To understand the effects of secondary minerals on changes in the mechanical properties of upper mantle rocks due to phase mixing, we conducted high-strain torsion experiments on aggregates of iron-rich olivine + orthopyroxene (opx) with opx volume fractions of fopx = 0.15, 0.26, and 0.35. For samples with larger amounts of opx, fopx = 0.26 and 0.35, the value of the stress exponent decreases with increasing strain from n ≈ 3 for γ ≲ 5 to n ≈ 2 for 5 ≲ γ ≲ 25, indicating that the deformation mechanism changes as strain increases. In contrast, for samples with fopx = 0.15, the stress exponent is constant at n ≈ 3.3 for 1 ≲ γ ≲ 25, suggesting that no change in deformation mechanism occurs with increasing strain for samples with smaller amounts of opx. The microstructures of samples with larger amounts of opx provide insight into the change in deformation mechanism derived from the mechanical data. Elongated grains align subparallel to the shear direction for samples of all three compositions deformed to lower strains. However, strain weakening with grain size reduction and the formation of a thoroughly mixed, fine-grained texture only develops in samples with fopx = 0.26 and 0.35 deformed to higher strains of γ ≳ 16. These mechanical and associated microstructural properties imply that rheological weakening due to phase mixing only occurs in the samples with larger fopx, which is an important constraint for understanding strain localization in the upper mantle of Earth.
Bibliographical noteFunding Information:
We thank A. Dillman, J. Tielke, M. Pec, C. Qi, and C. Meyers for valuable technical assistance and stimulating discussions. We also thank Y. Kouketsu for assistance in analysis of Raman spectroscopy results, T. Hiraga and M. Morishige for discussions of science, and L. Hansen for providing the original EBSD data in Hansen et al. ( 2012 , 2014 ). The manuscript was significantly improved by insightful comments from two anonymous reviewers. This research was supported by a JSPS Research Fellowship for Young Scientists 26‐4879 and the Japan Society for the Promotion of Science 18K13634 and 16K17832 (M. T.), NSF Grant EAR‐1755805 (D. L. K.), NASA Grant NNX11AF58G (M. E. Z), and NSF Grant EAR‐1345060 (M. E. Z.). EBSD and SEM analyses were carried out in the Institute of Technology Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program.
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- phase mixing
- rheological weakening