Apatite fission-track dating by LA-Q-ICP-MS imaging

Abstract

Obtaining accurate and precise apatite fission-track (AFT) ages depends on the availability of high-quality apatite grains from a sample, ideally with high spontaneous fission-track densities (c. >1.105 tracks.cm−2). However, many natural samples, such as bedrock samples from young orogenic belts or low-grade metamorphic samples with low U contents yield low spontaneous fission-track densities. Such apatites must be counted to avoid biasing the resultant FT age. AFT dating employing LA-Q-ICP-MS spot ablation works very well for grains with high spontaneous fission-track densities. This approach allows detection of potential U zoning, while also removing the need for an irradiation step and facilitating simultaneous acquisition of U-Pb and trace element data. The LA-Q-ICP-MS spot ablation thus offers several advantages compared to the External Detector Method (EDM). However, the spot ablation approach requires the counted area to mimic exactly the site and size of the laser spot, which for grains with low spontaneous fission-track densities (<105 tracks.cm−2), implies fewer track counts and impairs the precision of the resultant AFT age. Here we present an alternative approach to LA-Q-ICP-MS spot analysis of low fission-tracks density grains by generating a U distribution (238U/43Ca) map of the entire apatite surface by LA-Q-ICP-MS elemental mapping, which enables characterization of U zonation. The Monocle plugin for the Iolite LA-ICP-MS data reduction software is used to display elemental maps and extract mean 238U/43Ca values of the same area counted for the fission-tracks. A typical grain-mapping session takes <5 h to map 80 grains. The method was employed on the Durango and Fish Canyon Tuff apatite reference materials, six bedrock apatite samples with low fission-track densities (≤1.105 track.cm−2), and one bedrock apatite sample with high fission-track density (>1.106 track.cm−2) to assess the precision and accuracy of our approach. Most apatite samples investigated here were previously dated by the EDM or the LA-Q-ICP-MS ablation spot method. The AFT grain-mapping ages agree with previously published EDM or LA-Q-ICP-MS spot ablation ages at the 2σ level. For each apatite sample, we simultaneously acquired U-Pb age and trace element data (Mn, Sr, La, Ce, Sm, Eu, Gd, Lu); here again the data agree with literature constraints (when available) within uncertainties. The mapping approach is therefore a practical solution to low-temperature thermochronology studies facing apatite grains with low spontaneous fission-track densities, while also facilitating investigation of the spatial relationships between thermo- and geochronometric ages and grain chemistry.