Research into the mechanical properties of materials has been a focus of study for over a century leading to sophisticated advances in our understanding of not only materials of everyday use, but also minerals within the Earth and of other planetary bodies. The conditions within the Earth range over several thousands of Kelvins (K) in temperature and several million times atmospheric pressure (stated in gigapascals (GPa)). Adding to these extreme conditions, minerals under these conditions are also undergoing continuous microstructural evolution through processes such as phase transformation, plastic deformation, and dynamic recrystallization for which no satisfactory constrains have been achieved. Several experiments have focused on determining the various properties of minerals under lower mantle conditions using diamond anvil cells combined with synchrotron radiation. While enlightening, these have been relegated to either bulk statistical approaches or individual crystal studies, both limited in the achievable spatial resolution which is a crucial aspect when trying to scale from the microstructural level to large scale physical behavior. From a computational perspective, the processes in the Earth have also been a key area of interest with several modeling attempts attempting to constrain the active deformation mechanisms in minerals that would give rise to current observations of seismic anisotropy. Many models have proven insightful, but again focused on select regions and again lacked the spatial resolution to resolve the long-wavelength anisotropic structures of the lower mantle. This thesis aims to provide a route, both experimentally and computationally, to expand the current understanding of the behavior of lower mantle minerals on scales spanning from individual grain behavior to that of a polyphase aggregate. Quantitative information of the stress environment, phase distribution, as well as crystallographic orientations and microstructural relations are obtained across a range of mantle conditions using in-situ high energy synchrotron radiation combined with far field high energy diffraction microscopy, a.k.a. multigrain crystallography (MGC). Plasticity modeling is also performed under conditions of the lower most mantle to ascertain the active deformation mechanisms at play that could give rise to the long wavelength anisotropic structure observed near the core mantle boundary (CMB).In Chapter 2, MGC is applied to a sample of San Carlos olivine beginning at ambient conditions and through the olivine γ-ringwoodite phase transition. At high pressure and ambient temperatures, by measuring the evolution of individual Bragg reflection morphology, olivine shows profuse angular intensity broadening consistent with the onset of yielding at a measured stress of ~1.5 GPa, considerably lower than previously reported under similar conditions, which may have implications for constitutive models of lithospheric strength and dynamics. Furthermore, γ - ringwoodite phase was found to nucleate as μm (micrometer) to sub- μm grains imbedded with small amounts of a secondary phase at 15 GPa and 1000 °C. Using MGC, information of individual crystallites from a secondary unknown phase were extracted and refined where it was found to have a structure consistent with the phase recently named Poirierite formed from shock metamorphism and has been described in chondritic meteorites (Tomioka et al. 2021) but yet to be seen during high P-T experiments.
Chapter 3 continues deeper into the Earth’s mantle applying MGC to investigate the microstructural evolution during the formation of bridgmanite ((Mg,Fe)SiO3) and ferropericlase ((Mg,Fe)O) from olivine. A weaker ferropericlase forms with random orientations as a fine-grained (< μm - 2 μm ) interconnected matrix around a larger (2-9μm) and higher stressed bridgmanite phase; a configuration that has been implicated in slab stagnation as well as plume deflection in the upper part of the lower mantle through influences on the local viscocity structure surrounding subducting slabs. Pairs of individual twins in the bridgmanite phase are extracted with μm-scale spatial resolution confirming a previously reported {110} twinning relation previously only seen in ex-situ experiments which appears to act as a more energetically favorable mechanism of stress relaxation during grain growth compared to plastic deformation. When combined with the random orientations of ferropericlase, this twining mechanism in bridgmanite would act to reduce any seismic anisotropy from the aggregate.
Chapter 4 implements plasticity modeling to investigate the possible link between plastic deformation and phase transitions with the presence of seismic anisotropy at the base of the Earth’s lower mantle. To date there has yet to be consensus on the specific mechanisms that could give rise to the observed shear-wave anisotropic signatures at depths of 2500-2800 km. Strong anisotropy in magnesium post-perovskite (pPv) has been invoked, but different studies disagree on the dominant slip systems at play. In the model presented in this thesis, the most recent results from atomistic calculation and high-pressure deformation experiments are coupled with a realistic pyrolytic composition composed of 74% bridgmanite 17% and 9% calcium perovskite and a 3-dimensional geodynamic model, to compare the resulting deformation-induced anisotropy with seismic observations of the lowermost mantle. Additional complexities are included accounting for the P-T dependent forward and reverse phase transitions from bridgmanite to pPv. When including this feature, a strong textural inheritance is found between bridgmanite and pPv with (001) dominant slip, not seen when bridgmanite converts to pPv with (010) dominant slip. It is also seen that either dominant (001) or (010) slip can both explain the seismically observed anisotropy in colder regions where downwellings turn to horizontal flow, but only a model with dominant (001) slip matches seismic observations at the root of hotter large-scale upwellings.
