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,**/  /  3  ,**/  3  3 Petrological Characteristics of the ,**. Eruptive Depositsof Asama Volcano, Central Japan
Taketo SHIMANO῍,῍῍, Akiko IIDA῍, Mitsuhiro YOSHIMOTO῍,
Atsushi YASUDA῍ and Setsuya NAKADA῍
The ,**. eruption of Asama volcano, central Japan, was characterized by emergence of small amount of
andesite lava in the summit crater, strombolian explosions and a series of vulcanian explosions. There is a goodcorrelation between the degree of di#erentiation of glass in juvenile ash and repose times between successive
explosions. Glass in ash erupted after a long repose time is high in SiO, content and crystallinity. In contrast,
glass in ash from continuous strombolian explosions is less evolved chemically and is poorer in microlites. The
chemical compositions of the evolved glasses in the Qz-Ab-Or diagram suggest that the magma was degassed atlower pressures. This is supported by low H,O contents of the least vesicular rinds on bread crust bombs. In
addition, complexly deformed fragments of white-colored volcanic sediment containing high SiO, glass and silica
minerals, which probably originated from beneath the volcano, shows substantial heating evidenced by partial
melting of the sediment and further modiﬁcation by strong shearing along the conduit wall during magma ascent.
The eruptive sequence can be modeled, as follows ; +) magma emerged continuously in the summit crater,
accompanied by strombolian explosions (lava emergence and strombolian stage). ,) Magma became stagnant
and crystallized at a shallow level in the conduit (repose period and crystallizing stage). -) Vulcanian eruption
occurred when crystallizing magma was pressurized by addition of new magma and/or gas accumulation su$cient
to overcome the strength of a lava plug present below the crater (vulcanian stage). The ,**. eruption wasmarked by repetitions of this cycle and the conduit probably was choked when magma supply stopped. The
deformed nature of volcanic sediment entrapped as xenoliths suggests signiﬁcant shearing between viscous magma
and the conduit wall.
Key words : degassing, crystallization, repose interval, vulcanian eruption, Asama volcano+ῌ ῐ ῎ ῑ ῏/TJmn2oXY_ ?@AB%&'9(pqrHs!"#$%&'()*+,-,.&/01')23

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++-ῌ**-,  ¡¢ £¤¥ +ῌ+ῌ+ [¦§¨©ª«Earthquake Research Institute, University of Tokyo,+ῌ+ῌ+ Yayoi, Bunkyo-ku, Tokyo ++-ῌ**-,, Japan.
7¬ : 32*ῌ2/10 ­®¯°±²³´£µ¶ .+·[¦·¸¹¸©ªº»¼B

Present : The Center for Northeast Asian Studies,
Tohoku University, .+ Kawauchi, Aoba-ku, Sendai,
Miyagi 32*ῌ2/10, Japan.
Corresponding author : Taketo Shimanoe-mail : shimano@cneas.tohoku.ac.jp

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8 9

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Fig. +.

Locality map of Asama volcano.

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Table +. The ,**. eruption sequence summarizing
type of eruption, samples, and repose intervals of
the ,**. eruption.

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The repose intervals are calculated from eruption
times by Japan Meteorological Agency. explosive :vulcanian eruption, successive : strombolian eruption.
* : For the ﬁrst eruption, repose interval is assumed
to be equal to the duration of glowing in the crater
before the event. # : number of eruptions.

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317

Table ,. Classiﬁcation of ash particles and their
groundmass mineral assemblages.

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_$G5H%I /Table -. Relative amount of ash particle types foreach eruption. Small eruptions were not sampled.

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*¢& ¡*°h³¡kl°Constituents of lapilli in vulcanian eruptions.

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319

Fig. ,. (a) Pumiceous bread crust bomb erupted on Sep. +. (b) Pumices of the +12- plinian eruption (upperunit). (c) Lapilli erupted on Sep. ,3. (d) Dense block with white elongated xenoliths at the summit of Mt.
Asama (Yellow arrows indicate xenoliths and the white arrow indicates a pull-apart structure). SEM imagesof ash erupted on (e) Sep. +0, (f) Sep. +1, and (g) Sep. ,3. Scale bars are / cm for (a) to (c), ,* cm for (d),,** mm for (e) and (g), and + mm for (f). The ash particles of Sep. +1 are glassy and vesicular, whereas
many particles erupted on Sep. ,3 are crystalline and dense.

