The dependence of metal-silicate partitioning of moderately volatile elements on oxygen fugacity and Si contents of Fe metal: Implications for their valence states in silicate liquids

Abstract

The volatile siderophile elements are important tracers of the delivery of volatile elements to the Earth. Their concentrations in the bulk silicate Earth are a function of the relative timing of their accretion and their sequestration into the core: a comprehensive understanding of their metal-silicate partitioning behaviour is therefore required in order to infer the volatile element accretion history. We present new partitioning data between liquid metal and liquid silicate at 11 GPa for a suite of volatile siderophile elements: Ag, As, Au, Cu, Ge, P, Pb, Sb, Sn. We focus particularly on determining their valence states and the effects of Si on partitioning, which are required in order to extrapolate from experimental conditions to core-formation conditions. It was found that all elements have weak to strong positive interaction parameters with Si. At low fO2, redox equilibria dictate that the siderophile elements should become more siderophile. However, at low fO2, Si also partitions more strongly into the metal. Given the repulsive nature of the interaction between Si and the elements of interest, the increased Si concentration at low fO2 will counteract the expected increase in the partition coefficient, making these elements less siderophile than expected at very reducing conditions. This causes the linear relationship between fO2 and log(D) to become non-linear at low fO2, which we account for by fitting an interaction parameter between Si and the elements of interest. This has implications for the interpretation of experimental results, because the valence cannot be determined from the slope of log(D) vs. logfO2 if low fO2, high Si metal compositions are employed without applying an activity correction. This also has implications for the extrapolation of experimental partitioning data to core-formation conditions: reducing conditions in the early stages of core formation do not necessarily result in complete or even strong depletion of siderophile elements when Si is present as a light element in the core-forming metal phase.