The sulfur solubility minimum and maximum in silicate melt

Abstract

The behaviour of sulfur in magmas is complex because it dissolves as both sulfide (S2−) and sulfate (S6+) in silicate melt. Interesting aspects of the behaviour of sulfur are the solubility minimum (SSmin) and maxima (SSmax) observed with varying oxygen fugacity (fO2). We use a simple ternary model (silicate–S2–O2) to explore the varying fO2 paths where these phenomena occur. Both SSmin and SSmax occur when S2− and S6+ are present in the silicate melt in similar quantities owing to the differing solubility mechanisms of melt species containing these oxidation states of sulfur. At constant T, a minimum in dissolved total S content in vapour-saturated silicate melt (wmST) occurs along paths of increasing fO2 and either constant fS2 or constant P. For paths on which wmST is held constant with increasing fO2, the SSmin is expressed as a maximum in P. The SSmin occurs when the fraction of S6+ in the melt ([S6+/ST ]m) is 0.25 for constant fS2 and [S6+ /ST ]m ≈ 0.75 for constant wmST and P. A minimum in wmST is not encountered during closed-or open-system depressurization in the simple system we modelled. However, the SSmin marks a change from reduction to oxidation during degassing. Various SSmax occur when the silicate melt is multiply saturated with at least two phases: vapour, sulfide melt, and/or anhydrite. The SSmin and SSmax are potentially important features of magmatic process involving S, such as mantle melting, magma mixing, and degassing. These concepts influence calculations of the pressures of vapour-saturation, fO2, and SO2 emissions using melt inclusions.