Abundance and isotopic composition of helium in hydrothermal sulfides from the East Pacific Rise at 13 °N

Geochlmica et Cosmochimica

Acta, Vol. 60, No. I, pp. 87-93, 1996 Copyright 0 1996 Elsevier Science Lid Printed in the USA. All rights reserved 0016.7037/96 $15.00 + .OO

Pergamon

0016-7037(95)00357-6

Abundance

and isotopic composition of helium in hydrothermal from the East Pacific Rise at 13”N

PH. JEAN-BAPTISTE ’ and Y. FOUQUET’ ‘Laboratoire de Modklisation du Climat et de l’Environnement, CEA-Saclay, DSM/LMCE, %FREMER, (Received

DRO/GM, October

Centre 6,

de Brest, BP.70,

F-39280,

Plouzane

1994; accepted in revised form September

sulfides

F-91 191 Gif-&u-Yvette Cedex, France Cedex.

France

12, 1995)

Abstract-The helium isotopes composition of fluids trapped in hydrothermal sulfides from the East Pacific Rise ( 13”N) were measured. Two extraction techniques, i.e., crushing and heating, were tested and sulfides of different mineralogical types were studied. The comparison between the two extraction methods suggests that both should give similar results, provided the crushing efficiency is sufficiently high. Sulfides “He/4He ratio for crushed samples are in the range 6.85-8.10 times the atmospheric ratio (R, = 1.38 X 10 m6) and the measured helium concentration is of the order of 5.5 x 10 -’ cm’ STP per gram of trapped fluid. Comparison with the helium isotope composition of the hydrothermal vent fluids themselves shows that the mineral phases formed at the highest temperature (i.e., with less dilution by the ambient seawater) retain the helium information most effectively, with values almost indistinguishable from the endmember vent fluid. 1. INTRODUCTION

surveys of hydrothermal sites by different cruises at several years intervals (Campbell et al., 1988; Edmond et al., 1990; Baker, 1994; Butterfield, 1994; Massoth, 1994) also allow a closer look into the question of temporal variability. All these results show that significant variations of physical and chemical parameters may occur. Moreover, we know that “catastrophic” events such as a sudden release of large amounts of heat and fluid (Baker et al., 1987, 1995; Lupton et al., 1989, 1995; Von Damm et al., 1995 ) do happen in response to geophysical events or magmatic processes, indicating short-term changes in the volatile flux from the seafloor. As far as volatile species are concerned, pioneering work by Turner, Stuart, and their colleagues (Turner and Stuart, 1992; Stuart and Duckworth, 1993; Stuart et al., 1992, 1994) shows that analysis of fluid inclusions in hydrothermal mineral deposits could be of great interest for studying the gas composition of ancient hydrothermal vents and hence for reconstructing the fluid composition history and its temporal variability over the different timescales of interest. This study is an extension of the above work and reports preliminary analyses of helium isotopes in sulfide samples from hydrothermal edifices on the EPR at 13”N. Our objectives are threefold: ( 1) to compare the crushing and heating techniques, (2 ) to obtain an initial idea, at a single hydrothermal site, of the variability of the helium isotopic measurements due to the type of sulfide edifice and to sample mineralogy, and (3) to compare the results with the composition of the hydrothermal fluids.

Since the discovery of active hydrothermal vents at the Galapagos Spreading Center in 1977 (Corliss et al., 1979), a substantial number of hydrothermal sites have been located along mid-ocean ridges. The chemistry of the hydrothermal fluids from different sites shows large similarities, although significant differences may exist depending on the various geological and thermodynamical conditions encountered by the circulating fluids (Edmond et al., 1982; Campbell et al., 1988; Von Damm, 1990; Fouquet et al., 1991). The same remark also applies to the gas composition. As far as helium is concerned, the ‘He/4He ratio appears to be quite constant, with a mean value of around 8 5 1 X R,, (R,,, is the atmospheric ratio, equal to 1.38 X 10mh) first observed in the Galapagos Rift by Lupton et al. (1977) and Jenkins et al. (1978). The existing data also suggest that in most cases the helium concentration varies within a factor of two around a mean value of 3 x 10m5 cm7 STP/g (Welhan and Craig, 1983; Kim et al., 1984; Merlivat et al., 1984; Lupton et al., 1989, 1991; Sakai et al., 1990; Jean-Baptiste et al., 1991; Rudnicki and Elderfield, 1992; Ishibashi et al., 1994). However, it is not easy to distinguish between the part of the scatter that is due to the experimental dispersion, including analytical error and variability due to sampling, and the actual spatial variability. In contrast, very little is known as yet about the variability in time of the fluids’ composition, for different timescales going from a few days to the life span of a single vent or a hydrothermal site. Most of the analytical techniques for in situ monitoring of chemical parameters are still being developed. Therefore, continuous time-series of chemical parameters, as now being obtained for several subaerial volcanoes, are not yet fully available in the deep ocean. Nevertheless, time-series of physical parameters have been obtained, including temperature, light attenuation, mass flow measurements, seismic activity, or video recording (Little et al., 1988; McDuff and Delaney, 1989; McDuff et al., 1990; Tivey et al., 1990; Chevaldonne et al., 1991; Baker and Cannon, 1993). Repeated