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320     
 X

Lindslay (+33,) ?E +12- Y)eE Philips  PW,.**   (,**,)  + : / 2r +,*/* E\a/ (Fig. /b)0ῌ-  

   ! "#$%
 &'("),**. Y)*")

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<+

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0ῌ+   +12- Y)e)*F  {LM?b c7IJb6KLMK &'(IdK vu)eb<+Vr *..῍*./ wt.ῌ eNOK P\fgQhEQARS 3 i
+ TUj kl*g ¡¢£1X/¤¥$%8 4m:n (SiO, U

VE_r *.,/ wt.ῌ : ¦ G5§+

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1j   ^X ,**. ?l

321

Fig. -. Whole rock compositions of the products generated in the ,**. eruption in comparison with other rocks of
Asama volcano (Kurofu, Hotokeiwa, and Maekake Stages ; Takahashi et al., ,**-). White circles with SiO,
1* wt.ῌ are the compositions of white volcanic sediment.

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3  +0 01  3  + 2 3456(E </ =>=MN2 %OP(tuv%

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322

Fig. .. Comparison of the whole rock compositions among the products of the ,**. eruption excluding the
volcanic sediments (gray symbols are pre-,**. products in Maekake Stage ; Takahashi et al., ,**-).

ῒ  

  !"#$% 

I@A CDCb(I 9KL 9ῒ  &'()*+,-./012345(67(8 9:;8 <= >?
ῒ @A=B(CDC2~; 3 ^ +0ῌ+1 _O῏Ob(8 9?



EFGHF

O 3 ^ +0ῌ+1 _Ob(I ῎(8 GHI (Fig. ,f) X GH ῍sI
u*+ ῍s9 2%Ju*(I %J9KL MNF(I OP+Ixx 5O)-;

8 QRST%UVWOXYZ[\];8 9? CDC2 ῌ jGHF CDCm

9KL 3 ^ +0 _<`!"aObc(defg%J9KLhi# jkVW%lῐm ?\b;I %J

¡¢O

 % GH ῍s X©OI¦§῏ ª« dnopKL
1ῌ- ( !qῑῒ 9£¤¥d¦§῏¨ 9J!qr῍s t u*+ ῍svw5u*+
P 9 defg bcxMyz {|}

ῌ I I¨nopJ
1ῌ. 

¬ !­2?5 9 X©Ohº@; ,**. I?@?L »¼

323

Fig. /. Mineral compositions in the products of the ,**. and the +12- eruptions. (a) Anorthite content ofplagioclase. (b) Quadrilateral plot of pyroxenes (Lindsley, +32-). di : diopside, hd : hedenbergite, en :enstatite, fs : ferrosilite. Thick broken, solid, and thin broken lines are +***, +*/*, and ++**¹ isotherms,
respectively.

   Aramaki (+30+)!

"#$#%&' () "*+,-%./012341 56718 3 9 +- :%;<=>3 3

?Lqrs01  tu

vw

xy

OPz{ 4'"8|}y~y@;qjxyHp

M7"& m2 OP xy vw

9 + :?@@;ABCDEF'"G%H ,**- IJ

M7"XHS234'" 01 

K%LM7"2NOPQR%L

%H 4 pP  "

TU

S"8

 +* 9 ,3 :V , WDEX%HYNZ cm J[

@;A @\=>]^%_  `%@; abc22d1
S"8,**. I?L

QRe

fg'

" '  +* 9 ,3 :%QR%hd7"22hd71 (Fig. ,d)8 T&*+
  3p1H #$# =3  1@¡¢£¤&¥3

HS ¦§M718 ¨ J[©H@;Ae fH 3 9 ,- ,3 : +* 9 +* :Si7

 Ktª«%mS ¬S­61T H3

%?L "2 56718 V , WDEj%H 3 9pS  xy?L+ :?Lk@;q

3 9 ,- :Jk?Llo01T H3p'"  3 9 + :%mSn

3 9 ,- :Jk®d1¯°

3 9 ,- :J?@±² ³3 9 + :´ &µST

H #$#[¶%&·V%@=*+ ¸dHI´µ¶JK·!¶¸L¹º¶»K M¶NK¼O

324

Fig. 0. Glass compositions in comparison with (a) whole rock and groundmass compositions of the products in
the ,**. eruption. (b) Glass compositions of the +12- and the ,**. eruptions.



