Rheological Behavior of Clinopyroxene and Clinopyroxene-Olivine Aggregates under Hydrous Conditions: Implications for Dynamics of the Lower Crust and Upper Mantle

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

Kohlstedt 0126277 The investigators propose a laboratory investigation of the high-temperature, high-pressure rheological properties of clinopyroxene, an important mineral in both the lower crust and the upper mantle. Because even a small amount of water dramatically weakens nominally anhydrous minerals, emphasis will be given to the contribution of water to the viscosity of aggregates of clinopyroxene and clinopyroxene + olivine. 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. Recently, however, the investigators demonstrated that the viscosity of olivine-dominated aggregates is inversely proportional to OH concentration. The proposed research builds on this observation and on recently reported experiments on the deformation behavior of dry clinopyroxenite. Water concentration will be determined by micro-FTIR analysis, and deformation-produced microstructures will be analyzed using optical and electron microscopy. In order to extrapolate from laboratory to geologic conditions, a major goal will be to determine detailed flow laws describing viscosity as a function of water concentration as well as differential stress, grain size and temperature in both the diffusion creep and the dislocation creep regime. Recent analyses of the depth of earthquakes in continents and of the correlation between crustal thickness and elastic thickness have led to the conclusion that the lower crust, at least in some places, is stronger than the uppermost mantle. This point of view is contrary to most models of continental rheology such as the jelly-sandwich construction in which a weak lower crust lies between a strong upper crust and a strong upper mantle. Many models take the strength of the continental lithosphere to lie in the mantle beneath the Moho. Certainly, the relative strength of lower crust and upper mantle will 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 as in the mantle. Our recent experiments suggest that these two minerals have similar strengths under anhydrous conditions but that clinopyroxene is weaker when water is present. A careful investigation of the dependence of viscosity on water concentration for aggregates of clinopyroxene and clinopyroxene + olivine is critical in order to address these issues and to model the dynamical behavior of the lower crust and upper mantle.