Synthetic IIuid inclusions in natural quartz. III. Determination of phase equilibrium properties in the system H,O-NaCI to 1000C and 1500 bars

R.J. ~ODNAR* Chevron Oil Field Research Company, P. 0. Box 446. La Habra, CA 9063 1

and

C. W. BURNHAM andS.M. STERNER* Department of Geosciences, The Pennsylvania State University. Unive~ity Park, PA 16801

(Received January 10, 1985; accepted in revised.form April 9, 1985)

Abstract-Phase equilibria in the system H20-NaCl have been determined to 1000C and 1500 bars using synthetic fluid inclusions formed by healing fractures in inclusion-free Brazilian quartz in the presence of the two coexisting, immiscible H@-NaCt fluids at various temperatures and pressures. Petrographic and microthermometric analyses indicate that the inclusions trapped one or the other of the two fluids present. or mixtures of the two. Salinities of the two coexisting phases were obtained from heating and freezing studies on those inclusions which trapped only a single, homogeneous fluid phase.

Results of the present study are consistent with previously published data on the &O-NaCl system at lower temperatures and pressures. and indicate that the two-phase field extends well into the P-T range of most shallow magmatic-hydrothe~~ activity. As a consequence, chloride brines exsolved from many epizonal plutons during the process of second-boiling should immediately separate into a high-salinity liquid phase and a lower salinity vapor phase and produce coexisting halite-bearing and vapor-rich fluid inclusions. This observation is consistent with results of numerous fluid inclusion studies of ore deposits associated with shallow intrusions. particularly the porphyry copper deposits, in which halite-bearing and coexisting vapor- rich inclusions are ~ommonIy associated with the earliest stages of magmati~-hydrothe~al activity.

INTRODUCIION

THE SYSTEM H20-NaCl represents one of the most important binary fluid systems for understanding var- ious geologic processes. Numerous fluid inclusion analyses (ROEDDER. 1972,1984) have shown that NaCI is the dominant component of many aqueous fluids from a wide range of geologic environments, and many interpretations of fluid inciusion data are based on PVTX properties of the system H@-NaCI, Similarly, BURNHAM ( 1979) has shown that the H20-NaCI system provides an adequate model for predicting the fluid characteristics during the process of vapor separation (second-boiling) from a cooling silicic magma, and BURNHAM ( 198 1) has used this system to describe fluid evolution and related copper mineralization in the porphyry copper environment. Unfortunately, phase equilibria and PVT data for the H,O-NaCI system do not extend into the f-7 range associated with most magmati~-hydrotherma1 activity, thus necessitating extrapolation of the available data and precluding a rigorous interpretation of experimental and fluid in- clusion data. ROEDDER and BoDNAR(I~~O) have dis- cussed the numerous misinterpretations of fluid inclu- sion data that have resulted from this lack of experi- mental data.

The lack of experimental phase equilibria data on the H20-NaCI system at high temperatures and pres- sures and high NaCl concentrations may be attributed

* Present address: Department of Geological Sciences, Vir- ginia Polytechnic Institute and State University, Blacksburg, VA 2406 I.

to the extreme difficulty and complexity of the exper- imental techniques normally used to obtain such in- formation. The major limitation of these experimental techniques has been the inability to satisfactorily sam- ple the fluids, particularly those with NaCl concentra- tions in excess of room temperature saturation, 226 wt.% NaCl (cf:, SOURIRAJAN and KENNEDY. 1962). Recently, however, STERNER and BODNAR ( i 984) de- scribed a method of sampling fluids at elevated PTX conditions using synthetic fluid inclusions formed by healing fractures in quartz. These workers have shown that the synthetic fluid inclusions trap a representative sample of the fluid (or fluids) present at the time of their formation and maintain this sample during quenching to ambient conditions. Preliminary results (BODNAR and STERNER. ~~~~:STERNERC~U(.. 1984) have shown that by analyzing the synthetic inclusions using standard fluid inclusion techniques. phase equi- libria in various fluid systems could be easily deter- mined. The purpose of the present study is to extend the range of available phase equilibria data for the H20- NaCl system into the magmatic-hydrothermal PTX range using synthetic fluid inclusions in quartz.

PREVIOUS STUDIES

Numerous studies have been published on the phase equi- librium properties and vapor pressures of H20-NaC1 lluids. Nearly all of these studies have been limited to the low tem- perature region (

I861 K .i Hodnar. ( ti Burnham and S. M. Sterner

The most detailed stud! of phase eqtnhbna in the H,O- NaCl system is that of SOURIRAJAN nnd KENNEDY ( 1962). These workers determined the compositions of coexisting hq- uid and vapor to 700C and z 1200 bars. Unfortunately. be- cause of the design of their experimental system, they were unable to obtain information on fluids contaming greater than 26.4 wt.% NaCI. Thus, that portion of the H&I-NaCI phase diagram required to interpret results of heating measurements on halite-bearing fluid inclusions was not available. In addition. because of the rapid. nonsystematic changes in the phase re- lations near the upper temperature limit of their study. theu results may only be extrapolated to higher temperatures with a great deal of uncertainty.

The only studies published that mvesttgated the vapor pres- sures of H20-NaCI solutions having compositions greater than room temperature saturation (with the exception of KEEVII.~~ ( 1942) data, which is restricted to the NaCl solubility curvei are those of URusovA (1974. 1975) and URUSOVA and RAV- ICH (I 97 I ). In these studies. the vapor pressures of H*O-NaCi liquids with compositions of 10-59.9 wt.% NaCI were deter- mined up to 550C. but no information was provided on the compositions of the coexisting vapor phase.

.Although not directly related to the present study, GUNTER rt al. ( 1983) determined the liquidus in the H20-NaCI system up to pure NaCI. These data. combined with the vapor pressure data on fluids of the same composition obtained in the present study. permit a complete interpretation of results of heating measurements on halite-bearing inclusions from porphyv copper deposits (ROEDDER and BODNAR. 1980).

