Origin of hydrogen-nitrogen gas seeps, Oman

Applied Geochemistry, Vol. 8. pp. 1-8, 1993

(1883-2927/93 $5.00 + ,00 © 1992 Pergamon Press Ltd

Printed in Great Britain

Origin of hydrogen-nitrogen gas seeps, Oman YuJI SANO,* AKIKO URABE~ a n d HIROSHI WAKITA Laboratory for Earthquake Chemistry, Faculty of Science, The University of Tokyo, Buukyo-ku, Tokyo 113, Japan

and

HISAOWUSHIKI~ Department of Chemistry, Faculty of Science, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan (Received 27 September 1991; accepted in revised form 16 April 1992) Abstract--Six gas samples were collected from five thermal springs in the Semail Nappe ophiolite and

the calcareous (calcite and dolomite) Hajar Formation, northern Oman. The 3He/~He, 4He/~°Ne, 4°Ar/36Ar and 3SAr/36Arratios, chemical compositions (H2, N2, CO2, CI~, O2, Ar and He), and stable isotope compositions (6Du, 6DH~o, 613Cco~, 613CcH~, and 6ISNN:) are reported. Samples from the ophiolite region are significantly anoxic with major constituents of H2, Ct-I4 and N2, while those from calcite and dolomite regions are ordinary gas seeps, consisting of N2, CO, and/or O,. The former He-rich gas is characterized by relatively high 3He/4He ratio (0.4-4).8 R?~) with low He content (<5 ppm), atmospheric 4~Ar/~6Ar ratio, low N2/Ar ratio (<55) and high 61- NN~ value (~ 1%o). On the other hand, the latter N2-rich gas shows relatively low ~He/4He ratio (0.1-4).4 R~tm) with high He concentration (>300 ppm), slight radiogenic ~°Ar/36Ar ratio, high N2/Ar ratio (77-97) and low blSNN, value (<0%0). Observed bDH~value of -536%0 in H2-rich gas is distinguished from the literature value of -699%0 in the ophiolite region, giving discrepant isotope formation temperatures.

INTRODUCTION VARIOUS volatiles continuously emanate from the solid Earth over geological time (RUBEY, 1951; TUREKIAN, 1959). At present gas and fluid are being emitted through several geophysical settings such as volcanoes, geothermal springs, oil and gas fields, Artesian wells and deep-rooted faults. It is important to study the nature of the gases and fluids to understand the origin and evolution of the atmosphere, because the accumulation of the volatiles has made up the terrestrial atmosphere and ocean (RuBEY, 1951; BROWN, 1952). More practically their chemical and isotopic variations with time may provide useful information for seismological studies (WAKITAet al., 1980; SANO et al., 1986) and forecasting volcanic events (THOMAS and NAUGHTON, 1979; SANO et al., 1988). Investigations so far reported have revealed that major chemical compositions of gas and fluid emanating from the solid Earth are mostly N2, CO 2 and CH 4 in addition to H20, H2S and SO 2 (MOUREU, 1923; ZARTMAN et al., 1961; URABE et al., 1985). Among them, H 2 contents vary from < I ppm to Present address: *Institute of Geology and Mineralogy, Faculty of Science, Hiroshima University, Kagamiyama, Higashi Hiroshima 724, Japan. tTakano-Cho, Gifu 500 Japan. SEarth Sciences Laboratory, Institute of Physical and Chemical Research, Wako-shi, Saitama 351-01, Japan.

