Geochronology of Bedrock-hosted Secondary Iron Oxides and Shallow Crustal Fluid-flow and Brittle Deformation Histories

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Bedrock-hosted secondary Fe-oxide minerals are practically ubiquitous in faults, fractures, and localized high porosity zones in shallow crustal rocks. Unlike primary minerals, their formation is usually the result of reactions associated with fluid migration long after formation of the host rock. Radioisotopic dates and compositional information preserved in secondary Fe-oxides have the potential to reveal much about the timing and dynamics of regional fluid flow histories, as well as processes that create high permeability zones, like brittle deformation, in bedrock. Despite their potential, bedrock-hosted secondary oxides have proven difficult to date by radioisotopic means. However, new results show that, in many cases, (U-Th)/He dating provides apparently reliable and geologically consistent dates suggesting significant potential for this approach. This project will develop methods for radioisotopic dating and geochemical characterization of bedrock-hosted secondary iron-oxide minerals and focuses primarily on (U-Th)/He geo- and thermochronology of hematite and associated iron-oxide minerals in these environments, combined with characterization of trace element and oxygen isotopic compositions of the minerals. The goal is to use dates and compositions of these secondary phases to constrain the timing and conditions of brittle deformation that created the voids in which the minerals formed, and understand the timing and conditions of fluid flow that resulted in their precipitation. This project develops these approaches in a range of geologic settings, primarily in the western U.S., where results can be compared with other geologic and geochronologic constraints.Dates and compositions of secondary iron-oxides have the potential to improve the understanding of the timing and conditions of a wide range of important processes ranging from brittle fault movement, to groundwater migration, to atmospheric conditions, to ancient episodes of bedrock exposure. This project will also provide support for continued community use of the Arizona Radiogenic Helium Dating Lab, and support for a summer student workshop on low-temperature thermochronology, allowing visiting students to perform analyses and learn how to use and interpret thermochronology in geologic research.