Chloride depletions and enrichments in seafloor hydrothermal fluids: Constraints from experimental basalt alteration studies

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LETTER

Chloride depletions and enrichments in seafloor hydrothermal fluids: Constraints from experimental basalt alteration studies W. E. SEYFRRZD JR., MICHAELE. BERNDTand D. R. JANECK~ Departmentof Geology and Geophysicq University of Minnesota,~n~~~ MN 55455, U.S.A. (Received September 23, 1985;aoq&

in revised formFebruary 7, 1986)

Abstract-Na-K-&-Cl Buid was reactedwith diabaseat 400” and 425”C, 400 bars and fluid/rock mass ratioof 0.5 to assessthe relativemobility of dissolvedc1. Fluids!inm the presentexperimenurevealmlatively laxgetime and temperaturedependent&teases in c1, temperatureincreaseenhancesCl tentoval, whereas reactionprogresshas the opposite&ii These ebangcsare not due to boiling and/or phase separation. At 400” and 425Y, 400 bars,Cl removalfromm&ion is aowmmi& by Ca and Na lb&on, rqeetively. Both experimentspmduczd XnixaJ-layer ~~ clinmoisite and albit@?).olivine was not detazed by X-raydifhaction or p&ogqhic analysisof run produetsfrom eitherexpuiment, however,we propose that the time dependentchangesin dissolvedCl resultfrom olivine replacementby an Fe-hydroxychloride phase followed by hydration &cts. Quenched and washed alteratidnproducts,however, did not reveal anomaloiu Cl, which sqgests that the Cl-bear@ phase is charactenied by r&rogmdesolubility. This is consistent with the relative ma@ude of Cl fuation observedfor experimentsat both temperaturesand from rc3uJtsof an isobariccooling experiment. The experimentsprovideevidu~ forthe non-conservativebehaviorof Cl duringhydmthcrmalalteration of basalt.The dataarcparticuMy importantin light of the Cl-depletednatureof some ridgecresthot spring i&ii& and suggesttcmpuztafcs of fozmationforthesefluidsofapproximately410sC, assumingsub-seafloor v of 400 bars.In addition,the retrogradesolubility of the C&earing phasereapoasiblefor Cl ftxation during high tcmpcmturebasalt altuation, may help to exph+inCl enrichment in hot spring Buidswhich have eonduetiveiy cooled below 350°C. lNTRODUClION

data (VON DAMMet ol., 1985; BOWERSand TAYLCIR, 1986) that to model experimentally the formation of EXPERIMENTAL B.~.~ALT-SEAWA~ interaction at elend-member hydrothermal fluid issuing from black evated temperatures and pressures has proved usefm smoker vents along the East Pacific Rise (EPR) reqires in establishing the relative mobility of chemical comthat experiments be performed at temperatures gmater ponents during hy~~~~ altemtion of the oceanic than 35O*C, at pressures in the vicinity of 400 baq crust. This is reflected by the chemical similarity of and at fluid/basalt mass ratios of between 0.5 and 1.0. experimentally derived and natural ridge crest hydroFurthermore, basalt/diabase composition, mineralogy thermal fluid (Morn, 1983 and references therein; and texture as well as fluid chemistry for experiments SEYFRIEDand JANECKY,1985). Although similar in must rigorously duplicate the natural system (Sm many mspeetq the chemistries of these fluids do exhibit and JANECKY,1985). These conditions contrast with differences. For example, Cl concentrations in hot those in previous experimental studies, which used relspring fluids reveal significant gains and losses relative atively high fluid/rock mass ratios (SEYFRIEDand to seawater (VON DAMMet al., 1985; MECHARD 1 al., MOTI’L, 1982) and/or pressures exceeding 500 bars 19841, whereas experimental data typically indicate that (MO’ITLand HOLLAND,1978; HAJASH,1975; ROSENCl behaves conservatively, increasing in concentration BAUERand BISCHOFF,1983). in response to rock hydration (Mom and HOLLAND, The purpose of the present paper is to examine the 1978; SEYFIUEDand BISCHOFF,198 1; ROSENBAUER reactivity of Cl during hydrothermal alteration of baand BISCHOFF,1983). salt/diabase at conditions consistent with existing geoExperiments, however, can accurately represent logic and geochemical constraints for formation of endnatural processes only if the geologic model of the natmember hydrothermal fluid venting from EPR hot uraI process is valid. Thus, it is now becoming clear Spfings. from geophysical (SLEEPet al., 1984) and geochemicai Data presented here are a subset of results of a more extensive study involving alteration processes and mobility of heavy elements during basalt-solution interl Present a&em: Los Alamos National L~boratoks, L.os action in the supercritical region. SEYFRIEDand JAAlamos, NM 87545. NECKY( 1985) should be consulted for details on ex469

