strong heterogeneities of Swave speed in the uppermost mantle across the entire continent of Australia with a close relation to crustal geological features. For instance, the cratons in the western Australia usually have high S velocities (>4.70 km s-1), while the volcanic regions on the eastern margin of Australia are characterized by low S velocities (<4.40 km s-1). Exploiting an equivalent Pn inversion, we also determine the Vp/Vs ratios across the whole continent. We find that most of the uppermost mantle has Vp/Vs between 1.65 and 1.85, but with patches in central Australia and in the east with much higher Vp/Vs ratios. Distinctive local anomalies on the eastern margin may indicate the positions of remnants of mantle plumes.

Oxygen Fugacity of Mare Basalts and the Lunar Mantle Application of a New Microscale Oxybarometer Based on the Valence State of Vanadium

NASA Technical Reports Server (NTRS)

Shearer, C. K.; Karner, J.; Papike, J. J.; Sutton, S. R.

2004-01-01

The ability to estimate oxygen fugacities for mare basalts and to extend these observations to the lunar mantle is limited using bulk analysis techniques based on buffering assemblages or the valence state of iron. These limitations are due to reequilibration of mineral assemblages at subsolidus conditions, deviations of mineral compositions from thermodynamic ideality, size requirements, and the limits of the iron valence at very low fO2. Still, these approaches have been helpful and indicate that mare basalts crystallized at fO2 between the iron-w stite buffer (IW) and the ilmenite breakdown reaction (ilmenite = rutile + iron). It has also been inferred from these estimates that the lunar mantle is also highly reduced lying at conditions below IW. Generally, these data cannot be used to determine if the mare basalts become increasingly reduced during transport from their mantle source and eruption at the lunar surface and if there are differences in fO2 among mare basalts or mantle sources. One promising approach to determining the fO2 of mare basalts is using the mean valence of vanadium (2+, 3+, 4+, 5+) determined on spots of a few micrometers in diameter using synchrotron x-ray absorption fine structure (XAFS) spectroscopy. The average valence state of V in basaltic glasses is a function of fO2, temperature, V coordination, and melt composition. Here, we report the initial results of this approach applied to lunar pyroclastic glasses.

Zinc isotope systematics of subduction-zone magmas

NASA Astrophysics Data System (ADS)

Huang, J.; Zhang, X. C.; Huang, F.; Yu, H.

2016-12-01

Subduction-zone magmas are generated by partial melting of mantle wedge triggered by addition of fluids derived from subducted hydrothermally altered oceanic lithosphere. Source of the fluids may be sediment, altered oceanic crust and serpentinized peridotite/serpentinite. Knowledge of the exact fluid source can facilitate our better understanding of the mechanism of fluid flux, element cycling and crust-mantle interaction in subduction zones. Zinc isotopes have the potential to place a constraint on this issue, because (1) Zn has an intermediate mobility during fluid-rock interaction and is enriched in subduction-zone fluids (e.g., Li et al., 2013); (2) sediment, altered oceanic crust and serpentinite have distinct Zn isotopic compositions (Pons et al., 2011); and (3) the mantle has a homogeneous Zn isotope composition (δ66Zn = 0.28 ± 0.05‰, Chen et al., 2013). Thus, the Zn isotopic composition of subduction-zone magmas reflects the characteristics of slab-derived fluids of different sources. Here, high-precision Zn isotope analyses were conducted on igneous rocks from arcs of Central America, Kamchatka, South Lesser Antilles, and Aleutian. One rhyolite with 75.1 wt.% SiO2 and 0.2 wt.% FeOT displays the heaviest δ66Zn value of 0.394‰ (relative to JMC Lyon) that probably results from the crystallization of Fe-Ti oxides during the late-stage differentiation. The rest of rocks have Zn isotopic compositions (0.161 to 0.339‰) similar to or lighter than those of the mantle. In an individual arc, the δ66Zn values of rocks show broad negative correlations with Ba/Th and 87Sr/86Sr ratios, suggesting that the slab-derived fluids should have lighter δ66Zn as well as higher Ba/Th and 87Sr/86Sr ratios relative to the mantle. These features are in accordance with those of serpentinites. Thus, addition of serpentinite-derived 66Zn-depleted fluids into the mantle wedge can explain the declined δ66Zn of subduction-zone magmas. ReferenceChen et al. (2013) EPSL 369-370:34-42; Li et al. (2013) GCA 120:326-362; Pons et al. (2011) PNAS 108:17639-17643.

Stable isotope compositions of serpentinite seamounts in the Mariana forearc: Serpentinization processes, fluid sources and sulfur metasomatism

USGS Publications Warehouse

Alt, J.C.; Shanks, Wayne C.

2006-01-01

The Mariana and Izu-Bonin arcs in the western Pacific are characterized by serpentinite seamounts in the forearc that provide unique windows into the mantle wedge. We present stable isotope (O, H, S, and C) data for serpentinites from Conical seamount in the Mariana forearc and S isotope data for Torishima seamount in the Izu-Bonin forearc in order to understand the compositions of fluids and temperatures of serpentinization in the mantle wedge, and to investigate the transport of sulfur from the slab to the mantle wedge. Six serpentine mineral separates have a restricted range of ??18O (6.5-8.5???). Antigorite separates have ??D values of -29.5??? to -45.5??? that reflect serpentinization within the mantle wedge whereas chrysotile has low ??D values (-51.8??? to -84.0???) as the result of re-equilibration with fluids at low temperatures. Fractionation of oxygen isotopes between serpentine and magnetite indicate serpentinization temperatures of 300-375 ??C. Two late cross-fiber chrysotile veins have higher ??18O values of 8.9??? to 10.8??? and formed at lower temperatures (as low as ???100 ??C). Aqueous fluids in equilibrium with serpentine at 300-375 ??C had ??18O = 6.5-9??? and ??D = -4??? to -26???, consistent with sediment dehydration reactions at temperatures <200 ??C in the subducting slab rather than a basaltic slab source. Three aragonite veins in metabasalt and siltstone clasts within the serpentinite flows have ??18O = 16.7-24.5???, consistent with the serpentinizing fluids at temperatures <250 ??C. ??13C values of 0.1-2.5??? suggest a source in subducting carbonate sediments. The ??34S values of sulfide in serpentinites on Conical Seamount (-6.7??? to 9.8???) result from metasomatism through variable reduction of aqueous sulfate (??34S = 14???) derived from slab sediments. Despite sulfur metasomatism, serpentinites have low sulfur contents (generally < 164 ppm) that reflect the highly depleted nature of the mantle wedge. The serpentinites are mostly enriched in 34S (median ??34Ssulfide = 4.5???), consistent with a 34S-enriched mantle wedge as inferred from arc lavas. ?? 2006 Elsevier B.V. All rights reserved.

Magma genesis at Gale Crater: Evidence for Pervasive Mantle Metasomatism

NASA Astrophysics Data System (ADS)

Filiberto, J.

2017-12-01

Basaltic rocks have been analyzed at Gale Crater with a larger range in bulk chemistry than at any other landing site [1]. Therefore, the rocks may have experienced significantly different formation conditions than those experienced by magmas at Gusev Crater or Meridiani Planum. Specifically, the rocks at Gale Crater have higher potassium than other Martian rocks, with a potential analog of the Nakhlite parental magma, and are consistent with forming from a metasomatized mantle source [2-4]. Mantle metasomatism would not only affect the bulk chemistry but mantle melting conditions, as metasomatism fluxes fluids into the source region. Here I will combine differences in bulk chemistry between Martian basalts to calculate formation conditions in the interior and investigate if the rocks at Gale Crater experienced magma genesis conditions consistent with metasomatism - lower temperatures and pressures of formation. To calculate average formation conditions, I rely on experimental results, where available, and silica-activity and Mg-exchange thermometry calculations for all other compositions following [5, 6]. The results show that there is a direct correlation between the calculated mantle potential temperature and the K/Ti ratio of Gale Crater rocks. This is consistent with fluid fluxed metasomatism introducing fluids to the system, which depressed the melting temperature and fluxed K but not Ti to the system. Therefore, all basalts at Gale Crater are consistent with forming from a metasomatized mantle source, which affected not only the chemistry of the basalts but also the formation conditions. References: [1] Cousin A. et al. (2017) Icarus. 288: 265-283. [2] Treiman A.H. et al. (2016) Journal of Geophysical Research: Planets. 121: 75-106. [3] Treiman A.H. and Medard E. (2016) Geological Society of America Abstracts with Programs. 48: doi: 10.1130/abs/2016AM-285851. [4] Schmidt M.E. et al. (2016) Geological Society of America Abstracts with Programs. 48: doi: 10.1130/abs/2016AM-285651. [5] Filiberto J. and Dasgupta R. (2011) Earth and Planetary Science Letters. 304: 527-537. [6] Filiberto J. and Dasgupta R. (2015) Journal of Geophysical Research: Planets. 120: DOI: 10.1002/2014JE004745.

Not all Primordial Noble Gas Signatures are Associated with OIBs and Mantle Plumes - Mantle Heterogeneity, Primordial Shallow Sources and a Solar-like He, Ne Signature in an Ancient North American Craton

NASA Astrophysics Data System (ADS)

Ma, L.; Castro, M. C.; Hall, C. M.

2007-12-01

The presence of primordial He and Ne components in ocean island basalts (OIBs) as well as a mantle He/heat flux ratio lower than the production ratio near mid-ocean ridges have historically been used to support the existence of a two-layer mantle convection model. This would comprise a lower, primordial, undegassed reservoir from which He removal to the upper degassed mantle would be impeded. Arguments based on He and heat transport have been recently invalidated by Castro et al. (2005) and should no longer be used to justify the presence of two such distinct mantle reservoirs. Indeed, it was shown that such low He/heat flux ratios are expected and do not reflect a He deficit in the original crust or mantle reservoir. By contrast, the occurrence of a He/heat flux ratio greater than the radiogenic production ratio can only result from a past mantle thermal event in which the released heat has already escaped while the released He remains, and is slowly rising to the surface. Such a high He/heat flux ratio is present in shallow groundwaters of the Michigan Basin. We now present results of a new noble gas study conducted in the Michigan Basin, in which 38 deep (0.5-3.6km) brine samples were collected and analyzed for all noble gas abundances and isotopic ratios. As expected from previously computed shallow high He/heat flux ratios, both He and Ne isotopic ratios clearly indicate the presence of a mantle component. Of greater significance is the primordial, solar-like signature, of this mantle component. It is also the first primordial signature ever recorded in crustal fluids in a continental region. Because no hotspots or hotspot tracks are known in the area, it is highly unlikely for such primordial, solar-like signature to result from a mantle plume-related mechanism originating deep in the mantle. We argue that such a primordial signature can be explained by a shallow noble gas reservoir in the subcontinental lithospheric mantle (SCLM) beneath the Michigan Basin, possibly created by a mechanism similar to that proposed by Anderson (1998) for oceanic regions. Indeed, the Michigan Basin, located within the ancient North American craton (~1.1->2.5Ga), lies on a very thick U-Th depleted SCLM, possibly allowing preservation of a primordial, residual, mantle reservoir beneath the continental crust. Recent reactivation of the old mid-continent rift transecting the crystalline basement is likely responsible for the release of this primordial signature into the basin. The solar-like He and Ne signatures present in the Michigan Basin fluids not only suggest that a deep primordial mantle reservoir is not required to explain the presence of such components, they also point to a very heterogeneous mantle as previously suggested by Anderson (1998), Albarede (2005), and others. Consequently, the presence of a primordial noble gas signature, at least if observed in a continental region, should not be used to conclude at the existence of a deep mantle source and thus, of a hotspot as typically defined. The SCLM underneath ancient cratons is a great candidate for hosting primitive ancient mantle reservoirs. Arguments based on He/heat flux ratios as well as the presence of a primordial noble gas signature should not be used to support the existence of a lower, primordial, versus an upper, degassed mantle reservoir. Our study provides the first observational case for long-term primordial lithospheric storage. Anderson, 1998, Proc. Natl. Acad. Sci. USA, 95, 9087-9092. Albarede, 2005, AGU Monograph, 160, 27-46. Castro et al., 2005, EPSL, 237, 893-910.

