Systematics of metal-silicate partitioning for many siderophile elements applied to Earth’s core formation

Superliquidus metal-silicate partitioning was investigated for a number of moderately siderophile (Mo, As, Ge, W, P, Ni, Co), slightly siderophile (Zn, Ga, Mn, V, Cr) and refractory lithophile (Nb, Ta) elements. To provide independent constrains on the effects of temperature, oxygen fugacity and silicate melt composition, isobaric (3 GPa) experiments were conducted in piston cylinder apparatus at temperature between 1600 and 2600 °C, relative oxygen fugacities of IW−1.5 to IW−3.5, and for silicate melt compositions ranging from basalt to peridotite. The effect of pressure was investigated through a combination of piston cylinder and multi-anvil isothermal experiments between 0.5 and 18 GPa at 1900 °C. Oxidation states of siderophile elements in the silicate melt as well as effect of carbon saturation on partitioning are also derived from these results. For some elements (e.g. Ga, Ge, W, V, Zn) the observed temperature dependence does not define trends parallel to those modeled using metal-metal oxide free energy data. We correct partitioning data for solute interactions in the metallic liquid and provide a parameterization utilized in extrapolating these results to the P-T-X conditions proposed by various core formation models. A single-stage core formation model reproduces the mantle abundances of several siderophile elements (Ni, Co, Cr, Mn, Mo, W, Zn) for core-mantle equilibration at pressures from 32 to 42 GPa along the solidus of a deep peridotitic magma ocean (∼3000 K for this pressure range) and oxygen fugacities relevant to the FeO content of the present-day mantle. However, these P-T-fO₂ conditions cannot produce the observed concentrations of Ga, Ge, V, Nb, As and P. For more reducing conditions, the P-T solution domain for single stage core formation occurs at subsolidus conditions and still cannot account for the abundances of Ge, Nb and P. Continuous core formation at the base of a magma ocean at P-T conditions constrained by the peridotite liquidus and fixed fO₂ yields concentrations matching observed values for Ni, Co, Cr, Zn, Mn and W but underestimates the core/mantle partitioning observed for other elements, notably V, which can be reconciled if accretion began under reducing conditions with progressive oxidation to fO₂ conditions consistent with the current concentration of FeO in the mantle as proposed by Wade and Wood (2005). However, neither oxygen fugacity path is capable of accounting for the depletions of Ga and Ge in the Earth’s mantle. To better understand core formation, we need further tests integrating the currently poorly-known effects of light elements and more complex conditions of accretion and differentiation such as giant impacts and incomplete equilibration.     

Effect of pressure, temperature, and oxygen fugacity on the metal-silicate partitioning of Te, Se, and S: Implications for earth differentiation

We have measured liquid Fe metal-liquid silicate partitioning (D_i) of tellurium, selenium, and sulfur over a range of pressure, temperature, and oxygen fugacity (1-19 GPa, 2023-2693 K, fO₂ −0.4 to −5.5 log units relative to the iron-wüstite buffer) to better assess the role of metallic melts in fractionating these elements during mantle melting and early Earth evolution. We find that metal-silicate partitioning of all three elements decreases with falling FeO activity in the silicate melt, and that the addition of 5-10 wt% S in the metal phase results in a 3-fold enhancement of both D_Te and D_Se. In general, Te, Se, and S all become more siderophile with increasing pressure, and less siderophile with increasing temperature, in agreement with previous work. In all sulfur-bearing experiments, D_Te is greater than D_Se or D_S, with the latter two being similar over a range of P and T. Parameterized results are used to estimate metal-silicate partitioning at the base of a magma ocean which deepens as accretion progresses, with the equilibration temperature fixed at the peridotite liquidus. We show that during accretion, Te behaves like a highly siderophile element, with expected core/mantle partitioning of >10⁵, in contrast to the observed core/mantle ratio of ∼100. Less extreme differences are observed for Se and S, which yielded core/mantle partitioning 100- to 10 times higher, respectively, than the observed value. Addition of ∼0.5 wt% of a meteorite component (H, EH or EL ordinary chondrite) is sufficient to raise mantle abundances to their current level and erase the original interelement fractionation of metal-silicate equilibrium.     

Metal-silicate partitioning and constraints on core composition and oxygen fugacity during Earth accretion

We present the results of new partitioning experiments between metal and silicate melts for a series of elements normally regarded as refractory lithophile and moderately siderophile and volatile. These include Si, Ti, Ni, Cr, Mn, Ga, Nb, Ta, Cu and Zn. Our new data obtained at 3.6 and 7.7 GPa and between 2123 and 2473 K are combined with literature data to parameterize the individual effects of oxygen fugacity, temperature, pressure and composition on partitioning. We find that Ni, Cu and Zn become less siderophile with increasing temperature. In contrast, Mn, Cr, Si, Ta, Nb, Ga and Ti become more siderophile with increasing temperature, with the highly charged cations (Nb, Ta, Si and Ti) being the most sensitive to variations of temperature. We also find that Ni, Cr, Nb, Ta and Ga become less siderophile with increasing pressure, while Mn becomes more siderophile with increasing pressure. Pressure effects on the partitioning of Si, Ti, Cu and Zn appear to be negligible, as are the effects of silicate melt composition on the partitioning of divalent cations. From the derived parameterization, we predict that the silicate Earth abundances of the elements mentioned above are best explained if core formation in a magma ocean took place under increasing conditions of oxygen fugacity, starting from moderately reduced conditions and finishing at the current mantle-core equilibrium value.     

