Degree

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

Document Type

Dissertation

Abstract

The composition of Earth’s core remains unsolved, yet it carries critical implications for planetary evolution, thermal history, magnetic fields, and habitability. Planetary models derived from seismological observations, such as the Preliminary Reference Earth Model (PREM), indicate that Earth’s core is significantly less dense than pure iron, implying the presence of light elements, including sulfur, carbon, hydrogen, oxygen, and silicon. However, the exact composition and distribution of these elements remain largely unknown. In constraining Earth’s core composition, the density and compressional wave velocity of iron-rich alloys are often compared with seismologically derived models. However, no single iron–light-element alloy can simultaneously reproduce these observations, suggesting that Earth’s core is complex, multicomponent metallic liquid. Experimental data at such extreme conditions are scarce, and computational studies often overlook non-ideal mixing effects, even though atomic-scale interactions strongly influence thermodynamic and geophysical property estimations. Ignoring these effects can introduce systematic errors and result in unreliable compositional constraints. To address these challenges, this dissertation combines first-principles molecular-dynamics (FPMD) simulations, thermodynamic modeling, and data-driven analysis to quantify the effects of non-ideal mixing on the structure and density of Fe-Ni–light-element (LE) alloys under Earth’s core conditions. The first part investigates Fe-Ni-S liquids at pressures up to 330 GPa and temperatures to 5530 K, showing that sulfur induces strong non-ideal effects that reduce density and sound velocity relative to ideal mixing predictions. Building upon this, the second part extends the framework to all Fe-X binary systems (X = Ni, C, H, O, Si, S) along the core isentrope, linking microscopic structure to macroscopic density variations. The resulting multicomponent density model, constructed as a linear combination of non-ideality in binary systems, reproduces first-principles densities within ~0.1%, providing a physically grounded and computationally efficient tool for exploring Earth’s core compositions. Application of this model to seismic data constrains Earth’s outer-core composition to be moderately carbon-rich and sulfur-bearing, consistent with cosmochemical expectations. Overall, this study establishes a comprehensive framework that links atomic-scale thermodynamics to planetary-scale observables, advancing our understanding of how non-ideal mixing influences density, structure, and is important in understanding the composition in Earth’s and other terrestrial planetary cores.

Date

11-9-2025

Committee Chair

Wang, Jianwei

Download

Available for download on Saturday, October 31, 2026

Share

COinS