Factors influencing data reproducibility of fission-track (FT) thermochronology can be summarized into three main categories associated with data acquisition steps. (1) Sample preparation involves mineral separation, mounting, polishing and etching; (2) data revelation relates to instrumentation (microscope, LAICPMS, etc.) and software settings; and (3) execution depends on feature selection by the analyst. Previous committee reports and studies (Hurford A.J. 1990; Ketcham et al. 2009; Ketcham et al. 2015; Ketcham et al. 2018) have contributed significant insights into the reproducibility of fission-track data by comparing length and age measurements produced by several laboratories using their own preparation and revelation procedures. A recent attempt to isolate analyst-specific factors in length measurement using an image-based approach (Tamer et al. 2019) found that when two analysts observe the same feature and agree it is a valid track, measurement reproducibility was very good, though impacted by etching. Dispersion of individual length measurements was 0.7-1.0 µm (2 for weaker etching and 0.5-0.8 µm for stronger etching, but mean lengths were always within 0.1 µm of each other. Where the analysts disagreed more significantly, however, was in finding tracks and evaluating whether they were valid, sufficiently clear, and sufficiently etched for measurement, which led to differences of up to ~0.8 µm in mean track length. This study builds on the image-based approach to encompass more aspects of the measurement process and increase the number of analysts being compared. We will look at confined track selection in greater detail, and also study analyst decisions behind age determination, including the selection of the region of interest for counting, and identification of grain-surface features as tracks appropriate for counting. Reflected and transmitted light image stacks for 41 grains and graticules are available on a cloud platform Participants will carry out analyses of these images using their preferred approach, e.g. suitable analytical software, manual measurements or AI-based analysis. A limited license for FastTracks (v3.2) will be available for those who would like to participate but do not have measurement software. Analysts are asked to fill out a questionnaire about their fission track experience, conduct track density estimations, confined track length and Dpar measurements, and especially provide comments on all grains being analyzed or skipped. FastTracks users are asked to send the .xml files produced by the software, while other participants are asked to submit the results using a template. The results will be entirely anonymous unless the analyst states otherwise. The deadline for the submission of the results is June 1st, 2022. The results will be shared on 18th International Conference on Thermochronology.

We report a new series of experiments to explore the phenomenon of low-temperature annealing of fission tracks in apatite that feature a number of improvements over previous work. Grain mounts were pre-irradiated Cf to increase confined track detection and allow briefer thermal neutron irradiation. We co-irradiated and etched four apatite varieties (Durango, Fish Canyon, Renfrew, Tioga) over five time steps equally spaced from 3.66 to 15 ln(s). A length standard was co-etched with all experiments to ensure that subtle differences are within detection limits. Finally, we used a standard etching protocol, allowing the data to be co-modeled with extensive high-temperature data sets and recent analyses of induced tracks that underwent ambient-temperature annealing over year-to-decade time scales. Ambient-temperature annealing occurs at two different rates, with faster annealing at early stages that decreases to a slower rate that converges with empirical fanning linear or curvilinear models. The nature of this decrease varies among the apatite species examined, but no patterns could be determined. The fitted models make geological time-scale predictions consistent with those based on high-temperature data only, and also make predictions consistent with reasonable inferred low-temperature histories for all four apatite varieties. The empirical fanning curvilinear equation encompasses low-temperature annealing at month-to-decade time scales, but low-temperature annealing at shorter time scales may occur by a distinct mechanism. We consider but rule out annealing by radiation from short-lived activated isotopes. We also reconsider the notion of the initial track length, and the appropriate length for normalizing confined track length measurements.

We report a new series of step-etch experiments to reveal the influence of microscopy technique on track selection bias. Two different aliquots of induced tracks in Durango apatite were etched for 10-15-20-25-30s and for 20-25-30s in 5.5M HNO3 at 21°C. Three different track selection criteria were applied after the initial etch step of the etching procedure: (1) all tracks with reasonable measurability under transmitted and reflected light switch with 100x objective and 2.5X optovar magnification; (2) ”fully etched” tracks under transmitted light with 100x objective; and (3) ”fully etched” tracks under reflected light with 100x objective. Approach 1 was applied to both aliquots and the approaches 2 and 3 to the latter aliquot. Comparing the mean track lengths, approaches 2 and 3 result in similar values over all experiments, while approach 1 provides a ~0.4 μm higher in the first aliquot and ~0.3 μm lower in the second aliquot than approach 2 and 3 due to higher and lower variations of effective etch times. Comparing the c-axis angles, in the 0-30° range approach 3 provides a severely reduced fraction of tracks due to their weak appearance under reflected light. Furthermore, approach 2 provides ~%14 lower track densities comparing with approaches 1 and 3. We recommend using both transmitted and reflected light during entire track selection and measurement procedures. We are working to develop a new 2+-step etching procedure, where tracks are located after 10s of etching but measured after a second 10s etch step, resulting in better-controlled etching times while reducing the bias associated with analyst choice. Furthermore, this two-step etching procedure can be iterated for more etch-steps, by identifying newly-appeared tracks after each etch step and etching them for 10s more, to increase the number of measured tracks while maintaining consistent selection criteria.