Repetitive Duality of Rhyolite Compositions, Timescales, and Storage and Extraction Conditions for Pleistocene Caldera-forming Eruptions, Hokkaido, Japan

Abstract During the Early Pleistocene, numerous caldera-forming eruptions occurred in the southernmost Kurile arc (central Hokkaido, Japan), building an extensive pyroclastic plateau with an area >1600 km2. The arc remains active today, and proximity to populations and infrastructure makes understanding these magmatic systems a critical endeavor. We investigate three major caldera-forming ignimbrite eruptions: Biei (c. 2·0 Ma), Tokachi (c. 1·2 Ma), and Tokachi–Mitsumata (c. 1·0 Ma), with an emphasis on constraining the pressures of magma extraction and storage and the timescales of crystallization. Although all pumice glass compositions from the three eruptions are high-silica rhyolites (77–78 wt% SiO2), hierarchical clustering analysis of major and trace element glass data indicates that the Tokachi and Tokachi–Mistumata ignimbrites each have two distinct pumice populations (Type-1F and Type-2F). We find that these two distinct pumice types record pre-eruptive temperatures, extraction pressures, and crystallization timescales that are strikingly similar between the two eruptions. Using the rhyolite-MELTS geobarometer, we estimate that although all magma types from all three eruptions had storage pressures of 50–150 MPa (∼2–6 km), Type-1F magma was extracted from a deeper mush reservoir (200–450 MPa) compared with Type-2F (100–200 MPa). Pre-eruptive temperatures, constrained by plagioclase–liquid equilibration thermometry and rhyolite-MELTS, suggest that Type-1F magma in both eruptions was hotter (800–820 °C) compared with Type-2F (780–800 °C), but that both reached thermal equilibrium upon eruption (760–780 °C). Because zircon is observed only as inclusions and rarely in contact with glass, we conclude that all magmas were zircon-undersaturated, and thus zircon saturation temperatures, which are 60–100 °C lower than those estimated by the other three independent thermometers, underestimate magmatic temperatures. Using these temperatures as minimum estimates, diffusional relaxation times of Ti zonation in quartz, as revealed by cathodoluminescence (CL), give absolute maximum quartz residence times of