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Rare gas isotopic compositions in natural gases of Japan
Earth andplanetary Science Letters, 53 (1981) 175-188 Elsevier Scientific Publishing Company, Amsterdam - Printed
17.5 in The Netherlands
[31
RARE GAS ISOTOPIC COMPOSITIONS
IN NATURAL GASES OF JAPAN
KEISUKE NAGAO ’ ?*, NOBUO TAKAOKA
’ >**and OSAMU MATSUBAYASHI
’ Department
2
ofPhysics, Faculty of Science, Osaka University, Toyonaka, Osaka 560 (Japan) 2 Geological Survey of Japan, Tsukuba, Ibaragi 305 (Japan)
Received March 25,198O Revised version received December 29, 1980
Isotopic and elemental compositions of rare gases in various types of gas samples collected in the Japanese Islands were investigated. Excess 3He was found in most samples. Many samples showed a regionally uniform high 3He/4He ratio of about 7 times the atmospheric ratio. The He concentrations varied from 0.6 to 1800 ppm, and they were low in COz-rich gases and high in Nz-rich gases. Ne isotopic deviations from the atmospheric Ne were detected in most volcanic gases. The deviations and the elemental abundance patterns in volcanic gases can be explained by a mixing between two components, one is mass fractionated rare gases and the other is isotopically atmospheric and is enriched in heavy rare gas elements. Ar was a mixture of mass fractionated Ar, atmospheric Ar and radiogenic Ar, and the contribution of radiogenic 40Ar was small in all samples. Except for He, elemental abundance patterns were progressively enriched in the heavier rare gases relative to the atmosphere. Several samples were highly enriched in Kr and Xe relative to the abundance pattern of dissolution equilibrium of atmospheric rare gases in water. The component which is highly enriched in heavy rare gases may be released from sedimentary materials in the crust.
1. Introduction Early works on the isotopic compositions of terrestrial He were carried out for continental natural gases and minerals containing radioactive elements. Observed 3He/4He ratios were lower than the atmospheric ratio of 1.4 X 10e6 [32] because of the enrichment in radiogenic 4He from U and Th decay. However, recent isotopic analyses of He in volcanic gases [l-3] , oceanic basalts [4,5] and mantle-derived minerals [6-81 have indicated 3He/4He ratios up to 35 times the atmospheric ratio. Since the high ratios have been detected mainly in pillow basalts collected at oceanic ridges where new crust is forr%ed from the mantle, in rocks and minerals solidified in the mantle
* Present address: Okayama University of Science, Ridaicho l-l, Okayama 700, Japan. ** Present address: Department of Earth Sciences, Faculty Science, Yamagata University, Yamagata 990, Japan.
0012-821X/81/0000-0000/$
02.50
of
0 1981 Elsevier Scientific
and in samples from hot spots, the high ratios are regarded as a representation of the ratio in the mantle, and as evidence that primordial He is still emanating from the mantle [2]. Craig et al. [3] have pointed out that the 3He/ 4He ratios in volcanic gases from the subduction zone are regionally uniform and are about seven times the atmospheric ratio. In the Japanese Islands, they first detected a high 3 He/4He ratio of 6.2 times the atmospheric ratio in a fumarolic gas from the Hakone volcano [3]. Similarly high 3He/4He ratios, which confirm the uniform 3He/4He ratio at the subduction zone, were also found in gas samples from the earthquake fault region [9] and the geothermal area [lo]. The mantle He is also released via other types of naturally occurring gases as well as volcanic gas in the Japanese Islands. We have investigated the isotopic compositions of rare gases in various types of gases such as volcanic gases, hot-spring gases, free gas in low-temperature water pools, and geothermal well gases in the Japanese Publishing
Company
1
(ppm)
No.
Hot-spring gas HI 0.93 H2 1200 H3 58 H4 12 H5 14 H6 300 H7 940 H8 1300 H9 54
v9-2
Volcanic gas Vl v2-1 28 v2-2 23 V2-3 130 v24 V2-5 5.1 V2-6 4.1 V2-7 0.82 v3 11 v4 17 v5 42 V6 5.1 VI 210 V8 16 v9-1 14
4He
Sample
Rare gas elemental
TABLE
(x10-1)
3.1
1.4 150
32 9.3 48 40
0.44 33 20 32
3.0
1.6 1.5 3.1 2.2
5.9 5.2 4.6 4.5
5.3 3.1
4.3
2.0 4.1 3.0 5.1 3.9
1.8 3.2
6.4 4.6 4.8 3.4 3.9
2.0
15
130 40 6.1 12 25
3.5
2.8 3.9 5.2 6.0
84Kr/36Ar (x10-2)
20Ne/36Ar
9.7 48
4He/36Ar
and sample localities
0.66 8.3
4.6
0.083 0.44 6.2 0.41 8.5
0.58
(ppm)
36Ar
compositions 132Xe/36Ar
4.4 3.3 3.0 2.4
6.3 7.8
3.3
1.4 2.8 3.4 1.6 1.6
2.1
5.1
(x10-3)
Iwama Shiramine Yunomine Kawayu Arima Yumura Misasa Kaike Yubara
8.1 470 63 1.0 210 62 150 180 10
16
Tokachidake Showa-shinzan Showa-shinzan Showa-shinzan Showa-shinzan Showa-shinzan Showa-shinzan Showa-shinzan USU Nasudake Hakone Tateyama Kuju Kirishima Satsuma-Iwojima, Kuromoe Shita Satsuma-Iwojima, Kuromoe Ue
Locality
35 120 a 2.1 a lo a 15 a 86 a 2.0 a 0.61 a 650 a 91 23 22 64 24 9.6
4He/20Ne
.L
Hyogo Hyogo Tottori Tottori Okayama
Ishikawa Ishikawa Wakayama Wakayama
Kagoshima
Kagoshima
Hokkaido Hokkaido Hokkaido Hokkaido Hokkaido Hokkaido Hokkaido Hokkaido Hokkaido Tochigi Kanagawa Toyama Oita Miyazaki
Prefecture
-, 21, 28, 29, 30, 24, 25, 25, 30, 15,
Sep. Sep. Apr. Apr. Aug. Aug. Aug. Nov. Aug.
1978 1978 1978 1978 1978 1978 1978 1978 1978
1967
Aug. 12, 1974
Sep. 4, 1974 Aug. 12, 1958 Jul. 9, 1959 Jul. 1, 1959 Sep. 23, 1964 Oct. 6, 1965 Sep. 1, 1974 Oct. 2, 1977 Oct. 9, 1978 Jul. 24, 1970 Jul. 6, 1976 Sep. 30, 1978 Feb. 6, 1916 Dec. 30, 1978
Date
4.8
48 c
5.24
31.5
0.166
5.24
6.5
2.06
2.8
3.1 c
0.73
1.3
and Physics,
The Chemical
Rubber
therefore
spots”
solution,
Mitsubishi Kawasaki
Onuma Takenoyu
“Helium Shuzenji
Matsushiro,
Nigorikawa Matsushiro, COz-spring Yoshino Nameri Ishibotoke Kawanishi Ikeda
Reppu Yamakawa
Matsue Koyohara Yunotsu
with alkaline
0.318
23 0.72
2.3 0.75
0.27
180 c
were removed of Chemistry
1.7 c
a Nagao et al. [29]. bNagaoetal. [lo]. c Nagao et al. [ 261. For the volcanic gas samples except V3, V6 and V8, acid forming components original gas should be lower than those listed in this table. d Calculated with the rare gas elemental abundances in atmosphere (Handbook compositions.
Atmosphere d
Well gas for earthquake prediction Wl 39 w2 1.4
Geothermal prospecting well gas Gl 5.3 3.5 1.5 G2
s2
7.3 c
2.7 C
350 c
Soil gas Sl
.
12 780 30 110 180
3.6 2.4 6.3 2.5 5.3
4.6 140 8.2 26 17
5.9 4.3 7.0 4.3 7.5
0.22 0.32 0.073 4.1 0.63
3.7 1.8 2.8 2.3 0.95
200 160 120 75 7.0
1.0 45 0.60 110 11
2.5 6.4 3.9
L3 L4 L5 L6 L7
4.5 7.8 5.7
280 b 20 c
2.5 0.65 1.3
3.2 b 4.9 c
48 9.9 15
3.8 b 5.3 c
38 0.53 2.7
2.5 b 2.4 c
1800 5.3 40 31 29
Free gas in low-temperature water pool Ll 4.ob 0.058" 69b L2 5.3c 1.1 c 4.9 c
HI0 Hll H12 H13 H14
Co., 1966-1967)
and the isotopic
in the
Jan. -, 1977 Feb. 10, 1977
Oct. 5, 1976 Mar. 13, 1977
Jun. 13,1977 Nov. 11, 1977
the rare gas concentrations
Kanagawa Kanagawa
Akita Oita
Nagano Shizuoka
Hyogo Shimane
13,1977 24, 1978 23, 1978 23, 1978 6, 1978 1, 1978
Jun. Sep. Aug. Aug. Sep. Dec.
Nagano Nara Osaka Osaka
30, lY78 1, 1978 1, 1978 29, 1978 1, 1979
Oct. -, 1977
Nov. Dec. Dec. Dec. Jan.
Hokkaido
Shimane Shimane Oita Kagoshima
178
Islands. In this paper, we report the results of the isotopic compositions of He and other rare gases.
sured for samples V2-6, V3 and V.5. The Kr and Xe isotopic ratios were atmospheric within 2a, though they were not presented in this paper.
2. Samples and mass spectrometry
3.1. 3He/4He ratios and He concentrations
Collection methods of gas samples have been described elsewhere [lo-l 21. The localities and the dates of collection of gas samples are listed in Table 1. The gas samples are classified as follows: volcanic gas (V), free gas in hot spring (H), free gas in low-temperature water pool (L), geothermal prospecting well gas (G), and well gas for a study on earthquake prediction (W). Fig. 1 shows the sampling localities in the Japanese Islands and the classifications of samples. Details of the rare gas mass spectrometry have been described elsewhere [10,13]. The mass spectrometer is of a single focusing 90” sector type with 20cm radius of ion curvature and of stainless steel. A purification system made of stainless steel was connected to the mass spectrometer. The resolving power of the mass spectrometer was adjusted to about 600 to separate 3He from HD t H3 peaks. An electron acceleration voltage in the ion source was set at 40 volts to reduce the interference of 40Ar2+ and CO,’ ions and Ne isotopic ratios were corrected for these ions when they were not negligible. The sensitivities and mass discrimination coefficients for the rare gas isotopes were determined by measuring known amounts of atmospheric gases with the same procedure as applied to samples. The mass discrimination for 3He/4He ratio was determined by analysing a reference helium sample artifically prepared. The 3He/4He ratio was 1.34 X 10e4. An appearance of *‘NeH+ ion interfered the *lNe/**Ne ratio, as described previously [lo]. The *‘Ne/**Ne ratios were corrected for the *‘NeH+ ions with a relationship empirically determined between 2oNeH/20Ne and total amount of He and Ne introduced into the mass spectrometer.
