Thomas J. Williams á Philip A. CandelaPhilip M. Piccoli

Hydrogen-alkali exchange between silicate meltsand two-phase aqueous mixtures: an experimental investigation

Received: 22 August 1996 /Accepted: 5 February 1997

Abstract Experiments were performed in the three-phase system high-silica rhyolite melt + low-salinityaqueous vapor + hydrosaline brine, to investigate theexchange equilibria for hydrogen, potassium, and sodi-um in magmatic-hydrothermal systems at 800 °C and100 MPa, and 850 °C and 50 MPa. The K

aqm=meltH;Na and

Kaqm=meltH;K for hydrogen-sodium exchange between a va-

por + brine mixture and a silicate melt are inverselyproportional to the total chloride concentration (RCl) inthe vapor + brine mixture indicating that HCl/NaCland HCl/KCl are higher in the low-salinity aqueousvapor relative to high-salinity brine. The equilibriumconstants for vapor/melt and brine/melt exchange wereextracted from regressions of K

aqm=meltH;Na and K

aqm=meltH;K

versus the proportion of aqueous vapor relative to brinein the aqueous mixture (Faqv) at P and T, expressed as afunction of RCl. No signi®cant pressure e�ect on theempirically determined exchange constants was observedfor the range of pressures investigated. Model equilib-rium constants are: K

aqv=meltH;Na �vapor/melt� � 26��1:3� at

100 MPa (800 °C), and 19( � 7.0) at 50 MPa (850 °C);K

aqv=meltH;K � 14��1:1� at 100 MPa (800 °C), and 24(�12)

at 50 MPa (850 °C); K aqb=meltH;b �brine/melt� � 1:6��0:7�

at 100 MPa (800 °C), and 3.9(�2.3) at 50 MPa(850 °C); and K

aqb=meltH;K � 2:7��1:2� at 100 MPa

(800 °C) and 3.8(�2.3) at 50 MPa (850 °C). Values forK

aqv=meltH;K and K

aqb=meltH;K were used to calculate KCl/HCl

in the aqueous vapor and brine as a function of meltaluminum saturation index (ASI: molar Al2O3/(K2O+Na2O+CaO) and pressure. The model log KCl/

HCl values show that a change in melt ASI from pera-luminous (ASI = 1.04) to moderately metaluminous(ASI = 1.01) shifts the cooling pathway (in tempera-ture-log KCl/HCl space) of the aqueous vapor towardthe andalusite+muscovite+K-feldspar reaction point.

Introduction

The concentration of chloride in magmatic-hydrother-mal systems controls the partitioning of many cations,including H+ as HCl, between silicate melts and aqueousliquids and gases (Candela and Holland 1984; Whitney etal. 1985; Urabe 1985; Shinohara et al. 1989; Hedenquistand Lowenstern 1994; Candela and Piccoli 1995; Web-ster et al. 1989; Reed. 1995). Hydrogen chloride is amajor chloride-bearing species in volcanic gases (Sy-monds et al. 1988; Symonds et al. 1994) and aqueous ore-forming solutions (Hemley et al. 1992); however, thebehavior of HCl under high temperature magmatic-hy-drothermal conditions is not well understood.

Hydrogen chloride produced during magmatic aque-ous phase exsolution plays an important role in mag-matic-hydrothermal systems and their related geothermalsystems. The H+ competes with K, Na, Ca, Fe, and othercations including Cu and possibly Au, for the availablechloride in the aqueous phase. As the HCl concentrationincreases in the magmatic vapor or brine, the concentra-tion of Cu or other Cl-complexed ore metals will decreasefor a given initial Cl/water ratio in the melt (Candela andPiccoli 1995). As the magmatic volatile phases cool andmix with other natural waters, magmatically derived HCldrives alteration reactions, and can exert signi®cantcontrol on the precipitation of ore minerals. In geother-mal systems, the corrosive activity of HCl can degradetechnogenic and natural materials, and thereby increasethe total dissolved cations of a geothermal ¯uid.

A number of pioneering studies have laid the groundwork for our research. In a study of aqueous phase ±felsic melt equilibria, Holland (1972) presented evidencethat aqueous phases equilibrated with silicate melts were

Contrib Mineral Petrol (1997) 128: 114±126 Ó Springer-Verlag 1997

T.J. Williams (&)1 á P.A. Candela á P.M. PiccoliLaboratory for Mineral Deposits Research,Department of Geology,University of Maryland at College Park,College Park, MD 20742-4211, USA

1Present address:Department of Geological SciencesUniversity of Texas at El PasoEl Paso, TX 79968-0555, USA

Editorial responsibility: T.L. Grove

acidic at room temperature and pressure. Urabe (1985)demonstrated that the concentration of H+ in thequenched aqueous phase (assumed equal to the HClo

concentration at magmatic temperatures and pressures)was a function of the melt aluminosity. Shinohara (1987)demonstrated experimentally that pressure and meltcomposition control the HCl/total chloride (RCl) in theaqueous phase.

According to Candela (1990), the theoretical foun-dation for control of the aqueous phase HCl concen-tration in the magmatic volatile phase (MVP) involvesthe reciprocal exchange of chloride-complexed cations(i.e., H+, Na+ and K+) in the haplogranite ± aqueousphase system. In this paper, we report results of anexperimental study conducted with the phase assem-blage silicate melt + low-salinity vapor + hydrosalinebrine at 800 °C and 100 MPa, and 850 °C and 50 MPain the system NaAlSi3O8-SiO2-KNa)1-CaNa)2-HNa)1-H2O-Cl2O)1. The results of this investigation are pre-sented as hydrogen-sodium and hydrogen-potassiumexchange constants between a high-silica rhyolite and achloride-bearing aqueous mixture. The experiments wereperformed at pressures and temperatures within thevapor + brine immiscibility ®eld in the model systemNaCl + H2O (Sourirajan and Kennedy 1962). Whenquenched, the low-salinity aqueous vapor and thehydrosaline brine remix to form a single aqueous phase.The single phase quenched aqueous mixture and theglass were analysed.

Hydrogen-alkali exchange equilibria between silicatemelt, the aqueous vapor, and hydrosaline brine werederived form the analytical data by a series of regressionanalyses. Vapor-melt exchange constants were deter-mined by linear regression of the logarithm of theaqueous mixture/melt exchange constants for individualexperiments expressed as a function of the calculatedmass fraction of aqueous vapor/(vapor + brine) [F aqv].The mass fraction of aqueous vapor was calculated fromthe total chloride in the aqueous mixture (measured di-rectly), and the model chloride concentrations in thecoexisting vapor and brine at run T and P. The HCl,NaCl, and KCl concentrations in the brine were deter-mined by regression analyses of concentration in theaqueous mixture as a function of F aqv. Estimates of thebrine/melt exchange constants at a given T and P werethen calculated from the extrapolated brine compositionand the average melt composition of the experiment.The vapor/melt and brine/melt exchange constants in-dicate how melt composition a�ects the composition ofthe magmatic volatile phases exsolved from magmatic-hydrothermal systems.

