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Abstract: Unraveling the conditions at which Earth''s metallic iron core formed yields important information about Earth''s early accretion and differentiation history. Multi-variable statistical modeling of siderophile element partitioning between core-forming metallic liquids and silicate melts form the basis for physical models of core formation. While it seems clear that core segregation in a deep peridotitic magma ocean is generally consistent with many mantle siderophile element abundances, there is considerable disparity among extant physical models in terms of the key parameters of pressure, temperature and oxygen fugacity at which the core formed. Moreover, there is ongoing debate over whether simple single-stage equilibrium or more complex multi-stage accretion models are required by the partitioning data. Here we consider how variations in the statistical regression of partitioning data affect the outcomes of physical models for core formation. Taking extant experimental data sets for four well-studied siderophile elements (Ni, Co, W and V) as examples, we find that the regression model exerts a fundamental control on physical model outcomes. Further, the experimental data are currently too imprecise to discriminate among various single-stage and continuous core formation scenarios. Progress in the development of physical models requires better isolation of the independent variables that affect partition coefficients and verification of activity models at high pressure and temperature in order to reduce the global uncertainty in multi-variable statistical models. [Copyright &y& Elsevier]

Abstract: Tungsten partitioning between liquid metal and liquid silicate, D(W), from 1723–2673 K, 0.5–18 GPa, and over a wide range of metal and silicate compositions provides constraints on planetary core formation models. We find that W partitioning is extremely sensitive to the carbon content of the metal alloy, becoming about an order of magnitude more siderophile at carbon saturation. Activity-composition corrections based on interactions between W–C and Fe–C in metal solution shift calculated D(W) to more lithophile values, and calculated oxygen fugacities (fO2) to more oxidizing values, than uncorrected data. W exists as a highly charged cation in silicate solution, displaying a mixture of oxidation states from +4 to +6 in experiments at fO2 of ∼0.5–2 log units below the iron–wüstite buffer. At constant fO2, the average calculated valence decreases with pressure from ∼+5.5 at 0.5 GPa to ∼+4 at 11–18 GPa. As a result of its high oxidation state, W partitioning is strongly dependent on silicate melt polymerization and fO2. In contrast to some previous results, we find that D(W) may decrease slightly in response to increasing temperature in the pressure range of our experiments. Pressure exerts a non-monotonic effect: D(W) increases with pressure up to ∼3 to 4 GPa, but decreases at higher pressures. Previous models for the effects of pressure and temperature on W partitioning that conflict with our results appear to result from a conflation of the intensive parameters of pressure, temperature, and carbon content. The mantle abundance of W could have been set by single-stage metal–silicate equilibration along the liquidus in a deep peridotite magma ocean at pressures from 20–50 GPa, and at oxygen fugacities consistent with the mantle''s present iron budget (IW−2 to −2.5). Equilibration at higher pressure is viable if the core-forming metal contained a significant, but not unreasonable, abundance of carbon (∼2 wt.%). Recent continuous accretion models involving multi-stage metal-silicate equilibration in a deepening magma ocean with progressive oxidation of the silicate remain permissible given our new treatment of W partitioning data. [Copyright &y& Elsevier]

The birth and infancy of Earth was a time of profound differentiation involving massive internal reorganization into core, mantle and proto-crust, all within a few hundred million years of solar system formation (t0). Physical and isotopic evidence indicate that the formation of iron-rich cores generally occurred very early in planetesimals, the building blocks of proto-Earth, within about 3 million years of t0. The final stages of terrestrial planetary accretion involved violent and tremendously energetic giant impacts among core-segregated Mercury- to Mars-sized objects and planetary embryos. As a consequence of impact heating, the early Earth was at times partially or wholly molten, increasing the likelihood for high-pressure and high-temperature equilibration among core- and mantle-forming materials. The Earth''s silicate mantle harmoniously possesses abundance levels of the siderophile elements Ni and Co that can be reconciled by equilibration between iron alloy and silicate at conditions comparable to those expected for a deep magma ocean. Solidification of a deep magma ocean possibly involved crystal–melt segregation at high pressures, but subsequent convective stirring of the mantle could have largely erased nascent layering. However, primitive upper mantle rocks apparently have some nonchondritic major and trace element refractory lithophile element ratios that can be plausibly linked to early mantle differentiation of ultra-high-pressure mantle phases. The geochemical effects of crystal fractionation in a deep magma ocean are partly constrained by high-pressure experimentation. Comparison between compositional models for the primitive convecting mantle and bulk silicate Earth generally allows, and possibly favors, 10–15% total fractionation of a deep mantle assemblage comprised predominantly of Mg-perovskite and with minor but geochemically important amounts of Ca-perovskite and ferropericlase. Long-term isolation of such a crystal pile is generally consistent with isotopic constraints for time-integrated Sm/Nd and Lu/Hf ratios in the modern upper mantle and might account for the characteristics of some mantle isotope reservoirs. Although much remains to be learned about the earliest formative period in the Earth''s development, a convergence of theoretical, physical, isotopic and geochemical arguments is beginning to yield a self-consistent portrait of the infant Earth. [Copyright &y& Elsevier]

The connectivity of molten Fe–S in peridotite has been experimentally investigated by means of in situ electrical conductivity measurements at high temperatures and 1 GPa. Starting materials were powdered mixtures of peridotite KLB-1 with various amounts (0, 3, 6, 13, 19, 24 vol.%) of the 1 GPa eutectic composition in the Fe–FeS binary system. At temperatures above the eutectic point in the Fe–FeS system (∼980 °C) and below the solidus of KLB1 (∼1200 °C), molten Fe–S in a solid silicate matrix interconnects when the volume fraction is over ∼5%. Conductivity–temperature paths indicate that in the presence of partial silicate melting the connectivity of molten Fe–S in a peridotite matrix is inhibited. Based on observations of retrieved samples, the percolation threshold of Fe–S melts in the presence of low to moderate degrees of silicate melt is estimated at 13±2 vol.%. These results indicate that if the volume fraction of Fe-alloy in a planetesimal was initially greater than 5%, and if early heating by decay of radionuclides raised the temperature of the interior above the Fe-alloy melting point, initial metal segregation was controlled by permeable flow of molten iron alloy in a solid silicate matrix. These conditions were likely met by many terrestrial objects in the early solar nebula. Efficient removal of residual Fe-alloy (5 vol.%) from silicate requires high-degree melting of silicate so that metal can segregate as droplets. Giant impacts during the final stage of accretion of large planetary objects could supply the energy required for high-degrees of melting. Alternatively, if initial metal segregation were delayed until a planetary object grew to large size (∼1000 km in diameter), release of gravitational potential energy due to metal segregation could contribute enough heat to form a magma ocean. [Copyright &y& Elsevier]