Apatite fission track and (U-Th)/He data from Fennoscandia: An example of underestimation of fission track annealing in apatite

Abstract

Apatite fission track (AFT) data from cratons have commonly been interpreted in terms of major episodes of sedimentary burial and subsequent denudation, often in the absence of any remnant sedimentary rocks or other supporting geological evidence. Reconciling these interpretations with geologically documented denudation events or outcropping regional stratigraphy can be problematic. Also, (U-Th)/He ages from cratonic igneous rocks that are older than paired AFT data are commonly discarded as ‘too old’ even when the former are repeatable at the 2 σ confidence level. For example, AFT data from central Fennoscandia have been interpreted to reflect, first, burial by a Caledonian foreland basin several kilometres in thickness and, subsequently, its denudation. However, throughout Fennoscandia there is no direct evidence that such a deep, extensive foreland basin ever existed and indeed some geological evidence points to the contrary. Additionally, a significant number of (U-Th)/He ages from Fennoscandia are older than their AFT pairs but have been interpreted by their authors as ‘too old’ relative to the AFT ages and not geologically meaningful despite repeatable results. Here we propose a new interpretation of AFT data from Fennoscandia that closely reflects the observed data yet is compatible with the generally accepted geological history. Discarding the hypothesis of a thick Caledonian foreland basin, we suggest that 1) AFT ages in Finland are ‘too young’ rather than that the (U-Th)/He ages are ‘too old’; 2) repeatable sequences of (U-Th)/He ages are more likely to record the true denudation history of cratons; 3) the importance of radiation-enhanced annealing at low temperatures has been overlooked; and 4) Fennoscandian AFT data may be divided into an eastern domain strongly affected by radiation-enhanced annealing (where current AFT annealing models are invalid) and a western domain much less affected by radiation-enhanced annealing (where current modelling methods may appropriately be used to extract thermal histories). In principle, radiation-enhanced annealing is also pertinent to the interpretation of young AFT ages from apatites rich in α-emitter actinides. The construction of a properly calibrated annealing model that fully incorporates the effects of radiation-enhanced annealing is fundamentally important.