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

NASA Astrophysics Data System (ADS)

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

2013-12-01

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

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

NASA Astrophysics Data System (ADS)

Hamilton, W. B.

2002-12-01

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

Mantle xenoliths from Central Vietnam: evidence for at least Meso-Proterozoic formation of the lithospheric mantle

NASA Astrophysics Data System (ADS)

Proßegger, Peter; Ntaflos, Theodoros; Ackerman, Lukáš; Hauzenberger, Christoph; Tran, Tuan Anh

2016-04-01

Intraplate Cenozoic basalts that are widely dispersed along the continental margin of East Asia belong to the Western Pacific "diffuse" igneous province. They consist mainly of alkali basalts, basanites,rarely nephelinites, which are mantle xenolith-bearing, potassic rocks and quartz tholeiites. The volcanism in this area has been attributed to the continental extension caused by the collision of India with Asia and by the subduction of the Pacific Ocean below Asia. We studied a suite of 24 mantle xenoliths from La Bang Lake, Dak Doa district and Bien Ho, Pleiku city in the Gia Province, Central Vietnam. They are predominantly spinel lherzolites (19) but spinel harburgites (3) and two garnet pyroxenites are present as well. The sizes of the xenoliths range from 5 to 40 cm in diameter with medium to coarse-grained protogranular textures. Whole rock major and trace element analyses display a wide range of compositions. The MgO concentration varies from 36.0 to 45.8 wt% whereas Al2O3 and CaO range from 0.63 to 4.36 wt% and from 0.52 to 4.21 wt% (with one sample having CaO of 6.63 wt%) respectively. Both CaO and Al2O3 positively correlate with MgO most likely indicating that the sampled rocks were derived from a common mantle source experienced variable degrees of partial melting. Mineral analyses show that the rock forming minerals are chemically homogeneous. The Fo contents of olivine vary between 89.2 and 91.2 and the Mg# of orthopyroxene and clinopyroxene range from 89 to 92 and 89 to 94 respectively. The range of Cr# for spinel is 0.06-0.26. Model calculations in both whole rock and clinopyroxenes show that lithospheric mantle underneath Central Vietnam experienced melt extractions that vary between 2-7, 12-15 and 20-30%. The majority of the primitive mantle-normalized whole rock and clinopyroxene REE patterns are parallel to each other indicating that clinopyroxene is the main repository of the trace elements. Clinopyroxenes are divided into two groups: group A with concave upwards REE and (La/Yb)N < 1 suggesting various degrees of melt extraction and group B with (La/Yb)N ranging between 1 and 10. The group B in a mantle normalized trace element diagram shows negative Pb and Sr anomalies compared to their neighbour elements, which together with the general absence of hydrous phases, suggest variable interaction with percolating silicate melt(s). The primitive-mantle normalized highly siderophile element (HSE) concentration pattern show almost no fractionation among Ir, Ru and Pt with only slight depletion in Os suggesting very limited effect of metasomatism on the HSE contents. On the other hand, most of the samples display clear Re addition from the percolating melts preventing calculation of reliable rhenium depletion ages (TRD). However, one sample with depleted Pd and Re signature yield TRD of 1.0 Ga which can be interpreted as a minimum SCLM stabilization age in this area. Mantle xenoliths from Central Vietnam range from fertile to depleted compositions partly affected by metasomatic silicate melts. Re-Os isotopic composition reveals a Meso-Proterozoic minimum stabilization age of the lithospheric mantle.

Assimilation of sediments embedded in the oceanic arc crust: myth or reality?

NASA Astrophysics Data System (ADS)

Bezard, Rachel; Davidson, Jon P.; Turner, Simon; Macpherson, Colin G.; Lindsay, Jan M.; Boyce, Adrian J.

2014-06-01

Arc magmas are commonly assumed to form by melting of sub-arc mantle that has been variably enriched by a component from the subducted slab. Although most magmas that reach the surface are not primitive, the impact of assimilation of the arc crust is often ignored with the consequence that trace element and isotopic compositions are commonly attributed only to varying contributions from different components present in the mantle. This jeopardises the integrity of mass balance recycling calculations. Here we use Sr and O isotope data in minerals from a suite of volcanic rocks from St Lucia, Lesser Antilles arc, to show that assimilation of oceanic arc basement can be significant. Analysis of 87Sr/86Sr in single plagioclase phenocrysts from four Soufrière Volcanic Complex (SVC; St Lucia) hand samples with similar composition (87Sr/86Sr = 0.7089-0.7091) reveals crystal isotopic heterogeneity among hand samples ranging from 0.7083 to 0.7094 with up to 0.0008 difference within a single hand sample. δO18 measurements in the SVC crystals show extreme variation beyond the mantle range with +7.5 to +11.1‰ for plagioclase (n=19), +10.6 to +11.8‰ for quartz (n=10), +9.4 to +9.8‰ for amphibole (n=2) and +9 to +9.5‰ for pyroxene (n=3) while older lavas (Pre-Soufriere Volcanic Complex), with less radiogenic whole rock Sr composition (87Sr/86Sr = 0.7041-0.7062) display values closer to mantle range: +6.4 to +7.9‰ for plagioclase (n=4) and +6 to +6.8‰ for pyroxene (n=5). We argue that the 87Sr/86Sr isotope disequilibrium and extreme δO18 values provide compelling evidence for assimilation of material located within the arc crust. Positive correlations between mineral δO18 and whole rock 87Sr/86Sr, 143Nd/144Nd and 206,207,208Pb/204Pb shows that assimilation seems to be responsible not only for the isotopic heterogeneity observed in St Lucia but also in the whole Lesser Antilles since St Lucia encompasses almost the whole-arc range of isotopic compositions. This highlights the need for detailed mineral-scale investigation of oceanic arc suites to quantify assimilation that could otherwise lead to misinterpretation of source composition and subduction processes.

Formation and modification of chromitites in the mantle

NASA Astrophysics Data System (ADS)

Arai, Shoji; Miura, Makoto

2016-11-01

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

Dynamical links between small- and large-scale mantle heterogeneity: Seismological evidence

NASA Astrophysics Data System (ADS)

Frost, Daniel A.; Garnero, Edward J.; Rost, Sebastian

2018-01-01

We identify PKP • PKP scattered waves (also known as P‧ •P‧) from earthquakes recorded at small-aperture seismic arrays at distances less than 65°. P‧ •P‧ energy travels as a PKP wave through the core, up into the mantle, then scatters back down through the core to the receiver as a second PKP. P‧ •P‧ waves are unique in that they allow scattering heterogeneities throughout the mantle to be imaged. We use array-processing methods to amplify low amplitude, coherent scattered energy signals and resolve their incoming direction. We deterministically map scattering heterogeneity locations from the core-mantle boundary to the surface. We use an extensive dataset with sensitivity to a large volume of the mantle and a location method allowing us to resolve and map more heterogeneities than have previously been possible, representing a significant increase in our understanding of small-scale structure within the mantle. Our results demonstrate that the distribution of scattering heterogeneities varies both radially and laterally. Scattering is most abundant in the uppermost and lowermost mantle, and a minimum in the mid-mantle, resembling the radial distribution of tomographically derived whole-mantle velocity heterogeneity. We investigate the spatial correlation of scattering heterogeneities with large-scale tomographic velocities, lateral velocity gradients, the locations of deep-seated hotspots and subducted slabs. In the lowermost 1500 km of the mantle, small-scale heterogeneities correlate with regions of low seismic velocity, high lateral seismic gradient, and proximity to hotspots. In the upper 1000 km of the mantle there is no significant correlation between scattering heterogeneity location and subducted slabs. Between 600 and 900 km depth, scattering heterogeneities are more common in the regions most remote from slabs, and close to hotspots. Scattering heterogeneities show an affinity for regions close to slabs within the upper 200 km of the mantle. The similarity between the distribution of large-scale and small-scale mantle structures suggests a dynamic connection across scales, whereby mantle heterogeneities of all sizes may be directed in similar ways by large-scale convective currents.

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

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

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

1989-12-01

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

The Chlorine Isotope Composition of Earth’s Mantle

NASA Astrophysics Data System (ADS)

Bonifacie, M.; Jendrzejewski, N.; Agrinier, P.; Humler, E.; Coleman, M.; Javoy, M.

2008-03-01

Chlorine stable isotope compositions (δ37Cl) of 22 mid-ocean ridge basalts (MORBs) correlate with Cl content. The high-δ37Cl, Cl-rich basalts are highly contaminated by Cl-rich materials (seawater, brines, or altered rocks). The low-δ37Cl, Cl-poor basalts approach the composition of uncontaminated, mantle-derived magmas. Thus, most or all oceanic lavas are contaminated to some extent during their emplacement. MORB-source mantle has δ37Cl ≤ 1.6 per mil (‰), which is significantly lower than that of surface reservoirs (~ 0‰). This isotopic difference between the surface and deep Earth results from net Cl isotopic fractionation (associated with removal of Cl from the mantle and its return by subduction over Earth history) and/or the addition (to external reservoirs) of a late volatile supply that is 37Cl-enriched.

Magma Ocean Depth and Oxygen Fugacity in the Early Earth--Implications for Biochemistry.

PubMed

Righter, Kevin

2015-09-01

A large class of elements, referred to as the siderophile (iron-loving) elements, in the Earth's mantle can be explained by an early deep magma ocean on the early Earth in which the mantle equilibrated with metallic liquid (core liquid). This stage would have affected the distribution of some of the classic volatile elements that are also essential ingredients for life and biochemistry - H, C, S, and N. Estimates are made of the H, C, S, and N contents of Earth's early mantle after core formation, considering the effects of variable temperature, pressure, oxygen fugacity, and composition on their partitioning. Assessment is made of whether additional, exogenous, sources are required to explain the observed mantle concentrations, and areas are identified where additional data and experimentation would lead to an improved understanding of this phase of Earth's history.

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

NASA Astrophysics Data System (ADS)

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

2018-01-01

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

Multiple enrichment of the Carpathian-Pannonian mantle: Pb-Sr-Nd isotope and trace element constraints

NASA Astrophysics Data System (ADS)

Rosenbaum, Jeffrey M.; Wilson, Marjorie; Downes, Hilary

1997-07-01

Pb isotope compositions of acid-leached clinopyroxene and amphibole mineral separates from spinel peridotite mantle xenoliths entrained in Tertiary-Quaternary alkali basalts from the Carpathian-Pannonian Region of eastern Europe provide important constraints on the processes of metasomatic enrichment of the mantle lithosphere in an extensional tectonic setting associated with recent subduction. Principal component analysis of Pb-Sr-Nd isotope and rare earth element compositions of the pyroxenes is used to identify the geochemical characteristics of the original lithospheric mantle protolith and a spectrum of infiltrating metasomatic agents including subduction-related aqueous fluids and silicate melts derived from a subduction-modified mantle wedge which contains a St. Helena-type (HIMU) plume component. The mantle protolith is highly depleted relative to mid-ocean ridge basalt-source mantle with Pb-Nd-Sr isotope compositions consistent with an ancient depletion event. Silicate melt infiltration into the protolith accounts for the primary variance in the Pb-Sr-Nd isotope compositions of the xenoliths and has locally generated metasomatic amphibole. Infiltration of aqueous fluids has introduced radiogenic Pb and Sr without significantly perturbing the rare earth element signature of the protolith. The Pb isotope compositions of the fluid-modified xenoliths suggest that they reacted with aqueous fluids released from a subduction zone which had equilibrated with sediment derived from an ancient basement terrain. We propose a model for mantle lithosphere evolution consistent with available textural and geochemical data for the xenolith population. The Pb-Sr-Nd isotope compositions of both alkaline mafic magmas and rare, subduction-related, calc-alkaline basaltic andesites from the region provide important constraints for the nature of the asthenospheric mantle wedge and confirm the presence of a HIMU plume component. These silicate melts contribute to the metasomatism of the mantle lithosphere rather than being derived therefrom.

Relation of major volcanic center concentration on Venus to global tectonic patterns

NASA Technical Reports Server (NTRS)

Crumpler, L. S.; Head, James W.; Aubele, Jayne C.

