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basalt ratio
Earth and Planetary Science Letters, 34 (1977) 71-77 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
71
[1]
HYDROTHERMAL TRANSPORT OF HEAVY METALS BY SEAWATER: THE ROLE OF SEAWATER/BASALT RATIO WILLIAM SEYFRIED
Department of Geology, Stanford University, Stanford, Calif. (USAJ and JAMES L. BISCHOFF
U.S. Geological Survey, Menlo Park, Calif. (USA) Received October 13, 1976 Revised version received November 19, 1976
Seawater reacted with basaltic glass at 260°C and 500 bars under water-dominated conditions (50 : 1 water/rock ratio) efficiently leached and maintained heavy metals in solution. Cu, Zn, and Ba are transferred in significant proportions to the aqueous phase, while Fe and Mn attain concentrations of 45 and 20 ppm respectively as the basalt is completely made over to magnesian smectite. High metal solubility is a function of acidity maintained by large excess of dissolved Mg and equilibria with the alteration phase. Metal concentrations and relative proportions are consistent within limits required for metal-rich fluid which produced East Pacific Rise metalliferous sediments. Experiments mixing metal-bearing altered seawater and normal seawater were carried out as a qualitative indicator of sea-floor precipitation processes. Bulk composition of the precipitates are strongly influenced by mixing ratio. Precipitates range from silica-magnesiumrich under low dilution by seawater to essentially pure ferric hydroxide under conditions of high dilution.
1. Introduction Until recently, seawater-basalt interaction at midocean ridges was believed to be in all cases analogous to the Reykjanes geothermal system in Iceland, a system in which the flux of seawater is small relative to the amount of rock encountered and altered. Accordingly, experimental studies were heretofore carried out at relatively low water/rock ratios (~<10) such that the system was chemically dominated by the rock ([1 ], and references therein). Only at temperatures in excess of 400°C are significant concentrations of heavy metals such as Fe and Mn maintained in solution [2], temperatures unreasonably high to apply to through-flowing submarine geothermal system [3]. At lower temperatures ( 2 0 0 - 3 0 0 ° C ) the early stages of interaction are characterized by initial release of
heavy metals from the rock and a low pH, but with time the concentrations of the metals continuously decrease as the pH slowly rises [1,4]. These experiments suggest, therefore, that the interaction of seawater with basalt in a rock-dominated environment may not adequately solubilize heavy metals nor sufficiently separate them from A1 for sufficient lengths of time to act as a metal-bearing fluid for metalliferous sediments associated with seafloor spreading. Another point of view concerning the conditions prevailing during seawater-basalt interaction arises from mathematical modeling of heat budgets at spreading center [3,6,7]. These models infer that convecting seawater transfers a major portion of the heat released at the spreading center and that effective water/rock ratios (total amount of water which passes
72 degree of mixing of the altered seawater with bottom waters. Our second experiment was a qualitative attempt to assess the importance of the degree of such mixing on the sea-floor precipitation process.
through the geothermal system/total amount of rock altered) may be in the range of 5 0 - 1 0 0 : 1. Can such high ratios make a significant difference in metal solubility? At increasing water/rock ratios the proportions of chemical components in seawater become greater than those in the rock. For example, at a seawater/rock ratio o f 1, the bulk of H20, C1 and SO4 are in seawater, and the other components are in the rock. At a ratio of 50, the bulk of K, Na, and Mg as well as H20, C1- and SO4 are in seawater and only A1, Si, Fe and Mn in the basalt. Therefore, at greater seawater/rock ratios the bulk chemistry of seawater becomes increasingly controlled by temperature and pressure and less by reactions with basalt. Previous experiments with seawater alone at temperatures to 450°C (unpublished results) gave us reason to believe that seawater heated above 200°C is acidic and might possibly be an effective metal-leaching fluid when placed in contact with rock. We report here on an experiment designed to test this question, the results of which seem to affirm that metals are indeed efficiently solubilized. A further question, therefore, was pursued concerning the mode of precipitation of these metals on the sea floor. There are many variables that undoubtedly affect the process of precipitation during discharge. For example, the final composition of the altered seawater will, among many other variables, be affected by the rate and extent of retrograde reactions during the ascent of the fluid from the zone of highest temperatures through lower temperatures before final discharge. Another variable is the rate of discharge and
2. Experimental procedures Water-dominated basalt-seawater interaction. Four grams of fresh basaltic glass (from the Juan de Fuca Ridge, crushed and sieved to 4 3 - 6 3 / a m ) were reacted with 200 g of artificial SO4-free seawater (19.5%o chlorinity), at 260°C and 500 bars. SO4 was excluded from the chemical system since in all previous experiments SO 4 is essentially quantitatively removed as anhydrite in the early stages of interaction as all temperatures greater than 150°C. Thus, in natural systems, by the time seawater permeates to zones of maximum temperatures ( 2 0 0 - 3 0 0 ° ) , all the SO4 will have been removed. The deeper geothermal seawaters as Reykjanes, Iceland, contain less than 60 ppm SO 4 and support this inference [8]. The experimental equipment was the Dickson et al. [9], hydrothermal solution equipment which allows sampling of the altered seawater without disturbing the temperature and pressure of the experiment. Six 10-ml samples were taken during the 2400 hours of the experiment, by gas-tight syringe, membrane filtered (0.1/am) and subdivided into aliquots by weight for analysis. An aliquot was diluted and preserved for analysis of the major components Ca, Mg, K, Na and SiO2, and another was acidified with ultra-pure HNO 3 for analysis of the metals Fe, Mn, Zn,
TABLE 1 Concentration of selected dissolved species in artificial seawater reacted with Juan de Fuca basaltic glass at 260°C and 500 bars (Na, K, Ca, Mg, and SiO2 in parts per thousands; Fe, Mn, AI, Cu, Zn, and Ba in parts per million) Hours
pH *
Na
K
Ca
Mg
Start 60 228 384 576 1000 2400
8.0 4.15 4.24 4.50 4.65 4.58 4.65
10.1 10.2 10.3 10.3 10.3 10.3 10.3
0.363 0.371 0.368 0.373 0.371 0.370 0.370
0.373 1.153 1.569 1.559 1.481 1.497 1.420
1.210 0.636 0.320 0.294 0.350 0.350 0.351
* Measured at 25°C.
