Depleted Mantle Research Articles

Submarine Basaltic Glasses from the Galapagos Archipelago: Determining the Volatile Budget of the Mantle Plume

We report new major, trace and volatile element contents (H_2O, CO_2, F, S, Cl), and new Sr, Nd, Pb and He isotopes on submarine glasses from the Galapagos Archipelago from several dredging expeditions. Four groups are distinguishable on the basis of composition and geographical distribution: the Fernandina group (^3He/^4He > 22 R_A), which is similar to the less degassed primitive mantle; the Sierra Negra group (enriched Pb and Sr isotopes, ^3He/^4He = 8–20 R_A), produced by mixing the Floreana (HIMU-type) and Fernandina end-members; the Pinta group (high Δ7/4, Δ8/4 and Th/La ratios, ^3He/^4He = 6–9 R_A), an enriched mantle (EM)-type mantle indicative of recycled material in the source; and the depleted mantle (DM) group, characterized by an isotopic composition similar to mid-ocean ridge basalts (MORB). Only a single submarine glass with the isotopic composition of the Floreana end-member has been identified in the sample suite. Degassing has significantly lowered the glass CO_2 content with little effect on the H_2O concentration. Volatile data for oceanic basalts reveal that CO_2–H_2O gas–melt equilibration at eruption depth is common in ocean island basalts (OIB) and rare in MORB, suggesting different ratios of melt transport to bubble formation and gas–melt equilibration. The Galapagos glasses range from sulfide saturated to undersaturated, and a subset of samples indicate that S degasses at pressures ≤ 400 bars. Assimilation of hydrothermally altered material affected the volatile contents of a number of samples in the groups. Once shallow-level processes have been accounted for, we evaluate the volatile contents in the different Galapagos mantle sources. Ratios between volatile and refractory elements with similar incompatibilities are used to estimate the volatile budget of the Galapagos mantle plume. Most of the glasses from the Fernandina, Sierra Negra and Pinta groups have high volatile/refractory element ratios, whereas a few pristine DM group lavas have ratios similar to those measured in MORB. The volatile/refractory element ratios are consistent with previous reports for the high ^3He/^4He, HIMU and MORB components. The values measured for the Pinta group, however, are higher than those found in other OIB associated with the presence of recycled material (EM-type). Our data suggest that mixing between the different mantle components is pervasive throughout the archipelago, which acts to normalize the volatile data between the groups. The Fernandina component can be modeled by a 6–20% mixture of the high ^3He/^4He primitive mantle component with the MORB source, assuming a two-layered mantle and using existing estimates of helium concentrations. The resulting estimated volatile content and H/C mass ratio for the high ^3He/^4He primitive mantle are consistent with previous estimates, but calculated C/3He ratios are lower than the canonical ratio. This indicates the following: (1) the estimates require ∼20–50 times higher C or lower ^3He contents, which is difficult to reconcile with the measured volatile/refractory ratios in oceanic basalts; (2) the C/^3He ratio is not constant throughout the mantle; (3) an impact erosion model, rather than a two-layered mantle model, is more consistent with the relatively constant C/3He ratios observed in oceanic basalts, although it is unclear how representative oceanic basalts are of the lower mantle. The high volatile content of the high ^3He/^4He component will affect mantle dynamics and melt migration during plume–ridge interaction as this component would be predicted to be less viscous than the ambient mantle. The lower viscosity material would have an enhanced vertical upwelling, which could explain the buoyancy flux of the Galapagos plume without the need for a temperature anomaly. A lower viscosity, high ^3He/^4He component could also provide an explanation for the lack of high ^3He/^4He in Galapagos Spreading Center lavas erupting in the vicinity of the Galapagos plume.

Tracing Proterozoic arc mantle Hf isotope depletion of southern Fennoscandia through coupled zircon U–Pb and Lu–Hf isotopes

Constraints on the composition of the depleted mantle Sm–Nd and Lu–Hf crust formation ages have a long history of scientific debate. When calculating mantle extraction ages, and constructing crustal growth models, a linear evolution of incompatible trace elements in a depleted mantle since >4Ga is routinely used. Mantle depletion however varies regionally and over time and subduction of sediments and oceanic crust renders a mantle-wedge variously enriched relative to a modelled depleted mantle.Here we show that primitive mantle-derived subduction related gabbroic intrusions from southern Fennoscandia have Hf isotope compositions that are enriched relative to a MORB-like linear depleted mantle evolution curve. Extrapolation of primitive Paleoproterozoic gabbro suites enables the construction of a regional mantle evolution curve, providing improved constraints on model ages, crustal residence times and the fraction of juvenile versus reworked continental crust. Convergent margins are assumed to be one of the main sites of continental crust growth, and using an overly depleted mantle source yield model ages that are too old, and hence cumulative crustal growth models show too much crust generation early in the Earth's history. The approach of using the Hf isotope composition of zircon from primitive subduction related gabbroic intrusions as a proxy for mantle Hf isotope composition, piloted in this study, can be applied to other convergent margins.

