Numerical simulation of isothermal decompression-driven crystallization was performed for the basaltic melt of the 1707 eruption at Fuji volcano (SiO
2 ~ 51.5 wt.%) by using “rhyolite-MELTS” program under conditions of temperature of 1184–1094°C, initial melt H
2O content of 0.5–3.0 wt.%, initial pressure of 150 MPa and redox state of Ni-NiO oxygen buffer, respectively. During decompression, most abundantly crystallized phase is plagioclase and amounts of crystallized olivine, pyroxenes and magnetite depend on temperature. As initial temperature decreases from 1184°C to 1094°C, the onset pressures of degassing and crystallization (P
sat)increase from ~3 MPa to ~90 MPa and from ~2 MPa to ~40 MPa, respectively, and the final weight fraction of melt at 0.1 MPa (F
1atm) decreases from 0.93 to 0.4. The relation between melt fraction and pressure is chiefly controlled by P
sat and F
1atm, and both P
sat and F
1atm depend on temperature alone. This indicates that decompression-driven crystallization is essentially controlled by temperature of melt. During decompression, residual melt composition is not changed significantly at temperature higher than ~1120°C whereas melt SiO
2 contents increase up to ~55 wt.% at 1114°C and ~60 wt.% at 1094°C, respectively, chiefly due to magnetite crystallization. Changes in magma viscosities (η
magma) during decompression are estimated from melt composition, H
2O content, temperatue, and crystal volume fraction; it increases as pressure decreases at all temperature, but –dη
magma /dP drastically increases as temperature decreases. Rapid increases of η
magma and crystal volume fraction during decompression facilitate viscous fragmentation and preclude outgassing, resulting in more intense explosive eruption of H
2O-rich low-T basaltic melt. Present results suggest that preeruptive temperature may be an essential factor to control eruption dynamics of basaltic magma at Fuji volcano.
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