A Journey from Partition Coefficients in Melt Inclusions of Lunar Samples to the Prediction of Vibrational Modes under High P/T Conditions and the Thermodynamics of Sulfide-Mediated Redox Reactions in Sediments

摘要

The research topics in this dissertation vary from high-temperature, high-pressure processes in planetary science to low-temperature environmental problems on the Earth's surface. Chapters 2 and 3 focus on olivine, an important mineral not just in the Earth's interior but also on many planetary bodies. Its morphology, chemical composition, element distribution with another mineral/melt phase, and mineral properties provide valuable information about the magma's composition and physical conditions, cooling rates, and even the formation of the solar system. Chapter 2 systematically studies the element partitioning between host mineral olivine and its melt inclusions in lunar basalts. Results show that the partition coefficient of V (DV) is significantly higher, and DCr is lower in lunar basalts than in terrestrial basalts. The partitioning difference can better explain the element behavior in the two bodies. The results also show that the V/Cr ratio is constant in lunar basalts, which is essential for constraining concentrations of Cr (slightly volatile and siderophile) and V (slightly siderophile) in the bulk silicates of the Moon. Chapter 3 uses density functional theory (DFT) to simulate, for the first time, the vibrational properties of forsterite (Mg-end olivine) under high temperature and pressure simultaneously. Vibrational spectra (absorption infrared and Raman) are an essential tool for studying the mineral structure, composition, and thermodynamic properties that are sometimes challenging to obtain experimentally. This work aims to improve and enhance the IR and Raman databases in the entire P/T space to better understand the lattice dynamics of olivine at an atomic level. We show that lattice vibrational frequencies are just a function of lattice structures when anharmonicity is insignificant. Using the Quasi-Harmonic Approximation (QHA), we obtain the cell parameters under different temperatures and pressure, and from these, the infrared and Raman spectra. Good agreement is achieved between calculations and experiments for forsterite's vibrational frequencies and thermodynamic properties. Results show that the vibrational frequencies change approximately linearly with temperature/pressure. The slope of each mode is different depending on if they are associated with external lattice modes or internal stretching and bending within the SiO4 tetrahedra. Computational simulation is also an excellent way to study the thermodynamics, kinetics, and reaction mechanism of geochemical reactions in aqueous solutions. Chapter 4 determines the thermodynamics and electronic interactions of redox reactions between the organic reductant hydroquinone and inorganic oxidants Fe(III)/Mn(III) and their (co-)adsorption on a mackinawite cluster (Fe8S8). Iron sulfides can be critical mineral catalysts for electron transfer. The results show that the complexation and adsorption processes are mostly thermodynamically favorable, and whether there is spontaneous electron transfer from the reduced organic molecule or the mackinawite cluster to the metal oxidant is pH-dependent. Using the Gibbs free energy data computed for each species, we calculate the potential reduction shift of Fe3+/Fe2+ when adsorbed or complexed and derived the Eh-pH diagram. In short, the complexation and adsorption may induce remarkable changes in the reactivity of the metal and affect Fe and Mn cycling in various water bodies.