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Gypsum and halite from the Mid-Atlantic Ridge, DSDP Site 395
98
Earth and Planetary Science Letters, 42 (1979) 98-102 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [61
GYPSUM AND HALITE FROM THE MID-ATLANTIC RIDGE, DSDP SITE 395 J A M E S I. D R E V E R 1, J A M E S R. L A W R E N C E 2 and R O N A L D C. A N T W E I L E R 1 1 Geology Department, University of Wyoming, Laramie, WY 82071 (U.S.A.) 2 Lamont-Doherty Geological Observatoo,, Palisades, N Y 10964 (U.S.A.)
Received July 25, 1978 Revised version received October 2, 1978
Gypsum and halite crystals, together with saponite and phillipsite, were found in a vein in a basalt sill 625 m below the sea floor at DSDP Site 395A, located 190 km west of the crest of the Mid-Atlantic Ridge. The 6345 value of the gypsum (+19.4%o) indicates a seawater source for the sulfate. The 618 O values of the saponite (+19.9%~J) and phillipsite (+18.1%~,) indicate either formation from normal seawater at about 55°C or formation from 1sO_depleted seawater at a lower temperature. The gypsum (which could be secondary after anhydrite) was formed by reaction between Ca 2÷ released from basalt and SO42-in ckculating seawater. The halite could have formed when water was consumed by hydration of basalt under conditions of extremely restricted circulation. A more probable mechanism is that the gypsum was originally precipitated as anhydrite at temperatures above 60°C. As the temperature dropped the anhydrite converted to gypsum. The conversion would consume water, which could cause halite precipitation, and would cause an increase in the volume of solids, which would plug the vein and prevent subsequent dissolution of the halite.
1. Introduction The circulation of seawater through oceanic spreading centers has been invoked as a major process in the transport of heat [1], the genesis o f metal-rich sediments [2], and the regulation o f the major-elem e n t chemistry o f seawater [1,2]. A l t h o u g h a few samples of warm water emerging from the Galapagos spreading center have been collected and analyzed [3] the best m e t h o d presently available for determining the distribution o f t e m p e r a t u r e and chemical c o m p o s i t i o n in waters circulating in the oceanic crust is probably e x a m i n a t i o n o f the minerals p r o d u c e d by reaction o f the waters with basalt. In a previous paper [4] we suggested, on the basis o f isotopic analysis o f calcite and saponite, that the waters circulating through the basalt at Site 395 were relative cold (less than 15°C for calcites; less than 50°C for saponites), and that the circulation must have been relatively open, that is to say the effective ratio o f water to rock must have been high. In the present paper, however, we shall d o c u m e n t that, at least locally, secondary minerals were deposited at elevated tempera-
tures or under conditions o f highly restricted circulation.
2. Geological setting DSDP Site 395 is located a p p r o x i m a t e l y 140 km west o f the crest of the Mid-Atlantic Ridge at latitude 2 2 ° 4 5 . 3 5 ' N . Water depth was 4485 m. The age o f the crust at the site is a p p r o x i m a t e l y 7 m.y. (magnetic anomaly 4). The sample described here (395A-63-4 No. 1 L, a p p r o x i m a t e l y 625 m below sea floor) is a vein located 4.3 m above the base o f a 30 m thick sill composed o f plagioclase-olivine-clinopyroxene doleritic basalt. The sill is intensely fractured and alteration products are a b u n d a n t in the fractures [5].
3. Results The sample available for study was a vein which appeared to be filled with clear gypsum and m i n o r amounts o f saponite and phillipsite. The mineralogi-
99
Fig. 1. Scanning electron micrographs of vein material from sample 395A-63-4 No. 1L. (a), (b) Halite cube and gypsum needles on saponite substrate. (c), (d) Halite cube with saponite (right) and gypsum (left).