TOPOLOGY OF THE H,O-NaCI SYSTEM

The topology of the HQNaCl system (modified from MOREY. 1957) is shown schematically in Fig. i The H,O-NaCl system is a classic example of a binan system in which the solubility curve does not intersect the critical curve (MORES. 1957). The general geometry of this sytem can be attributed to the fact that NaCl

C

I

i

L/

has a much higher melting temperature f7i .\aiI). f :F I] and a much lower vapor pressure than fiJ1

The NaCl solubility curve ii. : I - \a< !, f-rg .

extends from the H,O-NaCI rutectic (1;. P .!C).XC I = 0.001 bar) to the triple pomt of NaCl 1 IINK~J 73 80 I C. f z ! bar]. Phenomenologlcali:\ the shayx of the soluhihty curve is esplarned b! rhc ~ndcnc\ .:j the addition of Hz0 to Increase the vapor pressure upor: moving into the binary field a\~ay liom the iI ~plc po:l\v of NaCl [71NaCI)]. The addition of II,C i hwt.~:i~. tends to lower the temperature along rhc ~i)luhilit*. curve, thus lowering the vapor pressure. i h-w oppob- ing eflects become equal at Go maximum prcssurc Critical CUP/=3 C (2)

NaC:

_.__ _.. _-.__-.-_______ T m-

FIG. I. DIstorted schematIc I-/ prqection 01 the H20-NaCI system modified from MERRY i 193 fi = H,O-NaCI eutectic ice + hydrohalite + liquid + vapor (T = -2O.gC. I = 0.001 bar. 73.2 wt.% NaCi! 7(H20) = Hz0 triple point (7 = 0.01 C, P = 0.006 bars); C(H*O) = Hz0 critical point ( I- 374C. I Z?(> bars); UNaCI) = NaCl triple point (7 = 801 C. P z I bar): C(NaCI) = NaCl critical point t 7. : 36OOi i 2 258 bars). Other symbols explaIned in text.

Synthetic fluid inclusions III 1863

the H@-NaCI eutectic (E) to the NaCl triple point [~NaCl)), the H20-NaCl melting curve extending from the eutectic (E) to the triple point of Hz0 [7(&O)], and the Hz0 liquid-vapor curve [L + V(H,O)] extending from the triple point of H,O [T(H,O)] to the critical point of Hz0 [C(H20)] (Fig. 1).

This large two-phase Iieid, which spans the entire composition range from pure Hz0 to pure NaCl, may be subdivided into smaller two-phase fields for specific H,O-NaCI compositions. Thus, as shown on Fig. 1, an HO-NaCl solution with composition (I) will split into a higher salinity liquid and a lower salinity vapor at temperatures and pressures within the field labeled L + V( 1). Similarly, the more saline composition (2) is immiscible in the field labeled L + V(2). The point at which each of these isopiethal loops become tangent to the critical curve [C(l) and C(2)] represents the ctit- ical temperature and pressure for that particular com- position. Additionally, the point of intersection of any two isopleths (and they may intersect only at a single point) defines the compositions of the two coexisting phases at that temperature and pressure. Thus. at the intersection of the L + I( 1) loop with the L + I(2) loop (1 % 2, Fig. l), a liquid of composition (2) is in equilibrium with a vapor of composition (1). At tem- peratures and pressures outside each isopiethai ioop, that particular composition exists as a single, homo- geneous fluid. For compositions in excess of room temperature saturation, such as composition (2), this one-phase fluid field is bounded at lower temperatures by the liquidus ](L + H) (2)]; at tem~ratu~s below the liquidus temperature, an N&l-saturated liquid is in equilibrium with solid halite.

EXPERIMENTAL PROCEDURE

Samples of the coexisting fluids at a given temperature and pressure were trapped as synthetic fluid inclusions by healing fractures in quartz. Quartz cores approximately 4 mm in di- ameter and 2-3 cm in length were cut from inclusion-free Brazilian quartz using a diamond coring device. The cores were placed into an oven at 350C for approximately one hour and, immediately upon removal from the oven, im- mersed in cold, distilled H20, causing the quartz to fracture, Numerous tests at various temperatures indicated that 350C was the optimum temperature for producing a large number of closely-spaced fractures without causing the core to disin- tegrate. The fractured cores were then placed into a vacuum oven at z 150C ovemi~t to remove all water intr~uc~ by the fracturing process.

The cleaned, dried, fractured cores were placed into 5 mm diameter platinum capsules, one end of which was closed by a flat disk with rolled edges (Le., Petri dish-shaped) welded into place. A small amount (usually a few milligrams) of crushed quartz or silica gel was added to the capsule to promote fracture healing and also to serve as a source of SiOl to produce overgrowths on the original quartz core. Finally. approximately IO-50 microliters of either 5, 10 or 20 weight percent NaCl solution which had been prepared earlier were injected into the capsule using a microliter syringe, and an end cap similar to that used on the bottom of the capsule was welded into place to seal the capsule.

The loaded capsules were weighed, placed in an oven at ~110C for several hours, and re-weighed to check for leaks. The capsules were then placed in either cold-seal type or in-

t P

T-

FIG. 2. Schematic P-T projection of a portion of the H2Q- NaCl system showing the various heating paths followed to reach final run conditions.

ternally-heated pressure vessels and taken to run conditions. Run conditions were chosen initially based on the results of SOURIRAJAN and KENNEDY ( 1962), and later on the results of previous runs, and were chosen to be within the two-phase held (2 and 4 on Fig. 2) for the HzQ-NaCl composition in the capsule. In addition, several capsules were run at temperatures and pressures outside the two-phase loop (I and 3 on Fig. 2) to verify the location of the solvus.

The P-T path followed during heating of the samples from ambient conditions to final run conditions was found to have an important effect on the types of inclusions formed. Initially, a path was followed such that temperature and pressure would increase smoothly to final run conditions. An example of this type of heating path is shown by path A in Fig. 2. This type of heating path was unsuitable because, even though the final run conditions were in the one-phase-fluid field in many cases (such as point 3, Fig. 2), inclusions were formed that indicated formation in a two-phase field. This probably results because various amounts of the liquid phase or the vapor phase, or both, were expelled from the still-open fracture as the sample crossed the two-phase field (L -t V, Fig. 2), thus changing the bulk composition of the H,O-NaCl mixture in the fracture. As a resuit of changing the bulk ~m~sition, the immiscible fluids faiied to re-homogenize upon leaving the two-phase field (i.e., the P-T conditions were still in the two-phase field for the new bulk composition in the fracture) and formed inclu- sions indicative of immiscibility, even though the P-T con- ditions were in the one-phase field for the ~rj~jna~com~sition loaded into the capsule.

