S1: Dynamics of igneous processes (Special Session)

[S1P-01] Injection of K₂O-rich magma into magma chambers beneath Myoko volcano

*Morihisa HAMADA¹, Estelle F. ROSE-KOGA², Kenneth T. KOGA², Kenji SHIMIZU¹, Takayuki USHIKUBO¹, Hideo HARADA³, Andereas AUER⁴, Yoshiaki YAMAGUCHI³ (1. JAMSTEC, 2. ISTO, CNRS-Universite d'Orleans, 3. Shinshu Univ., 4. Shimane Univ.)

Keywords:Myoko volcano, melt inclusion

Heterogeneous groundmass bearing higher-K₂O and lower-K₂O domains is often observed in volcanic rocks collected from the rear-arc region. This study characterizes concurrent occurrence of both higher-K₂O and lower-K₂O magmas preserved in quenched melt inclusions collected from Myoko volcano, a rear-arc volcano on the western margin of the northeast Japan arc. While the melt inclusions found in the scoriae from the Sekiyama eruption of Myoko volcano (43 ka) represent the whole-rock composition of Myoko’s lower-K₂O volcanic rocks, some melt inclusions found in the scoriae of the Kannoki eruption (41 ka) represent higher-K₂O domains in the groundmass. A possible explanation for such contrastive observations is that the mantle source beneath Myoko volcano is heterogeneous; the higher-K₂O magmas could be generated in a metasomatized mantle wedge, possibly veined by phlogopite and/or apatite-bearing dykes. Another possible explanation is that higher-K₂O magma is a product of partial melting of crustal rocks and its mingling with lower-K₂O magma before the Kannoki eruption. In either case, K₂O-rich magma batches were repeatedly injected into the main magma chambers and formed heterogeneous groundmass bearing higher-K₂O and lower-K₂O domains. Further geochemical studies are warranted to constrain the origin of K₂O-rich magma.