Modelling elements of microstructure in olivine

Thesis

To solve problems in the Earth and planetary sciences ranging from mantle convection and plate tectonics, magma dynamics, glacial-isostatic adjustment, post-seismic creep, seismic attenuation, and tidal dissipation, we require a deep understanding of the rheological, seismic, and chemical properties of silicate materials. Elements of microstructure, such as grain size and dislocation density, exert a strong control on these properties. In this thesis, I apply novel approaches to modelling the time evolution of these quantities.

I begin by examining grain-size evolution in the regime of normal grain growth, which is driven by surface energy reduction. I extend the canonical model of normal grain growth by limiting the interactions of a grain to its neighbourhood. In this extended model, heterogeneity in the local environments of grains is accounted for using a stochastic process. The model predicts the existence of a normal grain growth regime that evolves according to accepted grain-growth kinetics. Moreover, this regime is characterised by a grain-size distribution that agrees with observed distributions, in direct contrast to the distribution predicted by the canonical model.

I then examine dislocation-density evolution. I construct a new theory of the physical processes affecting dislocations: storage and recovery. The evolution of dislocation density is fundamentally connected to intracrystalline plastic deformation, and thus also makes predictions about viscosity. We calibrate our theoretical framework for olivine using data from steady-state deformation experiments. This model explains the empirical relationships among strain rate, applied stress, and dislocation density in disparate laboratory regimes. Indeed, it predicts the previously unexplained dependence of dislocation density on applied stress in olivine. The predictions of our model for geological conditions differ from direct extrapolations from experimental data. For example, the calibrated model predicts rapid, transient deformation in the upper mantle, consistent with recent measurements of post-glacial rebound.

The calibrated model of dislocation-density evolution sets out the hypothesis that dislocation recovery in olivine is controlled by pipe diffusion. This assumption is in direct contrast with previous work, which has assumed that dislocation recovery in olivine is controlled by lattice diffusion instead. The recovery process can be inferred from annealing experiments that track the decrease in dislocation density with time. Through statistical analysis of data from new annealing experiments, combined with previously published data from the literature, I determine that the dominant dislocation recovery process in olivine is likely controlled by pipe diffusion.

Authors: Breithaupt, T

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: recovery; olivine; plasticity; dislocation; dislocation creep; microstructure; annealing
• Subjects: Materials science; Microstructure; Olivine; Dislocations in crystals; Rheology; Earth sciences