East Pacific Rise at latitude 21°N: isotopic composition and origin of the hydrothermal sulphur

148

Earth and Planeta O' Science l.etter.~, 56 ( 1981 ) 148-156 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

IS]

East Pacific Rise at latitude 21 °N" isotopic composition and origin of the hydrothermal sulphur Michel Arnold and Simon M.F. Sheppard Centre de Recherches P#trographiques et G~ochimique~, B.P. 20, 54501 Vandoeu~;re-I&~NanO" (France)

Received May 14, 1981 Revised version accepted July 29, 1981

The sulphur isotope composition of 16 pyrite and chalcopyrite samples from recent sulphide deposits C C y a n a " project RITA) and active sulphide mineralisation ("Alvin"--project RISE) associated with hydrothcrmal sources at 3 8 0 ± 3 0 ° C on the East Pacific Rise at latitude 21 °N have been measured. The 34S/32S ratios are relatively uniform and essentially identical for both sites: 834S= + 1.4 to 3.0%0 (CDT), mean + 2.1%,. The sulphides were analysed after the majority of the very numerous micro-inclusions of anhydrite had been removed. Two independent physico-chemical analyses of the data demonstrate that about 90% of the sulphur was leached from the basaltic host rocks by the circulating seawater-hydrothermal fluids.

1. Introduction Recent sulphide deposits have been discovered on the East Pacific Rise off Mexico at latitude 21 °N, first in 1978 using the submersible " C y a n a " of the C Y A M E X expedition of the joint FrenchAmerican-Mexican project R I T A [1,2], and then in 1979 using the submersible "Alvin" of project RISE [3]. In the R I T A project area (Fig. 1) massive sulphide deposits were observed as mounds or hills up to 10 m high and averaging 5 m in diameter. No active hydrothermal sources were detected in this area. In the RISE project area 5 km or so along the axis to the southeast, at least 25 active hot sources within a band 200-300 m broad and 6 km long within the ~ 800 m broad central zone of freshest lava flows were observed. The hot waters which jet out of chimneys are loaded with suspended particulate material. These may be dominantly of sulphides (pyrrhotite, pyrite, sphalerite and Cu-Fe sulphides)--the high velocity ( m / s ) "black smokers" at temperatures of 380 ° - + - 3 0 ° C - - o r of mixed amorphous silica, barite and p y r i t e - - t h e more gentle emanating "white

smokers" at 32-330°C. Sediments which are dominantly composed of sphalerite, chalcopyrite and pyrite settle out around the vents. Many other sulphide, sulphate, silicate and oxide minerals have T

.$/jRito f~/'~

/

)1 / ~"--mse c~] _ ~,. \-t20°

3 / /

~2-~,8o ,o9o

/.

,o,"

. //

<..(_ / Rita

20 ° 54'N

2 / --/' Area /1 / -- -,aulphides /

~

/ /

I/

20 °

50'N

/

/

/

.,~ ",v~ 9 8 I

/

109°08'W

/

/ --980

"

~

Rise Area

~ Ventsvisitedby Alvin

L

109~06'W

IOWO4'W

109°02'W

Fig. 1. Location of the hydrothermal sulphide deposits analysed from the project RITA and RISE areas at 21°N on the East Pacific Rise [3,4]. Modified after map prepared by CNEXOCOB, 1980 in M. Fevrier (unpublished thesis).

0012-821X/81/0000-0000/$02.75 '¢ 1981 Elsevier Scicnti tic Publishing Company

149 been identified [3,4]. The spreading rate near 21 °N is around 3 c m / y r . These massive sulphide mounds and deposits of the East Pacific Rise have many things in common with the older massive sulphide deposits associated with ophiolite complexes (e.g. Cyprus, Notre Dame Bay, etc.). There are, however, certain differences such as the apparent greater abundance of sphalerite at the 21°N sites. The source of the sulphur could be from the reduction of marine sulphate, be leached from the basaltic host rocks or be a mixture of the two sources~ However, the sulphur source can only be satisfactorily defined if the physicochemical conditions of the sulphur depositing hydrothermal fluids are known. As this is the case for the East Pacific Rise, determination of the sulphur source here can then be applied to develop our genetic models with possible application to older ore deposits of this type.

