No. 96-16

1. Td (ºC) NaCl No. 96-16 (1) To sample in place (2) To avoid changes after sampling 350 P=110±5MPa 5mol%CO2 300 250 200 150 28 30 32 34 36 38 40 42 Concentration of starting solutions (wt. %) AN EXPERIMENTAL INVESTIGATION OF THE EFFECT OF CO2 ON NaCl SOLUBILITY IN HYDROTHERMAL SOLUTIONS USING SYNTHETIC FLUID INCLUSIONS NAGASEKI, Hiroki and HAYASHI, Ken-ichiro Department of Mineralogy, Petrology, and Economic Geology, Graduate School of Science, Tohoku University, Sendai, Miyagi, Japan Discussion Abstract Experimental technique: Synthetic fluid inclusions Results Synthetic fluid inclusions proposed by Sterner and Bodnar (1984) were chosen as an experimental technique. 42 The purpose of this paper is to evaluate the temperature dependence of the effect of CO2 on NaCl solubility in hydrothermal solutions. Many geochemists have reported sodium chloride solubility in water. However, the effect of CO2 has rarely been considered, even though most natural hydrothermal fluids have a composition of H2O-NaCl-CO2 system. Recently, Schmidt et al. (1995) have suggested that 5 mol % CO2 lower NaCl solubility by 1 weight percent around 320ºC, but its temperature dependence have not been considered. This study may contribute to improve the equations of state for the system H2O-NaCl-CO2, and describe hydrothermal system more precisely. Synthetic fluid inclusion proposed by Sterner and Bodnar (1984) was chosen for experimental technique. Fluid inclusions were synthesized in pre-fractured quartz core. Experimental solution contains 30-40 wt % NaCl and 5 mol % CO2. Silver oxalate (Ag2C2O4) was used as a CO2 source. The core, silica powder, and starting materials were loaded into a gold capsule and held for 7-14 days. After experiment, the core was sectioned, and halite dissolution temperatures (TmNaCl) of synthesized inclusions were measured using heating-freezing stage. Measured TmNaCl gave solubility of NaCl. A total of NaCl solubility experiments was successfully completed. A solubility curve of halite for 160-320ºC was obtained in CO2-bearing brine. It shows that halite solubility in CO2-bearing water was about one percent lower than that in CO2-free water (Sterner et al., 1988). This study determined TmNaCl as 331.4±6.6ºC for 40 wt % NaCl and 5 mol % CO2- bearing aqueous fluid. Our data is in close agreement with that of Schmidt et al. (1995). Schmidt and Bodnar (2000) have suggested that halite becomes about 1 wt % less soluble in 10 mol % CO2-bearing water than that of 5 mol %. Our result indicates that solubility of NaCl goes down further as an increase in CO2 concentration. <Microscopic observation at room temperature> : 0mol%CO2 (Sterner et al., 1988) : 0mol%CO2 (Potter et al. 1977) : 0mol%CO2 (Bodnar., 1994) : 5mol%CO2 (Schmidt et al., 1995) : 10mol%CO2 (Schmidt and Bodnar, 2000) : 5mol%CO2 (this study) <Feature of synthetic fluid inclusions > □◇○＋■ This figure shows the comparison of the solubility of NaCl obtained in this study with literature. It shows that halite solubility in CO2-bearing water was about one percent lower than that in CO2-free water (Sterner et al., 1988) within temperatures of this study (160-320 ºC). This study determined TdNaCl in composition of 40 weight NaCl as 331.4 ºC in 5 mol % CO2- bearing fluid. This data was in close agreement with data in Schmidt et al. (1995). Schmidt and Bodnar (2000) have suggested that halite becomes about 1% less soluble in 10 mol% CO2-bearing water than that of 5 mol%. This study indicated that solubility of NaCl goes down as an increase in CO2 concentration. 40 D) A) B) C) This method makes it possible to trap high pressure-temperature fluid in place. After sampling, thick quartz wall prevents fluid from oxidation by air, leakage of volatiles, or contamination. 