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; Fitoussi et al., 2009), the core contains significant amounts of Si, the important sidcrophile 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 similar to 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. (C) 2010 Elsevier Ltd. All rights reserved.