Theoretical considerations

HCl in the silicate melt and magmatic volatile phase

Candela (1990) modeled the systematics that control the HClconcentration in the magmatic volatile phase ± haplogranite H2O-

SiO2-NaAlSi3O8-KNa)1-HNa)1-Cl2O)1 system, where KNa)1,HNa)1 and Cl2O)1 represent exchange components. In the dis-cussion of our results, hydrogen ± sodium HNa)1, and hydrogen ±potassium exchange equilibria are treated identically; therefore tosimplify the discussion, only hydrogen ± sodium equilibria are de-tailed here. The condition governing the equilibrium concentrationof HCl in the magmatic aqueous phase can be represented by thechemical potential expression:

laqHNaÿ1 � lmeltHNaÿ1 : �1�By using an equivalent, but di�erent set of components, thisequilibrium can be represented by the balanced (empirical) ex-pression:

laqNaCl � 12

lH2O � lAl2O3� 6lSiO2

ÿ �melt� laqHCl � lmeltNaAlSi3O8�2�

The Al2O3 in this expression represents the Al2O3 activity inexcess of that needed to charge balance the Na2O, K2O, and CaOcomponents in the melt. That is, in the simple hydrous subalumi-nous haplogranite system, only four components are necessary:SiO2-NaAlSi3O8-KNa)1-H2O. If however, we wish to considermetaluminous compositions, then we must account for the devia-tion of the anhydrous constituents of the melt phase from thequartz-alkali feldspar plane. This is accomplished by adding thecomponent Al2O3 (note that Al2O3 is not a component of thesubaluminous haplogranite system, since Al2O3 is not indepen-dently variable in feldspar-stoichiometry phases). Linear algebraallows us to represent this additional component as Al2O3, or asHNa)1 (as long as H2O-SiO2-NaAlSi3O8 are among the othercomponents in the system), because:

Al2O3 � 2NaAlSi3O8 � 2HNaÿ1 ÿH2Oÿ 6SiO2: �2a�If we express this equation without negative stoichiometric coe�-cients, we can, as another alternative, express this component asH(fasc)melt � 1

2�H2O� Al2O3 � 6SiO2�melt � Component H(fasc)meltrepresents the ®ctive hydrous aluminosilicate component originallypresented by Candela (1990) as an algebraic thermodynamic vari-able. Note that this variable is a stoichiometric convenience; ther-modynamic components need not have structural signi®cance. ThatdlH2O � dlSiO2

� 0, is su�cient for the de®nition of H(fasc)melt, as:

H(fasc)melt � CmeltAl ÿ Cmelt

Na � CmeltK � 2Cmelt

Ca

� �: �3�

The exchange constant expression for equilibrium (2) is,

Kaqm;meltH;Na � Caqm

HCl � CmeltNa

CmeltH�fasc� � Caqm

NaCl

�4�

where the concentrations of HCl and NaCl are expressed as moles/kilogram of solution. The Cmelt

Na and CmeltH�fasc� represent the concen-

tration of sodium and the hydrous aluminosilicate component,respectively (these two components must be expressed in the samemole-based, units). Note that the hydrogen-sodium exchangeconstant in Eq. (4) is written in terms of the concentration of HCland NaCl in the aqueous mixture; an exchange constant of thistype is formulated for each experimental run. The same formalismis used to treat hydrogen-potassium exchange.

Also, note that if a system is not vapor saturated, or if the vaporcontains signi®cant dilutants (e.g., CO2), then the chemical poten-tial of water is lowered relative to the experimental system, and theequilibrium constant will be lower by a factor of the square root ofthe activity of water. Similarly, if the activity of silica in the melt ismuch below unity, then the equilibrium constant would be multi-plied by the cube of the activity of silica in the melt (see Eq. 2).

Immiscibility in the magmatic aqueous phase

The compositions of the starting aqueous solutions lie within theimmiscible two-phase ®eld at run temperature and pressure where alow-salinity aqueous vapor coexists with a hydrosaline liquid orbrine (see Fig. 1). Thus, at theP andT of interest here, the equilibria

115

occur among three phases: aqueous vapor + brine + silicate melt;¯uid inclusions found within the glass phase run products dem-onstrate that this is so. The vapor and brine assemblage in P-T-composition space is isobarically and isothermally invariant for agiven ratio of cations. In a melt + vapor + brine system, changesin the bulk chloride concentration are re¯ected by shifts in the massproportion of vapor to brine. The e�ect of KCl on aqueous solu-tion immiscibility was investigated by Cygan et al. (1993), Anderkoand Pitzer (1993), and others. Generally, the addition of KCl tendsto enlarge the vapor-brine immiscibility ®eld with the general e�ectof decreasing the RCl in the aqueous vapor and increasing RCl inthe brine. The e�ect of HCl on aqueous phase immiscibility atmagmatic temperatures has not been systematically studied; how-ever, Hemley et al. (1992) suggested that the e�ect on vapor-brinephase equilibria would be relatively small, and Shinohara andFujimoto (1994) show that the addition of HCl at a few hundredbars and 600 °C induces a (anticlockwise) rotation of the vaporlimb of the miscibility gap. Therefore, we use minimum values ofthe chloride concentration in the brine-saturated vapor fromBodnar et al. (1985) to model our system.

The coexisting vapor and brine are not stable at room tempera-ture and pressure, and the design of the experiments does not permitexchange reactions (e.g., Eq. (2)) between the aqueous vapor andsilicate melt and between the brine and the silicate melt to be moni-tored directly. When an experimental run was quenched, the vaporand the brine remixed to form the aqueous mixture (aqm). Thequenched aqueous mixture was then analyzed. Therefore, the

K aqm=melti;j given in Eq. (4) is an empirical chemical entity and is not a

true thermodynamic equilibrium constant. However, becausechanges in RCl shift the vapor to brine ratio, the behavior of

K aqm=melti;j as a function of vapor/vapor + brine provides insight into

the exchange equilibria for a given element pair, i and j, in the three-phase system silicate melt + aqueous vapor + hydrosaline brine.

Experimental and analytical methods

Experimental methods

The starting material (Table 2) used in the experiments was a high-silica rhyolite pumice (Bishop Tu�: silica contents ranging from 75±77 wt% Hildreth 1979) collected from Long Valley, California. The

starting material was ground in an agate mortar and pestle, ex-amined under a binocular microscope, and hand-picked for visibleimpurities and large opaque grains. Reasons for selecting a natu-rally occurring high-silica rhyolite as a starting material are detailedin Williams et al. (1995). Starting solutions were prepared fromreagent grade NaCl, KCl, and HCl dissolved in distilled and de-ionized water. Before being added to the starting solutions, theconcentrated reagent-grade HCl was titrated to determine itscomposition more precisely. The HCl/RCl of the solutions rangedfrom 0.3 to 0.03, and the starting NaCl/KCl of the solutions washeld constant at unity. Aqueous solution to glass ratios were heldconstant at 2 : 1.

Experiments were performed in externally heated, water-cooledrapid-quench cold-seal vessels (Candela and Holland 1984). Vesselswere pressurized with water and argon. At the end of an experi-mental run, the pressure vessel was removed from the furnace andtilted vertically. The experimental charge then slid to the cold endof the vessel, quenching to room temperature in a matter of

Fig. 1 Isothermal section of the system NaCl-H2O at 800 °C(modeled after Bodnar et al. 1985). The polythermal projection ofthe 3-phase vapor + brine + halite equilibrium onto the pressure-composition section is from Souririjan and Kennedy (1962). The opencircles represent the approximate wt% NaCl equivalent concentrationof the coexisting aqueous vapor and hydrosaline brine at 100 MPaand 800 °C

Table 1 List of abbreviations and symbols

melt silicate meltaqm quenched aqueous mixtureaqv low-salinity aqueous vaporaqb hydrosaline brineaq generalized aqueous phaseCj

i concentration of element i in phase j in moles/kgH (fasc) hydrous aluminosilicate component of the silicate melt

Kaqm=meltH ; X exchange constant for hydrogen-alkali exchange

between the silicate melt quenched aqueous mixture.X � Na or K

Kaqv=meltH ; X model exchange constant for hydrogen-alkali exchange

between the silicate melt and the low-salinity aqueousvapor. X � Na or K

Kaqb=meltH ; X model exchange constant for hydrogen-alkali exchange

between the silicate melt and the hydrosaline brine.X � Na or K

F aqv mass fraction ratio of aqueous vapor to vapor +hydrosaline brine

RCl total chlorine (in moles/kg of solution)I aqueous solution ionic strengthZ ionic chargeA Debye-Huckel solvent parameter at 25 °C and 1 bar

