ORIGINAL RESEARCH
published: 24 September 2019
doi: 10.3389/feart.2019.00249

Time Variation in the Chemical andIsotopic Composition of FumarolicGasses at Kusatsu-Shirane Volcano,Japan
Takeshi Ohba 1* , Muga Yaguchi 2 , Kana Nishino 1 , Nozomi Numanami 1 , Urumu Tsunogai 3 ,Masanori Ito 3 and Ryo Shingubara 3
1

Department of Chemistry, School of Science, Tokai University, Hiratsuka, Japan, 2 Meteorological Research Institute,
Tsukuba, Japan, 3 Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan

Edited by:Franco Tassi,
University of Florence, Italy
Reviewed by:Dmitri Rouwet,
National Institute of Geophysics
and Volcanology (INGV), Italy
Orlando Vaselli,
University of Florence, Italy*Correspondence:
Takeshi Ohba
volcano.ohba@gmail.com;
takeshi_ohba@tokai-u.jp
Specialty section:
This article was submitted toVolcanology,
a section of the journal
Frontiers in Earth Science
Received: 02 October 2018
Accepted: 05 September 2019
Published: 24 September 2019
Citation:
Ohba T, Yaguchi M, Nishino K,
Numanami N, Tsunogai U, Ito M and
Shingubara R (2019) Time Variationin the Chemical and Isotopic
Composition of Fumarolic Gasses
at Kusatsu-Shirane Volcano, Japan.
Front. Earth Sci. 7:249.
doi: 10.3389/feart.2019.00249

Minor seismicity may occur at volcanoes with hydrothermal system before a steam
eruption. To forecast any steam eruption, it is indispensable to detect and understandthe nature of this shallow seismicity. As the fumarolic gas resides in the hydrothermal
system, it may provide insights for elucidating the nature of any seismicity and thus
forecast steam eruptions. At Kusatsu-Shirane volcano Japan, intense seismic activity
took place in 2014 and 2018. To investigate the relationship between the seismicity
and gas chemistry, five fumarolic gas discharges have been repeatedly analyzed. Since
July 2014 to November 2017 a monotonic decrease in CO2 /H2 O, He/H2 O and N2 /H2 O
ratios was recorded in the fumarolic gasses located north of the summit of volcano,suggesting the decline of the magmatic component. On the contrary the CH4 /H2 O
ratio significantly increased during the seismically quiet period, indicating that reduced
conditions developed in the hydrothermal system, favoring the formation of CH4 . The
high N2 /He ratio in the quiet period indicates the addition of N2 , likely deriving from thecrustal rocks hosting hydrothermal reservoir. The N2 /He ratio in 2018 was significantly
lower than those recorded in 2014, indicating the evolution of magma with the progressof degassing. The δD(H2 O) and δ18 O(H2 O) values and the CO2 /H2 O ratios of fumarolicgas discharges were modeled with the following processes: generation of vapor phase
after the mixing between magmatic gas and a cold groundwater with meteoric origin,addition of vapor phase with meteoric origin, and partial condensation of water vapor
near surface. Only a single magmatic gas is necessary for the above modeling.
These data suggest that at Kusatsu-Shirane volcano the activation of seismicity wassynchronized with the increase of the magmatic component in the fumarolic gas. It is
postulated that the injection of magmatic gas increased the fluid pressure in the reservoir,
which triggered seismicity. The injection would have been triggered by a break of thesealing zone surrounding the degassing magma. The injection of magmatic gas can bedetected by monitoring the composition of the fumarolic gas, thus giving the possibility
to forecast any future seismicity.
Keywords: volcanic gas, seismic activity, hydrothermal system, magma, He

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Ohba et al.

Fumarolic Gases at Kusatsu-Shirane Volcano Japan

has been filled with an acidic lake water. Mt. Kusatsu-Shirane has
produced andesitic volcanic rocks with two age periods, which
are hereafter termed the old and young eras. Eruptions of the old
era terminated at 0.25 Ma (Kaneko et al., 1991). The recent active
young era began approximately 14,000 years ago (Hayakawa andYui, 1989). The pyroclastic cones occupying the summit area of
Mt. Kusatsu-Shirane (Figure 1) formed 14,000 years ago. The lava
flows extending from the summit area to the east slope of the
volcano were developed 3,000 to 5,000 years ago (Hayakawa andYui, 1989). The first historical eruption at Mt. Kusatsu-Shirane
took place in 1882. Before the eruption, there was a long dormant
period when the summit area was covered with groves (Ohashi,1914), and fish lived in the lake within Crater Mizugama (Mi in
Figure 1). The lake water within the Crater Yugama was acidic,
although the temperature of the water was similar to ambient
air temperature. From 1882 to the present day, Mt. KusatsuShirane has undergone several active periods and intervening
dormant periods. All of the historical eruptions were phreatic

INTRODUCTION
Minor seismicity episodically occurs at volcanoes withhydrothermal system. Such seismicity could be the precursor toa steam-driven explosive eruption (hereafter “Steam eruption”).
The steam eruption is generally small in scale (Barberi et al.,1992), and difficult to forecast, due to the scarcity of precursors.
Therefore, to forecast steam eruptions, it is indispensable
to determine the cause of the seismicity occurring in thehydrothermal system. The fumarolic gas observed at volcanoes
resides in the hydrothermal reservoir. Therefore, it may
be useful for elucidating the occurrence and the nature ofshallow seismicity.
Mt. Kusatsu-Shirane is an active volcano developed on the“volcanic front” (Sugimura, 1960) on Honshu Island, Japan andthe summit consists of several pyroclastic cones and craters(Figure 1). One of the craters, Crater Yugama (Yg in Figure 1,
the other acronyms used in the following text are listed in Table 1)

