Deformation Microstructures of Olivines from Deep Mantle Xenoliths: The Roles of Water and Dynamic Recrystallization

The presence and distributional heterogeneity of water in the upper mantle is critically important to our study of plate dynamics and continental evolution. Recent experimental results have shown that there are several discrete lattice-preferred-orientation (LPO) patterns in olivine, which vary as a function of water content and stress. These experimental data are part of our large laboratory dataset that considers the relationships between deformation mechanisms and water content of deformed, synthetic, olivine aggregates. To better understand the relationship between water content and deformation fabrics in natural rocks, we conducted comprehensive investigations of four sheared-lherzolite xenoliths from 5.5-5.7 GPa. The xenoliths in this study were collected from the Jagersfontein kimberlite in South Africa (kindly supplied by F.R. Boyd). Compositions were analyzed using EPMA, and depths were constrained using pyroxene thermobarometry (by F.R. Boyd). There is no obvious foliation/lineation pattern in any of the samples, so strain was defined by the shape-preferred-orientation of recrystallized olivines; therefore, the recorded strain reflects the orientations of recrystallized olivine grains, not the orientations of any visible macroscopic deformation textures. The finite strain for each sample was determined using an in-house digitization and software package that calculates a finite strain ellipsoid (FSE) from the elliptical projections of olivine grains in several thin-sections. Olivine LPO was measured using a TSL electron backscatter diffraction (EBSD) system on a Philips XL30 SEM. Dislocations were illuminated using the oxidation decoration technique. Water content was measured using Fourier-transform infrared (FTIR) spectroscopy. Although the strength and form of the LPO patterns vary, all samples show characteristically "dry" fabrics. The low water content is inferred to reflect the chemical environment of the rock immediately prior to its rapid deformation and exhumation. The absence of any "wet" fabrics determine an upper limit of ~400 ppm H/Si to the amount of water contained in this region of the upper mantle. FTIR analyses show only small absorption peaks from OH vibrations, suggesting that some water may have been lost during exhumation. Optical observation shows that the samples have experienced nearly complete dynamic recrystallization. There is a strong correlation between the LPO fabrics and the microscopic strain recorded by the FSE of the recrystalized grains. This suggests that the orientation of seismic anisotropy may deviate from the geometry defined by the macroscopic flow plane.