Sulfur_X: A Model of Sulfur Degassing During Magma Ascent
Shuo Ding, Terry Plank, Paul Wallace, Daniel J. Rasmussen
Abstract
Abstract The degassing of CO 2 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 CO 2 ‐H 2 O‐S trends observed in melt inclusions and provide widely conflicting results regarding the relationships between pressure and CO 2 /SO 2 in the vapor. In this study, we develop an open‐source degassing model, Sulfur_X, to track the evolution of S, CO 2 , H 2 O, 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 ) + H 2 O ( v ) H 2 S ( v ) + FeO (m), and RxnII. CaSO 4 (m) SO 2 ( v ) + O 2 ( 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 f O 2 and sulfur and iron redox states accurately in the system using electron balance and equilibrium calculations. Our results show that a typical H 2 O‐rich (4.5 wt.%) arc magma with high initial S 6+ /ΣS ratio (>0.5) will degas much more (∼2/3) of its initial sulfur at high pressures (>200 MPa) than H 2 O‐poor ocean island basalts with low initial S 6+ /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 CO 2 /S T ratio measured in high‐T volcanic gases in the run‐up to the eruption.