Magma generation process beneath volcanic front of Kyushu arc, southwest Japan

Abstract:

 We presents data for major and volatile (H₂O, CO₂, S, Cl) elements in olivine-hosted melt inclusions from Quaternary volcanoes (Aso, Kuju, Kirishima and Kaimon) along volcanic front of Kyushu arc, southwest Japan. Melt inclusion data are corrected for post-entrapment modifications including diffusive Fe-loss, H₂O loss and post-entrapment crystallization. The primitive magma compositions calculated from corrected melt inclusion data are used to estimate the degree of partial melting and compositions of slab-derived fluids beneath Kyushu volcanic front. The result show that magmatism of four volcanoes in Kyushu arc is divided into two groups (Group A and B). Group A indicates high K₂O contents in primitive magmas and in fluids at Aso and Kuju volcanoes, northern Kyushu arc. Group B indicates low K₂O contents in primitive magmas and in fluids at Kirishima and Kaimon volcanoes, southern Kyushu arc.

K₂O contents of Group A and B are impossible of explaining by degree of partial melting and the origin of hydrous mantle. High K₂O content in fluids is attributed to dehydration of phengite-bearing slab at deep depth (about 140 km) in Group A compared with shallow depth (about 100 km) in Group B. Phengite is dehydrated from submerging plate beneath 110 km depth. This study suggests that compositions of primitive magmas beneath Kyushu arc are reflected by the depth of slab and the kind of dehydrating hydrous minerals.