On the cause of the shortening of spontaneous fission tracks in certain minerals

Measurements are presented of the etchable ranges of spontaneous and induced fission tracks (via confined tracks) in a number of apatites from a variety of geological environments. The fission track ages of these apatites have also been determined. Spontaneous tracks are shorter than induced tracks in all cases, by amounts varying up to a maximum of 17% in an apatite with a fission track age of 256 My. Evidence is presented to show that this shortening is due to thermal annealing of spontaneous tracks over the fission track age of the sample, at low (ambient) temperatures. This effect is likely to be found in all apatites, and also in all samples of a number of other minerals, for which the above conclusions also hold. (author)