Partitioning of platinum-group elements in the Fe-Ni-S system and their fractionation in nature

The partitioning of minor amounts of platinum-group elements (PGEs: Ru, Rh, Pd, Os, Ir, and Pt) has been investigated in the Fe-Ni-S system at 1000 to 1400°C and low pressure. Revising previous experimental results indicating lack of specificity of PGE, distinctly different partitioning behavior is demonstrated for heavy and light platinoids; Os, Ir, and Pt are preferentially concentrated in alloy phases, whereas Pd is depleted in alloys and relatively enriched in sulfide liquid. Partition coefficients for Fe alloy/sulfide liquid at 1000°C are 1 to 2 for Pd (comparable to values for Ni), 30 to 110 for Rh, and in excess of 1000 for Os, Ir, and Pt. Ruthenium and, to a lesser extent, rhodium are the only PGEs with significant solubility in troilite coexisting with sulfide liquid. The marked fractionation of Pt from Pd in the presence of coexisting alloy and sulfide liquid is contrasted with the minimal fractionation of PGEs in the primary differentiation of planetary material and of Pt from Pd in the generation of basic magmas in the upper mantle. The characteristic fractionation patterns for PGEs in upper mantle and crustal rocks are most likely related to the presence of a combination of host phases (alloys, spinel, diopside) for the refractory PGEs (Os, Ir, and Ru) during partial melting in the upper mantle.