To remedy this problem, it was necessary to follow a heating path such that the pressure at any given temperature was well above the two-phase field (path B, Fig. 2). After stabilizing the temperature at the run temperature, the pressure was de- creased rapidly to the run pressure (path C, Fig. 2). This pro- cedure served two purposes. First, it avoided entering the two- phase field prematurely. Secondly, previous tests (STERNER and BODNAR, 1984) showed that the fractures begin to heal almost immediately at high temperatures, isolating pockets of the solution from the fluid surrounding the quartz core. Thus, in the time required to stabilize the temperature at a pressure above the two-phase field (usualty from I5 minutes to one hour), this procedure assured that at least some amount of fluid with the original bulk composition would be isolated in the fracture and available for subsequent inclusion for- mation upon reaching final run conditions. This procedure produced reproducible, consistent results, except for a very few F-T conditions.

For a very few run conditions (such as point 3, Fig. 2X even this high pressure path was not satisfactory because, during the final decrease in pressure to the run pressure, the path crossed the two-phase field (path D, Fig. 2). In order to avoid

I864 K, J. Bodnar. C. W. Bumham and S M. Sterner

crossmg the two-phase field to reach a run condition such as point 3, it was necessary to follow not only a high pressure path, but also to heat the sample beyond the run temperature before dropping the pressure, and finally the temperature. tO run conditions (path E, Fig. 2). Few data pomts were collected in this high temperature. low pressure region because. owing to the upper temperature limits of :he pressure vessels. the two-phase field could not be circumnavigated in most cases.

Samples were run for 2-10 days, with longer run durations corresponding to lower temperature runs. Upon completion. capsules were weighed to check for leakage and the samples removed from the capsules, cleaned and dried The cores were mounted in Lakeside cement in glass tubes. cut into disks approximately 1 mm thick and polished on both sides m preparation for petrographic examination and heating/freezing measurements. All heating and freezing determinations were made on a Fluid. Inc. adapted USGS gas-flow stage (WERRE (I al.. 1979) that was calibrated using synthetic fluid inclusions. Freezing temperatures were accurate to +O. 1 C; the accurac! of heating measurements ranged from approximately -L3C at 200C to approximately +lOC at temperatures above 600C. All measurements were made with the thermocouple tip located in the approximate center of the quartz disk. and the maximum distance of any measured inclusion from the thermocouple tip was approximately 2 mm; the errors quoted above have incorporated into them any thermal gradients be- tween the positions of inclusions and the thermocouple.

FLUID INCLIJSION CHARACIERISTICS

Fluid inclusions formed by healing fractures were concentrated along the edge of each disk where the fracturing was most intense. Occasionally. inclusions were observed along a fracture which cut across the entire disk and, rarely. primary inclusions were seen in overgrowths on the original quartz core. These latter inclusions are described in more detail below.

Samples run at P-T conditions m the one-fluid phase-field (such as point 1, Fig. 2) contained two-phase (liquid and vapor) inclusions that exhibited uniform phase ratios at room tem- perature. Other than an occasional freezing temperature de- termination to verify that these inclusions had the same salinity as that ofthe original HZO-NaCI solution loaded into the cap- sule, no additional measurements were made on this inclusion type.

Samples run in the two-fluid-phase-field (such as points _ and 4. Fig. 2) contained fluid inclusions exhibiting wide range in phase ratios at room temperature, with a fairly consistent bimodal distribution in individual samples. One end-member type of inclusion contained a vapor bubble. a halite crystal and liquid. Within a group ofsuch inclusions, the phase ratios appeared to be similar (Fig. 3A). This inclusion type trapped the high-salinity liquid phase present at run conditions. The other end-member type of inclusion contained only liquid and vapor at room temperature, And the vapor bubble ap peared to fill greater than 50 percent of the inclusion (Fig. 38). This type of inclusion trap@ the low-salinity vapor phase in equilibrium with high-salinity liquid at run conditions. The presence of these two end-member types of inclusions, of greatly different composition and density. in a single fracture. is evidence that the inclusions were trapped in the two-fluid- phase-field. Examples of coexisting halite-hearing and vapor- rich inclusions, representing trapping of the end-member compositions present at run conditions. are shown in Figs. X-F. Between these two end-member types of inclusions were inclusions with widely varying phase ratios; most of these appeared to represent original vapor-rich inclusions that

FIG. 3. Examples of typical fluid mclusions trapped in the two-phase (liquid + vapor) field in the system HLO-NaCl at various temperatures and pressures. Scale bar (shown in Fig. 3B) equals 50 micrometers and applies to photomicrographs 4-J. A. Halite-bearing inclusions representing trapping of the high-salinity liquid phase at 775C and 500 bars. R. Vapor. rtch inclusions representing trappmg ofthe low-salinitr vapor chase at 750C and IO00 bars. C. Vaoor-rich mclusion co existing with a halite-bearing incl&on. Inclusions were trapped in the two-phase-field at 850C and 1000 bars. D. Vapor-rich inclusion coexisting with a halite-bearing mclusion Inclusions were trapped in the two-phase-field at 800C and 500 bars. E. Vapor-rich inclusion coexisting with a halite- bearing inclusion. inclusions were trapped in the two-phase held at 800C and 1000 bars. F. Vapor-rich inclusions co- existing with halite-bearing inclusions. Inclusions were trapped m the two-phase-field at 825C and 1000 bars. G. Vapor-rich inclusions containing small halite-crystals (bottom right) in- dicating heterogeneous fluid entrapment at 825C and IO00 bars. These inclusions coexist with vapor-rich and halite- bearing inclusions, each of which trapped a single phase. H Vapor-rich inclusion containing a small halite crystal (top). mdicating heterogeneous fluid entrapment, coexisting with vapor-rich inclusions that trapped only the low-salinity vapor phase at 825C and 1000 bars. I. Vapor-rich inclusion con- taining a small halite crystal (center), indicating heterogeneous fluid entrapment at 800C and 1000 bars. This inclusion co.. exists with vapor-rich inclusions that trapped only the low. salinity vapor phase, and with halite-bearing inclustons that trapped only the high-salinity liquid phase. J. Vapor-rich in- clusion containing three small halite crystals, indicating het- erogeneous fluid entrapment at 750C and 1000 ban. This inclusion coexists with vapor-rich inclusions that trapped only the low-salinity vapor phase, and with a halite-bearing indu- sion that trapped only the high-salintty liquid phaw

Synthetic fluid illusions III 1865

FIG. 4. Mosaic constructed from numerous photomicrographs of a plane of inclusions formed at 750C and 1000 bars showing the distribution of inclusion types along the fracture. Note that halite-bearing inclusions are concentrated in the upper right of the photograph and vapor-rich inclusions are restricted to the lower left portion of the photograph. This distribution presumably indicates physical separation of the liquid and vapor phases within the fracture prior to final healing of the fracture. Bar scale equals 50 micrometers.

trapped mostly vapor with various amounts of high-salinity liquid (Figs. 3G-J).

