Crystal Size Distribution and Crystallization Process of the Ejecta from the September,**. Eruptions of the Asama Volcano, Central Japan

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1 /+ (,**0) +.30+,**.,**/. +.,**/ +, +- Crystal Size Distribution and Crystallization Process of the Ejecta from the September,**. Eruptions of the Asama Volcano, Central Japan Tatsuro TSUGANE, Kuniaki MAKINO, Yasuyuki MIYAKEand 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 (range): 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 (rangea-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 +,** ,.. km ,,*** -3*20,+ -++ Division of Environmental System Science, Graduate School of Science and Technology, Shinshu University, Asahi -++, Matsumoto -3*20,+, Japan. -3*20,+ -++,** * m -,**/ 3,-,3 Department of Geology, Faculty of Science, Shinshu University, Asahi -++, Matsumoto -3*20,+, Japan. Corresponding author: Tatsuro Tsugane ttsugane@yahoo.co.jp

2 50 3 +,- - Cashman and Marsh (+322) CSD (crystal size distribution) (L) (n) (Marsh, +322) Armienti et al., +33. ; Cashman, +322 ; Castro et al.,,**- ; Higgins, +330 ; Higgins and Roberge,,**- ; Maale et al., +323 ; Mangan, +33* ; +33, Hammer and Rutherford (,**,), Couch et al. (,**-) CSD,,+,**. 3 +,-,**/ 23 (,**/) 3 + /+* mm 3 + 3, ,- Fe Mg SiO, Fe, O - wt , , ,- (,**/),, a b Fig. + *.*,*.*. mm *.*,*.*+/ mm *.*+/ mm

3 ,**. 51 *.**/ mm *.*+ mm (Bates and Jackson, +321) Fig. +. Typical texture of groundmass in the pumice (a) and the scoria (b). Groudmass is composed of plagioclase (Pl), pyroxene (Px), oxide (Ox), quartz (Qtz), glass (Gl) and vesicle (Ves). The pumice and scoria have the di#erent proportion between glass and microlite. *.*- mm *.**/ mm *.*. mm *.*,*.*. mm *.*,*.*+/ mm *.*+/ mm H - GRS*. GRS*. SEM JSM-/-+*E-EDAX DX. (EDS) (,**.) (,**/) *.**. mm *.**. mm., mm + mm *.,/ mm (population density) *., mm, *.*. mm Fig., Fig. - / mm Fig. - *.- mm.+

4 52 Fig.,. Variation in population density over full range of grain size in the pumice. Fig. -. Variation in population density within **.,/ mm diameter for plagioclase (a) and pyroxene (b) and within **.*2 mm for oxide (c) in the pumice and scoria. Roman numerals indicate the range (Fig.,a) *.**- mm divisions (see text). (Fig. -a) *.**- mm CSD *.*. mm (Figs.,a, -a) *.*. mm LW 2. *.*. mm LW,/ +* *.+,/ mm *.+,/ mm (Fig.,a) *.+,/ mm *.+,/ mm a, *.+,/*.*. mm b

5 ,**. 53 Fig... Variation in population density within **.*2 mm diameter for plagioclase (a) and pyroxene (b) in the pumice and scoria. Roman numerals indicate the range divisions (see text). *., mm *.+,/ mm (Fig. -a) LW,* *.*+ mm (Fig..a) *.*.*.*+ mm a *.**- mm *.*+*.**- mm b, *.**- mm c (Fig. -a) a: (*.**++* mm. ) (Figs.,a, -a) *.+,/*., mm b: +./ a (Fig. -a) a : Fig..a *.*. mm PS+.- *.*+ mm PS. b: *.*+ mm c: *.**, mm (Fig..a)., CSD *.*- mm (Figs. -b,.b) *.*- mm LW / *.*- mm LW / CSD *.*- mm *.*1/ mm *.*1/ mm (Figs.,b, -b) *.*1/ mm *.*1/ mm a, *.*1/*.*- mm b *.+/ mm CSD *.*-*.*+/ mm a, *.*+/ mm b (Figs. -b,.b) a: (Fig.,b) *.+1/ mm (Fig. -b) b: a (Fig. -b) a: b (Fig. -b) PS/ (Fig..b) b: *.**, mm (Fig..b)

