Bull. Volcanol. Soc. Japan
Vol. .3 (,**.) No. ., pp. +23ῌ+33

ArticleFluorine and Chlorine Contents in the Products of the ++*2
(Tennin) Eruption of Asama Volcano
Minoru YOSHIDAῌ,ῌῌ and Ryuji AOYAGIῌ,ῌῌῌ
(Received, March +1, ,**. ; Accepted June ,2, ,**.)
Fluorine and chlorine have been determined for all kinds of the products of the ++*2 activity of Asama
volcano. The activity displayed various features that are characteristic of andesitic eruptions, i.e., scoria(pumice) fall (B-scoria L), intermediate-type pyroclastic ﬂow (the Oiwake pyroclastic ﬂow) and lava ﬂow (theKamino-butai lava ﬂow). Their F-Cl variation patterns are widely varied according to the natures of the eruption
in a similar way to those of the +12- activity of Asama (Yoshida and Tsuchiya, ,**.).
The B-scoria L samples show uniform and the highest Cl contents in average. The Kamino-butai lava ﬂow
samples have low and uniform Cl contents. These results coincide with those obtained by the study on the +12activity and may be explained by the enrichment of volatiles in the uppermost part of the magma column before
the eruption.
The F and Cl contents of Oiwake pyroclastic ﬂow samples show very large variances. Some Oiwake samples
take almost the same halogen contents as the B-scoria L samples. Whereas, the other samples show the lowest F
and Cl contents and the largest variances, and indicate strong release of gas at or after the e#usion. The detailed
examination of topographical distribution of the F and Cl contents shows that the Oiwake pyroclastic ﬂow iscomposed of many ﬂow units of high-halogen and of low-halogen contents and may suggest the transition of themode of the formation of the pyroclastic ﬂow with di#erent degassing conditions. In the low-halogen ﬂow units,volatiles might be mostly released in the early stage of the ﬂow.
Key words : ﬂuorine, chlorine, volatile contents, pyroclastic ﬂow, Asama volcano+. Introduction
Vesiculation of erupting magma plays an importantrole in formation and transportation of pyroclastic ﬂows
(e.g., Aramaki, +3/1 ; Aramaki and Yamasaki, +30-).
Hence it is expected that a study of volatile components
in pyroclastic deposits might provide useful information. Fluorine and chlorine are the next, most abundant volatile components after water in volcanic
rocks. Yoshida and Tsuchiya (,**.) analyzed ﬂuorineand chlorine on the volcanic products of the +12eruption of Asama volcano. Comparing with the resultsof model experiments for the behavior of F and Cl in
volcanic process (Yoshida, +30- ; +31/ ; +33*), theyreached the following conclusions. The transition of
the mode of the eruption from pumice fall to two typesof pyroclastic ﬂows followed by lava ﬂow may be intimately related to the volatile contents of the magma at
the time of e#usion. There might have been a verticalconcentration gradient of volatile components in themagma column. The upper part of the magma might

have high volatile contents and vesiculation in the ventbrought about pumice eruptions. Volatile contents ofthe magma, then, decreased until vesiculation took placeat the mouth of the vent and the expanding magma
overﬂowed from the crater while continuing to release
gas ; this could have produced an intermediate-type
pyroclastic ﬂow (the Agatsuma pyroclastic ﬂow). At
the last stage, the magma ﬂowed out without notablevesiculation as the Oni-oshidashi lava ﬂow. The semisolid mass formed around the fringe of the vent mightplug the vent and be thrown out with violent outburst ofcompressed gases. The fall of this material would havebrought about the Kambara pyroclastic ﬂow. It is theone and only report that discussed on the F and Cl
contents of pyroclastic ﬂow deposit as compared withpumice fall deposit and lava ﬂow erupted by the same
volcanic activity. In order to ascertain whether or not
the proposed mechanism is generally applicable, it isrequired to examine other examples.
Asama volcano has produced a number of pyroclastic

ῌ Department of Chemistry, Faculty of Science, Tokyo,1,ῌ*2,0.
ῌῌῌ Present address : +ῌ++ῌ,0, Mita, Meguro-ku, Tokyo

Institute of Technology, Ookayama, Meguro-ku,
Tokyo +/,ῌ2//+, Japan.
ῌῌ Present address : +ῌ/ῌ+0, Mama, Ichikawa-shi, Chiba+/-ῌ**0,.
Corresponding author : Minoru Yoshida

