Studying early Earth history is complicated by the fact that the rock record doesn’t extend past 4 Ga and our only record for the Hadean (>4 Ga) comes to us from detrital zircons from the Jack Hills in Western Australia. The Hadean zircon record extends back to almost 4.4 Ga and has revealed that the early Earth may have had liquid water, a felsic crust, plate boundary interactions, and possibly a biosphere. On the other hand, analyses of lunar and meteoritic samples are used to argue for a hellish Hadean Earth where frequent, large impactors repeatedly destroyed the crust. Indeed, these two models stand in direct contradiction. The focus of this thesis is to examine the evidence for these two models and ultimately propose a reconciliation based on a new interpretation of the chronology of the lunar samples used to constrain the impact history into the early Earth-Moon system.

In order to improve the understanding of zircon crystallization in igneous settings, we undertook experimental studies of zircon saturation which were analyzed using a novel ion imaging approach by a secondary ion mass spectrometer. This study confirmed the original model for zircon saturation, that it is a function of only temperature, melt composition, and Zr content. Indeed, the primary implication for the early Earth from this work is that zircons are much more likely to crystallize in a felsic rather than mafic magma and therefore simply the existence of Hadean zircons suggests a high likelihood for felsic Hadean magmatism.

The majority of the thesis focuses on the interpretation of 40Ar/39Ar ages of lunar and meteorite samples, specifically with regards to impact histories derived from compilations of such ages. The primary complication with lunar and meteorite 40Ar/39Ar ages is that the vast majority show evidence for later disturbances due to diffusive loss of 40Ar. To try and extract meaningful thermal histories from these samples, we undertook investigations of samples from Apollo 16 and the Jilin chondrite. We then used an extension of the multi-domain diffusion model that can model samples containing multiple activation energies (i.e., whole rock samples with multiple K bearing minerals) to propose that the 40Ar/39Ar system can be used to recover shock heating temperatures and durations.

Having shown the effects of diffusive 40Ar loss on the accuracy of 40Ar/39Ar dating, we then explored the question as to whether or not compilations of disturbed 40Ar/39Ar ages simply misestimate the timing of bombardment episodes or are fundamentally inaccurate. For this we created a simple numerical model that simulates a chosen impact history on a surface and then creates a histogram of 40Ar/39Ar plateau ages. Our results show that rather than simply misestimate timing, compilations of 40Ar/39Ar ages can lead to inferences of illusory bombardment episodes.

Finally, we examine the 40Ar/39Ar ages of suite of geochemically related Apollo 16 rocks to examine the effects of mixing and brecciation on the accuracy of inferred ages. By analyzing multiple rocks from each soil sample, we show that three out of six samples are not compatible with a single thermal history. That is to say, despite their close proximity during sampling and geochemical similarities, analyzed rocks in the soil sample have unique chronologies. Based on these findings, we developed a simple numerical model which shows that internal isochrons of mixed samples can yield erroneous ages while retaining a statistically acceptable mean squared weighted deviation (MSWD).

Detrital zircons from the Jack Hills (Yilgarn Craton, Western Australia) range from ca. 4.4 to 3.0 Ga in age and constitute the most complete known record for the pre-4 Ga (i.e., Hadean) Earth. Many past investigations have established the geochemistry of the Hadean zircons: their Hf isotope compositions suggest dominant sourcing from ancient felsic crust, while their low Ti crystallization temperatures (average ca. 680˚C) and commonly igneous internal zonation suggests granitic origins. In addition, their dominant mineral inclusion assemblage of quartz + muscovite, along with a large minority of zircons displaying heavy δ18O reminiscent of meta-sedimentary input, has led to their interpretation as largely sourced from S-type granites. Concordant Hadean zircons, however, make up only ca. 5% of the Jack Hills population, and the few investigations of the younger zircons have hinted at somewhat different provenances.

