Stable isotopic studies of japanese geothermal systems

Geothermics, Vol. 5, pp. 97-124. Pergamon Press, 1977. Printed in Great Britain

STABLE ISOTOPIC STUDIES OF JAPANESE GEOTHERMAL SYSTEMS H. S A K A I a n d O. M A T S U B A Y A Institute for Thermal Spring Research, Okayama University, Misasa, Tottori-Ken, Japan Almtraet--Stable isotopic studies on Arima type brines, Green Tuff type thermal waters and three volcanic systems, Hakone, lbusuki and Satsuma-lwojima, were reviewed with emphasis on the origins of the water and sulfur species in these systems. Of the three volcanicsystems. Hakone is a subaerml volcano consisting of calderas, central cones and a caldera lake, whereas lbusuki belongs to a caldera half-drowned in the ocean. Satsuma-lwojima is a volcanic island erupted within a drowned caldera ca. 40 km off the southern coast of Kyushu. Comparisons of the isotopic data of the waters and sulfur species from the three different volcanoes indicated that the waters of meteoric, oceanic and magmatic origins are involved in various ways and proportions in the volcanic activities. A considerable fraction of the volcanic sulfur species is sl~ownto be recyclicin origin. It was demonstrated that a combined use of chemical and isotopic data on thermal waters and dissolved sulfates would yield useful information on the hydrological aspects of many geothermal systems. INTRODUCTION It has been s h o w n that the J a p a n e s e thermal water systems isotopically a n d chemically are g r o u p e d into four m a j o r categories; that is, A r i m a type, G r e e n T u f f type, coastal type a n d volcanic type t h e r m a l waters ( M a t s u b a y a et al., 1973; Sakai a n d M a t s u b a y a , 1974). The characteristics of these four types o f t h e r m a l waters are s u m m a r i z e d in T a b l e I T h e i r locality is schematically s h o w n in Fig. 1.

•

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Fig. 1. Geothermal systems of Japanese Islands. The black spots in Fig. 1 are the m a j o r geothermal systems o f the Japanese Islands, m o s t , o f which are closely associated with Q u a t e r n a r y volcanic rocks a n d are classified as " v o l c a n i c type" in T a b l e 1. T h e rate o f heat flow within each o f these systems is greater t h a n 10 ~ cal/s a n d often 97

H. Sakai and O. Matsubaya

98 Table

I. F o u r

t y p e s o f t h e r m a l w a t e r s y s t e m s in J a p a n

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Geology

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Prc-Neogene plutonic and volcanic complex and metamorphic rocks

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Tottori Owani

Coastal

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Na-Ca CI 3 < CI < 20 g/kg

Ibusuki Shimogamo

Volcanic

Quaternary volcanic rocks

H SO~ H CI SO4 NaCI HCO s Mixed

Hakone Noboribetsu Tamagawa Satsuma lwojima

exceeds 107 cal/s. In coastal volcanic areas such as southern Kyushu and eastern lzu Peninsula, sea water is involved in water-rock interaction which produces isotopicatly and chemically unique hydrothermal systems of the coastal type. Thus, the coastal thermal system is a special case of the volcanic thermal waters. However, some coastal type systems exist, such as Shimogamo on the southern tip of Izu Peninsula (Mizutani and Hamasuna, 1972), which is not affiliated with Quaternary volcanisms. The solid lines of Fig. 1 indicate the outer margins of Green Tuff formations which are the marine clastic sediments of Miocene age with abundant pyroclastics and lava flows of the contemporaneous submarine volcanisms. The submarine formations are called Green Tuff formations due to the color of the propyritic alteration products. Green Tuff type thermal waters are generally found on the Japan Sea side of the submarine formations where volcanic activity has been low since the end of the Tertiary. The open circles in Fig. 1 are the most important of these systems. Note that Green Tuff formations underlie most of the Quaternary volcanic areas of Honshu and Hokkaido. Therefore, Green Tuff type waters sometimes occur in proximity to volcanic thermal waters in the central or backbone areas of Honshu and Hokkaido. Owani of Aomori-Ken is an example of this. The chemistry of Green Tuff type thermal waters is strongly controlled by interactions between hot meteoric waters and. the submarine formations forming Na-(Ca)-CI-SOg-type waters. The geothermal energy associated with this system is 105 to 106 cal/s and much less important than the volcanic and coastal thermal systems. Granitic rocks of Upper Cretaceous to Paleogene ages and Paleozoic metamorphosed rocks outcrop widely in outer southwest and northeast Honshu. Geothermal activities in these parts of Japan naturally are much less pronounced than those in the younger crusts. However, thermal and mineral waters of high salinity and of non-meteoric origin are found in these areas. Arima and Ishibotoke plotted in Fig. 1 are the most representative of them and are referred to the Arima type. Among these four types, volcanic systems including the coastal type are far more important than the other two from the geothermal point of view. although the latter two also have considerable importance in the study of water-rock interaction and genesis of ore solutions. Stable isotopic ratios of these thermal and volcanic systems have been measured rather extensively by several Japanese laboratories during the past decade. In the present paper, some important results of these studies will be reviewed with emphasis on the origins and hydrological features of the volcanic systems. A R I M A TYPE T H E R M A L A N D M I N E R A L WATERS Thermal and mineral waters at Arima of Kobe, Hyogo-Ken, are noted for their high salinity waters of N a - C a - C I - H C O 3 - t y p e chemistry and are found in close association with the Rokko granite-rhyolite complex of Upper Cretaceous to Paleogene ages (Nakamura, 1962). The Arima

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Fig. 3.6D vs &1sO plot of the therlnal and mineral waters at Arima, Takarazuka and Ishibotoke (modified after Matsubaya et al., 1974). Solid line is the meteoric water line for Japanese rains and shallow well waters. brines are grouped into three categories (Tsurumaki, 1965):(1) low temperature (t < 30°C), highly saline, carbonate-rich waters of N a - C a - C l - t y p e chemistry, (2) high temperature, highly saline waters of the same chemistry as (1) but of lower carbonate concentrations, and (3) highcarbonate waters of low temperature and low salinity. Typical chemistry of these waters is given in the Appendix. The water numbered 2 in the Appendix had the highest salinity and belongs to category (2). Waters of almost the same chemistry as category (1) of Arima are found on the eastern edge of the Rokko granite complex at Takarazuka, 8 km east of Arima and in the

100

H. Sakai and O. Matsubaya

Sanbagawa crystalline schist near Ishibotoke in Kawachinagano, ca. 30 km south of Osaka (Fig. 2) (Tsurumaki, 1965). Figures 3 and 4 are the plots of 6180 vs diD and of di]80 vs CI respectively, of the waters from Arima, Ishibotoke and Takarazuka. These plots indicate that the waters of categories ( 1) and (2) of Arima and those of the other two localities are, in their isotopic ratios and chemistry, surprisingly similar to each other. They are most simply considered to be mixtures of local meteoric waters and unique saline brine whose isotopic ratios significantly deviate from local meteoric values. The isotopic and chemical compositions of this brine may be obtained by extrapolation of.the mixing lines in these figures to an appropriate value of C1 assigned to the brine. A crossed circle in Fig. 4 is at the di~80 value of + 8°,,),, and the C I content equal to the highest salinity water of l keda (1955) and is taken to represent the deep-lying brine ( Matsubaya et al., 1973). The diD value of this brine is similarly estimated to be - 2 5 to - 3 0" ....... /00' ®/

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Figures 3 and 4 also reveal that the waters of category (3) of Arima are simply carbonate meteoric waters and have no significant contribution from the deep brine. Figure 5 is a plot of tritium content (in T R = 3H/1H × 10 ~8) vs diD value of these brines after Ikeda et al. (1974) and Kigoshi (unpublished). The T R in the saline waters decreases linearly with increasing diD values (or CI-content) but the waters of Arima lie on a different variation line from that of the other two localities. The diD value at TR=O is - 3 1 /oo/ 0 for Arima and - 4 4 % 0 for Ishibotoke and Takarazuka, respectively. The former value is in fair accord with the diD value estimated from the relationship between 6D value and C1 content as cited above. The data of ]keda et al. (1974) of Fig. 5 indicate that the Arima thermal and mineral waters are mixtures of the inferred deep-lying brine and meteoric waters of 6D = - 51 ~ o The meteoric waters associated with the brine are the oldest in tritium age and probably come from the deepest aquifer of this area. The Arima brines of categories (1) and (2) are similar to each other in their isotopic and chemical features except that the carbonate contents of (2) are much lower than (1). The thermal waters of (2) are found in a limited area of Arima. From these facts, it is suggested that the waters of (1) are primary in origin and become (2) by losing CO2 upon heating (Tsurumaki, 1965). On the other hand, the TR values of the carbonate waters of Arima are variable and higher than the present day meteoric waters, implying that meteoric aquifers of varying ages or depths contribute to carbonate waters.

