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Richard J Walker - One of the best experts on this subject based on the ideXlab platform.
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186 os 187 os and highly Siderophile Element abundance systematics of the mantle revealed by abyssal peridotites and os rich alloys
Geochimica et Cosmochimica Acta, 2017Co-Authors: James M.d. Day, Richard J Walker, Jessica M. WarrenAbstract:Abstract Abyssal peridotites are oceanic mantle fragments that were recently processed through ridges and represent residues of both modern and ancient melting. To constrain the nature and timing of melt depletion processes, and the composition of the mantle, we report high-precision Os isotope data for abyssal peridotites from three ocean basins, as well as for Os-rich alloys, primarily from Mesozoic ophiolites. These data are complemented by whole-rock highly Siderophile Element (HSE: Os, Ir, Ru, Pt, Pd, Re), trace- and major-Element abundances for the abyssal peridotites, which are from the Southwest Indian (SWIR), Central Indian (CIR), Mid-Atlantic (MAR) and Gakkel Ridges. The results reveal a limited role for melt refertilization or secondary alteration processes in modifying abyssal peridotite HSE compositions. The abyssal peridotites examined have experienced variable melt depletion (2% to >16%), which occurred >0.5 Ga ago for some samples. Abyssal peridotites typically exhibit low Pd/Ir and, combined with high-degrees of estimated total melt extraction, imply that they were relatively refractory residues prior to incorporation into their present ridge setting. Recent partial melting processes and mid-ocean ridge basalt (MORB) generation therefore played a limited role in the chemical evolution of their precursor mantle domains. The results confirm that many abyssal peridotites are not simple residues of recent MORB source melting, having a more complex and long-lived depletion history. Peridotites from the Gakkel Ridge, SWIR, CIR and MAR indicate that the depleted MORB mantle has 186Os/188Os of 0.1198356 ± 21 (2SD). The Phanerozoic Os-rich alloys yield an average 186Os/188Os within uncertainty of abyssal peridotites (0.1198361 ± 20). Melt depletion trends defined between Os isotopes and melt extraction indices (e.g., Al2O3) allow an estimate of the primitive mantle (PM) composition, using only abyssal peridotites. This yields 187Os/188Os (0.1292 ± 25), and 186Os/188Os of 0.1198388 ± 29, both of which are within uncertainty of previous primitive mantle estimates. The 186Os/188Os composition of the PM is less radiogenic than for some plume-related lavas, with the latter requiring sources with high long-term time-integrated Pt/Os. Estimates of primitive mantle HSE concentrations using abyssal peridotites define chondritic Pd/Ir, which differs from previous supra-chondritic estimates for Pd/Ir based on peridotites from a range of tectonic settings. By contrast, estimates of PM yield supra-chondritic Ru/Ir. The cause of enhanced Ru in the mantle remains enigmatic, but may reflect variable partitioning behavior of Ru at high pressure and temperature.
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186 Os– 187 Os and highly Siderophile Element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys
Geochimica et Cosmochimica Acta, 2017Co-Authors: James M.d. Day, Richard J Walker, Jessica M. WarrenAbstract:Abstract Abyssal peridotites are oceanic mantle fragments that were recently processed through ridges and represent residues of both modern and ancient melting. To constrain the nature and timing of melt depletion processes, and the composition of the mantle, we report high-precision Os isotope data for abyssal peridotites from three ocean basins, as well as for Os-rich alloys, primarily from Mesozoic ophiolites. These data are complemented by whole-rock highly Siderophile Element (HSE: Os, Ir, Ru, Pt, Pd, Re), trace- and major-Element abundances for the abyssal peridotites, which are from the Southwest Indian (SWIR), Central Indian (CIR), Mid-Atlantic (MAR) and Gakkel Ridges. The results reveal a limited role for melt refertilization or secondary alteration processes in modifying abyssal peridotite HSE compositions. The abyssal peridotites examined have experienced variable melt depletion (2% to >16%), which occurred >0.5 Ga ago for some samples. Abyssal peridotites typically exhibit low Pd/Ir and, combined with high-degrees of estimated total melt extraction, imply that they were relatively refractory residues prior to incorporation into their present ridge setting. Recent partial melting processes and mid-ocean ridge basalt (MORB) generation therefore played a limited role in the chemical evolution of their precursor mantle domains. The results confirm that many abyssal peridotites are not simple residues of recent MORB source melting, having a more complex and long-lived depletion history. Peridotites from the Gakkel Ridge, SWIR, CIR and MAR indicate that the depleted MORB mantle has 186Os/188Os of 0.1198356 ± 21 (2SD). The Phanerozoic Os-rich alloys yield an average 186Os/188Os within uncertainty of abyssal peridotites (0.1198361 ± 20). Melt depletion trends defined between Os isotopes and melt extraction indices (e.g., Al2O3) allow an estimate of the primitive mantle (PM) composition, using only abyssal peridotites. This yields 187Os/188Os (0.1292 ± 25), and 186Os/188Os of 0.1198388 ± 29, both of which are within uncertainty of previous primitive mantle estimates. The 186Os/188Os composition of the PM is less radiogenic than for some plume-related lavas, with the latter requiring sources with high long-term time-integrated Pt/Os. Estimates of primitive mantle HSE concentrations using abyssal peridotites define chondritic Pd/Ir, which differs from previous supra-chondritic estimates for Pd/Ir based on peridotites from a range of tectonic settings. By contrast, estimates of PM yield supra-chondritic Ru/Ir. The cause of enhanced Ru in the mantle remains enigmatic, but may reflect variable partitioning behavior of Ru at high pressure and temperature.
