16:30 〜 16:45
[SVC34-11] Melt inclusion insights into Kikai caldera magma evolution after the 7.3 ka caldera-forming eruption
キーワード：鬼界カルデラ、メルト包有物、マグマ進化
Caldera-forming eruptions in a caldera volcano generally occur at intervals of more than ten thousand years and are preceded by smaller-scale eruptions. The compositional transition of magmas is important for understanding mechanisms governing the activities of caldera volcanoes. We investigate changes in magmatic features from the last VEI-7 caldera-forming (the Kikai-Akahoya eruption, 7.3 ka; KA) to the succeeding eruptions at Kikai caldera volcano, mainly through chemical analyses of melt inclusions (MIs) in phenocrysts in volcanic products to clarify the geochemical evolution of magmas.
Kikai Caldera Volcano is located off the south coast of Kyushu Island and has a 20-km-diameter submarine caldera with inner and outer caldera rims; the inner rim was formed by the KA eruption. This eruption discharged a voluminous rhyolitic magma and a relatively small amount of andesitic magma. Since then, many post-caldera (post-KA) eruptions of basaltic-andesitic to rhyolitic magmas have occurred. Previous studies have recognized changes in the magmatic features in whole-rock composition from the KA eruption to the post-KA eruptions; for example, potassium content of the KA silicic product is higher in a given silica content than that of the post-KA one (Tatsumi et al., 2018; Hamada et al., 2023).
We analyzed MIs in phenocrysts from the KA pumice (71.6 wt.% SiO2 in whole-rock composition, pumice in the fallout event of the early eruption stage; Funakura pumice fall) and scoria (59.8 wt.% SiO2, scoria in the main pyroclastic flow event; Takeshima pyroclastic flow), post-KA lava dome rhyolite (70.0 wt.% SiO2), and post-KA Inamura-dake scoria (54.6 wt.% SiO2). Major and trace element compositions of the MIs, host phenocrysts, and groundmass were determined using FE-EPMA and LA-ICP-MS, respectively. Volatile concentrations in MIs were measured using SIMS.
The KA pumice and the post-KA rhyolite, which are the products from silicic magmas, have the MIs and their host phenocrysts with similar geochemical features; their MIs are alike in major and trace element compositions and show an overlapped range of volatile concentrations except for H2O, and the host phenocrysts of the MIs have similar compositional distribution. The MIs in the post-KA rhyolite have a little lower H2O concentration (~2-3 wt.%) than that in the KA pumice (~3-4 wt.%). On the other hand, the groundmass composition in the KA pumice is higher in potassium content than that of the post-KA rhyolite, which is the same tendency as the difference between the KA pumice and the post-KA in whole-rock composition. These features suggest that the post-KA silicic magma was generated by mixing of a new silicic magma with the unerupted KA silicic magma rather than complete replacement of a new magma after the KA eruption.
The MIs in the post-KA scoria have a wide range of composition (60-66 wt.% SiO2) and are more silicic than its groundmass (57 wt.% SiO2). In the KA scoria, the MIs also have a wide range of composition (63-68 wt.%) and composition of the groundmass is equivalent to that of the less silicic MIs. The MIs and groundmasses in the KA and post-KA scoriae show a single trend on compositional variation diagram, which can be explained by fractionation. The MIs in both the scoriae show an overlapped range of volatile element concentrations, particularly they have the same H2O concentration range (~2-3 wt.%). These indicate that, unlike the silicic magma, an injection of mafic magma has been repeated without significant compositional change through the KA and post-KA activities.
The example of Kikai Caldera volcano shows that after a caldera-forming eruption, a shallow magma system retained some amount of the magma for the next eruptions and was supplied with plural magmas which were compositionally changed and unchanged.
Kikai Caldera Volcano is located off the south coast of Kyushu Island and has a 20-km-diameter submarine caldera with inner and outer caldera rims; the inner rim was formed by the KA eruption. This eruption discharged a voluminous rhyolitic magma and a relatively small amount of andesitic magma. Since then, many post-caldera (post-KA) eruptions of basaltic-andesitic to rhyolitic magmas have occurred. Previous studies have recognized changes in the magmatic features in whole-rock composition from the KA eruption to the post-KA eruptions; for example, potassium content of the KA silicic product is higher in a given silica content than that of the post-KA one (Tatsumi et al., 2018; Hamada et al., 2023).
We analyzed MIs in phenocrysts from the KA pumice (71.6 wt.% SiO2 in whole-rock composition, pumice in the fallout event of the early eruption stage; Funakura pumice fall) and scoria (59.8 wt.% SiO2, scoria in the main pyroclastic flow event; Takeshima pyroclastic flow), post-KA lava dome rhyolite (70.0 wt.% SiO2), and post-KA Inamura-dake scoria (54.6 wt.% SiO2). Major and trace element compositions of the MIs, host phenocrysts, and groundmass were determined using FE-EPMA and LA-ICP-MS, respectively. Volatile concentrations in MIs were measured using SIMS.
The KA pumice and the post-KA rhyolite, which are the products from silicic magmas, have the MIs and their host phenocrysts with similar geochemical features; their MIs are alike in major and trace element compositions and show an overlapped range of volatile concentrations except for H2O, and the host phenocrysts of the MIs have similar compositional distribution. The MIs in the post-KA rhyolite have a little lower H2O concentration (~2-3 wt.%) than that in the KA pumice (~3-4 wt.%). On the other hand, the groundmass composition in the KA pumice is higher in potassium content than that of the post-KA rhyolite, which is the same tendency as the difference between the KA pumice and the post-KA in whole-rock composition. These features suggest that the post-KA silicic magma was generated by mixing of a new silicic magma with the unerupted KA silicic magma rather than complete replacement of a new magma after the KA eruption.
The MIs in the post-KA scoria have a wide range of composition (60-66 wt.% SiO2) and are more silicic than its groundmass (57 wt.% SiO2). In the KA scoria, the MIs also have a wide range of composition (63-68 wt.%) and composition of the groundmass is equivalent to that of the less silicic MIs. The MIs and groundmasses in the KA and post-KA scoriae show a single trend on compositional variation diagram, which can be explained by fractionation. The MIs in both the scoriae show an overlapped range of volatile element concentrations, particularly they have the same H2O concentration range (~2-3 wt.%). These indicate that, unlike the silicic magma, an injection of mafic magma has been repeated without significant compositional change through the KA and post-KA activities.
The example of Kikai Caldera volcano shows that after a caldera-forming eruption, a shallow magma system retained some amount of the magma for the next eruptions and was supplied with plural magmas which were compositionally changed and unchanged.