SiO2 <0.01 1.00 1.39 1.38 1.33 1.39 1.35
Fe
Mn
<0.01 30 33 38 40 42 45
<0.01 19 20 20 20 19 20
A1
Cu
Zn
Ba
<0.01 0.28 0.28 0.25 0.22 0.20 0.21
<0.01 0.01 0.02 0.19 0.38 0.50 0.38
<0.01 1.1 1.3 1.5 1.1 1.1 1.1
<0.01 0.04 0.09 0.155 0.135 0.135 0.135
73 TABLE 2 Chemical analysis of basaltic glass and alteration product before and after reaction with seawater at 260°C, 500 bars Basaltic glass (Juan de Fuca Ridge basalt)
Altered basaltic glass,2400 hr (smectite)
Smectite from altered basalt Mid-Atlantic Ridge* 39.7 16.5 15.6 12.0 0.5 0.8 1.7 1.8 0.06
SiO2 A1203 FeO ** MgO CaO Na20 K20 TiO2 MnO Ignition loss ***
50.10 14.80 12.61 7.00 11.10 2.57 0.14 1.28 0.22
43.9 15.4 12.7 16.44 1.2 1.2 <0.05 1.8 <0.1
0.10
6.6
Total
99.92
99.39
(-) 88.66
* Reported by Melson and Thompson [ 13 ]. ** Total iron expressed as FeO, solids dried at 100°C prior to analysis. *** Weight loss at 1000°C, includes loss of volatiles, and corrected for oxidation of FeO. (-) not analyzed.
Cu, Ba and A1. The pH was measured immediately by combination electrode at 25°C, and SiO2 was measured colorimetrically. Other species were analyzed by flame and flameless atomic absorption spectrophotometry. The solid material recovered from the reac, tion vessel at the termination of the experiment was composed of a uniformly blue-green smectite. Analysis
of the smectite was carried out after drying at 100°C and fusing with lithium metaborate. A similar procedure was followed for the unreacted basaltic glass. Si, A1, Fe, Mg, Ca, Na, K, Ti, and Mn were analyzed by atomic absorption spectrophotometry.
Interaction o f a l t e r e d seawater with normal seawater. A sample of altered seawater was collected at conditions at 2400 hours and filtered as above in gas-tight syringes. Three aliquots were then transferred to syringes containing measured amounts of normal seawater as follows: (1) 10 g altered to 10 g normal; (2) 5 g altered to 15 g normal; (3) 1 g altered to 39 g normal. A precipitate was visible within a few hours in (1) and (2), but not in (3). After 24 hours, fluid samples were taken of each of the three syringes, membrane filtered and analyzed as above.
3. Experimental results
Basalt-seawater interaction. Changes in the major components are characterized by a depletion of Mg, a drop in pH to 4.6, and increase in Ca and SiO2 (Table 1). In contrast with the earlier rock-dominated experiment [1 ], Mg depletion was not complete (from 1200 ppm down only to 350 ppm compared with 10 ppm in the earlier). Ca, K, and Ba are nearly quantitatively extracted from the rock to the aqueous phase. Fe, Mn, Zn and Cu increase sharply to values of 45, 20, 1.2 and 0.38 ppm respectively (Table 1), implying that a significant proportion of these elements,
TABLE 3 Results of mixing experiment between metal-bearing altered seawater and normal seawater (concentrations in parts per million)
Mg SiO2 Fe Mn pH
(1) 10 g altered seawater 10 g normal seawater
(2) 5 g altered seawater 15 g normal seawater
(3) 1 g altered seawater 39 g normal seawater
initial concentration of mixture
final concentration (24 hours)
initial concentration of mixture
final concentration (24 hours)
initial concentration of mixture
final concentration (24 hours)
820 600 22 9 -
790 130 0.2 7.8 6.4
1010 300 10 4.5 -
850 200 0.2 4.5 6.6
1220 30 1.0 0.45 -
1219 30 0.1 0.45 7.6
74 except Fe, have been solubifized. A1 remains relatively insoluble, never exceeding 0.28 ppm. The basaltic glass, analyzed after termination of the experiment was entirely made over to a magnesian smectite (Table 2). Its X-ray diffraction pattern exhibited characteristic basal spacings with a doo 1 at 14.5 A (50% humidity), expanding to 16.5 A after glycolation. The 06;33 reflection at 1.532 A indicated the smectite is trioctahedral.
Mixing experiment. Changes in the composition of the combined fluids allow assessment of the composition of the precipitate. Fe was essentially quantitatively precipitated in each of the three mixing ratios, whereas Mn in each case remained totally in solution (Table 3). SiO 2 and Mg were precipitated in the 1 : 1 and 1 : 3 mixtures, but apparently not in the 1 : 39 mixture (Table 3).
