Fission-track age calibration: Phi and Zeta, never the twain shall meet?

Abstract

The fission-track method was intended to be an autonomous dating method requiring two track counts and a neutron fluence measurement. It became clear that it was beset with methodological problems, and neutron fluence measurements were abandoned in favour of age standards. The coming of mass-spectrometric uranium measurements eliminated the need for irradiations altogether, and stimulated renewed efforts to establish fission-track dating as an autonomous method. This ran into similar problems as before, and recent publications signal a return to age standards. The attempts at independent dating nevertheless revealed that experts agree on the recommended fission constant and the need to consider experimental factors related to the track counts. In spite of that, the renewed reliance on standards makes that fission-track ages are again not independent, but relative, ages with an uncertain meaning, and from their nature untestable.We report an attempt at standardless and standard-based dating of the Durango and Duluth apatites using neutron irradiation and the external detector method. We start from the assumption that neutron fluence measurements are the most precise and accurate achievable in fission-track dating, a consensus on the recommended fission constant, and on the need to consider experimental factors. One factor (ζ0) accounts for the combined experimental influences; ζ0 can be calculated from identifiable contributions related to track registration, etching and counting; ζ0 can also be determined by experiment and calculated from the ages of standards. This gives three separate, consistent, dimensionless calibration factors that, in contrast to Z and ζ, are independent of the neutron fluence or the uranium content of a reference glass; ζ0 remains a personal factor, however.The standardless fission-track ages of the Durango age standard are in agreement with its reference age and the standardless and standard-based ages of the Duluth apatites are consistent with each other. Furthermore, if ζ0 is used to correct conventional Z- (and by implication ζ-) values for experimental effects, the results are consistent with their definitions in terms of physical parameters. Although ζ0 is a personal factor, the small number of measurements to date is consistent with each other. In contrast, annealing corrections based on the fossil and induced confined fission track lengths overestimate the reference age of Durango and produce incompatible standardless and standard-based ages of the dated apatites. We argue that this points to unresolved issues with the relationship between the age and the fossil track length, and suggest a connection with etching.Plots of the single-grain ages against uranium content and radial plots of the data exhibit conspicuous structure. However statistical factors alone account for the nature and magnitude of the observed correlations. This implies that in the case of the Duluth apatites there is no identifiable effect of radiation damage on the single-grain ages despite their high accumulated α-doses and protracted thermal histories. Spurious correlations must be addressed before attempting to interpret the observed data structure in terms of radiation damage, apatite composition or age components.