Low-temperature plastic yielding of olivine aggregates deformed at high pressures: New results leveraging a novel cell assembly for the Griggs Apparatus

Abstract:

Low-temperature plasticity of olivine is likely an important mechanism for accommodating a variety of deformational processes in Earth’s lithosphere, especially in ocean basins where the upper mantle bears most of the tectonic load. In general, at low temperatures, where chemical diffusion is inhibited, olivine plasticity is limited by dislocation glide. Dislocation pile-up, dislocation entanglement, and the limited availability of slip systems are expected to lead to strain hardening during low temperature deformation of polycrystalline olivine-rich mantle rocks. Without thermal recovery, plastic yielding may ultimately promote brittle failure.

To investigate these mechanisms, we ran a series of deformation experiments on olivine aggregates at high pressures (>2 GPa) and temperatures below 100 °C in a Griggs-type apparatus. The aggregates were initially synthesized by evacuated hot-pressing San Carlos olivine powders in a gas medium apparatus at 1250 °C and 300 MPa confining pressure, with a resulting grain size of 5-10 µm and porosity on the order of 0.1%. We developed a novel experimental cell for the Griggs-type apparatus, wherein unjacketed rectangular samples were sealed to high pressure and deformed in hydraulic oil confining media. This allowed for truly hydrostatic pressurization, excellent stress resolution, and full sample recovery. We performed a variety of constant rate and constant stress experiments, ending the experiment below the measured yield point (2-3 GPa), just beyond plastic yielding, and to ultimate brittle failure (in some cases up to differential stresses >4 GPa). After deformation, we observed topographic features on initially polished sample surfaces indicative of grain-scale plastic deformation, including the appearance of slip bands on grain surfaces and steps at grain boundaries. We also observed evidence of microcracking along grain boundaries, which may be initiated by elastic stress concentration due to plastic yielding of grains well-oriented for dislocation glide. Profilometry and EBSD mapping provide insights into low temperature plastic yielding and its relationship to brittle failure.