Microstructural and Micromechanical Evolution of Olivine Aggregates During Transient Creep

To examine the microstructural evolution that occurs during transient creep, we deformed samples of polycrystalline olivine to different strains that spanned the initial transient deformation. Two sets of samples with different initial grain sizes of 5 μm and 20 μm were deformed in torsion at T = 1,...

Full description

Saved in:
Bibliographic Details
Published in:Journal of geophysical research. Solid earth Vol. 129; no. 12
Main Authors: Wiesman, Harison S., Breithaupt, Thomas, Wallis, David, Hansen, Lars N.
Format: Journal Article
Language:English
Published: Washington Blackwell Publishing Ltd 01.12.2024
Subjects:
Angular resolution
Backscatter
Crystal defects
Crystal dislocations
Crystal lattices
Crystals
Deformation
Deformation effects
deformation experiments
Dislocation
Dislocation density
dislocations
Earth mantle
Earthquake prediction
Earthquakes
Elastic analysis
Elastic deformation
Electron backscatter diffraction
Evolution
geophysics
Glacier melting
Glaciers
Grain growth
Grain size
Heterogeneity
High temperature
HR‐EBSD
Microstructure
Olivine
probability
Rock
Rocks
Seismic activity
Shear strain
Solifluction
Strain
Strain analysis
Strain hardening
Strain rate
Stresses
transient creep
Viscosity
X-ray diffraction
ISSN:2169-9313, 2169-9356
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:To examine the microstructural evolution that occurs during transient creep, we deformed samples of polycrystalline olivine to different strains that spanned the initial transient deformation. Two sets of samples with different initial grain sizes of 5 μm and 20 μm were deformed in torsion at T = 1,523 K, P = 300 MPa, and a constant shear strain rate of 1.5 × 10−4 s−1, during which both sets of samples experienced strain hardening. We characterized the microstructures at the end of each experiment using high‐angular resolution electron backscatter diffraction (HR‐EBSD) and dislocation decoration. In the coarse‐grained samples, dislocation density increased from 1.5 × 1011 m−2 to 3.6 × 1012 m−2 with strain. Although the same final dislocation density was reached in the fine‐grained samples, it did not vary significantly at small strains, potentially due to concurrent grain growth during deformation. In both sets of samples, HR‐EBSD analysis revealed that intragranular stress heterogeneity increased in magnitude with strain and that elevated stresses are associated with regions of high geometrically necessary dislocation density. Further analysis of the stresses and their probability distributions indicate that the stresses are imparted by dislocations and cause long‐range elastic interactions among them. These characteristics indicate that dislocation interactions were the primary cause of strain hardening during transient creep in our samples. A comparison of the results to the predictions of three recent models reveals that the models do not correctly predict the evolution in stress and dislocation density with strain in our experiments due to a lack of previous such data in their calibrations. Plain Language Summary Forces from earthquakes and melting glaciers cause short‐term changes to the viscosity of rocks as they flow in Earth's mantle. Recent experiments have suggested that the extent of these changes is controlled by interactions among microscopic defects, called dislocations, within the lattices of crystals that make up the rocks. To be able to predict the effects of these changes on a global scale, we need to understand how the number of, and interactions among, these defects react to sudden changes in the forces driving deformation. To this end, we deformed rocks at high temperature and pressure and characterized the evolution of these defects during a change in stress. We found that the number of dislocations increased as the stress increased, as did the strength of interactions among dislocations. Our results give more support to the idea that interactions among dislocations control the evolution of the viscosity of the rocks. These results can be used to improve models that describe short‐term deformation of Earth's mantle. Key Points Dislocation density increases with strain during transient creep and is correlated with increases in intragranular stress heterogeneity The characteristics of intragranular stress heterogeneity indicate that long‐range interactions among dislocations control the stress evolution during transient creep Recent models were unable to reproduce the evolution of stress and dislocation density during transient creep, demonstrating a need to revisit these models
Bibliography:ObjectType-Article-1
SourceType-Scholarly Journals-1
ObjectType-Feature-2
content type line 14
content type line 23
ISSN:2169-9313
2169-9356
DOI:10.1029/2024JB029812