Annealing studies on radiation damages in biotite, apatite and sphene and corrections to fission track ages

Abstract

Fission track ages need to be corrected for the loss of fossil tracks due to geological and thermal annealing. The range distribution of full-length tracks can yield such corrections if calibration curves translating length reductions into density reductions are available. The study of the annealing characteristics of the three terrestrial minerals, i.e. biotite, apatite and sphene, as functions of temperature and time, has been completed from this point of view. A linear relation has been observed between etchable track length and track density, but slopes and intercepts of the curves tend to vary from mineral to mineral. Over-etching shows significant effect in retrieving partially faded tracks in the case of biotite only, and track density reduction always lags behind track length reduction. In apatite and sphene track density is reduced in the same proportion as track length when length reduction is ∼30 to 50%. For length reductions 50% the density reduction progressively outweighs the former. The possible reasons for such variations and the usefulness of the calibration curves are briefly discussed.