Fractionation of the platinum-group elements during mantle melting.

Experiments in sulfide-silicate systems demonstrate that two sulfide phases are stable in the asthenospheric upper mantle: a crystalline osmium-iridium-ruthenium-enriched monosulfide and a rhodium-platinum-palladium-enriched sulfide melt. During silicate melt segregation, monosulfide stays in the solid residue, dominating the noble metal spectrum of residual mantle. The sulfide melt is entrained as immiscible droplets in the segregating silicate melt, defining the noble metal inventory of the basaltic component.