Partitioning of sulfur and sulfur isotopes between fluids and melts - a key for understanding and monitoring of magmatic degassing

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Understanding of degassing of magmas is central to monitoring and forecasting of volcanic eruptions. Sulfur is a major volatile released from magmas after decompression and the improved understanding of sulfur isotope geochemistry is vital to volcanic hazard mitigation, because sulfur partitions strongly into the vapor in oxidized melt-vapor systems. The determi2 nation of the sulfur isotope geochemistry in gases from arc magmas is extremely useful to constrain sulfur fluxes in the crust and the underlying mantle wedge. However, this requires a good knowledge (1) of the sulfur isotope-characterization of primitive melts, (2) of the behavior of sulfur during decompression of magmas as well as during magma-fluid interactions and (3) of the distribution of sulfur isotopes between phases (minerals, melts, fluids). In the proposed research we want to investigate experimentally the distribution of sulfur and of sulfur isotopes between liquids and fluids both under near-equilibrium conditions (long term experiments) and under kinetic control (rapid decompression, rapid cooling). Base compositions for this pioneering study will be in the compositional range from dacite to andesitic basalt which have relevance for arc volcanism and can dissolve sufficient sulfur for reliable quantitative analyses. The experimental products will be analyzed with various analytical techniques including SIMS and Laser Ablation-ICPMS. One aim of the project will be to improve these techniques for measurement of sulfur isotopes at low sulfur concentrations in glasses. The experimental results will be used to interpret glass inclusion data which will be mainly obtained by our cooperation partner Charles Mandeville (American Museum of Natural History (AMNH), New York). Combination of the data will be used to develop degassing scenarios for selected volcanoes.