The dissolution behavior of volatile components in magmas is essential to model the volcanic process from the deep regions of magma generation and storage to the shallow regions of magma eruption and emplacement. Water, carbon dioxide, and sulfur compounds are the main volatile components in natural magmas, constituting in most cases more than 99% of the volcanic gases released before, during, and after eruption.We have developed a method to calculate the chemical equilibrium between a fluid phase in the C-O-H-S system and a silicate melt with composition defined by ten major oxides. The method is based on a previous model for the saturation of H2O-CO2 fluids  and on a sulfur solubility model  in silicate liquids. For the computation of the fugacities of components in fluids with complex composition we used the SUPERFLUID code . The model allows determining the partition of H2O, CO2, and S between the silicate liquid and the coexisting fluid, and the composition of the fluid phase in terms of H2O, CO2, SO2, and H2S, as a function of pressure, temperature, volatile-free liquid composition, oxygen fugacity, and total amount of each volatile component in the system. App lications are presented to several silicate liquids with rhyolitic and basaltic composition, oxygen fugacities in the range NNO ± 2, and pressure from a few hundred MPa to atmospheric, with the simplifying assumption that no reduced or oxidized sulfur-saturated solid or liquid phases nucleate or separate from the liquid-gas system. Results show the well-known minima in sulfur saturation contents as a function of oxygen fugacity, the reciprocal effects of volatiles on their saturation contents, and the complex relationships between saturation surface of a multicomponent fluid, liquid composition, volatile abundance, P-T conditions, and oxidation state. The method represents therefore a new powerful tool for the prediction of multicomponent gas-melt equilibria in magmas. REFERENCES  Papale P. (1999) Am. Mineral., 84, 477-492  Moretti R. and Ottonello G. This issue  Belonoshko A.B., Shi P. and Saxena S,K, (1992) Comp. Geosci., 18, 1267-1269. 1
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