Unexpected behaviour of fluid inclusions synthesized from silver

Chemical Geology 173 Ž2001. 159–177 www.elsevier.comrlocaterchemgeo

Unexpected behaviour of fluid inclusions synthesized from silver oxalate and an aqueous NaCl solution Yves Kruger ¨ ) , Larryn W. Diamond 1 Mineralogisch-Petrographisches Institut, UniÕersitat ¨ Bern, Baltzerstrasse 1, 3012 Bern, Switzerland Accepted 18 January 2000

Abstract Fluid inclusions were synthesized in quartz from silver oxalate ŽAg 2 C 2 O4 . and a 6-wt.% NaCl aqueous solution to produce a CO 2 –H 2 O–NaCl fluid, following the method of previous workers. Synthesis conditions of 500–6508C and 1500–4500 bar were chosen with the intention of trapping a homogeneous, ternary fluid. The resulting inclusions were analyzed by microthermometry and by Raman spectroscopy. Solid residues in the experimental capsules were analyzed after the syntheses by XRD and SEM-EDS. Several properties of the synthetic inclusions were unexpected: Tm Cla and Th tot vary over significant ranges and they correlate negatively; the calculated total molar volumes deviate significantly from data on the same bulk composition derived by Gehrig wGehrig, M., 1980. Phasengleichgewichte und pVT-Daten ternarer ¨ Mischungen aus Wasser, Kohlendioxid und Natriumchlorid bis 3 kbar und 5008C. Hochschul Sammlung Naturwissenschaft, Chemie, Band 1. Hochschul Verlag, Freiburgx in an optical cell, and from data derived from our fluid inclusions synthesized for comparison using the gas-loading technique of Frantz et al. wFrantz, J.D., Zhang, Y.-G., Hickmott, D.D., Hoering, T.C., 1989. Hydrothermal reactions involving equilibrium between minerals and mixed volatiles: 1. Techniques for experimentally loading and analyzing gases and their application to synthetic fluid inclusions. Chem. Geol. 76, 57–70x; all fluid inclusions contain a chlorargyrite ŽAgCl. daughter crystal; and HCOy 3 was detected in the aqueous phase of the inclusions by Raman spectroscopy, presumably reflecting high concentrations of NaHCO 3Žaq. . All these anomalous features can be traced back to effects caused by precipitation of insoluble AgCl and simultaneous formation of sodium oxalate upon loading the experimental capsules with silver oxalate and NaCl solution. Reconstruction of the chemical reactions that occurred during the hydrothermal syntheses suggests that incomplete redissolution of the AgCl during progressive quartz precipitation and fluid trapping yields inclusions with variable bulk properties. Nevertheless, quite accurate total homogenisation temperatures can be recovered by regressing the Th tot –Tm Cla correlations, but considerable effort is required. We conclude that silver oxalate should be used in combination with chloride solutions only with extreme caution, whereas the gas-loading technique provides an excellent alternative for the synthesis of CO 2 –H 2 O–NaCl fluid inclusions. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Synthetic fluid inclusions; Silver oxalate; Gas-loading; Chlorargyrite; System H 2 O–CO 2 –NaCl; P–V–T–X properties; Nahcolite; Raman )

Corresponding author. Fax: q41-31-631-48-43. E-mail addresses: [email protected] ŽY. Kruger ¨ ., [email protected] ŽL.W. Diamond.. 1 Present address: Institut fur ¨ Geowissenschaften, Montanuniversitat ¨ Leoben, Peter Tunner Strasse 5, 8700 Leoben, Austria.

0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 0 0 . 0 0 2 7 3 - 4

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Y. Kruger, L.W. Diamondr Chemical Geology 173 (2001) 159–177 ¨

1. Introduction Comprehensive knowledge of the P–V–T–X properties of the ternary CO 2 –H 2 O–NaCl system is necessary to determine quantitatively the composition and density of many natural fluid inclusions, and to constrain their formation conditions by reconstructing isochores. Knowledge of the fluid properties is also required to derive accurate thermodynamic parameters for phase-stability calculations. Various experimental methods have been used to obtain P–V–T–X data, and the synthetic fluid inclusion technique ŽSterner and Bodnar, 1984; Bodnar and Sterner, 1987. has become increasingly popular since the 1980s. Pure water and water–salt mixtures were studied initially ŽBodnar and Sterner, 1985; Bodnar et al., 1985; Zhang and Frantz, 1987; Sterner et al., 1988; Knight and Bodnar, 1989; and others., whereas the fluids investigated in the 1990s have become more complex through addition of volatiles such as CO 2 and CH 4 ŽKotel’nikov and Kotel’nikova, 1990; Sterner and Bodnar, 1991; Zhang and Frantz, 1992; Frantz et al., 1992; Shmulovich and Plyasunova, 1993; Schmidt et al., 1995; Lamb et al., 1996; and others.. The methods of producing CO 2 –H 2 O–NaCl fluid inclusions differ mainly by the way in which CO 2 is loaded into the noble metal capsules used for the syntheses. Solid oxalate compounds are convenient to handle at room temperature, and both oxalic acid ŽH 2 C 2 O4 P 2H 2 O. and silver oxalate ŽAg 2 C 2 O4 . are in use. At temperatures below 2008C these substances decompose, liberating CO 2 in known quantities. In the case of silver oxalate it is assumed that the metal liberated upon decomposition remains inert during the hydrothermal synthesis, as does the noble metal of the capsule walls. Frantz et al. Ž1989. developed a direct loading method in which CO 2 gas is condensed into the capsules using a liquid-nitrogen cold trap. An alternative but more cumbersome method is to load solid CO 2 ŽShmulovich and Plyasunova, 1993.. In previous studies Sterner and Bodnar Ž1991. and Bakker and Jansen Ž1991. used silver oxalate to synthesize binary CO 2 –H 2 O inclusions, and Bodnar and Sterner Ž1987., Chou Ž1988; DTA analysis., Diamond Ž1992., and Schmidt et al. Ž1995. com-

bined silver oxalate with aqueous chloride solutions. Shmulovich and Plyasunova Ž1993. were the first to mention that use of silver oxalate with chloride solutions causes problems during the subsequent microthermometric determination of clathrate AmeltingB. Earlier, Diamond Ž1992. reported ranges in Tm Cla within individual samples from 0.38C up to 1.68C, with most standard deviations below 0.288C. The largest ranges exceeded the instrumental and observational uncertainties associated with microthermometry, but their cause remained unexplained. Schmidt et al. Ž1995., on the other hand, worked with NaCl-saturated starting solutions and subsequently measured the melting temperature of halite as a monitor of salinity. Fluid inclusions were synthesized in this study from silver oxalate and a 6-wt.% aqueous NaCl solution. However, the results presented here demonstrate that silver oxalate is not a reliable reagent for producing ternary CO 2 –H 2 O–NaCl fluid inclusions. We present a thorough description and analysis of our experiments to show that silver does not behave inertly; it significantly affects the physico-chemical properties of the fluid mixture, resulting in a true quaternary system, CO 2 –H 2 O–NaCl–Ag. Our arguments are based in part on a comparison of the P–V–T–X properties derived from the synthetic fluid inclusions with the data of Gehrig Ž1980. and with fluid inclusions synthesized using gas-loading Žthe technique described by Frantz et al., 1989.. Gehrig’s work is still the most extensive in the CO 2 –H 2 O–NaCl system, and his experimental approach using an optical cell is independent from ours. A chemical characterization of the fluid inclusions and the reconstruction of chemical reactions before and during the hydrothermal experiments complement the microthermometric measurements, and, in turn, help to understand them. 2. Experimental method In this study we have essentially followed the synthesis procedure described by Bodnar and Sterner Ž1987.. 2.1. Capsule loading Pre-annealed gold tubes of 99.99% purity were used to make capsules of ca. 30 mm length, 3.5 mm