FIGURE 1 | Location of fumarolic gasses, W, C, E, S and M. The shaded regions overlapping the fumaroles indicate the region with geotherm. Yugama (Yg),
Mizugama (Mi) and Yumiike (Ym) are craters with lake water. Sirane (Sp), Ainomine (Ap) and Motoshirane (Mp) are pyroclastic cones. The inserted figure on the upper
left corner shows the location of Kusatsu-Shirane volcano on Island of Honshu, Japan with the volcanic front (Sugimura, 1960). The inserted figure on the lower left
shows the distribution of lava and pyroclastic deposits emitted from Kusatsu-Shirane volcano, which are younger than 1,4000 years (Uto et al., 1983). The yellowish
areas indicate the lava and pyroclastic deposit younger than 7,000 years (Hayakawa and Yui, 1989). The hypocenters of volcanic earthquake are distributed within
the two bold red circles (HP of Japan Meteorological Agency (JMA), https://www.data.jma.go.jp). The hypocenters of the volcanic earthquakes cluster at a depth
between sea level and up to 2000 m above sea level. A small steam explosion occurred at Er in January 2018. K is the position of hot spring water which contains
high concentration of Cl− . Ohba et al. (2000) suggested K represents the liquid phase of the hydrothermal system beneath Crater Yugama. The bold broken line
shows the ropeway. Thin broken curves indicate the ski slopes. The altitude interval of the contour line is 20 m. The thin solid curve indicates the car road.

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Ohba et al.

Fumarolic Gases at Kusatsu-Shirane Volcano Japan(Uto et al., 1983). Most of the historical eruptions occurred near
Crater Yugama. Following the first historical eruption, several
eruptions took place over the period 1897 to 1905, with aneruption in 1902 at Crater Yumiike (Ym in Figure 1). After an
eruption in 1905 there was a dormant period lasting 20 years.
The volcanic activity was maximized over the period 1925 to
1942. In 1932, Crater Yugama experienced the largest historicaleruption, with 20 cm of volcanic ash deposition at 2 km eastof Crater Yugama (Minakami et al., 1942). In 1976, a steam
eruption took place within Crater Mizugama. One year prior to
the eruption, an increase of the SO2 /H2 S ratio from the fumarolicgasses was observed (Ossaka et al., 1980). In 1982 and 1983,
steam eruptions occurred at Crater Yugama (Ossaka et al., 1997).
Lithics of 0.5–1 m in diameter were thrown over 500 m fromthe crater. Although the magnitude of the eruption was only
1 in the VEI scale (Global Volcanism Program, 2013), visitors
would certainly have been at risks. Fortunately, the eruption
occurred during winter when the access road was closed. In
1990, a large number of volcanic earthquakes occurred beneath
Crater Yugama although no eruption took place. After theseismic activity started, the pH of lake water in Crater Yugama
decreased, accompanying an increase of chloride concentration(Ohba et al., 1994, 2000). Ohba et al. (2008) estimated thatthe activity was caused by the break of sealing zone (Fournier,1999) and subsequent “dry degassing” (Giggenbach, 1997). Basedon the observation by Japan Meteorological Agency (JMA) two
hypocenter clusters of volcanic seismicity have been recognizedon the summit of volcano (Figure 1). One is a cluster beneath
Crater Mizugama (Mi) and another cluster is located east of
Ainomine peak (Ap). The hypocenters are distributed in therange of depth between sea level and the surface (up to 2000 m
above sea level; Figure 1). In January 2018, a small steam eruption
took place near Motoshirane peak (Er in Figure 1). This eruption
occurred in an unexpected place far from Crater Yugama. In
April 2018, a large number of volcanic earthquakes happened
beneath Crater Mizugama (Mi) and the east side of Ainomine
cone (Ap), although no eruption has yet occurred.
The unusual green–gray color of the lake water in Crater
Yugama attracts sightseers who climb to a belvedere situatedon the southern rim of the crater. The Crater Yugama is an
important resource for the local economy. In order to reveal
the relationship between the seismic activity and fumarolic gas
composition, a periodical (2 or 3 times per year) sampling
and analysis of five fumarolic gas discharges was carried out
since July 2014 until August 2018. In this study we aim toinvestigate the processes affecting the magmatic-hydrothermal
system beneath Crater Yugama on the basis of the chemical andisotopic composition of fumarolic gasses.

TABLE 1 | Acronyms used in the text.
Meaning

Yg

Crater Yugama

Mi

Crater Mizugama

Ym

Crater YumiikeSp

Peak of the Shirane cone

Ap

Peak of the Ainomine cone

Mp

Peak of the Motoshirane cone

Er

Eruptive point in 2018 near Ap and Mp

JMAJapan Meteorological Agency

W, C, E

Fumaroles located on the north of the Crater Yugama

M, S

Fumaroles located west and east slope of Kusatsu-Shirane
volcano, respectively

AET1

Apparent equilibrium temperature defined for the reaction among
H2 O, H2 S, SO2 and H2

AET2

Apparent equilibrium temperature defined for the isotope exchange
reaction between H2 O and H2

Mv

Magmatic gas derived from the degassing magma

Mc1

Components in the magmatic gas with high N2 /He ratio in the earlystage (until October 2015)

Mc2

Components in the magmatic gas with low N2 /He ratio in the latestage (after April 2018)

Hc

Components generated in the hydrothermal reservoir, such as CH4
and H2 S

Cc

Components extracted from the crustal rock hosting thehydrothermal reservoir

Lw

Local meteoric water

Vp

Vapor phase in the hydrothermal reservoir

Lp

Liquid phase in the hydrothermal reservoir

Lv

Water vapor generated from the local meteoric water with aconductive heating

Cd

Condensation of water vapor

En

Enthalpy used for the heating of groundwater

The total number of samples was 8 and 3 for the points S and
M, respectively. Gas sampling was carried out with a titanium
pipe that was inserted into the fumarolic orifice. One end of
the titanium pipe was connected to a rubber tube. The end of
the rubber tube was connected to a 120 ml Pyrex glass bottlewith air tight stop cock (Giggenbach, 1975), in which a 5 molar
20 ml KOH solution was put. The head-space of the gas bottle
was evacuated prior sampling. Water and acidic gasses wereabsorbed by the KOH solution. In the headspace, the residual
gasses (hereafter R-gas) such as N2 , O2 , Ar, He, H2 and CH4
were collected. Components absorbed in the KOH solution,such as H2 O, CO2 , H2 S and SO2 were analyzed according to
the method by Ozawa (1968). The ratio of SO2 /H2 S in the
fumarolic gas was determined using a KI-KIO3 solution (Ozawa,1968). The molar amount of R-gas was calculated by the gas
state equation accounting the head space volume of bottle andthe inner pressure in the head space at room temperature.
Based on the molar amount of H2 O, CO2 , H2 S, SO2 andR-gas, the relative concentration (%) of those components wascalculated (Table 2).
The R-gas was analyzed using two different gas
chromatographs with Ar and He as carrier gasses, hereafter

