Dacite-basalt magma interaction at Yakedake volcano, central Japan

The 2300 years B.P. eruption of the Yakedake volcano in central Japan consisted primarily of lava extrusion and dome growth in the summit area, and a repetitive gravitational collapse of the dome produced a series of block-and-ash flows known as the Nakao pyroclastic flow deposit (NPFD). Based on the geochemistry and mineralogy, the juvenile materials in the NPFD can be assigned to five groups: light-colored, porphyritic dacite (white dacite) showing little or no petrographic evidence of magma mixing, dark-colored hybrid andesite (black andesite) with disequilibrium phenocryst assemblages and textures, banded lava with streaks of white dacite and hybrid andesite, basaltic andesitic enclaves (hybrid enclave) having the same disequilibrium phenocryst assemblage as the hybrid andesite, and basaltic enclave (primitive enclave) lacking any evidence of magma mixing. Compositional data from the phenocrysts and whole rocks demonstrate that the juvenile materials of the NPFD preserve a magma mixing/mingling event between a basaltic magma (49.5 wt% SiO2, T ∼ 1075 °C from olivine-melt geothermometry), which is compositionally similar to the primitive enclave, and a dacitic magma (64.6 wt% SiO2, T = 790-800 °C from Fe-Ti oxide geothermometry), which is compositionally similar to the white dacite. The NPFD eruption was caused by an invasion of the basaltic magma into the preexisting, highly crystalline dacitic magma chamber. The composition of the dacitic magma remained constant throughout the eruption. However, heating by the basalt magma increased its temperature locally up to T ∼ 950 °C. Minor disruption and consecutive quenching of the replenishing basaltic magma may have formed the primitive enclave. Simultaneously, the replenishing basaltic magma entrained small amounts of the dacitic magma, producing a hybrid basaltic andesitic magma layer at the base of the chamber. The quenched part of the hybrid layer was preserved as the hybrid enclave. Finally, the simultaneous ascent of the dacitic magma and liquid interior of the hybrid layer through a common conduit promoted the mixing/mingling of the two magmas and caused the coeruption of diverse lava types.