Effective Etch Times and compositional effects on the etching of Fossil Fission Tracks in Geological Apatite Samples

Apatite fission-track modelling reconstructs the low-temperature histories of geological samples based on measurements of the lengths of etched confined fission tracks and counted surface tracks. We investigate the influence of the chemical composition of apatite on the etching of fossil confined fission tracks, and its consequences for the apatite fission-track method, to optimize the track-length distribution for modelling apatites with varying chemical compositions. The duration for which each confined track was etched can be calculated given the apatite etch-rate ν_R. We conducted step-etch experiments on samples with etch pit diameters (Dpar) spanning most of the range for natural apatites (1.4–4.6 μm), including four gem-quality apatites (Panasqueira, Slyudyanka, Brazil, Bamble) and fourteen samples from the igneous and metasedimentary basement of the Tian Shan, Central Asia, in or­der to determine the apatite etch rate v_R as a function of crystallographic orientation for each. To a first order, ν_R correlates with the size of the track intersections with the mineral sur­face for all hexagonal apatites. For the gem-quality apatites we fitted three-parameter exponential functions (v_R = a 𝜙’ × e ^b𝜙’ + c); a and b both exhibit a linear correlation with Dpar. Combin­ing these results gives one equation (v_R = a(Dpar) 𝜙’ × e ^{b(Dpar)𝜙’} + c) giving the apatite etch rate v_R in a giv­en orientation (ϕ’) for hexagonal apatites with a specified chemical com­position (Dpar). Bamble exhibits a different - bimodal - relationship between v_R and ϕ’. In all cases, including Bamble, there is a striking parallelity between the angular frequencies of horizontal con­fined tracks and the magnitude of the apatite etch rate v_R per­pendicular to the track axes. This shows that the sample of confined tracks selected for measurement and modelling is to a much greater degree de­termined by the etching properties of the apatite sample than by geometrical or subjective biases. The mean track-etch-rate v_T correlates with Dpar, so that tracks etch to their full lengths in a shorter time in faster etching apatites. The mean rate of length increase between etch steps, v_L, also correlates with Dpar. For the Tian Shan samples we use ν_R for calculating the effective etch time t_E of confined tracks measured after 20 to 60 s immersion in 5.5 M HNO₃ at 21 °C. Considering only tracks within a predetermined etch-time window for length measurement 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 modelling. If v_T correlates with Dpar as our data indicate, future studies need to investigate how such an effective etch time window should be scaled by chemical composition as well. An alternative approach for selecting an appropriate etch time for each sample is to look on the track length anisotropy. We finally compare thermal histories obtained with a conventional 20 s immersion protocol, without t_E selection, with those using the length of tracks within the range t_E = 15–30 s.