Equilibrium vs. Kinetic control of the CO2/SO2 ratio in the arc volcanic gases

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Degassing CO2 and S from arc volcanoes is fundamental to global climate, eruption forecasting, and cycling of volatiles through subduction zones. Particularly, changes in CO2/SO2 ratio in the volcanic gases could be a potential precursor to eruptions. Typically, the CO2/SO2 ratios in high-temperature volcanic gases are used to infer the pressure of degassing due to the different solubility and partitioning of CO2 and S species in vapor-saturated magmatic liquid. However, there are significant discrepancies in predicting S behavior and degassing pressures among different degassing models, primarily due to the scarcity of high-pressure experiments constraining S equilibrium degassing. Moreover, such an interpretation of volcanic gases relies on the conventional assumption that degassing occurs at equilibrium in basaltic melt. However, concentration gradients of sulfur and carbon dioxides have been commonly observed in olivine-hosted embayments and matrix glasses, indicating diffusive degassing of both S and CO2. Once disequilibrium degassing is accepted, due to potentially different diffusivities of CO2 and S in the basaltic melt, the interpretation of the CO2/SO2 ratios in high-temperature volcanic gases also requires a revisit. This project aims at investigating the equilibrium vs. kinetic control of the CO2/SO2 ratio in the arc volcanic gases and evaluating the hypothesis that kinetic degassing leads to elevated CO2/SO2 ratios prior to eruptions. The results will significantly improve understanding of sulfur behavior in basaltic-andesitic volcanic systems, instruct the interpretation of volcanic gas data and bear significance in constraining the ascent rates using volatile diffusion and petrological estimates of total S outgassing.This study employs both equilibrium and decompression experiments with piston cylinder apparatus and internally-heated pressure vessel to constrain the behavior of three major magmatic volatiles, CO2, H2O, and S, during equilibrium and kinetic degassing. First, equilibrium experiments between 1000MPa and 50 MPa with controlled fO2 and a basaltic andesite will be used to constrain the sulfur behavior during equilibrium degassing. The results will be used to test and improve existing degassing models and serve as a baseline to compare to decompression runs. Second, decompression experiments with buffered fO2 on the same bulk composition from 300MPa to various final pressures with slow and fast decompression rates will be used to investigate CO2-S in the melt, H2O/CO2, and CO2/SO2 ratio in the co-existing vapor during kinetic degassing. The experimental results can be used to test and improve existing kinetic degassing and bubble growth model. With updated equilibrium and kinetic degassing models, this study will construct region diagrams demonstrating the control of kinetic vs. equilibrium degassing on evolution of CO2/S and H2O/CO2 in the melt and in the co-existing vapor as changing decompression rates.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.