inner diameter and 4.0 mm outer diameter, sealed by welding at one end. Inclusion-free, synthetic quartz was cored to make rods of 2.9 mm diameter and 5 to 7 mm length. Following cleaning in HNO 3 and distilled water, the rods were heated to ca. 3508C and then quenched in distilled water, thereby inducing large numbers of narrow cracks without breaking the rods. The rods were dried in an oven. The 6-wt.% NaCl solution was prepared from analytical grade NaCl and high purity water. Fresh silver oxalate was made by reacting stoichiometric quantities of analytical grade silver nitrate and oxalic acid in water. After washing and filtering, the silver oxalate precipitate was dried and stored over phosphorous pentoxide in an opaque dessicator in order to avoid photochemical reactions and absorption of water. By sealing a known mass of silver oxalate in a gold capsule and then heating above the dissociation temperature, puncturing and re-weighing, it was confirmed that the silver oxalate releases more than 99% of the stoichiometric CO 2 . Each capsule was filled as follows, the total weight being redetermined on a Mettler AT20 balance Žto a precision of "10 mg. after each constituent had been added ŽAppendix A.. A quartz rod was dropped into the capsule together with some powdered quartz, to enhance crack healing during the hydrothermal experiment. Silver oxalate was added, and a small gold tube Žca. 15 mm length, 1.5 mm outer diameter. was inserted to keep the quartz rod in the upper part of the capsule during the hydrothermal run. The pre-mixed NaCl solution was loaded with a microliter pipette, and the top of the capsule was immediately crimped and closed by welding. The sealed capsule was weighed, placed into an oven at ca. 1008C overnight, then re-weighed to check for leakage. 2.2. Gas-loading technique A glass vacuum line for gas-loading was constructed according to the principles in Frantz et al. Ž1989., and our first experiments using this technique are presented here for comparison as a Asilver-freeB system. Details of the construction, volumetric calibration and operation of the gas-loading apparatus will be published elsewhere.

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A small gold capsule of 25 mm length, 2.5 mm outer diameter, and 2.0 mm inner diameter was completely filled with the NaCl solution and then immediately closed by crimping in a vice. The capsule was weighed before and after loading to determine the mass of NaCl solution. The small capsule was placed into a larger one with 3.5 mm inner diameter and 4.0 mm outer diameter together with the quartz rod and some quartz powder. This large capsule was connected to the glass vacuum line, a known quantity of CO 2 was condensed in using a liquid nitrogen cold trap, and the capsule was closed by welding. Upon heating in the autoclave during the subsequent experiments, the NaCl solution in the inner capsule expanded, resulting in an overpressure that opened the crimp and allowed the solution to escape and mix with the CO 2 gas.

2.3. Fluid composition All fluid inclusion syntheses were performed with the same starting composition, but at various pressures and temperatures. In order to permit comparison of our results with an independent data set, a bulk composition investigated by Gehrig Ž1980. was used: 88.57 mol% H 2 O, 9.69 mol% CO 2 and 1.74 mol% NaCl Ži.e. a salinity of 6.0 wt.% NaCl with respect to the H 2 O–NaCl subsystem.. The loading procedure using silver oxalate yields the desired composition to within "0.2 mol% CO 2 relative to the bulk fluid, while the error from weighing accuracy is negligible. The reproducibility of the gasloading technique is nearly perfect, and according to our calibrations the accuracy in determining the CO 2 concentration is "0.1 mol% for the specific bulk fluid composition used Žcorresponding to ca. "150 mg CO 2 .. The H 2 O–CO 2 content was checked after each hydrothermal run as follows. The capsule was first cleaned and weighed, then briefly immersed in liquid nitrogen to freeze the contents. Immediately after removal from the liquid nitrogen, the capsule was punctured and placed into a glass tube of known weight, then left overnight in an oven at 1208C to expel the volatiles. The glass tube thus collected any salt and quartz powder that sprayed out of the cap-

162


sule as it thawed and generated gas overpressure. The tube was then re-weighed and the loss of H 2 O– CO 2 was compared to the calculated values. 2.4. Hydrothermal apparatus and synthesis procedure The inclusions were synthesized in vertically positioned, cold-seal pressure vessels ŽNi–Cr–Co alloys NIMONIC 105 and Vacumelt ATS 340., the closure nut lying at the lower end. In this arrangement the gold capsule sits in the hot upper end of the autoclave, held in position by a nickel filler rod. The temperatures of the experiments were measured by NiCr–Ni ŽK-type. thermocouples fixed in the outer walls of the autoclaves. The thermocouples were calibrated using the melting points of high purity zinc Ž419.58C., aluminum Ž6608C. and NaCl Ž8018C., covering the temperature range of the experiments. The experimental temperature was kept constant by automatic furnace regulators to within "18C. Calibration of radial and vertical thermal gradients showed that the capsules were on average 88C cooler than the thermocouple temperatures, and the lower tips of the capsules were 38C cooler than the upper tips. The run temperatures reported in Table 1 are true values corrected for thermocouple calibration and thermal gradients, being certain to "38C. Argon was used as the pressure medium and was measured with a Heise bourdon-tube gauge Žmax. 7 kbar, readout scale subdivided to 10 bar.. Taking into account the manufacturer’s details, the readout precision of the instrument and our own comparative measurements in the hydrothermal laboratory of the Institute of Mineralogy and Petrology at ETH-Zurich, ¨ the maximum uncertainty in pressure is estimated at "20 bar. The experimental conditions cover the range from 1500 to 4500 bar and 5008C to 6508C. Under these conditions, all the experiments fall within the onephase fluid field ŽGehrig, 1980.. The runs were begun by pressurizing the cold autoclaves to 1000 bar and then inserting the autoclaves into pre-heated furnaces. Pressure was then increased stepwise until the final P–T conditions were reached after 30–40 min. After leaving the autoclaves at the desired P–T conditions for approximately 2 weeks, the runs were

terminated by reducing pressure and temperature quasi-isochorically, so as to minimize the pressure difference between the inclusions and the pressure medium, and thus avoid mechanical stress on the quartz host.

