Bull. Volcanol. Soc. Japan
Vol. .3 (,**.) No. ,, pp. 0/ῌ1,

ArticleFluorine and Chlorine Contents in the Products of the +12(Temmei) Eruption of Asama Volcano
Minoru YOSHIDAῌ,ῌῌ and Kentaro TSUCHIYAῌ,ῌῌῌ
(Received, June 0, ,**- ; Accepted March +, ,**.)
Fluorine and chlorine have been determined for all the products of the +12- activity of Asama volcano. The
activity displayed various features that are characteristic of andesitic eruptions, i.e., pumice falls, two types ofpyroclastic ﬂow (the Agatsuma and Kambara pyroclastic ﬂows), and the Oni-oshidashi lava ﬂow. Their F-Cl
variation patterns di#er from each other according to the nature of the eruption. The pumice samples have very
uniform and the highest Cl contents of all the eruptive products, despite their vesiculated texture. This is
explained by the enrichment of volatiles in the uppermost part of the magma column at the beginning of theeruption. The Agatsuma pyroclastic deposit samples show the lowest Cl and F contents and the largest
variations. An intense release of gas from the pyroclastic ﬂow after the e#usion is thought to have caused this.
The F and Cl contents of essential blocks in the Kambara pyroclastic ﬂow deposits, together with the ﬁeld
observations, suggest that the release of gas from the essential material contributed little to the ﬂuidization of thispyroclastic ﬂow. In conclusion, the transition from one mode of eruption to another observed in the +12- activityof Asama must be intimately related to the variation of the volatile contents of the magma at the time of thee#usion.
Key words : ﬂuorine, chlorine, volatile contents, pyroclastic ﬂow, Asama Volcano+. Introduction
Fluorine and chlorine are the next, most abundantvolatile components after water in volcanic rocks andare important constituents of high temperature volcanic
gases. Yoshida (+30- ; +31/ ; +33*) carried out modelexperiments to simulate the behavior of F and Cl during
the release of gas from magmas. He heated powderedigneous rocks in nitrogen ﬂows (atmospheric pressure)at temperatures from .** to +,**ῌ (Yoshida, +30-).
Chlorine was easier to release from rocks than ﬂuorine ;hence the (F/Cl)gas ratio was smaller than the (F/Cl)rockratio. Comparing the results with natural data, he
concluded that F is less volatile than Cl in the separation
process of the gas phase from the silicate melts. Then,he conducted further experiments for the reaction of
volcanic gas with volcanic rocks (Yoshida, +31/). The
gases released from powdered volcanic rocks on heating
at +***ῌ were passed through a column packed withrock grains (column temperature : 2/*ῌ+/*ῌ). Fluorine was easier to be ﬁxed by reaction with silicates thanchlorine. The F and Cl contents and the F/Cl ratio in
volcanic products will therefore be useful indicators of

the behavior of volatiles in volcanic processes.
Release of volatiles from magma is thought to play animportant role in the formation and transport ofpyroclastic ﬂows (e.g., Aramaki, +3/1 ; Aramaki andYamasaki, +30-). Hence it is expected that a study ofvolatile components in pyroclastic deposits might provide useful information. There exist, however, very
few systematic studies of the halogen contents ofpyroclastic deposits. All we have is a report on thechlorine content of the Aso pyroclastic deposits by
Iwasaki and Ono (+301).
The Temmei-activity of Asama volcano in +12- wasthe last large-scale eruption from the Maekake-yama
crater and manifested various characteristic features ofandesitic eruptions. Many old documents concerning
this activity have been preserved and their essential
information can be found in scientiﬁc literatures (e.g.,
Aramaki, +3/0 ; +30- ; +302).
The eruption started on May 3, +12- and in theearlier stage produced pumice and ash. Especiallyafter July ,/, the pumice eruptions occurred frequently
and violently. Total volume of the pumice and ash

῍ Department of Chemistry, Faculty of Science, Tokyo῍῍῍ Present address : National Institute of Advanced
Industrial Science and Technology, +0ῌ+, Onogawa,
Tsukuba-shi, Ibaraki -*/ῌ2/03, Japan.
Corresponding author : Minoru Yoshida

Institute of Technology, ,ῌ+,ῌ+, Ookayama,
Meguro-ku, Tokyo +/,ῌ2//+, Japan.
῍῍ Present address : +ῌ/ῌ+0, Mama, Ichikawa-shi, Chiba,1,ῌ*2,0, Japan.