The 3He/4He ratios are shown in Table 2 and Fig. 1 with the sample numbers and the sampling localities. With the exceptions of well gases Wl and W2, all samples were enriched in 3He relative to the atmospheric ratio of 1.4 X 10e6 [32]. The highest value of 3He/4He = (1.54 * 0.04) X lo-’ was found in Gl , a geothermal prospecting well gas at Onuma, Akita prefecture, northeastern district of Japan. 3He enrichments, about 7 times the atmospheric value, were found in many samples. This shows the emanation of He enriched in 3He is a common phenomenon in the Japanese Islands. The 3He/4He ratios for all gas samples are plotted against the 4He/20Ne ratios in Fig. 2. The mixing line represents a composition of mixture between atmosphere and He-enriched gas with a 3He/4He ratio of 1 .O X 1O-' .All data lie on or below the mixing line except the sample Gl whose 3He/4He ratio is significantly higher than the others. The 3He/4He ratios below the mixing line can be interpreted as an admixture of ratiogenic 4He accumulated in crustal rock. However, as will be discussed later, Ne and Ar in some volcanic gases are enriched in light isotopes because of a mass fractionation. Hence, the volcanic gases (e.g. V2-6) which do not fit the mixing line may be qualitatively interpreted as a rightward displacement caused by the mass fractionation because of a large mass difference between 4He and *‘Ne. A similar case has been reported for soil gas samples from the Nigorikawa geothermal area [lo]. Fig. 2 shows that there are no systematic differences in the 3He/4He and 4He/20Ne ratios among volcanic, hot-spring and low-temperature bubble gases. The upper limit to the 3He/4He ratios of these gases is approximately 1 .O X 1O-’ , about 7 times the ratio of atmospheric He, which confirms the previous work on the 3He/4He ratio in the subduction zone [1,3,9,10]. Most of the samples with high 4He/20Ne ratios contain He with high 3He/4He ratios and minor components of crustal He enriched in radiogenic 4He. On the other hand, samples Wl and W2 show low 3He/
3. Results Elemental abundances and isotopic compositions of He, Ne and Ar of the rare gases are listed in Tables 1 and 2. Errors cited in Table 2 are statistical ones (la). Isotopic compositions of Kr and Xe were mea-
179 TABLE 2 Isotopic compositions Sample No.
of He, Ne and AI 3He/4He
20Ne/22Ne
(X 10”) Vl v2-1 v2-2 V2-3 v2-4 v2-5 V2-6 V2-7 v3 v4 V5 V6 v7 V8 v9-1 V9-2 Hl H2 H3 H4 H5 H6 H7 H8 H9 HlO Hll H12 H13 H14 Ll L2 L3 L4 L5 L6 L7 Sl s2 Gl G2 Wl w2 Atmosphere
9.19 7.41 6.67 7.00 7.04 7.01 3.91 3.13 9.62 6.66 8.5 1 10.9 10.5 10.3 10.2 10.6 11.4 5.65 6.56 5.65 11.0 7.50 8.35 7.66 3.96 5.94 9.15 9.12 9.00 9.84 9.95 9.10 6.44 10.4 6.45 1.53 8.58 9.12 1.67 15.4 5.70 <0.3 0.86
(0.55) (0.31) (0.40) (0.19) (0.53) (0.53) (0.24) (0.32) (0.42) (0.29) (0.40) (0.8) (0.3) (0.7) (0.6) (0.5) (1.1) (0.27) (0.28) (0.37) (0.4) (0.18) (0.30) (0.11) (0.22) (0.28) (0.52) (0.57) (0.47) (0.56) (0.25) (0.69) (0.80) (0.3) (0.48) (0.06) (0.44) (0.58) (0.17) (0.4) (0.40) (0.32)
1.4 d
a a a a a a a a
b c
c
9.93 (0.08) 9.84 (0.05) 9.99 9.94 9.86 10.36 10.20 10.17 9.95 9.83 9.79 10.22 9.84 10.05
(0.04) (0.12) (0.05) (0.03) (0.03) (0.08) (0.07) (0.14) (0.06) (0.08) (0.06) (0.06)
21Ne/22Ne
3aAr/36Ar
(X10-y
(x10-3)
290 291
187 (1) 186 (1)
305 (2) 302.5 (1.2)
188 (3)
303
185 (2)
286.6 (1.6)
187 188 188 186 184
312.9 304.7 297.7 299.3 294.9
(6) (5)
301 (6) 289 (12) 292 (9) 292 (2) 301 (7) 299 (9) 294 (5) 296 (8) 287 (4) 289 (10) 287 (4) 287 (5)
(1) (1) (2) (1) (1)
40Ar/36Ar
(2)
(1.1) (1.5) (0.9) (1.7) (0.7)
186 (1)
289.9 (0.7)
9.89 (0.04) 9.76 (0.06) 9.75 (0.08)
282 292
(5) (8)
187 (2) 187 (1)
291.5 (2.1) 325.1 (1.2)
9.82 9.87 9.79 9.80 9.79 9.85 9.79 9.73 9.82 9.84 9.95 9.84 9.99 9.84 9.76 9.79 9.82 9.89
303 289
(6) (9)
290
(5)
188 185 187 187
303.3 297.6 301.7 304.9
(0.07) (0.06) (0.07) (0.06) (0.06) (0.07) (0.06) (0.06) (0.05) (0.05) (0.08) b (0.12) (0.05) (0.09) (0.08) (0.10) (0.07) (0.09)
(2) (2) (2) (2)
(1.0) (1.7) (2.1) (1.5)
294 (5) 284 (4) 293 (4) 292 (4) 287 (4) 298 (9) b 287 (6) 293 (3) 306 (7) 296 (5) 290 (6) 287 (6) 289 (10)
188 (2) 187 (2) 187 (2)
300.3 (1.6) 309.1 (1.5) 315.1 (2.0)
185 186 187 187 186 189 188 187
325 304 296.4 315.9 295.9 306.1 309.9 308
9.83 (0.05)
292
(5)
189 (2)
300.1 (1.8)
9.80 (0.04) 9.83 (0.04)
292
(4)
9.80 e
290 e
187 f
295.5 f
lo errors are shown in brackets. For example, 187 (2) means 187 f 2. a Nagao et al. [ 291. b Nagao et al. [lo]. ’ Nagao et al. (261. d Mamyrin et al. [32]. e Eberhardt et al. [ 331. f Nier [34].
(2) b (4) (1) (2) (1) (2) (2) (3)
(7) b (8) c (1.1) (1.4) (0.9) (1.2) (1.6) (9) c
180
Fig. 1. Distribution of 3He/4He in the Japanese Islands. Classification of samples are as follows: V = volcanic gas, H = hotspring gas, L = free gas in low temperature water pool, S = soil gas, G = geothermal well gas, W = well gas for earthquake prediction. Data sources: Ll [lo], V2, V3 [29], L2, Sl [26].
4He ratios of about 1U7 which is a typical of the 3He/4He ratio of crustal He. Samples Wl and W2 were collected in wells used for the detection of earthquake precursors in the Keihin district. This
area is covered with thick “Kant0 Loam”, volcanic ashes ejected from the Hakone and Fuji volcanoes, and there is no volcanic activity in this area. He concentrations showed wide variations, from 0.6 to 1800 ppm. High concentrations of more than 1000 ppm He were found in hot-spring gases H2, H8 and Hl 0. No significant correlation was found between the 3He/4He ratios and the He concentrations. The concentrations of He in natural gases and liquid samples have been measured to survey the He resources in Japan since 1922 [ 141. Systematic surveys were carried out from 1962 by the Geological Survey of Japan [ 15 ,161. These surveys showed that the natural gases in Japan could be grouped into three types according to their chemical compositions: CO*, Nz and CH4 types of gases. The He concentrations were highest in the N,-type gases, whereas in the COz-type gases they were very low. The He concentrations determined in this work are plotted against the COz and Nz concentrations in Fig. 3. Samples H6, H7 and Sl with He concentrations of several hundred parts per million are enriched in Nz. In contrast, samples L3 and L5 show high CO2 and very low He concentrations. This correlation
3H, &He Gl
0
IO-! 3xing line
/
l
I OH9
V2-6
10” Fig. 2. A plot of 3He/4He
H-441 V2-5
t
10’ against
4He/20Ne.
ld Data sources:
Ll [lo],
V2, V3 [29],
‘Hd‘“Ne
L2, Sl [26].
181 I
1
I
I
I
I
I
I
3.2. Isotopic compositions of Ne and Ar
I
lOOO-
Fig. 4 is a correlation diagram of 6(20/22) F(38/36). &(m/me) is defined by:
H7(
s(m/m,)
500-
z .E
l
L6
l ki3
50-
%142
l log
H5 L7
Hll 5JeL2
l-.L3
l
- l] x IO3
(1)
where R represents rare gas isotopic ratio of mass m and mo, and subscript “atm” means atmospheric. The mass fractionation line in Fig. 4 was determined from the soil gas samples collected at the Nigorikawa geothermal area [lo], whose rare gases were heavily mass fractionated. Large deviations from the atmospheric value and the mass fractionation line noted above are found in the volcanic gases Vl, V2-4, V2-6, V3, V4, V7 and V9-1. In these samples, the deviations of 20Ne/22Ne ratios from the atmosphere are larger than those of 3*Ar/36Ar ratios. This will be discussed later. All the Ar isotopic data are plotted in Fig. 5 with the S-values defined by equation (1). A mass fractionation line (slope = 2) through the atmospheric value was determined on the assumption that mass fractionation effect is twice as much on 40Ar/36Ar as on 3aAr/36 Ar. Fig. 5 indicates that the isotopic compositions of Ar can be understood as a mixture of atmospheric Ar, mass fractionated Ar and radiogenic 40Ar. Small enrichments in radiogenic 40Ar were observed in most of the samples. The highest 40Ar/ 36Ar ratio of 325 was measured in samples H2 and Ll .
OS1 l H6
floe-
= [(R/R,&
and
3.3. Elemental abundance patterns of rare gases
L5
0.5 -
0
t
100
20
:
:
00
40
:
:
60
80
60
:
:
40
:
:
20
100% N2
i
Rare gas elemental abundances relative to the atmospheric rare gases are shown in Figs. 6,7 and 8. F(m) is defined by:
OI co2
Fig. 3. A plot of He concentrations against N2 and CO2 concentrations. Data sources for CO2 and N2 concentrations: H7 [30],H3, H5, L3, L4, LS, L6 [15], Hll, kl2, L7 [16]. Data for L2 and Sl are from Wakita et al. [9].
between the He concentrations and the chemical compositions is consistent with the previous observations noted above.
