Most absolute dates for rocks are obtained by radiometric methods methods, which are based on the radioactive decay of certain chemical elements.
Most radiometric methods directly measure the isotopes and their decay products in rock samples.
The more tracks there are, the longer the uranium has been decaying for.
High temperatures remove the tracks from the crystal, so when the ash leaves the hot volcano, its fission track ‘clock’ is at zero.
Zircons contain trace amounts of uranium (up to 1000 ppm) and thus fission tracks form within the crystal lattice over time, the density of which is proportional to the uranium content of the zircon.
As in apatite, fission tracks in zircon shorten when heated, but significant effects require much higher temperatures than in apatite.
Track lengths show a steady decrease from lengths of approximately 15 µm in outcrop or near surface samples, to zero at about 125°C.
The AFT ages clearly postdate the Variscan exhumation history of the Bavarian Forest.
Thermal modeling reveals that the ages are best explained by a slight reheating of the basement rocks to temperatures within the apatite partial annealing zone during the middle and late Jurassic and/or by late Cretaceous marine transgression causing burial heating, which affected marginal low-lying areas of the Bohemian Massif and the Bavarian Forest.
Apatite Fission-Track Analysis (AFTA) is emerging as an important new tool for thermal history analysis in sedimentary basins.
At temperatures between approximately 20°C and 150°C over times of the order of 1 to 100 my, fission tracks in apatite are annealed.