Minoru YOSHIDA and Kentaro TSUCHIYA

66-

deposit amounts to *.+1 km . On August ., an intermediate type pyroclastic ﬂow (Aramaki and Yamasaki,+30-), the Agatsuma pyroclastic ﬂow, rushed down thenorthern slope and spread over an area of about ,* km,(volume : *.+ km-). A pyroclastic ﬂow of anothertype, the Kambara pyroclastic ﬂow, occurred the nextday, August /, with very strong detonations and caused
very serious disasters. Its deposit consists of large
essential blocks in the upper part of the ﬂow, while thelower part consists mainly of secondary materials.
Total volume of the deposit is estimated as *.*+ km-.
The activity ended with the e#usion of the Onioshidashi lava ﬂow (volume : *.+1 km-) poured out immediately after the Kambara pyroclastic ﬂow(Aramaki, +3/0 ; +30- ; +302).
All the products of the +12- eruption are andesites
with practically uniform chemical compositions. The
major and minor components vary a little despite thevery di#erent nature of the successive e#usions(Fukuoka and Aramaki, +320 ; Sato et al., +323). For
this reason, we presumed that the transition from one
type activity to another during the +12- activity wascaused by di#erence in mode of gas-release from themagma at the time of the eruption. We determined the
F and Cl contents in all kinds of volcanic products of
the +12- activity and examined their eruptive mechanism from this viewpoint.
,. Samples
The analyzed samples are the essential products of the+12- activity except the K-++ and K-+, samples (Table+). Sampling locations are shown in Fig. +. Pumicesamples were obtained from the exposure near the
Asama Volcano Observatory of the Earthquake Research Institute, University of Tokyo. They are
numbered from the bottom to the top of the deposit.
Samples A-+ to A-. of the Agatsuma pyroclastic ﬂow
were those of di#erent appearances collected at the sameoutcrop. That is, A-+ and A-, were taken from thecompact and porous part of a block resembling breadcrust bomb, and A-- and A-. were from the loose
matrix of the deposit. The other Agatsuma samples
were taken from di#erent parts of the deposit. Samplesof the Kambara pyroclastic ﬂow K-+ to K-- were takenfrom the same block, K-+ from the main part, K-, fromthe chilled margin, and K-- from the porous part. The
pairs K-0ῌK-1 and K-3ῌK-+* were also taken fromcorresponding parts of other blocks ; K-0 and K-+*
were from the main parts, K-1 from the glassy part andK-3 from the outside. Sample K-++ is the muddy
matrix of the secondary material in the lower part of the
ﬂow and K-+, is a mud-layer (, to - mm thick) thatcovered an essential block at the same locality.

-. Analytical method
Fluorine was determined by the method of Tsuchiya
et al. (+32/). A powdered sample is fused with Na, O,
in a nickel crucible. The cake is dissolved in water andﬂuoride in the solution is distilled as trimethylﬂuorosilane. The ﬂuoride concentration in the
distillate is determined with an ion selective electrode.
Total chlorine was determined by the method of
Iwasaki et al. (+3//) as improved by Ozawa (Yoshida
et al., +33.). A powdered sample is fused with Na,
CO- in a platinum crucible. The cake is dissolved inwater and the resulting suspension is centrifuged.
Chloride in the solution is determined by mercuric
thiocyanate-spectrophotometry (Tomonari, +30,).
Water-soluble chlorine is extracted by immersing therock powder overnight in water and measuring again bymercuric thiocyanate-spectrophotometry.

