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Abstract

Carbon is ubiquitous throughout the Earth’s surface, but plays an important role in the Earth’s interior as well. Carbon is known to influence a host of physical properties in minerals and melts, including melting temperature (Dasgupta et al., 2007), electrical conductivity (Gaillard et al., 2008), density (Solomatova and Caracas, 2021), and rheology (Stagno et al., 2018). However, much is still unknown about the deep Earth carbon cycle, from the amount of carbon contained in the Earth to the distribution of carbon between the upper mantle, lower mantle, and core. Improving our understanding of the phases that host carbon in the Earth and their potential interactions with other phases is crucial to understanding the processes that govern carbon distribution in the deep Earth. Equipped with this knowledge, we can better predict the fate of carbon in the Earth’s deep interior and explain observable geophysical phenomena. This thesis focuses on the fate of carbonates, which are regularly introduced into the Earth’s mantle in subducting slabs (Sano and Williams, 1996), and thus, serve as a constant source of carbon into the mantle. Previous studies of carbonates in the lower mantle focus on the stability of carbonates in isolation (Biellmann et al., 1993; Cerantola et al., 2017; Santos et al., 2019), but this research aims to investigate carbon stability in a more realistic petrologic context. In this thesis, I outline a possible delivery mechanism of carbonates into the lower mantle by reacting carbonates with iron alloys and exploring their stability fields. I then investigate the fate of carbonate phases in the lower mantle by examining the interaction of carbonate melts with silicate and metal melts. This research indicates that carbonate-silicate-metal melts could be parent melts for diamonds and iron carbides, as carbon forms complex polymers with high degrees of C-Fe bonding. Finally, this thesis concludes by investigating the densities and miscibilities of carbonate-silicate-metal melts as a possible explanation for ultralow velocity zones (ULVZs), which have been proposed to be gravitationally stable pockets of partial melt situated at the core-mantle boundary (Williams and Garnero, 1996).

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