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lithophile and Siderophile Element systematics of earth s mantle at the archean proterozoic boundary evidence from 2 4 ga komatiites
Geochimica et Cosmochimica Acta, 2016Co-Authors: I. S. Puchtel, Richard J Walker, Alan D. Brandon, Mathieu Touboul, R. W. Nicklas, V.s. Kulikov, Janne Blicherttoft, A. V. SamsonovAbstract:Abstract New Os isotope and highly Siderophile Element (HSE) abundance data, in combination with lithophile trace Element and Sm–Nd, Lu–Hf, and Hf–W isotope data, are reported for komatiitic basalts from the Vetreny Belt and tonalites from the adjacent Vodla Block in the Fennoscandian Shield. Komatiitic basalts define a Re–Os isochron with an age of 2407 ± 6 Ma and an initial γ187Os = +1.7 ± 0.2 (2 SE). The Pt–Os data for chromite separates yield an average initial e186Os = +0.03 ± 0.02 (2 SE). The 147Sm–143Nd and 176Lu–176Hf data for the komatiitic basalts give isochron ages and initial ratios of, respectively, 2403 ± 32 Ma and e143Nd = −0.90 ± 0.09, and 2451 ± 79 Ma and e176Hf = +0.4 ± 0.2 (2 SE). Bulk tonalites are characterized by average initial γ187Os, e143Nd, and e176Hf values of +304 ± 64, +1.8 ± 0.6, and +2.5 ± 1.6 (2 SE), respectively, when calculated for the ∼3.21 Ga age of the rocks. The komatiitic basalts and tonalites have μ142Nd values of, respectively, +0.5 ± 2.8 and −0.4 ± 5.2 (2 SD). By contrast, both the komatiitic basalts and tonalites exhibit positive 182W anomalies of +7.1 ± 4.5 and +12.6 ± 4.5 ppm (2 SD), respectively. The komatiitic basalts were derived from a komatiitic parental magma with ∼27 wt.% MgO; it was modified by both assimilation of the tonalites and fractional crystallization en route to the surface. Lithophile trace Element data constrain the degree of crustal contamination to be 4.0 ± 0.4%. Highly Siderophile Element abundance data indicate that crustal contamination must have had a negligible effect on the Os isotopic composition of the komatiitic parental magma. By contrast, the Nd, Hf, and W isotope systematics of the komatiitic parental magma were strongly modified as a result of assimilation of the tonalites. The positive initial e143Nd and e176Hf values of the tonalites indicate that they formed via melting of a precursor with time-integrated suprachondritic Sm/Nd and Lu/Hf. This precursor was most likely ancient mafic crust. The large positive 182W anomaly present in the tonalites requires that the precursor crust incorporated a primordial component with Hf/W that became fractionated, relative to the bulk mantle, within the first 50 Ma of Solar System history. The absolute HSE abundances in the mantle source of the Vetreny komatiite system are estimated to be 66 ± 7% of those in the present-day Bulk Silicate Earth. This observation, coupled with the normal 182W/184W composition of the komatiitic basalts, when corrected for crustal contamination (μ182W = −0.5 ± 4.5 ppm), indicates that the W-HSE systematics of the Vetreny komatiite system most likely were established as a result of late accretion of chondritic material to Earth. Our present results, combined with isotopic and chemical data available for other early and late Archean komatiite systems, are inconsistent with the model of increasing HSE abundances in komatiitic sources as a result of slow downward mixing into the mantle of chondritic material accreted to Earth throughout the Archean. The observed HSE concentration variations rather reflect sluggish mixing of diverse post-magma ocean domains characterized by variably-fractionated lithophile and Siderophile Element abundances.
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Section 4. Origin of Siderophile Elements in the Silicate Portions of Differentiated Planetary Bodies
Geochemical Perspectives, 2016Co-Authors: Richard J WalkerAbstract:Siderophile Element concentrations in the silicate portions of the Earth, Mars and Moon range from very well to rather poorly constrained. In this section I’ll provide an overview of what we know of Siderophile Element abundances in the silicate portions of these planetary bodies, as well as two
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use of hydrofluoric acid desilicification in the determination of highly Siderophile Element abundances and re pt os isotope systematics in mafic ultramafic rocks
Geostandards and Geoanalytical Research, 2016Co-Authors: James M.d. Day, Richard J Walker, Christopher L. Waters, Bruce F. Schaefer, Simon TurnerAbstract:Properly combining highly Siderophile Element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundance data, obtained by isotope dilution, with corresponding 187Os/188Os and 186Os/188Os measurements of rocks requires efficient digestion of finely-ground powders and complete spike-sample equilibration. Yet, because of the nature of commonly used methods for separating Os from a rock matrix, hydrofluoric acid (HF) is typically not used in such digestions. Consequently, some silicates are not completely dissolved, and HSE residing within these silicates may not be fully accessed. Consistent with this, some recent studies of basaltic reference materials (RMs) have concluded that an HF-desilicification procedure is required to fully access the HSE (Ishikawa et al. (2014) Chemical Geology, 384, 27–46; Li et al. (2015) Geostandards and Geoanalytical Research, 39, 17–30). Highly Siderophile Element abundance and Os isotope studies of intraplate basalts typically target samples with a range of MgO contents ( 18% m/m, or as mass fractions, 18 g per 100 g), in contrast to the lower MgO mass fractions (< 10 g per 100 g) of basalt and diabase RMs (i.e., BIR-1, BHVO-2, TDB-1). To investigate the effect of HF-desilicification on intraplate basalts, experiments were performed on finely ground Azores basalts (8.1–17 g per 100 g MgO) using a ‘standard acid digestion’ (2:1 mixture of concentrated HNO3 and HCl), and a standard acid digestion, followed by HF-desilicification. No systematic trends in HSE abundances were observed between data obtained by