Aqueous volatiles in hydrothermal fluids from the Main Endeavour Field, northern Juan de Fuca Ridge: temporal variability following earthquake activity

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Earth and Planetary Science Letters 216 (2003) 575^590 www.elsevier.com/locate/epsl

Aqueous volatiles in hydrothermal £uids from the Main Endeavour Field, northern Juan de Fuca Ridge: temporal variability following earthquake activity Je¡rey Seewald a;
, Anna Cruse a , Peter Saccocia b a

Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA b Department of Earth Science and Geography, Bridgewater State College, Bridgewater, MA 02325, USA Received 14 January 2003; received in revised form 29 August 2003; accepted 19 September 2003

Abstract The Main Endeavour Field, northern Juan de Fuca Ridge, experienced intense seismic activity in June 1999. Hydrothermal vent fluids were collected from sulfide structures in September 1999 and July 2000 and analyzed for the abundance of H2 , H2 S, CH4 , CO2 , NH3 , Mg and Cl to document temporal and spatial changes following the earthquakes. Dissolved concentrations of CO2 , H2 , and H2 S increased dramatically in the September 1999 samples relative to pre-earthquake abundances, and subsequently decreased during the following year. In contrast, dissolved NH3 and CH4 concentrations in 1999 and 2000 were similar to or less than pre-earthquake values. Aqueous Cl abundances showed large decreases immediately following the earthquakes followed by increases to near preearthquake values. The abundances of volatile species at the Main Endeavour Field were characterized by strong inverse correlations with chlorinity. Phase separation can account for 20^50% enrichments of CO2 , CH4 , and NH3 in low-chlorinity fluids, while temperature- and pressure-dependent fluid^mineral equilibria at near-critical conditions are responsible for order of magnitude greater enrichments in dissolved H2 S and H2 . The systematic variation of dissolved gas concentrations with chlorinity likely reflects mixing of a low-chlorinity volatile-enriched vapor generated by supercritical phase separation with a cooler gas-poor hydrothermal fluid of seawater chlorinity. Decreased abundances of sediment-derived NH3 and CH4 in 1999 indicate an earthquake-induced change in subsurface hydrology. Elevated CO2 abundances in vent fluids collected in September 1999 provide evidence that supports a magmatic origin for the earthquakes. Temperature^salinity relationships are consistent with intrusion of a shallow dike and suggest that the earthquakes were associated with movement of magma beneath the ridge crest. These data demonstrate the large and rapid response of chemical fluxes at mid-ocean ridges to magmatic activity and associated changes in subsurface temperature and pressure. 9 2003 Elsevier B.V. All rights reserved. Keywords: hydrothermal systems; Main Endeavour Field; aqueous volatiles; £uid^rock reaction; phase separation

1. Introduction * Corresponding author. Tel.: +1-508-289-2966; Fax: +1-508-457-2164. E-mail address: [email protected] (J. Seewald).

Hydrothermal convection at mid-ocean ridges is the primary mechanism for the exchange of

0012-821X / 03 / $ ^ see front matter 9 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0012-821X(03)00543-0

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heat and mass between seawater and the oceanic crust. The broad array of chemical and physical processes that modify seawater chemistry at or below the sea£oor results in the formation of large metal sul¢de deposits and creates chemical environments that support large and diverse biological communities. Phase separation and £uid^ rock interaction have been shown to play key roles in regulating vent £uid composition. In addition, magmatic degassing may represent an important source of aqueous volatiles. These processes in£uence the abundance of individual chemical species to varying degrees and are a sensitive function of temperature, pressure, and subsurface hydrology. Time series measurements provide important clues to understanding the formation of ridge crest hydrothermal £uids because they delineate both the gradual evolution of a system as it ages and the immediate response to discrete geologic events. Previous studies have demonstrated that £uid composition in some vents remains constant for as long as 16 years [1^3], while at other locations £uid composition can vary on a time scale of days to years [4^8]. In general, changes in subsurface hydrology and thermal structure that are induced by submarine volcanism and/or tectonism cause rapid variations in £uid composition. Volatile species represent a powerful tool for understanding £uid^rock reactions and sea£oor volcanism because their chemical behavior can be distinct from non-volatile ionic and neutral species. Hydrothermal activity and crustal generation along the Endeavour segment of the Juan de Fuca Ridge (Fig. 1) have been the focus of extensive research during the past 20 years. It has recently been reported that large increases in £uid output and temperature at sites of low-temperature di¡use £ow were observed along the Endeavour segment following earthquake activity in June 1999 [9]. Because £uid^rock interaction and phase separation are strongly dependent on temperature and crustal residence time, it can be expected that the £ow rate and temperature changes after the earthquakes were accompanied by variations in the abundance of dissolved gases in high- and low-temperature vents. Indeed, in situ and labo-

Fig. 1. Geologic map of the Main Endeavour Field, northern Juan de Fuca Ridge. Expanded view of the vent ¢eld is modi¢ed from Delaney et al. [44] and Robigou (1995) unpublished data.

ratory measurements of dissolved gases in hydrothermal £uids at the Main Endeavour Field (MEF; Fig. 1) reveal elevated concentrations of H2 , He, CH4 , CO2 , and H2 S in September 1999 relative to pre-earthquake values [8,10,11]. Here we report a dataset for the concentration of dissolved gases in hydrothermal £uids collected in September 1999 and July 2000 at MEF following the June 1999 earthquake. These data provide new insight regarding the thermal and hydrologic response of the oceanic lithosphere at mid-ocean ridges to earthquake-induced perturbations on short time scales. More generally, this comprehensive dataset for volatile species constrains the relative roles of phase separation, £uid^rock reactions, and magmatic activity in regulating vent £uid chemistry.