4. Discussion The results of seawater-basalt experiment at high water/rock ratios clearly indicate that the heavy metals are strikingly solubilized. Moreover, the loss of Ca, K, and SiO 2 from the basalt and the gain in Mg during alteration (Table 2) agrees well with the direction of major-element exchange observed in hydrothermally altered basaltic glass dredged from submarine ridge system [10-12]. The smectite formed in the experiment is an Fe-rich saponite and is similar to secondary smectite found in altered basalt from St. Peters and St. Pauls Rocks ([13], and Table 3). The enrichment of FeO and A1203 in our experimental smectite is produced by selective loss of other components to the fluid phase during alteration. How does the fluid derived in our experiment relate to that which deposited sea-floor metalliferous sediments? Bostr6m [14] calculated that a hydrothermal fluid would have to be debouching onto the ocean floor from submarine hot springs in the area of the East Pacific Rise at a rate of 0.01 litres/cm 2 per yqar and contain at least 10 ppm Fe and 3 - 5 ppm Mn to provide the metals required by the measured accumulation rates of these elements in the metalliferous sediment. These concentrations are in good agreement with those estimated by Mottl [2], based on flow rates re-
quired by the heat flow model of Wolery and Sleep [3]. The concentrations of Fe (45 ppm) and Mn (20 ppm), produced during our experiment (Table 1) are consistent with these limits, and are of the proper relative proportions as found in the metal-rich sediments [15]. Why should heavy metals be so much more soluble in seawater-dominated than in rock-dominated systems? During the earlier experiment in a rock-dominated system [1 ], the first few days of interaction were characterized by rapid release of heavy metals to levels close those observed in this present experiment, followed by a gradual reprecipitation of these metals into the alteration phase. The period of rapid and pronounced solubility was paralleled by an almost quantitative removal of Mg from the seawater and a corresponding drop in pH. After completion of Mg removal, the pH slowly rose, and the metals were gradually and continuously removed from solution. In the present experiment, Mg was only partially removed from seawater, presumably because there was insufficient capacity in the rock to "titrate" the seawater Mg to completion, and the pH remained low and apparently stabilized. We interpret these observations to indicate that seawater Mg is reacting with SiO2 released from the rock to form a "sepiolite"-type [Mg2Si306(OH)4 ] compound or alternatively a "brucite"-type [Mg(OH)2]'molecule constituting part of the final layered silicate alteration phase. Precipitation of such a molecule requires not only Mg and SiO 2, but O H - as well, taken from the hydrolysis of water. H+ equivalent to the amount of Mg precipitated is initially produced in the aqueous phase to maintain electrical neutrality. The H+ then attacks the rock to a degree dependent upon the water/rock ratio. Thus, the seawater becomes acid. In the rock-dominated situation, the mass of dissolved Mg is small relative to the mass of rock, and although the solution turns acid initially as Mg is being removed (and solubilizing metals in the process), Mg is finally quantitatively removed, and the following interaction is characterized by simple hydrolysis of silicates which produces OH-. The seawater then gradually turns alkaline and the initially released metals pass into the alteration phase. In the seawater-dominated system, there is excess Mg and the rock is completely made over to the "sepio lite-bearing" alteration phase. Thus, the excess dis-
75 solved Mg, silica and H+ reflect quasi-equilibrium with the alteration product and the pH is maintained at a slightly acid, steady state concentration, as are the metals in solution. What processes will take place if a fluid similar to that produced in our experiment, rich in Fe, Mn, and SiO2 were to discharge on the sea floor and interact with cold, oxygenated, and slightly alkaline seawater? The results of the mixing experiments provide some qualitative insight. Oxidation and rapid removal of dissolved Fe, presumably as ferric oxyhydroxide, is seen to take place rapidly and quantitatively in all three dilutions. In contrast, and regardless of the dilution factor, Mn remains in solution, demonstrating no tendency to precipitate as a discrete phase nor to co-precipitate with Fe even though excess 02 is present. Such efficient separation of Fe from Mn in aqueous systems has been observed in many natural systems and has commonly been attributed to the relatively sluggish oxidation of Mn 2+ at a pH <9 [16]. The basal metalliferous sediments associated with the active ridge systems in the eastern Pacific, however, are characterized by a strikingly consistent Fe/Mn ratio over large areas and through time [ 15,17], suggesting that quantitative fractionation of these metals is probably not taking place. In the natural environment, the relative time delay of Mn precipitation may not be significant, and the Mn that must eventually precipitate becomes ultimately mixed with the Fe compounds. The distribution of Fe and Mn may simply reflect physical mixing and homogenization processes which destroy the record of small-scale fractionation. The chemistry of the metal-rich sediments would also be modified by hydrogenous, biogenous, and detrital contributions depending on local accumulation rates [ 18]. The behavior of SIO2, Mg, and pH as the reacted seawater is diluted with increased amounts of seawater, is also of interest. In a mixture dominated by reacted seawater, SiO2 would not be greatly diluted and might be expected to precipitate upon cooling as amorphous SIO2. This situation is represented by the 1 : 1 ratio (Table 3), where SiO2 decreases from 617 to 130 ppm, a level presumably maintained by the solubility of amorphous SiO2. A slight removal of Mg is also apparent. The bulk precipitate in this case is dominated by amorphous SiO 2 and contains
perhaps 5-10% Fe, depending on the amount of water of hydration. The barite-opal precipitates of the Lau Basin [19], were possibly deposited under similar conditions. The dilution of altered seawater with three parts normal seawater (Table 3) displays significant removal of Mg, along with a less complete SiO 2 depletion and decrease in pH. Analyses of this mixture on subsequent days (data not shown) indicate a continuous adjustment of the Mg/Si ratio in the precipitated phase. Initially Mg rich and SiO 2 poor, the precipitate continued to extract SiO 2 from the solution until a constant value of 80 ppm SiO 2 was reached. At the same time, Mg and pH rose to 940 and 7.3, respectively. These developments can best be interpreted as the result of formation of a hydrated Mg silicate. In this case, the bulk precipitate was composed of approximately equal proportions of Mg and SiO2, with approximately 5-10% Fe depending on the degree of hydration. The hydrothermally deposited sediments of the Bauer Deep [20], are not only enriched in Fe and Mn but contain almost 50% SiO 2 and Mg greatly in excess of that contributed by detrital contamination. Bischoff and Rosenbauer [21] have attributed this enrichment to the precipitation of a Mg-SiO 2 phase as a consequence of the mixing between the hydrothermal fluid and ocean bottom water. A hydrothermal fluid which is rapidly mixed with large amounts of bottom water is depicted in the 1 : 39 mixture (Table 3). In this case, SiO 2 is not only diluted below amorphous SiO2 saturation but below that necessary to precipitate a Mg phase, and the hydrothermally derived SiO 2 simply remains in solution. The bulk precipitate in this case is apparently pure ferric hydroxide, and is analogous, perhaps, to the virtually SiO2-free and Fe-rich metalliferous sediments found at the crest of the East Pacific Rise [17], which contrast with those of the Bauer Deep. Depending on the mixing ratios at and near the sea floor, and neglecting the many other variables that might apply, the hydrothermal precipitate might range from almost pure amorphous SiO2, through a Mg silicate heavy-metal precipitate, to a relatively pure, heavy-metal precipitate. That metalliferous sediments of the Bauer Deep and East Pacific Rise display such remarkable uniformity of composition over geographically wide areas
76 implies that conditions of hydrothermal fluid generation, and also of sea floor discharge are remarkably uniform. Alternatively, mixing and distribution of precipitates by deep ocean currents must efficiently homogenize the composition before deposition.