3. Analytical methods Following retrieval from the autoclaves the quartz rods were cut into slices Žca. 0.5 mm thick. and polished on both faces. Microthermometry was carried out with a Linkam THMSG 600 heating–cooling stage Ž1993 manufacture. mounted on a Leitz Orthoplan microscope with Nikon M Plan 100r0.75 and Nikon Plan 40r0.55 objective lenses. Leitz Periplan GF 25 = oculars were used in conjunction with the 100 = objective lens to enhance visibility of the CO 2 homogenization ŽL q V ™ V.. Calibration was performed with synthetic H 2 O and CO 2 –H 2 O fluid inclusions. Selected inclusions were analyzed by Raman spectroscopy at the Institute of Mineralogy and Petrology, ETH-Zurich, using a Dilor LabRAM mi¨ croprobe equipped with a 633-nm He–Ne laser Ž25 mW power on sample.. Solid residues from some gold capsules were analyzed by XRD on a Phillips PW 1800 diffractometer, and partly by SEM-EDS ŽVoyager EDS system..

4. Results 4.1. Microthermometry Systematic microthermometric determinations of three phase-transitions were performed on 13–30 inclusions per experiment: clathrate dissociation ŽTm Cla ., partial homogenisation of the CO 2 phases ŽTh CO . and total homogenisation of the fluid phases 2 ŽTh tot .. Results are shown in Table 1. The purity of the CO 2 phases was checked via the melting temperature of CO 2 , and the absence of detectable CH 4 and N2 was confirmed by Raman spectroscopy. Clathrate always dissociates in the presence of aqueous liquid, CO 2 liquid and CO 2 vapor, and in the presence of two solid phases: chlorargyrite ŽAgCl.


163

Table 1 Synthetic fluid inclusion compositions, synthesis conditions and results of microthermometry Run no.

8 12 14 15 17 22 24 42 44 45 55 60 61 63

Fluid composition

Conditions of synthesis

Microthermometric measurements

Comparison of Th tot

XH 2O

XCO 2

X NaCl

Pf wbarx

Tf w8Cx

n

Tm Cla w8Cx

Th CO 2 ŽL q V ™ V. w8Cx

Th tot ŽL q V ™ L. w8Cx

Th )tot w8Cx

Th tot ŽGehrig. w8Cx

0.8865 0.8870 0.8873 0.8873 0.8842 0.8846 0.8859 0.8849 0.8878 0.8886 0.8858 0.8848 0.8855 0.8846

0.0962 0.0955 0.0952 0.0952 0.0984 0.0980 0.0967 0.0976 0.0948 0.0939 0.0968 0.0978 0.0971 0.0980

0.0173 0.0175 0.0175 0.0175 0.0174 0.0174 0.0174 0.0174 0.0175 0.0175 0.0174 0.0174 0.0174 0.0174

3820 2770 4500 3310 1850 2160 1670 3820 1670 3820 3820 3050 3780 2300

550 547 650 513 579 624 550 553 553 552 552 649 644 650

18 24 25 25 29 20 22 25 24 24 18 13 15 20

5.9–6.8 6.3–7.6 6.7–7.2 5.6–6.9 6.1–6.9 6.4–7.0 5.9–6.6 5.7–6.4 6.3–6.6 5.5–6.9 6.7–6.9 6.8–6.9 6.7–6.9 6.7–6.9

28.6 27.6 29.1 29.3 21.8 21.7 20.6 30.3 20.3 30.2 31.0 28.3 30.2 23.8

298.5–325.5 313.5–327.3 298.4–305.2 301.4–326.5 364.1–373.9 363.0–371.7 372.0–380.0 299.0–308.7 369.1–374.9 292.8–305.8 295.2–297.1 335.6–337.6 317.1–320.4 360.2–363.2

296 322 305 301 361 365 370 291 365 294 296 336 319 362

297" 2 324" 3 307" 2 298" 2 376" 3 375" 3 376" 3 298" 2 378" 3 298" 2 298" 2 345" 3 322" 3 375" 3

Run 8–45: Fluid inclusions synthesized from a 6-wt.% NaCl solution and silver oxalate. Run 55–63: CO 2 added by gas-loading. Tm Cla : temperature of clathrate dissociation; Th CO 2 : temperature of CO 2 homogenisation; Th tot : temperature of total homogenisation; n: number of inclusions measured; Th )tot : hypothetical temperature of total homogenisation for 6 wt.% NaCl equivalent Žsee text.; Th tot ŽGehrig.: corresponding value derived from Gehrig Ž1980. Žsee text..

and silver. Reproducibility of the measurements is better than "0.058C, and accuracy is "0.18C. According to Vlahakis et al. Ž1972. and Diamond Ž1992., Tm Cla was expected to be 6.8 " 0.18C for the 6.0-wt.% NaCl solution initially loaded into the capsules. However, measurements for individual samples fall within a remarkably large range of up to 1.38C ŽFig. 1.. Moreover, in every experiment except run 12, the scatter is not centered around the expected value of 6.88C, but is shifted to lower temperatures. The salinities of the synthesized inclusions in these runs therefore appear to be higher than that of the starting solution. A correlation between Tm Cla and the size of the inclusions can often be observed, whereby small inclusions ŽF 10 to 15 mm diameter. have lower Tm Cla than larger ones. Occasionally Tm Cla values also differ systematically from one healed crack to another. In all experiments CO 2 homogenizes upon heating to the vapor phase Žin the presence of chlorargyrite and silver.:

Precise measurements are difficult to obtain, owing to optical problems associated with the dew-point transition, and hence the phase-transition temperature is liable to be underestimated. To minimize this risk, measurements of CO 2 homogenisation were carried out only on strongly elongated flat inclusions. Care was taken to avoid optical artefacts during the measurements Žcf. Sterner, 1992. and in comparative experiments with the gas-loading method, CO 2 homogenisation values agree with the expected values to within 0.38C Žsee below.. The accuracy of these measurements is therefore estimated to lie within 0.28C for homogenisation above ca. 268C Žnear the critical temperature of 31.068C. and up to "0.48C at lower temperatures. In all of our synthetic inclusions the total homogenisation of the fluid phases occurred via a bubble-point transition to the liquid phase:

CO 2 liquidq CO 2 vapor q aqueous liquid

in the presence of chlorargyrite and silver. The chlorargyrite crystals become smaller upon heating, due to progressive dissolution, but no changes were ob-

™ CO2 vapor q aqueous liquid.

aqueous liquidq CO 2 fluid ™ aqueous liquid,

164


Fig. 1. Unexpected scattering of clathrate dissociation and total homogenisation temperatures and their correlation in synthetic fluid inclusions made from silver oxalate and a 6 wt.% aqueous NaCl solution. Note that the error bars for the measured homogenisation temperatures represent the range of reproducibility Ž"18C., not the accuracy of the measurements Ž"28C.. Open squares are Th )tot values Žsee text for explanation.. Open circles are values for comparison derived from Gehrig Ž1980.; the error bars represent the uncertainty resulting from the treatment of Gehrig’s data, as described in the text.


