Compositional and temperature effects on sulfur speciation and solubility in silicate melts

Abstract

We have determined the chemical speciation of dissolved sulfur and the sulfur concentration at fixed oxygen and sulfur fugacities for a wide range of silicate melt compositions (from Fe-rich basalt to dacite). Each melt was equilibrated at 1300°C and 1-atmosphere pressure at oxygen fugacities (fO2) between −1.67 and +1.6 log units relative to the Fayalite–Magnetite–Quartz (FMQ) buffer and absolute sulfur fugacities between −5.1 and −1.2 log units. The fO2 and fS2 of the experiments were controlled by using gas mixtures of CO–CO2–SO2. The speciation of sulfur in the quenched glasses was determined using both X-ray Absorption Near-Edge Spectroscopy (XANES), and from the dependence of equilibrium sulfur concentration on the fS2/fO2 ratio measured by secondary-ion mass spectrometry (SIMS) and electron microprobe.The speciation of dissolved sulfur in each melt undergoes an abrupt transformation from S2− to S6+ with increasing fO2, and this transition is shifted ∼0.5 log units higher in fO2 as melt FeO concentration increases from ∼5 wt% to ∼18 wt%. Since sulfide concentrations at constant fO2 and fS2 are consistently greater for more FeO-rich melts, the compositional effect on speciation may be explained by the well-known sensitivity of the sulfide capacity (CS2−) of the melt to FeO concentration.S6+/S2− ratios for the glasses exhibit a linear relationship with Fe3+/Fe2+, indicating that the redox couples for iron and sulfur can be directly related to one another. We used thermodynamic data to model the interrelationship between Fe and S oxidation states in terms of the equilibriumFeS+8FeO1.5=8FeO+FeSO4 Fitting the data to our experiments at 1300°C we obtained the following expression for the temperature-dependence of speciation:log⁡(S6+S2−)=8log⁡(Fe3+Fe2+)+8.7436×106T2−27703T+20.273 This equation fits the data for all our compositions and is also consistent with earlier results at 1050°C and 950°C. We used the interdependence of S and Fe oxidation states to infer electron transfer between Fe2+ and S6+ during quenching of glasses from Mauna Kea, Hawaii. The effect is sufficient to cause significant overestimation of equilibrium Fe3+/ΣFe in natural glasses and corresponding overestimate of fO2 by about 0.8 log units.Glasses equilibrated under the most oxidizing conditions (containing S6+ only) have equilibrium S concentrations that are negatively correlated with their mole fractions of tetrahedral (Si + Ti) cations.