Secular variations of carbon and helium isotopes at Izu-Oshima Volcano, Japan

Journalofvolcanology and geothermalresearch Journal of Volcanology and Geothermal Research 64 ( 1995) 83-94

Secular variations of carbon and helium isotopes at Izu-Oshima Volcano, Japan Yuji Sanoa, Toshitaka

Gamob, Kenji NotstC, Hiroshi Wakita”

‘Department of Earth and Planetary Sciences, Faculty of Science, Hiroshima University,Kagamiyama. Higashi Hiroshima, 724, Japan bOcean Research Institute, The Universityof Tokyo, Nakano-ku, Tokyo 164, Japan CLaboratoryforEarthquake Chemistry, Faculty ofScience, The Universityof Tokyo, Bunkyo-ku, Tokyo 113, Japan Received 2 November

1993; revised

version accepted20 May 1994

Abstract

Secuiar variations in 13C/‘*C ratios and chemical compositions of gas samples from October 1986 to July 1992 are reported from a 92-95 “C steam well located about 3 km north of Mt. Mihara, an active volcano on Izu-Oshima Island, Japan. The 613Cvalue steeply increased from - 2.97% (relative to PDB carbonate) in December 1986 to - l.l%o in March 1988 and then gradually decreased to - 1.75%~in July 1992. Over the same period, the CO2 content changed similarly with time, even though the experimental error is relatively large. These variations are consistent with helium isotope changes. Initially rapid and then slow enhancements of 3He/4He ratio, 6r3C value and CO, content are invoked by violent eruptions of Izu-Oshima volcano from 15 November to 18 December 1986. After the eruptive activity, depletion of magmatic gas emission and subsequent mixing with crustal fluids in the hydrothermal system may produce the gradual decreases of 3He/4He ratio, S13Cvalue and CO2 content. Taking into account the rates of these decreases, we suggest that helium and carbon isotope ratios will return to the situation of before the magmatic eruption within 15 years.

1. Intruduction

Secular variations in chemical and isotopic compositions of volcanic and hydrothermal gases may be useful to evaluate the present state of volcanic activity and to understand the behavior of magmatic fluid flow in geothermal systems (Noguchi and Kamiya, 1963; Thomas and Naughton, 1979; Faivre-Pierret, 1983; Oskarsson, 1984). Since noble gases are inert, they can retain important information concerning the origin of the magma, e.g., the Earth’s upper mantle or deep in the crust. Their isotope compositions are not distorted by subsequent chemical reac-

tions (Ozima and Podosek, 1983 ). Among noble gas isotopes, 3He is the most important parameter because of its primordial signature (Lupton, 1983; Mamyrin and Tolstikhin, 1984). Recently, secular variations in the 3He/4He ratios have been reported in volcanic and hydrothermal systems (Sano et al., 1988, 1990; Tedesco et al., 1990). It is generally accepted that helium isotope variation with time is sensitive to magmatic activity and may be of practical use for forecasting volcanic eruptions (San0 et al., 1991). On the other hand, it is difficult to assess the geochemical meaning of changes in chemical

0377-0273/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSiX037?-0273(94)00041-E

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Y. Sane et al. /Journal of Volcanology and Geothermal Research 64 (I 995) 83-94

composition of reactive gases, since they vary in time with temperature, with pH of coexisting water and/or water rock interactions in the hydrothermal system (Sano et al., 1994), although some chemical precursors were reported (Menyailov, 1975; Hirabayashi and Kusakabe, 1985 ). Contents of CO2 in hydrothermal gases may vary significantly when the gas resolves in groundwater or precipitates as carbonate. This physicochemical fractionation of helium to CO2 ratio was discussed by Marty et al. ( 1989 ). In addition, while looking for secular variations, it is not easy to distinguish the chemical change caused by magmatic activity from that produced by seasonal effects (Tedesco et al., 199 1). These de& tits may be overcome if stable isotope data are available and if they are treated together with noble gas isotope information. However, data on temporal variations of stable isotopes are sparse (Naughton and Terada, 1954; Matsuo et al., 1977). Allard ( 1983) reported on the evolution of 13C/i2C ratios of CO, in gases released by La Soufriere in Guadeloupe before, during and after the eruptive activity in 1976- 1977. The average isotopic characteristics of CO2 remained roughly unchanged in the sample. It was inferred that the 1976- 1977 eruption was not triggered or accompanied by a new upsurge of magmatic CO2 from below (Allard, 1983). We present here evidence of temporal variations in chemical composition and 13C/12C ratios in gas from a steam well on Izu-Oshima Island, Japan. Variations in 3He/4He and 4He/ 2”Ne ratios of the Dai-ichi Junior High School water well on the island are also given. Geochemical implications of stable isotope data are discussed based on the concurrent variations of 3He/4He and 4He/20Ne ratios so far reported (San0 et al., 1991). 2. Izu-Oshima volcano Mount Mihara, an isolated stratovolcano (elevation 758 m), is located on Izu-Oshima Island, about 110 km SSW of Tokyo in Sagami Bay of the Pacific Ocean (Fig. 1) . Tectonically, IzuOshima Island is located on the northernmost

