The effect of grain orientation versus the local stress environment on microstructures in polycrystalline San Carlos olivine
Abstract
The mechanical properties of olivine single crystals heavily influence the rheology of the upper mantle. However, single crystal properties cannot be fully factored into models of mantle convection without accounting for how the polycrystalline environment affects single crystal behavior. Intragranular stresses within single grains contained in a polycrystal are controlled by the combined effect of grain orientation and local stress fields. Heterogeneities in the stress field can be interpreted by the spatial distribution of specific microstructures, and are the main focus of our studies. Samples composed of polycrystalline San Carlos olivine were deformed using the D-DIA apparatus (located at the NSLS beamline X17B2) between 3.5 and 5GPa, and temperatures ranging from 700 to 900°C. Using a strain rate of 1.45x10-5s-1, samples were shortened by 15.5 and 25%. After recovery, samples were prepared and analyzed using an electron backscatter diffraction (EBSD) detector coupled with Channel 5's EBSD software. Results from EBSD orientation maps indicate a lack of correlation between Schmid factor and intragranular strain. Some of the lack of correlation can be explained by the incipient formation of kink bands in grains with low Schmid factors. Using Schmid factor maps we have identified a stage in the formation of kink bands prior to the development of the diagnostic sharp boundaries generally observed in kink bands. These "proto-kinks" have narrow alternating domains of discrete crystallographic orientations which rotate about a common axis. Rotations between alternating domains can vary between 1 and 7° in samples strained to 15.5%, and have been observed when the burgers vector lies within 35° of compression. Geometry of proto kinks allows for confirmation of slip system activity inside individual grains. In addition, the deviation between the macroscopic compressive stress and direction of maximum shortening for the kink bands provide a measure of the heterogeneity in the sample stress field. Results from new experimental conditions, as well as the impact of heterogeneous local stress environments on deformation, will be presented in further detail.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMMR23C2425C
- Keywords:
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- 3902 MINERAL PHYSICS / Creep and deformation;
- 3904 MINERAL PHYSICS / Defects;
- 3924 MINERAL PHYSICS / High-pressure behavior