HELIUM DIFFUSION IN NATURAL ZIRCON: RADIATION DAMAGE, ANISOTROPY, AND THE INTERPRETATION OF ZIRCON (U-Th)/He THERMOCHRONOLOGY

Accurate thermochronologic interpretation of zircon (U-Th)/He dates requires a realistic and practically useful understanding of He diffusion kinetics in natural zircon, ideally across the range of variation that characterize typically dated specimens. Here we present a series of date and diffusion measurements that document the importance of alpha dose, which we interpret to be correlated with accumulated radiation damage, on He diffusivity. This effect is manifest in both date-effective uranium (eU) correlations among zircon grains from single hand samples and in diffusion experiments on pairs of crystallographically oriented slabs of zircon with alpha doses ranging from similar to 10(16) to 101 alpha/g. We interpret these results as due to two contrasting effects of radiation damage in zircon, both of which have much larger effects on He diffusivity and thermal sensitivity of the zircon (U-Th)/He system than crystallographic anisotropy. Between 1.2 x 10(16) alpha/g and 1.4 x 10(18) alpha/g, the frequency factor, D-0, measured in the c-axis parallel direction decreases by roughly four orders of magnitude, causing He diffusivity to decrease dramatically (for example by three orders of magnitude at temperatures between 140 and 220 degrees C). Above similar to 2 x 10(18) alpha/g, however, activation energy decreases by a factor of roughly two, and diffusivity increases by about nine orders of magnitude by 8.2 x 10(18) alpha/g. We interpret these two trends with a model that describes the increasing tortuosity of diffusion pathways with progressive damage accumulation, which in turn causes decreases in He diffusivity at low damage. At high damage, increasing diffusivity results from damage zone interconnection and consequential shrinking of the effective diffusion domain size. Our model predicts that the bulk zircon (U-Th)/He closure temperature (T-c) increases from about 140 to 220 degrees C between alpha doses of 10(16) to 10(18) alpha/g, followed by a dramatic decrease in T-c above this dose. Linking this parameterization to one describing damage annealing as a function of time and temperature, we can model the coevolution of damage, He diffusivity, and (U-Th)/He date of zircon. This model generates positive or negative date-eU correlations depending on the extent of damage in each grain and the date-eU sample's time-temperature history.