The importance of sulfur in the generation of basaltic magmas

The fate of sulfur in magmatic systems influences a wide range of processes including chalcophile element behavior, magma redox evolution, and volcanic degassing. However, understanding sulfur behavior can be complicated by the presence of multiple sulfur valence states (S^2- and S⁶⁺) in silicate melts. The transition from S^2- to S⁶⁺ occurs over a narrow range of oxygen fugacities (f_O2) such that small changes in melt f_O2 may significantly impact sulfur transport and its partitioning between minerals, silicate melt, and vapor. In recent years, we have learned much more about the dependence of this transition on temperature and melt composition, but the effect of pressure remains poorly constrained.

Here, we present a new suite of experiments allowing us to quantify the effect of pressure on mafic silicate melts. We integrate these experimental results with existing calibrations to explore how sulfur-iron redox equilibria influences the generation of basaltic magmas. Using a simplified mantle melting and melt extraction model, we show that sulfur valence state remains relatively stable during mantle melting and extraction. However, we also show that sulfur and iron continuously exchange electrons during melt ascent, leading to a small but non-negligible change to f_O2 relative to the QFM buffer. This example showcases the importance of integrating sulfur-iron redox equilibria into petrologic models, and how we can leverage recent advances in analytical and experimental methodologies to do so accurately.