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Abstract To study the mechanical behavior of polymineralic rocks, we performed deformation experiments on two‐phase aggregates of olivine (Ol) + ferropericlase (Per) with periclase fractions (fPer) between 0.1 and 0.8. Each sample was deformed in torsion atT = 1523 K,P = 300 MPa at a constant strain rate to a final shear strain ofγ = 6 to 7. The stress‐strain data and calculated values of the stress exponent,n, indicate that Ol in our samples deformed by dislocation‐accommodated sliding along grain interfaces while Per deformed via dislocation creep. At shear strains ofγ < 1, the strengths of samples withfPer > 0.5 match model predictions for both phases deforming at the same stress, the lower‐strength bound for two‐phase materials, while the strengths of samples withfPer < 0.5 are greater than predicted by models for both phases deforming at the same strain rate, the upper‐strength bound. These observations suggest a transition from a weak‐phase supported to a strong‐phase supported regime with decreasingfPer. Aboveγ = 4, however, the strength of all two‐phase samples is greater than those predicted by either the uniform‐stress or the uniform‐strain rate bound. We hypothesize that the high strengths in the Ol + Per system are due to the presence of phase boundaries in two‐phase samples, for which deformation is rate limited by dislocation motion along interfacial boundaries. This observation contrasts with the mechanical behavior of samples consisting of Ol + pyroxene, which are weaker, possibly due to impurities at phase boundaries.
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Rheological Weakening of Olivine + Orthopyroxene Aggregates Due to Phase Mixing: Effects of Orthopyroxene Volume Fraction
Abstract 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 offopx = 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 fromn ≈ 3 for γ ≲ 5 ton ≈ 2 for 5 ≲ γ ≲ 25, indicating that the deformation mechanism changes as strain increases. In contrast, for samples withfopx = 0.15, the stress exponent is constant atn ≈ 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 withfopx = 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 largerfopx, which is an important constraint for understanding strain localization in the upper mantle of Earth.
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- Award ID(s):
- 1755805
- PAR ID:
- 10375662
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 125
- Issue:
- 9
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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