[S-VC34] 火山・火成活動および長期予測

[SVC34-05] 妙高火山のマグマ溜まりへのK₂Oに富むマグマの注入

*浜田 盛久¹、Rose-Koga Estelle²、Koga Kenneth²、清水 健二¹、牛久保 孝行¹、原田 英男³、Auer Andreas⁴、山口 佳昭³ (1.国立研究開発法人海洋研究開発機構、2.Institut des Sciences de la Terre d’Orléans, CNRS – UO-OSUC、3.信州大学、4.島根大学)

キーワード：妙高火山、メルト包有物、高K2Oマグマ、マグマ混交

Volcanic products of the rear-arc volcanoes are generally affected by crustal contamination effectively obscuring the origin of the magma. The investigation of melt inclusions is a useful approach to decipher magmatic processes beneath rear-arc volcanoes. In this study, we analyzed melt inclusions hosted by olivine, pyroxenes and plagioclase from Myoko volcano, a rear-arc volcano in central Japan, and developed hypotheses on the observed mixing of magmas variably enriched in K₂O. We focused on the Sekiyama eruption at 43 ka and the Kannoki eruption at 41 ka, which are two temporally close eruptions at the beginning of the present-day Stage IV volcanism of Myoko volcano. Whereas the melt inclusion compositions in the scoriae from the Sekiyama eruption (43 ka) overlap with the whole-rock compositions of Myoko’s medium-K₂O volcanic rocks, the melt inclusions from the subsequent Kannoki eruption (41 ka) exhibit both medium-K₂O and a cluster of high-K₂O to alkalic compositions. The F and Cl concentrations in the calculated primary melt compositions suggest that the metasomatized mantle sources responsible for both eruptions are different. We also discussed two possible explanations for the presence of high-K₂O to alkalic melts in the Kannoki scoriae. The first hypothesis is that the mantle wedge beneath Myoko volcano is heterogeneous; the high-K₂O to alkalic magma could be generated in a strongly metasomatized mantle source and mingled with ordinary medium-K₂O magma generated in a weakly metasomatized mantle source. The second hypothesis is that high-K₂O to alkalic magma is a product of the partial melting of crustal rocks and their mingling with ordinary medium-K₂O magma. In either case, magma mingling occurred before the Kannoki eruption and formed chemically heterogeneous groundmass and melt inclusions. Further geochemical studies are warranted to constrain the origin of such high-K₂O to alkalic magmas, which are commomnly observed in volcanic rocks from the rear-arc region.