My recent research focuses on the ultrafast dynamics of thermal phonon transport and phase transition mechanism of Earth materials under extreme conditions. I have successfully combined ultrafast optical pump-probe techniques with high-pressure diamond anvil cells to study thermal conductivity, elastic property, lattice dynamics, and phase transition of minerals at high pressures. In particular, my research accomplishments open up new routes to explore thermal conductivity of deep Earth materials with unprecedented measurement accuracy, enabling significant conceptual advances in the thermal evolution and geodynamics in Earth and planetary interiors. Some of my recent works are summarized as follows:
Thermal conductivity, sound velocity, and lattice dynamics of Earth and planetary materials: towards better understanding of the thermal history of Earth and icy body.
• Deep Earth materials
The complex thermo-chemical structure and dynamic processes of the Earth interior present us with a rich array of seismic and geodynamic phenomena that are not fully understood yet. Answering these fundamental questions requires knowledge of the physical properties of its constituting materials, such as thermal conductivity, sound velocity, and molecular bonding behaviors. We have studied the thermal conductivity of lower-mantle bridgmanite and ferropericlase up to the lowermost mantle pressures. Our precisely measured thermal conductivity of Fe-bearing bridgmanite at the lowermost mantle pressure is about twice smaller than the Mg-bridgmanite that was previously thought to represent most of the lower-mantle thermal conductivity. We further modeled our new data with maps of thermal and compositional anomalies of the lower mantle and found that thermal conductivity decreases by up to 50% within the large low shear-wave velocity provinces (LLSVPs), and the core-mantle boundary heat flux decreases accordingly.
In addition, we have also demonstrated strong influences of iron on the lattice thermal conductivity of ferropericlase and the dynamics of the entire lower mantle. For the first time, we observed a significant drop in the lattice thermal conductivity of an iron-rich ferropericlase (Mg0.44Fe0.56O), by a factor of 1.8, across the pressure-induced iron spin transition, indicating an enhanced iron substitution effect in the low-spin state. Combined with the bridgmanite data, our modeling offers a self-consistent radial profile of lower-mantle thermal conductivity, which shows a two-fold increase from top to bottom of the lower mantle. Importantly, such increase in thermal conductivity may delay the cooling of the core, while its decrease with iron content may enhance the dynamics of LLSVPs. Our findings further indicate that, if hot and strongly enriched in iron, the seismic ultra-low velocity zones have exceptionally low thermal conductivity, thus delaying their cooling.
• H2O, H2O-volatile mixture, and hydration effect
H2O is one of the most important materials in our universe, and therefore its physical properties under extreme conditions are critical to better understand many dynamics in Earth and icy body. We explored thermal conductivity, sound velocity, molecular vibrational frequencies, and bonding behavior of H2O-CH3OH mixtures under high pressure. Our results showed that the complex pressure evolution of Raman frequencies and intensities of CH3OH as well as the hysteresis of the water-ice VI phase transition suggests a pressure-induced segregation of CH3OH from ice VII. Moreover, we found that both thermal conductivity and sound velocity are reduced significantly due to the presence of CH3OH within the H2O. Our modeling further indicated that, for a potential icy super-Earth, both the heat conduction and convection power are substantially reduced due to the CH3OH-reduced thermal conductivity, demonstrating the important role of volatiles on the dynamics and thermal evolution within large icy planets and satellites.
In addition, Earth’s water cycle enables incorporation of water in mantle minerals that can influence many physical properties of the mantle. In particular, the lattice thermal conductivity of mantle minerals is critical to control the temperature distribution in Earth’s mantle and crust, which in turn may influence the evolution of plate tectonics. We have measured the thermal conductivity of hydrous and nominally anhydrous San Carlos olivine (Mg0.9Fe0.1)2SiO4 (Fo90) at high pressures. We found that the thermal conductivity of Fo90 incorporated with large amounts of water is substantially smaller than the nominally anhydrous Fo90 near the mantle transition zone. We further demonstrate that the hydration-reduced thermal conductivity of hydrous minerals can induce a temperature anomaly at the center of a subducting slab with hydrated oceanic crust, which slows down the olivine-wadsleyite-ringwoodite transformation rate and preserves the meta-stable olivine to greater depth of the transition zone.