The Role of Halogens, Sulfur, H2O, and CO2 in Volcanic and Plutonic Systems

International audienceIt has long been recognized that melt inclusions (MIs) have the potential for providing insight into how magmas and their volatiles evolve during cooling and crystallization. We analysed H2O, CO2, S and Cl contents of olivine-hosted MIs in several eruptive products of Mt. Etna (Italy), together with their major element composition, and iron speciation (Fe3+/ƩFe). Using these data, we investigated fractional crystallization, degassing and redox conditions variations occurring during magmatic ascent.MIs have composition extending from volatile-rich picro-basaltic toward volatile-poor evolved trachybasalt. The picro-basaltic MIs are extremely enriched in H2O and CO2 (up to 5.9 and 0.58 wt.% respectively). S content in Etnean MIs ranging from some ppm up to more than 4000 ppm, whereas the oxygen fugacity (fO2, estimated using XANES Fe3+/ƩFe ratios and oxybarometers models) ranges from NNO-0.9±0.1 to NNO+1.6±0.2, with NNO being the Nickel-Nickel Oxide buffer [1].Based on (i) S concentration and (ii) oxygen fugacity estimates in MIs, and using an empirical model predicting the variation of S partitioning between fluid and melt phases (DSfluid/melt) upon variations in P, T and fO2, our research suggests that during magmatic ascent in the Etnean system a fO2 decrease occurs, affecting the S behaviour. The empirical DSfluid/melt model coupled to MELTS code [2, 3] provides evidence of fO2 decrease during volatile exsolution and crystal fractionation of olivine, clinopyroxene, spinel and plagioclase, upon T and P decrease. The fO2 diminution is responsible for the increase in DSfluid/melt, and thus the decrease of S solubility in the melt and, consequently, for the increasing exsolution of S (which further enhances the reduction of fO2) from the magma before eruption. References[1] Gennaro, Paonita, Iacono-Marziano, Moussallam, Pichavant, Peters & Martel (2020), Journal of Petrology, 61, 10. [2] Ghiorso & Sack (1995), Contribution to Mineralogy and Petrology, 119, 197-212. [3] Smith & Asimow (2005), Geochemistry Geophysics Geosystems 6, Q02004