Knowledge of thermal conductivity and thermal diffusivity of the Earth's minerals under extreme conditions is important for understanding the physical and chemical processes and their evolution in the Earth. The rate of the heat transport through the mantle is crucial for the existence and stability of the Earth's magnetic field. The temperature distribution inside the Earth's mantle depends on the rate of heat transfer by convection, conduction, and radiation. An understanding of these processes requires knowledge of the thermal conductivity as a function of pressure and temperature.
In this project, we propose to determine the thermal conductivity of the Earth's key minerals under high P-T conditions by using optical spectroscopy in DACs (Diamond Anvil Cells) including pump-probe pulsed laser techniques. To determine the lattice thermal conductivity, we will measure the heat fluxes across the sample and their time history using time- and spatially resolved spectroradiometry and/or time-domain thermoreflectance (TDTR). Both continuous and pulsed laser techniques will be employed to access the thermal conductivity and diffusivity. To infer the radiative thermal conductivity, we will study the optical spectra of these mantle minerals in the ultraviolet-to-infrared spectral range at high P-T conditions (up to 130 GPa and 4000 K). Silicate perovskite and ferropericlase, the two dominant phases of the Earth's lower mantle, will be studied. Single crystals grown from pre-synthesized materials with a composition close to that in the Earth's mantle will be used as samples. We will also study the thermal conductivity of the postperovskite phase, synthesized by laser heating. To better understand the thermal transport and Earth's temperature profile near the Core-Mantle Boundary (CMB), we will measure the thermal conductivity of iron (using also electrical and optical conductivity methods). These experimental data will give a direct estimate of the radiative and conduction parts of the thermal conductivity, so they can be utilized in model calculations of the thermal processes in the Earth, thus providing a crucial test of these models and our current understanding of the Earth's interior.
This work will advance discovery and understanding by including graduate and undergraduate students as participants in the proposed research. A range of students, including area high school students, undergraduates, graduate students, and postdoctoral associates, will benefit from the scientific training at Carnegie that will be provided by participation in cutting-edge science that will be developed in the course of this work. We have developed collaborations with US and foreign Universities that allow us to train and incorporate graduate student research into our project. Moreover, we broaden participation of under-represented groups by establishing collaborations with Universities serving such groups and by including women and foreign postdoctoral associates (using exchange programs) into the research. Our project enhances infrastructure for research and education through several fruitful collaborations with the US and foreign Universities. We offer the use of our Carnegie optical facilities for our collaborators (and also NSF-supported programs such as COMPRES and the Carnegie Summer Intern Program, as well as the DOE-supported CDAC high-pressure center, headquartered at Carnegie).
Intellectual merits Knowledge of the depth dependent thermal conductivity of materials in the Earth’s and planetary interiors is the key to understanding the thermal history of the core and mantle and their dynamics, which are related to the processes of planetary accretion and differentiation, the time evolution of mantle and core temperatures, and the generation of the Earth’s magnetic field. The laboratory experiments, which have been performed in this work, are aimed to constrain the thermal conductivity of the Earth’s mantle and core materials at the pressure-temperature (P-T) conditions of the deep Earth’s interior, which are vitally important for theoretical modeling of the Earth’s interior including those which results in new features characterizing the mantle thermal convection. We have performed direct measurements of the conductive and radiative thermal conductivity of mantle minerals under extreme P-T conditions by using optical spectroscopy and pulsed laser techniques in diamond anvil cells (DAC)(Fig. 1). A transient (or flash) heating technique (THT)(Fig. 4) and time-domain thermoreflectance (TDTR)[1] have been used to determine the conductive thermal properties, while for the assessment of the radiative thermal conductivity we have measured the materials optical properties. The latter includes the use of pulsed laser based broad band optical spectroscopy (BBOS). The techniques developed in this work have been applied to study deep Earth and planetary materials subjected to high P-T conditions created in resistively and laser heated DAC. Major Results Lattice thermal conductivity. We have developed and used a transient-heating DAC technique to measure thermal diffusivity, which involves time-resolved radiometry combined with a pulsed infrared (IR) laser source. We have developed and utilized a fast pump-probe laser technique— time-domain thermoreflectance - to measure the thermal conductivity of MgO [1] and ferropericlase (FP) Mg0.94Fe0.06O at pressures up to 60 and 30 GPa at room temperature. Our results provide new constraints for pressure (depth) dependencies of the thermal conductivities of Fe bearing minerals and other materials. Specifically, we found that the Leibfreid-Schlomann equation works well to describe the pressure (depth) dependence of thermal conductivity of pure materials (e.g., MgO), however there is a large effect of alloying (e.g., Fe substitution), which remains to be studied comprehensively. We found that the lattice thermal conductivity of FP is 5.7(6) W/(m*K) at ambient conditions, which is almost 10 times smaller than that of pure MgO; however, it increases with pressure much faster (6.1(7)%/GPa vs 3.6%/GPa) (Fig. 2). The experimental results have been complemented by model calculations which take into account the effects of temperature and mass disorder on the deep Earth’s materials. The calculated thermal conductivity at the core-mantle boundary is smaller than the majority of previous predictions resulting in an estimated total heat flux of 10.4 TW, which is consistent with modern geomodeling estimates. Radiative thermal conductivity. We found that the radiative conductivity of silicate perovskite (PV) and FP is controlled by the amount of ferric iron, Fe3+, while changes of radiative thermal conductivity at the high-to-low spin transitions, pressure (depth), and also due to variation of temperature are relatively small (Fig. 3). However, our results show a substantial reduction with pressure of radiative thermal conductivity of the Earth’s minerals and also of iron-enriched dense silicate glasses, which were considered as a proxy to silicate melts [2] with implications for the evolution of the mantle such as generation and stability of thermo-chemical plumes in the lower mantle. On the other hand, the results for hydrous wadsleyite and ringwoodite up to a maximum of 26 GPa and 823 K show that the mantle transition zone may contribute significantly to heat transfer making it important in controlling geodynamic processes in Earth’s mantle [3]. Broader Impacts. The technical developments of this work enabled measurement of the lattice and radiative thermal conductivity of the deep Earth’s materials, knowledge of which are important for understanding of the Earth thermal history and dynamics. These have been also applied for a number of topics related to geochemistry (e.g., spin transition), physics and chemistry of materials relevant for planetary interiors and to physics and chemistry of materials at high pressures. A number of postdoctoral associates working on the projects have been trained in novel laser spectroscopy techniques and their applications to geo- planetary and material sciences. [1] D.A. Dalton, W.-P. Hsieh, G.T. Hohensee, D.G. Cahill, A.F. Goncharov, Effect of Mass Disorder on the Lattice Thermal Conductivity of MgO Periclase Under Pressure: Implication for the Deep Earth Heat Flow, Physics Reports, 3 (2013) 2400. [2] M. Murakami, A.F. Goncharov, N. Hirao, R. Masuda, T. Mitsui, S.-M. Thomas, C.R. Bina, High-pressure radiative conductivity of dense silicate glasses with implications for dark-magmas at Earth’s core-mantle boundary, Nature Communications, 5 (2013) 5428. [3] S.-M. Thomas, C.R. Bina, S.D. Jacobsen, A.F. Goncharov, Radiative heat transfer in a hydrous mantle transition zone, Earth and Planetary Science Letters, 357-358 (2012) 130–136.