190

Minoru YOSHIDA and Ryuji AOYAGI

ﬂows (Aramaki, +30- ; +302). The Oiwake pyroclastic
ﬂow is the largest intermediate-type pyroclastic ﬂow
during the activity of Maekake-yama, which is the
younger cone of Asama volcano. Total volume of the
Oiwake pyroclastic ﬂow is estimated to be *.0 km- andfairly larger than that of the Agatsuma pyroclastic ﬂow(*.+ km-) (Aramaki, +30-). It was formerly regarded
to have erupted in +,2+ (Koan) with the B-scoria(pumice) falls (the upper layers : *.+ km- and the lower
layers : *.. km-) and the Butai lava ﬂows (the Kaminobutai and the Shimono-butai : *.+ῌ*., km- in all) fromdescriptions of old documents (Aramaki, +30-). However, the stratigraphic relations between the B-scoria
falls and the historic relics of archeologically known age(Arai, +313), and the +.C-age of carbonized wood-chips
buried in the B-scoria falls and in the Oiwake
pyroclastic ﬂow deposits (Nakamura and Aramaki,+300) indicate that they are more reasonably assigned tobe the products of the ++*2 (Tennin) activity (Arai,+313 ; Aramaki, +32*).
Aramaki et al. (+323) carried out systematic sampling of rocks presumed to be the products of the ++*2
eruption and determined their major components.
They found that the major components of the lower
layers of the B-scoria (pumice) fall deposits (B-scoria
L), the Oiwake pyroclastic ﬂow deposits and theKamino-butai lava ﬂow ranged very narrowly (/3ῌ
0+ῌ SiO,). On the other hand, the upper layers of the
B-scoria fall deposits (B-scoria U), the Shimono-butai
lava ﬂow, the Maru-yama lava ﬂow and the Nishimaekake-yama ejecta show di#erent trends from theformer three groups in variation diagrams. They concluded that the latter groups are the products of theother eruptions.
All the products of the ++*2 activity are andesites
despite the di#erent nature of the successive e#usions.
The transition from one type e#usion to another issimilar to that in the +12- activity although the ++*2
activity lacked an equivalent for the Kambara
pyroclastic ﬂow in the +12- activity. Therefore, we
presumed that the transition is also caused by di#erencein the mode of gas-release from the magma at thee#usion. In this study, we determined the F and Cl
contents in all kinds of the volcanic products of the ++*2
activity and examined their formation mechanism fromthis viewpoint. Furthermore, Aramaki et al. (+323)
took so many samples of the Oiwake pyroclastic ﬂow
deposits that we expected to be able to obtain moredetailed information on the distribution of volatiles inintermediate-type pyroclastic ﬂow.
,. Samples
The analyzed samples are the essential products of the++*2 activity, i.e., ++ samples of the B-scoria L deposits,
2+ samples of the Oiwake pyroclastic ﬂow deposits (-2samples from the northern ﬂows : Oiwake N and .samples from the southern ﬂows : Oiwake S), and ++samples of the Kamino-butai lava ﬂow. The samplesof the B-scoria U deposits, the Shimono-butai lava ﬂow,
the Maru-yama lava ﬂow and the Nishi-maekake-yamaejecta, which were excluded from the volcanic productsof the ++*2 activity (Aramaki et al., +323), were also
analyzed. All the samples were systematically sampled
and analyzed for major components by Aramaki et al.
(+323). Sample numbers assigned by them arefollowed in this report. Fig. + gives the outline ofsampling locations.
-. Analytical method
Fluorine was determined by the method of Tsuchiya
et al. (+32/). A powdered sample is fused with Na, O, ina nickel crucible. The cake is dissolved and F in thesolution is distilled as trimethylﬂuorosilane. The ﬂuoride concentration in the distillate is determined with
an ion selective electrode.
Total Cl is determined by the method of Iwasaki et al.
(+3//) improved by Ozawa (Yoshida et al., +33.). A
powdered sample is fused with Na, CO- in a platinum
crucible. The cake is dissolved and the resulting suspension is centrifuged. Chloride concentration in thesolution is determined by the Hg(SCN), spectrophotometry (Tomonari, +30,). Water-soluble Cl is extracted by immersing the rock powder overnight inwater and determined by the Hg(SCN), spectrophotometry.
.. Results
Analytical results with various statistical parameters
are shown in Tables +a to +d for the ++*2 products and,a to ,c for the others. The errors in the determinationof both F and Cl are ῍+* mg/g. Water-soluble Cl contents in Japanese volcanic rocks are usually less than /*mg/g and can often be attributed to secondary contamination (Iwasaki et al., +3/1 ; Yoshida et al., +31+). In
the present case, most water-soluble Cl contents are lessthan /* mg/g and the following discussion is based on
the water-insoluble Cl contents which is calculated as
the di#erence between the total Cl and water-soluble Clcontents.
Halogen contents of volcanic rocks have approximately lognormal distributions (Iwasaki et al., +3/1 ;Iwasaki et al., +302 ; Yoshida et al., +31+), and it isusually appropriate to use logarithmic values for applying statistical analysis to the data for F and Cl contents.
Several samples of the Oiwake pyroclastic ﬂow have,
however, very low Cl contents, and when we takelogarithms of those values, the deviations from the othervalues are exaggeratedly emphasized. They are
regarded as outliers on account of lognormal distribution but not on account of normal distribution as will beFluorine and Chlorine Contents in the Products of the ++*2 (Tennin) Eruption of Asama Volcano

shown later. Those samples are supposed to be strongly degassed samples and cannot be ignored for discussion on the processes of gas-release. Therefore, we do
not reject those samples for some discussion and examined also on normal distribution together withlognormal distribution.
/. Discussion/ῌ+ Statistical examination of the di#erence of the
F and Cl contents with the type of eruption
The F and Cl contents are varied among the B-scoria
L deposits, the Oiwake pyroclastic ﬂow deposits, andthe Kamino-butai lava samples (Table + ; Fig. ,) in