served in the silver. Reproducibility of the measurements is within "18C, while accuracy is "28C. Th tot values scatter over ranges of up to 278C and a striking negative correlation with Tm Cla values was found ŽFig. 1.. 4.2. Chemical characterization of fluid inclusions In addition to CO 2 liquid, CO 2 vapor and an aqueous solution, the fluid inclusions synthesized from silver oxalate contain a round, transparent daughter crystal of chlorargyrite ŽAgCl., as mentioned above. The identity of this phase was verified by exposing the samples to UV radiation for several days. The crystals thus decomposed via a photochemical reaction, becoming irregular in form and opaque due to liberation of free silver ŽFig. 2.. According to visual estimates the volume proportions of phases within the fluid inclusions, including chlorargyrite, appear constant. Nevertheless, some variation in the total AgCl and NaCl concentrations must be assumed to explain the scatter in Tm Cla . Besides chlorargyrite, tiny crystals of accidentally

165

trapped silver are also present, best visible under reflected light where they appear bright white. Precipitation of chlorargyrite in the inclusions implies an excess of residual aqueous Naq. It therefore seems likely that charge balance is restored by formation of NaHCO 3 . Solid NaHCO 3 Žnahcolite. was indeed found in a few inclusions from one experiment Žrun no. 8., but in variable volume proportions. However, in fluid inclusions from the remaining runs, HCOy 3Žaq. was detected by Raman spectroscopy ŽFig. 3.. To our knowledge, the study by Beny and Feofanov Ž1993. is the only other instance in which HCOy 3Žaq. has been detected in fluid inclusions by Raman microprobe analysis. According to Davis and Oliver Ž1972., Anderson Ž1977. and Kruse and Franck Ž1982., HCOy 3Žaq. is not expected to be present in detectable quantities when CO 2 is dissolved in pure water at room temperature and at the internal pressure of the fluid inclusions. Although we have not quantified our Raman spectra, we therefore attribute the elevated, detectable content of HCOy 3 in the inclusions to the presence of significant amounts of dissolved NaHCO 3 .

Fig. 2. Žleft. Transmitted-light photomicrograph of a synthetic fluid inclusion containing a CO 2 vapor bubble, an aqueous solution and a chlorargyrite ŽAgCl. daughter crystal. Žright. The same inclusion after 24-h exposure to UV light. The AgCl has decomposed photochemically, leaving a residue of silver. Widths of the photographs correspond to ca. 80 mm.

166


Žcf. Erdey and Banyai, 1958, on the determination of ´ chloride and sulphide concentrations in aqueous solutions using Ag 2 C 2 O4 .. Therefore, as silver oxalate is in excess in all our experiments, reaction Ž1. is deduced to consume all the NaCl in the gold capsules before the syntheses are started. Thus, prior to heating and applying pressure, the sealed capsules actually contain a mixture of solid silver oxalate, sodium oxalate, chlorargyrite and water. Following synthesis of the fluid inclusions, the inner walls of the gold capsules were found to be completely coated with silver, and the quartz overgrowths on the rods were often found to be covered by flakes of gold. Residues of chlorargyrite and halite were identified. XRD and EDS analyses suggest that a third phase, appearing in fibrous aggregates ŽFig. 4., is sodium hydroxide ŽNaOH., but not nahcolite ŽNaHCO 3 .. No traces of oxalates were found, confirming that dissociation was complete at high temperature. This observation also agrees with weight-loss measurements of the CO 2 and H 2 O released by puncturing and drying the capsules following the syntheses. Fig. 3. Raman spectra of the aqueous solutions in four synthetic fluid inclusions in three different samples, plus a reference spectrum of host quartz. The broad peak at 1017 cmy1 is characterisX ŽDavis and tic of the C–OH stretching vibration n 5 Ž A . of HCOy 3 Oliver, 1972.. Acquisition time is 4=300 s. Only spectrum 8a is from an inclusion that contains a nahcolite ŽNaHCO 3 . daughter crystal.

4.3. Analysis of solid residues The solid residues in the gold capsules prior to and following the syntheses were analyzed in an attempt to elucidate the chemical reactions that occur before and during the hydrothermal syntheses. By simulating the capsule-loading procedure in laboratory beakers, chlorargyrite ŽAgCl. and Na 2 C 2 O4 were found to form instantaneously when the NaCl solution was added to silver oxalate, according to the reaction: Ag 2 C 2 O4Žs. q 2NaCl Žaq . ™ 2AgClx q Na 2 C 2 O4 .

Ž 1. This reaction was observed to run quantitatively to the right, regardless of the initial ratio of reactants

Fig. 4. Secondary-electron image of a fibrous mineral aggregate, probably NaOH, forming part of the solid residue in a gold capsule after a hydrothermal experiment. Na and O were identified by EDS Žthe sample was not coated.. Width of the image corresponds to 39 mm.


4.4. Total molar Õolumes of fluid inclusions Ignoring the presence of chlorargyrite, silver and dissolved NaHCO 3 , and assuming that the synthesis fluid is a ternary CO 2 –H 2 O–NaCl mixture, nominal total molar volumes of the fluid inclusions Ž Vtot . were calculated for P–T conditions of CO 2 homogenization ŽPh CO 2 , Th CO 2 . from the equation: Vtot s X Htot2 OyNaCl Õ Haq2 OyNaCl tot vap aq q XCO q f ÕCO , Ž 1 y f . VCO 2 2 2

ž

/

X Htot2 O – NaCl

Ž 2.

tot XCO 2

where and are the mole fractions of the H 2 O–NaCl binary subsystem and of the CO 2 in the bulk inclusion ŽAtotB .; Õ Haq2 0 – NaCl is the partial molar volume of the binary subsystem in the aquevap aq ous phase; VCO and ÕCO are the molar volume and 2 2 partial molar volume, respectively, of the CO 2 in the vapor and aqueous phases; and f is the molar partitioning coefficient of CO 2 between the two phases. tot Values for X Htot2 O – NaCl and XCO were taken from the 2 starting composition of the syntheses. Since the amount of dissolved CO 2 in the aqueous phase is small at the reference conditions, Õ Haq2 O – NaCl is assumed to be equal to the molar volume of the hypothetical CO 2-free solution, and values were extracted from the EOS of Archer Ž1992.. Values for vap VCO were obtained from data in Angus et al. Ž1976., 2 neglecting the trivial amount of H 2 O in the CO 2 vapor and assuming Ph CO 2 to be equal to that of aq liquid–vapor equilibrium in pure CO 2 . ÕCO was 2 3 treated as a constant value of 30 cm rmol, although according to Parkinson and De Nevers Ž1969. this value may be slightly underestimated. The only parameter in Eq. Ž2. which is not evaluated in a straightforward manner is the partitioning coefficient of CO 2 Ž f .: fs

aq XCO 2 aq Ž XCO

2

vap q XCO . 2

,

167

Since solubility measurements for CO 2 in aqueous NaCl solutions are rare in the range between 08C and 318C and 30 to 80 bar, we chose an indirect apaq via the Sechenov equaproach to determine XCO 2 tion: Km s

log Ž S0rS . m

or

Ss

S0 10 K m m

Ž 4.

where S is the required term for the solubility of CO 2 in aqueous NaCl solutions wg CO 2rkg H 2 O on a salt-free basisx, S0 is the solubility of CO 2 in pure water wg CO 2rkg H 2 Ox, K m is the salting-out coefficient and m is the salinity of the solution wmol NaClrkg H 2 Ox. Fig. 5 shows the solubility of CO 2 in water along the CO 2 liquid–vapor curve, based on our compilation of literature data. This curve compares well with the relation derived by Sterner and Bodnar Ž1991. from an older compilation by Dodds et al. Ž1956.. The salting-out coefficients, K m , for various salinities and temperatures are shown in Fig. 6. For some salt species K m is independent of salt concentration at constant temperature but K m for NaCl is a function of both temperature and salinity. Based on our literature compilation we assume that

Ž 3.

aq where XCO is the mole fraction of CO 2 dissolved in 2 vap the aqueous solution and XCO is the mole fraction 2 of CO 2 in the vapor bubble. In order to evaluate this we undertook an extensive review of literature data2 .

