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Simultaneous U–Pb and U–Th dating using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was performed on the ca. 0.1 Ma Toya tephra and the ca. 0.08 Ma SS14-28 U–Th zircon reference material. In U–Pb dating, both Th/U and Pa/U partitioning between magma and minerals were considered. In U–Th dating, both abundance sensitivity and molecular interferences on ²³⁰Th were reevaluated. As a result, the Toya tephra yielded an accurate weighted mean U–Pb age of 0.103 ± 0.029 Ma (2σ) using zircon and monazite. Conversely, the SS14-28 zircon yielded an inaccurate U–Pb age (0.25 ± 0.10 Ma), which was attributed to low ²⁰⁶Pb signal intensity. Both the Toya tephra zircon and the SS14-28 zircon yielded accurate U–Th model ages of 0.108 ± 0.014 Ma and 0.078 ± 0.007 Ma, respectively. The agreement of U–Pb and U–Th ages for Toya indicates that simultaneous U–Pb and U–Th dating is possible and viable. The inappropriate age of SS14-28 U–Pb age and appropriate U–Th model age also indicates it is preferable to apply both U–Pb and U–Th dating simultaneously for young (<0.4 Ma) zircons to check internal consistency. The proposed double dating approach may be especially useful for small grains when it otherwise would be impossible to obtain multiple ages from a single grain. By adopting simultaneous U–Pb and U–Th dating using LA-ICP-MS, zircon crystallization ages as old as 4.5 Ga to as young as 0.1 Ma (or even younger) can be obtained in a quick and cost-effective manner with a reasonable (~5% at 1σ) uncertainty. Full article

The focus of this study was the Baekdusan-Kita 1 (B-KY1) eruption during the late Middle Pleistocene. We identified B-KY1 tephra between the Toya and Aso-1 tephras in the ODP 794A core from the Japan Basin of the East Sea/Japan Sea. The stratigraphic position of the B-KY1 tephra correlated exactly with the first B-KY1 to be identified, in the 20EEZ-1 core from the Kita–Yamato Trough. However, B-KY1 tephra in the ODP 794A core showed a wide range of geochemical compositions. The textural characteristics of B-KY1 tephra in the ODP 794A core was characterized by higher content of fine-grained bubble-wall shards than that of the B-KY1 tephra in the 20EEZ-1 core. The difference in B-KY1 tephra between the two coring sites may reflect shifts in wind direction during volcanic eruptions. We refined the eruption age of the B-KY1 tephra by examining distinct sedimentary facies related to the start of the penultimate deglaciation of this region at ca. 135 ka. The findings of this study suggest that the compositional diversity of B-KY1 tephra may have been influenced by subsequent mixing of comendite and comenditic trachyte magma with injected pantelleritic magma during the late Middle Pleistocene Baekdusan volcanic eruption. Full article