191

almost similar manner to the +12- products (Yoshida
and Tsuchiya, ,**.). Any signiﬁcant correlation is
not found between SiO, (Aramaki et al., +323) andhalogen contents.
Dixon’s Q-test (Dixon, +3/-) is applied for eachgroup and some outliers are found in the Oiwake
pyroclastic ﬂow samples. The F-content of the +*,1῍+- sample is extremely high at the +ῌ signiﬁcance level.
The high F contents of this sample and of the 2+1῍+*sample, and possibly of the 2+1῍+, and 2+1῍,. samples,
may be attributed to secondary addition of F fromgas-phase like the A-/ sample of the Agatsuma
pyroclastic ﬂow in the +12- eruption (Yoshida andFig. +. An outline map showing the sampling locations of the volcanic products of the ++*2 and some other
Asama eruptions : BSLῌB-scoria deposit (lower) ; BSUῌB-scoria deposit (upper) ; ONῌOiwake northern
pyroclastic ﬂow ; OSῌOiwake southern pyroclastic ﬂow ; KBῌKamino-butai lava ﬂow ; SBῌShimono-butai
lava ﬂow ; MῌMaru-yama lava ﬂow ; NMῌNishi-maekake-yama ejecta.
--------Isopatch curves of the B-scoria fall deposit. (/ cm, +** cm and -** cm)

Minoru YOSHIDA and Ryuji AOYAGI

192

Table +a. F and Cl contents of the lower
B-scoria fall samples of the ++*2 eruptionof Asama volcano.
SampleF῍mgῌg῎

Cltotal῍mgῌg῎

ClH,O-sol.
῍mgῌg῎

F/Clinsol.
(atom)

LP-+-,
LP-+-LP-+-/
LP-+-1
LS---,
LS---.
LS---0LS-/-+
LS-/-.
LP-/-/
LS-/-1 bomb

,02
,/,
,/*
,/0
,.0
,00
,0,
,.2
,/,
,.,
,00

./*
.1*
.2*
..*
/+*
.3*
/,*
..*
.0*
/1*
.-*

+*
+*
+*
῏+*
-*
,*
,*
+*
+*
-*
+*

+4+.
+4*,
*433+4*3
*43/0
+4*0
*4312
+4*2
+4*.
*42-0
+4+2

a4m4a
s4d4c
m (log X)ds (log X)e
g4m4f,//42430,4.*1*4*+/,,//4+aArithmetic means.
Calculated for water-insoluble Cl.
cStandard deviationsdArithmetic means for log X.
eStandard deviations for log X.
fGeometric means.
b.0-40b
-.4.-b,400/b*4*-+.b.0,4/b

+4*-*4*3-0*4*+,//*4*.*//+4*,3Table +b. F and Cl contents of the Oiwake
northern pyroclastic ﬂow samples of the++*2 eruption of Asama volcano.
SampleF῍mgῌg῎

Cltotal῍mgῌg῎

ClH,O-sol.
῍mgῌg῎

F/Clinsol.
῍atom῎

0+3῍.
2+,῍1
2+1῍+
2+1῍,
2+1῍.
2+1῍/
2+1῍2
2+1῍3῍+2+1῍+*
2+1῍+,
2+1῍+.
2+1῍+/
2+1῍+02+1῍+12+1῍+22+1῍,,
2+1῍,.
2+2῍+
2+2῍.
2+2῍/
2+2῍1
2+2῍3
2+2῍+*
2+2῍+++*,0῍.
+*,0῍/+*,0῍2+*,0῍3+*,0῍+,+*,0῍+0+*,1῍+*,1῍0+*,1῍1+*,1῍3+*,1῍+*+*,1῍+++*,1῍++*,1῍+1

.*
+-0
,..
,3,
+,.
,+.
+*2
,**
./0
-3,
++.
+*0
+/2
+2,
+-*
--*
.*+
,*,
,/*
,/2
,+,
+32
30
+.*
,**
/.
2.
,22
,12
,**
,/,
+/*
+*,
3+
+*0
12
12*g
,32

+0*
.1*
./*
//*
..*
/-*
,.*
.1*
,/*
//*
.**
+3*
,-*
,,*
-,*
./*
/**
-1*
.-*
//*
.,*
.1*
/*
+0*
.3*
-0*
,+*
-2*
.1*
./*
/**
-2*
-0*
+-*
,1*
3*
1**
.2*

,*
,*
,*
,*
+*
.*
-*
-*
-*
.*
,*
,*
,*
-*
,*
,*
-*
,*
,*
,*
,*
,*
/*
-*
0*
+*
.*
,*
,*
-*
,*
,*
.*
.*
2*
-*
0*
,*