part of the Izu-Mariana volcanic arc caused by subduction of the Pacific plate. During the last 10,000 years, large-scale eruptive activities with some lo8 tons of volcanic products have occurred repeatedly once every 100-l 50 years (Nakamura, 1964; Tazawa, 1980). The historical activity of the present Izu-Oshima volcano, including Mt. Mihara was well documented since 7th century A.D. (Isshiki, 1984). The most recent magmatic eruption lasted from 15 November 1986 until 18 December 1986, with 5 x 10’ tons of lava erupted (SEAN Bull., 1986; Ida, 1987). Lava fountains and flows emerged, feeding a 1.5-km-long flow that advanced to within 1 km of Motomachi, the island’s main town. After one year of quiescence, small eruptions from the central crater occurred on the morning of November 20, 1987. Although small amounts of volcanic ash and steam were ejected, no lava flow was observed. The eruption was interpreted as the explosion of solidified lava on the surface of the central crater by the gas pressure accumulated in a cavity beneath the lava (Fukuyama and Takeo, 1990). Small eruptions occurred subsequently on 25 January 1988 with about 2 x 1O3 tons of volcanic ash (SEAN Bull., 1988). The most recent explosive event took place on 4 October 1990 with extensive ash fall over the northem half of Izu-Oshima Island (SEAN Bull., 1990). Several geophysical and geochemical monitorings have been undertaken (Oikawa et al., 1991; Yamaoka et al., 1991), even though the activity of the volcano became relatively calm.

3. Sampling and analysis Gas samples were collected at No. 3 steam well of the Oshima Onsen Hotel, located about 3 km north of the central cone of Mt. Mihara at the bottom of the inner slope of the caldera (Fig. 1). One end of the container was connected to a thick-walled tygon tube and the other to a manual pump. The container was then lowered into the well about 4 m below the ground surface. After air in the container was completely replaced by steam gas using the pump, the container was recovered and both valves were closed

Y. Sano et al. /Journal of Volcanologyand Geothermal Research 64 (1995) 83-94

I

85

Tokyo

D SC

Fig. 1. Sampling sites of steam wells at the Oshima Onsen Hotel and water well at Dai-ichi Junior High School in Oshima Island, Japan. Inset shows location of Oshima Island about 110 km SSW of Tokyo in the Sagami Bay, Pacific Ocean.

(San0 et al., 1991). The sampling method did not change during the entire sampling period of almost 6 years. Water samples were also collected in lead-glass containers by a similar method at the water well of Dai-ichi Junior High School, situated about 6.5 km NNW of the central cone (Fig. 1). Water samples were obtained by sucking it through the lead-glass container with the manual pump. An extra amount of water was passed through the container to ensure adequate flushing of residual air. In the laboratory, helium and neon in the water sample were extracted to the gas phase in a new

container using a high vacuum line (San0 et al., 1990). The helium and neon gases were purified using hot Ti-Zr getters and an activated charcoal trap held at the boiling temperature of liquid nitrogen. After helium abundance and 4He/2%Ie ratio measurement by a quadrupole mass spectrometer (QMG 112, Balzers ), helium was separated from neon by a cryogenic charcoal trap. The 3He/4He ratio was determined by a highprecision mass spectrometer (VG5400, VG Isotopes) installed at Laboratory for Earthquake Chemistry, University of Tokyo. Experimental details were described by Sano and Wakita

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Y. Sano et al. /Journal of Volcanologyand Geothermal Research 64 (I 995) 83- 94

