Thermal annealing of fission and ion tracks in epidote

Abstract

Fission tracks are used for geologic age-dating and for the reconstruction of thermal histories of Earth’s upper crust. However, there remains a gap in the understanding between the atomic-scale annealing mechanism of latent (unetched) fission tracks and the observations of etched tracks at the micrometer scale. This is because the structure of latent fission tracks is lost during the leaching process. We have conducted the first comparison of the thermal-annealing behavior of latent and etched tracks in epidote, using transmission electron microscopy (TEM) and optical microscopy, respectively. For high-resolution TEM observations, we used ion tracks instead of fission tracks to control the density of tracks, and we demonstrated that latent ion tracks are amorphous in epidote. The reduction in diameters of latent tracks is insignificant after thermal annealing at 800 °C for 24 h, indicating that the track diameter does not appear to change substantially until the final stage of annealing. The optical observations show that the parallel etched ion-induced tracks have a lower track density reduction rate at or below 500 °C and an accelerated reduction rate above 500 °C as compared with randomly oriented fission tracks. However, the two types of tracks display a comparable thermal-annealing behavior as evidenced by the fit of the two curves for the normalized density of ion and fission tracks as a function of temperature with the same equation but with different fitting parameters. Thus, ion-induced tracks can be used to simulate fission tracks in epidote and provide a basis for understanding the annealing of latent and etched tracks.