Within a given plane of inclusions, an area containing mostly halite-bearing inclusions can be seen grading into an area containing mostly vapor-rich inclusions (Fig. 4). It is in this mixing area between the two end-member types that in- clusions with widely varying phase ratios are most often ob- served. This distribution of inclusion types suggests that, fol- lowing initial splitting of the original homogeneous fluid into a high-salinity liquid and a low-salinity vapor in the partially healed fracture, separation, probably as a result of density differences, occurred prior to final healing of the fracture to form inclusions.

In addition to the secondary fluid inclusions formed by healing fractures in the quartz core, rare primary inclusions

This observation has important implications for the for- mation of various types of high-temperature ore deposits be- cause it suggests that these relatively low density, low salinity fluids are capable of transporting and depositing significant quantities of quartz. In addition, primary vapor-rich inclusions formed from H,O-NaCl fluids to which chalcopyrite had been added were observed to contain small chalcopytite crystals (BODNAR, unpub. data) similar to those seen in vapor-rich inclusions from porphyry copper deposits (c.J. BODNAR and BEANE, 1980). This suggests that, not only are these fluids capable of transporting silica, but also large amounts of ore metals. M. L. CRAWFORD (pets. comm., 1985) suggests that the presence of only vapor-rich inclusions in the overgrowths might not indicate that the overgrowths precipitated from a vapor phase, without the presence of the coexisting liquid, but, rather, could result from preferentially trapping the vapor phase from a liquid-vapor mixture. Because the liquid phase, when present, is the phase that wets the quartz surface. as is obvious from the behavior of inclusions containing both phases during heating tests, it is unlikely that inclusions would pref- erentially trap the non-wetting phase, i.e., the vapor. Note. however, that ROEDDER (1984, pp.28-3 1) describes several examples in which fluid-inclusions have apparently trapped the non-wetting phase with total exclusion of the wetting phase.

were observed in quartz overgrowths precipitated on the core during the run (Figs. 5A-D). Unlike the fractures, which con- tain both end-member types of inclusions as well as mixtures of the two, the overgrowths in a given sample contain only one type of inclusion, either the vapor-rich type or the halite- bearing variety. Most often, inclusions in the overgrowths are the vapor-rich type; very rarely are the halite-bearing type seen as primary inclusions.

Inclusions in the overgrowths are also generally much larger than those in fractures; inclusions in excess of I mm in max- imum dimension have been observed in the overgrowth quartz. Also, vapor-rich inclusions in the overgrowths appear to have more consistent phase ratios than those in fractures, and freezing temperatures of these inclusions exhibit a much smaller range as compared to similar inclusions in fractures.

Heating/freezing behavior

The compositions of the coexisting liquid and vapor phases at experimental conditions were obtained by measuring the temperatures of halite dissolution and ice melting of the halite-bearing and vapor-rich inclu- sions, respectively. These values were converted to sa- linities in weight percent NaCl using the equations of POITER et al. (1977, 1978). Because the inclusions were sufficiently large and the quartz clear, determination of temperatures of halite dissolution was easy. Deter- mining ice melting temperatures was more difhcult because many of the vapor-rich inclusions contained so little liquid that the exact point at which the last ice disappeared could not be observed clearly.

The underlying assumptions upon which this entire study is based are that (1) the inclusions trapped a representative, homogeneous sample of either one or the other of the two fluids (but not mixtures of the two) present at experimental conditions, and that (2) the inclusions maintained this composition during the

K J. Bodnar. c. W. Bumham and S. M. Sterner

I3

.

FIG. 5. Examples of primary fluid inclusions trapped III overgrowths on the original quartz core. Bar scale (shown on Fig. SD) equals 30 micrometers and applies to Fig. 5A-D. A. Primary, vapor-rich fluid inclusion in quartz overgrowth trapped at 800C and 1300 bars. The edge of the original quartz core is shown by the dark zone across the bottom of the photograph. B. Primary, vapor-rich fluid inclusion in quartz overgrowth trapped at 800C and 1000 bars. The edge of the original quartz core is represented by the dark zone across the bottom of the photograph. C. Primary. halite-bear- ing fluid inclusion in quartz overgrowth trapped at 850C and 1300 bars. The gray zone crossing the photograph diagonalI> represents the edge of the original quartz core. D. Two primary. halite-bearing fluid inclusions in quartz overgrowth trapped at 850C and 1300 bars. The gray area along the left side of the photograph represents the edge ofthe original quartz con

quench and did not re-equilibrate. or leak. or otherwise change this initial composition. Previous studies (STERNER and BODNAR, 1984; BODNAR and STERNER. 1984) have shown that assumption (2) is valid for vir- tually all PTX trapping conditions. Assumption f I ) ~5 not always valid for inclusions trapped in the two-phase field, particularly for the vapor-rich inclusions. How- ever, petrographic observations and results of heating freezing tests allow those inclusions that have trapped a single, homogeneous fluid phase to be distinguished from those that have not. Thus, the uniform phase ratios and temperatures of salt dissolution of hahte- bearing inclusions in a given sample are good evidence that these inclusions trapped only a single fluid phase.

The best evidence in support of the validity of the assumptions outlined above is provided hv the tem-

peratures of liquid-vapor (total) homopntr;itron ot thz halite-beating inclusions. When a fluid mclusion :I. formed in the two-phase liqurd-vapor held. : (1 31 II% equilibrium vapor pressure. the inciusron WI!:

homogenize at the suww temperature as its I~)rrnatrori temperature. However. this will happen onl> rfthe ir; elusion has trapped a homogeneous fluid and has IRV re-equilibrated or leaked during cooling. ii/lost ~)f thz inclusions formed in the present study cithcr had ht.- mogenization temperatures aho\e the upncr hmrt cr! the fluid inclusion stage (~650C) or ~er-~ i~~mrtf ;I! pressures which would have caused them to dtxreprtatc before reaching the homogenization temperatun: (~850 bars: LEROY. 1979). However. iiitic urnpie:, formed at temperatures

Synthetic fluid inclusions III 1867

bubbles in inclusions move rapidly in the direction of propagation of the shadow as it passes through inclu- sions. As soon as the shadow passes the inclusion, the bubble returns to its original position in the inclusion; this behavior is best observed in long, tubular inclusions with their long dimension-oriented perpendicular to the wave front. This phenomenon is reproducible both during heating and cooling through the (Y//II transition.