5. Summary and conclusions We feel the experimental results support the following conclusions: (1) In a water-dominated system (wate r/rock = 50 : 1) at 260°C and 500 bars seawater efficiently leaches heavy metals from basaltic glass and maintains them in solution at concentrations and relative proportions within theoretical limits required of a transport fluid for ridge-crest metalliferous sediments. In experimental and field observations of rock-dominated systems, heavy metals are initially released by the basalt only to be reprecipitated in alteration products. (2) The increased solubility of heavy metals in the seawater-dominated system is attributed to the large and excess reservoir of dissolved Mg. Mg2÷ is the single most active component in seawater reacting with basalt, precipitating with SiO 2 released by the rock and in the process releasing I4+ to the water. In the rock-dominated systems, the rock quantitatively titrates the dissolved Mg in the early stages of interaction, and only OH--producing hydrolysis reactions take place subsequently, causing the initially solubilized heavy metals to precipitate in the alteration phase. In water-dominated systems, the rock does not have the capacity to remove all dissolved Mg, and the pH is therefore buffered at a low value throughout and subsequent to interaction. (3) Results of the experiments mixing metal-bearing reacted seawater with normal seawater qualitatively reveal that the mixing ratio of reacted to normal exerts a fundamental control on the bulk composition of the precipitate. Fe is quantitatively precipitated under almost all mixing conditions. SiO2 constitutes a major proportion of the precipitate under conditions of low dilution (ca. 1 : 1), Mg silicate under intermediate (ca. 1 : 3), but both are kept below saturation at high ratios (ca. 1 : 40). "Hydrothermal" precipitates might thus range from almost pure amorphous SiO 2 to pure ferric hydroxide (with as-
sociate metals), depending on the mixing conditions alone. (4) The actual process of subsurface seawater circulation, interaction, and discharge on the sea floor must be uniform over large areas to account for the marked uniformity of sea-floor hydrothermal metal deposits along and adjacent to the equatorial East Pacific Rise.
Acknowledgements We aregrateful to F.W. Dickson, D.Z. Piper, M.J. Mottl and H.D. Holland for their comments, suggestions and discussion which greatly improved this manuscript. We would also like to thank W.G. Melson who kindly provided the basalt glass used in this study. This research was supported by N.S.F. Grant No. IDO74-12880-A01.
References 1 J.L. Bischoff and F.W. Dickson, Seawater-basalt interaction at 200°C and 500 bars: implication for origin of sea-floor heavy metal deposits and regulation of seawater chemistry, Earth Planet. Sci. Lett. 25 (1975) 385-397. 2 M.J. Mottl, Chemical exchange between seawater and basalt during hydrothermal alteration of the oceanic crust, Unpublished Ph.D. Thesis, Harvard Univ., Cambridge, Mass. 3 T.J. Wolery and N.H. Sleep, Hydrothermal circulation and geochemical flux at mid-ocean ridges, J. Geol. 84 (1976) 249-275. 4 W.E. Seyfried, J.L. Bischoff and F.W. Dickson, Basaltseawater interaction from 25-300°C and from 1-500 bars: an experimental study, Trans. Am. Geophys. Union 56 (1975) 1073. 5 M.J. Mottl, R. Corr and H.E. Holland, Trace element con. tent Reykjanes-Svartsengi thermal brines, Iceland, Geol. Soc. Am. 88th Annu. Meet., Abstracts with Programs 7 (1975) 206. 6 C.R.B. Lister, On the thermal balance of a mid-ocean ridge, R. Astron. Soc. Geophys. J. 26 (1972) 515-535. 7 D.L. Williams, R.P. Von Herzen, J.G. Schlater and R.N. Anderson, The Galapagos spreading center: lithospheric cooling and hydrothermal circulation, R. Astron. Soc. Geophys. J. 38 (1974) 587-608. 8 J. Tomasson and H. Kristmannsdottir, High-temperature alteration minerals and thermal brines, Reykjanes, Iceland, Contrib. Mineral. Petrol. 36 (1972) 123-134.