Late Paleozoic rift-related basalts from central Inner Mongolia, China

Late Paleozoic basaltic volcanism widely occurred in central Inner Mongolia, the southeastern part of the Central Asia Orogenic Blet (CAOB). Three volcanic pulses are identified by ion microprobe (SHRIMP II) zircon U-Pb analysis, which are Late Carboniferous (ca. 315Ma), the latest Carboniferous (ca. 303Ma) and early Permian (ca. 284Ma). Basalts in the earliest stage are characterized by flat HREE, depletion in Nb and Ta, MORB-like Th/Yb ratios and depleted mantle (DM)-like Hf isotopic ratios. These are similar to forearc basalts, that have been generated by melting of a fluxed depleted mantle in an extensional environment. The latest Carboniferous basalts (sample D1363) have no Zr-Hf anomalies, relatively radiogenic Hf isotopic ratios (εHf(t)=+12.1 to +19.7) and high δ18O values up to 8.5‰, VSMOW. These are likely to be magmas originated from decompressional melting of a depleted mantle that interacted with ocean floor pillow basalts during their ascending through the crust. The early Permian sample D1380 have variable 206Pb/238U zircon ages with an interpreted magma crystallization age of 284.4±2.5Ma. These zircons have scattered Hf isotope (εHf(t)=+5 to +14) and δ18O (1.8–11.0‰, VSMOW), which suggest the involvement of a relatively enriched mantle source. Overall the temporal evolution of the Late Paleozoic basaltic volcanism from central Inner Mongolia suggests a rift-related tectonic setting.

Mantle source heterogeneity and magmatic evolution at Carlsberg Ridge (3.7°N): constrains from elemental and isotopic (Sr, Nd, Pb) data

We present new major element, ICP-MS trace element, and Sr–Nd–Pb isotope data of basalts from four locations along the Carlsberg Ridge (CR), northern Indian Ocean. The basalts are low-K tholeiites with 7.52–9.51 wt% MgO, 49.40–50.60 wt% SiO2, 0.09–0.27 wt% K2O, 2.55–2.90 wt% Na2O, and 0.60–0.68 Mg#. Trace element contents of the basalts show characteristics similar to those of average normal MORB, such as LREE depleted patterns with (La/Sm)N ratio of 0.55–0.69; however, some samples are enriched in large-ion lithophile elements such as K and Rb, suggesting probable modification of the mantle source. Poor correlations between the compatible elements [e.g. Ni, Cr, and Sr (related to olivine, clinopyroxene and plagioclase, respectively)] and the incompatible elements (e.g. Zr and Y), and positive correlations in the Zr versus Zr/Y and Nb versus Nb/Y plots suggest a magmatic evolution controlled mainly by mantle melting rather than fractional crystallization. Our results extend the CR basalt range to higher radiogenic Pb isotopes and lower 143Nd/144Nd. These basalts and basalts from the northern Indian Ocean Ridge show lower 143Nd/144Nd and higher 87Sr/86Sr values than those of the depleted mantle (DM), defining a trend towards pelagic sediment composition. The Pb isotopic ratios of basalts from CR 3–4°N lie along the compositional mixing lines between the DM and the upper continental crust. However, the low radiogenic Pb of basalts from CR 9–10°N lie on the mixing line between the DM and lower continental crust. Since the Pb isotopic ratio of MORB would decrease if the source mantle was contaminated by continental lithospheric mantle, we suggest that CR contains continental lithospheric material, resulting in heterogeneous mantle beneath different ridge segments. The continental lithospheric material was introduced into the asthenosphere before or during the breakup of the Gondwana. These results support the long-term preservation of continental material in the oceanic mantle which would significantly influence the isotopic anomaly of the Indian Ocean MORB.

Elemental and Sr–Nd isotopic geochemistry of the Uradzhongqi magmatic complex in western Inner Mongolia, China: A record of early Permian post-collisional magmatism

The magmatic complex in Uradzhongqi, Inner Mongolia, is located in the western segment of the northern margin of the North China Craton (NCC). The dominant components in the complex include syenogranite, monzogranite, granodiorite, diorite and gabbro. Mafic microgranular enclaves (MMEs) are common in syenogranite and granodiorite. Zircon U–Pb dating shows that the ages of these rocks range from 283 to 270Ma, suggesting an early Permian emplacement. The syenogranite and monzogranite are peraluminous I-type granites, exhibiting conspicuous negative Eu anomaly, enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in high field strength elements (HFSE). The granodiorites, diorites and MMEs are metaluminous in composition, show high Al2O3, MgO and Fe2O3T contents and weak negative Eu anomaly, as well as LREE and LILE enrichment and HFSE depletion. The gabbros show weak positive Eu anomaly and slight REE differentiation. The Sr–Nd isotope compositions show that the source of mafic magma was depleted mantle (DM) with possible involvement of enriched mantle II (EM II), whereas the felsic magma was derived from the Archean lower crust. Petrographic observation and analytical results of mineralogy, geochronology, geochemistry and Sr–Nd isotopes indicate that the main petrogenesis of these magmatic rocks is the mixing of underplating mafic magma and felsic magma. Tectonically, the complex pluton was formed within a post-collisional regime, and the underplating in this area provides another piece of evidence for the vertical growth of the western segment of the northern margin of the NCC.