( 1988). After the measurement of helium and neon, the sample containers were stored in the laboratory. We have measured the chemical compositions of gas samples (COz, NZ, 02, and Ar) from No. 3 steam well of the Oshima Onsen Hotel by using a quadrupole mass spectrometer (MSQ 10 1, Ulvat). The gas extracted from the most recent water sample of Dai-ichi Junior High School was also analyzed. About 1 cm3 STP of the gas was admitted into a metal vacuum line. After measurement of sample pressure by a capacitance manometer, the gas was introduced into the quadrupole mass spectrometer through a highprecision variable leak valve. The mass number was measured and the ion current was detected by a digital voltmeter; the results were analyzed on a personal computer. The observed peak intensity was calibrated against that of in-house standard gases made by mixing of pure gas components. This experiment was done using gas samples in lead glass containers stored at the laboratory. Experimental errors of major components such as CO1 are between 7% and 10% ( 1a). estimated by repeated measurements of the standard during about one month. Experimental details are given elsewhere (Sano et al., 1992 ). The 13C/‘*C ratios of gas samples from No. 3 steam well of the Oshima Onsen Hotel have been measured using a conventional stable isotope mass spectrometer. Again the gas extracted from the most recent water sample was analyzed. About 3 cm3 STP of gas sample was admitted into a high vacuum glass line. Carbon dioxide in the sample was separated and purified using two traps held at boiling temperature of liquid nitrogen and dry-ice acetone bath, respectively. Since the CO2 was the most abundant component of the sample except for water vapor, we did not use a CuO furnace and a Pt catalyst for purilication. The “C/‘*C ratios of separated COz were measured using a stable isotope mass spectrometer (MAT250, Finigan) installed at the Ocean Research Institute, University of Tokyo. This experiment was also made using the gas samples after the chemical analysis during two months. Observed ‘3C/‘2C ratios were calibrated against a running standard, whose isotopic composition

has been precisely determined relative to CK- 13 laboratory standard, which had been calibrated against an international standard of PDB carbonate. Although the statistical error of each 13C/ “C ratio measurement was smaller than 0.05%0, we take the error of 0.l%o (one sigma) based on the reproducibility of a running standard relative to the CK-13 during this experiment.

4. Results and discussion Chemical compositions ( COz, NZ, 02, Ar and He) and 13C/12C ratios of gas samples from No. 3 steam well of the Oshima Onsen Hotel are listed in Table 1 together with the 3He/4He and 4He/ “Ne ratios referred from Sano et al. (1988, 199 1) and Faculty of Science, University of Tokyo ( 1992). Carbon isotopic compositions are expressed in the delta (S) notation, as parts per thousand (per mil, %o) deviation from the international standard of PDB carbonate. Experimental precision is less than _+0.1 %o. The CO2 and Nz contents vary significantly from 25% to 84% and from 15% to 64%, respectively. The O2 concentrations are less than 5% except for the oldest samples from before March 1987 and the most recent samples, suggesting that the atmospheric component is not significant in the samples. The major chemical components of gas extracted from the water sample of Dai-ichi Junior High School are COz=81%, N2= 17% and O2 = 2%. The 6l ‘C values of steam well gas samples range from - 2.97%0 to - 1.15%0, a little less but consistent with those of marine carbonate (Hoefs, 1980). The 613C value of gas extracted from the water sample is - 8.78%0, significantly lighter than those of the steam well. The 3He/4He and 4He/20Ne ratios of samples from water well at Dai-ichi Junior High School are listed in Table 2. Also indicated are hypothetical atmospheric and magmatic helium components. Calculations were made using observed 4He/20Ne ratios and assuming that one endmember is atmospheric noble gas dissolved in water at 25’ C and the other one is magmatic gas with an extremely high 4He/20Ne ratio (Sano et al., 1990). The 3He/4He and 4He/20Ne ratios of

81

Y. Sano et al. /Journal of Volcanologyand Geothermal Research 64 (199s) 83-94 Table 1 Secular variations of chemical compositions, Hotel, Izu-Qshima Island, Japan No.