standard acid digestion and HF-desilicification. Desilicification procedures using HF do not improve liberation of the HSE from Azores basalts, or some RMs (e.g., WPR-1). We conclude that HF-desilicification procedures are useful for obtaining total HSE contents of some young lavas, but this type of procedure is not recommended for studies where Re-Pt-Os chronological information is desired. The collateral effect of a standard acid digestion to liberate Os, followed by HF-desilicification to obtain Re and Pt abundances in samples, is that the measured Re/Os and Pt/Os may not correspond with measured 187Os/188Os or 186Os/188Os. Des donnees d'abondance des Elements fortement Siderophiles (HSE: Re, Pd, Pt, Ru, Ir, Os), obtenues par dilution isotopique, et combinees correctement avec les mesures sur des roches des rapports isotopiques correspondants 187Os/188Os et 186Os/188Os necessitent une digestion efficace de poudres finement broyees et une equilibration complete « spike »-echantillon. Cependant, en raison de la nature des methodes couramment utilisees pour la separation de l'Os a partir d'une matrice de roche, l'acide fluorhydrique (HF) n'est generalement pas utilise pour ces digestions. Par consequent, certains silicates ne sont pas completement dissous, et les HSE residant dans ces silicates peuvent ne pas etre entierement accessibles. En accord avec ceci, certaines etudes recentes de materiaux de reference basaltiques (RMs) ont conclu qu'une procedure de desilicifcation utilisant l’HF est necessaire pour recuperer entierement les HSE (Ishikawa et al. (2014) Chemical Geology, 384, 27–46; Li et al. (2015) Geostandards Geoanalytical Research, 39, 17–30). Les etudes des abondances en Elements fortement Siderophiles et des isotopes de l'Os dans des basaltes intraplaques ciblent typiquement des echantillons avec une gamme de teneurs en MgO (de 18% m/m, ou en fractions massiques,de 18 g/100 g), contrastant avec les plus faibles fractions de masse en MgO (<10 g/100 g) des basaltes et diabases de reference (RMs, c'est-a-dire BIR-1, BHVO-2, TDB-1). Pour etudier l'effet de la procedure de desilicification utilisant l’HF sur des basaltes intraplaques, des experiences ont ete realisees sur des basaltes finement broyes provenant des Acores (de 8,1 a 17 g/100 g de MgO) en utilisant une procedure de digestion acide «standard» (melange 2:1 de HNO3 concentre et d’HCl), et une digestion acide « standard », suivie par une desilicification utilisant l’HF. Aucune tendance systematique dans les abondances des HSE n'a ete observee entre les donnees obtenues par digestion acide « standard » et par desilicification a l’HF. Les procedures de desilicification utilisant l’HF n'ameliorent pas la liberation des HSE des basaltes des Acores, ou de certains materiaux de reference (par exemple, WPR-1). Nous concluons que les procedures de desilicification a l’HF sont utiles pour obtenir les teneurs totales en HSE de certaines laves recentes, mais ce type de procedure n'est pas recommande pour les etudes dans lesquelles des informations chronologiques Re-Pt-Os sont souhaitees. L'effet collateral d'une digestion acide «standard» permettant de liberer l'Os, suivie d'une desilicification a l’HF permettant d'avoir les abondances en Re et Pt des echantillons, est que les rapports Re/Os et Pt/S mesures ne peuvent pas correspondre aux rapports isotopiques 187Os/188Os ou 186Os/188Os mesures.
Igor S. Puchtel - One of the best experts on this subject based on the ideXlab platform.
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early earth differentiation investigated through 142 nd 182 w and highly Siderophile Element abundances in samples from isua greenland
Geochimica et Cosmochimica Acta, 2016Co-Authors: Igor S. Puchtel, Richard J Walker, Mathieu Touboul, H Rizo, Richard W Carlson, M F Horan, M Boyet, Minik T RosingAbstract:Abstract We report new data for W concentrations, stable W isotopic compositions, high-precision 182W/184W ratios, highly Siderophile Element (HSE) abundances and 187Re–187Os systematics in a suite of 3.8–3.3 Ga mafic and ultramafic rocks from the Isua supracrustal belt, and the Paleoarchean terrane in the northwestern part of the belt. These data are compared with published data for 146Sm–142Nd systematics in the same samples. The samples from the Isua supracrustal belt show well resolved excesses of 182W/184W of up to ∼21 ppm, consistent with previous W isotopic data reported by Willbold et al. (2011) . While there is abundant evidence that W was mobilized in the crust accessed by the Isua supracrustal suite, the isotopic anomalies are interpreted to primarily reflect processes that affected the mantle precursors to these rocks. The origin of the 182W excesses in these rocks remains uncertain. The Isua mantle source could represent a portion of the post-core-formation mantle that was isolated from late accretionary additions (e.g., Willbold et al., 2011 ). However, the combined 182W, Re–Os isotopic systematics and HSE abundances estimated for the source of the Isua basalts are difficult to reconcile with this interpretation. The W isotope variations were more likely produced as a result of fractionation of the Hf/W ratio in the mantle during the lifetime of 182Hf, i.e., during the first 50 Ma of Solar System history. This could have occurred as a result of differentiation in an early magma ocean. The Isua suite examined is also characterized by variable 142Nd/144Nd, but the variations do not correlate with the variations in 182W/184W. Further, samples with ages between 3.8 and 3.3 Ga show gradual diminution of 142Nd anomalies until these are no longer resolved from the modern mantle isotopic composition. By contrast, there is no diminishment of 182W variability with time, suggesting different mechanisms of origin and retention of isotopic variations for these two extinct-radionuclide isotope systems. The presence of 182W isotopic anomalies in rocks as young as 3.3 Ga, implies that early-formed, high Hf/W domains survived for more than 1 Ga in the convective mantle.