100 cal identifications were confirmed by X-ray diffraction and semi-quantitative X-ray energy spectrometry. The gypsum appeared to be a single crystal with dimensions approximately 20 × 10 X 5 ram. 5 mm represents the width of the vein; the true extent of the gypsum in the other directions could not be determined from the small fragment samples. By "single crystal" we mean that the material was completely clear and had the same cleavage directions throughout. The shape of the crystal boundaries was controlled by the shape of the fracture in the basalt; no crystal faces were visible. Examination with the scanning electron microscope (Fig. 1) showed the presence o f minor amounts o f halite in addition to the gypsum. The halite crystals are clearly intergrown with the smectite and gypsum, and are not a result of simple evaporation of seawater during handling. It is inconceivable that the gypsum could have been an artifact introduced during handling because approximately 500 ml of seawater would have to have evaporated to produce the observed amount of gypsum. Halite, however, is present in only trace amounts. The evidence that it is not an artifact is the textural relationships shown in Fig. 1. Samples of the gypsum, saponite and phillipsite were separated from the vein by hand picking for isotopic analysis (Table 1). The 634S of the gypsum is close to that of present-day seawater sulfate, indicating that the sulfur in the gypsum was probably derived from seawater. The ~180 value of the saponite (Table 1) indicates a temperature of formation of 55°C for formation from normal seawater (8180 = 0%~) (fractionation factor from Yeh and Savin [6]). The isotopic composition of the phillipsite indicates a similar tempera-
TABLE 1 Isotopic analyses of samples from veins containing gypsum
~345 (%0)
6180 SMOW(%0)
395A.63-4 No. 1L
Gypsum * Saponite Phillipsite
+19.4 +19.9 +18.1
395A-66-3, 125 em No. 12
Saponite * Determined by W.C. Shanks.
+21.3
ture, although the phillipsite-water fractionation factor is not well known. A small amount of gypsum was found intergrown with saponite in another vein (395A-66-3, 125 cm No. 12) from approximately 10.4 m below the base of the sill. The 6180 of saponite from this vein (Table 1) indicates a temperature of formation from normal seawater of 44°C.
4. Discussion Since boiling and evaporation are impossible under several kilometers of seawater, the "evaporite" minerals must have been formed by reactions between seawater and basalt. We shall present two possible mechanisms which are consistent with the data. Mechanism 1. This mechanism is based on reaction
between a fixed volume of water and a large volume of basalt at low (below 60°C) temperatures, with essentially no circulation of water. When basalt reacts with seawater at low temperatures, calcium is released from the basalt. Initially, the calcium reacts with carbonate species in seawater to form calcite. When all carbonate species are consumed, dissolved calcium will increase until gypsum precipitates. Halite precipitation cannot be explained by a similar mechanism. Alteration of basalt [7,8] causes either a slight uptake of sodium by the basalt or no significant uptake or actual release of sodium depending on water/rock ratio. There is no evidence that alteration of basalt could release sufficient sodium for seawater to become saturated with respect to halite. However, when basalt is altered, water is consumed by hydration reactions. Since chloride is not significantly involved in alteration reactions, the chloride concentration in the remaining water must increase as hydration reactions proceed. The remaining water could eventually become saturated with respect to halite, and halite would precipitate. Sayles and Manheim [9] invoked this mechanism to explain the increase in the chloride contents of some interstitial waters in marine sediments. The ~ 180 values of the saponites would be interpreted in this model as indicating not precipitation from normal seawater (alSO = 0%~) at slightly elevated temperatures, but precipitation at low temperatures (perhaps 20°C)
101 from seawater which had been depleted in 180 as a consequence of hydration reactions such as conversion of basalt to smectite. There are two problems with this mechanism. For halite saturation to be achieved, 90% of the original seawater would have to be removed by hydration reactions. If this happened at about 20°C, the mean 6x80 of the alteration products products (assuming the alteration product is mostly smectite) should be about 14%0. The calculation procedure is explained in Lawrence et al. [10]. The observed 6180 values of the saponites associated with gypsum (+ 19.9%~ and +21.3%0) are much higher than the calculated mean value. It is possible, of course, that the bulk of the phillipsite and smectite were formed earlier than the halite, before the water became highly depleted in 180. A second problem is that each millilitre of gypsum represents the sulfate in about 550 ml of seawater, so that to form a vein filled with gypsum, there must have been considerable movement of water on a local scale. On the other hand, there must have been essentially no mixing with ordinary seawater, otherwise halite saturation would not have been achieved. Isotopic studies of calcite veins from the same region of the core show evidence for extensive circulation of seawater whose isotopic composition was not significantly different from 0%~ [4].
