Structure and thermal annealing of latent fission tracks in apatite and zircon

We present our recent transmission electron microscopy (TEM) results on unetched, latent, fission tracks in apatite and zircon, two minerals that figure prominently in fission track thermochronology. The structure, morphologies (shape and radius), thermal annealing and orientation effects of latent fission tracks have been systematically investigated at the atomic-scale. We have studied both randomly orientated neutron-induced fission tracks and parallel tracks created by GeV ions (e.g., 2.2 GeV Au ions) or fission fragments (e.g., 80 MeV Xe ions). Our studies show that the very different thermal annealing behavior of fission tracks in zircon and apatite, as directly observed by in situ TEM, is a direct result of differences in the internal structure of the track -- the amorphous domain in zircon vs. the low atomic density void in apatite. We have imaged variations in radius and in situ thermal annealing along the entire lengths of latent tracks created by 80 MeV Xe ions in apatite. High-resolution analysis is performed to show how crystallographic orientation affects track shape and radius at the atomic-scale. These studies address long-standing issues raised by prominent researchers, such as Gleadow et al. (2002) and Jonckheere and Chadderton (2002), that is the need for atomic-scale characterization of latent track behavior in order to establish a quantitative basis for modeling fission track annealing under a variety of geologic conditions. Plan view HRTEM images of tracks induced by exposure to 2.2 GeV Au ions || c or ⊥ c of zircon. The track orientation (or the zone axis of the target material) was determined by indexing the inset of fast Fourier transform image for each figure.