Redox processes governing the chemistry of volcanic gases
IPGP - Îlot Cuvier
Richard W. Henley
The Australian National University
High temperature fumaroles provide us a glimpse into the otherwise inaccessible interiors of active volcanoes. Their dominant components are H2O, CO2, SO2 and H2S and H2 together with smaller amounts of HCl and HF and nitrogen compounds. Above about 500 °C, homogenous gas reactions, such as SO2(g) + 3H2(g) = H2S(g) + 2H2O(g) , determine the equilibrium speciation of these components as gas mixtures expand from fumarolic vents but little is known about what processes determine the relative abundances of H2(g) and sulfur. For example, control of their redox state (rH= XH2/XH2O) has generally been attributed to subsurface reactions involving accessory minerals, such as magnetite, or estimated through proxy buffer reactions (e.g.Ni/NiO), but these reactions have little buffer capacity at the 100km3 scale of active volcanic systems. By contrast, it is now evident, through combining analytical data from high temperature fumarole with observations on now deeply eroded arc volcanoes (‘porphyry Cu-Mo-Au deposits’) that the abundant and widely distributed phase, anhydrite (CaSO4) controls both the redox state and the total concentration of sulfur of volcanic gas mixtures within volcanic systems. Anhydrite-forming gas-solid reactions have been shown by experiment to be both fast and reversible. This talk will explore the systematics of these chemisorption reactions, and show their overall importance in controlling the flux of sulfur through eruptive cycles, reactive mass transport of trace elements inside volcanoes, and the formation of the world’s largest mineral deposits of Cu-Mo and Au.