Zircon U–Th–Pb dating using LA-ICP-MS: Simultaneous U–Pb and U–Th dating on the 0.1 Ma Toya Tephra, Japan

Simultaneous U–Pb and U–Th zircon dating using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was performed on the ~ 0.1 Ma Toya Tephra, Hokkaido, Japan. The method employs the concurrent measurement of 206Pb, 230Th, and 238U signals, which enables the determination of 238U–206Pb (U–Pb) and 238U–230Th (U–Th) ages simultaneously. The U–Pb ages revealed that the Toya Tephra contains zircons with ages that cluster at ~ 0.1 Ma (0.11 ± 0.01 Ma: error shown as 95% confidence level) and xenocrysts with ages > 2 Ma. Excluding zircons > 2 Ma, the U–Th method also gave an isochron age of ~ 0.1 Ma (108 ± 19 ka or 0.11 ± 0.02 Ma; error shown as 2σ). Therefore, both U–Pb and U–Th methods result in similar ages of ~ 0.1 Ma for the Toya Tephra, in agreement with the quartz TL age (104 ± 30 to 118 ± 30 ka) of Ganzawa and Ike (2011). It was revealed that the simultaneous U–Pb and U–Th zircon dating technique using LA-ICP-MS is easy to apply and is useful for Quaternary tephras in that it can give age information on the tephra itself and xenocrystic zircons in a quick and cost-effective manner. The double dating technique employed here has the merit that zircons yielding Quaternary U–Pb ages can be immediately cross-checked by the U–Th method. It was also revealed that the Toya Tephra magma crystallized zircons and experienced eruptions instantaneously at ~ 0.1 Ma, which is an important implication to decipher magmatic and eruptive processes for caldera-forming large volcanic systems.