2

Detailed results and discussion of this compilation exceed the scope of this article and will be published elsewhere.

Fig. 5. Solubility of CO 2 in water along the CO 2 liquid–vapor curve.

168


5. Discussion 5.1. Salinity

Fig. 6. K m values compiled from literature data, contoured for wt.% NaCl salinity.

K m for NaCl is independent of pressure in the range with which we are concerned. The Vtot values obtained from Eq. Ž2. were corrected for the volume expansion of the quartz host between the reference conditions of the calculation ŽTh CO , Ph CO . and the P–T conditions of synthesis 2 2 using experimental data in Raz Ž1983. and the EOS of Hosieni et al. Ž1985; adjusted according to footnote 3 in Sterner and Bodnar, 1991.. Corrections were also made for quartz expansion during microthermometric heating to Th tot at 1 atm external pressure. The calculated total molar volumes Ž Vtot . of the synthetic fluid inclusions are listed in Table 2, together with the measured Th CO 2 values and correvap . sponding CO 2 molar volumes ŽVCO . 2

Judging from the phase diagrams of Gehrig Ž1980. and the P–T conditions of the syntheses, all our fluid inclusions were trapped in the presence of only one fluid phase. Some of our experimental conditions exceed the P–T limits of Gehrig’s data, but the fact that the inclusion assemblages show constant phase proportions at room temperature excludes the possibility that they formed in a high-temperature region of fluid immiscibility, such as described by Frantz et al. Ž1992.. Nevertheless, the wide scatter in Tm Cla values proves that fluid of variable salinities was trapped in the inclusions. In a single fluid phase such variation can result only if spatial or temporal chemical gradients were present during the syntheses. Based on a reconstruction of the chemical reactions that may have occurred during the syntheses Žsee below., we attribute these salinity gradients to shifts in the equilibrium concentrations of the fluid-forming species during progressive healing of the fractures and entrapment of fluid inclusions in the host quartz. In order to test whether the salinity variation reflects incomplete mechanical mixing of the fluid during the syntheses, we made a test with experiment 45. After the run temperature was reached Ž5508C. the pressure was repeatedly cycled between 2 and 4 kbar to induce fluid convection. This method is routinely used in experiments in which solid halite is loaded, e.g. Schmidt et al. Ž1995.. No convincing reduction in the salinity variation resulted, as the lowermost diagram in Fig. 1 shows. The scatter of the Th tot values is interpreted to be a simple consequence of the variation in salinity, as reflected in the correlations in Fig. 1. The data of Gehrig Ž1980. show that, for given P–T conditions of fluid entrapment and constant CO 2 content, total homogenisation of the fluid shifts to higher temperatures and pressures with increasing salinity. This interpretation of the cause of the scatter in Th tot values is supported by the results from the gas-loading experiments ŽFig. 7., where Tm Cla matches the value expected for the initial salinity of the NaCl solution to within "0.18C, and the scatter in Th tot is only 2–38C. This is the same range of scatter we

28.6 Žq0.1 . 27.6 Žq0.2 . 29.1 Žq0.2 . 29.3 Žq0.3 . 21.8 Žy0.4 . 21.7 Žy0.3 . 20.6 Žq0.2 . 30.3 Žy0.1 . 20.3 Žq0.7 . 30.3 Žq0.2 . 30.9 – 31.0 28.3 Žy0.2 . 30.2 Ž0.0 . 23.8 Žy0.2 .

ŽL q V ™ V . w8C x

Th CO 2

146 156 140 137 210 211 221 122 224 122 103 149 124 192 Žy1 . Žy2 . Žy3 . Žy3 . Žq4 . Žq3 . Žy1 . Žq1 . Žy6 . Žy4 . Ž 0.0 . Žq2 . Ž 0.0 . Žq2 .

wcm 3 rmolx

va p V CO 2

28.8 29.5 28.2 28.0 34.4 34.4 34.9 27.0 34.7 26.6 25.4 29.3 27.2 32.8 Žq0.1 . Žq0.2 . Žq0.2 . Žq0.2 . Žy0.3 . Žy0.2 . Žq0.1 . Žy0.1 . Žq0.5 . Žq0.3 . Ž0.0 . Žy0.1 . Ž0.0 . Žy0.2 .

Th tot wcm 3 rmolx

Vtot

Molarvolumes of synthetic fluid inclusions

28.9 29.7 28.5 28.0 34.7 34.9 35.2 27.1 35.0 26.7 25.5 29.8 27.5 33.7 Žq0.1 . Žq0.2 . Žq0.2 . Žq0.2 . Žy0.3 . Žy0.2 . Žq0.1 . Žy0.1 . Žq0.5 . Žq0.3 . Ž0.0 . Žy0.1 . Ž0.0 . Žy0.2 .

Ž P – T exp.. wcm 3 rmolx

V tot

17.2 28.0 23.9 26.6 32.5 29.4 33.5 68.1 74.0 61.6 40.1 49.4 49.9 60.8

w% x

Fcap

31.0 29.8 30.8 31.0 23.5 23.6 23.1 31.0 22.5 31.0 31.01 28.0 30.1 23.6

ŽL q V ™ V . w8C x

Th CO 2

99 131 111 101 195 194 199 97 203 101 98 152 126 194

wcm 3 r molx

vap V CO 2

25.0 27.5 25.9 25.4 31.5 32.0 32.6 24.6 32.0 24.8 25.1 28.8 27.2 31.6

Th tot wcm 3 r molx

V tot

25.1 27.7 26.2 25.2 33.5 33.5 33.4 25.1 33.4 25.1 25.2 30.0 27.7 33.7


Vtot

Expected values according to the data in Gehrig Ž1980 .

y2.4 Žq0.1 . y2.2 Žq0.2 . y1.7 Žq0.2 . y1.7 Žq0.3 . y1.7 Žy0.4 . y1.9 Žy0.3 . y1.7 Žq0.1 . y0.7 Žy0.1 . y2.2 Žq0.7 . y0.7 Žq0.2 . ; y0.05 0.3 Žy0.2 . 0.1 Ž 0.0 . 0.2 Žy0.2 .

w8C x

DTh CO 2

47 25 29 36 15 17 22 25 21 21 5 y3 y2 y2

Žy1 . Žy2 . Žy3 . Žy3 . Žq4 . Žq3 . Žy1 . Žq1 . Žy6 . Žy4 . Ž0 . Žq2 . Ž0 . Žq2 .

wcm 3 rmolx

vap DV CO 2

Deviation from expected values DV tot

3.8 2.0 2.3 2.6 2.9 2.4 2.3 2.4 2.7 1.8 0.3 0.5 0.0 1.2

Ž q0.1 . Ž q0.2 . Ž q0.2 . Ž q0.2 . Ž y0.3 . Ž y0.2 . Ž0.0 . Ž y0.1 . Ž q0.5 . Ž q0.3 . Ž0.0 . Ž y0.1 . Ž0.0 . Ž y0.2 .

