Experimental Constraints on Magma Storage Conditions of Two Caldera‐Forming Eruptions at Towada Volcano, Japan

Towada volcano is an active volcano in northeast Japan, that caused two large caldera‐forming eruptions at 36 ka (episode N) and 15.5 ka (episode L). Petrological analyses and phase equilibrium experiments were performed on silicic endmember rhyolitic pumices from these two eruptions to constrain the pre‐eruptive magma storage conditions. A common mineral assemblage of plagioclase + orthopyroxene + clinopyroxene + magnetite + ilmenite was observed in both eruptions, whereas amphibole (hornblende) was observed only in the episode L pumice. Mineral thermometers presented temperatures of 832–883°C and 802–870°C for the episode N and L pumices, respectively, with an oxygen fugacity of ∼nickel–nickel oxide (NNO) + 1 in log units. The water‐saturation pressures evaluated using the plagioclase‐melt hygrometer were in the range of ∼100–200 MPa. Experiments at 100–350 MPa and 825–900°C under water‐saturated and NNO‐buffered conditions successfully reproduced the mineral assemblages in both pumices, except for magnetite. Further constraints by phase compositions and phase proportions resulted in the preferred storage conditions of 840–850°C and 150–170 MPa for both eruptions. The emergence of hornblende in the episode L magma was likely caused by the higher CaO content compared to the episode N magma and not by the difference in the storage conditions. The storage depths at ∼5–7 km coincide with the depth of the low seismic velocity beneath the present Towada volcano, suggesting possible magma accumulation at the same depths as in the past caldera‐forming eruptions. Plain Language Summary Experimental constraints on the pre‐eruptive storage conditions of voluminous SiO2‐rich magma are fundamentally important to mitigate the risks of serious future volcanic hazards. Towada volcano is an active volcano located in northeast Japan, that caused two large caldera‐forming eruptions 36 and 15.5 kyr ago. Hornblende is present only in the later eruption, potentially reflecting differences in magma storage conditions and/or bulk compositions. We conducted water‐saturated phase equilibrium experiments on the most SiO2‐rich pumices derived from two caldera‐forming eruptions at Towada volcano to find the pressure and temperature conditions that best explain the mineral assemblage, phase compositions, and phase proportions recorded in the natural pumice. Our preferred pre‐eruptive conditions were 840–850°C and 150–170 MPa for both eruptions under water‐saturated conditions at oxyg