Transfer of continental Mg, S, O and U to the mantle through hydrothermal alteration of the oceanic crust

Chemical Geology, 57 (1986) 1-15 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

1

TRANSFER OF CONTINENTAL Mg, S, O AND U TO THE MANTLE THROUGH HYDROTHERMAL ALTERATION OF THE OCEANIC CRUST* F. ALBARI~DE and A. MICHARD Centre de Recherches Pdtrographiques et Gdochimiques and E,cole Nationale Supdrieure de Gdologie, F-54501 Vandoeuvre-l~s-Nancy (France) (Accepted for publication July 4, 1986 )

Abstract Albar~de, F. and Michard, A., 1986. Transfer of continental Mg, S, 0 and U to the mantle through hydrothermal alteration of the oceanic crust. In: S. Deutsch and A.W. Hofmann (Editors), Isotopes in Geology - - Picciotto Volume. Chem. Geol., 57: 1-15. The chemistry of high-temperature vent fluids and deposits from the East Pacific Rise and other submarine hydrothermal fluids, the conditions prevailing during the hydrothermal process, and the calculation of fluxes associated with cycling of seawater through mid-ocean ridges are reviewed. Due to conductive cooling and low-temperature processes at the ridge flanks, only the hydrothermal flux of elements which are stripped out from seawater ( Mg, S, O, U) is known with a reasonable degree of confidence. A simple first-order model, in which these elements are transferred from the continental crust to the mantle through hydrothermal exchange at ridge crests, allows us to estimate the rate at which this transfer takes place. Mg should be 2-5 times less concentrated in the present-day crust than 4 Ga ago. Uptake of seawater sulfate during the hydrothermal process has to be qualitative and, through subsequent reduction, irreversible, as it is the only process which accounts for the high oxidation state of Fe in deep levels of the oceanic crust. A time constant of 100 Ma for continental sulfate removal indicates that S has to be massively reinjected into the continental crust by arc magmatism. Consequences for the secular evolution of sulfur isotopes in seawater are discussed. Progressive injection of oxidized basalt into the mantle explains why the terrestrial mantle is more oxidized than its extraterrestrial equivalents. U of the oceanic crust is increased by 20% during the hydrothermal process but loss of continental U through submarine hydrothermal alteration is negligible. However, hydrothermal leaching of Pb from the oceanic crust results in the subduction of oceanic layers with either high/l (altered basalts) and low # (hydrothermal and abyssal sediments) which may later contribute to the genesis of oceanic basalts with distinctive isotopic properties.

1. Introduction The discovery of hydrothermal activity at ridge crest turns out to be a major breakthrough in the understanding of major chemical *Contribution CRPG No. 635.

0009-2541/86/$03.50

exchanges between the mantle, the oceanic crust at spreading centers and seawater. This process is of prime importance in buffering the chemical composition of seawater for major constituents such as Mg ( E d m o n d et al., 1979a) or the isotopic evolution of oxygen ( Gregory and Taylor, 1981 ) and Sr (Albar~de et al., 1981 ) in the

© 1986 Elsevier Science Publishers B.V.

ocean. Hydrothermal alteration of the oceanic crust leads to subduction of lithospheric material which is significantly transformed relative to what solely magmatic processes at ridge crest would have produced. The return flow to the mantle of oxidized magmatic rocks enriched in Mg, U, depleted in Pb, transition elements, and eventually alkali elements must be taken into account for the generation of magmas at converging boundaries and for the long-term chemical evolution of the mantle. Thanks to water-rock interaction at ridge crest, irreversible leakage of a few elements from the continental crust through runoff and seawater towards the mantle (Mg, S, U) becomes a part of the global geochemical cycle and requires a dedicated appraisal. The purpose of this work is to review, in terms of flux and characteristic time, the state of knowledge for those hydrothermal exchanges which are relevant to the global mass balance of the oceanic crust-seawater system. It will be shown that some of these fluxes are strong enough to result in major changes of the composition of the mantle and continental crust for periods commensurate with the age of the Earth. Special attention will be paid to the chemical and isotopic effects which are expected from involvement of Mg, S, O and U in the hydrothermal alteration of the oceanic crust. Evolution of these elements in the continental crust through the geological time will be discussed in relationship with the known secular evolution of Sr and S isotopes in seawater. Effects of altered oceanic crust subduction will also be considered with respect to mantle geochemistry and isotope properties of basalts. 2. T h e c h e m i s t r y o f v e n t f l u i d s

Since the early discovery of warm hydrothermal vents in an axial ridge valley close to the Galdpagos Islands (Corliss et al., 1979), it has been thought that hydrothermal solutions represent seawater infiltrated at ridge flanks and spouted in the axial ridge valley after having reacted with rocks heated in the close vicin-

ity of the magma chamber. In spite of a considerable dilution by seawater simply reflected by a venting temperature of less than 20 ° C, Gal~pagos hydrothermal solutions enabled the following major thermal and chemical features of submarine hydrothermal systems to be outlined (Edmond et al., 1979a, b): (1) Comparison of the SiO2data with experimental isopleths indicate that water-rock interaction takes place at ~ 350°C and a few hundred meters below the seafloor. A useful parameter known as the water/rock ratio is used to assess the equivalent amount of water which exchanged heat or elements with 1 g of rock at equilibrium. It is well constrained by simple arguments on heat exchange (Corliss et al., 1979), removal of the most leachable alkali or alkaline-earth elements (Li, Rb, Ba) or equilibration of O, H and Sr isotopes (Craig et al., 1980; Albar~de et al., 1981 ) to lie within the range 1-5. (2) During the process, Mg and SO4 are quantitatively stripped off from seawater while alkali elements and those transition elements (Mn, Fe, Zn, Cu) which make most of the "hydrothermal sediments" well known on ridge flanks are strongly enriched relative to seawater. (3) The solutions are acid and sulfide-rich. (4) Large amounts of 3He (a primitive helium isotope) are dispersed in the overlying water column where they form hydrothermal plumes which can be traced over hundreds of kilometers; the conservative dilution of 3He by seawater may be used to infer world-wide elemental fluxes through the ridge hydrothermal system (Jenkins et al., 1978; Edmond et al., 1979a, 1982). The subsequent discovery of high-temperature hydrothermal vents and sampling of the nearly pure hydrothermal end-member on the East Pacific Rise (EPR) at 21 ° and 13°N (Spiess et al., 1980; H~kinian et al., 1983a, b), the Guaymas Basin (Lonsdale et al., 1980), and the Juan de Fuca Ridge (Von Damm and Bischoff, 1985) introduced the following additional features (Fig. 1 ) : (1) Anomalies of chlorine concentrations

Mn

Fe

/~1

H2S

H+

>10 5 10 4

H Y D R O T H E R M A L WATER SEAWATER

~

10 4

13°N

21°N

Ce

lO

Ba

4\.

si 10 2 /i • ~/}

?" \, ", . .

))

Li Rb

Yb

10

x\,

B Ca

H~

;,'

Br

CI

I

// \\.