(=0.5092)B Debye-Huckel solvent parameter at 25 °C and 1 bar

(=0.3283)_a e�ective ionic diameter (=6)

Table 2 Starting glass compositions (Bishop Tu� rhyolite collectedfrom Long Valley California) recalculated to an anhydrous basis(wt% of the oxides)

Microprobe XRFa ICPa

SiO2 76.4 (�1.5)b 76.4 76.6TiO2 0.04 (�0.2) 0.07 0.08Al2O3 14.0 (�0.4) 13.3 13.1FeO 0.52 (�0.03) 0.88 0.88MgO 0.10 (�0.01) 0.12 0.10CaO 0.46 (�0.1) 0.46 0.49MnO 0.00 (�0.02) 0.03 0.03Na2O 2.83 (�0.13) 2.95 2.93K2O 5.67 (�0.3) 5.66 5.75P2O5 NDc 0.02 0.02

Total 100.2 99.98 99.98

aAnalyses by Activation Laboratories Ltd., Ancaster, Ontario,CanadabUncertainty is �1sigmacNot determined

116

seconds. The pressure vessels were run sub-horizontally with thehot end tilted approximately 10° above the horizontal; Charles andVidale (1982) found that a tilted orientation minimized temperaturegradients in water-cooled rapid-quench vessels. Temperatures weremonitored with external Chromel-Alumel thermocouples, andpressure was monitored with bourdon tube gauges calibratedagainst a factory-calibrated Heise gauge.

Aliquots of glass starting material (75 mg for the 100 MPaexperiments; 55 mg for the 50 MPa experiments) were dried,weighed and loaded in platinum capsules. The aqueous startingmaterials (150 lL for the 100 MPa experiments; 110 lL for the 50MPa experiments) were added by micropipette. Capsules weresealed with a carbon-arc welder, and were weighed before and afterwelding to check for loss of aqueous solution; capsules were thenbaked at 110 °C for 24 hours to check for leaks. Immediately uponquenching the experimental run, the capsule was removed from thepressure vessel, cleaned and inspected for signs of failure or dam-age. The capsule was then weighed to check for leakage and piercedwith a stainless steel hypodermic needle; the aqueous solution wasdrawn into a plastic syringe. Great care was taken to minimize theelapsed time between quench and removal of the aqueous runproduct from the capsule. In our opinion, the combination of ourrapid quench, and our rapid processing of the aqueous run productfor analysis, limited back reaction of the quenched aqueous solu-tion with the run product glass. The aqueous solution was trans-ferred to a 10 milliliter volumetric ¯ask, diluted to volume withdistilled and deionized water, and the pH of the solution wasmeasured. After removal of the aqueous solution, the capsule wassplit open, then the quenched glass was removed, rinsed thoroughlywith distilled water, dried, and mounted in casting resin and pol-ished for electron probe microanalysis.

Analytical methods

Electron probe microanalyses of the silicate glass

Analyses of the silicate glass run products were conducted on aJEOL JXA 840A electron probe microanalyzer with a TracerNorthern 5500 Series II analyser. Quantitative wavelength dispersespectrometry was performed by using the following crystals: TAP(Si, Al, Mg, Na), LiF (Fe), and PET (Ca, K, Cl). Glasses wereanalyzed at an accelerating voltage of 15 kV and a 2.5 nA cupcurrent, with a 10 ´ 10 micron rastered beam. The X-rays gener-ated from Si, Al, K, Na, Fe, and Mg were counted for 30 secondsduring analysis of Yellowstone rhyolite (Si, Al, Fe, Ca, K), Ka-kanui hornblende (Mg), and jadeite standard (Na). A 60 secondcount-time was employed for chlorine analyses. Durango apatiteand scapolite were used as chlorine standards. Electron probe an-alytical uncertainties (1-sigma) for major and minor elements arelisted in Table 2. Raw intensities were corrected using the algo-rithm of Bence and Albee.

Sodium migration is a common problem in electron probe an-alyses of hydrated silicate glasses (Nielsen and Sigurdsson 1981).This problem was minimized by analyzing sodium ®rst, by using alow sample current, a wide rastered beam, and a short count-time(30 s). The low power beam and short count-time increased overallanalytical uncertainty but allowed for the collection of more ac-curate sodium data. A time-pro®le for sodium migration was per-formed by repeated analyses of the same point. Under theanalytical conditions of this study, sodium loss was within theanalytical uncertainty for an analysis of 30 seconds duration.

Determination of concentrations in the aqueous mixtures

All concentrations (K, Na, Fe and H+) for the quenched aqueousmixtures are given as moles/kilogram of solution. Analyses of thequenched aqueous phase for Na, K, and Fe were performed on aPerkin-Elmer 2380 atomic absorption spectrophotometer with an

air-acetylene ¯ame. Concentrations were determined by the methodof standard addition. Internal standard solutions were run as un-knowns to check analytical precision. Analytical precision foratomic absorption spectrophotometry (AAS) standards for the el-ements Na, K, and Fe in the aqueous mixtures was approximate-ly �0.5 for ca. 5 ppm (1r) aqueous solution.

The chloride concentration in the aqueous phase was deter-mined by using a titration chloridometer (Labconco Model 442-5100). Standardization was performed by titrating a 0.10 mole/kgNaCl standard solution. Analytical precision was approximate-ly �4% (1r). The concentration of hydrogen ions (H+) was de-termined by direct potentiometric measurement of the quench pHof the diluted aqueous mixtures. The H+ ion concentration atroom temperature and pressure yields the HCl concentration in theaqueous mixture at magmatic temperatures.

The determination of pH was made by using a combinationneedle-point glass electrode (Microelectronics MI-410) and a tem-perature compensated digital pH meter (Beckman pHI 40). Theelectrode was calibrated with two commercial pH bu�er solutions(pH = 4, potassium biphthalate bu�er, and a pH = 7, potassiumphosphate monobasic-sodium hydroxide bu�er) by using a two-bu�er linear calibration. Analytical uncertainty was �4% (1r).

The pH of the aqueous mixture is a measure of the hydrogenion activity and is related to the concentration by the activity co-e�cient, ci. Activity coe�cients were calculated by using the De-bye-Huckel equation written as:

logkH � ÿAkZ2I1=2

1� aBkI1=2: �5�

The activity coe�cient-corrected concentrations are listed as theCaqmH concentrations in Table 3.

Discussion of previous studies

Previous studies on HCl in silicate melt + two-phaseaqueous mixtures are few in number. In his experimentalstudy on aqueous phase acidity and melt aluminosity at750 °C and 160 to 350 MPa Urabe (1985) found that thequench pH was lower when the aqueous phase was inequilibrium with peraluminous melts as opposed to lessaluminous compositions. The HCl in one- and two-phase aqueous mixtures equilibrated with felsic meltswas investigated by Shinohara (1987) in a series of ex-periments at 810 °C and 0.6 kilobars (60 MPa) to 6.7kilobars (670 MPa) in the system silicate melt + Cl-bearing aqueous phase. His investigation spanned theregion from supercritical (single-phase) to subcritical(two-phase) behavior for the system NaCl + H2O. Inhis subcritical experiments, Shinohara analyzed thequenched aqueous mixture and did not directly measurethe coexisting vapor and brine. He presented his resultsas the ratio of aqueous HCl to the total aqueous chlo-ride Raq

HCl=RCl � fHCl=�NaCl�KCl� HCl�g.Shinohara used three synthetic starting glasses: one

was metaluminous, another peralkaline, and the thirdwas peraluminous. The bulk of his reported experiments(Shinohara 1987) were conducted with the metalumi-nous and peralkaline starting glasses. The peralkalineglass compositions would have bu�ered his equilibriumHCl concentrations at relatively low values. The factthat his highest HCl concentrations were associated withthe peraluminous starting compositions supports thishypothesis. The HCl concentrations presented in this

117

report are uniformly higher than those reported byShinohara (1987).

