Addressing Current Challenges in (U-Th)/He Thermochronology and Running Summer Student Workshops

University Of Arizona, Tucson AZ

Investigators

Abstract

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)." Intellectual Merit: Expanded use of (U-Th)/He and 4He/3He thermochronology in tectonic and geomorphic studies over the last decade have elucidated several first-order challenges facing their practical application and interpretation. Quantitative understanding of the phenomena behind these challenges is required to confidently integrate He dating with other approaches, develop realistic diffusion models, and robustly interpret thermal histories in general. This proposal focuses on addressing two primary challenges in zircon and apatite He dating. The first challenge is development of a robust kinetic model for He diffusion in zircon that incorporates anisotropy and the effects of radiation damage. Zircon He ages show two distinct types of correlations between age and parent nuclide concentrations, depending on the specific thermal histories of individual samples. This leads to a hypothesis whereby initially high and anisotropic diffusivity becomes progressively lower and more isotropic with increasing damage, followed by a rapid decrease in diffusivity after damage exceeds a critical threshold. Evaluation of this behavior may provide constraints on distinct parts of sample thermal histories. Kinetic calibration will be done through a series of crystallographically controlled He diffusion experiments and age analyses on zircons with varying degrees of radiation damage and thermal histories. The second challenge is to understand the origin of anomalous and highly dispersed apatite He ages in some samples. Detailed examination of detrital and bedrock samples documents formation of secondary phases in grain boundaries around at least some fraction of apatite crystals; in many cases these phases have U-Th concentrations comparable to or higher than the apatite. Such samples are likely to be affected by extracrystalline U-Th-bearing secondary phases in distinct ways, depending on the timing of secondary phase growth and whether some proportion of the phases are analyzed with the apatite. The secondary-phase hypothesis will be tested through a series of analyses to characterize in situ U-Th distributions in and around apatite in detrital and Laramide basement bedrock samples through SEM, ion-imaging, and induced-track imaging of thin sections. Methods to ameliorate or avoid the problem will be tested using grain abrasion and measurement of elements associated with the secondary phases on problematic samples. A final component of this study involves funding for continued support of two-week summer student workshops to be held each year for undergraduate and graduate students, allowing them to learn analytical and interpretational aspects of low-temperature thermochronology as applied to samples from their own research projects. Broader Impacts: This work will fund the research of two promising students who are members of underrepresented groups: PhD student Kendra Murray, and undergraduate Guleed Ali, and will provide partial funding for establishing FT analytical facilities for Research Scientist Dr. Stuart Thomson. The student workshops will provide direct training and research experience to a broad spectrum of students in the tectonics and geomorphology communities.