F(m) = (“X/36Ar)/(mX/36Ar)atm
(2)
where “‘X and subscript “atm” represent a rare gas element X of mass m and atmospheric, respectively. An abundance pattern which represents the dissolution equilibrium of atmospheric rare gases in lowtemperature water [ 17 ] is also shown for comparison. The elemental abundance patterns of Ne, Ar, Kr and Xe for most of the samples are progressively enriched in the heavier rare gases. For some samples, the abundance patterns are more enriched in the heavy
182
Fig. 4. A correlation diagram between *‘Ne/**Ne and 38Ar/ 36Ar. Errors (lo) are shown only for the volcanic gases. The mass fractionation line was determined from the Nigorikawa soil gases, for which A31 was the most fractionated [lo]. The mixing line represents the mixture between two components, one is isotopically and elementally fractionated (A31) and the other is enriched in heavy rare gas elements (Hll). Numerical figures along the mixing line are the ratios of 36Ar from A31 to total 36Ar in the mixture.
rare gases than those for the atmospheric rare gases dissolved in water. Significant enrichments in He were found in all samples analyzed for elemental abundances. As previously noted, He in these samples is
mainly mantlederived He enriched in 3He. The enrichments in He relative to the atmospheric composition is due to the escape of He from the atmosphere.
183
Fig. 5. A correlation diagram between 40Ar/36Ar and 38Ar/36Ar. The mass fractionation line was determined on the assumption that mass fractionation effect is twice as much on 40Ar/ 36Ar as on 38Ar/ 36Ar.
Fig. 6. Elemental abundance patterns and V3 are from Nagao et al. [ 291.
of volcanic
gases. V2
4. Discussion 4.1. He in the Japanese Islands High 3He/4He ratios relative to the atmospheric ratio of 1.4 X 10e6 [32] were observed in most of gas samples collected from various parts of the Japanese Islands. Fig. 2 shows following features: (1) a uniform 3He/4He ratio of about 1 .O X lo-’ as an upper limit is found with respect to the localities and sources of samples, and (2) the 3He/4He ratio of 1 .O X IO-’ is somewhat lower than the ratios found in the glassy margins of ridge basalts, which presumably represent the 3He/4He ratio of the upper mantle. Craig et al. [3] pointed out that the “He14He ratios of volcanic gases in circumPacitic continental margin are about 7 times the atmospheric ratio, ranging from 8 X 1Oe6 to 1 .I X 1O-' .Volcanic gases in the Kamchatka Peninsula also showed similar 3He/
4He ratios, of which the highest ratio was 1 .l X 10e5, 8.1 times the atmospheric value [ 11. The 3He/ 4He ratios determined in this work are consistent with the above ratios, with the exception of Onuma geothermal well gas Gl . In the Japanese Islands where an oceanic plate is subducting, mantle He is emanated via naturally out-flowing low-temperature gases without high heat flux, as well as via high heat flow areas such as volcanoes and hot springs. If upper mantle He has a uniform 3He/4He ratio of 1.5 X lo-‘, the 3He/4He ratio of 1 .O X lo-’ determined in this work suggests that mantle He is contaminated with radiogenic 4He to a uniform extent under the Japanese Islands. Craig et al. [3] have estimated the production of radiogenic 4He in the oceanic lithosphere and indicated that the 3He/4He ratios observed in the island arc could not be formed by releasing He from the subducting oceanic plate unless it contains an order of
184
1
ii -I
Fig. 7. Elemental abundance patterns of hot-spring gases.
more primordial He than has been observed in tholeiite glasses. In a Cretaceous deep-sea basalt near the southern end of the Bermuda Rise, which was estimated paleomagnetically to be 108 m.y. old [18], a 3He/4He ratio of (3 + 1) X 1O-6 was observed [ 191. The low 3He/4He ratio observed confirms their estimation. He, with the regionally uniform and high 3He/4He ratio, may come from the asthenosphere through the melted layer along the upper surface of the subduction plate [20]. The uniform 3He/4He ratio of about 1 .O X 10e5 may be produced in this layer by mixing of 3Heenriched mantle He and radiogenic 4He from the plate. Although clarification of the degassing mechanism is difficult at present, it is an important character to be considered that the 3He/4He ratios in various types of gas samples show regional uniformity and are somewhat lower than that of mantle He found in ridge basalts. A high 3He/4He ratio of (1.54 + 0.04) X 10m5, 11 times the atmospheric value, was found in Onuma
-I
Fig. 8. Elemental abundance patterns of free gases in Iowtemperature water pool, soil gas and geothermal prospecting wellgas. Data source: Ll [lo], L2, Sl [26].
magnitude
geothermal well gas Cl. This ratio is the highest of the ratios observed in this work and is identical with that found in oceanic ridge basalts [4,5]. This sample is the only one analyzed for the 3He/4He ratio from the northeastern district of Japan. If such a high ratio is found in other gases from this district, He degassing mechanism may be different from that in other regions. As can be seen in Fig. 1, high 3He/4He ratios were found in samples collected inside the volcanic front [2 11. Further isotopic analyses of He in samples outside the volcanic front are required to know whether He degassing is regionally different from that inside the volcanic front. Until the high 3He/4He ratios were ubiquitously found in the natural gases occurring in the Japanese Islands, He in gas and liquid samples observed in the previous He surveys was thought to be radiogenic He produced by theadecay of U and Th in crustal rocks!
185
especially in the granitic rocks underlying the area. The origin of He with a high 3He/4He ratio, however, should be attributed to the upper mantle. Fig. 3 indicates that the He concentration is higher in N2 -rich gases than in COz-rich gases, while the 3He/4He ratios in both types of gases are virtually the same. This indicates that N2 gas with high He concentrations can not be attributed to an atmospheric contamination or to a decomposition of organic materials in crustal rocks. It was proposed by Wakita et al. [9] that the COa-rich gas, L2, with 5 ppm He collected at the Matsushiro earthquake fault zone can be explained as the production of COZ by the chemical reaction of hydrochloric acid from a magma with calcareous matter in the crust. The low He concentrations in COz-rich gases can be also understood as the dilution of mantle He with the CO* gas produced by chemical reactions. This model will be discussed in section 4.4. 4.2. Ne in volcanic gases As previously reported [lo], the 21Ne/22Ne ratios were affected by the appearance of 2oNeH+ ions. Although the 21Ne/22Ne ratios in Table 2 have been corrected for the 2oNeH+ ions with the method described in section 2, the ratios are less reliable than the 20Ne/22Ne ratios. Hence, the 21Ne/22Ne ratios are not used in this discussion. Fig. 4 shows isotopic variations of 20Ne/22Ne and 3r’Ar/36Ar ratios in volcanic gases. Nigorikawa soil gas A3 1 and a mass fractionation line determined by the Nigorikawa samples [IO] are shown as an example of a mass fractionated sample. Since all Ne, Ar, Kr and Xe of Nigorikawa sample A3 1 were extremely mass fractionated from the atmospheric ones by a single step of mass fractionation [22], we assume A3 1 as a typical mass fractionated sample. When the volcanic gases are compared to the Nigorikawa gas, the isotopic variations of 20Ne/22Ne ratios are much larger than those of 38Ar/36Ar ratios, Rare gas elemental abundance patterns of volcanic gases shown in Fig. 6 indicate progressive enrichments in the heavier rare gases. The patterns are also different from that of the Nigorikawa soil gas A3 1, which was progressively depleted in the heavier rare gases, consistent with the light isotope enrichments in Ne, Ar, Kr and Xe [lo]. Hence the variations of rare gas isotopic and elemen-
tal compositions in volcanic gases are difficult to be understood as a result of simple mass fractionation of atmospheric rare gases. To understand the different extent of fractionation between 20Ne/22Ne and 3’Ar/36Ar, and the heavy rare gas enrichments, we considered a mixing of two rare gas components. One component is isotopically and elementally mass fractionated (e.g. A3 l), and the other is isotopically atmospheric and is enriched in heavy rare gas elements. The latter component was typical in bubble gases in hot springs (e.g. Hl 1) and low-temperature water pools as seen in Figs. 4,7 and 8. A mixing line in Fig. 4 represents a mixture of A3 1 and HI 1. Most of the volcanic gases lie on the mixing line within experimental error (lo). With this model, an elemental abundance pattern found in sample A3 1, with progressive depletion of heavier rare gas elements, is easily hidden by the admixing of rare gases whose pattern is characterized by the progressive enrichments in heavier rare gases. (When 36Ar from Hll is more than 10% of total 36Ar in the mixture, log F(132) for the mixture is positive value.) Hence, the mass fractionated abundance pattern as found in A3 1 may be difficult to observe in volcanic gases. As will be discussed in section 4.4, since the rare gas component in bubble gas sample such as Hl 1 is often accompanied with a high CO2 content, the heavy rare gases may be mainly released from the sedimentary materials in the crust. 613C values in volcanic gases from the Japanese Islands were similar to those in calcareous rocks [3 I]. The fact suggests the same origin for CO2 and heavy rare gases in both volcanic gases and C02-rich gases. The isotopic deviation of Ne in volcanic gases is not completely understood. It is, however, emphasized that Ne in most volcanic gases is istopically fractionated and is different from Ne in other types of gases, This fact may provide useful information about the degassing process of volcanic gases. 4.3. Isotopic compositions of Ar Since the 4oAr concentration in the earth’s interior depends on the K concentration and degassing rate, the 4oAr/36Ar ratio in the deep interior of the earth offers important information about the origin and evolution of the earth’s atmosphere. 40Ar/36Ar ratios in gas samples plotted in Fig. 5
186
range from atmospheric value of 295.5 to 325 except for mass fractionated Ar. Such relatively low 40Ar/ 36Ar ratios can be understood as a large atmospheric Ar contamination. Hence sufficient information about the 40Ar/36Ar ratio in the upper mantle beneath the Japanese Islands can not be derived from the 4oAr/36Ar ratios in gas samples analyzed in this work. 40Ar/36Ar ratios lower than the atmospheric value of 295.5 have been reported in volcanic rocks [23, 241 and CO?-rich soil gases [lo]. The low ratios have been interpreted as the results of mass fractionation of atmospheric rare gases [ 10,241. Ar isotopic compositions in three samples, Hl , V2-6 and V9-1, in Fig. 5 are also understood in terms of mass fractionation. Though a mechanism for the mass fractionation of atmospheric Ar is not clear, these data suggest that mass fractionated Ar in gas samples may be common. 4.4. Elemental abundances of rare gases Matsubayashi et al. [25] have shown that the elemental abundance patterns of fumarolic gases in the Japanese volcanoes Tokachi-dake, Showa-shinzan, Nasudake and Satsuma-Iwojima were between atmospheric rare gases and the fractionated atmospheric rare gases dissolved in water. They explained the pattern determined for the fumarolic gases as a mixture of atmospheric rare gases and the fractionated atmospheric rare gases presented in local low-temperature groundwater. This model, however, cannot be applied to samples whose elemental abundance patterns are more enriched in Kr and Xe than those for the atmospheric rare gas solubility in water. The enrichments in Kr and Xe suggest a different degassing process for the gas samples. Wakita et al. [9] proposed that COz-rich gas L2 found at the Matsushiro earthquake fault zone might be produced by a chemical reaction of hydrochloric acid from the magma with calcareous rocks in the crust. Nagao et al. [26] pointed out that the chemical reaction also explains the elemental abundance pattern in COz-rich gas L2 (Fig. 8) as the released rare gases from calcareous rocks. The heavy rare gases are probably enriched in sedimentary materials [27,28] because of a selective adsorption at their formation. The 132Xe/36Ar ratios for hot springs and low-temperature water pools are plotted against
coq
( %)
Fig. 9. A plot of 132Xe/36Ar to CO2 concentrations. concentrations are given in Fig. 3.
