Abstract: International audience ; Understanding the behavior of volatile elements at magmatic conditions is crucial to constrain the volcanic degassing phenomenon and its role during early planet formation and ore deposits genesis. Amongst other parameters it is critical to determine the speciation and partitioning of volatiles (C, O, H, S, Cl, N…) as a function of temperature, pressure, melt composition and oxygen fugacity. However, most experimental studies on the subject have been performed ex situ with part of the information on the composition and properties of the high P-T fluids lost upon quench. Speciation for instance cannot be determined using these methods. Here we will present an in situ apparatus that allows the direct measurement of volatilespeciation and fluid-melt partitioning to magmatic conditions (up to 1000°C and 1500 bars) in a volcanic degassing context using Raman spectroscopy. This setup consists of an autoclave originally designed for X-Ray absorption that has been adapted to meet the requirements of Raman spectroscopy. To show the efficiency of the setup, examples will be presented on the speciation of sulfur and selenium in supercritical conditions as a function of oxygen fugacity.
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