The solubility of sulphur in hydrous rhyolitic melts

Experiments performed at 2 kbar, in the temperature range 800-1000degreesC, with fO(2) between NNO-2.3 and NNO+2.9 (where NNO is the nickel-nickel oxide buffer), and varying amounts of sulphur added to hydrous metaluminous rhyolite bulk compositions, were used to constrain the solubility of sulphur in rhyolite melts. The results show that fS(2) exerts a dominant control on the sulphur solubility in hydrous silicate melts and that, depending on fO(2), a rhyolitic melt can reach sulphur contents close to 1000 ppm at high fS(2). At fO(2) below NNO+1, the addition of iron to a sulphur-bearing rhyolite magma produces massive crystallization of pyrrhotite and does not enhance the sulphur solubility of the melt. For a given fO(2), the melt-sulphur-content increases with fS(2). For fixed fO(2) and fS(2), temperature exerts a positive control on sulphur solubilities, at least for fO(2) below NNO+1. The mole fraction of dissolved sulphur exhibits essentially linear dependence on fH(2)S at low fO(2) and, although the experimental evidence is less clear, on fSO(2) at high fO(2). The minimum in sulphur solubility corresponds to the redox range where both fH(2)S and fSO(2) are approximately equal. A thermodynamic model of sulphur solubility in hydrous rhyolite melts is derived assuming that total dissolved sulphur results from the additive effects of H2S and SO2 dissolution reactions. The model reproduces well the minimum of sulphur solubility at around NNO+1, in addition to the variation of the sulphide to sulphate ratio with fO(2). A simple empirical model of sulphur solubility in rhyolitic melts is derived, and shows good correspondence between model and observations for high-silica rhyolites.