MECHANISMS OF HELIUM DIFFUSION FROM ZIRCON, AND THE TRANSITION TO THE METAMICT STATE

The relationship between time, temperature, and ⁴He diffusion from zircon has been employed by researchers for more than a decade as (U-Th)/He (ZHe) thermochronometry to investigate low-temperature thermal histories, although uncertainties remain regarding the effects of radiation damage on diffusivity. In geologic settings where zircons are exceptionally radioactive or have spent considerable time below damage annealing temperatures, many studies observe negative correlations between measured ZHe age and eU (eU = [U] + 0.23[Th]) from zircons with a shared thermal history, with age variations up to hundreds of millions of years. Such results highlight the need to improve our understanding of the effects of different types and states of radiation damage on ⁴He diffusion, and so to define a more robust forward model capable of accurate analysis of such thermal histories. This study presents step-heating experiments on zircons from the Hall Peninsula of Baffin Island, Nunavut, Canada, from an Archean (2.8 Ga, U-Pb) tonalite-orthogneiss complex using both radiogenic ⁴He and proton irradiation-induced ³He, alongside complementary Raman spectroscopic, laser ablation ICP-MS, and fission track (FT) analyses. ZHe ages range from ~1,400 - 200 Ma with a distinct log-linear age-eU correlation between approx. 120 - 500 ppm eU. Currently, dramatic increases in diffusivity are thought to occur exclusively after doses in excess of the metamictization threshold (approx. 2 - 4 × 10¹⁸ α-decays/g), manifest as decreases in activation energies (E_A), while the pre-exponential factor (D₀) remains constant above low doses. In our study, we observe anomalously young ZHe ages and lowered E_A and D₀ values in zircon for which the maximum possible doses are significantly lower than total metamictization, with Raman spectra suggestive of moderate crystallinity. We also observe a consistent and often linear relationship between diffusivity and total gas release in many experiments, manifest as increases in both E_A and D₀; such changes are most pronounced at time-temperature conditions at which FT damage anneals, suggesting such damage plays a significant role in diffusion at high doses. These data will be critical in the formation of a more robust ZHe model capable of retrieving more accurate thermal histories from high-damage zircon.