The temperature of the a/o transition has been found to vary significantly in natural quartz samples, from ~536 to 577C (SOSMAN, 1965) but Sosman states that . chances are 19 out of 20 that it will be between 572.2 and 574.1 C (p. 81). If we assume that this transition occurs at 573C in the quartz used in this study and that the difference between this temperature and the measured temperature (Fig. 6) is due to gradients in the stage and thermocouple errors, and add this difference to the measured homogenization temperatures, the average homogenization temperature for each sample corresponds almost exactly (g +2C) with the formation temperatures. Thus, these observations prove that the halite- bearing inclusions contain a sample of the high salinity liquid that was present at run conditions and have not t-e-equilibrated during the quench.

Unlike the halite-bearing inclusions, the vapor-rich inclu- sions do not appear to give consistent homogenization tem- peratures. These inclusions appear to homogenize over a wide range in temperature; furthermore, the apparent homogeni- zation temperature is usually 100 to 500C lower than the expected homogenization temperature. This behavior cannot be explained by heterogeneous fluid entrapment, as this would result in homogenization temperatures higher than the actual formation temperature. ROEDDER (1984) points out that &. The determination of Th L + V(V) will be accurate only when the steam inclusion has a narrow reentrant into which the last bit of fluid phase has been concentrated by capillarity

(p. 256). However, even vapor-rich inclusions in this study that had shapes such that the last bit of liquid could be seen to apparently disappear exhibited homogenization tem- peratures that were much too low. The apparenr homogeni- zation behavior of vapor-rich inclusions may be explained by considering the changes that occur within an inclusion as it is heated from room temperature to the homogenization tem- perature. At room temperature, the inclusion contains a liquid phase, generally of low salinity, and a vapor phase that is essentially a vacuum. As the inclusion is heated, the water vapor pressure increases as water is evaporated into the vapor phase, serving to increase the salinity of the remaining liquid phase. With continued heating, increasingly more water is partitioned into the vapor phase, and the liquid phase becomes progressively higher in salinity until, at the homogenization temperature, the last few molecules of liquid remaining must have the same salinity as that of the coexisting liquid at the temperature and pressure of formation. Because fluid inclu- sions represent, to a first approximation. isochoric systems, thisentire process must occur under constant volume. constant

2 The fact that the shadow or wave front moves back and forth across the sample with increasing and decreasing tem- perature would indicate that there is a temperature gradient across the sample. All fluid inclusion temperature measure- ments were taken with the tip of the thermocouple in the center of the quartz disk and, because the inclusions were various distances from the thermocouple tip (~2 mm) each requires a different gradient error correction which has not been applied. At 600C the maximum gradient between the tip of the thermocouple and any point in the sample is less than 10C based on earlier tests. However, the shape of the gradient field varies with each sample because of minor dif- ferences in sample width, thickness and shape. air-flow rate, and the absolute position of the disk in the sample chamber.

mass conditions. consistent with known PVTX characteristics of the H20-NaCI system.

This qualitative description of the heating behavior of vapor- rich inclusions has been quantified for an inclusion trapped in the two-phase field at 700C and 700 bars. At these con- ditions. a vapor containing approximately 2 wt.% NaCl is in equilibrium with a liquid with a composition of 60 wt.% NaCl (SOURIRAJAN and KENNEDY, 1962: this study). The change in composition of the liquid phase in the vapor-rich inclusion and the change in volume percent vapor with temperature are shown on Fig. 7. These paths were calculated assuming constant volume, constant mass conditions. As the temper- ature is increased from room temperature, the liquid phase composition remains fairly constant up to approximately 300C. as little water is partitioned into the vapor phase (Fig. 7). Above this temperature, however, the liquid salinity begins to increase rapidly, reaching the equilibrium value of 60 wt.% NaCl at the homogenization temperature (700C). The vol- ume percent vapor in the inclusion decreases initially from 75 to %70% at 300C. and then begins to increase rapidly above this temperature. This decrease followed by an increase in the volume percent vapor is a commonly observed phe- nomenon in vapor-rich inclusions and has been termed ho- mogenization with an inversion point by ERMAKOV (1965). At temperatures above 400C. the volume percent vapor in- creases progressively slower with increasing temperature and asymptotically approaches 100 percent vapor at 700C (Fig. 7).

When observing such a fluid inclusion on the heatingl freezing stage. one would note the rate of increase ofthe vapor phase until the liquid was no longer visible, and mentally extrapolate this rate of change to obtain the homogenization temperature. As shown on Fig. 7, the apparent homogenization temperature determined will depend upon the point on the volume percent vapor curve that the extrapolation originates, and for the inclusion shown the apparent homogenization temperature could be from ~300 to 150C be/oMs the actual

10

0

01

Temperature (C)

FIG. 7. Calculated volume percent vapor and liquid com- position as a function of temperature for a fluid inclusion that trapped the vapor phase in equilibrium with liquid at 700C and 700 bars. The calculations assume constant total mass and volume and are based on data of KHAIBULLIN and ROR- ISOV (1965, 1966). SOUR~RAIAN and KENNEDY (1962) and unpublished PVT data for HzO-NaCl (R. J. E%ODNAR, 1984, pets. commun.). Th Appurenf represents the temperature range over which the volume percent vapor in the inclusion is sufficiently large that. depending on inclusion size and shape, the inclusion might appear to homogenize.

I868 K. 1 Sodnar. C. U Rumham and S. M. Sterner

homogenization temperature. Because ot known wettmg characteristics, the liquid phase in aqueous inclusions must necessarily wet the inclusion walls. Even for a fairly large spherical inclusion 25 micrometers in diameter containing 99 volume percent vapor (which occurs approximately IWC below the homogenization temperature of the inclusion on Fig. 7) the thickness of the liquid rim would be only about 0.04 micrometers. and would not be visible even under the best optical conditions.

