EXPANDING APPLICATIONS OF ZIRCON (U-TH)/HE THERMOCHRONOMETRY BY EVALUATING VISUAL METAMICTIZATION AS A RELIABLE PROXY FOR ACCUMULATED RADIATION DAMAGE

Zircon (U-Th)/He (ZHe) thermochronometry is a common tool for reconstructing low-temperature and thus tectonic histories. Accumulation of radiation damage in zircon from actinide decay impacts He diffusion, the closure temperature of the ZHe system, and resulting ZHe dates. Variations in effective U (eU) concentration of zircon grains from samples that share a common time-temperature history are required to characterize ZHe date-eU relationships that help resolve complex geologic histories. Prior work demonstrated purposefully selecting grains with a spectrum of visual metamictization yields a range in intrasample eU values (Ault et al., 2018). In this previous study, although increasing metamictization generally corresponded with increasing eU in each sample, some samples yielded coupled negative ZHe date-eU trends reflecting coupled damage-diffusivity and other samples yielded uniform dates regardless of eU. This suggests He loss may be decoupled from damage in these samples and/or grain opacity does not capture accumulated damage.

Here we evaluate the visual metamictization approach and estimations of radiation damage dose in zircon grains from Archean and Proterozoic samples investigated in Ault et al. (2018), as well as a sample of the Miocene Punchbowl formation with likely Phanerozoic zircon grains. We compare the visual metamictization from dated grains, corresponding ZHe dates, and calculated alpha dose from U and Th decay equations assuming damage accumulates since the U-Pb crystallization age, and from thermal history modeling using the Guenthner (2021) DAAM code and Guenthner et al. (2013) and Ginster et al. (2019) annealing kinetics. These relationships reveal, as expected, that damage calculated from decay equations exceeds that from thermal history modeling for each grain and sample. Grain metamictization tracks damage regardless of calculation approach, even for the samples characterized by uniform ZHe dates regardless of eU. This suggests that visual metamictization provides a first order estimate of accumulated damage. Ongoing work involves comparing these damage estimates with damage inferred from Raman spectroscopy of new grains that have commensurate visual damage to evaluate the robustness of the metamictization approach and understand damage annealing.