Experimental Constraints on Magma Ascent During the Usu 2000 Eruption, Japan

The rate of magma ascent is important in determining eruption mechanisms. One way to estimate such rates is to reproduce volcanic pumice textures by decompression experiments, although such studies are few. Such experiments are under progress on the phreatomagmatic eruption of Usu volcano on 31 March 2000. Geophysical observations of this eruption revealed that the magma reached below West Nishiyama (0.5-2 km deep), the western foot of Usu volcano, after rising from depth beneath the summit without a change in ascent speed. A previous study (Reference) gave constraints on the timing of vesiculation, crystallization, and gas separation during ascent. The erupted magma was compositionally homogeneous. Variable vesicularity resulted from different times of quenching. Micropumices were quenched in various depths by aquifer (at 200m depth for deepest one), whereas pumices were not quenched. All juvenile material have similar groundmass microlite contents and types. Number density of bubbles in juvenile fragments first increases with vesicularity, and then decreases. Changes in the bubble size distribution may indicate variable nucleation events or rates. We are carrying out isobaric and decompression experiments in order to constrain conditions of the Usu eruption. Isobaric experiments constrain the pre-eruptive conditions of the magma. It is estimated that water pressure was less than 150 MPa at 900C and less than 200 MPa at 850C, based on the criterion that all phenocryst phases (Pl, Opx, Mt) are stable. Once pre-eruptive conditions are verified, decompression experiments will be carried out to replicate the bubble and microlite textures as a function of ascent rate. Furthermore, this study will evaluate the effect of crystals on bubble nucleation in order to explain the changing manner of bubble nucleation. (Reference) Suzuki, Y. and Nakada, S., 2002, Bull. Volcanol. Soc. Japan, in press.