77 9 F.W. Dickson, C. Blout and G. TunneU, Use of hydrothermal solution equipment to determine the solubility of anhydrite in water from 100°C to 275°C and from 1 bar to 100 bars pressure, Am. J. Sci. 261 (1963) 6 1 - 7 8 . 10 W.C. Melson, G. Thompson and Tj.H. van Andel, Volcanicism and metamorphism in the Mid-Atlantic Ridge, 22°N latitude, J. Geophys. Res. 73 (1968) 5925-5941. 11 A. Miyashiro, R. Shido and M. Ewing, Metamorphism in the Mid-Atlantic Ridge near 24 ° and 30°N, Philos. Trans. R. Soc. Lond. 268 (1971) 589-603. 12 J.R. Cann, Spilites from the Carlsberg Ridge, Indian Ocean, J. Petrol. 10 (1969) 1-19. 13 W.G. Melson and G. Thompson, Glassy abyssal basalt, Atlantic sea-floor near St. Pauls rocks: petrography and composition of secondary clay minerals, Geol. Soc. Am. Bull. 84 (1973) 703-716. 14 K. Bostr6m, The origin and fate of ferromanganoan active ridge sediment, Stockholm Contrib. Geol. 24 (1973) 2/ 149-243.
15 K. Bostr6m and M.N.A. Peterson, Precipitates form hydro thermal exhalations in the East Pacific Rise, ,Econ. Geol. 61 (1966) 1258-1265. 16 W. Stumm and J.J. Morgan, Aquatic Chemistry (WileyInterscience, New York, N.Y., 1970) 583. 17 D.S. Cronan, Basal metalliferous sediments from the eastern Pacific, Geol. Soc. Am. Bull. 87 (1976) 928-934. 18 G.R. Heath and J.R. Dymond, Genesis and diagenesis of metalliferous sediments from the East Pacific Rise, Bauer Deep and Central Basin, northwest Naeza plate, Geol. Soe Am. Bull. (in press). 19 K.K. Bertine and J.B. Keene, Submarine barite-opal rocks of hydrothermal origin, Science 188 (1975) 150. 20 F.L. Sayles, T.L. Ku and P.C. Bowker, Chemistry of ferromanganoan sediment of the Bauer Deep, Geol. Soc. Am. BuU. 86 (1975) 1423-1431. 21 J.L. Bischoff and B. Rosenbauer, Recent metalliferous sediment in the North Pacific manganese nodule area, Earth Planet. Sci. Lett. 33 (1977) 379-388.
71
[1]
HYDROTHERMAL TRANSPORT OF HEAVY METALS BY SEAWATER: THE ROLE OF SEAWATER/BASALT RATIO WILLIAM SEYFRIED
Department of Geology, Stanford University, Stanford, Calif. (USAJ and JAMES L. BISCHOFF
U.S. Geological Survey, Menlo Park, Calif. (USA) Received October 13, 1976 Revised version received November 19, 1976
Seawater reacted with basaltic glass at 260°C and 500 bars under water-dominated conditions (50 : 1 water/rock ratio) efficiently leached and maintained heavy metals in solution. Cu, Zn, and Ba are transferred in significant proportions to the aqueous phase, while Fe and Mn attain concentrations of 45 and 20 ppm respectively as the basalt is completely made over to magnesian smectite. High metal solubility is a function of acidity maintained by large excess of dissolved Mg and equilibria with the alteration phase. Metal concentrations and relative proportions are consistent within limits required for metal-rich fluid which produced East Pacific Rise metalliferous sediments. Experiments mixing metal-bearing altered seawater and normal seawater were carried out as a qualitative indicator of sea-floor precipitation processes. Bulk composition of the precipitates are strongly influenced by mixing ratio. Precipitates range from silica-magnesiumrich under low dilution by seawater to essentially pure ferric hydroxide under conditions of high dilution.
1. Introduction Until recently, seawater-basalt interaction at midocean ridges was believed to be in all cases analogous to the Reykjanes geothermal system in Iceland, a system in which the flux of seawater is small relative to the amount of rock encountered and altered. Accordingly, experimental studies were heretofore carried out at relatively low water/rock ratios (~<10) such that the system was chemically dominated by the rock ([1 ], and references therein). Only at temperatures in excess of 400°C are significant concentrations of heavy metals such as Fe and Mn maintained in solution [2], temperatures unreasonably high to apply to through-flowing submarine geothermal system [3]. At lower temperatures ( 2 0 0 - 3 0 0 ° C ) the early stages of interaction are characterized by initial release of
heavy metals from the rock and a low pH, but with time the concentrations of the metals continuously decrease as the pH slowly rises [1,4]. These experiments suggest, therefore, that the interaction of seawater with basalt in a rock-dominated environment may not adequately solubilize heavy metals nor sufficiently separate them from A1 for sufficient lengths of time to act as a metal-bearing fluid for metalliferous sediments associated with seafloor spreading. Another point of view concerning the conditions prevailing during seawater-basalt interaction arises from mathematical modeling of heat budgets at spreading center [3,6,7]. These models infer that convecting seawater transfers a major portion of the heat released at the spreading center and that effective water/rock ratios (total amount of water which passes
72 degree of mixing of the altered seawater with bottom waters. Our second experiment was a qualitative attempt to assess the importance of the degree of such mixing on the sea-floor precipitation process.
