A Re-examination of the Olivine-Spinel Microstructures Associated with Phase Transformation Faulting
Abstract
Thirty years ago a detailed mechanism for faulting associated with the olivine-spinel phase transformation was put forward by Green and Burnley (1989). The mechanism involved the propagation of lens-shaped bodies of fine-grained spinel dubbed "anticracks", linkage of anticracks to form shear zones filled with fine grained spinel and eventual faulting mediated by superplastic flow of fine grained spinel. Although the mechanical results have been duplicated by others, and most recently in the olivine-spinel system, with the addition of high resolution acoustic emissions, relatively little work has delved further into the associated microstructures and in particular to the formation and evolution of anticracks. We used electron backscatter diffraction to re-examine the microstructures in Burnley's original suite of experiments, as well as in unpublished hydrostatic experiments conducted in the gas piston cylinder apparatus. The examination of a large number of the spinel filled lens-shaped bodies demonstrates that they are predominantly composed of a small number of spinel crystals and in many cases composed of only one crystal. The lens-shaped bodies are associated with deformation gradients within their olivine host grains. However, it is not clear that the stress field around the lens-shaped bodies would be best described by an anticrack model. EBSD maps and backscattered electron images of faulted samples demonstrate that the majority of spinel in these samples occurs as fine grain aggregates distributed along grain boundaries, thus this is the most likely source of fine grained spinel found in shear zones.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMMR43A..07B
- Keywords:
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- 5112 Microstructure;
- PHYSICAL PROPERTIES OF ROCKS;
- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 8034 Rheology and friction of fault zones;
- STRUCTURAL GEOLOGY;
- 8159 Rheology: crust and lithosphere;
- TECTONOPHYSICS