6 54 Table +. Representative chemical compositions of constituent phases in the pumice and scoria. Ac : Acmite, CaTs : (Al-tschermakite)(Ti-tschermakite)(Fe-tschermakite)(Cr-tschermakite), AlCaTs : Al-tschermakite, TiCaTs: Ti-tschermakite, CrCaTs : Cr-tschermakite, DiHd : diopside-hedenbergite, EnFs : enstatite-ferrosilite, R, :Mg, Fe,,R - :Fe - Al - Cr -, Fe - : calculated from pyroxene stoichiometry. The pair phase to calculate the temperatures is as follows: +,, -3,.+*, /++, 03, 1+,, *.*/ *.*+ mm - a, b, (Figs.,c, -c) a: (Fig.,c) b: (Fig. -c) : *.*+ mm P S-/ (Fig. -c) *.**, mm (Figs. -c,.a) / Table + /+ Lindsley (+32-) 3++,**/ Fig. / +,*/* +,**3**

7 ,**. 55 Fig. /. Pyroxene components on a projection of the two-pyroxene geothermometer at + atom (after Lindsley, +32-). +,*-* +,+** /, Lindsley (+32-) /+ Putirka et al. (,**-) (+kb) + kb Fig. 0 +,*1.+,+/,, +, - + +,+/* 2223/- 3* / 3+2 Putirka et al. (,**-) +-/* K (+*11) /- Putirka (,**/) Fig. 0. Frequency in crystallization temperatures of clinopyroxene and plagioclase coexisting with melt in the pumice and the scoria. Temperatures are obtained at +kb from clinopyroxene-liquid assemblage (after Putirka et al.,,**-) and plagioclaseliquid (after Putirka,,**/). Pl-WR: core of plagioclase phenocryst -whole rock. Pl-Glass: plagioclase microlite and phenocryst rim-glass. Cpx-WR : core of clinopyroxene phenocryst-whole rock. Cpx-Glass: clinopyroxene microlite and phenocryst rim-glass. H, O H, O H, O H,O Putirka,,**/ ; Table, (B) 23 kb + kb Fig *-. /, +,*-* 0+ +* +* 20*

8 56 3* /3,, Marsh (+322) CSD +Gt G t CSD (Marsh, +322) a (Figs.,a,,b,,c, -a, -b) b a, Higgins (+330) Maale et al. (+323), Armienti et al. (+33.) ab CSD (Figs. -a, -b),**/ +,+/* +,+/* +,*-* +** Fig. 1 Fig. 1. Pyroxene (Px) enclosed in plagioclase (Pl) phenocryst observed in the pumice under an optical microscope. +,*,*+,*-* (Frost et al., +322) 3**+,*,* 3 + 3*+ 3,- 3,- 0, Lofgren (+31.) Couch et al. (,**-) (SiO, wt1+./), (T e#) Hammer and Rutherford (,**,) (SiO, wt11..) Larsen (,**/) (SiO, wt 1*.0)

9 ,**. 57 (,**/) SiO, 1* 02 a T eff -2 T eff -232 (Couch et al.,,**-) b +** 3*+ 3,- b b T eff 32 T eff 00 +-, (Couch et al.,,**-) Hammer and Rutherford (,**,) T eff+,* +** Hammer and Rutherford,,**, ; Couch et al.,,**- b b a *.**- mm *.**- mm +33, (Armienti et al., +33.) (Hammer and Rutherford,,**, ; Couch et al.,,**- ; Larsen,,**/) CSD Couch et al. (,**-) T eff **,** Hammer and Rutherford (,**,) T eff Larsen (,**/) T eff -/,.* Couch et al. (,**-) (Larsen,,**/).** Couch et al.,,**- T eff 00 3*+ 3,- 0- a, b Lofgren (+31.) CSD 0. 3*+ 3,- a, b a 0 mm, / mm,2 +. (Fig. 2) 3*+ 3,- a 3*+ 3,- 3*+ 3,-

10 58 Fig. 2. Accumulative microlite crystallinity against the grain size in the pumice and scoria. Pm: pumice, Sc : scoria, Pl: plagioclase, Px : pyroxene, Ox : oxide. 3*+ 3*+ 3,- - 3,- 3*+ - (Fig. 2) + Hammer and Rutherford (,**,), Couch et al. (,**-) 3*+ 3,- 21* 22* 3,- 3*+ 3,- (Fig. 3C) c CSD *.**, mm *.**, mm (Fig..a) 3*+ (Fig. 3C (-)) 3,- (Fig. 3C (,)) (Figs. 3C (,)-E (.)) Couch et al. (,**-), Larsen (,**/) (Fig. 3C (,), (-)) 2/* Couch et al. (,**-),.,.* +, Couch et al. (,**-) +/ mm /*. 0 mm, / mm

