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

<p>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 (<span class="inline-formula">&lt;</span> 300 ka). However, there is no consensus on how to calculate U-Th crystallization ages. To address this, we applied an optimized LA-ICP-MS U-Th-Pb double-dating strategy that simultaneously retrieves U-Th and U-Pb ages from the same zircon ablation volume. This dating routine increases confidence in crystallization ages across 150–300 ka, where the resolution of either dating technique alone is limited. We applied this strategy to the Kos Plateau Tuff, which spans this critical interval, and compared U-Th model age calculation approaches against the well-established U-Pb age calculations. U-Th model ages calculated using the two endmember approaches, either using a constant melt composition or a constant zircon-melt U <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="527256ea34e0af356380afd605ccefc0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-8-63-2026-ie00001.svg" width="8pt" height="14pt" src="gchron-8-63-2026-ie00001.png"/></svg:svg></span></span> Th fractionation factor (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>f</mi><mrow><mi mathvariant="normal">U</mi><mspace width="0.125em" linebreak="nobreak"/><mo>/</mo><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">Th</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="28pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="4d883c16a0cdf82d81af5c13a1b78aeb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-8-63-2026-ie00002.svg" width="28pt" height="14pt" src="gchron-8-63-2026-ie00002.png"/></svg:svg></span></span>), 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 methods in terms of accuracy and precision and are therefore recommended for eruption age estimates in volcanic U-Th zircon datasets.</p>