Sulfur_X: A model of sulfur degassing during magma ascent

19 The degassing of CO2 and S from arc volcanoes is fundamentally important to global climate, 20 eruption forecasting, ore deposits, and the cycling of volatiles through subduction zones. 21 However, all existing thermodynamic/empirical models have difficulties reproducing CO2-H2O22 S trends observed in melt inclusions and provide widely conflicting results regarding the 23 relationships between pressure and CO2/SO2 in the vapor. In this study, we develop an open24 source degassing model, Sulfur_X, to track the evolution of S, CO2, H2O, and redox states in 25 melt and vapor in ascending mafic-intermediate magma. Sulfur_X describes sulfur degassing by 26 combining separate sulfur partition coefficients for three equilibria: RxnI. FeS (m) + H2O 27 (v)→H2S (v) + FeO (m), RxnIa. FeS (m) + 1.5O2 (v) →SO2 (v) +FeO (m), and RxnII. 28 CaSO4(m)→SO2 (v) + O2 (v) + CaO (m), based on the sulfur speciation in the melt (m) and co29 existing vapor (v). Sulfur_X is also the first to track the evolution of fO2 and sulfur and iron 30 redox states accurately in the system using electron balance and equilibrium calculations. Our 31 results show that a typical H2O-rich (4.5 wt.%) arc magma with high initial S6+/ΣS ratio (>0.5) 32 will degas much more (~2/3) of its initial sulfur at high pressures (> 200 MPa) than H2O-poor 33 ocean island basalts with low initial S6+/ΣS ratio (<0.1), which will degas very little sulfur until 34 shallow pressures (<50 MPa). This new pressure-S relationship in the melt predicted by 35 Sulfur_X provides new insights into interpreting the CO2/ST ratio measured in high-T volcanic 36 gases in the run-up to eruption. 37 Plain Language Summary 38 Understanding the process by which CO2 and S are emitted from volcanoes, called degassing, is 39 important in interpreting the CO2/ST gas precursors to volcanic eruptions and quantifying the 40 total amount of climatically important gases released to the atmosphere. However, existing 41 models show significant discrepancies in predicting the behavior of sulfur during degassing. In 42 this study, we employ a new approach to describe sulfur behavior during magma degassing and 43 develop a new model, Sulfur_X, that successfully reproduces the distinct S, CO2, and H2O 44 records from different volcanoes. Sulfur_X shows that sulfur can either degas early at high 45 pressure or late at low pressure during magma ascent to the surface, depending on the initial 46 sulfur speciation and H2O contents in the magma. In addition, sulfur is one of the most 47 commonly measured volcanic gas components for volcano monitoring. Therefore, the predicted 48 compositional evolution of co-existing vapor by Sulfur_X during magma decompression bears 49 directly on the interpretation of CO2/ST ratio measured in high-T volcanic gases and the 50 development of eruption forecasts models. 51 52