38 <Setting of P-T conditions> Phase diagram of (H2O-35 wt% NaCl) - CO2 pseudobinary is shown. As the pressure rising, liquid- vapor coexisting region expands to the higher temperature. The higher pressure, boundaries of liquid- vapor coexisting regions make a convergence. Shaded area shows one-phase region at 110MPa. Composition of the inclusions should be equal to that of starting material. Therefore, solution should be homogeneous in experimental P-T. For example, figure says that the maximum CO2 concentration that homogeneous fluid is exist is about 6 mol% when T and P are 500ºC and 110MPa, respectively. To consider the constraint mentioned above, we decided experimental parameters as follows: Experimental temperature and pressure : 500ºC and 110MPa , respectively NaCl concentration: 30-40 weight percent,CO2 concentration: 5 mol %. 36 NaCl solubility (wt. %) 34 E) Examples of fluid inclusions in quartz synthesized in this study. These photographs shows that all fluid inclusions contain halite daughter mineral, and have the same composition. It means that fluid inclusion syntheses was completed successfully. Scale bars shows 20µm. A) 29.96 wt% NaCl + 5.0 mol % CO2. (run. 31) B) 31.94 wt% NaCl + 5.0 mol % CO2. (run. 43) C) 35.04 wt% NaCl + 5.0 mol % CO2. (run. 39) D) 39.96 wt% NaCl + 5.0 mol % CO2. (run. 27) E) 37.70 wt% NaCl + 5.0 mol % CO2. (run. 34) 32 30 Modified after Bowers and Helgeson (1983). Introduction <Synthesis of Fluid Inclusions> 200 350 150 250 300 Temperature (ºC) Keeping experimental P-T for 3-14 days in an autoclave the core, starting materials (H2O, NaCl, CO2 source) quartz powder (for acceleration of quartz recrystallization) Hydrothermal fluid in the system H2O-NaCl-CO2 is widely distributed on the earth; for instance, hydrothermal ore-forming fluid (Dugdale et al., 2001), hot spring (Imai, 2002), volcanic fluid (Chiodini et al., 2001), metamorphic fluid (Fu et al., 2001), carbonatite-related fluid (Genge et al., 2001), and submarine hydrothermal system (Kelley and Früh-green, 2001). Many geochemists have reported sodium chloride solubility in water above 100ºC (e.g. Benrath et al., 1937; Keevil, 1942; Potter et al., 1977; Chou, 1987; Sterner et al., 1988), however, the effect of CO2 on halite solubility have not been taken into consideration. Recently, Schmidt et al. (1995), Schmidt and Bodnar (2000) have suggested that 5 or 10 mol % CO2 lower NaCl solubility in 1 or 2 weight percent at about 320ºC. Because halite solubility curve considered the effect of CO2 have not been obtained, researchers have been forced to ignore the effect of CO2 on halite solubility in discussions about hydrothermal fluid. Thus, to obtain NaCl solubility curve make it possible to expand and improve equations of state for the system H2O- NaCl- CO2 (e.g. Bakker, 1999; Duan et al., 1995; Duan and Sun, 2003) and describe their history or behavior of hypersaline hydrothermal fluid more precisely. This is also useful for salinity measurement in fluid inclusions. Hypersaline fluid inclusions usually contain halite daugher crystal in room temperature when its NaCl concentration is above 26 weight percent. In this case, NaCl dissolution temperature (TdNaCl) is measured using heating- freezing stage. The TdNaCl is compared with published solubility curve (e.g. Sterner et al., 1988), and salinity is obtained. This study contributes to measure the salinity more precisely if CO2 concentrations are known using laser-Raman spectrometry, etc.. The purpose of this study is to evaluate an effect of CO2 on NaCl solubility in hydrothermal fluid with experimental approach. 3-4mm Conclusions quenching (using deionized water at room temperature) heating(450ºC) 2-4cm <Measurement of halite solubilities> Synthetic fluid inclusions in compositions of 30-40 wt% NaCl and 5 mol % CO2 were examined with microthermometric observation, and halite solubility curve in CO2-bearing fluid is obtained. This shows that 5 mol percent of CO2 lowers halite solubility by about one weight percent at 160-320 ºC. Halite solubilities in 5 mol% CO2-bearing solutions are 30.0 wt% in 166.0 ºC and 40.0 wt% in 331.4ºC. Halite solubility in condition of 40 wt% NaCl and 5 mol % CO2 obtained in this study is in close agreement with that of Schmidt et al. (1995). When 10 mol% CO2 added, halite is assumed to become less soluble according to Schmidt and Bodnar (2000) who have reported that solubility of NaCl in 10 mol% CO2-bearing