Lily Thompson

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, its position in the crystalline structure of hydrous phases greatly influences 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. I use synchrotron-based infrared (IR) spectroscopy to probe hydrogen bonding in mantle minerals at high-pressure to elucidate the complex relationship between structure, composition, pressure, and the phenomenon of hydrogen-bond symmetrization [e.g., Thompson et al., 2016b]. Supplementing experiments with DFT calculations to better explore these subtle phenomena, I aim to more completely describe the link between pressure-induced changes at the atomic scale and macro-scale material properties.

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

The solid inner core of Earth is composed of iron with the inclusion of light elements to compensate for the 2-5% difference between seismically obtained densities and the density of pure Fe at core pressure and temperature conditions. Hydrogen, the most abundant and lightest element in the solar system, plausibly contributes to this core density difference. To determine the likelihood that hydrogen exists in the Earth’s core, 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 mantle phases with liquid iron at the core-mantle boundary. I am currently investigating the outcome of these types of reactions by using in situ synchrotron X-ray diffraction to monitor reaction of hydrous silicate and iron at high pressure and temperature conditions. Once these reactions reach equilibrium, I thermally quench and recover 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].