The effect of Si on metal-silicate partitioning of siderophile elements and implications for the conditions of core formation

We have determined the liquid metal-liquid silicate partitioning of Ni, Co, Mo, W, V, Cr and Nb at 1.5 GPa/1923 K and 6 GPa/2123 K under conditions of constant silicate melt composition with variable amounts of Si in the Fe-rich metallic liquid. Partitioning of Ni, Co, Mo, W and V is sensitive to the Si content of the metal with, in all five cases, increasing Si tending to make the element more lithophile than for conditions where the metal is Si-free. In contrast, metal-silicate partitioning of Cr and Nb is, at constant silicate melt composition, insensitive to the Si content of the metal.The implications of our data are that if, as indicated by the Si isotopic composition of the silicate Earth ( [Georg et al., 2007] and [Fitoussi et al., 2009]), the core contains significant amounts of Si, the important siderophile elements Ni, Co, W and Mo were more lithophile during accretion and core formation than previously believed.We use our new data in conjunction with published metal-silicate partitioning results to develop a model of continuous accretion and core segregation taking explicit account of the partitioning of Si (this study) and O (from Ozawa et al., 2008) between metal and silicate and their effects on metal-silicate partitioning of siderophile elements. We find that the effect of Si on the siderophile characteristics of Ni, Co and W means that the pressures of core segregation estimated from these elements are ∼5 GPa lower than those derived from experiments in which the metal contained negligible Si (e.g., Wade and Wood, 2005). The core-mantle partitioning of Cr and Nb requires that most of Earth accretion took place under conditions which were much more reducing than those implied by the current FeO content of the mantle and that the oxidation took place late in the accretionary process. Paths of terrestrial accretion, oxidation state and partitioning which are consistent with the current mantle contents of Ni, Co, W, V, Cr and Nb lead to Si and O contents of the core of ∼4.3 wt.% and 0.15%, respectively.     

CO2 solubility in Martian basalts and Martian atmospheric evolution

To understand possible volcanogenic fluxes of CO₂ to the Martian atmosphere, we investigated experimentally carbonate solubility in a synthetic melt based on the Adirondack-class Humphrey basalt at 1-2.5 GPa and 1400-1625 °C. Starting materials included both oxidized and reduced compositions, allowing a test of the effect of iron oxidation state on CO₂ solubility. CO₂ contents in experimental glasses were determined using Fourier transform infrared spectroscopy (FTIR) and Fe³⁺/Fe^T was measured by Mössbauer spectroscopy. The CO₂ contents of glasses show no dependence on Fe³⁺/Fe^T and range from 0.34 to 2.12 wt.%. For Humphrey basalt, analysis of glasses with gravimetrically-determined CO₂ contents allowed calibration of an integrated molar absorptivity of 81,500 ± 1500 L mol⁻¹ cm⁻² for the integrated area under the carbonate doublet at 1430 and 1520 cm⁻¹. The experimentally determined CO₂ solubilities allow calibration of the thermodynamic parameters governing dissolution of CO₂ vapor as carbonate in silicate melt, K_II, (Stolper and Holloway, 1988) as follows: , ΔV⁰ = 20.85 ± 0.91 cm³ mol⁻¹, and ΔH⁰ = −17.96 ± 10.2 kJ mol⁻¹. This relation, combined with the known thermodynamics of graphite oxidation, facilitates calculation of the CO₂ dissolved in magmas derived from graphite-saturated Martian basalt source regions as a function of P, T, and f_O2. For the source region for Humphrey, constrained by phase equilibria to be near 1350 °C and 1.2 GPa, the resulting CO₂ contents are 51 ppm at the iron-wüstite buffer (IW), and 510 ppm at one order of magnitude above IW (IW + 1). However, solubilities are expected to be greater for depolymerized partial melts similar to primitive shergottite Yamato 980459 (Y 980459). This, combined with hotter source temperatures (1540 °C and 1.2 GPa) could allow hot plume-like magmas similar to Y 980459 to dissolve 240 ppm CO₂ at IW and 0.24 wt.% of CO₂ at IW + 1. For expected magmatic fluxes over the last 4.5 Ga of Martian history, magmas similar to Humphrey would only produce 0.03 and 0.26 bars from sources at IW and IW + 1, respectively. On the other hand, more primitive magmas like Y 980459 could plausibly produce 0.12 and 1.2 bars at IW and IW + 1, respectively. Thus, if typical Martian volcanic activity was reduced and the melting conditions cool, then degassing of CO₂ to the atmosphere may not be sufficient to create greenhouse conditions required by observations of liquid surface water. However, if a significant fraction of Martian magmas derive from hot and primitive sources, as may have been true during the formation of Tharsis in the late Noachian, that are also slightly oxidized (IW + 1.2), then significant contribution of volcanogenic CO₂ to an early Martian greenhouse is plausible.     

