Thermal annealing characteristics of fission tracks in natural zircons of different ages

Fission track (FT) thermochronometry using zircon has widely been applied to the resolution of a variety of geologic problems, for which the understanding of FT annealing behaviour is essential. Thermal annealing experiments were conducted on FTs in natural zircons having different ages (ranging from similar to 0.6 to similar to 70 Ma) and radiation damage levels. We measured horizontal confined track lengths on nine zircon concentrates separated from rapidly cooled volcanic rocks, after 1 hr annealing at 400-700 degrees C. As the annealing temperature increases, the observed tracks show a consistent and systematic length reduction in all samples, and the mean track lengths are hardly distinguishable among the nine samples for the same annealing step. Our results suggest that the thermal annealing characteristics at laboratory time-scale are concordant among the zircons, regardless of their ages, and that identical annealing kinetics may work for Late Mesozoic to Cenozoic zircons.