,,\

\\,

\\,

0.1

~,/,

\\\ ,\\

Fig. 1. Summary of enrichment or depletion factors of hydrothermal solutions relative to seawater at the 21 ° and 1 3 ° N hydrothermal vent fields (East Pacific Rise ). Source of data: Edmond et al. (1979, 1982 ) for 21 ° N and Michard et al. (1984) for 13¢N.

both positive (Michard et al., 1984) and negative (Kim et al., 1984) have been observed and may be as large as a factor 2 in both directions; they have been ascribed to magma degassing (Michard et al., 1984), phase separation ( Kim et al., 1984; Bischoff and Pitzer, 1985), or adsorption/desorption processes (Berndt et al., 1985). By simple charge compensation, these anomalies are associated with large excess or deficit of cations (Ca, Sr, K, Na, etc.). (2) pH (as measured at 20 ° C) of the hightemperature solutions may be as low as 3.1 as a result of Mg metasomatism (Bischoff and Dickson, 1975) and anhydrite or hydroxysulfate precipitation (Bischoff and Seyfried, 1978); their alkalinity is slightly negative ( Von Damm et al., 1983). (3) Like Mg and SO4, uranium has been shown to be quantitatively reduced and stripped off from seawater (Michard et al., 1983; Michard and Albar~de, 1985 ). Light rare-earth elements and Eu are enriched in hydrothermal solutions over seawater by two or three orders of magnitude; however, this apparently huge enrichment of hydrothermal solutions leaves the amount of rare-earth elements (REE) sequestered by the oceanic crust virtually unaffected nor does it need to be considered as a significant component of the oceanic REE balance (Michard et al., 1983; Michard and Albar~de, 1986). Beyond simple common sense arguments, seawater origin of hydrothermal solutions has been reasonably well established from several lines of evidence: (1) The C1/Br ratio of hydrothermal solutions is not distinguishable from that of seawater (Michard et al., 1984). Since these anions are not significantly incorporated into silicate minerals, this ratio assigns a marine origin to the solutions but also argues strongly against secondary boiling. (2) (~180 and ~D of hydrothermal solutions are higher by ~ + 1.5 and + 2.5%o, respectively, than the local bottom seawater (Craig et al., 1980; Welhan and Craig, 1983; Merlivat et al.,

EPR

r:_z :I,

Pacific Seawater

Guaymas

[ !

-

2

4

"

I J

I

;

Sediments

MO Rg 12

10

8

6

-4

2

0

6

8

10

12

ENd Fig. 2. ENadata for hydrothermal solutions at Guaymas, EPR 21°N (Piepgras and Wasserburg, 1985) and 13°N (Miehard and Albar~de, 1986). Cross in square refers to an anhydrite sample from a black smoker at 13 : N.

1984); this is interpreted as resulting from equilibrium reaction of seawater with a mineral assemblage containing both plagioclase and sheet silicates (smectite, chlorite) at a water/ rock ratio of ~ 1. (3) Nd isotopes in hydrothermal solutions and hydrothermal deposits from both the 13 ° and the 21 °N on the EPR have ~-values of +4 to + 6 (Piepgras and Wasserburg, 1985; Michard and Albar~de, 1986) (Fig. 2 ) whereas, for any meaningful water/rock ratio, reaction would be expected to transfer the basalt value (10-13) to the solution. The most obvious explanation is reaction of downwelling pore solutions beginning with sediments from the ridge flanks ( e < - 6), a process which has now received a direct support from REE data on interstitial waters (Elderfield et al., 1986): to date, these results represent the most compelling evidence for a recharge of hydrothermal vent systems occurring at some distance from the ridge axis. As the contact with seawater, vent solutions have been observed to precipitate a variety of sulfide and sulfate minerals which form the "smokers" or "chimneys" now documented from many sites of the world ridge system (e.g., H~kinian et al., 1980; Arnold and Sheppard, 1981; Haymon and Kastner, 1981; Styrt et al., 1981; Oudin, 1983; H~kinian and Fouquet, 1985). For base metals (Fe, Mn, Cu, Zn, Pb, etc.), H~kinian et al. (1983b) have shown that only a small part (10-20%) of the dissolved and particulate hydrothermal load is used to build smokers or sedimented as nearby sulfide-rich

deposits: the largest proportion is disseminated over distances variable for each element and form the ridge flank hydrothermal sediments. Hydrothermal Mn plumes detected from hydrocast data are now commonly used at the exploratory stage as far away as few kilometers from the vents (Lupton et al., 1980). From high to low temperature (a scale which in general implies an increasing dilution by seawater at subsurface level or within the chimney nozzle), mineral assemblages are dominated by Cu-Fe-sulfides (chalcopyrite), Zn-sulfides (sphalerite) or sulfates (anhydrite, barite). ~34S-values of +1.5-3.0%~ have been interpreted as resulting from either reduction of seawater sulfate, extraction of basaltic sulfide, or a combination of both processes (Styrt et al., 1981; Arnold and Sheppard, 1981, respec.). Although stockworks of sulfide minerals have been recognized in Deep-Sea Drilling Project (DSDP) cores (Honnorez et al., 1985), direct observation at E P R 21 ° and 13°N show that high-temperature solutions are vented clear of precipitates: indeed, thermodynamic calculations suggest that the only phase saturating 21 ° N solutions at 350 ° C is silica (Bowers et al., 1985 ). Assumption of saturation with any particular mineral phase (e.g., pyrite, pyrrhotite) should therefore be made with utmost care. Although severe experimental uncertainties plague temperature measurements even for most recent dives, high-temperature sulfide minerals are most certainly deposited at ternperatures as high as 350 ° and probably 380 ° C. Anhydrite and barite must precipitate upon mixing in commensurate proportions of the SO4-free high-temperature component with seawater as also required by Sr isotopes (Albar~de et al., 1981 ). The complementary relationships of hydrothermal solutions and oceanic metamorphic rocks is difficult to assess as most dredge rocks have undergone extensive low-temperature exchange in addition to hydrothermal alteration. Mottl (1983) suggests that rocks in equilibrium with hot-spring solutions at a

water/rock-ratio of ~1 contain a mineral assemblage dominated by albite + chlorite + actinolite + epidote+ sphene. For water/rock ratios in excess of 50, only chlorite, quartz and Fe-Ti-oxides are left. Probably, the best example of hydrothermally altered oceanic rocks is found in the 900-1350-m section of the DSDP Hole 504B (Anderson et al., 1982), i.e. at a depth which compares well with that inferred from SiO2 barometry of vent solutions. For most elements, chemical changes are consistent with hydrothermal solution data: loss of SiO2, CaO, gains of MgO, oxidation of Fe. STSr/S6Sr ratios are shifted from 0.7025 up to 0.7040, dlsO from +5.5 C"J'~c to less than +4.5%c and ~D from 0 to -50%c (Friedrichsen, 1985) in good agreement with predictions from isotopic equilibrium at ~ 300-400°C with a water/rock ratio close to unity. An important point which has been underestimated hitherto is the extensive oxidation of Fe in altered basaltic rocks, ferric iron being held primarily by epidote and Feoxides. The Fe3+/FeTotal ratio of the altered rocks in the 900-1350-m section may be as high as 0.75 (Alt and Emmermann, 1985) whereas in fresh mid-ocean ridge basalt (MORB) glasses this ratio is ~ 0.0%0.08 ( Carmichael et al., 1985) (Fig. 3). The most obvious source of oxygen is reduction of either sulfate anions in downflowing seawater (Shanks et al., 1981; McDuff and Edmond, 1982 ) or anhydrite upon reaction with ferrous silicates (Bischoff and Dickson, 1975; Mottl et al., 1978; Ohmoto et al..