Fig. +. Sampling locations of the rocks of the +12eruption of Asama volcano : A῍Agatsuma pyroclastic ﬂow ; K῍Kambara pyroclastic ﬂow ; O῍
Oni-oshidashi lava ﬂow ; P῍Pumice fall deposit ;A. V. O. : The Asama Volcano Observatory.
Isopatch curves of the pumice fall deposit.
(/ cm and +** cm)
Asama-yama is situated at +-2῎-+ῌE and -0῎,.ῌN.

Fluorine and Chlorine Contents in the Products of the +12- (Temmei) Eruption of Asama Volcano.. Results
Analytical results with various statistical parameters
are shown in Table +a to +d according to the type of
eruption. The errors in the determination of both F
and Cl are ῐ+* mg/g. Water-soluble Cl in Japanese
volcanic rocks are usually less than /* mg/g and canoften be attributed to secondary contamination(Iwasaki et al., +3/1 ; Yoshida et al., +31+). In thepresent case, most water-soluble Cl contents are lessthan /* mg/g and the discussion is based mainly on thewater-insoluble Cl which is calculated as the di#erence
between the total Cl and the water-soluble Cl.
Halogen contents of volcanic rocks have approximately lognormal distributions (Iwasaki et al., +3/1 ;Iwasaki et al., +302 ; Yoshida et al., +31+), and it ismore appropriate to use logarithmic values for applying
statistical analysis to the data for F and Cl contents.
We take logarithms of their contents for most discussion. The arithmetic mean values are also shown, because it is more commonly used as a statistical expression.
Arithmetic means(m) and standard deviations(s) oflog F and log Cl values (mg/g) for Japanese volcanic
rocks (nῑ+,2) are m (log F)ῑ,..2, s (log F)ῑ*.-and m (log Cl)ῑ,.+2/, s (log Cl)ῑ*./* (Yoshida, unpublished data). The m(log X) values for the Onioshidashi lava samples (Table +d) are included within
the range : m (log X)ῐs (log X) of Japanese rocks, so
we can say that the +12- Asama rocks have halogen
contents typical of Japanese volcanic rocks.
/. Discussion/ῌ+ Statistical examination of the di#erence of the
F and Cl contents with the type of eruption
The F and Cl contents of the pumices, the Agatsuma
and Kambara pyroclastic ﬂow deposits, and the Onioshidashi lava samples seem to be di#erent from one
another (Table + ; Fig. ,).
The F content of the A-/ sample is markedly di#erent
not only from the other Agatsuma samples but also
from all the other samples. Q-test for outliers (Dixon,+3/-) shows that the A-/ sample does not belong to thesame lognormal distribution as either that of the other
Agatsuma samples or that of all other samples at the +ῌ
signiﬁcance level. We therefore disregard this sample
in most of the following discussion.
Whether the mean logarithmic values m (log X) of
the groups di#er signiﬁcantly from one another has been
examined by t-test (Table ,). Also di#erences of thevariances ῍s (log X)῎, among the groups are examinedby F-test (Table -). 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.
The Cl contents of the pumice samples are very

