Japan Geoscience Union Meeting 2022

Presentation information

[J] Poster

S (Solid Earth Sciences ) » S-VC Volcanology

[S-VC29] Volcanic and igneous activities, and these long-term forecasting

Tue. May 31, 2022 11:00 AM - 1:00 PM Online Poster Zoom Room (25) (Ch.25)

convener:Takeshi Hasegawa(Department of Earth Sciences, College of Science, Ibaraki University), convener:Shimpei Uesawa(Central Research Institute of Electric Power Industry), Teruki Oikawa(GSJ, National Institute of Advanced Industrial Science and Technology ), convener:Koji Kiyosugi(Organization of Advanced Science and Technology, Kobe University), Chairperson:Shimpei Uesawa(Central Research Institute of Electric Power Industry)

11:00 AM - 1:00 PM

[SVC29-P08] Petrological study of Aso-2/1 andesitic rocks.

*Satoko Ishimaru1, Keiya Ishibashi2, Nobutatsu Mochizuki1 (1.Department of Earth and Environmental Science, Faculty of Advanced Science and Technology, Kumamoto University, 2.Department of Earth and Environmental Science, Faculty of Science, Kumamoto University)

Keywords:Aso-2/1 lava, andesite, phenocryst content, disequilibrium

We described petrological and geochemical features of andesite lava (Aso-2/1 lava) between Aso-2 and Aso-1 pyroclastic flow deposits: Hokamaki lava, Matoishi lava, Setaura lava and Akita lava. All of them contain plagioclase, clinopyroxene, orthopyroxene and titanomagnetite, and subordinate olivine only in Matoishi lava. Abundant dusty plagioclase phenocrysts are observed in Hokamaki and Matoishi lavas irrespective of the phenocryst size. Mg#s [= Mg/(Mg + Fe) cation ratio] of clinopyroxene and orthopyroxene in each lava is almost equal (ca. 0.70 and ca. 0.66, respectively) even though the whole-rock Mg# is variable: ca. 0.48 for low SiO2(ca. 55 wt.%: Hokamaki and Matoishi) lavas and 0.28-0.32 for high SiO2 (ca. 62 wt.%: Setaura and Akita) lavas. The disequilibria between mineral and whole-rock compositions indicates xenocrystic origin of minerals and/or magma mixing. Field observation and paleomagnetic data in addition to K-Ar ages indicate that the timing of high-SiO2 lava (Akita and Setaura) and low-SiO2 lava (Hokamaki and Matoishi) eruption was several hundred years before and several thousand years before Aso-2 event, respectively.
We discuss the possible mechanism(s) which made two types of Aso-2/1 andesitic magmas.