EVIDENCE FOR UNETCHABLE GAPS IN APATITE FISSION TRACKS

The apparent shape of etched confined fission tracks in apatite depend s on their crystallographic orientation, on the direction of observati on and on the distribution of longitudinal etching velocities along th e track. Overetched blade-like tracks, which are approximately perpend icular to the c-axis, sometimes exhibit an extremely discontinuous etc hing behaviour resulting from the existence of so-called ''unetchable gaps''. The full length of such tracks might not be revealed under nor mal etching conditions. In the context of thermal history analysis, th e occurrence of unetchable gaps is very important for the understandin g of the relationship between measured track-length distributions and areal track densities. In contrast to previous statements concerning D urango apatite, it can be shown that the phenomenon of fragmentation i s not restricted to short tracks with a length of < 11 mu m but may al so affect rather long tracks. During this study a significant number o f natural tracks with a clearly discontinuous etching behaviour and wi th a total length above 14 mu m has been found in samples from differe nt locations and with different cooling histories. In the investigated samples the percentage of obviously fragmented natural tracks varies between 3% and 8%. Fragmented tracks have also been found in the fresh induced populations of two samples. Therefore, the process of fading appears to be a combination of shortening and fragmentation even at th e early stages of annealing. Proportionality between the reductions of the areal track density and the etchable track length, as has been as sumed in some models dealing with projected track-length distributions , does not describe the true behaviour of strongly annealed apatites.