The along-track etching structure of fission tracks in apatite: Observations and implications

Abstract

Interpretation of apatite fission-track length data rests on the assumptions that etching fully reveals latent tracks, and that laboratory annealing of induced tracks constitutes a good proxy for geological annealing of spontaneous tracks. Recent work using improved step-etching procedures questions both of these assumptions. Step etching experiments interrogating the etching structure of tracks near and beyond their tips have shown that tracks are often not etched to their full extent of latent track damage in a standard single-step, 20-s etch, and that the etching structure in this outer region differs between fossil and induced tracks. In this study, we develop a more complete picture of track revelation for fossil and unannealed and annealed induced tracks in Durango apatite. Using first steps as early as 10 s after the beginning of etching, we discern differences in etching structure between various track types, and document a gradual fall-off in etching velocity from the track interior toward its tips. When effective etching time is constrained by selecting earlier-etching confined tracks for later analysis, mean lengths are longer, but length dispersion is not diminished. Although some proportion of unannealed tracks are under-etched after 20 s of etching, with increased levels of annealing the probability of an induced track being fully etched increases. We use an etch-anneal-etch procedure, consisting of partially etching tracks, annealing the apatite grains, and then continuing etching, to establish bulk etching rates at track tips. Bulk etching is effectively isotropic, as is etching velocity in the central portions of tracks, away from their tips. We use our data to compare two end-member possibilities for along-track etching structure, which provides a first step toward a deeper understanding of annealing and development of a more robust standard etching protocol.