Speciation of palladium and ruthenium in S-bearing, sulfide-undersaturated basaltic melt - implications for enrichment mechanisms of noble metals in magmatic sulfide melt

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Despite years of research, the processes governing the transfer of platinumgroup elements (PGE) from a silicate melt to a magmatic sulfide melt are not well understood. One parameter is the sulfide/silicate partition coefficient D, a concentration ratio that quantifies the affinity of a given element to sulfide in preference to silicate melt. Another parameter often invoked is the R factor. R is a measure of the silicate melt volume with which a sulfide melt must equilibrate in order to collect its PGE. Implicit in the R factor concept is the idea that high sulfide/silicate Ds can only be effective if the exsolved sulfide melt is given opportunity to come in contact with maximum amounts of silicate melt. Here, we will explore alternatives to the R factor model. By investigating the effect of S2- on the solubility of Pd and Ru in picritic silicate melt, we may quantify to what extent the PGE in silicate melt prefer the S2- ligand over O2-. If a preference exists toward the S2- ligand, exsolution of a sulfide melt will sequester all PGE-S molecules in silicate to the sulfide regardless of R. PGE enrichment in the sulfide then reflects the proportion of PGE associated with S2-, and it will be independent by physical conditions laid out by R factor model.