Apatite fission-track etch rates hold steady under annealing and ageing, new measurements show

For more than half a century, the fission-track method has allowed geologists to read the low-temperature thermal histories of rocks from microscopic damage trails left by uranium fission in minerals, with apatite the most intensively studied of them all. Yet one of the method’s foundational quantities has remained essentially unmeasured. Now a team led by Raymond Jonckheere of the Endogene Geologie group at Technische Universität Bergakademie Freiberg, together with colleagues from the Institute of Modern Physics of the Chinese Academy of Sciences in Lanzhou, reports direct measurements of the two velocities that govern how fission tracks are revealed under the microscope, and finds that several long-standing assumptions about them do not survive the evidence.

Etching is the step that makes the invisible visible. Apatite grains separated from a rock are mounted in resin, polished, and treated with acid so that channels open along the nanometre-wide damage trails left by fission fragments, enlarging them to micrometre diameters that can be counted and measured with an optical microscope. The process is described by two velocities: the track etch rate vT, the rate at which the etchant advances along the damaged lattice of the track itself, and the apatite etch rate vR, the rate at which undamaged apatite dissolves in other directions. Their values determine which grains are suitable for counting and dating, which tracks qualify for measurement and time-temperature modelling, and the characteristic shapes of etched tracks. Despite this importance, the authors note that actual measurements of vT and vR have long been lacking.

The new work, published in the journal Geochronology, provides numerical estimates of both quantities and probes the factors that control them. The central result concerns the behaviour of vT along a track. The team’s measurements support a constant core model, in which the track etch rate holds constant over most of the length of a track, rather than varying with the local density of damage. The evidence is visible directly in the microscope images: etched fission tracks in apatite show straight sides, lacking the curvature that a varying etch rate would produce. The authors argue that this means vT is unrelated to the local stopping power of the fission fragments or to the radius of the latent track, since etched confined tracks show no constriction or interruption in their midsections even where the stopping power and track radius are assumed to be near zero at the locus of the fissioned nucleus.

The first experiment used a single crystal of Durango apatite from Mexico, a well-characterised material with a fission-track age of about 31.5 million years. The researchers mounted a prism section in resin, polished it, and irradiated it perpendicular to the crystal’s c axis with 25.0 MeV per nucleon krypton-86 ions at the HIRFL facility in Lanzhou. Etching in 5.5 M nitric acid at 21 degrees Celsius revealed the ion tracks, and the team measured the diamond-shaped etch figures that develop where the tracks intersect a sealed natural crack running from the polished surface tens of microns into the crystal. From these measurements they calculated the effective etch time and the rate of etchant advance along each track. For the initial 20-second immersion, the track etch rate came out at roughly 60 micrometres per minute, a value comparable with earlier estimates for induced fission tracks in Durango apatite.

The experiment also yielded a practical finding about etching protocols. When the mount was etched in successive steps, the etchant advanced along the already-etched track sections at a high but finite rate, calculated at roughly 275 micrometres per minute, rather than instantaneously. This means that the summed immersion times of a step-etch experiment overestimate the effective cumulative etch time, by more than about 4 seconds for the 10-second step following the initial 20-second immersion in the krypton-track experiment. Step-etching therefore broadens the effective-etch-time distributions of ion and fission tracks, and the authors note that successive etch steps cause vT to be underestimated to an increasing extent.

To test whether partial annealing changes the track etch rate, the team examined four prism sections of Durango apatite that had been pre-annealed, neutron-irradiated, and then re-annealed to create induced-track populations with mean lengths of roughly 16, 14, 12, and 10 micrometres. All samples were etched for 20 seconds in 5.5 M nitric acid at 21 degrees Celsius, and the confined tracks were imaged and measured for length, orientation, width, cone angle, host-track intersection position, and Dpar, the size of the track’s surface intersection parallel to the c axis. The cumulative distributions of the track etch rates were the same after 10 hours of annealing at 240, 288, and 310 degrees Celsius as for unannealed induced tracks, despite mean length reductions of roughly 2, 4, and 6 micrometres. The authors state that this contradicts a previous finding that partial annealing of induced tracks at 235 to 280 degrees Celsius causes up to a doubling of vT.

