15:30 〜 17:00
[SVC35-P08] 阿蘇カルデラ外輪山に分布する溶岩および岩脈のK-Ar年代
キーワード：カルデラ、カリウム－アルゴン年代、阿蘇火山、外輪山、火山活動史
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 formation of caldera.
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 characterized by multiple effusive eruptions from several vents, and formed lava plateaus (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 of volcanic products is necessary. We, therefore, determined the K-Ar ages 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. In this method, the unknown concentration of 40Ar contained in a sample is determined by comparing its 40Ar peak intensity with that of a standard air sample whose 40Ar concentration is known. The isotopic composition of the initial Ar in the sample is determined by measuring the 38Ar/36Ar ratio without assuming that its 40Ar/36Ar ratio is equal to the modern atmospheric value of 296 (Orihashi et al., 2004).
K-Ar ages of the following lava and dike samples are newly obtained.
1) basalt lava from Takamorimachi, the southeastern caldera-rim: 760-660 ka, 2) andesite dike from Takamorimachi, the southeastern caldera-rim: 790-700 ka, 3) rhyolite lava from Sakanashi, the eastern caldera-rim : 490-430 ka, 4) andesite lava from Tateno, the western caldera-rim: 540-350 ka, 5) andesite dike from Tateno, the western caldera-rim: 290-250 ka.
This research was funded by the Integrated Program for Next Generation Volcano Research and Human Resource Development, Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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 characterized by multiple effusive eruptions from several vents, and formed lava plateaus (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 of volcanic products is necessary. We, therefore, determined the K-Ar ages 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. In this method, the unknown concentration of 40Ar contained in a sample is determined by comparing its 40Ar peak intensity with that of a standard air sample whose 40Ar concentration is known. The isotopic composition of the initial Ar in the sample is determined by measuring the 38Ar/36Ar ratio without assuming that its 40Ar/36Ar ratio is equal to the modern atmospheric value of 296 (Orihashi et al., 2004).
K-Ar ages of the following lava and dike samples are newly obtained.
1) basalt lava from Takamorimachi, the southeastern caldera-rim: 760-660 ka, 2) andesite dike from Takamorimachi, the southeastern caldera-rim: 790-700 ka, 3) rhyolite lava from Sakanashi, the eastern caldera-rim : 490-430 ka, 4) andesite lava from Tateno, the western caldera-rim: 540-350 ka, 5) andesite dike from Tateno, the western caldera-rim: 290-250 ka.
This research was funded by the Integrated Program for Next Generation Volcano Research and Human Resource Development, Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.