several percent, and natural gas seeps with He >50% are very sparse (WooD, 1970; NEAL and SqrANGER, 1983; COVENEYet al., 1987). This suggests that processes related to the production of H 2 are not as common as those for N2, CO 2 and CH 4 in the present Earth's crust. Reduced gas occurrences are generally found in regions of present-day serpentinization (NEAL and STANGER,1983; COVENEY et al., 1987; ABRAJANOet al., 1988) and in high temperature volcanic gases (ARNASONand SICURGEIRSSON,1968). Chemical reactions between meteoric water and ultramafic rocks at various depths in the crust may produce reduced gases with abundant H 2. The Semail Nappe of Oman is a typical ophiolite region comprised of basic rocks and partially to completely serpentinised hartzburgite. NEAL and SXAN~ER (1983) suggested that H2-rich gas emanating from ultramafic rocks of the Oman ophiolite is formed by low-temperature redox reactions in a closed groundwater environment. Efficient I-t2 production was attributed to the hydrogeological conditions in the area (NEAL and SXANGER, 1983). Noble gas isotope compositions, such as 3He/4He, 2°Ne/22Ne and 4°Ar/36Ar ratios, may provide constraints on the origin of volcanic gases and crustal fluids (OzIMA and PODOSEK, 1983; LuvroN 1983). Among them, the 3He/4He ratio is the most important parameter for identifying mantle-derived components (CLARKE et al., 1970). It is now well documented that 3He/4He ratios greater than that in air

2

Yuji Sano et al.

are c o m m o n l y f o u n d in volcanic a n d tectonically active regions, while geotectonically stable c o n t i n e n tal regions contain ratios less t h a n that in air (MAMYRIN a n d TOLSTIKHIN, 1984). T h e radiogenic 4He/4°Ar ratios of natural gases provide i n f o r m a t i o n on the U. T h a n d K c o n t e n t s of the average rocks a n d s e d i m e n t s in the crust, and the relative efficiency of extraction a n d migration from the source, because the 4He/a°Ar ratios were essentially equal to the p r o d u c t i o n ratio estimated from the radioactive decay of 23Su, 235U, 232Th and 4°K (ZARTMAN et al., 1961). O n the o t h e r h a n d , the c o m b i n a t i o n of stable isotope data such as d D values of H 2 and H 2 0 , and 613C values of C O 2 and C H 4 are useful in calculating f o r m a t i o n t e m p e r a t u r e s , assuming that isotope equilibrium is a t t a i n e d a n d s u b s e q u e n t isotope exchange is m i n i m a l (BoTTINGA, 1969; HOEFS, 1980). Thus n o b l e gases and stable isotope data, if well c o m b i n e d , may give i m p o r t a n t i n f o r m a t i o n on the origin and f o r m a t i o n of samples studied. B a s e d o n the o b s e r v e d 3He/4He a n d 4°Ar/36Ar ratios, chemical compositions a n d dlSNN2 values, this p a p e r reports work o n the origin of H2-N2 gas seeps in the Semail N a p p e ophiolite a n d H a j a r F o r m a t i o n of calcite and dolomite regions in n o r t h e r n O m a n . In addition, the f o r m a t i o n t e m p e r a t u r e of the H2-rich gases is estimated, taking into account the stable isotope data (ODH,, ODH~o, 613Cc0, and t~13CcH4 values).

EXPERIMENTAL

Six gas samples were collected in lead-glass containers with high vacuum stopcocks at both ends by water displacement from five thermal spring sites. Figure 1 shows sampling sites together with some geological information. The 3He/nile measurements were made within two months after sampling. Because the diffusion velocity of He in the leadglass is significantly slow at room temperature, penetration of atmospheric He may be negligibly small during the storage time. Sample locations, date, temperature and pH of the springs are listed in Table 1. In the laboratory, chemical compositions of major, minor and trace components (H 2, N 2, CO2, CH4, O 2, Ar and He) were analyzed using a gas chromatograph system. Peak heights of chemical components in the samples were calibrated against those of the in-house standard gases. Overall analytical errors were estimated to be <5% for the most samples. A precise description of the measurement was given elsewhere (UaABE et al., 1985). Purification of noble gases (He, Ne and Ar) was carried out by a two-stage Ti-Zr getter in a metallic vacuum line.

• 26"N,

24"N "iiiii!..............

.~#4 i ~ : ' 3 ........

#5

23"N

0, 56]E

~

1®kin 57"~.

58°E

59"E

FIG. 1. Location map of gas sampling sites in northern Oman with some geological information. Dotted and shadowed areas show the Semail Nappe ophiolite and the calcareous Hajar Formation rcgions, respectively.

