The Age of the Lower Tuff in the Utaosa Rhyolite, Teragi Cauldron, southwest Japan:Uranium‒lead dating of Late Pliocene zircon using femtosecond laser ablation-inductively coupled multi-collector ICP mass spectrometry

The Lower Tuff of the Utaosa Rhyolite, the Teragi Group, is a large-scale pyroclastic flow deposit with a volume of ~100 km3. Its eruption formed a volcanic-collapse structure called Teragi Cauldron. In this study, we present new uranium‒lead (U‒Pb) ages of zircons from a sample of welded pumice lapilli tuff from the Lower Tuff to determine the age of the eruption. U‒Pb isotopic data were collected using multiple-collector ICP-mass spectrometry coupled to a femtosecond laser ablation system for precise age determinations. Modified 207Pb method was used to calculate accurate U‒Pb ages with the correction of deviation derived from non-radiogenic lead and initial radioactive disequilibria. The resulting 238U‒206Pb ages of each spot analysis are ranging from 3.3 to 2.7 Ma, with a weighted mean age of 3.03±0.01 Ma (coverage factor k = 2, n = 70, MSWD = 2.6). The mean age overlaps within analytical uncertainties with a previously reported biotite K‒Ar age for the Lower Tuff. Our new U‒Pb age confirms that the caldera-forming eruption occurred at 3.1‒3.0 Ma.