1983). 3. C o m p u t a t i o n of e l e m e n t a l f l u x e s Lister (1972) was probably the first author to suggest that a significant fraction of the world heat flow is driven out of the oceanic crust by hydrothermal convection instead of conduction, thereby triggering several attempts to detect temperature anomalies in bottom seawater (Sclater and Klitgord, 1973; Crane and Normark, 1977). From a survey of heat flow data along the ridge system and comparison

I

I

I

I

.g .8

.. . ; . . . .

.?

....

",..

~-s [ HOLE 504BJ .2 MOR9 glasses

L_

SO0 - -

:OlO0

! llO0 DEPTH

1200

- 13100

(meters)

Fig. 3. DSDP Hole 504B: Fe3+/FeTotalratio of basalt sampies from the depth zone thought to be equivalent to the hydrothermal reaction zone at ridge axis (Alt and Emmermann, 1985). Comparison with fresh MORB glass values ( Carmichaelet al., 1985) suggeststhat an abundant source of oxygenis required. Reduction of anhydrite from downflowing seawater to sulfide balances reasonably oxidation of ferrous to ferric iron. with purely conductive models, Wolery and Sleep (1976) estimated this fraction at ~ 20% ( ~4.0.1019 cal. a - l ) . Taking Craig et al.'s (1975) estimate of the total 3He flux at middepth in the world-wide ocean (4 atoms cm -2 s -1) and assuming the 3He/heat ratio of the Gal~ipagos hydrothermal solutions to be representative of the whole submarine hydrothermal activity, Jenkins et al. (1978) arrived at a surprisingly similar figure ( 4.9' 10 r~ cal. a 1). Additional 3He data from the 21°N vent field later confirmed this contention (Lupton et al., 1980). E d m o n d et al. (1979a, 1982) used chemical data from vent solutions in conjunction with the assumption of conservative dispersal of hydrothermal heat and ~He in seawater to infer global fluxes of several elements. For some of t h e m (Mg for instance), this mass-balance calculation resulted in a much improved closure of the geochemical cycle and may be accepted as one of the major breakthroughs in our knowledge of the oceanic fluxes. Similarly, the evolution of STSr/SSSr in the ocean throughout geological time may be quantified more accurately thanks to an estimate of the Sr cycling through the ridge hydrothermal system

(1.10 l° mol a -1) (Albar~de et al., 1981). The STSr/SSSr ratio of 0.710 deduced from this calculation for the continental runoff (Albar~de et al., 1981 ) is in good agreement with independent estimates based on continental geothermal systems (Elderfield and Greaves, 1981 ) and major drainage basins (Wadleigh et al., 1985). SiO2 geothermobarometry of vent solutions suggests that the last major chemical exchanges between hydrothermal solutions and the plutonic part of the oceanic crust take place 0.5-3.5 km below the seafloor, a depth that recent seismic data correlate with the top of an axial magma chamber (Reid et al., 1977; Riedesel et al., 1982; McClain et al., 1985). Michard et al. (1984) place the last equilibration stage within a high thermal gradient fringe around intrusions and dykes within or somewhat below the sheeted complex which feeds the basaltic flows of the upper-crustal layers. However, computation of oceanic fluxes using the 3He/heat constraints usually suffers an intrinsic uncertainty: Sleep and Wolery (1978) among others, pointed out that a large fraction of the hydrothermal activity may escape detection due to warm porous flow, conductive cooling and extensive dilution by seawater even at ridge crest. These limitations were also pointed out by E d m o n d et al. (1979a) and Hart and Staudigel (1982) who realized that there is not enough K in the oceanic crust to account for the hydrothermal flow calculated from vent data. Michard and Albar~de (1985) arrived at the same conclusion for Pb. Sleep et al. (1985) recently assessed the fraction of heat loss through black smokers to 10-20% of the total hydrothermal heat loss. Low-temperature reaction with wall rocks, deposition of various minerals (sulfides, sulfates, carbonates, silicates, etc.) and clogging of the conduits likely strip the hydrothermal solutions from alkali, alkaline-earth or transition elements which are thereby retained in the oceanic crust. Examination of stockwork mineral assemblages confirms the importance of these secondary processes (Anderson et al., 1982). Therefore, fluxes inferred for elements

which are removed from the crust must be taken as maximum values. In contrast, the uptake of elements from seawater during water-rock hydrothermal interaction is much likely independent of the cooling and venting stages: the 9-Ma time which Edmond et al. (1979a) and Michard et al. (1983) suggest for complete renewal of seawater Mg, SO4 and U, respectively, by hydrothermal removal at ridge crest depends only on assumptions on the global hydrothermal heat and 3He flux estimate and not on the removal mechanism itself. Sleep et al. (1985) have lowered this renewal (residence) time to 4 Ma but this difference probably represents the maximum uncertainty allowed for this parameter. 4. U p t a k e o f m a g n e s i u m , sulfate, o x y g e n and u r a n i u m at ridge crest: a model and c o n s e q u e n c e s for c o n t i n e n t a l crust evolution The fate of Mg, SO4, O and U deserves special attention for they are the only species quantitatively stripped off from seawater during the hydrothermal process. The importance of the hydrothermal sink for the seawater budget of these elements has been stressed by Edmond et al. (1979a) but, on the scale of geological times, their residence time in the ocean is fairly short and their transfer from the continental crust to the altered oceanic crust and the mantle through river and seawater almost instantaneous. Rate and extent to which the hydrothermal uptake of these elements affects their content in the continental crust may be calculated in a rough manner by using a firstorder model of transfer from the continental crust to the mantle in which hydrothermal alteration is the rate-limiting step. To account for a range of different models of continental crust evolution ( see a discussion in Veizer and Jansen, 1979), the rate of crustal growth from the mantle will be taken as an adjustable parameter with two extreme models: a first model ( model A ) in which all the crustal

material was created in a single event at some time in the early Archean and subsequently evolved as a closed system and a second model (model B) in which crustal mass increases linearly with time. The evolution of the concentration C in the continental crust through time t is given by: C=Co exp ( - t / r )

(model A)

C = C 0 [ 1 - e x p (-t/r)]/(t/r)

(model B)

where Co is the concentration of the pristine crustal material presumed to be extracted from the mantle, and r is the time constant characterizing the transfer of the element under consideration from the continental crust to the mantle. It will be taken as being equal to its present-day value, i.e. the ratio of the amount of the element held by the continental crust to the hydrothermal uptake of this element at ridge crest. Masses, concentrations, and fluxes of interest for this calculation are reported in Table I and the results displayed in Fig. 4.

4.1. Magnesium The Mg time constant r is ~ 2.5 Ga and compares with the ~ 2-Ga mean age of continents indicated by the residence time of Nd (Jacobsen and Wasserburg, 1979) and suggests a dramatic decrease of crustal Mg concentration in the Earth's history. If the continental crust formed starting 4 Ga ago from basaltic melts containing 10% MgO, the MgO content of the present-day crust should not exceed 2% for model A (no-growth) and 5% for model B (linear growth). Whereas hydrothermal uptake of Mg does not increase the concentration of the oceanic crust by more than 0.3%, this flux is nevertheless quite significant for the evolution of the continental crust. Hydrothermal alteration of the oceanic crust may therefore be considered as a first-order cause of the Mg-poor character of the continental crust. Whether the Ca/Mg ratio of sediments, such as carbonates

8 TABLE I Parameters for calculations

Moles held by continental crust Hydrothermal flux (mola 1) Time constant, r (a)

Mg

S

U

1.9.1022 (a)

4.6"1020(b)

6.10 ~8(c)

7.5'1012 (d) 2.5' 109

4.0"1012 (d) 1.2' 108

2.2'106 (e) 2.7.10~°

(a) assuming2.3% as the average Mg crustal value (Krauskopf,1967) and a continental mass of 2-1025g (Heydemann,1969); (b) Holser and Kaplan (1966); (c) calculatedfrom heat flowconstraints (see text); (d) Von Damm et al. (1985b); (e) Michard et al. (1983).

