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  1. Abstract

    The degassing of CO2and 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/SO2in 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 H2S (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 is also the first to track the evolution offO2and 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/STratio measured in high‐T volcanic gases in the run‐up to the eruption.

     
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