ON THE USE OF 60-DEGREES-C COOLING AGES OBTAINED USING PROJECTED FISSION-TRACK LENGTHS IN APATITE

The estimation of fission-track ages using counts of projected track l engths, in particular, 60 degrees C cooling ages, is based on determin ing the proportion of tracks greater than a particular length. We show that errors on 60 degrees C cooling ages are significantly greater th an have been previously estimated; for example, a minimum error on suc h ages as they are conventionally determined, is +/- 22% at the 95% co nfidence level. In addition, the equation used to determine projected track length ages, involving a constant relationship between track sho rtening, conventional age reduction, and the proportion of projected t rack lengths greater than 10 mu m, is shown not to apply generally. It is recommended that the practise of estimating 60 degrees C cooling a ges using projected track length distributions be substantially re-eva luated.