Gabbroic Inclusions in the Yakeno Lava Flow, Fuji Volcano: Their Origin and Implications to the Magma Plumbing System beneath the Volcano

Abstract

Gabbroic inclusions newly found in the Yakeno lava flow, erupted about 1300 years ago on the northwestern flank of Fuji volcano, are divided into two groups; one consists of troctolite and the other consists of olivine gabbronorite and gabbronorite. Troctolite fragments are considered to originate from shallow dykes because of their mineral composition and high porosity. On the other hand, estimated equilibrium conditions of gabbronorite and olivine gabbronorite are 1020-1050°C and 250-380MPa, showing their deep origin; probably they had been caught in the Yakeno magma (i.e. the magma erupted as the Yakeno lava flow) at differentiated small magma bodies just above a substantial basaltic magma chamber of Fuji volcano. Pre-eruptive temperature and water content of the Yakeno magma are evaluated by olivine-liquid thermometer and plagioclase-liquid hygrometer to be 1090°C and 2.8wt%, respectively. So the viscosity of the Yakeno magma is calculated to be several tens Pascal second. Therefore the minimum ascent velocity required to lift the gabbroic fragments is ca.10m/h, implying that the Yakeno lava flow had ascent without significant pause en route to the surface.