SULFIDE AND SULFATE SATURATION OF DACITIC SILICATE MELTS AS A FUNCTION OF OXYGEN FUGACITY

Sulfur (S) is a key element in terrestrial magmatic processes and yet one of the most difficult to model due to its heterovalent chemistry and volatile nature. The maximum amount of S a silicate melt can dissolve before saturating with sulfide or sulfate changes with the prevailing redox state of the system and has important implications for the S budget of a system. Although a number of models have been developed to predict the S content of a silicate melt at either sulfide (under reducing conditions) or sulfate (under oxidizing conditions) saturation, only one model to date systematically assessed the sulfide-sulfate transition region at intermediate oxidation states (~ΔFMQ+1 to ~ΔFMQ+2). That model was developed using experimental data that constrain the S content at sulfide saturation and S content at sulfate saturation of basaltic silicate melts and its applicability to chemically evolved melts rests on the assumption that melt composition does not affect sulfide and/or sulfate solubility as the S in the melt changes from S^2- to S⁶⁺.

Here we report new experimental data to assess the effect of melt composition on sulfide and sulfate saturation in a dacitic silicate melt across the sulfide-sulfate transition. We present six experiments conducted using an H₂O-saturated natural dacitic silicate melt at 1000°C, 300 MPa, and oxygen fugacity encompassing the entire sulfide-sulfate transition (log fO₂ = ΔFMQ-0.7, ΔFMQ+0, ΔFMQ+0.5, ΔFMQ+1, ΔFMQ+1.75 and ΔFMQ+3.3). When our results are compared with previous studies, we are able to show that the existing model developed using a basaltic silicate melt does not fit the observations of sulfide and sulfate saturation at intermediate oxidation states (~ΔFMQ+1 to ~ΔFMQ+2) in a dacitic silicate melt. This discrepancy provides evidence that melt composition does affect how sulfide and sulfate dissolve in silicate melts and we propose a new model for the behavior of sulfide and sulfate saturation in evolved magmatic systems.