Mechanism 2. As the sill cooled, cracks formed and water of approximately seawater composition circulated through the cracks. Saponite, phillipsite, and anydrite precipitated and continued to precipitate as the system cooled. In the experiments of Bischoff and Dickson [2] anhydrite apparently precipitated when seawater alone was heated to 200°C. When seawater and basalt were heated together to 200°C, anhydrite was rapidly precipitated in large quantities by Ca 2÷ released from basalt combining with SO42- in the seawater. The equilibrium temperature for the gypsum/ anhydrite transition vnder present in-situ conditions is about 70°C in normal seawater, 60°C in seawater "evaporated" to gypsum saturation and 30°C in saturate NaCI solution, assuming the pressure on both solids and solution is hydrostatic. When the temperature dropped below about 60°C, the anhydrite transformed to gypsum, presumably by a solution/precipi-
tation mechanism. The effect of the transformation of anhydrite to gypsum is to consume approximately 0.78 ml of water per millilitre of anhydrite, and to cause an increase in the volume of the solid of approximately 59%. Uptake of water by gypsum formation in a confined system would cause the salinity of the remaining water to increase; the water would become supersaturated with respect to halite, which would then precipitate. The increase in volume of the solid phases associated with gypsum formation would completely plug the vein, so that there would be no further circulation of water. The halite would thus be preserved. The isotopic temperatures of the saponite and phillipsite in the vein are interpreted to represent some average of material formed above 55°C and material formed below 55°C from normal seawater (6180 = 0%0). Other saponite veins from the vicinity of the sill have 6180 values ranging from +9.8%0 to +24.3%0 (Lawrence and Drever, unpublished data), corresponding to formation temperatures from norreal seawater of 155°C to 30°C. It is impossible for the +9.8%0 value to have resulted from formation at temperatures below 60°C from seawater depleted in 180 as a consequence of reaction with basalt. There is thus good evidence for hot solutions depositing alteration products close to the sill. A calcite vein (395A63-4, 8 0 - 8 8 cm, piece 1E) from 70 cm above the vein containing halite had a 61 s O value of +30.81%o [4], which corresponds to a formation temperature of 14.3°C from normal seawater. If the seawater had been depleted in 180 by reaction with basalt, the calculated temperature would be lower. Formation at a temperature much below 14°C is most unlikely, so that, at some point in time, normal seawater was present in a crack in the sill close to the vein containing halite. There is always uncertainty in comparing alteration products in different veins, since they may have formed at different times from solutions of different compositions. The isotopic results show that in the vicinity of the vein containing halite, alteration products were formed at elevated temperatures, and alteration products were formed from seawater which was not depleted in aSO. Mechanism 2 fits this pattern, while mechanism 1 is less satisfactory in this regard. As mentioned previously, formation of a significant volume of a calcium sulfate mineral requires a large volume of seawater to provide the sulfate. Ac-
102 tive m o v e m e n t o f water must have been taking place w h e n the sulfate mineral was forming. We c a n n o t distinguish unequivocally which m o d e l is correct, although we feel that m e c h a n i s m 2 is m o r e plausible. T w o i m p o r t a n t general conclusions o f this study are that sulfate minerals do f o r m in spreading centers, and that c o n c e n t r a t e d brines may be present in the fractures in basalts at spreading centers. Brines might be i m p o r t a n t in the m o b i l i z a t i o n o f metals in this e n v i r o n m e n t , although the v o l u m e o f brine indicated by this study is very small.
Acknowledgements We thank W.C. Shanks for the sulfur isotope determ i n a t i o n . Financial support was provided by N S F grants G A - 3 3 5 0 3 , OCE 77-07950, OCE-76-81952 and OCE-75-02968. We are also grateful to the crew and scientific partly on the " G l o m a r Challenger".