Shinohara (1987) noted in his experiments that theRaqHCl=RCl of the aqueous phase had a positive dependence

on the melt ASI at pressures from 60 MPa to 670 MPa.Shinohara suggested that variations in the Na/Al in thesilicate melt controlled the compositional dependence ofthe Raq

HCl=RCl. The empirical observations of Shinoharaagree with the discussion by Candela (1990) of the the-oretical relationship between melt aluminosity and theHCl/NaCl ratio in the coexisting aqueous phase. In thatdiscussion, Candela showed that HCl/NaCl increaseswith increasing melt aluminosity. In addition, Shinoharasuggested that the di�erence in molar volume of HClrelative to NaCl, with changes in pressure, was a likelyexplanation for the decrease in HCl/RCl with increasingpressure.

In this report we present the data as exchange con-stants for the coexisting silicate melt + vapor + brine.To formulate the exchange constants we collected datafrom both the quenched aqueous mixture and the silicatemelt. The previous studies by Urabe and Shinoharacollected limited data on the chemistry of the silicatemelt and did not formulate their results as thermody-namic exchange constants. However, Shinohara didformulate a balanced reaction based on a quasi-crys-talline model from which he derived an apparent equi-librium constant expression that related his Raq

HCl=RCl tothe ratio of Na to Al in the silicate melt.

Experimental results

Hydrogen-alkali exchange betweenthe aqueous mixture and the silicate melt

Exchange constants, shown in Table 3, exhibit an in-verse dependence on RCl down to chloride concentra-tions of approximately 0.2 moles/kilogram. The increasein the exchange constants with decreasing RCl indicatesthat, in the polyphase assemblage silicate melt + low-salinity vapor + brine, HCl, KCl, and NaCl do notpartition in the same ratio between the vapor and brine;the HCl component prefers the low-salinity vapor overthe brine relative to KCl or NaCl. Absolute concentra-tions of NaCl, KCl, and HCl are elevated in the brinerelative to the vapor, due to the high chloride concen-tration in the brine; however, the ratios HCl/NaCland HCl/KCl are elevated in the vapor relative to thebrine. These experimental results are consistent withresults from other studies (at lower temperatures), suchas Cygan et al. (1993) and Shinohara and Fujimoto(1994).

At RCl values below approximately 0.2 moles/kilo-gram no brine was present, and element exchange in-volved only the melt and aqueous vapor (see Fig. 1). Inthese experiments, K aq=melt

H;Na and K aq=meltH;K exhibit wide

scatter and extend to anomalously low values. This be-havior is most evident in the K aq=melt

H;K values at 100 MPa

and is less apparent at 50 MPa total pressure. Experi-ment durations for the low RCl were not systematicallyshorter than experiments with higher RCl, and experi-mental procedures were not di�erent for the lowest RClexperiments. Shinohara (1987) noted that Na-K ionexchange occurred in his experiments as they cooledfrom magmatic temperatures through the range of sub-solidus hydrothermal temperatures. Those experimentswere quenched from 810 °C to temperatures less than200 °C over several minutes, which Shinohara proposedwas su�cient time for ion exchange to occur. In thepresent study, the rapid-quench technique cooled theexperimental charge in a matter of seconds makingsubsolidus ion exchange during quench kinetically un-likely. However, at low chloride concentrations, anyback reaction occurring with the rhyolite glass thatwould consume hydrogen ions (i.e., low temperaturehydrolysis reactions) before removal of the glass fromthe capsule would a�ect the aqueous phases with thelowest chloride and HCl concentrations but might benegligible at higher RCl. For this reason, experimentswith the lowest RCl, which were originally performed toestablish an exchange constant for the single-phase low-salinity vapor, were not considered acceptable. Toobtain exchange constants for the aqueous vapor, aregression analysis was performed on the two-phaseexchange constants (see values for RCl > 0:2 moles/kg inTable 3) versus F aqv which was calculated from themeasured RCl and the model chloride concentration inFormality (moles Cl/kilogram solution) of the aqueousvapor + brine from Bodnar et al. (1985). In va-por + brine space, F aqv � 1 at 100% vapor. By ex-amining Kaqm=melt

H;Na and the HCl, NaCl, and KClconcentrations in the aqueous mixture as a function ofF aqv, exchange constants for aqueous vapor/melt andhydrosaline brine/melt were determined. The derivationof F aqv is shown in the Appendix.

E�ect of pressure on K aqm=meltH;Na and K aqm=melt

H;K

In this investigation, K aqm=meltH;Na and K aqm=melt

H;K show little

variation with pressure. Shinohara (1987) reported a

strong pressure dependence in his RaqHCl=RCl between 60

MPa and 220 MPa with a decreasing pressure depen-dence from 220 to 670 MPa. The HCl/RCl values (forthe quenched aqueous mixture) from his investigationdemonstrate a negative dependence on pressure. Shin-ohara interpreted this pressure dependence to be afunction of the di�erence in the partial molar volumes ofHCl and NaCl, and HCl and KCl. The partial molarvolume di�erence was based on his quasi-crystallinemodel for melt + aqueous phase exchange equilibria.With decreasing pressure from 220 to 60 MPa, at810 °C, the chloride-bearing magmatic aqueous phasewill pass from a supercritical gas into a vapor + brinemixture. We suggest that the HCl/RCl (approximatelyequivalent to Shinohara's Raq

HCl=RCl) may be a function of

118

changes in the partition coe�cient for chloride, Daq=meltCl ,

The Daq=meltCl increases with increasing pressure (Webster

1992; Shinohara et al. 1989). Pressure variations in HCl/RCl could be a function of increasing total chloridepartitioned from the melt into the aqueous phase withincreasing pressure. However, as we have shown here,when the equilibria are expressed in the form of ex-change constants, the pressure dependence is reducedsigni®cantly. Generally, the proper formulation of par-titioning equilibria will minimize the dependence of theempirical constant on intensive parameters.

Uncertainty in the K aqm=melti;j

The uncertainty associated with K aqm=melti; j was calculated

by propagation of the uncertainties from the electron

probe analysis of the quenched silicate glass, the AASanalyses of the aqueous mixture (for Na and K), and thepH measurement of the aqueous phase. Uncertainties inthe probe analyses of the silicate glass were calculated bydetermining the 1-sigma standard deviation of the ana-lytical points on a given sample. The 1-sigma uncer-tainties in the AAS and pH analyses were determined byreplicate analyses of a standard solution (AAS), and acommercial bu�er (pH). The 1-sigma values were de-termined for concentrations in moles/kg of the elementsfor both aqueous solutions and glass.

The uncertainties in H(fasc) represent the propagateduncertainty in the electron probe analyses for Al, Na, K,and Ca. Small absolute uncertainties in Al, Na, K, or Cacan propagate through to produce higher relative un-certainty in H(fasc). The H(fasc) was the largest con-

tributor of uncertainty to the values of K aqm=melti;j .