proved impracticable to completely remove it. The dried sulphides were reacted with excess C u 2 0 at 1100°C to give SO 2 gas [6]. This SO 2 was purified and the yield measured manometrically to verify that the sulphur extraction had been quantitative. Isotopic compositions were measured initially on a McKinney-Nier type modified T H N 204 mass spectrometer and latter on a modified VG 602 type mass spectrometer. The 20 on the latter more precise mass spectrometer was better than 0.07%o. Two or more complete extractions were carried out on most samples. The data are reported as 634S values in permil relative to Canon Diablo troilite (CDT) using the McMaster University series of sulphide standards and values (277/6 =- 22.3; 277/2 = 5.9; 277/9 = - 18.7) as our reference [7].

3. Isotopic results 2. Samples Samples collected with the "Cyana" during the RITA project are generally heavily altered and oxidised. Eight sulphide-bearing samples were selected for isotopic analysis. These samples were provided by R, H6kinian and a more detailed description and chemical and mineralogical analysis is given in H6kinian et al. [4]. Nine samples from the RISE project given by F. Albar6de were also analysed. Initially each sample was crushed to < 0.1 mm and the sulphide mineral was separated by hand picking under a binocular microscope. During the course of the analyses the presence of microinclusions of a sulphate mineral (principally anhydrite) in the sulphide grains was revealed from the strontium isotope studies on some of these same samples [5]. This was confirmed by high magnification microscopy. Because anhydrite (or other sulphates) is typically enriched in 34S relative to associated sulphide in the marine environment it must be removed before isotopically analysing the sulphide. Hence the "sulphide concentrate" was reground and agitated ultrasonically in cold water. This purification method removed the major part of the trapped sulphate but it

The isotopic data for 17 sulphide samples are given in Table 1. The two analyses on the chalcopyrite from sample ALV981-R4-1 demonstrate the important influence of the micro-inclusions of anhydrite on the isotopic analysis. The 834S decreased from +2.8 to +2.3 as a consequence of the agitated cold water purification process. All the other data reported in Table 1 are for water purified sulphides. Part of the difference among samples is thought to be possibly a result of heterogeneities in the quantity of residual sulphate in the purified sulphides. Overall the sulphur isotopic composition of the 16 pyrite and chalcopyrite samples display a very restricted range of values from + 1.4 to3.0%o with a mean of +2.1%o (Fig. 2). The means of the 7 "Cyana" and 9 "Alvin" samples are the same within experimental error, 2.0 and 2.1%o respectively. For "Cyana" 6 of the 7 samples are pyrite but for "Alvin" only 1 of the 7 good mineral separates is a pyrite. Pyrites for !'Cyana" are both slightly enriched and slightly depleted in 34S relative to the chalcopyrite while for "Alvin" chalcopyrites are typically slightly enriched in 34S relative to the pyrite. The apparent pyritechalcopyrite sulphur isotope fractionation derived from the mean (RITA + RISE) pyrite and chal-

150 TABLE 1

Pyrite

Cha,copyrite

Sulphur isotope compositions of sulphide minerals Sample No.

Mineral *

"Cyana"--Project RITA CYP78-08-14-A2 py CYP78-12-38-A2 py CYP78-12-38-A7 cp CYP78-12-40-Aa py CYP78-12-40-B py CYP78-12-40-B' py CYP78-12-40-C5 py

1.5 3.0 1.8 2.4 1.4 2.5 1.5

"A lt'in" --Prt?ject RISE ALV980-R 12-1 ALV981-RI-I ALV981-R2-1 ALV98 l-R3-1 ALV981-R4- I' ALV98 I-R4-1 ALV981-RI5 ALV981 -R 18-1 ALV982-R l0 ALV982-RI5

2.0 2.3 2.3 2.9 2.8 2.3 1.7 2.4 1.6 1.8

py/cp cp cp cp/py cp *** cp py cp cp cp

* Abbreviations: py=pyrite; cp=chalcopyrite. ** Routine analytical reproducibility: ±0.1%0. *** Sample hand picked but not agitated in cold water (chalcopyrite plus few percent of anhydrite, see text).