MATERIALS AND METHODS
Fumarolic gasses had been sampled since July 2014 until August
2018, with a frequency of a few times per year at the points
W, C and E (Figure 1). The total number of samples was 10,
10 and 9 for the points W, C and E, respectively. Additionally
fumarolic gasses were sampled at the points S and M (Figure 1).

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TABLE 2 | Chemical and isotopic composition of fumarolic gasses with the apparent equilibrium temperatures.
R-gas
Location

Date

Temp

H2 O

CO2

H2 S

SO2

R-gas

He

H2

O2

N2

CH4

Ar

δDSMOW

δ18 OSMOW

δD(H2 )SMOW

AET1

AET2

◦C

%

%

%

%

%

%

%

%

%

%

%

h

h

h

◦C

◦C

4

W

July 23, 2014

92.4

95.38

4.19

0.39

0.00500.0421

0.084

0.062

0.062

99.61

0.000

0.178

−65

−6.7

−562

166

161

W

May 15, 2015

93.5

96.43

3.11

0.43

0.00360.0321

0.071

0.141

0.000

99.56

0.025

0.205

−69

−7.8

−560

177

165

W

October 15, 2015

94.2

97.42

2.05

0.50

0.00100.0254

0.054

0.137

0.020

99.52

0.043

0.227

−67

−7.1

−555

156

169

W

April 22, 2016

93.5

97.52

1.93

0.52

0.00690.0294

0.033

0.147

0.047

99.38

0.184

0.210

−78

−8.2

−536

180

194

W

October 28, 2016

93.4

98.26

1.31

0.40

0.00490.0248

0.022

0.108

0.025

99.05

0.230

0.560

−66

−7.0

−510

165

215

W

April 25, 2017

93.3

98.12

1.42

0.44

0.00450.0190

0.026

0.170

0.056

99.17

0.309

0.268

−81

−10.1

−562

169

168

W

November 10, 2017

93.6

98.42

1.13

0.43

0.00270.0173

0.028

0.133

0.034

99.28

0.203

0.324

−75

−8.1

−556

155

171

W

May 18, 2018

94.3

97.86

1.72

0.40

0.00470.0187

0.086

0.120

0.015

99.49

0.044

0.242

−74

−7.8

−626

160

106

W

June 19, 2018

94.8

97.14

2.42

0.43

0.00160.0113

0.251

0.223

0.058

99.05

0.048

0.367

−70

−7.6

−568

153

158

W

August 06, 2018

94.6

97.70

1.89

0.40

0.00110.0097

0.209

0.229

0.045

99.21

0.057

0.251

−73

−8.0

C

July 23, 2014

94.1

95.45

3.97

0.53

0.00720.0435

0.083

0.057

0.028

99.66

0.000

0.176

−77

−9.5

−538

165

191

C

May 15, 2015

93.4

96.80

2.76

0.41

0.00490.0298

0.074

0.080

0.015

99.66

0.008

0.167

−68

−7.2

−548

162

176

C

October 15, 2015

93.6

97.80

1.77

0.40

0.00210.0237

0.053

0.095

0.025

99.61

0.026

0.196

−70

−7.2

−533

153

193

C

April 22, 2016

97.2

97.80

1.75

0.42

0.00760.0258

0.034

0.087

0.052

99.47

0.145

0.209

−67

−5.4

−520

164

205

C

October 28, 2016

93.8

98.07

1.48

0.42

0.00500.0224

0.027

0.108

0.014

99.43

0.223

0.195

−66

−6.1

−482

162

247

C

April 25, 2017

93.6

98.21

1.34

0.42

0.00510.0194

0.025

0.112

0.035

99.36

0.233

0.231

−66

−6.3

−515

159

210

C

November 10, 2017

94.1

98.19

1.30

0.48

0.00380.0235

0.026

0.167

0.012

99.25

0.265

0.281

−75

−8.0

−546

172

182

C

May 18, 2018

94.3

97.86

1.73

0.40

0.00230.0183

0.089

0.089

0.020

99.51

0.055

0.238

−118

−16.3

147

145

June 19, 2018

94.0

97.56

2.00

0.43

0.00050.0104

0.222

0.158

0.074

99.31

0.042

0.198

−109

−15.1

August 6, 2018

94.5

97.80

1.78

0.41

0.00110.0114

0.165

0.165

0.034

99.22

0.042

0.375

−107

−14.6

E

May 15, 2015

93.4

96.94

2.74

0.29

0.00350.0252

0.095

0.169

0.000

99.53

0.017

0.192

−88

−10.9

−529

179

206

E

October 15, 2015

93.3

97.75

1.93

0.30

0.00190.0212

0.069

0.238

0.015

99.44

0.025

0.214

−79

−8.8

−551

177

179

E

April 22, 2016

94.1

97.78

1.84

0.35

0.00600.0246

0.043

0.206

0.033

99.41

0.104

0.208

−88

−10.0

−549

187

186

E

October 28, 2016

93.6

98.62

1.09

0.27

0.00260.0174

0.029

0.328

0.023

99.23

0.150

0.235

−79

−8.5

−525

185

206

E

April 25, 2017

93.8

98.53

1.11

0.34

0.00240.0167

0.027

0.387

0.039

99.11

0.204

0.232

−96

−12.9

−510

185

233

E

November 10, 2017

93.0

98.89

0.84

0.26

0.00270.0147

0.029

0.326

0.010

99.19

0.170

0.278

−84

−9.6

−557

180

175

E

May 18, 2018

94.8

98.40

1.28

0.30

0.00180.0171

0.057

0.316

0.029

99.12

0.074

0.401

−84

−9.9

−560

178

172

E

June 19, 2018

94.0

98.06

1.63

0.30

0.00120.0111

0.154

0.495

0.056

98.94

0.076

0.277

−77

−8.4

−564

175

165

E

August 6, 2018

93.9

98.18

1.53

0.28

0.00230.0109

0.135

0.432

0.055

99.05

0.055

0.277

−77

−8.3

S

July 22, 2014

94.5

95.17

3.42

1.34

0.01530.0500

0.031

0.065

0.000

96.79

1.65

1.46

−110

−18.5

S

November 14, 2014

93.9

94.23

4.36

1.34

0.00770.0656

0.031

0.056

0.014

97.68

1.23

0.99

S

April 25, 2017

94.1

94.71

3.44

1.77

0.00850.0731

0.023

0.225

0.010

96.69

1.66

1.39

−115

−19.1

−692

213

65

S

March 2, 2018

93.5

94.87

3.48

1.58

0.01240.0635

0.024

0.066

0.066

96.20

2.02

1.63

−115

−19.3

−626

174

122

S

April, 24, 2018