*4/--

a4m4a

,*34+
+-14/,,4,.*4,0.,
+1/4*

s4d4c
m(log X)ds(log X)e
g4m4f
a ῌf

-.-4.b+/+43+b,4./2b, h*4--0.b, h,214+b, h

*4/0.
+4*0
+4**4/-2
*42+/
*430*
*42.2
-421g
+4.*4/0*
+4+0
+4.*
+413
*42*3
+4.+4/3
+4*2
+4+.
*43*2
*4323
*42,+
ῐ+143g
,4*+
*4202
*4,22
*43,,
+4.3
+4+/
*4223
*432*
*4112
*4/3/
+423
+4*.
,4.,4,1+4,++40--h,413*1h*4*//3h*4,3*3/h+4+-1hSame as in Table +a.
Signiﬁcantly di#erent from the others with +ῌ level.
hCalculated on the assumption that Clinsol. in the sample
2+2῍+* is +* mg/g (lower limit of determination).
gFluorine and Chlorine Contents in the Products of the ++*2 (Tennin) Eruption of Asama Volcano
Table +c. F and Cl contents of the Oiwake
southern pyroclastic ﬂow samples of the++*2 eruption of Asama volcano.
SampleF῍mgῌg῎

Cltotal῍mgῌg῎

ClH,O-sol4
῍mgῌg῎

F/Clinsol4
῍atom῎

0+3῍0
0+3῍1
0+3῍2
2++῍+
2++῍0
2++῍1
2+,῍,
2+,῍.
2+,῍/῍2+-῍2+.῍+
2+/῍+
2+/῍2+/῍0
2+/῍++
2+/῍+02+/῍+12+/῍,*
2+/῍,+
2+0῍+
2+0῍2+0῍1
2,*῍,
2,*῍.
2,*῍/
2,*῍0
2,*῍2
2,*῍+*
2,+῍,
2,+῍.
2,+῍1
2,+῍+*
2,+῍++
2,+῍+,
2,+῍+12,+῍+32,,῍+2,,῍2,,῍/
2,,῍1
2,,῍2
2,,῍+*
2,,῍+-

,*.
,00
,/,
+.2
,/2
,/*
,/2
,/*
+0.
,0.
,*.
,0,
+/,
,0.
,1.
,/.
+,,
+-0
,,2
,01
,1.
,0.
+00
,0+
,.0
,,*
,02
,2.
,/,
,0*
,/0
,.0
-+2
,10
,/0
+32
,,0
,..
,-0
,+.
3.
,00
+/*

.0*
/,*
/+*
,.*
.2*
/-*
/+*
/**
3*
.1*
.2*
.1*
/**
/**
/**
/.*
+,*
-3*
,+*
//*
.2*
/,*
.-*
/0*
/+*
/.*
/**
/**
,-*
//*
,0*
./*
/0*
/,*
..*
-/*
/,*
.3*
./*
.3*
-*
/**
.2*

,*
.*
-*
,*
,*
,*
+*
.*
-*
-*
.*
,*
+*
-*
-*
+*
.*
-*
,*
-*
+*
+*
,*
-*
+*
+*
+*
+*
,*
-*
,*
-*
,*
+*
,*
+*
,*
,*
,*
,*
,*
+*
,*

a.m.a
s.d.c
m(log X)ds(log X)e
g.m.f

,-+4.
.341*
,4-/,
*4++,0
,,.43

a ῌf
g

.+242b+--43,b,4/0-b
*4-+01b-0/4.bSame as in Table +a.
Signiﬁcantly di#erent from the others with +ῌ level.

193

Table +d. F and Cl contents of the Kamino-butai
lava ﬂow samples of the ++*2 eruption of
Asama volcano.
SampleF῍mgῌg῎

Cltotal῍mgῌg῎

ClH,O-sol.
῍mgῌg῎

F/Clinsol.
῍atom῎*420/+4**432*
+4,0
+4*/
*43+/
*430+4*+
/4+*g
+4+,
*420/
+4*3
*4/13
+4*/
+4*3
*423.
,42/
*41*/
,4,.
*43/2
+4*3
*4300
*41/0
*43+3
*43+2
*411/
+4*,
+4*2
,4,.
*43-+433
+4*3
+4+*
+4*+
+4+.
+4*3
*42.*4303
+4*,
*42/*
+14/g
+4*+
*40*2

BU-U-,
BU-U-BU-U-1
BU-U-2
BU-U-3
BU-U-+*
BU-U-++
BU-U-+1
BU-U-+3
BU-U-,*
BU-U-,+

,0,
,..
,0*
,2+
,/*
,2*
,0*
,/0
,0,
,.0
,0*

..*
-,*
--*
..*
--*
.+*
-1*
-2*
.**
.**
.+*

,*
1*
/*
.*
-*
,*
,*
,*
-*
,*
-*

+4+0
+42,
+41+4-+
+4/0
+4-.
+4-3
+4-+4-,
+4,+
+4,2

a.m.a
s.d.c
m(log X)ds(log X)e
g.m.f

,0*4+
++43*
,4.+/
*4*+31,/342+4/1+,40*-/*4*0*+*4,/*1+
+4+.3

a ῌf

a ῌf

-/,41b/-4/*b,4/.,b*4*1*3b-.241b+4.*.
*4,+*/*4+.-,*4*0+1.
+4-3+

Same as in Table +a.

