thesis

Sound velocity profiles of lower mantle minerals : implications to geophysics and geochemistry of the deep Earth

Abstract

In this dissertation, I used heated diamond-anvil-cell techniques to simulate the environment of the Earth interior. Spectroscopic measurements, including Brillouin Light Scattering, Impulsive Stimulated Light Scattering and X-ray diffraction were performed to obtain elastic parameters of candidate lower mantle minerals, ferropericlase, bridgmanite and post-perovskite in a diamond anvil cell. Obtained single-crystal elastic constants, density, bulk and shear moduli allow us to derive seismic velocities, anisotropy, and lateral heterogeneity ratio to be compared with seismic observations. The results in this dissertation shed light on the observed seismic heterogeneity, anisotropy, and discontinuity within the Earth’s lower mantle. Thermoelastic modelling based on high P-T elasticity data from ferropericlase suggested that thermal perturbation dominates the lateral heterogeneity in upper part of the lower-mantle, but is not sufficient to explain the high lateral heterogeneity ratio in the lower-part of lower mantle. Chemical perturbations and/or spin transition effects can be the potential cause of seismic heterogeneity towards the lower part of lower mantle. By investigating the spin transition effects on the single-crystal elasticity of ferropericlase, we found drastically softened C₁₁ and C₁₂ within the spin transition, while C₄₄ is not affected. This leads to significant Vₚ/Vₛ reduction in the mid-lower mantle. Single-crystal elastic constants of two Al, Fe-bearing bridgmanite have been reliably resolved at high pressures. Our results suggested that coupled Fe and Al substitution can greatly reduce the Vₛ and slightly affect Vₚ and Vᵩ of bridgmanite. In addition, the enhanced Vₛ anisotropy of Fe and Al enriched bridgmanite may contribute to the seismic anisotropy in Earth’s lower mantle. The presence of Fe, Al enriched bridgmanite could exhibit distinct seismic features in Earth’s lower mantle. The existence of post-perovskite in the D" region is still uncertain. Direct Vₚ and Vₛ measurements performed on Fe-bearing post-perovskite that compared with extrapolated velocities of bridgmanite showed a Vₛ increase and Vₚ decrease across the phase boundary, which are consistent with the seismic discontinuities especially observed beneath the Cocos Plate to the first order. The measured velocity profiles of post-perovskite provide strong mineral physics constraints on the velocity profiles for the existence of Fe-bearing post-perovskite in the D" layer.Geological Science

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