Table 3 Elemental concentrations in the quenched aqueous mixture (aqm) and the quenched silicate glass [equivalent to the melt (mlt)],and values of exchange constants for hydrogen-sodium �Kaqm=melt

H;Na �, and hydrogen-potassium exchange �Kaqm=meltH;K � between the quenched

aqueous mixture (aqm) and the silicate melt (melt)

100 MPa and 800 °C

Sample no. 4 10 11 12 13 14 16 17 18 21 33 42 44

Caqma

H 0.31 0.75 0.56 0.1 0.05 0.39 0.68 0.48 0.32 0.01 0.33 0.97 0.06

H(fasc)b 0.31 0.5 0.45 0.33 0.28 0.19 0.38 0.16 0.16 0.4 0.29 0.35 0.4

CaqmNa 0.05 0.78 0.38 0.01 0.01 0.1 0.6 0.3 0.12 0.07 1.9 1.5 0.07

CmeltNa 1 0.82 0.88 0.94 0.91 0.97 0.71 0.88 0.93 0.91 0.81 0.78 0.95

CaqmK 0.12 1.2 0.82 0.06 0.05 0.28 1.20 0.46 0.21 0.03 0.91 1.1 0.05

CmeltK 1.04 1.19 1.16 1.12 1.15 1.11 1.35 1.49 1.33 1.14 1.32 1.37 1.11

RClc 0.7 3.4 2.4 0.2 0.1 0.8 3.8 1.9 0.6 0.1 3.2 3.1 0.2ASId 1.14 1.25 1.21 1.1 1.11 1.08 1.18 1.06 1.07 1.18 1.14 1.16 1.1

Kaqm=melta

H;Na 20.1 1.5 2.9 25.7 9.9 20.8 2.1 8.8 15.1 0.42 0.5 1.4 2.2

Kaqm=meltH;K 8.5 1.5 1.8 5.8 4.1 8.2 2 9.8 12.3 1.2 1.6 3.4 3.7

HCl/RCl 0.44 0.22 0.23 0.3 0.3 0.48 0.18 0.25 0.53 0.15 0.1 0.31 0.03

F aqvCl 0.96 0.7 0.8 n.c.e n.c 0.95 0.66 0.84 0.97 n.c. 0.71 0.72 n.c.

100 MPa and 800 °C 50 MPa and 850 °C

Sample no. 45 46 48 25 26 27 28 38 39 40 41

CaqmH 0.51 0.27 1.35 0.58 0.65 0.37 0.06 0.22 0.42 0.34 0.08

H(fasc) 0.29 0.09 0.22 0.27 0.16 0.14 0.44 0.17 0.05 0.28 0.35

CaqmNa 1.1 0.21 1.7 0.51 0.32 0.1 0.03 1.1 0.94 0.16 0.01

CmeltNa 0.96 1.14 0.88 0.89 0.96 1.08 0.93 0.83 0.97 0.96 1.03

CaqmK 0.85 0.22 1.4 0.74 0.39 0.15 0.01 0.88 0.38 0.19 0.01

CmeltK 1.1 1.1 1.56 1.23 1.32 1.22 1.08 1.5 1.56 1.24 1.15

RCl 3 0.9 4.4 3 1.6 0.9 0.1 3 1.5 0.5 0.1ASI 1.14 1.04 1.08 1.13 1.07 1.06 1.17 1.07 1.02 1.12 1.08

Kaqm=meltH;K 1.5 15.4 3.1 3.7 12 27.9 4.3 0.98 8.5 7.2 22.6

Kaqm=meltH;Na 2.2 14.1 6.8 3.6 13.5 21 15 2.2 33.8 7.8 25.3

HCl/RCl 0.17 0.3 0.3 0.19 0.4 0.41 0.3 0.07 0.28 0.68 0.03

F aqvCl 0.74 0.95 0.6 0.77 0.88 0.93 n.c. 0.77 0.89 0.97 n.c.

a Concentrations are moles/kg of quenched aqueous mixture or glass (=melt). Concentrations in the aqm determined by ¯ame atomicabsorption spectrophotometry, and glass pH electrode �Caqma

H �. Concentrations in the silicate glass determined by electron probe mi-croanalyzer (JEOL 840A)bFictive hydrous aluminosilicate component of the silicate melt. H (fasc) � Almelt ÿ �Namelt �Kmelt � 2Camelt�c Total chlorine in the quenched aqueous mixture in moles/kg of solution measured by autotitration chloridometerdAluminum saturation index of the quenched silicate glass. Calculated from the H(fasc) by using the following arithmetic relationship:ASI � �H (fasc)=R�Na�K� 2Ca�� � 1e not calculated

119

Uncertainties in K aqm=melti; j are reported as �1-sigma

uncertainty for the log values of K aqm=melti;j plotted on a

linear scale. Uncertainties in K aqm=melti; j are related to the

uncertainties in log K aqm=melti; j by:

��log K� � ��K�2:3 � K �6�

where ��log K� is the absolute uncertainty in

log K aqm=melti; j , and ��K�=K represents the relative un-

certainty in K aqm=melti; j .

Modeling of aqueous vapor/meltand hydrosaline brine/melt exchange constantsat 100 MPa, 800 °C and 50 MPa, 850 °C

Figures 2 and 3 display exchange constants for hydro-

gen-sodium (K aqm=meltH;Na ) and hydrogen-potassium

(K aqm=meltH;K ) exchange as log values versus F aqv at 100

MPa and 800 °C, and 50 MPa and 850 °C, respectively.The error bars represent 1-sigma uncertainty and thecon®dence envelope is at 95% con®dence.

Exchange constants K aqv=meltH;Na and K avm=melt

H; K are mod-eled by using a linear regression analysis of the logvalues of the empirical aqueous mixture/melt exchangeconstants as a function of F aqv. Aqueous vapor/meltexchange constants are modeled at F aqv � 1. Figure 2displays data collected at 100 MPa and 800 °C. Plots ofthe logarithm of the aqueous mixture/melt exchangeconstants versus F aqv are not necessarily linear. There-fore, the limiting slope should be used to de®ne valuesfor K aqv=melt

H;Na and K aqv=meltH;K ; however, in order to obtain

the best model for the exchange constant, the regressionanalysis should include the maximum number of datapoints that can be inferred to be linear. Fortunately, thecurvature of the plots is very low. We therefore per-formed a series of regressions including a progressivelygreater proportion of the data points at lower F aqv. The

data for K aqm=meltH;Na and K aqm=melt

H;K at F aqv � 0:6 areanomalously high relative to the other data and wereexcluded from the ®nal regression analysis. We foundthat the remaining points at F aqv � 0:66 and greaterproduced a regressed vapor/melt exchange constant with

a r2 value of approximately 0.8 for both K aqm=meltH;Na and

K aqm=meltH;K . We interpreted the anomalous point at

F aqv � 0:6 as representing the e�ects of the increasingmass fraction of brine. The model vapor/melt exchangeconstants are listed in Table 4.

Figure 3 displays the K aqv=meltH;Na and K aqv=melt

H;K as afunction of F aqv at 50 MPa and 850 °C. The smaller sizeof the data set at 50 MPa results in a less robust re-gression analysis; therefore, the model values for vapor/melt exchange constants at 50 MPa should be consideredin that light. Of the six data points in Table 3 at 50 MPa,®ve out of the six were used in the regression analyses(Fig. 3). The lower of the two data points at F aqv � 0:77was not used in the regression. This experiment producedHCl values in the quenched aqueous mixture that were

anomalously low relative to the other experiment atnearby values of F aqv (see Table 3). In the modeling ofthe brine/melt exchange constant, discussed below, thisexperiment was excluded from the regression analysisfor HCl in the brine. To maintain consistency, this da-tum was eliminated from the vapor/melt modeling aswell. However, these data are included in the tables, andwere not rejected from the data set; they have simplybeen left out of our regressions. The reader can easilyinclude these data in regressions if desired. The results ofthe regressions are listed in Table 4.

To calculate the K aqb=melti;j , the brine-melt exchange

constant, a model brine concentration for HCl, KCl,and NaCl must ®rst be determined. To model the con-centration of the brine components of interest, the F aqv

was again employed. Within the two-phase ®eld there isa linear relationship between the F aqv and the concen-trations of HCl, KCl, and NaCl with the concentrationsincreasing as a inverse function of the F aqv (see Ap-pendix). The empirical HCl, KCl, and NaCl concentra-tions (in the aqueous mixtures quenched from within the

Fig. 2 Log K aqm=mltH;Na (a) and log K aqm=mlt

H;K (b) versus F aqv at 100 MPa,800 °C. Plot displays the regression line used to model the values forlog K aqv=mlt

H;Na and log K aqv=mltH;K (aqueous vapor-melt exchange),

excluding the datum at F aqv � 0:6. Error bars are 1-sigma uncertaintyfor the log values of K. Envelope represents a 95% con®dence interval.Log K aqv=mlt

H;Na and log K aqv=mltH;K are modeled at F aqv � 0 (100% vapor).

