Conflicting (U-Th)/He and fission track ages in apatite: Enhanced He retention, not anomalous annealing behaviour

While initial studies showed a high degree of consistency between AFTA and apatite (U-Th)/He dating, an increasing number of studies are reporting apatite (U-Th)/He ages which are older than expected on the basis of apatite fission track data from the same region. We present data from a range of geological settings to document a systematic discrepancy between the two systems, which becomes more pronounced in samples with older fission track ages (except for samples with very low uranium contents). Results from a granite pebble and enclosing volcanogenic sandstone provide a well-controlled test case in which independent constraints are available on the underlying thermal history. These demonstrate that the progressive discrepancy between the two techniques arises not from anomalous fission track annealing behaviour, as has been suggested, but as a result of a change in the He retention properties of apatite. This change appears to be linked to the degree of accumulated radiation damage within the crystal lattice, although underlying mechanisms remain unclear. We suggest detailed experiments should be performed to delineate and quantify the processes responsible. Until this is achieved, we suggest that apatite (U-Th)/He studies should also incorporate apatite fission track data, as well as other low temperature indicators, in order to monitor the (U-Th)/He system response and to guard against the anomalous behaviour described here.