An activated charcoal trap was held at liquid N~ temperature in order to separate Ar from He and Ne. A mass spectrometer (6-60-SGA, Nuclide) was used for the measurements of 3He/4He, 4He/2~Ne, 4°Ar/36Ar and 3~Ar/36Ar ratios. For 3Hc/4He ratio analysis, resolving power of -600 at 1% of the ~eak height was attained for the complete separation of the -He beam from the H 3 and HD beams (SANO and WAKITA, 1985). Because He was not separated from Ne in this work, some uncertainty in the measured 3He/4He ratio may bc involved (RIsoN and CRAm, 1983; SANO et al., 1987). The conclusions of this work, however, are not substantially affected by the uncertainty. The isotope measurement of Ar was conducted separately on the same mass spectrometer under the condition of resolving power of -150. The He and Ar isotope ratios were corrected for mass discrimination on the basis of the measurement of the atmospheric He and At, respectively. Separation of the gas samples into pure gas component (H2, N2, CO 2 and CH4) and oxidation of CH 4 to CO 2 were made in a glass vacuum line using a trap held at liquid N2 temperature, another trap at acetone-liquid N, sherbet temperature, and a CuO furnace. The D/H, t3C/12C and 15N/14N ratios were measured by conventional mass spectrometers at the same time (Micromass 602C, VG Isotopes and MAT 250, Finnigan). Estimated total uncertainties of measurements for D/H of H 2, 13C/12Cof CO, and CH 4, and 15N/14N of N 2 were -+0.4, _+0.02 and -+(1.02~}~,,respectively. Experimental details of stable isotope measurements were given elsewhere (URABE, 1985).

Table I. Name, location, date, temperature and pH of gas seeps in Oman Location No. 1 2 3 4 5

Name

Longitude

Latitude

Date

Temperature (°C)

pH

Ain Al-Waddah Howqain Hammam Ain A1-Kesfah Ain A1-Shubaikhah Hammam AI-AIi

22°59'N 23°32'N 23°2YN 23°30'N 23°28'N

57°17'E 57°20'E 57°25'E 57°55'E 58°19'E

24 March 1983 22 September 1983 22 March 1983 22 March 1983 24 February 1984

32 42 45.7 52.1 67

9.5 9.5 6.5 6.3 6.3

Origin of H2-N2 gas seeps, Oman Table 2. Chemical compositions of gas seeps in Oman Name

H2 (%)

N2 (%)

Ain Al-Waddah Howqain Hammam Ain Al-Kesfah Ain AI-Shubaikhah Hammam AI-AIi

81 69 0.02 <-'5ppm <1 ppm

16 28 73 91 92

Air

0.5 ppm

78.1

No. la 2 3 4 5

CO2 (%) <0.1 <0.0l 1.5 4.8 <4.4 0.037

CH4 (%)

02 (%)

Ar (%)

He (ppm)

2.2 2.2 <0.002 0.0018 0.008

0.54 0.18 25 3.0 2.5

0.33 0.51 0.76 0.94 1.2

1.1" 3.4* 930 1200 300

1.7 ppm

20.9

0.934

5.24

*Estimated from a peak height of 4He in the helium isotope mass spectrometer. RESULTS AND DISCUSSION

The chemical compositions of the sampled gas seeps are listed in Table 2. Major components vary significantly from HE-rich gas (No. la) to N2-rich gas (No. 5). Sample No. 2 is intermediate between the HE-rich and Ne-rich gases. There are positive correlations among the N2, Ar, CO 2 and He concentrations, and negative correlations between the H 2 and N 2 contents and the CH 4 and CO 2 contents. The correlation coefficients of N2-Ar, NE-COE and N2-He contents are 0.94, 0.92 and 0.75, respectively, while those of H2-N 2 and CH4-CO 2 are -0.98 and -0.88, respectively. Because the HE concentration has a negative correlation with N2, H 2 also has negative correlations with the Ar, CO2 and He. Noble gas isotope compositions are listed in Table 3. The observed 3He/aHe and 4He/E°Ne ratios vary significantly from 0.094 to 0.786 Rat m ( w h e r e Rat m is the atmospheric 3He/4He ratio of 1.39 x 10-6) and from 0.47 to 940, respectively. There is a negative correlation between the 3He/nile and 4He/2°Ne ratios with a coefficient of -0.88. A sample with significant 0 2 (No. 3) has a large 4He/2°Ne ratio of 940, suggesting that the He in the sample is not compromised by air contamination. This is quite peculiar because abundant O E occurrence in volcanic and natural gases was generally regarded as indicating air contamination (MENYAXLOVet al., 1983; URABE et al., 1985). The 38Ar/36Ar ratios of all gas samples agree well with the atmospheric 38Ar/36Ar ratio within the experimental error margins, while the 4°Ar/36Ar ratios of some samples (Nos 3 and 4) show slight radiogenic values, that is, >295.5. It is noted that the true 4He/2°Ne and 4°Ar/36Ar ratios

