ZIRCON VISUAL METAMICTIZATION TRACKS EFFECTIVE RADIATION DAMAGE: IMPLICATIONS FOR ZIRCON (U-TH)/HE DATE-EU PATTERNS

Zircon (U-Th)/He (ZHe) thermochronometry quantifies the timing and tempo of low-temperature processes and is used to deconvolve tectonic and erosional histories. Accumulation and annealing of radiation damage from actinide decay in zircon impacts He diffusion and resulting ZHe dates. Resolving complex histories requires investigating ZHe date-effective U (eU) relationships in grains with a range of eU concentration from samples that share a common thermal history. Prior work demonstrated purposefully selecting grains with a spectrum of visual metamictization yields variable intrasample eU values (Ault et al., 2018). Increasing metamictization generally corresponded with increasing eU in each sample, but some samples yielded negative ZHe date-eU trends reflecting coupled damage and He diffusivity and other samples yielded uniform dates regardless of eU and increasing visual metamictization. This suggests He loss may be decoupled from damage in these samples and/or visual metamictization does not always capture effective radiation damage.

Here we evaluate the relationships between visual metamictization, effective damage calculated from Raman spectroscopy, and ZHe dates from a new suite of grains from some of the same samples investigated in Ault et al. (2018) and Armstrong et al. (2022). Zircon grains are from North American Archean and Proterozoic basement, and likely Phanerozoic detrital grains from the Miocene Punchbowl Formation. New ZHe data from each sample confirm increasing visual metamictization corresponds with increasing eU concentrations and data fall along previously reported date-eU trends, despite grains being selected by different analysts in different sessions. Raman-based alpha dose measurements from spot analyses (n = 480) on grain surfaces and interiors range from 3.19 x 10¹⁶ to 1.52 x 10¹⁹ (the maximum interpretable dose) a/g. Calculated effective damage increases with increasing visual metamictization regardless of the date-eU pattern and is pronounced in samples with negative date-eU trends. Complexities in metamictization-damage relationships reflect U, Th zoning and user grain selection bias. Overall, visual metamictization provides a qualitative estimate of effective damage and should be leveraged when selecting grains for ZHe thermochronometry.