マグマ上昇過程における発泡と結晶作用のタイミングー有珠山2000年噴火,マグマ水蒸気爆発の例ー

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A phreatomagmatic phase at Usu volcano, Hokkaido, on 31 March, 2000, issued essential dacite pumice and micropumice in ash. The magma ascent process of the eruption was investigated using these pumiceous samples. Compositions of individual phenocryst (magnetite, plagioclase, orthopyroxene) rims and groundmass (microlite plus glass, 75.0 wt.% SiO2) are uniform among the pumiceous fragments, showing chemically homogeneous melt before eruption. The water content in the melt before eruption is estimated to be around 5 wt.%, based on both indirectly estimated water content of glass inclusions in phenocrysts and the phase relationship. This means that the magma had been saturated with water at around 2kbar. The essential fragments vary in vesicularity (15-80 vol.%). The H2O content in groundmass glass decreases with increasing vesicularity. These facts suggest that variable vesicularity resulted from different quenching depths of the dacite magma in the aquifer, heterogeneity of H2O exsolution in dacite magma, or different response of H2O exsolution due to a changing ascent rate. The water content of groundmass glass corresponds to the solubility at lower than 0.3 kbar. However, the latter pressure is higher than the pressure under which the most plausible aquifer lies. Fragmentation of pumices into small pieces may have been caused by water quenching, however, highly vesiculated pumice of larger dimensions may have been formed without any interaction with water. Microlites in groundmass have a higher number density than phenocrysts, indicating that the former nucleated under high degree of supercooling during eruption. Chemical compositions of individual microlite species are homogeneous among essential fragments. This indicates that microlites had been crystallized under a similar physical condition and independently from quenching in the aquifer. Low water content in groundmass glass suggests the possibility of enough decompression-induced undercooling for the crystallization until quenching. However, constant microlite crystallinity among essential materials with various vesicularity can be explained by the completion of crystallization before the final decompression.

マグマ上昇過程における発泡と結晶作用のタイミングー有珠山2000年噴火,マグマ水蒸気爆発の例ー

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