1 2 3 6 10 11 14 17 18 20 23 26 27 29 30 31 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

Date

3-Ott-1986 3-Dec- I986 16-Jan-1987 12-Mar-1987 29-May-1987 26-Aug-1987 20-Nov-1987 24-Dec- 1987 24-Feb-1988 31-Mar-1988 5-Jul-1988 14-Nov-1988 l-Feb-1989 20-May-1989 6-Jul- I989 ItAug-1989 16-Nov- I989 W-Jan- 1990 26-Mar- 1990 2-Apr-1990 20-Apr- 1990 11-May- 1990 20-Jul- 1990 6-Qct-1990 I l-Dee-1990 1S-Jan- 199 1 26-Apr-1991 23-Jul-199 1 15act-1991 8-Feb- 1992 18-Apr-1992 1 l-Mar-1992 3-Jul-1992 Atmosphere

6r3C values, and 3He/4He and 4He/2c’Ne ratios in the steam well at Oshima Onsen

CO2

Nz

02

(%)

(Oh)

(%)

Ar (%)

25 24 50 60

61 64 41 34

13 11 7.9 6.1

0.77 0.74 0.5 0.39

80 84 82 83 82 84 81 79 81 80 80 75 76 72 73 72 76 75 75 71 70 68 71 67 60 57 61 64

18 15 17 16 17 16 17 19 17 17 18 22 21 23 23 24 21 22 21 25 25 27 24 28 33 36 33 30

1.6 1.3 1.3 1.3 1.5 1.3 2.3 2.4 1.9 2.3 2.6 3.5 3.3 4.2 3.8 4.0 3.3 3.5 3.7 3.7 4.2 4.7 4.4 5.2 6.2 7.5 6.1 6.0

0.18 0.13 0.15 0.14 0.15 0.13 0.17 0.19 0.16 0.18 0.18 0.24 0.22 0.27 0.26 0.26 0.25 0.22 0.24 0.28 0.30 0.30 0.28 0.33 0.39 0.43 0.36 0.34

0.03

78.1

20.9

6°C (Oh)

- 2.97 -2.74 -2.84 - 1.66 - 1.54 - 1.30 - 1.29 -1.15 -1.26 - 1.29 - 1.40 - 1.37 -1.37 -1.44 - 1.49 -1.50 -1.46

- 1.51 - 1.56 - 1.51 -1.81 - 1.82 -1.61 - 1.75

0.934

-7.6

3He/4He (Lx,)

4He/2c’Ne

He** (ppm)

1.71’ 1.89. 3.07* 3.61” 4.74. 4.74’ 5.02” 5.08” 5.19” 5.22* 5.49” 5.05” 5.05” 5.02’ 4.98” 4.89” 4.58” 4.45* 4.27” 4.18* 4.27’ 4.42” 4.59” 4.46” 4.25’ 4.05” 3.88” 4.06” 3.78* 3.29”

0.37’ 0.38* 0.49’ 0.62. 0.97’ 1.1‘ 1.8* 1.8* 1.5” 1.6” 1.8” 1.4” 1.3” 1.3’ 1.2” 1.2” 1.1x 0.98* 0.80f 0.93x 0.82’ 0.99” 1.1” 0.98X 0.76* 0.74” 0.76* 0.89” 0.75# 0.49#

5.3 5.2 4.3 4.3 3.6 3.8 4.1 4.8 4.2 4.2 4.1 4.2 4.4 3.7 3.8 3.9 4.5 3.8 3.8 4.3 3.8 4.4 4.2 4.1 3.7 3.9 4.0 4.4 4.4 4.1

3.54 3.65

0.71 0.58

4.0 3.8

1.00

0.314

5.24

* From Sano et al. (1988); “from Sano et al. (1991); Q from Fat. Sci., Univ. Tokyo ( 1992). *The contents were obtained from peak heights of 4He in QMS before the separation of helium from neon. Error of the measurement is estimated to be about 10% by repeated analysis of air standard.

water samples vary from 2.32 R,, (where R,,, is atmospheric 3He/4He ratio of about 1.4x 10-6) to 3.23 R,, and from 0.36 to 0.48, respectively. Both 3He/4He and 4He/20Ne ratios of the samples are apparently lower than those of No. 3 steam well of the Oshima Onsen Hotel, suggesting less magmatic contribution to the water samples.

4. I. Secular variations of CO, contents and their S13Cvalues Temporal changes of CO2 concentrations (middle ) and their J13Cvalues (bottom) in No. 3 steam well of the Oshima Onsen Hotel are shown in Fig. 2 together with concurrent ‘He/ 4He variations with time (top). Arrows indicate the dates of volcanic eruptions. The CO2 con-

Y. Sano et al. /Journal I$ Volcanologyand Geothermal Research 64 (1995) 83-94

88

Table 2 Secular variations of ‘He/“He and 4He/20Ne ratios in water well at Dai-ichi Junior High School, Izu-Oshima, Japan. No.