W. E. Seyfried Jr., M. E. Bemdt and D. R. Jane&y

470

perimental procedures, analytical techniques and the general mechanism of basalt alteration at near super-

critical temperatures

“_”

Expanmental conditions

and relatively low pressures.

EXPERIMENTAL Experiments were performed at 400” and 425”C, 400 ba& and a fluid/basalt mass ratio of 0.5 using flexible cell hydrothermal apparatus. The apparatus permits internally tiltered fluid samples to be periodiarlly withdrawn from a corrosionresistant and contamination-free reaction cell at experimental conditions (SEYFRIED and JANECKY,1985), thereby avoiding the ambiguous effects of quenching on solution chemistry. Unaltered diabase dredged from 23’N latitude on the MidAtlantic Ridge was utilized. Texturally this rock is subophitic and contains abundant plagioclase and clinopyroxene, minor olivine and orthopyroxene and trace magnetite. Rock fragments were ground until the entire sample passed through a 74 pm nylon sieve. Seawater was not used as a reactant for these experiments bccausc we feel that it does not represent adcquatcly the chemistry of fluid involved in deep-seated hydrotberma.l alteration of the oceanic crust. Alteration pmcesses reflectedby hot spring chemistry are more accurately modelled by a Na-Ca-K-CJ fluid of seawater ionic strength (SEYFRIELI and JANECKY,1985), as Mg and SD, arc known to be rapidly removed as seawater is heated in recharge zones. The concentrations of the species in the fluid used for our experiments were as follows: 0.45 m Na; 0.037 m Ca, 0.015 m K, 0.55 m Cl. RESULTS AND DISCUSSION Cl concentrations in solution are characterized by large time dependent deceases relative to starting fluid chemistry during both of the present experiments. This observation is highly significant in connection with the mode of formation of Cl depleted hot spring fluids and contrasts sharply with results of basalt/fluid interaction studies at relatively high fluid pressures and/or high fluid/basalt mass ratios. The present experiments show, for example, that Cl decreases more at 425”C, 400 bars than at 400”C, 400 bars, and earlier in an experiment rather than later (Fig. 1). At 400°C, the Cl decrease is attended by a large Ca decrease and increases in Na, K, and minor Fe. At 425”C, in contrast, the only species in solution to show an increase is Fe: Na, Ca, and K all decrease. The magnitude and time dependent behavior of chemical exchange during the experiment at 425°C however, indicates that the Cl decrease is primarily balanced by a decrease in Na, whereas the loss of Ca from solution is balanced by a gain in Fe (Fig 1). Fhrids from both experiments are relatively acidic (pH measured at 25’C), more so at 425°C than at 4OO”C, and become more acidic with reaction progress. Dissolved HIS appears to reflect pyrrhotite dissolution and pyrite precipitation, the latter being especially important at 425°C (SEYFMEDand JANECKY,1985), while dissolved SiOz concentrations are at or slightly below quartz saturation at experimental conditions. The magnitude of Cl loss from solution and cation response during the present experiments bracket closely analogous data from a typical hot spring system. For example, when plotted relative to the chemistry of the

-I 370

360

360

400

410

420

430

440

450

460

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Temperature(Da9 C) RG. 1. Change in concentration (mmolal) of Cl. Na, Ca, K, Fe, H2S, Si02 and pH during diabase-fluid interaction at a fluid/rock mass ratio of 0.5, 400 bars pressure, and tcmpcrature of 4OO’C(A) and 425°C (B). The solid, dotted and diional patterns retlect reaction times of approximately 50, 300 and 1800 hours respectively. For comparison purposes,

data are plotted (C) for the Hanging Garden vent at 21°N, EPR (VON DAMM et al., 1985). Dissolved Fe concentrations in (A) are not apparent owing to the scale setting. Them data, however, do not exceed a concentration of 0.5 mmolal (SEYFRIED and JANECKY,1985).