11 ,**. 59 Fig. 3. Schematic illustration of magma ascent history. The depth and volume of magmas are not to scale. The number in a parenthesis represents relative depth of the magma column. A) 3*+-magma left the magma chamber and ascended through the vent. B) 3*+-magma stopped under the crater. C) 3,--magma ascended through the vent and stopped beneath 3*+-magma. D) 3*+-magma erupted intermittently, accompanying with 3,--magma ascent. E) Lava cake was formed in the crater. 3,--magma ascended up to beneath the crater. +, 0/ (,**.) GPS,- km *.*. mm +.,+./+* 2 mms Couch et al. (,**-) T eff : -232 : 30/*. +*+..+* 2 mms 3*+ + 3*+ 3,- 3*+ 1 (Fig. 3A) (Fig. 3B (-)) 3, *+ (Fig. 3C (,)) 3 + 3* *+ (Fig. 3C (-)) 3, (Fig. 3D) 3*+ 3*+ 3,- (Figs. 3C (,)-E (.)) 3,- 1,**. 3,**. 3*+ 3,- - 3*+,**. B(,**--B-*+) (C) : +1/.*.-/

12 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 Mt. Etna eruption. Contrib. Mineral. Petrol., ++/,.*,.+.. 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.,*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,-*/. 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, +,-* +,.*. Couch, S., Sparks, R. S. J. and Carroll, M. R. (,**-). The kinetics of degassing-induced crystallization at Soufrie re Hills Volcano, Montserrat. J. Petrol.,.., +.11+/*,. Frost, B. R., Lindsley, D. H. and Andersen, D.J. (+322) Fe-Ti oxide-silicate equilibria; assemblages with fayalitic olivine. Amer. Mineral., 1-, 1,11.*. 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*, 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.,.., +.* (,**.),**. 3,**. +21 p. Kirkpatrick, R. J. (+311) Nucleation and growth of plagioclase, Makaopuhi and Alae lava lakes, Kilauea Volcano, Hawaii. Geol. Soc. Am. Bull., 22, Larsen, J. F. (,**/) Experimental study of plagioclase rim growth around anorthite seed crystals in rhyodacitic melt. Amer. Mineral., 3*,.+1.,1. Lindsley, D. H. (+32-) Pyroxene thermometry. Amer. Mineral., 02, Lofgren, G. (+31.) An experimental study of plagioclase crystal morphology: isothermal crystallization. Amer. J. Sci.,,1.,,.-,1-. Maale, S., Tumyr, O. and James, D. (+323) Population density and zoning of olivine phenocrysts in tholeiites from Kauai Hawaii. Contrib. Mineral. Petrol., +*+, :,**. 3 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/-*,. Marsh, B. D. (+322) Crystal size distribution (CSD) in rocks and the kinetics dynamics of crystallization : +. Theory. Contrib. Mineral. Petrol., 33,,11,3+. *. (,**/),**. 3 /* (+33,) -1,2/,3-. (,**.) GPS,**. 3,**.,*. p. (,**/),**. /* -*--+-. Putirka, K. D. (,**/) Igneous thermometers and barometers based on plagioclase liquid equilibria: tests of some existing models and new calibrations. Amer. Mineral., 3*, Putirka, K. D., Ryerson, F. J. and Mikaelian, H. (,**-) New igneous thermobarometers for mafic and evolved lava compositions, based on clinopyroxeneliquid equilibria. Amer. Mineral., 22, +/.,+//.. (,**.) ++* +/2+1.. Wager, L. R. (+30+) A note on the origin of ophitic texture in the chilled olivine gabbro of the Skaergaard intrusion. Geol. Mag., 32, -/--00. A+ A++ JSM-/-+*E, JSM-/,**LVP.* +** +** -/* +,***.* +** +** -/* 1/* Table A+ *., mm.*, A+,

13 ,**. 61 Table A+. Conditions utilizing for taking digital pictures of the pumice and scoria. * Images were photographed under reflected light and under transparent light with one- and crossed polarizer. (AdobePhotoshop1.*) Photoshop +**.* +** Table A+ RGB RGB (,//, *, *) red, (*, *,,//) blue, (*,,//, *) lime, (,//,,//, *) yellow, (,//, *,,//) magenta, (,//,,//,,//) white, (*,,//,,//) aqua aqua,. A+- +,, (GRS*., Visual Basic0.*) A+. + mm,, -, + mm - (Wager, +30+ ; Kirkpatric, +311) *.**+ mm