Melting of Earth Materials: Constraints from both Experiment and Density Functional Theory [electronic resource].

9781088381755

Ann Arbor : ProQuest Dissertations & Theses, 2019.

1 online resource (207 p.)

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Melting is a first-order phase transition with volume and entropy varying discontinuously. This discontinuity occurs along with dramatic changes in the physical properties of materials (e.g., elasticity, electrical conductivity, thermal conductivity). The melting behavior of minerals is of great interest in Earth science because of its essential role in understanding the thermochemical evolution of Earth and planets in general, including magma ocean and melting produced by Moon formation and impacts, seismic wave velocity reduction in the lowermost mantle, and rheological properties of mantle minerals. Here I integrate both high-pressure, high-temperature experiments and first-principles molecular dynamics simulations (FPMD) to study the melting of two important Earth materials, (Mg,Fe)O ferropericlase and FeO2Hx iron peroxide (P phase).Ferropericlase is semi-transparent and exhibits wavelength-dependent absorptivity and emissivity. In order to accurately measure the melting temperatures from its thermal spectrum collected in laser-heated diamond-anvil cell (LHDAC) experiments, I developed an inverse modeling method taking into account the radiative heat transfer process, wavelength-dependent absorption, and temperature gradients. This method, for the first time, explicitly considers the effects of wavelength-dependent absorption and has implications for temperature measurements of semi-transparent materials in LHDAC experiments.I successfully measured and corrected the melting temperatures of ferropericlase up to 83 GPa using this inverse modeling method. I found a pronounced melting temperature depression at pressures greater than ~40 GPa, where the electronic spin transition of Fe2+ occurs, creating local minima in the solidus and liquidus melting curves. This is the first time the effects of spin transition on melting have been proposed to behave in this manner. This melting depression can be explained within the framework of Lindemann’s Law for a Debye-like solid. The spin transition of iron from high to low reduces the molar volume and the bulk modulus of the crystal, resulting in a decrease in Debye frequency and consequently lowering the melting temperature. Thermodynamically, the melting depression may derive from a more negative Margules parameter for a liquid mixture of high- and low-spin endmembers than that of a solid mixture.The thermodynamic properties of the MgO-FeO system of MgO-FeO binary system were also resolved based on the experimental results and extrapolated to core-mantle boundary (CMB) conditions, based on which, the MgO-FeO binary phase diagram was constructed at 136 GPa. Implications for ultra-low velocity zones (ULVZs) and enrichment at CMB are discussed.Furthermore, the melting of ferropericlase is intimately related to its other physical properties. The relative viscosity profiles of ferropericlase based on the melting phase relation is calculated using homologous temperature scaling and exhibits a 10–100 times jump from ~750 km to ~1000–1250 km, before decreasing at greater depths, which offers a unified explanation for observed stagnating slabs and deflected plumes at these depths.The melting of P phase has been studied using the density functional theory at the lowermost mantle conditions. The results suggest that P phase is likely molten near the CMB and thus cannot be the source materials of ULVZs. Nevertheless, the molten products are characterized with a smaller density and bulk sound velocity compared to the isochemical P phase. As such, small amounts of liquid FeO2Hx could account for the observed seismic anomaly of ULVZs if stably maintained in the ULVZs.

Dissertations & Theses @ Yale University.

Books / Online / Dissertations & Theses

English

January 17, 2020

Thesis (Ph.D.)--Yale University, 2019.

Yale University. Geology and Geophysics.