Table ,a. F and Cl contents of the upper B-scoria
fall samples of Asama volcano.
SampleF῍mgῌg῎

Cltotal῍mgῌg῎

ClH,O-sol.
῍mgῌg῎

F/Clinsol.
῍atom῎US-+-+
US-+-US-+-/
US-+-0-b
US-+-1-w
US-+-2-bUS-+-+*-w
US-+-+*-b
US---,
US---.
US-+-+,-allw

,/2
,/,
,/2
,/,
,/,
,/.
,/1
,0+
,.*
,10
-/2g

.2*
./*
.2*
.-*
.0*
./*
.3*
/**
.2*
.2*
+*/g

,*
+*
,*
,*
,*
῏+*
+*
+*
῏+*
+*
,*

+4*/
+4*1
+4*/
+4+/
+4*1
+4*/
+4**
*433.
*43-+4+*1420g

a.m.a, hs.d.c, hm(log X)d, hs(log X)e, hg.m.f, h

,/04*
34*2
,4.*2
*4*+/,//43./24*b,-43.b,400*b*4*,-+b./14.bSame as in Table +a.
Signiﬁcantly di#erent from the others with +ῌ level.
hThe sample US-+-+,-allw is excluded as an outlier.
g+4*./*4*/3+*4*+20*4*,.2*+4*..

Minoru YOSHIDA and Ryuji AOYAGI

194

Table ,b. F and Cl contents of the Shimono-butai
lava ﬂow samples of Asama volcano.
SampleFmgῌg

Cltotalmgῌg

ClH,O-sol.
mgῌg

F/Clinsol.
atom

BU-L-+
BU-L-/
BU-L-0
BU-L-+,
BU-L-+BU-L-+.
BU-L-+0
BU-L-+2
BU-L-,,

,3.
-*,
,32
-+*
-*,
-*0
-*2
-*,
-*2

-/*
-/*
.,*
-2*
-.*
-0*
-3*
.**
.2*

-*
-*
,*
,*
,*
-*
,*
+*
,*

+41+
+410
+4-3
+40+
+410
+41+4//
+4./
+4,/

a.m.a
s.d.c
m(log X)ds(log X)eg.m.f-*-4/4,*,4.2,*4**1.1
-*-4-

a f

-0-4-b.141*b,4//1b*4*/.2b-0*41b+4/13*4+2.+*4+30*4*/,2+4/03

Table ,c. F and Cl contents of the Maru-yama lava
samples (Maru) and Nishi-maekake-yama
samples (MA) of Asama volcano.
Fmgῌg

Cltotalmgῌg

ClH,O-sol.
mgῌg

F/Clinsol.
atom

Maru-,
Maru-Maru-.

00
-2,
,/*

+.*
.0*
.3*

-*
+*
,*

+4+,
+4/2
*433-

MA-2+.-,-,w
MA-2+.-,-MA-2+.-,-/b
MA-2+.-,-/w
MA-2+.-,-/aMA-2+.-,-.
MA-2+.-,-0
MA-2+.-,-1

,22
,.*
,12
,-.
,.,
+,*
+*0
+*.

.0*
.0*
/**
/,*
.3*
.*
+*
.*

,*
,*
+*
+*
,*
,*
/
+*

+4,,
+4*,
+4*0
*42/0
*430+
++4,
-340
04.1

SampleSame as in Table +a.

Fig. ,. FClinsol. variation patterns for the samples of the ++*2 eruption of Asama volcano : B-scoria L
samples ; Oiwake N pyroclastic ﬂow samples ; Oiwake S pyroclastic ﬂow samples ; Kamino-butai
lava samples ;  Oiwake N 2+1ῌ+* sample. The sample Oiwake N +*,1ῌ+- is excluded from the ﬁgure.
Error bars.
Regression lines for
Oiwake N samples,: F*...//Cl -/./* (r*.1++)
Oiwake S samples,: F*.,-2-Cl +-+.0. (r*.0.,)
The sample Oiwake N 2+1ῌ+* is excluded from the calculation of the regression line.

