Fission-track etching in apatite: A model and some implications

Abstract

From their formation, fission tracks are complex structures, onto which their thermal histories come to be imprinted. Track etching leaves elongated voids whose lengths and orientations are used for reconstructing these histories. It is thus important to understand etching for interpreting track data. We revive an existing dissolution model which explains the geometries and dimensions of etched fission tracks in apatite. It implies that on continued etching the track contours come to reflect the minimum and maximum apatite etch rates, at the same time that all trace of the track structure is erased. We cannot derive valid etch rates from the dimensions of the track openings or from the length increase of step-etched confined tracks. The roundedness of the track tips is not a measure of etching progress. Understanding the contours of confined tracks does permit in most cases to calculate their true etch times. We propose to exploit this fact to set an etch-time window, and to model the confined-track data in this interval. The excluded measurements will be those of the least-etched and most-etched tracks. This numerical loss is offset by the fact that an etch-time window relaxes the requirement of a fixed immersion time, and a longer immersion multiplies the measurable confined tracks. This calls for no changes to existing procedures if the etch-time windows for different protocols give consistent results. The length data for apatites with different compositions could become comparable if their etch-time windows were linked to a compositional parameter.

Short communication: Experimental factors affecting fission-track counts in apatite

“…stant counts. Despite its correct prediction, the first model was deemed inapplicable because it failed on other counts (Jonckheere et al, 2019(Jonckheere et al, , 2022. In contrast to that model, in which no etched track is ever lost, the second model implies constant track counts because the rate at which tracks are added from surface etching equals that at which others are lost from the same cause.…”

“…The vectors radiate from the intersection of a prism plane P-P and a basal plane B-B, both perpendicular to the drawing plane. The elongate diamond shapes are the etch figures formed by the fastest etching faces after 10, 20 and 30 s etching, constructed using the model of Jonckheere et al (2019Jonckheere et al ( , 2022. The intersections of the elongate diamond shapes with the etched prism planes and basal planes give the sizes of the track openings in these surfaces.…”

“…A prism face perpendicular to the drawing plane intersects it along the line P-P running from left to right (dot dash). The diamond shapes are the etch figures formed by the fastest etching planes after 10 (solid), 20 (long dash) and 30 s etching (short dash; Jonckheere et al, 2019Jonckheere et al, , 2022. Their intersections with the etched basal surface (B-B) and prism surface (P-P) determine the sizes of the track openings (D BAS , D PAR ) in these surfaces.…”

“…Prism faces are not unproblematic for counting tracks and other orientations are not per se useless. Faster-etching surfaces, in which etch pits do not form at the track-surface intersections (Jonckheere et al, 2020(Jonckheere et al, , 2022 can indeed present practical advantages, in addition to the numerical advantage of including them. Their fission-track properties are the subject of ongoing studies.…”

On etching, selection and measurement of confined fission tracks in apatite

“…This underscores the importance of the etch times of individual tracks for modelling of the confined track length distribution. Aslanian et al (2021) and Jonckheere et al (2022) proposed a new etch model and measured the rates for Durango apatite etched in 5.5 M HNO3 at 21 °C. This permits to calculate the true duration (tE) for which an individual confined track has been etched from its orientation, shape and thickness.…”