copyrite isotopic compositions is 0.3. At 350°C the equilibrium fractionation between pyrite and chalcopyrite is 10001n O/py.c p = 1.2 [8]. Thus, although equilibrium may have been closely attained, in detail there is evidence for isotopic disequilibrium between some pyrites and chalcopyrites for both "Alvin" and "Cyana" samples. Also textural evidence on some of these samples suggests that disequilibrium was prevalent [4]. The mechanisms of precipitation of both the sulphides and sulphates were probably very complex. Sulphur isotope compositions of some other RITA samples have been reported by H6kinian et al. [4]. From their table 6, 5 sphalerite and pyrite samples have a range of +1.9 to 3.3%o, mean + 2.5%o and the three pyrite samples have a mean of +2.6%o (Fig. 2). They also give average values for 9 other analyses of pyrite, sphalerite and chalcopyrite (+3.5, +2.8 and +2.1%o, respectively). Thus these analyses and particularly their second group, which were not reported in detail, are on average 0.5-1.5%o enriched in 3% relative to our

Cyana

Alvin

(~34S (%o) **

Pyrite ['4]

~-6 CO

I

"6 4

I

1

2

3

0

1

I f

F-I

-

t

!

Lt I

[

2

3

~34S (%o) COT Fig. 2. Histogram plots of the sulphur isotope compositions of pyrite and chalcopyrite (Table 1) for the "Alvin" samples (RISE area) and "Cyana" samples (RITA area) and comparison with "Cyana" pyrite samples from table 6 in Hekinian et al. [4]. Histogram with light dashed line is for summation of all our °'Alvin" and "Cyana" sulphide samples.

values. The reasons for these differences, whether due to sample heterogeneity, presence of variable quantities of anhydrite and other sulphate microinclusions, difference in analytical techniques or even standardisation, is not presently known.

4. Origin of sulphur

4.1. Sulphur isotope properties of fluids There are two possible principal sources for the sulphur: (1) reduction of seawater sulphate with 634S~s = +20%o [9], and (2) a mantle source with ~34S= +0.5 ± 1%o [9-11] either as a direct magmatic contribution or indirectly during the leaching of the basaltic host rocks by the hydrothermal system. To determine which of these two sources is dominant or whether a mixed source is required the Sakai [12] and Ohmoto [13] approach is applied. The sulphur isotope composition of the parent fluid is calculated from the measured sulphur isotope composition of the sulphides for the pH, fo~, fs2, concentration of sulphur and salts, and temperature data for the fluids and assuming equilibrium among the different sulphur species.

151 The isotopic composition of sulphur in the fluid is given by [13]: ~34S'~s =

~j~34Sjxj

(1)

where 634Sj is the isotopic composition of the aqueous sulphur species j whose mole fraction is ~ . Because the sulphur isotope fractionation among the different sulphate species (HSO 4 , SO2 - , NaSO 4 , etc.) are assumed to be minor [8,13] equation (1) can be replaced by: 634SEs = 634SH2SaqXH2Saq "~ 6 34 S HS XHS

--~6 34 S ]~504 aqX,9s04 aq

(2)

where Xy~so • 4aq is the sum of the mole fractions of all the different sulphate species. When there is chemical and isotopic equilibrium among the different species the isotopic composition of speciesj is defined [14]: 834Sj = 834S~ S -[- X 1 A 1- j

+ x 2 A2_j + ... + X , 6 i j

(3)

where A i__j = 1000 In ai_ j. Under physico-chemical conditions were H2Saq is the dominant sulphur species the expression for the sulphur isotope composition of the solution reduces to: 8 34 SES = 834SH2S.q -~- Apy_H2Saq or ~

34

= ~345

S~s

H2Saq

-~-

A cp_H2Saq

(4) (5)

w h e r e Apy.H~S = 1000 In 0tpy.H2Saq , etc., is the frac-

tionation between the precipitated pyrite (or chalcopyrite) and the aqueous H2S species. At 350°C, Apy_H2s = + 1.0 z 0.2 and A~p.n2s = --0.1 + 0.2 [8]. Because isotopic equilibrium between pyrite and chalcopyrite is not generally observed the fractionation between the mean sulphur isotope composition of the sulphides ( + 2.1 -+ 0.5 %o, Table 1) and H2Saq is taken to be zero to a first approximation. Hence: oXS

o

OH2 S

For the more general case where both sulphate and sulphide species are present, equation (2), the mole fractions of the various sulphur species and their isotopic compositions can only be calculated

if there is equilibrium, or a very close approach, among the species at the temperature of interest [8,15]. The sulphate-sulphide exchange problem is now looked at in more detail because it is not evident that there is always chemical equilibrium and an effective isotopic exchange mechanism between sulphate and sulphide under hydrothermal conditions [ 15].