Terminology de®ned in Table 1

120

two-phase region) were regressed as a function of F aqv.The HCl, KCl, and NaCl concentrations in the brinewere determined at F aqv � 0. To calculate K aqb=melt

i;j ,moles of K/kg melt, moles Na/kg of melt, and H(fasc),were calculated from silicate glass run-products fromexperiments conducted within the two-phase ®eld. Thismethod is not as precise as the method used to extractdata for K vap=melt

H;NaÿK, however it yields a reasonable esti-mate of the brine/melt exchange constant.

Discussion

Estimation of log KCl/HCl and log NaCl/HClin the aqueous vapor and hydrosaline brine

The HCl concentration in the exsolving magmatic vol-atile phase determines (along with sulfurous gases) the

acidity of aqueous ¯uids at subsolidus temperatures andpressures, which, in turn, a�ects the type and relativeintensity of wall rock alteration and mineral precipita-tion. For simplicity, we will consider mainly thosealteration reactions involving potassium-bearing alum-inosilicate minerals. Potassium feldspar-bearing hostrocks can neutralize the acidity of the aqueous phase byway of hydrolysis reactions such as:

3KAlSi3O8 � 2HCl�aq� � KAl3Si3O10�OH�2� 6SiO2 � 2KCl�aq� �7�

or in the case of a sodium analog:

3NaAlSi3O8 � 2HCl�aq� � NaAl3Si3O10�OH�2� 6SiO2 � 2NaCl�aq� �8�

which can occur upon cooling of the aqueous phase(s).Both reactions (7) and (8) generate mica at the expenseof feldspar and decrease the HCl concentration in theaqueous phase. Mineral stability in the potassium alu-minosilicate system has been the subject of numerouspapers, notably those of Hemley (1959), Montoya andHemley (1975), Haselton et al. (1995), and Sverjenskyet al. (1991). Figure 4 is modi®ed from Haselton et al.(1995).

Log KCl/HCl (where KCl, NaCl and HCl representmoles/kilogram of solution) values for the low-salinityaqueous vapor and hydrosaline brine exsolved from amelt were calculated by using K aqv=melt

H;K and K aqb=meltH;K

from this study, given concentrations of potassium andsodium in the melt, and an aluminum saturation index.The potential for wall rock alteration by the exsolvingmagmatic volatile phase(s) can be examined as a func-tion of melt aluminosity. The calculated log KCl/HCl(see Table 5) of the vapor are lower relative to the brine.As melt aluminosity increases, KCl/HCl of both thevapor and brine coexisting with melt decreases.

Fig. 3 Log K aqm=mltH;Na (a) and log K aqv=mlt

H;K (b) versus F aqv at 50 MPa,850 °C. Plot displays the regression line used to model the values forlog K aqv=mlt

H;Na and log K aqv=mltH;K , excluding the datum at F aqv � 0:77.

This K value was excluded from the regression modeling of the HClbrine concentration because it had anomalously low HCl concentra-tions in the aqueous mixture. For consistency it is excluded in thisregression as well. Error bars are 1-sigma uncertainty for the log valuesof K. Envelope represents a 95% con®dence interval. LogK aqv=mlt

H;Na andlog K aqv=mlt

H;K are modeled at F aqv � 0 (100% vapor)

Table 4 Values for model exchange constants, Kaqv=meltH;Na ; Kaqv=melt

H;K ;Kaqb=meltH;Na and Kaqb=melt

H;K , for hydrogen-sodium and hydrogen-po-tassium exchange between the low-salinity aqueous vapor (aqv)and melt (melt), and hydrosaline brine (aqb) and melt (mlt) re-spectively. Uncertainties are �1r

100 MPa and 800 °C

Kaqv=melta

H;Na Kaqb=meltb

H;Na Kaqv=meltH;K Kaqb=melt

H;K

26(�1.3)3 1.6(�0.7) 14(�1.1) 2.7(�1.2)

50 MPa and 850 °C

Kaqv=meltH;Na Kaqb=melt

H;Na Kaqv=meltH;K Kaqb=melt

H;K

19(�7.0) 3.9(�2.3) 24(�12) 3.8(�2.3)

aKaqv=meltH;NaÿK are the model exchange constants for hydrogen-sodium

and hydrogen-potassium exchange between the silicate melt and thelow-salinity aqueous vaporbKaqb=melt

H;NaÿK are the model exchange constants for hydrogen-sodiumand hydrogen-potassium exchange between the silicate melt and thehydrosaline brinecUncertainty is �1r

121

Geologic implications

During decompression and/or crystalization of a magmaat low ( <150 MPa) pressures, a magma may saturatewith respect to a vapor, a brine, or both depending uponthe ratio of chlorine to water in the magma, the initialwater concentration, and the pressure of crystallization(Candela and Piccoli 1995). When two volatile phases areexsolving from a crystallizing magma, they may risethrough the magma toward the top of the chamber by avariety of mechanisms (Candela 1991; Candela and Ble-vin 1995). The ¯uid phases: vapor, brine and magma (anon-Newtonian poly-phase ¯uid) will tend to stratify

gravitationally, producing a lower density aqueous vaporcap overlying a higher density brine core trapped close to,and above the crystallizing magma. Of course, such anarrangement represents mechanical equilibrium in agravity ®eld, and equilibrium may be precluded by therelative rates of rise of the less dense volatile phases andthe crystallization of the magma. Due to its high densityrelative to both the vapor and the surrounding hydro-logical regime, the brine phase would tend to resist mi-gration away from the immediate vicinity of the magmachamber. Henley and McNabb (1978) have suggestedthat the high-density brine may pool at deeper levelsaround the crystalline magma or may collapse backthrough fractures that form during cooling and crackingwithin the sub-solidus pluton as the magma crystallizes.The low-density aqueous vapor may expand into distalportions of the fractured pluton as well as the surround-ing country rock. Upon cooling, the separated vapor andbrine phases would generally follow di�erent paths,thermally and chemically, resulting in di�erent alterationreactions. Adiabatic decompression of a brine-saturatedvapor will yield vapor that remains brine saturated, andprecipitates brine during rise, further stripping alkalichlorides from the vapor at the expense of HCl.

The equilibrium constants we have formulated(e.g., Eq. (4)) show that KCl/HCl and NaCl/HCl in themagmatic volatile phases increase proportional to theH(fasc), which is, in turn, proportional to the ASI ofthe melt; that is, more aluminous melts yielding morecorrosively aggressive aqueous solutions. The mineralequilibria shown in Fig. 4 can be used to de®ne the ratioof KCl to HCl (as a function of temperature at a given Pand 1 molal total chloride) at which an aqueous phase iscapable of e�ecting alteration of K-feldspar +/) mus-covite assemblages. As many authors have discussed(Blevin 1989; Sverjensky et al. 1991; Montoya andHemley 1975), regions of phyllic, andalusite-bearing,and argillic alteration can be assigned to speci®c ®eldson this diagram. Both phyllic and andalusite-bearingalteration can involve hydrolysis reactions that consumefeldspar to produce mica or aluminum silicates. Gener-ally, at temperatures above the K-feldspar + andalu-site + quartz + muscovite + water isobaric invariantpoint in this simple system, potassium enrichment offeldspar, and alteration of ferromagnesian minerals to