through the geothermal system/total amount of rock altered) may be in the range of 5 0 - 1 0 0 : 1. Can such high ratios make a significant difference in metal solubility? At increasing water/rock ratios the proportions of chemical components in seawater become greater than those in the rock. For example, at a seawater/rock ratio o f 1, the bulk of H20, C1 and SO4 are in seawater, and the other components are in the rock. At a ratio of 50, the bulk of K, Na, and Mg as well as H20, C1- and SO4 are in seawater and only A1, Si, Fe and Mn in the basalt. Therefore, at greater seawater/rock ratios the bulk chemistry of seawater becomes increasingly controlled by temperature and pressure and less by reactions with basalt. Previous experiments with seawater alone at temperatures to 450°C (unpublished results) gave us reason to believe that seawater heated above 200°C is acidic and might possibly be an effective metal-leaching fluid when placed in contact with rock. We report here on an experiment designed to test this question, the results of which seem to affirm that metals are indeed efficiently solubilized. A further question, therefore, was pursued concerning the mode of precipitation of these metals on the sea floor. There are many variables that undoubtedly affect the process of precipitation during discharge. For example, the final composition of the altered seawater will, among many other variables, be affected by the rate and extent of retrograde reactions during the ascent of the fluid from the zone of highest temperatures through lower temperatures before final discharge. Another variable is the rate of discharge and
2. Experimental procedures Water-dominated basalt-seawater interaction. Four grams of fresh basaltic glass (from the Juan de Fuca Ridge, crushed and sieved to 4 3 - 6 3 / a m ) were reacted with 200 g of artificial SO4-free seawater (19.5%o chlorinity), at 260°C and 500 bars. SO4 was excluded from the chemical system since in all previous experiments SO 4 is essentially quantitatively removed as anhydrite in the early stages of interaction as all temperatures greater than 150°C. Thus, in natural systems, by the time seawater permeates to zones of maximum temperatures ( 2 0 0 - 3 0 0 ° ) , all the SO4 will have been removed. The deeper geothermal seawaters as Reykjanes, Iceland, contain less than 60 ppm SO 4 and support this inference [8]. The experimental equipment was the Dickson et al. [9], hydrothermal solution equipment which allows sampling of the altered seawater without disturbing the temperature and pressure of the experiment. Six 10-ml samples were taken during the 2400 hours of the experiment, by gas-tight syringe, membrane filtered (0.1/am) and subdivided into aliquots by weight for analysis. An aliquot was diluted and preserved for analysis of the major components Ca, Mg, K, Na and SiO2, and another was acidified with ultra-pure HNO 3 for analysis of the metals Fe, Mn, Zn,
TABLE 1 Concentration of selected dissolved species in artificial seawater reacted with Juan de Fuca basaltic glass at 260°C and 500 bars (Na, K, Ca, Mg, and SiO2 in parts per thousands; Fe, Mn, AI, Cu, Zn, and Ba in parts per million) Hours
pH *
Na
K
Ca
Mg
Start 60 228 384 576 1000 2400
8.0 4.15 4.24 4.50 4.65 4.58 4.65
10.1 10.2 10.3 10.3 10.3 10.3 10.3
0.363 0.371 0.368 0.373 0.371 0.370 0.370
0.373 1.153 1.569 1.559 1.481 1.497 1.420
1.210 0.636 0.320 0.294 0.350 0.350 0.351
* Measured at 25°C.
SiO2 <0.01 1.00 1.39 1.38 1.33 1.39 1.35
Fe
Mn
<0.01 30 33 38 40 42 45
<0.01 19 20 20 20 19 20
A1
Cu
Zn
Ba
<0.01 0.28 0.28 0.25 0.22 0.20 0.21
<0.01 0.01 0.02 0.19 0.38 0.50 0.38
<0.01 1.1 1.3 1.5 1.1 1.1 1.1
<0.01 0.04 0.09 0.155 0.135 0.135 0.135
73 TABLE 2 Chemical analysis of basaltic glass and alteration product before and after reaction with seawater at 260°C, 500 bars Basaltic glass (Juan de Fuca Ridge basalt)
Altered basaltic glass,2400 hr (smectite)
Smectite from altered basalt Mid-Atlantic Ridge* 39.7 16.5 15.6 12.0 0.5 0.8 1.7 1.8 0.06
SiO2 A1203 FeO ** MgO CaO Na20 K20 TiO2 MnO Ignition loss ***
50.10 14.80 12.61 7.00 11.10 2.57 0.14 1.28 0.22
43.9 15.4 12.7 16.44 1.2 1.2 <0.05 1.8 <0.1
0.10
6.6
Total
99.92
99.39
(-) 88.66
* Reported by Melson and Thompson [ 13 ]. ** Total iron expressed as FeO, solids dried at 100°C prior to analysis. *** Weight loss at 1000°C, includes loss of volatiles, and corrected for oxidation of FeO. (-) not analyzed.
Cu, Ba and A1. The pH was measured immediately by combination electrode at 25°C, and SiO2 was measured colorimetrically. Other species were analyzed by flame and flameless atomic absorption spectrophotometry. The solid material recovered from the reac, tion vessel at the termination of the experiment was composed of a uniformly blue-green smectite. Analysis
of the smectite was carried out after drying at 100°C and fusing with lithium metaborate. A similar procedure was followed for the unreacted basaltic glass. Si, A1, Fe, Mg, Ca, Na, K, Ti, and Mn were analyzed by atomic absorption spectrophotometry.
Interaction o f a l t e r e d seawater with normal seawater. A sample of altered seawater was collected at conditions at 2400 hours and filtered as above in gas-tight syringes. Three aliquots were then transferred to syringes containing measured amounts of normal seawater as follows: (1) 10 g altered to 10 g normal; (2) 5 g altered to 15 g normal; (3) 1 g altered to 39 g normal. A precipitate was visible within a few hours in (1) and (2), but not in (3). After 24 hours, fluid samples were taken of each of the three syringes, membrane filtered and analyzed as above.