Evidence for high-pressure core-mantle differentiation from the metal-silicate partitioning of lithophile and weakly-siderophile elements

Liquid Fe metal-liquid silicate partition coefficients for the lithophile and weakly-siderophile elements Ta, Nb, V, Cr, Si, Mn, Ga, In and Zn have been measured in multianvil experiments performed from 2 to 24 GPa, 2023-2873 K and at oxygen fugacities of −1.3 to −4.2 log units relative to the iron-wüstite buffer. Compositional effects of light elements dissolved in the metal liquid (S, C) have been examined and experiments were performed in both graphite and MgO capsules, specifically to address the effect of C solubility in Fe-metal on siderophile element partitioning. The results were used to examine whether there is categorical evidence that a significant portion of metal-silicate equilibration occurred under very high pressures during core-mantle fractionation on Earth. Although the depletion of V from the mantle due to core formation is significantly greater than that of Nb, our results indicate that both elements have similar siderophile tendencies under reducing conditions at low pressures. With increasing pressure, however, Nb becomes less siderophile than V, implying that average metal-silicate equilibration pressures of at least 10-40 GPa are required to explain the Nb/V ratio of the mantle. Similarly the moderately-siderophile, volatile element ratios Ga/Mn and In/Zn are chondritic in the mantle but both volatility and core-mantle equilibration at low pressure would render these ratios strongly sub-chondritic. Our results indicate that pressures of metal-silicate partitioning exceeding 30-60 GPa would be required to render these element ratios chondritic in the mantle. These observations strongly indicate that metal-silicate equilibration must have occurred at high pressures, and therefore support core-formation models that involve deep magma oceans. Moreover, our results allow us to exclude models that envisage primarily low-pressure (<1 GPa) equilibration in relatively small planetary bodies. We also argue that the core cannot contain significant U as this would require metal-silicate equilibration at oxygen fugacities low enough for significant amounts of Ta to have also been extracted from the mantle. Likewise, as In is more siderophile than Pb but similarly volatile and also quite chalcophile it would have been difficult for Pb to enter the core without reversing the relative depletions of these elements in the mantle unless metal-silicate equilibration occurred at high pressures >20 GPa.     

Oxygen fugacity dependence of Os solubility in haplobasaltic melt

Os equilibrium solubilities were determined at 1350 °C over a wide range of oxygen fugacities (−12 < log fO₂ < −7) applying the mechanically assisted equilibration technique (MAE) at 10⁵ Pa (= 1 bar). Os concentrations in the glass samples were analysed using ID-NTIMS. Additional LA-ICP-MS and SEM analyses were performed to detect, visualize and analyse the nature and chemistry of “nanonuggets.” Os solubilities determined range at a constant temperature of 1350 °C from 0.63 ± 0.04 to 37.4 ± 1.16 ppb depending on oxygen fugacity. At the highest oxygen fugacities, Os³⁺ can be confirmed as the main oxidation state of Os. At low oxygen fugacities (below log fO₂ = −8), samples are contaminated by nanonuggets which, despite the MAE technique, were still not removed entirely from the melt. However, the present results indicate that applying MAE technology does reduce the amount of nanonuggets present significantly, resulting in the lowest Os solubility results reported to date under these experimental conditions, and extending the experimentally accessible range of fO₂ for these studies to lower values. Calculated metal/silicate melt partition coefficients are therefore higher compared to previous studies, making Os more siderophile. Neglecting the as yet unknown temperature dependence of the Os metal/silicate melt partition coefficient, extrapolation of the obtained Os solubilities to conditions for core-mantle equilibrium, results in a , while metallic alloy/silicate melt partition coefficients range from 1.4 × 10⁶ to 8.6 × 10⁷, in agreement with earlier findings. Therefore remains too high by 2-4 orders of magnitude to explain the Os abundance in the Earth’s mantle as result of core-mantle equilibrium during core formation.     

Experimental study of platinum solubility in silicate melt to 14 GPa and 2273 K: Implications for accretion and core formation in Earth

We determined the solubility limit of Pt in molten haplo-basalt (1 atm anorthite-diopside eutectic composition) in piston-cylinder and multi-anvil experiments at pressures between 0.5 and 14 GPa and temperatures from 1698 to 2223 K. Experiments were internally buffered at ∼IW + 1. Pt concentrations in quenched-glass samples were measured by laser-ablation inductively coupled-plasma mass spectrometry (LA-ICPMS). This technique allows detection of small-scale heterogeneities in the run products while supplying three-dimensional information about the distribution of Pt in the glass samples. Analytical variations in ¹⁹⁵Pt indicate that all experiments contain Pt nanonuggets after quenching. Averages of multiple, time-integrated spot analyses (corresponding to bulk analyses) typically have large standard deviations, and calculated Pt solubilities in silicate melt exhibit no statistically significant covariance with temperature or pressure. In contrast, averages of minimum ¹⁹⁵Pt signal levels show less inter-spot variation, and solubility shows significant covariance with pressure and temperature. We interpret these results to mean that nanonuggets are not quench particles, that is, they were not dissolved in the silicate melt, but were part of the equilibrium metal assemblage at run conditions. We assume that the average of minimum measured Pt abundances in multiple probe spots is representative of the actual solubility. The metal/silicate partition coefficients (D^met/sil) is the inverse of solubility, and we parameterize D^met/sil in the data set by multivariate regression. The statistically robust regression shows that increasing both pressure and temperature causes D^met/silto decrease, that is, Pt becomes more soluble in silicate melt. D^met/sil decreases by less than an order of magnitude at constant temperature from 1 to 14 GPa, whereas isobaric increase in temperature produces a more dramatic effect, with D^met/sil decreasing by more than one order of magnitude between 1623 and 2223 K. The Pt abundance in the Earth’s mantle requires that D^met/sil is ∼1000 assuming core-mantle equilibration. Geochemical models for core formation in Earth based on moderately and slightly siderophile elements are generally consistent with equilibrium metal segregation at conditions generally in the range of 20-60 GPa and 2000-4000 K. Model extrapolations to these conditions show that the Pt abundance of the mantle can only be matched if oxygen fugacity is high (∼IW) and if Pt mixes ideally in molten iron, both very unlikely conditions. For more realistic values of oxygen fugacity (∼IW − 2) and experimentally-based constraints on non-ideal mixing, models show that D^met/sil would be several orders of magnitude too high even at the most favorable conditions of pressure and temperature. These results suggest that the mantle Pt budget, and by implication other highly siderophile elements, was added by late addition of a ‘late veneer’ phase to the accreting proto-Earth.     