CO2
the CO2 concentrations in Fig. 9. In C02rich gases, relative elemental abundances are highly enriched in the heavy rare gases. Hence the model proposed by Nagao et al. [26] can be applied to the CO*-rich gases whose rare gas elemental abundance patterns are highly enriched in heavy rare gases. The elemental abundance patterns for Ne, Ar, Kr and Xe shown in Figs. 6,7 and 8 are characterized by the progressive enrichments in heavier rare gas elements. The patterns can be accounted for by the release of rare gases adsorbed on sedimentary materials or dissolved in water.
5. Conclusions (1) He in various types of naturally occurring gas samples collected in Japan showed high 3He/4He ratios, about 7 times the atmospheric ratio of 1.4 X lo*. The ratios were in harmony with those reported by Craig et al. [3] for circumPacitic volcanic gases and were relatively lower than the ratios reported for oceanic basalts [5]. These results suggest a regionally uniform contamination of radiogenic 4He in upper mantle He in the Japanese Islands. The He concentrations showed wide variations ranging from 0.6 to 1800 ppm and did not correlate with the 3He/4He
187
ratios. It was low in COz-rich gas and high in N,-rich gas. (2) Isotopic variations of Ne found in volcanic gases could not be explained as a simple mass fractionation of atmosphere. A mixing between two components can explain both the different amount of isotopic deviation between Ne and Ar, and the elemental abundance patterns in volcanic gases. One of these components is isotopically and elementally mass fractionated and the other is isotopically atmospheric and enriched in heavier rare gas elements. (3) Isotopic compositions of Ar in most samples can be explained in terms of the mixture of atmospheric Ar, radiogenic 40Ar and mass fractionated Ar . The contribution of radiogenic 40Ar, however, was small in gas samples collected in the Japanese Islands. Atmospheric Ar contamination is responsible for the low 40Ar/36Ar ratios. (4) Within the exception of He, elemental abundance patterns in all gas samples measured so far showed progressive enrichments in the heavier rare gases relative to the pattern for the atmosphere. Heavy enrichments in Kr and Xe relative to the pattern for dissolution equilibrium of atmospheric rare gases in low-temperature water were found in many samples. The enrichments may be explained by the release of heavy rare gases from sedimentary materials.
2
3
4
5
6
7
8
9
10
Acknowledgements 11
The authors wish to express their grateful thanks to Professors Sadao Matsuo of the Tokyo Institute of Technology and Yoshihiko Mizutani of Toyama University for kindly providing volcanic gas samples. Thanks are also expressed to many people for their generous assistance in the collection of gas samples. The authors are grateful to Dr. William Bison of the University of California at Berkeley for his useful suggestions especially for the interpretation of Ne isotopic variations and for the improvement in English of this paper.
12
13 14
15
16
References 17 1 I.L. Kamenskiy, A. Lobkov, E.M. Prasolov, N.S. Beskrovnyy, E.I. Kudryavtseva, G.S. Anufriyev and V.P. Pavlov,
18
Components of the upper mantle in the volcanic gases of Kamchatka (according to He, Ne, Ar, and C isotopy), transl. from Geokhimiya No. 5 (1976) 682-694. B.A. Mamyrin, I.N. Tolstikhin, G.S. Anufriyev and I.L. Kamenskiy, Abnormal isotopic composition of helium in volcanic gases, transl. from Dokl. Akad. Nauk USSR 184 (1969) 1197-1199. H. Craig, J.E. Lupton and Y. Horibe, A mantle helium component in circum-Pacific volcanic gases: Hakone, the Mariana, and Mt. Lassen, in: Terrestrial Rare Gases, E.C. Alexander, Jr. and M. Ozima, eds. (Cent. Acad. Publ. Japan,Tokyo, 1978) 3-16. J.E. Lupton and H. Craig, Excess 3He in oceanic basalts: evidence for terrestrial primordial helium, Earth Planet. Sci. Lett. 26 (1975) 133-139. H. Craig and J.E. Lupton, Primordial neon, helium and hydrogen in oceanic basalts, Earth Planet. Sci. Lett. 31 (1976) 369-385. N. Takaoka and M. Ozima, Rare gas isotopic compositions in diamonds, in: Terrestrial Rare Gases, E.C. Alexander, Jr. and M. Ozima, eds. (Cent. Acad. Publ. Japan, Tokyo, 1978) 65-70. I. Kaneoka, N. Takaoka and K. Aoki, Rare gases in mantle-derived rocks and minerals, in: Terrestrial Rare Gases, E.C. Alexander, Jr. and M. Ozima, eds. (Cent. Acad. Publ. Japan, Tokyo, 1978) 71-83. K. Saito, A. Basu and E.C. Alexander, Jr., Planetary-type rare gases in an upper mantle-derived amphibole, Earth Planet. Sci. Lett. 39 (1978) 274-280. H. Wakita, N. Fujii, S. Matsuo, K. Notsu, K. Nagao and N. Takaoka, “Helium Spots”: caused by a diapiric magma from the upper mantle, Science 200 (1978) 430-432. K. Nagao, N. Takaoka and 0. Matsubayashi, Isotopic anomalies of rare gases in the Nigorikawa geothermal area, Hokkaido, Japan, Earth Planet. Sci. Lett. 44 (1979) 82-90. Y. Mizutani, Chemical analysis of volcanic gases, J. Earth Sci. Nagoya Univ. 10 (1962) 125-134. S. Matsuo, M. Suzuku and Y. Mizutani, Nitrogen to argon ratio in volcanic gases, in: Terrestrial Rare Gases, E.C. Alexander, Jr. and M. Ozima, eds. (Cent. Acad. Publ. Japan, Tokyo, 1978) 17-25. N. Takaoka, A low-blank metal system for rare gas analysis, Mass Spectrosc. 24 (1976) 73-86. N. Yamada, Concentrations of He and other components in gas samples of Japan, Bull. Chem. Sot. Jpn. 43 (1922) 884-895 (in Japanese). K. Motojima, K. Shibata, M. Miyamura and T. Abe, Helium resources in the Osaka-Kobe area and the West Kii Peninsula region, Bull. Geol. Surv. Jpn. 20 (1969) 329342 (in Japanese). T. Hirukawa, Exploration for helium resources in Shimane prefecture, Bull. Geol. Surv. Jpn. 19 (1968) 671682 (in Japanese). E. Mazor, Geothermal tracing with atmospheric and radiogenic noble gases, Geothermics 5 (1977) 21-36. Geotimes 22, 6 (1977) 21-23.
188
19 N. Takaoka and K. Nagao, Rare-gas studies of Cretaceous deep-sea basalts, in: Initial Reports of the Deep Sea Drilling Project, Vols. LI, LII, LIII (1980) 1121-1126. 20 H. Okada, Fine structure of the upper mantle beneath Japanese island arcs as revealed from body wave analyses, Thesis, University of Hokkaido (1977). 21 A. Sugimura and S. Uyeda, Island Arcs: Japan and its Environs (Elsevier, Amsterdam, 1973) 246 pp. 22 I. Kaneoka, Rare gas isotopes and mass fractionation: an indicator of gas transport into or from a magma, Earth Planet. Sci. Lett. 48 (1980) 284-292. 23 G.B. Dalrymple, 40Ar/ 36Ar analyses of histric lava flows, Earth Planet. Sci. Lett. 6 (1969) 47-55. 24 D. Krummenacher, Isotopic composition of argon in modern surface volcanic rocks, Earth Planet. Sci. Lett. 8 (1970) 109-117. 25 0. Matsubayashi, S. Matsuo, I. Kaneoka and M. Ozima, Rare gas abundance pattern of fumarolic gases in Japanese volcanic area, in: Terrestrial Rare Gases, E.C. Alexander, Jr. and M. Ozima, eds. (Cent. Acad. Publ. Japan, Tokyo, 1978) 27-32. 26 K. Nagao, N. Takaoka, H. Wakita, S. Matsuo and N. Fujii, Isotopic compositions of rare gases in the Matsushiro earthquake fault region, Geochem. J. 14 (1980) 63-69.
27 R.A. Canalas, E.C. Alexander, Jr. and O.K. Manuel, Terrestrial abundance of noble gas, J. Geophys. Res. 73 (1968) 3331-3334. 28 F.P. Fanale and W.A. Cannon, Physical adsorption of rare gas on terrigeneous sediments, Earth Planet. Sci. Lett. 11 (1971) 362-368. 29 K. Nagao, N. Takaoka, S. Matsuo, Y. Mizutani and 0. Matsubayashi, Change in rare gas composition in the fumarolic gas of the Showashinzan volcano, Geochem. J. (in press). 30 B. Yamaguchi and Y. Kano, Concentrations of He and other components in gas samples of Japan, Bull. Chem. Sot. Jpn. 47 (1926) 452-454 (in Japanese). 31 S. Oana, Stable isotope studies on volcanic emanations, Bull. Volcanol. Sot. Jpn. 10 (1965) 233-237. 32 B.A. Mamyrin, G.S. Anufriyev, I.L. Kamenskiy and I.N. Tolstikhin, Determination of the isotopic composition of atmospheric helium, Geochem. Int. 7 (1970) 498-505. 33 P. Eberhardt, 0. Eugster and K. Marti, A redetermination of the isotopic composition of atmospheric neon, Z. Naturforsch. 20a (1965) 623-624. 34 A.O. Nier, A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon, and potassium, Phys. Rev. 77 (1950) 789-793.
