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

[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.