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102

H. Sakai and O. Matsubaya

possible range for the sulfates in Miocene sea waters, from which these minerals were precipitated. The sulfates, however, may not necessarily inherit the original 3180 values of Miocene sea water sulfates. Instead, the fix 80 values vary widely because of the oxygen isotopic exchange of sulfate with ore solutions of varying isotopic ratios and temperatures during the depositional and postdepositional processes of Kuroko ores (Sakai et al., 1970; Matsubaya and Sakai, 1973). Within the isotopic range of the Kuroko sulfates are found most of the sulfates in Green Tuff type thermal waters. Sulfates plotted by the solid circle in Fig. 6 are from thermal waters of inner southwest Honshu which are found in regions close to but definitely not belonging to Green Tuff formations. They are, without exception, lower in 634S than the Kuroko sulfates. From these facts, Sakai (1969) and Sakai and Matsubaya (1974) concluded that the sulfates in Green Tuff type +3,5----

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thermal waters are recycled oceanic sulfates of Miocene age which precipitated simultaneously with Green Tuff formations. Highly interesting in this connection is the low temperature (15 ~ 35°C) formation waters of Na-Ca-C1-SO4-type chemistry found in Green Tuff formations under the seafloor between Honshu and Hokkaido during the construction of Seikan Undersea Tunnel (Murozumi and Haraguchi, 1967; Mizukami, 1974). Figure 7 is the combined plot of the 6D vs C1- (left) and 6D vs 3180 (right) relationships of the formation waters (Mizukami et al., 1976) collected from the tunnel stretching ca. 4 km into the sea bed. Figure 7 indicates that the formation waters are mixtures of oceanic and local meteoric waters. However, a close inspection of the 6D vs CI- plot reveals that the mixing line does not go directly to the local meteoric waters (squares at C1- = 0). Instead, the waters of C1 less than 70 meq/1 show 6D values that are almost constant and similar to the local meteoric waters. This implies that aquifers of meteoric formation waters exist in the sea bed ca. 4 km offthe mainland Hokkaido and the formation waters of C1 > 70 meq/1 are formed by a mixing of the overlying sea water with these aquifers. The chemical compositions of the mixed and meteoric formation waters are controlled by saturation with gypsum and calcite in the Green Tuff formation and subsequent cation exchange reactions between the sea bed and the waters (Mizukami, 1975; Mizukami et al., 1976). The waters numbered 7 and 8 in the Appendix are typical of the meteoric and mixed formation waters. The 6348 and 61so values of the dissolved sulfates in these waters are essentially similar to those obser~,ed in Green Tuff-type thermal waters,

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although the former sulfates were made isotopically heavier than the latter by sulfate-reducing bacteria (Figure 6). Figure 7 also plots other waters of interest which are found in or closely related to Green Tuff formations of the respective areas (Sakai and Matsubaya, 1974; Matsubaya et al., 1975a). Note that the 6D vs C1 plots of these waters still retain the characteristics of the marine mixed formation waters such as observed in Seikan Undersea Tunnel. Except for Moritake, however, the 6'sO values significantly shift from the original values, presumably by rock-water interactions at higher temperatures. The chemical compositions also deviate in various ways from the low-temperature formation waters. r, H-2

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104

H. Sakai and O. Matsubaya

At Moritake (number 6 of the Appendix), for instance, thermal waters of 60-70 C are obtained from a Green Tuff formation rich in natural gases and oil. Sulfates in the waters are enriched in -~4S and ~80, reflecting the environment which favors the sulfate-reducing bacteria. The waters are saturated with anhydrite at ca. 120 C and more enriched in Ca and depleted in Mg than the mixed formation waters in Seikan Undersea Tunnel (number 8 of the Appendix). The Na/K ratio, on the other hand, is much lower in the former than in the latter, reflecting the higher temperature of the former waters. Although most of the thermal waters in Izu Peninsula are closely related to Green Tuff formations, those along the ocean coasts are considered to be mixtures of oceanic and local meteoric waters from the stable isotopic ratios and major element chemistry (Matsubaya et al.. 1973). Therefore, they should rather be treated as coastal thermal waters. However, we will discuss them here briefly because of the peculiar isotopic ratios of their sulfates. Open circles in Fig. 8 are sulfates from thermal waters of the eastern coast of Izu Peninsula (Sakai and Matsubaya, 1974). Note that they lie closely on a line of a slope (A634S/A61sO) = 0-66, extending from the oceanic sulfates towards the lower left. The isotopic ratios of the sulfates decrease steadily to the lower left as one moves southwards from the base of Izu Peninsula. Mizutani (1975) reported similar results on a greater number of samples collected from wider districts of the Izu Peninsula. He interpreted the systematic isotopic variation as follows; when gypsum or anhydrite precipitates from a limited amount of thermal waters, the remaining sulfates in solution would progressively become lighter, because the heavy isotopes are preferentially removed as gypsum and/or anhydrite. He experimentally showed that the observed slope of.the isotopic variation can be reproduced when anhydrite is fractionally crystallized from a sulfate solution at a temperature of 9ff~C. Thus, when a sulfate solution such as sea water penetrates into volcanic rocks, losing sulfates as it advances, the sulfates remaining in the solution would become progressively depleted in both 34S and 180 along a line similar to that shown in Fig. 8. Green Tuff formations are composed of submarine volcanic rocks of various compositions and marine clastic sediments and were most extensively accumulated on the sea beds of the middle Miocene age that now comprise the major parts of the backbone areas and the Japan Sea side of the Japanese Islands. The salts and water of volcanic and marine origins must have been trapped in the rocks by chemical reactions such as the precipitation of anhydrite and gypsum and the formation of clay minerals, by chemical absorption, and as interstitial solutions. Since the beginning of the Pliocene, Green Tuff regions started to be uplifted, forming the present geographic structures of the Japanese Islands. Volcanisms continued to be active within the most uplifted regions, especially along the present backbone areas. Green Tuff formations were then exposed to meteoric circulation, sometimes heated and acidified by volcanic activities. Hydrothermal to low temperature formation waters of various mixing ratios of meteoric and oceanic waters may have been formed widely. Marine environments may have been progressively replaced by meteoric ones at various rates. As has been shown above, the thermal and mineral waters found in the Green Tuff formations today still exhibit various stages of these processes.

VOLCANIC T H E R M A L WATERS AND GASES One of the most debated and unsettled problems about the active volcanic systems is the question as to whether any measurable contribution ofmagmatic components exist in the volcanic gases and geothermal fluids. Stable isotopic studies by Craig et al. (1956) indicated no measureable contribution of magmatic or, more properly speaking, "primary magmatic" water of fixed isotopic ratios in the geothermal and volcanic fluids. However, this does not necessarily deny a contribution of"magmatic waters" in these fluids. The "magmatic waters" here are defined as the waters of any origin that are in isotopic and chemical equilibrium with magma or equivalent states

Stable Isotopic Studies of Japanese Geothermal Systems

105

of silicate and carbonate rocks. Thus, meteoric waters may be able to penetrate into the crust deep enough to be "magmatic fluids". Although the rock-leaching reactions in geothermal reservoirs can provide a considerable portion of chloride and other dissolved salts in geothermal fluids (Ellis and Mahon, 1964), a small but definite quantity of"magmatic fluid" is also required in order to explain the high heat-flow in the volcanic areas. At Hakone, for instance, 2-3 x 107 cal of heat is carried to the surface for every second by 140 kg of NaCl-type thermal water and fumarolic gases (Oki and Hirano, 1970). A calculation by Fukutomi (1965) and Yuhara (1968) indicates, however, that only 10~oor so of the thermal energy can be explained by the conductive heat flow from the underlying magma chamber. The rest of the thermal energy must be carried by upward flow of aqueous fluid from the magma chamber. Yuhara (1968) showed that high enthalpy steam escaping from the magma chamber, amounting to ca.5% of the total output of the thermal waters and fumarolic gases at Hakone, would be sufficient for this purpose. On the other hand, Truesdell and Fournier (1975) proposed a deep hydrothermal system which is composed of brine of high temperature and salinity slowly convecting in the rocks just above the magma chamber and a much diluted hydrothermal system circulating above the brine. The heat and salts in the brine are transferred to the overlying hydrothermal system by mixing at the interface of the two water systems. The steam from the magma chamber should contain quantities of hydrogen chloride which will be neutralized through reactions with rocks. Thus, in either of the two models above, the neutral chloride-type water in volcanoes is considered to have a small contribution of the deep fluids. The origin of the sulfur species in volcanic and geothermal areas is also controversial. Recent sulfur isotopic studies of various hydrothermal ore deposits (Kajiwara, 1971; Ohmoto, 1972) and sulfur species in geothermal fluids (Kusakabe, 1974) indicated that 634S of the net-sulfur discharge from a volcano is often far greater than zerO~oo, or the inferred value for the sulfurs of the upper mantle origins, and that, in many cases, these sulfurs are recycled marine sulfates. Sulfates in Green Tuff-type thermal waters are examples of this. Sakai and Matsubaya (1974) maintained that the isotopically heavy sulfates in acid chloride-sulfate-type waters in Tamagawa and Beppu geothermal areas of Japan may also be recycled marine sulfates from the underlying Green Tuff formations. However, these sulfates can be formed also by the disproportionation of sulfur dioxide of magmatic origin (Iwasaki and Ozawa, 1960; Iwasaki et al., 1966). Sulfur isotopic evidence to support their view was presented by Oana and Ishikawa (1966) and will be discussed later. The origin of the volcanic gases and waters is of considerable interest from the viewpoint of geothermal energy exploration. In the following, stable isotopic studies of the three representative volcanic systems of Japan will be reviewed with special emphasis on the interpretation of the isotopic data with respect to the origins of the volcanic gases and waters. These systems are Hakone, Ibusuki and Satsuma-Iwojima. Of these, Hakone is a subaerial volcano of the central Honshu consisting ofcalderas and central cones. Ibusuki is located on the coast of Kagoshima Bay and belongs to a caldera which is "half-drowned" in Kagoshima Bay. Satsuma-Iwojima is a volcanic island within a completely drowned caldera on the continental shelf off the southern Kyushu. Comparative studies of the three volcanic systems each having different environments would be helpful to define and resolve the problems. HAKONE VOLCANIC THERMAL WATER SYSTEM Hakone volcano consists of two major central cones, Kamiyama and Komagatake, of Pleistocene age, still in fumarolic activity, Lake Ashi (a caldera lake) and a double caldera, and is underlain by Tertiary marine basement rocks (Green Tuffs) (Kuno et al., 1970). The majority of rocks of the volcano are andesitic to basaltic in composition. Figure 9 is a schematic diagram showing the geology and hydrology of the Hakone geothermal system after Oki and Hirano

