VARIABLE TIMESCALES AND MAGMA STORAGE CONDITIONS FOR THREE MAJOR PLEISTOCENE IGNIMBRITES, HOKKAIDO, JAPAN

During the Early Pleistocene, significant pyroclastic volcanism 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, including a nuclear power facility, makes understanding these magmatic systems a critical endeavor. We investigate three major caldera-forming ignimbrite eruptions: Biei (>50 km³), Tokachi (>70 km³), and Tokachi-Mitsumata (>130 km³), with an emphasis on constraining the final depths of magma storage and the timescales of crystallization and ascent. 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 two larger ignimbrites each have two distinct pumice populations (G1 and G2). Using the rhyolite-MELTS geobarometer, we estimate that final melt equilibration occurred at pressures of 50-175 MPa (~2-6 km) for all three eruptions. G1 and G2 have distinct equilibration pressures and temperatures, indicating complex magma storage. To investigate timescales of crystallization of the rhyolite melt, we modeled 1-D diffusional relaxation times of Ti zonation in quartz, as revealed by cathodoluminescence. A total of 220 zone boundaries were modeled on 94 quartz grains from five pumice clasts. More than 80% of the quartz (< 2.5 mm) have maximum core residence times of < 50 years. These short timescales represent the final crystallization of the buoyant melt-rich magma bodies that fed these large rhyolite eruptions. Interior and rim growth rates are significantly faster than those published for Bishop Tuff and two TVZ ignimbrite eruptions. Furthermore, G1 and G2 quartz, which are characterized by very different CL zoning patterns, have statistically different core growth times, suggesting that these two magma reservoirs had different crystallization timescales and processes. However, most quartz crystals from both G1 and G2 populations have high-Ti rims (10-130 µm) with identical maximum growth times of < 2 years. This suggests that for both magma bodies, the recharge or decompression event that triggered the final stage of crystallization occurred within two years the eruption.