  

wt.ῌ +,4 K  8&*2]"8#$8+ - MPa +,8U.//
 !" #$%&'()*
# +,- ./0(1234

5/6

 / +12- 8eOfgh=>z(

0" wx%yz(8%{&'"12*YY

 7 8!8 9 /

0K QRQ"Z&7ES> =7:;<+" 5@A/'3>YLM wx%yz(848  BCD ; @E8F/=> 3 ? ,-&12&'"v/*2]K 8"Z !" 5/G51H

&/ES>3>QRQ t12IJ!KLM/&z( $>LME5 6K

1ῌ/ 

 >

NOP#$ QRQEST8QRQ78U QRQ"Z& .- MPa 4 /ES> ^ ,**. 8h/%YVW1X>YLM Z8QRQ7/

8 =T7/)/7&3> ^8t%[QRQ8\ 2]"^_ `(N#$/ 12=>LM

F Frost and Lindsley (+33,) /& +,*/*a b(1ῌ0 N ()c#$F Housh and Luhr (+33+) /&,**.  =8()N" 8/K ^- wt.ῌ K

LM ,**. =8N*

#$ c() .d()*4 #$" ^_eOfgh

1> +12- 

8 (Figs. /, 0)
+12- 



()*#+,- #$9/:;>

Blundy and Cashman (,**+)

7;+$<5888ij>Y8 ¡¢!s> ()8N>VW1£=

T/" ,**.  8QRQ>  (Fig. 3) ¤" Blundy and Cashman (,**+) 8

8VWZ/K 8LM> An $'/¥?¦1§

 Qz-Ab-Or @E/

% 2]" 2]k8lm b( 8#

A¨>E©8$8noplm1LMq+ wt.ῌ 8)#$K .?¦8e«¬

lm1ro8LM
6  ,**.  ="!s"t#/$uvLM¡¢ª ,**. =8(" Qz῎-Ab῎-Or῎­4 B8?¦" Al, O- $'/¥?¦"vYY ,**. =8()#$C®

wx%yz(8%{&'1|; 98}(

(CIPW norm Corundum¯,ῌ) "°O*῍b(~2]8 - wt.ῌ /!)/*Y2] .*./

±D²8³E" Qz-Ab-Or 8+F Qz F0G/ ,**. 

Fig. 1.

Comparison of glass compositions among the products in the ,**. eruption.

325

°±²³´µ³¶·¸³¹´ º³»´¼

326

Fig. 2. Residual water content of the products
erupted on Sep. +, ,**., compared to that of the+12- eruption.

ῌ  (Johannes and Holtz, +330)
H, O  ternary minimum  +*ῌ  Qz-Or join  ῍

!"#$%

 (Johannes

and Holtz, +330) &%'() (Qz+0 Ab1/ Or3*.
wt.ῌ H, O ; +,-.
MPa 1

;

+*ῌ /00  ,**

234 (*.+ MPa)

56783 (23*9) :-."# <=->?@#A% .( BACD Qz E  F:G$;῍HIJ#

KL5 /M C%5NOPQRN0 ?.(STU1 1U;V?.(#

Fig. 3. Qz’῍Ab’῍Or’ ternary plot of glass compositions of (a) the products of the ,**. eruption
with (b) experimental results summarized by
Blundy and Cashman (,**+). Thick line :decompression induced crystallization path at T
¯23*9 with the initial melt composition ofQz῎+0 Ab῎1/ Or῎3 containing +*ῌ alkali-feldspar
crystals. The plagioclase crystals dissolve
before H,O saturation resulting in an Ab richmelt. After saturation with H, O, new plagioclase
crystallizes with decompression resulting enrichment of the melt in Qz῎. At +2 MPa, quartz
crystallizes, and the melt composition moves
along quartz-plagioclase cotectic line toward Qz῎Or῎ join with almost constant Qz῎ (ca. Qz῎/*).
With further decompression, the melt encountersthe quartz-tridymite phase boundary at +0 MPa,then reaches the ternary minimum at 3 MPa.
Thin line : quartz-feldspar cotectics at di#erent
labeled pressures (P¯PH,O). Filled circle :ternary minimum at each pressure, and * : at *.+MPa. Appearance of silica minerals in the
products of the ,**. eruption are labeled on the
thick broken line.

$ 83S5W XYZ<=?[\ ] ^ E  _   `  ? a  T   $  (Blundy and/M NOPQRN0 ?~ Qz῎E %klm

Cashman, ,**+)^E?wx ,**. ge(QQ4cHb()cdef +12- geh4i|%A-."# ST5}

jklm^E (Qz῎,*Ab῎1-Or῎1) no6#%^E5p

1ῌ1 

; Ti ,**. g(qr js^E6efE tklm)-.

