Exploring Earth's Core-Mantle Boundary with Multi-Technique Approaches

Citation

Dobrosavljevic, Vasilije V. (2022) Exploring Earth's Core-Mantle Boundary with Multi-Technique Approaches. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/v5xy-mt19. https://resolver.caltech.edu/CaltechTHESIS:05272022-200244272

Abstract

Earth's core-mantle boundary (CMB) is the most extreme interface of the planet's interior. It regulates the flow of heat out of the core and in doing so influences the two internal engines of our dynamic habitable planet: convection in the solid mantle and the magnetic geodynamo in the core. Seismic observations of the CMB have revealed a complex landscape of heterogeneous multi-scale structures that likely play key roles in Earth's internal dynamics and may hold memory of Earth's ancient past. Many details of the compositions and properties of these structures, however, are essentially unknown. In this thesis, I deploy a suite of experimental techniques and interdisciplinary approaches to constrain the temperature and phase relations of the CMB, properties that affect dynamics of the mantle and core. In particular, I focus my study on ultralow velocity zones (ULVZs) - the most extreme and perhaps least well understood structures in the lowermost mantle. I first quantitatively show that these structures, originally posited to be areas of partial melt, can be well explained as solid FeO-rich formations given seismic, geodynamic, and mineralogical constraints. To further explore the viability of such solid FeO-rich structures, I develop a multi-technique approach combining two in-situ synchrotron-based methods, one sensitive to crystal structure and another to atomic dynamics, to study the high-pressure melting of iron-bearing materials. With this approach, I place new constraints on the core-mantle boundary temperature by measuring the melting temperature of a candidate core-forming alloy (Fe_0.8Ni_0.1Si_0.1) at high pressures, finding that the addition of silicon to an Fe_0.9Ni_0.1 core can reduce CMB temperatures to ~3500 K. I then measure the melting of Fe_0.94O, finding a melting temperature of 4140 ± 110 K at CMB pressure, demonstrating the stability of solid FeO-rich ULVZs in the lowermost mantle. The melting experiments show strong agreement between the two independent techniques, helping to address sources of large discrepancies in previous high-pressure melting experiments. Reported high conductivity for iron-rich (Mg,Fe)O at CMB conditions may provide a mechanism for upwelling promoted by solid conductive ULVZs at roots of major hotspot plumes. As a whole, the thesis advances our understanding of the compositions and origins of ultralow velocity zones and, more broadly, the physical properties of Earth's core-mantle boundary region.

Thesis Files