solution used for our experiments, fluid from the Hanging Garden vent at 2 1“N, EPR (VON DAMM ef al., 1985) reveals a relative composition which falls between our 400 and 425°C data sets (Fig. 1). Although this is most obvious from the relative change in Na, Ca and Cl concentrations, virtually all other dissolved species are consistent with this interpretation. Diahase alteration and mineralization have been discussed by SEYFREDand JANECKY (1985). In general, loss of olivine and reduction in abundance and crystallinity of plagiociase feldspar are obvious changes to the primary mineralogy. Seeondary alteration phases are not abundant and/or well-cry+ talked as evidenced by X-ray diEaction data The data weal, for example, only weak reflections of a mixed-layer chloritesmectite phase and possibly albite. However, tremolite/actinelite (?) and clinozoisite were tentatively identihed by scanning electron microscopy. Petrographic study has confirmed the presence of clino zoisite as an alteration product of plagioclase. At low fluid/rock mass ratios, formation of even a small amount of alteration is reflected by large compositional changes in solution. Thus, chemical changes in the fluid phase provide the most sensitive means of estimating alteration processes. Cl removal mechanism Two processes are typically considered whenever substantial changes occur to the bulk chemistry of a fluid under hydrothermal conditions: (I) phase separation on intersection of the two phase boundary of the solution phase; and, (2) formation of a phase or phases significantly enriched in key components. Phase Separation. SOURIFUJAN and KENNEDY ( 1962) and more recently BISCHOFF and ROSENBAUER ( 1984) have evaluated the NaCl-Hz0 system at elevated temperatures and pressures. The latter study specifica.Ily

Chloride in seafloor hydrothermal fluid the critical composition curve and two phase boundary of seawater and a 3.2% NaCl solution, which was used as an analogue for seawater in the high temperature/pressure region. Intersection of the two phase boundary for a NaCl-rich solution at a supercritical temperature occurs at a specific pressure(which is a function of the salinity of the solution), and causes separation of a small amount of brine, while the bulk of the fluid becomes only slightly less saline. At 400 bars, a temperature of approximately 450°C is necessary to cause phase separation of a 3.2% NaCl solution (Fig. 2). Thus, phase separation cannot be called upon to explain the change in bulk fluid chemistry observed during the present experiments. Mineral Formation. The magnitude of Cl removal from solution during the present experiments is considerable, especially when considered in terms of formation of an alteration phase. For example, assuming no hydration, early stage Cl removal from solution at 400’ and 425°C 400 bars (Fig. 1), yields whole rock Cl enrichments of approximately 400 and 2000 ppm respectively, and undoubtedly much higher values for specific alteration phases. In comparison, common hydrous minerals from altered oceanic crust, such as chlorite, smectite and epidote, contain less than 150 ppm Cl (IT0 et al., 1983), while some amphiboles have been reported to contain as much as 4% Cl (VANKO, 1984; HONNOREZand FIRST, 1975; PRICHARD and CANN, 1982). The Cl-rich amphiboles, however, typdetermined

20 0

6

-20

:PH

-40 -80 -80 -100

Cl

Na

Ca

K

Ci

Na

Ca

K

Cl

Na

Ca

K

Fe H$

SiO, pH(25)

FIG. 2. Twophase curve of seawater (3.2% NaClsolution) as a function of temperature and pressure (BISCHOFT and R+ SENBAUER, 1984). Symbols designate pressure and temperati conditions utilized for the present experiments.