#tj!-."#A%$%%Guv?wx,**. g(klm^E"T<=->:

)S S efCD"#

3  +.῍+0 e5CDt$; ty z{?@#A|; jklm^Eje?;#3  +0῍+1 e5T-.":BA"# T5}

? ¡¢d£¤,¥¦(qA2~# §¨©e5-.%ef

5pT N^E Oz῎-(Ab῎-Or῎H5no Qz῎/* 5p; 1x*V ?

.(_KT;G234 / MPaklm)EHª# T e

40 5p%G'->1

"W-."«¬5%$%­®$QQ

 

(Table -)T:Ge"A> ,**. B c§X«     ' 3(+

! "#$%&) SiO, *+ &,-.!
)0 SiO, ),-.!

km 2AH &KL' )&M,&H N ,O   PH8y PQP FH 86$ 8yXV0: +.῍+//" 3 ( +0῍+1/1  3 ( ,-

327

F4

06jDJV : H Q ,&52

RSFG F86=H VT)S

3004561 7

&@:.&n F.

 586    

0 SiO, ) 1/ wt.ῌ

7+9 :&   ;<='  @A ' BC>

>?&
A&D

3 ( +/

0YZ

 

[

86$ .)U  F;<0 (Fig. +*) 57;<=>>&(Fig. ++)  0

 !?9) !?0

JK8"  ;<=># LM86$NOPQP
:&% &T&D

&E'  (Fig. +,) U'(=)0
V8W 6R'XYZ[)&*M\]^+  3 ( ,-23)0XYZ[0`Da,V -b0) .5%c  /d Le0

9 9 1c2PQ

Pf#)01+PQP86     Gb&D

= ; 86=H p1[ m .5^:  +&)8";<=>!
?0

_&LNMPX `1

PQP)C AJ&`a ' )86

   &D

;<4; G H 2I& 0 F_1 PQP

F\>H "9 ]&1n > 

+.

 RS

ct]uXZ-=H l++ (0E1 1

=H GPQP UV#&0 +86$N) 1 PQPWUX& F=

0 3 ( ,-23)

n R 5 PQP6jDJV_¡ :bc: , ¡ 23 F3 ( ,-23H .)0 2

PQP&dn Qe40¢1c&9 :& 1clf)0&,-.!

/J   Qz῎-Ab῎-Or῎£j) ternary minimum ) Lemg' (Fig. 3) 0 j) Qz῎-Ab῎-Or῎£&¤ ¥, c=h&n i  R

1c3gch4&i 5'  6j p13gcj¦06jAJ&PQP`a)PQP& kTc F7c8 ; 1cl2

 kTc ak=h lml' 

77/JH 3gh4&2no)0  c§Xpq &PQP)PQP9m)11n  1 3 ( +.῍+/Cr ¨n )p1©\ sª V &D0 2PQP ©\«t p&PQP`auw1D"l 5X1¨np9&

86XYZ[0_1  9)Cv"0&D

 (Fig. +,)  3 ( +/0 ¬G A> ,**. B9m PQP&D)0 ;<=>0:Vn1+ 7)

!?E'5 3 ( +.

V15,6;)4o)4n (Fig. ++) p1qr  s<)=ct]uv(&w: 3 ( +0+>)4 V 3 ( +.῍+/&0 3 (

86PQPLV& sk?x =c

 w C 1­I

x®I&

86r¯0n «t' yz4   ¬¡&D
06° {§X± ±0\]^7:5 E&5PQP)C.²³{ 86 P´µ&' {5D
)4¶p

0@: 
1ῌ2,**. 2)8yV A> ,**. BPQP)C

2   

A> ,**. B)0.|UX&,z{02jp17)4n

·n  .-&0=ct]u(Fig. +-) PQP06j km 2DPQPE5&F

 F}0 [H lXZ-Qi

G 5 |&0V)C }~ ¸  ~>;<=>U  ¹C0=

% /HI)4 V PQP06jAJ F+>º&5   c»¼

328

Fig. +*.

 

Temporal change in glass compositions of brown ash particles. Error bars are +s.

 ,**. 



329

Fig. ++. Relationship between repose intervals of eruptions and glass compositions of brown ash particles. Error bars
for compositions are +s. Nov. +. ash may have a much larger error because few data are available for averaging.

