Disequilibrium sulfur degassing - a mixed volatile bubble-growth model

Volcanic eruptions are driven by bubble growth in magma, caused by the exsolution of volatiles within the melt. The most important magmatic volatiles are H2O and CO2, as they are the most abundant and exert the largest control on bubble growth. As a result, most experimental work involving magma degassing involves a simplified H2O-only, or H2O-CO2 system. However, the most important magmatic volatile used in volcano monitoring is SO2, because it is much easier to identify as unambiguously volcanogenic compared to CO2 and H2O. Because of the relative scarcity of research into sulfur degassing, most interpretations made from SO2 emission data assume equilibrium conditions; however, given the relatively slow diffusion of sulfur in silicate melts, it is likely that disequilibrium sulfur degassing is common in natural systems.
In this study, a mixed volatile bubble-growth model has been adapted to include H2O, CO2 and S, and accounts for the kinetic limitations of degassing, allowing disequilibrium behaviour to be modelled. In order to account for the effect of sulfur speciation on sulfur solubility, techniques from the degassing model Sulfur_X have been implemented. This new bubble-growth model simulates degassing of a one-, two- or three-volatile system in basalts, and can determine under what conditions disequilibrium degassing occurs.
Results suggest that disequilibrium degassing of sulfur occurs within the range of decompression rates found in natural basaltic systems, suggesting that it may be more prevalent than previously thought, and could be affecting observed emissions. By recreating the conditions of Fagradalsfjall 2021, it has been concluded that equilibrium degassing occurred during effusive episodes, and significant disequilibrium degassing occurred during fountaining episodes, at an estimated decompression rate of∼1 MPa/s. Results from the model agree very well with natural data, supporting the applicability of this new model to natural systems, and its use in volcano monitoring efforts.