We investigate the <4 Ga history of the Jack Hills zircons and their provenance(s), finding both important geochemical similarities and differences between the post-Hadean and Hadean populations. The average crystallization temperature of ca. 680˚C does not appear notably different from the Hadean population, indicating continued dominance of granitic protoliths. However, the younger zircons are overall both more radiogenic in Lu-Hf and have a more restricted, mantle-like δ18O distribution with no obvious evidence for meta-sedimentary magma sources. Further investigation of the sparsely populated time period 4.0-3.6 Ga reveals that this restriction in δ18O occurs fairly suddenly after 3.8 Ga. This time period is also marked by the disappearance of an ancient felsic crustal component (at ca. 3.7 Ga) and evidence for juvenile input from the mantle (at ca. 3.8 Ga), reminiscent of Hf isotope patterns seen in Phanerozoic subduction-related orogens. We interpret the Hf isotope record as evidence for subduction-related recycling of much of the ancient Hadean crust at ca. 3.8-3.7 Ga. A distinctive group of zircons with trace element geochemistry and internal textures consistent with metamorphic recrystallization occurs just before this point at ca. 3.91-3.84 Ga. The coincidence of this apparent event with our proposed subduction event soon thereafter and/or with the hypothesized Late Heavy Bombardment of the inner solar system are both interesting, but causal relationships are not entirely clear with the present evidence. Overall, however, the changeover from the prevailing Hadean provenance(s) to a different source(s) for the younger zircons occurs in a series of geochemical transitions between 3.9 and 3.7 Ga, likely reflecting important tectonic (or exogenic) events in the ancestral Jack Hills crust.

Investigation of the Hadean population itself reveals interesting patterns of post-Hadean alteration: we employ Xe isotopic systematics to investigate the zircons' original Pu/U ratios and later Xe loss histories. 244Pu and 238U spontaneously fission to produce characteristic isotopic components of Xe, while irradiation with thermal neutrons induces fission of 235U to create a third component. Deconvolution of fission Xe from irradiated zircons into these end-member components allows for estimation of both the original Pu/U of the zircons and the U-Xe age. Nearly all investigated zircons in this and a previous study have post-Hadean U-Xe ages, and in this study they range as young at ca. 1.8 Ga. This finding underscores the long history of post-Hadean thermal events that affected the zircons. Pu/U is a potential indicator for aqueous mobilization of the more soluble U, but the near ubiquity of subchondritic Pu/U in this population may be mostly due to the effects of Xe loss. The higher range of Pu/U in younger relative to older Hadean zircons in this and a previous study, coupled with other trace element indicators for more compositionally evolved melts, may however suggest that the Pu/U was partly controlled by magmatic processes. A larger set of samples with minimal Xe loss, however, would be needed to confirm this observation.

Finally, we have begun building a model of free subduction in order to test whether this process would be more or less likely to occur in a warmer mantle (as expected for the early Earth) - a contentious subject with various contradictory model results in the literature. Results for our initial Cartesian model are of uncertain applicability to the Earth given the ubiquity of 2-sided rather than 1-sided subduction in the models. However, the model results do suggest that for oceanic plates of modern thickness (ca. 100 km), warmer mantle temperatures may indeed enhance the tendency toward subduction. Thinner plates, as proposed by some workers, do not subduct as readily and are more likely to show slab breakoff events, while thicker plates subduct more readily than modern slabs. Slab geometry appears to be a function of both mantle temperature and the maximum lithospheric viscosity allowed by each model, and has implications for the preservation of subduction-related lithologies on the upper plate.