The origin of highly radiogenic Hf isotope compositions in 3.33 Ga Commondale komatiite lavas (South Africa)

Drill core samples from the 3.33Ga high-alumina-type Commondale komatiites from the southern Kaapvaal craton (South Africa) were analysed for major, trace elements and Hf- isotope compositions in order to place possible constraints on their mantle sources and the process and timing of mantle depletion. The initial εHf isotope values range from +6.5 to +9.1 indicating a strongly depleted mantle source. This contrasts with previously published initial εNd values on the same geologic unit that reflect only minor depleted mantle sources of +2. Trace element modelling and modelling of the Hf-Nd isotope composition of the mantle source indicate that the strongly incompatible element-depleted trace element patterns may be explained by a mantle source that was depleted to high-degrees (ca. 40%) in the garnet stability field prior to high- degree melting of the restitic mantle in the spinel stability field. The depletion in the garnet-stability field may have been related to a plume event that produced the low-alumina Barberton-type komatiites. However, the decoupled Hf-Nd isotope compositions cannot be explained by such a mechanism. We discuss several possible scenarios that may explain the trace element distribution as well as the isotope composition of the Commondale komatiites. Based on trace and isotope modelling, we consider three different models (1) the decoupled Hf-Nd isotope compositions resulted from a depleted mantle source overprint by melt-depleted garnet-pyroxenite components that originated from older crustal differentiation events and were delaminated into the mantle; (2) the decoupling resulted from subduction fluids that overprinted a highly depleted mantle source and added Th, Nd and other light-rare-Earth elements (LREE) shortly before eruption of the komatiites and triggered the high degree of melting; (3) a possible origin from a mantle source that experienced fractionation processes in an early magma ocean, followed by at least three mantle depletion events. The trace element and Hf-Nd isotope composition of the Commondale komatiites reveal that parts of the Archean mantle must have been highly heterogeneous due to komatiite extraction and possible delamination processes of lower crustal restites.

Combined bulk-rock Hf- and Nd-isotope compositions of Mesoarchaean metavolcanic rocks from the Ivisaartoq Supracrustal Belt, SW Greenland: Deviations from the mantle array caused by crustal recycling

Bulk-rock Lu-Hf and Sm-Nd isotope compositions, as well as major and trace element data are presented for metavolcanic rocks from the Mesoarchaean (ca. 3075 Ma) Ivisaartoq Supracrustal Belt in the Nuuk region of southern West Greenland. The εHft calculated at 3075 Ma range from +0.8 to +3.1 and the corresponding εNdt values range from +0.7 to +3.6, which forms an array that is displaced off the mantle array for these two isotopic systems. Primitive mantle normalized trace element plots of the metabasalts display negative Nb- and Ti-anomalies in combination with the elevated Th abundances, which is consistent with a subduction zone affinity as proposed by previous studies of this metavolcanic belt. No significant correlations are observed between the isotope compositions and proxies of shallow crustal contamination in the Ivisaartoq rocks, despite clear evidence for inherited Eoarchaean zircon [Polat et al. (2009a) Chemical Geology 268, 248-271], which would have dominated the bulk-rock Hf-isotope budget. Furthermore, the measured samples are less radiogenic than the estimate for the depleted mantle composition at 3075 Ma. The lack of isotope and trace element correlation suggests incomplete equilibration between the crustal contaminant and the parental Ivisaartoq melts. We prefer a petrogenetic model with some combination of slab-derived metasomatism of the mantle source region for the Ivisaartoq magmas, which homogenized their trace element contents, in combination with the incorporation of granitoid residue with unradiogenic Hf-isotope composition at higher degrees of partial melting and finally the eruption of mechanically entrained Eoarchaean crust without significant chemical equilibration. The geochemical arc-affinity and lower than depleted mantle (DM) isotope compositions of these metavolcanic rocks support the notion that crustal recycling and plate tectonics has been operating on Earth since at least the Mesoarchaean Eon.

