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Crystal Size Distribution and Crystallization Process of the Ejecta fromthe 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 pumicewith bread crust on September +st and scoria on September ,-rd. The textural and chemical analyses on thematerials 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 andscoria developed a characteristic population density like a bell with a peak at the grain size of *.**- mm, which isinterpreted to reﬂect a decrease in nucleation rate of plagioclase, in response to crystallization and establishmentof equilibrium during the time duration when the magma stayed in the vent. Magmatic temperatures estimatedfrom 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 inthe 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  + 

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Division of Environmental System Science, Graduate
School of Science and Technology, Shinshu University,
Asahi -ῌ+ῌ+, Matsumoto -3*ῌ20,+, Japan.
-3*ῌ20,+  -ῌ+ῌ+ ¤d
Department of Geology, Faculty of Science, Shinshu
University, Asahi -ῌ+ῌ+, Matsumoto -3*ῌ20,+, Japan.
Corresponding author : Tatsuro Tsuganee-mail : ttsugane@yahoo.co.jp¹N~[º»¼ º

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Fig. +. Typical texture of groundmass in the pumice(a) and the scoria (b). Groudmass is composedof 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.

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Fig. ,. Variation in population density over full rangeof grain size in the pumice.



(Fig. ,a)*.**- mm   Fig. -. Variation in population density within *ῌ*.,/mm diameter for plagioclase (a) and pyroxene (b)and within *ῌ*.*2 mm for oxide (c) in the pumiceand scoria. Roman numerals indicate the range
divisions (see text).

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Fig. .. Variation in population density within *ῌ*.*2
mm diameter for plagioclase (a) and pyroxene (b)in the pumice and scoria. Roman numerals indicate
the range divisions (see text).

#$%&ῌ LM#$%&῍+,-./01 *.*1/ mm YZ *.*1/
mm YZ2^_KgOjklm#r+ (Figs. ,b, -b)  *.*1/ mm #$%&'()*> *.*1/ mm #$%&ῌa, *.*1/ῌ*.*- mm

 *., mm 

 *.+,/#$%&ῌb +, $%&῍_f *.+/ mm _K

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mm #$%&῍b + (Figs. -b, .b),-./01 LῌW 23

6 ,*7 89:;

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GHI1JK LMNO P>!QRb@ca!

+, *.*+ mm STUV3!

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:; b@ca!

(*.**+d+* mme.) f gO#hijk-./01$%&ῌb _Kf:

uv-./0_Kb:nVoc  *.+,/ῌ*., mm YZ

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(Fig. .b),

³´µῒ¤¶·¸¹¤¢£º»¤g¼ º

54
Table +.

Representative chemical compositions of constituent phases in the pumice and scoria.

Ac : Acmite, CaTs : (Al-tschermakite)²(Ti-tschermakite)²(Fe-tschermakite)²(Cr-tschermakite),
AlCaTs : Al-tschermakite, TiCaTs : Ti-tschermakite, CrCaTs : Cr-tschermakite,
DiHd : diopside-hedenbergite, EnFs : enstatite-ferrosilite, R,² : Mg,²²Fe,², R-² : Fe-²²Al-²²Cr-²,
Fe-² : calculated from pyroxene stoichiometry.
The pair phase to calculate the temperatures is as follows : +ῌ,, -ῌ3, .ῌ+*, /ῌ++, 0ῌ3, 1ῌ+,, 2ῌ+-.

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 ῌ 

 !"#$%&'()*mm cdNO?PQR\] ^_g (Figs. -c, .a))+%, *.*/ *.*+ mm -.!/0 - 1

23456 7῏8*5 ῌa, ῌb, ῍%

/ (Figs. ,c, -c))u vwx_y!῍0 z]%{|}~| ῌa : 9:;<=>?  @AB*C!D1EF.G (Fig. ,c))  ῌb :

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ª«4 Fig. / ¬J) ­+x_ +,*/*®

Sn-o/ ( (Fig. -c)) ῐpI BBqῑr

!J ¯%0 +,**ῌ3**® x_ ¡°±t¶·¸¹ ,**. º¸E m»#¼VY_pq

55

Fig. /. Pyroxene components on a projection of thetwo-pyroxene geothermometer at + atom (after
Lindsley, +32-).