Fig. 4 The stability ®elds of K-feldspar (Kfs), muscovite (Ms), andandalusite (And) (quartz-saturated) in log KCl/HCl (represented asmoles/kilogram of solution) versus temperature space at 100 MPatotal pressure and 1 molal RCl after Haselton et al. (1995). The And-Kfs phase boundary (dashed line) is an extrapolation of the metastableextension, drawn schematically as a straight line. Model log KCl/HClfor three compositions from the Bamford Hill Complex (Blevin 1989)are plotted (1BMP biotite micropegmatite, CSGP Core Shop GranitePorphyry,MCG medium grained granite; all from Blevin, 1989; 2ASIaluminum saturation index calculated from whole-rock data fromBlevin 1989). 3For illustrative purposes, log KCl/HCl for a BishopTu� Rhyolite (BTR) at ASI = 1.01 and 1.001 (log KCl/HCl = 1.6)are plotted

Table 5 Calculated values oflog KCl/HCl, and log NaCl/HCl for the low-salinityaqueous vapor and thehydrosaline brine coexistingwith a silicate melt at 100 MPa,800 °C and 50 MPa, 850 °C.Log values are calculated fromthe modeled exchange constantsfor aqueous vapor-melt, andbrine-melt

100 MPa and 800 °C 50 MPa and 850 °C

Melt ASIa Vapor Brine Melt ASIa Vapor Brine

log KCl/HCl 1.01 0.6 1.3 1.01 0.3 1.11.04 0.0 0.7 1.04 )0.3 0.51.10 )0.4 0.3 1.10 )0.6 0.1

log NaCl/HCl 1.01 0.2 1.4 1.01 0.3 1.01.04 )0.4 0.8 1.04 )0.3 0.41.10 )0.8 0.4 1.10 )0.7 0.0

aAluminum saturation index

122

biotite (potassic alteration), consumes KCl relative toHCl, and therefore shifts the composition of the aqueoussolution toward the andalusite or muscovite (+quartz)stability ®elds. According to our results, a magmaticvolatile phase exsolved from a melt with ASI > 1.01produces andalusite plus quartz upon reacting with al-kali feldspar during cooling (ignoring the e�ect ofpressure) (Fig. 4). The reaction of K-feldspar to yieldquartz + andalusite can be written as:

2KAlSi3O8 � 2HCl�aq� � Al2SiO5 � 2KCl�aq� �H2O� 5SiO2 �9�Below the invariant point, the acid alteration reactioninvolving K-feldspar follows Eq. (6). The slope of thecurve in Fig. 4 indicates that as andalusite + quartzform at the expense of K-feldspar, KCl/HCl in theaqueous phase rises as the vapor cools and the reactionproceeds. The ratio KCl/HCl will continue to increaseuntil all K-feldspar is consumed, or until the isobaricinvariant point is reached where andalusite and K-feld-spar react with the vapor to form muscovite. At ASIvalues closer to unity, a magmatically derived aqueousvapor will intersect the K-feldspar + muscovite curvejust below the invariant point.

Andalusite alteration has been observed in many al-teration environments including the porphyry deposit atE1 Salvador (Gustafson and Hunt 1975). However, ElSalvador is a large and complex ore body; a simpler, andless well known deposit will serve our purposes betterhere. The tungsten-molybdenum-bismuth ore systemassociated with the Bamford Granite at Bamford Hill,Northeast Queensland, Australia (Blevin 1989) is a goodexample of such a deposit. Blevin observed alterationassemblages of andalusite + quartz, andalusite +feldspar + quartz, and andalusite + muscovite +quartz. Blevin suggested that the various andalusite al-teration assemblages were due to temperature di�erenceswith the andalusite + muscovite + quartz-bearing as-semblage having formed at lower temperatures. Thehigher temperature hydrothermal system at BamfordHill was una�ected by meteoric waters as evidenced byH and O isotopic data (Blevin 1989), and the magmaticsystem was relatively small, lacking overprinting ofmany separate magmatic hydrothermal events.

Whole-rock compositions for a series of dominantlyunaltered rocks from the Bamford Hill complex (Blevin1989) were used to calculate the KCl/HCl of the MVP.The ASI range of the Bamford Granite is approximately1.01 to 1.1. Log KCl/HCl values for Bamford Hill(Table 6) indicate that a cooled MVP would stabilizeandalusite over K-feldspar at temperatures below ap-proximately 550 °C (see Fig. 4).

These results suggest that volatiles, exsolved frommagmas now represented by granites covering a broadrange of ASI values (approximately 1.01 to 1.1), wouldinitially stabilize andalusite over K-feldspar during hy-drothermal alteration. Cooling of the aqueous vaporpresent at any given point over time (e.g., during ulti-mate plutonic consolidation) would produce a retro-grade muscovite alteration assemblage at the expense of

andalusite or K-feldspar-bearing assemblage. Blevin(1989), in fact, suggested that a lower temperature offormation of his muscovite-bearing assemblages relativeto those bearing andalusite was consistent with his data.In a simple system such as Bamford Hill, andalusitepersists due to a lack of overprinting. In more complexgeothermal, or ore-generative systems, later alterationmay be so pervasive that early andalusite is not easilydetectible. At El Salvador, andalusite occurs as part of anumber of alteration assemblages. However, at El Sal-vador, the andalusite alteration has been overprinted.Note, however, that muscovite-bearing alteration doesnot require meteoric water; cooling of a MVP shouldand does appear to e�ect aluminous alteration by virtueof its acidity.

Clearly, there is a degree of uncertainty in predictingwhether a given devolatilizing magma will yield volatilesthat e�ect andalusite-versus muscovite-bearing altera-tion products initially. The ASI of a granite is certainlydistinct from the ASI of the devolatilizing melt at anygiven time and/or position in a magma chamber. Also,our results are for a rather restricted range of meltcomposition, temperature and pressure. Furthermore,before the temperatures of andalusite + quartz ormuscovite + quartz generation are reached, the volatilephases can change in pressure, or change in compositiondue to exchange reactions, other alteration reactions, orby interaction with other aqueous reservoirs. We canstate, however, that: these results show that the KCl/HCl of the exsolving magmatic volatile phases are cri-tically dependent upon the ASI of the melt, and thatKCl/HCl values that are critical to determining thetemperature and mineralogy of alteration, occur withinthe normal range of ASIs(1.0±1.1) found for granites theworld over. In general, our data suggest that higher ASImelts should produce alteration at higher temperatures,relative to lower ASI melts, other factors being constant.

Table 6 Calculated value of log KCl/HCl for a hypothetical aqu-eous vapor in equilibrium with a silicate melt at 100 MPa and800 °C. Silicate melt compositions are unaltered whole-rock ana-lyses from the Bamford Hill Complex (from Blevin 1989). LogKCl/HCl values are calculated by using the model Kaqv=melt

H;K fromTable 4

Sample Log KCl/HCla ASIb

MCGc 0.5 1.01MD )0.1 1.04BMP )0.2 1.06CSGP 0.1 1.02SCG 0.5 1.01

aLog KCl/HCl is calculated using the model vapor/melt exchangeconstant, Kaqv=melt

H;K at 100 Mpa and 800 °C, and the total alkaliconcentrations and aluminum saturation index (ASI) of the Bam-ford Hill Complex sample of interest (from Blevin 1989)bAluminum saturation index. Bamford Hill Complex ASI calcu-lated from whole-rock analyses of Blevin (1989)cBamford Hill Complex (MCG medium grained granite, MD mi-crogranite dykes, BMP biotite micropegmatite, CSGP Core Shopgranite porphyry, SCG Sunny Corner granite, Blevin 1989)

123

Summary

Experimental results on HCl under magmatic conditionsprovides useful insight into magmatic aqueous ¯uidsexsolved from silicic magmas associated with porphyryand epithermal ore deposits. The composition of themagmatic volatile phases can in¯uence the chemistry oftemporally later aqueous phases as the magmatic systemcools, which in turn a�ects the solubility of chloride-complexed ore metals, and the type and style of hydro-thermal alteration.

