5:15 PM - 7:15 PM
[SGC37-P02] Noble gas geochemistry of melt inclusions in pumice of the latest magmatic eruption since 2022 of Ioto island (Iwo-Jima)
Keywords:Volcano, Noble gas, Melt inclusion, Ioto, Volcanic gas
Ioto Island (Iwo-Jima) is one of the active volcanic islands of the Izu-Bonin-Mariana island arc, located about 1,250 km south of Tokyo. The island is the summit of a stratovolcano with a height of around 2,000 m from the ocean floor. Motoyama, a broad volcanic cone on the east side of Ioto which erupted around 2.7 cal kBP, is covered with pyroclastic deposits and lava flows (Nagai and Kobayashi 2015). Suribachiyama, a monticule on the southwest of Ioto, experienced at least three magmatic eruptions after the Motoyama eruption, though the actual eruption date remains unknown (Ueda et al., 2018).
Ioto has been featured by repetitive phreatic eruptions in craters and fumarolic areas and very high geothermal activities throughout the island. In July 2022, a Surtseyan eruption occurred off the coast of Okinahama in the southern part of Ioto for the first time since the last magmatic eruption. Cock’s tail jets were frequently observed and volcanic ejecta washed up on the shore. In November 2023, there were temporary eruptive activities on the land with small-scale lava flows. Eruptive activities have been continuing with a dormant phase in between, and most recently, a magma eruption spewing pumice occurred in July 2024.
This study was conducted on the pumices which erupted during this series of eruptions. The bulk composition indicates that all pumices are trachydacite and trachyte, which are unusual at the volcanic front, and there was no heterogeneity among the samples. The proportion of phenocrysts (>300 µm) is less than 10%. Minerals in the pumices are olivine, clinopyroxene, plagioclase, Fe-Ti oxide, pyrrhotite, and apatite. Most of the olivine was corroded and less than 1 mm, whereas pyroxene was idiomorphic and some exceeded 1 mm in size. For the noble gas analysis, pumices were crushed by a high voltage pulsed power fragmentation system (Selfrag Lab in the National Institute of Polar Research) and sieved to separate them by the grain size of 0.5-1 mm. The fraction was separated by a hand magnet, water stirring, and heavy liquid (Sodium polytungstate) to concentrate the target portions. Olivines, pyroxenes were handpicked under a stereomicroscope, collected at 300-500 mg, and leached with nitric acid. Noble gases were extracted from the melt inclusions in minerals under ultra-high vacuum (<10-6 Pa) by striking with a nickel slug 300-3,000 times using solenoid coils. After purifying and separating the gases, noble gas isotopes were measured with a magnetic sector mass spectrometer (modified VG-5400) at the University of Tokyo.
3He/4He of olivines, pyroxenes were 3.3-4.7, 2.0-4.5 Ra (where atmospheric 3He/4He of 1.4×10-6 ≡ 1Ra), respectively. This value is lower than the volcanic gas values collected in 1997-2002, ranging from 4.7 to 5.6 Ra (Sumino et al., 2004) and also lower than the highest value of the volcanic gases (2024) of 6.0 Ra. This indicates that minerals of the pumice may have trapped low 3He/4He magma in the shallow magma chamber where crustal contamination occurs. Pumices of the 2022-2024 eruption contain heterogenous 3He/4He values in minerals, which reflect several magma evolution processes of Ioto.
Ioto has been featured by repetitive phreatic eruptions in craters and fumarolic areas and very high geothermal activities throughout the island. In July 2022, a Surtseyan eruption occurred off the coast of Okinahama in the southern part of Ioto for the first time since the last magmatic eruption. Cock’s tail jets were frequently observed and volcanic ejecta washed up on the shore. In November 2023, there were temporary eruptive activities on the land with small-scale lava flows. Eruptive activities have been continuing with a dormant phase in between, and most recently, a magma eruption spewing pumice occurred in July 2024.
This study was conducted on the pumices which erupted during this series of eruptions. The bulk composition indicates that all pumices are trachydacite and trachyte, which are unusual at the volcanic front, and there was no heterogeneity among the samples. The proportion of phenocrysts (>300 µm) is less than 10%. Minerals in the pumices are olivine, clinopyroxene, plagioclase, Fe-Ti oxide, pyrrhotite, and apatite. Most of the olivine was corroded and less than 1 mm, whereas pyroxene was idiomorphic and some exceeded 1 mm in size. For the noble gas analysis, pumices were crushed by a high voltage pulsed power fragmentation system (Selfrag Lab in the National Institute of Polar Research) and sieved to separate them by the grain size of 0.5-1 mm. The fraction was separated by a hand magnet, water stirring, and heavy liquid (Sodium polytungstate) to concentrate the target portions. Olivines, pyroxenes were handpicked under a stereomicroscope, collected at 300-500 mg, and leached with nitric acid. Noble gases were extracted from the melt inclusions in minerals under ultra-high vacuum (<10-6 Pa) by striking with a nickel slug 300-3,000 times using solenoid coils. After purifying and separating the gases, noble gas isotopes were measured with a magnetic sector mass spectrometer (modified VG-5400) at the University of Tokyo.
3He/4He of olivines, pyroxenes were 3.3-4.7, 2.0-4.5 Ra (where atmospheric 3He/4He of 1.4×10-6 ≡ 1Ra), respectively. This value is lower than the volcanic gas values collected in 1997-2002, ranging from 4.7 to 5.6 Ra (Sumino et al., 2004) and also lower than the highest value of the volcanic gases (2024) of 6.0 Ra. This indicates that minerals of the pumice may have trapped low 3He/4He magma in the shallow magma chamber where crustal contamination occurs. Pumices of the 2022-2024 eruption contain heterogenous 3He/4He values in minerals, which reflect several magma evolution processes of Ioto.