in sample No. 3 are presumably much higher than the measured values. Stable isotope compositions (D/H, 13c/IEc and 15N/faN ratios) were measured for selected samples and the results are summarized in Table 4. They are expressed in the delta (6) notation, as parts per thousand (per mille; %0) deviation from international standards. The D/H, 13C/12C and 15N/IaN ratios are relative to SMOW, PDB and atmospheric NE, respectively. The 6D value of - 5 3 6 % in the H 2-rich gas (No. lb) is significantly distinguished from the literature value of - 699%o in the region, as reported by NEAL and STANGER(1983). Either experimental artifacts or temporal change and local distribution of 6D value at the area are suggested. Note that the pH of the Semail Nappe sample in the present work is apparently lower than that of NEAL and STANGER (1983). This may be related to the discrepancy of 6D values. In order to resolve this problem, further studies including intensive sampling and repeated measurements of HE-rich gases are highly desirable.

NOBLE GAS ISOTOPES AND GEOLOGICAL SETTING

Oman is situated in the eastern part of the Arabian Peninsula. A widespread occurrence of ultramafic rocks forms the lower and most massive part of the Semail Nappe, comprising an extremely large area of ophiolites consisting of oceanic crust (basic) and upper mantle (ultramafic) lithologies, totalling up to 7 km in thickness (GLENNIE et al., 1974). Some samples (Nos la, l b and 2) were collected in the Semail Nappe ophiolite while the others (Nos 3, 4

Table 3. Noble gas isotope compositions of gas seeps in Oman No.

Name

3He/aHe (Ratm)

4He/2°Ne

3He/4Hecor (Ratm)

la lb 2 3 4 5

Ain AI-Waddah Ain AI-Waddah Howqain Hammam Ain AI-Kesfah Ain AI-Shubaikhah Hammam AI-Ali

0.454 + 0.011 0.421 + 0.025 0.786 + 0.014 0.094 + 0.001 0.126 + 0.004 0.379 + 0.012

1.6 1.9 0.47 940 500 9.0

0.32 0.30 0.32 0.09 0.12 0.36

Air

1

0.318

4°Ar/36Ar

38Ar/36Ar

300.0 + 4.0

0.1874_+0.0020

294.8 + 1.1 300.3 _+2.4 303.0 + 2.2 295.5 + 1.9

0.1874+ 0.0017 0.1866+ 0.0015 0.1873+ 0.0013 0.1876___0.0011

295.5

0.187

4

Yuji Sano et al. Table 4. Stable isotope compositions of gas seeps in Oman t~DH 2

dDH:o

613Cco:

dl3CcH.,

~I5NN:

No.

Name

(%0)

(%0)

(%0)

(%0)

(%o)

