[S-VC28] International volcanology

[SVC28-11] Experimental Constraints on Pre-eruptive Magma Storage Conditions of Two Caldera-Forming Eruptions at Towada Volcano, Japan

*中谷 貴之¹、工藤 崇¹、鈴木 敏弘¹ (1. 産業技術総合研究所地質調査総合センター)

キーワード：カルデラ、実験岩石学、珪長質マグマ、相平衡、角閃石、内熱式ガス圧装置

Large-scale caldera-forming eruptions are known to occur in time intervals of tens to hundreds of thousands of years, severely damaging human societies. Because these eruptions require a substantial volume of accumulated silicic magma in the crust, geophysical detection of a large eruptible magma body is desired to assess and mitigate the risks of future serious volcanic hazards. However, geophysical techniques fail to detect a large melt-dominated body beneath the active volcanic systems possibly because of the ephemeral nature of such a melt-rich eruptible magma. Therefore, experimental investigation on the magma storage conditions of past caldera-forming eruptions is an important approach to precisely constrain the possible pre-eruptive depth of voluminous silicic magma. Towada volcano is an active arc volcano located in northeast Japan, characterized by a large caldera lake with a diameter of ca. 10 km. At this volcano, two extensive eruptions, which are called eruptive episodes N and L, occurred at 36 ka and 15.5 ka, respectively. These eruptions discharged large-volume silicic magma (~20 km³ dense rock equivalent in each eruption), causing caldera formation. A common mineral assemblage of plagioclase + orthopyroxene + clinopyroxene + magnetite + ilmenite is observed in both eruptions. However, amphibole crystals are known to be present only in the later eruption, potentially reflecting the difference in magma storage conditions and/or bulk compositions. In this study, we conducted water-saturated phase equilibrium experiments on the most SiO₂-rich pumices derived from two caldera-forming eruptions at the Towada volcano to find the pressure and temperature conditions that best explain the mineral assemblage, phase compositions, and crystallinity recorded in the natural pumice. All the experiments were performed using an internally heated pressure vessel (smc-5000) with Ar gas pressure medium at Geological Survey of Japan. The oxygen fugacity was buffered with nickel–nickel oxide (NNO) because that estimated from the magnetite–ilmenite thermo-oxybarometry was NNO + 1 in log units for both eruptions. The mineral assemblages except for magnetite were successfully reproduced in our experiments, and our preferred pre-eruptive conditions were 840–850 °C and 150–170 MPa for both eruptions. We suggest the importance of subtle differences in the whole rock compositions to crystallize amphibole in the later eruption. The pre-eruptive storage depths of 6–7 km are consistent with the depth of the low seismic velocity signal beneath the present Towada volcano. This suggests that magma, if present, is stored at the same depth as in the caldera-forming period. This study was supported by the Nuclear Regulation Authority of Japan.