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Extraordinary phase separation and segregation in vent fluids from the southern East Pacific Rise
Earth and Planetary Science Letters 206 (2003) 365^378 www.elsevier.com/locate/epsl
Extraordinary phase separation and segregation in vent £uids from the southern East Paci¢c Rise K.L. Von Damm a; , M.D. Lilley b , W.C. Shanks III c , M. Brockington a , A.M. Bray a , K.M. O’Grady a , E. Olson b , A. Graham b , G. Proskurowski b , the SouEPR Science Party 1 a
Institute for the Study of Earth, Oceans and Space and Department of Earth Sciences, University of New Hampshire, Durham, NH 03824-3525, USA b School of Oceanography, University of Washington, Seattle, WA 98195, USA c U.S. Geological Survey, 973 Denver Federal Center, Denver, CO 80225, USA Received 8 April 2002; received in revised form 19 August 2002; accepted 13 November 2002
Abstract The discovery of Brandon vent on the southern East Pacific Rise is providing new insights into the controls on midocean ridge hydrothermal vent fluid chemistry. The physical conditions at the time of sampling (287 bar and 405‡C) place the Brandon fluids very close to the critical point of seawater (298 bar and 407‡C). This permits in situ study of the effects of near critical phenomena, which are interpreted to be the primary cause of enhanced transition metal transport in these fluids. Of the five orifices on Brandon sampled, three were venting fluids with less than seawater chlorinity, and two were venting fluids with greater than seawater chlorinity. The liquid phase orifices contain 1.6^1.9 times the chloride content of the vapors. Most other elements, excluding the gases, have this same ratio demonstrating the conservative nature of phase separation and the lack of subsequent water^rock interaction. The vapor and liquid phases vent at the same time from orifices within meters of each other on the Brandon structure. Variations in fluid compositions occur on a time scale of minutes. Our interpretation is that phase separation and segregation must be occurring ‘real time’ within the sulfide structure itself. Fluids from Brandon therefore provide an unique opportunity to understand in situ phase separation without the overprinting of continued water^rock interaction with the oceanic crust, as well as critical phenomena. B 2002 Elsevier Science B.V. All rights reserved. Keywords: black smokers; hydrothermal processes; mid-ocean ridges; East Paci¢c Rise; phase separation
* Corresponding author. Tel.: +1-603-862-0142; Fax: +1-603-862-0188. E-mail address: [email protected] (K.L. Von Damm). 1 SouEPR Science Party: M.D. Lilley (chief scientist), K.L. Von Damm (co-chief scientist), R. Collier, J. Cowen, R. Haymon, M.K. Tivey, D. Fornari, K. Nakamura, E. McLaughlin-West, T. Shank, E. Olson, A. Graham, G. Proskurowski, J. Kaye, A. Bray, M. Brockington, K. O’Grady, J. Hobson, J. Sarrazin, M. Sparrow, D. Hubbard, D. McGee, S. Brinson, B. Cushman.
0012-821X / 02 / $ ^ see front matter B 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 1 0 8 1 - 6
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1. Introduction Although phase separation, and subsequent segregation of the vapor and liquid phases, is now accepted as one of the major controls on the composition of hydrothermal £uids that vent on the mid-ocean ridges, rarely have both phases been sampled from the same chimney, and then only at times several years apart [1]. Most models of sea£oor hydrothermal £uid generation must therefore assume storage of one of these phases, usually the liquid phase or ‘brine’, within the oceanic crust [1,2]. An area which has been of longterm interest to the study of sea£oor hydrothermal systems is the ultra-fast spreading southern East Paci¢c Rise (full opening rate V15 cm/yr), due in part to the large 3 He plume [3,4] that is swept o¡ the ridge axis from this region and then becomes a large and distinctive feature of the mid-
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depth circulation in the South Paci¢c. While previous water column work suggested robust hydrothermal venting between 13‡30P and 18‡40PS [5], little work had actually been done south of V19‡S. Studies by Krasnov et al. [6] and Tufar [7] presented strong evidence for signi¢cant hydrothermal activity around 21‡S. In October 1998 we discovered a new hydrothermal ¢eld, the ‘Rapa Nui’ ¢eld, located at 21‡33.6^33.8PS and 114‡18.0PW, which contains at least nine individual vents including ‘Brandon’ vent (Fig. 1). Brandon, located at an Alvin depth of 2834 m (287 bar) on the ultra-fast spreading southern East Paci¢c Rise, has not only the hottest £uid temperatures yet measured in a submarine hydrothermal vent at 405‡C, but is therefore extremely close to the critical point for seawater [8]. Brandon also vented £uids that di¡ered by almost a factor of two in composition from ori¢ces less than 2 m apart on its chimney structure. Hence for the ¢rst time we have observed both the vapor and liquid phases venting simultaneously from a single structure.
2. Geological setting
Fig. 1 (Continued). (a) Map of the eastern Paci¢c Ocean, showing the location of the East Paci¢c Rise, including the location of the Rapa Nui hydrothermal ¢eld. Modi¢ed after Macdonald et al. [23]. (b) Detailed bathymetric map of the Rapa Nui ¢eld, including the location of Brandon vent, and other hydrothermal vents, at this site. The bathymetry is based on SeaBeam data collected during the SouEPR cruise.
The East Paci¢c Rise at 21‡S is spreading at a full rate of 15.2 cm/yr classifying it as an ultra-fast spreading center [9]. A detailed morphotectonic study of the rise from 21‡12P to 22‡40PS is reported by Krasnov et al. [6]. Located south of the overlapping spreading center at 20‡40PS, this section of axis has a deep axial valley of s 2800 m. Our data suggest that in the Rapa Nui hydrothermal ¢eld the axial trough is tectonically controlled, with several V10 m deep and V20 m wide steps leading from the edge (V2800 m) to the deepest part of the axial cleft (V2860 m). The walls of the steps are essentially vertical, exposing massive basalt £ows. Brandon occurs on the second step down into the cleft on the western wall. The sul¢de structure of Brandon is at least 11 m tall. It is a large structure that extends across the width of the bench. It was impossible to determine the exact footprint of the structure because of the quantity of intense black smoke being emitted as well as the strong updraft. There
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are tens of individual active ori¢ces on the structure.
3. Methods All samples were collected with either titanium ‘majors’ syringe, or evacuated titanium ‘gas tight’ water samplers from the submersible Alvin. In some cases these individual bottles were connected to the NOAA manifold sampler. Temperatures were measured with either the Alvin high temperature probe, the temperature probes built into the manifold sampler, or inductively coupled link (ICL) temperature probes. In the case of the ICLs, the temperature probe is attached to the snorkels on the water samplers, and the temperature data are observed and logged inside Alvin during sampling. In many cases more than one method was used to measure the temperature at the time an ori¢ce was sampled, and all temperature probes were intercalibrated to an NISTtraceable reference. It is known that Mg is essentially quantitatively removed from seawater during hydrothermal reactions, based on both experimental and ¢eld studies [10,11]. As some seawater is always entrained during sampling, we extrapolate all compositions to Mg = 0 mmol/kg and report these as the ‘end member’ or pure hydrothermal £uid
compositions [11]. Methods of water sample handling and analysis, and calculation of end member compositions are as in Von Damm [11], the method of boron determination as in Bray and Von Damm [12], and methods of isotopic analysis for oxygen, hydrogen and sulfur as in Shanks [13]. Table 1 summarizes the water samples collected and sample quality.
4. Results Five separate ori¢ces were sampled on the Brandon structure (Tables 1 and 2). Of the ¢ve ori¢ces, three vented £uids with chlorinities less than the local ambient seawater value, and two vented £uids with chlorinities greater than the local ambient seawater value. These variations are the result of phase separation, which we interpret to be occurring within the sul¢de structure itself. Phase segregation also occurs with the vapor phase venting from di¡erent ori¢ces than the liquid (or brine) phase. The brine ori¢ces are located lower on the chimney structure than the vapor ori¢ces, and are just a few meters away. The three vapor phase ori¢ces were all sampled more than once on the cruise, and each varied chemically during this time period (Table 2, Figs. 2 and 3). The two liquid phase ori¢ces were each sampled only once, and on di¡erent days. Within the pre-
Table 1 Number and quality of £uid samples from Brandon vent Ori¢ce Vapor ori¢ces Ba.1 Ba.2 Ba.3 Ba.5 Bc.4 Bc.5 Bd.4 Bd.5 Brine ori¢ces Bb.3 Be.5
Temp. (‡C)
Dive
Date
Number of samples
Minimum Mg (mmol/kg)
404 405 401 400 403 n.m. 401 405
3289 3290 3302 3306 3303 3306 3303 3306
19 Oct 98 20 Oct 98 3 Nov 98 7 Nov 98 4 Nov 98 7 Nov 98 4 Nov 98 7 Nov 98
5 4 6 2 6 3 6 3
1.03 3.67 2.12 24.8 3.08 1.20 1.33 9.95
368 376
3302 3306
3 Nov 98 7 Nov 98
6 3
5.36 7.08
n.m. = not measured, temperature probe unplugged.
