On the effects of ageing and annealing on the apatite and fission-track etch rates

Fission tracks in apatite must be etched for counting and measuring with an optical microscope. Track etching is described with the aid of two velocities: the track etch rate v _T and the apatite etch rate v _R . The efficiencies of track counts, the selection of confined tracks, and the distinctive shapes of etched tracks depend on the values of v _T and v _R ; v _R is also an indicator of the apatite composition and annealing kinetics. Despite their importance, measurements of v _T and v _R have been lacking. We here report measurements of v _T and v _R and investigate factors controlling them. The results support a constant core model, i.e., a constant v _T over most of the track length. The host-confined-track intersections, c  axis angles, and track-etch rates have near-identical distributions across all experiments. Neither lab annealing of induced tracks nor geological ageing of fossil tracks has a significant effect on v _T . Overall, for a one-step etch protocol, the lengths of fission tracks exhibit a weak positive correlation with their track etch rates and effective etch times. The effect is however too small to be significant for geological applications. When using a step-etch protocol, the etchant advances at a finite rate along tracks etched during preceding steps. This broadens the effective-etch-time and track-length distributions, while the summed immersion times overestimate the aggregate effect of the procedure. In contrast to v _T , v _R exhibits clear variation related to damage from self-irradiation in the case of fossil tracks and to a minor extent from neutron irradiation in the case of induced tracks. Greater v _R shorten the access times to confined tracks, increasing their effective etch times. This implies a need for caution when using the track openings for assessing annealing rates.