Knowledge of crustal architecture, and the distribution of rock types within, did not by itself provide geologists unambiguous evidence of its evolution. It was not until the advent of modern geochemical and petrologic methods in the 1960s that the mechanisms and timescales of continental crustal evolution began to be delimited. Chapter 1 of this thesis introduces the historical background of modern geological studies and places my three principal investigations in that broader context. These studies range from understanding interactions between climate, rock strength, topographic form, and erosion to unraveling the controls of magmatic processes which control trace element partitioning. Chapter 2 investigates how erosion rate, topography, and rock properties control the landform evolution in eastern Tibet. Tectonic deformation can influence spatiotemporal patterns of erosion by changing both base-level and the mechanical state of bedrock. Although base-level change and the resulting erosion are well understood, the impact of tectonic damage on bedrock erodibility has rarely been quantified. Eastern Tibet, a tectonically active region with diverse lithologies and multiple active fault zones, provides a suitable field site to understand how tectonic deformation controls erosion and topography. In this study, I quantified erosion coefficients using the relationship between millennial erosion rates and the corresponding channel steepness. This work shows a two-fold increase in erosion coefficients between basins within 15 km of major faults and those beyond 15 km, suggesting that tectonic deformation through seismic shaking and rock damage significantly affects eastern Tibet erosion and topography. It demonstrates a field-based, quantitative relationship between rock erodibility and fault damage, which has important implications for improving landscape evolution models. In Chapter 3, I investigate the influences of Zr stable isotope variations in samples from the Peninsular Ranges Batholith, southern California. Zr isotopic measurements were undertaken on zircon, titanite, biotite, amphibole, and whole rocks from the La Posta pluton together with trace element analyses and U-Pb ages to understand the controls on Zr isotope fractionation in igneous rocks, including temperature, co-crystallizing phases, and kinetic effects. Middle rare earth element (MREE) depletions are present that could indicate that a co- or formerly crystallizing phase impacted the subsequent Zr isotope composition of zircon. Large (>0.6 ‰) Zr isotope fractionations (expressed as δ94/90Zr) were found between titanite and zircon forming at approximately the same temperature. Using equilibrium fractionation factors calculated from ionic and ab initio models, we inferred the controls on Zr isotope evolution to include crystallization order, with titanite fractionation resulting in isotopically lighter melt and zircon fractionation resulting in isotopically heavier melt. While these models of Zr fractionation can explain δ94/90Zr variations of up to ~1.5‰, crystallization order, temperature and presence of co-crystallizing phases do not explain all aspects of the intracrystalline Zr isotopic distribution in zircons in the La Posta pluton. Without additional constraints, Zr stable isotopic investigations of zircons are not yet unambiguous proxies of magmatic evolution. While the Ti-in-Quartz thermobarometer is one of the most widely used trace element methods in the geosciences to simultaneously obtain temperature and pressure information, recent studies have called its accuracy into question. In Chapter 4, I present six new experimental results and review the critiques of this thermobarometer. With regard to the latter, the effects of secondary fluorescence, supersaturation, and quenching rates appear to provide first order explanations for the differences in the varied results. My results broadly agree with the earlier calibrations but further experimental and modeling of the many controls on Ti concentrations in quartz appear warranted.

Over the past several years, optically-stimulated luminescence signals (OSL) from quartz and infrared-stimulated luminescence (IRSL) from feldspar in bedrock have been investigated for their use in thermochronology. In this study, I propose using thermally-stimulated signals (thermoluminescence, or TL) from feldspar instead. Because TL is measured by gradually heating a sample, the luminescence emissions correspond to electron traps of increasing thermal stability. The primary goal of this dissertation is to describe how this signal can be optimally measured and interpreted to understand the recent thermal history of bedrock samples.

I first modified the laboratory luminescence reader to allow us to irradiate feldspar samples at a range of dose-rates, from 8.7 x 10^-5 to 1.2 x 10^-1 Gy/s to estimate the influence of dose-rate on the subsequent TL signals. Although less stable sites were not preferentially populated at higher dose-rates (an unexpected result), I did observe an increase in brightness at the lowest dose-rate, a result which may suggest that the dose-rate influences recombination or trapping competition probabilities. In natural TL signals, I observed the expected trend of greater site occupancy at lower measurement temperatures with higher dose-rates.

Next, I perform several experiments to relate the TL signal that I monitor to OSL and IRSL from feldspar. I propose that the TL signal preserves a more detailed structure of trapping stability than the optically-stimulated signals, which derive from the full range of TL stabilities. Moreover, preheating and phototransfer effects may redistribute trapped charge, leading to potential inaccuracies in IRSL or post-IR IRSL thermochronology techniques.

The kinetic parameters involved in natural and laboratory conditions are of primary importance when using TL signals to quantify thermal history. I develop a novel method for determining the activation energy, effective frequency factor, and kinetic order values for natural and regenerative TL signals. This method, termed `post-isothermal TL' analysis, reveals that for the blue-green emission of the low-temperature TL peak, the apparent trap depth in measured bedrock K-feldspar samples increases to a depth of about >=1.9 eV as measurement temperature increases, at which point it reaches a plateau in some samples. If this plateau value is the true depth of the trap, the frequency factors are measured to decrease as measurement temperature increases, an observation consistent with the recent conception that feldspar luminescence (IRSL and TL) results from excited-state tunneling to randomly-distributed centers.

Three archived drill cores were sampled at depths corresponding to burial temperatures ranging from -4.1 to 60.2 °C. The extracted feldspars from these samples yield TL signals that clearly relate to temperature. With higher ambient temperatures, there is a linear increase in the T1/2 value (measurement temperature at half-maximum emission intensity) and a reduction in signal intensity. This behavior can be replicated by isothermal treatments in the laboratory. I interpret this behavior as reflecting the continuum of trap lifetimes present in feldspar TL, an observation that I substantiate with additive dose experiments and a numerical model.