References 1 T.J. Wolery and N.H. Sleep, Hydrothermal circulation and geochemical flux at mid-ocean ridges, J. Geol. 84 (1976) 249-275.
2 J.L. Bischoff and F.W. Dickson, Seawater-basalt interaction at 200°C and 500 bars: implications for origin of seafloor heavy-metal deposits and regulation of seawater chemistry, Earth Planet. Sci. Lett. 25 (1975) 385 397. 3 J.M. Edmond, L.1. Gordon and J.B. Corhss, Chemistry of the hot springs on the Galapagos Ridge axis, EOS 58 (1977) 1177. 4 J.R. Lawrence, J.I. Drever and M. Kastner, Low temperature alteration of the basalts predominates at Site 395 of the Deep Sea Drilling Project, Proc. 2nd Int. Symp. on Water-Rock Interaction, Strasbourg, France (1977) 1-355 to 1-362. 5 W.G. Melson and P.D. Rabinowitz, eds., Initial Reports of the Deep Sea Drilling Project, Vol. 45 (in press). 6 H.W. Yeh and S.M. Savin, Mechanism of burial metamorphism of argillaceous sediments, 3. O-isotope evidence, Geol. Soc. Am. Bull. 88 (1977) 1321 1330. 7 G. Thompson, A geochemical study of the low-temperature interaction of seawater and oceanic igneous rocks, EOS 54 (1973) 1015-1019. 8 M.J. Mottl, R.F. Corr and H.D. Holland, Chemical exchange between sea water and mid-ocean ridge basalt during hydrothermal alteration: an experimental study, Geol. Soc. Am. 87th Annu. Meet. Abstr. Progr. 6 (1974) 879-880. 9 F.L. Sayles and F.T. Manheim, Interstitial solutions and diagenesis in deeply buried marine sediments: results from the Deep Sea Drilling Project, Geochim. Cosmoclaim. Acta 39 (1975) 103-127. 10 J.R. Lawrence, J. Gieskes and T.F. Anderson, Oxygen isotope material balance calculations, Leg 35, in: Initial Reports of the Deep Sea Drilling Project, Vol. 35 (1976) 507 512.
Earth and Planetary Science Letters, 42 (1979) 98-102 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [61
GYPSUM AND HALITE FROM THE MID-ATLANTIC RIDGE, DSDP SITE 395 J A M E S I. D R E V E R 1, J A M E S R. L A W R E N C E 2 and R O N A L D C. A N T W E I L E R 1 1 Geology Department, University of Wyoming, Laramie, WY 82071 (U.S.A.) 2 Lamont-Doherty Geological Observatoo,, Palisades, N Y 10964 (U.S.A.)
Received July 25, 1978 Revised version received October 2, 1978
Gypsum and halite crystals, together with saponite and phillipsite, were found in a vein in a basalt sill 625 m below the sea floor at DSDP Site 395A, located 190 km west of the crest of the Mid-Atlantic Ridge. The 6345 value of the gypsum (+19.4%o) indicates a seawater source for the sulfate. The 618 O values of the saponite (+19.9%~J) and phillipsite (+18.1%~,) indicate either formation from normal seawater at about 55°C or formation from 1sO_depleted seawater at a lower temperature. The gypsum (which could be secondary after anhydrite) was formed by reaction between Ca 2÷ released from basalt and SO42-in ckculating seawater. The halite could have formed when water was consumed by hydration of basalt under conditions of extremely restricted circulation. A more probable mechanism is that the gypsum was originally precipitated as anhydrite at temperatures above 60°C. As the temperature dropped the anhydrite converted to gypsum. The conversion would consume water, which could cause halite precipitation, and would cause an increase in the volume of solids, which would plug the vein and prevent subsequent dissolution of the halite.
