An Experimental Study of CPO Development and Anisotropic Creep of Polycrystalline Olivine

Abstract:

Deformation of olivine in the Earth’s upper mantle causes grains to rotate and align, producing a crystallographic preferred orientation (CPO) that imparts anisotropic physical properties to large regions of the mantle. We performed deformation experiments aimed at exploring the geometry and rate of CPO development in polycrystalline olivine aggregates and the influence of CPO on the anisotropic creep response. We deformed hot-pressed San Carlos olivine aggregates in a high-resolution gas-medium apparatus at temperatures of 1200° and 1250°C and a confining pressure of 300 MPa under dry conditions in a variety of deformation geometries, including axial compression, axial extension, shear on angled pistons, and torsion. Several specimens were deformed in multiple deformation geometries, varying the three-dimensional stress state as well as the CPO strength and geometry. Specimen microstructures were characterized using electron backscatter diffraction (EBSD) before and after each deformation experiment. EBSD maps were used to measure CPO, grain size and shape, and crystallographic misorientations between grains. By varying the deformation geometry and CPO, we are adding constraints to a model that will ultimately predict the high-temperature anisotropic creep response of dry olivine aggregates along complex three-dimensional strain paths. Although the upper mantle is typically modeled as isotropic polycrystalline olivine, numerical models and first-order calculations show that anisotropic creep strength could significantly modify the dynamics of a variety of geological processes, including the shape and internal flow structure of convection cells, the shape and timing of Rayleigh-Taylor instabilities, the rate of post-glacial rebound, and the thermal structure and melt production rate in the mantle wedge of subduction zones.