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Insights into early Earth from the Pt–Re–Os isotope and highly Siderophile Element abundance systematics of Barberton komatiites
Geochimica et Cosmochimica Acta, 2014Co-Authors: Igor S. Puchtel, Richard J Walker, Mathieu Touboul, Euan G. Nisbet, Gary R. ByerlyAbstract:Abstract Highly Siderophile Element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance and Pt–Re–Os isotopic data are reported for well-preserved komatiites from the Komati and Weltevreden Formations of the Barberton Greenstone Belt in South Africa. The Re–Os data for whole-rock samples and olivine and chromite separates define isochrons with ages of 3484 ± 38 and 3263 ± 12 Ma for the Komati and Weltevreden systems, respectively. The respective initial 187Os/188Os = 0.10335 ± 15 (γ187Os = +0.34 ± 0.15) and 0.10442 ± 4 (γ187Os = −0.14 ± 0.04) are well within the range defined by chondritic meteorites. When considered together with the Re–Os data for late Archean komatiite systems, these data indicate that the mantle sources of most Archean komatiites evolved with essentially uniform long-term Re/Os that is well within the chondritic range. By contrast, the initial 186Os/188Os = 0.1198283 ± 9 (e186Os = −0.12 ± 0.08) and 0.1198330 ± 8 (e186Os = +0.22 ± 0.07) for the Komati and Weltevreden systems, respectively, are outside of known chondritic evolution paths, indicating that the mantle sources of these two komatiite systems evolved with fractionated time-integrated Pt/Os. The new 186,187Os isotopic data for these early Archean komatiite systems, combined with published 142,143Nd and 176Hf isotopic data for these systems, are consistent with formation and long-term isolation of deep-seated mantle domains with fractionated time-integrated Sm/Nd, Lu/Hf, and Pt/Os ratios, at ca. 4400 Ma. These domains may have been generated as a result of late-stage crystallization of a primordial magma ocean involving Mg-perovskite, Ca-perovskite and Pt-alloys acting as the fractionating phases. The inferred fractionated mantle domains were sampled by the early Archean komatiites, but were largely mixed away by 2.7 Ga, as evidenced by uniform time-integrated Sm/Nd, Lu/Hf, and Pt/Os ratios inferred for the sources of most late Archean komatiite systems. The calculated total Pt + Pd abundances present in the sources of the early Archean komatiite systems fall only 7–14% short of those present in estimates for the modern primitive mantle. These are also within the range of the total Pt + Pd abundances present in the sources of late Archean komatiite systems, indicating little change in the HSE abundances in the Archean mantle between 3.5 and 2.7 Ga. The new HSE data for the early Archean komatiite systems may implicate late accretion of HSE to the mantle prior to completion of crystallization of a final terrestrial magma ocean, followed by sluggish mixing of diverse, post-magma ocean domains characterized by variably fractionated lithophile Element and HSE abundances.
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New insights into the Hadean mantle revealed by 182W and highly Siderophile Element abundances of supracrustal rocks from the Nuvvuagittuq Greenstone Belt, Quebec, Canada
Chemical Geology, 2014Co-Authors: Mathieu Touboul, Igor S. Puchtel, Jingao Liu, Jonathan O'neil, Richard J WalkerAbstract:Abstract Tungsten concentration and isotopic data, coupled with highly Siderophile Element (HSE) concentration and Os isotopic data for ≥ 3.66 billion year-old ultramafic, mafic, and felsic supracrustal rocks from the Nuvvuagittuq Greenstone Belt, were investigated to place additional constraints on the nature and origin of 182 W heterogeneities in the early Earth. The absolute and relative abundances of HSE in the mafic and ultramafic rocks are generally similar to those in modern rocks with comparable MgO contents. Further, most samples plot close to 3.8 to 4.4 Ga reference lines on a 187 Re– 187 Os isochron diagram, indicating that HSE abundances in most Nuvvuagittuq samples remained undisturbed by post-Eoarchean metamorphic events. All Nuvvuagittuq samples analyzed show well-resolved 182 W excesses, ranging from + 6 to + 17 ppm, compared with the modern isotopic composition of W. The observed level of HSE abundances, coupled with the 182 W enrichments of these rocks is seemingly inconsistent with their derivation from mantle that was isolated from a HSE-rich and 182 W-depleted late accretionary component. However, the absence of correlation between W and MgO contents, as well as variable W enrichment relative to Elements with similar incompatibilities suggest that the W in the Nuvvuagittuq samples involved fluid transport of the W in either the crust or the mantle, and that it has little genetic relationship with the HSE. Given the lack of evidence for extensive redistribution of W in the crust, the HSE and W Elemental and isotopic systematics of the Nuvvuagittuq rocks may be explained by a model whereby peridotitic mantle, with modern-like HSE abundances, was metasomatized by fluids derived from a 182 W-rich crustal component that had been recycled into the mantle via subduction or delamination. The source of the 182 W excess carried by this crustal component remains enigmatic. It was most likely inherited from either pre-late accretionary, or early-depleted parental mantle reservoirs.
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New insights into the Hadean mantle revealed by W-182 and highly Siderophile Element abundances of supracrustal rocks from the Nuvvuagittuq Greenstone Belt, Quebec, Canada
Chemical Geology, 2014Co-Authors: Mathieu Touboul, Igor S. Puchtel, Jingao Liu, Jonathan O'neil, Richard J WalkerAbstract:Tungsten concentration and isotopic data, coupled with highly Siderophile Element (HSE) concentration and Os isotopic data for = 3.66 billion year-old ultramafic, mafic, and felsic supracrustal rocks from the Nuvvuagittuq Greenstone Belt, were investigated to place additional constraints on the nature and origin of W-182 heterogeneities in the early Earth. The absolute and relative abundances of HSE in the mafic and ultramafic rocks are generally similar to those in modern rocks with comparable MgO contents. Further, most samples plot close to 3.8 to 4.4 Ga reference lines on a Re-187-Os-187 isochron diagram, indicating that HSE abundances in most Nuvvuagittuq samples remained undisturbed by post-Eoarchean metamorphic events. All Nuvvuagittuq samples analyzed show well-resolved W-182 excesses, ranging from + 6 to + 17 ppm, compared with the modern isotopic composition of W. The observed level of HSE abundances, coupled with the W-182 enrichments of these rocks is seemingly inconsistent with their derivation from mantle that was isolated from a HSE-rich and W-182-depleted late accretionary component. However, the absence of correlation between W and MgO contents, as well as variable W enrichment relative to Elements with similar incompatibilities suggest that the W in the Nuvvuagittuq samples involved fluid transport of the W in either the crust or the mantle, and that it has little genetic relationship with the HSE. Given the lack of evidence for extensive redistribution of W in the crust, the HSE and W Elemental and isotopic systematics of the Nuvvuagittuq rocks may be explained by a model whereby peridotitic mantle, with modern-like HSE abundances, was metasomatized by fluids derived from a W-182-rich crustal component that had been recycled into the mantle via subduction or delamination. The source of the W-182 excess carried by this crustal component remains enigmatic. It was most likely inherited from either pre-late accretionary, or early-depleted parental mantle reservoirs. (C) 2014 Elsevier B.V. All rights reserved.
