Abstract. Explosive eruptions involve mainly silicic magmas in which sulfur solubility and diffusivity are low. This inhibits sulfur exsolution during magma uprise as compared to more mafic magmas such as basalts. Silicic magmas can nevertheless liberate large quantities of sulfur as shown by the monitoring of SO2 in recent explosive silicic eruptions in arc settings, which invariably have displayed an excess of sulfur relative to that calculated from melt degassing. If this excess sulfur is stored in a fluid phase, it implies a strong preference of sulfur for the fluid over the melt under oxidized conditions, with fluid/melt partition coefficients varying between 50 and 2612, depending on melt composition. Experimentally determined sulfur partition coefficients for a dacite bulk composition confirm this trend and show that in volcanic eruptions displaying excess gaseous sulfur, the magmas were probably fluid-saturated at depth. The experiments show that in more reduced silicic magmas, those coexisting only with pyrrhotite, the partition coefficient decreases dramatically to values around 1, because pyrrhotite locks up nearly all the sulfur of the magma. Reevaluation of the sulfur yields of some major historical eruptions in the light of these results shows that for oxidized magmas, the presence of 1-5 wt % fluid may indeed account for the differences observed between the petrologic estimate of the sulfur yield and that constrained from ice core data. Explosive eruptions of very large magnitude but involving reduced and cool silicic magmas, such as the Toba or the Bishop events, release only minor amounts of sulfur and could have consequently negligible long-term (years to centuries) atmospherical effects. This redox control on sulfur release diminishes as the melt composition becomes less silicic and as temperature increases, because both factors favor more efficient melt sulfur degassing owing to the increased diffusivity of sulfur in silicate melts under such conditions.

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.