Degree

Doctor of Philosophy (PhD)

Department

Geology & Geophysics

Document Type

Dissertation

Abstract

Silicate melts have served as transport agents in the chemical and thermal evolution of Earth. Diffusional isotope effect in silicate melts is the key to interpret isotope variations in lots of geological samples. Isotopic mass dependence of diffusion is commonly expressed as (Di/Dj)=(mj/mi)^β, where Di and Dj are diffusion coefficients of two isotopes whose masses are mi and mj. However, how the dimensionless empirical parameter β depends on temperature, pressure, and composition remains poorly constrained. Viscosity and electrical conductivity are two fundamental dynamical properties of silicate melts needed to constrain melt distribution in Earth's interior but remain unclear for most of Earth’s mantle conditions. We presented a partial substitution, pseudo-mass approach in calculating β for major elements (e.g., Mg) in silicate melts based on first-principles molecular dynamics (FPMD) simulations. The calculated β for Mg isotopes decreases with decreasing temperature at zero pressure and decreases with increasing pressure along the isothermal curve. Based on the potential energy and interatomic forces generated by a deep neural network trained with ab initio data, we performed deep potential molecular dynamics (DPMD) simulations to evaluate β for Li and He isotopes in silicate melts. Our calculations show that the β for Li and He in silicate melts is temperature and composition dependent, and the extent of temperature dependence of β hinges upon melt composition. These calculated β values are further used to discuss isotope fractionation happening in geological events such as crystal growth, bubble growth, and magma ascending. We also investigated the viscosity and electrical conductivity of MgSiO3 melt at Earth’s mantle conditions based on the deep-learning potential method. The pressure dependence of the viscosity of MgSiO3 melt at low pressures (3 melt is found to first increase, then decrease with increasing pressure at all temperatures. These results have important implications for zones with seismic velocity anomalies and high electrical conductivity in Earth’s interior.

Committee Chair

Bao, Huiming

Available for download on Thursday, July 07, 2022

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