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Table 2a Brandon end member vent £uid compositions Ori¢ce
Temp.NaCl Cl Si (‡C) (wt%) (mmol/kg) (mmol/kg)
a b c d e
pHa
1.85 1.98 1.98 1.93 1.78 1.88 1.74 1.93
317 T 2 338 T 2 339 T 2 330 T 2 304 T 2 321 T 4 297 T 4 330 T 2
8.78 T 0.09 9.28 T 0.13 9.23 T 0.09 9.58 T 0.09 8.82 T 0.09 8.94 T 0.09 8.69 T 0.10 9.55 T 0.12
8.16 T 0.08 7.52 T 0.08 7.70 T 0.08 7.98 T 0.08 7.80 T 0.08 7.14 T 0.07 7.93 T 0.08 7.95 T 0.08
3.2 6 3.7 3.1 6 5.1 3.2 3.1 3.1 6 4.0
3.26 3.26 3.20
558 T 3 557 T 3 540
12.1 T 0.1 12.5 T 0.1 0.155
6.66 T 0.07 6.86 T 0.07 0
3.2 3.3 7.8
Na-measb Na-CBb K (mmol/kg) (mmol/kg) (mmol/kg)
Li Ca Sr Fe (Wmol/kg) (mmol/kg) (Wmol/kg) (Wmol/kg)
267 T 5 303 T 9 266 T 9 244 T 8 251 T 5 255 T 5 242 T 5 270 T 5
261 275 279 270 256 267 248 282
7.49 T 0.07 9.12 T 0.28 8.23 T 0.12 8.21 T 0.08 7.27 T 0.10 7.64 T 0.17 6.87 T 0.11 7.68 T 0.08
296 T 3 335 T 10 312 T 4 302 T 3 271 T 3 277 T 6 270 T 3 309 T 3
17.5 T 0.2 20.6 T 0.7 18.4 T 0.2 20.3 T 0.2 16.3 T 0.2 17.8 T 0.2 15.8 T 0.2 21.3 T 0.2
7 590 T 80 8 680 T 250 8 150 T 80 7 740 T 80 6 660 T 70 7 350 T 200 6 970 T 150 7 570 T 80
676 T 7 790 T 31 731 T 7 762 T 13 636 T 11 687 T 19 622 T 8 702 T 7
77 T 8 81 T 8 95 T 9 87 T 9 75 T 8 101 T 10 100 T 10 78 T 8
30.708 449 T 9 30.685 441 T 9 2.4 464
451 449 464
13.4 T 0.1 13.8 T 0.1 10.1
489 T 5 488 T 5 26
34 T 0.3 97.6 T 2.0 12 500 T 120 32.8 T 0.3 93.5 T 2.0 12 300 T 120 9.95 87 0
1300 T 10 1300 T 10 0
120 T 12 121 T 21 0
Alk (meq/kg)
30.688 6 30.547 30.842 6 0.121 30.607 30.911 30.815 6 30.054
49.5 T 1.0 62.5 T 2.4 53.9 T 1.0 54.1 T 1.0 48.9 T 1.0 73.8 T 3.5 47.2 T 1.0 63.1 T 1.0
Mn Znc Cuc SO4 (Wmol/kg) (Wmol/kg) (Wmol/kg) (mmol/kg)
48 T 5 51 T 5 45 T 5 81 T 8 51 T 5 81 T 8 45 T 5 54 T 5
Br B N18 O (Wmol/kg) (Wmol/kg)
1.92 T 0.06 3.21 T 0.06 1.73 T 0.18 2.91 T 0.06 3.63 T 0.33 2.75 T 0.66 2.63 T 0.19 9.53 T 0.2
87 T 9 1.59 T 0.08 105 T 11 30.50 T 0.14 0 28.0
ND
N34 S
600 T 11 680 T 20 560 T 17 570 T 17 500 T 15 520 T 16 490 T 15 550 T 17
434 T 13 431 T 13 433 T 13
0.65 T 0.1 1.1 T 1.0 0.6 T 0.1 0.71 T 0.1 3.6 T 1.0
5.5 T 0.2
426 T 13 432 T 13 429 T 13 432 T 13
0.57 T 0.1 0.7 T 1.0 0.98 T 0.1 0.77 T 0.1 1.7 T 1.0
880 T 26 890 T 27 840
462 T 13 465 T 13 412
0.7 T 0.1 1.1 T 1.0e 5.0 T 0.2 0.82 T 0.1 2.9 T 1.0e 4.2 T 0.2 0.00 0 20
5.4 T 0.2 5.0 T 0.2 5.3 T 0.2 4.8 T 0.2 5.4 T 0.2
25‡C, 1 atm. Na-meas = measured Na concentrations; Na-CB = Na concentrations calculated from the charge balance on the end member composition. Total [Zn] and [Cu] including dissolved phase and particles. n.m. = not measured, temperature probe unplugged. Based on one sample.
Table 2b Molar ratios for Brandon end member £uid compositions Ori¢ce
Temp. (‡C)
Vapor ori¢ces Ba.1 404 Ba.2 405 Ba.3 401 Ba.5 400 Bc.4 403 Bc.5 n.m.c Bd.4 401 Bd.5 405 Brine ori¢ces Bb.3 368 Be.5 376 Seawater 1.9
a b c
Cl (mmol/kg)
Si/Cl
H2 S/Cl
Na-measa /Cl
Na-CBa /Cl
K/Cl
Li/Cl (W/m)
Ca/Cl
Sr/Cl (W/m)
Fe/Cl (W/m)
Mn/Cl (W/m)
Zn/Clb (W/m)
Cu/Clb (W/m)
Br/Cl (W/m)
B/Cl (W/m)
Fe/H2 S Fe/Mn
Cu/Zn K/Na
Na/Ca Li/K (W/m)
Ca/Sr
317 338 339 330 304 321 297 330
0.0277 0.0275 0.0272 0.0290 0.0290 0.0279 0.0293 0.0289
0.0257 0.0222 0.0227 0.0242 0.0257 0.0222 0.0267 0.0241
0.842 0.896 0.785 0.739 0.826 0.794 0.815 0.818
0.830 0.812 0.824 0.801 0.837 0.831 0.837 0.841
0.0236 0.0270 0.0243 0.0249 0.0239 0.0238 0.0231 0.0233
0.934 0.991 0.920 0.915 0.891 0.863 0.909 0.936
0.0552 0.0609 0.0543 0.0615 0.0536 0.0555 0.0532 0.0645
0.156 0.185 0.159 0.164 0.161 0.230 0.159 0.191
23.9 25.7 24.0 23.5 21.9 22.9 23.5 22.9
2.13 2.34 2.16 2.31 2.09 2.14 2.09 2.13
0.243 0.240 0.280 0.264 0.247 0.315 0.337 0.236
0.151 0.151 0.133 0.245 0.168 0.252 0.152 0.164
1.89 2.01 1.65 1.73 1.64 1.62 1.65 1.67
1.37 1.28 1.28 0.00 1.40 1.35 1.44 1.31
0.93 1.15 1.06 0.97 0.85 1.03 0.88 0.95
11.23 10.99 11.15 10.16 10.47 10.70 11.21 10.78
0.62 0.63 0.47 0.93 0.68 0.80 0.45 0.69
0.0281 0.0301 0.0309 0.0336 0.0290 0.0300 0.0284 0.0284
15.3 14.7 14.5 12.0 15.4 14.3 15.3 12.7
39.5 36.7 37.9 36.8 37.3 36.3 39.3 40.2
0.354 0.330 0.341 0.375 0.333 0.241 0.335 0.338
558 557 540
0.0217 0.0224 0.0003
0.0119 0.0123 0.0000
0.805 0.792 0.859
0.808 0.808 0.859
0.0240 0.0248 0.0187
0.876 0.876 0.048
0.0609 0.0589 0.0184
0.175 0.168 0.161
22.4 22.1 0.0
2.33 2.33 0.00
0.215 0.217 0.000
0.156 0.189 0.000
1.58 1.60 1.56
0.83 0.83 0.76
1.88 1.79 ^
9.62 9.46 ^
0.73 0.87
0.0298 0.0313 0.0218
13.2 13.4 46.6
36.5 35.4 2.6
0.348 0.351 0.114
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Vapor ori¢ces Ba.1 404 Ba.2 405 Ba.3 401 Ba.5 400 Bc.4 403 Bc.5 n.m.d Bd.4 401 Bd.5 405 Brine ori¢ces Bb.3 368 Be.5 376 Seawater 1.9
H2 S (mmol/kg)
Meas = measured Na; CB = Na calculated from the charge balance on the end member composition. Total [Zn] and [Cu] including dissolved phase and particles. n.m. = not measured, temperature probe unplugged.
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Fig. 2. Three-dimensional bar graphs showing the chemical and temperature variation in the various Brandon ori¢ces versus days between sampling. The vapor ori¢ces (Ba, Bc, Bd) are designated by the darker patterns, and the brine ori¢ces (Bb, Be) are designated by the lighter patterns. While there is temporal variability observed in the individual ori¢ces, the di¡erence between the vapor and brine ori¢ces is more striking. (a) Chloride, (b) lithium, (c) temperature, (d) silica.
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Fig. 3. See description for Fig. 2. This ¢gure shows the variability for the transition metals Fe (a) and Mn (b), whose concentrations may be a¡ected by the concentration of H2 S (c). The lower Fe/Mn ratio (d) observed in the brine ori¢ces compared to the vapor ori¢ces may indicate reduced Fe concentrations relative to Mn, due to the precipitation of Fe^S phases.
