Axial‐type olivine crystallographic preferred orientations: the effect of strain geometry on mantle texture

Abstract

The effect of finite strain geometry on crystallographic preferred orientation (CPO) is poorly constrained in the upper mantle. Specifically, the relationship between shape preferred orientation (SPO) and CPO in the mantle rocks remains unclear. We analyzed a suite of 40 spinel peridotite xenoliths from Marie Byrd Land, west Antarctica. X-ray computed tomography allows for quantification of spinel SPO, which ranges from prolate to oblate shape. Electron backscatter diffraction analysis reveals a range of olivine CPO patterns, including A-type, axial-[010], axial-[100], and B-type patterns. Until now, these CPO types were associated with different deformation conditions, deformation mechanisms, or strain magnitudes. Microstructures and deformation mechanism maps suggest that deformation in all studied xenoliths is dominated by dislocation-accommodated grain boundary sliding. For the range of temperatures (779–1198 ºC), extraction depths (39–72 km), differential stresses (2–60 MPa), and water content (up to 500 H/106Si) of the xenolith suite, variations in olivine CPO do not correlate with changes in deformation conditions. Here we establish for the first time in naturally deformed mantle rocks that finite strain geometry controls the development of axial-type olivine CPOs; axial-[010] and axial-[100] CPOs form in relation to oblate and prolate fabric ellipsoids, respectively. Girdling of olivine crystal axes results from intracrystalline slip with activation of multiple slip systems, and grain boundary sliding. Our results demonstrate that mantle deformation may deviate from simple shear. Olivine texture in field studies and seismic anisotropy in geophysical investigations can provide critical constraints for the 3D strain in the upper mantle.