94.1

96.47

2.36

1.12

0.00640.0378

0.024

0.068

0.011

96.34

2.20

1.36

−113

−18.8

−655

157

96

S

May 18, 2018

94.8

96.62

2.20

1.13

0.00600.0385

0.022

0.072

0.005

96.33

2.22

1.35

−110

−18.3

−553

133

192

142

177
−615

171

130

168

S

June 19, 2018

94.9

96.49

2.35

1.11

0.00400.0392

0.028

0.068

0.013

96.58

1.98

1.33

−106

−17.5

S

August 6, 2018

94.7

96.42

2.41

1.13

0.00460.0359

0.036

0.074

0.022

96.56

2.03

1.28

−108

−18.0

159
−634

154

111

155

M

April 24, 2018

94.6

99.47

0.21

0.31

0.00100.00930.0083

1.16

0.042

95.73

1.57

1.49

−108

−17.6

−546

192

199

M

June 19, 2018

94.9

99.52

0.17

0.30

0.00240.00950.0063

1.19

0.090

95.65

1.63

1.43

−102

−16.7

−575

204

165

M

August 6, 2018

94.8

99.53

0.15

0.31

0.00170.00940.0078

1.16

0.105

95.80

1.60

1.32

−106

−17.2

199

Fumarolic Gases at Kusatsu-Shirane Volcano Japan

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CCOhba et al.

Fumarolic Gases at Kusatsu-Shirane Volcano Japanwas separated from the H2 peak of 17% STP. All R-gas samples
had a good separation between He and H2 peaks on the chart of
GC-Ar. By using a GC-He, the relative concentrations of N2 andAr were determined. In the GC-He a 6m-long Gaskuropack-54
column (GC Sciences Inc.) and a TCD detector were installed.
The temperature of column and detector was kept at −70 and
50◦ C, respectively. With this condition, N2 , O2 and Ar peaks
in normal atmospheric air were separated. For all the R-gas

GC-Ar and CG-He, respectively. The GC-Ar was used todetermine the concentration of He, H2 , O2 , N2 and CH4 . In the
GC-Ar, a 6m-long MS5A packed column and a TCD detector
were installed. The temperature of the column and detector was
kept at 50 and 100◦ C, respectively. The flow rate of Ar carriergas was 30 ml/min. In general, He in volcanic gas is difficult
to be analyzed due to the overlap of H2 peaks if the amount ofco-existing H2 is high. In this study, the He peak of 17ppm STP

FIGURE 2 | Time variation of CO2 /H2 O (A), He/H2 O (B), and N2 /H2 O (C) molar ratios calibrated on the left y-axis. Some data were adjusted to the y-axis range bymultiplying values indicated in figure. The red histogram calibrated on the right y-axis shows the monthly number of volcanic earthquakes observed by JMA.

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Fumarolic Gases at Kusatsu-Shirane Volcano Japan

FIGURE 3 | Time variation of H2 S/H2 O (A), CH4 /H2 O (B), and Ar/H2 O (C) molar ratios calibrated on the left y-axis. Some data were adjusted to the y-axis range bymultiplying values as indicated in figure. The red histogram calibrated on the right y-axis shows the monthly number of volcanic earthquakes observed by JMA.

normalized, so that the sum of X for He, H2 , O2 , N2 , CH4 andAr was 100% (Table 2). For the determination of the H2 O isotopic
ratio, the fumarolic gas was cooled with a double tubes condenser
made of Pyrex glass. The isotopic ratios of oxygen and hydrogenin the condensates were determined by an IR-laser cavity ring
down analyzer (Picarro Inc., L2120-i). The isotopic ratio of theH2 in R-gasses was determined by a continuous flow system
combined with a mass spectrometer (Thermo Fischer Scientific
Delta V) (Tsunogai et al., 2011).

samples a good separation among N2 , O2 and Ar peaks wasprovided. The concentration of Ar in the R-gasses, XAr was
calculated by,
XN2 RAr
XAr =
(1)
RN2
where R indicates the concentration of Ar or N2 determinedby GC-He. XN 2 indicates the N2 concentration determined by
GC-Ar. The concentration of components in the R-gasses was

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Fumarolic Gases at Kusatsu-Shirane Volcano JapanMay 2018. The CO2 /H2 O and He/H2 O ratios of the point S
were high in July 2014 until March 2018, and then dropped
in April 2018 followed by an increase in He/H2 O and slightincrease in CO2 /H2 O. The CO2 /H2 O and He/H2 O ratios of
M were much lower than the values of other fumarolic gasses.
The N2 /H2 O ratio of W, C and E slowly decreased since
July 2014 until April 2017, followed by a stable period until
May 2018 (Figure 2). After May 2018, the N2 /H2 O ratio of

RESULTS
The most abundant component in the fumarolic gasses was
H2 O, up to 93–95% (Table 2). The content of other species
was expressed as the molar ratio to H2 O in Figure 2. For the
points W, C and E the CO2 /H2 O and He/H2 O ratios show asimilar time variation; namely, they monotonically decreased
since July 2014 until November 2017 and then increased in

FIGURE 4 | Time variation of δD(H2 O) (A), δ18 O(H2 O) (B), and δD(H2 ) (C) calibrated on the left y-axis. The red histogram calibrated on the right y-axis shows themonthly number of volcanic earthquakes observed by JMA.