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Fig. 4. Phase relationships for seawater and Brandon £uids. The solid line is the two-phase curve for seawater, separating the liquid from liquid+vapor stability ¢elds. The dotted line separates the liquid+vapor from vapor+halite stability ¢elds. If a £uid crosses the two-phase curve below the critical point a vapor phase, containing a small amount of salt and higher gas contents, will be formed and this is referred to as either boiling or subcritical phase separation. If the solution crosses the two-phase curve above the critical point, a small amount of a high salinity and low gas content liquid (or brine) phase will be produced and this is referred to as either condensation or supercritical phase separation. Filled square = critical point for seawater (407‡C, 298 bar); open square = critical point for 2.2 wt% NaCl solution (400‡C, 280.7 bar); ¢lled circle = sampling conditions for Brandon vapors; open circles = sampling conditions for Brandon brines.
cision of the chemical measurements, the compositions of the brines are identical. The brines contain 1.6^1.9 times more chloride than the vapors (depending on the concentration chosen for the vapor phase), and this same enrichment factor holds true for most of the other chemical constituents. As a result of phase separation and chlorocomplexing, most cations (as well as bromide) are conservative with respect to chloride concentrations if phase separation is the only process occurring. Because the chloride concentrations in the brines are higher than those in the vapors, most of the other chemical species also have higher absolute concentrations. To determine if any relative loss or gain of a chemical species has occurred, e.g., as a result of processes other than phase separation, the elemental-to-Cl ratio must
be examined. Most of these elemental ratios are the same in the vapor and liquid phases, suggesting no other processes, such as water^rock reaction, have occurred after phase separation, the exceptions to this conservative behavior between the vapor and liquid phases in the Brandon £uids are H2 S, CO2 , H2 , Si, Zn and Fe. Boron is rather unique in its behavior as it is not chloro-complexed and at least in basalt hosted systems its concentration does not appear to be solubility controlled [12]. The boron concentrations di¡er by only V2% between the vapor and liquid phases, re£ecting the minimal e¡ects of phase separation on this hydroxyl-complexed element. The isotope ratios (O, D/H and S) also are the same in the vapor and liquid phases, and are within the range of values measured in other sea£oor hydrothermal systems [13]. Sulfur isotope fractionation between H2 SðaqÞ and H2 SðgÞ is small at room temperature and is expected to be insigni¢cant at high temperatures. The gases (H2 S, etc.) preferentially partition into the vapor phase, as can be seen in the higher concentrations of H2 S, CO2 , etc. in the vapor phase (Table 2, Fig. 3). The relative partitioning depends on the exact conditions of phase separation. In the case of H2 S, the di¡erences in concentration may also be a result of solubility controls from metal sul¢de formation, discussed in Section 5.2. The chemical data therefore suggest that phase separation has occurred with no subsequent overprinting by continued water^rock reaction with the predominately aluminosilicate oceanic crust, supporting our hypothesis that phase separation is likely occurring within the sul¢de structure itself.
5. Discussion Brandon £uid compositions are a result of seawater^rock interaction within the oceanic crust, followed by phase separation and segregation. Especially in the case of the liquid phase, which has a lower temperature, mineral solubility controls may also exist for some of the transition metals.
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5.1. Phase separation The exact conditions at which phase separation occurs will determine both the chlorinity and the relative amounts of the two phases [14]. As Cl is the major anion and Na the major cation, seawater is often treated as a 3.2 wt% NaCl solution for purposes of evaluating the conditions and net results of phase separation. Due to electroneutrality constraints, the Na concentration must equal the Cl concentration on a molar basis. The chloride content therefore determines the physical properties, such as density, of the solution. The measured temperature and pressure conditions (405‡C, 287 bar) at the time £uids from Brandon vent were sampled put them just below the critical point for seawater (407‡C, 298.5 bar) [8] (Fig. 4). As for pure water, at the critical point for a salt solution the compositions and physical properties of both the liquid and vapor phases are identical. Unlike the pure water case, in a salt solution the two-phase curve continues beyond the critical point. The closer to the critical point at which phase separation occurs, the more similar the compositions of the two phases. The temperature and pressure of the critical point for a £uid will also change with its salt content. As the £uids from the ¢ve ori¢ces sampled on the Brandon structure (Table 2, Figs. 2 and 3) di¡er in composition by a factor of 1.6^1.9, phase separation must be occurring at conditions that are either sub- or supercritical. For the case where the starting £uid had the same salt content as seawater, phase separation at temperatures of 400^410‡C and pressures of 280.1^305.7 bar, bracketing the critical point for seawater, would generate £uids with 1.47^3.84 wt% NaCl [14], which includes the entire range observed in £uids from Brandon. The starting £uid, however, may not have a seawater salt content. If hydrothermal £uid intersects the two-phase curve as it rises through the oceanic crust, it will begin to phase separate, and will then travel along the two-phase curve until it reaches the sea£oor. For example, if our starting £uid contained just 2.2 rather than 3.2 wt% NaCl, its critical point would be 400‡C and 280.7 bar [14]. (A 2.2 wt% NaCl solution was chosen because it is the minimal salt content measured in
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the Brandon £uids.) If the pressure on this 2.2 wt% NaCl £uid dropped to 280.0 bar, subcritical phase separation would occur, and the vapor would contain 1.35 wt% NaCl and the liquid 3.45 wt% NaCl. This also encompasses the range of £uid compositions sampled from Brandon. The small change in pressure needed to cause phase separation could occur within the V11 m in height of the Brandon sul¢de structure. Hence, because we have no means of determining the NaCl content of our £uid when phase separation began, we cannot discern if the phase separation occurred sub- or supercritically. However, in either case examined, the pressure^temperature conditions are very close to the measured conditions, and most are within the range observed in the structure itself. The Brandon £uid compositions also demonstrate that in the real systems, as has been shown previously in experimental studies [15,16], the net e¡ects of phase separation for the cation concentrations are conservative with respect to chloride. Berndt et al. [17] and Horita et al. [18] have experimentally studied hydrogen and oxygen isotope fractionation between liquid and vapor phases near the critical point of NaCl solutions. Based on these studies, little oxygen isotope fractionation is expected, but deuterium enrichment in the vapor up to 12.5x has been observed. However, the magnitude of the hydrogen isotope fractionation increases with the pressure di¡erential from the critical curve. Berndt et al. [17] derived the following equation from their experimental data: 1000 ln K vaporbrine ¼ 2:54 þ 2:87 log vP
where vP is the di¡erence in bars between the measured pressure and the critical pressure at the temperature of the experiment. According to Bischo¡ and Rosenbauer [8], the critical pressure for seawater at 405‡C is 293 bar, thus the pressure at the sea£oor sampling conditions is 6 bar below the critical curve pressure. Using the equation above, we calculate an expected ND enrichment in the vapor of 4.8x. This is not observed in our data. However, small errors in the sea£oor temperature measurements, pressure/depth conversion calculations, or a small amount of conductive cooling would introduce errors into this
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calculated fractionation. Phase separation within the Brandon structure at sea£oor pressure of 287 bar and at temperatures slightly above the measured 405‡C would not produce signi¢cant hydrogen isotope fractionation. 5.2. E¡ects of proximity to the critical point As hydrothermal £uids are seawater that has reacted within the oceanic crust, almost none of the dissolved species maintain their seawater concentrations or their element-to-element ratios (Table 2b). Essentially hydrothermal £uid compositions can be thought of as the net result of the composition of the starting seawater, the availability of elements from the rock, the complexation and transport of ions in solution, and water^rock equilibration. There is increasing evidence that the composition of most sea£oor hydrothermal £uids (early eruptive £uids being a notable exception) is controlled by the achievement of steady state or equilibrium between the £uids and an alteration mineral assemblage (e.g., [19]). At high temperature and pressure most of the cationic species in sea£oor hydrothermal £uids are present as chloro-complexes. As the critical point is approached complexation increases, until at the critical point all of the aqueous charged species should be present as neutral species complexes [20]. This would permit enhanced transport of species in the £uids as mineral solubilities would not be exceeded due to the completeness of complexation. One therefore expects higher concentrations of the cationic species Li, K, Na, Ca, Sr, Mn, Fe, as well as Br (which usually behaves conservatively with respect to Cl) in the higher chlorinity liquid phase, in comparison to the lower chlorinity vapor phase, while the gases (e.g., CO2 , H2 S, H2 ) will be preferentially fractionated into the vapor phase [21]. This is indeed what we observe in the Brandon £uids. Our interpretation is therefore that proximity to the critical point results in the very high concentrations in both the vapor and liquid phases of metals, such as Fe, which are usually limited by solubility controls of metal sul¢de phases. A signi¢cant di¡erence does occur between the temperatures of the vapor and liquid phase £uids
from Brandon, with the liquid phase £uids having substantially cooler measured exit temperatures. This means that the brines are not as proximal to the critical point as are the vapors, and less complexation is occurring. The observation that the brines vent physically lower on the structure than the vapors may suggest that as the vapors rapidly jet out, the brine phase settles due to its greater density lower in the structure, resulting in a slightly longer residence time for the brines within the sul¢de structure. This may allow the brines to cool conductively (the structure is bathed in 2‡C seawater, forming essentially an in¢nite heat sink) before they exit. A second possibility, allowed by the measured Mg (minimum 5.36 mmol/kg) and temperature data, is that a small amount of seawater ( 6 10%) mixes into the brines within the structure prior to sampling, resulting in the cooler temperatures. The very low densities of the £uids near the critical point could result in enhanced entrainment of seawater (although this does not appear to be an important process in the sampled vapors where measured Mg concentrations are as low as 1.03 mmol/kg, and almost half of this is attributable to the dead volume in the sampling bottles). As the solubilities of most metal sul¢des also decrease with decreasing temperature, it is possible that the brines may be losing some of their dissolved constituents to mineral precipitation. The chemical data from the Brandon brines (Table 2) suggest loss of Fe and H2 S, based on the decrease in the ratios of these species to Cl, and the decrease in the Fe/Mn ratio. With conditions so close to the critical point, quantitative geochemical models cannot be rigorously applied so other approaches must be used. Using a mass balance approach, based on the H2 S/Cl ratios, between 1.65 and 0.14 mmol/kg of H2 S is lost between the vapor and liquid phases. Some of the enrichment in the vapors is a result of phase separation, and therefore only some of the relative loss in the brines may be due to the precipitation of metal sul¢des. As the Fe levels are so high, and the relative amount of Fe loss low, the loss of Fe is best identi¢ed through the Fe/Mn ratio (Fig. 3) in combination with the Fe/Cl and Mn/Cl ratios. A slight decrease in the Fe/Cl ratio and a larger decrease in
