Bull. Volcanol. Soc. Japan
Vol. /0 (,*++) No. +, pp. +3ῌ,-

Letter

Zircon Fission-track Dating of the Hiyoriyama Cryptodome
at Kuttara Volcano, Southwestern Hokkaido, Japan
Yoshihiko GOTOῌ and Tohru DANHARAῌῌ
(Received, October ,+, ,*+* ; Accepted, February ,,, ,*++)
Zircon ﬁssion-track dating was applied to determine the age of the Hiyoriyama Cryptodome in theNoboribetsu Geothermal Field, Kuttara Volcano, southwestern Hokkaido, Japan. We dated two rock samples(HY-+* and HY-++) collected from the wall of an explosion crater at the summit of the cryptodome. A total of,*./ kg of dacite (HY-+*) and ,/./ kg of dacite (HY-++) was crushed, and +**. (HY-+*) and +**2 (HY-++)zircon grains were used to determine the relatively young ﬁssion-track ages. Dating was performed using theexternal detector method, and the ages were calculated from the densities of spontaneous and induced tracks in thewhole zircon grains. The obtained ages are +/ῌ. ka (HY-+*) and +.ῌ. ka (HY-++), which are the same withinerror. The dating results suggest that the cryptodome formed at ca. +/ ka.
Key words : Fission-track dating, zircon, Hiyoriyama Cryptodome, Noboribetsu Geothermal Field, Kuttara Volcano+. Introduction
Fission-track dating is a radiometric dating method
based on counting the number of damage trails left byﬁssion fragments in uranium-bearing minerals, such as
zircon and apatite, and in glasses (Fleischer et al., +31/ ;Hurford, +33* ; Wagner and Van den haute, +33,).
This method is commonly used for samples in the age
range of +*/ to +*2 years, but may be applied to younger
samples in the range of +*- to +*. years by measuring alarge number of mineral grains (Danhara, +33/ ; Wagner,+332 ; Kameyama et al., ,**/ ; Takagi et al., ,**1).
In the present study, we applied ﬁssion-track zircon
dating to a Quaternary subaerial dacite cryptodome at
Hiyoriyama (the Hiyoriyama Cryptodome) in the Noboribetsu Geothermal Field, Kuttara Volcano, southwestern Hokkaido, Japan. No previous study has reported
geochronological data for the cryptodome, and ﬁssiontrack dating of zircon from the cryptodome provides aninsight into the history of dome formation and theevolution of the Kuttara Volcano.
,. Hiyoriyama Cryptodome
The Hiyoriyama Cryptodome is located in the northern part of the Noboribetsu Geothermal Field, in thewestern part of Kuttara Volcano (Fig. +). The Kuttara
Volcano consists mainly of an andesitic stratovolcanothat reaches an elevation of /.3 m above sea level, with
῏College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology,
Mizumoto-cho ,1῍+, Muroran, Hokkaido */*῍2/2/,
Japan.

a small caldera (Lake Kuttara) on the summit. The
volcano evolved over the period 2*῍./ ka, involving
early silicic explosive activity and subsequent stratovolcano building associated with caldera collapse at .*ka (Katsui et al., +322 ; Yamagata, +33. ; Moriizumi, +332 ;Moriya, ,**-). The Noboribetsu Geothermal Field isinferred to have formed after the collapse of the caldera(Katsui et al., +322). The geothermal ﬁeld is approximately + km wide (northeast-southwest) and +./ km long(northwest-southeast).
The Hiyoriyama Cryptodome (Fig. ,) is elliptical inplan view, ranging in diameter from -/* m (northeastsouthwest) to //* m (northwest-southeast). It rises +-*m above the surrounding area, with the highest point
being -11 m above sea level. The surface of thecryptodome is covered with sediments up to +/ m thick(Katsui et al., +322). An explosion crater occurs at
the summit (Fig. -). The crater is //῍3/ m in size
(elongate northwest-southeast) and ,* m deep, and contains active fumaroles.
The Hiyoriyama Cryptodome consists of coherent
dacite that is well exposed on the wall of the summit
explosion crater, where it appears massive with columnar
joints spaced at intervals of +**῍+/* cm. The dacite isgrey and porphyritic, containing phenocrysts of plagioclase (῎. mm long, ,+῍,/ vol.ῌ), quartz (῎/ mm, 0῍
2 vol.ῌ), hypersthene (῎, mm, .῍0 vol.ῌ), trace
῏῏Kyoto Fission-Track Co. Ltd, Minamitajiri-cho, Omiya,
Kita-ku, Kyoto 0*-῍22-,, Japan.
Corresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp

20

Yoshihiko GOTO and Tohru DANHARA

Fig. ,. Photograph of the Hiyoriyama Cryptodome(viewed from the south). The cryptodome contains
an explosion crater at the summit, within which
are active fumaroles.