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Fumarolic Gases at Kusatsu-Shirane Volcano Japan(hereafter AET) was calculated assuming the equilibrium of thefollowing reaction.

W, C and E declined together in June 2018. The N2 /H2 O
ratio of S was high until March 2018, and then it dropped
after April 2018. The H2 S/H2 O ratio of W, C and E had nomonotonic increase or decrease through the period of this study(Figure 3A). The stable H2 S/H2 O ratio of S until March 2018
declined significantly in May 2018. A similar decrease can befound in the CH4 /H2 O ratio of S (Figure 3B). The CH4 /H2 O
ratio of W, C and E show a noticeable feature; namely, they
were not detected in July 2014 when the seismic activity was
high. They significantly increased in April 2016 until November
2017, followed a quick drop in May 2018. The Ar/H2 O ratioof S was almost higher than the ratios of W, E and C, except
October 2016. The Ar/H2 O ratio of M was high relative tothe ratios of W, E and C (Figure 3C). The δD and δ18 O of
H2 O in fumarolic gas (Figures 4A,B) were almost stable besidea drop of C after May 2018. The δD and δ18 O of S wereslightly increasing after April 2018. The δD(H2 ) in fumarolicgas (Figure 4C) of W, C, E was almost stable except thesmall rising in October 2016 and a drop of C in May 2018.
The δD(H2 ) of S was lower than the δD(H2 ) of W, C, E
over the whole period. The apparent equilibrium temperature

SO2 + 3H2 = H2 S + 2H2 O(2)AET is worth due to a potential measure for the temperatureof gas source. The equilibrium of the above reaction depends on
the total pressure of gas. The total pressure of gas was assumed tobe the saturation pressure of H2 O at AET. For the calculation,
the equation by Ohba et al. (2010) was used. The apparent
equilibrium temperature (AET) was also calculated assuming
the equilibrium of the following isotope exchange reaction.
HD + H2 O = H2 + HDO(3)For the calculation, the fractionation factor given by Richet
et al. (1977) was used. Hereafter, the AETs for the reactions(2) and (3) are designated to be ATE1 and AET2, respectively.
As shown in Figure 5, AET1 of W, C, E was stable overthe entire period. AET2 of W, C, E showed almost similarvalues each other. A weak rise of AET2 for W, C, E
occurred in 2016.

FIGURE 5 | Time variation of apparent equilibrium temperature [AET1 (A) and 2 (B) defined in the text] calibrated on the left y-axis. The red histogram calibrated on
the right y-axis shows the monthly number of volcanic earthquakes observed by JMA.

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FIGURE 6 | Time variation of CO2 /He (A), N2 /He (B) molar ratios calibrated on the left y-axis. The red histogram calibrated on the right y-axis shows the monthly
number of volcanic earthquakes observed by JMA.

DISCUSSION

N2 /He ratios of W, C and E are not constant throughout the study
period, and cooperative fluctuations are observed. Namely, theseratios showed high values from April 2016 to November 2017.
As a cause of this fluctuation, a) CO2 /He ratio and N2 /He ratioof magma component themselves fluctuated. Alternatively, b)
CO2 and N2 other than magmatic component were contaminatedduring April 2016 to November 2017. At first we consider the
possibility of a). As for the cause of the variation of the magmaticcomponent, a change due to magma degassing is possible. Alongwith magma degassing, the He-N2 -CO2 ternary composition will
be governed by the solubility of each gas to magma. Namely,
as degassing progresses, gasses with low solubility are selectively
removed, and gas components with high solubility become
gradually prominent. The solubilities of He, N2 and CO2 inmagma are approximately 2.5 × 10−7 (mol g−1 bar−1 ) (Carroll
and Webster, 1994), 1.3 × 10−9 (mol g−1 bar−1 ) (Humbert
et al., 1998) and 1.3 × 10−8 (mol g−1 bar−1 ) (Giggenbach,1996), respectively. As the degassing progresses, the magmaticcomponent is relatively rich in He having the highest solubility
and is relatively deficient in N2 . Actually, as shown in Figure 7,
from July 2014 (position-A) to November 2017 (position-B),

Multiple Components With Different
Origins in the Fumarolic Gasses
For the source of He in fumarolic gas, two candidates are possible.
One is He derived from a degassing magma. The other one is the
He derived from the radioactive U and Th decay chain. According
to Sano et al. (1994), the 3 He/4 He ratio in the fumarolic gas at
Kusatsu-Shirane volcano was high as 8Ratm , where Ratm is theatmospheric 3 He/4 He ratio, suggesting that the He in fumarolicgas originates in a degassing magma. Sano et al. (1994) recognizeda negative correlation between 3 He/4 He ratio and δ13 C of CO2
in fumarolic gas and bubble gas associated with hot springdischarges, suggesting an addition of CO2 with high δ13 C, likely
derived from a limestone contained in the basement rock situatedbeneath Kusatsu-Shirane volcano (Uto et al., 1983). As shownin Figure 2, the variation pattern of He/H2 O, CO2 /H2 O andN2 /H2 O ratios resemble each other for W, C and E, namely, theratios monotonically decreased since July 2014 until November
2017, suggesting that CO2 and N2 would also originate in adegassing magma. According to Figure 6, the CO2 /He and theFrontiers in Earth Science | www.frontiersin.org

