This effect is manifest in both date-effective uranium (e U) 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 ∼10 α/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.The key to the He thermochronometry method is the recognition that at elevated temperatures characteristic of the crust at a depth of a few km, helium diffuses from common U, Th-bearing minerals as rapidly as it is supplied by radioactive decay.
Thermal histories are important for example for studying tectonics, especially associated with mountain range exhumation, for paleogeomorphology, and for resource exploration (e.g., hydrocarbon maturation).
The technique we have been developing is based on the radioactive decay of uranium and thorium to He, a dating method proposed more than a century ago but never widely utilized.
The role of crystallographic structure in influencing these differences among diffusivities was evaluated using the maximum aperture approach of Cherniak and Watson (2011), in which crystallographic structures are sectioned along possible diffusion directions and the maximum interstitial apertures in each 'slice' in the structure are identified.
Preliminary results show that observed differences in diffusivities are consistent with the size of the smallest maximum aperture along each diffusion direction.
Our model predicts that the bulk zircon (U-Th)/He closure temperature ( above this dose.