4.2. Sulphate-sulphide exchange Even for hydrothermal systems it is the rupture of the SO42- tetrahedron which is the rate determining step in the attainment of equilibrium between S vl and S II. Certain experimental studies [ 1 6 - 1 8 ] have shown that sulphate-sulphide equilibrium can be very rapidly attained at temperatures greater than 250°C under some relatively extreme conditions (rn~s and mu+ high, for example). Similarly equilibrium is attained at temperatures of about 250°C in the natural acid hotspring-type systems at Volcano, Italy [14]. However, the extent of sulphate-sulphide exchange under conditions like those for the sources at 21°N is less clear. For this reason the experimental data at 2 0 0 - 5 0 0 ° C for the systems basalt-seawater, basalt-Na-K-Ca-C1 and andesite-seawater of Mottl and Holland [19] and Mottl et al. [20] are analysed in this section to define as precisely as possible the behaviour of sulphate and sulphide. There are however important differences between the experimental and natural systems which must not be underestimated. For example, the experimental systems are closed systems, in contrast to the natural systems which are open. The sulphate present in the initial seawater ( ~ 0.028 mole/kg H20 ) remains in contact with the basalt during the experiment to temperatures greater than 300°C. In natural systems, however, a large proportion of the seawater sulphate is lost by precipitation during the thermal ascent of the seawater as it descends down through the basaltic crust, as discussed below. Once the sulphate-sulphide" system becomes reversible--certainly at 4 0 0 ° C - - t h e fo2 is no longer controlled by the quartz-magnetite-fayalite buffer but by the much more complex system that includes anhydrite, an oxygen donor. This explains the generally observed association pyrite-hematite-

152

magnetite at 400°C [20]. Thus at temperature higher than 300°C the departure of the experimental system from the natural system becomes too large to make a direct comparison between them meaningful. For this reason the experimental data for temperatures greater than 300°C are not considered here. The experiments at 300°C pose another problem because there is only a trend towards and not attainment of sulphate-sulphide equilibrium. The ratio Sn/S vI measured where SIz/S v l = (ZS~qSo~ia)/Y.So~id is compared with the same ratio calculated from the observed mineralogical association assuming the attainment of equilibrium (see Appendix for method of calculation). The ratio ( s I l / s V X ) c a l c / ( S I I / s V I ) m e a s c a n be used as an indicator of the degree of attainment of equilibrium and has the value 1.0 at equilibrium. Because the final observed mineralogy is by the pyrite-magnetite boundary and the fo, is given (10 -32 atm) [20] t h e SII/sVI ratios can be determined. The measured and calculated s n / s w ratios are plotted in Fig. 3a, b and c for the systems andesite-seawater, basalt-K-Na-Ca-C1 solution and basalt-seawater. The data for the systems andesite-seawater and basalt-K-Na-Ca-Cl solution are particularly relevant because initially for the former the sulphur is principally present in the oxidised state (S vx) as the andesite is depleted in sulphur and for the

/-

-- L o g ~ o 2 (Atm.) "~-... 28

30

"'"-....'~j~} ....... ~ .(10-9)

- L ° g # o 2 (Atm.)

T

.~\\"

= 300"C

'm[S~5!2{.410-3mkgl' H20

.......~ ' ~ . ~ , ' ~ /

30

34

i~"~0--]~ $poy 2 "

~ ]0--9-~'tm"'~m~ "~! .' ........ ........fO2: _ri"""'"" ?_+ t gZ]0-321tm'•

36

\ I Z

I 4

I 6

I- L ° g ¢ o 9 (Atm) --~k ~

|

12 pH

= 300"C = 0.54 mTS= 5.44.10-3m kg-1H20

I

10-11< ~ 2 ~ < 10"9Atm.* ~ -

I 10

T

~ -

34 -10-II

fay.

~ 8

=

f3o pPYo

....

~

f02 = 10-32Arm.* "'"'--.

I ~m

I

I

I

2

4

6

,

"'".....