67

uniform and are higher than the others at the +ῌ
signiﬁcance level. On the other hand, their F contents
are almost the same as those of the Kambara pyroclastic
deposit and the Oni-oshidashi lava ﬂow. The Agatsuma pyroclastic samples are in striking contrast to thepumice samples, even though they erupted next to thepumice eruptions. They show the lowest F and Cl
contents, the highest F/Cl ratio and the largest variations in these values. The essential blocks of theKambara pyroclastic ﬂow have the highest Cl contents
after those of the pumice samples. The variance of
their Cl contents is only slightly larger than those of thepumice and the Oni-oshidashi samples, and distinctly
smaller than those of the Agatsuma samples. The Onioshidashi lava samples have the second lowest Cl contents, while the variances are comparable to those of thepumice samples. The statistical examinations abovementioned indicate that F and Cl were di#erentiated todi#erent degrees during the process of magma e#usion.
/ῌ, Di#erence of the Cl contents between the pumice
and the lava
One might attempt to explain the di#erence of the Cl
contents between the pumice and the Oni-oshidashi
samples as reﬂecting a greater amount of Cl released
from the lava ﬂow after the e#usion than from thepumice. This explanation is based on the view thatpumice is thrown up in the air and quenched as glass,whereas minute crystals grow in lava ﬂow during cooling and Cl is expelled from melts as minerals crystallizedout then escape to the air. Iwasaki and Ono (+301)
found that the Cl contents in the Aso-ῌ pyroclastic
ﬂow deposits depend on crystallinity of the matrix ofrocks. They explained the results by the mechanism
above-mentioned and thought that Cl was emitted
through secondary fumaroles. Furthermore, emissionsof great amounts of halogen-rich gases were observed
for about +/ years in the Valley of Ten Thousand
Smokes which was formed from the pyroclastic depositsof the +3+, activity of Katmai volcano in Alaska(Hildreth, +32- ; Zies, +3,3). These deposits have verylarge volume (Aso-III : ῌ2* km- ; Katmai : ῌ,* km-)and their inner parts could maintain high temperature
for a long time. Thus, release of Cl by the above
mechanism is very plausible.
Gas emissions from lava ﬂows have also been observed, and those high temperature gases have quite
high Cl contents (e.g., Iwasaki, +31*). However, theamounts of the gases are small, and the gas-emission has
only a short duration, as observed in the +32--Miyakejima and +320-Izu-Oshima eruptions (Hirabayashi etal., +32. ; Hirabayashi et al., +322). In accordancewith these results, the Cl contents of the lava samples ofthese eruptions have a rather small variance(Hirabayashi et al., +32. ; Yoshida et al., +33.). The
volume of the Oni-oshidashi lava ﬂow is estimated to beMinoru YOSHIDA and Kentaro TSUCHIYA

68

Table + a. F and Cl contents of the pumice fall samplesof the +12- eruption of Asama volcano.
SampleF(mg/g)

P-+
P-,
P-P-.
P-/
P-0
P-1

,1*
-+*
-+*
-**
-**
,3*
-+*

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

,3240
+.40.
,4.1/
*4*,+3,324/

Cltotal(mg/g)ClH O-sol.
(mg/g)

F/Clinsol.
(atom)

.3*
/.*
//*
/-*
/-*
/-*
/.*

῍+*
῍+*
+*
+*
+*
+*
῍+*

+4*+4*1
+4*1
+4*2
+4*2
+4*.
+4*0

,

/,.4-b+24+-b,41+3b*4*+/,b/,-40b+4*0,*4*+3/*4*,0,*4**2*+4*0,

These samples are provided by Prof. S. Aramaki.
aArithmetic means.
bCalculated for water-insoluble Cl.
cStandard deviationsdArithmetic means for log X.
eStandard deviations for log X.
fGeometric means.

Table + c. F and Cl contents of the Kambara
pyroclastic ﬂow deposit samples of the +12eruption of Asama volcano (K-+ to +* aretaken from essential blocks).
Samplegῌ K- +
῎ K- ,h῍ K- -iK- .
K- /ῌ K- 0῍ K- 1jK- 2
k
ῌ K- 3
῍ K-+*

F(mg/g)
,2*
,1*
,0*
,3*
,0*
,3*
,3*
,/*
-+*
-2*

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

,224*
-14*0
,4./0
*4*/+2
,2/42

K-++l
K-+,m

-.*
-0*

Cltotal(mg/g)ClH O-sol.
(mg/g)

F/Clinsol.
(atom)

.0*
.2*
-0*
.0*
.-*
.-*
..*
-3*
.3*
..*

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

+4,.
+4+*
+4/,
+4,0
+4+0
+4,3
+4-/
+4,+4,+40/,.+/4*b.,4,-b,40+0b*4*.1*b.+-4*b
-**
-,*

