11:00 AM - 1:00 PM
[SVC29-P09] Whole-rock Chemical Compositions and Sr Isotopic Ratios of Volcanic Products before and after Aso-1, Aso Volcano, Central Kyushu
Keywords:Aso caldera, Sr isotopic ratio, pre Aso volcanic rocks, Koga lava, Hokamaki lava, Aso-1 ignimbrites
Aso volcano is located on the volcanic front at the northern end of the Kyushu-Ryukyu arc, and four large pyroclastic eruptions ranging from about 266 ka (Aso-1) to 89 ka (Aso-4) formed a large caldera. The volcanic activity can be divided into three stages in chronological order: the pre-Aso volcanic rocks (ca. 400-800 ka), the caldera-forming stage, and the post-caldera stage (e.g., Ono and Watanabe, 1985).
With some exceptions, volcanic products of the caldera-forming stage and the post-caldera stage are enriched in liquid-phase enrichment elements such as K2O, but this is not the case for the Pre-Aso volcanic rocks. The upper limit of Sr isotopic ratios varies with time, with a lower limit of about 0.704 throughout the entire periods, and the reported upper limits of Sr isotopic ratios for different time periods are 0.704462 for pre-Aso volcanic rocks, 0.70422 for essential materials of pyroclastic flow deposits, and 0.704355 for volcanic products of post-caldera, except for some (e.g., Hunter, 1988; Furukawa et al, 2008; Miyoshi et al., 2011; Shinmura et al., 2013). It is highest in the pre-Aso, becomes lower in the caldera-forming stage, and rises again in the post-caldera stage. In this study, we measured the whole-rock chemical compositions and Sr isotopic ratios of six volcanic rocks from the Pre-Aso volcanic rocks before and after the Aso-1 eruption, and tried to clarify the changes in magma composition at the beginning of the entire Aso caldera eruption.
We collected three samples of pre-Aso volcanic rocks (Watanabe et al., 2021) from a stream on the approach to the Iwato Shrine in the Tateno Valley, two samples of Koga lava (Tajima et al., 2017) with an age of 275 ± 11 ka from a part of the east wall of the caldera, and one sample of Hokamaki lava (Aso-1/2 lava, Watanabe et al., 2021) from near the Seta Shrine in the Tateno Valley. All rocks are andesite. The chemical composition of the whole rock was determined by an X-ray fluorescence spectrometer installed at the Kitakyushu Museum of Natural History & Human History, and the Sr isotope ratio was determined by a thermal ionization mass spectrometer (TIMS) (Finnigan MAT262) at Tsukuba University and a TIMS (Thermo Fischer Scientific TRITON Plus TIMS) at Kumamoto University. The major elemental compositions of SiO2 wt.%, K2O wt.%, and Sr isotopic ratios (87Sr/86Sr) were 57-61, 2.3-3.0, 0.70422-0.70424 for the Pre-Aso volcanic rocks, 55-58, 1.8-2.7, 0.70405-0.70421 for the Koga lava, and 60, 2.9, 0.70399 for the Hokamaki lava, respectively.
The K2O content in Koga and Hokamaki lava tends to be high relative to the SiO2 content as in the post-caldera volcanic products. The Sr isotopic ratio of the Koga lava is almost equal to the upper limit of the Aso-1 pyroclastic flow deposits, and that of the Hokamaki lava is around the lower limit of the volcanic products from the pre-Aso volcanic rocks to the post-caldera stage. In addition, the amount of K2O to SiO2 is high in all three samples, which is not a characteristic of general pre-Aso volcanic rocks, but a characteristic of the post-caldera period. The Sr isotopic ratios of all three samples are slightly higher than the upper limit of the Aso-1 pyroclastic flow deposits, and much lower than the upper limit of 0.70446 for the entire Pre-Aso volcanic rocks, which is consistent with the post-caldera period characteristics. The age of the pre-Aso volcanic rocks in this study from the Tateno Valley, western part of Aso caldera, has not been measured, but in the area where these rocks are distributed, the pre-Aso volcanic rocks are in contact with Aso-1 and Aso-2 (Watanabe et al., 2021). Therefore, it is thought that these rocks are relatively young among the pre-Aso volcanic rocks and were active at a time close to that of Aso-1, and magma with similar properties existed underground in the western part of the caldera before the Aso-1 eruption, and that there was activity to erupt these magma before Aso-1 like Koga lava in the eastern part of caldera.
