Widespread intra-crystalline 4He redistribution in zircon revealed by in-situ analysis

Zircon is widely acknowledged as the ultimate geo-/thermochronometer for its widespread application in U-Pb, fission-track and (U-Th)/He dating, among others. However, it is also characterized by complex crystal zonation, which has implications for radiometric analyses. The in-situ technique for (U-Th)/He thermochronology, based on two sequential spot analyses using laser ablation, is particularly susceptible to the impact of crystal zonation not only because of possible compositional heterogeneity in the analyzed volume, but also because of the long travelling distances of radiogenic 4 He atoms during alpha decay. By performing repeat in-situ (U-Th)/He analyses of crystals with a well-constrained thermochronological history, we demonstrate that the latter effect can generate substantial and non-systematic dispersion under unfavorable analytical conditions, with apparent ages multiple times older than U-Pb age. Combining multiple LA-ICP-MS analytical rounds with Raman spectroscopy mapping, we show that redistribution of 4 He during alpha decay from more radioactive to less radioactive crystal zones seems to have a more pronounced impact in controlling date dispersion than intracrystalline diffusive migration. This is further supported by simulating alpha redistribution for a range of zonation patterns. We also show empirically that changes in spot geometry have an impact on (U-Th)/He dates, confirming previous modelling results. While our results act as a cautionary signal for the future development of the in-situ (U-Th)/He technique, dispersion can be significantly reduced with relatively straightforward adaptations. We highlight the need to estimate the magnitude of date dispersion due to intra-crystalline 4 He redistribution in order to accurately describe the uncertainty of the obtained dataset.