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Insights into early Earth from the Pt-Re-Os isotope and highly Siderophile Element abundance systematics of Barberton komatiites
Geochimica et Cosmochimica Acta, 2014Co-Authors: Igor S. Puchtel, Richard J Walker, Mathieu Touboul, Euan G. Nisbet, Gary R. ByerlyAbstract:Highly Siderophile Element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance and Pt-Re-Os isotopic data are reported for well-preserved komatiites from the Komati and Weltevreden Formations of the Barberton Greenstone Belt in South Africa. The Re-Os data for whole-rock samples and olivine and chromite separates define isochrons with ages of 3484 +/- 38 and 3263 +/- 12 Ma for the Komati and Weltevreden systems, respectively. The respective initial Os-187/Os-188 = 0.10335 +/- 15 (gamma Os-187 = +0.34 +/- 0.15) and 0.10442 +/- 4 (gamma Os-187 = -0.14 +/- 0.04) are well within the range defined by chondritic meteorites. When considered together with the Re-Os data for late Archean komatiite systems, these data indicate that the mantle sources of most Archean komatiites evolved with essentially uniform long-term Re/Os that is well within the chondritic range. By contrast, the initial Os-186/Os-188 = 0.1198283 +/- 9 (epsilon Os-186 = -0.12 +/- 0.08) and 0.1198330 +/- 8 (epsilon Os-186 = +0.22 +/- 0.07) for the Komati and Weltevreden systems, respectively, are outside of known chondritic evolution paths, indicating that the mantle sources of these two komatiite systems evolved with fractionated time-integrated Pt/Os. The new 186,187 Os isotopic data for these early Archean komatiite systems, combined with published Nd-142,Nd-143 and Hf-176 isotopic data for these systems, are consistent with formation and long-term isolation of deep-seated mantle domains with fractionated time-integrated Sm/Nd, Lu/Hf, and Pt/Os ratios, at ca. 4400 Ma. These domains may have been generated as a result of late-stage crystallization of a primordial magma ocean involving Mg-perovskite, Ca-perovskite and Pt-alloys acting as the fractionating phases. The inferred fractionated mantle domains were sampled by the early Archean komatiites, but were largely mixed away by 2.7 Ga, as evidenced by uniform time-integrated Sm/Nd, Lu/Hf, and Pt/Os ratios inferred for the sources of most late Archean komatiite systems. The calculated total Pt + Pd abundances present in the sources of the early Archean komatiite systems fall only 7-14% short of those present in estimates for the modern primitive mantle. These are also within the range of the total Pt + Pd abundances present in the sources of late Archean komatiite systems, indicating little change in the HSE abundances in the Archean mantle between 3.5 and 2.7 Ga. The new HSE data for the early Archean komatiite systems may implicate late accretion of HSE to the mantle prior to completion of crystallization of a final terrestrial magma ocean, followed by sluggish mixing of diverse, post-magma ocean domains characterized by variably fractionated lithophile Element and HSE abundances. (C) 2013 Elsevier Ltd. All rights reserved.
Michael J. Drake - One of the best experts on this subject based on the ideXlab platform.
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Metal-silicate equilibrium in a homogeneously accreting earth: new results for Re
Earth and Planetary Science Letters, 1997Co-Authors: Kevin Righter, Michael J. DrakeAbstract:The accretion and early differentiation of the Earth is the starting point of earth history. The abundances of metal-seeking (Siderophile) Elements in the mantle are a powerful probe of those events. It has long been known that Siderophile Element abundances in the Earth's mantle are too high to have resulted from metal-silicate equilibrium at near surface conditions. This mismatch provided support for the idea that the Earth accreted heterogeneously. We report new experimental results for the highly Siderophile Element Re and show that metal-silicate partition coefficients decrease with increasing temperature (at fixed pressure and relative oxygen fugacity). Calculations using these and previously published experimental results indicate that the abundances of the moderately Siderophile Elements Fe, Ni, Co, Mo, W, P and, most importantly, the highly Siderophile Element Re, in Earth's upper mantle are consistent with early equilibration between metal and silicate liquid at the base of a deep (800–1000 km) magma ocean. Reconciliation of mantle abundances of moderately and highly Siderophile Elements with high temperature and pressure metal-silicate equilibrium would obviate the need for heterogeneous accretion. These new results indicate that the Earth accreted homogeneously, rather than heterogeneously, or that evidence for heterogeneous accretion was erased by early high temperature and pressure melting events.
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Core Formation in Earth's Moon, Mars, and Vesta
Icarus, 1996Co-Authors: Kevin Righter, Michael J. DrakeAbstract:Stimulated by new experimental results on metal/silicate partitioning of Elements at elevated temperatures and pressures, we have revisited the question of core formation in Earth's Moon, Mars, and Vesta (the probable source of the eucritic meteorites). Earlier studies suggested metal/silicate equilibrium in Mars, but led to the paradox that Mars accreted homogeneously while the Earth accreted heterogeneously. Using new elevated pressure and temperature metal/silicate partition coefficients, we show that abundances of the moderately Siderophile Elements in the mantles of the Moon, Mars and Vesta are consistent with early magma oceans on these bodies. The most successful model for explaining the lunar mantle Siderophile Element abundances requires a core of 5% of the mass of the Moon (500-km radius). Siderophile Element abundances in Mars are consistent with intermediate pressure metal–silicate equilibrium, as has also been recently suggested for the Earth. The most successful model for explaining the martian Siderophile Element abundances requires a bulk planetary composition that has greater than CI chondritic abundances of the moderately Siderophile Elements, and a core of 30% of the mass of Mars. Siderophile Element abundances in the mantle of Vesta are consistent with low pressure liquid metal–liquid silicate equilibrium, as expected for an asteroid-sized body, and a core of 10% of the mass of Vesta. Comparison of our best-fit oxygen fugacities for Vesta with thermodynamic calculations of oxygen fugacity for silicate-bearing iron meteorites indicates that parts of the inner solar system were homogeneous with respect to redox state at 4.5 Ga, approximately 2 log fO2units below the Fe–FeO buffer—much higher than estimates for the solar nebula. Similar comparisons for Mars and the Earth indicate that these bodies have undergone oxidation since 4.5 Ga, because the oxygen fugacities associated with metal–silicate equilibrium in both Mars and the Earth–Moon system are much lower than those recorded in martian and terrestrial basaltic and periodotitic samples. The oxidation on Mars is most likely due to atmospheric effects, whereas Earth's much wider range of oxygen fugacities must be due to both atmospheric and plate tectonic effects.