2. SAMPLE

DESCRIPTIONS

All samples were collected in the axial graben of the East Pacific Rise ( 13”N) as shown in Fig. 1. HR-02-01 is a section of a massive inactive hydrothermal chimney collected during the Hero cruise in 1991 with the Nautile. Its mineralogy is dominated by sphalerite, pyrite, and marcasite. The CY samples were taken during the Cy&herm (1982) and Geocyarise (1984) cruises with the Cyann submersible. Their detailed mineralogical description, along with their chemical composition is given in Fouquet et al. (1988) and sum87

Ph. Jean-Baptiste

88

and Y. Fouquet To inlet system

12"46

\

SE SEAMOUNT

12"42 N

n 0

Active $acttve

Sine

HIGH

, FIG. 2. Schematic

1. Bathymetric map showing the topography of the studied area (I 2”50N, East Pacific Rise) and the location of the samples.

drawing

of the crusher.

NC.

of around 2-4 g, was baked under vacuum for several hours at a temperature between 50 and 120°C (see Table 1 for details) to drive out water vapor. Then the sample was placed on the inlet line inside a crusher and pumped out to IO-’ to IO-’ mbar. Samples were crushed in a custom-fabricated stainless steel container with a bellows welded to a high vacuum flange with a copper gasket (Fig. 2) and the released helium was introduced in the mass spectrometer using our standard procedure, briefly described below. In a second run, measurements were performed on the crushed material in order to study the helium residuals. These measurements were also carried out directly on the mass spectrometer inlet, by plac-

marized in Table 1. CY-82-13-4 comes from an inactive chimney consisting mainly of pyrite and sphalerite. CY84-13-l is the inner zone of a Cu-rich active chimney (T = 335°C) made of chalcopyrite along the central conduit and pyrite + anhydrite towards the outside. Sample CY84-38 is the inner part of a Zn-rich active chimney (T = 283°C) showing small crystals of pyrite overgrown by Zn sulfides. 3. EXPERIMENTAL

METHODS

Gas extraction was performed using the mass spectrometer inlet system. Prior to extraction, each sample, consisting of a solid section

TABLE

sulphide Fouquet

Sample HR-02-01

CY-82-13-4

CY-84-13-I

CY-84-38

I, Mineralogy

samples from et al., 1988).

Type Inactive Fe-Zn chtmney mactwe Fe-Zn chimney active Cu- rich chimney active Zn chunney

chemical East Pacific

and bulk

13”N,

Temperature

= 200-250°C

335°C

283°C

composition of the selected hydrothermal Rise. (detailed description is available in

Mineralogy &Py, Ma, @ b&2,%, cw Qy, Py. Id, Bn, Cv rYz?Jm, Cpy

Sp-sphalerite; Pypyrite; Cpy-chalcopyrite; marcasite; Id-idaite; Bn-bomite; Cv-covellite.

CU 0 12

Fe 32.00

Zn 9.05

Ca 0.02

S 39.40

SiO2 17 40

0.10

30.50

16.30

009

40.70

1.25

30 80

28.30

0 08

2.50

31 80

I 43

0.45

12.20

28.40

10.80

31.70

0 18

silica;

Ma-

Po-pyrrhotite;

Op-opahne

Helium TABLE 2. Helium Pacific Rise).