1. Introduction The circulation of seawater through oceanic spreading centers has been invoked as a major process in the transport of heat [1], the genesis o f metal-rich sediments [2], and the regulation o f the major-elem e n t chemistry o f seawater [1,2]. A l t h o u g h a few samples of warm water emerging from the Galapagos spreading center have been collected and analyzed [3] the best m e t h o d presently available for determining the distribution o f t e m p e r a t u r e and chemical c o m p o s i t i o n in waters circulating in the oceanic crust is probably e x a m i n a t i o n o f the minerals p r o d u c e d by reaction o f the waters with basalt. In a previous paper [4] we suggested, on the basis o f isotopic analysis o f calcite and saponite, that the waters circulating through the basalt at Site 395 were relative cold (less than 15°C for calcites; less than 50°C for saponites), and that the circulation must have been relatively open, that is to say the effective ratio o f water to rock must have been high. In the present paper, however, we shall d o c u m e n t that, at least locally, secondary minerals were deposited at elevated tempera-
tures or under conditions o f highly restricted circulation.
2. Geological setting DSDP Site 395 is located a p p r o x i m a t e l y 140 km west o f the crest of the Mid-Atlantic Ridge at latitude 2 2 ° 4 5 . 3 5 ' N . Water depth was 4485 m. The age o f the crust at the site is a p p r o x i m a t e l y 7 m.y. (magnetic anomaly 4). The sample described here (395A-63-4 No. 1 L, a p p r o x i m a t e l y 625 m below sea floor) is a vein located 4.3 m above the base o f a 30 m thick sill composed o f plagioclase-olivine-clinopyroxene doleritic basalt. The sill is intensely fractured and alteration products are a b u n d a n t in the fractures [5].
3. Results The sample available for study was a vein which appeared to be filled with clear gypsum and m i n o r amounts o f saponite and phillipsite. The mineralogi-
99
Fig. 1. Scanning electron micrographs of vein material from sample 395A-63-4 No. 1L. (a), (b) Halite cube and gypsum needles on saponite substrate. (c), (d) Halite cube with saponite (right) and gypsum (left).
100 cal identifications were confirmed by X-ray diffraction and semi-quantitative X-ray energy spectrometry. The gypsum appeared to be a single crystal with dimensions approximately 20 × 10 X 5 ram. 5 mm represents the width of the vein; the true extent of the gypsum in the other directions could not be determined from the small fragment samples. By "single crystal" we mean that the material was completely clear and had the same cleavage directions throughout. The shape of the crystal boundaries was controlled by the shape of the fracture in the basalt; no crystal faces were visible. Examination with the scanning electron microscope (Fig. 1) showed the presence o f minor amounts o f halite in addition to the gypsum. The halite crystals are clearly intergrown with the smectite and gypsum, and are not a result of simple evaporation of seawater during handling. It is inconceivable that the gypsum could have been an artifact introduced during handling because approximately 500 ml of seawater would have to have evaporated to produce the observed amount of gypsum. Halite, however, is present in only trace amounts. The evidence that it is not an artifact is the textural relationships shown in Fig. 1. Samples of the gypsum, saponite and phillipsite were separated from the vein by hand picking for isotopic analysis (Table 1). The 634S of the gypsum is close to that of present-day seawater sulfate, indicating that the sulfur in the gypsum was probably derived from seawater. The ~180 value of the saponite (Table 1) indicates a temperature of formation of 55°C for formation from normal seawater (8180 = 0%~) (fractionation factor from Yeh and Savin [6]). The isotopic composition of the phillipsite indicates a similar tempera-
TABLE 1 Isotopic analyses of samples from veins containing gypsum
~345 (%0)
6180 SMOW(%0)
395A.63-4 No. 1L
Gypsum * Saponite Phillipsite
+19.4 +19.9 +18.1
395A-66-3, 125 em No. 12
Saponite * Determined by W.C. Shanks.
+21.3
ture, although the phillipsite-water fractionation factor is not well known. A small amount of gypsum was found intergrown with saponite in another vein (395A-66-3, 125 cm No. 12) from approximately 10.4 m below the base of the sill. The 6180 of saponite from this vein (Table 1) indicates a temperature of formation from normal seawater of 44°C.
