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including H2O and S, are known. The model is calibrated against three main experimental data bases consisting
in both dry and hydrous silicate melts. Its prime goal is to calculate the f S2 of hydrous basalts that currently lack
experimental constraints of their sulphur solubility behaviour. Application of the model to Stromboli, Vesuvius, Vulcano
and Etna eruptive products shows that the primitive magmas found at these volcanoes record f S2 in the range
0.1-1 bar. In contrast, at all volcanoes the magmatic evolution is marked by dramatic variations in f S2 that spreads
over up to 9 orders of magnitude. The f S2 can either increase during differentiation or decrease during decompression
to shallow reservoirs, and seems to be related to closed versus open conduit conditions, respectively. The calculated
f S2 shows that the Italian magmas are undersaturated in a FeS melt, except during closed conduit conditions,
in which case differentiation may eventually reach conditions of sulphide melt saturation. The knowledge of f S2, fO2
and fH2O allows us to calculate the fluid phase composition coexisting with magmas at depth in the C-O-H-S system.
Calculated fluids show a wide range in composition, with CO2 mole fractions of up to 0.97. Except at shallow
levels, the fluid phase is generally dominated by CO2 and H2O species, the mole fractions of SO2 and H2S rarely exceeding
0.05 each. The comparison between calculated fluid compositions and volcanic gases shows that such an
approach should provide constraints on both the depth and mode of degassing, as well as on the amount of free fluid
in magma reservoirs. Under the assumption of a single step separation of the gas phase in a closed-system condition,
the application to Stromboli and Etna suggests that the main reservoirs feeding the eruptions and persistent volcanic
plumes at these volcanoes might contain as much as 5 wt% of a free fluid phase. Consideration of the magma
budget needed to balance the amounts of volatiles emitted in the light of these results shows that the amount of nonerupted
magma could be overestimated by as much as one order of magnitude.
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