Microstructural Recovery of Experimentally Deformed Olivine Rocks
Abstract
Although a number of experimental studies have examined the microstructures of olivine aggregates deformed to high shear strains, relatively few have investigated the recovery of these features after the stress has relaxed, as occurs in the Earth. Deformation experiments on olivine aggregates result in grain size refinement, formation of strong crystallographic preferred orientations (CPOs), and development of intragranular crystallographic distortion. To predict transient microstructural changes that occur under mantle conditions, one must understand both how an imposed shear stress generates deformation microstructures and how those microstructures recover when the stress is relaxed. Therefore, we performed a series of deformation and subsequent static recovery experiments on highly dense olivine aggregates to investigate this problem. Several fine-grained, evacuated hot-pressed San Carlos olivine aggregates were deformed in torsion to high shear strains in a gas-medium high-resolution apparatus at 300 MPa and 1250°C. The deformed samples were then cut to produce several subsamples, packed in San Carlos olivine powder, and statically annealed in the same apparatus at 300 MPa and 1250°C or 1300°C for up to 21 hours. Radial and tangential sections of the deformed and annealed samples were analyzed by electron backscatter diffraction (EBSD).
We identified two regimes of microstructural recovery, a continuous (normal) recovery regime and a discontinuous (abnormal) recovery regime. In the case of continuous recovery, the grain-size distribution remained roughly log-normal as the mean grain-size gradually increased. This behavior was accompanied by reduction in intragranular crystallographic distortion and weakening of the shape preferred orientation imposed by deformation. Overall, fabric strength and orientation did not change considerably during static recovery. In the case of discontinuous recovery, a few strain-free grains that nucleated locally consumed surrounding grains, becoming very large and resulting in a bimodal grain-size distribution. These abnormal grains, which have almost no internal distortion, have long, straight boundaries that swept through matrix grains. Abnormal grains were frequently not aligned with the CPO, therefore overprinting the CPO with time.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMMR31B0083M
- Keywords:
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- 3625 Petrography;
- microstructures;
- and textures;
- MINERALOGY AND PETROLOGYDE: 8004 Dynamics and mechanics of faulting;
- STRUCTURAL GEOLOGYDE: 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICSDE: 8160 Rheology: general;
- TECTONOPHYSICS