I

8

t

I

10

I

12 pH

mt

SII/SVirnes.= 0 . 0 ~ R " 32

.

mt

run 4B, 4E* ":~/./r un 4D* lI/SVlcal. = 0.36 - - y - -

Fig. 3, fo2-PH diagrams [13] for the experimental data [19,20] comparing the ratios SU/S vl measured for the experiment and s n / s vl calculated from the observed mineralogical association (see text and Appendix): (a) andesite-seawater, (b) basalt-NaK-Ca-CI solution, (c) basalt-seawater.

hm

"

_.

32

36

I

T = 300"C l = 0.72 mTS = t.g1.10"-3"mkg'lH2 O

""'"",

latter sulphur is only present in the reduced state. These systems can thus be considered to represent the reactants and the products respectively of the reaction: ~

2

i

4

I

6

!/

8

I

10

pH

SO42 + 2 H + = H 2 S . q + 2 0 2

153

whose extent of equilibration is to be determined for T = 300°C, t = 172 days. The ratio (SH/SVt)~,I~/(SH/SW) . . . . is 1/8 and 1//7 for the andesite-seawater and basalt-K-NaCa-C1 solution systems, respectively. For the basalt-seawater system (run 1C, t = 236 days [20]) the value for the same ratio is similar at 1/12. Thus for the three different experiments the measured SO42- concentration is 7 to 12 times lower than the theoretical equilibrium value. In natural systems under similar pH, mzs and T conditions equilibrium is probably quite closely attained where the time of fluid-rock interaction is much longer. Temperatures above 300°C, as observed in some of the submarine springs, work even more strongly in favour of the attainment of equilibrium. Thus the ~34S value for such systems can only be strongly enriched in 34S relative to basaltic sulphur isotopic compositions when seawater sulphate participates in the reaction.

isotopic composition of either + 20 (Fig. 4) or 0.0 (Fig. 5). Fig. 4 shows that the observed mean sulphur isotope composition of the sulphides ( + 2.1%o) is only compatible with a purely marine sulphate origin if the hydrothermal solution is simultaneously oxidising and very acid. Neither of these conditions are observed. At 350°C the observed association pyrite + chalcopyrite -t- cubanite is stable at f s , - - 10-9 atm [21] but not under oxidising conditions if the pH is slightly less than 8. The basalt-hydrothermal solution system is reducing because the "black smoker" fluids are rich in pyrite, pyrrhotite and Cu-Fe sulphides as they mix with the more oxidising seawater. The fluids thus plot by the pyrite-pyrrhotite boundary in Fig. 4. Sulphides of marine sulphate origin should have values of +20%~ which are not observed. In contrast, Fig. 5 shows that the 834S values of the sulphides are only compatible with a dominantly magmatic or leached basaltic sulphur origin

4. 3. 21°N hydrothermal system The chemistry of the fluids from the East Pacific Rise sources is broadly comparable to that of the basalt-seawater experimental systems [3,20], and these define the following: the ionic strength 1 = 0.6, T = 350°C, mzs "~ 10-2 moles/kg H 2 0 . The pH-fo~ diagrams (Figs. 4 and 5) are used to test the two hypotheses: (1) hydrothermal sulphur is of marine origin (834S = +20%o), and (2) hydrothermal sulphur is of magmatic or basaltic origin (834S = + 0 . 5 -+ 1%o). The principles of the construction of these pH-fo ~ diagrams are given in Ohmoto [13] and are not repeated here. For both models the isotopic composition of HzSaq and ~.SOaaq are calculated for several points on the stability boundary of pyrite using a simplified form of relation (3) and [8,18]: 1000 In aHS_H2S.q = --0.75 -4- 0.15

T l

- Log f O 2 (Atm.) 22

= 350"C = 0.6

mZS= 634Sis

lO-2m kg-lH20 = + 2 0 Z.

(Oceanic Sulfur) 103 L n caSH 2 . ~ S O 4 = - 19.1

24 ,,, -10 (Log T/$2) --,.9 - , [+3.61 26

~"~-=~,?":"{

28

I I ~344sSSH2aq lcalculated isotopic

SSO4aq /

values

nee,to' hm

i '- ,?-L "-,, "-¢-.,'-~.'-,,, "-" I+ 4.3I , -,~+ ~3.4~ I ' h, ',:x,,{ usl

iiil;iiiiii'~iiiiiiriiiiiii~-~;;iii;;;ii-ii;iiiiiiiiiiiiiiiiiii~,~+2blil ..... r ..... p o -___~,__ .... -. . . . .