2. Geologic setting The MEF is located at 47‡57PN and 129‡05PW

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on the northern end of the Juan de Fuca Ridge (Fig. 1). This region of the ridge crest is spreading at an intermediate rate (6 cm/yr full rate) and is presently in an extensional mode [12]. Hydrothermal activity extends for about 400 m along the western wall of the approximately 800 m wide axial valley. Vent locations are closely associated with bounding normal faults that provide conduits for £uids to reach the sea£oor [13,14]. Mineral precipitation has resulted in the formation of large, steep-sided sul¢de structures that are up to 30 m in width at their base and 20 m high. In general, hydrothermal £uids vent from narrow chimneys and beneath overhanging £anges located on the sides of sul¢de structures. Lowertemperature di¡use venting from basalt along faults and ¢ssures is also common [14]. The extent to which subsurface magmatic activity at MEF in£uences hydrothermal circulation is presently unclear. Geophysical surveys have been interpreted to indicate the absence of a largescale axial magma chamber [15,16]. Wilcock and Delaney [17] have postulated that heat is extracted from hot rock via a downward-propagating cracking front model [18]. Although the 1999 earthquakes have been attributed to amagmatic tectonic activity [9], the possibility of a subsurface dike intrusion or replenishment of a small magma chamber cannot be discounted. Bohnenstiehl et al. [19] have suggested that the 1999 seismic data are consistent with magma movement and results of a recent seismic re£ection study indicate the presence of magma below the ridge axis [20]. Numerous ROV and submersible dives since the earthquakes indicate no recent volcanic activity at MEF. The chemistry of vent £uids prior to the 1999 earthquakes has been studied in detail by Butter¢eld et al. [2] and Lilley et al. [10]. These studies emphasized the importance of phase separation in regulating £uid composition and provided compelling evidence for the interaction of hydrothermal £uids with organic-bearing sediments. The source of these sediments, however, is unconstrained. Data for £uids collected between 1984 and 1998 indicate that the composition of £uids at Endeavour remained relatively constant the 15 years prior to the 1999 earthquakes [2,8].

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3. Methods Hydrothermal £uid samples were collected in titanium gas-tight isobaric £uid samplers [21]. During the 1999 cruise, £uids were also collected in non-gas-tight titanium ‘major’ bottles [22], but these £uids were used only for analyses of NH3 , Cl, and H2 S. For individual sul¢de structures characterized by £ange- and chimney-style venting, £uids were collected from both types of ori¢ce if su⁄cient samplers were available. In 1999, only one gas-tight sample was collected at each structure, while in 2000, structures were sampled in triplicate. Fluids were collected at a structure in 2000 that we have tentatively identi¢ed as Peanut based on water depth and structure morphology. Owing to poor navigation, however, there is a possibility that these £uids are from the Bastille structure. Temperatures were measured with the Alvin high-temperature probe. Processing of £uids occurred within hours of submersible recovery. Fluids collected in isobaric gas-tight samplers were analyzed for the abundance of both volatile and non-volatile species according to the techniques described in [21,23]. Dissolved NH3 concentrations were determined by ion chromatography. Analytical precision (1c) for CO2 and H2 S is 9 10%. Similar precision is associated with the CH4 and H2 analyses in 1999 but signi¢cant improvements in the analytical method reduced this value to 9 5% in 2000 except for concentrations 6 0.1 mmol/l. An analytical precision of 9 5 and 9 2% are associated with the NH3 and Cl determinations, respectively. Because the £uid samplers have a ¢nite dead volume that is ¢lled with bottom seawater prior to deployment, and seawater entrainment occurs during sampling to varying degrees, the composition of endmember £uid venting at the sea£oor is determined by extrapolating the measured concentration of an individual species to zero Mg using a least squares linear regression of vent £uid and seawater compositions. This approach is justi¢ed by laboratory experiments demonstrating near-quantitative removal of Mg from seawater during high-temperature seawater^basalt interaction [24]. Fluids collected from di¡erent ori¢ces

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on a given sul¢de structure were regressed as a group to yield a single endmember composition. Although endmember compositions for the 1999 volatile data were extrapolated from a single gastight sample, and therefore may have greater un-

certainty, measured abundances of volatile species in these £uids agree with values determined in situ using chemical sensors [11] and for £uids collected in evacuated titanium gas-tight bottles [25]. Reported endmember volatile concentrations at

Table 1 Measured abundances and extrapolated endmember concentrations of dissolved gases, Cl, and Mg in hydrothermal £uids collected from the Main Endeavour Field Vent-Year

Sample

Structure

Hulk-1999

M3468A M3468B M3468C M3468D BGT3478 Endmember BGT-3591-2 BGT-3591-3 BGT-3591-4 Endmember M3470A M3470B Endmember BGT-3590-2 BGT-3590-3 BGT-3590-4 Endmember BGT3470 M3470C M3470D Endmember BGT3474 M3474A Endmember M3474B BGT3480 Endmember BGT-3592-2 BGT-3592-3 BGT-3592-4 Endmember BGT-3593-2 BGT-3593-3 BGT-3593-4 Endmember BGT-3594-2 BGT-3594-3 BGT-3594-4 Endmember

