Transformation-induced faulting in magnesium germanate: Nucleation of shear failure, implications for brittle failure, transformation kinetics and reaction interface morphology

Transformation-induced faulting remains the leading candidate mechanism thought to produce deep-focus earthquakes. Studies of the phenomenon have concentrated on the first-order characteristics of the mechanism, i.e. the required strain-rate/temperature/pressure conditions, but many details remain poorly understood. This dissertation contains observations, measurements and calculations which further characterize and refine our understanding of the details of shear localization in transformation-induced faulting associated with the olivine-spinel transformation under stress at high pressure and temperature. Our experiments were conducted in a Griggs-style deformation apparatus, and most analyses were performed via SEM observations. We present confirmation of the analogy between anticrack formation and shear failure and dilatant brittle shear failure. The size distribution of anticracks, their nucleation sites, and the formation of shear zones at 30° to compression are now well documented. We present results from experiments which quantify the rates of bulk transformation under load and under hydrostatic conditions for areas of preexisting strain or damage. Also presented are observations of a newly-discovered class of transformation microstructure which leads to shear failure in materials with larger (∼150 μm) grain sizes. The appearance of these new microstructures is investigated as a function of loading and strain. We conclude that the thin bands of spinel phase observed in SEM images are most likely cross-sections of planar, fine-grained zones of transformation which have been localized by the transformation of high-strain, crystallographically-controlled surfaces in the deformed olivine, which allow slip on the individual bands and coalesce to lead to specimen-scale shear failure. We also report mechanical “erosion” and damage of pyroxene inclusion grain surfaces in contact with the olivine to spinel transformation. We present calculations that this phenomenon is due to stresses induced by the local volume loss, assisted by a temperature increase generated by the exothermic transformation. Also reported is a new technique using directional fractal dimension measurements to identify the statistical nature of interfaces of structures associated with anticrack formation, coalescence, and coarsening, which we then relate to microphysical processes operating to create these structures.