1993-01-01

Global analysis of Magellan image data indicates that a major concentration of volcanic centers covering about 40 percent of the surface of Venus occurs between the Beta, Atla, and Themis regions. Associated with this enhanced concentration are geological characteristics commonly interpreted as rifting and mantle upwelling. Interconnected low plains in an annulus around this concentration are characterized by crustal shortening and infrequent volcanic centers that may represent sites of mantle return flow and net downwelling. Together, these observations suggest the existence of relatively simple, large-scale patterns of mantle circulation similar to those associated with concentrations of intraplate volcanism on earth.

Numerical modeling the genetic mechanism of Cenozoic intraplate Volcanoes in Northeastern China

NASA Astrophysics Data System (ADS)

Qu, Wulin; Chen, Yongshun John; Zhang, Huai; Jin, Yimin; Shi, Yaolin

2017-04-01

Changbaishan Volcano located about 1400 km west of Japan Trench is an intra continental volcano which having different origin from island arc volcanoes. A number of different mechanisms have been proposed to interpret the origin of intraplate volcanoes, such as deep mantle plumes, back-arc extension and decompressional partial melting, asthenosphere upwelling and decompressional melting, and deep stagnant slab dehydration and partial melting. The recent geophysical research reveals that the slow seismic velocity anomaly extends continuously just below 660 km depth to surface beneath Changbaishan by seismic images and three-dimensional waveform modelling [Tang et al., 2014]. The subduction-induced upwelling occurs within a gap in the stagnant subducted Pacific Plate and produces decompressional melting. Water in deep Earth can reduce viscosity and lower melting temperature and seismic velocity and has effects on many other physical properties of mantle materials. The water-storage capacity of wadsleyite and ringwoodite, which are the main phase in the mantle transition zone, is much greater than that of upper mantle and lower mantle. Geophysical evidences have shown that water content in the mantle transition zone is exactly greater than that of upper mantle and lower mantle [Karato, 2011]. Subducted slab could make mantle transition zone with high water content upward or downward across main phase change surface to release water, and lead to partial melting. We infer that the partial melting mantle and subducted slab materials propagate upwards and form the Cenozoic intraplate Volcanoes in Northeastern China. We use the open source code ASPECT [Kronbichler et al., 2012] to simulate the formation and migration of magma contributing to Changbaishan Volcano. We find that the water entrained by subducted slab from surface has only small proportion comparing to water content of mantle transition zone. Our model provide insights into dehydration melting induced by water transport out of the mantle transition zone associated with dynamic interactions between the subducted slab and surrounding mantle. References Karato, S. (2011), Water distribution across the mantle transition zone and its implications for global material circulation, EARTH PLANET SC LETT, 301(3), 413-423. Kronbichler, M., et al. (2012), High accuracy mantle convection simulation through modern numerical methods, GEOPHYS J INT, 191(1), 12-29. Tang, Y., et al. (2014), Changbaishan volcanism in northeast China linked to subduction-induced mantle upwelling, NAT GEOSCI, 7(6), 470-475.

Tracking the source of the enriched martian meteorites in olivine-hosted melt inclusions of two depleted shergottites, Yamato 980459 and Tissint

NASA Astrophysics Data System (ADS)

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

2015-05-01

The apparent lack of plate tectonics on all terrestrial planets other than Earth has been used to support the notion that for most planets, once a primitive crust forms, the crust and mantle evolve geochemically-independent through time. This view has had a particularly large impact on models for the evolution of Mars and its silicate interior. Recent data indicating a greater potential that there may have been exchange between the martian crust and mantle has led to a search for additional geochemical evidence to support the alternative hypothesis, that some mechanism of crustal recycling may have operated early in the history of Mars. In order to study the most juvenile melts available to investigate martian mantle source(s) and melting processes, the trace element compositions of olivine-hosted melt inclusions for two incompatible-element-depleted olivine-phyric shergottites, Yamato 980459 (Y98) and Tissint, and the interstitial glass of Y98, have been measured by Secondary Ionization Mass Spectrometry (SIMS). Chondrite-normalized Rare Earth Element (REE) patterns for both Y98 and Tissint melt inclusions, and the Y98 interstitial glass, are characteristically light-REE depleted and parallel those of their host rock. For Y98, a clear flattening and upward inflection of La and Ce, relative to predictions based on middle and heavier REE, provides evidence for involvement of an enriched component early in their magmatic history; either inherited from a metasomatized mantle or crustal source, early on and prior to extensive host crystallization. Comparing these melt inclusion and interstitial glass analyses to existing melt inclusion and whole-rock data sets for the shergottite meteorite suite, defines mixing relationships between depleted and enriched end members, analogous to mixing relationships between whole rock Sr and Nd isotopic measurements. When considered in light of their petrologic context, the origin of these trace element enriched and isotopically evolved signatures represents either (1) crustal assimilation during the final few km of melt ascent towards the martian surface, or (2) assimilation soon after melt segregation, through melt-rock interaction with a portion of the martian crust recycled back into the mantle.

Geochronological and geochemical assessment of Cenozoic volcanism from the Terror Rift region of the West Antarctic Rift System

NASA Astrophysics Data System (ADS)

Rilling, Sarah E.

The work presented in this dissertation explains results from three different methods to determine the relation between tectonism and rift-related volcanism in the Terror Rift region of the West Antarctic Rift System (WARS). Alkaline lavas from seven submarine features, Beaufort Island and Franklin Islands, and several locations near Mt Melbourne were dated by 40Ar/39Ar geochronology and analyzed for elemental and isotopic chemical signatures. Each chapter addresses a different aspect of the hypothesis that the presence of volatiles, primarily H2O or CO2, in the magma source has led to anomalously high volumes of magmatism after rift-related decompressional melting rather than requiring an active mantle plume source. Chapter 2 provides the temporal framework, illustrating that the sampled features range in age from 6.7 Ma to 89 ka, post-dating the main Miocene age phase of Terror Rift extension. Chapter 3 illustrates the traditional enriched elemental and isotopic chemical signatures to support the overall homogeneity of these lavas and previously analyzed areas of the WARS. This chapter also provides a new model for the generation of the Pb isotopic signatures consistent with a history of metasomatism in the magma source. Chapter 4 provides an entirely new chemical dataset for the WARS. The first platinum group element (PGE) abundances and extremely unradiogenic Os isotopic signatures of Cenozoic lavas from Antarctica provide the strongest evidence of melting contributions from a lithospheric mantle source. The combined results from these three studies consistently support the original hypothesis of this dissertation. New evidence suggests that WARS related lavas are not related to a mantle plume(s) as previously proposed. Instead, they are generated by passive, decompressional melting of a source, likely a combination of the asthenospheric and lithospheric mantle, which has undergone previous melting events and metasomatism.

Generation, ascent and eruption of magma on the Moon: New insights into source depths, magma supply, intrusions and effusive/explosive eruptions (Part 1: Theory)

NASA Astrophysics Data System (ADS)

Wilson, Lionel; Head, James W.

2017-02-01

We model the ascent and eruption of lunar mare basalt magmas with new data on crustal thickness and density (GRAIL), magma properties, and surface topography, morphology and structure (Lunar Reconnaissance Orbiter). GRAIL recently measured the broad spatial variation of the bulk density structure of the crust of the Moon. Comparing this with the densities of lunar basaltic and picritic magmas shows that essentially all lunar magmas were negatively buoyant everywhere within the lunar crust. Thus positive excess pressures must have been present in melts at or below the crust-mantle interface to enable them to erupt. The source of such excess pressures is clear: melt in any region experiencing partial melting or containing accumulated melt, behaves as though an excess pressure is present at the top of the melt column if the melt is positively buoyant relative to the host rocks and forms a continuously interconnected network. The latter means that, in partial melt regions, probably at least a few percent melting must have taken place. Petrologic evidence suggests that both mare basalts and picritic glasses may have been derived from polybaric melting of source rocks in regions extending vertically for at least a few tens of km. This is not surprising: the vertical extent of a region containing inter-connected partial melt produced by pressure-release melting is approximately inversely proportional to the acceleration due to gravity. Translating the ∼25 km vertical extent of melting in a rising mantle diapir on Earth to the Moon then implies that melting could have taken place over a vertical extent of up to 150 km. If convection were absent, melting could have occurred throughout any region in which heat from radioisotope decay was accumulating; in the extreme this could have been most of the mantle. The maximum excess pressure that can be reached in a magma body depends on its environment. If melt percolates upward from a partial melt zone and accumulates as a magma reservoir, either at the density trap at the base of the crust or at the rheological trap at the base of the elastic lithosphere, the excess pressure at the top of the magma body will exert an elastic stress on the overlying rocks. This will eventually cause them to fail in tension when the excess pressure has risen to close to twice the tensile strength of the host rocks, perhaps up to ∼10 MPa, allowing a dike to propagate upward from this point. If partial melting occurs in a large region deep in the mantle, however, connections between melt pockets and veins may not occur until a finite amount, probably a few percent, of melting has occurred. When interconnection does occur, the excess pressure at the top of the partial melt zone will rise abruptly to a high value, again initiating a brittle fracture, i.e. a dike. That sudden excess pressure is proportional to the vertical extent of the melt zone, the difference in density between the host rocks and the melt, and the acceleration due to gravity, and could readily be ∼100 MPa, vastly greater than the value needed to initiate a dike. We therefore explored excess pressures in the range ∼10 to ∼100 MPa. If eruptions take place through dikes extending upward from the base of the crust, the mantle magma pressure at the point where the dike is initiated must exceed the pressure due to the weight of the magmatic liquid column. This means that on the nearside the excess pressure must be at least ∼19 ± 9 MPa and on the farside must be ∼29 ± 15 MPa. If the top of the magma body feeding an erupting dike is a little way below the base of the crust, slightly smaller excess pressures are needed because the magma is positively buoyant in the part of the dike within the upper mantle. Even the smallest of these excess pressures is greater than the ∼10 MPa likely maximum value in a magma reservoir at the base of the crust or elastic lithosphere, but the values are easily met by the excess pressures in extensive partial melt zones deeper within the mantle. Thus magma accumulations at the base of the crust would have been able to intrude dikes part-way through the crust, but not able to feed eruptions to the surface; in order to be erupted, magma must have been extracted from deeper mantle sources, consistent with petrologic evidence. Buoyant dikes growing upward from deep mantle sources of partial melt can disconnect from their source regions and travel through the mantle as isolated bodies of melt that encounter and penetrate the crust-mantle density boundary. They adjust their lengths and internal pressure excesses so that the stress intensity at the lower tip is zero. The potential total vertical extent of the resulting melt body depends on the vertical extent of the source region from which it grew. For small source extents, the upper tip of the resulting dike crossing the crust-mantle boundary cannot reach the surface anywhere on the Moon and therefore can only form a dike intrusion; for larger source extents, the dike can reach the surface and erupt on the nearside but still cannot reach the surface on the farside; for even larger source extents, eruptions could occur on both the nearside and the farside. The paucity of farside eruptions therefore implies a restricted range of vertical extents of partial melt source region sizes, between ∼16 and ∼36 km. When eruptions can occur, the available pressure in excess of what is needed to support a static magma column to the surface gives the pressure gradient driving magma flow. The resulting typical turbulent magma rise speeds are ∼10 to a few tens of m s-1, dike widths are of order 100 m, and eruption rates from 1 to 10 km long fissure vents are of order 105 to 106 m3 s-1. Volume fluxes in lunar eruptions derived from lava flow thicknesses and surface slopes or rille lengths and depths are found to be of order 105 to 106 m3 s-1 for volume-limited lava flows and >104 to 105 m3 s-1 for sinuous rilles, with dikes widths of ∼50 m. The lower end of the volume flux range for sinuous rilles corresponds to magma rise speeds approaching the limit set by the fact that excessive cooling would occur during flow up a 30 km long dike kept open by a very low excess pressure. These eruptions were thus probably fed by partial melt zones deep in the mantle. Longer eruption durations, rather than any subtle topographic slope effects, appear to be the key to the ability of these flows to erode sinuous rille channels. We conclude that: (1) essentially all lunar magmas were negatively buoyant everywhere within the crust; (2) positive excess pressures of at least 20-30 MPa must have been present in mantle melts at or below the crust-mantle interface to drive magmas to the surface; (3) such pressures are easily produced in zones of partial melting by pressure-release during mantle convection or simple heat accumulation from radioisotopes; (4) magma volume fluxes available from dikes forming at the tops of partial melt zones are consistent with the 105 to 106 m3 s-1 volume fluxes implied by earlier analyses of surface flows; (5) eruptions producing thermally-eroded sinuous rille channels involved somewhat smaller volume fluxes of magma where the supply rate may be limited by the rate of extraction of melt percolating through partial melt zones.