  

 +,*-*  +,+**  
! "#$%/ῌ, ῌ
Lindsley (+32-)  &'( )*"#+,-% ./ /῍+ -01
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<&  =>?@ "-

Fig. 0. Frequency in crystallization temperatures ofclinopyroxene and plagioclase coexisting with meltin 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 ofplagioclase phenocryst -whole rock. Pl-Glass :plagioclase microlite and phenocryst rim-glass.
Cpx-WR : core of clinopyroxene phenocryst-wholerock. Cpx-Glass : clinopyroxene microlite andphenocryst rim-glass.

#<ABC DEF<G1HI
JI K LMNO?@4-<P*QRSTF (+ ῌ kb) - < +  H, O   H, O '

kb #U>&D  VW Fig. 0 ! $!F 9o#:;

\ H, O

89XYZ[# \-%]2:;3^ Epq-Qz _`# 7  +,*1.῍+,+/, #¡"#¢£-% "2S ""- H, O""-abcG1d , d% + dZ' ¤Putirka, ,**/ ; Table , (B) ¥¦[#e]$ , fghi jk--&D§]2 "¨1$89῍:;

- fgml$njk-"# $( + d89<&# #<ABC DEF<opq- :;_ - rZ[

G1HIJI K

st-3^_`:# uv-%"kb #©TF-%- < + kb #&D#-% 2] 89XYYEwx 

2 VW Fig. 0 ! &DF2yz+,+/* #=>{"#uv#| f}V~y

2 # 89XY=>289

<P*Q 2῍3 ῌ

9oXYZ[VY_  \%]2:;
3^_`#U> # 3.+῍+*-. #CS12
/῍, -ª«2#-9oXYYEwx

 ' :;#  +,*-* #@uv-%( 2 0῍+ -ª«

=>2 - 222῍3/-¬( 9o¬­K1YExS2Q 3+2 -% #"%- 3^_`K1 +*ῌ ®¯2



3*. Z [- 221῍3./- Putirka et al. (,**-) 89῍:; <&

:;#&D u4y  +-/* K (+*11) -%



LDE2

# VY_ "4t#- ""-|# 

# 7

A 9o VY_ CS%2] :# 2 8/ῌ-  ῌ

9῍:;

Putirka (,**/) 9o῍:; <& 8  #7

9# 9oXYZ[ 9o VY_ =>2 " <&' +* <&#°



±

 

²]2$7³[&D"]2 9o ´µ

- 20*῍

F¸GH4IJ¹=4|*º»4 ¼ º56

3*+  21-  2./῍3,,  213

0 



0ῌ+ ῌMarsh (+322)   

 CSD 

 

!"#$%&'  ()*+ῌGt 
 G ,  ++

 t -./'01 +2 
3 45- 6 ,!"7 89:; CSD <8 +2  =>?.

(Marsh, +322)

@A BC?῍
DEFῌG!

Fig. 1. Pyroxene (Px) enclosed in plagioclase (Pl)phenocryst observed in the pumice under an optical
microscope.

)H HC?
IJH. DEFῌa 

2, !"785P?.- G "KLMN  O-) 7 S NUN5! +B,H.
/P"8 QRC (S (Figs. ,a, ,b,?/! UN S N5_-

,c, -a, -b) T' S N UNDEFῌb  G

+()DEFῌa OV

0_O27' +,*,*῍+,*-* -+2 W , L6! 7  " UN_S N῍ _ =>?.

8E

XYB,H.+  Higgins (+330) Z!"w7" 5_a&῍a

7 6  Maalῌe et al. (+323) ![ -a&_ (Frost et al., +322) C?^P

W + G\]^P _-RR`'2 W

abE&-c+"7 gh +i()W  ,d"ef!
# G2S

3**῍+,*,*

N5_<7 " ,'5!P"7 i2\]'+ $!7- j 8 ODEFῌ

Z 3  + 9:90MNB,!

 Armienti et al. (+33.) -Z! 2 a 3*+  3  ,- 9:90B

k$la&m,no%p q,!  3,- ¡<7no` - 01Cr", '+ B

0ῌ,,H. =>?.