Mantle depletion and metasomatism recorded in orthopyroxene in highly depleted peridotites

Although trace element concentrations in clinopyroxene serve as a useful tool for assessing the depletion and enrichment history of mantle peridotites, this is not applicable for peridotites in which the clinopyroxene component has been consumed (~25% partial melting). Orthopyroxene persists in mantle residues until ~40% melting and it is therefore this mineral that offers petrological insights into the evolution of refractory peridotites. Major and trace element concentrations in orthopyroxene±clinopyroxene from two spinel facies harzburgitic xenolith suites from New Zealand are examined. Samples from Cape L'Evique (CLEV) on Chatham Island contain traces of clinopyroxene (<2 modal %) but a suite from Lake Moana (MOA) in the South Island is devoid of this mineral. When compared with modelled orthopyroxene trace element budgets, which are constructed from a review of published source modes, melting modes and element/melt partition co-efficients, the measured orthopyroxene rare earth element data in both suites generally indicate minimums of 25–30% partial melting. These results are consistent with co-existing elevated Mg# in olivine (mostly 91.4 to 93.0) and orthopyroxene (mostly 91.3 to 93.6), high spinel Cr# (commonly >45) and low orthopyroxene Al2O3 (generally <3.1wt%). However, comparison of modelled and measured orthopyroxene compositions shows that all samples, even the most refractory, have undergone metasomatism by small volume light rare earth element-bearing agents. Measured orthopyroxene Ti concentrations show that the metasomatic agent that affected the CLEV suite carried Ti, but that the MOA suite metasomatiser was Ti-poor. Orthopyroxene trace elements in the inspected rocks are therefore partly decoupled from the major element abundances, with the results demonstrating that even highly refractory peridotites can record evidence for mantle metasomatism.

Osmium isotope compositions of detrital Os-rich alloys from the Rhine River provide evidence for a global late Mesoproterozoic mantle depletion event

We report osmium isotopic compositions for 297 mantle-derived detrital Ru–Os–Ir alloy grains found in gold and platinum-group mineral bearing placers of the Rhine River. These alloys were likely formed as a result of high degree melting in the convective mantle and derived from residual Paleozoic mantle peridotites in the Alps of Central Europe that were accreted as part of a collage of Gondwana-derived ‘Armorican’ terranes before the Variscan Orogeny. The 187Os/188Os isotope ratios of the Os-rich alloys show a wide distribution, with two modes at 0.1244 and 0.1205. These two modes correspond to rhenium depletion ages, interpreted to correspond with episodes of high-degree mantle melting, at ∼0.5 and ∼1.1 Ga. The data confirm the ability of the oceanic mantle to preserve evidence of ancient melting events. Our new data, in combination with published data on Os-rich alloys from the Urals and Tasmania and with data for abyssal peridotites, indicate a geographically widespread record of a major global Late Mesoproterozoic (1.0–1.2 Ga) high-degree melting event in Paleozoic oceanic mantle rocks. This model age peak is essentially absent from the crustal record of Central-Western Europe, but does coincide with the apparent peak in global continental crust zircon ages at this time. Thus, high-degree mantle melting peaking in the 1.0–1.2 Ga interval may have affected a large part of Earth's mantle. This interval occurred during a period of relative super-continental stability, which may have been accompanied in the oceanic realm by rapid seafloor spreading and extensive subduction, and by unusually high activity of mantle plumes forming two active mantle superswells.

Unravelling the record of Archaean crustal evolution of the Bundelkhand Craton, northern India using U–Pb zircon–monazite ages, Lu–Hf isotope systematics, and whole-rock geochemistry of granitoids

Combined whole-rock, U–Pb zircon and monazite dating, in situ zircon (MC)-SF-ICP-MS Lu–Hf-isotope data are reported for more than 20 Archaean granitoid samples to constrain magmatic events and their sources in the Bundelkhand Craton, northern India. The results are crucial for understanding the extent of reworking vs juvenile crustal growth during Palaeoarchaean–Neoarchaean and also provide new information on the temporal gap between Palaeoarchaean TTG and Neoarchaean granodiorite–granite magmatism. These data reveal that the Bundelkhand Craton was affected by at least four discrete episodes of Palaeoarchaean TTG magmatism between 3.55Ga and 3.20Ga at regular intervals of 100–150 Myr. After a period of about 500 Myr of near magmatic shutdown, it was followed by a relatively short-lived, but more voluminous granitoid emplacement event during the Neoarchaean at 2.58–2.50Ga, during which minor granodioritic gneisses and voluminous granites intruded. The new data also provide evidence for a metamorphic overprint at ca. 3.20Ga, and a Pan-African tectonothermal overprint at 0.50–0.60Ga.Hf-isotope and whole-rock geochemical data of the TTGs suggest a complex process of crust formation and stabilisation during the Palaeoarchaean, mainly by the reworking of older mafic and felsic crust between 3.55 and 3.20Ga. The CHUR parallel array for the TTGs may indicate that these rocks were formed by partial melting of an oceanic crust that was derived from chondritic mantle sources, shortly before formation of TTG. This interpretation is inconsistent with Hf-isotope data from worldwide sources, which provide evidence for a successive mantle depletion since at least 4.0Ga, resulting in εHft of about +5.0 at 3.0Ga. Intense melting of the heterogeneous TTG crust during the Neoarchaean resulted in the coeval formation of granodioritic gneisses and voluminous granites, and subordinate hybrid granites with a minor contribution of melts from an enriched mantle source, affected by subduction-zone processes prior to 2.55Ga.

