[Title]
Timing of vesiculation and crystallization during magma ascent
-Example of the phreatomagmatic phase in Usu 2000 eruption-

[Authors]
Yuki SUZUKI＊, and Setsuya NAKADA＊

[Abstract]
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.

Key word：magma ascent, phreatomagmatic eruption, vesiculation, microlite, volatile exsolution
__________
＊ Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyou-ku,
Tokyo, 113-0032, Japan