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4km

Fig. 9. Hakone volcano (modified after Kuno et ~;/.. 1970 and Oki and Hirano, 1970). AQ (top figure): major geothermal aquifer Numbers: sample numbers of Matsubaya et al. (1973) L: lake deposits CC: central cone lavas Triangle: summit of the central cones P: pumice flows YS: young somma lavas OSz, OS,,, B2, T2: other Tertiary and Pleistocene formations older than Hakone volcano M2: Yugashima formation (Miocene).

(1970). Thermal waters of various chemistry issue by themselves or are obtained by deep drilling along the Hayakawa valley which cuts open the old caldera wall on the eastern flank and drains into Sagami Bay. Oki et al. (1968a) and Oki and Hirano (1970) distinguished the following four types of thermal waters based on the chemical compositions and hydrogeological informatiin: (1) acid sulfate-type waters (H-SO4) which are associated with the fumarolic activity in the central cones, (2) bicarbonate-sulfate-type waters (HCO3-804) which are low in C1 and are obtained around the central cones, especially from the western slope of Kamiyama, (3) sodium chloride type waters (NaC1) which are characterized by high-temperature waters and distribute only on the eastern flank of Kamiyama, and (4) sodium chloride-bicarbonate-sulfate-type waters (NaCIHCO3-504) which are also found only on the eastern flank of Kamiyama and further to the east. Typical examples of these four types of waters are given in the Appendix (from number 10 to 13 in the above order). The last type of water is further divided into subgroups a and b, depending on whether they are from the Quaternary volcanic rocks (subgroup a) or found in the basal Tertiary rocks (b). The. temperature of these waters generally increases with the increasing content of C1. The waters are considered to be mixtures of NaCl-and H C O 3 - S O 4 type waters. A major aquifer of this system lies within the zone of unconformity between the central cones and the Tertiary basement rocks (Fig. 9). Most of the thermal waters within the caldera come from this aquifer. Waters from the groundwater aquifers and Lake Ashi at the western caldera flow eastwards in the major aquifers (Oki et al., 1968b). According to Oki and Hirano (1970), the groundwaters of HCO3-SO4-type chemistry flow into the high temperature regions of the central cones from the western caldera, where they are impregnated with the NaCl-rich magmatic fluids, and then appear as the NaC1- and the mixed type thermal waters at the eastern side of the central

Stable Isotopic Studies of Japanese Geothermal Systems

107

cones. The acid sulfate-type waters are formed in shallower aquifers on the western slope of Kamiyama with surface oxidation of hydrogen sulfide and sulfur. They consider that the acid waters are neutralized and mixed with the HCO3-rich groundwaters, forming the HCO3-SO4-type waters. Figure 10 plots the isotopic ratios of the thermal waters (Matsubaya et al., 1973) and the yearly averages of Sukumo River, Lake Ashi and two NaCl-type thermal waters (Matsuo, written communication). "Average rain" in Fig. 10 represents the yearly average precipitation within the Hakone caldera and was estimated from the isotopic values of rain waters collected at 6 stations of different altitude within the caldera. (Matsuo, written communication). Unlike the shallow HSO4-type waters at Owakudani, the steam condensates from the wells of this area of ca. 100 m depth are highly enriched in 180 and D compared to the local meteoric waters. The yearly average values of 6D = --24.5 and 6~sO = + 1.4~o were obtained for one of them by Matsuo and his associates (Matsuo, written communication). As is seen from Fig. 10, the waters of Lake Ashi, Sukumo River and the average rain closely fit a line of slope of approx. 6, indicating that the first two waters are enriched in D and 180 due to the kinetic evaporation of rain waters at the surface (Craig et al., 1963). Many caldera lakes of Japan show a similar enrichment in the heavy isotopes relative to their respective local meteoric waters. It will be shown later that the heavy lake waters may be used as a tracer to follow the hydrological features of certain caldera systems.

•

' ~ Sokume~,eT~411 4 ~

~

-10

SO

SZC)

C

1--I -s's "

Average ~ Rain" / ~

[~ /

-40

'~(Lake Ashi '

~$D=gSISO.I,.S

C) 0 • []

NaCI-type Mixed-type

H-SO4-type

-go

HCOz-SO4-type

-8

Fig. 10. 6 D vs t~zsO plot for the thermal waters at Hakone volcano. Data points indicated by number are from Matsubaya et al. (1973) (numbers correspond to those in Fig. 9) and others from Matsuo et a1.(1972) and their unpublished data (see text). See text for RSW (respective surface water) and average rain.

The thermal waters within the Hakone caldera, especially the NaC1- and NaCI-HCO3-SO4-type waters, are moderately enriched in the heavy isotopes along a line of an approximate slope of 2. As shown in Fig. 11, the 6~80 values of the two groups of water are roughly proportional to their Cl-content. These facts suggest that the waters are predominantly meteoric in origin but contain a small amount of NaCl-rich brine of a high oxygen isotopic ratio but of less significant enrichment in deuterium. Matsuo and his group (Matsuo, written communication) consider that the isotopic value of the meteoric component in the above mixing model should lie at the intersection of the two lines, one connecting the average rain, Sukumo River and Lake Ashi and the other drawn through the thermal waters. Their best estimate on this "representative surface water" within the caldera (RSW) has 6D = --50.7 and 6 ' 8 0 = -8"3~oo as indicated in Fig. 10. Quite a few fresh water samples from shallow wells in the caldera show the isotopic values close to RSW. That RSW is isotopically intermediate between the average rain and Lake Ashi may suggest some contribution of the heavy water of Lake Ashi into the thermal water aquifer, as was

108

H. S a k a i and O. M a t s u b a y a -5

C) NaCI- type water (]) subgroup a of NaCI-HC03-S04 -6

o

50

53

O to_ 7

-8 I

I

5OO

IOQO

1500

Cl PPM

Fig. 11. Relationshipbetween~8OandClcontentinNaCl-typeandthemixedtype(subgroupa)watersofHakone(data after Matsubaya et a1.,1973). suggested from the hydrological model of Oki and Hirano (1970). Alternatively, Matsuo and his group suggest that Lake Ashi should have no significant contribution to RSW, because surface waters obtained at a higher elevation than Lake Ashi also show isotopic values similar to RSW. The isotopic enrichment of RSW may be partly due to direct evaporation of rainwaters at surface but more important would be evaporation of sub-surface water through plant leaves (Matsuo, oral communication). Also difficult to answer is the question as to the origin and the character of the other component which brought about the high concentration of 180 and NaC1 in the thermal waters. Based on the assumption that the NaCl-type waters, in average, are composed of 10~o of the deep chloride-rich fluid and 9050 of the average rain and that all the chloride came from the fluid, Matsuo et al. (1972) estimated that the deep fluid at Hakone is - 25,°,00in 6D and + 3~,,o in 6lsO. Because the average concentration of CI in the NaCt-type waters is from 2000 to 2500 ppm, the fluid should contain 20,000-25,000 ppm of C1 according to this model. The estimated value of 6D of the deep fluid is much heavier than the local precipitation and rather similar to the volcanic gases at Satsuma-Iwojima (Matsuo et al., 1975; Matsubaya et al., 1975b) which will be discussed later in this paper and to the fluids in isotopic equilibrium with the hydrous minerals in some granites (Kuroda et al., 1974). Matsuo et al.(1972) emphasized the fact that the Hakone volcano is underlain by the Tertiary submarine formations, suggesting that the uprising magmatic fluids picked up the heavy waters and salts from the submarine formation. They also suggested that the steams from Owakudani area represent the vapor in equilibrium with the deep fluid. Figure 8 plots the 6345 and 6180 values of sulfates from the four waters (Matsubaya et al., 1973 and some unpublished data of this laboratory). The 634S values of sulfate in acid waters are similar to those of the native sulfur in Owakudani fumarolic areas (-5.2 a n d - 8.2~o ) and suggest that the sulfates were formed by the. surficial oxidation of volcanic sulfur and hydrogen sulfide. On the other hand, the NaCI-HCO3-SOa-type waters have the heavier sulfates, the 6345 values rapidly increasing with the increasing distance from the central cones (compare Figs. 9 and 8). The three heaviest sulfates are found in the waters from the basement Green tuff formation and are considered to be similar in origin to the sulfates in Green Tuff-type thermal waters discussed before. These waters are also similar to the Green Tuff-type thermal waters in that they are significantly depleted in Mg 2+ and K + compared to the subgroup a of the mixed-type waters. However, a close inspection of Fig. 10 would reveal that the 6180 values of these waters are slightly shifted from RSW, suggesting a small contribution of the NaCl-type waters in them.

