Independent fission-track ages (ϕ-ages) of proposed and accepted apatite age standards and a comparison of ϕ-, Z-, ζ- and ζ0-ages: Implications for method calibration

The results of 94 determinations of the independent fission-track ages (ϕ-ages) of proposed and accepted apatite age standards show that the ages of the accepted standards are consistent with their reference ages but that those of the proposed age standards are not. The FCT and DUR data have been used to calculate Z-, ζ- and ζ0-calibration factors. The ζ0-values agree with independent measurements and with calculated values. The ϕ-, Z-, ζ- and ζ0-ages (tϕ, tZ, tζ and tζ0) of 71 samples dated in the course of geological investigations are, on the whole, consistent, although systematic deviations from the mean (tM) exceed the statistical uncertainties. These are, however, ≤ 2% and traceable to statistical variation, including counting statistics and an extra-Poisson component resulting from somewhat inconsistent track identification criteria. The overall agreement between tZ, tζ and tζ0 implies that measurements of (ρs/ρi), ρd and ϕ are reproducible but says nothing about their intrinsic accuracies. The fact that tϕ also agrees with tZ, tζ and tζ0 (tϕ/tM = 1.000 ± 0.001) is inconsistent with the ∼ 10% length reduction of confined spontaneous tracks in the standards; the same applies to the tϕ/tREF-ratio (mean: 1.009 ± 0.010). It is concluded that the agreement between tϕ and tREF and between tϕ and tZ, tζ and tζ0 is an indication of the fact that the calculated ages are indeed accurate. This implies that the ∼ 10% reduction of the spontaneous track lengths in FCT and DUR has no measurable effect on their fission-track ages, to within the < 1% resolution of the present datasets.