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Fluids affect many processes that occur in the Earth's interior including partial melting, magmatic and volcanic processes, rock and mineral deformation and transformation kinetics. In addition, fluids have important effects on phase equilibria and the physical and chemical properties of Earth materials. In recent years it has been shown that large amounts of water can be stored in the Earth's interior because the solubility of H₂O in minerals of the Earth's mantle that are nominally water-free (anhydrous) is often significant. This means that water cycled from the oceans and atmosphere into the mantle by subduction can reside in the interior for long periods of time. A quantification of the water cycle is not only important for understanding the water budget of the Earth but is also critical for our knowledge of the physical and chemical properties of the Earth's mantle because these are greatly affected by water dissolved in the constituent minerals.

Research at the Bayerisches Geoinstitut is concerned with many of these issues. Studies reported here are concerned not only with the solubilities of H₂O in mantle minerals but also with the details of exactly how hydrogen is incorporated into the respective crystal structures. As well as studying water-bearing minerals using X-ray diffraction and spectroscopic techniques, investigating the kinetics of hydration and dehydration of minerals can also provide important information about the crystal defects involved in hydrogen incorporation. The role of water in magmatic and volcanic processes is also under investigation. Recent research at the Bayerisches Geoinstitut has shown that aqueous fluids and silicate melts evolve to a single supercritical fluid at high pressures and temperatures – an observation that has many consequences for our understanding of magmatic processes in the Earth. Continuing work reported here shows that such supercritical behaviour is likely to be important during the formation of pegmatites – coarse-grained granitic intrusions that are a major source of economic ore deposits. Understanding how and when water exsolves from magma as bubbles during volcanic eruptions will greatly aid in predicting the occurrence and nature of such eruptions. Further studies have investigated the chemical properties of fluids at high pressures and temperatures which will lead to a better thermodynamic characterisation of fluid-bearing systems. The chemical species present in aqueous carbonate-bearing fluids at high pressures and temperatures, have been studied in-situ using Raman spectroscopy on samples contained in the diamond anvil cell. Finally, results are presented on the partitioning of elements between fluids and melts obtained using a novel technique involving the analysis of synthetic fluid inclusions contained in quenched silicate melt.