+4-*,
*4+00.
*4+++/*4*/-*
+4,3-

+*
+*

,4+1
,4+3

aῌf

Same as in Table + a.
Samples bound by braces are taken from the same blocks.
hChilled margin. i Porous part. j Glassy part.
k
The outer part. l Matrix of muddy secondary materials.
m
Mud layer adhering to an essential block.
g

Table + b. F and Cl contents of the Agatsuma
pyroclasticﬂow deposit samples of the +12eruption of Asama volcano.
SampleF(mg/g)

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

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

a.m.a, gs.d.c, gm (log X)d, gs (log X)e, g
g.m.f, g

,,242
-1430
,4-/.
*4*1*+,,/43

Cltotal(mg/g)ClH O-sol.
(mg/g)

F/Clinsol.
(atom)

.+*
+3*
,3*
-**
-2*
..*
-.*
,2*
,.*

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

+4*+
,420
+4/3
+4/3
.4+2
+4+2
+4*/
,4,.
+412

,

,2+4-b
2.4*2b
,4.-+b*4+-3*b,0342b+400,*40-/.
*4+3.0*4+/3++4/0/

Samples A-/ to A-3 are provided by Prof. S. Aramaki.
aῌfSame as in Table + a.
g
The sample A-/ is omitted as an outlier.

Table + d. F and Cl contents of the Oni-oshidashi
lava samples of the +12- eruption of Asama
volcano.
SampleF(mg/g)

O-+
O-,
O-O-.
O-/
O-0
O-1
O-2

,2*
,2*
,2*
-+*
-+*
-+*
,2*
-+*

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

,3/4*
+04*.
,4.03
*4*,-0,3.4.

Cltotal(mg/g)ClH O-sol.
(mg/g)

F/Clinsol.
(atom)

-/*
-2*
-2*
.+*
-2*
.**
--*
.**

῍+*
,*
,*
-*
+*
+*
+*
-*

+4.3
+4./
+4./
+4/,
+4/0
+4.2
+40+4/0,-0,4/b,+4,+b,4//3b*4*,0+b-0,4,b+4/,**4*0-*4+2+/*4*+13++4/+3Samples O-, to O-2 are provided by Prof. S. Aramaki.
aῌfSame as in Table + a.

Fluorine and Chlorine Contents in the Products of the +12- (Temmei) Eruption of Asama Volcano

69

Table ,. Di#erence among the mean logarithmic valuesm (log X) of the groups examined by t-testa.
F:
Pumice ῌ Oni-oshidashi ῌ Kambara ΐ Agatsuma
Clinsol. :Pumice ΐ Kambara ΐ Oni-oshidashi ῏ Agatsuma
F/Clinsol. :Agatsuma ῌ Oni-oshidashi ΐ Kambara ΐ Pumiceΐ means that the di#erence is signiﬁcant at the +ῌ level ; ῏means that the di#erence is signiﬁcant at the /ῌ level ; ῌmeans that the di#erence is not signiﬁcant at the /ῌ level.

a

Table -. Di#erence among the logarithmic variancesῌs (log X)῍, of the groups examined by F-testa.

Fig. ,. F-Clinsol. variation patterns for the samplesof the +12- eruption of Asama volcano : ῐ῎
Pumice samples ; ῒ῎Agatsuma pyroclastic ﬂow
samples ; ῑ῎Kambara pyroclastic ﬂow samples ;῍ ῎ Oni-oshidashi lava samples ; (ῒ) ῎ A-/
sample which is omitted as an outlier.
Error bars