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the Fe/Mn ratio suggest that some Fe is lost in the liquid relative to the vapor phases. Mass balance constraints suggest the maximum Fe loss is 0.1 mmol/kg. Based on the stoichiometry of the minerals likely to precipitate, this suggests that the maximum loss of H2 S due to mineral precipitation is V0.2 mmol/kg. Surprisingly it does not appear that Cu is lost between the vapor and liquid phases based on their respective Cu/Cl ratios, but some Zn does appear to be lost. Fe-, Cu- and Zn-sul¢de minerals are most common in chimneys. Cu is generally assumed to be the most temperature sensitive of the three, but at these high temperatures even for the liquid phase, loss of Cu does not appear to be occurring. Zn is, however, known to be more sensitive to losses caused by mixing. The loss of Zn is also indicated by the increase in the Cu/Zn ratio. A striking feature of the Brandon £uids is their unusually high Fe/Mn ratios (Fig. 3). The ratio of 9.5^11.2 is signi¢cantly higher than what is usually observed in sea£oor hydrothermal £uids, which cluster around the 3:1 ratio typically found in metalliferous sediments. Unusually high ratios have also been noted in hydrothermal £uids that are formed immediately following volcanic eruptions, which also have temperatures close to 400‡C [11]. The ratios are high in both cases due to unusually high levels of Fe, not low levels of Mn. Iron sul¢de minerals are usually the dominant minerals in structures formed by high temperature £uids on the mid-ocean ridges, and the £uids are often found to be at, or close to, saturation with these minerals. We suggest that the explanation for the high Fe/Mn ratios, as well as high Fe/Cl ratios, is the proximity of these £uids to the critical point. Under these conditions the almost complete complexation of Fe by Cl, and almost complete association of H2 S allows greater transport of Fe in these £uids, which is usually limited by the solubility of Fe^S minerals. Hemley et al. [22] noted that under some conditions decreasing pressure can result in increased metal solubility at isothermal conditions, which is a result of the increasing strength of aqueous complexes. Proximity to the critical point provides a previously undiscussed mechanism for transport of cations, especially transition metals,
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in sea£oor hydrothermal systems. Likely this is the correct explanation, or at least an important part of the explanation, for why many metals have enhanced transport at the high temperatures that are often associated with eruptive/magmatic events, rather than being a result of the more reducing nature of the £uids as previously proposed [1]. If proximity to the critical point is common, it will lead to enhanced £uxes of transition metals compared to what has been previously calculated. 5.3. Evidence for ‘real time’ phase separation While the ratios to Cl are generally constant between the vapor and liquid phase £uids, there is more scatter in the data than is expected (Table 2b). Given the acidic pH of these £uids, CO2 should be relatively unreactive in comparison to H2 and H2 S. Examination of the CO2 data versus either Mg (Fig. 5a) or Cl shows much greater scatter than the analytical error for individual samples collected from the same ori¢ce at the same time (i.e., sequentially one sample right after the other). While this scatter is also seen for other chemical constituents, it is perhaps most striking for CO2 . Usually, for a given sampling time at a given ori¢ce, the analytical data for all of the samples are ¢t to a least squares linear regression versus Mg, and the end member is calculated for each constituent [11]. In the case of CO2 (Fig. 5a) this ¢t is unusually noisy, and in the case of Cl (Fig. 5b) there is a sample from Ba.2 that falls distinctly o¡ the trend. If instead end member concentrations for CO2 and Cl are calculated for each sample, end member CO2 versus end member Cl now de¢nes a well-constrained line (Fig. 5c). If a similar plot is made for end member K versus end member Cl (Fig. 5d) the sample that falls o¡ of the measured Cl versus measured Mg trend (Fig. 5b) now lies on a mixing line between the vapor and liquid phase compositions. This suggests that in addition to the inter-day variability in composition, the £uids actually change composition on a time scale of minutes, the time needed to collect successive water samples. This is in contrast to most vents, where changes in vent £uid chemistries are rarely observed on a time scale of days, and more commonly on a scale
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Fig. 5. Property^property plots for various chemical parameters from the ori¢ces on the Brandon vent structure. SW designates seawater, B designates Brandon vent, a^e designate the individual ori¢ces, 1^5 designate the dive to Brandon on which that sample was collected. See Table 2 for more detailed description, and text for more detailed discussion. (a) Measured CO2 versus measured Mg, (b) measured Cl versus measured Mg, (c) end member CO2 calculated for each sample versus end member Cl calculated for each sample for the Ba ori¢ce only, (d) end member K calculated for each sample versus end member Cl calculated for each sample.
of years and sometimes decades. The maximum Cl for the CO2 versus Cl trend is 376 mmol/kg (Fig. 5c), which is equivalent to 2.2 wt% NaCl, consistent with this being a possible starting £uid composition. The Ba.2 sample that is o¡ the Cl versus Mg trend provides evidence, in this case, for incomplete segregation of the brine from the vapor phase as it is best explained as containing 54^71% vapor phase and 46^29% liquid phase,
depending on the composition chosen for the vapor phase. If such a model is invoked, our Cl ranges from 270 to 559 mmol/kg, or 1.58 to 3.26 wt% NaCl. While we still cannot distinguish if we are sampling above or below the critical point, these observations are additional strong evidence that the Brandon £uids undergo phase separation right at the sea£oor. We do not know the nature of the heat source
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that drives the hydrothermal circulation observed at Brandon vent. Several of the nearby vents (Fig. 1) are also very hot, with Preston, Roadrunner and Wallace having measured £uid temperatures s 380‡C, suggesting that the heat source extends at least 100 m along strike. There are no signs of fresh lavas, snowblower vents, or microbial blooms indicative of a recent volcanic eruption in the vicinity of Brandon. Neither are the silica contents of the £uids de¢nitive as to what the depth of water^rock reaction may be, beyond suggesting that the heat source is very shallow (only hundreds of meters below the sea£oor) at this site.
6. Conclusions Brandon vent, at a greater depth (2834 m) than has been previously studied on a magmatically robust and fast spreading mid-ocean ridge, vents £uids that are close to the critical point for seawater. This provides enhanced complexation, and hence enhanced transport of transition metals in these £uids. To explain the chemical data, the Brandon £uids must phase separate essentially instantaneously within the sul¢de structure at the sea£oor. Typically after phase separation occurs in sea£oor hydrothermal systems, water^rock reaction within the aluminosilicate oceanic crust continues, and overprints the results of phase separation. This is partly evidenced by di¡ering element-to-Cl ratios in the two phases. The Brandon £uids, as evidenced by their generally conservative elemental ratios, therefore provide an unique opportunity to examine the e¡ects of phase separation alone. While we cannot de¢nitely determine if these £uids phase separate sub- or supercritically, the compositions of these £uids provide new insights into the processes of phase separation, and the overall controls on the compositions of sea£oor hydrothermal vent £uids.
Acknowledgements We thank the master, o⁄cers and crew of the R/V Atlantis and the Alvin group for helping to make the SouEPR cruise a success. We would like
377
to dedicate this paper to the memory of Brandon Lilley. We thank J. Bischo¡ and R. Mills for their comprehensive reviews of this paper. This research was funded by the U.S. National Science Foundation through OCE9419156 (K.V.D.) and OCE9417121 (M.D.L.).[BOYLE]
References [1] K.L. Von Damm, L.G. Buttermore, S.E. Oosting, A.M. Bray, D.J. Fornari, M.D. Lilley, W.C. Shanks III, Direct observation of the evolution of a sea£oor ‘black smoker’ from vapor to brine, Earth Planet. Sci. Lett. 149 (1997) 101^112. [2] K.L. Von Damm, Systematics of and postulated controls on submarine hydrothermal solution chemistry, J. Geophys. Res. 93 (1988) 4551^4561. [3] J.E. Lupton, H. Craig, A major 3 He source on the East Paci¢c Rise, Science 214 (1981) 13^18. [4] J.E. Lupton, Hydrothermal helium plumes in the Paci¢c Ocean, J. Geophys. Res. 103 (1998) 15853^15868. [5] E.T. Baker, T. Urabe, Extensive distribution of hydrothermal plumes along the superfast spreading East Paci¢c Rise, 13‡30P^18‡40PS, J. Geophys. Res. 101 (1996) 8685^ 8695. [6] S. Krasnov, P. Poroshina, G. Cherkashev, E. Mikhalsky, M. Maslov, Morphotectonics, volcanism and hydrothermal activity on the East Paci¢c Rise between 21‡12PS and 22‡40PS, Mar. Geophys. Res. 19 (1997) 287^317. [7] W. Tufar, Investigation of modern hydrothermal complex massive sul¢de ore formation in the west Paci¢c and in the Mediterranean Sea, DeRidge Newsl. 1 (1995) 25^30. [8] J.L. Bischo¡, R.J. Rosenbauer, Liquid-vapor relations in the critical region of the system NaCl-H2 O from 380‡ to 415‡C: A re¢ned determination of the critical point and two-phase boundary of seawater, Geochim. Cosmochim. Acta 52 (1988) 2121^2126. [9] D.F. Naar, R.N. Hey, Recent Paci¢c-Easter-Nazca plate motions, in: J.M. Sinton (Ed.), Evolution of Mid-Ocean Ridges, IUGG Symp. 8, AGU Geophys. Monogr. 57 (1989) 9^30. [10] J.L. Bischo¡, F.W. Dickson, Seawater-basalt interaction at 200‡C and 500 bars: implications for origin of sea£oor heavy-metal deposits and regulation of seawater chemistry, Earth Planet. Sci. Lett. 25 (1975) 385^397. [11] K.L. Von Damm, Chemistry of hydrothermal vent £uids from 9‡^10‡N, East Paci¢c Rise: ‘Time zero’, the immediate posteruptive period, J. Geophys. Res. 105 (2000) 11203^11222. [12] A.M. Bray, K.L. Von Damm, The role of phase separation and water-rock reactions in controlling the boron content of mid-ocean ridge hydrothermal vent £uids, Geochim. Cosmochim. Acta (2002) in revision. [13] W.C. Shanks III, Stable isotopes in sea£oor hydrothermal
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[14]
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K.L. Von Damm et al. / Earth and Planetary Science Letters 206 (2003) 365^378 systems: Vent £uids, hydrothermal deposits, hydrothermal alteration, and microbial processes, in: J.W. Valley, D.R. Cole (Eds.), Stable Isotope Geochemistry: Rev. Mineral. Geochem. 43 (2001) 469^525. J.L. Bischo¡, Densities of liquids and vapors in boiling NaCl-H2 O solutions: A PVTX summary from 300‡ to 500‡C, Am. J. Sci. 291 (1991) 309^338. J.L. Bischo¡, R.J. Rosenbauer, Phase separation in sea£oor geothermal systems an experimental study of the e¡ects of metal transport, Am. J. Sci. 287 (1987) 953^978. M.E. Berndt, W.E. Seyfried Jr., Boron, bromine, and other trace elements as clues to the fate of chlorine in midocean ridge vent £uids, Geochim. Cosmochim. Acta 54 (1990) 2235^2245. M.E. Berndt, R.R. Seal II, W.C. Shanks III, W.E. Seyfried Jr., Hydrogen isotope systematics of phase separation in submarine hydrothermal systems: Experimental calibration and theoretical models, Geochim. Cosmochim. Acta 60 (1996) 1595^1604. J. Horita, D.R. Cole, D.J. Wesolowski, The activity-composition relationship of oxygen and hydrogen isotopes in aqueous salt solutions; III, Vapor-liquid water equilibra-
[19]
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tion of NaCl solutions to 350‡C, Geochim. Cosmochim. Acta 59 (1995) 1139^1151. A.M. Bray, K.L. Von Damm, Controls on the alkali metal composition of mid-ocean ridge hydrothermal £uids: constraints from the 9^10‡N East Paci¢c Rise time series, Geochim. Cosmochim. Acta (2002) in revision. H.C. Helgeson, E¡ects of complex formation in £owing £uids on the hydrothermal solubilities of minerals as a function of £uid pressure and temperature in the critical and supercritical regions of the system H2 O, Geochim. Cosmochim. Acta 56 (1992) 3191^3207. S.E. Drummond Jr., Boiling and Mixing of Hydrothermal Fluids: Chemical E¡ects on Mineral Precipitation, Ph.D. Dissertation, Pennsylvania State University, University Park, PA, 1981, 381 pp. J.J. Hemley, C.L. Cygan, W.M.. d’Angelo, E¡ect of pressure on ore metal solubilities under hydrothermal conditions, Geology 14 (1986) 377^379. K.C. Macdonald, R.M. Haymon, S. Miller, J.-C. Sempere, P.J. Fox, Deep tow and SeaBeam studies of dueling propagating ridges on the East Paci¢c Rise near 20‡40PS, J. Geophys. Res. 93 (1988) 2875^2898.
