How chalcophile is rhenium? An experimental study of the solubility of Re in sulphide mattes

Abstract

Although the Re/Os isotopic system has proved to be a valuable aid in understanding the evolution of the Earth's mantle, interpretation of the data is currently hindered by limited knowledge of the high-temperature geochemical behaviour of Re. In particular, the extent to which Re in the mantle is hosted by sulphide or silicate phases is poorly known. We report the results of an experimental study of the solubility of Re in sulphide melts (called here "mattes") coexisting with a Re-rich Re-Fe alloy in the system Fe-Re-S-O over a range of fO2, fS2 and temperatures, which allow extrapolation to conditions pertinent to the Earth's mantle. The solubility of Re in mattes increases with increasing fS2, with Re dissolving as Re4+ at high fS2 and Re0 at low fS2. The effect of fO2 is negligible except at high fO2 where O in the matte becomes important. At constant fS2, an increase in temperature leads to an increase in the solubility of Re0 in the matte, but a decrease in the solubility of Re4+. These results, coupled with data for the Re solubility in silicate melts taken from the literature, allow the calculation of Re matte/silicate-melt partition coefficients (DRematte/sil) for a range of conditions. The calculated DRematte/sil show a large dependence on fO2, and a lesser dependency on fS2, with Re behaving as a chalcophile element for relatively reduced MORB-type mantle and as a lithophile element for the oxidised sources of island-arc basalts. The sensitivity of Re to fS2 and fO2 reconciles the apparent discrepancies between previous estimates of this parameter, which can vary by more than five orders of magnitude within the range of fS2 and fO2 covered by terrestrial basaltic magmas.