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

地幔中的热对流驱动着板块构造。这个过程将地球内部的热量，即地核和地幔，传送到地表。通过地幔的热传输对于维持地核的地球发电机和保护地球表面免受太阳风影响的磁场至关重要。地幔动力学取决于对流、传导（接触）和辐射（如光）的传热速率。因此，量化地幔和地核材料的热导率对于理解地球的热系统、动力学和演化至关重要。然而，这是具有挑战性的，因为地球深处普遍存在极端的压力和温度。在这里，研究人员测量了下地幔矿物和核心富铁合金的导热性。他们对合成材料进行了实验，这些材料被压缩在两个相对的钻石的尖端，从而产生相应的高压。他们使用高功率激光加热标本并改变其温度。导电和辐射热特性是利用该团队先前开发的最先进的光谱技术提取的。该项目逐渐揭示了极端条件下热传输的物理特性。它推进了地球科学领域，以及与潜在能源应用相关的材料科学领域。它为华盛顿卡内基研究所的一名博士后提供支持和培训，并向本科生和高中生提供服务。该项目还促进了与欧洲科学家的国际合作。地球内部材料的热导率是控制地球热历史和动力学的关键参数。热性质限制了行星吸积和分异、地幔和地核的热演化以及地球磁场的产生等过程。在这里，研究小组的重点是更准确地限制通过外核和核幔边界（CMB）的热流。将激光加热金刚石砧槽（DAC）实验与深部地球温度剖面模拟相结合。该团队应用了瞬态加热和宽带光谱学，这是他们之前开发的两种新技术；这些可以量化热边界层的导电性和辐射性。研究人员开发了一种新技术——“脉冲电导率”技术，该技术与瞬态加热相结合，可以量化外核的导热性。这些实验是在高铁合金（包括熔体）上进行的，以及在大体积装置中或在DAC中原位合成的高质量相关矿物（例如，桥菱石的单晶）。起始材料是在气体混合气动悬浮激光炉中熔合在一起的高度均匀的玻璃。项目成果将提供通过核心和CMB的热通量的准确和一致的估计。这些结果对理解CMB现今的热通量、地球的热历史和下地幔底部的热传输机制（如通过超羽流）具有重要意义。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.