Crystallographic Preferred Orientation of Olivine in Sheared Partially Molten Rocks: The Source of the “a-c Switch”

Qi, C and Hansen, LN and Wallis, David and Holtzman, BK and Kohlstedt, DL (2018) Crystallographic Preferred Orientation of Olivine in Sheared Partially Molten Rocks: The Source of the “a-c Switch”. Geochemistry, Geophysics, Geosystems, 19 (2). pp. 316-336. ISSN 15252027 DOI https://doi.org/10.1016/j.matchar.2016.04.008

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Abstract

© 2018. American Geophysical Union. All Rights Reserved. To investigate the mechanism that produces the crystallographic preferred orientations (CPO) characteristic of sheared partially molten rocks of mantle composition, we analyzed the microstructures of samples of olivine plus 7% basaltic melt deformed in torsion to shear strains as large as γ = 13.3 Electron backscattered diffraction (EBSD) observations reveal a CPO characterized by a weak a-c girdle in the shear plane that develops by γ = 4. This CPO, which exhibits a slightly stronger alignment of 001 than 100 axes in the shear direction, changes little in both strength and distribution with increasing stress and with increasing strain. Furthermore, it is significantly weaker than the CPO observed for dry, melt-free olivine aggregates. Orientation maps correlated with grain shape measurements from tangential, radial, and transverse sections indicate that olivine grains are longer along 001 axes than along 100 axes and shortest along 010 axes. This morphology is similar to that of olivine grains in a mafic melt. We conclude that the weak a-c girdle observed in sheared partially molten rocks reflects contributions from two processes. Due to their shape-preferred orientation (SPO), grains rotate to align their 001 axes parallel to the flow direction. At the same time, dislocation glide on the (010)100 slip system rotates 100 axes into the flow direction. The presence of this CPO in partially molten regions of the upper mantle significantly impacts the interpretation of seismic anisotropy and kinematics of flow.

Item Type: Article
Uncontrolled Keywords: NILAREP
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
07 - Gold Open Access
Journal or Publication Title: Geochemistry, Geophysics, Geosystems
Volume: 19
Page Range: pp. 316-336
Identification Number: https://doi.org/10.1016/j.matchar.2016.04.008
Depositing User: Sarah Humbert
Date Deposited: 07 Apr 2022 11:49
Last Modified: 07 Apr 2022 11:49
URI: http://eprints.esc.cam.ac.uk/id/eprint/4841

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