Apatite fission-track age correction and thermal history analysis from projected track length distributions

Abstract

The frequency distribution of fission-track lengths in an apatite reflects the mineral's thermal history since the time of its last crossing into the partial annealing zone (PAZ). Apatite thermal history analysis based on confined track length measurements is an already well-established tool in hydrocarbon exploration. In a less direct though more easily accessible way the thermal history is also reflected in the frequency distribution of the projected lengths of tracks intersecting the apatite surface. A mathematical description of the probability distribution of the projected length of surface tracks (PTLD, projected track length distribution) corresponding to different types of thermal history is presented and a new method for correcting thermally lowered fission-track ages is proposed. This correction method gives the time of entry into the PAZ at ∼120°C but also the time when the sample left the PAZ to enter the total track stability zone (at ∼60°C). An age function is subsequently defined which allows us to distinguish between different types of thermal history. Experimental verification shows that, although the proposed approach is less precise compared to existing methods based on projected track lengths, it is possible to distinguish between different thermal histories provided that a sufficient number of tracks are measured. On the other hand, the age function provides information relative to the samples' complete trajectory through the PAZ whereas existing methods are limited to temperatures below the closing temperature of the fission-track system.