Continued Investigation of Rheological Behavior of Clinopyroxene and Clinopyroxene-Olivine Aggregates Under Hydrous Conditions: Implications for Crustal and Mantle Dynamics

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++This proposal involves an experimental investigation of the high-temperature, high-pressure strength of aggregates of clinopyroxene, an important mineral in both the lower crust and the upper mantle, and of aggregates of clinopyroxene plus olivine. Recent analyses of earthquake depth distributions and gravity anomalies on continents conclude that the lower crust is, in general, stronger than the uppermost mantle. This point of view is contrary to the jelly-sandwich model of the continental lithosphere in which a weak lower crust lies between a strong upper crust and a strong upper mantle such that the strength of the continental lithosphere lies in the mantle rather than in the crust. This research projects explores the proposition that the relative strengths of lower crust and upper mantle depend on the presence or absence of water. Little is known about the strength of clinopyroxene relative to that of olivine, even when the two phases are present in the same rock. Recent experiments suggest that clinopyroxene is stronger than olivine under anhydrous conditions but weaker than olivine in hydrous environments. A careful investigation of the water concentration dependence of the viscosity of clinopyroxene and clinopyroxene + olivine rocks is critical in order to address these issues and to model the geodynamical and geochemical behavior and evolution of the lower crust and upper mantle. The effect of water on the viscosity of rocks composed of clinopyroxene and clinopyroxene + olivine is emphasized, because even a small amount of water dramatically weakens nominally anhydrous minerals. In most studies, the water-weakening effect has been treated as an 'on-off' process - if water is present, rocks are weak; if water is absent, rocks are strong. Recent experiments demonstrate that the viscosity of olivine-rich rocks is a factor of more than 10 smaller in water-rich environments (e.g., the mantle wedge above a subducting slab) than in water-under saturated settings (e.g., mantle beneath a mid-ocean ridge). The proposed research builds on this observation, on published investigations of the deformation behavior of dry clinopyroxene-rich rocks, and on our initial results that reveal a significantly larger water-weakening effect in clinopyroxene than in olivine. To apply laboratory results to processes occurring at depth in the Earth, detailed flow laws are essential in order to describe strain rate (i.e., viscosity) as a function of water concentration as well as differential stress, grain size, temperature, and pressure in both the diffusion and the dislocation creep regimes. Water concentrations are determined by micro-infrared analyses, microstructures produced by deformation are analyzed using optical and electron microscopy, and fabrics are measured using electron backscattered diffraction.