温泉科学（J. Hot Spring Sci.），60，410‑417（2011）

原著

Dissolved Inorganic Carbon Rich in Mantle Componentof Hot Spring Waters from the Hitoyoshi Basin in
a Non-volcanic Region of Central Kyushu, Japan
Shinji OHSAWA1）, Takuya SAKAI2）, Makoto YAMADA1）,
Taketoshi MISHIMA1）, Shin YOSHIKAWA3）
and Tsuneomi KAGIYAMA3)(Received Oct. 4, 2010, Accepted Dec. 20, 2010)

九州中部の非火山地域に位置する人吉盆地のマントル成分に富んだ温泉水の溶存無機炭素
大沢信二1），酒井拓哉2），山田 誠1），三島壮智1），吉川 慎3），鍵山恒臣3)

要旨

九州地方の阿蘇火山と霧島火山の間に位置する非火山地域の地下に深部流体を検出するために，地球化学的な広域温泉調査を実施した．調査した温泉の内 8ヶ所において，全溶存無機炭素（DIC）を含めた主要溶存化学成分の濃度を測定し、水の同位体組成（δD とδ18O）および DIC の炭素安定同位体比（δ13C）を得ることができたが，同時に溶存希ガス成分（He とNe）の化学・同位体データも取得することができた．希ガスデータの同位体システマテイッ
クスを検討した他，C/3H 比とδ13C の関係から DIC の炭素の起源推定を行ったところ，霧島火山の北方数十 km にある人吉盆地内において，沈み込み帯の火山性 CO2 とは異なるマント
ル由来のヘリウムを伴ったマントル成分に富む DIC を含む温泉の湧出があることを認めた．
キーワード：温泉，ヘリウム，全溶存無機炭素，同位体，起源，人吉盆地

Abstract
To detect deep-seated aqueous fluid in the non-volcanic region between Aso and1)

Beppu Geothermal Research Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Noguchibaru, Beppu, Oita 874‑0903, Japan. 1）京都大学大学院理学研究科附属地
球熱学研究施設 〒874‑0903 大分県別府市野口原．
2）Graduate School of Education, Oita University, 700 Dannoharu, Oita 870‑1192, Japan. 2）大分大学大学院
教育学研究科教科教育専攻理科教育専修 〒870‑1192 大分県大分市大字旦野原 700.
3）
Aso Volcanological Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto
University, Minami-Aso, Aso, Kumamoto 869‑1404, Japan. 3）京都大学大学院理学研究科附属地球熱学研
究施設火山研究センター 〒869‑1404 熊本県阿蘇郡南阿蘇村河陽 5280.

410

第 60 巻（2011）

九州中部の非火山地域に位置する人吉盆地のマントル成分に富んだ温泉水の溶存無機炭素Kirishima volcanoes of Kyushu District, we conducted a geochemical investigation of hot
springs over a wide area. Although the main geochemical data obtained were major
chemical components, dissolved inorganic carbon (DIC), water isotope compositions (δD andδ18O), and δ13C of DIC, we also obtained chemical and isotopic data of rare gases (He andNe), from eight sites of the hot springs in this area. From isotope systematics of the raregas data and consideration of source carbon of DIC based on the relation between C/3He
ratio and δ13C, we concluded that hot spring waters bearing DIC extremely rich in mantle
component with mantle-derived He, which diﬀer from subduction-related volcanic CO2, aredischarged from the Hitoyoshi Basin located several tens of kilometers north of Kirishima
volcano.
Key words : Hot spring, Helium, Dissolved inorganic carbon, Isotope, Origin, Hitoyoshi Basin

1.

Introduction

Kyushu District in Japan is crowded with active Quaternary volcanoes, but it includes anextensive non-volcanic area between Aso and Kirishima volcanoes (Fig. 1). This area mainlycomprises Mesozoic formations aligned mainly southwest-northeast, although Miocene igneousrocks are scattered, as presented in Fig. 1. Many people have investigated the problem of why
no active volcano exists in this area. Some suppositions have been presented (e.g., Horikoshi,
1979 ; Kobayashi, 1986 ; Yoshida and Seno, 1992). However, the problem has not been resolved.
Recently, a high crustal conductive zone was reportedly found in this area (Kagiyama andMunekane, 2006). Referring to a distribution map of isopleths of the Curie point depth (Okubo
1989), it can be thought that no igneous body with temperature greater than 500℃ lies to a

Fig. 1 Localities of hot springs investigated in this study (●) with related geologic information.
Open circles (○) denote other hot springs examined simultaneously in this study. Distributionof limestone masses is cited from the seamless digital geological map of Japan 1 : 200,000
produced by Geological Survey of Japan, AIST (Feb 1, 2010 version). Distributions of Miocene
granitic and volcanic rocks are traced from Fig. 3. 25 in Regional Geology of Japan Part 9
Kyushu (Editorial Committee of Kyushu, 1992). The isopleths of depth of subducting plate [top]are cited from Ishihara and Yoshida (1992).

