Radiative Thermal Conductivity of Upper Mantle and Transition Zone Minerals

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The structure and dynamics of our planet's interior depend crucially upon heat flow and thus upon the thermal conductivity of its constituents. Thermal conductivities of Earth's mantle materials, such as olivine, majorite, wadsleyite and ringwoodite, are important parameters for geodynamic models of mantle convection. Differences in radiative thermal conductivities of these minerals will affect models of lithospheric geotherms, subduction dynamics, and the structure of lithospheric slabs. However, there is a lack of experimental data about radiative thermal conductivities, especially at the high pressures and temperatures that exist in the deep interior. This experimental study aims to complement evolving theoretical frameworks, extend ongoing projects to all major constituents of the Earth's mantle, and indirectly measure the temperature-pressure variation of radiative thermal conductivity using in-situ optical spectroscopy. A primary goal of the project is to further our understanding of a major driving force of nature - heat flow within the Earth. This project builds on recent in-situ optical measurements, in which two major transition zone phases, hydrous wadsleyite and hydrous ringwoodite, were studied for the first time at simultaneous high pressure and high temperature. The PI reported radiative thermal conductivities of 1.5 Wm-1K-1 for hydrous wadsleyite and 1.2 Wm-1K-1 for hydrous ringwoodite at transition zone conditions. For anhydrous wadsleyite and ringwoodite radiative conductivities were analytically derived to be 40% and 33% higher, respectively. The results indicate that the transition zone may contribute significantly to heat transfer in the mantle and demonstrate the importance of radiative heat transfer in controlling geodynamic processes. The current project is an extension of those measurements to study olivine, majorite, akimotoite and phase D. The effect of sample thickness and compositional changes (such as iron and water concentrations) on thermal conductivity properties of majorite, akimotoite, phase D and olivine and its high-pressure polymorphs ringwoodite and wadsleyite will be systematically investigated. The experiments are very timely, since theoretical models are in development and experimental data are needed to support such models. This research is of highly collaborative nature, and will take advantage of the expertise and facilities of laboratories across the nation, as well as internationally. This work will illuminate a fundamentally interdisciplinary topic of deep concern to a broad audience, including mineral physicists, seismologists, geodynamicist, geochemists, and planetary scientists. This study will provide a valuable learning experience for undergraduate students, as well as important data, which will contribute to our knowledge of heat transfer in the Earth and eventually lead to refined thermal models of the Earth's interior.