I

;_ , ~ ¢ -

,'~+39.2)

.......

~

" A

m t + CL~

i

i "/

f°Y

8

10

1000 In aH2s. _so' = -- 19.1 and taking as before ~345H2 S ~ 8345py ~-- 834Scp. The important influence can be seen of variations in mineralogy, p H a n d / o r fo~ on the isotopic composition of the sulphide or sulphate minerals precipitating from a fluid with a bulk sulphur

2

4

6

12 pH

Fig. 4. f o z p H diagram at 350°C for model with marine sulphate sulphur (834S= +20%c) as the sulphur source. The sulphur

isotope composition of the precipitated sulphide, [], and sulphate, ( ) , are given for a number of selected fozpH values and/or mineralogical associations.

154

-

Log 2'IJC(~ , _ (Atm.)

T 1

= 350"C =0.6 mYS = 10-2 rn kg -1 H 2 0 &34S][ s = 0.0 "/.. (Basaltic Sulfur) 103 Ln aSH_-~5SO4 = - 19.1 [ I &34SSH2aq /calculated isotopic ( ) 534SsSO4aq values

22

24 ""'. ,~l, F , ...., "--2-/UlLOg "tS2J

.

",--9]]?.... "I-'1641 neutral

~4.~

""'<[ +2 7)

__

,.. ,, ,,x3.~. ]] ....

: t

,. ~.~.. --..j-.. , "',. ,t," "'-..-. "" -.

28

1-15.751 i-' ""'~:q,'~(+335).. ;'. ' """."'~,L~," [-8.531

30

-cp + py

"!

"~T" ]~....(+1057) "

':iiiiiiiii:~iiii[ii[[ii[iiii~i!!iiiiii:]iiiiiiiiii:,iiiiiiiiii!!~ I+O]i[ ...... [ ............ _'x_.......... i ...~_____"_;,~ "(+ 1~ 2) .......

~ 10.01 . ~

/

32 k- .........

[\

!_+_1tl) . . . . . . .

_

/

i

l

i pmt

/

I

1

I

2

4

6

__it

.....

f°Y ~

8

....

q_z

I 10

solubility of sulphate in the solutions. As the temperature of seawater increases during its descent through the crust, sulphate from the solution is precipitated out as anhydrite, etc., and thus the sulphate concentration in the fluid decreases. This anhydrite is effectively removed from the system and is not available for re-dissolution after reduction of dissolved sulphate starts to take place at higher temperatures. In basalt-seawater experiments at 200°C and 500 bars the sulphate concentration tends towards 0.62× 10 -3 mole/kg HzO for interaction times of greater than 149 days [23]. This is equivalent to about 59 mg of marine sulphate per kg H 2 0 which can be reduced by the basalts by the time the solution has descended to the 200°C isotherm by a reaction such as: 11 Fe 2SiO4 + 2 SO42- + 4 H +

__

I 12 pH

Fig. 5. fo2-pH diagram at 350°C for model with basaltic sulphur (834S=0%o) as the sulphur source. The sulphur isotope composition of the precipitated sulphide, [ ], and sulphate, (), are given for a number of selected fo2-pH values and/or mineralogical associations. For another value of ~348~s, for example + 1.6%o, the 834SH2s,q and ~34Szso,oq values are increased by 1.6%o from those given on the figure.

under the fo2 conditions defined by the mineralogy. Using the mean isotopic composition an upper limit of 8% marine sulphate is permissible. The maximum A34S value of +3.0%0 of a sulphide extends this up to 13%. For the observed mineral assemblage the sulphur isotopic composition of the sulphide mineral is insensitive to variations in pH.