Fe-Ni and Mn-Cr isotopic systems in sulfides from unequilibrated enstatite chondrites

In situ measurements of ⁶⁰Fe-⁶⁰Ni and ⁵³Mn-⁵³Cr isotopic systems with an ion microprobe have been carried out for sulfide assemblages from unequilibrated enstatite chondrites (UECs). Evidence for the initial presence of ⁶⁰Fe has been observed in nine sulfide inclusions from three UECs: ALHA77295, MAC88136, and Qingzhen. The inferred initial (⁶⁰Fe/⁵⁶Fe) [(⁶⁰Fe/⁵⁶Fe)₀] ratios show a large variation range, from ∼2 × 10⁻⁷ to ∼2 × 10⁻⁶. The sulfide inclusions with high Fe/Ni ratios yield (⁶⁰Fe/⁵⁶Fe)₀ ratios of ∼(2-7) × 10⁻⁷, similar to most of the (⁶⁰Fe/⁵⁶Fe)₀ values of troilite and pyroxene observed in unequilibrated ordinary chondrites (UOCs). Inclusions with high inferred (⁶⁰Fe/⁵⁶Fe)₀ ratios (∼1-2 × 10⁻⁶) have low Fe/Ni ratios and the magnitude of the ⁶⁰Ni excesses is similar in two MAC88136 assemblages in spite of a difference of a factor of two in their Fe/Ni ratios. The inferred high (⁶⁰Fe/⁵⁶Fe)₀ ratios were probably the result of Fe-Ni re-distribution in the sulfides during later alteration processes.The ⁵³Mn-⁵³Cr system was measured in five of the sulfide assemblages that were examined for their ⁶⁰Fe-⁶⁰Ni systematics. The ⁵³Mn-⁵³Cr isochrons yielded variable initial (⁵³Mn/⁵⁵Mn) [(⁵³Mn/⁵⁵Mn)₀] ratios from ∼(2-7) × 10⁻⁷. There is no obvious correlation between the (⁶⁰Fe/⁵⁶Fe)₀ and (⁵³Mn/⁵⁵Mn)₀ ratios. The variable ⁵³Mn-⁵³Cr isochrons probably also indicate later disturbance to the isotopic systems in these sulfides. Even though no chronological information can be extracted from the ⁶⁰Fe-⁶⁰Ni and ⁵³Mn-⁵³Cr systems in these UEC sulfides, our results indicate that ⁶⁰Fe was present in the enstatite chondrite formation region of the early Solar System.     

Internally consistent thermodynamic data for magnesium sulfate hydrates

Experimental studies on the stability of several Mg-sulfate hydrates including epsomite (MgSO₄·7H₂O), hexahydrite (MgSO₄·6H₂O), starkeyite (MgSO₄·4H₂O), and kieserite (MgSO₄·H₂O) as a function of temperature and relative humidity are in poor agreement with calculations based on thermodynamic properties of these substances taken from the literature. Therefore, we synthesized four different MgSO₄ hydrates and measured their enthalpies of formation by solution calorimetry at T = 298.15 K. The resulting enthalpies of formation from the elements are:

•

Δ_fH⁰₂₉₈ (epsomite) = −3387.7 ± 1.3 kJmol⁻¹

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Δ_fH⁰₂₉₈ (hexahydrite) = −3088.1 ± 1.1 kJmol⁻¹

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Δ_fH⁰₂₉₈ (sanderite, MgSO₄·2H₂O) = −1894.9 ± 1.3 kJmol⁻¹

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Δ_fH⁰₂₉₈ (kieserite) = −1612.4 ± 1.3 kJmol⁻¹

Using mathematical programming (MAP) techniques, standard thermodynamic values consistent both with our calorimetric data and previously published humidity brackets could be derived:

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Epsomite: Δ_fH⁰₂₉₈ = −3388.7 kJmol⁻¹, S⁰₂₉₈ = 371.3 Jmol⁻¹ K⁻¹, Δ_fG⁰₂₉₈ = −2871.0 kJmol⁻¹

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Hexahydrite: Δ_fH⁰₂₉₈ = −3087.3 kJmol⁻¹, S⁰₂₉₈ = 348.5 Jmol⁻¹ K⁻¹, Δ_fG⁰₂₉₈ = −2632.3 kJmol⁻¹

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Starkeyite: Δ_fH⁰₂₉₈ = −2496.1 kJmol⁻¹, S⁰₂₉₈ = 259.9 Jmol⁻¹ K⁻¹, Δ_fG⁰₂₉₈ = −2153.8 kJmol⁻¹