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Metal‐silicate thermochemistry at high temperature: Magma oceans and the “excess Siderophile Element” problem of the Earth's upper mantle
Journal of Geophysical Research: Planets, 1993Co-Authors: Christopher J. Capobianco, John H. Jones, Michael J. DrakeAbstract:Recent theoretical considerations indicate that Earth should have been at least partly molten at the end of accretion and perhaps sufficiently to produce a magma ocean. Metal-silicate partition coefficients applicable to a magma ocean (3000 K-4000 K and up to 130 GPa) would be valuable to test such models against well-known mantle Siderophile Element abundances, but they have not been measured. However, an extrapolation of existing low temperature data to these extreme conditions has recently been attempted [Murthy, 1991]. Murthy's results seem to account for many apparent excesses of Siderophile Elements in the mantle. We examined his extrapolation method and found it to be inconsistent with published temperature dependencies for metal-silicate partition coefficients. We also attempted an extrapolation to magma ocean temperatures, based on known chemical behavior for several Elements, and using published metal-silicate partition coefficients. If the chemistry quantified by the low-temperature data is applicable at high temperature, an important assumption, then our results indicate that high temperature alone will not help ameliorate the excess Siderophile Element problem of the upper mantle, in contrast to the conclusions of Murthy [1991]. For most Elements, a modest increase in Siderophile behavior is predicted with rising temperature if an iron-wustite redox buffer is paralleled. But long-range extrapolation of experimental data containing even modest experimental errors is hazardous. Extrapolated high-temperature partition coefficients can differ by orders of magnitude for a given Element, even though the input from independent studies is consistent within quoted errors. Direct experimental measurements for at least some of the Siderophile Elements will be necessary to accurately assess Siderophile Element behavior in a magma ocean. The excess Siderophile Element problem of the Earth's upper mantle remains unsolved.
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metal silicate thermochemistry at high temperature magma oceans and the excess Siderophile Element problem of the earth s upper mantle
Journal of Geophysical Research, 1993Co-Authors: Christopher J. Capobianco, John H. Jones, Michael J. DrakeAbstract:Recent theoretical considerations indicate that Earth should have been at least partly molten at the end of accretion and perhaps sufficiently to produce a magma ocean. Metal-silicate partition coefficients applicable to a magma ocean (3000 K-4000 K and up to 130 GPa) would be valuable to test such models against well-known mantle Siderophile Element abundances, but they have not been measured. However, an extrapolation of existing low temperature data to these extreme conditions has recently been attempted [Murthy, 1991]. Murthy's results seem to account for many apparent excesses of Siderophile Elements in the mantle. We examined his extrapolation method and found it to be inconsistent with published temperature dependencies for metal-silicate partition coefficients. We also attempted an extrapolation to magma ocean temperatures, based on known chemical behavior for several Elements, and using published metal-silicate partition coefficients. If the chemistry quantified by the low-temperature data is applicable at high temperature, an important assumption, then our results indicate that high temperature alone will not help ameliorate the excess Siderophile Element problem of the upper mantle, in contrast to the conclusions of Murthy [1991]. For most Elements, a modest increase in Siderophile behavior is predicted with rising temperature if an iron-wustite redox buffer is paralleled. But long-range extrapolation of experimental data containing even modest experimental errors is hazardous. Extrapolated high-temperature partition coefficients can differ by orders of magnitude for a given Element, even though the input from independent studies is consistent within quoted errors. Direct experimental measurements for at least some of the Siderophile Elements will be necessary to accurately assess Siderophile Element behavior in a magma ocean. The excess Siderophile Element problem of the Earth's upper mantle remains unsolved.
John H. Jones - One of the best experts on this subject based on the ideXlab platform.
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Signatures of the highly Siderophile Elements in the SNC meteorites and Mars: a review and petrologic synthesis
Chemical Geology, 2003Co-Authors: John H. Jones, Clive R. Neal, James C. ElyAbstract:We have evaluated the highly Siderophile Element (HSE) signatures of the martian (SNC) meteorites using new and literature data. These Ir and Os concentrations correlate with the Mg# [molar Mg/(Mg+Fe)], Cr and Ni, suggesting that olivine or chromite acts as a host for compatible Siderophiles. Our analysis agrees with others who have suggested that the martian mantle has chondritic relative abundances of Siderophiles. We also agree that, unlike the Sr and Nd isotopic systems, there is no evidence from Os isotopes for crustal assimilation. Comparisons of the Siderophile Element ratios of ALH 84001 to younger SNCs give no indication of a change in the martian Siderophile Element pattern over time. D 2002 Published by Elsevier Science B.V.
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neutron activation analysis of multiple 10 100 μg glass samples from Siderophile Element partitioning experiments
Geochimica et Cosmochimica Acta, 1996Co-Authors: David J. Lindstrom, John H. JonesAbstract:Abstract Experimental studies of the metal/silicate partitioning of Ir have produced two sets of results differing by a factor of 10 6 ( Jones and Drake, 1986 ; O'Neill et al., 1995 ). In an attempt to understand this discrepancy, we have performed instrumental neutron activation analyses of multiple glass chips from these Siderophile Element partitioning experiments. From seven to ten individual chips (~ 10–100 μ g each) were analyzed from each experimental glass. The major target was Ir, which was detected at concentrations ranging from 690 ppb (ng/g) to less than 1 ppb. Detection limits are less than 10 −13 grams Ir. Almost all of the run products appear to be homogeneous with respect to lithophile Elements, but most vary considerably in their Ir concentrations. It appears that most of the experiments analyzed so far contain small amounts of metal in the glass separates. One experiment from Jones and Drake (1983) appears to be homogeneous, but in fact may not be. More recent experiments are definitely not homogeneous, and require the existence of a metallic phase distinctly different from the bulk solid metal or liquid metal/sulfide phases also present in the charges. Iridium contents of the glass in the stirred crucible experiment studied ( O'Neill et al., 1996 ) decreased continuously over a period of several months, even as the oxygen fugacity was increasing, suggesting that the experiment never achieved equilibrium. Iridium contents in multiple samples of the glasses varied over factors of 2 to 3x. Further subdivision of one low-Ir glass did not result in a reduction in the variation of Ir between aliquots. Thus, the variation in Ir concentration appears to remain fairly constant, irrespective of the scale of the sampling.
