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3.6 Fluids and their interaction with silicate melts and minerals

Although H2O- and CO2-rich fluids occur only in small quantities in the Earth's crust and mantle, they are very important for transport processes, for magma generation and for the kinetics of geochemical processes. Unlike crystalline phases, fluids cannot be quenched from high pressure and temperature without major changes in structure and speciation occurring upon quenching. Therefore, many studies of fluid properties and of the interaction of fluids with other geomaterials are only possible by in-situ measurements carried out directly at high pressure and temperature. Current research on fluids at the Geoinstitute is focussed on the following topics:

- Fluid inclusions. Fluid inclusions form in natural minerals crystallizing in the presence of a free fluid. Analysis of natural fluid inclusions is therefore probably the most important tool for gaining insights into the kind of fluids present during geologic processes. The interpretation of fluid inclusion data, however, is not always straightforward, as fluid inclusions can change composition over geologic time due to reequilibration and because current methods of investigation do not yet allow complete access to all of the information stored in the fluid inclusion.

- Solubility and speciation of water in silicate melts. Silicate melts easily dissolve several weight % of water which is present in the melt both as molecular water and as OH groups. The formation of OH groups depolymerizes the melt structure and therefore greatly reduces viscosity and increases diffusivity in the melt phase. For this reason, the equilibrium between OH groups and molecular H2O is of great importance for understanding the physical and also the chemical properties of hydrous magmas. The total amount of water that can be dissolved in silicate melts essentially determines the solidus temperature of a rock and thereby largely controls the amount of melt present in the crust and mantle.

- Solubility of water in nominally anhydrous minerals. The Earth´s mantle consists mainly of nominally anhydrous minerals which do not contain any hydroxyl groups in their chemical formula. However, all of these minerals can contain appreciable concentrations of OH defects which are likely to have a major influence on such properties as electrical conductivity and mechanical strength. Moreover, the amounts of water that can be dissolved in nominally anhydrous minerals are comparable to or larger than the average water content in the upper mantle. Therefore, the solubility of water in these minerals essentially controls the stability of a free fluid phase in the interior of the Earth.

Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
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