Fig. +. Location of the Hiyoriyama Cryptodome in
the Noboribetsu Geothermal Field, Kuttara Volcano,southwestern Hokkaido, Japan.

amounts of augite (ῌ+ mm) and opaque minerals (ῌ*./ mm), and rare hornblende (ῌ*., mm). The groundmass (0,῍00 vol.ῌ) is granophyric, containing silica
minerals, feldspars, and opaque minerals of ῌ*.+ mm
across. Table + lists the whole-rock major element
chemical composition of the dacite, which contains 1*
wt.ῌ SiO,, -./ wt.ῌ Na,O, and +./ wt.ῌ K,O.
-. Fission-track age determination
We determined ﬁssion-track ages for two rock samples(HY-+* and HY-++). Sample HY-+* is a fresh, coherent
dacite that was collected from the eastern wall of the
explosion crater at the summit of the Hiyoriyama
Cryptodome (Fig. -). A total of ,*./ kg of the dacite
was crushed, and about -*** zircon grains wereseparated using conventional heavy liquid and magnetic
techniques. Relatively large zircon grains with planar
crystal faces were separated for ﬁssion-track dating,with +**. grains being analyzed. The zircon grains are
uniformly short, prismatic, colorless, and *.+῍*./ mmlong (Fig. .).
Sample HY-++ is a fresh, coherent dacite collectedfrom the northeastern wall of the explosion crater (Fig.
-). A total of ,/./ kg of the dacite was crushed, andabout -*** zircon grains were separated. Relatively
large zircon grains with planar crystal faces were separated for ﬁssion-track dating, with +**2 grains being
analyzed. The zircon grains separated from HY-++ are

Fig. -. Locations of rock samples (HY-+* and HY++) used for ﬁssion-track dating.

identical in shape and size to those from HY-+*.
The zircon grains were mounted in a PFA sheet(Danhara et al., +33-) and etched with KOH : NaOH
eutectic etchant at ,,/῍ for /+ hours (HY-+*) or 0,
hours (HY-++). The etching time is appropriate forthe appearance of isotropic ﬁssion tracks along various
crystal axes (see Iwano et al., +33, ; Danhara, +33/).
Figure / shows examples of ﬁssion tracks in the zircon
grains. The zircon grains were then packed for irradiation between NIST-SRM0+, glass dosimeters. Diallyl
phthalate (DAP) plastic detectors (Yoshioka et al.,,**-) were used for induced-track counts of zircon andZircon Fission-track Dating of the Hiyoriyama Cryptodome at Kuttara Volcano, Southwestern Hokkaido, Japan

21

Table +. Whole-rock major-element compositions ofdacites from the Hiyoriyama Cryptodome, as
determined by X-ray ﬂuorescence (Rigaku RIX,***) at Shimane University, Japan, following
the analytical method proposed by Kimura and
Yamada (+330).

Fe,O-*῍total iron as Fe,O-.
loss on ignition.

L.O.I.῍

Fig. /. Photomicrographs of ﬁssion tracks in zircon
grains. (A) Zircon grain number ++3 in sample
HY-+*, (B) zircon grain number -22 in sample
HY-+*. Fission tracks (FT) are shown by solid
arrows. Also shown is the orientation of thec-axis for each zircon crystal.

Fig. .. Representative zircon crystals used for ﬁssiontrack dating (sample HY-+*). Photomicrographs
taken after etching.