Jurassic zircons from the Southwest Indian Ridge.

The existence of ancient rocks in present mid-ocean ridges have long been observed but received less attention. Here we report the discovery of zircons with both reasonably young ages of about 5 Ma and abnormally old ages of approximate 180 Ma from two evolved gabbroic rocks that were dredged from the Southwest Indian Ridge (SWIR) in the Gallieni fracture zone. U–Pb and Lu–Hf isotope analyses of zircons were made using ion probe and conventional laser abrasion directly in petrographic thin sections. Young zircons and their host oxide gabbro have positive Hf isotope compositions (εHf = +15.7–+12.4), suggesting a highly depleted mantle beneath the SWIR. The spread εHf values (from−2.3 to−4.5) of abnormally old zircons, together with the unradiogenic Nd-Hf isotope of the host quartz diorite, appears to suggest an ancient juvenile magmatism along the rifting margin of the southern Gondwana prior to the opening of the Indian Ocean. A convincing explanation for the origin of the unusually old zircons is yet to surface, however, an update of the theory of plate tectonics would be expected with continuing discovery of ancient rocks in the mid-oceanic ridges and abyssal ocean basins.

Miocene-Pliocene mantle depletion event in the northern Fossa Magna, western NE Japan

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. Less radiogenic 176Hf/177Hf and 143Nd/144Nd, high Zr/Hf, and little or no Hf anomaly characterize the Early Miocene volcanism in the northern Fossa Magna region. The mantle wedge consisted of chemically heterogeneous mantle source. Based on out isotope proxies, we propose that during the onset of subduction, influx of hot asthenospheric mantle provided sufficient heat to partially melt newly subducting sediment. Geochemical modeling demonstrates that slab-derived melt mixed with mantle wedge produces the observed isotopic and trace elemental characteristics. 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. Then, the mantle wedge was gradually cooled during the Middle Miocene to the Pliocene 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. The Late Miocene and Pliocene volcanism at the post stage of the back-arc opening is best explained by a partial melting of subducted metasediment saturated with trace quantities of zircon and rutile.

Crustal thickness anomalies in the Indian Ocean inferred from gravity analysis

AbstractOceanic crustal thickness is a critical parameter for understanding the magmatic, tectonic and hydrothermal processes at mid‐ocean ridges. Gravity anomalies can be mapped across the oceans through satellite altimetry, and these data hold valuable information about the nature and spatial variation of oceanic crust. Residual mantle Bouguer anomaly (RMBA) reflects variations in crustal thickness and crustal and mantle densities. It was simply treated as an upper mantle gravity component and a crustal gravity component in our model of a lithospheric depth. The upper mantle gravity component results from variations in mantle temperature, mantle porosity and mantle depletion due to partial melting initiating at depths of 80–100 km. The crustal gravity component results from variations in crustal thickness. RMBA contributed by mantle temperature variation of 200 K, mantle depletion of 5% and mantle porosity of 5% are 46, 16 and 21 mGal, respectively. Upper mantle gravity variations do not exceed 46 mGal, and variations in oceanic crustal thickness or deeper mantle effects have to be considered for areas with RMBA variations greater than 46 mGal. Assuming a standard RMBA of a reference crust is 0 mGal, we partitioned the RMBA in the Indian Ocean into three categories: ‘negative’ (less than −46 mGal), ‘normal’ (between −46 and 46 mGal) and ‘positive’ (&gt;46 mGal), corresponding to thick, normal and thin oceanic crust, respectively. Thick oceanic crustal anomalies are associated with hotspots or plumes and are distributed along hotspot tracks. Prominent thin oceanic crustal anomalies are observed along fracture zones, around Rodriguez Triple Junction (RTJ) and Australian‐Antarctic Discordance (AAD). Besides the possible effects of fracture zones, oceanic core complex or some other enigmatic deep mantle factors must be responsible for the positive gravity anomalies and crustal thinning. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Constraints on core formation in Vesta from metal–silicate partitioning of siderophile elements

