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Magma degassing processes are commonly elucidated by studies of melt inclusions in erupted phenocrysts and measurements of gas discharge at volcanic vents, allied to experimentally constrained models of volatile solubility. Here we develop an alternative experimental approach aimed at directly simulating decompression-driven, closed-system degassing of basaltic magma in equilibrium with an H^C^O^S^Cl fluid under oxidized conditions (f O2 of 1•0^2•4 log units above the Ni^NiO buffer). Synthetic experimental starting materials were based on basaltic magmas erupted at the persistently degassing volcanoes of Stromboli (Italy) and Masaya (Nicaragua) with an initial volatile inventory matched to the most undegassed melt inclusions from each volcano. Experiments were run at 25^400 MPa under super-liquidus conditions (11508C). Run product glasses and starting materials were analysed by electron microprobe, secondary ion mass spectrometry, Fourier transform infrared spectroscopy, Karl-Fischer titration, Fe 2þ /Fe 3þ colorimetry and CS analyser. The composition of the exsolved vapour in each run was determined by mass balance. Our results show that H 2 O/ CO 2 ratios increase systematically with decreasing pressure, whereas CO 2 /S ratios attain a maximum at pressures of 100^300 MPa. S is preferentially released over Cl at low pressures, leading to a sharp increase in vapour S/Cl ratios and a sharp drop in the S/Cl ratios of glasses. This accords with published measurements of volatile concentrations in melt inclusion and groundmass glasses at Stromboli (and Etna). Experiments with different S abundances show that the H 2 O and CO 2 contents of the melt at fluid saturation are not affected. The CO 2 solubility in experiments using both sets of starting materials is well matched to calculated solubilities using published models. Models consistently overestimate H 2 O solubilities for the Stromboli-like composition, leading to calculated vapour compositions that are more CO 2-rich and calculated degassing trajectories that are more strongly curved than observed in experiments. The difference is less acute for the Masaya-like composition, emphasizing the important compositional dependence of solubility and meltv apour partitioning. Our novel experimental method can be readily extended to other bulk compositions.

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