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Neutron activation analysis of multiple 10–100 μg glass samples from Siderophile Element partitioning experiments
Geochimica et Cosmochimica Acta, 1996Co-Authors: David J. Lindstrom, John H. JonesAbstract:Abstract Experimental studies of the metal/silicate partitioning of Ir have produced two sets of results differing by a factor of 10 6 ( Jones and Drake, 1986 ; O'Neill et al., 1995 ). In an attempt to understand this discrepancy, we have performed instrumental neutron activation analyses of multiple glass chips from these Siderophile Element partitioning experiments. From seven to ten individual chips (~ 10–100 μ g each) were analyzed from each experimental glass. The major target was Ir, which was detected at concentrations ranging from 690 ppb (ng/g) to less than 1 ppb. Detection limits are less than 10 −13 grams Ir. Almost all of the run products appear to be homogeneous with respect to lithophile Elements, but most vary considerably in their Ir concentrations. It appears that most of the experiments analyzed so far contain small amounts of metal in the glass separates. One experiment from Jones and Drake (1983) appears to be homogeneous, but in fact may not be. More recent experiments are definitely not homogeneous, and require the existence of a metallic phase distinctly different from the bulk solid metal or liquid metal/sulfide phases also present in the charges. Iridium contents of the glass in the stirred crucible experiment studied ( O'Neill et al., 1996 ) decreased continuously over a period of several months, even as the oxygen fugacity was increasing, suggesting that the experiment never achieved equilibrium. Iridium contents in multiple samples of the glasses varied over factors of 2 to 3x. Further subdivision of one low-Ir glass did not result in a reduction in the variation of Ir between aliquots. Thus, the variation in Ir concentration appears to remain fairly constant, irrespective of the scale of the sampling.
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Metal‐silicate thermochemistry at high temperature: Magma oceans and the “excess Siderophile Element” problem of the Earth's upper mantle
Journal of Geophysical Research: Planets, 1993Co-Authors: Christopher J. Capobianco, John H. Jones, Michael J. DrakeAbstract:Recent theoretical considerations indicate that Earth should have been at least partly molten at the end of accretion and perhaps sufficiently to produce a magma ocean. Metal-silicate partition coefficients applicable to a magma ocean (3000 K-4000 K and up to 130 GPa) would be valuable to test such models against well-known mantle Siderophile Element abundances, but they have not been measured. However, an extrapolation of existing low temperature data to these extreme conditions has recently been attempted [Murthy, 1991]. Murthy's results seem to account for many apparent excesses of Siderophile Elements in the mantle. We examined his extrapolation method and found it to be inconsistent with published temperature dependencies for metal-silicate partition coefficients. We also attempted an extrapolation to magma ocean temperatures, based on known chemical behavior for several Elements, and using published metal-silicate partition coefficients. If the chemistry quantified by the low-temperature data is applicable at high temperature, an important assumption, then our results indicate that high temperature alone will not help ameliorate the excess Siderophile Element problem of the upper mantle, in contrast to the conclusions of Murthy [1991]. For most Elements, a modest increase in Siderophile behavior is predicted with rising temperature if an iron-wustite redox buffer is paralleled. But long-range extrapolation of experimental data containing even modest experimental errors is hazardous. Extrapolated high-temperature partition coefficients can differ by orders of magnitude for a given Element, even though the input from independent studies is consistent within quoted errors. Direct experimental measurements for at least some of the Siderophile Elements will be necessary to accurately assess Siderophile Element behavior in a magma ocean. The excess Siderophile Element problem of the Earth's upper mantle remains unsolved.
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metal silicate thermochemistry at high temperature magma oceans and the excess Siderophile Element problem of the earth s upper mantle
Journal of Geophysical Research, 1993Co-Authors: Christopher J. Capobianco, John H. Jones, Michael J. DrakeAbstract:Recent theoretical considerations indicate that Earth should have been at least partly molten at the end of accretion and perhaps sufficiently to produce a magma ocean. Metal-silicate partition coefficients applicable to a magma ocean (3000 K-4000 K and up to 130 GPa) would be valuable to test such models against well-known mantle Siderophile Element abundances, but they have not been measured. However, an extrapolation of existing low temperature data to these extreme conditions has recently been attempted [Murthy, 1991]. Murthy's results seem to account for many apparent excesses of Siderophile Elements in the mantle. We examined his extrapolation method and found it to be inconsistent with published temperature dependencies for metal-silicate partition coefficients. We also attempted an extrapolation to magma ocean temperatures, based on known chemical behavior for several Elements, and using published metal-silicate partition coefficients. If the chemistry quantified by the low-temperature data is applicable at high temperature, an important assumption, then our results indicate that high temperature alone will not help ameliorate the excess Siderophile Element problem of the upper mantle, in contrast to the conclusions of Murthy [1991]. For most Elements, a modest increase in Siderophile behavior is predicted with rising temperature if an iron-wustite redox buffer is paralleled. But long-range extrapolation of experimental data containing even modest experimental errors is hazardous. Extrapolated high-temperature partition coefficients can differ by orders of magnitude for a given Element, even though the input from independent studies is consistent within quoted errors. Direct experimental measurements for at least some of the Siderophile Elements will be necessary to accurately assess Siderophile Element behavior in a magma ocean. The excess Siderophile Element problem of the Earth's upper mantle remains unsolved.
Kevin Righter - One of the best experts on this subject based on the ideXlab platform.
