4:15 PM - 4:30 PM
[SVC30-09] Petrogenesis of silicic and mafic magma in the Aso volcano on the basis of melt inclusion and groundmass analyses
Keywords:Aso caldera, Melt inclusion, Silicic magma, Crystal mush, Petrology, Large-scale eruption
The Aso volcano in Kyushu, southwest Japan, has a large caldera (18×25 km) and repeated VEI 6-8 silicic eruptions four times in the last 270,000 years, known as Aso-1, Aso-2, Aso-3, and Aso-4. These large-scale eruptions are typical zoned ignimbrites; they ejected silicic magma (pumice) in the early stages and ended with crystal-rich, more mafic magma (scoria). These eruptive activities suggest that the mafic magma is commonly present in the voluminous silicic magma system and should play an important role in the system. This study aims to understand the processes that generate such compositionally zoned eruptions, focusing on the Aso-2, Aso-3, and Aso-4 eruptions using major and trace element compositional analysis of melt inclusion and groundmass in the ejecta.
The analytical results show that the melt inclusion and groundmass in the pumices and scoriae are classified into HK-S, HK-M, and MK types. The HK-S type is dacitic to rhyolitic glasses belonging to the High-K series found in the Aso-2, 3, and 4 pumices and shows an upward slope from MREE to HREE in REE pattern normalized by Aso’s basalt. The HK-M type is basaltic andesitic to dacitic glasses belonging to the High-K series found in the Aso-2, 3, and 4 scoriae and shows flat REE pattern from MREE to HREE. The MK type is andesitic to dacitic glasses belonging to Medium-K series found in the Aso-3 and 4 scoriae, and MREE patters of the Aso-3 and 4 MK glasses are different. The HK-S and HK-M glasses are interpreted as melts of the silicic and mafic magmas ejected by the large-scale eruptions, respectively, because the high K2O contents in the HK-S and HK-M glasses are similar to whole rock composition of the pumices and scoriae. On the other hand, the MK glass is a melt not directly derived from the silicic and mafic magmas in the low K2O content.
We examine whether the REE pattern of the HK-S and HK-M melts can be explained by differentiation of a single mafic source. The assumption of the single source material is based on results of previous works showing that Sr isotopic compositions of the silicic and mafic magmas are the same. The examination method is calculations of melt composition during crystallization of the Aso’s basaltic magma using distribution coefficients in previous studies and crystallized mineral phases. The mineral phases are determined by two independent data: crystallization experiments of basaltic magma in previous studies and a mass balance calculation that reproduces compositional change from the Aso’s basalt to the HK-S melt in this study. The calculation results indicate that the REE patterns of the HK-M melt are produced by crystallization of the basaltic magma with a small amount of amphibole and/or partial melting of basaltic rock, which has the same composition as the Aso’s basaltic magma, dissolving a large amount of amphibole. On the other hand, the REE pattern of the HK-S melt is produced by only crystallization with apatite. The MK melt is interpreted to be generated by melting crystal mush composed of plagioclase and apatite with/without amphibole since the MK melt is poor in K2O content and rich in these mineral components.
We propose the following magma generation and evolution processes based on the petrogenesis of the three melt types: (1) The HK-M magma was generated by the crystallization of the basaltic magma and/or melting of the basaltic rock at the lower crust. (2) Shallow reservoir was formed by supply of the HK-M magma from the lower crust. The HK-M magma in the shallow reservoir was differentiated into the HK-S magma by crystallization. (3) The MK magma was generated by the melting of crystal mush in the shallow reservoir by heat from newly supplied hot magma. This study shows that the mafic magma was generated by both the crystallization and melting processes and then evolved into the silicic magma. The simultaneous occurrence of the two generation processes is efficient in producing magma and reasonable for the voluminous silicic magmatism.
The analytical results show that the melt inclusion and groundmass in the pumices and scoriae are classified into HK-S, HK-M, and MK types. The HK-S type is dacitic to rhyolitic glasses belonging to the High-K series found in the Aso-2, 3, and 4 pumices and shows an upward slope from MREE to HREE in REE pattern normalized by Aso’s basalt. The HK-M type is basaltic andesitic to dacitic glasses belonging to the High-K series found in the Aso-2, 3, and 4 scoriae and shows flat REE pattern from MREE to HREE. The MK type is andesitic to dacitic glasses belonging to Medium-K series found in the Aso-3 and 4 scoriae, and MREE patters of the Aso-3 and 4 MK glasses are different. The HK-S and HK-M glasses are interpreted as melts of the silicic and mafic magmas ejected by the large-scale eruptions, respectively, because the high K2O contents in the HK-S and HK-M glasses are similar to whole rock composition of the pumices and scoriae. On the other hand, the MK glass is a melt not directly derived from the silicic and mafic magmas in the low K2O content.
We examine whether the REE pattern of the HK-S and HK-M melts can be explained by differentiation of a single mafic source. The assumption of the single source material is based on results of previous works showing that Sr isotopic compositions of the silicic and mafic magmas are the same. The examination method is calculations of melt composition during crystallization of the Aso’s basaltic magma using distribution coefficients in previous studies and crystallized mineral phases. The mineral phases are determined by two independent data: crystallization experiments of basaltic magma in previous studies and a mass balance calculation that reproduces compositional change from the Aso’s basalt to the HK-S melt in this study. The calculation results indicate that the REE patterns of the HK-M melt are produced by crystallization of the basaltic magma with a small amount of amphibole and/or partial melting of basaltic rock, which has the same composition as the Aso’s basaltic magma, dissolving a large amount of amphibole. On the other hand, the REE pattern of the HK-S melt is produced by only crystallization with apatite. The MK melt is interpreted to be generated by melting crystal mush composed of plagioclase and apatite with/without amphibole since the MK melt is poor in K2O content and rich in these mineral components.
We propose the following magma generation and evolution processes based on the petrogenesis of the three melt types: (1) The HK-M magma was generated by the crystallization of the basaltic magma and/or melting of the basaltic rock at the lower crust. (2) Shallow reservoir was formed by supply of the HK-M magma from the lower crust. The HK-M magma in the shallow reservoir was differentiated into the HK-S magma by crystallization. (3) The MK magma was generated by the melting of crystal mush in the shallow reservoir by heat from newly supplied hot magma. This study shows that the mafic magma was generated by both the crystallization and melting processes and then evolved into the silicic magma. The simultaneous occurrence of the two generation processes is efficient in producing magma and reasonable for the voluminous silicic magmatism.