17.5 in The Netherlands
[31
RARE GAS ISOTOPIC COMPOSITIONS
IN NATURAL GASES OF JAPAN
KEISUKE NAGAO ’ ?*, NOBUO TAKAOKA
’ >**and OSAMU MATSUBAYASHI
’ Department
2
ofPhysics, Faculty of Science, Osaka University, Toyonaka, Osaka 560 (Japan) 2 Geological Survey of Japan, Tsukuba, Ibaragi 305 (Japan)
Received March 25,198O Revised version received December 29, 1980
Isotopic and elemental compositions of rare gases in various types of gas samples collected in the Japanese Islands were investigated. Excess 3He was found in most samples. Many samples showed a regionally uniform high 3He/4He ratio of about 7 times the atmospheric ratio. The He concentrations varied from 0.6 to 1800 ppm, and they were low in COz-rich gases and high in Nz-rich gases. Ne isotopic deviations from the atmospheric Ne were detected in most volcanic gases. The deviations and the elemental abundance patterns in volcanic gases can be explained by a mixing between two components, one is mass fractionated rare gases and the other is isotopically atmospheric and is enriched in heavy rare gas elements. Ar was a mixture of mass fractionated Ar, atmospheric Ar and radiogenic Ar, and the contribution of radiogenic 40Ar was small in all samples. Except for He, elemental abundance patterns were progressively enriched in the heavier rare gases relative to the atmosphere. Several samples were highly enriched in Kr and Xe relative to the abundance pattern of dissolution equilibrium of atmospheric rare gases in water. The component which is highly enriched in heavy rare gases may be released from sedimentary materials in the crust.
1. Introduction Early works on the isotopic compositions of terrestrial He were carried out for continental natural gases and minerals containing radioactive elements. Observed 3He/4He ratios were lower than the atmospheric ratio of 1.4 X 10e6 [32] because of the enrichment in radiogenic 4He from U and Th decay. However, recent isotopic analyses of He in volcanic gases [l-3] , oceanic basalts [4,5] and mantle-derived minerals [6-81 have indicated 3He/4He ratios up to 35 times the atmospheric ratio. Since the high ratios have been detected mainly in pillow basalts collected at oceanic ridges where new crust is forr%ed from the mantle, in rocks and minerals solidified in the mantle
* Present address: Okayama University of Science, Ridaicho l-l, Okayama 700, Japan. ** Present address: Department of Earth Sciences, Faculty Science, Yamagata University, Yamagata 990, Japan.
0012-821X/81/0000-0000/$
02.50
of
0 1981 Elsevier Scientific
and in samples from hot spots, the high ratios are regarded as a representation of the ratio in the mantle, and as evidence that primordial He is still emanating from the mantle [2]. Craig et al. [3] have pointed out that the 3He/ 4He ratios in volcanic gases from the subduction zone are regionally uniform and are about seven times the atmospheric ratio. In the Japanese Islands, they first detected a high 3 He/4He ratio of 6.2 times the atmospheric ratio in a fumarolic gas from the Hakone volcano [3]. Similarly high 3He/4He ratios, which confirm the uniform 3He/4He ratio at the subduction zone, were also found in gas samples from the earthquake fault region [9] and the geothermal area [lo]. The mantle He is also released via other types of naturally occurring gases as well as volcanic gas in the Japanese Islands. We have investigated the isotopic compositions of rare gases in various types of gases such as volcanic gases, hot-spring gases, free gas in low-temperature water pools, and geothermal well gases in the Japanese Publishing
Company
1
(ppm)
No.
Hot-spring gas HI 0.93 H2 1200 H3 58 H4 12 H5 14 H6 300 H7 940 H8 1300 H9 54
v9-2
Volcanic gas Vl v2-1 28 v2-2 23 V2-3 130 v24 V2-5 5.1 V2-6 4.1 V2-7 0.82 v3 11 v4 17 v5 42 V6 5.1 VI 210 V8 16 v9-1 14
4He
Sample
Rare gas elemental
TABLE
(x10-1)
3.1
1.4 150
32 9.3 48 40
0.44 33 20 32
3.0
1.6 1.5 3.1 2.2
5.9 5.2 4.6 4.5
5.3 3.1
4.3
2.0 4.1 3.0 5.1 3.9
1.8 3.2
6.4 4.6 4.8 3.4 3.9
2.0
15
130 40 6.1 12 25
3.5
2.8 3.9 5.2 6.0
84Kr/36Ar (x10-2)
20Ne/36Ar
9.7 48
4He/36Ar
and sample localities
0.66 8.3
4.6
0.083 0.44 6.2 0.41 8.5
0.58
(ppm)
36Ar
compositions 132Xe/36Ar
4.4 3.3 3.0 2.4
6.3 7.8
3.3
1.4 2.8 3.4 1.6 1.6
2.1
5.1
(x10-3)
Iwama Shiramine Yunomine Kawayu Arima Yumura Misasa Kaike Yubara
8.1 470 63 1.0 210 62 150 180 10
16
Tokachidake Showa-shinzan Showa-shinzan Showa-shinzan Showa-shinzan Showa-shinzan Showa-shinzan Showa-shinzan USU Nasudake Hakone Tateyama Kuju Kirishima Satsuma-Iwojima, Kuromoe Shita Satsuma-Iwojima, Kuromoe Ue
Locality
35 120 a 2.1 a lo a 15 a 86 a 2.0 a 0.61 a 650 a 91 23 22 64 24 9.6
4He/20Ne
.L
Hyogo Hyogo Tottori Tottori Okayama
Ishikawa Ishikawa Wakayama Wakayama
Kagoshima
Kagoshima
Hokkaido Hokkaido Hokkaido Hokkaido Hokkaido Hokkaido Hokkaido Hokkaido Hokkaido Tochigi Kanagawa Toyama Oita Miyazaki
Prefecture
-, 21, 28, 29, 30, 24, 25, 25, 30, 15,
Sep. Sep. Apr. Apr. Aug. Aug. Aug. Nov. Aug.
1978 1978 1978 1978 1978 1978 1978 1978 1978
1967
Aug. 12, 1974
Sep. 4, 1974 Aug. 12, 1958 Jul. 9, 1959 Jul. 1, 1959 Sep. 23, 1964 Oct. 6, 1965 Sep. 1, 1974 Oct. 2, 1977 Oct. 9, 1978 Jul. 24, 1970 Jul. 6, 1976 Sep. 30, 1978 Feb. 6, 1916 Dec. 30, 1978
Date
4.8
48 c
5.24
31.5
0.166
5.24
6.5
2.06
2.8
3.1 c
0.73
1.3
and Physics,
The Chemical
Rubber
therefore
spots”
solution,
Mitsubishi Kawasaki
Onuma Takenoyu
“Helium Shuzenji
Matsushiro,
Nigorikawa Matsushiro, COz-spring Yoshino Nameri Ishibotoke Kawanishi Ikeda
Reppu Yamakawa
Matsue Koyohara Yunotsu
with alkaline
0.318
23 0.72
2.3 0.75
0.27
180 c
were removed of Chemistry
1.7 c
a Nagao et al. [29]. bNagaoetal. [lo]. c Nagao et al. [ 261. For the volcanic gas samples except V3, V6 and V8, acid forming components original gas should be lower than those listed in this table. d Calculated with the rare gas elemental abundances in atmosphere (Handbook compositions.
Atmosphere d
Well gas for earthquake prediction Wl 39 w2 1.4
Geothermal prospecting well gas Gl 5.3 3.5 1.5 G2
s2
7.3 c
2.7 C
350 c
Soil gas Sl
.
12 780 30 110 180
3.6 2.4 6.3 2.5 5.3
4.6 140 8.2 26 17
5.9 4.3 7.0 4.3 7.5
0.22 0.32 0.073 4.1 0.63
3.7 1.8 2.8 2.3 0.95
200 160 120 75 7.0
1.0 45 0.60 110 11
2.5 6.4 3.9
L3 L4 L5 L6 L7
4.5 7.8 5.7
280 b 20 c
2.5 0.65 1.3
3.2 b 4.9 c
48 9.9 15
3.8 b 5.3 c
38 0.53 2.7
2.5 b 2.4 c
1800 5.3 40 31 29
Free gas in low-temperature water pool Ll 4.ob 0.058" 69b L2 5.3c 1.1 c 4.9 c
HI0 Hll H12 H13 H14
Co., 1966-1967)
and the isotopic
in the
Jan. -, 1977 Feb. 10, 1977
Oct. 5, 1976 Mar. 13, 1977
Jun. 13,1977 Nov. 11, 1977
the rare gas concentrations
Kanagawa Kanagawa
Akita Oita
Nagano Shizuoka
Hyogo Shimane
13,1977 24, 1978 23, 1978 23, 1978 6, 1978 1, 1978
Jun. Sep. Aug. Aug. Sep. Dec.
Nagano Nara Osaka Osaka
30, lY78 1, 1978 1, 1978 29, 1978 1, 1979
Oct. -, 1977
Nov. Dec. Dec. Dec. Jan.
Hokkaido
Shimane Shimane Oita Kagoshima
178
Islands. In this paper, we report the results of the isotopic compositions of He and other rare gases.
sured for samples V2-6, V3 and V.5. The Kr and Xe isotopic ratios were atmospheric within 2a, though they were not presented in this paper.
2. Samples and mass spectrometry
3.1. 3He/4He ratios and He concentrations
Collection methods of gas samples have been described elsewhere [lo-l 21. The localities and the dates of collection of gas samples are listed in Table 1. The gas samples are classified as follows: volcanic gas (V), free gas in hot spring (H), free gas in low-temperature water pool (L), geothermal prospecting well gas (G), and well gas for a study on earthquake prediction (W). Fig. 1 shows the sampling localities in the Japanese Islands and the classifications of samples. Details of the rare gas mass spectrometry have been described elsewhere [10,13]. The mass spectrometer is of a single focusing 90” sector type with 20cm radius of ion curvature and of stainless steel. A purification system made of stainless steel was connected to the mass spectrometer. The resolving power of the mass spectrometer was adjusted to about 600 to separate 3He from HD t H3 peaks. An electron acceleration voltage in the ion source was set at 40 volts to reduce the interference of 40Ar2+ and CO,’ ions and Ne isotopic ratios were corrected for these ions when they were not negligible. The sensitivities and mass discrimination coefficients for the rare gas isotopes were determined by measuring known amounts of atmospheric gases with the same procedure as applied to samples. The mass discrimination for 3He/4He ratio was determined by analysing a reference helium sample artifically prepared. The 3He/4He ratio was 1.34 X 10e4. An appearance of *‘NeH+ ion interfered the *lNe/**Ne ratio, as described previously [lo]. The *‘Ne/**Ne ratios were corrected for the *‘NeH+ ions with a relationship empirically determined between 2oNeH/20Ne and total amount of He and Ne introduced into the mass spectrometer.
