HIGH-TEMPERATURE STABILITY OF LAURITE AND Ru Os Ir ALLOY AND THEIR ROLE IN PGE FRACTIONATION IN MAFIC MAGMAS

Abstract

Experiments were performed to establish the stability limits and mineral – sulfide melt partitioning behavior of the platinum-group minerals laurite (Ru,Os,Ir)S2 and Ru–Os–Ir alloy as functions of temperature and sulfur fugacity. Ruthenium-doped experiments yielded laurite up to ~1275 °C at log f (S2) of −2.0, with laurite replaced by Ru alloy at higher temperature. Similar results were observed at log f (S2) of −2 and −1.3 for compositions doped with multiple PGE, except that Ru–Os–Ir alloy coexisted with laurite at T ≤ 1265°C. 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 (S2). Concentrations of PGE in sulfide liquid saturated with laurite ± lloy were found to be: Ru: 1–13 wt%; Os and Ir: ≤ 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.