In-situ mechanical testing and characterization of olivine grain boundaries

The mechanics of olivine deformation play a key role in long-term planetary processes, such as the response of the lithosphere to tectonic loading or the response of the solid Earth to tidal forces, and in short-term processes, such as the evolution of roughness on oceanic fault surfaces or postseismic creep within the upper mantle. Many previous studies have emphasized the importance of grain-size effects in the deformation of olivine. However, most of our understanding of the role of grain boundaries in the deformation of olivine is inferred from comparison of experiments on single crystals to experiments on polycrystalline samples.

To directly observe and quantify the mechanical properties of olivine grain boundaries, we use high-precision mechanical testing of synthetic forsterite bicrystals with well characterised interfaces. We conduct in-situ micropillar compression tests at high-temperature (700°C) on low-angle (13° tilt about [100] on (015)) and high-angle (60° tilt about [100] on (011)) grain boundaries. In these experiments, the boundary is contained within the micropillar and oriented at 45° to the loading direction to promote shear along the boundary. In these in-situ tests, we observe differences in deformation style between the pillars containing the grain boundary and the pillars in the crystal interior. In-situ observations and analysis of the mechanical data indicate that pillars containing the grain boundary consistently support elastic loading to higher stresses than pillars without a grain boundary. Moreover, only the pillars without a grain boundary display evidence of sustained plasticity and slip-band formation. Post-deformation advanced microstructural characterization (STEM) confirms that under the conditions of these deformation experiments, sliding did not occur along the grain boundary. These observations support the hypothesis that grain boundaries are stronger than the crystal interior. 

These experiments on small deformation volumes allow us to qualitatively explore the differences between the crystal interior and regions containing grain boundaries. Overall, the variation in strain and temperature in our small scale experiments allows fundamental investigation of the response of well characterised forsterite grain boundaries to deformation.