title: The role of grain boundaries for the deformation and grain growth of olivine at upper mantle conditions
Creators: name: Ferreira, Filippe
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: Olivine
subject: grain boundaries
subject: deformation
subject: upper mantle
subject: GBPD
subject: GBCD
subject: EBSD
subject: TEM
subject: dislocation
subject: diffusion
subject: creep
subject: grain-boundary sliding
subject: 550

description: 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. This would improve our knowledge of large-scale geodynamic processes and interpretations of geophysical observations such as electrical conductivity and seismic anisotropy. In this work, I examined the evolution of olivine grain boundaries during experimental deformation and their impact on deformation in the dislocation-accommodated grain-boundary sliding (disGBS) regime. Previous studies suggest that disGBS may be the main deformation mechanism for olivine in most of Earth’s upper mantle. I used electron backscattered diffraction and transmission electron microscopy data to investigate the micromechanics involved in disGBS, with a focus on the interactions between dislocations and grain boundaries. These interactions are to date only poorly understood and not yet quantitatively investigated in olivine. The results presented here suggest that grain boundaries play a major role in moderating deformation in the disGBS regime. I present observational evidence that the rate of deformation is controlled by assimilation of dislocations into grain boundaries. I also demonstrate that the ability for dislocations to transmit across olivine grain boundaries evolves with increasing deformation. Finally, I show that dynamic recrystallization of olivine creates specific grain boundaries, which are modified as deformation progresses. The effective contribution of grain-boundary processes 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. Experimental data on grain growth kinetics of olivine is to date restricted to pressures of up to 1.2 GPa. To evaluate the effects of pressure on grain growth of olivine, I performed grain growth experiments at pressures ranging from 1 to 12 GPa using piston-cylinder and multi-anvil apparatuses. I found that the olivine grain-growth rate is reduced as pressure increases. The results presented here suggest that grain-boundary diffusion is the main process of grain growth at high pressure. Based on the experimental results, I present an equation which includes the effect of an activation volume on the grain growth of olivine. The results indicate 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. I present estimations of viscosity as a function of depth considering the grain-size evolution predicted here. They indicate that the viscosity is approximately constant at the deep upper mantle conditions.