•

Kieserite: Δ_fH⁰₂₉₈ = −1611.5 kJmol⁻¹, S⁰₂₉₈ = 126.0 Jmol⁻¹ K⁻¹, Δ_fG⁰₂₉₈ = −1437.9 kJmol⁻¹

Additionally, heat capacity measurements and standard entropy determinations of several magnesium sulfate hydrate minerals from the literature are analyzed and judged against estimates obtained from a linear combination of the heat capacities of MgSO₄ and hexagonal ice. The results of the MAP analysis are compared to these estimates to conclude that heat capacity and entropy correlate well with the number of waters of hydration. However, even the good correlation is not good enough to capture the fine variations in these properties. Consequently, their experimental measurement is inevitable if reliable thermodynamic data are sought. Our MAP thermodynamic data show that epsomite, hexahydrite, and kieserite have stability fields in the T-%RH space. Starkeyite is metastable. Although no MAP data could have been derived for pentahydrite (MgSO₄·5H₂O) and sanderite, their transient existence suggest that both of them are metastable as well.     

Silicon isotopes in meteorites and planetary core formation

The silicon (Si) isotope compositions of 42 meteorite and terrestrial samples have been determined using MC-ICPMS with the aim of resolving the current debate over their compositions and the implications for core formation. No systematic δ³⁰Si differences are resolved between chondrites (δ³⁰Si = −0.49 ± 0.15‰, 2σ_SD) and achondrites (δ³⁰Si = −0.47 ± 0.11‰, 2σ_SD), although enstatite chondrites are consistently lighter (δ³⁰Si = −0.63 ± 0.07‰, 2σ_SD) in comparison to other meteorite groups. The data reported here for meteorites and terrestrial samples display an average difference Δ³⁰Si_{BSE−meteorite∗} = 0.15 ± 0.10‰, which is consistent within uncertainty with previous studies. No effect from sample heterogeneity, preparation, chemistry or mass spectrometry can be identified as responsible for the reported differences between current datasets. The heavier composition of the bulk silicate Earth is consistent with previous conclusions that Si partitioned into the metal phase during metal-silicate equilibration at the time of core formation. Fixing the temperature of core formation to the peridotite liquidus and using an appropriate metal silicate fractionation factor (ε ∼0.89), the Δ³⁰Si_{BSE−meteorite∗} value from this study indicates that the Earth core contains at least 2.5 and possibly up to 16.8 wt% Si.     

Mn-Cr systematics of the early Solar System revisited

We have reinvestigated the Mn-Cr systematics in a number of primitive meteorites, differentiated planetesimals and terrestrial planets in order to address the chronology of the early stages of protoplanetary disk evolution and planetary formation. Our analytical procedure is based on the assumption of terrestrial abundances for ⁵⁰Cr and ⁵²Cr only; recognizing that a data reduction scheme based on Earth-like ⁵⁴Cr/⁵²Cr abundances in all meteorites is not tenable. Here we show that initial ε_⁵³Cr compositions of ⁵⁴Cr-rich and ⁵⁴Cr-poor acid leach fractions in the primitive carbonaceous chondrite Orgueil differ by 0.9ε, reflecting primordial mineral-scale heterogeneity. However, asteroidal processing effectively homogenized any ε_⁵³Cr variations on the planetesimal scale, providing a uniform present-day solar ε_⁵³Cr=0.20±0.10. Thus, our ⁵³Mn-⁵³Cr data argue against the previously suggested ⁵³Mn heliocentric gradient. Instead, we suggest that inner Solar System objects possessed an initially homogeneous ⁵³Mn/⁵⁵Mn composition, which determined by two independent means is estimated at (6.28 ± 0.66) × 10⁻⁶. Our revised Mn-Cr age for Ste. Marguerite (SM) metamorphism of 4562.9 ± 1.0 Ma is identical to the Pb-Pb age of SM phosphates. Using this age, we confirm that mantle differentiation of the eucrite parent body occurred 4564.9 ± 1.1 Ma ago, and revise the time interval between this event and CAI formation to 2.2 ± 1.1 Ma. We also constrain metamorphism in carbonaceous chondrites of type 2 and 3 to have occurred between 1 and 6 Ma after CAI formation. The ⁵³Mn-⁵³Cr correlation among chondrites, planetesimals and terrestrial planets (the eucrite parent body, Mars and Earth) provides evidence for Mn/Cr fractionation within the protoplanetary disk recorded by all precursor materials of the terrestrial planets and primitive asteroids. This fractionation appears to have occurred within 2 Ma of CAI formation.     

Trace element partitioning in the Fe-S-C system and its implications for planetary differentiation and the thermal history of ureilites

Element partitioning in metal-light element systems is important to our understanding of planetary differentiation processes. In this study, solid-metal/liquid-sulfide, liquid-metal/liquid-sulfide and solid-metal/troilite partition coefficients (D) were determined for 18 elements (Ag, As, Au, Co, Cr, Cu, Ge, Ir, Ni, Os, Pd, Pt, Mo, Mn, Re, Ru, Se and W) in the graphite-saturated Fe-S-C system at 1 atm. Compared at the same liquid S concentration, the solid/liquid partition coefficients are similar to those in the Fe-S system, but there are systematic differences that appear to be related to interactions with carbon dissolved in the solid metal. Elements previously shown to be “anthracophile” generally have larger solid/liquid partition coefficients in the Fe-S-C system, whereas those that are not have similar or smaller partition coefficients in the Fe-S-C system. The partitioning of trace elements between C-rich and S-rich liquids is, in most cases, broadly similar to the partitioning between solid metal and S-rich liquid. The highly siderophile elements Os, Re, Ir and W are partitioned strongly into the C-rich liquid, with D ? 100. The partition coefficients for Pt, Ge and W decrease significantly at the transition to liquid immiscibility, while the partition coefficient for Mo increases sharply. The bulk siderophile element patterns of ureilite meteorities appear to be better explained by separation of S-rich liquid from residual C-rich metallic liquid at temperatures above the silicate solidus, rather than by separation of S-rich liquid from residual solid metal at lower temperatures.     

