Shuo Ding, Terry Plank, Paul Wallace, Daniel Rasmussen

The degassing of CO2 and S from arc volcanoes is fundamentally important to global climate, eruption forecasting, and 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 co-existing vapor in ascending mafic-intermediate magma. Unlike previous models, Sulfur_X describes sulfur degassing by combining separate sulfur partition coefficients for three relevant equilibria: RxnI. FeS (m) + H2O (v)→H2S (v) + FeO (m), RxnIa. FeS (m) + 1.5O2 (v) →SO2 (v) +FeO (m), and RxnII. CaSO4(m)→SO2 (v) + O2 (v) + CaO (m), based on the sulfur speciation in the melt (m) and co-existing vapor (v). Sulfur_X tracks the evolution of fO2 and sulfur and iron redox states 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). Resulting from this new pressure-S relationship in the melt, the predicted CO2/ST evolution of co-existing vapor by Sulfur_X also provides new insights into interpreting the CO2/ST ratio measured in high-T volcanic gases.