Modeling the behavior of sulfur in magmatic systems from source to surface: Application to Whakaari/White Island, Aotearoa New Zealand, and Etna, Italy

Our understanding of the role of volcanoes in the global sulfur cycle and how volcanic gas emissions can be used to monitor volcanoes is limited by the complex interactions between hydrothermal systems and volcanic sulfur emissions. Hydrothermal systems influence the amount and speciation of volcanogenic sulfur, which is ultimately released to the surface and atmosphere via a range of physicochemical processes. To understand the effect of the hydrothermal system on surface emissions, we model the magmatic-hydrothermal systems at Whakaari/ White Island, Aotearoa New Zealand, and Etna, Italy. We quantify the magmatic sulfur inputs using mass balance and MELTS modeling (thermodynamic model of crystallization); model the effects of degassing using Sulfur_X (an empirical model of melt-gas equilibria); and model the influence of the hydrothermal system using CHIMXPT and EQ3/6 (thermodynamic and kinetic models of gas+water +/- rock reactions), which we compare to measured plume and fumarole compositions. We find that the sulfur inputs can broadly equal sulfur outputs over long timescales. However, the hydrothermal system can modulate the total mass of sulfur released and its H2S/ SO2 ratio on shorter timescales, especially as the system evolves from water- to gas-dominated through the development of dry, gas-dominated pathways.