Implications of uranium and thorium zoning for zircon laser-ablation (U-Th)/He age interpretation

Laser-ablation zircon (U Th)/He (LA-ZHe) dating is a promising thermochronological technique. Its advantages compared to conventional whole-grain dating are the possibility to analyse many grains in short succession and at high spatial resolution, the ability to analyse imperfect grains, and its compatibility with other dating methods such as U Pb geochronology. For accurate geological interpretation, it is important to understand what factors affect LA-ZHe data. We therefore acquired LA-ZHe dates of four monotonically cooled granitic bedrock samples from the Southern Canadian Cordillera. Our results show greater date dispersion for the LA-ZHe than for data from the whole-grain (U Th)/He technique. Cathodoluminescence images and models of He diffusion and α-implantation suggest U and Th zonation to be the main driver of LA-ZHe date variation. We find that date variation strongly depends on the zonation type: the LA-ZHe dates of grains with oscillatory zoning or unzoned grains are relatively homogeneous and match the expected whole-grain dates. In contrast grains with core-rim zoning or mixed domains exhibit large within-grain date variation and significant deviations from the whole-grain dates. We apply this observation to the sample-age calculation and inverse thermal-history modelling. Sample age calculated fromLA-AHe data from unzoned grains or those with oscillatory zoning are comparable to those obtained with the whole-grain technique. Thermal-history models from LA-ZHe data allow first-order geological interpretation with differences to the whole-grain results likely arising from LA-ZHe date uncertainty and limited whole-grain analyses. In addition, we discuss how the LA-ZHe technique benefits from the simultaneous acquisition of mineral-chemical and U Pb geochronological data.