Date

3He/4He

4He/20Nc

Helium component

(Rat, )

24-Feb-1988 28-Apr-1988 23-Jun-1988 6-Ott-1988 l-Feb-1989 6-Ott-1990 4-Jul-1992

2.57 2.66 2.40 2.32 2.36 2.61 3.23

tents of samples collected before and immediately after the major eruption (Nos. 1 and 2 ) are identical to about 25%. Since then the CO* contents have increased steeply and reached their maximum value of 84% in the period between November 1987 and July 1988. The variation agrees well with those of 3He/4He ratios, although experimental error is relatively large. The enhancement of mantle-derived helium in the well was attributed to the influence of magmatic eruptions on the hydrothermal system. The time lag between the eruptions and the helium enhancement could reflect the travel time of the magmatic component through fissures or channels occurring in the volcanic edifice and the basement (Sano et al., 1988). Even though experimental error of CO2 measurement is relatively large, identical variation of CO2 contents with helium isotopes suggests that not only mantle helium but also magmatic CO2 were derived from the volcanic eruptions. Since the CO2 is major components of the samples, the gas may act as a carrier of mantle helium from the conduit to the steam well. The COz contents then decreased almost linearly until February 1992. Although there was a peak in the 3He/4He ratio during March-December 1990 (Sano et al., 199 I ), no meaningful change of CO2 contents was found within the experimental error margin for that period. Secular variations of COz contents may be not so sensitive as the 3He/4He ratio to volcanic activity. This will be discussed in a later section. Generally, temporal changes of 613C values in

0.45 0.46 0.48 0.42 0.40 0.41 0.36

Air

Magma

61% 60% 57% 65% 69% 67% 76%

39% 40% 43% 35% 31% 33% 24%

No. 3 steam well of the Oshima Onsen Hotel show a similar trend to the 3He/4He ratios and CO2 contents, although there is a small difference. The 613C value was almost constant at about - 2.8%0in the period from December 1986 to March 1987. Then the value increased significantly with time and reached its maximum of - 1.15%0 in March 1988. There seems to be a small time lag between the enhancement of 613C value and CO* contents. The reason of the lag is not well understood and has to be clarified in future work. Since March 1988 the 613C value decreased monotonically until July 1992. The recent variation of 6r3C values agrees well with those of the 3He/4He ratios and CO, contents. The steep increase of 6’ 3C value may be attributable to the influence of magmatic eruptions on hydrothermal systems, which is similar to helium isotopes. There was no meaningful change of 6r3C value in March-December 1990. Again secular variations of 613C value may not be so sensitive as the 3He/4He ratio to volcanic activity. This may partly be related with the origin of COz, which will be discussed in the next section. 4.2. Recent trends of 3He/4He ratios, CO, contents and 613C values In order to predict a volcanic eruption by geochemical monitoring, it is important to accumulate background data of chemical and isotopic compositions during relatively quiescent periods. Various data from July 1988 to July 1992 in No. 3 steam well of the Oshima Onsen Hotel

Y. Sano et al. /Journal of Volcanologyand Geothermal Research 64 (1995) 83-94

89

3He/4He (t) = (6.56 2 0.24)

Oshima Omen Hotel

CO1 (t)=(100.1+12.8) - (0.0166~0.0071)~t S13C (t)=-(0.942+0.198) 1986

’

1987

1988

1989

1990

1991

I

o,

1992

-=

1986

1987

t,t

1988

t,

I

1989

1990

1991

199;

1989

1990

1991

1992

-l2

-

-2-

U 2 9

-3.

-43 1986

1987

1988

Year Fig. 2. Secular variations of helium isotope ratios at No. 3 steam well of the Oshima Omen Hotel and at the water well of Dai-ichi Junior High School (top); CO1 contents (middle) and 613C values (bottom) at the steam well. Arrows indicate the dates of volcanic eruptions. Error bars in CO2 contents and 6°C values are two standard deviations.

