Experimental constraints on the chemical and physical states of Earth’s lower mantle

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Fu, Suyu

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Earth’s lower mantle is the most voluminous layer of our planet, extending from approximately 670 to 2900 km in depth. However, there are still plenty of enigmatic phenomena in the region from geochemical, seismological, and geodynamic observations, such as the lower-mantle mineralogy and temperature, seismic anisotropy, origin of ultra-low velocity zones (ULVZs), and water reservoir. In this dissertation, I used diamond anvil cells (DACs) coupled with laser and X-ray spectroscopic techniques, such as Brillouin light scattering, impulsive stimulated light scattering, and X-ray diffraction, to experimentally measure properties of candidate lower-mantle minerals under relevant pressure and temperature (P-T) conditions, including sound velocity, density, melting behavior, and water solubility. These results were applied to better understand these aforementioned scientific enigmas on the lower-mantle chemistry and physics. Bridgmanite [(Mg,Fe,Al),(Fe,Al,Si)O3] and ferropericlase [(Mg,Fe)O] are the two most abundant minerals in the lower mantle. Investigating effects of Fe and Al substitution as well as Fe spin crossover on their elastic properties and melting behavior is of particular importance to our unraveling the physical signature and dynamic history of Earth’s deep interior. Here, I measured sound velocity, density, and single-crystal elasticity of bridgmanite at high pressure and room temperature. Based on results on polycrystalline Fe- and (Al,Fe)-bearing bridgmanite, I quantitatively evaluated Fe and Al effects on its high-pressure elasticity. By comparing mineral physics models on the velocity and density of candidate mienrals with one-dimensional seismic profile, my results yield an internally consistent lower-mantle mineralogy and adiabatic goetherm. In addition, the elastic anisotropy of deformed bridgmanite under relevant P-T and stress conditions could plausibly explain seismically-observed shear wave radial anisotropy at the topmost lower mantle near subducted slabs. Furthermore, I conducted high P-T melting experiments on ferropericlase up to ~120 GPa and ~5400 K in laser-heated DACs across the spin crossover. Results show that the existence of dense iron-rich low-spin residual (Mg,Fe)O melt over geodynamic evolution could produce characteristic seismic signatures of ULVZs at the lowermost mantle. Abundant water could be transported to Earth’s deep interior via subduction and dehydration processes, however, water reservoir in the lower mantle is still poorly constrained due to limited and inconclusive literuature studies on water solubities in candidate minerals. High-quality single-crystal (Al,Fe)-bearing bridgmanite were grown from hydrous melt in a Kawai-type apparatus. Reliable measurements on its water concentration show as high as ~1020(±70) ppm wt water in the structure. The high water solubity in bridgmanite can greatly affect our understanding of lower-mantle geophysics, such as a deep-mantle water reservoir and dehydration melting at the topmost lower mantle.


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