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Experiments were performed to determine the sulfur solubilities of hydrous basalts from Vesuvius, Etna and Stromboli (Italy). The melts were equilibrated at 1050 and 1200 °C with H 2 O and sulfur (added as pyrrhotite), and at pressures ranging from 250 to 2000 bar. Most experiments were performed under oxidising conditions (NNO + 2), and a few under reducing conditions (NNO − 1), with melt water contents of 0.5–3.5 wt.%. Sulfur contents in glasses were determined by electron microprobe and range from 860 up to 6700 ppm. No compositional effect is found between the three alkali basaltic melts. The fugacities of S-bearing species were derived using an MRK equation of state applied to an O–H–S fluid, knowing H 2 and H 2 O fugacities, and range from 50 up to 3000 bar. A thermodynamic species-based model is derived from our results along with available data in the literature, assuming that sulfur dissolution results from the additive contributions of both H 2 S and SO 2 dissolution reactions. Compared to similar models developed for silicic melts, basalt compositions requires the incorporation of an Fe term, which accounts for the strong association between Fe and S in silicate melts, and considers the elevated Fe content of mafic melts. The model shows that, at any fixed f S 2 , the sulfur solubility in hydrous basalt displays a pronounced minimum around NNO, the position of which depends on temperature. The minimum in sulfur solubility coincides with the redox range were the abundance of S 2 in the fluid reaches its maximum compared to either H 2 S or SO 2 species. Such a minimum in solubility is in agreement with experimental constraints at 1 bar under carefully controlled f O 2 and f S 2 . Calculated proportions of dissolved species in the melt depend on the prevailing f S 2 and f O 2 , being in general agreement with available spectroscopic models. Calculations of gas saturation pressures, which classically consider only H 2 O and CO 2 dissolved volatiles, are strongly affected by S-bearing species. At f O 2 close to, or higher than, NNO + 1, omission of sulfur species may result in underestimates of gas saturation pressures of 1 kbar or more. The same happens at f O 2 below NNO − 1.

International audience; Deformation experiments involving hydrous magmas of different compositions (basalt andhaplotonalite) have been performed in a Paterson press at 300 MPa, in the temperature range600°C-1020°C, with water-saturated melts, during 2-4 hours. Prior to deformation the twoend-member magmas were annealed at either 950°C or 1000°C, yielding magmas with crystalcontents in the range 31-53 wt% and 2 sets of viscosity contrasts. Under the experimentalconditions investigated (i.e. moderate shear rates 950°C. In the temperature range 950-985°C a few mixing and minglingtextures occur, though both end-members essentially retain their physical integrity. It is onlyat, or above, 1000°C that a dramatic jump in mingling efficiency happens, corresponding to acrystal fraction of 45 vol%. Textures include entrainment of mafic crystals into the felsicmagma, mafic-felsic banding, enclave formation, diffusion-induced interface, the latter only over limited distances (< 300 microns) due to the short run durations. In the most strainedparcels of interacting magmas, complex mixing/mingling textures were produced, similar tothose observed in volcanic and plutonic rocks in arc settings. The experiments show thatmixing between hydrous felsic and mafic magmas takes place at around 1000°C, atemperature which is almost 200°C lower than mixing under dry conditions. Magma mixingis commonly invoked as a trigger for volcanic eruptions; our experiments suggest that sucheruptions can be driven by small (~15°C) temperature fluctuation in the reservoir. Our resultsalso suggest that slow replenishment of a felsic reservoir by mafic inputs will likely result instratification between end-members rather than in a homogeneous mixture.

Ion probe measurements offer the advantage of in situ determination of water content with a few micrometres spatial resolution. After a brief review of ion microprobe procedures used for the determination of water concentration in silicates, we examine three important effects on analytical precision: (1) the effect of energy filtering on the background; (2) the variation of the H/Si ratio with time; and (3) the dependence of the H/Si ratio on the composition of the matrix, which requires the use of standards close in chemical composition to the unknowns. Finally, applications of this method to the determination of water in glasses quenched from water-saturated melts, to experimental products consisting of a mixture of glass and crystals, and to natural melt inclusions illustrate the advantages and the limitations of the ion microprobe.