It is now widely accepted that the asteroid 4-Vesta has an Fe-rich metallic core, but the composition of the core and the conditions prevailing during core–mantle differentiation are poorly constrained. In light of new constraints on Vesta’s geophysical and geochemical properties obtained by the DAWN mission, we have re-examined the conditions at which core–mantle differentiation in Vesta may have occurred by linking the estimated mantle depletions of siderophile elements P, Co, Ni, Cu, Ga, Ge, Mo and W in the vestan mantle to newly derived predictive equations for core–mantle partitioning of these elements. We extend the number of elements previously considered in geochemical modeling of vestan core formation and use published metal–silicate partitioning data obtained at low pressures to characterize the dependence of metal/silicate partition coefficients (D) on pressure, temperature, oxygen fugacity and composition of the silicate and metallic melt. In our modeling we implement newly derived mantle depletions of P, Co, Ni and Ga through analysis of published HED meteorite analyses and assess two contrasting bulk compositional models for Vesta.Modeling results using Monte Carlo simulations constrain vestan core formation to have occurred at mildly reducing conditions of approximately 2log units below the iron–wüstite (IW) buffer (ΔIW=−2.05±0.20) if the two most likely bulk compositions (binary mixtures of H+CM or H+CV chondritic meteorites) are considered, assuming a temperature range between 1725 and 1850K and a sulfur-free pure Fe core. If the core is assumed to be sulfur-rich (15wt.% S) as predicted by the latter bulk compositional models, observed depletions for all eight siderophile elements can be simultaneously satisfied at ΔIW=−2.35±0.10 and 1725–1850K for the H+CV bulk composition and ΔIW=−2.30±0.15 and 1725–1850K for the H+CM bulk composition. More reducing conditions are not consistent with the observed siderophile element depletions in the vestan mantle, independent of the sulfur content of the vestan core and of the bulk compositional model chosen.Our analysis shows that a previously proposed shallow magma ocean on Vesta during core formation is not consistent with the observed mantle depletion of Ga for the two considered bulk compositions irrespective of core composition. Instead, our results are consistent with the existence of a deep magma ocean during core formation on Vesta, requiring >50–100% mantle melting.

Geochemistry of platinum-group elements and mineral composition in chromitites and associated rocks from the Abdasht ultramafic complex, Kerman, Southeastern Iran

The Abdasht complex is a major ultramafic complex in south-east Iran (Esfandagheh area). It is composed mainly of dunite, harzburgite, podiform chromitites, and subordinate lherzolite and wehrlite. The podiform chromitites display massive, disseminated, banded and nodular textures. Chromian spinels in massive chromitites exhibit a uniform and restricted composition and are characterized by Cr# [=Cr/(Cr+Al)] ranging from 0.76 to 0.77, Mg# [=Mg/(Mg+Fe2+)] from 0.63 to 0.65 and TiO2<0.2wt.%. These values may reflect crystallization of the chromian spinels from boninitic magmas. Chromian spinels in peridotites exhibit a wide range of Cr# from 0.48 to 0.86, Mg# from 0.26 to 0.56 and very low TiO2 contents (averaging 0.07wt.%). The Fe3+# is very low, (<0.08wt.%) in the chromian spinel of chromitites and peridotites of the Abdasht complex which reflects crystallization under low oxygen fugacities.The distribution of platinum group elements (PGE) in Abdasht chromitites displays a high (Os+Ir+Ru)/(Rh+Pt+Pd) ratio with strongly fractionated chondrite-normalized PGE patterns typical of ophiolitic chromitites. Moreover, the Pd/Ir value, which is an indicator of PGE fractionation, is very low (<0.1) in the chromitites.The harzburgite, dunite and lherzolite samples are highly depleted in PGE contents relative to chondrites. The PdN/IrN ratios in dunites are unfractionated, averaging 0.72, whereas the harzburgites and lherzolites show slightly positive slopes PGE spidergrams, together with a small positive Ru anomaly, and their PdN/IrN ratio averages 2.4 and 2.3 respectively. Moreover, the PGE chondrite and primitive mantle normalized patterns of harzburgite, dunite and lherzolite are relatively flat which are comparable to the highly depleted mantle peridotites.The mineral chemistry data and PGE geochemistry indicate that the Abdasht chromitites and peridotites were generated from a melt with boninitic affinity under low oxygen fugacity in a supra-subduction zone setting. The composition of calculated parental melts of the Abdasht chromitites is consistent with the differentiation of arc-related magmas.

A Late Cretaceous (ca. 90 Ma) kimberlite event in southern India: Implication for sub-continental lithospheric mantle evolution and diamond exploration

We report groundmass perovskite U–Pb (SIMS) ages, perovskite Nd isotopic (LA-ICPMS) composition and bulk-rock geochemical data of the Timmasamudram diamondiferous kimberlite cluster, Wajrakarur kimberlite field, in the Eastern Dharwar craton of southern India. The kimberlite pipes gave similar Mesoproterozoic ages of 1086±19Ma (TK-1, microcrystic variant) and 1119±12Ma (TK-3). However, a perovskite population sampled from the macrocrystic variant of TK-1 gave a much younger Late Cretaceous age of ca. 90Ma. This macrocrystic kimberlite phase intrudes the Mesoproterozoic microcrystic phase and has a distinct bulk-rock geochemistry. The Nd-isotope composition of the ~1100Ma perovskites in the cluster show depleted εNd(T) values of 2.1±0.6 to 6.7±0.3 whereas the ~90Ma perovskites have enriched εNd(T) values of −6.3±1.3. The depleted-mantle (DM) model age of the Cretaceous perovskites is 1.2Ga, whereas the DM model age of the Proterozoic perovskites is 1.2 to 1.5Ga. Bulk-rock incompatible trace element ratios (La/Sm, Gd/Lu, La/Nb and Th/Nb) of all Timmasamudram kimberlites show strong affinity with those from the Cretaceous Group II kimberlites from the Bastar craton (India) and Kaapvaal craton (southern Africa). As the Late Cretaceous age of the younger perovskites from the TK-1 kimberlite is indistinguishable from that of the Marion hotspot-linked extrusive and intrusive igneous rocks from Madagascar and India, we infer that all may be part of a single Madagascar Large Igneous Province. Our finding constitutes the first report of Cretaceous kimberlite activity from southern India and has significant implications for its sub-continental lithospheric mantle evolution and diamond exploration programs.

