Fission-track annealing in apatite: Track length measurements and the form of the Arrhenius plot

Abstract

Previous fission-track annealing studies have described the reduction in fission-track density in terms of a series of fanning lines on an Arrhenius plot. This has been interpreted in terms of a range of activation energies corresponding to different degrees of annealing, with activation energies varying by a factor of 2 or 3 from complete retention to total erasure. New High precision measurements of confined track lengths in annealed Durango apatite, however, seem to be described by a single activation energy or only a very narrow range of energies (about 30%), implying a near parallelism of lines for various degrees of length reduction in an Arrhenius plot. Borehole studies have shown that different apatite grains respond to the same annealing conditions to differing degrees. Electron microprobe analyses of these apatites indicate that the annealing properties of individual grains are strongly controlled by their Cl/F ratio. The interpretation of laboratory annealing studies, and to a lesser extent borehole studies, in terms of fanning Arrhenius plots may be understood as the result of the superposition of a series of near parallel Arrhenius plots corresponding to the range of compositions present, each characterised by different activation energies.