la lb 4 5

Ain AI-Waddah Ain AI-Waddah Ain AI-Shubaikhah Hammam AI-AIi

-34.5

0.9

-536

0.6 - 10.7 -9.6

Air

-8

and 5) were from the Hajar Formation (calcite and dolomite) (Fig. 1). The gas samples emanating from the ophiolite are H2-, CH 4- and N2-rich, while the latter are N2-, CO 2- and O2-rich, as was well documented by NEAL and STANGER(1983). The ophiolite spring waters have lower temperature and higher pH values than those from calcite-dolomite springs (Table 1). The differences between these two groups are now discussed with regard to noble gas isotopes. Based on the 3He/4He and 4He/2°Ne data so far accumulated (LuPTON, 1983; MAMYRINand TOLSTIKraN, 1984), the He isotope variation in most terrestrial gases and fluids can be explained in terms of mixing of three major components (e.g. SANO and WAKITA, 1985; SANO et al., 1987) when excluding deep mantle component such as Hawaiian fluids (e.g. CRAIG and LUPTON, 1976; KODERA et al., 1988) and Icelandic gases (e.g. POLYAKet al., 1976; SANOet al., 1985). The end-members are atmospheric He with 3He/4He ratio of 1 Ratm, radiogenic He with that of 0.01 Ratm. Figure 2 shows a correlation between the 3He/4He and 4He/2°Ne ratios in the Oman samples. The samples are located within the domain defined by the mixing of the three end-members, suggesting that the canonical framework of He isotope systematics is applicable to the present Oman samples. More precisely, there is a difference of 3He/4He ratio between the samples in the Semail Nappe and

10

.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Mantle .....

Air[

",,

#1a

0#5

"",,~lb

o

#4 o

"~ o.1

,,

~3 4

"'"... 0.01 0.1

.... 1

........ , 10

.

Radiogenic , "~""7-7:rqP.v" 100 1000

4He/2°Ne FIG. 2. Correlation between the 3He/4He and 4He/2°Ne ratios in the samples studied. Dashed lines show the mixing line between mantle He and atmospheric He and between radiogenic He and atmospheric He.

-0.5 0 -47

0

those in the Hajar Formation. Assuming that the 4He/2°Ne ratios of radiogenic and mantle He components are significantly higher than that of air, it is possible to correct atmospheric He contamination as follows: (3He/4He)cor = [(3He/4He)obs - r]/(1 - r) r = (4He/2°Ne)air/(4He/2°Ne)obs where (3He/aHe)cor and (3He/aHe)obs denote the corrected and observed 3He/4He ratios, and (4He/2°Ne)air and (4He/Z°Ne)obs are the atmospheric and observed 4He/Z°Ne ratios, respectively (CRAIGet al., 1978). The corrected 3He/4He ratios are listed in Table 3. It is noted that Semail Nappe samples (Nos la, lb, and 2) indicate higher corrected 3He/4He ratios than those of the Hajar Formation (Nos 3 and 4) except for sample No. 5. An ophiolite succession generally consists of peridotite, gabbro, pillow basalt, and the deep-marine sediments that formed on the oceanic crust, and is believed to have been scraped off a subducting plate and plastered against the upper plate. Geological study defined the time span within which the main phase of overthrusting of the Semail Nappe occurred as being late Campanian to early Maastrichtian (GLENNIE etal., 1974). The rare sediments in primary normal contact with the lavas of the Semail ophiolite are locally baked cherts and lime mudstones of Cenomanian and Coniacian age (GLENNIE et al., 1974). The Semail Nappe may have been generated between 95 and 98 Ma probably above a newly initiated intra-oceanic subduction zone (ROBERTSON and SEARLE, 1990). TORGERSEN a n d J E N K I N S (1982) reported that magmas which become isolated from their source will begin to accumulate crustal He by in situ U and Th series decay thus lowering the He ratio in a magma aging process. Taking glassy tholeiite magma with U and Th contents of 0.1 and 0.18 ppm, respectively, and typical He contents, the 3He/4He ratio decrease can be obtained. The 3He/4He ratio would be 0.24 Rat m after 10l° a (TORGERSEN and JENKINS, 1982) and this value is compatible with corrected 3He/4He ratios for Semail Nappe samples (Nos la, lb, and 2), when considering the uncertainty of the magma aging effect. Thus relatively high 3He/4He ratios in Semail Nappe samples are attributable to some remnant of magmatic He in the mantle sequence rocks.

Origin of H2-N2 gas seeps, Oman 1.2

?

].0

9 Air

¸

0.8 ~Z 0.6

~

0.4e~

0.20.0

'

,

'~~ '" .

,

294

,

,

#1a°

#3 r r - ' t~

I .

J

298

.