Flange Flange Chimney Chimney Flange

339 339 341 341 347

Chimney Flange Flange

na 120 120

Flange Flange

350 350

Flange Flange Flange

341 341 341

Exc. chimneyb Exc. chimney Exc. chimney

368 368 368

Flange Flange

375 375

Chimney Chimney

377 379

Chimney Flange Flange

na 342 342

Di¡use £ow Chimney Chimney

40 367 367

Chimney Chimney Chimney

382 382 382

Hulk-2000

Dante-1999

Dante-2000

Bastille-1999

Cantilever-1999

Sully-1999

LOBO-2000

SpM-2000

Peanut-2000d

Seawater

Temperature (‡C)

H2 (mmol/l) na na na na 0.31 0.33 0.20 0.083 0.076 0.23 na na 0.52a 0.29 0.27 0.28 0.29 0.55 na na 0.62 0.55 na 0.70 na 0.036 0.96c 0.14 0.25 0.31 0.29 0.04 0.48 0.53 0.54 0.21 0.43 0.23 0.47 0

H2 S CH4 (mmol/l) (mmol/l) 2.8 6.2 6.5 6.0 7.0 7.4 5.3 1.9 1.7 5.8 7.8 11 13 7.3 8.9 7.3 8.3 20 2.2 12 22 24 20 25 19 3.6 20 7.3 5.7 6.8 8.6 0.9 13 14 14 6.0 13 6.5 14 0

a

na na na na 1.4 1.6 1.4 0.5 0.49 1.5 na na na 1.3 1.3 1.3 1.4 1.5 na na 1.7 1.7 na 1.7 na 0.061 1.7c 1.2 1.0 1.2 1.4 0.11 1.5 1.7 1.7 0.56 1.2 0.61 1.3 0

CO2 NH3 Cl Mg (mmol/kg) (Wmol/kg) (mmol/kg) (mmol/kg) na na na na 51 53 26.5 11.9 10.8 29 na na na 26.0 23.9 23.9 26 55 na na 61 68 na 71 na 6 100c 20.5 18.0 20.4 24 4.3 31.7 32.1 34 12.9 25.2 14.0 27 2.3

159 411 499 493 479 503 420 122 122 447 398 383 529 418 415 426 442 555 158 262 602 623 541 645 635 19 666 383 296 354 431 41 404 446 453 173 402 194 430 1

484 438 422 433 447 426 484 520 518 477 446 457 419 474 463 459 461 239 438 398 207 53.5 109 38.6 61.8 534 31 468 468 472 452 521 391 377 373 452 367 449 347 543

32.2 9.05 1.49 2.36 2.82 0 5.63 35.6 35.6 0 13.3 14.9 0 2.58 2.76 2.79 0 5.84 36.7 29.0 0 2.43 8.11 0 2.49 52.9 0 7.41 14.9 9.77 0 48.6 5.12 1.62 0 29.7 5.34 28.3 0 54.0

Value from Ding et al. [11] Excavated chimney. c Due to signi¢cant seawater entrainment in the gas-tight sample, higher uncertainties are associated with these data. d This vent is tentatively identi¢ed as Peanut based on water depth and structure morphology, but owing to poor navigation, there is a high degree of uncertainty with the location of these £uid samples. The possibility exists that these £uids may have been from the Bastille structure. b

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in a £ange at Hulk yielded 120‡C and a di¡use £uid at SpM had a temperature of 40‡C (Table 1). The low temperature reported for Hulk likely re£ects problems with temperature probe placement during measurement. 4.1. Chloride Endmember Cl concentrations varied from 31 to 426 mmol/kg £uid in 1999 and from 347 to 477 mmol/kg £uid in 2000 (Table 1, Fig. 2). Relative to values prior to the 1999 earthquakes, Cl concentrations at Hulk, Dante and Sully were signi¢cantly lower in the 1999 £uids (Fig. 3). Approximately one year later, Cl concentrations at Hulk and Dante had rebounded to values that were slightly lower and higher, respectively, relative to pre-1999 values. Chloride concentrations at Bastille in 1999 and LOBO and SpM in 2000 were higher than the pre-1999 concentrations (Fig. 3). Regardless of the year sampled, all Cl concentrations are depleted relative to bottom seawater (543 mmol/kg £uid). In general, vent £uids in Fig. 2. Plot of (a) Cl versus Mg, (b) NH3 versus Mg, (c) CH4 versus Mg, (d) CO2 versus Mg, (e) H2 versus Mg, and (f) H2 S versus Mg in vent £uids collected at the MEF in 1999 and 2000.

Sully are highly uncertain due to entrainment of substantial seawater during sampling.

4. Results Vent £uids from MEF in 1999 and 2000 contained high concentrations of dissolved gases and were characterized by a broad range of salinities (Table 1, Fig. 2). In contrast to data collected prior to 1999 that demonstrated near-constant £uid composition over a 15 year period [2,8], compositions in 2000 had changed substantially relative to those measured in 1999. Fluid composition also varied signi¢cantly between structures, but remained constant in £uids collected from multiple ori¢ces/£anges on an individual structure. The majority of measured temperatures fell within the range 339^382‡C although a single measurement

Fig. 3. Measured concentrations of aqueous volatiles and Cl at selected vents in the MEF before and after the 1999 seismic activity. The pre-1999 data are from Butter¢eld et al. [2] and Lilley et al. [8,10].