411

大沢信二，酒井拓哉，山田誠，三島壮智，吉川慎，鍵山恒臣

温泉科学

depth of around 10 km under this area ; it will be considered alternatively that aqueous ﬂuid isseated latently. Accordingly, we investigated many hot springs in this area seeking deep-seatedaqueous ﬂuid and obtained a stable carbon isotope ratio (δ13C) of dissolved inorganic carbon(DIC=CO2(aq)+HCO3−+CO32−), together with concentrations of major chemical components
including DIC and water isotope composition (δD and δ18O). We also collected water samples
to obtain geochemical data of dissolved rare gases (He and Ne), in cases where atmospheric-air
contact to hot spring water was avoidable at sampling. Eventually, eight samples were obtained.
Examining the relation between the δ13C and C/3He ratio on the samples in conformity toprevious reports (Sano and Marty, 1995 ; Sano
Ohsawa and Yusa, 2001 ; Yamamoto2006 ; Mao2009 ; Ohba

1997 ; Nishio

2001 ; Deines, 2002 ; Sumino

1998 ; Sato1999 ;

2004 ; Doğ an2010), we found DIC extremely rich in mantle component

accompanied with mantle-derived He in hot spring waters discharged from the Hitoyoshi Basin.
Therefore, we expressly report this result herein before the presentation of other geochemical
data.

2.

Sampling and analyses

Hot spring waters, for analyses of dissolved inorganic carbon (DIC) and dissolved rare gases(He and Ne) and also for their isotope measurements, were collected from wells at eight hot
spring sites (solid circles T, AR, AX, AI, AE, AN, B, and E in Fig. 1). Samples for DIC werestored in a CO2 gas-tight plastic bottle (Barex@, Nikko Hansen & Co., Ltd., Japan), following theexample of Ohsawa(2002). Hot spring waters for rare gases were introduced directly in asoft copper pipe (outside diameter, 9.5 mm ; inside diameter, 7.9 mm ; length, 30 cm ; capacity,
about 40 mL) avoiding air-contamination ; they were sealed using special clamps, following theexample of Morikawa(2008).

Concentrations of DIC were determined using a CO2-gas electrode with an ion meter after
all carbonate species in the sample water were converted into CO2 (aq) by addition of sulfuric
acid. Stable carbon isotope ratios (δ13C) of DIC were obtained by transferring the generated
CO2 gas through addition of phosphoric acid to SrCO3 precipitated from the sample water to amass spectrometer at the Stable Isotope Laboratory of IGNS, New Zealand. Concentrations of
dissolved He, Ne, and 3He were analyzed using mass spectrometry at the Geo-Science Laboratory, Japan after extracting dissolved gases from the water samples in the copper pipe.
Maximum errors of the analyses were roughly estimated as [He]±1.0%, [Ne]±2%, [3He]±1.5%,[DIC]±3% (at ca. 50 mg/L), and δ13C±0.2‰.

3.

Results and discussion

Chemical and isotopic data obtained from the samples are presented in Table 1 with information showing the well depth and water temperature. The 3He/4He value is calculated fromconcentrations of 3He and He considering 4He to be the greater part of He. The measured
3

He/4He ratios normalized by the atmospheric value (1.39×10−6) are shown in Fig. 2 against

4

He/20Ne ratios, which are substituted from He/Ne ratios. All samples in this diagram are412

第 60 巻（2011）

九州中部の非火山地域に位置する人吉盆地のマントル成分に富んだ温泉水の溶存無機炭素Table 1 Chemical and isotopic data of dissolved inorganic carbon (DIC) and dissolved noble
gases of hot spring waters investigated in this study.
No.

Latitude(N)

Longitude
(E)

T
AR
AX
AI
AE
AN
B
E

32 14 57
32 12 34
32 13 33
32 23 2232 13 25
32 43 30
32 40 132 36 32

130 44 48
130 45 47
130 43 2
131 19 52
131 9 49
131 20 48
130 44 1
130 53 0

Depth ofWater
DIC δ13C-DIC
well (m) temp. (℃) (mg/L)
(‰)
1000
?
400
1500
1400
1000
996
1000

38.2
49.6
42.3
37.1
36.5
27.8
35.5
29.6

520
360
560
435
740
105
109
45

−10.0
−13.9
−9.0
−3.7
−4.5
−13.3
−13.5
−16.3

He
(ccSTP/g)

Ne(ccSTP/g)

3.53×10−5
9.43×10−6
3.34×10−4
9.39×10−7
2.80×10−6
2.73×10−7
6.82×10−8
6.14×10−7

1.13×10−6
2.15×10−7
6.00×10−7
2.90×10−8
1.01×10−7
2.29×10−7
1.78×10−7
3.98×10−7

3

He/4He

4.92×10−6
1.02×10−5
1.97×10−6
9.98×10−7
9.04×10−7
3.67×10−7
9.60×10−7
3.68×10−7

distributed in a region that is explainable interms of a mixing model with three endmember components : upper mantle gas,crustal gas containing a large amount ofradiogenic 4He, and atmospheric air. Two
thick lines connecting atmospheric air tomantle gas and to crustal gas in Fig. 2 are
the mixing lines. Sample AR is close to theupper mixing line to the mantle end member.

