Helium and methane measurements in hydrothermal fluids from the mid-Atlantic ridge: The Snake Pit site at 23°N

Earth and Planetary Science Letters, 106 (1991) 17-28 Elsevier Science Publishers B.V., A m s t e r d a m

17

[XLeP]

Helium and methane measurements in hydrothermal fluids from the mid-Atlantic ridge: the Snake Pit site at 2 3 ° N P. Jean-Baptiste a, J i . Charlou b, M. Stievenard a, J.P. Donval b, H. Bougault b and C. Mevel c Laboratoire de Gdochimie Isotopique, D S M - C E A /Saclay, 91191 Gif sur Yvette cedex, France b Ddpartement Gdoseiences Marines-DERO/GM, Ifremer, Centre de Brest BP70, 29287 Plouzane, France c Laboratoire de Pdtrologie et structure des roches mdtarnorphiques, Universitd Pierre et Marie Curie, 4, PI. Jussieu, 75252 Paris cedex 05, France Received September 6, 1990; revised version accepted June 10, 1991

ABSTRACT Methane and helium isotopes concentrations have been measured in hydrothermal fluids of black smoker vents at the Snake Pit area (MAR, 2 3 ° N ) as part of the Hydrosnake cruise with the submersible Nautile. The samples contain large quantities of helium and methane with e n d m e m b e r values of 4.6 x 10 -5 cm 3 S T P / g for helium and 1.38 x 10 -3 cm 3 S T P / g for methane. The helium isotopic ratio 3 H e / a H e is 8.4 times the atmospheric ratio. These values are strikingly similar to those already obtained on the East Pacific Rise. The C H 4 / 3 H e ratio is also quite similar ( C H 4 / 3 H e = 2.6 x 106) suggesting a non-biogenic origin of this gas in the fluids. Assuming that the range of variation of the 3He concentration data obtained so far from various tectonic settings is close to the genuine range of values encountered on mid-ocean ridges, the global mass flow involved in the hydrothermal circulation at the ridge's axis is estimated between 1000 and 3000 m 3 of water/sec, leading to a residence time of the global ocean with respect to the hydrothermal circulation between 15 and 45 million years. This estimate implies that, in addition to the individual vents (smokers), there is an extensive diffuse circulation of hydrothermal fluids along the ridges, in agreement with recent observations concerning heat budgets at hydrothermal sites in the Eastern Pacific.

1. Introduction Over the past ten years, hydrothermal activity has been discovered and studied at numerous locations, in relation with various tectonic settings: fast spreading ridges (Galapagos Spreading Center (GSC), East Pacific Rise (EPR) [1-6]), back-arc basins and subduction zones of the Western Pacific (Lau, N. Fiji, Woodlark, Manus and Celebes basins, Mariana Trough [7-11]). Water column samples collected in hydrothermal areas usually show large 3He and C H 4 anomalies due to the input of hydrothermal fluids at the seafloor, highly enriched with regard to these gases [12-20]. The GSC, where the first ever direct observation of hydrothermal vents was made in 1977 [21], and the EPR, have been extensively studied. The helium analysis of these fluids shows a fairly steady 0012-821X/91/$03.50

isotopic composition with 3 H e / 4 H e around eight times the atmospheric ratio, in agreement with the isotopic composition of the oceanic basalts at the same location (Table 1). Helium concentrations in these hydrothermal fluids show values around one thousand times the oceanic background, with a range of variation from one site to another of about a factor of 2 to 3 (Table 1). Methane concentrations in hydrothermal vents on the EPR are of the order of 10 -3 to 3 × 10 -3 cm 3 S T P / g (Table 1). This methane seems to originate mainly from the mantle [22-25]. Unlike helium however, due to its various possible sources and sinks in the deep ocean, the concentrations of methane in different geological settings can be much more variable, as displayed in Table 1. In spite of extensive survey and of some early evidence of hydrothermal activity [26-28], it is