3. Experimental results
Basalt-seawater interaction. Changes in the major components are characterized by a depletion of Mg, a drop in pH to 4.6, and increase in Ca and SiO2 (Table 1). In contrast with the earlier rock-dominated experiment [1 ], Mg depletion was not complete (from 1200 ppm down only to 350 ppm compared with 10 ppm in the earlier). Ca, K, and Ba are nearly quantitatively extracted from the rock to the aqueous phase. Fe, Mn, Zn and Cu increase sharply to values of 45, 20, 1.2 and 0.38 ppm respectively (Table 1), implying that a significant proportion of these elements,
TABLE 3 Results of mixing experiment between metal-bearing altered seawater and normal seawater (concentrations in parts per million)
Mg SiO2 Fe Mn pH
(1) 10 g altered seawater 10 g normal seawater
(2) 5 g altered seawater 15 g normal seawater
(3) 1 g altered seawater 39 g normal seawater
initial concentration of mixture
final concentration (24 hours)
initial concentration of mixture
final concentration (24 hours)
initial concentration of mixture
final concentration (24 hours)
820 600 22 9 -
790 130 0.2 7.8 6.4
1010 300 10 4.5 -
850 200 0.2 4.5 6.6
1220 30 1.0 0.45 -
1219 30 0.1 0.45 7.6
74 except Fe, have been solubifized. A1 remains relatively insoluble, never exceeding 0.28 ppm. The basaltic glass, analyzed after termination of the experiment was entirely made over to a magnesian smectite (Table 2). Its X-ray diffraction pattern exhibited characteristic basal spacings with a doo 1 at 14.5 A (50% humidity), expanding to 16.5 A after glycolation. The 06;33 reflection at 1.532 A indicated the smectite is trioctahedral.
Mixing experiment. Changes in the composition of the combined fluids allow assessment of the composition of the precipitate. Fe was essentially quantitatively precipitated in each of the three mixing ratios, whereas Mn in each case remained totally in solution (Table 3). SiO 2 and Mg were precipitated in the 1 : 1 and 1 : 3 mixtures, but apparently not in the 1 : 39 mixture (Table 3).
4. Discussion The results of seawater-basalt experiment at high water/rock ratios clearly indicate that the heavy metals are strikingly solubilized. Moreover, the loss of Ca, K, and SiO 2 from the basalt and the gain in Mg during alteration (Table 2) agrees well with the direction of major-element exchange observed in hydrothermally altered basaltic glass dredged from submarine ridge system [10-12]. The smectite formed in the experiment is an Fe-rich saponite and is similar to secondary smectite found in altered basalt from St. Peters and St. Pauls Rocks ([13], and Table 3). The enrichment of FeO and A1203 in our experimental smectite is produced by selective loss of other components to the fluid phase during alteration. How does the fluid derived in our experiment relate to that which deposited sea-floor metalliferous sediments? Bostr6m [14] calculated that a hydrothermal fluid would have to be debouching onto the ocean floor from submarine hot springs in the area of the East Pacific Rise at a rate of 0.01 litres/cm 2 per yqar and contain at least 10 ppm Fe and 3 - 5 ppm Mn to provide the metals required by the measured accumulation rates of these elements in the metalliferous sediment. These concentrations are in good agreement with those estimated by Mottl [2], based on flow rates re-
quired by the heat flow model of Wolery and Sleep [3]. The concentrations of Fe (45 ppm) and Mn (20 ppm), produced during our experiment (Table 1) are consistent with these limits, and are of the proper relative proportions as found in the metal-rich sediments [15]. Why should heavy metals be so much more soluble in seawater-dominated than in rock-dominated systems? During the earlier experiment in a rock-dominated system [1 ], the first few days of interaction were characterized by rapid release of heavy metals to levels close those observed in this present experiment, followed by a gradual reprecipitation of these metals into the alteration phase. The period of rapid and pronounced solubility was paralleled by an almost quantitative removal of Mg from the seawater and a corresponding drop in pH. After completion of Mg removal, the pH slowly rose, and the metals were gradually and continuously removed from solution. In the present experiment, Mg was only partially removed from seawater, presumably because there was insufficient capacity in the rock to "titrate" the seawater Mg to completion, and the pH remained low and apparently stabilized. We interpret these observations to indicate that seawater Mg is reacting with SiO2 released from the rock to form a "sepiolite"-type [Mg2Si306(OH)4 ] compound or alternatively a "brucite"-type [Mg(OH)2]'molecule constituting part of the final layered silicate alteration phase. Precipitation of such a molecule requires not only Mg and SiO 2, but O H - as well, taken from the hydrolysis of water. H+ equivalent to the amount of Mg precipitated is initially produced in the aqueous phase to maintain electrical neutrality. The H+ then attacks the rock to a degree dependent upon the water/rock ratio. Thus, the seawater becomes acid. In the rock-dominated situation, the mass of dissolved Mg is small relative to the mass of rock, and although the solution turns acid initially as Mg is being removed (and solubilizing metals in the process), Mg is finally quantitatively removed, and the following interaction is characterized by simple hydrolysis of silicates which produces OH-. The seawater then gradually turns alkaline and the initially released metals pass into the alteration phase. In the seawater-dominated system, there is excess Mg and the rock is completely made over to the "sepio lite-bearing" alteration phase. Thus, the excess dis-