²³´|µ¶·|¸¹|ºµ

330

|»µ¼!

Fig. +,. Temporal change in glass compositions (SiO, vs. TiO,) for all ash particles erupted in ,**.. The shadedpart shows the compositional range of ash particles before the erupted date of interest. The broken line shows
the liquid line of descent.

     Qzῌ-Abῌ-Orῌ" Qzῌ/*!

#$%&#'()*+,῍ῌ[\] ^_`abcdef ghi_jk8lmnopq ghjkTrdefs=tufU;8lvw-( ./.01 23456)

f xyz{|}~E '*R

7 89:;<=>??@  8l s=tTB 

'()*-( 8A./.BC6

L 7 E*) 9:D;./.01 23EFG6H

V   ~

456) I JJ KLMNO  +¡¢£E Johns Hopkins

P QPRS'() )( ./.=

*T1U
()*

PVWX=KL YZ

jk Thomas Wright ¤¥¦L¡§4

L ¨©ª«*¬ ­s®¯7 de °k±°kd,**.  rF sQ ¼m

Fig. +-. Model of the ,**. Asama eruption (seetext for details).

 ,**. 


 

:



  +02***,


ῌ ῏ ῎ ῍ ! " ! # $ ! %& ' ! ()(,**/),**.ῌ,**/ * +,
 /* -. Aramaki, S. (+30+) Sillimanite and cordierite from volcanic xenoliths. Am. Mineral., .0, ++/.ῌ++0/.
/0 (+33-)+ : /**** 12
120 1 3+32,  . 5 ,0
/0 ! &!4" (+32,)  ,1 -,+ῌ-,+.
#67
8 9:
Blundy, J. and Cashman, K. (,**+) Ascent-driven
crystallisation of dacite magmas at Mount St. Helens,+32*ῌ+320. Contrib. Mineral. Petrol., +.*, 0-+ῌ0/*.
Couch, S., Sparks, R. S. J. and Carroll, M. R. (,**-) The
kinetics of degassing-induced crystallization at Soufriere
Hills Volcano, Montserrat. J. Petrol., .., +.11ῌ+/*,.
Dobson, P. F., Epstein, S. and Stolper, E. M. (+323) Hydrogen isotope fractionation between coexisting vaporand silicate glasses and melts at low pressure. Geochim.
Cosmochim. Acta., /-, ,1,-ῌ,1-*.

331

Frost, B. R. and Lindsley, D. H. (+33,) Equilibria among
Fe-Ti oxides, pyroxenes, olivine, and quartz : Part II.
Application. Am. Mineral., 11, +**.ῌ+*,*.
Hammer, J. E. and Rutherford, M. J. (,**,) An experimental study of the kinetics of decompression-induced
crystallization in silicic melt. J. Geophys. Res., +*1,+*.+*,3/,**+JB***,2+.
Housh, T. B. and Luhr, J. F. (+33+) Plagioclase-melt equilibria in hydrous systems. Am. Mineral., 10, .11ῌ.3,.
Johannes, W. and Holtz, F. (+330) Petrogenesis and experimental petrology of granitic rocks. Springer, Berlin
Heidelberg, --/ p.
Lindsley, D. H. (+32-) Pyroxene thermometry. Am. Mineral., 02, .11ῌ.3-.
;< $ (,**-) *=% >  ?3&' 12 ,2-ῌ.0-.
 ! @(AB ! # $ ! )C ! *+ ,(,**/)  ,**.  DEF  D -G H
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n!/0 (,**-) !op
Eq rF
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22.
4|)!}cM!  (,**,) `~a X NOOP Hs !Q
RtNNO : - S!0 S!++ S`~aT
NOU .I V/Q?3' 2 ,0ῌ-0.
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8rR Z
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W ! iI ! X=  ! @(AB ! :;UV(,**/b) [9# \,**.  3 5 +/ῌ+2 #  /* -. ]V ! /^_ ! /= ! o&` ! a ,**.  3 5
P!("!# $ (,**/) + # o /* xxxῌxxx
0b\(cw!23 J!/01!%& '! !
¡n ! d ¢ ! )C ! # $ ! e¤¥£!*+ , (,**/) ,**.   /* xxxῌxxx
¦§e¨©?*hij!#WXY5!/0 (+331) fªFD«=+12-  ¯°±] %¬­®
., ,2+ῌ,31.
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