The Solubility of Sulphur in Hydrous Rhyolitic Melts

Experiments performed at 2 kbar, in the temperature range 800-1000 C, with fO 2 between NNOÀ2Á3 and NNOþ2Á9 (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 NNOþ1, the addition of iron to a sulphurbearing 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 NNOþ1.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 H 2 S and SO 2 dissolution reactions.The model reproduces well the minimum of sulphur solubility at around NNOþ1, 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.