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浅間火山2004年噴火噴出物の鉱物粒径分布とマグマの結晶化過程

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(3)       . . ,**/  .  +.  ,**/  +,  +- . Crystal Size Distribution and Crystallization Process of the Ejecta from the September ,**. Eruptions of the Asama Volcano, Central Japan Tatsuro TSUGANE῍, Kuniaki MAKINO῍῍, Yasuyuki MIYAKE῍῍ and Kou TAKAHASHI῍ The September ,**. eruptions of Asama volcano, central Japan, ejected essential materials such as pumice with bread crust on September +st and scoria on September ,-rd. The textural and chemical analyses on the materials reveal the crystallization processes in a deep magma chamber and a shallow vent. Two distinct stages of crystallization can be recognized in size distributions and morphology of plagioclase phenocryst and microlite both in the pumice and scoria. First stage (rangeῌ) : In a deep magma chamber, pyroxene phenocryst began to crystallize out at ++/*, and then pyroxene and plagioclase continued to nucleate and grow slowly. Second stage (range῍) is divided into two sub-stages for pyroxene or three (range῍a-c) for plagioclase. ῍a : Magma left the chamber and rose slowly through the vent with ever increasing nucleation rate. ῍b-c : In a shallow vent beneath the crater, numerous plagioclase microlites like swallow-tailed shape precipitated rapidly under a high undercooling condition induced by decompression. Plagioclase microlite in the pumice and scoria developed a characteristic population density like a bell with a peak at the grain size of *.**- mm, which is interpreted to reflect a decrease in nucleation rate of plagioclase, in response to crystallization and establishment of equilibrium during the time duration when the magma stayed in the vent. Magmatic temperatures estimated from plagioclase-glass equilibrium decreased to 2/* before the September +st eruption. The similarity in crystal size distribution of the pumice and scoria implies that they had a common ascent history, although groundmass in the scoria has lower crystallinity than that in the pumice, suggesting that the magma of the Sept. ,-rd eruption stood lower in the magma column than the Sept. +st magma. Key words : Asama volcano, crystal size distribution, microlite, pumice, scoria. +ῌ ῐ ῎ ῑ ῏ ,**.  3  + . ^23 !4567(8 90*0 HI%TdOPe !"#$%&'()*!+,%. f05g7(hijk4lmnop@; TdOP4. - +, ., . /0123 !4567(8 9:;. qrqrse7 tuPv5g7(ijkP$Lwva. !,% <=:>!?@; . km AB!+CDE. nopvMR( xy z ,**.8 .( +0 {|%. 0. FAG0 HI4JK7(8 3  + 0123. !,0LM. !?}~O€4Q;‚:(9v4ƒU„:_ HI%N+OP,. † V0w‡% +3* ˆ m- v\‡>;:_<n 1‰. QR(4 ST0 H>U';:(8 V0W!+"#%X. z ,**/8 V0W 3  ,-  ,3 123 !45. YMR(Z[\](4 3  +. ^23M !4. 67(8 Š !,0LM HI%N+OP,QR(4. - /567_AJ +2 .,0* ,,*** /$`ab c. k‹Œ0 H>Ž‚:(8. -3*ῌ20,+ ‘’“ -ῌ+ῌ+ ” •–•–—˜–™š›œžŸk ¡œ–¢£ Division of Environmental System Science, Graduate School of Science and Technology, Shinshu University, Asahi -ῌ+ῌ+, Matsumoto -3*ῌ20,+, Japan. -3*ῌ20,+ ‘’“ -ῌ+ῌ+. ” •––¤dœ–œ Department of Geology, Faculty of Science, Shinshu University, Asahi -ῌ+ῌ+, Matsumoto -3*ῌ20,+, Japan. Corresponding author : Tatsuro Tsugane e-mail : ttsugane@yahoo.co.jp.

