How does the hot core of a volcanic plume control the sulfur speciation in volcanic emission?

Abstract

Volcanic eruptions inject sulfur into the atmosphere mainly in form of SO2 and H2S. The ratio of these species (H2S/SO2), which is usually used as the mirror of the oxidation state of the source magma, varies significantly according to the type of activity, tectonic setting etc. This study aims to investigate the role of the hot core of plinian and sub‐plinian volcanic plumes (T > 600°C) in sulfur speciation based on the thermodynamic equilibrium in this temperature range. We consider the hot core as a box model in which 1000°C magmatic gas and 25°C ambient air are mixed and show that it functions as a hot oxidizing reactor for S species. Processes inside the hot core usually decrease the H2S content of the system but can either increase or decrease SO2 depending on initial oxidation state. Thus the SO2 injected into the atmosphere is not essentially generated directly from the magma but it can be produced in the hot core as the result of H2S oxidation. Besides, volcanic cloud compositions do not mirror the source conditions. Considering three types of tectonic settings (convergent plate, divergent plate and hot spot) we propose that H2S emission is more likely under reduced conditions in divergent plate and hot spot volcanic settings.