The continental crust is a chemically distinct geochemical reservoir of the bulk silicate Earth that has formed near continuously over the last 4 billion years and is preserved by its intrinsic buoyancy relative to its oceanic counterpart. Rare earth element patterns imply that continental crust is formed at subduction zone settings and yet the primary melts produced by flux melting of the mantle here are basaltic in composition. An additional process is necessary to differentiate these melts to the Si-rich character of continental crust. The models proposed for differentiation includes fractional crystallization and partial melt extraction, which differ greatly in the resulting lower crustal compositions and the timing of formation relative to Si-rich melts. Here we propose that the investigation of the lower crust composition and timing provides one way to distinguish between these two mechanisms. A study was undertaken to characterize lower crustal xenoliths that are inferred to be related to granitoid plutons from the Southern Sierra Nevadas; the Dinkey Creek and Red Lake granodiorites specifically. Thirteen lower to mid-crustal xenoliths were analyzed for major element chemistry and U-Pb geochronology. We suggest that the broad range in chemistry seen in these lower crustal xenoliths can only be derived by the multiple steps and continuous evolution of fractional crystallization. The ages of these xenoliths record magmatism contemporaneous with the upper crustal granitoid plutons. The mafic lower crustal rocks yield zircon U-Pb dates that are younger than their upper felsic counterparts. This is interpreted to reflect a difference in temperature and crystallization sequence anticipated for mafic and felsic bodies. A summary is developed for the genesis of these plutons and their corresponding mafic cumulate, leading to the generation of crust matching current bulk continental crust estimates.

Isotopes of uranium and their radioactive decay products are important timekeepers for geologic and planetary processes, capable of recording precise dates across an enormous range of timescales and environments. This dissertationencompasses four distinct studies that employ the U decay systems to explore both time and physical processes in various geologic and planetary systems. While the focus of the chapters vary in both geologic and temporal setting, the studies fall under a unifying theme of pairing geochronologic methods with models that relate chronologies to Earth system and asteroidal processes.

The first two chapters use U-Pb thermochronology to measure high-temperature cooling processes in deep time (million-to-billion-year timescales). The first chapter interprets the cooling dates of U-Pb thermochronometers from middle-to-lower crustal xenoliths with thermal models to reconstruct the thermal history of the Superior craton, constraining the timing, temperature, and lithospheric response of the region to mantle plume heating 1.1 billion years ago. The second chapter interprets Pb-Pb cooling dates of phosphate minerals in LL ordinary chondrites with thermal models to reconstruct the size and timeframes of accretion of the LL parent planetesimal, revealing a large (>150 km diameter) asteroidal body that accreted rapidly after the formation of its constituent chondrule particles.

The latter chapters use intermediate decay products of the ²³⁸U-series to explore subglacial conditions over the Pleistocene. The third chapter explores physical and chemical weathering processes in the hyperarid polar environment of Taylor Valley, Antarctica and confirms that Taylor Glacier actively comminutes sediment at its base, challenging canonical assumptions that glaciers in the McMurdo Dry Valleys are non-erosive. The fourth chapter dates subglacial melting events beneath the northern Laurentide Ice Sheet and reveals their coincidence with Heinrich events, recurring episodes of voluminous iceberg discharge into the North Atlantic during the last glacial period. This terrestrial record of basal ice sheet processes corroborates subsurface ocean warming as the primary stimulus for Heinrich events and identifies subglacial aquifers and permafrost as important reservoirs involved in deglacial perturbations of the Atlantic Ocean uranium budget.

Collectively, the studies herein span timeframes that exceed the age of the Earth and peer into deep planetary interiors as well as the geosphere-cryosphere interface, each applying U decay systems in distinct ways to explore both geologic time and process.

The Superior craton represents the largest exposure of Archean crust on Earth and can offer insights into the long-term evolution of cratonic geotherms. U-Pb geo- and thermochronology of zircon and rutile from lower and mid-crustal xenoliths record lower crustal crystallization, metamorphism, and long-term cooling prior to surface exhumation within a Jurassic-aged kimberlite. Archean igneous zircon LA-ICPMS ages range from 2751 21 to 2341 41 Ma, indicating a prolonged record of zircon crystallization. Preliminary U-Pb LA-ICPMS rutile data has a far wider range of ages from 2660 140 to 324 17 Ma and are interpreted to record the long-term relaxation of the cratonic geotherm. Nitrogen aggregation data collected from diamonds exhumed within the Jurassic and a nearby older Proterozoic (~1.1 Ga) kimberlite pipe suggest at least two major diamond-forming events after craton formation. These data require that a thermal pulse caused by the Keweenawan Midcontinent Rift at ~1.1 Ga reverted the mostly high N-aggregated IaB diamonds to graphite but did not perturb the deep crust (~50 km depth). As the geotherm relaxed after the thermal pulse, the diamond stability window was reoccupied and a second diamond-forming event took place before later Jurassic eruption of poorly nitrogen-aggregated type IaA diamonds near Attawapiskat, Ontario. Maintenance of Archean rutile U-Pb cooling ages in middle-crustal amphibolite limits the magnitude of reheating.

With more than 70m of sea level equivalent ice stored in the polar ice sheets, sea level forecasting is heavily reliant on projections of ice sheet response to changes in global climate. One way that Earth scientists have approached this problem is to look back at past warm periods to determine how terrestrial ice mass changed in during previous climatic events. In Antarctic, however, there is an added complexity that 97.6% of the modern continent is covered by ice, which restricts access to the geologic record. Without terrestrial archives of Antarctic ice sheet evolution, it is challenging to parameterize the dominant processes that govern ice sheet sensitivity to climate and the environmental effects of ice loss. In this dissertation, I applied geochronologic, isotopic, elemental, and spectroscopic analyses to Antarctic subglacial chemical precipitates – a novel terrestrial record of basal conditions – to investigate the processes that link climate change, Antarctic ice motion, and the hydrologic system at the ice-bed interface. Collectively, this work expands our understanding of Antarctic evolution on centennial to millennial timescales and establishes Antarctic subglacial precipitates as climate archives analogous to speleothems.The first two chapters investigate the physical processes associated with subglacial hydrology and ice motion. By applying geochronologic and geochemical analyses to a group of precipitates that formed over tens-of-thousands of years during the Late Pleistocene, we showed that the continent-wide Antarctic subglacial hydrologic system responds rapidly (within 60 yrs.) to millennial-scale climate events, with more intense subglacial flushing during warm periods and diminished basal meltwater flow during cold periods. This close coupling between climate and subglacial hydrologic activity requires changes to Antarctic ice surface slope caused by hundreds of meters of thinning at the ice sheet margins. These studies provide evidence that the Antarctic gains and loses ice during millennial-scale climate cycles and indicate that subglacial meltwater flushing drive higher ice velocities that promote ice thinning and grounding line retreat. The latter chapters focus on the mechanisms that control the chemical composition of subglacial waters, to help discern the environmental effects of Antarctic ice loss on geologic timescales. The third chapter uses stable isotope measurements ( carbon and oxygen) on a suite of 49 subglacial precipitates to show that microbial activity mobilizes fossil carbon stored in rocks and sediment throughout the Antarctic continent. This respired CO2 drives a continent-wide silicate weathering cycle that mobilizes elements from the subglacial bedrock, which may play an important role in fertilizing the Southern Ocean ecosystem. Chapter 4 established 10 kyr record of trace metal cycling beneath the East Antarctic Ice Sheet (EAIS) measured in a subglacial precipitate that formed across glacial termination III. This record implies that climate modulated changes in subglacial flushing intensity regulate the mobilization of redox sensitive trace metals on geologic timescales.