Improving crystallization and eruption age estimation using U-Th and U-Pb dating of young volcanic zircon

Abstract. Quantifying timescales and establishing robust eruption chronologies is critical for understanding the evolution and hazards of volcanic systems. U–Th disequilibrium dating on zircon is especially valuable for young and active systems (<300 ka). However, there is no consensus on how to calculate U–Th crystallization ages. To address this, we developed a new LA-ICP-MS U–Th–Pb double-dating technique that simultaneously retrieves U–Th and U–Pb ages from the same zircon ablation volume. This method increases confidence in crystallization ages across 150–300 ka, where the resolution of either method alone is limited. We applied this method to the Kos Plateau Tuff, which spans this critical interval, and compared U–Th model age approaches against the well-established U–Pb age calculations. Model ages calculated using the two endmember approaches, either a constant melt composition or a constant zircon–melt U/Th fractionation factor (f_U/Th), yield similar age spectra when well-estimated values are used. In this context, it is essential to evaluate whether the measured groundmass glass or whole-rock composition truly reflects the zircon-forming melt. This can be assessed by comparison with the youngest isochron intercept on the secular equilibrium line, which provides an independent melt composition estimate. We also evaluated eruption age estimation methods using synthetic U–Th datasets, with increasing uncertainty toward older ages. Bayesian models, particularly those with uniform priors, consistently outperformed weighted mean approaches in terms of accuracy and precision and are therefore recommended for eruption age estimates in volcanic U–Th zircon datasets.