The role of iron in the Earth's deep interior

Liu, Jin, Ph. D.
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Iron represents almost one third of the total mass of our planet, more than any other element existing in the Earth. Knowledge of the physical and chemical properties of iron-bearing phases at high pressure and temperature (P-T) is crucial for understanding the thermal-chemical state and evolution of our planet. In this dissertation, I employed the diamond anvil cell (DAC) and synchrotron radiation facilities (e.g., X-ray diffraction and inelastic X-ray scattering spectroscopies) to study the phase stability, sound velocity and/or elasticity of representative iron-bearing phases of the mantle and core, namely ferromagnesite and iron alloys. In the Earth's mantle, iron-bearing magnesite [(Mg,Fe)CO₃] (hereafter called ferromagnesite) has been commonly proposed to be a potential deep-carbon carrier. Studying the spin transition and phase stability of ferromagnesite at high P-T is necessary for our understanding of the deep-carbon storage and the global carbon cycle of the Earth. Based on X-ray diffraction results, the spin crossover in ferromagnesite broadens and shifts toward higher pressures at elevated temperatures up to 1200 K. The rhombohedral ferromagnesite (phase I) is found to transform into a new orthorhombic high-pressure phase (phase II) up to the lower-mantle conditions of approximately 120 GPa and 2400 K. It is conceivable that the high-spin phase I undergoes spin transition into the low-spin phase I approximately at 1400 km, and below 1900 km the high-pressure phase II becomes stable as a major deep-carbon carrier at the deeper parts of the lower mantle. In the Earth's core, the primary constituent is iron that is alloyed with a certain amount of light elements. Studying the velocity-density profiles and elasticity of iron and iron-rich alloys at high P-T is essential for establishing satisfactory geophysical and geochemical models of the core. Based on the measured velocity-density-pressure relationships of bcc-Fe and Fe-Si alloy at high P-T, a strong velocity reduction is found at elevated temperatures. Furthermore, velocity-density profiles of hcp-Fe₀.₈₅Ni₀.₁₀Si₀.₀₅ alloy have been investigated up to 147 GPa using multiple complementary experimental techniques. The derived ρ-V[subscript P] and ρ-V[subscript D] profiles of hcp-Fe₀.₈₅Ni₀.₁₀Si₀.₀₅ exhibit concave curvatures with increasing pressure. The velocity-density profiles and Poisson's ratio of the hcp-Fe alloyed with 5 (±2) wt. % Si and 5% Ni at 6000 K could match seismic observations of the inner core, indicating that silicon can be a potential major light element that satisfies geophysical constraints of the Earth's core.