Other measured distributions proved equally insensitive to annealing. The angular distribution of the tracks remained unchanged after partial annealing, as did the distribution of the intersection points between host tracks and confined tracks. The median effective etch time of the unannealed tracks was only 2 seconds longer than that of the most heavily annealed sample, a difference the authors show is consistent with the roughly 6-micrometre mean-length difference between the two samples given the average track etch rate of about 120 micrometres per minute. The Dpar distributions did shift systematically, with medians decreasing from 1.89 to 1.80 micrometres with increasing annealing, which the authors interpret as a thermal relaxation of irradiation-induced strain from atomic recoils. Under an Arrhenius assumption, this small shift corresponds to the annealing of roughly one in a thousand of the point defects created by irradiation over the 10-hour treatment.

The third strand of the investigation turned to geological time. The team studied FC1 and AS3 apatites from the Duluth intrusive complex in Minnesota, reference materials with fission-track ages of roughly 820 to 882 million years and 924 to 989 million years respectively. These old samples have long low-temperature thermal histories and have accumulated extensive radiation damage, the most important factor responsible for ageing; the total accumulated alpha dose for FC1 is estimated at about 7 times 10 to the 16 alpha particles per gram, based on published data, although the fraction of that damage surviving to the present is unknown. The researchers annealed an aliquot of each sample for 24 hours at 450 degrees Celsius, irradiated it with thermal neutrons at the BR1 reactor in Mol, Belgium, and compared fossil tracks in the untreated material with induced tracks in the irradiated aliquot, etching both for 20 seconds under the same conditions.

Once again, the track etch rate proved remarkably stable. The vT distributions of all the Duluth samples were almost identical, and the roughly 5 percent difference between the means for fossil and induced tracks was judged too small to be significant. Fossil tracks in both samples had undergone more than 20 percent shortening, yet ageing, or geological low-temperature annealing, appears to have had a minimal effect on the etch rates of the fossil tracks. In sharp contrast, the apatite etch rate vR showed marked variation. Dpar measurements indicated that the apatite etch rate differs by 7 to 8 percent between FC1 and AS3 and by 16 to 17 percent between fossil and induced tracks, with the fossil-track values greater in both samples. The authors attribute this to distributed radiation damage from alpha decay of uranium, thorium, and samarium isotopes and from collateral recoils, accumulated over geological time, which increases the overall apatite etch rate. The effect was even more pronounced in the widths of the track-surface intersections, which roughly doubled while Dpar increased by less than 20 percent, consistent with slow-etching crystal faces being more susceptible to radiation damage than fast-etching ones.

The practical consequences reach into how thermal histories are reconstructed. Because a greater apatite etch rate shortens the time the etchant needs to reach confined tracks through the host track, confined tracks in fast-etching apatites have longer effective etch times, which the authors say implies a need for caution when using track openings or Dpar for assessing annealing rates. Within a single etching step, however, the variation of vT and effective etch time between tracks added only about 0.1 to 0.3 micrometres to the lengths of the fastest-etching or best-etched tracks, an effect the authors conclude is too small to be significant for geological applications. Overall, the measurements show that the track etch rate is little dependent on the track-forming process and is little affected by either laboratory or geological annealing, while the apatite etch rate, besides its dependence on composition, is somewhat affected by annealing and in some samples by ageing. The team reports 3588 values of vT, which they describe as the only estimates in existence apart from predetermined etch-rate profiles fitted to 28 track lengths, and their revised picture carries implications for the interpretation of track etching, track selection, and the estimation of thermal histories from apatite fission tracks.

Subject of Research: Measurements of the track and apatite etch rates that govern fission-track dating in apatite and the effects of annealing and ageing on them.

Article Title: On the effects of ageing and annealing on the apatite and fission-track etch rates

Article References: Jonckheere, R., Trilsch, F., Liu, J., Zhai, P., & Nagel, T. (2026). On the effects of ageing and annealing on the apatite and fission-track etch rates. Geochronology, 8(4), 645-659. https://doi.org/10.5194/gchron-8-645-2026

Image Credits: AI Generated

DOI: 10.5194/gchron-8-645-2026

Keywords: apatite, fission-track dating, etch rates, annealing, radiation damage, geochronology, thermal history, Durango apatite, confined tracks, Dpar, ion irradiation, thermochronology