Analytical improvements and a machine-learning assessment of age dispersion in laser-ablation (U-Th)/He zircon dating

Abstract. Laser-ablation (U-Th)/He dating offers major advantages over conventional whole-grain dissolution analysis, including spatially resolved sampling, minimal sample preparation, and much faster data acquisition. Despite these advantages, the method’s full potential is held back by issues including the lack of standardized, fully documented analytical protocols, and age dispersion that commonly exceeds the propagated analytical uncertainty. In this study, we document a complete LA-(U-Th)/He workflow for zircon, combining ultra-high-vacuum laser extraction, white-light-interferometric pit-volume measurement, and time-resolved LA-ICP-MS parent-nuclide quantification, and introducing two methodological refinements: a dynamic multi-phase oxide normalization for the parent concentration calculation, and parent-source depth weighting to account for alpha redistribution. Applied to 811 spots in Fish Canyon Tuff (FCT) zircon, the workflow yields a median age of 28.32 Ma (σ_MAD = 3.85 Ma; 13.6 %), in agreement with the published reference age, and reduces dispersion by a factor of ~1.7 relative to conventional ²⁹Si- or ⁹¹Zr-based reductions. To identify the drivers of the remaining dispersion, we train a gradient-boosted regression model on 33 simultaneously measured per-spot parameters. He pit volumes emerge as the strongest predictor of age residuals, highlighting volume-correlated analytical and geometric effects as the largest identifiable and potentially correctable source, while elevated ²⁰⁸Pb concentrations flag inclusion-related disturbance and provide a practical spot-screening criterion. The measured degree of zonation within the ICP-MS profile on the other hand carries no independent predictive information. Instead, a simple two-layer zonation model shows that U-Th zonation in the grain volume removed during polishing can alone generate age offsets exceeding the entire residual dispersion, a mechanism confirmed by the contrast in dispersion between zoned FCT zircon (13.6 %) and the compositionally homogeneous LGC megacryst (1.5 %). The residual dispersion is therefore not an instrumental limitation, and further gains in precision will require a new generation of methods that characterize or circumvent the polished-away parent-source volume.