Subsolidus convection in the mantle of Earth is the driving mechanism behind plate tectonics and provides a central framework linking geophysical, geochemical, petrological, hydrological, and biological processes within the system. Seismic observations have revealed mantle heterogeneities in wide-ranging scales from less than tens of to thousands of kilometers. Understanding the origins and dynamics of these anomalies is critical to advance our knowledge on how mantle convection operates and coevolves with the surface system. This dissertation attempts to constrain the past, present and future of mantle dynamics with lines of evidence from seismology, geodynamics, petrology, geochemistry, and astrophysics. Above Earth’s core, two continent-sized large low shear velocity provinces (LLSVPs) beneath Africa and the Pacific Ocean were seismically detected decades ago. Yet their origin, composition, detailed morphology and influence over mantle convection remain elusive. First, I propose the two LLSVPs may represent the mantle remnants of the Moon-forming impactor Theia. I show that the mantle of Theia is intrinsically denser than Earth’s mantle and would have sunk and accumulated into LLSVP-like structures in the deepest mantle after 4.5 billion years. Second, I examined the maximum height of the two LLSVPs and determined that the African LLSVP is ~1,000 km higher than the Pacific counterpart. Using geodynamic simulations, I find the height of a stable LLSVP is mainly controlled by its density and the ambient mantle viscosity. With ~1,000 numerical experiments, I conclude that the origin of the great height difference between the LLSVPs is that the African LLSVP is less dense, and thus less stable than the Pacific LLSVP. Next, I numerically identified another dynamic scenario accounting for the vastly different height of the two LLSVPs, which is caused by catastrophic sinking of accumulated subducted slabs at the 660-km boundary. Last, targeting one ancient carbonatite above the African LLSVP, I show that lithium isotopes in humite measured by nanoscale secondary ion mass spectrometry was able to uncover the signature of a subducted oceanic crust in its magma source, which may return from the interior to the surface by mantle plumes.

Lithium (Li) is a trace element in kerogen, but the content and isotopic distribution (δ7Li) in kerogen has not previously been quantified. Furthermore, kerogen has been overlooked as a potential source of Li to sedimentary porefluids and buried sediments. Thus, knowing the content and isotopic composition of Li derived from kerogen may have implications for research focused on the Li-isotopes of buried sediments (e.g., evaluating paleoclimate variations using marine carbonates).The objective of this work is to better understand the role of kerogen in the Li geochemical cycle. The research approach consisted of 1) developing reference materials and methodologies to measure the Li-contents and δ7Li of kerogen in-situ by Secondary Ion Mass Spectrometry, 2) surveying the Li-contents and δ7Li of kerogen bearing rocks from different depositional and diagenetic environments and 3) quantifying the Li-content and δ7Li variations in kerogen empirically in a field study and 4) experimentally through hydrous pyrolysis. A survey of δ7Li of coals from depositional basins across the USA showed that thermally immature coals have light δ7Li values (–20 to – 10‰) compared to typical terrestrial materials (> –10‰) and the δ7Li of coal increases with burial temperature suggesting that 6Li is preferentially released from kerogen to porefluids during hydrocarbon generation. A field study was conducted on two Cretaceous coal seams in Colorado (USA) intruded by dikes (mafic and felsic) creating a temperature gradient from the intrusives into the country rock. Results showed that δ7Li values of the unmetamorphosed vitrinite macerals were up to 37‰ lighter than vitrinite macerals and coke within the contact metamorphosed coal. To understand the significance of Li derived from kerogen during burial diagenesis, hydrous pyrolysis experiments of three coals were conducted. Results showed that Li is released from kerogen during hydrocarbon generation and could increase sedimentary porefluid Li-contents up to ~100 mg/L. The δ7Li of coals becomes heavier with increased temperature except where authigenic silicates may compete for the released Li. These results indicate that kerogen is a significant source of isotopically light Li to diagenetic fluids and is an important contributor to the global geochemical cycle.