These preliminary calculations are consistent with the results of numerous fluid inclusion studies of por- phyry copper deposits in which homogenization tem- peratures of vapor-rich inclusions presumably coex- isting with halite-bearing inclusions are several hundred degrees lower than the high salinity inclusions (c:!., REYNOLDS and BEANE, 198.5). This obviously has se- rious implications concerning genetic models for por- phyF copper deposits which are based on such fluid inclusion data. For example, if the coexisting vapor- rich and halite-bearing inclusions do not appear to ho- mogenize at the same temperature. which is required if they represent an immiscible pair. one would inter- pret these results to indicate that the inclusions rep- resent two distinct fluids. perhaps having different or- igins. introduced at different times. Moreover, lacking evidence that the inclusions were trapped at their equi- librium vapor pressures. the homogenization temper- atures would necessarily represent minimum formation temperatures, and an incorrect (and unnecessaryi pressure correction would be applied to the inclusions to obtain trapping temperatures

DISCUSSION OF RESUI;IX

The results of 550 fluid Inclusion measurements from 15 samples are summarized in Table I. Shown on Table I are the experimental run temperatures and pressures and the range in salinities of the halite-bearing and vapor-rich inclusions in each sample. calculated

TABLE i. Compositions of the coexisting liquid and vapor phases In the system H1O-NaC1 determined from synthetic inclusions.

T(OC) P (bars)

550 500 600 500 650 500 700 500 750 500 775 500 800 500 825 500 625 750 700 750 775 750 800 750 650 1000 700 1000 750 1000 800 1000 825 1000 850 1000 900 1000

1000 1000 700 1100 800 1100 800 1200 800 1300 850 1300

Liquid CompositioIl Wt.% N&l)

48.7-49.2 57.6-59.8 65.9-67.1 69.4-70.2 69.9-71.7 70.5-73.2 75.1-76.7 75.3-77.0 48.3-49.0 59.5-59.7 65.0-67.6 66.8-67.7 34.5-36.8 48.5-49.2 55.5-56.5 59.8-61.9 62.0-64.5 66.1-66.6 67.2-68.8 71.8-72.9 43.7-44.7 57.8-59.4 52.1-53.8 50.0-51.2 56.5-57.9

17 0.4- 3.2 6 21 0.4-15.6 8 8 9

30 19 6

II 12 10 15

I4 16 18 26 11 10 12

vapor Compoaitloo {Wt.% NsCl) No. -

--_ --- 0 __- u

1.7- 7.3 3 -__ 0 _-_ 0

1.9 1 --_ i)

1.6- 4.8 6 1.4- 1.7 10 8.3-19.0 29 4.2-12.2 13 3.5-12.9 12 1.9- 6.9 22 1.4- 9.9 21 1.6- 9.3 18

--_ 0 2.6-21.7 17

__- 0 _-_ 0 --_ 0

5.9- 8.8 h -_- 0

900 u :I

;q

FIG. 8. Compositions of the coexlstmg phases In the H,i>- NaCl system as a function of temperature at a constant pres- sure of 1000 bars determined from synthetic fluid inclusions. Also shown for comparison are five data points from SW- RIRAJAN and KENNEDY ( 1%: 1

from the equations of POTTER CI ui. ( 197 :i. I 978 j. in- cluded are the number of fluid inclusions ofrach type that were measured in each sample.

The 1000 bar isobar was studied m detali and the results, including all the measured data points. arc shown on Fig. 8. The results from the halite-beanng inclusions (right limb) generalI> fall withln :I narrou range of salinities and the data project smoothly to the minimum on this loop (critical point) determined h;, SOURIRAJAN~~~ KENNEDY (1962). Asdescribed ear- lier, the range in calculated salinities ofthe halite-bear- ing inclusions is probably a result of thermal gradients in the fluid inclusion heating/freezing stage.

Salinities of the vapor-rich inclusions (left hmb) e,x. hibit a much wider range in calculated salinities LL\ compared to the halite-bearing inclusions ~2s noted earlier, the vapor-rich inclusions also appear to have much less consistent phase-ratios at room temperature, suggesting that some (or all) of the vapor-rich inclusions trapped a small amount of liquid along with the vapor. Two other lines of evidence also support this obser- vatlon. First. if vapor-rich inclusions trap various amounts of high-salinity liquid. the freezing data would show a range in salinities with the lowest salinity coi. responding to that inclusion which trapped the smallest amount of liquid. Salinity data for three groups of va- por-rich inclusions formed at temperatures and pres. sures for which salinity data are available from SOL:- RIRAJAN and KENNEDY (1967) are shown trn Fig. Y. Even though the salinities of the vapor-rich mclusions span a wide range. no salinities below the value reported by Sourirajan and Kennedy were obtained. This cw dence suggests. but does not prove. that the salimt) range is a result of heterogeneous trapping and that the minimum salinity most closely approximates the equilibrium composition.

Synthetic fluid inclusions Ill IS69

1.5. < S + K

T. S. > S + K

+ fJJ 4 I

2 700%. !iOO Bars 625 C, 7.50 Bara

ov I I I I I I 0 2 4 6 a 10 12 14

Minimum Salinity This Study (Wt % NaCi)

FIG. 9. Comparison of the minimum salinity obtained from vapor-rich inclusions in this study (T.S.) (black dots) with the salinity of the vapor phase reported by SOURIRAJAN and KENNEDY ( 1962) (S + K) at the same tern~ratu~ and pres- sure. The total range in salinity obtained from the vapor-rich inclusions is indicated by the horizontal lines.

The second piece of evidence suggesting heteroge- neous trapping is provided by the vapor-rich inclusions found in overgrowths. As reported earlier, overgrowths were found to contain either vapor-rich inclusions or halite-bearing inclusions, but rarely both. Thus, an overgrowth containing only vapor-rich inclusions pre- sumably precipitated from the vapor-phase and there was no liquid available that could have been trapped in the inclusions with the vapor (see footnote 1). A single inclusion found in an overgrowth from a sample run at 650C and 1000 bars had a salinity of 8.3 wt.% NaCI: this value is in good agreement with the salinity of g7 wt.% NaCl reported by SOURIRAJAN and KEN- NEDY (1962). Similarly, seventeen inclusions in frac- tures from a sample run at 1000C and 1000 bars had a range in salinities of 2.6-21.7 wt.% NaCl (Fig. 8; Table 1). However, three inclusions from the over- growth zone of this sample had freezing temperatures of -1.6 ?I O.IC, corresponding to salinities of 2.6- 2.9 wt.% NaCI, suggesting that this value represents the equitib~um vapor com~sition, and that higher salinities obtained from vapor-rich inclusions in frac- tures are a result of heterogeneous fluid entrapment.

