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[SGC37-13] H2O- and Ca-Enriched Basaltic Melt Associated with the 2021 Explosive Eruption of Fukutoku-Oka-no-Ba
Keywords:Fukutoku-Oka-no-Ba, Pumice, Explosive eruption, Melt inclusion
Introduction: Various factors have been proposed to trigger explosive eruptions, such as rapid magma ascent coupled with low degassing rates (e.g., Zhang, 1999), magma–water or magma–ice interactions (e.g., Swanson et al., 2012), increased magma viscosity due to crystallization (e.g., Lavallée et al., 2007), and high volatile contents (e.g., Sable et al., 2006). In island-arc settings, the most common eruption trigger is the injection of hot, volatile-rich magma from deep levels into shallow magma reservoirs (Hilley et al., 2022). In this study, we examine the chemical characteristics of a basaltic melt of deep origin—implicated in the 2021 eruption of Fukutoku-Oka-no-Ba in the Izu-Bonin-Mariana (IBM) arc (e.g., Yoshida et al., 2022, 2023)—by analyzing the major and volatile element compositions in melt inclusions hosted within pumice phenocrysts.
Samples: We selected three pumice samples exhibiting distinct appearance in colors (amber, gray, and black), collected from Amami Oshima, Kikai Island, and Okinawa Island, respectively. The amber pumice displayed two markedly distinctive domains (bright and dark), which were separated at their boundary for individual analysis. Hand-picked clinopyroxene, orthopyroxene, and plagioclase crystals from crushed samples were polished. While melt inclusions in clinopyroxene from the gray pumice were generally homogeneous, those from the other samples commonly exhibited post-entrapment crystallization and heterogeneity. To accurately determine volatile element concentrations in these heterogeneous inclusions, we homogenized them by heating to approximately 1300–1380°C on a heating stage followed by quenching.
Methods: CO2 concentrations in shrinkage bubbles within the melt inclusions were measured using Raman spectroscopy. Major element compositions of the glass were determined by electron probe microanalyzer (EPMA). The concentrations of H2O, CO2, F, Cl, S, and P2O5 were measured using SIMS following the methodology established by Shimizu et al. (2017).
Results and Discussion: A SiO2 versus (Na2O+K2O) diagram classifies the studied melt inclusions into distinct trachytic and basaltic groups, in general agreement with previous studies (e.g., Yoshida et al., 2022; Maeno et al., 2022). The basaltic melt inclusions are characterized by a lower Al2O3 concentration (~13 wt.%) and higher CaO (~15 wt.%) and H2O (~5 wt.%) contents compared to those from other IBM arc volcanoes. The average CO2 density in the shrinkage bubbles is approximately 0.014g/cm3, which is an order of magnitude lower than values observed in eruptions where CO2 plays a major role in explosivity (e.g., 0.1-0.3 g/cm3 at Sunset Crater, Arizona; Allison et al., 2021). Meanwhile, the glass in the melt inclusions contains up to ~300 ppm CO2, comparable to other IBM arc volcanoes. Previous reports from the southern Bonin arc have identified lavas and melt inclusions with highly radiogenic Pb isotopic signatures (e.g., Li et al., 2024; Ishizuka et al., 2008; Tatsumoto, 1966), suggesting that subducted oceanic plateaus with HIMU-type characteristics may be involved. Notably, Li et al. (2024) documented CaO concentrations up to 12.65wt.% in melt inclusions. Therefore, the basaltic melt associated with the 2021 eruption of Fukutoku-Oka-no-Ba may also derive from a subducted oceanic plateau exhibiting HIMU-type geochemical features. Future work will involve measurements of trace element concentrations and Pb isotopic ratios to further constrain the origin of these basaltic melts.
Samples: We selected three pumice samples exhibiting distinct appearance in colors (amber, gray, and black), collected from Amami Oshima, Kikai Island, and Okinawa Island, respectively. The amber pumice displayed two markedly distinctive domains (bright and dark), which were separated at their boundary for individual analysis. Hand-picked clinopyroxene, orthopyroxene, and plagioclase crystals from crushed samples were polished. While melt inclusions in clinopyroxene from the gray pumice were generally homogeneous, those from the other samples commonly exhibited post-entrapment crystallization and heterogeneity. To accurately determine volatile element concentrations in these heterogeneous inclusions, we homogenized them by heating to approximately 1300–1380°C on a heating stage followed by quenching.
Methods: CO2 concentrations in shrinkage bubbles within the melt inclusions were measured using Raman spectroscopy. Major element compositions of the glass were determined by electron probe microanalyzer (EPMA). The concentrations of H2O, CO2, F, Cl, S, and P2O5 were measured using SIMS following the methodology established by Shimizu et al. (2017).
Results and Discussion: A SiO2 versus (Na2O+K2O) diagram classifies the studied melt inclusions into distinct trachytic and basaltic groups, in general agreement with previous studies (e.g., Yoshida et al., 2022; Maeno et al., 2022). The basaltic melt inclusions are characterized by a lower Al2O3 concentration (~13 wt.%) and higher CaO (~15 wt.%) and H2O (~5 wt.%) contents compared to those from other IBM arc volcanoes. The average CO2 density in the shrinkage bubbles is approximately 0.014g/cm3, which is an order of magnitude lower than values observed in eruptions where CO2 plays a major role in explosivity (e.g., 0.1-0.3 g/cm3 at Sunset Crater, Arizona; Allison et al., 2021). Meanwhile, the glass in the melt inclusions contains up to ~300 ppm CO2, comparable to other IBM arc volcanoes. Previous reports from the southern Bonin arc have identified lavas and melt inclusions with highly radiogenic Pb isotopic signatures (e.g., Li et al., 2024; Ishizuka et al., 2008; Tatsumoto, 1966), suggesting that subducted oceanic plateaus with HIMU-type characteristics may be involved. Notably, Li et al. (2024) documented CaO concentrations up to 12.65wt.% in melt inclusions. Therefore, the basaltic melt associated with the 2021 eruption of Fukutoku-Oka-no-Ba may also derive from a subducted oceanic plateau exhibiting HIMU-type geochemical features. Future work will involve measurements of trace element concentrations and Pb isotopic ratios to further constrain the origin of these basaltic melts.