ON THE EVOLUTION OF VOLATILES AND OXIDATION STATE IN MULTI-COMPONENT DEGASSING MAGMAS

The study of volcanic degassing requires the definition of gas-melt equilibria in order to relate observed gas compositions to magma PTX conditions. This knowledge, unless prevented by scrubbing of magmatic gases by hydrothermal systems at shallower levels, rests on the constitution of saturation models of the fluid phase in melts, which should also account for the evolution of both volatiles and oxidation state in a multicomponent degassing magma. In the geochemical literature of fluid systems, the oxidation state is often carefully evaluated in terms of ratio of moles of species of altervalent elements, such as CO/CO2, H2/H2O and H2S/SO2 with oxygen fugacity as by-product. On the other hand, in the petrological literature, the magmatic oxidation state is often given as fO2, usually expressed as difference in log-units to a solid buffer assemblage, such as QFM or NNO. Many petrological calculations then assume oxygen fugacity as the parameter controlling the oxidation state of magmas, implying that redox couples furnished by abundant altervalent elements such as iron and sulfur are governed by the fugacity of molecular oxygen, a quantity which is actually undetectable. The composition of gas released in the C-H-O-S-silicate melt system is obviously influenced by this assumption of constant oxygen fugacity along the PTX pathways of system evolution. Also the interpretation of surface data, such as those resulting from remote-sensing of plume emissions or in-situ sampling of exhaling vents, may be affected by this assumption when recasting petrological information on the feeding magmatic system. In this study we show that application of FeO/Fe2O3 and H2S/SO2 buffers results in PTX paths of gas exsolution which differ sensibly from those obtained at constant oxygen fugacity. These complex pathways of gas release and oxidation state evolution cannot be simply calculated or extrapolated, but must be carefully modeled, being strongly related to the non-linearity intrinsic in calculations whenever confident solubility models are joined in a unique saturation algorithm. The study is based on the model of Moretti et al. (2003), implemented for the calculation of the ferric to ferrous iron ratio under hydrous conditions in the framework of an ionic-polymeric approach which resolves the controversies of literature about the oxidation state of hydrated magmas. REFERENCES: Moretti et al. (2003) in Volcanic degassing (Oppenheimer et al., eds.), Geol. Soc. London Spec. Publ., 213, 81