The 3He/4He ratios are shown in Table 2 and Fig. 1 with the sample numbers and the sampling localities. With the exceptions of well gases Wl and W2, all samples were enriched in 3He relative to the atmospheric ratio of 1.4 X 10e6 [32]. The highest value of 3He/4He = (1.54 * 0.04) X lo-’ was found in Gl , a geothermal prospecting well gas at Onuma, Akita prefecture, northeastern district of Japan. 3He enrichments, about 7 times the atmospheric value, were found in many samples. This shows the emanation of He enriched in 3He is a common phenomenon in the Japanese Islands. The 3He/4He ratios for all gas samples are plotted against the 4He/20Ne ratios in Fig. 2. The mixing line represents a composition of mixture between atmosphere and He-enriched gas with a 3He/4He ratio of 1 .O X 1O-' .All data lie on or below the mixing line except the sample Gl whose 3He/4He ratio is significantly higher than the others. The 3He/4He ratios below the mixing line can be interpreted as an admixture of ratiogenic 4He accumulated in crustal rock. However, as will be discussed later, Ne and Ar in some volcanic gases are enriched in light isotopes because of a mass fractionation. Hence, the volcanic gases (e.g. V2-6) which do not fit the mixing line may be qualitatively interpreted as a rightward displacement caused by the mass fractionation because of a large mass difference between 4He and *‘Ne. A similar case has been reported for soil gas samples from the Nigorikawa geothermal area [lo]. Fig. 2 shows that there are no systematic differences in the 3He/4He and 4He/20Ne ratios among volcanic, hot-spring and low-temperature bubble gases. The upper limit to the 3He/4He ratios of these gases is approximately 1 .O X 1O-’ , about 7 times the ratio of atmospheric He, which confirms the previous work on the 3He/4He ratio in the subduction zone [1,3,9,10]. Most of the samples with high 4He/20Ne ratios contain He with high 3He/4He ratios and minor components of crustal He enriched in radiogenic 4He. On the other hand, samples Wl and W2 show low 3He/
3. Results Elemental abundances and isotopic compositions of He, Ne and Ar of the rare gases are listed in Tables 1 and 2. Errors cited in Table 2 are statistical ones (la). Isotopic compositions of Kr and Xe were mea-
179 TABLE 2 Isotopic compositions Sample No.
of He, Ne and AI 3He/4He
20Ne/22Ne
(X 10”) Vl v2-1 v2-2 V2-3 v2-4 v2-5 V2-6 V2-7 v3 v4 V5 V6 v7 V8 v9-1 V9-2 Hl H2 H3 H4 H5 H6 H7 H8 H9 HlO Hll H12 H13 H14 Ll L2 L3 L4 L5 L6 L7 Sl s2 Gl G2 Wl w2 Atmosphere
9.19 7.41 6.67 7.00 7.04 7.01 3.91 3.13 9.62 6.66 8.5 1 10.9 10.5 10.3 10.2 10.6 11.4 5.65 6.56 5.65 11.0 7.50 8.35 7.66 3.96 5.94 9.15 9.12 9.00 9.84 9.95 9.10 6.44 10.4 6.45 1.53 8.58 9.12 1.67 15.4 5.70 <0.3 0.86
(0.55) (0.31) (0.40) (0.19) (0.53) (0.53) (0.24) (0.32) (0.42) (0.29) (0.40) (0.8) (0.3) (0.7) (0.6) (0.5) (1.1) (0.27) (0.28) (0.37) (0.4) (0.18) (0.30) (0.11) (0.22) (0.28) (0.52) (0.57) (0.47) (0.56) (0.25) (0.69) (0.80) (0.3) (0.48) (0.06) (0.44) (0.58) (0.17) (0.4) (0.40) (0.32)
1.4 d
a a a a a a a a
b c
c
9.93 (0.08) 9.84 (0.05) 9.99 9.94 9.86 10.36 10.20 10.17 9.95 9.83 9.79 10.22 9.84 10.05
(0.04) (0.12) (0.05) (0.03) (0.03) (0.08) (0.07) (0.14) (0.06) (0.08) (0.06) (0.06)
21Ne/22Ne
3aAr/36Ar
(X10-y
(x10-3)
290 291
187 (1) 186 (1)
305 (2) 302.5 (1.2)
188 (3)
303
185 (2)
286.6 (1.6)
187 188 188 186 184
312.9 304.7 297.7 299.3 294.9
(6) (5)
301 (6) 289 (12) 292 (9) 292 (2) 301 (7) 299 (9) 294 (5) 296 (8) 287 (4) 289 (10) 287 (4) 287 (5)
(1) (1) (2) (1) (1)
40Ar/36Ar
(2)
(1.1) (1.5) (0.9) (1.7) (0.7)
186 (1)
289.9 (0.7)
9.89 (0.04) 9.76 (0.06) 9.75 (0.08)
282 292
(5) (8)
187 (2) 187 (1)
291.5 (2.1) 325.1 (1.2)
9.82 9.87 9.79 9.80 9.79 9.85 9.79 9.73 9.82 9.84 9.95 9.84 9.99 9.84 9.76 9.79 9.82 9.89
303 289
(6) (9)
290
(5)
188 185 187 187
303.3 297.6 301.7 304.9
(0.07) (0.06) (0.07) (0.06) (0.06) (0.07) (0.06) (0.06) (0.05) (0.05) (0.08) b (0.12) (0.05) (0.09) (0.08) (0.10) (0.07) (0.09)
(2) (2) (2) (2)
(1.0) (1.7) (2.1) (1.5)
294 (5) 284 (4) 293 (4) 292 (4) 287 (4) 298 (9) b 287 (6) 293 (3) 306 (7) 296 (5) 290 (6) 287 (6) 289 (10)
188 (2) 187 (2) 187 (2)
300.3 (1.6) 309.1 (1.5) 315.1 (2.0)
185 186 187 187 186 189 188 187
325 304 296.4 315.9 295.9 306.1 309.9 308
9.83 (0.05)
292
(5)
189 (2)
300.1 (1.8)
9.80 (0.04) 9.83 (0.04)
292
(4)
9.80 e
290 e
187 f
295.5 f
lo errors are shown in brackets. For example, 187 (2) means 187 f 2. a Nagao et al. [ 291. b Nagao et al. [lo]. ’ Nagao et al. (261. d Mamyrin et al. [32]. e Eberhardt et al. [ 331. f Nier [34].
(2) b (4) (1) (2) (1) (2) (2) (3)
(7) b (8) c (1.1) (1.4) (0.9) (1.2) (1.6) (9) c
180
Fig. 1. Distribution of 3He/4He in the Japanese Islands. Classification of samples are as follows: V = volcanic gas, H = hotspring gas, L = free gas in low temperature water pool, S = soil gas, G = geothermal well gas, W = well gas for earthquake prediction. Data sources: Ll [lo], V2, V3 [29], L2, Sl [26].
4He ratios of about 1U7 which is a typical of the 3He/4He ratio of crustal He. Samples Wl and W2 were collected in wells used for the detection of earthquake precursors in the Keihin district. This
area is covered with thick “Kant0 Loam”, volcanic ashes ejected from the Hakone and Fuji volcanoes, and there is no volcanic activity in this area. He concentrations showed wide variations, from 0.6 to 1800 ppm. High concentrations of more than 1000 ppm He were found in hot-spring gases H2, H8 and Hl 0. No significant correlation was found between the 3He/4He ratios and the He concentrations. The concentrations of He in natural gases and liquid samples have been measured to survey the He resources in Japan since 1922 [ 141. Systematic surveys were carried out from 1962 by the Geological Survey of Japan [ 15 ,161. These surveys showed that the natural gases in Japan could be grouped into three types according to their chemical compositions: CO*, Nz and CH4 types of gases. The He concentrations were highest in the N,-type gases, whereas in the COz-type gases they were very low. The He concentrations determined in this work are plotted against the COz and Nz concentrations in Fig. 3. Samples H6, H7 and Sl with He concentrations of several hundred parts per million are enriched in Nz. In contrast, samples L3 and L5 show high CO2 and very low He concentrations. This correlation
3H, &He Gl
0
IO-! 3xing line
/
l
I OH9
V2-6
10” Fig. 2. A plot of 3He/4He
H-441 V2-5
t
10’ against
4He/20Ne.
ld Data sources:
Ll [lo],
V2, V3 [29],
‘Hd‘“Ne
L2, Sl [26].
181 I
1
I
I
I
I
I
I
3.2. Isotopic compositions of Ne and Ar
I
lOOO-
Fig. 4 is a correlation diagram of 6(20/22) F(38/36). &(m/me) is defined by:
H7(
s(m/m,)
500-
z .E
l
L6
l ki3
50-
%142
l log
H5 L7
Hll 5JeL2
l-.L3
l
- l] x IO3
(1)
where R represents rare gas isotopic ratio of mass m and mo, and subscript “atm” means atmospheric. The mass fractionation line in Fig. 4 was determined from the soil gas samples collected at the Nigorikawa geothermal area [lo], whose rare gases were heavily mass fractionated. Large deviations from the atmospheric value and the mass fractionation line noted above are found in the volcanic gases Vl, V2-4, V2-6, V3, V4, V7 and V9-1. In these samples, the deviations of 20Ne/22Ne ratios from the atmosphere are larger than those of 3*Ar/36Ar ratios. This will be discussed later. All the Ar isotopic data are plotted in Fig. 5 with the S-values defined by equation (1). A mass fractionation line (slope = 2) through the atmospheric value was determined on the assumption that mass fractionation effect is twice as much on 40Ar/36Ar as on 3aAr/36 Ar. Fig. 5 indicates that the isotopic compositions of Ar can be understood as a mixture of atmospheric Ar, mass fractionated Ar and radiogenic 40Ar. Small enrichments in radiogenic 40Ar were observed in most of the samples. The highest 40Ar/ 36Ar ratio of 325 was measured in samples H2 and Ll .
OS1 l H6
floe-
= [(R/R,&
and
3.3. Elemental abundance patterns of rare gases
L5
0.5 -
0
t
100
20
:
:
00
40
:
:
60
80
60
:
:
40
:
:
20
100% N2
i
Rare gas elemental abundances relative to the atmospheric rare gases are shown in Figs. 6,7 and 8. F(m) is defined by:
OI co2
Fig. 3. A plot of He concentrations against N2 and CO2 concentrations. Data sources for CO2 and N2 concentrations: H7 [30],H3, H5, L3, L4, LS, L6 [15], Hll, kl2, L7 [16]. Data for L2 and Sl are from Wakita et al. [9].
between the He concentrations and the chemical compositions is consistent with the previous observations noted above.
