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 (delta S-34) of fumarolic and plume gases collected at Mount Etna volcano during 2008-2009. While low-temperature fumaroles are affected by post-magmatic processes that modify the pristine isotopic signature, high-temperature and plume gases allow establishment of a delta S-34 range of similar to 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 delta S-34 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 delta S-34 value in both the melt and gaseous phases. This modeling required assessment of the fractionation factor (alpha(gas-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 (Delta NNO >= 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 delta S-34 to 1.0 +/- 1.5%. It is remarkable that the assessed redox conditions-which are significantly more oxidizing than previously thought-are able to explain why the dominant sulfur species measured in the Etnean plume is SO2.