Fluorine and Chlorine Contents in the Products of the ++*2 (Tennin) Eruption of Asama Volcano

Tsuchiya, ,**.). The log Cl-values of the 2+,῍/῍-,
2+/῍+1, 2,,῍2 and 2+2῍+* samples, the F/Cl-values ofthe 2+,῍/῍-, 2,,῍2, 2+1῍+* and 2+2῍+* samples, andthe log(F/Cl)-values of the 2+,῍/῍-, 2,,῍2 and 2+2῍+*samples are regarded to di#er at the +ῌ signiﬁcancelevel from the other samples of each group.
The signiﬁcances of the dissimilarity in the arithmetic
mean values m and the mean logarithmic values m(logX) among the groups were examined by t-test (Table

Table -. Di#erence of the arithmetic means and of themean logarithmic values m(log X) among the
volcanic products of the ++*2 eruption and someothers examined by t-testa.
Fb, f :
Kamino-butaiῌB-scoria LῑOiwake S῏Oiwake N
B-scoria UῌB-scoria L
Shimono-butaiῑKamino-butailog Ff :Kamino-butaiῌB-scoria LῑOiwake S῏Oiwake N
B-scoria UῌB-scoria L
Shimono-butaiῑKamino-butai
Clinsol.f :B-scoria LῐOiwake S῏Kamino-butaiῌOiwake N
B-scoria LῑKamino-butai
B-scoria LῌB-scoria U
Shimono-butaiῌKamino-butai-). Also di#erences of the variances s and ῍s(log X)῎,
among the groups were examined by F-test (Table .).
The values distinguished as outliers by Q-test were
excluded in these examinations. The results support
the deduction from Tables +a-+d and Fig. , that the F
and Cl contents of the volcanic products strongly
depend upon the type of eruption.
/ῌ, The F and Cl contents of the scoria and the lava
The Cl contents of the B-scoria L samples are uniform and the arithmetic mean is higher than those of theothers. On the other hand, their F contents are almost
the same as those of the Kamino-butai lava ﬂow. The
Kamino-butai lava samples show lower Cl contents anda little higher variance than those of the B-scoria L
samples. This relation coincides with that between the
pumice and the Oni-oshidashi lava ﬂow in the +12activity. The results support the conclusion of the
Yoshida and Tsuchiya’s report (,**.) that Cl (andperhaps also H, O) may have been enriched in theuppermost part of the magma column at the beginningof the eruption, and vesiculation in the vent would have
Table .. Di#erence of the variances s, and of ῌs(log X)῍,
among the volcanic products of the ++*2 eruption
and some others examined by F-testa.
Fb, f :
Oiwake NῑOiwake SῑKamino-butaiῌB-scoria L
B-scoria UῌB-scoria L
Kamino-butai῏Shimono-butailog Clinsol.c, f :B-scoria LῌOiwake S῏Kamino-butaiῌOiwake N
B-scoria LῑKamino-butai
B-scoria LῌB-scoria U
Shimono-butaiῌKamino-butailog Ff :Oiwake NῑOiwake SῑKamino-butaiῌB-scoria L
B-scoria UῌB-scoria L
Kamino-butai῏Shimono-butai

F/Clinsol.d, f :Kamino-butai῏Oiwake NῌOiwake SῌB-scoria L
Kamino-butaiῑOiwake S
B-scoria UῌB-scoria L
Shimono-butaiῐKamino-butai

Clinsol.f :Oiwake SῌOiwake NῑKamino-butaiῌB-scoria L
B-scoria LῌB-scoria U
Kamino-butaiῌShimono-butailog Clinsol.c, f :Oiwake NῑOiwake SῌKamino-butai῏B-scoria L
Oiwake SῑB-scoria L
B-scoria LῌB-scoria U
Kamino-butaiῌShimono-butailog (F/Clinsol.)e, f :Kamino-butaiῑOiwake NῌOiwake SῌB-scoria L
B-scoria UῌB-scoria L
Shimono-butaiῐKamino-butaiῑmeans that the di#erence is signiﬁcant at the +ῌ level ; ῏means that the di#erence is signiﬁcant at the /ῌ level ; ῐmeansthat the di#erence is signiﬁcant at the +*ῌ level ; ῌmeans thatthe di#erence is not signiﬁcant at the +*ῌ level.
b
The sample +*,1῍+- is excluded as an outlier.
c
The samples 2+,῍/῍-, 2+/῍+1, 2,,῍2 and 2+2῍+* are excluded
as outliers.
d
The samples 2+,῍/῍-, 2,,῍2, 2+1῍+* and 2+2῍+* are excluded
as outliers.
e
The samples 2+,῍/῍-, 2,,῍2 and 2+2῍+* are excluded as
outliers.
f
The sample US-+-+,-allw is excluded as an outlier.

195

,

a

F/Clinsol.d, f :Oiwake NῌOiwake S῏Kamino-butai῏B-scoria L
Oiwake NῑKamino-butai
B-scoria LῌB-scoria U
Kamino-butaiῌShimono-butailog(F/Clinsol.)e, f :Oiwake NῑOiwake S῏Kamino-butaiῌB-scoria L
Oiwake SῑB-scoria L
B-scoria LῌB-scoria U
Kamino-butaiῌShimono-butaiaῌfSame as in Table -.

