富士火山,箱荒沢坑で発見された白色火山灰層の起源

抄録

Tsuya (1962) described a white pumiceous ash layer in the Hakoarezawa tunnel located at the southwest flank of Mt. Fuji, and ascribed it to an acidic magma activity of the older Fuji Volcano. We found a thin-section labeled “Hakoaresawa pumice lens 1,960m” in rock-samples left by the late Professor Hiromichi Tsuya, and analyzed it by electron microprobe. The white pumice is composed largely of flakes of colorless glass together with a few fragmented plagioclase micro-phenocrysts coated by the glass and free crystals of orthopyroxene. Chemical composition of the glass (SiO₂=78.1, Al₂O₃=12.2, CaO=1.1, Na₂O=3.7, K₂O=3.3(wt%)) is identical to that of Aira-Tn (AT) ash, a widespread volcanic deposit erupted from Aira caldera about 26,000 years ago. Composition of the plagioclase is also similar to that in AT ash. Considering some other similarities between the white pumice and AT ash such as their refractive indexes of glass, we conclude that the white pumice is AT ash, and not a product of the older Fuji volcano.