On Confined-Track Samples for Constraining Thermal Histories

Fission-track modelling rests on etching, counting and measuring the lattice damage trails from uranium fission. The tools for interpreting fission-track data are advanced but the results are never better than the data. Confined-track samples must be an adequate size for statistical analysis, representative of the track population, consistent with the model assumptions and with the calibration data. Geometrical and measurement biases are understood and can be dealt with up to a point. The interrelated issues of etching protocol and track selection are however more difficult to untangle. Our investigation favours a two-step protocol. The duration of the first step is inversely proportional to the apatite etch rate, so that different apatites etch to the same Dpar. A long immersion reveals many more confined tracks, terminated by basal and prism faces. This allows consistent length measurements and permits to orient each track relative to the c-axis. Long immersion times combined with deep ion irradiation reveal confined tracks deep inside the grains. Provided it is long enough, the precise immersion time is not important if the effective etch times of the selected tracks are calculated from their measured widths. Then, whether the sample is mono- or multi-compositional, we can, post hoc, select tracks with the desired properties. The second part of the protocol has to do with the fact that fossil tracks in geological samples appear to be under-etched compared to induced tracks etched under the same conditions. This should be assumed if the semi-axes (lC, lA) of a fitted ellipse plot above the induced-track line. In that case an additional etch can increase the track lengths to a point where they are consistent with the model based on lab-annealing of induced tracks, a condition for valid thermal histories. Here too, it is possible to select a subset with effective etch times consistent with the model if the widths of confined tracks are measured along with their lengths and orientations.