471

ically reflect alteration of basalt/gabbro at temperatures of Ween 550” and 750°C (ITO and ANDERSON,1983; SPEAR, 198 1), and may require the presence of a Clrich brine to achieve high Cl concentrations. For example, VANKO(1984) reported amphibole containing 4% Cl from the Mathematician Ridge (Eastern Pacific), which was associated with quartz bearing fluid inclusions characterized by salinities approaching 26 equivalent wt. % NaCl. Based on temperature and salinity requirements, therefore, one can preclude formation of a Cl-rich amphibole to account for Cl changes in solution during our experiments. Moreover, amphibole was not detected as an alteration product in the present experiments, and phases which were (clinozoisite and chlorite/smectite), failed to contain Cl. Thus. mineral formation as a mechanism to account for observed changes in solution chemistry during the experiments is possible only if the phase involved is characterized by retrograde solubiiity causing it to dissolve during the quench procedure. Anhydrite (BLOUNT and DICUSON, 1969) and magnesium hydroxy sulfate hydrate (MHSH) and brucite (JANECKYand SEYFRIED,1983), are examples of minerals routinely encountered in hydrothermal systems, both natural and experimental, which behave in this way. Retrograde solubility of the inferred Cl-bearing phase has been tested by monitoring temperature dependent changes in solution chemistry during step-wise isobaric cooling of an experiment initially maintained at a temperature-pressure condition sufficient to cause Cl removal from solution. This was accomplished at conditions similar to those utilized for the 425°C experiment discussed previously, but diabase was replaced with a crystalline basalt from the East Pacific Rise (SEYFRIEDand JANECKY,1985). Early stage Cl removal from solution during the retrograde experiment (Table I) was not as great as that observed with the diabase (Fig. I ), indicating that rock chemistry, mineralogy and texture play an important role in the magnitude and mechanism of Cl removal from solution. Nevertheless, decreasing temperature to 35O”C, then to 25O”C, caused large increases in the Cl concentration in solution (Table I), reflecting both rock hydration and a Cl release process. The relative importance of hydration and Cl release was determined by monitoring the Cl/Br ratio. Br was added to the experiment such that the Cl/Br ratio prior to reaction with basalt was similar to that of seawater (Table 1, Fig. 3). Basalt alteration at 425°C caused the ratio to decrease indicating that the process responsible for Cl removal from solution also accommodates Br, but in lesser amounts; that is, the Cl/Br ratio of the Cl-rich phase is greater than that of the fluid from which it formed, a result attributable to the relatively large size of the Br ion. On cooling to 350°C, the Cl/Br ratio remains virtually unchanged, while the Cl concentration increases noticeably (Fig. 3). This can best be explained by rock hydration (approximately 5%). Further cooling to 250°C, however, produces a significant and rapid increase in the Cl/Br ratio and the Cl concentra-

W. E. Seyfiied Jr., M. E. Berndt and D. R. Jan&y

472 Table1

Dissolved Cl (umolel) and ClDr ratio during isoberic (400 bars) coaling experiment Taapereture (oc) 25 425 350 250 *Time in hours experiment

Reectfon time* 0 313 458 463

Cl 547 521 584 649

from start

Cl/Br 650 623 625 662

of

tion in solution (Table 1, Fig. 3). This requires dissolution of a Cl-rich mineral with a distinctive and relatively high Cl/Br ratio, into a fluid reservoir depleted by previous sampling. Calculations indicate that at this stage of the experiment (250°C) the fluid/rock mass ratio has been reduced by a factor of 2 from that which existed at the start. Thus, changes in solution chemistry caused by rock hydration and dissolution processes are exaggerated relative to the effects of similar processes at earlier stages of the experiment, Nevertheless, the data indicate that the phase responsible for Cl removal from solution during this experiment at 425”C, 400 bars, and, presumably, during the constant temperature experiments as we! (Fig. I), is indeed characterized by retrogmde solubility and dissolves on cooling below 350°C. Thus, it is not surprising that a Cl-bearing mineral was not detected by analysis of the quenched and washed products of the experiments. Considering the retrograde solubiity of the Cl-rich phase and the chemistry of the fluid from which it precipitates, also Cl-rich, the task of designing an experiment to isolate the phase will be difficult, and requisite chemical and minrralogic data for the phase may have to be obtained by means other than direct analysis. EDMONDet n!. ( 1979) suggsted formation of an Fe/ Mg-hydroxy chloride phase to account for the relatively low Cl concentrations in warm spring solutions issuing from vents in the vicinity of the Galapagos Spreading Center, because minerals similar to this have been reported or inferred to occur in nature, specifzaUy in partially altered ultramafic rocks (RUCKLIDGEand PATTERSON,1977; EARLY, 1958), or derived experimentally during hydrothermal alteration of olivine at elevated temperatures and pressures (Pan d al., 1972; JANECKY, 1982). Owing to the relatively, simple structure, an Fe/Mg-hydroxy chloride phase could pmcipitate homogeneously from solution, or form as a replacement product of a highly unstable primary mineral, such as olivine, thereby providing a responsive and effective means of removal of Cl. One of the more intriguing aspects of the Cl removal pm is the rapid rate at which it occurs (Fig. 1). In fact, this, along with other time dependent changes in solution chemistry, led us to evoke an adsorption model (BERNDT er al., 1985), because adsorption is typically characterized by relatively weak chemical and electrosta tic forces which one would expect to respond rapidly and reversibly to alteration e8bcts. Mineral formation, however, and not adsorption is suggested by results of the isobaric cooling

