The effect of iron and aluminum substitution on the physical properties of bridgmanite in Earth’s mantle

Thesis

Using a combination of in situ high-pressure X-ray diffraction (XRD), density functional theory (DFT) calculations, and thermodynamic modeling, this work explores the influence of Fe and Al substitution on the elastic and thermodynamic properties of bridgmanite, a key mineral in the Earth’s lower mantle. The results indicate that Fe and Al substitutions significantly affect bridgmanite’s compressibility and stiffness. DFT calculations show that increasing Fe and Al content leads to a decrease in bulk modulus (K₀) under ambient conditions, suggesting a more compressible lattice. However, high-temperature calculations reveal that the softening effect diminishes at elevated temperatures, with the (Mg_0.75,Fe_0.25)(Si_0.75,Al_0.25)O₃ (FeAl25) composition becoming stiffer than the other studied compositions at temperatures above 1000 K. Experimental XRD data on sintered polycrystalline bridgmanite, in contrast, suggest an increase in K₀ with higher Fe and Al content at static conditions, possibly due to extrinsic factors such as sample heterogeneity and grain size. Thermodynamic modeling using Perple X predicts that increasing Fe-Al content stabilizes bridgmanite over a broader range of pressure-temperature conditions, particularly in deeper mantle regions. The modeling also suggests that Fe-Al substitutions could alter seismic velocities and V_p/V_s ratios, contributing to seismic anomalies such as those observed in Large Low Shear Velocity Provinces (LLSVPs) and Ultra-Low Velocity Zones (ULVZs).

Authors: Kulka, BL

• DOI: 10.5287/ora-yxobr5z0x
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: equation of state; synchrotron XRD; Earth dynamics; bridgmanite; mantle elasticity; Fe-Al substitution; bulk modulus; iron substitution; aluminum substitution; density functional theory; Earth structure; lower mantle mineralogy
• Subjects: Materials Sciences; High-Pressure Physics; Planetary Science; Earth Sciences; Geophysics; Mineral Physics