Mechanical Behavior, Microstructural Evolution, and Phase Mixing in Olivine Plus Ferropericlase Aggregates

Abstract

Plate tectonics is unique to Earth in our solar system. Deformation is localized into narrow boundaries, such as subduction zones, and is driven by mantle convection. However, the underlying physics occurring at these boundaries is not clear: How does plate tectonics initiate? What allows it to persist over geological timescales? Why does deformation occur along highly localized regions? Due to the presence of fine-grained, polymineralic rocks called mylonites at shear zones, it is likely that both grain size reduction and the presence of multiple mineral phases play a significant role in these localization processes. In this dissertation I explore the influence of secondary phases on the rheological behavior and microstructural evolution of two-phase aggregates. First, I introduce mechanical and microstructural data from large strain laboratory experiments on two-phase samples of olivine plus ferropericlase. The results from these experiments are analyzed to determine the dominant deformation mechanism in each sample and are compared to their single-phase counterparts and previous studies on two-phase materials. Second, I present results from static annealing experiments on previously deformed samples, which demonstrate that phase boundary pinning hinders grain growth and preserves fine grain sizes over long time scales. Finally, I summarize microstructures from deformed samples in which large domains of ferropericlase were initially isolated from one another in the olivine matrix. These microstructures are examined to determine the mechanisms responsible for phase mixing and how phase mixing results in the formation of shear bands, a feature of localized deformation.