Christopher J. Capobianco

Partitioning of ruthenium, rhodium, and palladium between spinel and silicate melt and implications for platinum group element fractionation trends

Abstract High temperature (1450 and 1300°C), one atmosphere trace element partitioning experiments were made in the CaO-MgO-SiO 2 -Al 2 O 3 system to investigate the chemical compatibility of Ru, Rh, and Pd in magnesium aluminate spinels coexisting with silicate melts. Spinel/melt partition coefficients indicate that Rh and Ru are highly compatible with values near 90 and 20, respectively. These high partition coefficients contrast sharply with partitioning behavior for Pd under similar conditions. Palladium is rather incompatible with partition coefficients of less than 0.02. Platinum group elements, generally considered a geochemically coherent group, may thus be fractionated with respect to one another when spinel is a magmatic phase.

Experiments on crystal/liquid partitioning of Ru, Rh and Pd for magnetite and hematite solid solutions crystallized from silicate melt☆

Experiments to characterize crystal/melt partitioning of Ru, Rh and Pd between Fe-oxides (magnetite and hematite solid solutions) and silicate melt are reported for oxygen fugacities imposed by CO2 decomposition. Oxides were equilibrated near 1275°C at 1 atm with silica-saturated melts in the compositional system “FeO”CaOAl2O3SiO2±MgO±Cr2O3±TiO2. Ru and Rh are strongly compatible while Pd is slightly incompatible. Partition coefficients (elemental weight ratios) for Ru are variable but range from 100 to > 4000 depending, in a poorly defined way, on the oxide crystal composition. We report enhanced compatibility for Ru compared to previous work for spinels in an Fe-free system. Rh compatibility is also enhanced compared to the Fe-free system, but to a lesser degree. However, Rh partition coefficients (grand average is 250±120) are more uniform than those measured for Ru. Pd is slightly incompatible in magnetite and hematite (grand average is 0.7 ± 0.3) in contrast to the Fe-free system where Pd is highly incompatible. The differences in compatibility among Ru, Rh and Pd suggest that spinels could play a role in determining platinum-group element (PGE) fractionation trends, in particular, for rocks crystallizing at high oxygen fugacity where it is most likely that the dissolved PGEs are present as oxidized species. We also propose a reaction mechanism to explain the common observation of platinum-group minerals (PGMs) included within spinel phases. Finally, our data suggest that Ru, at least, should be concentrated in spinels crystallizing in Earths atmosphere, either within micrometeroids, or from atmospheric crystallization of impact ejecta.

V, Cr, and Mn in the earth, moon, EPB, and SPB and the origin of the moon - Experimental studies

Abstract The abundances of V, Cr, and Mn inferred for the mantles of the Earth and Moon decrease in that order and are similar, but are distinct from those inferred for the mantles of the Eucrite Parent Body (EPB) and Shergottite Parent Body (SPB). This similarity between Earth and Moon has been used to suggest that the Moon is derived substantially or entirely from Earth mantle material following terrestrial core formation. To test this hypothesis, we have determined the partitioning of V, Cr, and Mn between solid iron metal, S-rich metallic liquid, and synthetic basaltic silicate liquid at 1260°C and one bar pressure. The sequence of compatibility in the metallic phases is Cr > V > Mn at “high” oxygen fugacity (just below the iron-wustite buffer) and V > Cr > Mn at low oxygen fugacities. Solubilities in liquid metal always exceed solubilities in solid metal. These partition coefficients suggest that the abundances of V, Cr, and Mn do not reflect core formation in the Earth. Rather, they are consistent with the relative volatilities of these elements. The similarity in the depletion patterns of V, Cr, and Mn inferred for the mantles of the Earth and Moon is a necessary, but not sufficient, condition for the Moon to have been derived wholly or in part from the Earths mantle.

Metal‐silicate thermochemistry at high temperature: Magma oceans and the “excess siderophile element” problem of the Earth's upper mantle

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]. Murthys 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 Earths upper mantle remains unsolved.