CONTINENT Mg, S AND U

-

I EVOLUT,ONOUETOC.EM,OAL ALTERATION

o

L

~1~

~

A

! I I

-- J

4 "

0{30

1.0 ~1~

4

3 t2 Age,

1

6

7

0.8 g

A

o 0.6 \ \

0 U

C = Co exp (-t/T)

B o

\

o4 k , ~

B~-~ ~

9to

°°otioe ~ 1

2

3

4

5

t/T

Fig. 4. Evolution of concentration in continental crust through time with two different models of crustal growth ( inse t) : suddenseparation early in the Archean (modelA ) or linear growth rate (modelB). Symbols and arrows refer to the depletion of the continental crust expected after 4 Ga. or shales, has varied or not through geological time (Vinogradov and Ronov, 1956; Van Moort, 1973 ) is a question which is still the subject of an active debate (see Holland, 1984, Chapters 5 and 6, for a recent account and references). Estimating the average Mg content of the Archean continental crust is a highly uncertain task as we have little idea of what the average Archean crust looked like. Reference to the sediment is not much more attractive as the bias of the average Mg sediment concentration rel-

ative to the average crust in an open system is also a major uncertainty. However, the present calculation suggests t h a t Mg depletion of the continental crust through geological time is probably far from being of trivial extent. An intriguing companion question is the apparent lack of relative Fe excess relative to Mg in the continental crust: Fe, which is almost equally concentrated as Mg in basaltic rocks, has a rather low concentration in crustal rocks and is not presently leaking to the mantle through hydrothermal systems. Veizer (1978) found indeed an increase of the Fe/A12Q ratio in Archean carbonates but suggests t h a t it probably relates to changing oxygen fugacity in the atmosphere and the hydrosphere through time. An alternative interpretation would be to emphasize the early formation of the crust during the Archean (whence model A is a better approximation t h a n model B ) when the atmosphere was more reducing t h a n today: ferrous Fe was efficiently removed from the crust as a soluble species then precipitated on the ocean floor as thick banded iron formations and, due to the high specific gravity of these rocks, dragged massively down to the mantle at subduction zones. 4.2. S u l f u r a n d oxygen

The fate of sulfur during the hydrothermal process is the subject of a long-standing debate. It is now commonly held t h a t sulfate precipi-

tation above 150°C severely limits sulfate ingress into the high-temperature reaction zone (Bischoff and Dickson, 1975). Direct reduction of seawater sulfate by basaltic ferrous iron is preferred by McDuff and Edmond (1982) and indeed laboratory experiments suggest that this reaction is fairly rapid (Shanks et al., 1981). This led Von Damm et al. (1985a) to the contention that sulfate hydrothermal mass balance is a kinetic problem: if seawater is rapidly heated above 150°C, 304 will be efficiently reduced and stored within the oceanic crust as sulfide minerals (with the exception of the small fraction which would be emitted by black smokers); in the case of a more sluggish flow and heating, most of the sulfate will be precipitated along the pathway to the high-temperature zones. In the latter case, off-ridge axis dissolution of anhydrite when temperature drops below 150 ° C would account for the scarcity of this mineral in deep oceanic bore-holes (Anderson et al., 1982). This anhydrite precipitation-dissolution process has been suggested to control the oxygen isotope composition of seawater sulfate (Holland, 1984, p. 474). Each of these models fails to take into account the oxidation state of the oceanic crust. Fe from the hydrothermally altered rocks observed in the deep part of DSDP Hole 504B (Alt and Emmermann, 1985) (Fig. 3) or, in general, in ophiolites is oxidized to 50-80%. Such an extensive oxidation is demonstrated by the occurrence of epidote, hematite or magnetite in those rocks and must be balanced with oxygen addition from seawater. Water reduction is certainly an inappropriate reaction as the maximum hydrogen ( + methane) flux calculated for the EPR 21 °N vent solutions from Welhan and Craig's data (1983) is 2.1011 mol a 1, i.e. two orders of magnitude smaller than required by Fe oxidation. Anhydrite precipitated at low temperature is likely to be reduced upon reaction with ferrous iron from basaltic rocks, sulfide minerals, or hydrothermal solutions (Bischoffand Dickson, 1975; Mottl et al., 1978). Although ~34S-values as high as + 9.6%c were

known from sulfides in altered mid-oceanic basalts (Grinenko et al., 1975), this process has also been recently advocated to account for rather high fi34S-values in kuroko ores (Ohmoto et al., 1983) and Juan de Fuca Ridge hydrothermal deposits (Shanks and Seyfried, 1985 ). If all seawater sulfate entering the system [ ~ 4' 1012 mol a-1 according to Edmond et al. (1979a) and Von Damm et al. (1985a)] is stripped off from the solution and not returned to seawater, this flux provides enough oxygen to oxidize ~ 40% of the Fe held by a 5-km-thick basaltic/gabbroic layer with 10% FeO. The semi-quantitative agreement between the flux of sulfate and the demand of oxygen to oxidize Fe to the extent which is observed in deep oceanic crust suggests that a large fraction of precipitated anhydrite is reduced within the hydrothermal system prior to cooling of the crust which therefore behaves as a sink for seawater sulfate. The strength of this sink cannot be far from Edmond et al.'s (1979a) estimate which, probably not coincidentally, equals the sulfate removal from the continental crust by runoff. The hydrothermal flux derived by Edmond et al. (1979a) may be combined with an estimate of the amount of sulfur (both reduced and oxidized) sequestered by continental crust to derive the time constant T characteristic of removal through hydrothermal processes. Using the estimate of 4.6'102o mol of Holser and Kaplan (1966) for continental sulfur, a T-value of ~ 100 Ma is derived. Both models A and B suggest therefore that for any acceptable initial concentration of sulfur in the continental crust, this element should have been quantitatively transferred from the continental crust to the mantJe. This major imbalance requires that a hidden return flow of sulfur from the oceanic crust to the continental crust exists with a fast turnover rate. Arc volcanics are known to be sulfur-rich and fi34S measured in anhydrite microphenocrysts fi'om E1 Chich6n may be as high as +9.2%~ (Rye et al., 1984). Up to now, the pre-

10

ferred interpretation of those high 534S-values favoured incorporation of sedimentary sulfur (evaporites) at shallow level in the continental crust. Recently, however, Ueda and Sakai (1984) reported a 534S-value of +9.8%0 for a basalt dredged in the intraoceanic Mariana arc. R.S. Harmon (pers. commun., 1985) has measured the sulfur isotope composition of mafic volcanics from continental (Aegean Sea) or intraoceanic arcs (Lesser Antilles, Marianas) and found 534S-value as high as + 15 to + 20%0. Such high values in arcs which may not be suspected to contain significant amounts of evaporites at depth (in particular for the Mariana arc) have the implication that sulfur added to the oceanic crust through hydrothermal alteration is outgassed from the oceanic crust by the process which generates arc magmas. Arc magmas are known to be often much more oxidized than ridge tholeiites (Gill, 1981, p. 109). Although subduction of oxidized oceanic crust with its sediments may account for an oxygen fugacity higher in the terrestrial mantle than in its extraterrestrial equivalents (Moon, meteorites; Basaltic Volcanism Study Project, 1981, pp. 378-381), the hydrothermal flux of oxygen remains orders of magnitude lower than the amount of Fe held by the mantle. This interpretation of the sulfur mass balance has major implications for understanding the secular evolution of 534S in oceanic sulfate and atmospheric oxygen pressure through the Phanerozoic. Some similarity between the evolution of STSr/S6Sr and 534S (Fig. 5) in seawater has been pointed out by Brass (1976), Holland (1984, p. 537) and Holser and Magaritz (1984). Extrapolation of present-day flux and isotopic exchange rate of Sr at ridge crest back in time suggests that minima of the STSr/S6Sr secular evolution curve may coincide with periods of increased submarine hydrothermal activity (Albar~de et al., 1981 ). If the secular evolution of STSr/S~Sr and 5a4S are somehow coupled, a common regulation mechanism calls certainly upon cycling of seawater through mid-ocean ridges. Fluctuations of sulfate reduction by

.