ῌ*.+1 km- (Aramaki, +3/0). This is a little larger
than those of the +32--Miyakejima (ῌ*.*+ km-) and+320-Oshima (ῌ*.*- km-) lava ﬂows, but far smaller
than those of the Aso and Katmai pyroclastic ﬂows.
The rate of cooling of the Oni-oshidashi lava ﬂow might
be comparable to those of the Miyakejima and Oshimalava ﬂows and far faster than those of the Aso andKatmai ﬂows. Then, it can be concluded that the lossof Cl after e#usion did not a#ect markedly the Cl
content of the Oni-oshidashi lava ﬂow. The uniformityof the Cl content in the Oni-oshidashi samples supports
this view.
Pumice must certainly be quenched shortly after thee#usion. But, judging from its well-vesiculated texture
and the violent eruption by which pumice is thrown out,
we should say that large amounts of gas were released ator just before the time of the eruption. In order tointerpret the higher Cl content of the pumice samples
than that of the lava, we must suppose that the pumicegenerating magma had considerably higher Cl content
than the lava-generating magma. Sekine et al. (+313)
studied near-liquidus phase relations in dry and watersaturated high pressure conditions on products ofandesitic eruptions including the +12- Asama activity.
They concluded that at the beginning of the eruption,

F:
Agatsuma ῌ Kambara ῌ Oni-oshidashi ῌ Pumice
Agatsuma ῏ Oni-oshidashi
Clinsol. :Agatsuma ΐ Kambara ῌ Oni-oshidashi ῌ Pumice
F/Clinsol. :Agatsuma ΐ Kambara ΐ Oni-oshidashi ῌ Pumiceΐ means that the di#erence is signiﬁcant at the +ῌ level ; ῏means that the di#erence is signiﬁcant at the /ῌ level ; ῌmeans that the di#erence is not signiﬁcant at the /ῌ level.
athe uppermost part of the magma column should havebeen saturated with water and the H,O content was
lower in the lower part of the magma column. Our
results are consistent with theirs and strongly indicate avertical gradient of the Cl content similar to that of
H,O. On the other hand, the di#erence in the F content between the pumice and lava samples is small andthe behavior of F and Cl might be di#erent during thevertical migration of volatiles in the magma column.
/ῌ- Variation in the F and Cl contents of the
pyroclastic ﬂow deposits
When we consider that the vertical change of the Cl
content is gradual, the magma from which theAgatsuma pyroclastic ﬂow was generated should have
had the second highest Cl contents. However, the Cl
and even the F contents of the Agatsuma samples are
the lowest, and the F/Cl ratios have the highest valuesof all the eruption products. Furthermore, variationsof the F and Cl contents, and the F/Cl ratios are
considerably larger than the others (Fig.,, Table -).
Putting all these observations together, we can deduce
an intense release of Cl and F from the Agatsuma
pyroclastic ﬂow after its e#usion. This is consistent
with the surface ﬂow of high-temperature, gas-releasing,fragmental material. After rather long duration of the