*) he

hne

isotopes

1 Bakmg temperature 2: Pumping time prior

composition

in hydrothermal

sulfide

of the crushed

89

deposits

samples

(hydrothermal

sulfides-

13”N, East

and time to inlet

ing the sulfide powder in a corundum tube and extracting the residual gases by heating with a furnace at IOOOY for about 10 min. The mass spectrometer inlet is a stainless steel line with high vacuum valves, ensuring a low helium blank (<6 X IO-” cm3 STP). It is equipped with a water vapor trap and a charcoal U trap, both operated at 77 K, and a SAES getter for reducing the hydrogen level. No neon trap was used in these experiments. As already discussed by several authors (Rison and Craig, 1983; Lott and Jenkins, 1984; Sano and Wakita, 1988). if the Ne/He ratio in the samples is quite different from that in the air standard, the measured ‘He/+He ratio can be biased up to IO%, depending on the actual pressure in the mass spectrometer source (Sano and Wakita, 1988). In agreement with the latter study, we find that below 2 X 10wJ Pa (roughly corresponding to one aliquot of air standard), our spectrometer behaves quite linearly and the ‘He/JHe ratio is constant within the experimental error. In hydrothermal fluids, Ne concentration roughly corresponds to a 2°C air-saturated seawater (Kennedy, 1988) so that the NelHe ratio in our samples should be very low and therefore, it is easy to remain well inside the linear domain. ‘He/“He measurements were made on a VG 3000 mass spectrometer described previously (Jean-Baptiste et al., 1992) and used in routine operation for oceanic helium samples. 4He is measured using a Faraday collector while ‘He+ ions are detected in the pulse counting mode on an electron multiplier. The resolving power is sufficient for a complete separation of the ‘He + peak from its HD + neighbor. For this type of sample, the uncertainty in the 4He measurements ( lo) is within 20.8% while the error in the isotopic ratio ‘He14He (10) is

of the order of 20.05 x R,,,, (The actual error corresponding to each individual measurement is indicated in the tables.) The water content of some samples was also measured to derive helium concentrations. The sample was sealed in the same crusher as above and placed on a stainless steel line. The crushing assembly and the line were baked at 120°C under vacuum for several hours. Then the line was isolated, the sample was crushed, and the released water vapor was transferred to a small glass bulb immersed in liquid nitrogen. As crushing results in a large surface area increase, the crusher was heated with a flame during the transfer procedure to prevent adsorption of the water onto newly produced powder. After 1 h, the bulb was flame-sealed. The amount of water was measured precisely by weighing on an electronic balance with a theoretical precision of 51 yg. The actual maximum uncertainty in our measurements was estimated to be around 210 pg. which corresponds to a precision of ~3%. Water blanks performed in the same way were found to be negligible. 4. RESULTS The helium results for the crushed samples are listed in Table 2. The duration and temperature of the initial pre-crush baking under vacuum are also given along with the pumping time on the mass spectrometer introduction line prior to analysis. In the range of values explored, there is no detectable

90

Ph. Jean-Baptiste

and Y. Fouquet

5.5 t 2 X lo-’ cm’STP of He/g of water (this value is obtained from the mean helium value of the seven helium experiments HR-02-01 samples and from the mean water value of the two samples of the same type mentioned above). This value compares favorably with the helium concentration observed in hydrothermal fluids themselves (Table 4). 5. DISCUSSION

The helium isotopic ratio measured for the various samples for crushed material appears to be quite reproducible, with a dispersion ( 1g) less than 0.1 X R,,, in most cases (Table 2). In the largest series of measurements, carried out on HR-0201 sulfides, the dispersion ( la ) for five samples is similar (0.1 X R,,,, ). Two samples, however, (no 1 and 4) show a significant departure from the average value, respectively, 7.19 and 9.90 x R,,, , with no clear explanation for this. As the samples were cut from a relatively large piece of material from a massive sulfide edifice, this may reflect a concentration gradient in the material. This explanation also holds for CY84-13-l samples, where a clear distinction can be made between the interior of the pipe and the outer rim of the chimney, the latter showing a lower isotopic ratio and a lower helium concentration probably due to dilution by ambient seawater. However, for the HR-02-01 samples, this explanation is contradicted by the results for total helium obtained when taking into account the isotopes fraction released by both crushing and heating (Table 3 ) In this case, the global 1He/4He ratios of the two samples become 8.02 and 8.38 X R,,,, respectively, which are close to the average value. Table 3 shows that the amount of helium released by crushing is only a fraction of the helium content of the sulfide. In the case of the HR-02-01 samples, this released fraction is less than 20% of the total. The total helium isotopic ratio is also affected, with values somewhat different from those recorded for the crushed fraction alone. For the chalcopyrite samples (CY-84-13-l), the crushing efficiency is much higher, over 60%, and the isotopic ratios do not differ significantly between the two extraction methods. To understand why the crushing efficiency was not satisfactory, several samples were sieved to establish the grain-size distribution. For HR-02-01 samples, it appears that only 40% of the powder was smaller than 200 pm and 25% larger than 1 mm. For CY84-13-l samples, these percentage are, respectively, 52 and 15%, in agreement with the higher degassing efficiency. Although the actual size of the fluid inclusions in these samples is not known, the results suggest that the low degassing efficiency may originate from the poor performance of our

