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

Chao Qi, Lars N. Hansen, David Wallis, Benjamin K. Holtzman, David L. Kohlstedt

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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.

Original languageEnglish (US)
Pages (from-to)316-336
Number of pages21
JournalGeochemistry, Geophysics, Geosystems
Issue number2
StatePublished - Feb 2018

Bibliographical note

Funding Information:
This work benefited greatly from stimulating discussions with Takehiko Hiraga. We thank to Jessica Warren for inspiring discussions, the use of SEM and EBSD facilities at the Department of Plant Biology in Carnegie Institution of Stanford University, and the use of EBSD data processing software. We are also grateful to Yan Liang and Clint Conrad for providing the alkali basalt, to Mark Zimmerman for his help with this work, and to Matej Pecˇ and Cameron Meyers for their help with collecting EBSD data. Discussions with Max Bezada and Joseph Byrnes are appreciated. We appreciate the constructive comments from Shun-ichiro Karato, Holger Stu€nitz and an anonymous reviewer. This work was supported by NSF grants OCE-1459717 and EAR-1520647 to DLK and NERC grant NE/M000966/1 to LNH. Data presented in this paper are accessible via the data repository for the University of Minnesota (http://hdl.

Publisher Copyright:
© 2018. American Geophysical Union. All Rights Reserved.


  • CPO
  • SPO
  • olivine
  • partial melt
  • seismic anisotropy


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