Comptes Rendus. Géoscience (Jul 2022)
Redox behavior of degassing magmas: critical review and comparison of glass-based oxybarometers with application to Etna volcano
Abstract
Glass/melt inclusions are repositories of polybaric magma physiochemical interactions. They are fundamental to retrieve the conditions of fluid saturation and magma oxidation state. The latter is usually reported as oxygen fugacity, which is a key thermodynamic variable: involved in such polybaric physiochemical interactions, $f\mathrm{O}_{2}$ affects melt saturation properties and fixes the speciation of fluids discharged by magmatic systems either up to volcanic craters or feeding hydrothermal systems. Several oxybarometers have been proposed, based on iron and sulfur oxidation states. In this study I summarize their main features and show their performances for 1-bar and fluid-saturated andesitic and basaltic melts. I then show that any assessment of magma redox behavior that does not account for the melt reactivity, driven by melt polymerization and the so-called oxygen electrode in association with iron or sulfur redox couples, may fail in determining the actual contribution of the composition to the observed variations of iron and sulfur oxidation states, resulting in potentially biased $f\mathrm{O}_{2}$ estimates. In the case of Etna volcano, by using a well-established ionic-polymeric approach we show that a major control is the dehydration taking place during magmatic differentiation and magma ascent. Dehydration determines a slight iron oxidation down to around 2.5 wt% in dissolved water, but further dehydration taking place up to the surface results in iron reduction. This dual behavior, related to a change of trivalent iron speciation, is reproduced by reactive ionic-polymeric approaches and for the Etna basalt it is predicted also for constant $f\mathrm{O}_{2}$. Such a modulation of the redox state due to water dissolution cannot be reproduced by empirical formulations.
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