Laboratory investigation of mechanisms for phase mixing in olivine+ferropericlase aggregates

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Abstract

To investigate the role of grain boundary pinning and the mechanisms by which phase mixing occurs during deformation of polymineralic rocks, we conducted high-strain torsion experiments on samples consisting of olivine plus 30 vol% ferropericlase. Experiments were performed in a gas-medium deformation apparatus at 1524K and 300 MPa. Samples were deformed to outer radius shear strains of up to γ (R)=14.1. The value of the stress exponent and the small grain sizes of our samples indicate that our two-phase material deformed by dislocation-accommodated grain boundary sliding. In samples deformed to 1<γ <7, elongated clusters of ferropericlase grains form thin layers in the olivine matrix, and small grains of ferropericlase appear at olivine grain boundaries and three- and four-grain junctions. By γ ≈14, a well-distributed mixture of small ferropericlase grains among the olivine grains developed. Microstructures exhibit similarities to both mechanical and chemical models proposed to describe the processes leading to phase mixing. Our results provide evidence for grain size reduction during phase mixing that results in a grain size significantly smaller than the value predicted by the single-phase recrystallization piezometer for olivine. Thus, phase mixing provides a mechanism for the persistent weakening of rocks that is important for developing and maintaining shear zones necessary for plate tectonics.

Original languageEnglish (US)
Article number20170417
JournalPhilosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
Volume376
Issue number2132
DOIs
StatePublished - Nov 13 2018

Bibliographical note

Funding Information:
Data accessibility. This article has no additional data. Authors’ contributions. H.S.W. carried out the experiments, performed the data analysis, and drafted the manuscript. M.E.Z. and D.L.K. conceived of and helped to design the study and provided revisions of the manuscript. All authors read and approved the manuscript. Competing interests. We declare we have no competing interests. Funding. This study was funded by NASA grant NNX15AL53G (D.L.K.), NSF grant EAR-1755498 (M.E.Z.). Part of this work was carried out in the Characterization Facility at the University of Minnesota - Twin Cities, which receives partial support from NSF through the MRSEC program. Acknowledgements. The authors thank A. Dillman L. Hashim C. Meyers and C. Qi for valuable discussion about and assistance with experiments. We would also like to thank the reviewers for their insightful comments and detailed reviews. We thank the Characterization Facility at the University of Minnesota - Twin Cities for the use of their SEM.

Publisher Copyright:
© 2018 The Author(s) Published by the Royal Society. All rights reserved.

Keywords

  • Ferropericlase or magneseowüstite
  • Mechanical weakening
  • Olivine
  • Phase mixing
  • Torsional deformation
  • Zener pinning

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