water is lower than that of 5 mol% CO2 by about 1 wt. %. sealing with welding Au capsule TdNaCl polished section for microscopic observation Quartz core drying TdNaCl of these inclusions were measured. This table shows the measurement of TdNaCl in H2O-NaCl-5%CO2 solution in this study. <CO2 source: use of silver oxalate> Silver oxalate (Ag2C2O4) was used in this study. Silver oxalate was chosen to handle easier than dry ice (Shmulovich and Plyasunova, 1993) or using gas pipette to load gas directly (Frantz et al., 1989). Experimental condition Ambient condition Silver nitrate(AgNO3) T : formation temperature of fluid inclusions (ºC) P : formation pressure of fluid inclusions (MPa) salinity : NaCl concentration of starting solution in wt% relative to H2O. CO2 : CO2 concentration of the solution in mol% relative to H2O. TdNaCl : NaCl dissolution temperature (ºC). σ : standard deviation of TdNaCl. n : number of measured inclusions. Ag2C2O4 (Silver oxalate) precipitation CO2 generation mixing Heating! Experimental apparatus : High pressure-temperature autoclave Decomposes at about 140ºC (Ag2C2O4→2Ag＋2CO2) Oxalic acid(H2C2O4) <specification> A schematic diagram of an experimental apparatus equipped in this study. Maximum Temperature: 650ºC Maximum Pressure: 145MPa (Krüger and Diamond, 2001) References <Determination of halite solubility> 100 Bakker, R. J. (1999) Chem. Geol. 154, 225-236. Benrath, A., Gjedebo, F., Schiffers, B., and Wunderlich, H. (1937) I. Zeitschr. anorg. und alleg. Chemie 231, 285-297. Bodnar, R. J. (1994) Geochim. Cosmochim. Acta 58, 1053-1063. Bowers, T. S. and Helgeson H. C. (1983) Geochim. Cosmochim. Acta 47, 1247-1275. Chiodini, G., Marini, L. and Russo, M. (2001) Geochim. Cosmochim. Acta 65, 2129–2147. Chou, I. M. (1987) Geochim. Cosmochim. Acta 51, 1965-1975. Duan, Z., Møller, N., and Weare, J. H. (1995) Geochim. Cosmochim. Acta 59, 2869-2882. Duan, Z. and Sun, R. (2003) Chem. Geol. 193, 257-271. Dugdale, A. L., and Hagemann, S. G. (2001) Chem. Geol. 173, 59–90. Frantz, J. D., Zhang, Y. -G., Hickmott, D. D., and Hoering, T. C. (1989) Chem. Geol. 76, 57-70. Fu, B., Touret, J. L. R. and Zheng, Y. F. (2001) J. Metamolphic Geol. 19, 531-547. Genge M. J., Balme M., and Jones A. P. (2001) J. Volcanol. Geotherm. Res. 106, 111-122. Imai, H. (2002) Shigen Chishitsu, 52, 69-75. Keevil, N. B. (1942) J. Am. Chem. Soc. 64, 841-850. Kelley, D. S. and Früh-green, G. L. (2001) Geochim. Cosmochim. Acta 65, 19, 3325–3346. Krüger, Y., and Diamond, L. W. (2001) Chem. Geol. 173, 159–177. Potter, R. W., Babcock, R. S., and Brown, D. L. (1977) J. Res. U. S. Geol. Survey 5, 389-395. Schmidt, C., Rosso, K. M., and Bodnar, R. J. (1995) Geochim. Cosmochim. Acta 59, 3953-3959. Schmidt, C., and Bodnar, R. J. (2000) Geochim. Cosmochim. Acta 64, 3853-3869. Shmulovich, K. I., and Plyasunova, N. V. (1993) Geochem. Int. 30, 53-71. Sterner, S. M., and Bodnar, R. J. (1984) Geochim. Cosmochim. Acta 48, 2659-2668. Sterner, S. M., Hall, D. L., and Bodnar, R. J. (1988) Geochim. Cosmochim. Acta 52, 989-1005. Vanko, D. A., Bodnar, R. J. and Sterner, S. M. (1988) Geochim. Cosmochim. Acta 52, 2451-2456. Halite solubility was measured by microthermometry. Hypersaline fluid inclusions usually contain halite daughter crystal when its NaCl concentration is above 26 weight percent. NaCl dissolution temperature (TdNaCl) was measured using heating- freezing stage. There is positive correlation between temperature and halite solubility in aqueous fluid (e.g. Vanko et al., 1988). Hence, halite daughter crystal dissolves gradually as temperature increases, and finally dissolves completely. We recorded this temperature as TdNaCl at this time. Because halite concentration of a fluid inclusion equals halite solubility in TdNaCl, halite solubility is obtained in this way. NaCl dissolution temperature 80 This figure shows NaCl dissolution temperatures of NaCl-H2O-CO2 fluid inclusions synthesized at T=500 ºC, P=110.5±5.8MPa, and 5 mol% CO2 graphically. 60 NaCl solubility (wt. %) 40 Cooling Heating experimental temperature Ambient temperature 20 Temperature (ºC) 0 200 400 600 800