苦鉄質マグマの温度が減圧結晶作用と噴火ダイナミクスに及ぼす影響：伊豆大島1986年噴火の玄武岩質安山岩メルトの例

抄録

Numerical simulations of isothermal decompression-driven crystallization were performed for the basaltic andesite melt of the 1986 eruption at Izu-Oshima volcano (SiO₂ = 54.4 wt.%) by “rhyolite-MELTS” program under the conditions of temperatures of 1132–1079°C, initial melt H₂O contents of 1–4 wt.%, initial pressure of 200 MPa and fO₂ at Ni-NiO buffer, respectively. The starting melt composition is similar to a groundmass glass composition of microlite-poor strombolian scoria from the A vent and also a whole rock composition of phenocryst-poor sub-plinian scoria from the B vents of the 1986 eruption. The results show that starting pressure of crystallization, and increasing rate and final value of crystallinity increase as temperature decreases. In addition, assemblage, abundance and order of crystallization of mineral phases change with temperature. The temperature-dependent changes of crystallization behaviors induce change of melt SiO₂ content-crystallinity-pressure paths. As a result, increasing rates and final values of both melt viscosity and relative viscosity, and hence those of bulk viscosity of magma increase as temperature decreases. Increases of crystallinity and magma viscosity inhibit outgassing and also facilitate magma fragmentation during magma ascent, resulting in violently explosive eruption. Present results suggest that temperature difference between magmas from the A and B vents is a key factor to induce the contrasting eruption styles observed during the 1986 eruption at Izu-Oshima volcano.