the glass dosimeter. These samples were irradiated in apneumatic tube of the JRR-. reactor (HY-+*) or JRR- reactor (HY-++) at the Japan Atomic Energy Agency(JAEA).
The ﬁssion-track ages were measured using the external detector method (ED, ; Danhara et al., +33+, ,**-)and employing the standardization of ﬁssion-track calibration recommended by the International Union of
Geological Sciences (IUGS) Subcommission on Geochronology (Hurford, +33*). The ages were calibrated
by the zeta calibration approach (Hurford and Green,+32-) using a zeta factor of -/*ῌ- yr cm, determinedfrom the known age of the standards (Danhara et al.,,**-) for HY-+*, and a zeta factor of -1+ῌ- yr cm,(Danhara and Iwano, ,**3) for HY-++. The data were
examined to determine whether the grains belonged to asingle population, using the c, test with a statisticalsigniﬁcance of /ῌ (Galbraith, +32+ ; Green, +32+).
The ages were calculated using the densities of spontaneous and induced tracks for all zircon grains, regardless of the presence or absence of spontaneous tracks.
.. Results and discussion
Table , lists the results of ﬁssion-track dating. Sample
HY-+* yields +1 spontaneous tracks in the +**. zircon

22

Yoshihiko GOTO and Tohru DANHARA
Table ,.

Fission-track zircon ages of the Hiyoriyama Cryptodome.

r and N represent the track density and total number of ﬁssion tracks counted, respectively. Analyses were
performed using the external detector method (ED , ; Danhara et al., ,**-) applied to the natural crystal surfaces.
A NIST-SRM0+, standard glass was used as a dosimeter. P (c,) is the probability of obtaining the c, value for n
degrees of freedom (n῏number of crystals - + ; Galbraith, +32+). r is the correction coe$cient between rs and ri.
U is the uranium content. Zircon grains were irradiated using the pneumatic tube of reactor unit JRR-. (for HY+*) or JRR-- (for HY-++) at the Japan Atomic Energy Agency (JAEA). The age was calculated using a zeta
calibration factor zED ,῏-/*῍- (+s) for HY-+* (Danhara et al., ,**-), and zED ,῏-1+῍- (+s) for HY-++ (Danhara
and Iwano, ,**3). Ages are expressed in ka (+*- years) with an error range of +s.

grains, with a density of spontaneous tracks of 2.1/῎+*, cmῌ,. The density of induced tracks for zircon
grains is +..,῎+*0 cmῌ,. Because the zircon grains
were separated from a coherent dacite, all the grains are
considered to belong to a single population. The c, test
yields a value of 33ῌ ; therefore, the ﬁssion-track age
can be calculated using the densities of spontaneous andinduced tracks for all +**. zircon grains. Sample HY+* yields a ﬁssion-track age of +/῍. ka.
Sample HY-++ yields +, spontaneous tracks from the+**2 zircon grains, with a density of spontaneous tracksof 0./2῎+*, cmῌ,. The density of induced tracks forthe zircon grains is ,.-,῎+*0 cmῌ,. The c, test yields avalue of 33ῌ ; consequently, the ﬁssion-track age can becalculated using the densities of spontaneous and induced tracks for all +**2 zircon grains. Sample HY-++
yields a ﬁssion-track age of +.῍. ka. The ages obtained
from HY-+* (+/῍. ka) and HY-++ (+.῍. ka) are thesame within error. The weighted average of the two ages
is +/῍- ka, suggesting the Hiyoriyama Cryptodome
formed at ca. +/ ka.
The Noboribetsu Geothermal Field is inferred tohave formed after .* ka, following caldera collapse andassociated with post-caldera volcanism of the Kuttara
Volcano (Katsui et al., +322 ; Yamagata, +33. ; Moriizumi,+332 ; Moriya, ,**-). The ﬁssion-track ages obtained
for the Hiyoriyama Cryptodome (+/῍. and +.῍. ka)
are consistent with the eruption history of the Kuttara
volcano. The average uranium contents of the zircon
crystals used for ﬁssion-track dating are +1* ppm (HY+*) and +.* ppm (HY-++) (Table ,), which are within
the standard range for zircons in volcanic rocks (mode,+**῍,** ppm ; Danhara et al., ,**.). The present
results show that ﬁssion-track dating is applicable indetermining the age of formation of late Quaternary
volcanic domes.
Acknowledgements
This research was sponsored by the Ministry of Edu-

cation, Culture, Sports, Science and Technology of Japan(MEXT), and supported ﬁnancially by the MuroranInstitute of Technology. We thank Y. Toriguchi, Y.
Sekiguchi, S. Takahashi, and H. Ito (Muroran Instituteof Technology) for help in the ﬁeld. H. Iwano (Kyoto
Fission-Track Co. Ltd) is thanked for technical support.
We are grateful to H. Ito (Central Research Institute ofElectric Power Industry) and K. Watanabe (Kyushu
University) for reviewing the manuscript. N. Hasebe(Kanazawa University) is thanked for editing the manuscript.
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