Fission track lengths in the apatite annealing zone and the interpretation of mixed ages

Abstract

Measurements of confined fission tracks in apatites from deep boreholes show that their mean length is reduced, and the length distribution becomes progressively broader through the fission track annealing zone. At subsurface temperatures of around 100°C and above, the length distribution characteristically becomes very broad without a pronounced peak. The lengths of the longest tracks remain essentially constant at all stages of natural annealing observed in the boreholes. This pattern is similar to that found in laboratory annealing of spontaneous fission tracks in apatite from an outcrop sample of the same formation. These observations show that confined track lengths can be used as important indicators of the type of thermal history that a sample has experienced in the temperature zone of increasing track stability. The length distributions reported here provide a basis for interpretation of fission track ages that might otherwise be ambiguous. An apatite age which results from a uniform slow-cooling history will have a broad, negatively-skewed length distribution, while a bimodal distribution provides clear evidence of a two-stage history involving partial annealing by a discrete thermal event. The apparent fission track age associated with a bimodal length distribution will be a “mixed” age intermediate between the original age and that of the later event.