Stable Isotopic Studies o f Japanese Geothermal Systems

109

The 634S and ~180 values of the sulfates from the NaCl-type and the subgroup a of the mixed type thermal waters are intermediate between the two types of sulfates mentioned above. The isotopic separation between So ] - and H20 in the two NaCl-type waters is 6.8 and 8"8~o, respectively, corresponding to isotopic temperatures of 190° and 160°C, respectively (see Sakai, 1977). The silica contents of these waters are around 300 ppm (Oki and Hirano, 1970) and equivalent to a silica temperature of ca. 200°C. However, the highest observed temperature of the major geothermal aquifer was 120°C (Oki and Hirano, 1970). Therefore, the dissolved sulfates may not have been equilibrated with the waters. As a matter of fact, the flow rate of water in the major aquifer is of the order of a few km/yr as is calculated from the average inclination of the water table and the permeability of the aquifer (Oki and Hirano, 1970). Then, the residence time of a water mass moving in the aquifer is only five years or so, whereas the halftime for the isotopic exchange reaction between sulphate and water in an aquifer ofpH = 7 and a temperature of 100°C would be 100 years or so (Sakai, 1977). Origin of the sulfates in the NaCl-type waters is of particular interest because of the inferred magmatic components in these waters. If the sulfates in these waters are taken to be magmatic in origin, the 634S values of +4 and + 7~o may be taken as the maximum value of the magmatic sulfur, because some lighter fraction must have escaped as hydrogen sulfide at Owakudani. It is possible that the isotopic temperatures mentioned above may represent the temperatures of deeper aquifers in which the NaCl-type waters stay before they enter into the major aquifer. Alternatively, the sulfates might essentially be formed by surface oxidation of volcanic sulfur at the fumarolic areas (Oki and Hirano, 1970; Kusakabe, 1976). The secondary sulfates may be made isotopically heavier through the partial reduction by bacteria (Kusakabe 1976). As is shown in Fig. 8, two isotopically heavy sulfates were found, one in a groundwater (56) and the other in a thermal water of HCO3-SOg-type (58). Note that both are from the western caldera and not directly related to the Tertiary basement rocks. Summarizing the foregoing discussion, the hydrothermal system at Hakone is composed of approx. 90~o of the dilute meteoric groundwaters and 10~o of the chloride-rich deep fluid of possible magmatic origin. Lake Ashi, the caldera lake, maintains water of higher isotopic ratios than the average rain in the caldera. The meteoric component (RSW) in the thermal waters is isotopically intermediate between Lake Ashi and the average rain. This might suggest that the heavy lake water contributes to the hydrothermal system, as is favored by the hydrogeological features of this area. However, isotopic evidence exists which questions this assumption. Thus, the hydrological role of Lake Ashi within the Hakone caldera remains a subject of controversy. If 10~oof the NaCl-type waters are magmatic, the annual output of the fluid at Hakone would be ca. 4 × 10s tons. If the water content of the magma of Hakone is assumed to be 2-4~ in weight, the amounts of the magma outgassed since the Pleistocene (10 6 yr) would be 2-1 x 103 km 3, which is not incompatible with the size of the Hakone volcano. Matsuo et al. (1972) suggested that the deep fluid was significantly enriched in D as well as 180 compared to the local precipitation. However, the isotopic and chemical characters of the deep fluid are still obscure. If any magmatic sulfur is being discharged from this volcano at present, it would be lighter than + 7~oo and could be much closer to zero. On the other hand, sulfates from the basement Green Tuff formation are +20~o or heavier and the thermal waters from the basement rocks can be regarded as the Green Tuff-type thermal waters of Sakai and Matsubaya (1974). COASTAL THERMAL WATERS AT IBUSUKI IN ATA. CALDERA, SOUTHERN KYUSHU Like Shimogamo oflzu Peninsula (Mizutani and Hamasuna, 1970), Ibusuki is one of the typical coastal thermal water systems of Japan (Matsubaya et al., 1973; Sakai and Matsubaya, 1974). It is formed by mixing of meteoric and oceanic waters and by chemical interactions between the

110

H. Sakai and O. Matsubaya

thermal waters and volcanic rocks. As is shown in Fig. 12, lbusuki and related thermal water systems are closely associated with Quaternary volcanic rocks erupted within Ata caldera, one of the gigantic calderas of Kyushu (Fig. 13). The eastern half of the caldera is beneath Kagoshima Bay, while the western half is noted for the typical volcanic landscapes dotted by a caldera craters and central cone-volcanoes. A caldera lake, Lake Ikeda is ca. 4 + 3 km 2 in surface area and the water level is 66 m above sea level (Fig. 15). However, because the water is ca.250 m deep, the bottom is 190 m below sea level. Another lake, Lake Unagi, is smaller and shallower than Lake Ikeda but is higher in elevation. Two lakes have no river outflow. However, because their salinity is low (ca. 10 ppm in CI), they must have underground drainage, as will be discussed later. Although the Quaternary volcanism of this area might have started 106 yr before present day (B.P.), the oldest rocks dated by the ~4C-method were 24,500 yr B.P. (Ui, 1967). The rocks betbre this age were grouped as pre-Ata and Ata pyroclastic and lava flows in Fig. 12. Numerous eruptions of andesitic composition followed them on a rather small scale until an intense eruption took place at ca. 4000 yr B.P., forming a small caldera, or Lake lkeda (Ui, 1967). The rocks erupted

•

Lake

Ikeda

- - Ata C a i d e r ~ a ~ : ~ ima Bay

,

ii

Fig. 12. l busuki geothermal system and Ata caldera (modified after Ota (1966) and Ui (1967), see Fig. 13 for locality and Fig. 15 for cross-section). PA: pre-Ata and Ata pyroclastic and lava flows: PAl: post-Ata and Ikeda pyroclastic flows and lavas; PI: post-Ikeda pyroclastic and lava flows; AL: alluvial deposits; L: lake deposits. For PA, PAl and PI, see text.

in this period are post-Ata and Ikeda pyroclastic and lava flows in Fig. 12. The formation of Mt. Kaimondake is the latest, greatest event since that time. At present, however, no volcanic activity is observed within Ata caldera except weak fumarolic activities near Lake Unagi and several other spots. Thermal waters of neutral Na-Ca-Cl-type chemistry and low content of sulfate and bicarbonate are widely found along the coast of Kagoshima Bay and the coastal alluvial plains. High-temperature aquifers seem to lie in andesite lava flows underlying the coastal plains at 60-100 m depths (Hatae et al., 1965). Unlike other more active volcanic systems, however, no acid water occurs at present.

Stable Isotopic Studies of Japanese Geothermal Systems

111

~ S A K ~ I MCALDERA ;AIRA

SATSUMA- IWOJ I k ~ _

r~KE-S.,MA

SHOWA'IWOJIMA

J

1 )

f /

Fig. 13. Map showing three gigantic calderas of southern Kyushu. The 6D values of the thermal waters at Ibusuki and in the vicinity are plotted against the C1 concentration (at the left) and against the 6180 values (right) in Fig. 14. The rain waters on the coast of Lake Ikeda and the waters from coastal shallow wells and the two crater lakes were collected monthly for a period of a year. The isotopic ratios of these waters are also plotted in Fig. 14. Note that the lakes are significantly enriched in the heavy isotopes along a line slope 5 or the kinetic evaporation line of Craig et al. (1963). 0

200

C l - m mole/I

400

600

//, a 6D vs. Cl-

Sq)a"~4er So=8 S"~o+"rts

/ -,e

o~.O

,

,s. _~-~ .~, I

o/o

,,.

o°~

//

,

o

o /

•

• ~,

..~ ~-~..~..

"

LaKe Ikeda b 6D vs. 61~10

/ °/m

I/ ......

-so

"*l

'_.

,"

t

~

o - * - - -

.

--average rain

_/0

i

e Metolx~ wmtlr

-~

-,'*

i o

+~

Fig. 14.6D vs CI and 6D vs 6 ~80 plots for the thermal waters at Ibusuki and the vicinity(after Sakai and Matsubaya, 1974) and their unpublished data). Isotopic range for Lake Ikeda is a yearly variation from August, 1974. Averagerain for a yearly weighted average for the same period. Figure 14 clearly indicates that the Ibusuki thermal waters are mixtures of sea water and meteoric water. However, the isotopic values of the meteoric water component are neither similar to those of the coastal shallow well waters nor to the average rain water. They are rather intermediate between the heavy lake waters and the coastal well waters. This implies that either or both of the lakes feed the coastal aquifers through underground drainage, although they have no river outflow as mentioned before. The stable isotopic data, therefore, demonstrated clearly that the Ibusuki and related thermal waters are unique mixtures of the oceanic, coastal meteoric and caldera lake waters.

