Dilution drives deep degassing of sulfur in hydrous magmas

Sulfur (S) is thought to degas deep from hydrous magmas (e.g., arc basalts), in contrast to water-poor magmas where S degasses at very shallow depths (e.g., Kīlauea, mid-ocean ridge basalts). Our modelling of degassing shows this occurs for magmas that are both reduced (i.e., S is present predominantly as H2S in the vapor and dissolved sulfide in the melt) and oxidised (i.e., SO2 in the vapor and dissolved sulfate in the melt). Even deeper degassing occurs for silicate melts containing both dissolved sulfide and sulfate due to the sulfur solubility minimum. Additionally, deep degassing of a fictive, inert, and ideal gas species that behaves like a noble gas but with a similar solubility to S occurs in hydrous magmas. We show that dilution by additional gas species (i.e., H2O and CO2) in the vapor phase is the main driver of deep degassing of S, which can be further enhanced for magmas at the sulfur solubility minimum. Dilution is the same effect that drives deeper degassing of H2O and CO2 in melts containing both compared to pure-H2O or -CO2; deep degassing of H2O or He in CO2-rich systems; and reduces the S content in gases during hydrous degassing. This mechanism does not require a direct effect of H2O (or CO2) on the solubility of S, although H2O does decrease the solubility of S, contributing to the deep degassing of S. The dilution effect impacts all volatile species, with its magnitude depending on the relative solubilities and concentrations of the different volatiles present.