Solubility of Os and Ir in sulfide melt: Implications for Re/Os fractionation during mantle melting

Abstract

Although both rhenium (Re) and osmium (Os) are highly siderophile elements (HSE), they show contrasting geochemical behaviors during partial melting of the mantle - Re is mildly incompatible whereas Os appears to be compatible. This fundamental difference, unique among commonly used isotopic chronometers, causes large variations of Re/Os in oceanic basalts. However, which mantle phases control the geochemical behavior of these elements during partial melting is controversial. Sulfide is typically regarded as a major host for these elements, but recent studies have shown that silicate phases and spinel may also play a role. Here we report the results of an experimental study on the solubilities of Os and Ir in sulfide melts (or mattes) over a large range of oxygen (fO2) and sulfur (fS2) fugacities at 1300 °C. Our experiments indicate that the solubilities of Os and Ir in mattes increase with increasing fS2, with both Os and Ir dissolving as trivalent species at high fS2 and metallic species at low fS2. The effect of fO2 on Os and Ir solubilities appears to be related to oxygen being dissolved into the matte at more oxidizing conditions. Our results coupled with solubility data for Os and Ir in silicate melts have enabled matte/silicate melt partition coefficients for these elements (Dimatte/sil) to be calculated. Assuming a relative oxygen fugacity equal to the quartz-fayalite-magnetite redox equilibrium (i.e. QFM) and a sulfur fugacity of 10-0.5 bar, calculated DOsmatte/sil is ~104 and DIrmatte/sil is ~106. The low solubilities of Os and Ir in silicate melts, coupled with their high matte/silicate melt partition coefficients, suggest that Os and Ir in fertile mantle with ~200 ppm S are held in the mantle matte phase. However, we show that the empirical range of Re/Os in mid-ocean ridge basalts (MORB) can only be reproduced when sulfide is exhausted by high degrees of partial melting, leaving Os-Ir-rich metal alloy in the mantle residue.