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FIGURE 7 | He-N2 -CO2 ternary molar composition of fumarolic gas. The position of A, B, and C indicate the composition in July 2014, November 2017 and August
2018, respectively.

it is highly likely Cc. In Figure 7, it is considered that the change
of A to B, the CO2 /N2 ratio of the additional component wasconstant. Therefore, CO2 as an additional component is also
regarded as crustal component (Cc). It is presumed that theCH4 /N2 ratio of the additional component increased because theredox state of the hydrothermal reservoir became reduced. As acause of the reduction, the decrease in the flow rate of magmaticgas enriched in SO2 is possible.
According to the conventional theory (Kita et al., 1993), theHe-N2 -Ar ternary composition of volcanic gasses is controlled by
the tectonic environment where the target volcano is located. For
example, in subduction zones volcanoes are strongly influenced
by a subducting oceanic plate. Nitrogen supplied from theoceanic plate is taken into the magma and the N2 /He ratio ofthe volcanic gas is high (Sano et al., 2001). On the other hand,the volcanoes located in the back arc and hot spots have low
N2 /He ratio because the influence of N supplied from the oceanic
plate is small or negligible. Figure 9 plots the He-N2 -Ar ternarycomposition of the fumarolic gas. Distribution of fumarolic gas
on the figure is not consistent to the conventional theory. W, E
and C were in the area specific to some Mediterranean volcanoes,
such as Vulcano, Campli Flegrei and Milos (Giggenbach, 1997)
around July 2014 (position-A), but moved to the position of
Circum-Pacific Rim volcanoes by November 2017 (position-B).
After that, again it moved to a region of the Mediterranean
volcanoes (position-C), but with N2 /He ratios lower than theposition-A in July 2014. The shift from A to B can be explainedby the addition of N2 as crustal component (Cc). HereafterN2 temporally enriched, contradicting the prediction in the caseof a). Therefore, the possibility of a) is low. Next, we consider
the possibility of b). CH4 is not a magmatic component, it isthought to be produced through the reduction of CO2 by Fe2+
contained in the rocks (Giggenbach, 1997). Here, the component
generated in the hydrothermal system is defined as hydrothermal
component (Hc). CH4 is a typical Hc. As other Hc, H2 S is alsomentioned. H2 S can be generated by reduction of SO2 . The
CO2 /He and N2 /He fluctuation of W, C and E (Figure 6) aresimilar to the variation pattern of CH4 /H2 O ratio (Figure 3B).
This similarity suggests that CO2 and N2 other than magmaticcomponent may have been added to W, C and E during April
2016 to November 2017. The trajectory of He-N2 -CH4 ternarycomponent fluctuation has an interesting pattern (Figure 8). In
Figure 8, the composition of W, C, E was in the position of A in
July 2014. Hereafter, the composition at the position-A is defined
to be Mc1. The composition of W, C and E gradually moved to thedirection of CH4 corner and reached the position-B in November
2017. The trajectory from A to B depicts a convex upward curve.
This indicates that at the beginning, the CH4 /N2 ratio of theadditional component to the composition of A was low and theCH4 /N2 ratio of the additional component later increased. The
composition in the position B changed toward another position
C instead of returning to A. CH4 and N2 are likely to havedifferent origins as components added in the process from A to
B. Here, CO2 , N2 derived from carbon and nitrogen contained inhost rocks hosting hydrothermal reservoirs are defined as crustal
component (Cc). As a source of N2 as the additional component,

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FIGURE 8 | He-N2 -CH4 ternary molar composition of fumarolic gas. The position of A, B, and C indicate the composition in July 2014, November 2017 and August
2018, respectively.

FIGURE 9 | He-N2 -Ar ternary molar composition of fumarolic gas. The position of A, B, and C indicate the composition in July 2014, November 2017 and August
2018, respectively. Cp, Me, Ma represent the typical endmember composition of gas from volcanoes on circum-Pacific volcanic belt, Mediterranean sea and hot
spots with mantle origin, respectively (Giggenbach, 1997).

the component at C is defined as Mc2. The fumarolic gas
at A and C contain little CH4 . Therefore, Mc1 and Mc2 arethought to represent the magmatic component. Mc2 is poorin N2 relative to Mc1. This feature may have occurred withmagma degassing, because N2 is less soluble than He and CO2 .

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Based on the above discussion, the evolution of hydrothermal
reservoir, which is the source of fumaroles W, C and E, isschematically shown in Figure 10. In Period I, magmatic gas
is transported to hydrothermal reservoir passing through thesealing zone (Fournier, 1999). The composition of the magmatic

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FIGURE 10 | Conceptual model for the evolution of magma hydrothermal system beneath Crater lake Yugama.

Let us now examine the possibility that the compositionalchange of the fumarole occurred as a result of the earthquake.
Such a situation can occur, for example, when a fluid is confinedin a hydrothermal system, and the host rock is fractured byseismic activity and the fluid is released. The fluid shouldinteract with the host rock. Generally, rocks are rich in Fe2+ ,which acts as a reducing agent and converts CO2 to CH4(Giggenbach, 1997). Therefore, the fluid confined in the spaceshould contain CH4 . Moreover, CO2 and N2 , which are crustal
components contained in the host rock, will be extracted into
the fluid. The fluid interacting with the host rock should be thusrich in hydrothermal components CH4 and crustal components
CO2 and N2 . If such fluids are released by seismic activity,
the CH4 /H2 O, CO2 /He, and N2 /He ratios will increase justafter the seismic event. Since such changes are opposite to
the actual observations, the possibility that seismic activity is
responsible for the compositional change of fumarole is denied.
Therefore, we postulate that the magmatic gas was injected into
the hydrothermal system, triggering the seismicity.