.

.

.

//

/t 3°

,/ /

J 25

/

!

~34S

I ~Claypool et 87Sr/86Srl \ \ ~

0707-

~j/

20

I

I~ ,:Koepnick et al., 1985)~5 /

0.706 - t-- T- c~- 7 7 I [ F] r, [ i > ~ T 100 200 300

~lsT 400

o I 500

c

q

TIME (Ma) Fig. 5. Comparison of secular evolution curves of ~:~4S ( Claypool et al., 1980) and 87Srff6Sr ( Koepnick et al., 1985 ).

changing submarine activity affects the oxygen level in the atmosphere and the general sulfate/sulfur mass balance at the surface of the Earth. Present-day hydrothermal uptake of oxygen by hydrothermal oxidation of basalts amounts to 8"1012 mol a 1 if oceanic sulfate entering the oceanic ridges is quantitatively reduced. This oxygen flux is small compared to short-term fluxes controlled by organic activity (Walker, 1977, Ch. 3; Garrels and Lerman, 1981), but oxygen removal from the dissolved oceanic sulfate and carbonate and from the atmosphere should be complete in 15 Ma unless the limestone/reduced organic carbon system acts as an oxygen buffer. Significant variations of the 534S in marine sulfate associated with fluctuations of submarine hydrothermal activity are therefore expected since both ultimate regulation mechanisms (erosion of carbonate and reduced sedimentary carbon and reinjection of sulfide/sulfate from subduction zone volcanoes) have longer characteristic times of 100-200 Ma. An integrated quantitative model of the S-O-C system which would extend that

11

of Garrels and Lerman (1981) for seawater cycling at mid-ocean ridges is beyond the scope of this work. Nonetheless, there is strong evidence from vent solutions and altered oceanic rocks that hydrothermal ridge processes control to some extent the external S-O-C system. 4.3. Uranium The analysis of U in warm Gal~ipagos vents (Edmond et al., 1979b; Krishnaswami and Turekian, 1982) provided the first direct evidence that this element is absent from the submarine hydrothermal solutions. Michard et al. (1983), Chen et al. (1983), Chen and Wasserburg (1985), Michard and Albar~de (1985) measured U concentrations in vent solutions from the EPR at 13 ° and 21 °N and the Juan de Fuca Ridge and found that the high-temperature component ( > 320°C) is essentially free of U. Hence, 3.3 ppb U are quantitatively stripped off from seawater upon transfer through the 350 °C isotherm, likely as a consequence of reduction during the hydrothermal process. A formal reaction could be written as: UO~ + +2Fe 2÷ + 4 H + U 4+ +2Fe 3+ + 2 H 2 0 which indicates that abundance of ferrous iron and low pH should convert massively uranyl to quadrivalent uranium with a rather low solubility much similar to Th and REE. The alkalinity drop should also enhance massive decomplexation of uranyl species by carbonate ions, which again displaces the above reaction to the right. As continental crust is strongly zoned for heat sources (Birch et al., 1968), its mean concentration in U is better estimated from indirect arguments. It will be assumed that ~ ~ of the continental heat flow from stable areas, i.e. ~ 50 ~W m 2, is produced by radioactive sources (U, Th, K) in the 25-km-thick continental crust. Approximately 40% of the crustal radiogenic heat is released by U. This would correspond to

a mean crustal U concentration of ~ 1 ppm (mostly concentrated in the few uppermost kilometers) and a ~-value of ~ 10 Ga. Hence, removal of U from continental crust through time is almost negligible. U concentration of seawater is kept at a rather low level likely not through hydrothermal removal at ridge crest but most likely by carbonate precipitation which cycles this element back to the continental crust. In contrast, the U content of the oceanic crust increases by ~20% solely through the hightemperature hydrothermal activity at midocean ridges (Michard et al., 1983). An alternative form of U uptake from seawater is lowtemperature alteration of basalts and sediments (Aumento, 1971). The strength of this sink is highly uncertain and estimates vary from 0.1" 108to 5" l0 s mol a-~ (Bloch, 1980; Hart and Staudigel, 1982). The depth at which oceanic crust undergoes U gain by low-temperature alteration is presently not known, neither is the proportion of this flux which actually represents U stripped at low temperature from seawater in the recharge zone of the axial ridge systems. Global U fluxes are therefore probably not known to better than an order of magnitude. Hydrothermal alteration has a potentially major relevance to basalt genesis because the altered oceanic basalts will be subducted with an excess of U and a large deficit ofPb ( Michard and Albar~de, 1985). A significant fraction of Pb contained in the oceanic crust will be removed at ridge crest, even if redeposition of sulfide upon conductive cooling of vent solutions limits the efficiency of the process. As shown by Pb isotope data from the EPR (Dasch, 1981), lead extracted by hydrothermal alteration is transferred to the ridge flank metalliferous sediments. The altered oceanic crust presents therefore two layers of contrasting l~: the high-/~ basaltic layer overlain by a veneer of low-/l hydrothermal+abyssal sediments. Following the original suggestion of Hofmann and White (1980), Michard and Albar~de (1985) have calculated that altered oceanic basalts

12

stored in the mantle for a period of 1 Ga or so may reproduce the Pb isotopic properties of the source of some ocean island basalts. Michard et al. (1986) have attributed the existence of a component with a very low 2°6pbff°4Pb ratio in basalts from the Rodrigues Triple Junction (Indian Ocean) to the presence of hydrothermal and abyssal sediments in their mantle source.

5. D i s c u s s i o n a n d c o n c l u s i o n s Some assumptions of the present model are probably crude, even in comparison with our ignorance of some parameters: (1) the continental crust "box" is assumed to be homogeneous and the concept of average continental crust composition differs obviously from the every-day concentration level familiar to the geochemist; (2) even if the crustal growth rate may be reasonably bracketed by models A and B, the assumption that primitive crustal material extracted from the mantle is of constant composition is crude. Recycling of continental crust in the mantle is neglected; (3) the present-day seawater cycling rate through mid-ocean ridges is uncertain, perhaps within a factor 2. The assumption that its present-day value may be extrapolated back in time is not well documented as the global heat flow or ridge length may have significantly changed in the past. However, the statement that, in the course of the Earth's history, Mg from the continental crust has decreased significantly, S has to be recycled massively through subduction zones, and U/Pb ratio of the subducted oceanic crust is highly heterogeneous, reflects robust orderof-magnitude conclusions. Their revision would require that some of the assumptions which are considered as "hard facts" of the hydrothermal research should be abandoned.

