ERUPTION MECHANISMS AND ROCK TEXTURES

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(1) High pressure and high temperature experimental studies :Phase stability experiments on Unzen dacite showed that the pressure of groundmass pargasite crystallization requires more than 60-100 MPa at 880 C, suggesting they crystallized at depth of more than 3 km. Chlorine content of the groundmass pargasite indicates that degassing of 2% of water from the magma had occurred before the groundmass crystallization, indicating the initial water content of the dacite is ca. 6 wt.%.Preliminary vesiculation experiment through depressurization of hydrous melts were conducted, and compared with natural pumiceous ejecta of Kikai-Akahoya plinian erutpion, 6500 yr. b.p., in terms of vesicle size distribution and number density of vesicles in a unit mass of melt.Another suite of experiments were conducted on the 1707A.D.basalt of Fuji volcano, to determine the Ca/(Ca+Na) ratio of liquidus plagioclase as a function of water content of the melt, yielding an equation An=6* (H2Owt%)+67 at 190 MPa. Using this relationship, we successfully obtained the degassing/mixing history of magmas from the compositional zoning of plagioclase phenocyrsts in the 1707 as well as 864 A.D.basalts.(2) Analyses of natural rock texturesCompositional zonings of phenocrysts (plagioclase and homblende) in the Unzen 1991-1995 dacite showed both evidences of magma mixing at the rim of phenocrysts and of volatile input/degassing of magmas in the core of phenocrysts. Latter point is deduced from the minor element distributions (Fe, Mg, K in plagioclase and Cl in homblende), and provide new evidence concerning the behavior of volatiles in the magma chamber.(3) numerical modeling.We calculated the flow rate of the 1991 dome eruption of Unzen volcano usuing Hagen-Poiseuille law. Strong dependence of the viscosity of magma on the water content results in much faster uprise velocity of the dacite magma at depth more than 700 meters.