in interpreting AFTA data?

Conversion of raw fission track data into thermal history solutions is the core of the AFTA technique. In order to ensure that these solutions are as precise and accurate as possible, it is essential to use the most reliable algorithms for annealing kinetics, and to allow explicitly for compositional influences on annealing rates. In addition, it is important that the basic data are collected in a way that allows these aspects of the technique to be taken into account. The example shown here illustrates the possible errors that can be introduced by failing to allow for these factors.

This plot shows some results from an outcrop sample of Triassic sandstone from NE England. Fission track ages in individual apatite grains from a single sample are plotted as a function of Chlorine content. Low-Cl grains are more easily annealed than their high Cl counterparts. This is expressed by the way in which the fission track ages of low-Cl grains have been significantly reduced while higher Cl grains have undergone very little age reduction. The result is a significant spread in single grain ages within a single sample. It is vital that this effect should be allowed for in extracting thermal history solutions from these data.

Using Geotrack's multi-compositional annealing kinetic model as described in Information Sheet 99/2 to define the best-fit thermal history from the full dataset (also taking into account the track length data and their variation with Cl content), we find the solution shown above. The sample began to cool from a maximum paleotemperature around 100C between 75 and 50 Ma, while a later cooling episode is also revealed. This interpretation is consistent with extensive evidence for Early Tertiary cooling across Northern England, and provides a geologically reasonable interpretation.

If, however, we had analysed only fission track ages from the higher Cl grains, while track length data came dominantly from lower Cl grains, and if we also ignore the influence of Cl content, we would get the above solution, which makes very little sense geologically. Analytical procedures in which fission track ages are measured in grains with highest track densities, while lengths are measured in all grains, could easily produce such data, while compositional effects are largely ignored in most other fission track laboratories. Geotrack's techniques are designed specifically to allow explicitly for compositional effects at every stage of the process, and provide the most reliable thermal history solutions available.

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