Confined fission track revelation in apatite: how it works and why it matters

Abstract. We present a new model for the etching and revelation of confined fission tracks in apatite, based on step etching measurements that demonstrate variable along-track etching velocity, vT(x). We define two end-member model forms: Constant-core, with a central zone of constant etching rate that then falls off toward track tips; and Linear, in which etching rates fall linearly from the midpoint to the tips. Based on these, we construct a characterization of confined track revelation that encompasses all of the relevant processes, including penetration and thickening of semi-tracks from the polished grain surface, intersection of confined tracks, and analyst selection of which tracks to measure and which to bypass. Both model forms are able to fit step-etching data from five sets of paired experiments of fossil tracks and unannealed and annealed induced tracks, supporting the correctness of our approach and providing a series of insights into the theory and practice of fission-track thermochronology. Etching rates for annealed induced tracks are much faster than those for unannealed induced and spontaneous tracks, impacting the relative efficiency of both confined track length and density measurements, and suggesting that high-temperature laboratory annealing may induce a transformation in track cores that does not occur at geological conditions of partial annealing. The model quantifies how variation in analyst selection criteria, summarized as the ratio of along-track to bulk etching velocity at the track tip (vT/vB), likely plays a first-order role in the reproducibility of confined length measurements. It also shows that a large proportion of tracks that are intersected are not measured, indicating that length biasing is an insufficient statistical model for predicting the relative probability of detection of different track populations. The vT(x) model provides an approach to both optimizing etching conditions and linking track length measurements across etching protocols.