Rare gas isotopes and parent trace elements in ultrabasic-alkaline-carbonatite complexes, Kola Peninsula: identification of lower mantle plume component

NASA Astrophysics Data System (ADS)

Tolstikhin, I. N.; Kamensky, I. L.; Marty, B.; Nivin, V. A.; Vetrin, V. R.; Balaganskaya, E. G.; Ikorsky, S. V.; Gannibal, M. A.; Weiss, D.; Verhulst, A.; Demaiffe, D.

2002-03-01

During the Devonian magmatism (370 Ma ago) ∼20 ultrabasic-alkaline-carbonatite complexes (UACC) were formed in the Kola Peninsula (north-east of the Baltic Shield). In order to understand mantle and crust sources and processes having set these complexes, rare gases were studied in ∼300 rocks and mineral separates from 9 UACC, and concentrations of parent Li, K, U, and Th were measured in ∼70 samples. 4He/3He ratios in He released by fusion vary from pure radiogenic values ∼108 down to 6 × 104. The cosmogenic and extraterrestrial sources as well as the radiogenic production are unable to account for the extremely high abundances of 3He, up to 4 × 10-9 cc/g, indicating a mantle-derived fluid in the Kola rocks. In some samples helium extracted by crushing shows quite low 4He/3He = 3 × 104, well below the mean ratio in mid ocean ridge basalts (MORB), (8.9 ± 1.0) × 104, indicating the contribution of 3He-rich plume component. Magnetites are principal carriers of this component. Trapped 3He is extracted from these minerals at high temperatures 1100°C to 1600°C which may correspond to decrepitation or annealing primary fluid inclusions, whereas radiogenic 4He is manly released at a temperature range of 500°C to 1200°C, probably corresponding to activation of 4He sites degraded by U, Th decay. Similar 4He/3He ratios were observed in Oligocene flood basalts from the Ethiopian plume. According to a paleo-plate-tectonic reconstruction, 450 Ma ago the Baltica (including the Kola Peninsula) continent drifted not far from the present-day site of that plume. It appears that both magmatic provinces could relate to one and the same deep-seated mantle source. The neon isotopic compositions confirm the occurrence of a plume component since, within a conventional 20Ne/22Ne versus 21Ne/22Ne diagram, the regression line for Kola samples is indistinguishable from those typical of plumes, such as Loihi (Hawaii). 20Ne/22Ne ratios (up to 12.1) correlate well with 40Ar/36Ar ones, allowing to infer a source 40Ar/36Ar ratio of about 4000 for the mantle end-member, which is 10 times lower than that of the MORB source end-member. In (3He/22Ne)PRIM versus (4He/21Ne)RAD plot the Kola samples are within array established for plume and MORB samples; almost constant production ratio of (4He/21Ne)RAD ≅ 2 × 107 is translated via this array into (3He/22Ne)PRIM ∼ 10. The latter value approaches the solar ratio implying the non-fractionated solar-like rare gas pattern in a plume source. The Kola UACC show systematic variations in the respective contributions of in situ-produced radiogenic isotopes and mantle-derived isotopes. Since these complexes were essentially plutonic, we propose that the depth of emplacement exerted a primary control on the retention of both trapped and radiogenic species, which is consistent with geological observations. The available data allow to infer the following sequence of processes for the emplacement and evolution of Kola Devonian UACC: 1) Ascent of the plume from the lower mantle to the subcontinental lithosphere; the plume triggered mantle metasomatism not later than ∼700 to 400 Ma ago. 2) Metasomatism of the lithosphere (beneath the central part of the Kola Peninsula), including enrichment in volatile (e.g., He, Ne) and in incompatible (e.g., U, Th) elements. 3) Multistage intrusions of parental melts, their degassing, and crystallisation differentiation ∼370 Ma ago. 4) Postcrystallisation migration of fluids, including loss of radiogenic and of trapped helium. Based on model compositions of the principle terrestrial reservoirs we estimate the contributions (by mass) of the plume material, the upper mantle material, and the atmosphere (air-saturated groundwater), into the source of parent melt at ∼2%, 97.95%, and ∼0.05%, respectively.

Laboratory-based electrical conductivity at Martian mantle conditions

NASA Astrophysics Data System (ADS)

Verhoeven, Olivier; Vacher, Pierre

2016-12-01

Information on temperature and composition of planetary mantles can be obtained from electrical conductivity profiles derived from induced magnetic field analysis. This requires a modeling of the conductivity for each mineral phase at conditions relevant to planetary interiors. Interpretation of iron-rich Martian mantle conductivity profile therefore requires a careful modeling of the conductivity of iron-bearing minerals. In this paper, we show that conduction mechanism called small polaron is the dominant conduction mechanism at temperature, water and iron content conditions relevant to Mars mantle. We then review the different measurements performed on mineral phases with various iron content. We show that, for all measurements of mineral conductivity reported so far, the effect of iron content on the activation energy governing the exponential decrease in the Arrhenius law can be modeled as the cubic square root of the iron content. We recast all laboratory results on a common generalized Arrhenius law for iron-bearing minerals, anchored on Earth's mantle values. We then use this modeling to compute a new synthetic profile of Martian mantle electrical conductivity. This new profile matches perfectly, in the depth range [100,1000] km, the electrical conductivity profile recently derived from the study of Mars Global Surveyor magnetic field measurements.

Horizontal mantle flow controls subduction dynamics.

PubMed

Ficini, E; Dal Zilio, L; Doglioni, C; Gerya, T V

2017-08-08

It is generally accepted that subduction is driven by downgoing-plate negative buoyancy. Yet plate age -the main control on buoyancy- exhibits little correlation with most of the present-day subduction velocities and slab dips. "West"-directed subduction zones are on average steeper (~65°) than "East"-directed (~27°). Also, a "westerly"-directed net rotation of the lithosphere relative to the mantle has been detected in the hotspot reference frame. Thus, the existence of an "easterly"-directed horizontal mantle wind could explain this subduction asymmetry, favouring steepening or lifting of slab dip angles. Here we test this hypothesis using high-resolution two-dimensional numerical thermomechanical models of oceanic plate subduction interacting with a mantle flow. Results show that when subduction polarity is opposite to that of the mantle flow, the descending slab dips subvertically and the hinge retreats, thus leading to the development of a back-arc basin. In contrast, concordance between mantle flow and subduction polarity results in shallow dipping subduction, hinge advance and pronounced topography of the overriding plate, regardless of their age-dependent negative buoyancy. Our results are consistent with seismicity data and tomographic images of subduction zones. Thus, our models may explain why subduction asymmetry is a common feature of convergent margins on Earth.

Intraplate mantle oxidation by volatile-rich silicic magmas

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

Martin, Audrey M.; Médard, Etienne; Righter, Kevin

The upper subcontinental lithospheric mantle below the French Massif Central is more oxidized than the average continental lithosphere, although the origin of this anomaly remains unknown. Using iron oxidation analysis in clinopyroxene, oxybarometry, and melt inclusions in mantle xenoliths, we show that widespread infiltration of volatile (HCSO)-rich silicic melts played a major role in this oxidation. We propose the first comprehensive model of magmatism and mantle oxidation at an intraplate setting. Two oxidizing events occurred: (1) a 365–286 Ma old magmatic episode that produced alkaline vaugnerites, potassic lamprophyres, and K-rich calc-alkaline granitoids, related to the N–S Rhenohercynian subduction, and (2)more » < 30 Ma old magmatism related to W–E extension, producing carbonatites and hydrous potassic trachytes. These melts were capable of locally increasing the subcontinental lithospheric mantle fO2 to FMQ + 2.4. Both events originate from the melting of a metasomatized lithosphere containing carbonate + phlogopite ± amphibole. The persistence of this volatile-rich lithospheric source implies the potential for new episodes of volatile-rich magmatism. Similarities with worldwide magmatism also show that the importance of volatiles and the oxidation of the mantle in intraplate regions is underestimated.« less

Piecewise delamination of Moroccan lithosphere from beneath the Atlas Mountains

NASA Astrophysics Data System (ADS)

Bezada, M. J.; Humphreys, E. D.; Davila, J. M.; Carbonell, R.; Harnafi, M.; Palomeras, I.; Levander, A.

2014-04-01

The elevation of the intracontinental Atlas Mountains of Morocco and surrounding regions requires a mantle component of buoyancy, and there is consensus that this buoyancy results from an abnormally thin lithosphere. Lithospheric delamination under the Atlas Mountains and thermal erosion caused by upwelling mantle have each been suggested as thinning mechanisms. We use seismic tomography to image the upper mantle of Morocco. Our imaging resolves the location and shape of lithospheric cavities and of delaminated lithosphere ˜400 km beneath the Middle Atlas. We propose discontinuous delamination of an intrinsically unstable Atlas lithosphere, enabled by the presence of anomalously hot mantle, as a mechanism for producing the imaged structures. The Atlas lithosphere was made unstable by a combination of tectonic shortening and eclogite loading during Mesozoic rifting and Cenozoic magmatism. The presence of hot mantle sourced from regional upwellings in northern Africa or the Canary Islands enhanced the instability of this lithosphere. Flow around the retreating Alboran slab focused upwelling mantle under the Middle Atlas, which we infer to be the site of the most recent delamination. The Atlas Mountains of Morocco stand as an example of large-scale lithospheric loss in a mildly contractional orogen.

Intraplate mantle oxidation by volatile-rich silicic magmas

NASA Astrophysics Data System (ADS)

Martin, Audrey M.; Médard, Etienne; Righter, Kevin; Lanzirotti, Antonio

2017-11-01

The upper subcontinental lithospheric mantle below the French Massif Central is more oxidized than the average continental lithosphere, although the origin of this anomaly remains unknown. Using iron oxidation analysis in clinopyroxene, oxybarometry, and melt inclusions in mantle xenoliths, we show that widespread infiltration of volatile (HCSO)-rich silicic melts played a major role in this oxidation. We propose the first comprehensive model of magmatism and mantle oxidation at an intraplate setting. Two oxidizing events occurred: (1) a 365-286 Ma old magmatic episode that produced alkaline vaugnerites, potassic lamprophyres, and K-rich calc-alkaline granitoids, related to the N-S Rhenohercynian subduction, and (2) < 30 Ma old magmatism related to W-E extension, producing carbonatites and hydrous potassic trachytes. These melts were capable of locally increasing the subcontinental lithospheric mantle fO2 to FMQ + 2.4. Both events originate from the melting of a metasomatized lithosphere containing carbonate + phlogopite ± amphibole. The persistence of this volatile-rich lithospheric source implies the potential for new episodes of volatile-rich magmatism. Similarities with worldwide magmatism also show that the importance of volatiles and the oxidation of the mantle in intraplate regions is underestimated.

Devonian magmatism in the Timan Range, Arctic Russia - subduction, post-orogenic extension, or rifting?

NASA Astrophysics Data System (ADS)

Pease, V.; Scarrow, J. H.; Silva, I. G. Nobre; Cambeses, A.

2016-11-01

Devonian mafic magmatism of the northern East European Craton (EEC) has been variously linked to Uralian subduction, post-orogenic extension associated with Caledonian collision, and rifting. New elemental and isotopic analyses of Devonian basalts from the Timan Range and Kanin Peninsula, Russia, in the northern EEC constrain magma genesis, mantle source(s) and the tectonic process(es) associated with this Devonian volcanism to a rift-related context. Two compositional groups of low-K2O tholeiitic basalts are recognized. On the basis of Th concentrations, LREE concentrations, and (LREE/HREE)N, the data suggest two distinct magma batches. Incompatible trace elements ratios (e.g., Th/Yb, Nb/Th, Nb/La) together with Nd and Pb isotopes indicate involvement of an NMORB to EMORB 'transitional' mantle component mixed with variable amounts of a continental component. The magmas were derived from a source that developed high (U,Th)/Pb, U/Th and Sm/Nd over time. The geochemistry of Timan-Kanin basalts supports the hypothesis that the genesis of Devonian basaltic magmatism in the region resulted from local melting of transitional mantle and lower crust during rifting of a mainly non-volcanic continental rifted margin.