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Extraordinary phase separation and segregation in vent £uids from the southern East Paci¢c Rise K.L. Von Damm a; , M.D. Lilley b , W.C. Shanks III c , M. Brockington a , A.M. Bray a , K.M. O’Grady a , E. Olson b , A. Graham b , G. Proskurowski b , the SouEPR Science Party 1 a
Institute for the Study of Earth, Oceans and Space and Department of Earth Sciences, University of New Hampshire, Durham, NH 03824-3525, USA b School of Oceanography, University of Washington, Seattle, WA 98195, USA c U.S. Geological Survey, 973 Denver Federal Center, Denver, CO 80225, USA Received 8 April 2002; received in revised form 19 August 2002; accepted 13 November 2002
Abstract The discovery of Brandon vent on the southern East Pacific Rise is providing new insights into the controls on midocean ridge hydrothermal vent fluid chemistry. The physical conditions at the time of sampling (287 bar and 405‡C) place the Brandon fluids very close to the critical point of seawater (298 bar and 407‡C). This permits in situ study of the effects of near critical phenomena, which are interpreted to be the primary cause of enhanced transition metal transport in these fluids. Of the five orifices on Brandon sampled, three were venting fluids with less than seawater chlorinity, and two were venting fluids with greater than seawater chlorinity. The liquid phase orifices contain 1.6^1.9 times the chloride content of the vapors. Most other elements, excluding the gases, have this same ratio demonstrating the conservative nature of phase separation and the lack of subsequent water^rock interaction. The vapor and liquid phases vent at the same time from orifices within meters of each other on the Brandon structure. Variations in fluid compositions occur on a time scale of minutes. Our interpretation is that phase separation and segregation must be occurring ‘real time’ within the sulfide structure itself. Fluids from Brandon therefore provide an unique opportunity to understand in situ phase separation without the overprinting of continued water^rock interaction with the oceanic crust, as well as critical phenomena. B 2002 Elsevier Science B.V. All rights reserved. Keywords: black smokers; hydrothermal processes; mid-ocean ridges; East Paci¢c Rise; phase separation
* Corresponding author. Tel.: +1-603-862-0142; Fax: +1-603-862-0188. E-mail address: [email protected] (K.L. Von Damm). 1 SouEPR Science Party: M.D. Lilley (chief scientist), K.L. Von Damm (co-chief scientist), R. Collier, J. Cowen, R. Haymon, M.K. Tivey, D. Fornari, K. Nakamura, E. McLaughlin-West, T. Shank, E. Olson, A. Graham, G. Proskurowski, J. Kaye, A. Bray, M. Brockington, K. O’Grady, J. Hobson, J. Sarrazin, M. Sparrow, D. Hubbard, D. McGee, S. Brinson, B. Cushman.
0012-821X / 02 / $ ^ see front matter B 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 1 0 8 1 - 6
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1. Introduction Although phase separation, and subsequent segregation of the vapor and liquid phases, is now accepted as one of the major controls on the composition of hydrothermal £uids that vent on the mid-ocean ridges, rarely have both phases been sampled from the same chimney, and then only at times several years apart [1]. Most models of sea£oor hydrothermal £uid generation must therefore assume storage of one of these phases, usually the liquid phase or ‘brine’, within the oceanic crust [1,2]. An area which has been of longterm interest to the study of sea£oor hydrothermal systems is the ultra-fast spreading southern East Paci¢c Rise (full opening rate V15 cm/yr), due in part to the large 3 He plume [3,4] that is swept o¡ the ridge axis from this region and then becomes a large and distinctive feature of the mid-
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depth circulation in the South Paci¢c. While previous water column work suggested robust hydrothermal venting between 13‡30P and 18‡40PS [5], little work had actually been done south of V19‡S. Studies by Krasnov et al. [6] and Tufar [7] presented strong evidence for signi¢cant hydrothermal activity around 21‡S. In October 1998 we discovered a new hydrothermal ¢eld, the ‘Rapa Nui’ ¢eld, located at 21‡33.6^33.8PS and 114‡18.0PW, which contains at least nine individual vents including ‘Brandon’ vent (Fig. 1). Brandon, located at an Alvin depth of 2834 m (287 bar) on the ultra-fast spreading southern East Paci¢c Rise, has not only the hottest £uid temperatures yet measured in a submarine hydrothermal vent at 405‡C, but is therefore extremely close to the critical point for seawater [8]. Brandon also vented £uids that di¡ered by almost a factor of two in composition from ori¢ces less than 2 m apart on its chimney structure. Hence for the ¢rst time we have observed both the vapor and liquid phases venting simultaneously from a single structure.
2. Geological setting
Fig. 1 (Continued). (a) Map of the eastern Paci¢c Ocean, showing the location of the East Paci¢c Rise, including the location of the Rapa Nui hydrothermal ¢eld. Modi¢ed after Macdonald et al. [23]. (b) Detailed bathymetric map of the Rapa Nui ¢eld, including the location of Brandon vent, and other hydrothermal vents, at this site. The bathymetry is based on SeaBeam data collected during the SouEPR cruise.
The East Paci¢c Rise at 21‡S is spreading at a full rate of 15.2 cm/yr classifying it as an ultra-fast spreading center [9]. A detailed morphotectonic study of the rise from 21‡12P to 22‡40PS is reported by Krasnov et al. [6]. Located south of the overlapping spreading center at 20‡40PS, this section of axis has a deep axial valley of s 2800 m. Our data suggest that in the Rapa Nui hydrothermal ¢eld the axial trough is tectonically controlled, with several V10 m deep and V20 m wide steps leading from the edge (V2800 m) to the deepest part of the axial cleft (V2860 m). The walls of the steps are essentially vertical, exposing massive basalt £ows. Brandon occurs on the second step down into the cleft on the western wall. The sul¢de structure of Brandon is at least 11 m tall. It is a large structure that extends across the width of the bench. It was impossible to determine the exact footprint of the structure because of the quantity of intense black smoke being emitted as well as the strong updraft. There
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are tens of individual active ori¢ces on the structure.
3. Methods All samples were collected with either titanium ‘majors’ syringe, or evacuated titanium ‘gas tight’ water samplers from the submersible Alvin. In some cases these individual bottles were connected to the NOAA manifold sampler. Temperatures were measured with either the Alvin high temperature probe, the temperature probes built into the manifold sampler, or inductively coupled link (ICL) temperature probes. In the case of the ICLs, the temperature probe is attached to the snorkels on the water samplers, and the temperature data are observed and logged inside Alvin during sampling. In many cases more than one method was used to measure the temperature at the time an ori¢ce was sampled, and all temperature probes were intercalibrated to an NISTtraceable reference. It is known that Mg is essentially quantitatively removed from seawater during hydrothermal reactions, based on both experimental and ¢eld studies [10,11]. As some seawater is always entrained during sampling, we extrapolate all compositions to Mg = 0 mmol/kg and report these as the ‘end member’ or pure hydrothermal £uid
compositions [11]. Methods of water sample handling and analysis, and calculation of end member compositions are as in Von Damm [11], the method of boron determination as in Bray and Von Damm [12], and methods of isotopic analysis for oxygen, hydrogen and sulfur as in Shanks [13]. Table 1 summarizes the water samples collected and sample quality.
