Sulfur isotopic compositions of fumarolic and plume gases at Mount Etna (Italy) and inferences on their magmatic source

Here we report new data on the sulfur isotopic compositions (δ34S) of fumarolic and plume gases collected at Mount Etna volcano during 2008-2009. While low-temperature fumaroles are affected by postmagmatic processes that modify the pristine isotopic signature, high-temperature and plume gases allow establishment of a δ34S range of ∼0±1‰ for magmatic SO2. We compared our data with those from S dissolved in primitive melt inclusions from 2002 lava and in whole rocks that erupted during the past two thousand years. Such a comparison revealed that δ34S is systematically lower for magmatic gases than for sulfur dissolved in the melt. We modeled how isotopic fractionation due to magma degassing process may vary δ34S value in both the melt and gaseous phases. This modeling required assessment of the fractionation factor (αgas-melt). The most recent measurements on the oxidation state of sulfur in basaltic melt inclusions indicate that nearly all S is dissolved as sulfate (S6+), which would be possible in oxidized magmatic systems (ΔNNO≥1). Under these conditions the exsolved gaseous phase is depleted with respect to the melt and the proposed model fits both gas and melt data, and constrains the Etnean magmatic δ34S to 1.0±1.5‰. It is remarkable that the assessed redox conditionswhich are significantly more oxidizing than previously thoughtare able to explain why the dominant sulfur species measured in the Etnean plume is SO2.