H. Sakai and O. Matsubaya

112

-300m

" ~ ter

0 -300

....

~:'~':':":"".... 5km

Thermal Mixed

ground w a t e r

w a t e r (-1 , -11 I (-5,-361

Fig. 15. Schematiccross-section of lbusuki geothermal system. Figures in brackets are ~D and 6~80 values of water. Unlike subaerial volcanic systems, the coastal thermal waters are much more saline, the dissolved salts mostly being derived from the oceanic waters (number 9 in the Appendix). However, an inspection of the Appendix would show that the thermal waters are significantly depleted in Mg 2÷ and SO42- and enriched in Ca 2÷ compared to the simple mixtures of meteoric and oceanic waters. The Mg-loss is due to the rock-water reactions of the type shown by equation

(1) volcanic rocks (glass) + 2 Mg 2÷ + HEO Mg-chlorite + Ca z÷ + K ÷ + H ÷ + H4SiO4 T h e C a 2+

released by this reaction would precipitate as anhydrite for the first part Ca 2+ + 5 0 4 2 - - . C a S O 4

(1)

(2)

but as sulfate in the solution is used up, it would accumulate in the solution. The K +and H ÷ would be consumed to form potassium feldspars or clay minerals by hydrolysis of silicate glasses. Thus, the final solution would become Na-Ca-Cl-type solutions (Mizutani and Hamasuna, 1972; Matsubaya et al., 1973; Sakai and Matsubaya, 1974). Recent laboratory experiments by Bischopf and Dickson (1975) indicated that reactions of this sort readily take place between basalt and sea water at a temperature of 200°C. The (~34S and 3180 values of dissolved sulfates in these thermal waters are mostly similar to those of oceanic sulfates (Sakai and Matsubaya 1974). However, as shown in Fig. 8, the sulfate in Fushime of l busuki has (~345 = -~- 16"3 and 6~80 = + 5-1 ~oo and is considerably lighter than the oceanic sulfates (•345 = + 20 and 6180 = + 9"5Yoo)iThis was interpreted by Sakai and Matsubaya (I 974) as a result of the mixing of the lighter supergene sulfate into oceanic sulfate during the rockwater interaction. Mizutani (1975), on the other hand, indicated, as mentioned earlier, that fractional crystallization of anhydrite from a mass of sea water migrating through hot volcanic rocks would produce thermal waters having lighter sulfates than the original oceanic sulfates. Such a mechanism may also explain the isotopic ratios of the Fushime sulfates. In either case, the fi34S values of sulfates in the coastal thermal waters of Ibusuki are strongly influenced by oceanic sulfates and do not permit us to discuss the sulfur isotopic ratios of magmatic origin in this volcanic system. As was the case for Lake Ashi of Hakone and the two lakes of Ibusuki, many caldera and crater lakes are more or less enriched in the heavy isotopes compared to the local precipitation, rivers and groundwaters. At Ibusuki, the isotopically heavy lake waters were shown to serve as a good tracer for the analysis of the geothermal system within the caldera. Therefore, the caldera lakes deserve further attention here. In the following, it is shown that the isotopic balance calculation of a caldera lake may be used to estimate the water balance of the lake between precipitation, evaporation and drainage.

Stable I s o t o p i c Studies o f J a p a n e s e G e o t h e r m a l S y s t e m s

113

Isotopic and material balances of any lake may be described by the following equations dN/dt

= nm-

and d ( R N ) / d t

ne-

= n~.R~, -

(3)

nout

neRe -

(4)

no,tRo.,

where N = mass of water in the lake at time = t; n = mass of water being taken in or out of the lake within a unit time; subscripts, in, e, and out, indicate rain, evaporation and drainage, respectively; R = the isotopic ratios of water ( D / H or 180/160) in the lake at time = t; Ri,~ Reand Rou, = the isotopic ratios of the subscribed waters, respectively. At a steady state, the two equations are equal to zero. Recalling the definition of the delta values, the isotopic ratios of a lake at a steady state can be expressed as follows:

nin(t~in--~L s) ~- ne(6e-- (~Ls)

(5)

where 6L s denotes the 5 values of the lake water at steady state. At steady state, the isotopic ratios of the vapor leaving the lake surface are controlled by the temperature of the surface water, the humidity of the air and the isotopic ratios of air moisture and is expressed as follows after Craig and Gordon (1965)

6~- 6d

h(6d-

6~)/(1 + 6~3 -

= e, = 1 +

6L s

(6) ( l - - h ) { 1 + Cc*(p,.L/p) } + A t

where h = humidity relative to the saturation humidity at the lake surface, 5A = the 6 value of the atmospheric moisture, e = the kinetic isotopic effect of evaporation, A t = (1 - h) (p i / p - 1), p = the total transport resistance of H21°O in air, pi = that of H D 1 6 0 or H2180 in air, p~,L = that for the heavy isotopic water molecules in water phase, and a* = the equilibrium vapor pressure ratio of the heavy to light water. It should be noted that the present steady state corresponds to the transient model of Craig and Gordon's model, because in the latter no rain-input was considered. The e for oxygen at room temperature is 14%o at h = 75% according to Craig and Gordon (1965). The controlled evaporation experiments by Gonfiantini (1965) suggest the value to be 18~oo at 15°C and h = 70%. From these results and taking 1/at* to be ca. 1.009, the value of At is estimated to lie between 5 and 8%° for the climatic conditions prevailing at Ibusuki (annual average temperature and humidity: 18°C and 75~o). Taking that 6 ~, = - 6 " 8 % 0 and 6 ~ = - 15% o, and using the value of p ~, d p = 0.2 (Sofer and Gat, 1975), equations (5) and (6) yield that ca. 77-85% of the rainfall on Lake Ikeda and its recharge area leaks out of the lake through the underground channel, while this value would increase to 85-90% in Lake Unagi. If there is no leakage from these lakes, the steady state condition would be 6e = 6i, Therefore, from equation (6), the 6180 values of both of the lakes would be stabilized at + 0.64%o (At = + 5%o) to + 3.5% o (At = + 8%0) which is much heavier than the observed values. The underground leakage of these lakes is further supported by the fact that the waters are low in salinity. If there is no loss of water except by evaporation, the salts in the rains should be accumulated in the lake. The chloride concentration of a caldera lake in a steady state with respect to both water and chloride can be determined by the following equation n ou, C L~ = n ~, C i,,

(7)

where C f l and C~, denote the chloride contents of the lake and rain waters, respectively. The relationship can be derived by the similar calculation described above. The CI content of Lake Unagi is ca. 10 ppm. Rain waters of Japan contain ca. 7 ppm of CI in coastal areas to less than 1 ppm in far inland areas (Miyake, 1965). If the C1 content of the rains at Ibusuki is taken to be 5-2 ppm, for instance, the leakage of Lake Unagi would be 50-20% of the rain-input.

114

H. S a k a i and O. M a t s u b a y a

According to weather records, the average annual rainfalls from 1941 to 1970 at Kagoshima near Ibusuki was 2400 mm. We measured 2380 mm rainfalls at Lake Ikeda during a year from August of 1974. The area of Lakes Ikeda and Unagi including their recharge areas is 26 and 2.6 km 2, respectively. Therefore, the annual rain-input amounts to 6 x 107 and 6 × 106 tons for Lakes Ikeda and Unagi, respectively. Ibusuki and the vicinity are one of the most popular hot spas of West Japan and the thermal waters have been heavily pumped up for commercial use. The amount of the thermal waters utilized in this area would roughly be 3_+ 1 × 106 tons per year according to the personal communication from Mr. Ego of Ibusuki City. The town of Yamagawa, near lbusuki, annually takes ca. 2 × 10° tons of the water of Lake Unagi for tapwater. These figures suggest that these lakes provide more than enough waters to support the geothermal systems along the coastal areas. However, note that the amount of the tapwater used by Yamagawa is sufficiently large to affect the isotopic ratios of Lake Unagi. Figure 15 summarises the hydrological features of the Ibusuki areas discussed so far. The sources of the thermal waters on the coast are threefold; sea water, coastal shallow groundwater and caldera lakes. The majority of the salts in these thermal waters including sulfur species are from the sea water, although the chemical compositions are modified by the hydrothermal interactions with rocks. VOLCANIC GASES AND THERMAL WATERS AT SATSUMA-IWOJIMA, A VOLCANIC ISLAND OFF SOUTHERN KYUSHU Satsuma-Iwojima is a volcanic island located ca. 40 km south of the Ata caldera discussed above (Fig. 13). Together with a neighboring island, Takeshima, it belongs to Kikai caldera of Matumoto (1943). As schematically shown in Fig. 16, the island consists of a caldera wall, a volcano (Yahazudake) older than the caldera wall, and two Quaternary cone-volcanoes of andesitic composition. One of them, Iwodake, is still active in high-temperature fumarolic and thermal water activities, whereas the other discharges only thermal waters.

