Date of Award:

5-2026

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Geosciences

Committee Chair(s)

Alexis K. Ault

Committee

Alexis K. Ault

Committee

Dennis L. Newell

Committee

Donald E. Penman

Committee

Anthony R. Lowry

Committee

Jonathan S. Caine

Abstract

The iron oxide mineral hematite commonly forms from groundwater near Earth’s surface and can preserve a history of fluid flow, fault slip, and landscape evolution. This record can be decoded using (U-Th)/He thermochronology, a dating method based on the temperature-sensitive ingrowth of helium from uranium and thorium decay. Because later heating and natural variation in hematite crystals can produce ambiguous (U-Th)/He results, I integrate dating with tools that reveal differences in hematite chemistry and textures. I apply these techniques to two distinct settings: ancient granitic rocks in Colorado and an active fault zone in the southern San Andreas fault (SSAF) system in California.

In Colorado, hematite (U-Th)/He dates from Boulder Canyon (BC) and Turkey Creek Canyon (TCC) were affected by a known episode of sediment burial that heated rocks above ~100 °C, causing helium loss. This heating overprints crystallization ages in the dated hematite samples, which averaged helium ingrowth across many crystals of different sizes. At BC, a revised (U-Th)/He approach, or 4He/3He thermochronology, shows the largest hematite crystals resisted helium loss and preserve a ~450-million-year-old crystallization age. TCC hematite comprised smaller crystals that lost helium more readily, yielding younger (U-Th)/He dates that postdate an independent formation age inferred from ancient magnetic signals. Additional (U-Th)/He dates from martite, a special variety of hematite, indicate TCC rocks reached near-surface conditions by ~1.04 billion years ago, where exposure to oxygen was sufficient to “rust” select iron-bearing minerals, forming martite. In contrast, BC rocks resided deeper for longer and did not undergo the same degree of “rusting.”

In the SSAF, paired (U-Th)/He and uranium-series isotope measurements show that hematite-forming groundwater carried uranium and thorium isotopes in disequilibrium, which causes standard (U-Th)/He dates to be too young or old. At the same time, the extremely small hematite crystals in these samples likely allowed some uranium to be lost during radioactive decay, which partly canceled the disequilibrium effect. Together, these results show how rocks in active fault zones exchange radioactive elements with circulating fluids, and that uranium disequilibrium and crystal size need to be accounted for when dating young, fault-related hematite.

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