Chlorine in amphibole: intracrystalline diffusion and partitioning between amphibole and silicate melt

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Chlorine (Cl) is an abundant volatile element in the interior and outer layers of the Earth and other planets (e.g. Mars), which plays important roles in a variety of geological and environmental processes within a wide range of temperature and pressure. In hydrothermal systems, Cl concentrations can be traced using fluid inclusions. However, as a volatile element, Cl is usually partly released (with fluids) from magmatic systems and there are only little phases which can be used as tracer for Cl concentrations in natural magmatic systems. Amphibole is a common halogen-hosting mineral which exists in a large range of rock systems and of geological conditions, and which can serve as an important tracer of Cl evolution. However, because of the complex structure and chemistry of amphiboles and of the large compositional variation of melt which can coexist with amphibole, the partitioning of Cl between amphibole and melt is still poorly understood. Two key issues will be addressed in this proposal with series of experiments aimed at constraining (1) the partitioning of Cl between amphibole and silicate melt (KCl), and (2) the diffusion rate of Cl in amphibole (DCl). The partitioning of Cl between amphibole and silicate melt will be investigated in a large pressure and temperature range within the stability of amphibole (100-700 MPa and 750-1000 °C) for basaltic to rhyodacitic systems. Preliminary studies show that the bulk composition of natural systems may influence strongly the partitioning of Cl. Using 12 different starting materials, we plan to constrain the main compositional parameters controlling the partitioning of Cl (crystal chemistry of amphibole and melt composition). The experiments on Cl partitioning will be complemented by challenging experiments aimed at understanding the diffusivity of this element in amphibole. Large amphibole crystals will be used as starting materials and the experiments will be designed to achieve Cl/OH exchange between amphibole and melt without other cation exchange. The results of this study are of particular interest for volcanic systems, but have the potential to be applied to magmatic processes on Earth and other planets. The partitioning of Cl between amphibole and melt is crucial to estimate accurately Cl contents in magmas based on amphibole composition and to interpret quantitatively processes occurring in magmatic systems. Chlorine diffusivity in amphibole is crucially needed to extract time scales of geological processes in magmatic systems using Cl concentration profiles in amphiboles.