Interpretation of apatite (U–Th)/He ages and fission track ages from cratons

Apatite (U–Th)/He ages from southeast Sweden have been interpreted as proving that the region has remained stable through Mesozoic and Cenozoic times, with no more than 100 metres of sedimentary cover throughout this time. However, we suggest this interpretation cannot be sustained, and present an alternative interpretation involving deposition of 1 km of Mesozoic cover, which explains the observed ages at least as well as the original models. Inconsistencies between apatite fission track (AFT) data and apatite (U–Th)/He ages from this and other regions of Fennoscandia have been explained in terms of fission track ages in these apatites being reset by a non–thermal “radiation–enhanced” annealing process, on the basis that fission track ages are younger than expected on the basis of the “known geological evolution” of the area. We dispute this line of reasoning, and suggest that AFT ages from the region can be readily understood in terms of normal thermal annealing processes. Other evidence put forward to support the concept of “radiation–enhanced” annealing can be explained more simply in other ways. We suggest the discrepancy between AFT and (U–Th)/He ages arises because the He retention properties of apatite change as the amount of He retained within the apatite increases, and further suggest that greater attention should be paid to diffusion of helium in different apatite species.