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2000, however, were su⁄ciently high for phase separation to be occurring at the sea£oor (Fig. 4). 4.2. Aqueous volatiles

Fig. 4. Phase relations in the system NaCl^H2 O at conditions typical of ridge crest hydrothermal systems. The dashed line indicates seawater salinity and the symbols represent vent £uids collected at the MEF in 1999 (solid circles) and 2000 (open circles). Requisite data for construction of this ¢gure are from Bischo¡ [33] and Bischo¡ and Pitzer [45].

the southernmost regions of the ¢eld had higher measured temperatures and lower salinities than £uids to the north. Similar temperature and salinity trends were noted for £uids collected at MEF prior to the 1999 earthquakes [2]. It has been wellestablished that the large Cl variations in these £uids re£ect phase separation in subsurface reaction zones [2,10]. Water depth at MEF is 2200 m, which corresponds to a sea£oor pressure of 220 bar. At this pressure a minimum temperature of 378‡C is required to induce phase separation of seawater [26] (Fig. 4). If it is assumed that the substantial Cl depletions indicate phase separation, then the observation that the majority of measured temperatures are lower than that required for phase separation at or below the sea£oor (Fig. 4) provides compelling evidence for varying degrees of subsurface cooling of MEF £uids prior to venting. Cooling of hydrothermal £uids may occur by a variety of mechanisms that include adiabatic expansion during ascent [27], conductive heat loss through the walls of sul¢de structures [28], and subsurface mixing of cool seawater and/or lower-temperature hydrothermal £uids. Measured temperatures at Sully and Cantilever in 1999 and Peanut in

Carbon dioxide was the most abundant dissolved gas in all £uids, with endmember values varying from 24 to 100 mmol/kg £uid (Table 1, Fig. 2). Aqueous H2 S, the next most abundant volatile species, varied from 5.8 to 25 mmol/kg £uid. Dissolved CH4 , H2 , and NH3 were present at substantially lower concentrations that varied from 1.3 to 1.7, 0.23 to 0.96, and 0.48 to 0.71 mmol/kg £uid, respectively. Total NH3 is included here and in subsequent discussions as an aqueous volatile because at the temperature and pH conditions of ridge crest hydrothermal £uids it exists predominantly as the neutral and volatile species NH3 [29]. Based on observed temporal variability, the dissolved gases can be separated into two groups. Whereas dissolved concentrations of CO2 , H2 , and H2 S increased dramatically at Hulk and Dante in the 1999 samples relative to pre-1999 values, abundances of CH4 and NH3 remained the same or decreased (Fig. 3). In the 2000 samples, CO2 , H2 , and H2 S abundances decreased relative to 1999 but were similar to or greater than pre-1999 values, while CH4 and NH3 continued to decrease relative to 1999 and pre-1999 £uids (Fig. 3). The dissolved concentrations of all gases decrease systematically with increasing chlorinity (Fig. 5). In contrast to measured NH3 , CH4 , H2 S, and H2 concentrations that in 1999 and 2000 de¢ne a single trend as a function of Cl, the 1999 CO2 abundances were signi¢cantly higher for a given Cl relative to the 2000 £uids. The extent of volatile enrichment as a function of Cl is species-dependent as indicated by H2 and H2 S concentrations that increase by a factor of 3^5 in the low-Cl £uids and CO2 , CH4 , and NH3 concentrations that increase by only a factor of 1.2^ 1.5.

5. Discussion Information regarding the reactivity and sour-

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Fig. 5. Variations in the endmember concentrations of aqueous volatiles as a function of Cl in hydrothermal £uids from the MEF before and after the 1999 seismic activity. The pre-1999 data are from Butter¢eld et al. [2] and Lilley et al. [8,10].

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ces of chemical species under hydrothermal conditions can be used to constrain subsurface processes at Endeavour. For example, the abundance of CO2 , H2 , and H2 S can be regulated by partitioning during phase separation and/or the solubility of carbonate, sul¢de, and redox-dependent Fe minerals, while abundances of sediment-derived CH4 and NH3 are likely in£uenced by phase separation alone because they are not readily incorporated into solid alteration phases. Magmatic degassing and basalt leaching may also in£uence aqueous CO2 concentrations. High concentrations of CH4 , NH3 , and Br at MEF relative to other bare rock systems provide evidence that vent £uid chemistry is also in£uenced by reaction with organic-bearing sediments that are either buried by basalt £ows or located at the sea£oor in distal recharge zones [2,10,30]. Based on an assessment of systematic variations in the correlation of alkali metals and B as a function of Cl concentration, Butter¢eld et al. [2] concluded that interaction with sediment was occurring prior to phase separation. 5.1. Fluid^rock equilibria A striking feature of vent £uid chemistry at MEF is the strong negative correlations of aqueous H2 and H2 S with Cl. Experimental and theoretical studies have demonstrated that dissolved concentrations of H2 and H2 S are regulated by £uid^mineral equilibria during seawater^basalt alteration at near-critical conditions [28,31,32]. In general, concentrations of H2 and H2 S during equilibration of £uids with anhydrite and Febearing oxides, sul¢des, and aluminosilicates in basalt increase in response to increased temperature and decreased pressure (Fig. 6). Moreover, mineral-bu¡ered concentrations of these species are extremely sensitive to small variations in temperature and pressure in the near-critical region characteristic of subsea£oor reaction zones. Chloride concentrations produced by phase separation show a similar sensitivity to variations in temperature and pressure at near-critical conditions (Fig. 4). Thus, £uids undergoing phase separation and £uid^mineral equilibria at relatively high temperature or low pressure will be enriched in H2 and

Fig. 6. Variations in (a) dissolved H2 S and (b) dissolved H2 as a function of temperature during laboratory experiments reacting crystalline basalt, a pyrite^pyrrhotite^magnetite redox bu¡er, and an anorthite^epidote^anhydrite^magnetite^ pyrite mineral assemblage with a seawater-salinity £uid at 400 bar [28,31,32,46]. For each mineral assemblage, concentrations of H2 S and H2 increase with decreasing pressure and salinity. The shaded regions are constrained by the range of concentrations and minimum temperatures observed at the MEF in 1999 and 2000. Maximum temperatures are unconstrained but can be estimated from concentrations of dissolved gases predicted for the mineral assemblages shown.