,

#4i n

I ,

i

302

I

306

40Ar/36Ar FIG. 3. Correlation between the 3He/4He and 4°Ar/36Ar ratios in the samples studied. Bars represent measurement error (_+1 c0. Solid line shows the mixing line between air and radiogenic noble gas.

The carbonates of the Hajar Formation are believed to have been deposited for the most part in shallow water, which may have been very shallow, perhaps supratidal or even emergent during the time span Mid-Permian to Cenomanian (GLENN~E et al., 1974). Recently, RABUet al. (1990) reported that the Hajar Formation consists of dominantly carbonate deposits ranging in age from Late Permian to Late Cretaceous. Because it is a sedimentary deposit, the initial content of magmatic He at the time of formation was negligibly small. Thus, irrespective of the formation age, the 3He/erie ratio may be radiogenic. Corrected 3He/4He ratios for the Hajar Formation are very low, with a value of - 0 . 1 Ratm except for No. 5, which is consistent with the idea of a sedimentary origin. Anyway, the discrepancy of the 3He/4He ratio between the two groups, the Semail Nappe and Hajar Formation, agrees well with that of the chemical compositions and their occurrences. The 4°Ar/36Ar ratios in the Semail Nappe samples appear to be different from those in the Hajar Formation. The former samples have atmospheric Ar while the latter are slightly radiogenic. The observed 3He/4He ratios are plotted against the 4°Ar/36Ar ratios in Fig. 3. There is a negative correlation between 3He/4He and 4°Ar/36Ar ratios even though the experimental error of 4°Ar/36Ar ratios is large, suggesting a mixing trend between air and crustal noble gases. The content of radiogenic 4°Ar in the sample is estimated as follows: (40Ar)rad = (Ar) × [(4°Ar/36Ar)cor - 295.5]/[4°Ar/36Ar)cor + 1] (4°Ar/36Ar)cor = (4°Ar/36Ar)obs + 591 X [0.187 -- (3SAr/36Ar)obs]/0.187 where (40Ar)rad and (Ar) denote the content of radiogenic 4°Ar and total Ar in the sample, and (40Ar/36Ar) .... (4°Ar/36Ar)obs and (38Ar/36Ar)obs are the 4°Ar/36Ar ratio corrected for mass fractionation with the slope of 2.0, observed 4°Ar/36Ar and

5

3SAr/36Ar ratio, respectively. The radiogenic 4°Ar concentrations in the sample Nos 3 and 4 can be calculated because 4°Ar/36Ar ratios of the other samples agree with that of the atmosphere within the margin of experimental error and it is impossible to estimate the 4°Ar contents using the equations above. The radiogenic 4°Ar contents are - 1 5 0 ppm in No. 3 and 200 ppm in No. 4. The radiogenic 4He/4°Ar ratio of natural gases is a useful parameter to investigate relative abundance of U and Th to K in the reservoir rocks and sediments because 4He and 4°Ar are radioactive decay products of 235U, 238U and 232Th, and 4°K, respectively. ZARTMAN et al., (1961) reported that the total range of radiogenic 4He/4°Ar ratio in natural gases from North America is 1.6-130, with most samples having values between 6 and 25. The range of values was essentially equal to the production ratio of U, Th and K in average igneous rocks and a wide variety of sedimentary rocks. The radiogenic 4He/4°Ar ratio of samples Nos 3 and 4 can be estimated based on the He and radiogenic 4°Ar contents. Values of 6.2 for No. 3 and 6.0 for No. 4 agree well with the range given by ZARTMANetal. (1961). This suggests that the relative abundance of U and Th to K in Hajar Formation rocks is identical to the average igneous rocks and sedimentary rocks in North America and that the relative efficiency of extraction and migration of gases from the source is similar.

