In situ determination of sulfur speciation in fluids at high temperature and pressure at controlled redox conditions: implications for magma degassing and formation of magmatic ore deposits

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Sulfur-bearing gases or fluids are produced in tremendous amounts during degassing of magmas in nature, but are also released during fining of melts in industrial glass production. Since high temperature fluids are often not quenchable, central issue of the proposed project is to study such fluids in situ at high temperature and pressure using Raman spectroscopy as analytical tool. For doing so, we develop a new sapphire-based cell which allows abrupt or continuous changes of pressure at constant temperature, operating at pressures up to 200 MPa and temperatures up to 800°C. Oxygen fugacity (fO2) is a key parameter determining the stability of sulfur species in the fluid. In the experiments the redox conditions will be either pre-adjusted by the loaded fluid components or controlled by adding a specially designed buffer capsule filled with Fe/FeO, Co/CoO, Ni/NiO or other redox pairs. For comparison, the hydrothermal diamond anvil cell technique will be used to study fluids under isochoric conditions at higher pressures and densities. At conditions at which the speciation in the solution is known, these experiments are also aimed at determination of relative Raman scattering cross-sections relevant to this study, and to test the applicability of literature data. In order to facilitate interpretation of the Raman spectra, we want to start with relative simple hydrous systems containing only one sulfur component (e.g., H2SO4, H2S, MgSO4, Na2SO4, Na2S). It is planned to approach more complex magmatic fluids by using combinations of different sulfur sources or source of intermediate oxidation state (Sx, Na2S2O3, Na2SO3) and by adding silicate materials (e.g. quartz, rhyolite) as solids to the cell. The results of our studies will be useful for modeling of magma degassing and formation of magmatic ore deposits.