H2 S and depleted in Cl relative to £uids undergoing the same processes at lower temperatures and higher pressure. However, a low-salinity £uid produced by phase separation alone will also have increased abundances of H2 and H2 S due to vapor phase partitioning. As will be shown below, the magnitude of H2 and H2 S enrichments as a function of decreasing Cl concentration at MEF indicates £uid^mineral equilibria as the dominant control on the aqueous abundance of these species. The absolute concentrations of H2 and H2 S in

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MEF £uids are similar to those observed during laboratory experiments and predicted by theoretical models, and can be used to constrain £uid temperatures in deep-seated reaction zones prior to cooling during ascent. Comparison of results from basalt and mineral-bu¡ered experiments conducted at 500 bar yield estimated subsurface temperatures at MEF of 340^425‡C (Fig. 6). 5.2. Phase separation Phase separation can play a critical role in regulating vent £uid chemistry because aqueous volatiles are preferentially partitioned into the vapor phase. The extent of volatile enrichment in low-Cl vapors, however, is strongly dependent on the temperature and pressure of phase separation and £uid salinity. Incipient phase separation of seawater at temperatures below its critical point (407‡C) involves the formation of a Cl-depleted vapor bubble and a liquid phase of essentially unmodi¢ed chlorinity (Fig. 4). Under supercritical conditions, incipient phase separation is characterized by the condensation of a high-salinity brine droplet, which produces a residual phase that is generally referred to as vapor, although it may have a density and salinity very similar to the original unseparated £uid. As the extent of subor supercritical phase separation increases due to decreased pressure or increased temperature, both salinity and density decrease in the vapor and increase in the liquid (Fig. 4). 5.2.1. Volatile enrichment The abundance of volatiles in the vapor phase under physical and chemical conditions in reac-

6 Fig. 7. Predicted enrichment factors (solid lines) for aqueous volatiles as a function of vapor chlorinity and partition coef¢cients (KD ) during phase separation of seawater at (a) 380‡C, (b) 390‡C, (c) 400‡C, (d) 410‡C, and (e) 420‡C. The extent of enrichment continues to decrease at temperatures s 420‡C. Due to the absence of experimental data at nearseawater salinities at 410 and 420‡C, enrichment factors are interpolated (dash-dot lines). The symbols represent enrichment factors calculated for aqueous volatiles in vent £uids from the MEF in 1999 and 2000 (see text). Requisite data for the construction of this ¢gure are from Bischo¡ [33].

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tions zones at Endeavour can be quantitatively assessed by considering phase relations in the system Na^Cl^H2 O. The term enrichment factor is used here to describe the degree of volatile enrichment in a vapor phase relative to the concentration in the bulk £uid prior to phase separation. Enrichment factors for phase separation of seawater-chlorinity £uids in a closed system are calculated as a function of vapor phase salinity, partition coe⁄cients for volatile species distribution between vapor and liquid, and temperature from data provided in Bischo¡ [33] (Fig. 7). Partition coe⁄cients (KD ) are de¢ned by the relationship : K D ¼ mvapor =mliquid

ð1Þ

where mvapor and mliquid represent the molal concentration of a given species in the vapor (lowsalinity) and liquid phase (brine), respectively. Although gas partitioning is poorly constrained at near-critical conditions, results of laboratory phase separation experiments indicate KD values in the range 4^6 at 388‡C and 240 bar [34], and reveal that partitioning of gases into the vapor phase is far from complete. Under subcritical conditions, the predicted enrichment factors in Fig. 7 terminate at the maximum salinity possible for a vapor along the two-phase boundary of seawater at the indicated temperatures (Fig. 4). A similar termination does not occur under supercritical conditions because all salinities less than seawater are possible in the vapor phase (Fig. 4). The graphs shown in Fig. 7 are constructed for a closed system at constant temperature. Accordingly, formation of lower-chlorinity vapors re£ects the e¡ects of decreasing pressure as £uid travels along the two-phase boundary for seawater. Examination of Fig. 7 indicates that the vapor phase is substantially enriched in volatiles for KD values greater than unity during the initial stages of subcritical phase separation of seawater in a closed system. As phase separation continues and lower-salinity vapors are produced, volatile concentrations in the vapor phase decrease rapidly and eventually approach those of the starting £uid composition (enrichment factor equal to 1), regardless of the KD value. Thus, the common assumption that vapors produced by subcritical