Acknowledgements This work has been made possible by the financial support from the Actions Thdma-

tiques Programm6es CNRS "G6ochimie-M6tallog6nie" and "Oc6anographie Chimique" and also from the Centre National pour l'Exploitation des Oc6ans (now IFREMER) through the incentive actions of H. Bougault and J. Francheteau. A1 Hofmann corrected the manuscript for a few major mistakes and dusted out some French colloquialisms. Many of the ideas discussed in this work have benefited from the expert advice of Gil Michard. Russ Harmon is gratefully acknowledged for making his ~34S data on arc volcanics available.

References Albar~de, F., Michard, A., Minster, J.F. and Miehard, G., 1981. S7Srff6Sr ratios in hydrothermal waters and deposits from the East Pacific Rise at 21"N. Earth Planet Sci. Lett., 55: 229-236. Alt, J.C. and Emmermann, R., 1985. Geochemistry of hydrothermally altered basalts: Deep Sea Drilling Project Hole 504B, Leg 83. Init. Rep. Deep-Sea Drill. Proj., 83: 249-262. Anderson, R.N., Honnorez, J., Becker, K., Adamson, A.C., Alt, J.C., Emmermann, R., Kempton, P.D., Kinoshita, H., Laverne, C., Mottl, M.J. and Newmark, R.L., 1982. DSDP Hole 504B, the first reference section over 1 km through layer 2 of the oceanic crust. Nature (London), 300: 589-594. Arnold, M. and Sheppard, S.M.F., 1981. East Pacific Rise at latitude 21 ° N: isotopic composition and origin of the hydrothermal sulphur. Earth Planet. Sci. Lett., 56: 148-156. Aumento, F., 1971. Uranium content of mid-oceanic basalts. Earth Planet. Sci. Lett., 11: 90-94. Basaltic Volcanism Study Project, 1981. Basaltic Volcanism on the Terrestrial Planets. Pergamon, New York, N.Y., 1286 pp. Berndt, M.E., Seyfried, W.E. and Janecky, D.R., 1985. Behaviour of C1 during hydrothermal alteration of basalt in the low pressure supercritical region. EOS (Trans. Am. Geophys. Union ), 66 ( 46 ) : 921 (abstract). Birch, F., Roy, R.F. and Decker, E.R., 1968. Heat flow and thermal history in New England and New York. In: E. an-Zen (Editor), Studies of Appalachian Geology. Interscience, New York, N.Y., pp. 437-451. Bischoff, J.L. and Dickson, F.W., 1975. Seawater-basalt interaction at 200°C and 500 bars: implications for seafloor heavy-metal deposits and regulation of seawater chemistry. Earth Planet. Sci. Lett., 25: 385-397. Bischoff, J.L. and Pitzer, K.S., 1985. Phase relations and

13 adiabats in boiling seafloor geothermal systems. Earth Planet. Sci. Lett., 75: 327-338. Bischoff, J.L. and Seyfried, W.E., 1978. Hydrothermal chemistry of seawater from 25 ° C to 350 ° C. Am. J. Sci., 278: 838-860. Bloch, S., 1980. Some factors controlling the concentration of uranium in the world ocean. Geochim. Cosmochim. Acta, 44: 373-377. Bowers, T.S., von Damm, K.L. and Edmond, J.M., 1985. Chemical evolution of mid-ocean ridge hot springs. Geochim. Cosmochim. Acta, 49: 2239-2252. Brass, G.W., 1976. The variation of the marine STSr/S~Sr ratio during Phanerozoic time: Interpretation using a flux model. Geochim. Cosmochim. Acta, 40: 721-730. Carmichael, I.S.E., Christie, D.M. and Langmuir, C.H., 1985. Ferric/ferrous ratios and oxygen fugacities of MORB glasses. EOS (Trans. Am. Geophys. Union ), 66: 1108 (abstract). Chen, J.H. and Wasserburg, G.J., 1985. U, Th, and Pb isotopes in hydrothermal fluids from the Juan de Fuca Ridge. EOS (Trans. Am. Geophys. Union), 66:929 (abstract). Chen, J.H., Wasserburg, G.J., von Damm, K.L. and Edmond, J.M., 1983. Pb, U, and Th in hot springs on the East Pacific Rise at 21 °N and Guaymas Basin, Gulf of California. EOS (Trans. Am. Geophys. Union), 64: 724 (abstract). Claypool, G.E., Holser, W.T., Kaplan, I.R. and Zak, I., 1980. The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation. Chem. Geol., 28: 199-260. Corliss, J.B., Dymond, J., Gordon, L.I., Edmond, J.M., von Herzen, R.P., Ballard, R.D., Green, K., Williams, D., Bainbridge, A., Crane, K. and van Andel, Tj.H., 1979. Submarine thermal springs on the Galdpagos rift. Science, 203: 1073-1083. Craig, H., Clarke, W.B. and Beg, M.A., 1975. Excess 3He in deep water on the East Pacific Rise. Earth Planet. Sci. Lett., 26: 125-132. Craig, H., Welhan, J.A., Kim, K., Poreda, R. and Lupton, J.E., 1980. Geochemical studies of the 21°N EPR hydrothermal fluids. EOS (Trans. Am. Geophys. Union), 61:992 (abstract). Crane, K. and Normark, W.R., 1977. Hydrothermal activity and crustal structure of the East Pacific Rise. J. Geophys. Res., 82: 5336-5348. Dasch, E.J., 1981. Lead isotopic composition of metalliferous sediments from the Nazca plate. Geol. Soc. Am. Mem., 154: 199-209. Edmond, J.M., Measures, C., Mangum, B., Grant, B., Sclater, F.R., Collier, R., Hudson, A., Gordon, L.I. and Corliss, J.B., 1979a. On the formation of metal-rich deposits at ridge crests. Earth Planet. Sci. Lett., 46: 19-30. Edmond, J.M., Measures, C., McDuff, R.E., Chan, L.H.,