NITROGEN-TO-ARGON RATIO AND blSN VALUE The abundance ratio of N2 to atmospheric Ar with the 4°Ar/36Ar ratio of 295.5 has been used as a clue to the origin of N2-rich gases (ZARTMANet al., 1961: MATSUOet al., 1978). The presence of N2 in natural gases from North America was attributed to the mixing of atmospheric N2 and non-atmospheric N2. The latter component was explained by the gas produced by the bacterial decomposition of N2-bearing components or release of N2 from decomposition of organic compounds by chemical reactions, although the process to make decomposition of organics was not well understood (ZARTMAN et al., 1961). The N2/Ar ratios of several volcanic gases were compiled by MATSUOetal., (1978). Excess N2/Ar ratio in island arc volcanic gases was attributed to N2 gas derived from subducted sediments. Observed N2/Ar ratios in the Oman samples vary from 48 to 97. In some of the samples (Nos 1, 2 and 5), the ratios are in the range between atmospheric N2/Ar ratio (83.6) and air-saturated water (ASW, 38). The other samples (Nos 3 and 4) have small but significant N 2 excesses ( - 1 . 2 times air, bearing in mind that some of the Ar is radiogenic). In identifying the origin of excess N2, it is noted that there is a significant positive correlation between He content and the N2/Ar ratio (Fig. 4). This implies that excess N 2 in these samples accompanies the He. The cor-

6

Yuji Sano et al.

rected 3He/4He ratios of these samples (Nos 3 and 4) are - 0 . 1 Ratm, indicating that a magmatic contribution to the He in the samples is no more than 1%. Thus it is inferred that the excess N 2 in the Hajar Formation samples is mostly crustal and/or organic in origin. Additional information as to the the origin of the N 2 is provided by the isotope composition (SWEENEY et al., 1978; HOEFS, 1980). Organic N in present biogenic materials such as marine plants, animals and sediments shows a wide range of dlSN values from - 8 to +23%0. The inorganic OISN value may be +7 + 2%0, taking into account data for igneous rocks (ScALAN, 1959) and granites (ZHANC, 1988). There are controversial discussions concerning the 615N value of mantle-derived N 2 (BECKER and CLAYTON, 1977; SAKAI et al., 1984; JAVOV et al., 1984; EXLEY et al., 1986/1987). Taking most recent results, however, it is almost certain that the dISN value of the upper mantle component (MORB) is ~+10%o and not so different from the crustal component (SANo and PILLINGER, 1990). Figure 5 shows a correlation between the N j A r ratio and 615N value. Sample Nos la and 5 have compositions consistent with mixtures of air and air-saturated water. In contrast, sample No. 4 has excess N 2 with a negative 615N value. Considering the negative 615N value, the excess N 2 may be attributed to organic N, and not to crustal igneous rocks or magmatic in origin, if the fractionation effect on the isotope is negligible.

FORMATION TEMPERATURE OF Hz-RICH GAS

The negative correlations between H2 and N2 and between H a and He strongly suggests a discrepant origin of H 2 from N2 and He in gas seeps in Oman. Because the He and N a are attributed to crustal and/or organic origin, the H 2 should be derived from the other origins such as magmatic. The 6D value of -536%0 in H2-rich gas (No. la), however, does not

10000#4 1000

J

#5

E e-~

&

m

q

#3

100

10 ¸ #2 #1a 1, . ~ , 40 50

@Air

.

,

60

•

,

,

70

80

,

90

•

100

110

Nz/Ar FIG. 4. Correlation between the He contents and N2/Ar ratios in the samples studied. Bars represent measurement error (_+1 ty).

2 "

1 Aw nU'a z

~_~.#5 eAir

0

i

-1

"230

40

50

(50 " 70

80

90

I(~)" l l 0

N2/Ar Fro. 5. Correlation between the dtSN values and N2/Ar ratios in the samples studied. Bars represent measurement error (+1 a). ASW shows data of air-saturated water at 20°C.

