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 fO 2, fS 2 and temperatures, which allow extrapolation to conditions pertinent to the Earth's mantle. The solubility of Re in mattes increases with increasing fS 2, with Re dissolving as Re 4+ at high fS 2 and Re 0 at low fS 2. The effect of fO 2 is negligible except at high fO 2 where O in the matte becomes important. At constant fS 2, an increase in temperature leads to an increase in the solubility of Re 0 in the matte, but a decrease in the solubility of Re 4+. 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 ( D Re matte/sil) for a range of conditions. The calculated D Re matte/sil show a large dependence on fO 2, and a lesser dependency on fS 2, 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 fS 2 and fO 2 reconciles the apparent discrepancies between previous estimates of this parameter, which can vary by more than five orders of magnitude within the range of fS 2 and fO 2 covered by terrestrial basaltic magmas.