gas was Mc1. The fluid pressure in the reservoir was high,and earthquakes occurred frequently. Kusatsu-Shirane volcano
experienced a seismic crisis in 1990 and 1991. Based on thetemporal changes of the lake water in Crater Yugama, Ohba et al.
(2008) modeled the magma-hydrothermal system. In 1990 and
1991, the chloride concentration of lake water increased quickly
accompanying the decrease in pH of lake water, suggesting an
input of HCl enrich magmatic gas to the hydrothermal reservoir
beneath Crater Yugama through the sealing zone surrounding
the degassing magma. In 2014, a chloride increase and pH drop,
similar to those in 1990, were observed in the lake water (inpreparation for publication). Considering the similarity between
the activity in 1990 and 2014, an input of magmatic gas throughthe sealing zone was assumed in the Period-I. In period II, due
to the regrowth of sealing zone, the release of magmatic gas
was reduced. The hydraulic pressure in the reservoir decreased
and the occurrence of the earthquake was suppressed. During
this period, the flow rate of SO2 supplied to the reservoir would
decrease, the fluid in the reservoir gradually became reductive,
and CH4 was generated by the reductive reaction of CO2 . In
addition, carbon and nitrogen contained in host rock of reservoir
were extracted to the hydrothermal fluid as CO2 or N2 . In
Period III, magmatic gas whose composition is Mc2 again passed
through the sealing zone. The fluid pressure in the reservoir
again increased, causing frequent earthquakes. The N2 /He ratioof Mc2 decreased relative to the N2 /He ratio of Mc1, due to theprogress of degassing.

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Formation Process of Fumarolic Gas
The distribution of fumarolic gasses (W, C, and E) on theδD vs. δ18 O plane (Figure 11) coincides with the slope of
the isotopic fractionation between water and steam at 100◦ C.
A partial condensation of water vapor near the surface of theground is presumed to produce the observed distribution. The
isotope ratio of water vapor before condensation will be around12

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FIGURE 11 | δD and δ18 O of H2 O in fumarolic gas with the modeled vapor phase in hydrothermal reservoir. The isotopic fractionation between water and vapor withvarious temperatures is inserted.

the upper right of the distribution. As a model to generate water
vapor for the explanation of isotopic ratios of W, C and E, weconsidered a mixing of high-temperature magmatic gas (Mv) andcold meteoric groundwater (Lw). Numerical values necessary forthe model calculation are listed in Table 3. As a result of mixingof Mv and Lw, water vapor (Vp) and coexisting hot water (Lp)are generated. In the model calculation, the temperature of Vp
and Lp is necessary. Considering the AET1 and 2 of W, C andE, 200◦ C was adopted as an appropriate and delimitating value
for the temperature of Vp and Lp. In this model, CO2 /H2 O andstable isotopic ratios of Mv should be input and CO2 /H2 O andstable isotope ratios of Vp and Lp can be outputted. Equations
necessary for calculation are, as follows:
HMv f + HLw (1 − f ) = HVp g + HLp (1 − g)(4)

δMv f + δLw (1 − f ) = δVp g + δLp (1 − g)(5)

CMv f + CLw (1 − f ) = CVp g + CLp (1 − g)(6)

δLp + 1000α=δVp + 1000
β=

CVp
CLp

on the amount of H2 O. Alpha (α) is the isotopic fractionation
factor between liquid and vapor phase in terms of D/H and
18 O/16 O ratios (Horita and Wesolowski, 1994). Beta (β) is the

TABLE 3 | Parameters used for the model calculation generating Vp.
Term

(7)
(8)

where H, δ, and C represent enthalpy, the δ-notated isotope ratio,
and the CO2 /H2 O value, respectively. The phases are indicatedby subscripts (Mv, Lw, Vp, and Lp). f and g denote the mixing
fraction of Mv and the generating fraction of Vp, respectively.
In general, g is not equal to f. The value of f and g is based

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13

SymbolValue

Unit References

Temperature of Mv

900

◦C

Temperature of Lw

15◦C

Enthalpy of Mv

HMv

4391

kJ/kg The Japan Society of MechanicalEngineers [JSME] (1999)

Enthalpy of Lw

HLw

64

kJ/kg The Japan Society of MechanicalEngineers [JSME] (1999)

Enthalpy of Vp at200◦ C

HVp

2790

kJ/kg The Japan Society of MechanicalEngineers [JSME] (1999)

Enthalpy of Lp at
200◦ C

HLp

855

kJ/kg The Japan Society of MechanicalEngineers [JSME] (1999)

δ18 O of Mv

δMv

8

δ18 O of Lw

δLw−11.83

h

δMv

−25

h

Ohba et al. (2000)

δD of Mv
δD of Lw

δLw

−79.3

h

CMv

0.008

h

Ohba et al. (2000)

CO2 /H2 O molar ratioof Mv
CO2 /H2 O ratio of Lw

CLw0

D/H and 18 O/16 O
fractionation factorαHorita and Wesolowski (1994)

CO2 /H2 O
distribution
coefficientβGiggenbach (1980)

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The isotopic ratios of S and M are similar. The positionsof S and M are clearly different from W, C and E. Although
the similarity in isotopic ratios between S and M, they havequite different CO2 /H2 O ratios (Figure 12). By considering theCO2 /H2 O ratio in addition to the isotopic ratio, the formation
mechanism of S and M is elucidated. In Figure 12, the scaled CO2
fraction is taken on the horizontal axis. The scaled CO2 fraction
is defined by the following equation.

distribution coefficient between vapor and liquid phase in termsof CO2 /H2 O ratio (Giggenbach, 1980). Equations 4, 5, and 6
describe the conservation of enthalpy, isotopic ratio, and theamount of CO2 , respectively. Equations 7 and 8 describe theequilibrium distribution of stable isotope and CO2 , respectively,
between Vp and Lp. Combing Eqs. 4 through 8, the isotopic ratio
and CO2 /H2 O ratio for Vp can be calculated. As a result of thecalculation, the isotopic ratio of Vp is represented by a line in
Figure 11. Because the degree of freedom is left in the mixing
ratio of Mv and Lw, Vp is expressed as the line. Comparing W, C,
and E with the high isotopic ratios near the line of Vp, Vp that isthe basis of W, C and E is considered to occur when the mixing
ratio of Mv is 0.3–0.4. It seems that this mixing ratio was stablefrom 2014 to 2018.

