My research harnesses the tools of mineral physics to produce constraints for the fields of geophysics, geodynamics, and planetary science—enhancing our understanding of the distribution and cycling of volatile species in planetary interiors.

How is hydrogen stored in the deep Earth?

Hydrogen is known to have profound impacts on the melting temperature, rheology, electrical conductivity, and atomic diffusivity of geological materials. Yet despite decades of research on the subject, there is little consensus on the quantity or location of hydrogen within the deep Earth. I investigate which hydrogen-bearing phases are stable under the pressure-temperature (P-T) conditions applicable to varied regions of the Earth’s interior. By pairing phase stability constraints with equations of state, I inform seismic observations to help refine our understanding of the planet’s interior.

I use synchrotron X-ray diffraction (XRD) to pair phase stability constraints with equations of state, informing seismic observations in order to refine our understanding of our planet’s interior [e.g., Thompson et al., 2016a]. Furthermore, recent advances in computational techniques render density functional theory-based (DFT) calculations increasingly important in the pursuit of determining the stability and properties of high-pressure phases. I perform DFT calculations that are complementary to my experimental determinations, as well as using calculations to determine additional geophysical constraints (e.g. sound velocities) to benchmark against seismic observations [e.g., Thompson et al., 2017].

How does hydrogen influence material properties?

Despite hydrogen’s diminutive size, understanding its position in the crystalline structure of hydrous phases can be potent in understanding the behavior of the bulk material as a whole. For example, the phenomenon of pressure-induced hydrogen bond symmetrization is reported to have a significant influence on a mineral’s bulk modulus and its pressure derivative. Additionally, a strong negative correlation has been established between OH-stretching frequency and H/D isotopic fractionation factor, potentially providing insight into the difference in observed δD of seawater (0‰) and mineral assemblages of upper mantle origin (~80‰).

Due to the small scattering cross section of a hydrogen atom, its detection from powder X-ray diffraction is daunting. To explore hydrogen bonding in deep Earth phases I employ three complementary techniques: (1) Fourier transform infrared (FTIR) spectroscopy, (2) Raman spectroscopy, and (3) DFT calculations. Synchrotron-based FTIR spectroscopy enables the direct, in situ detection of hydrogen bonding in hydrous Earth materials at extreme conditions, allowing us to elucidate the complex relationship between structure, composition, pressure, and the phenomenon of hydrogen-bond symmetrization [e.g., Thompson et al., 2016b]. Raman spectroscopy provides a secondary means of exploring chemical bonding and can be employed in conjunction with high pressures without the necessity of a synchrotron source. These two spectroscopic techniques naturally dovetail to create a more holistic picture of the pressure and composition-induced changes to the bonding in hydrous phases. Lastly, to create a theoretical framework for interpreting my experimental work, I perform DFT-based calculations to model O-H bonding as a function of pressure.

Is hydrogen one of the light elements in the Earth’s core?

The Earth’s core is composed primarily of iron, but also contains light elements that cause differences between the seismically determined densities and velocities of the core and the properties of pure iron at core conditions. Hydrogen, the most abundant and lightest element in the solar system, plausibly contributes to this core density difference. To determine the likelihood that the Earth’s core contains hydrogen we must clarify two key areas of uncertainty: (1) a path by which hydrogen is introduced into the core, and (2) the influence of hydrogen on the geophysical parameters of iron alloys.

A likely pathway of hydrogen into Earth’s core is the reaction of hydrogen bearing phases with liquid iron at the core-mantle boundary. To explore this further, I use in situ synchrotron X-ray diffraction to monitor the reaction of hydrous silicate and iron at high pressure and temperature conditions. Once these reactions reach equilibrium, I thermally quench the samples and evaluate the resultant phase assemblages using ex situ analytical techniques.

Determining the influence of hydrogen on the seismic signature of iron alloys requires X-ray diffraction combined with nuclear resonance inelastic X-ray scattering (NRIXS). NRIXS enables the determination of the partial phonon density of states of iron-bearing phases at pressures exceeding those of the Earth’s outer core. I use these methods to determine the influence of hydrogen on the density and sound velocities of the iron alloys, constraining the amount to which hydrogen may contribute to the light element budget of the Earth’s core [e.g., Thompson et al., 2018].