Th tot wcm 3 rmolx

DV tot

3.8 2.0 2.3 2.8 1.2 1.4 1.8 2.0 1.6 1.6 0.3 y0.2 y0.2 0.0

Ž q0.1 . Ž q0.2 . Ž q0.2 . Ž q0.2 . Ž y0.3 . Ž y0.2 . Ž0.0 . Ž y0.1 . Ž q0.5 . Ž q0.3 . Ž0.0 . Ž y0.1 . Ž0.0 . Ž y0.2 .


Run 8–45: Fluid inclusions synthesized from a 6-wt.% NaCl solution and silver oxalate. Run 55– 63: CO 2 added by gas-loading. va p Th CO 2 : temperature of CO 2 homogenisation; V CO : molar volumes of CO 2 in vapor; V tot ; total molar volume of the fluid at Th tot and at P – T conditions of synthesis. 2 Fcap : degree of capsule filling at P – T conditions of synthesis Žsee text.. Calculated maximum possible values for correction of the deviations in CO 2 content Žsee Table 1 . are given in parentheses.

8 12 14 15 17 22 24 42 44 45 55 60 61 63

Run no.

Table 2 Calculated molar volumes of synthetic fluid inclusions in Table 1, and comparison with molar volumes derived from data in Gehrig Ž1980 .

Y. Kruger, L.W. Diamondr Chemical Geology 173 (2001) 159–177 ¨ 169

170


Fig. 7. Clathrate dissociation and total homogenisation temperatures of synthetic fluid inclusions made from a 6-wt.% aqueous NaCl solution and CO 2 gas Žgas-loading technique.. Note that the error bars for the measured homogenisation temperatures represent the range of reproducibility Ž"18C., not the accuracy of the measurements Ž"28C.. No unexpected scattering was found. Open squares are mean Th tot values, open circles are comparative values derived from Gehrig Ž1980.; the error bars represent the uncertainty resulting from our treatment of Gehrig’s data.

found in syntheses with pure H 2 O, and it therefore seems to represent the ultimate precision achievable with synthetic fluid inclusions. The difference in slopes of the fitted lines in Fig. 1, however, cannot be explained by variation in NaCl content alone. Differences are evident even between replicate syntheses performed under the same P–T conditions Žruns 24, 44 and 42, 45.. This feature is most likely due to variable proportions of the dissolved salt species in the inclusions, including NaHCO 3 , AgCl and probably HCl, but why these proportions should in fact vary between replicates is not clear. 5.2. Physical properties of the inclusions Obviously, the synthetic fluid inclusions synthesized from silver oxalate are best described as a quaternary system, CO 2 –H 2 O–NaCl–Ag, rather than

the intended CO 2 –H 2 O–NaCl ternary. The question now arises as to what way and to what extent the deviations in composition influence the physical properties of the fluid. In other words, can valid physico-chemical properties for the CO 2 –H 2 O–NaCl system be derived from fluid inclusions synthesized from silver oxalate? To answer these questions the homogenisation temperatures and the calculated total molar volumes of the synthetic fluid inclusions are now compared with the data of Gehrig Ž1980..

5.2.1. Comparison of total-homogenisation temperatures A direct comparison would require fluid inclusions with a salinity of 6 wt.% NaCl, corresponding to Tm Cla of 6.88C. As our inclusions contain other electrolytes besides NaCl, the calculated salinities were treated as NaCl equivalent. Because few of our


171

of our Th tot measurements Ž"28C; Fig. 8.. However, with increasing temperature, i.e. increasing total molar volume of the fluid, the deviations from Gehrig’s homogenisation temperatures also become larger Žup to ca. 138C.. An additional correction for the volume change of the inclusions, based on the difference between internal and confining pressure during microthermometry, as described Schmidt et al. Ž1998., would increase the deviations from Gehrig’s homogenisation temperatures even further, by 58C to 108C.

Fig. 8. Total homogenisation temperatures, Th tot , of the synthetic fluid inclusions, compared to the reference data of Gehrig Ž1980.. Shaded band represents the uncertainty in Gehrig’s Th tot values due to our treatment of his data.

measurements show the expected Tm Cla , the fitted lines in Fig. 1 were used to define hypothetical homogenisation temperatures ŽTh )tot . at Tm Cla s 6.88C Žopen squares in Fig. 1.. For comparison, Th tot values were derived from Gehrig’s Ž1980. data by constructing isochores through the P–T conditions of our experiments, and intersecting them with the isoplethic bubble curve. The molar volumes from Gehrig’s raw data were corrected for the thermal expansion of the optical cell using his recommended equation and plotted in P–V space to derive the isochores and the locus of the bubble curve. To render these AtrueB isochores applicable to the synthetic fluid inclusions, an additional correction for the volume change of quartz was applied. The estimated uncertainty in Th tot resulting from the treatment of Gehrig’s data is "2–38C ŽTable 1.. Fig. 1 shows a comparison of the predicted homogenisation temperatures from Gehrig Žopen circles. with the Th )tot values Žopen squares. derived from the fluid inclusions. In all these experiments Gehrig’s homogenisation temperatures are higher than our Th )tot values, although by variable degrees. Deviations of similar magnitude are also visible in the syntheses using gas loading ŽFig. 7.. In the temperature range between 3008C and 3258C the deviations are within the uncertainty resulting from our treatment of Gehrig’s data and from the accuracy

5.2.2. Comparison of molar Õolumes In order to allow a direct comparison with our calculated molar volumes, some of Gehrig’s isochores must be extrapolated above 5508C, the upper limit of his measurements. Eq. Ž2. was rearranged to vap calculate VCO from Gehrig’s Ž1980. corrected Vtot 2 values ŽTable 2., and a correction was applied to account for the volume change of quartz from the P–T conditions of synthesis to room temperature. Fig. 9 shows that the Vtot values for the experiments using silver oxalate are consistently higher than those expected from Gehrig’s data. Quantitative comparison shows that the volumetric deviations between the silver-bearing and the pure CO 2 –H 2 O–

Fig. 9. Total molar volumes of synthetic fluid inclusions at the P – T conditions of the syntheses, compared to the independent reference data of Gehrig Ž1980.. Shaded band represents the range of systematic deviation of inclusions synthesized with silver oxalate Žcf. Fig. 10.. Note that runs 55–63, for which the gas-loading technique was used, are in good agreement with the reference.

