Towards more uniform confined-track data for modelling apatite fission-track thermal histories

Apatite fission-track modelling is a well-established method in geosciences. It is based on numerical mod­els calibrated against data collected using different etching conditions and applied to geologi­cal data measured under still other conditions. It is known that the etching and meas­urement protocols affect the track lengths and, in consequence, also the mod­elled thermal histories. Step-etching combined with measurements of the confined tracks widths permit calculating of their individual effective etch times (Aslanian et al., 2021), which better characterize their true state and the significance of their length than the immersion time. We applied this approach to basement and boulder samples from the German Naab, for which there are detailed and reliable independent burial and exhumation histories since the Permian. The apatite mounts were step etched for 10-20-30 s. We re-measured the lengths and widths of fossil and induced confined tracks after each step. We calculated the apatite etch rates as a function of orientation to the c-axis and used the results to estimate the effective (true) etch time of each confined track at each stage. Furthermore, we split the confined-track data into two sets, corresponding to etch time windows of 0-15 s (A) and 15-30 s (B). The mean induced track length is 14.5 µm for (A) and 16.5 µm for (B). The modelling results are different but show considerable overlap, indicating rapid exhumation around 200 Ma, a prolonged residence in the partial annealing zone and accelerated exhumation starting around ~60 Ma (A) or somewhat later (B; ~30 Ma). The overall Tt-trends are somewhat different; (A) shows gradual cooling through the partial annealing zone, whereas for (B), most Tt-paths show initial cooling to near-surface temperatures followed by reheating before final rapid cooling. Further investigation must establish which etch time windows are appropriate for fossil and induced tracks, and if these are identical. The research was funded by the EU/MEYS (CZ.02.2.69/0.0/0.0/19_074/0014756). References: Aslanian C., Jonckheere R., Wauschkuhn B., Ratschbacher L., 2021. A quantitative description of fission-track etching in apatite. American Mineralogist, 106, 518-526. Fig. 1 The inversion models for different etching conditions. A: etch time 0-15s, B: etch time 15-35s.