Studies on the reduction of radon plate-out

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

Bruemmer, M.; Nakib, M.; Calkins, R.

The decay of common radioactive gases, such as radon, produces stable isotopes by a sequence of daughter particles with varied half-lives. These daughter particles are a significant source of gamma, neutron, and alpha (α) particle backgrounds that can mimic desired signals in dark matter and neutrinoless double beta decay experiments. In the LUMINA Laboratory at Southern Methodist University (SMU), studies of radon plate-out onto copper samples are conducted using one of XIA’s first five UltraLo 1800 alpha counters. We present results from investigations into various mitigation approaches. A custom-built copper holder (in either plastic or metal) has been designed andmore » produced to maximize the copper’s exposure to {sup 220}Rn. The {sup 220}Rn source is a collection of camping lantern mantles. We present the current status of control and experimental methods for addressing radon exposure levels.« less

Metal-rich meteorites from the aubrite parent body

NASA Technical Reports Server (NTRS)

Casanova, I.; Mccoy, T. J.; Keil, K.

1993-01-01

Three metal-rich meteorites - Mt. Egerton, Horse Creek, and LEW 88055 - were studied and it is suggested that they formed in the aubrite parent body. LEW 85369 and 88631 may also have a common origin, but these rocks have not yet been studied in detail. This body was probably heated to about 1600 C by a very strong heat source. While molten, metal agglomerated into sizeable nodules which never segregated efficiently to form a core, but were trapped in the silicate mantle. Different clasts and lithologies in aubrites solidified and cooled under local equilibrium conditions of oxygen fugacity, and with different thermal histories. Impacts mixed clasts from throughout the parent body, creating the typical aubrite breccias.

Dehydration of subducting slow-spread oceanic lithosphere in the Lesser Antilles.

PubMed

Paulatto, Michele; Laigle, Mireille; Galve, Audrey; Charvis, Philippe; Sapin, Martine; Bayrakci, Gaye; Evain, Mikael; Kopp, Heidrun

2017-07-10

Subducting slabs carry water into the mantle and are a major gateway in the global geochemical water cycle. Fluid transport and release can be constrained with seismological data. Here we use joint active-source/local-earthquake seismic tomography to derive unprecedented constraints on multi-stage fluid release from subducting slow-spread oceanic lithosphere. We image the low P-wave velocity crustal layer on the slab top and show that it disappears beneath 60-100 km depth, marking the depth of dehydration metamorphism and eclogitization. Clustering of seismicity at 120-160 km depth suggests that the slab's mantle dehydrates beneath the volcanic arc, and may be the main source of fluids triggering arc magma generation. Lateral variations in seismic properties on the slab surface suggest that serpentinized peridotite exhumed in tectonized slow-spread crust near fracture zones may increase water transport to sub-arc depths. This results in heterogeneous water release and directly impacts earthquakes generation and mantle wedge dynamics.

Dehydration of subducting slow-spread oceanic lithosphere in the Lesser Antilles

PubMed Central

Paulatto, Michele; Laigle, Mireille; Galve, Audrey; Charvis, Philippe; Sapin, Martine; Bayrakci, Gaye; Evain, Mikael; Kopp, Heidrun

2017-01-01

Subducting slabs carry water into the mantle and are a major gateway in the global geochemical water cycle. Fluid transport and release can be constrained with seismological data. Here we use joint active-source/local-earthquake seismic tomography to derive unprecedented constraints on multi-stage fluid release from subducting slow-spread oceanic lithosphere. We image the low P-wave velocity crustal layer on the slab top and show that it disappears beneath 60–100 km depth, marking the depth of dehydration metamorphism and eclogitization. Clustering of seismicity at 120–160 km depth suggests that the slab’s mantle dehydrates beneath the volcanic arc, and may be the main source of fluids triggering arc magma generation. Lateral variations in seismic properties on the slab surface suggest that serpentinized peridotite exhumed in tectonized slow-spread crust near fracture zones may increase water transport to sub-arc depths. This results in heterogeneous water release and directly impacts earthquakes generation and mantle wedge dynamics. PMID:28691714

Kaersutite-bearing xenoliths and megacrysts in volcanic rocks from the Funk Seamount in the souhtwest Indian Ocean

NASA Technical Reports Server (NTRS)

Reid, Arch M.; Le Roex, Anton P.

1988-01-01

The petrography, mineral chemistry, and whole-rock compositions of volcanic rocks dredged from the Funk Seamount, located 60 km NW of Marion Island in the southwestern Indian Ocean, are presented together with the mineral chemistry of their inclusions. On the basis of these characteristics, the possible relationships between the Funk Seamount's volcanic rocks and the megacrysts and xenoliths in these rocks are discussed. It is argued that the Funk Seamount lavas derive from a similar mantle source region as that of the Marion Island and Prince Edward Island hotspot lavas. The geochemical signature of these lavas implies derivation from a source that is enriched (e.g., in Ti, K, P, and Nb) over the depleted mantle source regions for the adjacent mid-ocean ridge basalts.

Hydrogen Isotope Geochemistry of Mariana Trough Lavas

NASA Astrophysics Data System (ADS)

Oleary, J.; Kitchen, N.; Eiler, J.

2002-12-01

Basaltic lavas from the Marianas trough vary in water content from values similar to mid-ocean ridge basalts (MORBs) to ten times those values. These variations plausibly reflect addition of subducted water to the mantle wedge, but must also reflect variations in extent of melting and crystallization-differentiation. We report hydrogen isotope data for 18 samples of lavas from the Mariana trough; these measurements, when combined with other geochemical data, constrain the relative proportions of subducted vs. 'primitive' water in their mantle sources. Previous measurements of the hydrogen isotope composition of Mariana trough lavas [1] found a correlation between dD and measured water content, consistent with two-component mixing between water in the ambient MORB source and water from the subducted slab, but include only four samples, only two of which have known major and minor element geochemistry. Our purpose is to confirm this result and expand it to include a more representative sampling. Our measurements made use of a recently developed technique for on-line stepped heating, water reduction and hydrogen isotope mass spectrometry [2]. This method is appropriate for relatively small samples of basaltic glass (ca. 100 μg to 1 mg) and up to 10 analyses can be performed per day. Its principle advantages for our purposes are that it can be applied to even small or glass-poor samples and it is fast enough to permit replication of all data and analysis of relatively large numbers of standards. Hydrogen isotope compositions of Mariana trough lavas vary between -74 per mil and -34 per mil (SMOW); this compares with a range of -46 to -32 per mil for related lavas in [1] and is similar to the previously observed range for back-arc-basin basalts generally (-70 to -32 per mil). Two-thirds of our sample suite span a small range in dD (-40+/-4 ). We suggest this average is the most representative value for back arc basin basalts measured to-date. Our data are inconsistent with the correlation between dD and measured water content suggested for back-arc basin basalts by [ref], even considering only lavas spanning a small range in MgO. This suggests one or both of two things: (1) melting and/or crystallization differentiation produce variations in water abundance unrelated to the abundance and dD of water in the mantle source; (2) there are three or more reservoirs in the mantle wedge of the Mariana arc, all of which differ in dD and water content (i.e., such that data do not define a simple line in a plot of dD vs. 1/H2O). The first of these is significant, but there is also evidence for the second. In particular, dD values decrease monotonically with increasing abundance of highly incompatible trace elements and with increasing La/Sm and K2O/H2O ratios; the lowest dD sample in our suite is an enriched basalt (La/Sm = 3.6) with an 'arc like' K2O content (0.71 wt. %). These data suggest that water in the mantle wedge of the Mariana arc is derived from three sources: ambient water common to the MORB source (ca. 0.02 wt. % H2O; dD ~ -65 to -75 per mil), subducted water (dD ~ -30 per mil) and an enriched source having high abundances of water and other incompatible trace elements and a dD value of ca. -80 per mil. [1] Poreda, 1985, EPSL 73, 244-254 [2] Eiler and Kitchen, 2001, GCA 65, 24, 4467-4479

Source Parameters for Moderate Earthquakes in the Zagros Mountains with Implications for the Depth Extent of Seismicity

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

Adams, A; Brazier, R; Nyblade, A

2009-02-23

Six earthquakes within the Zagros Mountains with magnitudes between 4.9 and 5.7 have been studied to determine their source parameters. These events were selected for study because they were reported in open catalogs to have lower crustal or upper mantle source depths and because they occurred within an area of the Zagros Mountains where crustal velocity structure has been constrained by previous studies. Moment tensor inversion of regional broadband waveforms have been combined with forward modeling of depth phases on short period teleseismic waveforms to constrain source depths and moment tensors. Our results show that all six events nucleated withinmore » the upper crust (<11 km depth) and have thrust mechanisms. This finding supports other studies that call into question the existence of lower crustal or mantle events beneath the Zagros Mountains.« less

Lunar ferroan anorthosites and mare basalt sources - The mixed connection

NASA Technical Reports Server (NTRS)

Ryder, Graham

1991-01-01

Global overturn of a hot, gravitationally unstable lunar mantle immediately following the solidification of a magma ocean explains several characteristics of lunar petrology. Lunar mare basalt sources are inferred to be depleted in europium and alumina. These depletions are consensually attributed to complementary plagioclase floating from a magma ocean. However, in contrast to the mare basalt source parent magma, the ferroan anorthosite parent magma was more evolved by virtue of its lower Mg/Fe ratio and Ni abundances, although less evolved in its poverty of clinopyroxene constituents, flat rare earth pattern, and lower incompatible element abundances. The europium anomaly in mare sources is inferred to be present at 400 km depth, too deep to have been directly influenced by plagioclase crystallization. Massive overturning of the post-magma ocean mantle would have carried down clinopyroxene, ilmenite, and phases containing fractionated rare earths, europium anomalies, and some heat-producing radionuclides.

Piecewise Delamination Drives Uplift in the Atlas Mountains Region of Morocco

NASA Astrophysics Data System (ADS)

Bezada, M. J.; Humphreys, E.; Martin Davila, J.; mimoun, H.; Josep, G.; Palomeras, I.

2013-12-01

The elevation of the intra-continental Atlas Mountains of Morocco and surrounding regions requires a mantle component of buoyancy, and there is consensus that this buoyancy results from an abnormally thin lithosphere. Lithospheric delamination under the Atlas Mountains and thermal erosion caused by upwelling mantle have each been suggested as thinning mechanisms. We use seismic tomography to image the upper mantle of Morocco by inverting teleseimic p-wave delay times, complemented with local delays, recorded on a dense array of stations in the Iberian peninsula and Morocco. A surface wave model provides constraint on the shallower layers. We determine the geometry of lithospheric cavities and mantle upwelling beneath the Middle Atlas and central High Atlas, and image delaminated lithosphere at ~400 km beneath the Middle Atlas. We propose discontinuous delamination of an intrinsically unstable Atlas lithosphere, enabled by the presence of anomalously hot mantle, as a mechanism for producing the imaged structures. The Atlas lithosphere was made unstable by a combination of tectonic shortening and eclogite loading during Mesozoic rifting and Cenozoic magmatism. The presence of hot mantle, sourced from regional upwellings in northern Africa or the Canary Islands, enabled the mobilization of this lithosphere. Flow around the retreating Alboran slab focused upwelling mantle under the Middle Atlas, where we image the most recent delamination. The Atlas Mountains of Morocco stand as an example of mantle-generated uplift and large-scale lithospheric loss in a mildly contractional orogen.

Mantle Flow Induced by Subduction Beneath Taurides Mountains

NASA Astrophysics Data System (ADS)

Hui, H.; Sandvol, E. A.; Rey, P. F.; Brocard, G. Y.

