Variability of apatite fission-track annealing kinetics: II. Crystallographic orientation effects

A method is presented that permits the length of any horizontal, confined f ission-track inclined at a specified angle to the crystallographic c axis i n apatite to he converted to an equivalent track length parallel to the cry stallographic c axis. The model is based on the results of annealing experi ments for six selected apatites (five calcian fluorapatites and Durango apa tite) representing a subset of the 15 total apatite specimens studied. An i terative process of calculation is required to project fission-track length s onto the c axis and computer source code implementing the solution to thi s problem is presented. This method of projecting apatite fission-track len gths onto the crystallographic c axis is shown to remove effectively fissio n-track length variation within single fission-track populations due to ani sotropic track-length reduction fur all 15 apatites studied. In addition, a model is developed that offers predictions that closely reproduce publishe d experimental data concerning the relationship between fission-track densi ty (etched fission tracks per unit area of apatite surface) and the arithme tic mean fission-track length. Finally, it is shown that natural fission-tr ack populations exhibit fission-track length anisotropy similar to that of fission-track populations created and annealed in the laboratory. This obse rvation implies that the same process by which apatite fission tracks annea l in the laboratory is responsible for annealing of apatite fission tracks in the geological environment.