Grain-Boundary Plasticity and the Strength of the Upper Mantle

The rheology of the Earth’s upper mantle is controlled by the plasticity of olivine-rich rocks. Despite considerable efforts to better understand their plasticity, experimental flow laws fail to explain important geodetic and geophysical observations, such as fast post-seismic surface displacements or the “elusiveness” of the lithosphere-asthenosphere boundary beneath cratons. The plasticity of olivine single crystals is well understood and was experimentally quantified to mantle pressures and temperatures (e.g., Bai et al., 1991, JGR, 96, 2441-2463; Raterron et al., 2012, PEPI, 200-201, 105-112). The plasticity of aggregates, however, involves additional mechanisms, and the fundamental question of the amount of strain accommodation at grain boundaries remain unanswered.

In this study, we compare the plasticity of olivine aggregates deformed experimentally at mantle conditions (Durham et al., 2009, PEPI, 172, 67-73; Hilairet et al., 2012, JGR, 117, B01203 ; Bollinger et al., 2013, PEPI, 228, 211-219) to that of single crystals and demonstrate that strain at grain boundaries can be orders of magnitude larger than intracrystalline strain. We further show that the proportion of grain-boundary strain decreases with increasing temperature and stress. Applied along mantle geotherms, our results shows that grain boundary plasticity is dominant in the shallow mantle. The strength of olivine in the lithosphere can be more than one order of magnitude lower than predicted by classical flow laws (Hirth and Kohlstedt, 2003, AGU Geophys. Monogr, 138, 83–105). In the deep upper mantle, grain boundary plasticity vanishes and strain is mostly accommodated within the grains.