Magma plumbing system and eruption mechanism in recent eruptions of Haruna volcano, central Japan

In last tens of thousands years, volcanic activity in Haruna produced at least five lava domes along with pyroclastics deposits (Oshima, 1986). For future eruptions, author revealed eruption trigger and magma plumbing system petrologically, focusing on a latest eruption in the middle of 6th century. The eruption started with Plinian phase (1.2 km3; Arai, 1993), followed by pyroclastic flow, and ended with lava dome emplacement. White pumices are found in all units of Plinian and flow phases, while gray and banded pumices are limited to the first Plinian phase. Involved magmas are felsic magma (60 wt% in SiO2; with phenocrysts of orthopyroxene, amphibole, Fe-Ti oxides and plagioclase; ca. 850C), aphyric mafic magma (52wt% in SiO2; ca. 1130C) and phyric mafic magma (with olivine and clinopyroxene; ca. 1130C). Mixing between the felsic and mafic magmas formed gray part of pumice and dome lava. The mafic endmember is aphyric in the pumice and is phyric in lava. The felsic magma appeared without mixing in middle of the eruption, but that in the first Plinian phase has heating record (-880C). Olivine (-100micrometer) in gray part of pumice is from the aphyric mafic magma. Their cores have similar compositions as olivine phenocryst rims in lava, indicating that aphyric mafic magma had the same composition as melt part of the phyric mafic magma and mafic magmas are from a storage system. Melts program yields more than 4kbar for olivine and clinopyroxene to be the first liquidus phases in melt with aphyric magma composition. On the other hand, storage pressure of felsic magma can be ca. 4 kbar. Thus, the felsic reservoir locates over shallower depth and is separated from the mafic reservoir. Separation of melt part from a phyric mafic magma ascending toward felsic reservoir may have caused earlier supply of the aphyric mafic magma. The felsic magma was nearly rigid due to high phenocryst abundance (ca. 50 vol%). So, the vent opening by the mixed and heated magmas with lower viscosity made the felsic magma erupt.