Abstract. We present a new model for the etching and revelation of
confined fission tracks in apatite based on variable along-track etching
velocity, vT(x). Insights from step-etching experiments and theoretical
energy loss rates of fission fragments suggest two end-member etching
structures: 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. From these, we construct a
characterization of confined track revelation that encompasses all relevant
processes, including penetration and widening of semi-tracks etching in from
the polished grain surface, intersection with and expansion of confined
tracks, and analyst selection of which tracks to measure and which to
bypass. Both etching structures are able to fit step-etching data from five
sets of paired experiments of fossil tracks and unannealed and annealed
induced tracks in Durango apatite, 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 etched track tip (vT/vB), likely plays a first-order
role in the reproducibility of confined length measurements. It also
accounts for and provides an estimate of the large proportion of tracks that
are intersected but not measured, and it shows how length biasing is likely to
be an insufficient basis for predicting the relative probability of
detection of different track populations. The vT(x) model provides an
approach to optimizing etching conditions, linking track length measurements
across etching protocols, and discerning new information on the underlying
structure of fission tracks.
Confined fission-track revelation in apatite: how it works and why it matters