2017-12-01

GPS data of Anatolian Plateau shows westward plate motion with respect to the Eurasian plate at a rate of approximately 20 mm/yr, however, the fast direction of shear-wave splitting data in Anatolian Plateau is dominantly northeast-southwest, with significant variations around the central Taurides Mountains. To address the decoupling between the deformation in the crust and in the mantle, we explore the mantle strain pattern beneath Anatoian Plateau. Numerical models of the African plate subducting beneath the Taurides have been constructed with the open source code Underworld by Louis Moresi and the Lithospheric Modeling Recipe by EarthByte Group. We have constructed a 2-D model with dimension of 400km × 480km with 60km thick plate subducting into the mantle. In our numerical model, we observe a poloidal component of the mantle flow around the edge of the subducting plate, which could be explained by straight-forward corner flow. The horizontal component of mantle flow above the subducting plate may explain the shear-wave splitting pattern that is nearly perpendicular to the trench at Anatolia. We are also working on 3-D models with dimension of 400km×400km×480km with the subducting plate width 100km. The asthenospheric mantle below the subducting plate exhibits a flow parallel to the trench, then rotates around the edge of the plate and becomes perpendicular to the trench. This mantle flow pattern may explain the shear-wave splitting directions in central Anatolia.

The genetic link between the Azores Archipelago and the Southern Azores Seamount Chain (SASC): The elemental, isotopic and chronological evidences

NASA Astrophysics Data System (ADS)

Ribeiro, Luisa Pinto; Martins, Sofia; Hildenbrand, Anthony; Madureira, Pedro; Mata, João

2017-12-01

New geochemical, isotopic (Sr-Nd-Hf-Pb) and K-Ar data, are presented here on samples from the Southern Azores Seamount Chain (SASC) located south of the Azores Plateau. The SASC also includes the Great Meteor, Small Meteor and Closs seamounts, morphologically connected by a saddle at - 4100 m deep. We conclude that the SASC are characterized by a narrow isotopic variability that falls within the Azores isotopic field. Although each seamount has its own isotopic signature, their mantle source must comprise four local mantle end-members, three of which are common to the Azores, e.g. Plato isotopic signature results from the mixing between HIMU and N-MORB while Great Meteor signature results from this mix with the Azores Common Component (AzCC). A fourth end-member with high 208Pb/204Pb and decoupled Th/U ratios (Δ8/4 up to 59.2) is identified on Great Meteor northern flank. New K-Ar ages on Plato (33.4 ± 0.5 Ma) and Small Hyeres (31.6 ± 0.4 Ma) show nearly coeval volcanism, which is contemporaneous with the E-MORBs erupted at the MAR, drilled on oceanic crust with 30-34 Ma (DSDP82). This study endorses the genetic link between the Azores Archipelago and the SASC to the long-term activity of the Azores plume and the large-scale ridge-hotspot interaction, contributing to better constrain the temporal-spatial evolution of this region of the North Atlantic.

Latest Pleistocene crustal cannibalization at Baekdusan (Changbaishan) as traced by oxygen isotopes of zircon from the Millennium Eruption

NASA Astrophysics Data System (ADS)

Cheong, Albert Chang-sik; Sohn, Young Kwan; Jeong, Youn-Joong; Jo, Hui Je; Park, Kye-Hun; Lee, Youn Soo; Li, Xian-Hua

2017-07-01

The silicic volcanism of Baekdusan (Changbaishan), which is on the border between North Korea and China, was initiated in the Late Pleistocene and culminated in the 10th century with a powerful (volcanic explosivity index = 7) commendite-trachyte eruption commonly referred to as the "Millennium Eruption." This study presents oxygen isotope data of zircon in trachydacitic pumices ejected during the Millennium Eruption, together with whole-rock geochemical and Sr-Nd-Pb isotopic data that manifest once again the A-type and EM1 affinities of the Millennium Eruption magma. The zircon crystals, dated by previous studies at ca. 12-9 ka, show a moderate inter-grain variation in δ18O from 3.69‰ to 5.03‰. These values are consistently lower than the normal mantle range, and interpreted to have resulted from the digestion of meteoric-hydrothermally altered intracaldera rocks in the shallow magma chamber beneath Baekdusan just prior to the crystallization of the zircons, rather than from derivation from low-δ18O sources deep in the mantle. The whole-rock geochemical/isotopic considerations suggest that the magma mainly self-cannibalized the earlier erupted volcanic carapace around the magma chamber. This study highlights the usefulness of zircon oxygen isotopes for characterizing past volcanic activity that has now been commonly eroded away and implies that the generation of Yellowstone-type low-δ18O magma is not a rare phenomenon in large-volume silicic eruptions.

Insights into Along Strike Variability in the Lau Back Spreading Center and Tonga Arc from Bodywave Tomography

NASA Astrophysics Data System (ADS)

Adams, A. N.; Wiens, D.; Barklage, M.; Conder, J. A.; Wei, S. S.; Cai, C.

2016-12-01

The Lau Backarc Spreading Center (LBSC) and the Tonga Arc offer an excellent location to study the complex interactions between magma production in subduction arcs and backarcs. Although the LBSC is often considered to be an archetype of backarc spreading centers, the system exhibits major along strike changes in surficial and subsurface characteristics - including rift morphology, spreading and subduction rates, rift-arc separation, magma production, and crustal thickness. These variations, together with geochemical evidence, suggest that mixing of arc and backarc magmas may occur at depth beneath the southern LBSC, where the backarc spreading center and the Tonga Arc are most proximal. To investigate magma production and transport beneath the LBSC and the Tonga Arc, this study jointly inverts arrivals from local and teleseismic earthquakes at 51 OBS and 16 land stations to create P- and S-wave upper mantle velocity models. Results from this study show that low velocity zones associated with the LBSC and Tonga Arc are distinctly separated in the north, but merge to a single low velocity zone in the south, supporting prior geochemical evidence for a common source of arc and backarc magmas in the south. Low velocities beneath the LBSC tilt westward with depth, consistent with predictions from numerical models for asymmetrical melting in the mantle wedge. Beneath the central LBSC, low velocities extend to depths of 300 km, suggesting a deep source for melt in some regions.

Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars.

PubMed

Franz, Heather B; Kim, Sang-Tae; Farquhar, James; Day, James M D; Economos, Rita C; McKeegan, Kevin D; Schmitt, Axel K; Irving, Anthony J; Hoek, Joost; Dottin, James

2014-04-17

The geochemistry of Martian meteorites provides a wealth of information about the solid planet and the surface and atmospheric processes that occurred on Mars. The degree to which Martian magmas may have assimilated crustal material, thus altering the geochemical signatures acquired from their mantle sources, is unclear. This issue features prominently in efforts to understand whether the source of light rare-earth elements in enriched shergottites lies in crustal material incorporated into melts or in mixing between enriched and depleted mantle reservoirs. Sulphur isotope systematics offer insight into some aspects of crustal assimilation. The presence of igneous sulphides in Martian meteorites with sulphur isotope signatures indicative of mass-independent fractionation suggests the assimilation of sulphur both during passage of magmas through the crust of Mars and at sites of emplacement. Here we report isotopic analyses of 40 Martian meteorites that represent more than half of the distinct known Martian meteorites, including 30 shergottites (28 plus 2 pairs, where pairs are separate fragments of a single meteorite), 8 nakhlites (5 plus 3 pairs), Allan Hills 84001 and Chassigny. Our data provide strong evidence that assimilation of sulphur into Martian magmas was a common occurrence throughout much of the planet's history. The signature of mass-independent fractionation observed also indicates that the atmospheric imprint of photochemical processing preserved in Martian meteoritic sulphide and sulphate is distinct from that observed in terrestrial analogues, suggesting fundamental differences between the dominant sulphur chemistry in the atmosphere of Mars and that in the atmosphere of Earth.

Helium Flux from the Earth's Mantle as Estimated from Hawaiian Fumarolic Degassing.

PubMed

Naughton, J J; Lee, J H; Keeling, D; Finlayson, J B; Dority, G

1973-04-06

Averaged helium to carbon dioxide ratios measured from systematic collections of gases from Sulphur Bank fumarole. Kilauea, Hawaii, when coupled with estimates of carbon in the earth's crust, give a helium flux of 1 x 105 atoms per square centimeter per second. This is within the lower range of other estimates, and may represent the flux from deep-seated sources in the upper mantle.

Evaluating Crustal Contamination Effects On The Lithophile Trace Element Budget Of Shergottites, NWA 856 As A Test Case

NASA Technical Reports Server (NTRS)

Brandon, A. D.; Ferdous, J.; Peslier, A. H.

2017-01-01

The issue of whether crustal contamination has affected the lithophile trace element budget of shergottites has been a point of contention for decades. The evaluation has focused on the enriched shergottite compositions as an outcome of crustal contamination of mantle-derived parent magmas or, alternatively, the compositions of these stones reflect an incompatible trace element (ITE) enriched mantle source.

a Steady Thermal State for the Earth's Interior

NASA Astrophysics Data System (ADS)

Andrault, D.; Monteux, J.; Le Bars, M.; Samuel, H.

2015-12-01

Large amounts of heat are permanently lost at the surface yielding the classic view of the Earth continuously cooling down. Contrary to this conventional depiction, we propose that the temperature profile in the deep Earth has remained almost constant for the last ~3 billion years (Ga) or more. The core-mantle boundary (CMB) temperature reached the mantle solidus of 4100 (+/-300) K after complete crystallization of the magma ocean not more than 1 Ga after the Moon-forming impact. The CMB remains at a similar temperature today; seismological evidences of ultra-low velocity zones suggest partial melting in the D"-layer and, therefore, a current temperature at, or just below, the mantle solidus. Such a steady thermal state of the CMB temperature excludes thermal buoyancy and compositional convection from being the predominant mechanisms to power the geodynamo over geological time. An alternative mechanism to produce motion in the outer core is mechanical forcing by tidal distortion and planetary precession. The conversion of gravitational and rotational energies of the Earth-Moon-Sun system to core motions could have supplied the lowermost mantle with a variable intensity heat source through geological time, due to the regime of core instabilities and/or changes in the astronomical forces. This variable heat source could explain the dramatic volcanic events that occurred in the Earth's history.

Slab melting beneath the Cascade Arc driven by dehydration of altered oceanic peridotite

NASA Astrophysics Data System (ADS)

Walowski, K. J.; Wallace, P. J.; Hauri, E. H.; Wada, I.; Clynne, M. A.

2015-05-01

Water is returned to Earth’s interior at subduction zones. However, the processes and pathways by which water leaves the subducting plate and causes melting beneath volcanic arcs are complex; the source of the water--subducting sediment, altered oceanic crust, or hydrated mantle in the downgoing plate--is debated; and the role of slab temperature is unclear. Here we analyse the hydrogen-isotope and trace-element signature of melt inclusions in ash samples from the Cascade Arc, where young, hot lithosphere subducts. Comparing these data with published analyses, we find that fluids in the Cascade magmas are sourced from deeper parts of the subducting slab--hydrated mantle peridotite in the slab interior--compared with fluids in magmas from the Marianas Arc, where older, colder lithosphere subducts. We use geodynamic modelling to show that, in the hotter subduction zone, the upper crust of the subducting slab rapidly dehydrates at shallow depths. With continued subduction, fluids released from the deeper plate interior migrate into the dehydrated parts, causing those to melt. These melts in turn migrate into the overlying mantle wedge, where they trigger further melting. Our results provide a physical model to explain melting of the subducted plate and mass transfer from the slab to the mantle beneath arcs where relatively young oceanic lithosphere is subducted.

Evolution of depleted mantle: The lead perspective

NASA Astrophysics Data System (ADS)

Tilton, George R.

