194-3 New Insights into the Damage-Diffusivity Relationship in Zircon (U-Th)/He Thermochronology from Step-Heating Experiments in Highly Radiation Damaged Zircon Crystals

Session: Broad Applications of Thermochronology to Understanding Geologic Rates and Processes Through the Sedimentary Record (Posters)

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Abstract:

Zircon (U-Th)/He (ZHe) thermochronology is a useful tool for understanding the burial and erosion history of basins and source regions, and sedimentary provenance. The utility of the method depends upon a robust understanding of the kinetics of He diffusion in zircon. A commonly used model for interpreting ZHe dates relies on a parameterization of the relationship between radiation damage and He diffusivity: the zircon radiation damage accumulation and annealing model (ZRDAAM). Despite its use in a wide variety of geologic settings, including sedimentary basins, several persistent issues and inconsistencies with the model remain unresolved. In this presentation, we further explore specific issues surrounding the model’s “roll-over point” and predictions of the damage-diffusivity relationship in highly self-irradiated crystals (defined here as crystals with an alpha dose greater than ~2 × 10¹⁸ α/g). We present new He diffusion data from step-heating experiments that addresses these current knowledge gaps. Our samples consist of five large, gem-quality, Sri Lankan zircon crystals that are homogenous in their U and Th concentrations, and well-characterized for their degree of damage with a range in alpha dose from 2.53 to 5.01 × 10¹⁸ α/g. Activation energies for these samples decrease with increasing radiation damage from 130 to 51 kJ/mol. Frequency factors are also negatively correlated with radiation damage and range from 7.1 × 10^–3 cm²/s to 6.5 × 10^–5 cm²/s. These data agree to first order with the current ZRDAAM kinetic parameterization, but several important observations from these data suggest that a refinement to ZRDAAM is likely necessary. We highlight here two key observations from our data: 1) non-Arrhenius behavior in the initial fractions of gas release, with characteristics that change as a consequence of the degree of radiation damage, and 2) our highest-damaged samples show two separate linear trends in Arrhenius space, each with distinct activation energies and frequency factors. We suggest that these observations could be better explained with a multi-path model for the damage-diffusivity relationship. To that end, we also present multi-path model results that reproduce the particular Arrhenius relationships of our samples, and fit the trends between radiation damage and bulk kinetic parameters. Finally, we show some potential geologic implications of these different kinetics, with a specific focus on ZHe applications to the sedimentary record.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8930

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