MN S Nak$la& '¢

MN sKDEFῌa4ῌb MN CSD 6& (Figs. -a, -b)  't-(6cu+ Tv'

++  M p 2  B

7O +2 S Nak$la&O)£78CY,¤¥¦gH."7

Lofgren (+31.) §;;N ;wx), Kwx),-y<7 w¤¥ S N UNB

z6&' {|*4+ ,**/} + C?O~H. ¨![©7 "m+ =?C!

,sKw_ P

Couch et al. (,**-)  ª>?! @;«¬A

-''UNC?.?. UN__ /SUN῍0_1C?^P?.

(SiO, wtῌ­1+./) B_®w¤¥¯

/S"7 G° V® C± ® , L D

UN_ O 23 +,+/*   ¤²¨![ (³Te#) )£ 'UN O +,+/* 5!P"7©7´µ!

. 6

 S

=>?

N5_ G(#¤¥ Hammer and Rutherford(,**,) @;«¬A (SiO, wtῌ­11..) H+,*-*  /SUN5_. Larsen (,**/) @¶a·a&«¬A (SiO, wtῌ 3 +** y7

­1*.0) <7"¨![6no¤¥¯7  T!

Fig. 1 S N UN

S N-UN4

w¨E7""7d ,**. effJ; _ <\<A&B
 (,**/) 

-2῍32Q  +**῍,** eAl/2G

 

 YHZ 67A I#. Hammer and Ruth-$  %#&'

erford (,**,) PTeff -.Q  +02 e Larsen (,**/)   
 !"#()*

57

SiO, +,"-" 1* 02ῌ 

PTeff -/Q  ,.* eH2G* tI A. !"# / 012#" 3&iJ U'245 67 8901:;KL M H2G 3;<=>?=@A&BCD*

;N2>I}#"! S EFGH῍a 2<=>?

Couch et al. (,**-) 32

dUVAIl/2.  @IO

=@IJ9* <=>?=@A&012K 

P;&I  (Larsen, ,**/)

;LMNOPTeff -2Q RMN

K, .** e2 P;&2KV;M O PT eff -2῍32Q J*!

IST"A >I9" YCouch et al., ,**- PTeff(Couch et al., ,**-) U  A FGHῌb KV

00Q 3Z ¡[ 3*+ 3,- !<\< WB"^

E%.VWX Y +** 6o2%" El/-no2}eZ .[!

%#AIl/ .[ 3*+ 3,- !<\<FGHῌb 2]B"^IWX_

Q " AIl/2G P;&¢ A

`#EW$2.[ ! I %>` ! 2.

FGH῍b  E2A I%&2'a <=>?=@b()*.`0ῌ-   

£FGH῍a, ῍b <=>?=@ $cd b(;<=>?=@LMN E2R%#S T¤.E.r)*.

PTeff I 32Q e2fJ LMNPTeff 00Q`$Ed!Kt.`$£EWB"^o

g +-,Q 01 R.MN01J9.

J*! I Lofgren (+31.) ¥U 01

+,2 Hammer and Rutherford

V#%2T" £<=>?=@E;

(Couch et al., ,**-)(,**,)  LMNOPTeffh+,*Q b(;<=>?=@IJ*! S !"#
01Ai%# b(;<=>?

=@` ,"-"<\<Ijk2.X l
m2 +**Q 5XB"^-no2E #T" ! 2K<=>?=@ +,2WB"^oJ !
W¦§G¨&2©`$cdE

I%

E} !"E+,ª«J
£

Cu#"

W¦§G¨& CSD ;rX2W

X ;Kt.>E . !" !¬ ./p U  B"^I0pX.  A1I<=>?=@Y-no;2]Z

KV_`1Iq2WX.I YHammer and   @2KP;&­®I ¯

Rutherford, ,**, ; Couch et al., ,**- .rZ !"FGr[#".! 2 * Cu#" £ W¦

H῍b 2XWA 23s/§G¨&$FGH«°I;KVE;\2}

v <\

!

<Lw.MN5 2Kxy  v tz

0ῌ.

Kt.WB"^L24u2

?{ CuI|E67}~t l8 !Kt

E]±.
  3,-

3*+ 

A ; £FGH.FGH῍b 2] FGH῍a  <\<UV%#9

ῌa, ῌb ,"-"l^_c[

" :"^1E5 1E4,$

I FGH῍a KV;².[

_ .