4. Discussion Since boiling and evaporation are impossible under several kilometers of seawater, the "evaporite" minerals must have been formed by reactions between seawater and basalt. We shall present two possible mechanisms which are consistent with the data. Mechanism 1. This mechanism is based on reaction
between a fixed volume of water and a large volume of basalt at low (below 60°C) temperatures, with essentially no circulation of water. When basalt reacts with seawater at low temperatures, calcium is released from the basalt. Initially, the calcium reacts with carbonate species in seawater to form calcite. When all carbonate species are consumed, dissolved calcium will increase until gypsum precipitates. Halite precipitation cannot be explained by a similar mechanism. Alteration of basalt [7,8] causes either a slight uptake of sodium by the basalt or no significant uptake or actual release of sodium depending on water/rock ratio. There is no evidence that alteration of basalt could release sufficient sodium for seawater to become saturated with respect to halite. However, when basalt is altered, water is consumed by hydration reactions. Since chloride is not significantly involved in alteration reactions, the chloride concentration in the remaining water must increase as hydration reactions proceed. The remaining water could eventually become saturated with respect to halite, and halite would precipitate. Sayles and Manheim [9] invoked this mechanism to explain the increase in the chloride contents of some interstitial waters in marine sediments. The ~ 180 values of the saponites would be interpreted in this model as indicating not precipitation from normal seawater (alSO = 0%~) at slightly elevated temperatures, but precipitation at low temperatures (perhaps 20°C)
101 from seawater which had been depleted in 180 as a consequence of hydration reactions such as conversion of basalt to smectite. There are two problems with this mechanism. For halite saturation to be achieved, 90% of the original seawater would have to be removed by hydration reactions. If this happened at about 20°C, the mean 6x80 of the alteration products products (assuming the alteration product is mostly smectite) should be about 14%0. The calculation procedure is explained in Lawrence et al. [10]. The observed 6180 values of the saponites associated with gypsum (+ 19.9%~ and +21.3%0) are much higher than the calculated mean value. It is possible, of course, that the bulk of the phillipsite and smectite were formed earlier than the halite, before the water became highly depleted in 180. A second problem is that each millilitre of gypsum represents the sulfate in about 550 ml of seawater, so that to form a vein filled with gypsum, there must have been considerable movement of water on a local scale. On the other hand, there must have been essentially no mixing with ordinary seawater, otherwise halite saturation would not have been achieved. Isotopic studies of calcite veins from the same region of the core show evidence for extensive circulation of seawater whose isotopic composition was not significantly different from 0%~ [4].
Mechanism 2. As the sill cooled, cracks formed and water of approximately seawater composition circulated through the cracks. Saponite, phillipsite, and anydrite precipitated and continued to precipitate as the system cooled. In the experiments of Bischoff and Dickson [2] anhydrite apparently precipitated when seawater alone was heated to 200°C. When seawater and basalt were heated together to 200°C, anhydrite was rapidly precipitated in large quantities by Ca 2÷ released from basalt combining with SO42- in the seawater. The equilibrium temperature for the gypsum/ anhydrite transition vnder present in-situ conditions is about 70°C in normal seawater, 60°C in seawater "evaporated" to gypsum saturation and 30°C in saturate NaCI solution, assuming the pressure on both solids and solution is hydrostatic. When the temperature dropped below about 60°C, the anhydrite transformed to gypsum, presumably by a solution/precipi-
