17:15 〜 18:45
[SVC30-P14] 阿蘇カルデラ形成前の多様なマグマとその活動史
キーワード：カルデラ、カリウム－アルゴン年代、阿蘇火山、マグマ
The geochronological and geochemical data of pre-caldera volcanic rocks are expected to provide us key information about magmatic evolution process and magma plumbing system before the caldera-forming eruptions.
Aso volcano, situated in central Kyushu, produced four gigantic caldera-forming pyroclastic eruptions (Aso-1 to Aso-4) between 270 and 90 ka. On the other hand, the pre-caldera volcanism (before 270 ka) is mainly characterized by multiple thick lava flow eruptions from several vents (Ono and Watanabe, 1985; Furukawa et al., 2009). The drastic change of eruption style probably reflects the change of magma-plumbing system beneath caldera. To clarify the detailed temporal change of the magma-plumbing system, the absolute age dating and geochemical analysis of volcanic products are necessary. Therefore, we determined the K-Ar ages and the isotopic compositions for several samples of the pre-caldera volcanic rocks from Aso volcano.
The argon isotopic ratio was measured using a noble-gas mass spectrometer MS-III (modified VG-5400) in the Research Center for Advanced Science and Technology, The University of Tokyo. In this study, the radiogenic 40Ar contents of samples were determined by using the unspiked method. The Sr and other isotopic compositions were measured by Neptune Plus MC-ICP-MS at Kyushu university.
The K-Ar ages of the basaltic lava and andesitic dike samples from the southeastern part of the caldera are relatively old (740-690 ka). The K-Ar ages of the other andesitic and rhyolitic lava samples concentrated between 500 and 380 ka. The temporal changes of geochemical compositions of the pre-caldera volcanic rocks are not clear. The heterogeneous Sr isotope ratios of the volcanic rocks indicate that crustal assimilation was predominant in the pre-caldera stage of Aso.
Aso volcano, situated in central Kyushu, produced four gigantic caldera-forming pyroclastic eruptions (Aso-1 to Aso-4) between 270 and 90 ka. On the other hand, the pre-caldera volcanism (before 270 ka) is mainly characterized by multiple thick lava flow eruptions from several vents (Ono and Watanabe, 1985; Furukawa et al., 2009). The drastic change of eruption style probably reflects the change of magma-plumbing system beneath caldera. To clarify the detailed temporal change of the magma-plumbing system, the absolute age dating and geochemical analysis of volcanic products are necessary. Therefore, we determined the K-Ar ages and the isotopic compositions for several samples of the pre-caldera volcanic rocks from Aso volcano.
The argon isotopic ratio was measured using a noble-gas mass spectrometer MS-III (modified VG-5400) in the Research Center for Advanced Science and Technology, The University of Tokyo. In this study, the radiogenic 40Ar contents of samples were determined by using the unspiked method. The Sr and other isotopic compositions were measured by Neptune Plus MC-ICP-MS at Kyushu university.
The K-Ar ages of the basaltic lava and andesitic dike samples from the southeastern part of the caldera are relatively old (740-690 ka). The K-Ar ages of the other andesitic and rhyolitic lava samples concentrated between 500 and 380 ka. The temporal changes of geochemical compositions of the pre-caldera volcanic rocks are not clear. The heterogeneous Sr isotope ratios of the volcanic rocks indicate that crustal assimilation was predominant in the pre-caldera stage of Aso.