The iron spin crossover in ferropericlase and its seismic effects in Earth’s lower mantle

Thesis

In the study of Earth’s deep interior, mineral physics and seismology are two principal and closely linked disciplines. Seismology allows us to image the interior structure of our planet, but the interpretation of such observations requires a detailed understanding of the elastic properties of deep Earth materials, which govern the propagation velocity of seismic waves. Experimental and computational mineral physics provide constraints on the physical properties of minerals existing at the extreme pressure and temperature conditions of Earth’s interior. The second most abundant mineral in the lower mantle is the magnesium-iron oxide ferropericlase. Iron in ferropericlase undergoes a pressure-induced spin crossover, where its electronic configuration changes from a high-spin to a low-spin state, leading to marked changes to elastic properties. The objectives of this thesis are twofold: firstly, to constrain the elastic properties of ferropericlase across the iron spin crossover as a function of temperature and iron content, and secondly, to characterize the signal of the spin crossover in seismic observations.

The compressibility of ferropericlase at ambient and high temperatures is determined using novel high-pressure, high-temperature diamond-anvil cell experiments in combination with synchrotron X-ray diffraction measurements. New experimental results for different compositions are combined with literature data and machine learning techniques are utilized to predict the relationship between pressure, iron content and ferropericlase properties. In addition, the compression behaviour of ferropericlase in a multi-phase assemblage approximating lower mantle rocks is investigated. The seismic expression of the spin crossover in seismic tomography models and travel-time measurements is characterized using synthetic models and wave propagation simulations, thus providing tools for mapping of the spin crossover in the lower mantle in future studies. Evidence for the occurrence of the spin crossover in Earth’s deep interior is presented through comparison of synthetic results to observational seismic data.

Authors: Trautner, VE

• DOI: 10.5287/ora-14kyogz1m
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: lower mantle; high-pressure experiments; iron spin transition; ferropericlase
• Subjects: Mineral physics; Earth sciences