Experimental Investigation of the Anisotropic Viscosity of Deformed Dunites with a Pre-existing Crystallographic Preferred Orientation

Cameron Meyers, University of Minnesota

Solid state deformation of Earth’s mantle rocks allows cooling of the planet’s interior through convection, driving plate tectonics. Therefore, the mechanical properties of mantle rocks are key to understanding tectonic phenomena such as earthquake generation and magma production. Under high temperatures and pressures found in Earth’s interior, viscous flow of rocks causes the constitutive mineral grains to rotate and align to form a crystallographic preferred orientation (CPO) that imparts anisotropic material properties to the bulk rock. Most models of mantle flow do not account for the anisotropic viscosity resulting from CPO development, which may result in large modeling errors. We performed high-pressure, high-temperature deformation experiments on mantle materials to quantify the magnitude of viscous anisotropy in the Earth’s mantle.

Naturally deformed mantle rocks with a preexisting CPO were experimentally deformed in axial compression. Oriented dunite cores (rock composed predominantly of olivine, the primary constituent of the upper mantle) collected from a high-strain shear zone in the Josephine Peridotite were dehydrated in a controlled CO/CO2 atmosphere at 1,200 C. These were subsequently deformed in a high-resolution gas-medium apparatus under constant strain-rate conditions in three strain-rate steps at 1,250 C under 300 MPa confining pressure to a maximum strain of 12 percent. Stress-strain rate data were used to fit a power-law, which showed a stress exponent of n≈3.5 for all rock orientations, indicating deformation by dislocation creep. In addition, grain orientation maps were collected on deformed samples using electron backscatter diffraction (EBSD) to quantify the CPO. We found that flow law parameters scale with the mean Schmid Factor (and therefore the resolved shear stress) on known dislocation slip systems. The mechanical behavior observed for oriented polycrystalline samples compares well with estimates based on effective medium models calculated from published single-crystal flow laws for olivine.

Abstract Author(s): C.D. Meyers, D.L. Kohlstedt