are suitable for this purpose. General trends of decreasing 3He/4He ratios, COz contents and 6i3C values are ascribed to depletion of magmatic gas emission and subsequent mixing with atmospheric or crustal fluid in the hydrothermal system. When we take four samples from July 1988 to May 1989, the average CO2 contents is 81.25 8.1% (error is 20). The value is apparently higher than those of the most recent four samples with 60.5 ? 6.1%. Statistically linear regression analysis was made for trends of 3He/ 4He ratios, CO2 contents and S13C values. Calculated results are as follows:

where t is elapsed days since January lst, 1986. The 3He/4He ratios, CO2 contents and 613C values are expressed in the unit of R,,, % and %o, respectively. Errors assigned to the values are 2~. Decreases of the 3He/4He ratios, COZ contents and 6 ’ 3C values can be statistically different from a zero time rate of change. It is noted that the calculation takes into account the weight of error of each determination. Correlation coefficients of 3He/4He ratios, CO* contents and 6 ’ 3C values are again statistically significant with the values of -0.962, -0.944, and -0.907 at the 99.5% confidence level, respectively. Assuming that trends of the recent secular variations will remain constant over several decades, we can estimate the number of days required to restore the chemical and isotopic situations before the magmatic eruptions of November-December 1986. The 3He/4He ratio, CO2 content and S’TC value before the anomalous changes related to the eruptions are about 25% and - 2.8%0, respectively. Taking 1.8 Ln, into account the above equations, the number of days required for the 3He/4He ratio, CO2 content and 613Cvalue to return to preeruption level are 3580 ? 380, 4520 + 1930, and 5630 +_1880 (two sigma), respectively. Within 15 years at maximum, the chemical and isotopic signature of No. 3 steam well of the Oshima Onsen Hotel will return to the situation before the eruptions. Thus, a magmatic eruption invokes a long-term influence of more than ten years on the near-by hydrothermal system. According to the historical records of volcanic activity of Izu-Oshima Volcano (Isshiki, 1984)) medium size eruptions with volcanic products of some 10’ tons have repeatedly occurred once in every 30-40 years in addition to large-scale

90

Y. Sane et al. /Journal of Volcanologyand Geothermal Research 64 (1995) 83-94

eruptions with 100-l 50 years recurrence interval. This means that the 3He/4He ratio, CO1 content and 613C value will restore the signature before the eruptions of November-December 1986 within about half period of recurrence time in medium size eruptions. It is necessary to continue the geochemical monitoring to verify this idea. 4.3. Origin of carbon in the CO2 The 613C value has often been used to identify the origin of carbon in natural gas samples (Schwartz, 1969; Hoefs, 1980). It is now well documented that mid-ocean ridge basalt (MORB) glasses have S13C values between -4%a and -9%0 with an average of - 6.5%0 (Javoy et al., 1986; Marty and Jambon, 1987). The value of about - 6.5% is considered to represent the deep-seated carbon in the Earth’s upper mantle. In contrast, 613C values of crustal carbon vary significantly. Hoefs ( 1980) summarized that crustal carbon may originate from two major sources, marine limestone and organic carbon from sedimentary rocks. The former has an average S13Cvalue of near 0960while the latter indicates a value less than - 20%0. Observed 613C values in No. 3 steam well of the

Oshima Onsen Hotel vary from -2.97%0 to - 1 . 15%0, suggesting incorporation of MORBtype carbon with marine limestone. However, mixing of carbon from organic sediment with limestone in addition to some fractionation processes can also produce the observed Sr3C values. It is impossible to identify the origin of carbon based on the 613C value only. The C02/3He ratio coupled with the 3He/4He ratio gives another constraint on the origin of carbon in natural gases (Marty and Jambon, 1987; Poreda et al., 1988). Two-component mixing was taken into account to explain origin of CO2 in hydrothermal gases (Poreda et al., 1988 ) . End-members were assigned to low-3He/ 4He and high-C0,/3He gas presumably resulted from decarbonation reactions and high-3He/4He and low-C0,/3He gas derived from the upper mantle. Although this method can identify the origin of carbon as magmatic or non-magmatic, it is difficult to distinguish carbon in organic sediment from limestone carbon. Sano and Marty ( 1992) reported that a combination of 613C value and C02/3He ratio is useful to identify the origin of carbon in volcanic and geothermal gases in subduction zones. This method may be applicable to the present samples. The C02/3He ratios are estimated by ob-

Fig. 3. Correlation diagram between CO#He ratio and 6°C value for steam well gases in the Oshima Onsen Hotel (0 ) and high-temperature volcanic gases in subduction zones ( A). Also indicated are data for MORB-type, limestone, and sedimentary carbon (Sano and Marty, 1992). Lines show mixing lines among the three endmembers.

