Increasing the resolution of geological temperature-time (T,t-) paths from apatite fission-track modelling

A common etching protocol for fission tracks in apatite is 20 s immersion in 5.5 M HNO3 at 21 °C. The duration that each confined track has been etched is, however unknown. Step-etching and track width measurements permit to calculate their individual etch times (Aslanian et al., in press). This approach helps mitigate part of the measurement-related uncertainties and offers closer access to the track structure, which, in fossil tracks, reflects the combined effect of track formation and geological annealing. Strategy, as mentioned above, improves the resolution of apatite fission-track modelling (Novakova et al., 2020).

We studied samples from the Naab region on the western border of the Bohemian Massif. We measured the tracks lengths and widths after 10 s etching. We then etched the samples for 10 s more and re-measured the same tracks, where possible, as well as tracks that appeared for the first time. After this, we added the third 10 s step and measured all visible tracks. The track lengths and widths increase with each step. For the tracks that were etched after 10 s, the mean lengths increased from 10.8 μm (10 s) to 12.7 μm (20 s) and 15.5 μm (30 s). Tracks first visible after 20 s increased in length from 11.7 μm (20 s) to 13.6 μm (30 s). Tracks first measured after 30 s had a mean length of 13.9 μm. Their mean track width increased from 0.7 μm (10 s) to 1.2 μm (20 s) and 2.1 μm (30 s). The corresponding values for tracks first observed after 20 s are 1.0 μm (20 s) and 1.9 μm (30 s). Tracks measured first after 30 s have a mean width of 1.9 μm. The provisional mean calculated effective etch time 9.6 s for 10 s etching, 15.8 s for 20 s etching and 24.8 s for 30 s etching. Our data suggests the relation between track length and effective etch time is similar regardless of the etching step the track was first noticed.

The research was funded by the EU/MEYS (CZ.02.2.69/0.0/0.0/19_074/0014756).

Aslanian, C., et al. A quantitative description of fission-track etching in apatite. Am. Miner. in press.

Novakova, L., et al. 2020. Thermal history modelling of the western margin of the Bohemian Massif using high-resolution apatite fission-track thermochronology. EGU 2020-15152.