experiment. Thus, we now believe that formation of a divalent cation-hydroxy chloride phase provides the most likely explanation for Cl removal from solution during the present experiments, and for certain portions of the submarine geothermal system, as reflected by the Cl depleted nature of some hot spring fluids. Because the fluid used for our experiments was initially Mg-free and became Fe-rich during reaction with diabase (more so at 425°C than 400°C; Fig. l), the Clrich phase is probably also Fe-rich. To illustrate this we will adopt the stoichiometry of the Fe-hydroxy chloride phase reported by RUCWDGE and PATTERSON( 1977); that is, Fe2(0H)&I, as an analogue for the Cl-rich phase which formed during our experiments. The solubility of all minerals decreases with ining temperature in the low pressure supercritical region owing to changes in the physical nature of the water solvent (NORTON, 1984; SEYFIUED and JANECKY, 1985). Undoubtedly, Fe-hydroxy chloride hydrolysis behaves similarly, as suggested by the retrograde solubility of the phase. Thus, the reaction: 2Fe”

+ 3H20 + Cl- = Fe2(OH),Cl + 3H+

(1)

proceeds from left to right with increasing temperature at constant pressure, accounting for greater Cl fixation at 425”C, than at 4OO”C, 400 bars, even though the pH at the higher temperature is decidedly more acidic. The source of Fe for Fe-hydroxy chloride formation is either the solution, as illustrated by reaction 1, or, an Fe-rich solid phase, such as magnetite, or more likely the fayalite component of olivine. Olivine is an important component of diabase, and was replaced totally by hydrous Mg and Fe-rich alteration phases during the present experiments, both at 400” and 425°C.

Hydration w

600: 600

I

1

1

630

600

650

700

Cl (mmolal) FIG. 3. Cl/Br molal ratio relative to Cl concentration in solution for isobaric (400 bars) cooling experiment. Experi-

ment utilized a cr@alline basalt from the East Pacific Rise and fluid similar to that used for other experimentstinitial fluid/rock mass ratio of 0.5). Changes in the Cl/Bf ratio eau beusedto hydration from mineral xeaction &kc& For example, o~%ngG diffkrences in ionic radii, Cl and Br

pahtion differentlybetweensolution and 8 solid phase containingthese species Thus, the Ci/Brratioin solutionhnges during phase formation (425OC) and dissolution (350’250°C). phase dissolution at temperatures below 350°C provides evidence for re$mgr& solutility. Cl/k ratio is unaffectedbyhydration,whichdomiaatosthe~~~in~fiom 425°C to 350°C.