Y. Sano et al. /Journal of Volcanologyand Geothermal Research 64 (1995) 83-94

served 3He/4He ratios, and helium and COz.concentrations. In the calculation, atmospheric contamination in ‘He is corrected using the 3He/ 4He and 4He/2”Ne ratios. Fig. 3 shows the correlation diagram between the S13C value and C02/3He ratio in the steam well gas. Also indicated are high-temperature volcanic gases in subduction zones and endmembers of carbon such as MORB and limestone. Calculations based on mass balance equations by Sano and Marty ( 1992) suggest that a major part of the carbon in the steam well samples is derived from marine limestone under the assumption that elemental and isotopic fractionation due to production of the steam phase are minimal. Interaction between acidic magmaticderived volatiles (i.e. rich in HCl, H2S and S02) and local country rocks may have lead to the decarbonation of the limestones (San0 et al., 1994). Therefore, the CO2 is not primarily derived from the MORE&type mantle and secular variations of CO2 contents and 6 l 3C values may not be so sensitive as the helium isotopes to volcanic activity. The average MORB-type and sedimentary organic carbon contents are about 6.3% and 4.9%, respectively. It is noted that the samples before the magmatic influence show a relatively large contribution of organic carbon of about 10%. This is clearly seen in Fig. 3 where samples Nos. 2,3 and 6 have less carbon-l 3 than the majority. The eruptions of November-December 1986 have affected the rapid increase in the proportion of MORB-type carbon in the steam well, even though the experimental error is relatively large. The gradual decrease of Mom-type carbon is due to depletion of the magmatic gas emanating from the conduit and subsequent mixing with crustal fluids in the hydrothermal system. 4.4. Correlationbetween3He/4He and N,/Ar ratios The abundance ratio of nitrogen to argon has been used as an indicator for the origin of nitrogen in natural gases (Zartman et al., 196 1; Sano et al., 1993), volcanic gases (Matsuo et al., 1978; Kiyosu, 1986) and sedimentary rocks (San0 and Pillinger, 1990). According to Zartman et al.

91

( 196 1 ), the presence of nitrogen in natural gases from North America was attributed to the mixing of atmospheric nitrogen and a non-atmospheric component. The latter component was explained by gas produced by bacterial decomposition of nitrogen-bearing components or release of nitrogen from decomposition of organic compounds by chemical reactions. The N,/Ar ratios of several volcanic gases were compiled by Matsuo et al. ( 1978). Excess N2/Ar ratios relative to atmospheric conditions in the volcanic gases were attributed to nitrogen derived from subducted sediments. Kiyosu ( 1986) suggested that original magmatic gases in Northeastern Japan have a N2/Ar ratio of about 4000. The secular variation of the N2/Ar ratio may be informative to evaluate volcanic activity. Matsuo et al. ( 1978) reported a yearly decrease in the N2/Ar ratio of high-temperature gases from the A- 1 fumarole of Showa-shinzan, Japan. There was a significant correlation between the temperature of fumaroles and the N2/Ar ratio. A decrease in the ratio was explained by an increase in the contribution of atmospheric components to the original magmatic gases. Temporal changes of N,/Ar ratio in No. 3 steam well of the Oshima Onsen Hotel are harmonized with those of the 3He/4He ratio, even though the experimental error is relatively large. The N,/Ar ratio reached a maximum value of 123 in July 1988 and then gradually decreased to 8 1 in July 1992, which is consistent with the atmospheric value. When we take four samples from July 1988 to May 1989, the average N2/Ar ratio is 107.32 10.7 ( error 2~). The value is statistically higher than those of the most recent four samples with 84.8 + 8.5. Linear regression analysis of N2/Ar ratios from July 1988 to July 1992 gives a decreasing rate of 0.0152+0.0094 per day (error 20). Again the decrease is statistically different from a zero time rate of change. The recent trend may be attributable to an increase in the contribution of atmospheric components. Fig. 4 indicates a correlation between the 3He/ 4He and N2/Ar ratios in the steam well gases. A curved line is calculated by mixing of air and magmatic gas with the 3He/4He ratio of 6 R,,, and the N,/Ar ratio of infinity. The observed