4. Results Five separate ori¢ces were sampled on the Brandon structure (Tables 1 and 2). Of the ¢ve ori¢ces, three vented £uids with chlorinities less than the local ambient seawater value, and two vented £uids with chlorinities greater than the local ambient seawater value. These variations are the result of phase separation, which we interpret to be occurring within the sul¢de structure itself. Phase segregation also occurs with the vapor phase venting from di¡erent ori¢ces than the liquid (or brine) phase. The brine ori¢ces are located lower on the chimney structure than the vapor ori¢ces, and are just a few meters away. The three vapor phase ori¢ces were all sampled more than once on the cruise, and each varied chemically during this time period (Table 2, Figs. 2 and 3). The two liquid phase ori¢ces were each sampled only once, and on di¡erent days. Within the pre-
Table 1 Number and quality of £uid samples from Brandon vent Ori¢ce Vapor ori¢ces Ba.1 Ba.2 Ba.3 Ba.5 Bc.4 Bc.5 Bd.4 Bd.5 Brine ori¢ces Bb.3 Be.5
Temp. (‡C)
Dive
Date
Number of samples
Minimum Mg (mmol/kg)
404 405 401 400 403 n.m. 401 405
3289 3290 3302 3306 3303 3306 3303 3306
19 Oct 98 20 Oct 98 3 Nov 98 7 Nov 98 4 Nov 98 7 Nov 98 4 Nov 98 7 Nov 98
5 4 6 2 6 3 6 3
1.03 3.67 2.12 24.8 3.08 1.20 1.33 9.95
368 376
3302 3306
3 Nov 98 7 Nov 98
6 3
5.36 7.08
n.m. = not measured, temperature probe unplugged.
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Table 2a Brandon end member vent £uid compositions Ori¢ce
Temp.NaCl Cl Si (‡C) (wt%) (mmol/kg) (mmol/kg)
a b c d e
pHa
1.85 1.98 1.98 1.93 1.78 1.88 1.74 1.93
317 T 2 338 T 2 339 T 2 330 T 2 304 T 2 321 T 4 297 T 4 330 T 2
8.78 T 0.09 9.28 T 0.13 9.23 T 0.09 9.58 T 0.09 8.82 T 0.09 8.94 T 0.09 8.69 T 0.10 9.55 T 0.12
8.16 T 0.08 7.52 T 0.08 7.70 T 0.08 7.98 T 0.08 7.80 T 0.08 7.14 T 0.07 7.93 T 0.08 7.95 T 0.08
3.2 6 3.7 3.1 6 5.1 3.2 3.1 3.1 6 4.0
3.26 3.26 3.20
558 T 3 557 T 3 540
12.1 T 0.1 12.5 T 0.1 0.155
6.66 T 0.07 6.86 T 0.07 0
3.2 3.3 7.8
Na-measb Na-CBb K (mmol/kg) (mmol/kg) (mmol/kg)
Li Ca Sr Fe (Wmol/kg) (mmol/kg) (Wmol/kg) (Wmol/kg)
267 T 5 303 T 9 266 T 9 244 T 8 251 T 5 255 T 5 242 T 5 270 T 5
261 275 279 270 256 267 248 282
7.49 T 0.07 9.12 T 0.28 8.23 T 0.12 8.21 T 0.08 7.27 T 0.10 7.64 T 0.17 6.87 T 0.11 7.68 T 0.08
296 T 3 335 T 10 312 T 4 302 T 3 271 T 3 277 T 6 270 T 3 309 T 3
17.5 T 0.2 20.6 T 0.7 18.4 T 0.2 20.3 T 0.2 16.3 T 0.2 17.8 T 0.2 15.8 T 0.2 21.3 T 0.2
7 590 T 80 8 680 T 250 8 150 T 80 7 740 T 80 6 660 T 70 7 350 T 200 6 970 T 150 7 570 T 80
676 T 7 790 T 31 731 T 7 762 T 13 636 T 11 687 T 19 622 T 8 702 T 7
77 T 8 81 T 8 95 T 9 87 T 9 75 T 8 101 T 10 100 T 10 78 T 8
30.708 449 T 9 30.685 441 T 9 2.4 464
451 449 464
13.4 T 0.1 13.8 T 0.1 10.1
489 T 5 488 T 5 26
34 T 0.3 97.6 T 2.0 12 500 T 120 32.8 T 0.3 93.5 T 2.0 12 300 T 120 9.95 87 0
1300 T 10 1300 T 10 0
120 T 12 121 T 21 0
Alk (meq/kg)
30.688 6 30.547 30.842 6 0.121 30.607 30.911 30.815 6 30.054
49.5 T 1.0 62.5 T 2.4 53.9 T 1.0 54.1 T 1.0 48.9 T 1.0 73.8 T 3.5 47.2 T 1.0 63.1 T 1.0
Mn Znc Cuc SO4 (Wmol/kg) (Wmol/kg) (Wmol/kg) (mmol/kg)
48 T 5 51 T 5 45 T 5 81 T 8 51 T 5 81 T 8 45 T 5 54 T 5
Br B N18 O (Wmol/kg) (Wmol/kg)
1.92 T 0.06 3.21 T 0.06 1.73 T 0.18 2.91 T 0.06 3.63 T 0.33 2.75 T 0.66 2.63 T 0.19 9.53 T 0.2
87 T 9 1.59 T 0.08 105 T 11 30.50 T 0.14 0 28.0
ND
N34 S
600 T 11 680 T 20 560 T 17 570 T 17 500 T 15 520 T 16 490 T 15 550 T 17
434 T 13 431 T 13 433 T 13
0.65 T 0.1 1.1 T 1.0 0.6 T 0.1 0.71 T 0.1 3.6 T 1.0
5.5 T 0.2
426 T 13 432 T 13 429 T 13 432 T 13
0.57 T 0.1 0.7 T 1.0 0.98 T 0.1 0.77 T 0.1 1.7 T 1.0
880 T 26 890 T 27 840
462 T 13 465 T 13 412
0.7 T 0.1 1.1 T 1.0e 5.0 T 0.2 0.82 T 0.1 2.9 T 1.0e 4.2 T 0.2 0.00 0 20
5.4 T 0.2 5.0 T 0.2 5.3 T 0.2 4.8 T 0.2 5.4 T 0.2
25‡C, 1 atm. Na-meas = measured Na concentrations; Na-CB = Na concentrations calculated from the charge balance on the end member composition. Total [Zn] and [Cu] including dissolved phase and particles. n.m. = not measured, temperature probe unplugged. Based on one sample.
Table 2b Molar ratios for Brandon end member £uid compositions Ori¢ce
Temp. (‡C)
Vapor ori¢ces Ba.1 404 Ba.2 405 Ba.3 401 Ba.5 400 Bc.4 403 Bc.5 n.m.c Bd.4 401 Bd.5 405 Brine ori¢ces Bb.3 368 Be.5 376 Seawater 1.9
a b c
Cl (mmol/kg)
Si/Cl
H2 S/Cl
Na-measa /Cl
Na-CBa /Cl
K/Cl
Li/Cl (W/m)
Ca/Cl
Sr/Cl (W/m)
Fe/Cl (W/m)
Mn/Cl (W/m)
Zn/Clb (W/m)
Cu/Clb (W/m)
Br/Cl (W/m)
B/Cl (W/m)
Fe/H2 S Fe/Mn
Cu/Zn K/Na
Na/Ca Li/K (W/m)
Ca/Sr
317 338 339 330 304 321 297 330
0.0277 0.0275 0.0272 0.0290 0.0290 0.0279 0.0293 0.0289
0.0257 0.0222 0.0227 0.0242 0.0257 0.0222 0.0267 0.0241
0.842 0.896 0.785 0.739 0.826 0.794 0.815 0.818
0.830 0.812 0.824 0.801 0.837 0.831 0.837 0.841
0.0236 0.0270 0.0243 0.0249 0.0239 0.0238 0.0231 0.0233
0.934 0.991 0.920 0.915 0.891 0.863 0.909 0.936
0.0552 0.0609 0.0543 0.0615 0.0536 0.0555 0.0532 0.0645
0.156 0.185 0.159 0.164 0.161 0.230 0.159 0.191
23.9 25.7 24.0 23.5 21.9 22.9 23.5 22.9
2.13 2.34 2.16 2.31 2.09 2.14 2.09 2.13
0.243 0.240 0.280 0.264 0.247 0.315 0.337 0.236
0.151 0.151 0.133 0.245 0.168 0.252 0.152 0.164
1.89 2.01 1.65 1.73 1.64 1.62 1.65 1.67
1.37 1.28 1.28 0.00 1.40 1.35 1.44 1.31
0.93 1.15 1.06 0.97 0.85 1.03 0.88 0.95
11.23 10.99 11.15 10.16 10.47 10.70 11.21 10.78
0.62 0.63 0.47 0.93 0.68 0.80 0.45 0.69
0.0281 0.0301 0.0309 0.0336 0.0290 0.0300 0.0284 0.0284
15.3 14.7 14.5 12.0 15.4 14.3 15.3 12.7
39.5 36.7 37.9 36.8 37.3 36.3 39.3 40.2
0.354 0.330 0.341 0.375 0.333 0.241 0.335 0.338
558 557 540
0.0217 0.0224 0.0003
0.0119 0.0123 0.0000
0.805 0.792 0.859
0.808 0.808 0.859
0.0240 0.0248 0.0187
0.876 0.876 0.048
0.0609 0.0589 0.0184
0.175 0.168 0.161
22.4 22.1 0.0
2.33 2.33 0.00
0.215 0.217 0.000
0.156 0.189 0.000
1.58 1.60 1.56
0.83 0.83 0.76
1.88 1.79 ^
9.62 9.46 ^
0.73 0.87
0.0298 0.0313 0.0218
13.2 13.4 46.6
36.5 35.4 2.6
0.348 0.351 0.114
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EPSL 6490 27-1-03 Cyaan Magenta Geel Zwart
Vapor ori¢ces Ba.1 404 Ba.2 405 Ba.3 401 Ba.5 400 Bc.4 403 Bc.5 n.m.d Bd.4 401 Bd.5 405 Brine ori¢ces Bb.3 368 Be.5 376 Seawater 1.9
H2 S (mmol/kg)
Meas = measured Na; CB = Na calculated from the charge balance on the end member composition. Total [Zn] and [Cu] including dissolved phase and particles. n.m. = not measured, temperature probe unplugged.