Oxygen fugacity dependence of Ni,Co, Mn,Cr, V,and Si partitioning between liquid metal and magnesiowüstite at 9–18 GPa and 2200°C

The oxidation states of Ni, Co, Mn, Cr, V and Si in magnesiowüstite have been determined in metal-oxide distribution experiments using a multi anvil apparatus at 9 and 18 GPa and 2200°C as a function of oxygen fugacity. Despite limitations to control oxygen fugacity by applying conventional buffering methods in high pressure experiments, a wide range of redox-conditions (3 log bar units) has been imposed to the metal-oxide partitioning experiments by varying the Si/O ratio of the starting material. The oxygen fugacity was calculated according to the Fe-FeO equilibrium between the run products. The ability to impose different oxygen fugacities by varying the starting material is confirmed by the large variation of element partitioning coefficients obtained at constant pressure and temperature. The calculated valences at both pressures investigated are divalent for Co, Mn, V and 4+ for Si. The results for Cr (∼2.5+) and Ni (∼1.5+) indicate non-ideal mixing of Ni and Cr in at least one of the product phases. Because the application of 1 bar activity coefficients for Ni and Cr in metal alloys does not change these valences, non-ideal mixing in magnesiowüstite or significantly larger non-ideal mixing properties of Ni and Cr in metal alloys at high pressure are likely to be responsible for the apparent valences. Omitting such non-ideal mixing properties when extrapolating high-pressure element partitioning data may be significant. The elements Cr, V and Mn become siderophile (D_M^met/ox > 1) at 9–18 GPa and 2200°C at oxygen fugacities below IW-2.7 to IW-3.7. Considering, in addition, the influence of temperature, the depletion of Cr, Mn and V in the Earth’s mantle may be due, at least partly, to siderophile behavior at high pressure and temperature.     

Spectroscopic analysis (FTIR, Raman) of water in mafic and intermediate glasses and glass inclusions

Micro-Raman spectroscopy, even though a very promising technique, is not still routinely applied to analyse H₂O in silicate glasses. The accuracy of Raman water determinations critically depends on the capability to predict and take into account both the matrix effects (bulk glass composition) and the analytical conditions on band intensities. On the other hand, micro-Fourier transform infrared spectroscopy is commonly used to measure the hydrous absorbing species (e.g., hydroxyl OH⁻ and molecular H₂O) in natural glasses, but requires critical assumptions for the study of crystal-hosted glasses. Here, we quantify for the first time the matrix effect of Raman external calibration procedures for the quantification of the total H₂O content (H₂O_T = OH⁻ + H₂O_m) in natural silicate glasses. The procedures are based on the calibration of either the absolute (external calibration) or scaled (parameterisation) intensity of the 3550 cm⁻¹ band. A total of 67 mafic (basanite, basalt) and intermediate (andesite) glasses hosted in olivines, having between 0.2 and 4.8 wt% of H₂O, was analysed. Our new dataset demonstrates, for given water content, the height (intensity) of Raman H₂O_T band depends on glass density, reflectance and water environment. Hence this matrix effect must be considered in the quantification of H₂O by Raman spectroscopy irrespective of the procedure, whereas the parameterisation mainly helps to predict and verify the self-consistency of the Raman results. In addition, to validate the capability of the micro-Raman to accurately determine the H₂O content of multicomponent aluminosilicate glasses, a subset of 23 glasses was analysed by both micro-Raman and micro-FTIR spectroscopy using the band at 3550 cm⁻¹. We provide new FTIR absorptivity coefficients (ε₃₅₅₀) for basalt (62.80 ± 0.8 L mol⁻¹ cm⁻¹) and basanite (43.96 ± 0.6 L mol⁻¹ cm⁻¹). These values, together with an exhaustive review of literature data, confirm the non-linear decline of the FTIR absorptivity coefficient (ε₃₅₅₀) as the glass depolymerisation increases. We demonstrate the good agreement between micro-FTIR and micro-Raman determination of H₂O in silicate glasses when the matrix effects are properly considered.     