Y. Sane et al. /Journal of Volcanologyand Geothermal Research 64 (1995) 83-94

92

Oshima Onsen Hotel

50

75

loo

125

150

N2/Ar Fig. 4. Correlation diagram between 3He/4He and N,/Ar ratios for gases of steam wells in the Oshima Onsen Hotel. The atmospheric value is also indicated. Error bars in NJAr ratios are two standard deviations. The curved line shows mixing of air and magmatic gas with high 3He/4He and NJAr ratios.

data agree well with the line within the experimental margin, suggesting that two-component mixing is plausible. In order to assess the origin of excess N2/Ar ratio relative to atmosphere, the isotopic composition of nitrogen is highly desirable. 4.5. Secular variations of 3HePHe and 4He/20Ne ratios at water well

A rapid increase of groundwater temperature by up to 20°C was observed at the Koshimizu and Otsu water wells in Motomachi area from September 1987 to August 1988 (Takahashi et al., 1988). The variations was attributed to addition of thermal water into a shallow aquifer lying below the area, which may have been induced by magmatic eruptions of November-December 1986. At the Dai-ichi Junior High School well, a similar but relatively small increase of water temperature of about 8.8”C was reported by Takahashi et al. ( 1988). In contrast no meaningful change in the 3He/4He and 4He/20Ne ra-

tios was found at the water well during February 1988 to February 1989 (Fig. 2). On the other hand, an apparent increase of the 3He/4He ratio was observed from February 1989 to July 1992. The physical mechanism of the time lag between magmatic heat and 3He input into the water well is not well understood at present, even though data are interesting. The time lag of about 2 months between magmatic eruptions of November-December 1986 and the increase of 3He/4He ratios at the No. 3 steam well of the Oshima Onsen Hotel was already ascribed to the travel time of the magmatic component from the conduit to the sampling site. The velocity of the component was calculated to be several tens of meters per day (Sano et al., 1988 ). If this is applicable to the increase of 3He/ 4He ratios at the water wells in Motomachi area, the time lag should be about 4 months, since the distance from the cone is two twice that of the steam well. However, this is not the case for water wells. The time lag is estimated to be about 26 months for 3He/4He enhancement at the Dai-ichi Junior High School well. The velocity of the high 3He/4He fluid would be about 8 meters per day. Again, the discrepance in travel time between No. 3 steam well and the School water well is not well understood physically. Anyway, these are the first data showing different travel time of magmatic fluid in the same volcanic-geothermal system.

5. Conclusions Several conclusions can be drawn concerning the secular variations of CO2 contents, 613C values and 3He/4He ratios at the No. 3 steam well and the School water well in Oshima Island from the foregoing discussion: ( 1) Secular variations of CO* contents and 613C values in No. 3 steam well of the Oshima Onsen Hotel agree well with that of 3He/4He ratios, although their variations are not so clear as that of helium isotopes. Rapid increases of CO2 contents and 613C values after the eruptions of November-December 1986 suggest that the CO2 may act as a carrier of mantle helium from the conduit to the site.

Y. Sano et al. /Journal of Volcanologyand Geothermal Research 64 (1995) 83-94

(2) Recent trends of 3He/4He ratios, CO2

contents and S13Cvalues in the steam well show a monotonous decrease, which may be caused by a decrease of magmatic gas emission at the source and subsequent mixing with crustal components in the hydrothermal system. If the decreasing rates remain constant, the chemical and isotopic signature of the steam well will restore to the situation before the eruptions within 15 years. ( 3 ) The origin of carbon in CO2 of the steam well gas was considered based on the 613C value and the C02/3He ratio. Mass balance calculation revealed that a major part of the carbon (>80%) is derived from marine limestone. Contributions of MORB-type carbon and sedimentary organic carbon are about 6.3% and 4.9% on average, respectively. (4) A significant correlation was found between the 3He/4He and N2/Ar ratios in the steam well, suggesting two-component mixing of air and magmatic gas with high 3He/4He and N2/Ar ratios. The gradual decrease of the N,/Ar ratio since July 1988 is attributable to an increase in the contribution of atmospheric components. (5) A significant increase of the 3He/4He ratio was found at the Dai-ichi Junior High School water well since February 1989. The time lag of about 26 months between the beginning of the magmatic eruptions ( 1986) and the start of the increase is ascribed to the travel time from the conduit to the site. The estimated velocity of about 8 m/day is much smaller than that of several tens m/day for the No. 3 steam well. Acknowledgments

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