© 1991 - Elsevier Science Publishers B.V. All rights reserved

18

P. JEAN-BAPTISTE ET AL.

only recently that high-temperature vents were studied on the mid-Atlantic Ridge (MAR). The initial discovery was made in July 1985 (8 years after the discovery of hydrothermal vents at the Galapagos Spreading Center) on the T A G area near 26 ° N [29]. The Snake Pit site is the second location on the M A R where black smokers have been found. This discovery was made in December 1985 from the drill ship Joides Resolution on Leg 106 [30]. Due to its quite different tectonic setting from the E P R (slow spreading rate, greater d e p t h s . . . ) , it is very interesting to compare the chemistry and gas composition of the hydrothermal fluids in both cases. With respect to the 3He concentration, there is a marked contrast at the basin scale between the Pacific and the Atlantic oceans. As shown by the GEOSECS data in the Pacific [31] as well as from the work by L u p t o n and Craig at 1 5 ° S [32], the hydrothermal 3He injected at the ridge crest spreads westwards across the deep Pacific basin and gives a high 3He b a c k g r o u n d with a mean 33He value under the thermocline of around 17%.

Conversely, the Atlantic deep waters are m u c h less enriched in 3He [33,34], with a corresponding 33He value a r o u n d 2%. The first-order cause that one might suspect for this large discrepancy is the m u c h lower spreading rate of the M A R c o m p a r e d to the EPR, which is likely to p r o d u c e a lower 3He input in the deep Atlantic. This weaker 3He input is indeed confirmed by the study of the 3He and 14C balance between the main deep ocean and the atmosphere, which suggests a 3He flux in the deep Atlantic lower than in the Pacific by a factor of 6 [35]. A second i m p o r t a n t reason lies also in the fact that the Atlantic ocean is m u c h more efficiently ventilated due to the process of deep-water formation both in its N o r t h e r n and Southern polar regions. In the open ocean, m e t h a n e is characterized by a 50% supersaturation in surface layers where the biogenic production is important. Then, C H 4 contents decrease regularly with depth to 10 8 cm 3 S T P / g at 1000 m. In deep waters, typical C H 4 b a c k g r o u n d s are about 8 × 10 -9 c m 3 S T P / g in the Atlantic (4 × 10 -9 c m 3 S T P / g in the Pacific) [36,37]. A l o n g the mid-Atlantic Ridge (MAR),

TABLE 1 Helium isotopic ratio, 4He and C H 4 endmember concentrations and CH4/3He ratio in hydrothermal fluids from various environments. References for these data are given below each set of measurements Location Guaymas basin Galapagos Rift East Pacific R. 11° N/13 o N/21 ° N Juan de Fuca R. Mid-Atlantic R. 23 ° N Red Sea brines Mariana Trough Mean value: EPR basalts MAR basalts MAR "popping rocks"

R/Rat m

8.04 [75] 6.7-7.8 [2,12] 7.5-8.34 [5,24,53,70] 7.7-8.1 [6,15,54-56] 8.4 [this work] 8.7-8.77 [18,19] 8.6 [74] 8.1 +0.4 7.7-10.4 [77,78] 8.0-9.0 [24,52] 8.4 [24]

4He

CH 4

CHa/3He

( X 105 cm3 STP/g)

( x 103 cm3 STP/g)

( × 10 6)

270-370 [76]

3100 [23] 12.4-42.0 [72] 1.4-7.0 [14,24,49,71]

2.0-6.0 [5,24,71] 1.3-2.3 [54] 3.8-4.9 [this work] 1.4-2.7 [18,191 3.2+ 1.5 0.52 [24] 0.64 [241 1.15 [24]

0.6-2.6 [24,49,70] 1.8-2.6 [73] 1.38 [this work]

0.10 [24] 0.052 [24] 0.28 [24]