phase separation are always highly enriched in volatile species is not appropriate for £uids that experience high degrees of phase separation under closed conditions prior to brine removal elsewhere in the system. In open system environments typical of ridge crest hydrothermal convection cells, the extent of volatile enrichment during subcritical phase separation will be dependent on the ef¢ciency of phase segregation during £uid ascent, which in turn will be regulated by the permeability structure of the crust and £uid buoyancy. Phase separation of seawater at temperatures above the critical point produces vapor phase gas enrichments that di¡er substantially from those generated under subcritical conditions (Fig. 7). The vapor phase produced when the two-phase boundary is ¢rst encountered will show no gas enrichment because the formation of an in¢nitely small brine droplet has an insigni¢cant e¡ect on the composition of the residual vapor. However, as the amount of brine increases with increased phase separation, vapor phase gas concentrations rapidly increase to maxima before decreasing to the initial seawater concentrations in low-chlorinity vapors. Maximum volatile enrichments in a vapor phase produced from phase separation of supercritical seawater increase with decreasing temperature, reaching a value of approximately 2 at near-critical conditions, and are substantially smaller than enrichments possible under subcritical conditions (Fig. 7). To evaluate the e¡ects of phase separation on the abundance of aqueous volatiles present at vastly di¡erent concentrations, enrichment factors were determined for MEF vent £uids by assuming that gas-enriched Cl-depleted £uids are derived by phase separation of a seawater-salinity hydrothermal source £uid of constant composition (Fig. 7). The systematic decrease of individual volatile abundances with increasing Cl concentration is consistent with £uids being generated from a common source. Enrichment factors were calculated by normalizing measured gas concentrations to values in a hypothetical source £uid determined from a linear extrapolation of dissolved gas and Cl concentrations to seawater chlorinity. Because the measured concentrations of CO2 in 1999 and 2000 de¢ne o¡set subparallel trends as a function

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of Cl (Fig. 5e), the data were normalized to separate source £uids for each year. MEF vent £uids are characterized by similar enrichment factors for CO2 , CH4 , and NH3 that increase systematically to values between 1.2 and 1.5 as Cl concentrations decrease to 30 mmol/kg (Fig. 7). Enrichment factors of this magnitude are consistent with phase separation under sub- or supercritical conditions at temperatures 9 420‡C. Enrichment factors for H2 , and H2 S, however, reach values as high as 3^5 in the lowest-chlorinity £uids, which are substantially greater than values calculated for CO2 , CH4 , and NH3 (Fig. 7). If the observed enrichments for all dissolved gases were the result of phase separation alone, then the KD values for H2 and H2 S would have to be substantially greater than those for CO2 , CH4 , and NH3 . Such chemical behavior is not supported by experimental and theoretical data that indicate CH4 , NH3 , CO2 , H2 , and H2 S partition into the vapor phase to a similar extent during phase separation at temperatures typical of MEF [34^36]. Moreover, except for the near-seawater-salinity £uids, the levels of enrichment for H2 and H2 S are greater than values possible during supercritical phase separation (Fig. 7d,e). Enrichments of this magnitude are possible during phase separation under subcritical conditions but the lowest-salinity £uids would require temperatures 9 380‡C and depths at or near the sea£oor. This scenario appears unlikely at MEF since the large volumes of conjugate brine that are produced under such conditions have not been observed. Thus, the large di¡erence in the extent of volatile enrichments supports the idea that £uid^mineral equilibrium is the dominant process that regulates aqueous H2 and H2 S concentrations. A key implication of this result is that enrichments caused by pressure- and temperature-dependent £uid^ mineral equilibrium at near-critical conditions are substantially greater than the relatively minor vapor phase enrichments for CO2 , CH4 , and NH3 during phase separation. 5.2.2. Models for the evolution of £uid chemistry Phase separation at Endeavour may be induced by increasing temperature during descent/recharge or decreased pressure during up£ow. In either

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case, ascent of a highly buoyant vapor phase may result in continued phase separation as the £uid travels a temperature^pressure^composition path that de¢nes the two-phase boundary. Although phase separation in ridge crest hydrothermal systems may be initiated under sub- or supercritical conditions, all subsequent phase separation of a segregated vapor during ascent along the two-phase boundary is by de¢nition supercritical due to changes in the critical point of the vapor as salinity decreases (see Fig. 4). Two models of £uid evolution can be evaluated to account for the variations of dissolved CO2 , CH4 and NH3 with chlorinity at MEF. The ¢rst involves separation of a vapor phase followed by segregation and ascent to the sea£oor without mixing. In the absence of subsurface mixing, the range of chlorinities and volatile enrichment should re£ect the extent of phase separation and/or e⁄ciency of segregation. The predicted enrichment factors shown in Fig. 7 indicate that vapor phase volatiles enrichments should increase with increasing chlorinity under conditions of constant temperature and partition coe⁄cient, especially under subcritical conditions. Although the temperature variability and absolute values of partition coe⁄cients for liquid^vapor gas partitioning during phase separation at Endeavour are poorly constrained, the absence of increased CO2 , CH4 , and NH3 abundances in the mid- and higher-chloride £uids suggests that a model involving vapor phase partitioning without mixing may not be responsible for the systematic variation of dissolved gases with chlorinity. Chlorinity^temperature relationships also suggest that a no-mixing model may not be appropriate for MEF £uids. The occurrence of £uids at Sully, Cantilever, and Peanut with salinities and temperatures that are consistent with phase separation at near-sea£oor pressures (Fig. 4) suggests that phase segregation is an e⁄cient process that allows £uid salinity to respond rapidly to changes in temperature and pressure. Measured Cl concentrations and volatile enrichment factors in the most dilute £uids at Cantilever and Sully are consistent with generation of a vapor during subor supercritical phase separation and segregation. Generation of £uids with near-seawater chlorin-

