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- Title
Sulfur_X: A Model of Sulfur Degassing During Magma Ascent.
- Authors
Ding, Shuo; Plank, Terry; Wallace, Paul J.; Rasmussen, Daniel J.
- 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) → $\to $ H2S (v) + FeO (m), and RxnII. CaSO4(m) → $\to $ 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. Plain Language Summary: Understanding how CO2 and S are emitted from volcanoes, called degassing, is important in interpreting the CO2/ST gas precursors to volcanic eruptions and quantifying the total amount of gases released into the atmosphere that are climatically important. However, existing models show significant discrepancies in predicting the behavior of sulfur during degassing. In this study, we employ a new approach to describe sulfur behavior during magma degassing and develop a new model, Sulfur_X, that successfully reproduces the distinct S, CO2, and H2O degassing behavior recorded in melts from different volcanoes. Sulfur_X shows that sulfur can either degas early at high pressure or late at low pressure during magma ascent to the surface, depending on the initial sulfur speciation and H2O contents in the magma. In addition, sulfur is one of the most commonly measured volcanic gas components used for volcano monitoring. Therefore, the predicted compositional evolution of co‐existing vapor by Sulfur_X during magma ascent bears directly on the interpretation of CO2/ST ratio measured in high‐T volcanic gases and the development of eruption forecast models. Key Points: Sulfur_X is a new open‐source magma degassing model that accurately predicts the volatile and redox evolution of ascending arc magmasSulfur_X shows that sulfur can start degassing in the lower crust or near‐surface, depending on the initial S6+/ΣS and H2O in the meltThe vapor compositions predicted by Sulfur_X can be used to interpret the CO2/ST ratios in high‐T volcanic gases, an eruption precursor
- Subjects
SULFUR; VOLCANIC gases; MAGMAS; VOLCANIC eruptions; SUBDUCTION zones; ORE deposits; SIDEROPHILE elements; MERCURY vapor
- Publication
Geochemistry, Geophysics, Geosystems: G3, 2023, Vol 24, Issue 4, p1
- ISSN
1525-2027
- Publication type
Article
- DOI
10.1029/2022GC010552