The Role of Grain Boundaries in Low-Temperature Plasticity of Olivine Revealed by Nanoindentation

The rheological properties of olivine influence large-scale, long-term deformation processes on rocky planets. Studies of the deformation of olivine at low temperatures and high stresses have emphasized the importance of a grain-size effect impacting yield stress. Laboratory studies indicate that aggregates with finer grains are stronger than those with coarser grains. However, the specific interactions between intracrystalline defects and grain boundaries leading to this effect in olivine remain unresolved. In this study, to directly observe and quantify the mechanical properties of olivine grain boundaries, we conduct nanoindentation tests on well characterized bicrystals. Specifically, we perform room-temperature spherical and Berkovich nanoindentation tests on a subgrain boundary (13 degrees, [100]/(016)) and a high-angle grain boundary (60 degrees, [100]/(011)). These tests reveal that plasticity is easier to initiate if the high-angle grain boundary is within the deformation volume, whereas the subgrain boundary does not impact the initiation of plasticity. Additionally, the high-angle grain boundary acts as a barrier to slip transmission, whereas the subgrain boundary does not interact with dislocations in a measurable manner. We suggest that the distribution of grain-boundary types in olivine-rich rocks might play a role in generating local differences in mechanical behavior during deformation.