`T"

"#<=>?=@H³; ,"-" 0 mm,


2<

p
; *.**- mm U;M
v

A

 *.**- mm 6o/ mm,´µTI <=>?=@A2

,2ῌ +.ῌ¶2.´I} (Fig. 2) !"#·2=X.V>`* !Kt.>

E #

?@A YBo +33,Z @A (Armienti etB´²2dCD*Ial., +33.) .rEC#"

01'D

,

3*+ <\<

3,- <\< vA&<\< v¸¹2} FGH῍a  8I  (Hammer and Rutherford, ,**, ; Couch et al.,A KVE W !" 3*+ <\<,**- ; Larsen, ,**/) %# CSD 2f#">

8I 3,- <\<KV _`#Iº%2W$2}[ ! »`* !"a¼

Kt2T"
Couch et al. (,**-) LMN2J ;AeE&F/,Ai PTeff I

3*+ <\<I"u b v 2] 3,- <\<c#"b} %# v "^2X%[

%E

5¶6·78¸9·¹:º»·w¼ º

58

7;Z[ p ae.Ch% ($ gBw 3*+

789:7; iZ
Q#rs(F3,-B.Chd% (Fig. 3C)789:7; = WXYῌc ( 12.34
CSD j d *.**, mm ( 56(12*.**, mm s((wiZ 

3456 (w_% (Fig. .a)  12 !"#  $ 3456 # _ # 789:7; %@I8
Fig. 2. Accumulative microlite crystallinity against
the grain size in the pumice and scoria. Pm :pumice, Sc : scoria, Pl : plagioclase, Px : pyroxene,
Ox : oxide.

8 d% "d j s Q$ F




&@

qQ%.Chd% Ak (Fig. 3C (-)) ( rs(# 3*+'3 t(B 12 !" [E i k( ,)KB

  3*+   

 3,-%k (Fig. 3C (,)) ('3

 789:7;,Z[. F¡*+u( ¢    !"#$%  &' (Figs. 3C (,)-E (.)) ¢'3,# "() *+,-%". /(%
012.3456
?@
F
%  )

¢,#(3456  88

789:7;< =>GH ¤¥I -%. 789:7; GHI



< = F

 IJK-% ""( L3;MNOPHQ%R S
(.T-% . 3,-

,UVWXYῌ
. 12

[

( F



[Q# - \]^7

789:7; R S] ae.f(@% gB 3,3*+

Q# - \]aB,UV¦-B%Q$

5

iZSp

 )

789:7;..Zk(Fig. 3C (,), (-)) ( p,xB%.Chd%

©Q$|2 Z[p ( ) p ae.Ch /dª«0d

)

 2/* ¬ [.­(@%«¬d[( ]®, 789:7;

 12.3456 iZ @_jB% "

789:7; GHI%".  " ".d.Chd%
(Fig. 2)w_%

". p¨ E(%".d [|2ῌ}~pr(< = 12.3456(a.bcd ( [¢

Couch et al. (,**-), Larsen (,**/)

789:7;< =  @_% `(% 789:7;

 wsB] §GH UV3*+

789:7;,Z[.-%3456

] 3456

89:7;

GHI ¢ ]Id£ABCD-% Ek(

789:7;GHlHmn,oB% " Q$iZ +A  k( p q Chd% 
Hammer and Rutherford (,**,), Couch et al. (,**-)

< =d¯% Couch et al. (,**-) d°±(  iZ-%( ,.ῌ,.* ]

B%".d 12 3456

E

,UV¬ B t-%( ]  + 1d , \]2(($ 

789:7;

Q#rs( tumn-v iZ Q#

R S ²²Q)]34-%.Chd%.

w_%".xB% ( y j iZ< =d 789:7; lH] ¯% £mn( r jQ%z{%|2ῌ}~prd|2
789:7; iZSp  3*+5

"³ª8 dc Couch et al. (,**-)
 GHI 789:7;

. 3,-

< =

°±(

+/ mm .% ´/*. ]µ )d­-%.12.3456.()  21*  22* (% 3,-wp(%  )   (789:

789:7;<=) 0 mm, / mm ( F 2(iZ ae  ´ tµ BdE" ,**. 3E_¼_#j$%9,-,NO 

59.70AB ; + F1G E
 / 9!"i<55  =9=09X9 ,)*IJ)K#j$%&Dk 3*+ ,-,9 3,- ,-,7 ' =l#L< 3*+ ,,,-,. 1 1#270BX % ;m
70AB(Fig. 3A) (nEo)*] (Fig. 3B(-)) '< 3,- ,-, 2 1+ῌ#2,-,.