agree with the dD values of mantle materials ranging from - 5 0 to -80%0 (KYSER, 1986). NEAL and STANGER (1983) reported that H2-rich gases in the Semail Nappe ophiolite region may be formed inorganically within the relatively low temperatures of the hydrosphere during both shearing of ultramafic rocks and by post-serpentinization chemical degradation. The idea is compatible with the data and discussion presented here. NEAL and STANGER(1983) suggests that the formation temperature of H2-rich gases in the Semail Nappe ophiolite region is about 20-50°C, based on the 5D value of the gas and assuming that isotope equilibrium was attained between H a and HaO at the time of H2 formation and little or no subsequent isotope exchange. The dD values of - - 7 0 0 % 0 are for the most D-depleted H 2 gas found in nature except for H2-N 2 well gas in Kansas with 5D values of - 7 4 0 to -836%0 (COVENEY e t al., 1987). The observed dD value of -536%0 in this study is distinguished from data by NEAL and STANGER (1983). The value, if combined with the dD value of +0.6%0 in the spring water associated with the gas (Table 4), gives very high formation temperature of -300°C. This temperature is also significantly higher than the other H2-rich gases in regions of present-day serpentinization. For example, H2-formation temperature was estimated to be -10°C in Kansas, North America (COVENEY et al., 1987) and -140°C in Zambales ophiolite, Philippines (ABRAJANO el al., 1988). In contrast, the temperature reported here resembles those in geothermal areas such as found in Iceland (ARNASON, 1977) and New Zealand (LYON, 1974). There is, however, no indication of any geothermal activity in the area studied. The inferred high HE-H20 equilibration temperature warrants further study. The lack of CO2 in H2-rich gases (Nos la and 2) and the negligible occurrence of CH4 in Na-rich gases (Nos 3, 4, and 5) suggest that CO 2 in Ha-rich gases

Origin of H2-N 2 gas seeps, Oman has been completely consumed under highly reducing conditions by the following chemical reaction: C O 2 q- 4 H : = C H 4 + 2 H 2 0 . The necessary H 2 in this reaction is produced from water decomposition by ferrous iron oxidation reactions in the groundwater system (NEAL and STANDER, 1983). Assuming that the dil3C value of the consumed C O 2 in H2-rich gas is similar to the dia3C value of C O 2 in N2-rich gases (Nos 4 and 5; dil3C = ~ - 1 0 % o ) and that isotope equilibrium was attained between C O 2 and C H 4 at the time of C H 4 formation, a formation temperature for the C H 4 gas in the system of 300°C is estimated. This temperature agrees well with the formation temperature determined from the H : - H : O system. It is not very likely that both di13C values of C O 2 and C H 4 and diD values of H 2 and H 2 0 were attributable to experimental artifacts in the measurement and that formation temperatures agree by chance, because these are independent data sets. Although there are not enough data to object to relatively lower formation temperature by NEAL and STANGER (1983), the present estimate is consistent within the framework of H and C isotope systems.

CONCLUSIONS Several conclusions can be drawn concerning the origin of H e - N 2 gas seeps in northern Oman: 1. There are significant differences of chemical and isotope compositions between gas seeps from the calcareous Hajar Formation region and those from the Semail Nappe ophiolite region such as He contents, 3He/Rile ratios, 4°Ar/36Ar ratios, N2/Ar ratios, and dilSN values. 2. The excess N 2 relative to A r in air and/or A S W in gas seeps from the Hajar Formation may be attributed to organic N 2. Considering the accompanied H e with low 3He/nile ratio, the magmatic contribution to the N 2 is negligibly small. 3. The chemical and isotope compositions of the H2-rich gases in the Semail Nappe are consistent with production from water decomposition by ferrous iron oxidation reactions in the groundwater system as was suggested by NEAL and STANGER(1983). 4. A formation temperature of the H2-rich gas is estimated to be - 3 0 0 ° C , assuming that isotope equilibrium between H : and H 2 0 was attained and any subsequent isotope exchange was insignificant. A similar formation temperature is estimated from di13C values assuming C H 4 and C O : equilibrium. Discrepancy between the present temperature and that reported by NEAL and STANGER (1983) will be resolved by future intensive sampling and repeated measurements of diD values. Acknowledgements--The authors are indebted to Drs S.

Matsuo (Tokyo Institute of Technology) and H. Sakai (The University of Tokyo) for use of mass spectrometers for H, C

7

and N isotope measurements. Dr M. Kennedy and an anonymous reviewer provided useful comments on the manuscript. Revision was made at Laboratoire MAGIE, Universit6 Pierre et Marie Curie when Y. SanD was supported in France by an associate professorship from the university. Editorial handling: J. L. Bischoff.

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