Sulfur_X: A Model of Sulfur Degassing During Magma Ascent

Abstract

The degassing of CO2 and S from arc volcanoes is fundamentally important to global climate, eruption forecasting, ore deposits, and the cycling of volatiles through subduction zones. However, all existing thermodynamic/empirical models have difficulties reproducing CO2-H2O-S trends observed in melt inclusions and provide widely conflicting results regarding the relationships between pressure and CO2/SO2 in the vapor. In this study, we develop an open-source degassing model, Sulfur_X, to track the evolution of S, CO2, H2O, and redox states in melt and vapor in ascending mafic-intermediate magma. Sulfur_X describes sulfur degassing by parameterizing experimentally derived sulfur partition coefficients for two equilibria: RxnI. FeS (m) + H2O (v) (Formula presented.) H2S (v) + FeO (m), and RxnII. CaSO4(m) (Formula presented.) SO2 (v) + O2 (v) + CaO (m), based on the sulfur speciation in the melt (m) and co-existing vapor (v). Sulfur_X is also the first to track the evolution of fO2 and sulfur and iron redox states accurately in the system using electron balance and equilibrium calculations. Our results show that a typical H2O-rich (4.5 wt.%) arc magma with high initial S6+/ΣS ratio (>0.5) will degas much more (∼2/3) of its initial sulfur at high pressures (>200 MPa) than H2O-poor ocean island basalts with low initial S6+/ΣS ratio (<0.1), which will degas very little sulfur until shallow pressures (<50 MPa). The pressure-S relationship in the melt predicted by Sulfur_X provides new insights into interpreting the CO2/ST ratio measured in high-T volcanic gases in the run-up to the eruption.