Effect of the FCC to HCP Phase Transition on Trace Element Partitioning Between Metal and Sulfide Melt

Most of what we understand about the chemical behavior of iron alloys, even at high pressure, pertains to the fcc phase. However, it is widely thought that the relevant structure in the Earth's core is hcp, not fcc. In this study we aim to understand the effect of the fcc-hcp transition on siderophile element partitioning between metal and coexisting sulfide melt. This is important, for example, in evaluating models in which Re-Os-Pt isotope fractionations are attributed to partitioning between the Earth's inner and outer core. Experiments were doped with trace elements Ni, Re, Os, Ir, and Pt, which partitioned between Fe-Ru alloys and sulfide melt. Most experiments were performed at 1 bar in sealed silica tubes in a tube furnace, and some experiments were performed at 6 GPa in a multi-anvil press. The fcc-hcp transition was investigated by varying the Ru content of the experiments; the metal is fcc at Ru-poor compositions but hcp at higher Ru contents. The sulfur content of the melt varied with temperature and with bulk composition. The run products were characterized by electron microprobe, and abundances of the trace elements in both metal and melt were determined by laser ablation ICP-MS. The effect on partitioning of the phase transition can be distinguished from compositional effects because a range of Ru contents was studied. Our Ru-free dataare in good agreement with previously published data in the Fe-S system at 1 bar. However, our highest-Ru compositions show significant differences in their D values, attributable to the phase transition in the metal.