Lead isotopic systematics of massive sulphide deposits in the Urals: Applications for geodynamic setting and metal sources

Lead isotopic compositions of 61 samples (55 galena, one cerussite [PbCO3] and five whole ore samples) from 16 Volcanic Hosted Massive Sulphide (VHMS) deposits in the Urals Orogeny show an isotopic range between 17.437 and 18.111 for 206Pb/204Pb; 15.484 and 15.630 for 207Pb/204Pb and 37.201 and 38.027 for 208Pb/204Pb. Lead isotopic data from VHMS deposits display a systematic increase in ratios across the Urals paleo-island arc zone, with the fore-arc having the least radiogenic lead compositions and the back-arc having the most radiogenic lead. The back arc lead model ages according to Stacey–Kramers model are close to the biostratigraphic ages of the ore-hosting volcano-sedimentary rocks (ca. 400Ma). In contrast, less radiogenic lead from the fore-arc gives Neoproterozoic (~700Ma) to Cambrian (480Ma) lead model ages with low two-stage model μ values of 8.8 (parameter μ=238U/204Pb reflects the averaged U/Pb ratio in the lead source), progressively increasing stratigraphically upwards to 9.4 in the cross-section of the ore-hosting Baymak–Buribai Formation. The range of age-corrected uranogenic lead isotopic ratios of the volcanic and sedimentary host rocks is also quite large: 206Pb/204Pb=17.25–17.96; 207Pb/204Pb=15.48–15.56, and generally matches the ores, with the exception of felsic volcanics and plagiogranite from the Karamalytash Formation being less radiogenic compare to the basaltic part of the cross-section, which would potentially imply a different source for the generation of felsic volcanics. This may be represented by older Neoproterozoic oceanic crust, as indicated by multiple Neoproterozoic ages of mafic–ultramafic massifs across the Urals. The relics of these massifs have been attributed by some workers to belong to the earlier Neoproterozoic stage of pre-Uralian ocean development. Alternative sources of lead may be Archean continental crust fragments/sediments sourced from the adjacent East-European continent, or Proterozoic sediments accumulated near the adjacent continent and presently outcropping near the western edge of Urals (Bashkirian anticlinorium). The contribution of Archean rocks/sediments to the Urals volcanic rock formation is estimated to be less than 0.1% based on Pb–Nd mixing models.The most radiogenic lead found in VHMS deposits and volcanics in the Main Uralian Fault suture zone, rifted-arc and back-arc settings, show similar isotopic compositions to those of the local Ordovician MORBs, derived from highly depleted mantle metasomatized during dehydrational partial melting of subducted slab and oceanic sediments. The metasomatism is expressed as high Δ 207Pb/204Pb values relative to the average for depleted mantle in the Northern hemisphere, and occurred during the subduction of oceanic crust and sediments under the depleted mantle wedge. A seemingly much younger episode of lead deposition with Permian lead model ages (ca. 260–280Ma) was recorded in the hanging wall of two massive sulphide deposits.

Depleted Mantle-sourced CFB Magmatism in the Jurassic Africa–Antarctica Rift: Petrology and 40Ar/39Ar and U/Pb Chronology of the Vestfjella Dyke Swarm, Dronning Maud Land, Antarctica