With some exceptions, volcanic products of the caldera-forming stage and the post-caldera stage are enriched in liquid-phase enrichment elements such as K2O, but this is not the case for the Pre-Aso volcanic rocks. The upper limit of Sr isotopic ratios varies with time, with a lower limit of about 0.704 throughout the entire periods, and the reported upper limits of Sr isotopic ratios for different time periods are 0.704462 for pre-Aso volcanic rocks, 0.70422 for essential materials of pyroclastic flow deposits, and 0.704355 for volcanic products of post-caldera, except for some (e.g., Hunter, 1988; Furukawa et al, 2008; Miyoshi et al., 2011; Shinmura et al., 2013). It is highest in the pre-Aso, becomes lower in the caldera-forming stage, and rises again in the post-caldera stage. In this study, we measured the whole-rock chemical compositions and Sr isotopic ratios of six volcanic rocks from the Pre-Aso volcanic rocks before and after the Aso-1 eruption, and tried to clarify the changes in magma composition at the beginning of the entire Aso caldera eruption.
We collected three samples of pre-Aso volcanic rocks (Watanabe et al., 2021) from a stream on the approach to the Iwato Shrine in the Tateno Valley, two samples of Koga lava (Tajima et al., 2017) with an age of 275 ± 11 ka from a part of the east wall of the caldera, and one sample of Hokamaki lava (Aso-1/2 lava, Watanabe et al., 2021) from near the Seta Shrine in the Tateno Valley. All rocks are andesite. The chemical composition of the whole rock was determined by an X-ray fluorescence spectrometer installed at the Kitakyushu Museum of Natural History & Human History, and the Sr isotope ratio was determined by a thermal ionization mass spectrometer (TIMS) (Finnigan MAT262) at Tsukuba University and a TIMS (Thermo Fischer Scientific TRITON Plus TIMS) at Kumamoto University. The major elemental compositions of SiO2 wt.%, K2O wt.%, and Sr isotopic ratios (87Sr/86Sr) were 57-61, 2.3-3.0, 0.70422-0.70424 for the Pre-Aso volcanic rocks, 55-58, 1.8-2.7, 0.70405-0.70421 for the Koga lava, and 60, 2.9, 0.70399 for the Hokamaki lava, respectively.
The K2O content in Koga and Hokamaki lava tends to be high relative to the SiO2 content as in the post-caldera volcanic products. The Sr isotopic ratio of the Koga lava is almost equal to the upper limit of the Aso-1 pyroclastic flow deposits, and that of the Hokamaki lava is around the lower limit of the volcanic products from the pre-Aso volcanic rocks to the post-caldera stage. In addition, the amount of K2O to SiO2 is high in all three samples, which is not a characteristic of general pre-Aso volcanic rocks, but a characteristic of the post-caldera period. The Sr isotopic ratios of all three samples are slightly higher than the upper limit of the Aso-1 pyroclastic flow deposits, and much lower than the upper limit of 0.70446 for the entire Pre-Aso volcanic rocks, which is consistent with the post-caldera period characteristics. The age of the pre-Aso volcanic rocks in this study from the Tateno Valley, western part of Aso caldera, has not been measured, but in the area where these rocks are distributed, the pre-Aso volcanic rocks are in contact with Aso-1 and Aso-2 (Watanabe et al., 2021). Therefore, it is thought that these rocks are relatively young among the pre-Aso volcanic rocks and were active at a time close to that of Aso-1, and magma with similar properties existed underground in the western part of the caldera before the Aso-1 eruption, and that there was activity to erupt these magma before Aso-1 like Koga lava in the eastern part of caldera.