1983-07-01

Isotopic data have established that, compared to estimated bulk earth abundances, the sources of oceanic basaltic lavas have been depleted in large ion lithophile elements for at least several billions of years. Various data on the Tertiary-Mesozoic Gorgona komatiite and Cretaceous Oka carbonatite show that those rocks also sample depleted mantle sources. This information is used by analogy to compare Pb isotopic data from 2.6 billion year old komatiite and carbonatite from the Suomussalmi belt of eastern Finland and Munro Township, Ontario that are with associated granitic rocks and ores that should contain marked crustal components. Within experimental error no differences are detected in the isotopic composition of initial Pb in either of the rock suites. These observations agree closely with Sr and Nd data from other laboratories showing that depleted mantle could not have originated in those areas more than a few tenths of billions of years before the rocks were emplaced. On a world-wide basis the Pb isotope data are consistent with production of depleted mantle by continuous differentiation processes acting over approximately the past 3 billion years. The data show that Pb evolution is more complex than the simpler models derived from the Rb-Sr and Sm-Nd systems. The nature of the complexity is still poorly understood.

Petrology of Hualalai volcano, Hawaii: Implication for mantle composition

USGS Publications Warehouse

Clague, D.A.; Jackson, E.D.; Wright, T.L.

1980-01-01

Hualalai is one of five volcanoes whose eruptions built the island of Hawaii. The historic 1800-1801 flows and the analyzed prehistoric flows exposed at the surface are alkalic basalts except for a trachyte cone and flow at Puu Waawaa and a trachyte maar deposit near Waha Pele. The 1800-1801 eruption produced two flows: the upper Kaupulehu flow and the lower Huehue flow. The analyzed lavas of the two 1800-1801 flows are geochemically identical with the exception of a few samples from the toe of the Huehue flow that appear to be derived from a separate magmatic batch. The analyzed prehistoric basalts are nearly identical to the 1800-1801 flows but include some lavas that have undergone considerable shallow crystal fractionation. The least fractionated alkalic basalts from Hualalai are in equilibrium with mantle olivine (Fo87) indicating that the Hawaiian mantle source region is not unusually iron-rich. The 1800-1801 and analyzed prehistoric basalts can be generated by about 5-10% partial fusion of a garnet-bearing source relatively enriched in the light-rare-earths. The mantle underlying the Hawaiian Islands is chemically and mineralogically heterogeneous before and after extraction of the magmas that make up the volcanoes. ?? 1980 Intern. Association of Volcanology and Chemistry of the Earth's Interior.

Slab melting beneath the Cascades Arc driven by dehydration of altered oceanic peridotite

USGS Publications Warehouse

Walowski, Kristina J; Wallace, Paul J.; Hauri, E.H.; Wada, I.; Clynne, Michael A.

2015-01-01

Water is returned to Earth’s interior at subduction zones. However, the processes and pathways by which water leaves the subducting plate and causes melting beneath volcanic arcs are complex; the source of the water—subducting sediment, altered oceanic crust, or hydrated mantle in the downgoing plate—is debated; and the role of slab temperature is unclear. Here we analyse the hydrogen-isotope and trace-element signature of melt inclusions in ash samples from the Cascade Arc, where young, hot lithosphere subducts. Comparing these data with published analyses, we find that fluids in the Cascade magmas are sourced from deeper parts of the subducting slab—hydrated mantle peridotite in the slab interior—compared with fluids in magmas from the Marianas Arc, where older, colder lithosphere subducts. We use geodynamic modelling to show that, in the hotter subduction zone, the upper crust of the subducting slab rapidly dehydrates at shallow depths. With continued subduction, fluids released from the deeper plate interior migrate into the dehydrated parts, causing those to melt. These melts in turn migrate into the overlying mantle wedge, where they trigger further melting. Our results provide a physical model to explain melting of the subducted plate and mass transfer from the slab to the mantle beneath arcs where relatively young oceanic lithosphere is subducted.

Contrasting volcanism in Hawaiʻi and the Galápagos

USGS Publications Warehouse

Poland, Michael P; Harpp, Karen S.; Mittelstaedt, Eric; d'Ozouville, Noémi; Graham, David W.

2014-01-01

The archipelagos of Hawai‘i and the Galápagos originated at mantle hotspots, yet the volcanoes that make up the island chains differ in most respects. Some of the most important differences include the dynamics of magma supply, characteristics of magma storage and transport, morphology, and compositional and structural evolution. Of particular significance in the Galápagos is the lack of well-developed rift zones, which may be related to higher rates of pre-eruptive inflation compared to Hawai‘i, and the absence of widespread flank instability—a common feature of Hawai‘i's volcanoes. The close proximity of the Galápagos to a mid-ocean-ridge system may account for many of the differences between Hawaiian and Galápagos volcanoes. The Galápagos archipelago is built on young, thin oceanic crust, which might allow for contemporaneous growth of numerous volcanoes, and its volcanoes are fed by a mix of plume and asthenospheric melt sources. Hawaiian volcanoes, in contrast, grew in the middle of the Pacific Plate on older, thicker crust, where localized changes in mantle and lithosphere structure and composition did not exert dominant control over volcano evolution.

Yellow-bellied marmot and golden-mantled ground squirrel responses to heterospecific alarm calls

PubMed

Shriner

1998-03-01

When two species have predators in common, animals might be able to obtain important information about predation risk from the alarm calls produced by the other species. The behavioural responses of adult yellow-bellied marmots, Marmota flaviventris, and golden-mantled ground squirrels, Spermophilus lateralis, to conspecific and heterospecific alarm calls were studied to determine whether interspecific call recognition occurs in sympatric species that rarely interact. In a crossed design, marmot and squirrel alarm calls were broadcast to individuals of both species, using the song of a sympatric bird as a control. Individuals of both species responded similarly to conspecific and heterospecific anti-predator calls, and distinguished both types of alarms from the bird song. These results indicate that both marmots and squirrels recognized not only their own species' anti-predator vocalizations, but also the alarm calls of another species, and that these vocalizations were discriminated from an equally loud non-threatening sound. These findings suggest that researchers ought to think broadly when considering the sources of information available to animals in their natural environment. Copyright 1998 The Association for the Study of Animal Behaviour Copyright 1998 The Association for the Study of Animal Behaviour.

Prodigious degassing of a billion years of accumulated radiogenic helium at Yellowstone

USGS Publications Warehouse

Lowenstern, Jacob B.; Evans, William C.; Bergfeld, D.; Hunt, Andrew G.

2014-01-01

Helium is used as a critical tracer throughout the Earth sciences, where its relatively simple isotopic systematics is used to trace degassing from the mantle, to date groundwater and to time the rise of continents1. The hydrothermal system at Yellowstone National Park is famous for its high helium-3/helium-4 isotope ratio, commonly cited as evidence for a deep mantle source for the Yellowstone hotspot2. However, much of the helium emitted from this region is actually radiogenic helium-4 produced within the crust by α-decay of uranium and thorium. Here we show, by combining gas emission rates with chemistry and isotopic analyses, that crustal helium-4 emission rates from Yellowstone exceed (by orders of magnitude) any conceivable rate of generation within the crust. It seems that helium has accumulated for (at least) many hundreds of millions of years in Archaean (more than 2.5 billion years old) cratonic rocks beneath Yellowstone, only to be liberated over the past two million years by intense crustal metamorphism induced by the Yellowstone hotspot. Our results demonstrate the extremes in variability of crustal helium efflux on geologic timescales and imply crustal-scale open-system behaviour of helium in tectonically and magmatically active regions.

Mantle evolution on Mars: Constraints from Lu-Hf and Sm-Nd isotope systematics of SNC meteorites

NASA Astrophysics Data System (ADS)

Scherer, E. E.; Kurahashi, E.; Mezger, K.

2012-12-01

The long-lived 176Lu-176Hf and 147Sm-143Nd isotope systems are commonly employed to track the evolution of complementary mantle and crust reservoirs. The four elements involved are refractory and lithophile, and thus their relative abundances are not expected to have been changed by accretion or core formation. Subsequent silicate differentiation processes, however, e.g., the formation of crust by extraction of melts from the mantle, will fractionate Lu/Hf and Sm/Nd. This typically leaves a depleted mantle with higher Lu/Hf and Sm/Nd values than those of the undifferentiated, presumably chondritic parental reservoir. On the other hand, these same values in crustal rocks tend to be lower than those of their source. (Apparent exceptions are the Martian shergottites, which tend to have lower Lu/Hf as expected, but Sm/Nd higher than their presumed sources. Such decoupling of the two isotope systems may be explained by two-stage melting [e.g., 1, 5].) The ensuing chemical variability among secondary and later generation silicate reservoirs causes their isotopic compositions (e.g., 176Hf/177Hf and 143Nd/144Nd) to diverge from that of the bulk silicate planet over hundreds of millions of years. The resulting isotopic diversity preserved (SNC) meteorites is being used to constrain the differentiation history, melting mineralogy, and dynamics of the Martian mantle [e.g., 1-8]. However, interpretations based on the initial isotope compositions of Hf and Nd strongly depend on the accuracy of crystallization ages. The ages of shergottites in particular are debated (e.g., [3,4,7]). To resolve this issue and gain a better understanding of Martian mantle evolution, we are investigating the Lu-Hf and Sm-Nd systematics of bulk SNC meteorites and constructing internal (mineral) isochrons. Eleven bulk Martian meteorites (5 shergottites, 4 nakhlites, and 2 chassignites) were digested without prior leaching in high-pressure autoclaves for 5 days. Initial ɛ176Hf and ɛ143Nd values range from +49.0 to +51.3 and +34.7 to +47.8, respectively, for depleted shergottites, -13.3 to -17.4 and -6.3 to -6.5 for enriched shergottites, +1.8 to +14.5 and +13.4 to +15.6 for nakhlites, and +16.3 to +24.9 and +15.3 to +15.4 for chassignites. These data agree well with those of earlier studies [e.g., 2-8]. The initial ɛ143Nd of nakhlites and chassignites vary little relative to ɛ176Hf. The time-integrated 176Lu/177Hf and 147Sm/144Nd of all investigated source reservoirs form a narrow trend within the terrestrial MORB + OIB field, with the sources of nakhlites and chassignites located between those of depleted and enriched shergottites. Shergottites themselves display a large range of 147Sm/144Nd with higher values at a given 176Lu/177Hf relative to MORB + OIB. Evidence for both young (474-150 Ma) and old (> 4 Ga) shergottite ages will be evaluated and the effects of these disparate ages on models of silicate differentiation on Mars will be illustrated. [1] Borg et al., (1997) GCA 61:4915-4931. [2] Blichert-Toft J. et al. (1999) EPSL 173:25-39. [3] Bouvier A. et al. (2005) EPSL 240:221-233. [4] Bouvier A. et al. (2008) EPSL 266:105-124. [5] Debaille V. et al. (2008) EPSL 269:186-199. [6] Debaille V. et al. (2009) Nature Geosci. 2:548-552. [7] Shafer J. T. et al. (2010) GCA 74:7307-7328. [8] Lapen T. J. et al. (2010) Science 328:347-351.

Toward a Greater Kerguelen large igneous province: Evolving mantle source contributions in and around the Indian Ocean

NASA Astrophysics Data System (ADS)

Olierook, Hugo K. H.; Merle, Renaud E.; Jourdan, Fred

2017-06-01

The link between the Kerguelen large igneous province and several moderately-voluminous magmatic domains emplaced on continental crust near the relict triple junction of eastern Gondwana remains tentative. In particular, linking Sr-Nd-Pb isotopic ratios of the 90,000 km2 submerged Naturaliste Plateau at the relict triple junction of eastern Gondwana to the Kerguelen LIP were difficult due to previous age estimates of ca. 100 Ma. Sericite hydrothermal plateau ages as old as 127.6 ± 0.6 Ma indicate that the volcanism on the plateau began at or prior to ca. 128 Ma, which is > 25 m.y. older than previous estimations. These ages are closely matched by the then-nearby ca. 140-130 Ma Comei, 137-130 Ma Bunbury, 124 Ma Wallaby Plateau and 118-117 Ma Rajmahal-Bengal-Sylhet magmatic provinces. The Sr-Nd-Pb isotopic characteristics of the majority of these ca. 140-117 Ma circum-eastern Gondwana magmatic provinces display only source contributions from the depleted asthenosphere and lithosphere with negligible contribution from the Kerguelen mantle plume. The Comei Province shows a direct plume-related melt signature, probably because it sits directly in the center of the modeled plume head position at 140-130 Ma. We suggest that the Kerguelen mantle plume provided the additional heat necessary to melt the asthenosphere and lithosphere of the circum-eastern Gondwanan magmatic provinces. Only after the motion of the Kerguelen plume head into the nascent Indian Ocean at ca. 100-95 Ma does a significant melt contribution from the Kerguelen mantle plume become evident in the isotopic signature, a signal that persists until the present-day. Despite differences in source contributions over time, it is clear that the Kerguelen mantle plume is necessary for the production of all the circum-eastern Gondwana magmatic domains, which we propose should be referred to as the Greater Kerguelen Large Igneous Province.