Collier, R., Grant, B., Gordon, L.I. and Corliss, J.B., 1979b. Ridge crest hydrothermal activity and the balances of the major and minor elements in the ocean: the Gal~pagos data. Earth Planet. Sci. Lett., 46: 1-18. Edmond, J.M., von Damm, K.L., McDuff, R.E. and Measures, C.I., 1982. Chemistry of hot springs on the East Pacific Rise and their effluent dispersal. Nature (London), 297: 187-191. Elderfield, H. and Greaves, M.J., 1981. Strontium isotope geochemistry of Icelandic geothermal systems and implication for sea water chemistry. Geochim. Cosmochim. Acta, 45: 2201-2212. Elderfield, H., Kennedy, H., Pagett, R. and Ridout, P., 1986. A preliminary study of rare earth elements in pore waters of deep-sea sediments. (Can.) Dep. Environ., Ottawa, Ont., Rep. (in press). Friedrichsen, H., 1985. Strontium, oxygen, and hydrogen isotope studies on primary and secondary minerals in basalts from the Costa Rica Rift, Deep Sea Drilling Project Hole 504B, Leg 83. Init. Rep. Deep-Sea Drill. Proj., 83: 289-295. Garrels, R.M. and Lerman, A., 1981. Phanerozoic cycles of sedimentary carbon and sulfur. U.S. Natl. Acad. Sci., Proc., 78: 4652-4656. Gill, J., 1981. Orogenic Andesites and Plate Tectonics. Springer, Berlin, 392 pp. Gregory, R.T. and Taylor, H.P., 1981. An oxygen isotope profile in a section of Cretaceous oceanic crust, Samail ophiolite, Oman: Evidence for 180 buffering of the oceans by deep ( > 5 km) seawater-hydrothermal circulation. J. Geophys. Res., 86: 2737-2755. Grimaud, D., Michard, A. and Michard, G., 1984. Compo~ sition chimique et composition isotopique du strontium dans les eaux hydrothermales sous-marines de la dorsale Est Pacifique ~ 13 ~ Nord. C. R. Acad. Sci., Paris, 299: 865-870. Grinenko, V.A., Dimitriev, L.V., Migdisov, A.A. and Sharas'kin, A.Y., 1975. Sulfur contents and isotope compositions for igneous and metamorphic rocks from midocean ridges. Geochem. Int., 12: 132-137. Hart, S.R. and Staudigel, H., 1982. The control of alkalies and uranium in seawater by ocean crust alteration. Earth Planet. Sci. Lett., 58: 202-212. Haymon, R.M. and Kastner, M., 1981. Hot spring deposits on the East Pacific Rise at 21 °N: preliminary description of mineralogy and genesis. Earth Planet. Sci. Lett., 53: 363-381. H~kinian, R. and Fouquet, Y., 1985. Volcanism and metallogenesis of axial and off-axial structures on the East Pacific Rise near 13 °N. Econ. Ecol., 80 (2): 221-249. H~kinian, R., F~vrier, M., Bischoff, J.L., Picot, P. and Shanks, W.C., 1980. Sulfide deposits from the East Pacific Rise near 21 °N. Science, 207: 1433-1444. H~kinian, R., F6vrier, M., Avedik, F., Cambon, P., Char-

14 lou, J.-L., Needham, H.D., Raillard, J., Boul~gue, J., Merlivat, L., Moinet, A., Manganini, S. and Lange, J., 1983a. East Pacific Rise near 13°N: geology of new hydrothermal fields. Science, 219: 1321-1324. Hdkinian, R., Francheteau, J., Rdnard, V., Ballard, R.D., Coukroune, P., Cheminde, J.L., Albar~de, F., Minster, J.-F., Charlou, J.L., Marty, J.C. and Boul~gue, J., 1983b. Intense hydrothermal activity at the axis of the East Pacific Rise near 13°N: submersible witnesses the growth of sulfide chimney. Mar. Geophys. Res., 6: 1-14. Heydemann, A., 1969. Tables. In: K.H. Wedepohl (Editor), Handbook of Geochemistry, Ch. 12 I. Springer, Berlin, pp. 376-412. Hofmann, A.W. and White, W.M., 1980. The role of subducted oceanic crust in mantle evolution. Carnegie Inst. Washington, Yearb., 79: 477-483. Holland, H.D., 1981. The Chemical Evolution of the Atmosphere and Oceans, Princeton University Press, Princeton, N.J., 584pp. Holser, W.T. and Kaplan, I.R., 1966. Isotope geochemistry of sedimentary sulfates. Chem. Geol., 1: 93-135. Holser, W.T. and Magaritz, M., 1984. Chemical events near the Permian-Triassic boundary. EOS (Trans. Am. Geophys. Union), 65 (16 ): 297 {abstract). Honnorez, J., Alt, J.C., Honnorez-Guerstein, B.M., Laverne, C., Muehlenbachs, K., Ruiz, J. and Saltzman, E., 1985. Stockwork like sulfide mineralization in young oceanic crust: Deep Sea Drilling Project Hole 504B. Init. Rep. Deep-Sea Drill. Proj., 83: 263-282. Ito, E., Harris, D.M. and Anderson, A.T., 1983. Alteration of oceanic crust and geologic cycling of chlorine and water. Geochim. Cosmochim. Acta, 47: 1613-1624. Jacobsen, S.B. and Wasserburg, G.J., 1979. The mean age of mantle and crustal reservoirs. J. Geophys. Res., 84: 7411-7428. Janecky, D.R. and Seyfried, W.E., 1983. The solubility of magnesium-hydroxide-sulfate-hydrate in seawater at elevated temperatures and pressures. Am. J. Sci., 283: 831-860. Jenkins, W.J., Edmond, J.M. and Corliss, J.B., 1978. Excess :~He and 4He in Gal~ipagos submarine hydrothermal waters. Nature (London), 272: 156-158. Kim, K.-R., Welhan, J.A. and Craig, H., 1984. The hydrothermal vent fields at 13 ° N and 11 ° N on the East Pacific Rise: ALVIN 1984 results. EOS (Trans. Am. Geophys. Union), 65:973 (abstract). Koepnick, R.B., Burke, W.H., Denison, R.E., Hetherington, E.A., Nelson, H.F., Otto, J.B. and Waite, L.E., 1985. Construction of the seawater 87Sr/S6Sr curve for the Cenozoic and Cretaceous: supporting data. Chem. Geol. (Isot. Geosci. Sect. ), 58: 55-81. Krauskopf, K., 1967. Introduction to Geochemistry. McGraw-Hill, New York, N.Y., 721 pp. Krishnaswami, S. and Turekian, K.K., 1982.2~8U, 226Ra

and 21°pb in some vent waters of the Galfipagos spreading center. Geophys. Res. Lett., 9 (8): 827-830. Lister, C.R.B., 1972. On the thermal balance of an oceanic ridge. Geophys. J.R. Astron. Soc., 26: 515-535. Lonsdale, P.F., Bischoff, J.L., Burns, V.M., Kastner, M. and Sweeney, R.E., 1980. A high-temperature hydrothermal deposit in the seabed at a Gulf of California spreading center. Earth Planet. Sci. Lett., 49: 8-20. Lupton, J.E., Klinkhamer, G.P., Normark, W.R., Haymon, R., Macdonald, K.C., Weiss, R.F. and Craig, H., 1980. Helium-3 and manganese at the 21 ° N East Pacific Rise hydrothermal site. Earth Planet. Sci. Lett., 50:115-127. McClain, J.S., Orcutt, J.A. and Burnett, M., 1985. The East Pacific Rise in cross section: a seismic model, J. Geophys. Res., 90 (B10) : 8627-8639. McCulloch, M.T., Gregory, R.T., Wasserburg, G.J. and Taylor, H.P., 1981. Sm-Nd, Rb-Sr, and lsO/1~O systematics in an oceanic crustal section: evidence from the Samail ophiolite. J. Geophys. Res., 86: 2721-2735. McDuff, R.E. and Edmond, J.M., 1982. On the fate of sulfate during hydrothermal circulation at mid-ocean ridges. Earth Planet. Sci. Lett., 57: 117-132. Merlivat, L., Andrie, C. and Jean-Baptiste, P., 1984. Distribution des isotopes de l'hydrog~ne, de l'oxyg~ne et de l'helium dans les sources hydrothermales de la ride EstPacifique h 13°N. C.R. Acad. Sci., Paris, 299: 1191-1196. Michard, A. and Albar~de, F., 1985, Hydrothermal uranium uptake at ridge crests. Nature (London), 317: 344-346. Michard, A. and Albar~de, F., 1986. The REE content of some hydrothermal fluids. Chem. Geol., 55: 51-60. Michard, A., Albar~de, F., Michard, G., Minster, J.F. and Charlou, J.L., 1983. Rare-earth elements and uranium in high-temperature solutions from East Pacific Rise vent field (13°N). Nature (London), 303: 795-797. Michard, G., Albar~de, A., Michard, A., Minster, J.-F., Charlou, J.-L. and Tan, N., 1984. Chemistry of solutions from the 13 ° N East Pacific Rise hydrothermal site. Earth Planet Sci. Lett., 67: 297-307. Michard, A., Montigny, R. and Schlich, P., 1986. Geochemistry of the mantle beneath the Rodrigues Triple Junction and the South-East Indian Ridge. Earth Planet. Sci. Lett., 78: 104-114. Mottl, M.J., 1983. Metabasalts, axial hot springs, and the structure of hydrothermal systems at mid-ocean ridges. Geol. Soc. Am. Bull., 94: 161-180. Mottl, M.J. and Holland, H.D., 1978. Chemical exchange during hydrothermal alteration of basalt by seawater, I. Experimental results for major and minor components of seawater. Geochim. Cosmochim. Acta, 42:1103-1115. Ohmoto, H., Mizukami, M., Drummond, S.E., Eldridge, C.S., Pisutha-Arnond, V. and Lenagh, T.C., 1983. Chemical processes of Kuroko formation. Econ. Geol., Mem. No. 5, pp. 570-604.