Samples B, AN, and E are on thelower mixing line from atmospheric air tocrustal gas. Samples T, AX, AE, and AI
Fig. 2 Plot of 3He/4He versus 4He/20Ne ratios forwater samples obtained for this study. The
upper line represents the mixing line between
air and upper mantle components ; the lower
one is a mixing line for air and crust components. The medium thickness lines show
trajectories for mixing between air and deeporiginated gas for various mantle helium
components of 10, 20 and 50%.

contain all the components, but mixing proportions of atmospheric air component arealmost zero, as determined using a calculation method of contribution of the three
end members presented by Shimizu(2005). By this calculation, contributions ofmantle He on sample AR, T, AX, AE, andAI are estimated as 91%, 44%, 18%, 8%, and

9%, respectively. Those of atmospheric He on samples B, AN, and E are shown as 83%, 27%,
and 21%, respectively.
Figure 3 portrays a diagram showing the C/3He ratio as a function of δ13C value presented
by Sano and Marty (1995). It is used to be applied to obtain a rigid constraint on the origin of
DIC for hot spring water samples. This ﬁgure shows data of eight water samples (T, AR, AX,
AI, AE, AN, B, and E) together with mantle, sedimentary organic carbon and marine carbonate(Sano and Marty, 1995 ; Nishio

1998 ; Deines, 2002), and subduction-related fumarolic gases

from Kirishima and Aso volcanoes (Sato1999 ; NEDO, 1989). All samples show mixing fea-tures of mantle, sedimentary organic carbon and marine carbonate components. However,contributions of the components vary and can be divided roughly into three groups. Samples B,413

大沢信二，酒井拓哉，山田誠，三島壮智，吉川慎，鍵山恒臣温泉科学

Fig. 3 Correlation between C/3He ratio and δ13C of DIC for water samples in this study.
In addition, model end members for mantle (M-a : Sano and Marty, 1995 ; M-b :Deines, 2002 ; M-c : Nishio et al., 1998), marine carbonate (Sano and Marty, 1995)
and sedimentary organic carbon [mean value of Sano and Marty (1995), Deines(2002) and Nishio(1998)] are shown in this ﬁgure. The tie lines of the modelend members are mixing lines.

AN, and E are rich in the sedimentary organic carbon component. These samples containremarkable amounts of atmospheric He as presented above, showing that the organic carbon isprobably be derived from soil. Samples AR, T, and AX are rich in the mantle component,
whereas samples AE and AI are not rich in the mantle component but rich in the marine
carbonate component. This distinction on the C/3He―δ13C diagram is approximately consistent
with the He isotope systematics described above. Moreover, samples AE and AI have features
similar to those of subduction-related volcanic CO2.
We examined the geographical distribution of geochemically diﬀerentiated hot springs. Hot
springs B, AN, and E are located in the northwestern part of the studied area occupied by old
geologic bodes accompanied with many limestone areas shown in Fig. 1 (Sambagawa andChichibu Belts). The contributions of the marine carbonate component on DIC of these hotspring waters are not negligible, perhaps because of the small limestone bodies. Hot springs AI
and AE, whose DIC are rich in the marine carbonate component more than hot springs B, AN,
and E, are located in the southeastern part of the studied area. Because the surface distributionof limestone is scarce in this area, the DIC rich in the marine carbonate component of these hotspring waters (AI and AE) is originated in greater depths, probably derived from dehydrated
ﬂuid from the subducting Philippine-Sea Plate such as Umeda(2006) assumed at the Kii

Peninsula of SW Japan. Because hot spring AE is situated along a distribution of Miocene
volcanic rocks, the possibility exists that such a DIC is residual magmatic CO2 in old igneous
body, but if so, the same feature ought to appear on hot spring AN. Hot springs AR, T, and AX,414

第 60 巻（2011）

九州中部の非火山地域に位置する人吉盆地のマントル成分に富んだ温泉水の溶存無機炭素whose DIC are rich in the mantle component, are situated in the Hitoyoshi Basin, which isthought to be a tectonic basin (e.g., Chida, 2000). The southern part of Kyushu, including this basin area from Pliocene to Pleistocene, was on a regional stress field by extensional or transtensional tectonics of NW―SE or NNE―SSW direction (Yamaji

2003). The high contribu-

tions of the mantle component to DIC and He of hot spring waters in the Hitoyoshi Basin suggest that a ﬂuid channel for uprising of mantle-derived CO2 and He must be formed in the crustunder this basin area.

Acknowledgments
Owners of hot springs at all sampling sites are acknowledged for permitting sample
collection. We also thank workers of the Geo-Science Laboratory, Japan, and the Stable Isotope
Laboratory of IGNS, New Zealand, for their technical assistance. This research was partially
supported by a Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for
Scientiﬁc Research (B) 19310116. Anonymous referees provided useful comments that led to considerable improvements in this manuscript.
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