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To understand the eruptive history, structure, and magmatic evolution of Yotei Volcano, southwest Hokkaido, Japan, we investigated the geology and petrology of tephras located around the base of the volcano. We identified 43 tephra units interbedded with soils (in descending stratigraphic order, tephras Y1–Y43), and four widespread regional tephras. Ten radiocarbon ages were obtained from soils beneath the Yotei tephras. On the basis of petrologic differences and, the stratigraphic positions of thick layers of volcanic ash soil, indicative of volcanic stratigraphic gaps, the Yotei tephras are divided into four groups (in ascending stratigraphic order): Yotei tephra groups I, II-1, II-2, and II-3. We calculated the age of each eruptive deposit based on the soil accumulation rate, and estimated the volume of each eruption using isopach maps or the correlation between eruption volume and the maximum thickness at ~ 10 km from the summit crater. The results regarding eruptive activity and the rate of explosive eruptions indicate four eruptive stages at Yotei Volcano over the last 50,000 years. Stage I eruptions produced Yotei tephra group I between ca. 54 cal. ka BP and up to at least ca. 46 cal. ka BP, at relatively high average eruption rates of 0.07 km 3 dense-rock equivalent (DRE)/ky. After a pause in activity of ca. 8000 years, Stage II-1 to II-2 eruptions produced Yotei tephra groups II-1 and II-2 from ca. 38 to ca. 21 cal. ka BP at high average eruption rates (0.10 km 3 DRE/ky), after a pause in activity of 2000–3000 years. Finally, after another pause in activity of 4000–5000 years, Stage II-3 eruptions produced Yotei tephra group II-3 from ca. 16.5 cal. ka BP until the present day, at low average eruption rates (0.009 km 3 DRE/ky). Whole-rock geochemical compositions vary within each tephra group over the entire eruption history. For example, group I and II-3 tephras contain the lowest and highest abundances, respectively, of K 2 O, P 2 O 5 , and Zr. Group II-1 has the highest abundances of Zr and Y. These trends indicate that the explosive activity was controlled by an evolving magma system. An understanding of these temporal changes in the chemical composition of the magma will enable future correlations of tephras with volcanic edifices, thereby revealing the full eruptive history and structure of Yotei Volcano, and constraining the timing of geological events in the region. [ABSTRACT FROM AUTHOR]

Zircon double-dating utilises (U–Th)/He dating coupled with U–Th disequilibrium or U–Pb dating to determine eruption ages for volcanic rocks between ca. 2 ka to 1 Ma. This approach depends on understanding the crystallisation history of each zircon crystal analysed. For lack of better constraints, zircon crystallisation is generally assumed to be represented by a single crystallisation age, which is routinely determined on the rim of the grain by spot analyses. While zircon crystallisation is often protracted, interrogating the crystallisation history of a zircon crystal usually requires grinding the grain, which can introduce uncertainty to the alpha ejection (Ft) correction, critical for accurate (U–Th)/He ages. Grinding a zircon crystal to exactly 50% of its original width, to a plane of symmetry, leaves the Ft correction factor unchanged relative to that of the whole crystal. This is verified by a new computer program – GriFt, which also allows the calculation of accurate Ft correction factors for a range of different grinding depths, opening the opportunity to measure both the core and rim crystallisation ages and integrate these into a more robust disequilibrium correction of (U–Th)/He data. The feasibility of this approach is tested here in a case study of zircon crystals with protracted crystallisation histories from the Shikotsu-Toya volcanic field in Hokkaido, Japan. A maximum of 15% difference in overall eruption age is calculated between rim- and core-corrected (U–Th)/He ages. Eruption ages were determined for two tephras – Kimobetsu 1 (59–79 ka) and Kimobetsu 2 (96 ± 5 ka, 2σ). The geological implication from these dates is that a regionally important tephra, Toya, may be younger (<96 ± 5 ka) than previously reported (109 ± 3 ka). In addition, the maximum eruption ages determined from crystallisation age distributions calculated for samples from eruptions at Shikotsu and Kuttara (48 ± 17 and 49 ± 21 ka, respectively) are within uncertainty of previous measurements (44–41 ka and >43 ka, respectively). [ABSTRACT FROM AUTHOR]