2.6 [this work] 0.8 [231 0.5 [741 1.7-2.5 [24] 0.7 [24] 2.1 [24]

HELIUM AND METHANE MEASUREMENTS IN HYDROTHERMAL FLUIDS AT SNAKE PIT

anomalies have been measured in several locations, associated with other geochemical tracers: the results obtained in samples taken from the T A G area ( 2 6 ° N ) [38] and from stations located every 10 miles between 13 and 2 6 ° N along the M A R during the Ridelente cruise [39] show significant C H 4 anomalies similar in amplitude to those detected on known sites along the East Pacific Rise (EPR). These anomalies suggest the existence of hot venting at different places on the M A R and have already allowed the localization of active sites [40]. C H 4 and 3He vertical profiles were obtained by us from the Ridelente cruise at the exact location of the Snake Pit site a few months before the present study. The results show an anomaly for both tracers centered at 3400 m depth, that is, around 300 m above the seafloor (Fig. 1A). This anomaly originates from the active hydrothermal vents located immediately below. A typical EPR plume--Hydrofast c r u i s e / 1 3 ° N [16]--is shown on Fig. l b for comparison: at Snake Pit, the level reached by the plume is higher than the one at 1 3 ° N on the EPR, in agreement with previous observations at the T A G hydrothermal field [41] and with the turbulent entrainment model of Pacific and Atlantic plumes by Speer and Rona [42]. Also, the magnitude of the C H 4 and 83He tracers anomalies is significantly lower than that on the EPR suggesting, as already discussed above at the basins scale, a more rapid dilution with low tracers background ambient waters. Furthermore, the C H 4 / 3 H e ratio in the water column is comparatively lower with a value C H 4 / 3 H e = 0.5 × 1 0 6 instead of 1.5 × 10 6 to 7 × 1 0 6 at 11, 13 and 2 1 ° N on the EPR (Table 1). Keeping this in mind, we were very interested in obtaining the first direct measurements of methane and helium isotopes in the M A R vents fluids in order to find answers to the questions raised both by the similarities and the differences observed in the water column data (at global and local scales) between the Atlantic and Pacific oceans. CH 4

2. The Snake Pit hydrothermal site

The Hydrosnake cruise was aimed at studying the general tectonic setting of the Snake Pit area

[A]

Deltn

19

HELIUM-3

(Jl)

METHANE [ 1 0 - % m 3 S T P / g )

~ee~,, .~,..1...~,..~..

I O_ W 0

3Tee

RIDELENTE

[B]

2

Dolta

HELIUM-3 4o 60

(~) 8B

( MAR-23hD

METHANE

1~eT

. . . . . . .

( 10"9cm 3 STP/g) . . . . . . .

~ge~F

w T b-n hi O

27~

HYDROFAST ( EI=~ - 13N)

Fig. 1. 8 3 H e a n d m e t h a n e w a t e r c o l u m n profiles: (A) at the S n a k e Pit ( M i d - A t l a n t i c Ridge). Ridelente cruise, s t a t i o n R D 8 7 / H Y - 2 7 (23 o 2 1 ' 5 4 N , 4 4 o 5 6 ' 9 9 W ) , b o t t o m d e p t h = 3680 m.

(B) at 13°N (East Pacific Rise). Hydrofast cruise [16], station HF-19 (12°43'29N, 103° 55'20W), bottom depth = 2632 m.

as well as the chemistry of the hydrothermal vents fluids. The hydrothermal field is located in the M A R K zone at 23°22.08'N, 4 4 ° 5 7 . 0 0 ' W on the midAtlantic Ridge, some 25 km south of the Kane Fracture Zone (KFZ) (Fig. 2), on an elongated dome which is part of an axial neovolcanic ridge, 400 to 600 m above the floor of the rift valley. During the Hydrosnake cruise, ten dives were made on the summit of this dome, to characterize its structure, to locate precisely the vents as well as to observe and sample sulfides, hydrothermal fluids and biological communities. A detailed geological map of the area was drawn from Nautile observations [43]. The neovolcanic ridge is a con-

