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Geochronology Advances in geochronological science
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Preprints
https://doi.org/10.5194/gchron-2020-31
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gchron-2020-31
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  20 Oct 2020

20 Oct 2020

Review status
This preprint is currently under review for the journal GChron.

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

Richard A. Ketcham and Murat T. Tamer Richard A. Ketcham and Murat T. Tamer
  • Department of Geological Sciences, Jackson School of Geoscience, University of Texas, Austin, TX 78712, USA

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.

Richard A. Ketcham and Murat T. Tamer

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Richard A. Ketcham and Murat T. Tamer

Data sets

Step-etching data for confined fission tracks in Durango apatite Richard Ketcham and Murat Taner Tamer https://doi.org/10.18738/T8/MPWDZI

Model code and software

ketcham/vTx-model: First release of vTx-model code Richard Ketcham https://doi.org/10.5281/zenodo.4064290

Richard A. Ketcham and Murat T. Tamer

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Latest update: 25 Nov 2020
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Short summary
We introduce a new model of confined fission-track revelation, which simulates the etching process and accounts for variable along-track etching rates. This complete characterization explains many observations, including community difficulty in obtaining consistent track length measurements. It also provides a quantitative basis for optimizing etching procedures, and ultimately deriving more robust and reproducible fission-track ages and thermal histories.
We introduce a new model of confined fission-track revelation, which simulates the etching...
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