Sulfur sequestration and redox equilibria in volcanic gases

Abstract

Although it has long been recognized that sub-volcanic gas-solid reactions involving iron play a major role in determining the redox state (RH ¼ log f H2 f H2O ) of discharging gases, the crucial role played by calcium has only recently been recognized through high temperature gas-solid experimental studies. These show that SO2, which dominates H2S in high temperature (> ~600 °C) volcanic gas mixtures, rapidly and efficiently reacts with abundant plagioclase and other calcic rock-forming minerals to simultaneously deposit anhydrite and release reduced sulfur. Coupled with sulfide deposition, these reactions control the total sulfur content of the gas phase through the very low solubilities of sulfides and anhydrite and, via coupled multicomponent reactions, determine the redox state and the H2S/SO2 ratio of the volcanic gas mixtures. Multi-component thermochemical modelling of gas-solid equilibria and titration-precipitation reactions along adiabatic expansion pathways from magma to surface confirm that for fumarole gas mixtures with outlet temperature >400 °C, RH is primarily controlled by anorthite - pyroxene - anhydrite - sulfide reactions, irrespective of their tectonic location, state of volcanic activity, gas discharge temperature and the composition of the gas mixtures released from the magma. Anhydrite and sulfide deposition through gas-solid reactions result in extensive sulfur sequestration as gas mixtures expand from the magma to the surface as is observed in the many large scale ‘porphyry’ Cu-Mo-Au deposits exposed in ancient, now dissected, volcanoes in magmatic arcs. These volcano-scale alteration processes also imply that high temperature (>600 °C) volcanic gases have C/S ratios that may have been increased by this process relative to their original magmatic values. The corollary is that current estimates of the total sulfur released annually from the mantle may be significantly underestimated.