Evolution of Microstructural Properties in Sheared Iron­Rich Olivine

Abstract

Iron­rich olivine is mechanically weaker than olivine of mantle composition, ca. Fo90, and thus is more amenable to study under a wide range of laboratory conditions. To investigate the effects of iron content on deformation­produced crystallographic preferred orientation (CPO) and grain size, we analyzed the microstructures of olivine samples with compositions of Fo70, Fo50 and Fo0 that were deformed in torsion under either anhydrous or hydrous conditions at 300 MPa. Electron backscatter diffraction (EBSD) observations reveal a transition in CPO from D­type fabric, induced by dislocation glide on both the (010)[100] and the (001)[100] slip systems, at low strains, to A­type fabric, caused by dislocation glide on the (010)[100] slip system, at high strains for all of our samples, independent of iron content and hydrous/anhydrous conditions. A similar evolution of fabric with increasing strain is also reported to occur for Fo90. Radial seismic anisotropy increases with increasing strain, reaching a maximum value of ∼1.15 at a shear strain of ∼3.5 for each sample, demonstrating that the seismic anisotropy of naturally deformed olivine­rich rocks can be well approximated by that of iron­rich olivine. Based on EBSD observations, we derived a piezometer for which recrystallized grain size decreases inversely with stress to the ∼1.2 power. Also, recrystallized grain size increases with increasing iron content. Our experimental results contribute to understanding the microstructural evolution in the mantle of not only Earth but also Mars, where the iron content in olivine is higher.