Iron-rich 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 deformation-produced 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 D-type fabric, induced by dislocation glide on both the (010) and the (001) slip systems, at low strains, to A-type fabric, caused by dislocation glide on the (010) 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 olivine-rich rocks can be well approximated by that of iron-rich 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.
Bibliographical noteFunding Information:
The authors thank to Lars Hansen, Zachary Michels, Amanda Dillman, Cameron Meyers and Harison Wiesman for stimulating discussions. The authors are also grateful to Lars Hansen and Zachary Michels for sharing their codes for analyzing orientation data obtained by EBSD. Comments from Andrew Cross, Haemyeong Jung, an associate editor and two anonymous reviewers are appreciated. This research was supported by the Key Research Program of the Institute of Geology and Geophysics, CAS, grant IGGCAS‐201905 (to CQ), National Key R&D Program of China grants 2018YFC1504203 and SQ2017YFSF040025 (to YHZ), KOPRI project PE21050 (to DK), and NSF grant EAR‐1755805 (to DLK). EBSD analyses were done in the SEM laboratory at Korea Institute for Rare Metals (KIRAM), and in the laboratory at Beijing office of Oxford Instruments company.
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- crystallographic preferred orientation
- grain size