20

structional feature made up of pillow lavas and lavas tubes. Its centre is formed by a N - S oriented 40-100 m wide axial graben bounded by two horsts. Results on the morphology, mineralogy and chemistry of the sulfides from different parts of the mound, establishing the mineralogical and geochemical zonation, are presented elsewhere [44]. Hydrothermal fluids were sampled from two separated vents sites with temperatures of 330 and 345°C. The first one, called "Ruches", consisted of a large hydrothermal mount capped by several large chimney complexes comprising both active (black smokers) and inactive vents as well as dif-

P. J E A N - B A P T I S T E

E T AL.

fuse flows. The second site, called "Clou", was discovered only 150 m away in the SW direction, on a very steep scarp. Both areas showed high biological activity. Our measurements of the major and trace elements in those fluids (Table 2) are in good agreement with previous data on the same hot fluids sampled one year before by Campbell and his colleagues [45]. The Snake Pit fluids have almost the same mineral composition and temperatures as fluids collected in the T A G hydrothermal area on the M A R [45] or along the EPR [46-48]. As recently discussed by Campbell et al. [45], this means that the factors controlling the chemistry of

3ite 649 ODP

(Snake Pit)

45 °20 45°00 44°40 Fig. 2. Seabeam map of the eastern intersection of the Mid-Atlantic Ridge rift valley and the Kane Fracture Zone, showing the location of the Snake Pit hydrothermal field, from Detrick et al. [79].

HELIUM

AND METHANE

MEASUREMENTS

IN HYDROTHERMAL

21

FLUIDS AT SNAKE PIT

the m a j o r elements in h y d r o t h e r m a l fluids are similar in all cases, d e s p i t e the s t r u c t u r a l differences.

3. Gas sampling method O u r s a m p l e s were t a k e n using 0.8 liter t i t a n i u m syringes o p e r a t e d f r o m the submersible. T h e s e t i t a n i u m syringes have b e e n suspected, in the past, of reacting with the h o t a n d acid h y d r o t h e r m a l solutions a n d of releasing gases such as h y d r o g e n [49]. H o w e v e r that m a y be, their p e r f o r m a n c e is a b s o l u t e l y correct with r e g a r d to the analysis of helium and methane. T h e h y d r o t h e r m a l fluids are c o n u n o n l y highly s u p e r s a t u r a t e d with various gases. C o n s e q u e n t l y , the recovery of the total a m o u n t of gases f r o m the syringes is a very critical process a n d requires special care. I n o r d e r to c a r r y out the gas s a m p l i n g p r o c e d u r e with the m a x i m u m safety a n d to pre-

vent a n y loss of gas while t r a n s f e r r i n g the fluid, we d e v e l o p e d a specially d e s i g n e d i n s t r u m e n t (Fig. 3). T h e s y s t e m consists of a n e v a c u a t e d c y l i n d e r w i t h a v o l u m e W~ ----2.5 liters. I n s i d e this stainless steel c o n t a i n e r a s e c o n d P V C c y l i n d e r is fitted in o r d e r to p r e v e n t the fluid f r o m reacting with the m e t a l walls. This allows, after the gas s a m p l i n g is c o m p l e t e d , collecting the fluid for s u b s e q u e n t c h e m i c a l analysis. B o t h on the top a n d on the walls of the v a c u u m c h a m b e r , small v o l u m e aliquots ( 3 / 8 " glass o r c o p p e r tubing) are conn e c t e d b y C a j o n u n i o n to collect a small fraction of b o t h gas a n d liquid p h a s e s (see Fig. 3).

