Isotopic and Trace Element Investigations of Crustal Processes
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Isotopic and Trace Element Investigations of Crustal Processes


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.

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