A novel comparison between the annealing kinetics of induced and spontaneous fission tracks in zircon

Abstract

The annealing kinetics of fission tracks in zircon is dependent on the quantity of alpha recoil damage accumulated. This effect must be better understood to improve zircon fission track age interpretation and to enable the safe inference of thermal histories. We conducted annealing experiments on zircon from Poços de Caldas (ZPC, Brazil) to compare the annealing kinetics of freshly induced fission tracks in pre-heated zircon with the annealing kinetics of spontaneous fission tracks. For reference, we also conducted annealing experiments on spontaneous fission tracks of the Nisatai Dacite (NST) zircon, for which annealing data are available in the literature. ZPC has an age of 81.4 Ma (FT) while NST is younger (21 Ma), but has a higher uranium content, leading to a similar magnitude of alpha recoil damage. Our results indicate that the freshly induced fission tracks in previously annealed zircons are more resistant to annealing than the spontaneous fission tracks in the ZPC sample during initial stages of annealing. However, under advanced annealing conditions, the annealing kinetics of both types of fission tracks appear to converge. It is yet unclear whether this is due to differences in track formation in a damaged structure, by the alpha recoil impacting already formed tracks or by a combination of both. The annealing datasets were used to fit annealing models, whose geological extrapolations yield higher annealing temperatures for the induced than for the spontaneous fission tracks. When a 1 Ma geological natural annealing dataset is included in the fitting, the predicted annealing temperatures fit better to geological constraints, at least in part, mitigating the alpha recoil damage effects.