Meteorites provide an opportunity to reconstruct the history of the SolarSystem. Differentiated meteorites, also called achondrites, are the result of melting and differentiation processes on their parent body. Stable isotopic compositions of differentiated meteorites and their components have added to the understanding of physical parameters, such as temperature, pressure, and redox conditions relevant to differentiation processes on planetesimals and planets in the early Solar System. In particular, Fe and Si isotopes have proven to be useful in advancing the understanding of physical and chemical processes during planetary accretion and subsequent evolution. In this work, I developed a new method to simultaneously purify Fe and Si from a single aliquot of sample while ensuring consistently high yields and accurate and precise isotopic measurements. I then measured the Fe isotope compositions and Si contents of metals from aubrite meteorites to infer the structure and thermal evolution of their asteroidal parent body. Thereafter, I determined the combined Si and Fe isotope compositions of aubrite metals and the Horse Creek iron meteorite, and compared the magnitude of Si and Fe isotope fractionation factors between metal and silicates for both enstatite chondrites and aubrites to estimate the effect of high-temperature core formation that occurred on the aubrite parent body. I additionally assessed whether correlated Si and Fe isotope systematics can be used to trace core formation and partial melting processes for the aubrite parent body, angrite parent body, Mars, Vesta, Moon, and Earth. Finally, I measured the combined Fe and Si isotope composition of a variety of ungrouped achondrites and brachinites that record different degrees of differentiation under different redox conditions to evaluate the role of differentiation and oxygen fugacity in controlling their Fe and Si isotope compositions. Taken together, this comprehensive dataset reveals the thermal evolution of the aubrite parent body, provides insights into the factors controlling the Fe and Si isotope compositions of various planetary materials, and helps constrain the bulk starting composition of planets and planetesimals.

The redox conditions of Earth have been changing since proto-Earth’s accretion from the solar nebula. These changes have influenced the distribution and partitioning of volatile elements between the atmosphere and the mantle (Righter et al., 2020; Stagno and Fei, 2020. Though oxygen fugacity fO2 is arguably not the main factor for phase stability at certain pressure-temperature conditions (McCammon, 2005), it can influence which phases are stable, especially within a closed system such as the ones presented in this study. Despite the importance of controlling fO2 for interpreting the history of planetary bodies, there have been no methods to control the redox conditions in the laser-heated diamond anvil cell (LHDAC). This thesis has examined the feasibility for controlling redox conditions in the LHDAC using a mixture of Ar and H2 for insulation media. The experiments of this study were carried out at the GSECARS sector of the Advanced Photon Source at Argonne National Laboratory. In this study, α-Fe2O3 (hematite), ε-FeOOH (CaCl2-type), and Fe3O4 (magnetite) starting materials were used for probing changes of redox conditions. Experiments were also conducted with a pure Ar-medium for ε-FeOOH at the same pressure-temperature conditions of the hydrogen-bearing medium in order to provide a reference point for data which has uncontrolled redox conditions for an initially Fe(2+)-free material. The results for the ε-FeOOH starting material in Ar show transformation to ι-Fe2O3 (Rh2O3(II)-type) at 30.0 GPa and 1900 K, while in Ar + H2 it transformed to Fe5O7 with minor FeH (dhcp) at 30.0 GPa and 1850 K. For α-Fe2O3 in Ar + H2, it was found to convert to ε-FeOOH, Fe5O7, Fe5O6, and FeH (dhcp) at 36.5 GPa and 1800 K. For Fe3O4 in Ar + H2, it was found to convert to Fe4O5 (CaFe3O5-type), Fe5O6, and minor FeH (fcc) at 26.0 GPa and 1800 K. These results demonstrate that H in an Ar medium can promote the conversion of some Fe(3+) to Fe(2+) and Fe(0). However, the formation of ε-FeOOH in the α-Fe2O3 starting material suggests that H may participate in the chemical reaction of iron oxides.