75 solved Mg, silica and H+ reflect quasi-equilibrium with the alteration product and the pH is maintained at a slightly acid, steady state concentration, as are the metals in solution. What processes will take place if a fluid similar to that produced in our experiment, rich in Fe, Mn, and SiO2 were to discharge on the sea floor and interact with cold, oxygenated, and slightly alkaline seawater? The results of the mixing experiments provide some qualitative insight. Oxidation and rapid removal of dissolved Fe, presumably as ferric oxyhydroxide, is seen to take place rapidly and quantitatively in all three dilutions. In contrast, and regardless of the dilution factor, Mn remains in solution, demonstrating no tendency to precipitate as a discrete phase nor to co-precipitate with Fe even though excess 02 is present. Such efficient separation of Fe from Mn in aqueous systems has been observed in many natural systems and has commonly been attributed to the relatively sluggish oxidation of Mn 2+ at a pH <9 [16]. The basal metalliferous sediments associated with the active ridge systems in the eastern Pacific, however, are characterized by a strikingly consistent Fe/Mn ratio over large areas and through time [ 15,17], suggesting that quantitative fractionation of these metals is probably not taking place. In the natural environment, the relative time delay of Mn precipitation may not be significant, and the Mn that must eventually precipitate becomes ultimately mixed with the Fe compounds. The distribution of Fe and Mn may simply reflect physical mixing and homogenization processes which destroy the record of small-scale fractionation. The chemistry of the metal-rich sediments would also be modified by hydrogenous, biogenous, and detrital contributions depending on local accumulation rates [ 18]. The behavior of SIO2, Mg, and pH as the reacted seawater is diluted with increased amounts of seawater, is also of interest. In a mixture dominated by reacted seawater, SiO2 would not be greatly diluted and might be expected to precipitate upon cooling as amorphous SIO2. This situation is represented by the 1 : 1 ratio (Table 3), where SiO2 decreases from 617 to 130 ppm, a level presumably maintained by the solubility of amorphous SiO2. A slight removal of Mg is also apparent. The bulk precipitate in this case is dominated by amorphous SiO 2 and contains
perhaps 5-10% Fe, depending on the amount of water of hydration. The barite-opal precipitates of the Lau Basin [19], were possibly deposited under similar conditions. The dilution of altered seawater with three parts normal seawater (Table 3) displays significant removal of Mg, along with a less complete SiO 2 depletion and decrease in pH. Analyses of this mixture on subsequent days (data not shown) indicate a continuous adjustment of the Mg/Si ratio in the precipitated phase. Initially Mg rich and SiO 2 poor, the precipitate continued to extract SiO 2 from the solution until a constant value of 80 ppm SiO 2 was reached. At the same time, Mg and pH rose to 940 and 7.3, respectively. These developments can best be interpreted as the result of formation of a hydrated Mg silicate. In this case, the bulk precipitate was composed of approximately equal proportions of Mg and SiO2, with approximately 5-10% Fe depending on the degree of hydration. The hydrothermally deposited sediments of the Bauer Deep [20], are not only enriched in Fe and Mn but contain almost 50% SiO 2 and Mg greatly in excess of that contributed by detrital contamination. Bischoff and Rosenbauer [21] have attributed this enrichment to the precipitation of a Mg-SiO 2 phase as a consequence of the mixing between the hydrothermal fluid and ocean bottom water. A hydrothermal fluid which is rapidly mixed with large amounts of bottom water is depicted in the 1 : 39 mixture (Table 3). In this case, SiO 2 is not only diluted below amorphous SiO2 saturation but below that necessary to precipitate a Mg phase, and the hydrothermally derived SiO 2 simply remains in solution. The bulk precipitate in this case is apparently pure ferric hydroxide, and is analogous, perhaps, to the virtually SiO2-free and Fe-rich metalliferous sediments found at the crest of the East Pacific Rise [17], which contrast with those of the Bauer Deep. Depending on the mixing ratios at and near the sea floor, and neglecting the many other variables that might apply, the hydrothermal precipitate might range from almost pure amorphous SiO2, through a Mg silicate heavy-metal precipitate, to a relatively pure, heavy-metal precipitate. That metalliferous sediments of the Bauer Deep and East Pacific Rise display such remarkable uniformity of composition over geographically wide areas
76 implies that conditions of hydrothermal fluid generation, and also of sea floor discharge are remarkably uniform. Alternatively, mixing and distribution of precipitates by deep ocean currents must efficiently homogenize the composition before deposition.
5. Summary and conclusions We feel the experimental results support the following conclusions: (1) In a water-dominated system (wate r/rock = 50 : 1) at 260°C and 500 bars seawater efficiently leaches heavy metals from basaltic glass and maintains them in solution at concentrations and relative proportions within theoretical limits required of a transport fluid for ridge-crest metalliferous sediments. In experimental and field observations of rock-dominated systems, heavy metals are initially released by the basalt only to be reprecipitated in alteration products. (2) The increased solubility of heavy metals in the seawater-dominated system is attributed to the large and excess reservoir of dissolved Mg. Mg2÷ is the single most active component in seawater reacting with basalt, precipitating with SiO 2 released by the rock and in the process releasing I4+ to the water. In the rock-dominated systems, the rock quantitatively titrates the dissolved Mg in the early stages of interaction, and only OH--producing hydrolysis reactions take place subsequently, causing the initially solubilized heavy metals to precipitate in the alteration phase. In water-dominated systems, the rock does not have the capacity to remove all dissolved Mg, and the pH is therefore buffered at a low value throughout and subsequent to interaction. (3) Results of the experiments mixing metal-bearing reacted seawater with normal seawater qualitatively reveal that the mixing ratio of reacted to normal exerts a fundamental control on the bulk composition of the precipitate. Fe is quantitatively precipitated under almost all mixing conditions. SiO2 constitutes a major proportion of the precipitate under conditions of low dilution (ca. 1 : 1), Mg silicate under intermediate (ca. 1 : 3), but both are kept below saturation at high ratios (ca. 1 : 40). "Hydrothermal" precipitates might thus range from almost pure amorphous SiO 2 to pure ferric hydroxide (with as-
sociate metals), depending on the mixing conditions alone. (4) The actual process of subsurface seawater circulation, interaction, and discharge on the sea floor must be uniform over large areas to account for the marked uniformity of sea-floor hydrothermal metal deposits along and adjacent to the equatorial East Pacific Rise.
Acknowledgements We aregrateful to F.W. Dickson, D.Z. Piper, M.J. Mottl and H.D. Holland for their comments, suggestions and discussion which greatly improved this manuscript. We would also like to thank W.G. Melson who kindly provided the basalt glass used in this study. This research was supported by N.S.F. Grant No. IDO74-12880-A01.
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