Crystal Chemistry and Seismic Wavespeeds of Dense Oxyhydroxides: Hydrogen Transport in Earth's Lower Mantle

Citation

Strozewski, Benjamin Thomas (2025) Crystal Chemistry and Seismic Wavespeeds of Dense Oxyhydroxides: Hydrogen Transport in Earth's Lower Mantle. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/tejk-h629. https://resolver.caltech.edu/CaltechTHESIS:05202025-040444350

Abstract

In this thesis, I perform a thorough investigation of the electronic and elastic properties of the dense oxyhydroxide (Al,Fe)-phase H (Al_0.84Fe³⁺_0.07Mg_0.02Si_0.06OOH). This phase represents a realistic composition in a solid solution which has been hypothesized to carry 'water', in the form of hydrogen, to the lowermost depths of Earth's mantle. Its propensity for water storage and elastic properties are affected by hydrogen bond symmetrization and a high-low spin crossover of Fe³⁺ atoms, respectively. In order to determine changes in hydrogen bonding environment, I use synchrotron infrared spectroscopy and Raman spectroscopy, which identify O-H vibrational modes in the crystal structure and changes in their frequencies with pressure. These vibrational modes indicate that (Al,Fe)-phase H likely stores additional hydrogen as defects and that hydrogen bonds are disordered at ambient pressure due to the substitution of cations of different valence states. I find that hydrogen atoms become dynamically disordered across sites at 10 GPa and conclude that hydrogen bond symmetrization in (Al,Fe)-phase H takes place at 35 GPa. I use powder X-ray diffraction to constrain the equation of state of this phase, providing fundamental constraints on its incompressibility and density at high pressures. I complement this equation of state with study of the electronic environment around the Fe atoms via nuclear resonant forward scattering in order to constrain the spin crossover of Fe³⁺ atoms between 48 and 63 GPa. I use nuclear resonant inelastic X-ray scattering measurements to determine the seismic wavespeeds of (Al,Fe)-phase H to 120 GPa, the base of the lowermost mantle. The measured seismic wavespeeds are incorporated into whole-rock models which suggest that (Al,Fe)-phase H contributes to seismic heterogeneity in the mid-mantle and that hydrous metabasalt containing (Al,Fe)-phase H could contribute to seismic anomalies associated with the edges of large, low, shear velocity provinces in the lowermost mantle as it heats during descent in the lowermost mantle. The combined results of this thesis elucidate a complete compression pathway during transport of a dense oxyhydroxide into the lower mantle in the context of changes in its electronic and elastic properties. I offer several observables which may be used to detect the presence of this phase in subducted metabasalt and comment on the implications for hydrogen storage in the deep Earth.

Thesis Files