Thermal conductivity of lower mantle minerals and outer core alloys studied by combined fast pulsed laser and optical spectroscopy techniques

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Thermal convection in the Earth’s mantle drives plate tectonics. This process transports heat from within the planet, in its core and mantle, to the surface. Heat transport through the mantle is crucial for maintaining the geodynamo in the Earth's core, and the magnetic field which shields the surface from the solar wind. Mantle dynamics depends on the rate of heat transfer by convection, conduction (contact), and radiation (e.g., light). Quantifying the thermal conductivity of mantle and core materials is, thus, critical for understanding Earth’s thermal system, dynamics, and evolution. It is, however, challenging because of the extreme pressures and temperatures prevailing in Earth’s deep interior. Here, the researchers measure the thermal conductivity of lower-mantle minerals and core Fe-rich alloys. They carry out experiments on synthetic materials compressed at the tips of two opposing diamonds, which produces the relevant high pressures. They use high-power lasers to heat up the specimens and vary their temperature. Conductive and radiative thermal properties are extracted using state-of-the-art spectroscopic techniques previously developed by the team. The project gradually unveils the physics of thermal transport at extreme conditions. It advances the Earth Sciences field, as well as adjacent fields in Materials Sciences with potential energy applications. It provides support and training to one postdoctoral associate at Carnegie Institution of Washington, and outreach towards undergraduate and high-school students. The project also fosters an international collaboration with European scientists. The thermal conductivity of materials in Earth’s interior is a key parameter in controlling the thermal history and dynamics of the planet. Thermal properties constrain processes involved in planetary accretion and differentiation, the thermal evolution of mantle and core, and the generation of Earth’s magnetic field. Here, the team focusses on constraining more accurately the heat flow through the outer core and core-mantle boundary (CMB). Experiments in the laser-heated diamond anvil cell (DAC) are combined with modeling of deep Earth temperature profiles. The team applies transient heating and broad band optical spectroscopy, two novel techniques they previously develop; these allow quantifying the conductive and radiative conductivities of the thermal boundary layer. The researchers develop a new technique – the "pulsed electric conductivity" technique – which applied in combination with transient heating allows quantifying the thermal conductivity of the outer core. These experiments are performed on Fe-rich alloys (including the melts), and high-quality relevant minerals (e.g., single crystals of bridgmanite) synthesized in large-volume devices or in situ in the DAC. The starting materials are highly homogeneous glasses fused together in a gas-mixing aerodynamic levitation laser furnace. The project outcomes will provide accurate and consistent estimates of the heat flux through the core and the CMB. These results have strong implications for the understanding of the present-day heat flux at the CMB, the thermal history of Earth and heat transport mechanisms at the bottom of the lower mantle (e.g., via superplumes).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.