E V o-l0

-I

20-

3

l/T(

FIG.

(after

3. Helium Boschmann

K>

diffusion coefficient et al., 1984).

~10~

in various

sulfide

effect of these parameters on the measured helium the isotopic ratio, which is consistent with the low of helium in sulfides (Fig. 3). This encouraging agreement with the observations made by Turner

minerals

amount or diffusivity result is in and Stuart

(1992).

The mean ‘He/4He ratio and helium concentration per gram of material, for the different types of sulfides studied, are also shown in Table 2. The ‘He/4He and “He concentrations measured when heating some of the crushed samples at 1000°C to look for residuals are given in Table 3, along with the crushing efficiency, total helium amount, and corresponding isotopic ratio. The degassing efficiency at 1000°C was checked by reextracting the remaining helium for an additional 10 min: this was found to be around 0.6% of the first extraction. In addition to these helium isotope measurements, the water contents of two HR-02-01 samples, each around 3 g, were determined by weighing as described above and are 301 ,ug and 383 pg of water/g of sulfide. This leads to a helium concentration in the hydrothermal fluid inclusions of the order of

TABLE

3.

Helium isotopes components released successively

by crushing

and heating

of selected

samples. CRUSHING Sample

HEATING

14W

RfRatm

Helium

recov3y (%)

x lOhn3/g l-R-02-01 HR-m-01 HR-02-01

(1) (4) (7)

mean value (1 sigma) I CY84-13-l

(1)1

7 19Iko.05 9 9Oxko.05 7.99ko.05

8.36 (1.4) 7.71kO.05

3.13kO.03 1.65+0.01 2.3OkO.02

I

2.36 (0.7)

1 5.65kO.04

TOTAL

Rmatm.

14Hal

R/Ratm

L4W

x 108,,3/g

17.3% 12.4% 194%

8.19ItO.02 8.161tO.03 8.47f0.03

I

16.4 (3.5)

I

8.27 (0.2)

]

61.1%

[ 7.8OkO.05

15.oofo. 11.7M0.09 9.56f0.08

I

12.1 (2.7)

( 3.6OkO.03

x lOhn3/g 1

8 o-to. 1 8.4fO.l 8.4*0.1

1s.1+0.1 13.3kO.l 11.9kO.l

I

8.3 (0.2)

I

14.4 (3.3)

1

7.7+O.l

1

9.220.1

91

Helium in hydrothermal sulfide deposits crusher, with compaction of the sample preventing complete crushing. Hydrothermal fluids have been sampled on several occasions at 13”N. Helium results were obtained by Merlivat et al. ( 1984) at three different vents (Chandelier, Chainette, Cathedrale), and by Kim et al. ( 1984). The data are given in Table 4 and Fig. 4 along with some unpublished 3He/4He measurements we obtained at 13”N, with the submersible Nautile during the HERO cruise ( 1991). Also recalled in the table are our sulfides data and MORB data available in the same area (Hiyagon et al., 1992). Unfortunately, most of the data reported in Table 4 do not allow a precise direct comparison between the hydrothermal fluids and sulfides since they were collected at different times. However, CY-84- 13- 1 sulfide sample (Table 4, first row) comes from the inner pipe of “Chandelier” hydrothermal chimney where fluid samples were collected during the same dive in 1984. In this interesting case, the “He14He value of the sulfide sample is almost indistinguishable from the value recorded in the hydrothermal fluid itself. This is a confirmation that mineral phases which precipitate in equilibrium with the endmember fluid do retain the helium isotopic information, even after a ten-year storage of the sulfide sample. Within a single hydrothermal site (i.e., 13”N), the Table 2 results show a significant variability in the helium isotopic ratio, depending on the type of sulfides studied, with values ranging from 6.85 to 8.1 x R,,, . This variability appears to exceed substantially that observed in the hydrothermal fluids themselves (see Table 4). For instance, CY-82-13-4 samples have a low 7He/4He ratio (6.85 X R,,, ), compared to the other types of samples. Their helium content is also much lower (0.5 X lo-* cm’/g), suggesting a possible influence of some radiogenic helium produced by the in situ decay of U and Th. The typical U-Th content of hydrothermal sulfide chimneys on the EPR is in the range 1- 10 ppm for U and O.l- 1 ppm for Th (Lalou and Brichet, 1982,1987). However the age of sulfide edifices sampled between 12’48 and 12”52 N in the central graben is very young, ranging from 0 to 20