Data were presented as exchange constant expres-sions (K aqm=melt

H;Na and K aqm=meltH;K ) and revealed an impor-

tant factor governing hydrogen-alkali exchange in thethree-phase system felsic silicate melt + aqueous va-por + hydrosaline brine. Exchange constants K aqm=melt

H;Naand K aqm=melt

H;K are inversely proportional to the RClcon®rming that HCl/NaCl and HCl/KCl ratios arehigher in the low-salinity aqueous vapor relative to high-salinity brine. In addition, we found little in the way of apressure e�ect on the empirically determined exchangeconstants. Modeling of hydrogen-alkali exchange be-tween the vapor and the silicate melt was performed byconducting a series of linear regressions of K aqm=melt

H;Na andK aqm=melt

H;K versus F aqv, a mass-balance variable that ex-presses the ratio of vapor to vapor plus brine in theexperimental system. To model hydrogen-alkali ex-change between the hydrosaline brine and the silicatemelt, linear regressions were performed by using HCl,KCl, and NaCl concentrations in the aqueous mixturesversus F aqv. Model K aqv=melt

H;Na � 26 at 100 MPa (800 °C),and 19 at 50 MPa (850 °C), and K aqv=melt

H;K � 14 at 100MPa (850 °C), and 24 at 50 MPa (850 °C). For bri-ne + melt equilibria, model K aqb=melt

H;Na � 1:6 at 100 MPa(800 °C), and 3.9 at 50 MPa (850 °C), andK aqb=melt

H;K � 2:7 at 100 MPa (800 °C) and 3.8 at 50 MPa(850 °C).

Log KCl/HCl values for the low-salinity aqueousvapor and hydrosaline brine were calculated as a func-tion of melt ASI by using K aqv=melt

H;K and K aqb=meltH;K . Ex-

amination of the predicted log KCl/HCl values revealthat they are lower in the aqueous vapor relative to thebrine; however, when placed in the context of melt ASI,the di�erences in log KCl/HCl (and log NaCl/HCl)values at di�erent melt aluminosities for any given vaporor brine are greater than the di�erence between anaqueous vapor and brine pair at ®xed melt ASI. Themodel log KCl/HCl values show that a change in meltASI from peraluminous (ASI = 1.04) to moderatelymetaluminous (ASI = 1.01) composition shifts thecooling pathway (in temperature-log KCl/HCl space) ofthe aqueous vapor toward the andalusite + musco-vite + K-feldspar reaction point. This result indicatesthat, in aqueous phase-saturated systems, moderatedi�erences in melt composition (in this case melt ASI)would shift the composition of the aqueous vapor (orbrine) and therefore the sequence and style of hydro-thermal alteration.

Appendix

Calculation of F aqv

F aqv is expressed as:

F aqv � mass�aqv�=mass�aqv� aqb�: �A1�The chloride concentration in the quenched aqueousphase is measured directly with an autotitration chlo-ridometer. In the system NaCl + H2O, the chlorideconcentrations in the coexisting aqueous vapor and inthe brine are ®xed. Therefore, changes in the total chlo-ride concentration of the quenched run-product liquidre¯ect variations in the relative proportion of aqueousvapor to brine (e.g., F aqv) at run P and T. We can relatethe measured chloride concentration (as moles/kilogramsolution) in the quenched aqueous run product, andmodel chloride concentrations in the vapor and brine(Bodnar et al. 1985), to the vapor brine ratio as:

CaqmCl � �F aqv � Caqv

Cl � � ��1ÿ F aqv� � CaqbCl � �A2�

where CaqvCl and Caqb

Cl represent the chlorine concentra-tion in the low-salinity aqueous vapor and the hydro-saline brine respectively. Vapor and brine concentrationsare from synthetic ¯uid inclusion measurements byBodnar et al. (1985) selected for the temperature andpressure of interest. Equation (A2) can be rearranged to®nd the value of F aqv:

F aqv � �CaqmCl ÿ Caqb

Cl ��Caqv

Cl ÿ CaqbCl �

: �A3�

F aqv varies between 1 (100% vapor) and 0 (100% brine).The chlorine concentrations for the low-salinity aqueousvapor and the hydrosaline brine are listed in Table A1.

Modeling of HCl, KCl, and NaCl concentrationsin the hydrosaline brine

The concentrations of HCl, KCl, and NaCl in thehydrosaline brine were determined by relating the em-pirical concentrations in the quenched aqueous phase tothe F aqv (see above). Figure A1a±f are plots of aqueousmixture concentrations versus F aqv. Concentrations inthe brine were determined by linear regression of theanalytical concentrations in the quenched aqueous so-lution and calculating the concentrations at F aqv � 0(100% brine). Model concentrations are listed in Ta-ble A2. The model brine concentrations could then beused to calculate the exchange constants, K aqb=melt

H;Na andK aqb=melt

H;K (brine-melt exchange).

124

Calculation of log KCl/HCl and log NaCl/HClin the low-salinity aqueous vapor and the hydrosalinebrine using the model values of the vapor-meltand the brine melt exchange constants

Log KCl/HCl and log NaCl/HCl values for the vaporand brine were calculated by using the appropriate val-ues for K aqv=melt

H;Na ;K aqb=meltH;Na , and K aqv=melt

H;K , for vapor-meltand brine-melt exchange.

HClaqv

KClaqv� K aqv=melt

H;K �CmeltH�fasc�CmeltK

�A4�

where HClaqv � HCl in the aqueous vapor, andKClaqv � KCl in the aqueous vapor, and Cmelt

H�fasc� is the®ctive hydrous aluminosilicate component of the silicatemelt. Cmelt

H�fasc� is a function of the melt aluminum satu-ration index:

CmeltH�fasc� � H�fasc� � RAlkmelt � �ASIÿ 1� �A5�where RAlkmelt � Cmelt

K � CmeltNa � Cmelt

Ca , and the ASI isthe aluminum saturation index of the melt or rock of

Table A1

100 MPa, 800 °C 50 MPa, 825 °C

Caqva;b

Cl 0.33 0.07

CaqbCl 10.4 13

aConcentrations in moles/kg solutionbFrom Bodnar et al. (1985)

Fig. A1a±f Quenched aqueous mixture (aqm) concentrations ofKCl, HCl, and NaCl (as moles/kilogram of solution) versus F aqv at100 MPa, 800 °C and 50 MPa, 850 °C. Regression analysis was

performed on each data set and the brine concentration �CaqbX �

modeled at F aqv � 0 (100% brine). 1-sigma uncertainty is approxi-mately the diameter of the plotted circles

Table A2

Pressure andtemperature

Caqba

H CaqbK Caqb

Na

100 MPa, 800 °C 2.1 (�0.2)b 3.4 (�0.1) 3.9 (�0.4)50 MPa, 850 °C 1.6 (�0.1) 3.1 (�0.1) 2.2 (�0.3)

aModel concentration of the element (H, K, or Na) in the hydro-saline brine. Concentration determined by linear regression analy-sis of the concentration of the element of interest in the quenchedaqueous mixture as a function of F aqv

b1-r uncertainty in the regressed estimate of the concentration inthe hydrosaline brine

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interest. Substituting (A5) into Eq. (A4) and rearrangingproduces:

logKCl

HCl� log

1

K aqv=meltH;K

� CmeltK

RAlkmelt�ASIÿ 1� : �A6�

The ASI may be the empirical value for a whole-rock orglass composition, or may be selected arbitrarily to testthe e�ects of compositional variability.

Equation (A6) and the K values from this in-vestigation may be used to determine the KCl/HCl (orNaCl/HCl) for any given whole-rock or glass that has anASI > 1.

Acknowledgments Support for this project was provided by theNational Science Foundation [EAR 9018870 (PAC) and EAR9204671 (PAC)], and by the Department of Energy [DEF6079ID13025 (PAC and PMP)]. We would like to acknowledgeGary Cygan and Jim Webster for providing thoughtful and con-structive reviews of the manuscript. Thanks are also in order toMark Frank who reviewed an initial draft of the manuscript.

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