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°, [100]/(016)) and a high‐angle grain boundary (60°, [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. Plain Language Summary Olivine is the main mineral constituent of Earth's upper mantle. Consequently, the physics of olivine deformation govern the response of Earth's tectonic plates to tectonic forces. Laboratory studies on the deformation of olivine indicated that the size of the crystals in a sample subjected to deformation impacts the strength of the sample. However, the causes of this effect remain unresolved. In this study, we directly observe and quantify the impact of the interfaces between crystals on the deformation of olivine. We conduct microscale experiments at room temperature with diamond tips. Our experiments reveal that some interfaces can act as sources of the crystal defects that facilitate deformation, as well as obstacles to moving crystal defects, whereas other interfaces do not generate a measurable impact. We suggest that the distribution of these interfaces between crystals in olivine‐rich rocks may impact the response of tectonic plates to tectonic forces. Key Points Nanoindentation experiments on a high‐angle grain boundary (60° misorientation)