DIRECT TESTING OF FORSTERITE BICRYSTALS VIA in-situ MICROPILLAR EXPERIMENTS AT 700 • C

Abstract

The mechanics of olivine deformation play a key role in long-term planetary processes, including 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 post-seismic creep within the upper mantle. Previous studies have emphasized the importance of grain-size effects in the deformation of olivine. 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, as there are no direct studies of the mechanical properties of individual grain boundaries in olivine. In this study, we use high-precision mechanical testing of synthetic forsterite bicrystals with well characterized interfaces to directly observe and quantify the mechanical properties of olivine grain boundaries. We conduct in-situ micropillar compression tests at high-temperature (700• C) on bicrystals containing low-angle (4• tilt about [100] on (014)) and high-angle (60• tilt about [100] on (011)) boundaries. During the in-situ tests, we observe differences in deformation style between the pillars containing the grain boundary and the pillars in the crystal interior. In the pillars containing the grain boundary, the interface is oriented at ∼ 45• to the loading direction to promote shear. In-situ observations and analysis of the mechanical data indicate that pillars containing the grain boundary consistently support elastic loading to higher stresses than the pillars without a grain boundary. Moreover, the pillars without the grain boundary sustain larger plastic strain. Post-deformation microstructural characterization 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 relative to the crystal interior at these conditions.