The solubility of palladium and ruthenium in picritic melts: 2. The effect of sulfur

Vera Laurenz, Raúl O.C. Fonseca, Chris Ballhaus, Klaus Peter Jochum, Alexander Heuser, Paul J. Sylvester

Research output: Contribution to journalArticle

40 Scopus citations

Abstract

The platinum-group elements (PGE) are exceptional tracers of planetary differentiation and therefore are useful discriminants of different high-temperature processes. Many of the initial efforts to understand how PGE behave during mantle melting involved experiments where PGE solubility in silicate melt was measured. The results of these experiments showed that PGE dissolve in silicate melts as oxide species, and that their saturation depends on oxygen partial pressure (pO2). However, the majority of these experimental studies were conducted on FeO and sulfur-free melt compositions that little resemble natural magmas. In particular, the role of sulfur has been downplayed because of its lower availability as a ligand to the PGE in silicate melt when compared to oxygen. However, magmatic sulfides from oceanic basalts and layered intrusions are greatly enriched in the PGE relative to the silicate melts in equilibrium with them, suggesting that the PGE are strongly chalcophile. The question then is whether the PGE are actively scavenged into sulfide melt after it exsolves from a silicate melt; or if the PGE are already associated with S2- in a silicate melt prior to sulfide saturation.Here we report the results of experiments where the solubilities of Ru and Pd have been measured in natural picritic melt, as a function of pO2 and in the presence of sulfur. Results show that the presence of sulfur in the silicate melt enhances the solubility of Ru in picrite by over an order of magnitude. These results can be used to quantify the preference that PGE have to associate with sulfur relative to oxygen. For example, Ru was found to be 880±410 times more likely to associate with sulfur than with oxygen at pO2 around the fayalite-magnetite-quartz redox equilibrium. The preferential association of the PGE with S2- ligand indicates that once a sulfide melt exsolves, it will already be enriched in PGE precluding the need for it to actively scavenge PGE from a silicate melt.

Original languageEnglish
Pages (from-to)172-183
Number of pages12
JournalGeochimica et Cosmochimica Acta
Volume108
DOIs
StatePublished - May 1 2013

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