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 S in silicate melts, it is likely that disequilibrium S degassing is common in natural systems. 

In this contribution, we explore the nature of sulfur degassing in magmatic systems. We extend a bubble growth model to include sulfur (creating a general framework that could be used to incorporate other volatile phases). Preliminary results indicate that sulfur has a negligible influence on bubble growth physics under typical eruption conditions, but, importantly, it shows that sulfur degassing is typically in disequilibrium in basaltic melts. This runs contrary to the typical assumption of equilibrium degassing when interpreting SO2 emissions, which has implications for the use of SO2 data as a proxy for magma degassing.