Siderophile geochemistry of Ga, Ge, and Sn: cationic oxidation states in silicate melts and the effect of composition in iron–nickel alloys

We report a series of metal–silicate partitioning experiments for Ga, Ge, and Sn to characterize the dependence of the partition coefficient, D, on oxygen fugacity, fO2. These were isothermal (1260°C) and isobaric (1 bar) experiments using a silicate composition that approximates a eucritic meteorite. It is well known that elements such as Ni, which exist in only one valence state under redox conditions of planetary interest, produce linear trends on log D vs log fO2 diagrams. For our experiments on Ga, Ge, and Sn, however, large deviations from linearity were evident and seemed to suggest unusual changes in the oxidation states for these elements in the silicate melt. But such an inference would have been mistaken because the metallic phase of these experiments, Ni–Fe alloys, was not of constant composition; instead, the Ni/Fe ratio was varied systematically to control oxygen fugacity. Although Ni and Fe form alloys that are not far from ideal, binary interactions between the trace elements and either major metallic component, Ni or Fe, are not necessarily similar. Using only information obtained from the literature on binary mixing properties among the metallic components, Fe, Ni, Ga, Sn, and Ge, a simple thermodynamic solution model was formulated to calculate the activity coefficients for the metallic components in our experimental system. It was found that, despite the slight deviations from ideality for Ni–Fe alloys, large differences exist between the way Ni interacts with trace elements and the way Fe does. Activity coefficients calculated from the thermodynamics of the metallic solution rationalized the experimentally derived log D vs log fO2 plots. When the independently derived activity coefficients for the trace elements in the alloys (γ) are used to plot log γ KD vs log fO2, the unusual oxygen fugacity dependencies can be fully reconciled with the expected valences of 3+ for Ga, 4+ for Ge, and predominantly 4+ at high oxygen fugacities changing to predominantly 2+ at low oxygen fugacities for Sn. If a planetary metal–silicate system evolves with increasing oxygen fugacity, the increasing Ni content of the metal will maintain the siderophile character of Ga, Ge, and Sn because these elements are more stable dissolved in Ni compared to Fe.

Metal-silicate partitioning of nickel and cobalt: The influence of temperature and oxygen fugacity

We report new metal-silicate melt partitioning experiments for Co and Ni. One atmosphere, gas-mixing (H2CO2 or CO-CO2) experiments were run at 1300, 1425, and 1550°C and at oxygen fugacities between air and near or below the iron-wustite (IW) buffer. Bulk melt composition, at the diopsideanorthite-forsterite eutectic, was constant in all runs. Metals in these experiments were platinum alloys permitting a large oxygen fugacity range to be investigated while maintaining fixed bulk melt composition. Since activity-composition relations of Pt-Ni and Pt-Co alloys are reasonably well known, we calculated equilibrium constants for the metal-silicate partitioning reactions. Variations of the equilibrium constants as a function of experimental parameters allowed us to monitor the chemical behavior of Ni and Co in the silicate melt. Nickel and Co behave as divalent species in the silicate melt over the range of conditions investigated. However, differences in solubility between low and high oxygen fugacity could not be entirely explained by a simple oxidation-reduction equilibrium constant. Differences in melt solubility were correlated with which gas-mixing system was used, H2-CO2 gave larger NiO solubilities at low oxygen fugacity than CO-CO2. The difference in melt solubility was attributed to minor amounts of melt-dissolved volatile species, perhaps H2O. Along the IW buffer, Fe metal-silicate partition coefficients for Ni and Co decrease with increasing temperature, but strengths of the temperature dependencies, even in this simple system, appear to vary with experimental conditions. Such differences highlight the danger of extrapolation to conditions far beyond the experimental regime. For example, extrapolation to magma ocean temperatures (3000–4000°C) results in uncertainties larger than the order of magnitude difference in siderophility between Ni and Co.