196

Minoru YOSHIDA and Ryuji AOYAGI

brought about pumice eruption.
/ῌ- Variation in the F and Cl contents of the
pyroclastic ﬂow deposits
The Oiwake pyroclastic deposit samples exhibit thelargest variation in the products of the ++*2 activity andrelease of Cl and F after the e#usion from intermediatetype pyroclastic ﬂow is indicated. Generally speaking,tendency for the distribution of F and Cl is consistent
with that for the Agatsuma pyroclastic ﬂow in the +12eruption. Nevertheless, the F and Cl contents of considerable numbers of the Oiwake pyroclastic ﬂow
samples are within the range : m(X)ῌ-s(X) of the
B-scoria L samples unlike the relation between theAgatsuma pyroclastic ﬂow samples and the pumice fallsamples in the +12- activity. We name these samples
high-halogen samples. Aramaki (+30-) pointed outthat the Oiwake deposits show no welding or indurationin many exposures as di#er from the Agatsuma deposits.
He attributed the di#erence to the temperature at thetime of settling. Then, he concluded that the materialof the Oiwake pyroclastic ﬂow might be verticallyejected and cooled, deposited on the ﬂank not far fromthe crater, and then ﬂowed down. The high-halogen
samples, which have the F and Cl contents almost thesame as those of the B-scoria L samples, might beproduced by this mechanism.
On the other hand, the other Oiwake pyroclastic ﬂow
samples have low Cl and F contents. We name them
low-halogen samples. Yoshida and Tsuchiya (,**.)
thought the formation mechanism of the Agatsuma
pyroclastic ﬂow in the +12- eruption as follows. The
vertical change of the Cl content in the magma column
might be gradual and after the pumice eruption, somewhat less volatile-rich magma would come up to thevent. The magma would have vesiculated near themouth of the vent and overﬂowed without being
quenched. In this case, volatiles would be released byvesiculation after the e#usion, and the F- and Cl-poor,
heterogeneous deposits could be produced. The
Oiwake low-halogen samples might be resulted by asimilar cause and are consistent with the surface ﬂow ofhigh-temperature, gas-releasing, fragmental material.
As a whole, the distribution range of the halogen contents of the Oiwake pyroclastic ﬂow became wider thanthat of the Agatsuma pyroclastic ﬂow.
/ῌ. Examination on the topographical distributionof the F and Cl contents in the Oiwake
pyroclastic ﬂow deposits
The samples of the southern ﬂows (Oiwake S) show
higher F and Cl contents than those of the northernﬂows (Oiwake N) in average. Correlations between F
and Cl content both for the Oiwake S and N samples are
signiﬁcant (Fig. ,). The slopes of the regression lines
for the two ﬂows are signiﬁcantly di#erent, but the lines
converge at the distribution area of the scoria samples.

This indicates that they were derived from a similar
magma but conditions of gas-release were di#erent.
Detailed topographical distribution of the F and Cl
contents in the Oiwake pyroclastic ﬂow is illustrated in
Figs. -a and -b. In these ﬁgures, the halogen contents
are divided into 2 classes based on the arithmetic means
and the standard deviations of the F and Cl contents in
the B-scoria L samples. The classes are expressed withthe lengths of sides of rectangles, the F contents withthe height and the Cl contents with the width. Any
relations cannot be recognized between the halogen
contents and the distances from the crater to sampling
points. The low-halogen and high-halogen groups
rather seem to line up roughly parallel with the streamline of the ﬂows. According to Aramaki (+30-), theOiwake pyroclastic ﬂow may be consisted of many small
ﬂow units of a few meters thick and a few hundred
meters long. Then, these results may suggest that the
F and Cl contents will reﬂect the degassing condition ofeach ﬂow unit and volatiles might be mostly released at
the early stage of the ﬂow. Some ﬂow units indicate
very intense gas-release as compared with those of theAgatsuma pyroclastic ﬂow. We cannot clearly explain
the condition to cause such a strong degassing, but
imagine that addition of external water might play arole as presumed from the experiment by Sugiura(+302). He reported that H, O vapor accelerates remarkably the volatilization of halogens from volcanic
rocks on heating.
Putting all these observations together, we candeduce a transition of the mode of formation of the
Oiwake pyroclastic ﬂow. After repetition of the scoria
eruptions, volatile contents of the magma decreased.
Then, scoriae were not highly ejected, rained mainly
over the ﬂank near the crater, and ﬂowed down likepumice ﬂow. They were cooled enough not to releasevolatiles after the eruption. The ﬂow units of the highhalogen group might be produced in this stage. Further decrease of volatiles led to the next stage in which
the magma vesiculated near the mouth of the vent andthe gas-releasing magma overﬂowed without being
quenched. This would result in the ﬂow units of the
low-halogen group, which are similar to the Agatsuma
pyroclastic ﬂow of the +12- eruption. High-halogen
samples were not found in the samples of the Agatsuma
pyroclastic ﬂow unlike the Oiwake pyroclastic samples(Yoshida and Tsuchiya, ,**.). The di#erence may
indicate that the +12- eruption lacked the pumice ﬂow
stage, perhaps due to the di#erence in scale of the two
eruptions (Aramaki, +30-). Another possibility is thatthe number of analyzed samples (3) of the Agatsuma
ﬂow was not large enough to ﬁnd high-halogen samples.
/ῌ/ The F and Cl contents of the samples excluded
from the ++*2 activity
The samples of the groups excluded from the ++*2

