On the thermal stability of fossil and neutron-induced fission-tracks in natural titanite

Results are presented of 625 degrees C and 675 degrees C-annealing experiments on fossil and neutron-induced fission-tracks in 44 natural-titanite samples from the Fish Canyon Tuff. Both track density and track length decrease in a complex manner with annealing time. Possible causes are: (a) a phase transition Juring annealing, (b) parallel annealing of recoil-tracks or (c) differences in the defect structure along the tracks at the atomic scale. The relationship between the decrease of track density and the reduction of track length is similar to that in apatite, i.e. proportional up to similar to 35%, followed by a collapse of track density without a comparable reduction of track length. The latter is ascribed to the appearance of unetchable gaps. The initial decrease is not exactly 1:1, partly as a result of a decrease of track counting efficiency rig with decreasing track length. The mean plateau-age of all samples before the break in slope at similar to 35% is 31.8 +/- 0.6 Ma, i.e. similar to 15% higher than the reference age of the Fish Canyon Tuff (27.8 +/- 0.2 Ma). It is thought that this results neither from inaccurate measurement of the thermal neutron fluence, nor from a different degree of metamictisation of the crystals with spontaneous and with induced tracks, but from unidentified systematic factors affecting the track-density measurements. The systematic differences between fossil and induced tracks in these experiments call into question the generality of the principle of equivalent time, that underlies all fission-track temperature time path modelling. (C) 2000 Elsevier Science B.V. All rights reserved.