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NaCl fluids can be explained if the solubility of CO 2 in the aqueous phase at room temperature is up to twice as high as that in the ternary CO 2 –H 2 O–NaCl fluid. The enhanced solubility is attributed to higher concentrations of NaHCO 3Žaq. , as evidenced by the Raman detection of HCOy 3Žaq. . Aqueous CO 2 solubilities of this magnitude imply that the CO 2 content in the vapor bubble at Th CO 2 is reduced by up to 2 mol% versus the CO 2 –H 2 O–NaCl system. However, the size of the bubble does not decrease proportionately because of the much higher molar volume of CO 2 vapor compared to CO 2Žaq. and NaHCO 3Žaq. . Thus, the elevated molar volumes of CO 2 result in total molar volumes that are too high. Our evidence for elevated CO 2 solubilities is not conclusive, but in the following we show by way of exclusion that, to our understanding, it is the only plausible explanation for our observations. First, the elevated CO 2 molar volumes could conceivably have been due to errors in microthermometry. Underestimates of at least 0.78C would be required to explain the discrepancies Žcompare column DTh CO 2 in Table 2. but this is well outside our uncertainty for the dew-point transition. In the comparative experiments with the gas-loading method, CO 2 homogenisation agrees with the expected values to within 0.38C, whereas the experiments with silver oxalate yield Th CO 2 values which are markedly too low. Second, the good agreement achieved with gasloading shows that our choice of values for parameters in Eq. Ž2., including the CO 2 solubilities derived from the compiled literature data, cannot be the cause of any miscalculation of Vtot . A third possibility is that some inclusions formed before all the CO 2 was liberated by thermal decomposition of the oxalate species. This would lead to lower CO 2 contents in the inclusions than expected, resulting in elevated molar volumes of the CO 2 vapor phase at room temperature and, in turn, Th CO 2 Ždew-point. values that are apparently too low. This explanation is unlikely because strong scattering of Th CO 2 values would be expected in individual samples if variable amounts of CO 2 had been trapped as a function of time, and presumably some inclusions would show the expected Th CO 2 , but these features are not observed. We recall that weight-loss tests following puncturing and drying the capsules after

the syntheses, and the observation that oxalates are not present in the capsule residues, both confirm that decomposition of oxalate was complete at least by the end of the syntheses. Note that the effect of compositional variations associated with the loading procedure is far below the magnitude with which we are concerned ŽTable 2.. Fourth, the discrepancies in bulk molar volume may reflect errors in the nominally applied P–T conditions of the experiments. Here again, the good agreement of the gas-loading experiments and other tests demonstrate that there were no systematic errors in measuring the applied pressure and temperature. Nevertheless, it is conceivable that the effective fluid pressure was not equal to the applied pressure. If the amount of fluid loaded were insufficient to support the capsule walls at the P–T conditions of synthesis, the capsules may have collapsed against the quartz rods, leaving the fluid in the quartz fractures at some lower effective pressure than that applied externally. To explore this possibility we calculated the degree of filling of the capsules Ž Fcap . from the following equation:

Fcap s

Vfluid n fluid Vcap y Vqtz

100

w%x ,

Ž 5.

where Vfluid is the total molar volume of the fluid wcm3rmolx at P and T of the synthesis according to Gehrig Ž1980., n fluid is the number of moles of fluid in the capsule, Vcap is the volume of the undeformed gold capsule wcm3 x, and Vqtz is the volume of the quartz rod wcm3 x at P and T of the synthesis. Values of Fcap are listed in Table 2. In Fig. 10 the degrees of capsule filling are vap compared to DVCO , the difference between our CO 2 2 molar volumes and those calculated from the data of Gehrig Ž1980.. It is likely that in some experiments Žruns 8, 15 and perhaps 14. the volume of fluid in the capsule was indeed too low to have allowed vap pressure equilibration and therefore the DVCO val2 ues are far too high. However, the other capsules show a spread in deviations Žshaded band. that is independent of the degree of filling, from which it can be inferred that the expected pressure was effectively attained. Fig. 10 also shows that, for the


173

From the residues it is clear that all the sodium oxalate also decomposes during the syntheses, presumably in a redox reaction involving water, as follows: Na 2 C 2 O4 q 2H 2 O ™ 2NaHCO 3 q H 2 .

Fig. 10. Relation between the degree of filling of the gold vap capsules at the run conditions and DVCO , the difference in CO 2 2 molar volumes between the synthetic fluid inclusions and the values derived from Gehrig’s Ž1980. data. Runs 8, 15, and possibly 14, did not contain enough fluid to permit pressure equilibration Žsee text.. Shaded band represents the observed deviations in CO 2 molar volumes of fluid inclusions synthesized from silver oxalate and the NaCl solution. Runs 55–63, using the gas-loading technique, is in good agreement with the data of Gehrig Ž1980. vap Ž DVCO s 0.. 2

dimensions of the capsules and quartz rods used, a minimum filling degree of 30% is necessary to ensure pressure equilibration. The gas-loading experivap ments in Fig. 10 lie very close to DVCO s 0, as also 2 vap seen in Fig. 9. The error bars for DVCO in run 55 2 are large because the molar volume of CO 2 varies strongly near the measured Th CO 2 value of 318C. 5.3. Reconstruction of the chemical reactions On the basis of the chemical characterization of the inclusions and of the dry residues in the gold capsules, an attempt is now made to reconstruct qualitatively the chemical reactions that may take place during the synthesis of fluid inclusions from silver oxalate and the NaCl solution. In the reactions that follow, phases that have been identified in the analyses are underlined. From reaction Ž1. above, it was deduced that the sealed gold capsules contain a mixture of AgCl Žs. , Ag 2 C 2 O4Žs. , Na 2 C 2 O4 , and H 2 O before the syntheses are started. Upon heating, the excess silver oxalate decomposes below 2008C to form CO 2 : Ag 2 C 2 O4Žs. ™ 2Ag Žs. q 2CO 2 Žaq . .

Ž 6.

Ž 7.

Both liberated species should then react with the AgCl, whether it is present as the solid phase below its liquidus temperature Že.g. the melting in the H 2 O–Ag–AgCl subsystem is at 4668C at 2000 bar, according to Chou and Frantz, 1977. or dissolved as AgCl Žaq.8 at higher temperature: 2NaHCO 3Žaq . q 2AgCl (Žaq . q H 2 Žaq .

™ 2H 2 O q 2CO2 Žaq . q 2NaCl (Žaq . q 2Ag Žs.( .

Ž 8.

According to calculations made with SUPCRT92 ŽJohnson et al., 1992., the right side of reaction Ž8. is thermodynamically favored at the run conditions, and exactly enough hydrogen is produced via reaction Ž7. to drive it to completion. However, since we observe excess reactants Ž chlorargyrite and NaHCO 3Žaq. . in the fluid inclusions at room conditions, we conclude that the reaction in fact did not run to completion. The most likely mechanisms of hindering the progress of reaction are reduction of hydrogen fugacity and of pH. Hydrogen may have been lost by diffusion through the capsule walls. The use of argon as an external pressure medium in the presence of Ni–NiO filler rods imparts an extremely low hydrogen fugacity under the run conditions, providing a sink for hydrogen from the capsules, whereas the liberation of H 2 in reaction Ž7. markedly raises the hydrogen fugacity within the capsule. If loss by diffusion were fast enough, reduction of AgCl via reaction Ž8. could have been stalled or even reversed, leading to variable fluid inclusion salinities during progressive crack-healing of the quartz. No direct evidence for hydrogen loss was found following the syntheses, as the potential weight-loss of the capsules lies outside the certainty of our determinations. According to Frantz and Eugster Ž1973. and Chou and Frantz Ž1977. the pair of species Ag Žs.8 and

174


AgCl Žaq.8 produces low pH values even at low hydrogen fugacities: ( 2AgCl (Žaq . q H 2 Žaq . ™ 2Ag Žs. q 2HCl (Žaq .

