[S-VC34] Volcanic and igneous activities, and these long-term forecasting

[SVC34-05] Mingling of medium-K magma and high-K to alkalic magma beneath Myoko volcano in central Japan: Insights from analysis of melt inclusions

*Morihisa Hamada¹, Estelle F. Rose-Koga², Kenneth T. Koga², Kenji Shimizu¹, Takayuki Ushikubo¹, Hideo Harada³, Andreas Auer⁴, Yoshiaki Yamaguchi³ (1.Japan Agency for Marine-Earth Science and Technology, 2.Institut des Sciences de la Terre d’Orléans, CNRS – UO-OSUC, 3.Shinshu University, 4.Shimane University)

Keywords:Myoko volcano, Melt inclusion, High-K2O magma, Magma mingling

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.