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ity, however, requires temperatures near or above the critical point of seawater because subcritical phase separation and segregation alone (i.e. no mixing) at temperatures below 400‡C can only produce very dilute vapor phases (Figs. 4 and 7). If a no-mixing model is to explain the generation of near-seawater-chlorinity £uids, measured vent temperatures that are consistently below the two-phase boundary require extensive conductive heat loss. Because pressure conditions consistent with the generation of near-seawater salinities in a vapor phase during phase separation are only encountered at signi¢cant depths below the sea£oor (Fig. 4), and phase segregation appears to be an e⁄cient process in near-sea£oor environments, the conductive heat loss must occur within the crust, not within highly conductive sul¢de structures at the sea£oor. Owing to the low thermal conductivity of the oceanic crust, conductive heat loss to crustal rocks is not an e⁄cient mechanism to cool hydrothermal £uids [37]. A system of nested hydrothermal convection cells could function as a radiator and advectively remove heat from high-temperature vent £uids at MEF, causing them to migrate o¡ the two-phase boundary, but there is no evidence for the existence of such £uids. The second model for generation of MEF vent £uids involves segregation of a low-salinity vapor during phase separation followed by mixing with a cooler hydrothermal £uid of seawater salinity during ascent to the sea£oor. An analogous process involving mixing of a high-Cl brine with a hydrothermal £uid of seawater salinity was proposed by Von Damm and Bischo¡ [38] to account for the chemistry of high-chloride £uids venting at the South Cleft segment of the Juan de Fuca Ridge. Mixing may occur at a millimeter scale in a ‘cracking front’ as a vapor generated upon contact of hydrothermal £uid with freshly exposed hot rock ascends and mixes with an overlying seawater-salinity hydrothermal £uid, or at a larger scale by conductive heating at the base of a hydrothermal reservoir. In both cases, the overlying seawater-salinity £uid represents the source £uid for vapors prior to phase separation. Within the context of a mixing model, linear trends of increasing volatiles with decreasing Cl represent

mixing lines between a low-chlorinity gas-rich vapor and a high-chlorinity relatively gas-poor £uid from which the vapor was derived. The low-salinity £uids at Cantilever and Sully likely represent a vapor phase that has experienced minimal or no mixing. Mixing of relatively hot low-salinity £uid with a cooler seawater-salinity hydrothermal £uid is consistent with Cl-depleted £uids venting at temperatures below the two-phase boundary of seawater (Fig. 4) and does not require conductive heat loss within the oceanic crust. This model suggests that, except for the most Cl-depleted £uids that have experienced little or no mixing, the absolute abundance of conservative species such as CO2 , CH4 , NH3 and Cl may not accurately re£ect temperature and pressure conditions during phase separation. The data presented here do not allow accurate determination of the temperature and pressure conditions during generation of vapor prior to mixing. That the MEF data de¢ne a single trend for the abundance of each gas as a function of Cl, however, suggests uniform vapor composition throughout the entire MEF. This observation suggests phase separation is not occurring under subcritical conditions because the extent of volatile enrichment is highly sensitive to small spatial variations in temperature and pressure that are likely to exist at MEF (Fig. 7). In contrast, relatively uniform vapor phase composition is expected during phase separation under supercritical conditions at temperature 9 420‡C because the range of enrichment factors is limited to values 6 2. Seyfried et al. [32] utilized a theoretical model of £uid £ow and Cl partitioning to estimate a temperature of 418‡C for phase separation at MEF in 1999. Accordingly, the extent of volatile enrichment in conjunction with the linear variability as a function of Cl supports a model involving generation of low-chlorinity vapor under supercritical conditions followed by mixing with hydrothermal source £uids during ascent. Subsurface mixing at MEF also has implications for the abundances of H2 and H2 S. The strong inverse correlations of these species with Cl are consistent with the mixing of a volatilerich vapor phase produced by £uid^mineral equilibria at relatively high temperatures or low pres-

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sures with a seawater-salinity hydrothermal source £uid containing lower amounts of dissolved gas. The slight curvature in the trends for H2 and H2 S concentrations as a function of Cl (Fig. 5) suggests reequilibration of £uids following mixing. 5.3. Sources of CO2 Although phase separation increases the concentration of CO2 to a relatively small extent in low-chlorinity £uids at MEF, its overall abundance is largely controlled by the availability of sources. Carbon dioxide may be released to solution during magma degassing, rock leaching, and dissolution of sedimentary carbonates. The carbon isotope composition of CO2 at MEF, however, indicates a predominantly mantle source and precludes a signi¢cant contribution from sedimentary material [23]. Leaching of basaltic rocks could have contributed substantial quantities of mantle-derived CO2 to £uids at MEF, but requires water/rock ratios to have decreased by more than a factor of 2 to account for the entire CO2 increase in 1999 without a magmatic contribution. Decreased water/rock ratios would be accompanied by a dramatic increase in the aqueous abundance of mobile trace elements such as Li, Rb, and Cs. Relatively constant Cl-normalized abundances of these species before and after the 1999 earthquakes [2,32] suggest that water/rock ratios remained constant. Thus, magma degassing is the most likely cause for the increases in aqueous CO2 in 1999. Direct evidence for magma degassing in ridge crest environments is presently lacking, but is well-documented in sub-aerial and shallow marine environments [39^42]. Carbon dioxide di¡using through the roofs of shallow magma chambers or released from recently intruded dikes may be entrained in ridge crest hydrothermal £uids and delivered to the sea£oor. Lilley et al. [8] attribute the large increases in measured CO2 concentrations at MEF in 1999 to magma movement beneath the ridge crest and suggest that the seismic swarm was magmatic in origin, and not a tectonic event as originally suggested [9]. Our data are consistent with the interpretation of Lilley et al.