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Fig. 2. Three-dimensional bar graphs showing the chemical and temperature variation in the various Brandon ori¢ces versus days between sampling. The vapor ori¢ces (Ba, Bc, Bd) are designated by the darker patterns, and the brine ori¢ces (Bb, Be) are designated by the lighter patterns. While there is temporal variability observed in the individual ori¢ces, the di¡erence between the vapor and brine ori¢ces is more striking. (a) Chloride, (b) lithium, (c) temperature, (d) silica.
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Fig. 3. See description for Fig. 2. This ¢gure shows the variability for the transition metals Fe (a) and Mn (b), whose concentrations may be a¡ected by the concentration of H2 S (c). The lower Fe/Mn ratio (d) observed in the brine ori¢ces compared to the vapor ori¢ces may indicate reduced Fe concentrations relative to Mn, due to the precipitation of Fe^S phases.
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Fig. 4. Phase relationships for seawater and Brandon £uids. The solid line is the two-phase curve for seawater, separating the liquid from liquid+vapor stability ¢elds. The dotted line separates the liquid+vapor from vapor+halite stability ¢elds. If a £uid crosses the two-phase curve below the critical point a vapor phase, containing a small amount of salt and higher gas contents, will be formed and this is referred to as either boiling or subcritical phase separation. If the solution crosses the two-phase curve above the critical point, a small amount of a high salinity and low gas content liquid (or brine) phase will be produced and this is referred to as either condensation or supercritical phase separation. Filled square = critical point for seawater (407‡C, 298 bar); open square = critical point for 2.2 wt% NaCl solution (400‡C, 280.7 bar); ¢lled circle = sampling conditions for Brandon vapors; open circles = sampling conditions for Brandon brines.
cision of the chemical measurements, the compositions of the brines are identical. The brines contain 1.6^1.9 times more chloride than the vapors (depending on the concentration chosen for the vapor phase), and this same enrichment factor holds true for most of the other chemical constituents. As a result of phase separation and chlorocomplexing, most cations (as well as bromide) are conservative with respect to chloride concentrations if phase separation is the only process occurring. Because the chloride concentrations in the brines are higher than those in the vapors, most of the other chemical species also have higher absolute concentrations. To determine if any relative loss or gain of a chemical species has occurred, e.g., as a result of processes other than phase separation, the elemental-to-Cl ratio must
be examined. Most of these elemental ratios are the same in the vapor and liquid phases, suggesting no other processes, such as water^rock reaction, have occurred after phase separation, the exceptions to this conservative behavior between the vapor and liquid phases in the Brandon £uids are H2 S, CO2 , H2 , Si, Zn and Fe. Boron is rather unique in its behavior as it is not chloro-complexed and at least in basalt hosted systems its concentration does not appear to be solubility controlled [12]. The boron concentrations di¡er by only V2% between the vapor and liquid phases, re£ecting the minimal e¡ects of phase separation on this hydroxyl-complexed element. The isotope ratios (O, D/H and S) also are the same in the vapor and liquid phases, and are within the range of values measured in other sea£oor hydrothermal systems [13]. Sulfur isotope fractionation between H2 SðaqÞ and H2 SðgÞ is small at room temperature and is expected to be insigni¢cant at high temperatures. The gases (H2 S, etc.) preferentially partition into the vapor phase, as can be seen in the higher concentrations of H2 S, CO2 , etc. in the vapor phase (Table 2, Fig. 3). The relative partitioning depends on the exact conditions of phase separation. In the case of H2 S, the di¡erences in concentration may also be a result of solubility controls from metal sul¢de formation, discussed in Section 5.2. The chemical data therefore suggest that phase separation has occurred with no subsequent overprinting by continued water^rock reaction with the predominately aluminosilicate oceanic crust, supporting our hypothesis that phase separation is likely occurring within the sul¢de structure itself.
5. Discussion Brandon £uid compositions are a result of seawater^rock interaction within the oceanic crust, followed by phase separation and segregation. Especially in the case of the liquid phase, which has a lower temperature, mineral solubility controls may also exist for some of the transition metals.
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5.1. Phase separation The exact conditions at which phase separation occurs will determine both the chlorinity and the relative amounts of the two phases [14]. As Cl is the major anion and Na the major cation, seawater is often treated as a 3.2 wt% NaCl solution for purposes of evaluating the conditions and net results of phase separation. Due to electroneutrality constraints, the Na concentration must equal the Cl concentration on a molar basis. The chloride content therefore determines the physical properties, such as density, of the solution. The measured temperature and pressure conditions (405‡C, 287 bar) at the time £uids from Brandon vent were sampled put them just below the critical point for seawater (407‡C, 298.5 bar) [8] (Fig. 4). As for pure water, at the critical point for a salt solution the compositions and physical properties of both the liquid and vapor phases are identical. Unlike the pure water case, in a salt solution the two-phase curve continues beyond the critical point. The closer to the critical point at which phase separation occurs, the more similar the compositions of the two phases. The temperature and pressure of the critical point for a £uid will also change with its salt content. As the £uids from the ¢ve ori¢ces sampled on the Brandon structure (Table 2, Figs. 2 and 3) di¡er in composition by a factor of 1.6^1.9, phase separation must be occurring at conditions that are either sub- or supercritical. For the case where the starting £uid had the same salt content as seawater, phase separation at temperatures of 400^410‡C and pressures of 280.1^305.7 bar, bracketing the critical point for seawater, would generate £uids with 1.47^3.84 wt% NaCl [14], which includes the entire range observed in £uids from Brandon. The starting £uid, however, may not have a seawater salt content. If hydrothermal £uid intersects the two-phase curve as it rises through the oceanic crust, it will begin to phase separate, and will then travel along the two-phase curve until it reaches the sea£oor. For example, if our starting £uid contained just 2.2 rather than 3.2 wt% NaCl, its critical point would be 400‡C and 280.7 bar [14]. (A 2.2 wt% NaCl solution was chosen because it is the minimal salt content measured in
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the Brandon £uids.) If the pressure on this 2.2 wt% NaCl £uid dropped to 280.0 bar, subcritical phase separation would occur, and the vapor would contain 1.35 wt% NaCl and the liquid 3.45 wt% NaCl. This also encompasses the range of £uid compositions sampled from Brandon. The small change in pressure needed to cause phase separation could occur within the V11 m in height of the Brandon sul¢de structure. Hence, because we have no means of determining the NaCl content of our £uid when phase separation began, we cannot discern if the phase separation occurred sub- or supercritically. However, in either case examined, the pressure^temperature conditions are very close to the measured conditions, and most are within the range observed in the structure itself. The Brandon £uid compositions also demonstrate that in the real systems, as has been shown previously in experimental studies [15,16], the net e¡ects of phase separation for the cation concentrations are conservative with respect to chloride. Berndt et al. [17] and Horita et al. [18] have experimentally studied hydrogen and oxygen isotope fractionation between liquid and vapor phases near the critical point of NaCl solutions. Based on these studies, little oxygen isotope fractionation is expected, but deuterium enrichment in the vapor up to 12.5x has been observed. However, the magnitude of the hydrogen isotope fractionation increases with the pressure di¡erential from the critical curve. Berndt et al. [17] derived the following equation from their experimental data: 1000 ln K vaporbrine ¼ 2:54 þ 2:87 log vP
where vP is the di¡erence in bars between the measured pressure and the critical pressure at the temperature of the experiment. According to Bischo¡ and Rosenbauer [8], the critical pressure for seawater at 405‡C is 293 bar, thus the pressure at the sea£oor sampling conditions is 6 bar below the critical curve pressure. Using the equation above, we calculate an expected ND enrichment in the vapor of 4.8x. This is not observed in our data. However, small errors in the sea£oor temperature measurements, pressure/depth conversion calculations, or a small amount of conductive cooling would introduce errors into this
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calculated fractionation. Phase separation within the Brandon structure at sea£oor pressure of 287 bar and at temperatures slightly above the measured 405‡C would not produce signi¢cant hydrogen isotope fractionation. 5.2. E¡ects of proximity to the critical point As hydrothermal £uids are seawater that has reacted within the oceanic crust, almost none of the dissolved species maintain their seawater concentrations or their element-to-element ratios (Table 2b). Essentially hydrothermal £uid compositions can be thought of as the net result of the composition of the starting seawater, the availability of elements from the rock, the complexation and transport of ions in solution, and water^rock equilibration. There is increasing evidence that the composition of most sea£oor hydrothermal £uids (early eruptive £uids being a notable exception) is controlled by the achievement of steady state or equilibrium between the £uids and an alteration mineral assemblage (e.g., [19]). At high temperature and pressure most of the cationic species in sea£oor hydrothermal £uids are present as chloro-complexes. As the critical point is approached complexation increases, until at the critical point all of the aqueous charged species should be present as neutral species complexes [20]. This would permit enhanced transport of species in the £uids as mineral solubilities would not be exceeded due to the completeness of complexation. One therefore expects higher concentrations of the cationic species Li, K, Na, Ca, Sr, Mn, Fe, as well as Br (which usually behaves conservatively with respect to Cl) in the higher chlorinity liquid phase, in comparison to the lower chlorinity vapor phase, while the gases (e.g., CO2 , H2 S, H2 ) will be preferentially fractionated into the vapor phase [21]. This is indeed what we observe in the Brandon £uids. Our interpretation is therefore that proximity to the critical point results in the very high concentrations in both the vapor and liquid phases of metals, such as Fe, which are usually limited by solubility controls of metal sul¢de phases. A signi¢cant di¡erence does occur between the temperatures of the vapor and liquid phase £uids
