Effective Etch Times of Fossil Fission Tracks in Geological Apatite Samples and Impact on Temperature-Time Modeling

Abstract

Apatite fission-track modeling reconstructs the low-temperature histories of geological samples based on measurements of the lengths of etched confined fission tracks and counted surface tracks. The duration for which each confined track was etched can be calculated from its width given the apatite etch-rate νR. We measured νR as a function of crystallographic orientation for fourteen samples from the igneous and metasedimentary basement of Tian Shan, Central Asia, to optimize the track-length distribution for modeling the thermal histories of apatites with varying chemical compositions. To first order, νR scales with the size of the track intersections with the mineral surface in the range of Dpar = 1.4–2.6 µm. We use νR for calculating the effective etch time tE of confined tracks measured after 20–60 seconds of immersion in 5.5 M HNO3 at 21°C. Considering only tracks within a predetermined etch-time window improves the reproducibility of the track-length distributions. Because an etch-time window allows excluding under- and over-etched tracks, sample immersion times can be optimized to increase the number of confined tracks suitable for modeling. Longer immersion times also allow the longest-etched tracks to develop a clear geometrical outline from which the orientation of the apatite c-axis can be inferred. We finish by comparing thermal histories obtained with a conventional 20-second immersion protocol, without tE selection, with those using the length of tracks within the range of tE = 15–30 seconds. Overall, the alternative models fit better to independent AHe data than the conventional ones.