5:15 PM - 7:15 PM
[SVC36-P02] Deep Magma Plumbing System of Fukutoku-Oka-no-Ba Volcano: Constraints from Melt Inclusions
★Invited Papers
Keywords:Fukutoku-Oka-no-Ba, melt inclusion, magma plumbing system, alkaline rock
Fukutoku-Oka-no-Ba (FOB) is one of submarine volcanoes of the Izu-Bonin-Mariana arc. The volcano erupted ~0.1 km³ (Maeno et al., 2022) of magma during the latest explosive eruption on 13 August 2021. The main erupted material consists of gray aphyric trachytic pumice containing phenocryst-rich dark enclaves. Phenocrysts in the dark enclaves are classified into two groups: Group-A minerals are homogeneous and have more evolved compositions, whereas Group-B minerals are chemically zoned and have cores of less evolved compositions. The depth of chamber A, where the main trachytic magma and Group-A minerals are stored, was previously estimated to be 3±1 km. However, derived depths and origins of Group-B minerals remain unclear; they may retain information on the deep magma plumbing system beneath FOB. This study performed textural observations and chemical analyses for Group-B minerals and their melt inclusions (MIs) in FOB 2021 pumice to clarify the deep magma plumbing system beneath FOB.
Drift pumice collected from Okibara Beach, Miyake-Jima was used in this study. Dark enclaves were collected from crushed pumice and processed into polished thin sections. Backscattered electron (BSE) imaging and element distribution map were performed using FE-SEM at Shizuoka University, and chemical analyses of minerals and MIs were conducted using EPMA at the Earthquake Research Institute, The University of Tokyo.
Group-B minerals are chiefly composed of phenocrysts of clinopyroxene (CPX) of Mg# [=100Mg/(Mg+Fe)] ~63-93, with less abundant olivine of Mg# ~82-90 and rarely plagioclase. These minerals have compositional zoning in their rims. MIs hosted in CPX and olivine were chemically classified into three groups: Group-B1 MIs are hosted in CPX and have alkaline compositions with SiO2~50-65 wt% and CaO~7-2 wt%. Group-B2 MIs are also in CPX but have subalkaline compositions with SiO2~50-61 wt% and CaO~10-6 wt%. Group-B3 MIs are hosted in olivine and have subalkaline compositions of SiO2~49-50 wt% and CaO~15-19 wt%. These results indicate that four independent chambers of alkaline (A, B1) and subalkaline (B2, B3) magmas exist beneath FOB.
The equilibrium temperatures and H2O contents were estimated for Group-B1 and B3 melts to be ~1010 °C, ~7.2 wt% and ~982 °C, ~7.1 wt%, respectively. The estimated melt H2O contents correspond to the H2O saturation pressures and depths of ~168 MPa and ~6.2 km for Group-B1 and ~488 MPa and ~18 km for Group-B3, respectively, assuming a crustal density of 2750 kg/m³. The depth of the B2 magma chamber is inferred to be between the B1 and B3 chambers based on the texture and sulfur contents of MIs and the compositions of host minerals. The depths of the B1 and B3 chambers are comparable with the upper crust-middle crust (~6-10 km) and middle crust-lower crust boundaries (~18 km), respectively, suggesting that the depths of the magma chambers are controlled by crustal density structure.
To examine the petrogenetic relationships of Group-B magmas, fractional crystallization simulations were performed using MELTS assuming Group-B3 melt or the basanitic melt erupted in adjacent Minami-Ioto as the parental magma. The results show that it is difficult to generate Group-B1 and B2 melts from Group-3 melt, whereas the Minami-Ioto basanitic melt can produce B1 melts. This suggests the petrogenetic relation between Group-B1 and Minami-Ioto basanitic magmas.
The pre-eruptive process of the FOB 2021 eruption is proposed as follows; Before the 1904 eruption, basanitic magma was supplied from the Minami-Ioto volcano to form magma chambers B1 and A. Magma or fluid was supplied through the four chambers just before the 2021 eruption although the time the B2 and B3 magma chambers were formed is unclear. This magma/fluid intrusion likely caused partial oxidation (Yoshida et al., 2023) and overpressure in the A magma chamber, which triggered the 2021 eruption.
Drift pumice collected from Okibara Beach, Miyake-Jima was used in this study. Dark enclaves were collected from crushed pumice and processed into polished thin sections. Backscattered electron (BSE) imaging and element distribution map were performed using FE-SEM at Shizuoka University, and chemical analyses of minerals and MIs were conducted using EPMA at the Earthquake Research Institute, The University of Tokyo.
Group-B minerals are chiefly composed of phenocrysts of clinopyroxene (CPX) of Mg# [=100Mg/(Mg+Fe)] ~63-93, with less abundant olivine of Mg# ~82-90 and rarely plagioclase. These minerals have compositional zoning in their rims. MIs hosted in CPX and olivine were chemically classified into three groups: Group-B1 MIs are hosted in CPX and have alkaline compositions with SiO2~50-65 wt% and CaO~7-2 wt%. Group-B2 MIs are also in CPX but have subalkaline compositions with SiO2~50-61 wt% and CaO~10-6 wt%. Group-B3 MIs are hosted in olivine and have subalkaline compositions of SiO2~49-50 wt% and CaO~15-19 wt%. These results indicate that four independent chambers of alkaline (A, B1) and subalkaline (B2, B3) magmas exist beneath FOB.
The equilibrium temperatures and H2O contents were estimated for Group-B1 and B3 melts to be ~1010 °C, ~7.2 wt% and ~982 °C, ~7.1 wt%, respectively. The estimated melt H2O contents correspond to the H2O saturation pressures and depths of ~168 MPa and ~6.2 km for Group-B1 and ~488 MPa and ~18 km for Group-B3, respectively, assuming a crustal density of 2750 kg/m³. The depth of the B2 magma chamber is inferred to be between the B1 and B3 chambers based on the texture and sulfur contents of MIs and the compositions of host minerals. The depths of the B1 and B3 chambers are comparable with the upper crust-middle crust (~6-10 km) and middle crust-lower crust boundaries (~18 km), respectively, suggesting that the depths of the magma chambers are controlled by crustal density structure.
To examine the petrogenetic relationships of Group-B magmas, fractional crystallization simulations were performed using MELTS assuming Group-B3 melt or the basanitic melt erupted in adjacent Minami-Ioto as the parental magma. The results show that it is difficult to generate Group-B1 and B2 melts from Group-3 melt, whereas the Minami-Ioto basanitic melt can produce B1 melts. This suggests the petrogenetic relation between Group-B1 and Minami-Ioto basanitic magmas.
The pre-eruptive process of the FOB 2021 eruption is proposed as follows; Before the 1904 eruption, basanitic magma was supplied from the Minami-Ioto volcano to form magma chambers B1 and A. Magma or fluid was supplied through the four chambers just before the 2021 eruption although the time the B2 and B3 magma chambers were formed is unclear. This magma/fluid intrusion likely caused partial oxidation (Yoshida et al., 2023) and overpressure in the A magma chamber, which triggered the 2021 eruption.