F(m) = (“X/36Ar)/(mX/36Ar)atm
(2)
where “‘X and subscript “atm” represent a rare gas element X of mass m and atmospheric, respectively. An abundance pattern which represents the dissolution equilibrium of atmospheric rare gases in lowtemperature water [ 17 ] is also shown for comparison. The elemental abundance patterns of Ne, Ar, Kr and Xe for most of the samples are progressively enriched in the heavier rare gases. For some samples, the abundance patterns are more enriched in the heavy
182
Fig. 4. A correlation diagram between *‘Ne/**Ne and 38Ar/ 36Ar. Errors (lo) are shown only for the volcanic gases. The mass fractionation line was determined from the Nigorikawa soil gases, for which A31 was the most fractionated [lo]. The mixing line represents the mixture between two components, one is isotopically and elementally fractionated (A31) and the other is enriched in heavy rare gas elements (Hll). Numerical figures along the mixing line are the ratios of 36Ar from A31 to total 36Ar in the mixture.
rare gases than those for the atmospheric rare gases dissolved in water. Significant enrichments in He were found in all samples analyzed for elemental abundances. As previously noted, He in these samples is
mainly mantlederived He enriched in 3He. The enrichments in He relative to the atmospheric composition is due to the escape of He from the atmosphere.
183
Fig. 5. A correlation diagram between 40Ar/36Ar and 38Ar/36Ar. The mass fractionation line was determined on the assumption that mass fractionation effect is twice as much on 40Ar/ 36Ar as on 38Ar/ 36Ar.
Fig. 6. Elemental abundance patterns and V3 are from Nagao et al. [ 291.
of volcanic
gases. V2
4. Discussion 4.1. He in the Japanese Islands High 3He/4He ratios relative to the atmospheric ratio of 1.4 X 10e6 [32] were observed in most of gas samples collected from various parts of the Japanese Islands. Fig. 2 shows following features: (1) a uniform 3He/4He ratio of about 1 .O X lo-’ as an upper limit is found with respect to the localities and sources of samples, and (2) the 3He/4He ratio of 1 .O X IO-’ is somewhat lower than the ratios found in the glassy margins of ridge basalts, which presumably represent the 3He/4He ratio of the upper mantle. Craig et al. [3] pointed out that the “He14He ratios of volcanic gases in circumPacitic continental margin are about 7 times the atmospheric ratio, ranging from 8 X 1Oe6 to 1 .I X 1O-' .Volcanic gases in the Kamchatka Peninsula also showed similar 3He/
4He ratios, of which the highest ratio was 1 .l X 10e5, 8.1 times the atmospheric value [ 11. The 3He/ 4He ratios determined in this work are consistent with the above ratios, with the exception of Onuma geothermal well gas Gl . In the Japanese Islands where an oceanic plate is subducting, mantle He is emanated via naturally out-flowing low-temperature gases without high heat flux, as well as via high heat flow areas such as volcanoes and hot springs. If upper mantle He has a uniform 3He/4He ratio of 1.5 X lo-‘, the 3He/4He ratio of 1 .O X lo-’ determined in this work suggests that mantle He is contaminated with radiogenic 4He to a uniform extent under the Japanese Islands. Craig et al. [3] have estimated the production of radiogenic 4He in the oceanic lithosphere and indicated that the 3He/4He ratios observed in the island arc could not be formed by releasing He from the subducting oceanic plate unless it contains an order of
184
1
ii -I
Fig. 7. Elemental abundance patterns of hot-spring gases.
more primordial He than has been observed in tholeiite glasses. In a Cretaceous deep-sea basalt near the southern end of the Bermuda Rise, which was estimated paleomagnetically to be 108 m.y. old [18], a 3He/4He ratio of (3 + 1) X 1O-6 was observed [ 191. The low 3He/4He ratio observed confirms their estimation. He, with the regionally uniform and high 3He/4He ratio, may come from the asthenosphere through the melted layer along the upper surface of the subduction plate [20]. The uniform 3He/4He ratio of about 1 .O X 10e5 may be produced in this layer by mixing of 3Heenriched mantle He and radiogenic 4He from the plate. Although clarification of the degassing mechanism is difficult at present, it is an important character to be considered that the 3He/4He ratios in various types of gas samples show regional uniformity and are somewhat lower than that of mantle He found in ridge basalts. A high 3He/4He ratio of (1.54 + 0.04) X 10m5, 11 times the atmospheric value, was found in Onuma
-I
Fig. 8. Elemental abundance patterns of free gases in Iowtemperature water pool, soil gas and geothermal prospecting wellgas. Data source: Ll [lo], L2, Sl [26].
magnitude
geothermal well gas Cl. This ratio is the highest of the ratios observed in this work and is identical with that found in oceanic ridge basalts [4,5]. This sample is the only one analyzed for the 3He/4He ratio from the northeastern district of Japan. If such a high ratio is found in other gases from this district, He degassing mechanism may be different from that in other regions. As can be seen in Fig. 1, high 3He/4He ratios were found in samples collected inside the volcanic front [2 11. Further isotopic analyses of He in samples outside the volcanic front are required to know whether He degassing is regionally different from that inside the volcanic front. Until the high 3He/4He ratios were ubiquitously found in the natural gases occurring in the Japanese Islands, He in gas and liquid samples observed in the previous He surveys was thought to be radiogenic He produced by theadecay of U and Th in crustal rocks!
185
especially in the granitic rocks underlying the area. The origin of He with a high 3He/4He ratio, however, should be attributed to the upper mantle. Fig. 3 indicates that the He concentration is higher in N2 -rich gases than in COz-rich gases, while the 3He/4He ratios in both types of gases are virtually the same. This indicates that N2 gas with high He concentrations can not be attributed to an atmospheric contamination or to a decomposition of organic materials in crustal rocks. It was proposed by Wakita et al. [9] that the COa-rich gas, L2, with 5 ppm He collected at the Matsushiro earthquake fault zone can be explained as the production of COZ by the chemical reaction of hydrochloric acid from a magma with calcareous matter in the crust. The low He concentrations in COz-rich gases can be also understood as the dilution of mantle He with the CO* gas produced by chemical reactions. This model will be discussed in section 4.4. 4.2. Ne in volcanic gases As previously reported [lo], the 21Ne/22Ne ratios were affected by the appearance of 2oNeH+ ions. Although the 21Ne/22Ne ratios in Table 2 have been corrected for the 2oNeH+ ions with the method described in section 2, the ratios are less reliable than the 20Ne/22Ne ratios. Hence, the 21Ne/22Ne ratios are not used in this discussion. Fig. 4 shows isotopic variations of 20Ne/22Ne and 3r’Ar/36Ar ratios in volcanic gases. Nigorikawa soil gas A3 1 and a mass fractionation line determined by the Nigorikawa samples [IO] are shown as an example of a mass fractionated sample. Since all Ne, Ar, Kr and Xe of Nigorikawa sample A3 1 were extremely mass fractionated from the atmospheric ones by a single step of mass fractionation [22], we assume A3 1 as a typical mass fractionated sample. When the volcanic gases are compared to the Nigorikawa gas, the isotopic variations of 20Ne/22Ne ratios are much larger than those of 38Ar/36Ar ratios, Rare gas elemental abundance patterns of volcanic gases shown in Fig. 6 indicate progressive enrichments in the heavier rare gases. The patterns are also different from that of the Nigorikawa soil gas A3 1, which was progressively depleted in the heavier rare gases, consistent with the light isotope enrichments in Ne, Ar, Kr and Xe [lo]. Hence the variations of rare gas isotopic and elemen-
tal compositions in volcanic gases are difficult to be understood as a result of simple mass fractionation of atmospheric rare gases. To understand the different extent of fractionation between 20Ne/22Ne and 3’Ar/36Ar, and the heavy rare gas enrichments, we considered a mixing of two rare gas components. One component is isotopically and elementally mass fractionated (e.g. A3 l), and the other is isotopically atmospheric and is enriched in heavy rare gas elements. The latter component was typical in bubble gases in hot springs (e.g. Hl 1) and low-temperature water pools as seen in Figs. 4,7 and 8. A mixing line in Fig. 4 represents a mixture of A3 1 and HI 1. Most of the volcanic gases lie on the mixing line within experimental error (lo). With this model, an elemental abundance pattern found in sample A3 1, with progressive depletion of heavier rare gas elements, is easily hidden by the admixing of rare gases whose pattern is characterized by the progressive enrichments in heavier rare gases. (When 36Ar from Hll is more than 10% of total 36Ar in the mixture, log F(132) for the mixture is positive value.) Hence, the mass fractionated abundance pattern as found in A3 1 may be difficult to observe in volcanic gases. As will be discussed in section 4.4, since the rare gas component in bubble gas sample such as Hl 1 is often accompanied with a high CO2 content, the heavy rare gases may be mainly released from the sedimentary materials in the crust. 613C values in volcanic gases from the Japanese Islands were similar to those in calcareous rocks [3 I]. The fact suggests the same origin for CO2 and heavy rare gases in both volcanic gases and C02-rich gases. The isotopic deviation of Ne in volcanic gases is not completely understood. It is, however, emphasized that Ne in most volcanic gases is istopically fractionated and is different from Ne in other types of gases, This fact may provide useful information about the degassing process of volcanic gases. 4.3. Isotopic compositions of Ar Since the 4oAr concentration in the earth’s interior depends on the K concentration and degassing rate, the 4oAr/36Ar ratio in the deep interior of the earth offers important information about the origin and evolution of the earth’s atmosphere. 40Ar/36Ar ratios in gas samples plotted in Fig. 5
186
range from atmospheric value of 295.5 to 325 except for mass fractionated Ar. Such relatively low 40Ar/ 36Ar ratios can be understood as a large atmospheric Ar contamination. Hence sufficient information about the 40Ar/36Ar ratio in the upper mantle beneath the Japanese Islands can not be derived from the 4oAr/36Ar ratios in gas samples analyzed in this work. 40Ar/36Ar ratios lower than the atmospheric value of 295.5 have been reported in volcanic rocks [23, 241 and CO?-rich soil gases [lo]. The low ratios have been interpreted as the results of mass fractionation of atmospheric rare gases [ 10,241. Ar isotopic compositions in three samples, Hl , V2-6 and V9-1, in Fig. 5 are also understood in terms of mass fractionation. Though a mechanism for the mass fractionation of atmospheric Ar is not clear, these data suggest that mass fractionated Ar in gas samples may be common. 4.4. Elemental abundances of rare gases Matsubayashi et al. [25] have shown that the elemental abundance patterns of fumarolic gases in the Japanese volcanoes Tokachi-dake, Showa-shinzan, Nasudake and Satsuma-Iwojima were between atmospheric rare gases and the fractionated atmospheric rare gases dissolved in water. They explained the pattern determined for the fumarolic gases as a mixture of atmospheric rare gases and the fractionated atmospheric rare gases presented in local low-temperature groundwater. This model, however, cannot be applied to samples whose elemental abundance patterns are more enriched in Kr and Xe than those for the atmospheric rare gas solubility in water. The enrichments in Kr and Xe suggest a different degassing process for the gas samples. Wakita et al. [9] proposed that COz-rich gas L2 found at the Matsushiro earthquake fault zone might be produced by a chemical reaction of hydrochloric acid from the magma with calcareous rocks in the crust. Nagao et al. [26] pointed out that the chemical reaction also explains the elemental abundance pattern in COz-rich gas L2 (Fig. 8) as the released rare gases from calcareous rocks. The heavy rare gases are probably enriched in sedimentary materials [27,28] because of a selective adsorption at their formation. The 132Xe/36Ar ratios for hot springs and low-temperature water pools are plotted against
coq
( %)
Fig. 9. A plot of 132Xe/36Ar to CO2 concentrations. concentrations are given in Fig. 3.
