16:15 〜 16:30
[SGC32-09] Helium and carbon isotope ratios of fumarolic gases from Iwojima volcano, Izu-Ogasawara arc, Japan revisited: Significant contribution of subducted volatiles?
キーワード：ヘリウム同位体、硫黄島、火山ガス、沈み込み
The magma of Iwojima (Ioto) volcano, located on the southernmost part of the Izu-Ogasawara arc, is characterized by magma chemistry different from that of the other volcanoes in the arc, suggesting a more significant contribution of a subducted component (e.g., Bloomer et al., 1989). The island is also known for substantial crustal deformation, resulting in continuous uplift and active geothermal activity throughout the island with frequent phreatic eruptions (Nagai and Kobayashi, 2015). To elucidate the unusual magma genesis and the behavior of volatiles in vigorous hydrothermal activity, fumaroles and hot-spring gases were sampled from 1997 to 2001 and in August 2017, November 2022, and November 2023, and noble gases and carbon isotope ratios were measured.
Noble gas and carbon isotopic characteristics of Iwojima magma are quite distinct from other volcanoes in the Izu-Ogasawara-Mariana arc: (1) low 3He/4He ranging from 5.6 Ra to 6.0 Ra, where Ra denotes the atmospheric 3He/4He, (2) enrichment in heavy noble gases, and (3) a heavy carbon isotope ratio (+2.0 ‰) almost equivalent to the highest value for marine carbonates. These unusual features strongly suggest an involvement of slab-derived volatiles in the magma (Sumino et al., 2004). Enhanced slab-derived carbon and helium emissions estimated from carbon dioxide emissions at the island (Notsu et al., 2005) indicate a significant input of slab-derived volatiles to the mantle wedge. As for its origin, Sumino et al. (2004) invoked the unusual tectonic setting around Iwojima: (1) subduction of the Ogasawara Plateau, which brings a significant volume of thick crustal blocks into the mantle wedge beneath Iwojima, (2) flattened subduction as a consequence of the buoyancy of the subducted Ogasawara Plateau, and (3) the extension of the back-arc basin in the Mariana Trough has migrated northward into the back-arc region of this island.
The phreatic eruptions on the island have continued for the past 1,000 years (Nagai and Kobayashi, 2015). However, the submarine eruption in August 2022 on the southeast marine caldera margin of the island was a magmatic eruption, suggesting that the volcanic activity may be moving to a different stage. The 3He/4He of fumaroles and hot spring gases sampled in November 2022 on the island's north side, where phreatic eruptions have frequently occurred until August 2022, showed no significant changes from the previous observations of around 5.0-5.6 Ra. However, an unprecedentedly high value of 6.0 Ra was observed at Iwogaoka in the center of Motoyama, a lava dome forming the central part of the island. In November 2023, the high 3He/4He at Iwogaoka was kept high, and a higher value (5.9 Ra) than before 2022 was also observed at Ohyudani, located at the eastern edge of the Motoyama lava dome.
Based on the distribution of epicenters and observations of the ejecta of previous phreatic eruptions, the center of the hydrothermal system is thought to be located beneath the north of the island. Therefore, even if the helium derived from the magma that ascent to the shallow depth in 2022 has a higher 3He/4He than 6.0 Ra, its contribution would not have yet been observed in the hydrothermal reservoir, where the 3He/4He would be lowered by the contamination by 4He derived from surrounding basement rock. On the other hand, the magmatic contribution may have been more significant at Iwogaoka, where the influence of the hydrothermal reservoir is relatively small, implying future changes in the 3He/4He of fumarolic and hot spring gases at other sites could occur. To clarify the relation of the 3He/4He increase at Iwogaoka with the magmatic eruption since 2022, we are preparing an analysis of olivines separated from the pumice ejecta of the eruption.
Noble gas and carbon isotopic characteristics of Iwojima magma are quite distinct from other volcanoes in the Izu-Ogasawara-Mariana arc: (1) low 3He/4He ranging from 5.6 Ra to 6.0 Ra, where Ra denotes the atmospheric 3He/4He, (2) enrichment in heavy noble gases, and (3) a heavy carbon isotope ratio (+2.0 ‰) almost equivalent to the highest value for marine carbonates. These unusual features strongly suggest an involvement of slab-derived volatiles in the magma (Sumino et al., 2004). Enhanced slab-derived carbon and helium emissions estimated from carbon dioxide emissions at the island (Notsu et al., 2005) indicate a significant input of slab-derived volatiles to the mantle wedge. As for its origin, Sumino et al. (2004) invoked the unusual tectonic setting around Iwojima: (1) subduction of the Ogasawara Plateau, which brings a significant volume of thick crustal blocks into the mantle wedge beneath Iwojima, (2) flattened subduction as a consequence of the buoyancy of the subducted Ogasawara Plateau, and (3) the extension of the back-arc basin in the Mariana Trough has migrated northward into the back-arc region of this island.
The phreatic eruptions on the island have continued for the past 1,000 years (Nagai and Kobayashi, 2015). However, the submarine eruption in August 2022 on the southeast marine caldera margin of the island was a magmatic eruption, suggesting that the volcanic activity may be moving to a different stage. The 3He/4He of fumaroles and hot spring gases sampled in November 2022 on the island's north side, where phreatic eruptions have frequently occurred until August 2022, showed no significant changes from the previous observations of around 5.0-5.6 Ra. However, an unprecedentedly high value of 6.0 Ra was observed at Iwogaoka in the center of Motoyama, a lava dome forming the central part of the island. In November 2023, the high 3He/4He at Iwogaoka was kept high, and a higher value (5.9 Ra) than before 2022 was also observed at Ohyudani, located at the eastern edge of the Motoyama lava dome.
Based on the distribution of epicenters and observations of the ejecta of previous phreatic eruptions, the center of the hydrothermal system is thought to be located beneath the north of the island. Therefore, even if the helium derived from the magma that ascent to the shallow depth in 2022 has a higher 3He/4He than 6.0 Ra, its contribution would not have yet been observed in the hydrothermal reservoir, where the 3He/4He would be lowered by the contamination by 4He derived from surrounding basement rock. On the other hand, the magmatic contribution may have been more significant at Iwogaoka, where the influence of the hydrothermal reservoir is relatively small, implying future changes in the 3He/4He of fumarolic and hot spring gases at other sites could occur. To clarify the relation of the 3He/4He increase at Iwogaoka with the magmatic eruption since 2022, we are preparing an analysis of olivines separated from the pumice ejecta of the eruption.