4. Experimental procedure O n b o a r d the ship, the syringe is c o n n e c t e d to the e v a c u a t e d d e g a s s i n g c h a m b e r . Then, the fluid is injected in the c o n t a i n e r , l e a d i n g to a t w o - p h a s e system: a gas p h a s e on the t o p a n d a liquid p h a s e

TABLE 2 Chemical composition of endmember fluids at Snake Pit and comparison with other data from the Mid-Atlantic Ridge. TAG and MARK data are from ref. [45] Standard

seawater Temperature ( o C) pH Alkalinity (#eq)

2 7.8 2300

TAG [45] 26 o N 321/390

MARK 1/2 [45] 23 o N

This work 23 o N

350/335 3.9/3.7 - 6 4 / - 243

345 3.8 - 563

18.2/18.3 5.9/5.9 559/559 0/0 518/530 510/509 843/849 23.6/23.9 10.5/10.8 0/0 9.9/10.5 50/51 38.5/38 177/181 2180/1832 491/493 17/10 50/47 5.3/5

18 2.7 559 0

Element

SiO2 (mM) H2S (mM) CI (mM) SO4 (mM) B (~M) Na (m M) Li (/zM) K (mM) Rb (~tM) Mg (raM) Ca (m M) Sr (/~M) Be (nM) Cs (nM) Fe (#M) Mn (~tM) Cu (#M) Zn (pM) A1 (pM) Fe/Mn Sr(87/86)

0.16 0 541 28 419 465 26 9.8 1.3 53 10.2 87 < 0.02 2.2 < 0.001 < 0.001 0.007 < 0.01 0.005 0.7091

22 659 0 584 411 17 10 0 26 99 100 1640 1000

0.7029

3.7/4.4 0.7028

546 1030 24 11.9 0 10 50 2121 443 12 47 4.8 0.7030

22

P. JEAN-BAPTISTE ET AL. glass tubes

making any assumption as to the gases' solubility values: Ng

chy = w-- 7 ×

to syringe Fig. 3. Schematic view of the equipment used for degassing the fluids and collecting the gas aliquots.

still containing a small fraction of the gases. The glass tubes on the top of the cylinder are used to sample aliquots of the gas phase and 3 / 8 " copper tubes on the lateral wall are filled with the liquid phase. The glass tubes (volume Vg = 1.9 cm3), made of Corning glass 1724, are sealed off with a flame. The copper tubes (volume Vl -=- 8 cm 3) are clamped with a pinch-off tool of the same type as that which we routinely use for our t r i t i u m / h e l i u m analysis on oceanographic samples. Both fluid and gas samples were sent back to the lab for gas analysis. From the measured amount of gas (here helium and methane) in the liquid and gas aliquots, respectively N 1 and Ng, and knowing precisely the volume of those aliquots, Vl and Vg, of the hydrothermal fluid sample Why and of the vacuum container W~, it is possible to derive the actual gas concentration Chy in the original fluid without

(Wc - Why )

vg

Nl

+ v,

Instead of a complete degassing of the sample, the sampling of both phases makes this equipment very simple to operate at sea; thus, no pumping equipment is needed to transfer the gases from the water to another part of the installation where the gas is collected. This technique is also advantageous with respect to the blank, especially the helium blank which can be kept completely negligible in spite of its high diffusivity. The only sensitive part with respect to the blank was the relatively high leak rate of the syringe piston o'ring. This problem was eliminated by placing the syringe in a water bath during all of the whole procedure. Helium isotopes measurements were carried out at Saclay (CEA) with a V G 3000 mass spectrometer, using our routine analysis procedure developed for oceanic studies [50]. The accuracy on the 3He excess for oceanic samples (63He in % = ( R / R a - 1) × 100 with R = isotopic ratio of the sample and R a = atmospheric ratio) is within _+0.3% and the relative standard deviation of the absolute 4He determination is around 0.3%. Due to the large helium concentrations encountered in hydrothermal samples, these need to be reduced with calibrated expansion volumes, prior to their injection in the mass spectrometer. With this different procedure, the accuracy on the isotopic ratio, expressed as R / R a is within _+0.1, with a relative error in the total helium amount around 6%. C H 4 analyses were performed in Brest (Ifremer) by gas chromatography: the tubes with the samples were connected to a manifold which was previously evacuated. After evacuating the dead space, the gas was equilibrated in the analytical line. Then, 1 cm 3 aliquots were injected with a Valco valve into a chromatographic column filled with Porapak Q. C H 4 w a s analysed using a flame ionization detector. The accuracy for C H 4 m e a s u r e m e n t s was within around 5%. Since the volume of the different sampling tubes, of the manifold and of the analytical section were known precisely, measurements made on the chromatographic loop (1 cm 3) could be related to the amount of gas in the sample. System calibrations