The Jurassic Vestfjella dyke swarm at the volcanic rifted margin of western Dronning Maud Land represents magmatism related to the incipient Africa–Antarctica rift zone; that is, rift-assemblage magmatism of the Karoo continental flood basalt (CFB) province. Geochemical and Nd–Sr isotopic data for basaltic and picritic dyke samples indicate diverse low-Ti and high-Ti tholeiitic compositions with εNd(180 Ma) ranging from +8 to –17. Combined with previously reported data on a subcategory of ferropicritic dykes, our new data facilitate grouping of the Vestfjella dyke swarm into seven geochemically distinct types. The majority of the dykes exhibit geochemical affinity to continental lithosphere and can be correlated with two previously identified chemical types (CT) of the wall-rock CFB lavas and are accordingly referred to as the CT1 and CT3 dykes. The less abundant Low-Nb and High-Nb dykes, a relatively enriched subtype of CT3 (CT3-E) dykes, and dykes belonging to the depleted and enriched ferropicrite suites represent magma types found only as intrusions. The chemically mid-ocean ridge basalt (MORB)-like Low-Nb and the depleted ferropicrite suite dykes represent, respectively, relatively high- and low-degree partial melting of the same overall depleted mantle (DM)-affinity source in the sublithospheric mantle. In contrast, we ascribe the chemically ocean island basalt (OIB)-like High-Nb dykes and the enriched ferropicrite suite dykes to melting of enriched components in the sublithospheric mantle. Geochemical modelling suggests that the low-Ti affinity CT1 and CT3, and high-Ti affinity CT3-E magma types of Vestfjella dyke may predominantly result from mixing of DM-sourced Low-Nb type magmas with <10 wt % of crust- and lithospheric mantle-derived melts. U/Pb zircon dating confirms synchronous emplacement of CT1 dykes and Karoo main-stage CFBs at 182·2 ± 0·9 and 182·2 ± 0·8 Ma, whereas two 40Ar/39Ar plagioclase plateau ages of 189·2 ± 2·3 Ma (CT1) and 185·5 ± 1·8 Ma (depleted ferropicrite suite), and a mini-plateau age of 186·9 ± 2·8 Ma (CT3-E) for the Vestfjella dykes raise the question of whether the onset of rift-zone magmatism could predate the province-wide c. 179–183 Ma main stage of Karoo magmatism. Notably variable Ca/K spectra suggest that younger 40Ar/39Ar plagioclase plateau ages of 173, 170, 164, and 154 Ma are related to crystallization of secondary minerals during the late-stage tectono-magmatic development of the Antarctic rifted margin. The occurrence of rare MORB- and OIB-like magma types in Vestfjella and along the African and Antarctic rifted margins suggests melting of geochemically variable depleted and enriched sublithospheric mantle beneath the Africa–Antarctica rift zone. Our models for the Vestfjella dyke swarm indicate that the voluminous lithosphere-affinity low-Ti and high-Ti rift-assemblage tholeiites could have been derived from MORB-like parental magmas by contamination, which implies sublithospheric depleted mantle as the principal source of the CFB magmas of the Africa–Antarctica rift zone.

Geochemistry, Geochronology, Sr‐Nd Isotopic Compositions of Jiang Tso Ophiolite in the Middle Segment of the Bangong‐ Nujiang Suture Zone and Their Geological Significance

AbstractThe Jiang Tso ophiolite, situated in the middle segment of the Bangong‐ Nujiang Suture Zone, is a part of the easternmost Qieli Lake ophiolite subzone and is close to the south of Pung Lake ophiolite. The rock association of Jiang Tso ophiolite is relatively complete and is mainly composed of metamorphic peridotite, gabbro and diabase. Comparing with N‐MORB, the ophiolite is high in Mg and low in Ti, K, Na, P, and is depleted in Nb, Ta, Hf, Th and enriched in Rb, Sr and Ba. Geochemical characteristics of the Jiang Tso ophiolite indicate it is of a supra‐subduction zone type formed in the spreading ridge of back arc basin. The SHRIMP U‐Pb dating of zircons from the gabbro yielded a weighted average age of 188.1±4.1 Ma (MSWD=1.4), indicating the Jiang Tso ophiolite was formed in the late stage of early Jurassic. The Sr, Nd isotopic compositions show that the Tethyan mantle domain is the depleted mantle (DM), with enriched mantle domain II (EM II). They have the same Sr, Nd isotopic composition with the India Ocean MORB type.

Reinforcing the origin of volcanic rocks from the Massif Central through the isotopic composition of lead and strontium

The Massif Central in France hosts a large volume of Tertiary to Recent continental alkaline volcanism, with evidence of crustal contamination of the differentiated magmas. Detailed investigations on feldspathic basalts and basanite rocks from the Cantal Volcano exposed in the Allanche basin by lead and strontium isotopes i) reveal the role of the mantle components Depleted Mantle (DM) and Low Velocity Component (LVC, corresponding to the European asthenospheric reservoir) end-members based on 208Pb/204Pb and 206Pb/204Pb isotopic ratios, and ii) show contamination by meta-igneous granulites from the lower crust when using 207Pb/204Pb vs. 206Pb/204Pb ratios, as only part of the samples plot along the Northern Hemisphere Reference Line (NHRL) mixing line between DM and HIMU (High U/Pb Mantle) end-members. Cross plotting of Pb–Sr isotopes rules out recycled sediment and/or igneous granulite as potential crustal contaminant for the Cantal basalts, but suggests mixing between the low-velocity component (LVC) and EMI (Enriched Mantle I)-pelagic sediments. Replaced in the regional context of the Massif Central, we confirm the existence of several mantle sources and different crustal contaminations in the magmas of the Cantal Volcano.