High-temperature stability of laurite and Ru-Os-Ir alloy and their role in PGE fractionation in mafic magmas

Experiments were performed to establish the stability limits and mineral - sulfide melt partitioning behavior of the platinum-group minerals laurite (Ru,Os,Ir)S-2 and Ru-Os-Ir alloy as functions of temperature and sulfur fugacity. Ruthenium-doped experiments yielded laurite up to similar to 1275 degreesC at log f(S-2) of -2.0, with laurite replaced by Ru alloy at higher temperature. Similar results were observed at logf(S-2) of -2 and -1.3 for compositions doped with multiple PGE, except that Ru-Os-Ir alloy coexisted with laurite at T less than or equal to 1265 degreesC. Laurite and alloy from these latter experiments were found to be Ru-Os-Ir-rich and Pt-Pd-poor, with grains of alloy containing more Os and Ir than laurite, and both phases becoming (Os + Ir)-rich with increasing f(S-2). Concentrations of PGE in sulfide liquid saturated with laurite + alloy were found to be: Ru: 1-13 wt%; Os and Ir: less than or equal to 0.6 wt%; Pt and Pd: 0.5-4 wt%. The finding that laurite and Ru-Os-Ir alloy are stable at chromian-spinel-based liquidus temperatures indicates that the commonly observed inclusion of these phases in chromian spinel can be interpreted as a primary magmatic texture. High solubilities for Ru in molten sulfide, combined with low intrinsic abundances of the PGE in igneous rocks, suggest that crystallization of laurite or Ru-Os-Ir alloy in the presence of immiscible sulfide liquid is unlikely. Thus, the extent to which PGE-bearing accessory minerals will affect the final distribution of the PGE within an igneous body may be strongly linked to when, or if, saturation in sulfide liquid occurs.