HELIUM AND METHANE MEASUREMENTS IN HYDROTHERMAL FLUIDS AT SNAKE PIT

23

TABLE 3 D i s s o l v e d h e l i u m isotopes a n d m e t h a n e c o n c e n t r a t i o n s of the S n a k e Pit s a m p l e s Sample

Mg (/×mole/g)

3He (cm 3 S T P / g )

4He (cm 3 S T P / g )

HS-88-11-1A HS-88-11-1B HS-88-10-1 HS-88-5-1 HS-88-3-1 HS-88-8-1 HS-88-6-1

46.1 46.1 24.4 5.3 49.8 49.8 51.5

7.67 x 10-11 7.13 x 10-11 2.62 x 10-10 5 . 1 3 X 1 0 -1° 2.14 × 10-11

6.51 × 10 - 6 6.09X10 6 2.27X10 5 4.39X10 5 -

0.72X10 3 1.26X10 3 0.23X 10 4 0.9 × 1 0 0.2 X10 - 4

-

4

1.88 X 10 - 6

were made by injecting known values of three calibration standard mixtures (Alphagaz standards 2.2 p p m + 2%; 10 p p m + 2%; 10,000 p p m + 2% in ultra pure helium).

CH 4 (cm 3 S T P / g )

the ambient seawater endmember and the pure hydrothermal fluid. The slope of the correlation . . . . 3 4. gives the isotopic ratio H e / He of the hydrothermal endmember: 3 H e / 4 H e = 8.4 times the atmospheric ratio. Table 4 lists the percent hydrothermal endmember for each sample (deduced from the magnesium concentration) as well as the calculated isotopic ratio and helium concentration of the pure hydrothermal fluid. Samples HS 88l l - I A and HS 88-11-1B are duplicates and illustrate the overall accuracy of our measurements. Similarly, methane concentrations are correlated with the magnesium content (Fig. 4). The mixing line gives a C H 4 endmember of 1.38 x 1 0 - 3 cm 3 S T P / g , corresponding to 1 0 6 times the deep Atlantic ocean background. The C H 4 c o n c e n t r a t i o n s in the Snake Pit fluids are quite similar to those found at 2 1 ° N [24] and 1 3 ° N [49] on the EPR. There is no difference between the two

5. The helium and methane results The results are listed in Tables 3 and 4. The magnesium concentration of the sampled fluids was used as an indicator of the dilution factor of the hydrothermal e n d m e m b e r with ambient seawater. As indicated in Fig. 4, the helium concentrations are linear with the magnesium content (mixing line). The extrapolation of the 4He concentration to zero magnesium concentration gives a 4He endmember value of 4.6 x 10 -5 cm 3 S T P / g . Figure 5 shows the 3He concentration versus 4He for the samples. The values show a linear correlation still corresponding to the mixing line between

TABLE 4 E n d m e m b e r m e t h a n e a n d h e l i u m isotopes d a t a in our S n a k e Pit h y d r o t h e r m a l fluids. T h e p e r c e n t e n d m e m b e r of each s a m p l e is d e d u c e d from the m e a s u r e d m a g n e s i u m content. All t e m p e r a t u r e s were r e c o r d e d from the Nautile w i t h the s a m e t h e r m o c o u p l e p r o b e a n d electronic package. A c c u r a c y on t e m p e r a t u r e m e a s u r e m e n t s was w i t h i n + 2% f r o m 100 to 4 0 0 ° C Sample