Fig. -. Topographic distribution of F and Cl contents of the Oiwake pyroclastic ﬂow. a) Northern ﬂows. b) Southern ﬂows. The lengths ofsides of rectangles exhibit the class of the halogen contents ; mF and mCl are the arithmetic means, and sF and sCl are the standard
deviations of the F and Cl contents in the B-scoria L samples. The height for the F content῍I : ῑ3, mg/g (ῑmFῐ+.sF)ῌ II : 3,῎+,1 mg/g
(mFῐ+.sF῎mFῐ+*sF), III : +,2῎+0- mg/g (mFῐ+*sF῎mFῐ0sF), IV : +0.῎,** mg/g (mFῐ0sF῎mFῐ,sF), V : ,*+῎,-0 mg/g (mFῐ,sF῎
mF῏,sF), VI : ,-1῎,1, mg/g (mF῏,sF῎mF῏.sF), VII : ,1-῎-*3 mg/g (mF῏.sF῎mF῏0sF), VIII : ῒ-*3 mg/g (ῒmF῏0sF). The width
for the Cl content῍I : ῑ3* mg/g (ῑmClῐ++sCl), II : 3*῎+/1 mg/g (mClῐ++sCl῎mClῐ3sCl), III : +/2῎,,/ mg/g (mClῐ3sCl῎mClῐ1sCl), IV :,,0῎,3- mg/g (mClῐ1sCl῎mClῐ/sCl), V : ,3.῎-0+ mg/g (mClῐ/sCl῎mClῐ-sCl), VI : -0,῎.,3 mg/g (mClῐ-sCl῎mClῐsCl), VII : .-*῎.32
mg/g (mClῐsCl῎mCl῏sCl), VIII : ῒ.32mg/g (ῒmCl῏sCl).

Fluorine and Chlorine Contents in the Products of the ++*2 (Tennin) Eruption of Asama Volcano
197

198

Minoru YOSHIDA and Ryuji AOYAGI

activity by Aramaki et al. (+323), i.e., the B-scoria U,
the Shimono-butai lava ﬂow, the Maru-yama lava ﬂow,
and the Nishi-maekake-yama ejecta, are also analyzed
for F and Cl. The Shimono-butai lava ﬂow samples
have distinctly higher F contents than the Kamino-butai
samples and is consistent with the conclusions from thepetrological studies. On the other hand, we cannot
ﬁnd any signiﬁcant di#erence in the halogen contents
between the B-scoria U and the B-scoria L samples.
But, the US-+-+,-allw sample has halogen contents very
di#erent from the other B-scoria U samples and thissample must be reexamined for its assignment. We
cannot make any discussion on the Maru-yama lava
ﬂow and the Nishi-maekake-yama ejecta because theyshow very large variations even within a small numberof samples.
0. Conclusion
The F and Cl contents of successive phases of the++*2 eruption of Asama volcano lead to the following
conclusions as compared with the +12- activity.
The transition of the mode of eruption from scoria(pumice) fall to intermediate-type pyroclastic ﬂow andthen to lava ﬂow may be intimately related to thevolatile contents of the magma at the time of eruption
almost similar to that reported for the +12- eruption.
There may have been a vertical concentration gradientof Cl as well as H, O in the magma column before theeruption. The upper part of the magma had highvolatile contents and vesiculated in the vent. The
vesiculation in a limited space would have brought
about violent explosions ejecting scoriae highly. At thelast stage of the scoria eruption, the eruption became
weaker to eject the material not so highly and rained
over the ﬂank near the crater and piled scoriae ﬂowed
down. The volatile contents of the magma decreased
further as to vesiculate at the mouth of the vent and thegas-releasing magma overﬂowed from the crater without being quenched. The volatiles might be mostly
released at the early stage of the ﬂow. The Oiwake
pyroclastic ﬂow may be consisted of many ﬂow units ofthese two eruption stages. At the last stage, Cl-poor
magma ﬂowed out without notable vesiculation as the
Kamino-butai lava ﬂow.
Acknowledgement
We would like to express our hearty thanks to Professor Shigeo Aramaki of Department of Geology andMineralogy, Hokkaido University (at present, Emeritus
Professor of the Earthquake Research Institute, University of Tokyo), Professor Masaki Takahashi of Department of Earth Science, Ibaraki University (at present,
Professor of the Department of Applied Earth Sciences,
Nihon University) and Dr. Yasuo Ichikawa of the
Earthquake Research Institute, University of Tokyo (atpresent, Oyo Chisitsu Co.). They generously provided
us the systematically collected samples of the ++*2 andthe other eruptions and also gave us valuable suggestionand discussion from geological and petrological viewpoints.
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