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- Title
Stress and Strain Rate Evolution During the Finite Strain Deformation of a Weak Ferropericlase Grain by Diffusion Creep: Implications for Shear Localization in the Lower Mantle.
- Authors
Cho, H. E.; Karato, Shun‐ichiro
- Abstract
We investigate the finite deformation of a weak inclusion embedded in a strong matrix. The goal is to understand the possible causes for shear localization that would explain the presence of geochemical heterogeneity in the Earth's lower mantle. The weak phase in the lower mantle is ferropericlase (Fp), and the strong phase is bridgmanite (Br). We herein consider deformation by diffusion creep for which some supporting evidence is available. When an inclusion deforms by diffusion creep, deformation itself modifies its geometry and stress distribution. The modified stress distribution in turn leads to strain‐dependent rheological properties due to the changes in diffusion path length and the stress gradient that drives diffusion flux. We investigate the evolution of deformation of a weak inclusion (i.e., Fp‐like weak grain embedded in Br‐like strong matrix) using the Eshelby theory (for stress and strain rate fields) combined with a theory of diffusional mass transport caused by the gradient in the normal stress within the inclusion. We found that finite strain leads to a significant strain weakening under simple shear deformation but not under axial deformation. Since diffusion creep is thought to be a dominant mechanism of the lower mantle rheology, the results of the present study provide a basis for investigating the nature of shear localization and its important implication for the preservation of geochemical heterogeneity and the distribution of seismic anisotropy in the lower mantle. Plain Language Summary: Localized deformation is a possible explanation for the preservation of geochemical heterogeneity in the lower mantle. The lower mantle is made of two major phases: bridgmanite (Br [∼70%]) and ferropericlase (Fp [∼20%]) (and other minor phases). Lab data show that Fp is much weaker than Br. We investigate the deformation of a weak Fp‐like inclusion embedded in a strong Br‐like matrix as a function of strain. We consider finite strain deformation by diffusion creep. Deformation of a weaker phase evolves with strain caused by the evolution of internal stress (stress acting on the inclusion) and the shape of an inclusion. A substantial weakening of inclusion is observed particularly for shear deformation. These results provide an important implication for the preservation of geochemical heterogeneity and the distribution of seismic anisotropy in the lower mantle of the Earth. Key Points: Diffusion creep rate is highly sensitive to the shape of the Fp grain (embedded in a Br matrix) and boundary normal stress stateIn the setting of a two‐phase mixture, the diffusion creep rate of Fp dramatically increases under remote simple shear deformationThe strain‐dependent diffusion creep of Fp may lead to shear localization that forms the weak boundary layer within the lower mantle
- Subjects
FERROPERICLASE; DEFORMATIONS (Mechanics); SHEAR zones; HETEROGENEITY; ANISOTROPY
- Publication
Journal of Geophysical Research. Solid Earth, 2022, Vol 127, Issue 1, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2021JB022673