HS-88-11-1A HS-88-11-1B HS-88-10-1 HS-88-5-1 HS-88-3-1 HS-88-8-1 HS-88-6-1 Best e s t i m a t e

Fluid temp. (oC)

%endmember

330 330 330 330 330 325 345

15 15 55 90.3 6 5.6 4.9

(3He/4He)Hy

(4Helium)H (×10 5cm~'STP/g)

CH 4 (xl0-3cm

8.60 8.52 8.36 8.47 8.40

4.3 4.04 4.06 4.87 3.76

1.29 1.40 0.39 1.46 0.40

2.75 2.46 0.93

8.4 + 0.1

4.6 + 0.3

1.38

2.57

(3He/4He)atm

3 STP/g)

CH4/3He (×10 -6)

24

P. J E A N - B A P T I S T E E T AL. 2,0

O_ 1 . 6

03

(/) nE

mE 30 4 ° A 20

,,

7 d 0.5

,~

~r

/

5--

0.0

20 40 MAGNESIUM ( 10-6mo I e / g )

,,,

60

Fig. 4. 4He (triangles) and CH4 (squares) versus magnesium. sampled sites (Clou and Ruches) since all points fall on the same mixing line. Figure 6 shows the relationship between C H 4 and 3He for the samples at Snake Pit. The slope of 0.60-

O. 40

mE o

0.20-

0,00

......

~6' . . . . . . .

HELIUM-4

"~ .......

~

.......

&~ . . . . . . .

(10"6cm 3 STP/g)

Fig. 5.3He-4He relationship.

1,6" %

N D_ O3 nE 1.0 o

Ld Z

~b- 0 . 5 W IE

0.0 0.00

HELIUM-3

( lO-'~cm a S Y P / g )

Fig. 6. CH4-SHe

relationship.

the linear correlation gives a CH4//3He ratio of 2.6 X 10 6. This value is to be compared to the value of 0.5 × 10 6 derived from the water column data. The lower value in the water column above the vents emphasizes the non-conservative nature of methane in the marine environment and its oxydation in the plume [51]. The oxygen concentration in the deep Atlantic (5.7 x 10 -3 cm 3 S T P / g [39]) is higher than in the Pacific (2.5 × 10 3 cm 3 S T P / g [16]) and probably increases the methane removal.

~o

6. Discussion H y d r o t h e r m a l C H 4 may be derived from multiple carbon sources, particularly in geologically complex hydrothermal systems: thermal breakdown of complex hydrocarbons at moderate to high temperatures ( > 100°C), bacterial production at low temperatures, outgassing of juvenile carbon as C H 4 from the mantle, inorganic synthesis in reactions at high temperatures ( > 300400°C) involving CO z and H 2 or other hydrocarbon molecules which may be derived from various sources [23,25]. The Snake Pit fluids circulate through young hot basaltic material without interaction with organic-rich sediments. Therefore, the C H 4 enr i c h m e n t may not be due to a thermocatalytic decomposition or oxidation of organic matter, but is much more likely produced by an outgassing of juvenile carbon and via high-temperature inorganic synthesis as on the EPR. Isotopic 8a3CH4 measurements may be required to strengthen this hypothesis, however, the abiogenic origin is supported by the C H 4 / 3 H e ratio (2.6 X 10 6) which is similar to the ratio found on the EPR (Table 1) and also to the ratio measured in MAR basalts (0.7 x 106) or MAR popping rocks (2.1 × 106) [24]. Our helium results are the first reported for pure hydrothermal fluids on the MAR. The isotopic ratio 3 H e / 4 H e is close to the typical value found on intermediate or high spreading rates ridges (see Table 1). This isotopic ratio is also in agreement with the 3 H e / 4 H e ratio of the midAtlantic ridge basalts reported in the literature [24,521. The 3 H e / h e a t ratio is calculated considering our measured 3He concentration and a tempera-