[ [ [ 1

:lera

Fig. 16. Map showing geology and geothermal activity of Satsuma-lwojima (see Fig. 13 for locality).

Chemical and stable isotopic compositions of the thermal waters and volcanic gases have been studied extensively by Kamada (1963, 1972), Kamada et aL (1974), Matsuo et al. (1975) and Matsubaya et al. (1975b) and are briefly summarized in the Appendix and Table 2. Table 2 also indicates for comparison the chemical and isotopic compositions of volcanic gases of some other active andesite volcanoes of Japan. The fumarolic gases are high in temperature, ranging from 120 to 900°C and rich in HC1, SO2 and HF. The concentrations of heavy metals are also notably high, molybdenum hydroxide and iron oxide being deposited from the gases on hot rocks around high-

Stable Isotopic Studies of Japanese Geothermal Systems

115

Table 2. Chemical and isotopic compositions of volcanic gases of Japan Volcano

Satsuma-lwojima ill

Fumarole Coa 15) t °C sr),~ HF 6D%0 61"O 63,S SO2 H,S XS 161 6,~C CO2

A--3 247 3"3 11'5 1'6 10'7 1/10 of HCI

K--3 144

-27"2 + 6"6

-25"6 + 7"6

+ 11"6 + 9"9 +12"1 -

5-5

Nasudakel3)

Kuju (2)

A--1 835 2-5 8"8 0'45 0'62 ca.

Ku--I 120 4-4 2"0 3"1 I "4 --

Ku--2 340 4.7 13.9 3.4 2"7 --

K--5 500 5-5 1-24 23"3 0'66 --

- 6'9 + 9'1

'-40"3 + 3"5

-29'8 + 7'2

---

---

+ 12"8 + 6"5 +11"7

+ 13'7 + 2"4 +12"5

+ 7"8 + 2"2 + 4"5

+ 5'8 + 4'5 + 5"6

+ 10"6 + 7"3 + 6"9

+ 10"7 -- 0"6 + 2"2

-

-

-

-

5"2

1'9 4'6 0.37 0"3

5.4

8-5

8.9

.

M--2b 352 1.7 0"88 3"5 0"24 0"04

.

Showa-Shinzanl4l

M--3 332 2"3 0'2 1"5 0" I 1 --

.

0.004 0" 12 0-25 0"036

A--I 722 3.4 0" 14 0-008 0-42 0"19

-59"5 - 6'3

--

--

----

+9"2 +4"3 +4"5

+4"4 -+4"4

.

I~ 347

5"I

.

(I) Matsubaya et at. (1975b). (2) K u - - I , K u - - 2 : Matsubaya e t aL (1975h); K - - 5 : collected in 1960, unpublished data by S. Oana, H. Sakai and M. L. Jensen (19611. (3) M - - 2 b : same as K--5 ofKuju; Kusakabe'et al. 0970). (4) Same as K - - 5 of Kuju. (5) Gas compositions are in 1000 x (volume ratio to water vapor). (6) Measured isotopic ratio for bulk-sulfur (SO2 + H2S + S:).

temperature fumarolic vents (Yoshida et al., 1972). In lower temperature areas, amorphous silica is deposited abundantly from gases through the hydrolysis of silicon tetrafluoride (Yoshida et al., 1966)

(8)

SiF,, + 2 H 2 0 ---¢ SiO 2 + 4HF

Figure. 17 plots the 6D and 6180 values of fumarolic gases and thermal waters after Matsuo et al. (1975) and Matsubaya et al. (1975b~ Rains and shallow well waters at Nagahama (see Fig. 16) were collected monthly for a period of a year and analyzed for their isotopicratios. The results are also plotted in this figure. Numerical values in brackets in Figure 17 are the tritium contents in T/H x 1018 which were measured by Ikeda et al. (1975). TabM,3. 6180 values of fresh and altered rocks from Satsuma-Iwojima (after Matsubaya et al., 1975b) No.

Sample description

6' SO%o

whole rock

plagioclase*

fresh rocks I 2 3 4

lwodake lava, andesite (IW74S-13) old somma lava, andesite (IW74S-251 Welded tuff of somma (IW74S-311 Showa-lwojima lava, andesite (IW74S-31)

+ + + +

6.7 6.8 7.0 6-6

+ 6"3 ----

+ 7.4 + 10-1 + 11.3 -1-11.6 + 11.1 + 25.0 + 13.2 + 12.5 + 18.3 + 29.3

+6.7 + 18.6 -+ 15"1 + 14.8. -------

altered rocks from Iwodaka 5 6 7-1 7-2 7-3 8 9 10 11 12

andesite (IW74S,11 andesite (IW74S-37)

andesite, white outer part (IW74S-3) .gray middle part of 7-1 tuner part of 7-1 white siliceous rocks (IW74S-4)t bottom sediment of the crater (IW74S-91 ash around high-temperature fumarole (IW74S-96) white rock coated with molybdenum blue (1W74S-137) silica sublimate (IW74S-138)

*phenocryst. *crystalline phases are a-cristobalite and a-tridymite only: opale is seen under microscope.

The isotopic values of the fumarolic gases, especially from the high-temperature ones (900-500°C), converge within narrow ranges around 6D = - 25 and 6180 = +6-6%0. Because the 61 sO values of the fresh andesite of this island are from + 6.6 to + 7.0~o as listed in Table 3, the fumarolic gases are in isotopic equilibrium with the andesite magma as was suggested by Matsuo et al. (1975). On the other hand, the low-temperature fumarolic gases are further enriched in the heavy isotopes of both hydrogen and oxygen compared to the high-temperature ones. The slope of the enrichment is similar to that expected between water and vapor at ca. 150°C (Bottinga and

116

H. Sakai and O. Matsubava

.....ge_,a,O

.,o..h,-,

r

average ground water -50

:, her

a

Fumarolic

gases

•

® 100-

250°C

low HCI

O 100 - 2 5 0 ° C

SO0 ~ 900 ~C

high HCI

Fig. 17. 6 D vs 6 ] 80 plot for the volcanic gases and thermal waters at Satsuma-lwojima (after Matsubaya et al., 1975b and after Matsuo et al. 1975 (triangles and Kitabira gas) ). Average rain and groundwater: yearly averages from August, 1974 (unpublished data, this laboratory). Figure in brackets: tritium content in T/H 10 TM (after Ikeda et al., 1975).

Craig, 1968), indicating the isotopic fractionation to be due to the condensation-evaporation of the gases at near surface of the fumarolic areas. Andesite rocks within the fumarolic areas have been attacked by the acid fumarolic gases and, to a varying extent, altered to siliceous rocks. As shown in Table 3, the altered rocks are often extremely enriched in 180 compared to the fresh andesite. A white siliceous rock numbered 8 in Table 3, for instance, represents the most strongly altered rocks of this area, consisting of acristobatite, a-tridymite and opal. The isotopic fractionation factor of ca. 19-16~o between this rock and the fumarolic gases would correspond to an isotopic temperature of ca. 150-200°C on the quartz-water isotopic thermometer (O'Neit and Taylor, 1967). Silica sublimate in Table 3 (number t2) is formed by reaction (8). The isotopic ratio suggests that the reaction would have proceeded at even lower temperatures. These examples indicate that isotopically heavy alteration products have been accumulated in low temperature zones of the volcano through the oxygen isotopic exchange between andesite and magmatic water of highly enriched isotopic ratios. Note also that plagioclase phenocrysts are more susceptible to the isotopic exchange than whole rocks. Taylor (1971, 1974) has shown that hydrothermal and late-magrnatic alteration under the influence of meteoric water often produce alteration zones significantly depleted in 180. The data in Table 3 are the first demonstration that the reverse is taking place on the surface of active volcanoes. The sulfur isotopic ratios of SO2 and H2S from the high temperature fumarolic gases are + 12 and +9 to + 11~o, respectively (Ueda, 1976). The fractionation factors between the two sulfur species also suggest that they are in isotopic equilibrium with each other at a high temperature. This implies that the observed isotopic ratios of the sulfur dioxide or the weighed average of the two species represent the 634S values of the bulk-sulfur in the andesite magma. As compared in Table 2, the 634S value of the bulk-sulfur at Satsuma-Iwojima is + 12~ooand is much higher than other andesite volcanoes. The origins of the volcanic gases of this island are of particular interest, because of their extraordinary isotopic ratios and chemical compositions. The 6D values of the fumarolic gases are much higher than the major groundwater aquifers at Nagahama and the average annual precipitation (Fig. 17). The oxygen isotopic ratios indicate the vapors to be in isotopic equilibrium with andesite at a magmatic temperature. These facts combined with the high heat flow on this island strongly suggest that a considerable portion of these gases are "magmatic" in origin. The average 6D value of-257,~o of this "magmatic water"is much higher t h a n - 4 0 t o - 60~oo which are