Chloride in seafloor hydrothermal fluid Thus, early stage Cl removal from solution (Fig. 1), may correlate with early stage abundance of ohvine. Cl removal from solution owing to mineral formation and/or replacement requires cation hation reactions to occur simultaneously so as to maintain an electrically neutral fluid. For example, assuming olivine serves as a source of Fe for Fe-hydroxy chloride formation during our experiments, Cl removal from solution at 400°C and 425°C is compensated by Ca++ and Na+ fixation respectively, as follows: 400°C 400 bars:

1.8Ca.,Na.~A11.,0~+ 4(Mg.,~Fe.&SiO~ + 0.77Ca++ Pkgioclase (An&

Olivine (Fo.&

+ 6HzO + Cl- = Mg&i,O,r,(OH), + Canal& Clincchlore

(2)

Fe-hydroxy chloride 425°C. 400 bars; 3.0Ca,,Na.gU1.~4 + 4(Mg.,,Fe.&SiO, + Na+ + 2.2Si02

Plsgioctase (An,,,)

Olivine (For,)

+ 6HzO + CI- = M~~O~~OH~ + C!a&O,&OH) c%nochlore

Clillomisite

+ ZNaAlSi& + FaOH)&l Albite

(3)

Fe-hydroxy ChlOli&

by coupling appropriate reactions based on our recognition of primary and alteration mineralogy, we can distribute Cl, and Na and Ca, between several phases, and account, reasonably well, for the direction and magnitude of chemical exchange, as reflected by solution chemistry (Fig 1) during the experiments. An important assumption made, however, is that Fe for Fe-hydroxy chloride formation is provided by the fayalite component of olivine. This is somewhat tenuous, and justified solely by our knowledge of olivine reactivity under hydrothermal conditions, which is invariably rapid, the rate of Cl removal for solution during these experiments, which is also rapid (Fii l), and the association of ohvine with Fe-hydroxy chloride in partiahy serpentinized ultramafic rocks (RUCKIJDGE and hTl-ERSON, 1977). A remaining point which needs to be explained is the conspicuous increase in concentrations of most dissolved species with reaction pm folIowing the first sample at approximately 50 hours (Fig. 1). The systematic increase in Na and Cl, for example, at 400° and 425°C is di&ult to account for by an alteration process, because thii would require dissolved species to respond similarly in experiments cWed by different temperatuns, different pH vah~es, and different alteration assemblages. However, we believe a less complicated explanation for the changes involves Thus,

progressive hydration of unaltered portions of diabase following an early period of rapid alteration characterized by eation and anion fixation reactions. For example, assuming Cl fixation ceases during this early reaction period, then, subsequent hydration of 2-396 at W”C, and 3-496 at 42S’C, accounts for the increases in Cl concentration in solution (Fig. 1). The estimates of hydration are maximum values because they assume the flui~~k mass ratio has remained constant, which it certainly has not, since fluid was removed by sampling. Thus, early stage chemical exchange followed by late stage hydration provides the most satisfactory explanation for the time dependent changes in solution chemistry during the present experiments CONCLUSlONS AND IMPLICATIONS

Clinozoisite

+ FQ(OH)$ZI + O.S4Na*+ 0.79SQ

473

Results of experiments at 400” and 425°C 400 bars and fluid/rock mass ratio of 0.5 have identified an important component of basalt altemtiou not encountered by previous experimental studies at higher pressures and fluid/rock mass ratios. Our data reveal significant Cl removal from solution at both 400” and 425°C owing to formation of a still poorly defined Cl-bearing phase, possibly Fe-hydroxy chloride. High tempemtures and relatively low pressures may act to stabilize the phase, even for conditions which are decidedly acidic. The quenched and washed products of experiments, however, fail to reveal anomolous Cl, which suggests that the Cl-bearing phase is characterized by retrograde solubility. This is consistent with the relative magnitude of Cl fixation observed for experiments at both temperatures and from results of an isobaric cooling experiment. Cl removal from solution is a rapid process, possibly restricted to the earliest stages of reaction. At 400” and 425% the process is accompanied by Ca and Na fixation and clinozoisite and clinozoisite-albite formation, respectively. The rapid rate of formation of the Cl-bearing mineral suggests that the phase either precipitates directly from solution, or more likely, replaces a highly unstable primary mineral such as olivine. Olivine is an important component of diabase, is particularly reactive under hydrothermal conditions, and has been linked to formation of Fe/Mg-hydroxy chlorides in nature. ~u~~o~, that Cl removal &om solution is more noticeable at extremely low, rather than moderate or high fluid/rock mass ratios and limited to early-stage alteration, suggests involvement of a rock derived mineral, particularly one not abundantly present. Subsequent to the Cl removal stage of alteration, however, dissolved Cl con~n~tio~ tend to increase owing to hydration. In submarine geothermal systems, conditions most likely to result in Cl removal from solution are best satisfied in a region close to the top of a relatively shallow sub-seafloor magma chamber, where prograde conditions dominate reaction between com~sition~ly evolved seawater and continuously exposed fresh rock for decidedly short periods of time (SEYFRIED and JANECKY,1985; LIST’ER, 1982).

