A NEW MODEL FOR DECIPHERING ZIRCON (U-TH)/HE DATES FROM RADIATION DAMAGED CRYSTALS

Zircon (U-Th)/He (zircon He) dating has become a widely used thermochronologic technique in tectonic and geomorphologic studies. Results from this technique have traditionally been interpreted with a single set of ⁴He diffusion kinetics obtained from a handful of crystals. However, assuming constant kinetics can lead to erroneous conclusions and incongruent data if the assumption is not valid. Here we present grain-specific diffusion kinetics from a series of diffusion measurements that show how the specific alpha dose of a given zircon, which we interpret to be correlated with accumulated radiation damage, influences its He diffusivity. Our approach consists of step-heating diffusion experiments on pairs of crystallographically oriented slabs of zircon with alpha doses ranging from ~10¹⁶ to 10¹⁹ α/g. Results from these experiments lead to the following observations: from 1.2 * 10¹⁶ α/g to 1.4 * 10¹⁸ α/g, He diffusivity at a given temperature decreases by three orders of magnitude, but as alpha dose increases from ~2 * 10¹⁸ α/g to 8.2 * 10¹⁸ α/g, He diffusivity increases by about nine orders of magnitude. We parameterize both the initial decrease and subsequent increase in diffusivity with alpha dose with a function that describes these changes in terms of increasing abundance and size of intracrystalline radiation damage zones and resulting effects on the tortuosity of He migration pathways and dual-domain behavior. This is coupled to another parameterization describing damage annealing as a function of time and temperature (as in the RDAAM of Flowers et al. (2009)), and the combined model allows us to calculate the coevolution of damage, He diffusivity, and (U-Th)/He date of a zircon. As a demonstration of its ability to constrain time-temperature histories, we show model results for zircon (U-Th)/He datasets from the Longmen Shan in China and the western US Cordillera. Datasets that were previously considered to be overly complex and inconclusive can now be used to constrain detailed thermal histories, in some cases for single hand samples.