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
Final Report Abstract
A new spectroscopic cell was developed to study hydrothermal and magmatic fluids at temperatures up to 750 °C and pressures up to 200 MPa. Pressure and temperature can be independently varied and thus, enables simulation of decompression of fluids during ascent in the Earth’s crust. The cell has sufficient space inside to add a small capsule to study interaction of fluids with solid materials. This assembly may be used to control sulfur and/or oxygen fugacity in the fluid. However, mass transfer between the capsule and duration of the high T experiments is limited so that equilibrium conditions can be adjusted only for specific conditions. Combining the cell with Raman spectroscopy is a powerful method to study homogeneous reactions in fluids with high temporal resolution, depending mainly on the available Raman spectrometer. Measurements on 3 molar ammonium sulfate solutions demonstrate the importance of fluid density for stabilization of ionic species. Ammonium was found to have high thermal stability under conditions of the Earth’s crust. Even at temperatures of 700 °C in presence of an oxidant, the lifetime of NH3 is at least several hours. The experiments performed in our studies have also implications concerning the evolution of sulfur species during degassing of magmas and subsequent ascent of fluids to the Earth’s surface. Formation of SO2 in sulfate-bearing fluids, as observed in a study of Keppler (2012), seems to depend strongly on the prevailing redox conditions in the system. In presence of strongly reducing agents (such as ammonium) sulfide species will be formed while at moderate redox conditions, SO2 is the reaction product. Confining pressure will also play a crucial role for the reaction.
Publications
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(2018) A new optical cell for in situ Raman spectroscopy, and its application to study sulfur-bearing fluids at elevated pressures and temperatures. American Mineralogist 103 (3) 418–429
Dietrich, Marcel; Behrens, Harald; Wilke, Max
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(2013): Towards in situ determination of sulfur speciation in fluids at high P-T at controlled redox conditions, 23rd Goldschmidt Conference, Florence, Italy
M. Dietrich, H. Behrens, C. Schmidt, M. Wilke
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(2015): In situ determination of sulfur speciation in fluids at high P-T and controlled redox conditions, GeoBerlin, Annual Meeting of DGGV and DMG, Berlin, Germany
M. Dietrich, H. Behrens, C. Schmidt, M. Wilke
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Raman spectroscopic quantification of sulfur species in aqueous fluids:Ratios of relative molar scattering factors of Raman bands of H2S, HS−, SO2, HSO4−, SO42−, S2O32−, S3− and H2O at ambient conditions and information on changes with pressure and temperature. Chem. Geol. 467 (2017) 64-75
C. Schmidt, T.M. Seward