474

W. E. Seytiied Jr., M. E. Bemdt and D. R. Jane&y

The similarity between the chemistry of end-member hydrothermal fluid issuing from, for example, Hanging Garden vent (VON DAMM et al., 1985) and a fluid derived experimentally at a temperature of between 400” and 425”C, 400 bars, by reaction with a nonglassy basalt (see SEYFRIEDand JANECKY, 1985) at a fluid/basalt mass ratio of 0.5, is more than coincidental (Fig l), and suggests that both systems are affected by similar processes. However, as pointed out by BIG CHOPP and ROSENBAUER(1984), temperatures this high at a sub-seafloor depth consistent with a total pressure of 400 bars, require that the fluid must lose heat on ascent in amounts greater than permitted by adiabatic decompression alone to vent at the seafloor at only 350°C. Either vent fluid tempemtures have been underestimated by 30_4O”C, or heat has been lost by conduction or sub-sea&n mixing processes. In contrast to the formation of Cl depleted hot spring fluids, some relatively low temperature hot springs reveal Cl enrichment. For example, fluids issuing from NGS vent at 2 1ON (VON DAMM et al., 1985) and vents at ~~“N(MIcHARD~~uL 1985),EPR,arecharacterixed by Cl concentrations approximately 10 and 30% greater than seawater, and temperatures of 273’ and 317”C, mspectively. VON DAMM et al. (1985) proposed conductive cooling and basalt hydration to account for the relatively low temperature and high Cl for NGS vent fluids, whereas, RICHARD et al. (1984) proposed magmatic processes to account for the relatively high Cl at 13“N. Although hydration undoubtedly contrib utes to the Cl concentration of these fluids, particularly for NGS vent fluids, mtmgmde alteration/dissolution procesms may also be important. Direct and indirect evidence exists to indicate that the NGS vent system is the oldest of the vents at 2 1ON WON DAMM et al., 1985); an aged hydrothermal system, which may permit fluid cooled by conduction to react with rock previously altered at higher temperatures. Similar arguments may apply to the system at 13”N. Thus, if a Cl-rich mineral analogous to that inferred to have formed during our experiments and during high temperature hydrothermal alteration at 2 1“N, as reflected by the existence of Cl depleted hot spring fluids in the immediate proximity of the NGS vent, is part of the higb temperature assemblage, it would surely dissolve on cooling, and release Cl to solution. The extent to which retrogmde alteration could affect the chlorinity of ridge crest hot springs, however, would depend on the magnitude of Cl fixation during the prograde phase of alteration, and on temperature, residence time and fluid/altered rock ratio during the retrograde process For example, the amount of Cl fixed in solid phases during the present experiments was 4002000 ppm, thus a 30% increase in dissolved Cl by a dissolution process alone, would require fluid/altered rock ratios of between 0.1 and 0.4. It is not yet clear from the little that is known of mtrogmde alteration processes at mid-ocean ridges, whether thrid/altered rock ratios such as these are reasonable, or even possible. As shown by our isobaric cooling experiment

(Fig. 3), however, at fluid/altered rock ratios less than 0.5, the effects of hydration and Cl-phase dissolution can result in considerable enrichment of dissolved Cl. Acknowledgements-We wish to thank Drs. R. McDuK, M. Mottl, and an anonymous reviewer for their thoughtful critiques of an earlier version of the manuscript. We are also gmtetbl to Dr. Pat Shanks for his thorough reviewand detailed editorial comments on more than one previous version of the paper. Part of this paper was written while the senior author was on sabbatical leave at the United States Geological Survey, Reston, Va. Funds were provided by NSF grants GCE8400676 and DCE-8542276.

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