EMPIRICAL CONSTRAINTS ON THE EFFECTS OF RADIATION DAMAGE ON HE DIFFUSION KINETICS IN ZIRCON

Paleozoic and Precambrian zircons from continental interiors, where long-term erosion rates may be exceedingly low, can accumulate radiation damage for hundreds of millions of years before cooling through He closure. Zircon (U-Th)/He datasets from such environments are often overdispersed and difficult to interpret due to the effects of radiation damage on He diffusion kinetics. The purpose of this study is to empirically assess the impact of radiation damage on He diffusion in zircons from the McClure Mountain syenite of South-Central Colorado. Zircon (U-Th)/He dates for this Cambrian body are dispersed over 240 million years, with higher U+Th zircons yielding younger dates. We estimate the alpha dose at the time of He closure for each zircon grain using the weighted mean titanite (U-Th)/He date minus each zircon (U-Th)/He date as the damage integration time. The titanite (U-Th)/He date serves as a proxy for the time at which temperatures are assumed to be sufficiently low to inhibit damage annealing. Alpha doses investigated range from 4.2 x 10¹⁶ to 1.3 x 10¹⁸ α/g. Thermal modeling demonstrates that the syenite cooled slowly through the zircon He partial retention zone at a rate of 0.133 ± 0.074 ^oC/Ma. Results indicate a simple linear relationship between alpha dose and He closure temperature, as implied by the thermal model, and between alpha dose and the natural log of the diffusivity constant, lnD₀, such that He diffusivity increases with increasing damage. These empirical observations are not consistent with existing experimental data (Guenthner et al., 2013, American Journal of Science). For the same alpha dose range, Guenthner et al. found D₀ to decrease with increasing damage, resulting in increased He retentivity at low damage. At the highest damages investigated in this study, closure temperatures predicted by Guenthner et al. and those based on the McClure Mountain syenite cooling history differ by ~40 ^oC, suggesting that additional study, both experimental and empirical, is merited to fully understand the effects of radiation damage on He diffusion kinetics in zircon.