A grain-by-grain comparison of apatite fission-track analysis by LA-ICP-MS and the External Detector Method

Abstract

Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is increasingly used in fission-track analysis to determine the uranium content of host mineral specimens, particularly apatite. Fission-track dating by LA-ICP-MS (LAFT) has several advantages over the conventional External Detector Method (EDM), particularly in terms of sample turn-around times and the fact that neutron irradiations and the handling of radioactive materials are no longer necessary, while providing a similar level of in-situ information about parent nuclide ( 238U) concentrations. In addition, it facilitates the simultaneous measurement of multiple isotopes for double or triple-dating approaches or compositional characterisation. While it is often implicitly assumed that the EDM and LAFT fission-track dating approaches produce equivalent results, this assertion has yet to be adequately tested. We present an extensive dataset of apatite fission track results from 17 samples representing a large range of fission-track ages (~0–2 Ga), 238U concentrations (0.14–410 ppm) and thermal histories that were analysed grain-by-grain using both techniques in order to investigate whether they yield concordant results during routine fission-track analysis. Apart from a few outliers, our data show that 238U concentrations measured by the EDM and LAFT techniques yield indistinguishable results across at least three orders of magnitude when a similar calibration system against rapidly cooled standards (e.g., Durango) is used. Comparison of single grain pooled and central ages reveals that LAFT ages are within error of EDM ages for apatite fission track standards such as Fish Canyon Tuff or Durango, as well as for a range of other samples whose shorter mean confined track lengths (<13 μm) and broader track distributions indicate they experienced more complex cooling histories. The most important conclusion here is that both the conventional EDM and LAFT methods can be expected to yield identical results for the breadth of ages, 238U concentrations, and underlying thermal histories commonly found in real world apatites. Importantly, the aggregate empirical calibrations for EDM and LAFT mask an underlying assumption that the mean etchable range of fission fragments is a constant having the mean value observed for spontaneous tracks in age standards such as the Durango apatite. Given that this assumption is known to be false in the great majority of samples, it is our view that empirically derived EDM and LAFT fission-track ages are best considered as model ages and that there should be greater clarity about the assumptions involved in their calculation.