Solubility and speciation of S in hydrous magmas: Effects of composition and redox conditions at 1050°C and 200 MPa

Knowledge on the behaviour of sulphur in natural magmas provides important constraints on geochemical and geophysical processes occurring in magmatic and hydrothermal systems. The partitioning of S between silicate melts and coexisting fluids is mainly controlled by the transformations of S species into sulphide or sulphate form due to changes in the redox conditions of the system. The saturation of a silicate melt in respect of sulphide- or/and sulphate-bearing phase is responsible for the maximum concentrations of dissolved sulphur. Hence, the composition of the silicate melt and activities of melt components, especially Fe and Ca, are critical parameters controlling S partitioning. Here, we present experimental data on the solubility of S in rhyolitic to basaltic magmas equilibrated with aqueous S-bearing fluids of constant bulk composition (5 wt.% H2O and 1 wt.% S in the system) at 1050°C, 200 MPa and redox conditions corresponding to that of logfO2~QFM-2 to QFM+3 (where QFM is quartz-fayalite-magnetite oxygen buffer). For studied melt compositions, the solubility of S systematically increases with increasing fO2 and depolymerisation of the silicate melt. At logfO2 < ~QFM, S solubility is controlled by the stability of FeS phase while at logfO2 > ~QFM+2, CaSO4 is a stable phase. Both S-bearing phases may coexist at intermediate fO2 values. The largest difference in solubilities of sulphide and sulphate S is observed for basaltic melts, where the concentration of sulphide S is 0.07 wt.%, being approximately ten times lower than the concentration of sulphate S of about 0.6 wt.%. Other melt compositions show lower concentration differences between dissolved S species, having, however, similar positive dependence on fO2. Thus, the partition coefficients of S between fluid and silicate melt vary as a function of melt composition and redox conditions. The largest partition coefficients (>1000) are determined for reduced silicic melts. For all investigated melt compositions, the increase in the concentrations of dissolved S is continuous in the fO2 range from QFM to QFM+2 which is consistent with continuous change in S speciation as a function of the redox state. However, it is remarkable that the sufide-sulfate transition occurs in fO2 range of less than 0.5 log units in silicic melts whereas this range approaches about two log units fO2 in basaltic melts. This have an effect on the solubility dependence of S on fO2 in different melt compositions, i.e., S solubility is silicic melts shows more pronounced dependence on fO2. The experimental data obtained for a wide compositional range and different redox conditions can be used to predict the concentrations of dissolved S, to estimate the prevailing redox conditions in natural magmas saturated with S-bearing phase(s) and to calibrate the existing thermodynamic models.