70

Minoru YOSHIDA and Kentaro TSUCHIYA

pumice eruption, somewhat less volatile-rich magma
would come up to the vent. The magma would havevesiculated at the mouth of the vent and would overﬂow
without being ejected vertically. In this case, themagma would maintain high temperature and volatiles
would be released by vesiculation after the e#usion, andthe F- and Cl-poor, heterogeneous deposits could be
produced.
Incidentally we can o#er a plausible reason for ahigh-F value in the A-/ sample. The model experiments for the reaction of volcanic gas with silicate rock
(Yoshida, +31/ ; +33* ; Yoshida et al., +302) show that
F is preferentially ﬁxed on the rock grains in a waterinsoluble form, while a smaller amount of Cl is ﬁxed
mainly in a water-soluble form. The A-/ sample wasobtained from the surface of the ﬂow (Aramaki, private
communication). The gas of high halogen contents
released from the pyroclastic ﬂow might react with arock at the surface where the temperature is relatively
low. The fact that the A-/ sample contains a slightly
higher water-soluble Cl may support this explanation.
The Kambara pyroclastic ﬂow has a quite di#erent
appearance from the Agatsuma pyroclastic ﬂow(Aramaki, +3/0). Huge essential blocks are found in
and around a channeled area which ends in a large
amphitheater in the upper course of the ﬂow where
deposits from the past activity are strongly eroded.
Excavation of the buried village at Kambara-mura hasrevealed that the deposit in the lower course consists
mostly of low-temperature, dry debris from secondary
sources and contains only a small proportion of essential
fragments (Aramaki et al., +320). The essential blocks
are thought to have had high temperature at the timethey were emplaced, for their direction of remnant
magnetism is parallel to the present ﬁeld, and theirsmooth glassy skins have expansion cracks. Geologicalstudies (Suzuki et al., +320 ; Aramaki et al., +320)
reached the following conclusion. During the pumice
eruption, the magma might be partly solidiﬁed aroundthe fringe of the vent. The semisolid mass formed inthis way might plug the vent temporarily after thee#usion of the Agatsuma pyroclastic ﬂow and be thrownout with a violent outburst of compressed gases. The
huge essential blocks ejected from the crater rushed
down the steep slope or were thrown up and landed
directly on the present site of the amphitheater(Aramaki, +30-). The blocks deeply scoured the olddeposits and transferred their kinetic energy to the
excavated materials which constitute the major portionof the lower course of the ﬂow.
The F and Cl contents and their variations in theessential blocks of the Kambara pyroclastic ﬂow indicate that the amount of gas released from the blocks
after the e#usion was insigniﬁcant compared with thatof the Agatsuma deposit. However, variations of thecontents within a block, coupled with the inner structures, suggest that the temperatures of the blocks werehigh enough to cause some migration of volatiles within
it. While, the mud layer adhering to the blocks has
almost the same F content as the matrix of the deposit
near the block and secondary enrichment of F, such asthat observed in the A-/ sample, is not seen. We
estimate, therefore, that the temperature of the blocks
was around 2**ῌ at the time of deposition, because
halogen can migrate to a small extent at that temperature (Yoshida, +30-). By all accounts, the release ofhigh temperature gas from the essential materials might
not play an important role in ﬂuidization of theKambara pyroclastic ﬂow. This deduction is consistent with the conclusion reached in the geological study
mentioned above.
0. Conclusions
Determinations of F and Cl contents of the productsof successive phases of the +12- eruption of Asama
volcano lead to the following conclusions.
The transition of the mode of eruption from pumice
fall to pyroclastic ﬂows and then to lava ﬂow may be
intimately related to the volatile content of the magmaat the time of eruption. There may have been a vertical concentration gradient of Cl, as well as H, O in themagma column before the eruption. The upper part ofthe magma had high volatile contents and vesiculated in
the vent. The vesiculation in a limited space would
have brought about violent explosions ejecting wellvesiculated pumice. After repetition of the pumice
eruptions, the volatile content of the magma decreased
until vesiculation took place at a high level in the ventand the expanding magma overﬂowed from the crater
while continuing to release gas. This could have produced the Agatsuma pyroclastic ﬂow. At the laststage, the magma ﬂowed out without notable vesiculation as the Oni-oshidashi lava ﬂow. A semisolid mass,
which was formed around the fringe of the vent during
the pumice eruption, might plug the vent temporarily
and be thrown out with a violent outburst of compressed gases. The fall of this material may havebrought about the Kambara pyroclastic ﬂow.
Acknowledgements
We would like to express our hearty thanks to Professor Shigeo Aramaki of the Earthquake Research Institute, University of Tokyo (at present, Emeritus Professor) for valuable suggestion and discussion from geological and petrological viewpoints, and for kind instructions in the sampling of the Kambara and Agatsuma
pyroclastic ﬂow deposits. Also he generously provided
us with many samples. We wish to thank Dr. F.
LeGuern for constructive suggestions to improve themanuscript. We are also grateful to Dr. A. McBirney

Fluorine and Chlorine Contents in the Products of the +12- (Temmei) Eruption of Asama Volcano

who kindly revised the English in detail.
This study was supported in part by Grant in Aid for
Fundamental Scientiﬁc Research from the Ministry of
Education, Science and Culture.
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72

Minoru YOSHIDA and Kentaro TSUCHIYA

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