CO2
the CO2 concentrations in Fig. 9. In C02rich gases, relative elemental abundances are highly enriched in the heavy rare gases. Hence the model proposed by Nagao et al. [26] can be applied to the CO*-rich gases whose rare gas elemental abundance patterns are highly enriched in heavy rare gases. The elemental abundance patterns for Ne, Ar, Kr and Xe shown in Figs. 6,7 and 8 are characterized by the progressive enrichments in heavier rare gas elements. The patterns can be accounted for by the release of rare gases adsorbed on sedimentary materials or dissolved in water.
5. Conclusions (1) He in various types of naturally occurring gas samples collected in Japan showed high 3He/4He ratios, about 7 times the atmospheric ratio of 1.4 X lo*. The ratios were in harmony with those reported by Craig et al. [3] for circumPacitic volcanic gases and were relatively lower than the ratios reported for oceanic basalts [5]. These results suggest a regionally uniform contamination of radiogenic 4He in upper mantle He in the Japanese Islands. The He concentrations showed wide variations ranging from 0.6 to 1800 ppm and did not correlate with the 3He/4He
187
ratios. It was low in COz-rich gas and high in N,-rich gas. (2) Isotopic variations of Ne found in volcanic gases could not be explained as a simple mass fractionation of atmosphere. A mixing between two components can explain both the different amount of isotopic deviation between Ne and Ar, and the elemental abundance patterns in volcanic gases. One of these components is isotopically and elementally mass fractionated and the other is isotopically atmospheric and enriched in heavier rare gas elements. (3) Isotopic compositions of Ar in most samples can be explained in terms of the mixture of atmospheric Ar, radiogenic 40Ar and mass fractionated Ar . The contribution of radiogenic 40Ar, however, was small in gas samples collected in the Japanese Islands. Atmospheric Ar contamination is responsible for the low 40Ar/36Ar ratios. (4) Within the exception of He, elemental abundance patterns in all gas samples measured so far showed progressive enrichments in the heavier rare gases relative to the pattern for the atmosphere. Heavy enrichments in Kr and Xe relative to the pattern for dissolution equilibrium of atmospheric rare gases in low-temperature water were found in many samples. The enrichments may be explained by the release of heavy rare gases from sedimentary materials.
2
3
4
5
6
7
8
9
10
Acknowledgements 11
The authors wish to express their grateful thanks to Professors Sadao Matsuo of the Tokyo Institute of Technology and Yoshihiko Mizutani of Toyama University for kindly providing volcanic gas samples. Thanks are also expressed to many people for their generous assistance in the collection of gas samples. The authors are grateful to Dr. William Bison of the University of California at Berkeley for his useful suggestions especially for the interpretation of Ne isotopic variations and for the improvement in English of this paper.
12
13 14
15
16
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18
Components of the upper mantle in the volcanic gases of Kamchatka (according to He, Ne, Ar, and C isotopy), transl. from Geokhimiya No. 5 (1976) 682-694. B.A. Mamyrin, I.N. Tolstikhin, G.S. Anufriyev and I.L. Kamenskiy, Abnormal isotopic composition of helium in volcanic gases, transl. from Dokl. Akad. Nauk USSR 184 (1969) 1197-1199. H. Craig, J.E. Lupton and Y. Horibe, A mantle helium component in circum-Pacific volcanic gases: Hakone, the Mariana, and Mt. Lassen, in: Terrestrial Rare Gases, E.C. Alexander, Jr. and M. Ozima, eds. (Cent. Acad. Publ. Japan,Tokyo, 1978) 3-16. J.E. Lupton and H. Craig, Excess 3He in oceanic basalts: evidence for terrestrial primordial helium, Earth Planet. Sci. Lett. 26 (1975) 133-139. H. Craig and J.E. Lupton, Primordial neon, helium and hydrogen in oceanic basalts, Earth Planet. Sci. Lett. 31 (1976) 369-385. N. Takaoka and M. Ozima, Rare gas isotopic compositions in diamonds, in: Terrestrial Rare Gases, E.C. Alexander, Jr. and M. Ozima, eds. (Cent. Acad. Publ. Japan, Tokyo, 1978) 65-70. I. Kaneoka, N. Takaoka and K. Aoki, Rare gases in mantle-derived rocks and minerals, in: Terrestrial Rare Gases, E.C. Alexander, Jr. and M. Ozima, eds. (Cent. Acad. Publ. Japan, Tokyo, 1978) 71-83. K. Saito, A. Basu and E.C. Alexander, Jr., Planetary-type rare gases in an upper mantle-derived amphibole, Earth Planet. Sci. Lett. 39 (1978) 274-280. H. Wakita, N. Fujii, S. Matsuo, K. Notsu, K. Nagao and N. Takaoka, “Helium Spots”: caused by a diapiric magma from the upper mantle, Science 200 (1978) 430-432. K. Nagao, N. Takaoka and 0. Matsubayashi, Isotopic anomalies of rare gases in the Nigorikawa geothermal area, Hokkaido, Japan, Earth Planet. Sci. Lett. 44 (1979) 82-90. Y. Mizutani, Chemical analysis of volcanic gases, J. Earth Sci. Nagoya Univ. 10 (1962) 125-134. S. Matsuo, M. Suzuku and Y. Mizutani, Nitrogen to argon ratio in volcanic gases, in: Terrestrial Rare Gases, E.C. Alexander, Jr. and M. Ozima, eds. (Cent. Acad. Publ. Japan, Tokyo, 1978) 17-25. N. Takaoka, A low-blank metal system for rare gas analysis, Mass Spectrosc. 24 (1976) 73-86. N. Yamada, Concentrations of He and other components in gas samples of Japan, Bull. Chem. Sot. Jpn. 43 (1922) 884-895 (in Japanese). K. Motojima, K. Shibata, M. Miyamura and T. Abe, Helium resources in the Osaka-Kobe area and the West Kii Peninsula region, Bull. Geol. Surv. Jpn. 20 (1969) 329342 (in Japanese). T. Hirukawa, Exploration for helium resources in Shimane prefecture, Bull. Geol. Surv. Jpn. 19 (1968) 671682 (in Japanese). E. Mazor, Geothermal tracing with atmospheric and radiogenic noble gases, Geothermics 5 (1977) 21-36. Geotimes 22, 6 (1977) 21-23.
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19 N. Takaoka and K. Nagao, Rare-gas studies of Cretaceous deep-sea basalts, in: Initial Reports of the Deep Sea Drilling Project, Vols. LI, LII, LIII (1980) 1121-1126. 20 H. Okada, Fine structure of the upper mantle beneath Japanese island arcs as revealed from body wave analyses, Thesis, University of Hokkaido (1977). 21 A. Sugimura and S. Uyeda, Island Arcs: Japan and its Environs (Elsevier, Amsterdam, 1973) 246 pp. 22 I. Kaneoka, Rare gas isotopes and mass fractionation: an indicator of gas transport into or from a magma, Earth Planet. Sci. Lett. 48 (1980) 284-292. 23 G.B. Dalrymple, 40Ar/ 36Ar analyses of histric lava flows, Earth Planet. Sci. Lett. 6 (1969) 47-55. 24 D. Krummenacher, Isotopic composition of argon in modern surface volcanic rocks, Earth Planet. Sci. Lett. 8 (1970) 109-117. 25 0. Matsubayashi, S. Matsuo, I. Kaneoka and M. Ozima, Rare gas abundance pattern of fumarolic gases in Japanese volcanic area, in: Terrestrial Rare Gases, E.C. Alexander, Jr. and M. Ozima, eds. (Cent. Acad. Publ. Japan, Tokyo, 1978) 27-32. 26 K. Nagao, N. Takaoka, H. Wakita, S. Matsuo and N. Fujii, Isotopic compositions of rare gases in the Matsushiro earthquake fault region, Geochem. J. 14 (1980) 63-69.
27 R.A. Canalas, E.C. Alexander, Jr. and O.K. Manuel, Terrestrial abundance of noble gas, J. Geophys. Res. 73 (1968) 3331-3334. 28 F.P. Fanale and W.A. Cannon, Physical adsorption of rare gas on terrigeneous sediments, Earth Planet. Sci. Lett. 11 (1971) 362-368. 29 K. Nagao, N. Takaoka, S. Matsuo, Y. Mizutani and 0. Matsubayashi, Change in rare gas composition in the fumarolic gas of the Showashinzan volcano, Geochem. J. (in press). 30 B. Yamaguchi and Y. Kano, Concentrations of He and other components in gas samples of Japan, Bull. Chem. Sot. Jpn. 47 (1926) 452-454 (in Japanese). 31 S. Oana, Stable isotope studies on volcanic emanations, Bull. Volcanol. Sot. Jpn. 10 (1965) 233-237. 32 B.A. Mamyrin, G.S. Anufriyev, I.L. Kamenskiy and I.N. Tolstikhin, Determination of the isotopic composition of atmospheric helium, Geochem. Int. 7 (1970) 498-505. 33 P. Eberhardt, 0. Eugster and K. Marti, A redetermination of the isotopic composition of atmospheric neon, Z. Naturforsch. 20a (1965) 623-624. 34 A.O. Nier, A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon, and potassium, Phys. Rev. 77 (1950) 789-793.