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(237) :9!>FG !. 60.     Armienti, P., Paresci, M. T., Innocenti, F. and Pompilio, M. (+33.) E#ects of magma storage and ascent on the kinetics of crystal growth ; the case of the +33+ῌ3- Mt. Etna eruption. Contrib. Mineral. Petrol., ++/, .*,ῌ.+.. Bates, R. L. and J. A. Jackson (+321) Glossary of geology, -rd Edition. American Geological Institute. Cashman, K. V. (+322) Crystallization of Mount St. Helens dacite : a quantitative textural approach. Bull. Volcanol., /*, +3.ῌ,*3. Cashman, K. V. and Marsh, B. D. (+322) Crystal size distribution (CSD) in rocks and the kinetics dynamics of crystallization : ,.Makaopuhi lava lake. Contrib. Mineral. Petrol., 33, ,3,ῌ-*/. Castro, J. M., Cashman, K. V. and Manga, M. (,**-) A technique for measuring -D crystal-size distributions of prismatic microlites in obsidian. Amer. Mineral., 22, +,-*ῌ +,.*. Couch, S., Sparks, R. S. J. and Carroll, M. R. (,**-). The kinetics of degassing-induced crystallization at Soufrie ◊re Hills Volcano, Montserrat. J. Petrol., .., +.11ῌ+/*,. Frost, B. R., Lindsley, D. H. and Andersen, D. J. (+322) Fe-Ti oxide-silicate equilibria ; assemblages with fayalitic olivine. Amer. Mineral., 1-, 1,1ῌ1.*. Hammer, J. E. and Rutherford, M. J. (,**,) An experimental study of the kinetics of decompression-induced crystallization in silicic melt. J. Geophys. Res., +*1(B+), ,*,+, doi : +*. +*,3/,**+JB***,2+. Higgins, M. D. (+330) Magma dynamics beneath Kameni volcano, Greece, as revealed by crystal size and shape measurements. J. Volcanol. Geotherm.Res., 1*, -1ῌ.2. Higgins, M. D. and Roberge, J. (,**-) Crystal size distribution (CSD) of plagioclase and amphibole from Soufriere Hills volcano, Montserrat : evidence for dynamic crystallisation/textural coarsening cycles. J. Petrol., .., +.*+ῌ+.++.  

(238)                

(239) ! "#$ % (,**.) &'() ,**.  3 *(+,+ -,+./01 ()23 ,**. 45"3 678 +21 p. Kirkpatrick, R. J. (+311) Nucleation and growth of plagioclase, Makaopuhi and Alae lava lakes, Kilauea Volcano, Hawaii. Geol. Soc. Am. Bull., 22, 12ῌ2.. Larsen, J. F. (,**/) Experimental study of plagioclase rim growth around anorthite seed crystals in rhyodacitic melt. Amer. Mineral., 3*, .+1ῌ.,1. Lindsley, D. H. (+32-) Pyroxene thermometry. Amer. Mineral., 02, .11ῌ.3-. Lofgren, G. (+31.) An experimental study of plagioclase crystal morphology : isothermal crystallization. Amer. J. Sci., ,1., ,.-ῌ,1-. Maalῌe, S., Tumyr, O. and James, D. (+323) Population density and zoning of olivine phenocrysts in tholeiites. from Kauai Hawaii. Contrib. Mineral. Petrol., +*+, +10ῌ +20. 9:;<=!> ? @ : ABC &'() ,**.  3 *+,+DE1 ( )1 Mangan, M. T. (+33*) Crystal size distribution systematics and the determination of magma storage times : the +3/3 eruption of Kilauea volcano, Hawaii. J. Volcanol. Geotherm. Res., .., ,3/ῌ-*,. Marsh, B. D. (+322) Crystal size distribution (CSD) in rocks and the kinetics dynamics of crystallization : +. Theory. Contrib. Mineral. Petrol., 33, ,11ῌ,3+. !>FG !:9HI! J"#K$LMN9"2&'() *. (% OPQRS (,**/) &'() ,**.  3 *(+T ,UVWX1 ()8 /*8 ---ῌ-.0. $&Y (+33,) '

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Fig. 0 . Frequency in crystallization temperatures of clinopyroxene and plagioclase coexisting with melt in the pumice and the scoria
Fig. 1 . Pyroxene (Px) enclosed in plagioclase (Pl) phenocryst observed in the pumice under an optical microscope.
Fig. 3 . Schematic illustration of magma ascent history.

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