X-Ray Absorption Spectroscopy of Uranium in Zircon and the Redox Evolution of the Continental Crust

Author: Houchin, Shane Kenyon

Year: 2026

Degree: Dissertation (Ph.D.)

Advisor: Tissot, Francois L. H.

Committee Members: Eiler, John M.; Tissot, Francois L. H.; Bucholz, Claire E.; Jackson, Jennifer M.

Option: Geology

Abstract

Zircon is one of the most important minerals for reconstructing Earth history as it can preserve its primary chemical and isotopic composition through weathering, metamorphism, and crustal recycling. While zircon U-Pb geochronology, trace-element analysis, and isotope geochemistry have transformed our understanding of crustal evolution, the ability to recover magmatic redox conditions directly from zircon remains limited. Oxygen fugacity (ƒO₂) is a thermodynamic parameter used to quantify magma redox, and imparts first-order controls on melt chemistry, mineral stability, volatile speciation, and the long-term coevolution of Earth’s interior and surface environments. This thesis develops and applies a zircon-based oxybarometer using uranium X-ray absorption near-edge structure (U XANES) spectroscopy to measure U oxidation states in zircon as a proxy for magmatic ƒO₂.

The first part of this work establishes the analytical and experimental foundation for U XANES zircon oxybarometry. Measurements of well-characterized zircon demonstrate that U M₄-edge XANES provides a sensitive and reproducible measure of average U valence and shows that zircon can incorporate uranium across its full range of oxidation states, from entirely U^IV to pure U^VI. An empirical calibration is then developed using the U M₄-edge spectra of samples with independently constrained ƒO₂, allowing zircon U valence to be used as a quantitative proxy for magma redox.

The second part of the thesis applies this new oxybarometer to questions in early Earth geology and long-term crustal evolution. U XANES analyses of Jack Hills zircon provide improved constraints on Hadean and Archean magma redox states and indicate relatively oxidized magmatic conditions on the early Earth. Additionally, metamorphic Jack Hills zircon domains reveal low-temperature, high-pressure metamorphism at 3.35 Ga, consistent with an early onset of mobile-lid tectonics. The final chapter applies the U XANES oxybarometer to zircon grains spanning Earth history, from ~4200 to 55 Ma, producing a robust archive of continental redox evolution through time. This record reveals that major changes in magma redox coincide with transitions in tectonomagmatic processes, mantle dynamics, and the nature of the continental crust. Together, these results expand the breadth of information that can be extracted from zircon to include magmatic oxygen fugacity and provides a new framework for linking mineral-scale redox information to the evolution of Earth’s lithosphere, atmosphere, and habitability.