70AB  ,+m7-p 2 1#
2 3 12 3*+ ,-, #' (Fig. 3C(,))< 3 1 +  3*+ ,-,VqE0  3 1+.S+2 El..Er 3*+ ,-,s ;' (Fig. 3C (-))  3,- ,-,4#'599LM< 3 1 +.S+2 /0E(Fig. 3D) %Eo)61tFguv0QM;E+ 3*+ ,-,wx yz 5 =2
  3*+ ,-,99Q7 ;' 3,- ,-,QEo)*]7 ; (Figs. 3C (,)-E (.)) 3 1 ,- 
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 thevent. B) 3*+-magma stopped under the crater.
C) 3,--magma ascended through the vent andstopped 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.

E <1,**. 3 3 1E

{9|}~NO#j0  NO(4$%0< 5NO(
4$%R,)*IJ)K56 7(
NOO ,-,.9 E
8,-,NO 0 9 <,**. 3E:,-, 3*+ ,-,9 3,- ,-, ;h,-,.09  < -  
0QM;7  L=' NO00
n E0< +ῌ,  

0ῌ/ 

 (,**.)  GPS 
 !



"# $# ,ῌ- km 

{ |}~t>? 3*+ ,-,@ A B ,)*IJ)K0O 59
9 'nLM X<%%&'()*+,-,./0  1 1#23



45,-, 67 8 590:0X CDE: ;'< 5 =0>?@ῌAB =9FGH0IM;'.'< qJ

C ;D9 3 1 + E + F1GKLRMN& @3¡P0'.i  4H,)*IJ)K *.*. mm'¢O< lDL£¤' < 

LM< 559NO0PQ9 R

PC ,**. 3¥I@¦*KQ

4 +.,S+./T+*U2 mmῌs 9L< Couch et al. (,**-) P J§R¨5©; B(,**--B-*+) ª«,)*IJ)KVNO0WX;R YZ

E"RES¬ TUV9=EWX­[\ ,-,]'^_ `aTeff : -2ῌ32b J?

YU `+Z®¯T[e° :±\²³´Pe +*U+..T+*U2 mmῌs 

]µ¶ `­^ (C) : +1/.*.-/e ªE"_

0;'< 5PQMNO­>?0`a EW `·b¸¹T[e° 0º

9fg(gh ;' 3*+ ,-,,-,c»;'.'<c)d : 30ῌ/*.

: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.+..
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678 +21 p.
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from Kauai Hawaii. Contrib. Mineral. Petrol., +*+, +10ῌ+20.
9:;<=!> ? @ :ABC &'() ,**.  3 *+,+DE1 ()1Mangan, M. T. (+33*) Crystal size distribution systematics
and the determination of magma storage times : the +3/3
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J"#K$LMN9"2&'() *. (%OPQRS (,**/) &'() ,**.  3 *(+T,UVWX1 ()8 /*8 ---ῌ-.0.
$&Y (+33,) ' Z)[([+\]^_`)*UV
WX1 ()8 -18 ,2/ῌ,3-.
ab + (,**.) GPS cdef&'() ,**.  3 *(I+gPgh,1 ()23 ,**. 45"3
678 ,*. p.
 /0 1i-  j  k)l  .
(,**/) &') ,**. (-mn+(-+2oUp
qr,3s1 ()8 /*8 -*-ῌ-+-.
Putirka, K. D. (,**/) Igneous thermometers and barometers based on plagioclase t liquid equilibria : tests ofsome existing models and new calibrations. Amer. Mineral., 3*, --0ῌ-.0.
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4u (,**.) 08 5vwxy z{6()+()r,| } ~2-v[2U WX}1 7T8 ++*8 +/2ῌ+1..
Wager, L. R. (+30+) A note on the origin of ophitic texture
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Geol. Mag., 32, -/-ῌ-00.

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