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Carbon in deep Earth from high-pressure and high-temperature studies of the Fe-C system

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Title:Carbon in deep Earth from high-pressure and high-temperature studies of the Fe-C system
Authors:Lai, Xiaojing
Contributors:Chen, Bin (advisor)
Geology and Geophysics (department)
Date Issued:2019
Publisher:University of Hawai'i at Manoa
Abstract:Carbon in deep Earth, as well as the nature and extent of deep carbon cycles, is essential for understanding the physical and chemical evolution of habitable planets. This dissertation includes three projects to address carbon in the Earth’s deep interior from the perspectives of high-pressure mineral physics.
The first project (Chapter 3) comprises experimental and computational results on the structural evolution of iron-nickel liquids alloyed with carbon at high pressures. Our X-ray diffraction experiments up to 7.3 gigapascals (GPa) demonstrate that Fe90Ni10 liquids alloyed with 3 and 5 wt.% carbon undergo a polyamorphic liquid structure transition at approximately 5 GPa. Corroborating the experimental observations, our first-principles molecular dynamic calculations reveal that the structural transitions result from the marked prevalence of three-atom face-sharing polyhedral connections in the liquids at >5 GPa. The structure and polyamorphic transitions of liquid iron-nickel-carbon alloys govern their physical and chemical properties and may thus cast fresh light on the chemical evolution of terrestrial planets and moons.
The second project (Chapter 4) concerns the high-pressure thermoelastic properties of one of the Earth’s inner core candidates, iron carbide, Fe7C3. In this study, we performed synchrotron-based single-crystal X-ray diffraction experiments using an externally-heated diamond anvil cell to determine the crystal structure and thermoelastic properties of Fe7C3 up to 80 GPa and 800K. Our diffraction data indicate that Fe7C3 adopts an orthorhombic structure under the experimentally investigated conditions. The pressure-volume-temperature relations of Fe7C3 were obtained by fitting the high-temperature Birch-Murnaghan equation of state. We also observed an anisotropic elastic response to changes in pressure and temperature along the different crystallographic directions. Fe7C3 has strong anisotropic compressibilities with the linear moduli Ma > Mc > Mb from zero pressure to core pressures at 300K, revealing the b axis to be the most compressible. The thermal expansion of c3 is approximately four times larger than that of a3 and b3 at 600K and 700K, implying that high temperature may significantly influence the elastic anisotropy of Fe7C3. Therefore, the effect of high temperature needs to be considered when using Fe7C3 to explain the anisotropy of the Earth’s inner core.
The third project (Chapter 5) examines the high-pressure phase stability and melting behavior of the Fe-C-(H) system by both multi-anvil press and diamond anvil cell experiments. Metallic iron reacted with an organic C-H compound, which served as carbon and hydrogen source, under conditions of high pressure and temperature. With excess C-H compound, Fe carbide and molecular hydrogen formed first from the reaction at high pressures and relatively low temperature. With increasing temperature, Fe hydride and diamond were found to form. With excess Fe, by contrast, the presence of hydrogen depressed the melting temperature of the Fe-C system, such that the eutectic melting temperatures for both the Fe-C and Fe-C-H systems are below the mantle geotherm. Those Fe-rich melts may facilitate the cycling of subducted carbon and hydrogen in the deep mantle. These melts may provide a necessary melt environment for the growth of macro diamonds in deep mantle; they could also serve as a potential reservoir for both carbon and hydrogen in the mantle.
Description:Ph.D. Thesis. Ph.D. Thesis. University of Hawaiʻi at Mānoa 2019
Pages/Duration:117 pages
Rights:All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections: Ph.D. - Geology and Geophysics

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