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Moderately and slightly Siderophile Element constraints on the depth and extent of melting in early Mars
Meteoritics & Planetary Science, 2011Co-Authors: Kevin Righter, Nancy L. ChabotAbstract:– The thermal history of Mars during accretion and differentiation is important for understanding some fundamental aspects of its evolution such as crust formation, mantle geochemistry, chronology, volatile loss and interior degassing, and atmospheric development. In light of data from new Martian meteorites and exploration rovers, we have made a new estimate of Martian mantle Siderophile Element depletions. New high pressure and temperature metal–silicate experimental partitioning data and expressions are also available. Using these new constraints, we consider the conditions under which the Martian mantle may have equilibrated with metallic liquid. The resulting conditions that best satisfy six Siderophile Elements—Ni, Co, W, Mo, P, and Ga—and are consistent with the solidus and liquidus of the Martian mantle phase diagram are a pressure of 14 ± 3 GPa and temperature of 2100 ± 200 K. The Martian mantle depletions of Cr and V are also consistent with metal–silicate equilibration in this pressure and temperature range if deep mantle silicate phases are also taken into account. The results are not consistent with either metal–silicate equilibrium at the surface or at the current-day Martian core–mantle boundary. Recent measurements and modeling have concluded that deep (∼17 GPa or 1350 km) mantle melting is required to explain isotopic data for Martian meteorites and the nature of differentiation into core, mantle, and crust. This is in general agreement with our estimates of the conditions of Martian core formation based on Siderophile Elements that result in an intermediate depth magma ocean scenario for metal–silicate equilibrium.
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Depletion of Vandium in Planetary Mantles: Controlled by Metal, Oxide, or Silicate?
2006Co-Authors: Kevin RighterAbstract:Vanadium concentrations in planetary mantles can provide information about the conditions during early accretion and differentiation. Because V is a slightly Siderophile Element, it is usually assumed that any depletion would be due to core formation and metal-silicate equilibrium. However, V is typically more compatible in phases such as spinel, magnesiowuestite and garnet. Fractionation of all of these phases would cause depletions more marked than those from metal. In this paper consideration of depletions due to metal, oxide and silicate are critically evaluated.
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Metal-Silicate Partitioning of Siderophile Elements and Core Formation in the Early Earth*
Annual Review of Earth and Planetary Sciences, 2003Co-Authors: Kevin RighterAbstract:▪ Abstract Accretion models for the Earth and terrestrial planets are based on the distribution of Siderophile (iron-loving) Elements between metal and silicate. Extensive experimental studies of the partitioning of these Elements between metallic liquid and silicate melt have led to a better understanding and a more sophisticated application to planetary problems. Siderophile Element metal/silicate partition coefficients are a function of temperature, pressure, oxygen fugacity, and metal and silicate composition. Quantification of these effects for a limited subset of Siderophile Elements has led to the idea that early Earth had a 700-km or deeper magma ocean. This new understanding of Siderophile Element partitioning has also led to applications to the kinetics of metal-silicate equilibrium, links to the timing of core formation, and a better understanding of core formation and metal-silicate equilibrium in the Moon and Mars. Key issues for future consideration include the role of water in early Earth, ...
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Does the Moon Have a Metallic Core?: Constraints from Giant Impact Modeling and Siderophile Elements
Icarus, 2002Co-Authors: Kevin RighterAbstract:Abstract The issue of whether the Moon has a small metallic core is reexamined in light of new information: improved dynamical modeling, new constraints on core size, and high temperature and pressure metal–silicate partition coefficients. Addressed specifically is the question of whether the Moon's Siderophile Element budget can be explained by derivation of the Moon from a differentiated impactor or proto-Earth (stage 1), followed by formation of a small metallic core within the Moon (stage 2). If the Moon is made of mantle material from either a “hot” impactor or a “warm” impactor or proto-Earth, a small metallic core (0.7 to 2 mass%) is predicted. If the Moon is made from mantle material from a “hot” proto-Earth, the lunar mantle would be more depleted in W or Re than is observed. Scenarios in which the Moon is made from impactor or proto-Earth mantle material that has equilibrated with metal at low pressures and temperatures (“cold” scenarios) would yield a much larger metallic core than observed. Finally, the greater depletions of Ni, Mo, and Re in the Moon (relative to the Earth) can be explained by low PT and reduced metal–silicate equilibrium in an impactor without later core formation in the Moon (i.e., no stage 2), but depletions of Co, Ga, and W cannot. Altogether, geochemically unlikely or geophysically inadequate non-metallic core alternatives, substantial geophysical evidence for a metallic core, and the successful models presented here for Siderophile Element depletions all favor the presence of a small lunar metallic core. Previous geochemical objections to an impactor origin of the Moon are eliminated because Siderophile Element concentrations in the lunar mantle are consistent with separation of a small core from a bulk Moon derived from impactor mantle material.
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Metal-silicate equilibrium in a homogeneously accreting earth: new results for Re
Earth and Planetary Science Letters, 1997Co-Authors: Kevin Righter, Michael J. DrakeAbstract:The accretion and early differentiation of the Earth is the starting point of earth history. The abundances of metal-seeking (Siderophile) Elements in the mantle are a powerful probe of those events. It has long been known that Siderophile Element abundances in the Earth's mantle are too high to have resulted from metal-silicate equilibrium at near surface conditions. This mismatch provided support for the idea that the Earth accreted heterogeneously. We report new experimental results for the highly Siderophile Element Re and show that metal-silicate partition coefficients decrease with increasing temperature (at fixed pressure and relative oxygen fugacity). Calculations using these and previously published experimental results indicate that the abundances of the moderately Siderophile Elements Fe, Ni, Co, Mo, W, P and, most importantly, the highly Siderophile Element Re, in Earth's upper mantle are consistent with early equilibration between metal and silicate liquid at the base of a deep (800–1000 km) magma ocean. Reconciliation of mantle abundances of moderately and highly Siderophile Elements with high temperature and pressure metal-silicate equilibrium would obviate the need for heterogeneous accretion. These new results indicate that the Earth accreted homogeneously, rather than heterogeneously, or that evidence for heterogeneous accretion was erased by early high temperature and pressure melting events.