15 Oudin, E., 1983. Hydrothermal sulfide deposits of the East Pacific Rise (21°N) - - Descriptive mineralogy. Mar. Mining, 4: 39-72. Piepgras, D.J. and Wasserburg, G.J., 1985. Strontium and neodymium isotopes in hot springs on the East Pacific Rise and Guaymas Basin. Earth Planet. Sci. Lett., 72: 341-356. Reid, I.D., Orcutt, J.A. and Prothero, W.A., 1977. Seismic evidence for a narrow zone of partial melting underlying the East Pacific Rise at 21 °N. Geol. Soc. Am. Bull., 88: 678-682. Riedesel, M., Orcutt, J.A., McDonald, K.C. and McClain, J.S., 1982. Microearthquakes in the black smoker hydrothermal field, East Pacific Rise at 21 ~N. J. Geophys. Res., 87: 613-623. Rye, R.O., Luhr, J.F. and Wasserman, M.D., 1984. Sulfur and oxygen isotopic systematics of the 1982 eruptions of E1 Chich6n volcano, Chiapas, Mexico. J. Volcanol. Geotherm. Res., 23: 109-123. Sclater, J.G. and Klitgord, K.D., 1973. A detailed heat flow, topographic, and magnetic survey across the Gal~pagos spreading center at 86°W. J. Geophys. Res., 78: 6951-6965. Seyfried, W. and Bischoff, J.L., 1977. Hydrothermal transport of heavy metals by seawater: the role of seawater/basalt ratio. Earth Planet. Sci. Lett., 34: 1-77. Shanks, W.C. and Seyfried, W.E., 1985. Sulfur isotopic fractionation, anhydrite dissolution, and sulfate reduction: Juan de Fuca Ridge and other seafloor hydrothermal vent systems. EOS (Trans. Am. Geophys. Union), 66 ( 46 ) : 928-929 (abstract). Shanks III, W.C., Bischoff, J.L. and Rosenbauer, R.J., 1981. Seawater sulfate reduction and sulfur isotope fractionation in basaltic systems: interaction of seawater with fayalite and magnetite at 200-350°C. Geochim. Cosmochim. Acta, 45: 1977-1995. Sleep, N.H. and Wolery, T.J., 1978. Egress of hot water from midocean ridge hydrothermal systems: some thermal constraints. J. Geophys. Res., 83: 5913-5922. Sleep, N.H., Morton, J.L. and Wilson, D.S., 1985. Nearaxial thermal structure and hydrothermal circulation. EOS (Trans. Am. Geophys. Union), 66(46): 920 (abstract). Spiess, F.N., McDonald, K.C., Atwater, T., Ballard, R.D., Carranza, A., Cordoba, D., Cox, C., Diaz-Garcia, V.M., Francheteau, J., Guerrero, J., Hawkins, J., Haymon, R., Hesler, R., Juteau, T., Kastner, M., Larson, R., Luyendick, B., McDougall, J.D., Miller, S., Normark, W.R., Orcutt, J. and Rangin, C., 1980. East Pacific Rise: hot springs and geophysical experiments. Science, 207: 1421-1432. Styrt, M.M., Brackmann, A.J., Holland, H.D., Clark, B.C.,

Pisutha-Arnond, V., Eldridge, C.S. and Ohmoto, H., 1981. The mineralogy and the isotopic composition of sulfur in hydrothermal sulfide/sulfate deposits on the East Pacific Rise, 21 ° N latitude. Earth Planet. Sci. Lett., 53: 382-390. Ueda, A. and Sakai, H., 1984. Sulfur isotope study of Quaternary volcanic rocks from the Japanese Islands arc. Geochim. Cosmochim. Acta, 48: 1837-1848. Van Moort, J.C., 1973. The magnesium and calcium contents of sediments especially pelites, as a function of age and degree of metamorphism. Chem. Geol., 12: 1-37. Veizer, J., 1978. Secular variations in the composition of sedimentary carbonate rocks, II. Fe, Mn, Ca, Mg, Si and minor constituents. Precambrian Res., 6: 381-413. Veizer, J. and Jansen, S.L., 1979. Basement and sedimentary recycling and continental evolution. J. Geol., 87: 341-370. Vinogradov, A.P. and Ronov, A.B., 1956. Evolution of the chemical composition of clays of the Russian Platform. Geochemistry (U.S.S.R.), 2: 123-129. Von Damm, K.L. and Bischoff, J.L., 1985. Southern Juan de Fuca Ridge hot spring chemistry. EOS (Trans. Am. Geophys. Union), 66 (46) : 926 ( abstract ). Von Damm, K.L., Grant, B. and Edmond, J.M., 1983. Preliminary report on the chemistry of hydrothermal solutions at 21 ° North-East Pacific Rise. In: P.A. Rona, K. BostrSm, L. Laubier and K.L. Smith, Jr. (Editors}, Hydrothermal Processes at Seafloor Spreading Centers. Plenum, New York, N.Y., pp. 369-389. Von Damm, K.L., Edmond, J.M., Grant, B., Measures, C.I., Walden, B. and Weiss, R.F., 1985a. Chemistry of submarine hydrothermal solutions at 21 N , East Pacific Rise. Geochim. Cosmochim. Acta, 49: 2197-2220. Von Damm, K.L., Edmond, J.M., Measures, C.I. and Grant, B., 1985b. Chemistry of submarine hydrothermal solutions at Guaymas Basin, Gulf of California. Geochim. Cosmochim. Acta, 49: 2221-2237. Wadleigh, M.A., Veizer, J. and Brooks, C., 1985. Strontium and its isotopes in Canadian rivers: fluxes and global implications. Geochim. Cosmochim. Acta, 49: 172%1736. Walker, J.C.G., 1977. Evolution of the Atmosphere. Macmillan, New York, N.Y., 318 pp. Welhan, J.A. and Craig, H., 1983. Methane, hydrogen and helium in hydrothermal fluids at 21°N on the East Pacific Rise. In: P.A. Rona, K. BostrSm, L. Laubier and K.L. Smith, Jr. (Editors), Hydrothermal Processes at Seafloor Spreading Centers. Plenum, New York, N.Y., pp. 391-409. Wolery, T.J. and Sleep, N.H., 1976. Hydrothermal circulation and geochemical flux at mid-ocean ridges. J. Geol., 84: 249-275.