ScF(CO2 ) =50C
50C + 1(9)where C is the CO2 /H2 O molar ratio. In Figures 12A,B,
the vertical axis shows δD and δ18 O, respectively. First, thevapor (Vp) in reservoir condensed slightly, then mixed with

FIGURE 12 | Correlation between isotopic ratios and CO2 /H2 O molar ratio for fumarolic gasses and modeled vapor phase. ScF(CO2 ) is the scaled CO2 fraction in
H2 O-CO2 binary system.

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FIGURE 13 | Conceptual model for the magma hydrothermal system at Kusatsu-Shirane volcano.

reservoir. The compositional difference among the fumarolic gas
at M, S and the group of W, C and E suggests the existence ofindividual hydrothermal reservoir for the gasses at S and M. In
total, three reservoirs are necessary at Kusatsu-Shirane volcano.
The vapor in hydrothermal reservoir beneath Crater Yugama
discharges at surface as fumarolic gas after receiving a vapor
condensation. The vapor in the reservoir beneath fumarole
S is mixed with Lv and then appears on the ground surface
after a heavy vapor condensation near the surface. The vapor
in the reservoir beneath fumarole M undergoes mixing with alarge fraction of Lv then appears on the surface as fumarolicgas. The fumarolic gas M is highly affected by the atmosphericN2 and Ar (Figures 7, 9), which were carried by the meteoric
water vapor (Lv).

a water vapor generated from the local meteoric water (Lv).
A groundwater circulating in crust could be heated conductively.
The groundwater gets enthalpy through the heating. The
existence of Lv in Kusatsu Shirane volcano was demonstrated
by Ohwada et al. (2003). The fumarolic gas S is considered tohave formed from the mixture followed by a heavy condensation.
The fumarolic gas M has the CO2 concentration much lower
than that of S. The low CO2 concentration is due to the large
mixing fraction of Lv. After the mixing little condensation took
place. Some arbitrariness remains in the above model such as,
the temperature of Vp and Lp, the exact isotopic ratio of Lv etc.
However, this model successfully demonstrates the isotopic ratio
and the CO2 /H2 O ratio of all the fumarolic gasses of Kusatsu
Shirane volcano. In the model, a single magmatic gas (Mv) isrequired with the interaction of local meteoric water and thecondensation of water vapor.

CONCLUSION
Magma Hydrothermal System Beneath
Kusatsu Shirane Volcano

As a result of repeated collecting and analyzing fumarolic gas
at Kusatsu Shirane volcano, a close relationship was foundbetween the seismic activity and the chemical composition of
fumarolic gas near the summit (W, C, and E). When seismic
activity was active, He and CO2 increased, and when seismic
activity fell, CH4 and N2 increased. These components increase
and decrease in response to the injection of magmatic gas tothe hydrothermal reservoir. The N2 /He ratio of the fumarolicgas near the summit varies with two factors. When the crustal
component is added to the hydrothermal reservoir, this ratio
rises. As magma degassing progresses, this ratio decreases
inversely. It is estimated that the fumarolic gasses of KusatsuShirane volcano are generated by a single magmatic gas with theinteraction of meteoric groundwater. The fumarolic gas (S) at
the eastern slope has a relatively high CO2 concentration due

Figure 13 schematically shows the estimated magmahydrothermal system in Kusatsu-Shirane volcano. First,
there is a single degassing magma emitting magmatic gasses (Mv)through the sealing zone. Mv mixes with the cold groundwaterwith meteoric origin (Lw), then vapor (Vp) and thermal water(Lp) are generated in hydrothermal reservoir. As shown in
Figures 7, 9, the fumarolic gas at the point M is significantly
affected by atmospheric components relative to the fumarolic gas
at the point W, C, E and S. As shown in Figure 8, the fumarolicgasses at the points M and S are enriched in CH4 relative tothe gasses at W, C and E. There is a compositional similarity
among the gasses at W, C and E, suggesting the fumarolicgasses W, C and E are derived from a common hydrothermal

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to considerable vapor condensation. The fumarolic gas (M) on
the western slope contains CO2 with low concentration due tothe large contribution of water vapor generated from a meteoric
groundwater, consistent to the high concentration of atmosphericN2 and Ar. These data suggest that at Kusatsu-Shirane volcano
the activation of seismicity was synchronized with the increaseof magmatic components in fumarolic gas. It is postulated thatthe injection of magmatic gas increased the fluid pressure in thereservoir, which triggered the seismicity. The injection would
be triggered by a break of the sealing zone surrounding thedegassing magma. The injection of magmatic gas can be detectedby monitoring the composition of the fumarolic gas, thus giving
the possibility to forecast any future seismicity.

FUNDING
This research was supported by the Japanese Ministry of
Education, Culture, Sports, Science and Technology, under grantof the Integrated Program for Next Generation Volcano Researchand Human Resource Development 2016 to 2018, also under
its Earthquake and Volcano Hazards Observation and Research
Program 2015 to 2018, the Japan Society for the Promotion of
Science (JSPS) KAKENHI Grant Number 15K12485 in 2015 to2017, the Earthquake Research Institute, The University of Tokyo
Joint Usage/Research Program 2015 to 2018, and the General
Research Institute of Tokai University 2016 to 2018.

ACKNOWLEDGMENTS
AUTHOR CONTRIBUTIONS
We sincerely thank the funders for their funding support
for this research, the reviewers OV and DR for their efforts
in reviewing and improving the quality of the manuscript,
and the editor FT and Dr. Valerio Acocella for handling theeditorial process.

TO drafted the manuscript. TO, MY, KN, and NN sampled
fumarolic gasses and analyzed them. UT, MI, and RS analyzed
fumarolic gas samples. All authors read and approved thefinal manuscript.

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Conflict of Interest: The authors declare that the research was conducted in theabsence of any commercial or financial relationships that could be construed as apotential conflict of interest.
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