0

0

-8

-9

-/

-6

Log ,@ C 4He 1 ( c rn3STP/g

1

FIG. 4. ‘He vs. “He relationship for hydrothermal sulfides (East Pacific Rise, 13”N) compared to hydrothermal vents fluids and MORB at 13”N. Open squares are our results for crushed samples and solid squares are for crushed + heated samples. Vent endmember values are from Merlivat et al. (1984) (open triangles) and Kim et al.

(1984)

(solid

triangles).

Tilted

squares represent

hydrothermal

fluids

measurements from the Hero cruise (Ph. Jean-Baptiste, unpubl. data). MORB data (stars) are from Hiyagon et al. (1992).

years in most cases (Lalou et al., 1985 ), therefore ruling out any significant radiogenic contribution. From the diffusion coefficient of helium in sulfides (Fig. 3, after Boschmann et al., 1984) and assuming that ‘He and 4He diffusivities vary inversely with the square root of their atomic weight, a simple diffusion calculation also shows that there should be no detectable isotopic fractionation in the chimney wall over the typical age of these hydrothermal chimneys. Hence, the var-

TABLE 4. Comparison between the helium isotc $c composition of hydrothermal ventings, sulfide fluids inclusions and basalts in the vicinity o 13”N, East Pacific Rise.

1nclus10ns EPRI 3”N Chandelier CY84-13-l (thts work) EPR l3”N (this work) CY-82-13-4 CY-84-38 HR-02-01 EPR 13-N EPR l3N-1 EPR 13N-2

sulfides (1984)

1

I 7 68+0 05

sulfides (1982) (1984) (1991) basalts [4)

I

1

I

1 8 I *08 1 8 74 8.17

1

55i2

References: [I] = Merlivat et al., 1984; [2] = Kim et al., 1984; [3] = Jean-Baptiste, unpubl. data; [4] = Hiyagon et al., 1992. Note that vent fluids and sulfides data reported for Chandelier (first row) correspond to samples collected on the same hydrothermal chimney at the same time (1984).

rl

-4

-5

92

Ph. Jean-Baptiste

ation in the isotopic ratio between different samples is most likely related to dilution of the pure hydrothermal endmember by ambient seawater. This was already noted concerning the isotopic shift between the inner and outer part of sample CY84-13-1 earlier in the discussion. The same explanation holds for sample CY-82-13-4: it shows a ‘He/“He ratio out of the range of the hydrothermal fluids in the area, but this Zn-rich chimney is also characterized by the absence of a central conduit and was formed from water diffusing at somewhat lower temperatures (200-250°C)) that is more diluted by ambient seawater. This implies that when using sulfides to reconstruct the characteristics of the hydrothermal fluids, care must be taken to select appropriate samples, i.e., those, such as chalcopyrite, where the mineralogical analyses demonstrate the presence of fluids with the highest temperatures. 6. CONCLUSION

We have compared helium isotopes measurements in fluid inclusions from hydrothermal sulfides with data obtained directly for hydrothermal fluids. These observations suggest that helium isotopes in fluid inclusions are representative of those in vent fluids, provided sulfide samples are well characterized and adequately selected. These preliminary results first suggest that it may be possible to study volatile species at contemporary hydrothermal sites from sulfides alone, whenever fluid samples are not available. The possibility of extending the study of helium isotopes back in time is supported by the retentivity of helium in sulfides: using the diffusivity data available in the literature (Boshmann et al., 1984), one can calculate that the helium concentrations in a typical hydrothermal chimney are not significantly affected, even over times of 100 My. As already suggested by Turner and Stuart ( 1992), this raises the possibility of utilizing hydrothermal sulfides for studying the past degassing history of the earth. Edirorial

handling:

D. E. Fishet REFERENCES

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sulfide

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93

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