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

NASA Astrophysics Data System (ADS)

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

2008-12-01

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

Nature of the Mantle Sources and Bearing on Tectonic Evolution in the West Antarctic Rift System

NASA Astrophysics Data System (ADS)

Mukasa, S. B.; Rilling-Hall, S.; Marcano, M. C.; Wilson, T. J.; Lawver, L. A.; LeMasurier, W. E.

2012-12-01

We collected samples from subaerial lava flows and dredged some Neogene basanitic lavas from seven volcanic edifices in the Ross Sea, Antarctica - a part of the West Antarctic Rift System (WARS) and one of the world's largest alkaline magmatic provinces - for a study aimed at two principal objectives: (1) Geochemical interrogation of the most primitive magmatic rocks to try and understand the nature of the seismically abnormal mantle domain recently identified beneath the shoulder of the Transantarctic Mountains (TAM), the Ross Sea Embayment and Marie Byrd Land; and (2) Using 40Ar/39Ar geochronology to establish a temporal link between magmatism and tectonism, particularly in the Terror Rift. We have attempted to answer the questions of whether magmatism is due to a hot mantle or wet mantle, and whether rifting in the area triggered magmatic activity or vice versa. Results show that the area does not have an age-progressive hotspot track, and the magmatism post-dates the main phase of extension along the Terror Rift within the WARS, which supports a decompression-melting model without the benefit of a significant thermal anomaly. In fact, preliminary volatile measurements on olivine-hosted melt inclusions have yielded water concentrations in excess of 2 wt%, indicating that flux melting was an important complementary process to decompression melting. The major oxide compositions of lavas in the WARS are best matched to experimental melts of carbonated peridotite, though garnet pyroxenite can also be a minor source. The Pb and Nd isotopic systems are decoupled from each other, suggesting removal of fluid-mobile elements from the mantle source possibly during the long history of subduction along the Paleo-Pacific margin of Gondwana. Extremely unradiogenic 187Os/188Os ranging to as low as 0.1081 ± 0.0001 hints at the involvement of lithospheric components in generation of magmas in the WARS.

Volatile composition of microinclusions in diamonds from the Panda kimberlite, Canada: Implications for chemical and isotopic heterogeneity in the mantle

NASA Astrophysics Data System (ADS)

Burgess, Ray; Cartigny, Pierre; Harrison, Darrell; Hobson, Emily; Harris, Jeff

2009-03-01

In order to better investigate the compositions and the origins of fluids associated with diamond growth, we have carried-out combined noble gas (He and Ar), C and N isotope, K, Ca and halogen (Cl, Br, I) determinations on fragments of individual microinclusion-bearing diamonds from the Panda kimberlite, North West Territories, Canada. The fluid concentrations of halogens and noble gases in Panda diamonds are enriched by several orders of magnitude over typical upper mantle abundances. However, noble gas, C and N isotopic ratios ( 3He/ 4He = 4-6 Ra, 40Ar/ 36Ar = 20,000-30,000, δ 13C = -4.5‰ to -6.9‰ and δ 15N = -1.2‰ to -8.8‰) are within the worldwide range determined for fibrous diamonds and similar to the mid ocean ridge basalt (MORB) source value. The high 36Ar content of the diamonds (>1 × 10 -9 cm 3/g) is at least an order of magnitude higher than any previously reported mantle sample and enables the 36Ar content of the subcontinental lithospheric mantle to be estimated at ˜0.6 × 10 -12 cm 3/g, again similar to estimates for the MORB source. Three fluid types distinguished on the basis of Ca-K-Cl compositions are consistent with carbonatitic, silicic and saline end-members identified in previous studies of diamonds from worldwide sources. These fluid end-members also have distinct halogen ratios (Br/Cl and I/Cl). The role of subducted seawater-derived halogens, originally invoked to explain some of the halogen ratio variations in diamonds, is not considered an essential component in the formation of the fluids. In contrast, it is considered that large halogen fractionation of a primitive mantle ratio occurs during fluid-melt partitioning in forming silicic fluids, and during separation of an immiscible saline fluid.

Mantle amphibole control on arc and within-plate chemical signatures: Quaternary lavas from Kurdistan Province, Iran

NASA Astrophysics Data System (ADS)

Kheirkhah, M.; Allen, M. B.; Neill, I.; Emami, M. H.; McLeod, C.

2012-04-01

New analyses of Quaternary lavas from Kurdistan Province in west Iran shed light on the nature of collision zone magmatism. The rocks are from the Turkish-Iranian plateau within the Arabia-Eurasia collision. Compositions are typically basanite, hawaiite and alkali basalt. Sr-Nd isotope values are close to BSE, which is similar to Quaternary alkali basalts of NW Iran, but distinct from a depleted source melting under Mount Ararat. The chemical signatures suggests variable melting of two distinct sources. One inferred source produced melts with La/Nb from~3.5 to~1.2, which we model as the result of depletion of amphibole during ≤1% melting in the garnet stability field. We infer phlogopite in the source of potassic lavas from Takab. Lithosphere delamination or slab break-off mechanisms for triggering melting are problematic, as the lithosphere is~150-200km thick. It is possible that the negative dT/dP section of the amphibole peridotite solidus was crossed as a result of lithospheric thickening in the collision zone. This explanation is conditional upon the mantle source being weakly hydrated and so only containing a small proportion of amphibole, which can be exhausted during small degrees of partial melting. Our model maybe viable for other magmatic areas within orogenic plateaux, e.g. northern Tibet. Depletion of mantle amphibole may also help explain larger scale transitions from arc to within-plate chemistry in orogens, such as the Palaeogene Arabia-Eurasia system.

The Contribution of Recycled Crust to Mantle Inventories of Trace elements, Hydrogen, and Carbon

NASA Astrophysics Data System (ADS)

Hirschmann, M. M.

2008-12-01

It is clear that crustal recycling has had a profound impact on the non-volatile trace element budget of the mantle, but its impact on mantle carbon and hydrogen are less well-understood. If an active crust recycling mechanism such as plate tectonics has operated since early in Earth history, and if magmatic production has diminished through time according to the decay in heat production, then the mass of recycled crust may dominate the mantle inventory of many trace elements. For example, Earth evolution models suggest time- integrated crust production equal to 7-15% of the mantle, and this accounts for ~25 to >100% of the mantle inventory of LREE and HFSE elements, depending on the mean concentration of these elements in the average crust produced. A key question is the role of recycling in the budgets of H and C. Consideration of the near-surface reservoirs and fluxes of C and H indicates that these principal volatiles have residence times of billions of years, and so they may be grouped with continental crust as a single long-lived near-surface geochemical reservoir (NSGR) that results from extraction from the mantle by melting combined with selective return to the mantle by subduction. The primitive mantle-normalized mass concentrations of H and C and the NSGR are equal to 90-200 and 1.5-18, respectively, with the primitive mantle inventories of H and C as the chief uncertainty. When the NSGR is plotted on a compatibility diagram, H and C form extreme positive and negative anomalies relative to their mineral/melt partition coefficients, meaning that there is much more H and much less C in the NSGR than would be predicted based solely on their magmatic flux from the mantle. The most straightforward interpretation is that H subduction is highly inefficient, but that recycled C amounts to at least half and possibly dominates the mantle C budget. This interpretation is supported by H/C mass ratios of the mantle sources inferred from undegassed oceanic basalts (H/C=0.75±0.25), which are substantially lower than that for the NSGR (H/C=1.95±0.15).

Geochemical Diversity of the Mantle: 50 Years of Acronyms

NASA Astrophysics Data System (ADS)

Hart, S. R.

2014-12-01

50 years ago, Gast, Tilton and Hedge demonstrated that the oceanic mantle is isotopically heterogeneous. 28 years ago, Zindler and Hart formalized the concept of geochemical mantle components, with an attendant, to some, odious, acronym soup. Work on a marriage of mantle geochemistry and dynamics continues unabated. We know unequivocally that the mantle is chemically heterogeneous; we do not know the scale lengths of these heterogeneities. We know unequivocally that these heterogeneities have persisted for eons (Gy); we do not know where they were formed or where they are stored. Through the kind auspices of the Plume Model, we plausibly have access to the whole mantle. The most accessible and well understood mantle reservoir is the upper depleted MORB mantle (DMM). Classically, this mantle was depleted by extraction of oceanic and continental crust from a "chondritic" bulk silicate Earth. In this post-Boyet and Carlson world, the complementary enriched reservoir may instead be hidden in the deepest mantle. In this case, DMM will become an endangered acronym. Hofmann and White (1982) argued that radiogenic Pb mantle (HIMU) is re-cycled ocean crust, and this is a comfortably viable model. It does require some ad hoc chemical manipulations during subduction. Given 2 Gy of aggregate mantle strains, the mafic component in HIMU may be of small length scale (< 50 m), possibly subsumed into the dominant peridotitic lithology. This mantle species is globally widespread. Enriched mantles (EM1 and EM2) almost certainly reflect recycling of enriched continental material. This was splendidly verified by Jackson et al (2007), with 87Sr/86Sr in Samoan EM2 lavas up to 0.721. The lithology and length scale of EM1 and EM2 is unconstrained. EM1 is globally present; EM2 is confined to the SW Pacific hotspots. FOZO is a work in progress; many would like to see it become extinct! The trace element signatures of HIMU and FOZO mantles have been constrained using melting models; in both cases the spidergrams are "enriched" with peaks at Nb-Ta of 2x and 4x bulk silicate earth, respectively, but with quite different shapes. As is typical with OIB, the derived source compositions are incompatible with the isotopic signatures, requiring a fairly recent "enrichment" event (possibly auto-metasomatism).

Quantifying mantle structure and dynamics using plume tracing in seismic tomography

NASA Astrophysics Data System (ADS)

O'Farrell, K. A.; Eakin, C. M.; Jackson, M. G.; Jones, T. D.; Lekic, V.; Lithgow-Bertelloni, C. R.

2017-12-01

Directly linking deep mantle processes with surface features and dynamics is a complex problem. Hotspot volcanism gives us surface observables of mantle signatures, but the depth and source of the mantle plumes feeding these hotspots are highly debated. To address these issues, it is necessary to consider the entire journey of a plume through the mantle. By analyzing the behavior of mantle plumes we can constrain the vigor of mantle convection, the net rotation of the mantle and the role of thermal versus chemical anomalies as well as the bulk physical properties such as the viscosity profile. To do this, we developed a new algorithm to trace plume-like features in shear-wave (Vs) seismic tomography models based on picking local minima in the velocity and searching for continuous features with depth. We applied this method to recent tomographic models and find 60+ continuous plume conduits that are > 750 km long. Approximately a third of these can be associated with known hotspots at the surface. We analyze the morphology of these continuous conduits and infer large scale mantle flow patterns and properties. We find the largest lateral deflections in the conduits occur near the base of the lower mantle and in the upper mantle (near the thermal boundary layers). The preferred orientation of the plume deflections show large variability at all depths and indicate no net mantle rotation. Plate by plate analysis shows little agreement in deflection below particular plates, indicating these deflected features might be long lived and not caused by plate shearing. Changes in the gradient of plume deflection are inferred to correspond with viscosity contrasts in the mantle and found below the transition zone as well as at 1000 km depth. From this inferred viscosity structure, we explore the dynamics of a plume through these viscosity jumps. We also retrieve the Vs profiles for the conduits and compare with the velocity profiles predicted for different mantle adiabat temperatures. We are able to constrain the average temperature anomaly of the conduits to be around 150 K. We use these thermal anomalies in conjunction with our measured plume tilts/deflections to further explore the dynamics of plume conduits in the lower mantle and transition zone.