from Brandon, with the liquid phase £uids having substantially cooler measured exit temperatures. This means that the brines are not as proximal to the critical point as are the vapors, and less complexation is occurring. The observation that the brines vent physically lower on the structure than the vapors may suggest that as the vapors rapidly jet out, the brine phase settles due to its greater density lower in the structure, resulting in a slightly longer residence time for the brines within the sul¢de structure. This may allow the brines to cool conductively (the structure is bathed in 2‡C seawater, forming essentially an in¢nite heat sink) before they exit. A second possibility, allowed by the measured Mg (minimum 5.36 mmol/kg) and temperature data, is that a small amount of seawater ( 6 10%) mixes into the brines within the structure prior to sampling, resulting in the cooler temperatures. The very low densities of the £uids near the critical point could result in enhanced entrainment of seawater (although this does not appear to be an important process in the sampled vapors where measured Mg concentrations are as low as 1.03 mmol/kg, and almost half of this is attributable to the dead volume in the sampling bottles). As the solubilities of most metal sul¢des also decrease with decreasing temperature, it is possible that the brines may be losing some of their dissolved constituents to mineral precipitation. The chemical data from the Brandon brines (Table 2) suggest loss of Fe and H2 S, based on the decrease in the ratios of these species to Cl, and the decrease in the Fe/Mn ratio. With conditions so close to the critical point, quantitative geochemical models cannot be rigorously applied so other approaches must be used. Using a mass balance approach, based on the H2 S/Cl ratios, between 1.65 and 0.14 mmol/kg of H2 S is lost between the vapor and liquid phases. Some of the enrichment in the vapors is a result of phase separation, and therefore only some of the relative loss in the brines may be due to the precipitation of metal sul¢des. As the Fe levels are so high, and the relative amount of Fe loss low, the loss of Fe is best identi¢ed through the Fe/Mn ratio (Fig. 3) in combination with the Fe/Cl and Mn/Cl ratios. A slight decrease in the Fe/Cl ratio and a larger decrease in
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the Fe/Mn ratio suggest that some Fe is lost in the liquid relative to the vapor phases. Mass balance constraints suggest the maximum Fe loss is 0.1 mmol/kg. Based on the stoichiometry of the minerals likely to precipitate, this suggests that the maximum loss of H2 S due to mineral precipitation is V0.2 mmol/kg. Surprisingly it does not appear that Cu is lost between the vapor and liquid phases based on their respective Cu/Cl ratios, but some Zn does appear to be lost. Fe-, Cu- and Zn-sul¢de minerals are most common in chimneys. Cu is generally assumed to be the most temperature sensitive of the three, but at these high temperatures even for the liquid phase, loss of Cu does not appear to be occurring. Zn is, however, known to be more sensitive to losses caused by mixing. The loss of Zn is also indicated by the increase in the Cu/Zn ratio. A striking feature of the Brandon £uids is their unusually high Fe/Mn ratios (Fig. 3). The ratio of 9.5^11.2 is signi¢cantly higher than what is usually observed in sea£oor hydrothermal £uids, which cluster around the 3:1 ratio typically found in metalliferous sediments. Unusually high ratios have also been noted in hydrothermal £uids that are formed immediately following volcanic eruptions, which also have temperatures close to 400‡C [11]. The ratios are high in both cases due to unusually high levels of Fe, not low levels of Mn. Iron sul¢de minerals are usually the dominant minerals in structures formed by high temperature £uids on the mid-ocean ridges, and the £uids are often found to be at, or close to, saturation with these minerals. We suggest that the explanation for the high Fe/Mn ratios, as well as high Fe/Cl ratios, is the proximity of these £uids to the critical point. Under these conditions the almost complete complexation of Fe by Cl, and almost complete association of H2 S allows greater transport of Fe in these £uids, which is usually limited by the solubility of Fe^S minerals. Hemley et al. [22] noted that under some conditions decreasing pressure can result in increased metal solubility at isothermal conditions, which is a result of the increasing strength of aqueous complexes. Proximity to the critical point provides a previously undiscussed mechanism for transport of cations, especially transition metals,
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in sea£oor hydrothermal systems. Likely this is the correct explanation, or at least an important part of the explanation, for why many metals have enhanced transport at the high temperatures that are often associated with eruptive/magmatic events, rather than being a result of the more reducing nature of the £uids as previously proposed [1]. If proximity to the critical point is common, it will lead to enhanced £uxes of transition metals compared to what has been previously calculated. 5.3. Evidence for ‘real time’ phase separation While the ratios to Cl are generally constant between the vapor and liquid phase £uids, there is more scatter in the data than is expected (Table 2b). Given the acidic pH of these £uids, CO2 should be relatively unreactive in comparison to H2 and H2 S. Examination of the CO2 data versus either Mg (Fig. 5a) or Cl shows much greater scatter than the analytical error for individual samples collected from the same ori¢ce at the same time (i.e., sequentially one sample right after the other). While this scatter is also seen for other chemical constituents, it is perhaps most striking for CO2 . Usually, for a given sampling time at a given ori¢ce, the analytical data for all of the samples are ¢t to a least squares linear regression versus Mg, and the end member is calculated for each constituent [11]. In the case of CO2 (Fig. 5a) this ¢t is unusually noisy, and in the case of Cl (Fig. 5b) there is a sample from Ba.2 that falls distinctly o¡ the trend. If instead end member concentrations for CO2 and Cl are calculated for each sample, end member CO2 versus end member Cl now de¢nes a well-constrained line (Fig. 5c). If a similar plot is made for end member K versus end member Cl (Fig. 5d) the sample that falls o¡ of the measured Cl versus measured Mg trend (Fig. 5b) now lies on a mixing line between the vapor and liquid phase compositions. This suggests that in addition to the inter-day variability in composition, the £uids actually change composition on a time scale of minutes, the time needed to collect successive water samples. This is in contrast to most vents, where changes in vent £uid chemistries are rarely observed on a time scale of days, and more commonly on a scale
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Fig. 5. Property^property plots for various chemical parameters from the ori¢ces on the Brandon vent structure. SW designates seawater, B designates Brandon vent, a^e designate the individual ori¢ces, 1^5 designate the dive to Brandon on which that sample was collected. See Table 2 for more detailed description, and text for more detailed discussion. (a) Measured CO2 versus measured Mg, (b) measured Cl versus measured Mg, (c) end member CO2 calculated for each sample versus end member Cl calculated for each sample for the Ba ori¢ce only, (d) end member K calculated for each sample versus end member Cl calculated for each sample.
of years and sometimes decades. The maximum Cl for the CO2 versus Cl trend is 376 mmol/kg (Fig. 5c), which is equivalent to 2.2 wt% NaCl, consistent with this being a possible starting £uid composition. The Ba.2 sample that is o¡ the Cl versus Mg trend provides evidence, in this case, for incomplete segregation of the brine from the vapor phase as it is best explained as containing 54^71% vapor phase and 46^29% liquid phase,
depending on the composition chosen for the vapor phase. If such a model is invoked, our Cl ranges from 270 to 559 mmol/kg, or 1.58 to 3.26 wt% NaCl. While we still cannot distinguish if we are sampling above or below the critical point, these observations are additional strong evidence that the Brandon £uids undergo phase separation right at the sea£oor. We do not know the nature of the heat source
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that drives the hydrothermal circulation observed at Brandon vent. Several of the nearby vents (Fig. 1) are also very hot, with Preston, Roadrunner and Wallace having measured £uid temperatures s 380‡C, suggesting that the heat source extends at least 100 m along strike. There are no signs of fresh lavas, snowblower vents, or microbial blooms indicative of a recent volcanic eruption in the vicinity of Brandon. Neither are the silica contents of the £uids de¢nitive as to what the depth of water^rock reaction may be, beyond suggesting that the heat source is very shallow (only hundreds of meters below the sea£oor) at this site.
6. Conclusions Brandon vent, at a greater depth (2834 m) than has been previously studied on a magmatically robust and fast spreading mid-ocean ridge, vents £uids that are close to the critical point for seawater. This provides enhanced complexation, and hence enhanced transport of transition metals in these £uids. To explain the chemical data, the Brandon £uids must phase separate essentially instantaneously within the sul¢de structure at the sea£oor. Typically after phase separation occurs in sea£oor hydrothermal systems, water^rock reaction within the aluminosilicate oceanic crust continues, and overprints the results of phase separation. This is partly evidenced by di¡ering element-to-Cl ratios in the two phases. The Brandon £uids, as evidenced by their generally conservative elemental ratios, therefore provide an unique opportunity to examine the e¡ects of phase separation alone. While we cannot de¢nitely determine if these £uids phase separate sub- or supercritically, the compositions of these £uids provide new insights into the processes of phase separation, and the overall controls on the compositions of sea£oor hydrothermal vent £uids.
Acknowledgements We thank the master, o⁄cers and crew of the R/V Atlantis and the Alvin group for helping to make the SouEPR cruise a success. We would like
377
to dedicate this paper to the memory of Brandon Lilley. We thank J. Bischo¡ and R. Mills for their comprehensive reviews of this paper. This research was funded by the U.S. National Science Foundation through OCE9419156 (K.V.D.) and OCE9417121 (M.D.L.).[BOYLE]
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EPSL 6490 27-1-03 Cyaan Magenta Geel Zwart