Stable Isotopic Studies of Japanese Geothermal Systems

117

the 6D values of waters in volcanic gases from Surtsey Island, Iceland, from the ocean basalts near Hawaii and hydrous minerals of upper mantle origins and are currently inferred as "primary magmatic fluids" (White, 1974; Taylor, 1974). On the other hand, the volcanic steam condensates from White Island, New Zealand, at the time of high activity, are very similar to those from Satsuma-Iwojima, showing 6D and 6180 values around - 1 0 to - 2 0 and 0 to +4~oo, respectively and CI content up to 5000 ppm (Stewart and Hulston, 1976; Giggenbach, 1976). The two volcanic islands have many other common features in geology, geochemistry and tectonics. Close comparison of their isotopic and chemical features would be quite fruitful. As in one of the models advanced by Stewart and Hulston (1976), the high 6D values of this island may be a result of contamination of the andesite magma by waters of marine origins at near surface environments. However, the possibility also exists that the andesite magma was originally impregnated with the fluid of the observed isotopic ratios when it formed underneath the Japanese Islands. In this context, one of the high-temperature fumarolic gases was examined for tritium content. Unfortunately, however, the duplicate runs gave 4 and 0 TR and failed to give a definite value (personal communication from Tanaka). The high 634S values of the net-sulfur discharge from this island also set a certain limitation on the possible origins of the magmatic gases. The isotopic values are unusually high compared with those assigned for the sulfurs of the upper mantle origins. Sulfurs in some basalts dredged from the Atlantic Ocean Ridge have 634S values of + i%o (Kanehira et al., 1973). Tholeiitic basalts in Germany yielded - 2 to + 1~oo (Schneider, 1970). The high values of this island most likely come from the mixing of marine heavy sulfates of Tertiary to Recent ages into lighter sulfur of upper mantle origin. It is again not clear whether the andesite magma had already been enriched in the heavy isotope when it was formed, or contaminated during or after its rise to the surface. It should also be noted that the 634S values of net-sulfur discharges from other andesite volcanoes in Table 2 vary from + 2.2 at Nasudake, central Honshu, to ca. + 7~oo at Kuju, central Kyushu, but are always heavier than zero. This may suggest that" all the volcanic sulfur of the Japanese Islands, especially in association with andesite volcanoes, are more or less contaminated with heavy sulfurs. Ueda (1976) also noted that volcanic native sulfurs as well as net-sulfur discharges collected from various volcanoes of Japan have varying isotopic ratios from - 8 to + 3~oo, showing a weak trend that the sulfurs from the western volcanic belt are heavier than those from the eastern one. These variations may also be indicative of the shallow origins of the volcanic sulfur species. However, it may also be a result of the complicated isotopic fractionations during the oxidation-reduction cycles of sulfur in volcanic activity. Therefore, care must be taken when the 6345 values of volcanic sulfur deposits are related to those of the net-sulfur discharge from volcanoes. The thermal waters of Satsuma-Iwojima may be grouped into two types which differ significantly from each other in isotopic and chemical compositions. The first is the neutral chloride-type waters at Sakamoto, Nagahama and Showa-Iwojima. Sakamoto is located on the outside of the caldera wall, while Nagahama is associated with the non-active cone-volcano adjacent to Iwodake. Showa-Iwojima is a tiny cone-volcano erupted above the sea in 1934 ca. 2 km east of Satsuma-Iwojima. These waters are simply a heated sea water (Showa-Iwojima and Nagahama) or a heated mixture of oceanic and meteoric water as is evident from their isotopic and chemical compositions (Fig. 17 and numbers 17-19 of the Appendix). The second type is of the acid sulfate-chloride-type chemistry and associated with the active cone, Iwodake. Thermal waters at Higashi, Kitabira and Heikeno-jo belong to this type. Contrary to the neutral chloride waters, the acid thermal waters contain as much as 10,000 ppm or more of sulfate against a chloride concentration of 2000 ppm or so. This indicates that the acidity of these waters is due to sulfuric acid.

H. Sakai and O. Matsubaya

118

Recently, nine wells of 100-140 m depths were drilled around the eastern foot oflwodake by the feasibility study team on the electric power generation at volcanoes sponsored by the Ministry of Trade and Commerce. Chemical and isotopic studies of the cores and well waters obtained from these holes revealed the following three interesting facts regarding the origins of the thermal waters mentioned above. Firstly, acid sulfate-chloride-type thermal waters similar to both Heikeno-jo and Higashi occur accompanying abundant deposits of alunite at around sea level or slightly below. Secondly, neutral chloride-type thermal waters such as Sakamoto also exist at similar depths. Lastly, gypsum and anhydrite, though much less abundant than alunite, occur in the zones overlying the alunite deposits. Significant difference in the isotopic ratios exists between the sulfates in the acid thermal waters and alunite on one hand and those in the neutral thermal waters on the other. As plotted in Fig. 18, the latter sulfates are almost the same as the oceanic sulfate around this island, whereas the others are much heavier in 6~80 and slightly lighter in 634S than the oceanic sulfates. 25

Sea water

/

well water

(~J ~)

20 well water

30

co

A

*

DEJ:~

2O 15-

,poe (#

D O

.10

Ground w a t e r

•

N e u t r a l chloride type Acid sulfate t y p e

• (~)

Alunlte H2SO 4 in sulfur (insert) Gypsum ( i n s e r t )

s

o

10,

20,

30 10

,;

2;

8180 of S042Fig. 18. 634S vs 6~sO of sulfates from Satsuma-Iwqiima (after Ueda, 1976). Well water denotes sulfates from acid sulfate-chloride (two solid circles)--and neutral chloride (two open circles)-type waters from the shallow drill holes around the lwodake (see text). These are not plotted in the insert.

Kamada (1963) suggested that sulfates in the acid thermal waters of this island are formed by the disproportionation of volcanic sulfur dioxide (Iwasaki and Ozawa, 1960)

or

SO 2 + H20 -. HzSO 3 3 H 2 S O 3 --~ 2 H 2 S O 4 + S + H 2 0 4 H 2 S O 3 --~ 3 H 2 S O 4 + HzS

(9) (lO) (ll)

The sulfuric acid thus formed is significantly enriched in 348 compared to S o r H 2 S owing to the sulfur isotopic fractionation (Oana and Ishikawa, 1968). At the same time, oxygen isotopic exchange would also take place between sulfate and water, the sulfate being enriched in 1so. Taking the 6345 of the sulfur dioxide to be + 12~oo (Table 2) and the isotopic fractionation factors between sulfate and hydrogen sulfide to be 1.020 at 327°C and 1-029 at 227°C (Sakai, 1968), sulfuric acid of di34S = + 17.0 to + 19.2~ o would be formed by equation (11). The observed values of + 19.5 to + 20"0~0o are compatible with the model, if the uncertainties in the fractionation factor and temperature are taken into account. Further evidence to support this model is the fact that sulfuric acid is often found trapped in coarse crystalline sublimate of sulfur around the low-temperature fumarolic areas. As plotted in

Stable Isotopic Studies of Japanese Geothermal Systems

119

the insert of Fig. 18, these sulfates are quite heavy in both 634S and 6180 (unlike the supergene sulfate of Hakone, Fig. 8). This implies that they are produced by reaction (10) or (11), the higher enrichment facl ors reflecting the fact that they are formed at lower temperatures than those in the acid thermal waters. All these facts strongly support the view that the isotopically heavy sulfuric acid is being actively formed at or near the surface of the volcano, attacks andesite rocks and deposits alunite in the shallow zones surrounding the volcano. Unlike the sulfates in the acid thermal waters and alunite, those of gypsum and anhydrite from the overlying zones are much lighter, the lightest fi34S values being similar to those expected for H 2 or S formed by reaction (11) or (10) (also compare with the supergene sulfate of Hakone). They, therefore, are the surface oxidation products of volcanic sulfide and sulfur including those formed by the disproportionation and would balance the heavy sulfates. The foregoing results and discussion indicate that there are three types of waters involved in the hydrothermal systems of this island, that is, oceanic, meteoric and fumarolic (or "magmatic"). Three types of sulfates are also distinguished: oceanic sulfates, the disproportionation product and supergene sulfates. These waters and sulfates are characterized by their own isotopic and chemical compositions and thus be utilized in the analyses of the hydrological and geochemical problems of this island. As examples, inventory calculations for the acid thermal waters are given in Table 4 and will be discussed in the following. The first example is Higashi-1 (number 4 in Table 4) which was collected by Matsubaya et al. (1975b). As can be seen from Fig. 17, this water is on the mixing line between oceanic and Nagahama groundwater. Model-1 (number 6) is a mixture of oceanic and meteoric waters, in which the mixing ratio was so chosen as to give the same 6D value as Higashi- 1. Comparison of the calculated values of ~80-,C1- and SO4-contents of the model mixture with the observed values of Higashi-1 would indicate that the model mixture is in perfect accord in 6180 and moderate accord in C1 with Higashi-1, but ninety percent of the sulfate must come from other sources than sea water, that is, from the sulfuric acid formed by the disproportion. However, the raw fumarolic gases of high temperatures cannot be directly added into the groundwater, because it would also significantly increase the 1sO and C1 contents of the mixture. Someof the fumarolic gases of low temperatures show HC1 contents as low as 1/100 of the high-temperature ones (see Table 2). They can be mixed into the groundwater without increasing the C1 content. However, it cannot solve the problem of 6180. Therefore, sulfur dioxide must be separated from HCl-rich condensates and then added to the groundwaters. Table 4. Mixingmodelsfor acid thermal waters at Higashiand Heikeno-jo No.