Assuming that the minimum salinity obtained from vapor-rich inclusions and the average salinity obtained from halite-bearing inclusions most closely approxi- mate the equilib~um vapor and liquid compositions at a given temperature and pressure, data from all 25 samples have been combined with data from KEEVIL ( 19423. SOURIRAJAN and KENNEDY f 1962). and URU- SOVA ( 1975) and smoothed graphically to obtain the phase equilibrium properties of HZO-NaCI up to 1500 bars and iOOOC. These results are plotted as isobaric, isothermal and isoplethal projections on Figs. IO. 1 I,

I b I I b I so 100

Weight Percent NaCI

FIG. 10. Isobaric (T-X) projection of the coexisting phases in the &O-NaCI system determined from synthetic fluid in- clusions. The projection of the critical curve is from SOURI- RAJAN and KENNEDY f 1962) and PITZER (1984).

and 12. respectively. Also shown on these figures are the projection of the critical curve from SOURIRAJAN and KENNEDY ( 1962) and PITZER ( 1984) and the HzO- NaCl solubility curve from KEEVIL ( 1942). The actual data points have heen omitted for clarity, but are shown on Fig. 13 where the results obtained here are compared with previously published data (see below).

2.0 / &XrltiUl I CUWO

1.5

ii Y E 3 1.0 ::

L 0

0.5

0 0 so 100 Weight Percenl N&t

FIG. I I. Isothermal (P-X) projection of the coexisting phases in the H,O-NaCl system obtained from synthetic fluid inclu- sions and data from the iiterature. The solubility curve (L + C f NaCI) is from SOURIRAJAN and KENNEDY (1962). the critical curve is from SOURIRAJAN and KENNEDY (1962) and PITZER (19S4) and the critical point of NaCl [C(NaCI)] is from PITZER ( 19X4).

1870 K J. Hodnar.

Synthetic fluid inchtsions 111 1871

with previously determined lower temperature iso- therms and with vapor compositions. in particular. four data points along the liquid limb ofthe 700C isotherm define a line that extrapolates smoothly, in a manner consistent with other isotherms, to both the critical curve and the NaCl solubility curve. These observations all serve to indicate that liquid compositions obtained from halite-bea~ng inclusions accurately represent PTX characteristics of the high-salinity region of the H20-NaCI system.

Fewer data were obtained on vapor phase compo- sitions in this study. and these data are thought to be less accurate than liquid phase data for reasons outlined previously. Vapor phase compositions of inclusions formed at 700C and pressures of 500 and 1000 bars are compared to data from !SOURIRAIAN and KENNEDY ( 1962) on Fig. 13. Although the two data points indicate slightly higher salinities than those reported by Sou- rirajan and Kennedy, the agreement is quite good. Also shown on Fig. 13 are the vapor phase compositions along the 800 and 1000C isotherms obtained from this study. It is not possible to determine the accuracy of these data, but, given the close agreement between the 700C data and those of SOURIRAJAN and KEN- NEDY (1962) and the fact that a line fit through three of the 800C data points projects smoothly to the crit- ical curve and into the liquid field, the data are thought to closely approximate the vapor compositions at these higher temperatures.

As described earlier (Fig. 9), four vapor phase com- positions obtained from fluid inclusions showed close agreement with data of SOURIR~AN and KENNEDY ( 1962). and no salinities less than the Sourirajan and Kennedy values were found. If this behavior holds at higher temperatures, the vapor phase compositions shown for 800C and 1000C necessarily represent ~~u~j~zim values, with the actual composition being some unknown amount below the measured value. This then suggests a rather unusual. but perhaps not unexpected, shape for the H20-NaCl solvus. The Sou- rirajan and Kennedy data indicate that above 600C the vapor phase composition moves to progressively higher NaCI concentrations with increasing tempera- ture and decreasing pressure (SOURIRAJAN and KEN- NEDY, 1962; their Figs. 14 and 19). A rough extrapo- lation of the Sourirajan and Kennedy data to higher

3 Most previous studies to determine phase equilibria and vapor pressures of pure H20-NaCl were conducted in stainless-steel pressure vessels. At the elevated temperatures and chloride concentrations of these studies. the amount of iron dissolved into solution from the walls of the pressure vessels may have been as high, or higher, than the amount of silica in solution in the present study. Thus, WHITNEY CY ui. ( 1979) reported iron concentrations ranging from 0.0 I to 0.25 molal in I N chloride solutions equilibrated with synthetic quartz monzonite at 400-700C. Similarly, BURNHAM (I 982) reported iron concent~tions of SO. I molal in I modal chloride solutions in equilibrium with granodiorite at 8ZO-900C and I kb, and further suggested that the iron concentration should increase rapidly as total chloride increases in the range 650- 750C and 2 kb.

temperatures suggests that the vapor phase limb of the 800C isotherm should cross the 700C isotherm at z 1000 bars: at all pressures below this value the 800C isotherm should lie at higher NaCl concentrations than the 7OOC isotherm. However. as shown on Fig. 13. the 800C isotherm determined in this study lies at lower NaCl concentrations than the 700C isotherm to pressures at least aslow as 500 bars. Even the 800C. 750 bar data point. which does not fall on the proposed isotherm, has a salinity lower than the Sourirajan and Kennedy value at 700C and this same pressure (Fig. f 3). This apparent discrepancy between the Sourirajan and Kennedy data. extrapolated. and the higher tem- perature data of this study cannot be reconciled given the present data. However. if these higher temperature results do adequately describe the vapor compositions in the H20-NaCI system, they indicate that the soivus widens at higher temperatures, rather than becoming smaller as an extrapolation of the Sourirajan and Ken- nedy data would indicate.

In summary, the results of the present study are in reasonable agreement with data obtained by previous workers. In particular, data from halite-bearing inclu- sions provide an accurate representation of phase equilibria in the system I-120-NaCl up to 1000C and 1500 bars.

