The effect of pressure on sulfur valence state in mafic silicate melts

• Spectroscopic measurements of melt S 6+ /ΣS as a function of f O2 at 1.5 and 3 GPa. • Increasing pressure preferentially stabilizes S 6+ but this effect is modest. • Crustal pressure melt S 6+ /ΣS broadly represents melt S 6+ /ΣS at mantle pressures. In silicate melts the transition from S 2- to S 6+ occurs over a narrow range of oxygen fugacities ( f O2 ) such that small changes in melt f O2 may significantly impact sulfur transport, sulfur mineral/melt partitioning, and sulfur vapor/melt partitioning. The dependence of this transition on pressure remains poorly constrained. Here we present analyses of hydrous sulfur-bearing basaltic-andesite glasses equilibrated at f O2 values that span the S 2- to S 6+ transition at 1380 °C at 1.5 and 3 GPa. We have quantified sulfur valence state using S-XANES and estimated f O2 from glass Fe 3+ / ΣFe ratios determined using Fe-XANES and Pt x Fe 1-x capsule alloy compositions. We find that the effect of pressure on sulfur valence state is modest at constant absolute f O2 . From 1.5 GPa to 3 GPa, the absolute log f O2 of the transition from S 2- to S 6+ (defined as log f O2 tr ans ) increases by 0.30±0.19 log units. This corresponds to an empirical pressure dependence for absolute log f O2 tr ans of 0.20±0.13/GPa, a pressure dependence for log f O2 tr ans of -0.47±0.13/GPa relative to the QFM buffer, and a pressure dependence for log f O2 tr ans of -0.14±0.13/GPa relative to the FFM buffer, respectively. These results are consistent with previously published thermodynamic calculations. We apply our results to simplified mantle melting and melt extraction models and find that melt S 6+ /ΣS remains relatively constant from 2 to 0.1 GPa. This implies that, unlike the case of Fe 3+ / ΣFe, pressure is not a strong control on melt S 6+ /ΣS, and that S 6+ /ΣS measured in primitive, undegassed volcanic glasses broadly reflects the S 6+ /ΣS of melts in their source regions. The corollary is that, while S-free liquids decompress approximately parallel to the QFM buffer, liquids with significant S 6+ contents will not. In mafic melts, we predict that sulfate is present under the relatively reducing conditions found at mid-ocean ridges. For particularly sulfate-rich magmas, a failure to consider S-Fe redox exchange during ascent could lead to a slight overestimate of f O2 on the order of 0.2 log units relative to QFM.