Water contents in nominally anhydrous minerals in oceanic peridotite

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Olivine, ortho- and clinopyroxene are the main constituents of the Earth`s upper mantle. Although these minerals are nominally anhydrous, they can host traces of water, predominantly as OH defects, with concentrations of hundreds to thousands ppm H2O. These amounts of hydrous components have a disproportionately large effect on the physical and chemical properties of minerals and rocks. They enhance deformation, increase ionic diffusion and electrical conductivity in the asthenosphere and also may influence the melting behaviour. So far, predominantly xenoliths were used in order to measure the water contents in mantle minerals. No data are available für the suboceanic mantle. However, the knowledge of water contents is very important für implications on mantle rheology, convection and plate tectonics. The focus of this project, therefore, is to determine the water concentrations in oceanic peridotite. The unique samples taken at the Mid-Atlantik ridge (ODP legs 153 and 209) shall be investigated in this context. These peridotites are well suited for this purpose: (¡) They preserve relics of olivine and pyroxenes of sufficient size for measurements. (¡¡) They reveal different fabrics, from undeformed to strongly sheared, which is very fortunate in order to assess to weakening effect of water on mineral deformation.