To improve kinetic models for apatite fission-track annealing, we present new experimental annealing data that complement previously published data. To determine the degree of annealing of induced tracks, surface density (ρ), and mean horizontal-confined track lengths (l), were measured, both for basal and randomly oriented faces. Our annealing data were obtained by submitting an apatite sample collected in Itambé, Bahia, Brazil, to 46 different isothermal treatments where temperature ranged from 150 to 600 °C (duration of 1, 10, 100, and 1000 h). To compare the behavior of Itambé to Durango apatite, the latter was also annealed for 1 h in 9 isothermal experiments at temperatures between 240 and 380 °C. Our results show that the l/l0 values in Durango are systematically smaller than those in Itambé sample, both in basal and random faces. The curves depicting relative track density reduction, ρ/ρ0, and relative mean confined track length reduction, l/l0, as a function of time and temperature, are similar for ρ/ρ0 > ~0.5, but different for ρ/ρ0 < ~0.5. In this interval, ρ/ρ0 can be measured but the measurement of l/l0 is very difficult because the confined tracks become undetectable. Measurements of ρ/ρ0 and l/l0 for tracks revealed in basal surfaces are systematically lower (but this difference is <3%) than those in randomly oriented ones.

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