Ž 9.

although the pH is not buffered if hydrogen fugacity varies. This implies that a considerable amount of chlorine is bound as HCl Žaq.8, possibly retarding formation of NaCl. Evidence for low pH during the runs is seen in the mobilization of gold within the capsules. The gold found on the quartz overgrowths probably precipitated as the aqueous chloride complexes ŽZotov et al., 1991. decomposed during quenching. In contrast, the rods recovered from runs using gas-loading show no gold precipitates. The final observation to be addressed is the presence of NaHCO 3 in the inclusions and its absence from the solid residues. Upon opening the capsules after the experiments, escape of CO 2 may cause NaHCO 3 to decompose: NaHCO 3Žaq . ™ NaOH Žaq . q CO 2Ž g . ≠ ,

Ž 10 .

leaving only NaOH as a solid phase once the remaining water evaporates. All the reactions presented here, except for reaction Ž1., which was observed directly, have been reconstructed according to the phases identified after the experiments. No additional experiments were undertaken to examine and verify the reactions of this model in detail. Nevertheless, the qualitative understanding provided by this discussion is sufficient to draw conclusions on the reliability of silver oxalate as a reagent for the synthesis of chloridebearing fluid inclusions.

6. Conclusions The main aim of this article is to draw attention to the fact that syntheses of fluid inclusions with silver oxalate and salt solutions can produce serious problems and unexpected results, if the intention is to produce ternary CO 2 –H 2 O–NaCl mixtures of known composition. The succession of hydrothermal reactions that occurs during synthesis of the fluid inclusions is triggered by the precipitation of insoluble chlorar-

gyrite ŽAgCl. in the capsules prior to run-up. On heating to the run conditions, some chlorargyrite dissolves to form aqueous NaCl and solid silver, but considerable amounts remain until the end of the runs. The failure to reduce all the Agq to Ag 0 during the syntheses is also reflected in the presence of chlorargyrite daughter crystals in all the fluid inclusions. The shift in aqueous salinity of the inclusions at room temperature appears to be irreversible, inasmuch as our attempt to restore the ternary composition within the fluid inclusions by decomposing the chlorargyrite daughter crystals under strong UV radiation was not successful: clathrate dissociation was reduced to even lower temperatures than before. Eq. Ž8. suggests it may be possible to complete the reduction of Agq to Ag 0 prior to inclusion formation by imposing a higher external hydrogen fugacity than the Ar–Ni–NiO medium used in our experiments, but this may also lead to the production of unwanted methane. In view of these problems we recommend that silver oxalate be used only with extreme caution. Prior to drawing conclusions regarding the P–V–T–X properties of ternary CO 2 –H 2 O–NaCl fluids, the synthesized fluid inclusions should be carefully examined for the presence at room temperature of Ž chlorargyrite daughter crystals and HCOy 3Žaq. or nahcolite., and for the existence of negative correlations between Th tot and Tm Cla . We have shown that, at least for the single isopleth investigated, quite accurate total homogenisation temperatures for the ternary CO 2 –H 2 O–NaCl system can be recovered by regressing the observed Th tot –Tm Cla correlations through the initial salinity of the H 2 O–NaCl solution loaded into the experimental capsules Žthe ATh )tot methodB ., although considerable effort is required. Finally, we have demonstrated, by comparison with Gehrig’s Ž1980. independent data, that the gasloading technique of Frantz et al. Ž1992. is an excellent alternative for the synthesis of CO 2 –H 2 O–NaCl fluid inclusions.

Acknowledgements We are grateful to Adrian Liechti for technical support in the hydrothermal laboratory and to Ruth Mader ¨ for support and advice in the chemical labora-


tory. We also thank Marco Herwegh for SEM-EDS analysis, Giancarlo Rizzoli for XRD analysis, and Urs Mader, Rolf Brunner and Steve Burns for loan ¨ of equipment and helpful discussions. We appreciate the help of colleagues at the Institute of Mineralogy and Petrology, ETH-Zurich: Eric Reusser kindly pro¨ vided access to the laser Raman microprobe and assisted with analysis, and Sven Girsperger kindly enabled comparative pressure measurements to cali-

175

brate our Heise manometer. We thank Christian Schmidt, I-Ming Chou, an anonymous reviewer and the editor Robert Bodnar for their corrections, criticisms and suggestions that helped us improve the manuscript. This work was financed by Swiss Science Foundation ŽSNF. Grant 21-45639.95 to Larryn Diamond and laboratory facilities were supported by SNF Grant 20-31.152.91 to Tjerk Peters.

Appendix A. Weights of constituents and lengths of capsules used in fluid inclusion syntheses

Run no. 8 12 14 15 17 22 24 42 44 45

Gold capsule wgx 1.909348 1.933652 1.711440 1.789986 2.002078 2.061436 1.969056 1.918178 1.971114 1.671392

Gold tube wgx – 0.163040 0.157320 0.180286 0.191860 0.174612 0.166828 0.219046 0.180414 0.132324

Quartz rod wgx 0.136670 0.167834 0.124110 0.122624 0.169994 0.126660 0.195438 0.121052 0.124240 0.106312

Quartz powder wgx 0.003210 0.003064 0.003646 0.004236 0.003372 0.003520 0.003364 0.002826 0.004234 0.009886

Silver oxalate wgx 0.024628 0.031244 0.026276 0.035174 0.031758 0.032448 0.031024 0.100316 0.076290 0.072928

6-wt.% NaCl solution wgx 0.028416 0.036448 0.030888 0.041354 0.036018 0.036970 0.035872 0.114728 0.090182 0.087090

Length of capsule wmmx 29 28 25 28 29 30 29 30 29 25

Gas loading 55

Outer capsule wgx 2.176851

Inner capsule wgx 0.615670

Quartz rod wgx 0.098894

Quartz powder wgx 0.003043

CO 2 wgx

6-wt.% NaCl solution wgx 0.072580

Length of capsule wmmx 33

60 61 63

2.263674 2.242279 2.262651

0.682950 0.724736 0.720400

0.095660 0.095710 0.095642

0.002740 0.003664 0.002672

0.079708 0.083514 0.083612

35 34 34

For the gas-loading experiments, the inner gold capsule was filled with NaCl solution. In exp. 55 the mass of CO 2 was calculated from the Van der Waals equation, based on pressure difference measured in a known volume. In the other three experiments the amount of CO 2 loaded was also checked after the runs by puncturing the capsules and determining H 2 O–CO 2 by weight loss. The lengths of gold cap-

0.018236 "0.000150 0.020238 0.021029 0.021279

sules have been used to calculate the degree of capsule filling ŽTable 2..

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