Abstract
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.
Highlights
Apatite fission-track confined lengths remain of great interest because of their capacity to record detailed thermal histories (Malusa and Fitzgerald, 2019; Gallagher, 2012; Ketcham et al, 2018)
This divergence leads to continuing uncertainty in the fidelity of induced tracks annealed in the laboratory as proxies for spontaneous ones annealed at geological conditions over geological timescales
All mathematical treatments of track revelation, biasing, and the relationship between confined track length and track density portray latent tracks as line segments in space and presume that the probability of a track being measured is equivalent to its probability of being intersected by an etchant pathway (Galbraith and Laslett, 1988; Galbraith et al, 1990; Laslett et al, 1984, 1982; Dakowski, 1978; Jonckheere and Van Den Haute, 1999; Ketcham, 2003). This simplification does not consider time and effectively assumes that all tracks are etched to their full extents, or at least that all tracks are likely to become fully etched once they are intersected. We demonstrate that these assumptions are incorrect and that this shortcoming impacts apatite fission-track (AFT) thermochronology in multiple ways, from reproducibility of confined track length measurements to the efficiency of track revelation that underlies age determinations
Summary
Apatite fission-track confined lengths remain of great interest because of their capacity to record detailed thermal histories (Malusa and Fitzgerald, 2019; Gallagher, 2012; Ketcham et al, 2018). Measurements of laboratory-annealed spontaneous and induced fission tracks designed to test the principle of equivalent time, which posits that track annealing behavior is determined by length alone and not prior thermal history (Duddy et al, 1988), indicate that their behavior subtly but certainly differs (Wauschkuhn et al, 2015). Work on using ion tracks for identification of cosmic ray particles (e.g., Green et al, 1978; Price and Fleischer, 1971; Price et al, 1967, 1973) measured etch rates at varying locations along implanted tracks, linking them to the ionization rate, or the rate at which an ion transfers energy to the medium it is passing through These studies established that if vT can be determined with sufficient precision at two points along a particle path, the atomic number of the particle can be uniquely identified (Price and Fleischer, 1971). The details of each fission pair vary, with either the heavier or lighter fragment initially losing energy more quickly, there is a general pattern of relatively slow change in energy loss rate toward the center of a track and faster as the enhanced etching limit is approached
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