HELIUM AND METHANE MEASUREMENTS IN HYDROTHERMAL FLUIDS AT SNAKE PIT

ture of 330°C. This ratio is equal to 3.2 X 1 0 7 atoms/cal and compares well with the values found for the East Pacific rise [5,12,15,24,49,5356]. The 4He concentration in the pure hydrothermal fluid is also surprisingly close to the value found elsewhere on mid-ocean ridges (see Table 1). As already noted, the chemistry of the MAR vents of the T A G and M A R K zones shows striking similarities with that of the EPR fluids. This similarity can be extended to the helium isotope concentrations and to the 3He/heat ratio. This suggests that in both cases the circulating hydrothermal fluids have been subjected to similar chemical and thermodynamic conditions. The distribution pattern of 3He in the deep ocean, as inferred from the GEOSECS measurements [31], strongly stresses the major role played by the ridges in the 3He flux. This is especially clear in the Pacific ocean. In a recent paper by Torgersen [57] the helium fluxes from the various reservoirs are re-estimated or reviewed. The contribution of the mid-ocean ridges to the total oceanic 3He production is estimated to be around 95%. An even greater value (97%) has been proposed by Sano [58]. Hence, if we assume on one hand, that the range of variation of the 3He concentrations measured so far on various spreading centers (high or medium spreading rates as in the eastern Pacific, slow spreading rate as in the present study) is likely to represent its natural range of variation on mid-ocean ridges (see Table 1), and if we consider on the other hand the average rate of 3He outgassing from the seafloor determined by Clarke, Beg and Craig (~ --- 3 3He a t o m s / s / c m 2 of the whole earth surface) [59-61], then we find that the global hydrothermal mass flow is in the range 1000 to 3000 m3/s. This corresponds to a residence time of the world ocean with regard to the hydrothermal circulation between 15 and 45 million years. Those figures are consistent with the magnesium cycle, which is one of the simplest geochemical cycles with respect to the hydrothermal circulation since the hydrothermal fluids are totally magnesium free. Taking a magnesium concentration in seawater of 54.2 x ] 0 - 6 m o l e / g and a rate of river discharges of 5.5 x 1012 m o l e / y [62,63], one calculates that the hydrothermal removal rate of Mg 2+ is between 30 and 90% of the river inputs, in agreement with magnesium cycle

25

studies [64-66]. Finally, when compared to the flow rate delivered by a single black smoker (typically 10 -3 m3/s [67]), our result implies that, in addition to the individual hot vents, an extensive diffuse circulation does take place along the ridge's axis, as pointed out by recent heat flux studies on the Endeavour Segment [68] and on the Juan de Fuca Ridge [69].

7. Conclusion The concentrations of the helium isotopes and methane in pure hydrothermal fluids from the Snake Pit area on the MAR (23 ° N) show strong similarities with previous measurements, already made on fast spreading ridges (EPR, G S C . . . ) . Hence the similarities already observed for the chemistry of the fluids seem to be also valid for these volatile species. Taking advantage of this similarity for spreading centers with very different tectonic settings, we have assumed a natural range of variation of the 3He concentration in hydrothermal fluids that leads to a global estimate of the hydrothermal circulation between 1000 and 3000 m3/s. The order of magnitude of this mass flow is consistent with the magnesium budget and implies, along with high-temperature hydrothermal vents, the existence of an intense diffuse circulation at the ridge's axis.

Acknowledgments The authors wish to thank the captain and crew of the R / V Nadir and the Nautile team for outstanding support and cooperation. We are also grateful to L. Merlivat and H. Craig for their critical review and constructive comments for improving the manuscript.

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