The Development Of Olivine Textures In Complex Deformation Geometries

Abstract

The analysis of crystallographic preferred orientations (CPOs) in olivine is a crucial tool in our understanding of the Earth’s upper mantle. The development of CPOs is controlled by the activation of slip systems, which are sensitive to various thermochemomechanical conditions such as stress, temperature, and water content. Laboratory experiments have offered insight on the conditions in which different olivine CPOs develop. However, the relationship between these thermochemomechanical conditions and CPOs is complicated. Recent studies have challenged some of our current understanding on olivine CPO development and have brought attention to the importance of kinematics. Here, we aim to experimentally investigate the role of kinematics in the development of olivine CPOs, particularly pertaining to complex deformation geometries involving simultaneous pure and simple shear. To explore this topic, cylindrical samples of dry, polycrystalline San Carlos olivine were deformed in either simultaneous extension and torsion or simultaneous shortening and torsion at a temperature of 1523 K and confining pressure of 300 MPa in a Paterson gas-medium apparatus at the University of Minnesota. Following deformation, the CPO was measured at different sections along the sample radius using electron backscatter diffraction analysis. For each section, the kinematic vorticity number, the equivalent strain, the J-index, and the M-index were determined. The fabric index angle was also calculated to classify the resulting CPO for each section. In total, two samples were deformed in extension and torsion and three samples in shortening and torsion. Of the two extension and torsion experiments, one sample localized thus allowing for multiple sections along the length to be analyzed. Moreover, the samples subjected to extension and torsion went out to axial strains of 0.15, 0.26, and 0.36 and to shear strains between 1.5 and 2.2. These samples generally produced stronger textures and displayed an evolution from C-type to E-type to D-type CPO with increasing kinematic vorticity. The presence of an E-type CPO aligns with numerical simulations of CPO development in simultaneous extension and simple shear geometries and suggests that olivine does not necessarily develop as a function of water content. Conversely, the samples deformed in shortening and torsion went to axial strains of -0.12, -0.28, and -0.35 and shear strains between 1.0 and 1.7. These samples had weaker textures and typically evolved from an AG-type to an A-type CPO with increasing kinematic vorticity, which also reasonably aligns with numerical simulations of this given geometry. Although different textural evolutions were observed between these two deformation geometries, the fabric index angles began to converge as kinematic vorticity increased for both sets of experiments, ultimately resulting in textures expected in simple shear. The geometries achieved in this study were then simulated using a modified director textural model. The simulations produced from this model reasonably reproduced the CPO characteristics observed in the experimental samples, but the CPO classification, as determined by the fabric index angle, did not align as well. All in all, the findings presented in this study highlight the role of kinematics in CPO development, emphasize the caution needed when interpreting the thermochemomechanical conditions of the mantle from seismic anisotropy and exhumed peridotites, and contribute valuable information on how we can use olivine CPOs as a field tool.