Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Oral

S (Solid Earth Sciences) » S-VC Volcanology

[S-VC43] Volcanic and igneous activities, and these long-term forecasting

Sun. May 20, 2018 10:45 AM - 12:15 PM A05 (Tokyo Bay Makuhari Hall)

convener:Teruki Oikawa(GSJ, National Institute of Advanced Industrial Science and Technology), Takeshi Hasegawa(Department of Earth Sciences, College of Science, Ibaraki University), Daisuke MIURA(一般財団法人 電力中央研究所 地球工学研究所 地圏科学領域, 共同), Nobuo Geshi(Geological Survey of Japan, The National Institute of Advanced Industrial Science and Technology), Chairperson:Hasegawa Takeshi(Department of Earth Sciences, College of Science, Ibaraki University), Maeno Fukashi

11:15 AM - 11:30 AM

[SVC43-07] Contributions of crust-derived mafic/intermediate and felsic magmas to the Aira catastrophic caldera-forming eruption inferred from geochemical characteristics of essential clasts

*Ayumu Nishihara1, Yoshiyuki Tatsumi2,1, Keiko Suzuki-Kamata2,1, Katsuya Kaneko1, Jun-Ichi Kimura3, Qing Chang3, Hironobu Hinata4 (1.Department of Planetology, Graduate School of Science , Kobe University, 2.Kobe Ocean-Bottom Exploration Center, 3.Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology, 4.Kobe Municipal Tarumi Junior High School)

Keywords:Aira caldera, caldera-forming eruption, Sr isotope ratio, magma mixing

In order to understand the origin of voluminous felsic magmas causing catastrophic caldera-forming (CCF) eruptions, we examined geochemical and petrographical characteristics of essential clasts with variable compositions contained in the Ito ignimbrite erupted at ~30 ka from Aira CCF eruption. The composition of the cores of plagioclase phenocrysts in both white and dark-colored rhyolitic pumice clasts showed a bimodal distribution with peaks at ~An85 and ~An40, while those within andesitic scoria clasts exhibited a unimodal distribution peaked at ~An80. Strontium isotopic composition of the high-An70-90 and low-An30-50 plagioclase cores were 87Sr/86Sr = 0.7068±0.0008 and 0.7059±0.0002, respectively. These observations may lead to a conclusion that the voluminous rhyolitic magma that caused Aira CCF eruption formed by mixing of the andesite and the felsic magmas which crystallized high- and low-An plagioclase phenocrysts, respectively. The andesite magma that may be derived from a less differentiated, possibly mafic magma shows lines of evidence for assimilation of the upper crustal basement rocks such as the Miocene granitic intrusions (87Sr/86Sr = 0.7119-0.7245) and/or sedimentary rocks of the Shimanto Group (87Sr/86Sr = ~0.7145). This suggests mixing of the andesite and the felsic magmas occurred at a shallow upper crustal level. The rhyolite magma has far low 87Sr/86Sr = ~0.7059 than that of the basement rocks whereas has a similar Sr isotope composition with the least assimilated andesite magma. Both the felsic and mafic magmas could have been derived from a same source rock, possibly the lower crust material beneath Aira caldera.