Locality ~D

I 2

Meteoric water Sea water Volcanic gas II~ H i g a s h i - I (2) 5 Higashi--213} 6 Model--ll4) 7 M o d e l - - 2 ~5) Heikeno-jo t6J Kitabira-- I (7) 10 Model--3 t8)

Isotopic ratios of water 6 ~BO

- 37.0 0.0 -25-0 - 32-7 -35.0 -32"7 -35-4 -18-6 - 25.8 -18.5

(I) Average of the high-temperature fumarolic gases (Ueda, 1976). (2) Matsubaya et al. (1975b) (3) Matsuo et aL (1975) (4) 0-884 meteoric water + 0-116 sea water. (5) 0-58 meteoric water + 0-42 volcanic gases. (6) Matsubaya el al. (19751)). (7) Isotopic data by Matsuo et aL (1975); CI and SO, by Kamada (1963). (8) 0-283 sea water + 0.717 Kitabira--I. (9)-63"S of volcanic sulfur dioxide.

- 6-0 0.0 +6.6 -5.3 -2-0 -5.3 -4.3 --2.2 - 3.4 -2.4

mmole/I CI 2 535 370 52-0 43.7 63.9 52"0 174.9 32.6 174.9

SO4 I 28-1 533 72.0 n.d. 8-3 72.8 114.0 148.3 114-1

Isotopic ratios of sulfate 6 ~sO

6a4S'~'~. + 16.9 +21.5 + 1 2 (9) + 19.4

+ 12.2 +10-0 -+ 16-4

+20.0

+16.9

120

It. S a k a i and O. M a t s u b a v a

Model-2 in Table 4, on the other hand, is a mixture of the high-temperature fumarolic gases and the groundwater, with a mixing ratio that gives the same C1 content as Higashi-l. The mixture is similar in isotopic and chemical compositions to Higashi-2, one of the Higashi waters collected b 3 Matsuo et al. (1975) in 1961. The mixing ratio so chosen is 13"11~;fumarolic gases in the mixture. Assuming the enthalpy of the high-temperature gases to be 600 kcal/kg and the temperature of the groundwater to be 2OC, the mixture should have a temperature of ca. 100 C, which is compatible with the observed underground temperatures from the wells mentioned above. Heikeno-jo discharges its thermal water into the sea from the northern cliff of the island. However, because the exit of the water is always below sea level except at ebb tide, sea water contamination at the time of sampling was inevitable. Heikeno-jo is geographically in close association with Kitabira fumarolic area which lies ca. 300 m above Heikeno-jo on the slope of lwodake (Fig. 16). Kitabira thermal waters issue from this fumarolic area and drain towards Heikeno-jo, suggesting that the Heikeno-jo thermal water collected by us is a mixture of the Kitabira thermal water and sea water. The CI and SO4 contents of Kitabira thermal waters were 32.6 and 148.3 mmole/l in 1961 (Kamada, 1963). Model-3 in Table 4 was made by mixing sea water (number 3) and Kitabira-1 (number 9) so as to give the same CI content as the observed water of Heikeno-jo (number 8). Excellent agreement between Heikeno-jo and model-3 supports the mixing model, although the good agreement ~hould partly be taken as fortuitous. The t~348 and (5~sO values of the Heikeno-jo sulfate were measured at +20.0 and + 16-91~,;o, respectively (Ueda, 1976). Those of sea water sulfates around this island are + 21.5 and + 10.0",i.... respectively, according to Ueda (1976). The previous calculation indicates that Heikeno-jo is composed of 28-3°,oo sea water and 71.7"; o Kitabira-1. The balance calculation using these figures gives the 63% and 6280 values of sulfuric acid in Kitabira-1 at + 19.4 and + 19'61~,,o, respectively, which are to be compared with the observed values of + 19.4 and + 16.4°;,, for the sulfate of Higashi- 1. To summarise, the volcanic gases at Satsuma-lwojima are neither pure meteoric, nor completely magmatic in origin. Significant contribution to the sulfur species as well as to the,water from the sources of marine or marine connate origins are strongly suspected, although the data are still lacking that help to construct a complete picture of their origins. Stable isotopic studies of the thermal waters and the drill cores indicate that three types of sulfate exist: the majority of the sulfates in acid sulfate-chloride-type waters come from the disproportionation of the fumarolic sulfur dioxide, whereas the neutral chloride-type waters contain oceanic sulfates. The third sulfate is the supergene sulfate formed by the surface oxidation of the volcanic hydrogen sulfide and sulfur and isotopically is much lighter than the other two types of sulfate. It so happened that the 634S values of the first two sulfates were quite similar to each other, but they can be distinguished by their 6180 values. The generation of such heavy sulfates in volcanic areas would have an important bearing on the origin of the isotopically heavy sulfates observed in other volcanic areas such as Tamagawa, Beppu and Kusatsu-Shirane of Japan, in all of which thermal waters of the acid-chloride-sulfate-type abundantly occur.

CONCLUSIONS Stable isotopic studies combined with chemical and geologic information indicated that the characters of the geothermal and volcanic systems are strongly affected by the geological and geochemical environments with which the systems are associated. Green Tuff-type thermal waters are heated meteoric waters which acquired their salts from the submarine formations rich in fossil marine salts of Miocene age. Some waters in Green Tuff formations present the isotopic evidence that the fossil sea waters still remain in the formations. On the other hand Arima-type waters from

Stable Isotopic Studies o f Japanese Geothermal S y s t e m s

121

pre-Neogene granitic and metamorphic rocks contain the highly saline, N a - C a - C 1 - H C O 3 - t y p e brine of non-meteoric origin. O f the three volcanic systems discussed, H a k o n e is a sub-aerial volcano consisting of a double caldera, a caldera lake and several active cone-volcanoes. The hydrothermal systems within the calderas are predominantly meteoric in water origin. However, ca. 10~o of the water and the majority of the dissolved chloride ions are considered to be of deep origin from geophysical evidence. The isotopic and chemical characters of the deep fluid are not well known, although certain evidence suggests the fluid to be highly enriched in chloride and deuterium. Ibusuki is in the Ata caldera, the eastern half of which underlies the sea water of the Kagoshima Bay. The geothermal systems developed on the coast of the Bay contain up to 6 0 ~ of sea water. Any contribution from the magmatic or deep fluids to the geothermal brines is masked by the oceanic components and can be neither identified nor denied from the isotopic and chemical data. The third volcano, Satsuma-Iwojima, is a volcanic island located ca. 40 km to the south of Ibusuki and belongs to the Kikai caldera which is completely " d r o w n e d " in the ocean. The isotopic and chemical evidence indicates that the oceanic, local meteoric and magmatic waters contribute in varying proportions to the waters and chemicals in the volcanic gases and thermal waters which are actively discharged from the island. The high-temperature volcanic gases of this island have a 6D value of - 2 5 ~ o and suggest a possibility that the "magmatic fluids" along the Japanese Islands may be heavier than " p r i m a r y magmatic fluids". The stable isotopic evidence reviewed in this paper also indicated that a significant part of the sulfur species in hydrothermal waters and volcanic gases are recyclic in origin. Oceanic sulfates and marine sedimentary sulfates presumably are the most important sources for this. A wide range of variation in the 634S of native sulfur deposits of volcanic origin in Japan (Ueda, 1976) also suggests the shallow origin of the volcanic sulfurs. In order to estimate the magmatic contribution to the volcanic sulfurs, the •345 values of sulfur in volcanic and plutonic rocks of the Japanese Islands have to be collected. At Satsuma-Iwojima, it was demonstrated that sulfuric acid isotopically similar to the modern sea water sulfates is being formed by the disproportionation of the volcanic sulfur dioxide of 6345 = + 12%o. The geothermal systems within large calderas such as those at H a k o n e and Ibusuki are hydrologically controlled, to a varying extent, by the caldera lakes. The lake waters commonly and measurably are enriched in D and 180 compared to the local meteoric waters owing to the isotopic and mass balances between rain-input, kinetic evaporation and drainage. The isotopically heavy waters can be used to trace the hydrological features within the calderas. This was best demonstrated at Ibusuki in the Ata caldera. It was shown that the isotopic balance as well as chloride balance of a caldera lake m a y be used to analyze the water balance of the lake and the geothermal systems within the caldera. Acknowledgements---Many of the data used in this review, especiallythose of Satsuma-Iwojima, were obtained by joint

research from 1973 to 1975 by the present authors, M. Kusakabe, Y. Matsuhisa, A. Ueda and A. Sasaki. S. Matsuo, M. Kusakabe and Y. Oki kindly provided us with valuable information and data on Hakone volcanicsystem.N. Kishima and K. Hattori critically read the manuscript and helped us to improve it in various aspects. The present authors are greatly indebted to these colleaguesfor their enthusiasm and helpfuldiscussion on the subject. We also wish to expressour sincere thanks to S. Sakai and S. Masbima for their help in producing the manuscript and to T. Nogi for her technical assistanceat various stages of the project. REFERENCES

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H. S a k a i a n d O. M a t s u b a y a

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