Metal-silicate partitioning of cesium: Implications for core formation

Here we present the first set of metal-silicate partitioning data for Cs, which we use to examine whether the primitive mantle depletion of Cs can be attributed to core segregation. Our experiments independently varied pressure from 5 to 15 GPa, temperature from 1900 to 2400 °C, metallic sulfur content from pure Fe to pure FeS, silicate melt polymerization, expressed as a ratio of non-bridging oxygens to tetrahedrally coordinated cations (nbo/t) from 1.26 to 3.1, and fO₂ from two to four log units below the iron-wüstite buffer. The most important controls on the partitioning behavior of alkalis were the metallic sulfur content, expressed as X_S, and the nbo/t of the silicate liquid. Normalization of X_S to 0.5 yielded the following expressions for D-values as a function of nbo/t: log D_Na = −2.0 + 0.44 × (nbo/t), log D_K = −2.4 + 0.67 × ( nbo/t), and log D_Cs = −3.2 + 1.17 × (nbo/t). Normalization of nbo/t to 2.7 resulted in the following equations for D-values as a function of S content: log D_Na = −4.1 + 6.4 × X_S, log D_K = −7.7 + 13.9 × X_S, and log D_Cs = −12.1 + 23.3 × X_S.There appears to be a negative pressure effect up to 15 GPa, but it should be noted that this trend was not present before normalization, and is based on only two measurements. There is a positive trend in cesium’s metal-silicate partition coefficient with increasing temperature. D_Cs exhibits the largest change and increased by a factor of three over 500 °C. The effect of oxygen fugacity has not been precisely determined but in general, lowering fO₂ by two log units resulted in a rise in all D-values of approximately an order of magnitude. In general, the sensitivity of partition coefficients to changing parameters increased with atomic number.The highest D-value for Cs observed in this study is 0.345, which was obtained at nbo/t of 2.7 and a metal phase of pure FeS. This metallic composition has far more S than has been suggested for any credible core-forming metal. We therefore conclude that the depletion of Cs in Earth’s mantle is either caused by radically different behavior of Cs at pressures higher than 15 GPa or is not related to core formation. Even so, we have shown that a planet with a sufficient S inventory may incorporate significant amounts of alkali elements into its core.     

The LaPaz Icefield 04840 meteorite: Mineralogy, metamorphism, and origin of an amphibole- and biotite-bearing R chondrite

The R chondrite meteorite LaPaz Icefield (LAP) 04840 is unique among metamorphosed, non-carbonaceous chondrites in containing abundant OH-bearing silicate minerals: ∼13% ferri-magnesiohornblende and ∼0.4% phlogopite by volume. Other minerals include olivine (Fo₆₂), orthopyroxene (En₆₉Fs₃₀Wo₁), albite (An₈Ab₉₀Or₂), magnetite, pyrrhotite, pentlandite, and apatite. Ferromagnesian minerals are rich in Fe³⁺, as determined by Mössbauer spectrometry and electron microprobe chemical analyses. Fe³⁺/Fe_tot values are olivine ?5%, amphibole 80%, phlogopite 65%, and magnetite 42%. Mineral compositions are nearly constant across grains and the section, except for a small variability in amphibole compositions reflecting the edenite exchange couple (^ANa + ^IVAl ↔ ^A□ + Si). These mineral compositions, the absence of Fe-Ni metal, and the oxygen isotope data support its classification as an R (Rumuruti) chondrite. LAP 04840 is classified as petrologic grade 5, based on the chemical homogeneity of its minerals, and the presence of distinctly marked chondrules and chondrule fragments in a fine-grained crystalline matrix. The mineral assemblage of LAP 04840 allows calculation of physical and chemical conditions at the peak of its metamorphism: T = 670 ± 60 °C from a amphibole-plagioclase thermometer; P_H2O between 250 and 500 bars as constrained by the assemblage phlogopite + orthopyroxene + olivine + feldspar and the absence of diopside; P_CO2 unconstrained; f_O2 at QFM + 0.5 log units; . The hydrogen in LAP 04840 is very heavy, an average δD value of +3660 ± 75‰ in the magnesiohornblende. Only a few known sources of hydrogen have such high δD and are suitable sources for LAP 04840: ordinary chondrite phyllosilicates (as in the Semarkona chondrite), and insoluble organic matter (IOM) in ordinary chondrites and CR chondrites. Hydrogen from the IOM could have been released by oxidation, and then reacted with an anhydrous R chondrite (at high temperature), but it is not clear whether this scenario is correct.     

Constraints on core formation from Pt partitioning in mafic silicate liquids at high temperatures

A new high temperature piston cylinder design has enabled the measurement of platinum solubility in mafic melts at temperatures up to 2500 °C, 2.2 GPa pressure, and under reducing conditions for 1-10 h. These high temperature and low f_O2 conditions may mimic a magma ocean during planetary core formation. Under these conditions, we measured tens to hundreds of ppm Pt in the quenched silicate glass corresponding to , 4-12 orders of magnitude lower than extrapolations from high f_O2 experiments at 1 bar and at temperatures no higher than 1550 °C. Moreover, the new experiments provide coupled textural and compositional evidence that noble metal micro-nuggets, ubiquitous in experimental studies of the highly siderophile elements, can be produced on quench: we measure equally high Pt concentrations in the rapidly quenched nugget-free peripheral margin of the silicate as we do in the more slowly quenched nugget-bearing interior region. We find that both temperature and melt composition exercise strong control on and that Pt⁰ and Pt¹⁺ may contribute significantly to the total dissolved Pt such that low f_O2 does not imply low Pt solubility. Equilibration of metal alloy with liquid silicate in a hot primitive magma might not have depleted platinum to the extent previously believed.     