How glaciers erode is an active research question that can be informed by feldspar TL thermochronology. I collected bedrock samples along vertical and longitudinal profiles within the Rock Creek glacial valley in the Beartooth Mountains of Montana to investigate their thermal history. Using the relationship observed with the drill core samples, I can predict the ambient temperature of these samples from their T1/2 values. The calculated values are indistinguishable from the historical record of mean annual air temperature from the local weather station. The shapes of the resulting TL signals imply that the samples are in disequilibrium, i.e., that the signals are still growing through time. By measuring the single-aliquot regenerative (SAR) dose-responses of the T1/2 values, I estimated the maximum time that each sample has been at its current surface temperature. These ages correlate with periods of local glacial activity and offer insight into the erosional mechanisms involved. In particular, I observe post-glacial high-elevation plateau erosion, which supports a key prediction of the glacial buzzsaw hypothesis.

Although a maximum age is useful, a more desirable solution is a continuous T - t history. The final chapter pursues this goal with samples taken from the Yucaipa Ridge tectonic block (YRB) of the San Bernardino Mountains in Southern California. I introduce a multiple-aliquot additive-dose (MAAD) measurement protocol that can be used to estimate the degree of dose saturation as a function of measurement temperature, n/N(T). This MAAD TL n/N(T) method capitalizes on the earlier observation of feldspar TL, that site stability increases with measurement temperature. Using the same kinetic model used to describe the drill core samples, I simulate two previously-proposed geologic cooling scenarios for the YRB (as constrained by apatite (U - Th)/He ages and catchment-averaged cosmogenic 10Be denudation rates) and the model is found to be sensitive enough to discriminate between them. I then measure MAAD TL signals for several YRB samples, convert these to n/N(T) functions, and use Monte Carlo simulations to invert for each sample's thermal history. The resulting T - t histories from these five samples tell an internally consistent story of samples nearest the fault-parallel valley cooling recently and those nearer the ridge having been at present temperature for a longer time. Despite the vertical relief being only about 0.4 km between the highest and lowest samples, the difference in trap saturation is significant, suggesting that this technique may be well suited to resolving Quaternary landscape evolution. I interpret the exhumation histories of these samples to reflect a combination of post-uplift relaxation of isotherms and a lagged erosional response in the form of fluvial downcutting.

Tectonic models of continental orogens seek to reconstruct the deformation processes associated with large-scale continental collision events. Geographically broad field surveys provide two spatial dimensions, and geochronology provides a third temporal dimension. The missing fourth dimension, most critical to understanding crustal evolution throughout collision, is the crustal thickening history of the orogen. Over the past few decades, however, accurate methods for estimating crustal thickness from the rock record have not fully emerged. Recently, indirect proxies for Moho depth have been developed using whole rock composition: trace element ratios Sr/Y, La/Yb, and Gd/Yb, and stable Nd isotopes. While these proxies can show changes in apparent depth based on transitions in whole-rock chemistry, these proxies are not immune to unconstrained effects on composition, including source melt composition and phase relations, or systematic changes in the intensive variables controlling the proxy. The application of an accurate and precise empirical thermobarometer to plutonic rocks can provide the depth of magma generation and assimilation, and thus the minimum depth of the Moho and thickness of the crust. The Gangdese batholith in southern Tibet, spanning ca. 200 Ma of pre- and syn-orogenic history, provides a continuous record of silicic magmatism associated with the subduction and collision history of southern Tibet. This work reconstructs whole-rock and zircon P-T-X-t histories of Gangdese granitoids spanning 225 Ma – 18 Ma from a transect of plutons 0 – 110km north of the Indus-Tsangpo Suture, the southern margin of Eurasia, near Lhasa, Tibet at 92�E. I compare the utility of indirect geochemical proxies and thermobarometry, and discuss the relationship between the thermo-chemical histories of complex plutonic growth processes, and crustal thickening during orogenesis. High-resolution in-situ thermobarometry of quartz inclusions in zircon suggest rapid crustal thickening began ca. 25 Ma earlier than indicated by indirect isotopic and geochemical proxies. Pluton formation occurred throughout the lower three-quarters of the Tibetan Crust, representing a large zone of plutonic activity that weakened the lower crust and contributed significantly to total orogenic heat budget. These results provide constraints for geodynamic models of intracontinental deformation and reveal the limitations and utility of various barometers and pseudobarometers in granites.