Dilution drives deep degassing of sulfur in hydrous basaltic magmas

Ery Catherine Hughes, Edward M. Stolper

Sulfur (S) is thought to degas at higher pressures from hydrous magmas (e.g., arc and some ocean island basalts) relative to water-poor magmas, from which S degasses at lower pressures (e.g., Kīlauea, mid-ocean ridge basalts). Our modelling of degassing shows this occurs for magmas that are either reduced (i.e., S is present predominantly as H₂S in the vapor and dissolved sulfide in the melt) or oxidised (i.e., dominantly SO₂ in the vapor and dissolved sulfate in the melt). Degassing at even higher pressures occurs for silicate melts containing both dissolved sulfide and sulfate due to the “sulfur solubility minimum”. Additionally degassing of a fictive, inert, and ideal gas species that behaves like a noble gas but with a similar overall solubility function (i.e., similar concentration dissolved in vapor-saturated melt at the same conditions) to S mimics the higher pressure initiation of degassing for hydrous magmas and the overall behavior of S. We show that dilution by additional gas species (i.e., H₂O and CO₂) in the vapor phase is the main driver of relatively high-pressure degassing of S, which can be further enhanced for magmas at the sulfur solubility minimum. Dilution is the same effect that drives: (1) higher pressure degassing of H₂O and CO₂ in melts containing both compared to pure-H₂O or -CO₂; (2) higher pressure degassing of H₂O or He in CO₂-rich systems; and (3) a decrease in the S content in gases during hydrous degassing. This mechanism does not require a direct effect of H₂O (or CO₂) on the solubility function of S, although H₂O does decrease the concentration of S dissolved in the melt at given set of conditions contributing to higher pressure degassing of S. Although our emphasis is on the significant impact of dilution on S degassing, the dilution effect impacts all volatile species, with its magnitude depending on the relative solubility functions and concentrations of volatile components present in the magma.