4.4. Anhydrite and the sulphur balance An independent limit on the seawater sulphate contribution can be derived from the sulphate solubility data. When seawater enters hot basaltic rocks a major part of the initially dissolved sulphate is precipitated as anhydrite. Heating seawater itself to 300°C precipitates about 60% of the total sulphur as sulphate [22]. In basalt-seawater systems the release of Ca reduces even further the

-, FeS 2 + 7 Fe304 + 11 SiO2aq + 2 H 2 0 The quantity of fayalite consumed is very small as confirmed by observation on altered oceanic basalts [24,25]. The 59 mg of sulphate can generate 20 mg of sulphide sulphur with 634S~ +20%0. The isotopic composition of the sulphide is essentially that of the bulk sulphur isotopic composition of the solution, as can be seen from Fig. 4, under fo~ conditions defined by the basalts-quartzmagnetite-fayalite buffer. The reduction of sulphate by the basalts will be quantitative if the reduction is kinetic and quasi-quantitative if the process is reversible. Hence to generate sulphide with ~ 3 4 8 = +2.1 the seawater hydrothermal fluid must leach 224 mg of sulphur with 834S = +0.5 from the basalts per kg H 2 0 in addition to the 20 mg of reduced sulphur of marine origin. This quantity is quite realistic and the marine sulphur contribution is 8% of the total sulphur. Basaltic glasses reacted with initially sulphur depleted solutions liberated 64 mg S at 300°C and 597 mg S at 500aC [20].

5. C o n c l u s i o n s

This analysis of the sulphur isotope data for hydrothermal sulphides from both the project RITA and project RISE areas on the East Pacific

155

Rise indicates that the marine sulphate contribution is probably about 10% of the total sulphur in the hydrothermal solutions. A more refined analysis is not considered warranted until more detailed information is available on a number of important aspects of the hydrothermal system including measurements on the total sulphur content of the hydrothermal solutions which issue from the black and white vents and w a t e r / r o c k ratio data. Similarly such an analysis will have to take into account possible Rayleigh distillation effects during the sulphate precipitation processes, the assumed small fractionations between basaltic sulphide and the aqueous sulphide species, and the sulphur isotope composition of the residual sulphur remaining in the basalts after leaching. The source of the hydrothermal fluids has been assumed to be of seawater origin by analogy with hydrogen and oxygen isotope studies of oceanic and ophiolitic rocks [26-28]. Although volatiles of mantle origin occur in the RISE vents [3] there are currently no data on whether there is either a significant, measurable magmatic water or magmatic sulphur contribution. Nevertheless more detailed data here are considered unlikely to modify the interpreted minor importance of marine sulphate to the total hydrothermal sulphur budget. However, more sulphur isotope data on wellcharacterised samples from oceanic environments are required (1) to verify the generality of the 21°N hydrothermal systems, (2) to resolve the apparent differences reported for the sulphur isotope composition of the sulphides, (3) to define possible differences among the vents, and (4) to help elucidate the nature of the complex precipitation processes.

Acknowledgements

J. Francheteau is thanked for proposing the study of the RITA sulphides and F. Albarrde ("Alvin") and R. Hrkinian ("Cyana") for providing the samples. The technical expertise of B. Jacquier and the support of C.N.E.X.O.. (contract No. 79/5967) and C.N.R.S. are gratefully acknowledged.

Appendix--Calculation of the ratio S n / S vt ms'~ -- mH2Saq + m i l S msvl

(A-l)

mHSO,- + m s o 2

Noting the pH values generally measured relation (A-l) simplifies to: m s n -msvt

mHzSaq

(A-2)

mHSO4- + m s o ~

at 300°C [29]: a H +a so~ -- 10 - 7.06

(A-3)

aHsof

and: aso: a2+

/g2a.2s q

-

1048.55

(A-4)

combining (A-2), (A-3) and (A-4) gives: ms" - 10 -,,8.55 Yso~- a~ + mSvl

.,jrH2 Saq f(~22

10 -- 7"06~HSO~

×

1+ YSO~ a l l +

The activity coefficients, ~'i, are calculated by the Delta approximation method [29]. The mineralogical association observed at the end of the experiment defines the values for fo2 and aH. (Fig. 3a, b and c).

References

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Note added in proof After submitting our manuscript Styrt et al. [30] have published sulphur isotope analyses of sulphides from some of the same and similar "'Alvin" sites. Their data for pyrite and chalcopyrite range from + 1.3 to +4.1%c with a mean of about 2.9 and are thus typically enriched in 34S relative to our values. The explanation of this difference, whether due to the presence of micro-inclusions of anhydrite (see above), sample heterogeneity or analytical techniques is not currently known. We note. however, that the less 34S-rich the sulphides the smaller is the necessary seawater sulphate contribution to the total hvdrothermal sulphur.