The oxygen-isotope composition of chondrules and isolated forsterite and olivine grains from the Tagish Lake carbonaceous chondrite

The oxygen-isotope compositions (obtained by laser fluorination) of hand-picked separates of isolated forsterite, isolated olivine and chondrules from the Tagish Lake carbonaceous chondrite describe a line (δ¹⁷O = 0.95 * δ¹⁸O − 3.24; R² = 0.99) similar to the trend known for chondrules from other carbonaceous chondrites. The isolated forsterite grains (Fo_99.6-99.8; δ¹⁸O = −7.2‰ to −5.5‰; δ¹⁷O = −9.6‰ to −8.2‰) are more ¹⁶O-rich than the isolated olivine grains (Fo_39.6-86.8; δ¹⁸O = 3.1‰ to 5.1‰; δ¹⁷O = −0.3‰ to 2.2‰), and have chemical and isotopic characteristics typical of refractory forsterite. Chondrules contain olivine (Fo_97.2-99.8) with oxygen-isotope compositions (δ¹⁸O = −5.2‰ to 5.9‰; δ¹⁷O = −8.1‰ to 1.2‰) that overlap those of isolated forsterite and isolated olivine. An inverse relationship exists between the Δ¹⁷O values and Fo contents of Tagish Lake isolated forsterite and chondrules; the chondrules likely underwent greater exchange with ¹⁶O-poor nebular gases than the forsterite. The oxygen-isotope compositions of the isolated olivine grains describe a trend with a steeper slope (1.1 ± 0.1, R² = 0.94) than the carbonaceous chondrite anhydrous mineral line (CCAM_slope = 0.95). The isolated olivine may have crystallized from an evolving melt that exchanged with ¹⁶O-poor gases of somewhat different composition than those which affected the chondrules and isolated forsterite. The primordial components of the Tagish Lake meteorite formed under conditions similar to other carbonaceous chondrite meteorite groups, especially CMs. Its alteration history has its closest affinities to CI carbonaceous chondrites.     

The partitioning behavior of As and Au in S-free and S-bearing magmatic assemblages

The partitioning of As and Au between rhyolite melt and low-salinity vapor (2 wt% NaCl eq.) in a melt-vapor-Au metal ± magnetite ± pyrrhotite assemblage has been quantified at 800 °C, 120 MPa and f_O2=NNO. The S-bearing runs have calculated values for the fugacities of H₂S, SO₂ and S₂ of logf_H2S=1.1, logf_SO2=-1.5, and logf_S2=-3.0. The ratio of H₂S to SO₂ is on the order of 400. The experiments constrain the effect of S on the partitioning behavior of As and Au at magmatic conditions. Calculated average Nernst-type partition coefficients (±1σ) for As between vapor and melt, , are 1.0 ± 0.1 and 2.5 ± 0.3 in the S-free and S-bearing assemblages, respectively. These results suggest that sulfur has a small, but statistically meaningful, effect on the mass transfer of As between silicate melt and low-salinity vapor at the experimental conditions. Efficiencies of removal, calculated following Candela and Holland (1986), suggest that the S-free and S-bearing low-salinity vapor can scavenge approximately 41% and 63% As from water-saturated rhyolite melt, respectively, during devolatilization assuming that As is partitioned into magnetite and pyrrhotite during second boiling. The S-free data are consistent with the presence of arsenous acid, As(OH)₃ in the vapor phase. However, the S-bearing data suggest the presence of both arsenous acid and a As-S complex in S-bearing magmatic vapor. Apparent equilibrium constants, , describing the partitioning of As between melt and vapor are −1.3 (0.1) and −1.1 (0.1) for the S-free and S-bearing runs, respectively. The increase in the value of with the addition of S suggests a role for S in complexing and scavenging As from the melt during degassing.The calculated vapor/melt partition coefficients (±1σ) for Au between vapor and melt, , in S-free and S-bearing assemblages are 15 ± 2.5 and 12 ± 0.3, respectively. Efficiencies of removal (Candela and Holland, 1986) for the S-free melt, calculated assuming that magnetite is the dominant Au-sequestering solid phase during crystallization (Simon et al., 2003), suggest that magmatic vapor may scavenge on the order of 72% Au from a water-saturated melt. Efficiencies of removal calculated for the S-bearing assemblage, assuming pyrrhotite and magnetite are the dominant Au-sequestering solid phases, indicate that vapor may scavenge on the order of 60% Au from the melt. These model calculations suggest that the loss of pyrrhotite and magnetite from a melt, owing to punctuated differentiation during ascent and emplacement, does not prohibit the ability of a rhyolite melt to generate a large-tonnage Au deposit. Apparent equilibrium constants describing the partitioning of Au between melt and vapor were calculated using the mean values for the S-free and S-bearing assemblages; only S-bearing data from runs longer than 400 h were used as shorter runs may not have reached equilibrium with respect only to vapor/melt partitioning of Au. The values for are −4.4 (0.1) and −4.2 (0.2) for the S-free and S-bearing runs, respectively. These data suggest that the presence of S does not affect the mass transfer of Au from degassing silicate melt to an exsolved, low-salinity vapor in a low-f_S2 assemblage (i.e., pyrrhotite-magnetite at NNO) at the experimental conditions reported here. Efficiencies of removal are calculated and used to model the mass transfer of Au from a crystallizing silicate melt to an exsolved, low-salinity vapor phase. The calculations suggest that the model, absolute tonnage of Au scavenged and transported by S-free and S-bearing vapors, from a crystallizing melt, would be comparable and that the time-integrated flux of low-salinity vapor could be responsible for a significant quantity of the Au in magmatic-hydrothermal ore deposits.