The role of grain boundaries for the deformation and grain growth of olivine at upper mantle conditions

Crystal defects such as vacancies, dislocations and grain boundaries are central in controlling the rheology of the Earth’s upper mantle. Their presence influences element diffusion, plastic deformation and grain growth, which are the main microphysical processes controlling mass transfer in the Earth’s lithosphere and asthenosphere. Although substantial information exists on these processes, there is a general lack of data on how these defects interact at conditions found in the Earth’s interior. A better understanding of processes occurring at the grain scale is necessary for increased confidence in extrapolating from laboratory length and time scales to those of the Earth. We examined the evolution of olivine grain boundaries during experimental deformation and their impact on deformation in the dislocation-accommodated grain- boundary sliding (disGBS) regime. This may be the main deformation mechanism for olivine in most of Earth’s upper mantle. Our results suggest that grain boundaries play a major role in moderating deformation in the disGBS regime. We present observational evidence that the rate of deformation is controlled by assimilation of dislocations into grain boundaries. We also demonstrate that the ability for dislocations to transmit across olivine grain boundaries evolves with increasing deformation. Lastly, we show that dynamic recrystallization of olivine creates specific grain boundaries, which are modified as deformation progresses. This might affect electrical conductivity and seismic attenuation in the upper mantle. The effective contribution of grain-boundary processes (such as disGBS) on the rheology of the upper mantle is correlated to the amount of grain boundaries in upper mantle rocks, that is, their grain-size distribution and evolution. The grain-size distribution in the Earth’s mantle is controlled by the balance between damage (recrystallization under a stress field) and healing (grain growth) processes. However, grain growth, one of the main processes controlling grain size, is still poorly constrained for olivine at conditions of the upper mantle. To evaluate the effects of pressure on grain growth of olivine, we performed grain growth experiments at pressures ranging from 1 to 12 GPa using piston-cylinder and multi-anvil apparatuses. We found that the olivine grain-growth rate is reduced as pressure increases. Our results suggest that grain-boundary diffusion is the main process of grain growth at high pressure. Based on extrapolation of our experimental results to geological time scales, we suggest that at deep upper-mantle conditions (depths of 200 to 410 km), the effect of pressure on inhibiting grain growth counteracts the effect of increasing temperature. We present estimations of viscosity as a function of depth considering the grain-size evolution predicted here. Our estimations suggest that viscosity is almost constant at the deep upper mantle, which corroborates postglacial-rebound observations.