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[8] and provide additional evidence for a shortlived perturbation of the chemical system in response magmatic activity. 5.4. Temporal variability and subsurface hydrology Temporal variability of MEF vent £uid chemistry between 1999 and 2000 suggests that the 1999 seismic event may have been associated with injection of a dike at shallow levels of the crust. A decrease in Cl concentrations immediately after the earthquakes, in conjunction with an increase in H2 and H2 S concentrations, points to an increased supply of heat, a conclusion that is consistent with increased temperatures of diffuse venting and £uid output [9]. Venting of £uids characterized by extreme Cl depletions in 1999 provides evidence for phase separation in shallow regions of the crust where relatively low pressures favor formation of low-Cl vapors (see Fig. 4) that may travel the short distance to the sea£oor without undergoing extensive mixing. Increases in measured Cl concentrations across the MEF and the absence of extremely dilute vapors in 2000 are consistent with phase separation at greater depths (higher pressure) in response to rapid cooling of a dike. The decreased dependence of Cl abundance on measured temperature in 2000 relative to 1999 (Fig. 8) supports this idea because a transition to phase separation at greater depth increases the Cl content of the vapor at a given temperature (Fig.

Fig. 8. Variations in aqueous Cl concentration as a function of measured temperature in vent £uids from the MEF in September 1999 and July 2000.

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4), and increases the likelihood of mixing with seawater-salinity £uids during up£ow, especially if £ow rates have decreased in response to a diminished heat source. Similar Cl variation was observed after a volcanic eruption at 9^10‡N on the East Paci¢c Rise and was attributed to a deepening of the reaction zone in response to rapid cooling of a near-surface dike [5]. Signi¢cant decreases in the abundances of CH4 and NH3 relative to pre-earthquake £uids for a given Cl concentration (Fig. 5) indicate a decrease in the contribution of sediment-derived species to seawater-salinity source £uids at MEF. Although data are not available for NH3 , constant CH4 concentrations immediately before 1999 [8] and more decreases following the earthquakes indicate a change in subsurface hydrology that reduced £uid^sediment interaction. That the data from 1999 and 2000 de¢ne a single trend as a function of Cl suggests that the sedimentary contribution of CH4 and NH3 to the source £uid did not change substantially after the earthquakes. The small degree of scatter associated with the correlation of CH4 and NH3 with Cl regardless of vent structure (Fig. 5) indicates phase-separated vent £uids were derived from source £uids having a uniform composition across the MEF. Kelley et al. [43] distinguish two £ow geometries that may be feeding vents at the Mothra hydrothermal ¢eld at the southern end of the Endeavour segment. The ¢rst involves narrow up£ow regions fed by discrete reaction zones that produce vent £uids of varying composition on the scale of an individual vent ¢eld. The second involves an upwelling system that is fed by a common reservoir of hydrothermal £uid. Vent clusters associated with the latter system may be characterized by chemical similarities, but near-surface mixing and/or cooling may modify the composition of the common source £uid. Our data suggest that the second model of Kelley et al. [43] is most appropriate for MEF. In this case, large variations in £uid composition across the ¢eld from north to south may re£ect di¡erences in the depth of phase separation and variable amounts of subsurface mixing between dilute vapors and seawater-salinity hydrothermal £uids. The near constancy of CH4 and NH3 concentrations in the source £uid prior

to phase separation suggests that the sedimentary source of these species is not local to each vent, but may be located in recharge zones distal to the MEF.

6. Summary Time series sampling of hydrothermal vent £uids at MEF following seismic activity in June 1999 reveals large and rapid changes in £uid composition. These variations contrast with nearsteady-state £uid composition prior to the earthquakes, and re£ect substantial changes in the permeability and thermal structure of the oceanic crust. In general, the abundances of all dissolved volatiles were characterized by a strong inverse correlation with Cl concentrations. These trends can be accounted for by mixing of a Cl-poor vapor generated at depth during phase separation with an evolved hydrothermal £uid of seawater chlorinity during up£ow. The Cl dependence of dissolved gases also points to a model in which a common source £uid feeds reaction zones throughout the MEF, with intervent chemical heterogeneities introduced by locally variable mixing and cooling processes. Lower concentrations of sediment-derived species in MEF vent £uids following the earthquakes re£ect a step change in subsurface hydrology that reduced the interaction of £uids with sediments. Although large variations in the abundance of non-volatile species such as Cl and major cations result from phase separation, volatile partitioning results in only 20^ 50% enrichments of CH4 , NH3 , and CO2 in vapor phases at MEF. Phase separation is not responsible for 300^500% enrichments of H2 S and H2 observed in these £uids. Instead, concentrations of H2 S and H2 are regulated by temperatureand pressure-dependent £uid^mineral equilibria at near-critical conditions. Substantial increases in the abundance of dissolved CO2 in vent £uids collected in September 1999 support a magmatic origin for the earthquake activity earlier in the year. Temperature^ salinity relationships also suggest that the 1999 seismic activity may have been accompanied by intrusion of a dike that did not reach the sea£oor.

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In addition to modifying the subsurface hydrologic system, dike intrusion represents an e¡ective means to transport heat to shallow regions of the oceanic crust and induce phase separation at relatively low pressures. Results of this study demonstrate the sensitivity of vent £uid composition to variations in subsurface temperature, pressure, and permeability and the utility of time series monitoring to elucidate cause-and-e¡ect relationships in ridge crest hydrothermal systems.

Acknowledgements The e¡orts and expertise of the Alvin Group and o⁄cers and crew members of R/V Atlantis are greatly appreciated. We thank William Seyfried for the opportunity to participate in the 1999 cruise to MEF. Numerous and insightful comments by Meg Tivey greatly improved early versions of this article. Michael Berndt, Meg Tivey, Dane Percy, Karen Hurley, and Hallie Marbet provided invaluable assistance during the collection of these £uids. Funds for this research were provided by NSF Grants OCE-9906752, OCE-961442, OCE-9911472, and a Green Technology Award from WHOI. WHOI contribution no. 10991.[BOYLE]

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