Soret separation of highly siderophile elements in Fe–Ni–S melts: Implications for solid metal–liquid metal partitioning

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Soret separation of melts consisting of Fe (+ 1–10 wt.% Ni) with added C, Si or S and trace highly siderophile elements (HSE; Ru, Rh, Pd, Re, Os, Ir, Pt and Au) was investigated in experiments done at 2 GPa, T hot ~ 2000 °C and T hot−T cold ~ 250 °C. Experiments with added C and Si produced homogeneous samples, whereas those with S resulted in significant compositional gradients, with S enriched at the hot end of the sample, and Fe and HSE enriched at the cold end. The magnitude of the separation of the HSE is not the same for each, but varies in the order (smallest to largest) Pd~Au<Rh<Ru~Pt<Ir~Re~Os. We show that the observed distribution of HSE within experiments is consistent with the variation in activity coefficients for these elements as predicted by the non-metal avoidance model of Jones and Malvin (1990). Thus, our results indicate that the melt composition effect on solid metal–liquid metal partitioning can be captured in a single thermal diffusion experiment. The magnitude of the melt composition effect, as judged by the Jones–Malvin interaction parameter, β, varies with metal size in a manner similar to solid metal–liquid metal partition coefficients, suggesting like origins of the HSE “selectivity” for Fe-rich solid or liquid structures.