EFFEm OF DISSOLVED SiOt

Phase equilibria data obtained in this study have been compared to and interpreted in terms of the pure H20-NaCI system. Actually, however, the data reported here more accurately describe the system H,O-NaCI- SiOz. The effect of SiO2 on phase equilibria in the HzO- NaCl system is unknown, but the close agreement of the present results with previously pubtished data on the pure H*O-Nazi system3 suggests that the effects are minimal.

The degree to which SiOZ might alter phase relations in the H20-NaCl system should be a function of the amount of silica in solution. The solubility of quartz in water has been determined by a number of workers (and POTTER (1982) lo generate a single equation re- lating quartz solubiiity to temperature and specific volume of HZ0 up to 900C and ~O,O~ bars. Using this equation, the solubility of quartz in water ranges from a minimum of 0.054 wt.% (825C. 500 bars) to a maximum of 0.453 wt.% (65OC, 1000 bars) over the P-T range of this study.

The soiubility of quartz in H20-NaCI solutions has been determined over a much more limited range of temperatures and pressures (and NaCl concentrations) (FOURNIER a al., 1982; ANDERSON and BURNHAM, 1967; NOVGORODOV, 1977). Although there is some di~gr~ment concerning the mechanism of quartz solubility in chloride solutions, there is general agree- ment that the solubility in chloride solutions is slightly higher than in pure HzO. Thus, NOVGORODOV (1977)

1871 K. J. Bodnar. C. W. Bumham and S. M. Sterner

reports that quartz solubility at 700C and I.5 kb in- creases from 0.68 wt.% in pure Hz0 to 0.03 wt.9 in a 33.6 wt% NaCl solution. His results also suggest that above ~10-15 wt.% NaCI. the solubility of quartz is essentially independent of NaCl concentration. A% DERSON and B~JRNHAM (1983. their Fig. 3) show a similar flattening of the solubility curve with increasing KC! concentration at 700C and 4 kb. Assuming that the solubility of quartz in H&I-NaCI solutions over the PTX range of this study is on the order of I wt.:; or less (except, perhaps, in the 900C and 1000C runs. where it is probably higher), SiOZ should. therefore, be expected to have little effect on the position of the solvus.

GEOLOGIC IMPLICATIONS

The most important feature ofthe H20-NaCl system at high temperatures and pressures from a geologic standpoint is the large PTX range over which two fluid phases may coexist. Stated differently, most fluids of reasonable bulk composition entering the shallow magmatic-hydrothermal P-T regime would be within the immiscibility field and would split into a higher salinity liquid phase in equilibrium with a lower salinity vapor phase. The extent of the two-phase region beyond the P-T limits of the present study is unknown. al- though PITZER ( 1984). based on theoretical calcula- tions, predicts that this field may extend to ~360OY and reach a maximum pressure of ~-2.5 kb at a tem- perature of ~2000C.

An important application of the results of this study is in understanding fluid evolution in the porphyry copper deposits. Coexisting halite-bearing inclusions and vapor-rich inclusions are nearly ubiquitous in these deposits, and understanding the origin of these inclu- sions is of more than academic interest because they have been shown to be closely associated with the min- eralization process and have been used as an explo- ration tool in the search for new deposits (NASH. 1976: BODNAR. I98 1).

Following emplacement of a typical granodioritic magma at shallow levels in the Earths crust. the melt will become saturated in an aqueous phase in response to crystallization of anhydrous phases and the lower pressures in this environment. BIJRNHAM (I 979) has shown that vapor separation. or second-boiling, will occur in the range 800 to 1000C. depending on the initial water content of the magma and the confining pressure. BURNHAM ( 1979) has further suggested that the salinity ofthe initial aqueous fluid to separate from this magma will be on the order of a few to perhaps 20 wt.% NaCl equivalent. At 800-1000C and pres- sures of 0.5-l .5 kb. most of this composition range is in the immiscibility field in the H20-NaCI system (Figs. 10 and 1 I ), and this aqueous phase will immediately split into a high-salinity liquid and a low-salinity vapor to produce the halite-bearing and vapor-rich inclusions so characteristic of the porphyry copper deposits. Fol- lowing immiscibility. these two fluids of vastly different densities may separate physically. with the low salinity

vapor moving upward into the shallower p(*rtlon5 ci! the system and the liquid moving down along thr outer edges of the stock (HENLEY and MCNABH. I?rX FOURNIER. 1983). The majont!: of the metals will k partitioned mto the relative11 small \~olurn~~ studtec which have shown that areas 01 highest gradz copper mineralization in the porphyc copper deposit3 art coincident with areas of highest percentage l>i halite bearing inclusions (c.,r_. MCK~R~ and N.ASH. i 9?4)

The PTS range over which tlutd ~mnnsc.iblht) oc curs in the porphyry copper envlronmant m&i! hc> dc- termined using the results of [he preszn! \~ud? ano abundant data In the literature on composiir~~nr and homogenization temperatures of halite-beanng mciu- sions. Compositions of halite-heanng mclustons from porphyry-type deposits are nearly always In ~hr rangs 30-70 wt.%) NaCl equivalent. and the majontb ofthew are between 30-50 wt.%. Homogenization tempera- tures of these inclusions generalI> are in the range ?SO- 600C. although temperature5 01 ?OO( (ROED DER. 1971). 800C(RE~~o~n.~and BEANL-. iYXS)and even up to 1 oo(jc (WILSON t? Ii 19X0: ~!AS I OE and EADINGTON. 1982) are not uncommon. According to Figs. 10 and I 1. these data suggest pressures ranging from a few hundred bars to approximateI> I500 bars for the formation of these inclusions. with most m the 500-1000 bar range. consistent with the eplzonal na- ture of these systems.

One of the most important results of this study was the observation that vapor-rich lnclustons appear to homogenize at temperatures several hundred degrees below the actual homogenizatron temperature. Fur- thermore, mass balance-volume balance calculations indicate that this behavior IS exactly what one would predict. based on PVTX data for the system H$?-NaCl Similarly. these data indicate that the difference bc- tween the apparent and actual homogenrzatron tem- peratures should Increase as the actual homogemzation temperature increases. Therefore. it should nof be as- sumed that coexisting halite-bearing and vapor-nch Inclusions are not contemporaneous if the vapor-rich Inclusions appear to homogenize at a much lowc~ temperature, particularly if homogenization tcmper- atures of the halite-bearing Inclusions arc :-h(iiiC

-I(,knr~n,lc,~~~~,,l~rrtv-Kevlews 01 311 carller verskrn the I ! C (kc. logical Surve>.

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