tation mechanism. The effect of the transformation of anhydrite to gypsum is to consume approximately 0.78 ml of water per millilitre of anhydrite, and to cause an increase in the volume of the solid of approximately 59%. Uptake of water by gypsum formation in a confined system would cause the salinity of the remaining water to increase; the water would become supersaturated with respect to halite, which would then precipitate. The increase in volume of the solid phases associated with gypsum formation would completely plug the vein, so that there would be no further circulation of water. The halite would thus be preserved. The isotopic temperatures of the saponite and phillipsite in the vein are interpreted to represent some average of material formed above 55°C and material formed below 55°C from normal seawater (6180 = 0%0). Other saponite veins from the vicinity of the sill have 6180 values ranging from +9.8%0 to +24.3%0 (Lawrence and Drever, unpublished data), corresponding to formation temperatures from norreal seawater of 155°C to 30°C. It is impossible for the +9.8%0 value to have resulted from formation at temperatures below 60°C from seawater depleted in 180 as a consequence of reaction with basalt. There is thus good evidence for hot solutions depositing alteration products close to the sill. A calcite vein (395A63-4, 8 0 - 8 8 cm, piece 1E) from 70 cm above the vein containing halite had a 61 s O value of +30.81%o [4], which corresponds to a formation temperature of 14.3°C from normal seawater. If the seawater had been depleted in 180 by reaction with basalt, the calculated temperature would be lower. Formation at a temperature much below 14°C is most unlikely, so that, at some point in time, normal seawater was present in a crack in the sill close to the vein containing halite. There is always uncertainty in comparing alteration products in different veins, since they may have formed at different times from solutions of different compositions. The isotopic results show that in the vicinity of the vein containing halite, alteration products were formed at elevated temperatures, and alteration products were formed from seawater which was not depleted in aSO. Mechanism 2 fits this pattern, while mechanism 1 is less satisfactory in this regard. As mentioned previously, formation of a significant volume of a calcium sulfate mineral requires a large volume of seawater to provide the sulfate. Ac-
102 tive m o v e m e n t o f water must have been taking place w h e n the sulfate mineral was forming. We c a n n o t distinguish unequivocally which m o d e l is correct, although we feel that m e c h a n i s m 2 is m o r e plausible. T w o i m p o r t a n t general conclusions o f this study are that sulfate minerals do f o r m in spreading centers, and that c o n c e n t r a t e d brines may be present in the fractures in basalts at spreading centers. Brines might be i m p o r t a n t in the m o b i l i z a t i o n o f metals in this e n v i r o n m e n t , although the v o l u m e o f brine indicated by this study is very small.
Acknowledgements We thank W.C. Shanks for the sulfur isotope determ i n a t i o n . Financial support was provided by N S F grants G A - 3 3 5 0 3 , OCE 77-07950, OCE-76-81952 and OCE-75-02968. We are also grateful to the crew and scientific partly on the " G l o m a r Challenger".
References 1 T.J. Wolery and N.H. Sleep, Hydrothermal circulation and geochemical flux at mid-ocean ridges, J. Geol. 84 (1976) 249-275.
2 J.L. Bischoff and F.W. Dickson, Seawater-basalt interaction at 200°C and 500 bars: implications for origin of seafloor heavy-metal deposits and regulation of seawater chemistry, Earth Planet. Sci. Lett. 25 (1975) 385 397. 3 J.M. Edmond, L.1. Gordon and J.B. Corhss, Chemistry of the hot springs on the Galapagos Ridge axis, EOS 58 (1977) 1177. 4 J.R. Lawrence, J.I. Drever and M. Kastner, Low temperature alteration of the basalts predominates at Site 395 of the Deep Sea Drilling Project, Proc. 2nd Int. Symp. on Water-Rock Interaction, Strasbourg, France (1977) 1-355 to 1-362. 5 W.G. Melson and P.D. Rabinowitz, eds., Initial Reports of the Deep Sea Drilling Project, Vol. 45 (in press). 6 H.W. Yeh and S.M. Savin, Mechanism of burial metamorphism of argillaceous sediments, 3. O-isotope evidence, Geol. Soc. Am. Bull. 88 (1977) 1321 1330. 7 G. Thompson, A geochemical study of the low-temperature interaction of seawater and oceanic igneous rocks, EOS 54 (1973) 1015-1019. 8 M.J. Mottl, R.F. Corr and H.D. Holland, Chemical exchange between sea water and mid-ocean ridge basalt during hydrothermal alteration: an experimental study, Geol. Soc. Am. 87th Annu. Meet. Abstr. Progr. 6 (1974) 879-880. 9 F.L. Sayles and F.T. Manheim, Interstitial solutions and diagenesis in deeply buried marine sediments: results from the Deep Sea Drilling Project, Geochim. Cosmoclaim. Acta 39 (1975) 103-127. 10 J.R. Lawrence, J. Gieskes and T.F. Anderson, Oxygen isotope material balance calculations, Leg 35, in: Initial Reports of the Deep Sea Drilling Project, Vol. 35 (1976) 507 512.