Bodnar (1993): Introduction to aqueous fluid systems. This is chapter 4 from: I. Samson, A. Anderson, and D. Marshall: F
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Bodnar RJ (2003) Introduction to aqueous fluid systems. In I. Samson, A. Anderson, & D. Marshall, eds. Fluid Inclusions: Analysis and Interpretation. Mineral. Assoc. Canada, Short Course 32, 81-99.
CHAPTER 4. INTRODUCTION TO AQUEOUS -ELECTROLYTE FLUID INCLUSIONS Robert J. Bodnar The Bubble Factory Virginia Tech Blacksburg, VA 24061 USA [email protected] INTRODUCTION Aqueous fluids containing various amounts of salts are common in many geologic environments. In most environments, NaCl, KCl or CaCl2 is the dominant salt, but MgCl2 or LiCl can be present in significant amounts in some environments. Before the 1960s, essentially all gas -free aqueous inclusions were interpreted using PVTX data for H2O (cf. Kennedy 1950, Skinner 1953, Kalyuzhnyy 1960), for two reasons. First, fluid inclusion microthermometric and analytical techniques were rather primitive, precluding the possibility of determining which salts were present or even the total salt content. Secondly, even if the nature of the salts in the inclusions could be determined, the PVTX data for salt solutions needed to interpret the microthermo metric results were almost non-existent. The 1960s saw the introduction of a reasonably accurate cooling stage for measuring ice-melting temperatures of aqueous inclusions (Roedder 1962), and the application of these data to determine salinities of fluid inclusions (Roedder 1963). At about this same time, workers were beginning to determine the composition of inclusions using a variety of bulk extraction techniques (Roedder 1958; see also Roedder 1990). Also, in 1962, the classic paper by Sourirajan & Kennedy (1962), describing the PTX properties of the H2O-NaCl system at elevated temperatures and pressures was published. Finally, techniques were available to estimate salinities of aqueous inclusions, and PTX data were available to interpret the results. The next three decades saw a significant increase in the number of studies of the H2O-NaCl system, as well as other aqueous-salt systems, improving the accuracy of available data and extending the database to significantly higher temperatures, pressures and salinities. In 1977, Potter & Brown (1977) published a summary of available PVT data for H2O-NaCl to 500°C and 2,000 bars, allowing workers to estimate isochores for aqueous inclusions. Hilbert (1979) extended these data to
600°C and 4,000 bars, and Bodnar (1985) extended the range of PVT data to salinities of 70 wt.%. The best-studied binary aqueous electrolyte system is H2O-NaCl, owing to its importance not only in geologic studies, but also in many industrial and engineering applications. PVTX data for H2O-NaCl have been used to interpret results from fluid inclusions which show no detectable gases during normal microthermo metric and/or crushing analysis, and for those inclusions which show first melting of ice near the H2O-NaCl eutectic temperature (-21.2°C). Properties of H2O-NaCl are also commonly used to interpret microthermometric data from inclusions with much lower first melting temperatures (indicating the presence of cations other than Na or K), owing to the lack of PVTX data for most other aqueous electrolyte systems. In this chapter PVTX properties of aqueous electrolyte systems are summarized and the application of these data to interpretation of aqueous fluid inclusions is presented. Because data for the H2O-NaCl system are more complete than those for other aqueous electrolyte systems, this system will be described in detail. The methodology and problems associated with obtaining microthermometric data have been discussed in detail by other workers (Roedder 1984, Goldstein & Reynolds 1994, Goldstein 2003) and are not included here. H2O-NaCl SYSTEM PTX Topology of the H2O-NaCl System The H2O-NaCl system is an example of a binary system in which the solubility curve does not intersect the critical curve (Morey 1957); that is, the system exhibits a critical curve that is continuous between the critical points of the two end-members. The H2O-NaCl system is characterized by a large region of PTX space in which fluid immiscibility, represented by coexisting higher salinity liquid and lower salinity vapor, is possible (Fig. 4-1). The H2O-NaCl liquidvapor two-phase region is bounded by:
FIG.4-1. Distorted, schematic PT projection of the H2O-NaCl liquid-vapor envelope. TPH 2O = H2O triple point (T = 0.01ºC, P = 0.006 bar); CPH 2O = H2O critical point (T = 374.1ºC, P = 220 bars); TP NaCl = NaCl triple point (T = 801ºC, P = approx. 1 bar); CP NaCl = NaCl critical point (T = approx. 3327ºC, P = approx. 235 bars); E = eutectic point (L + V + I + HH, T = -21.2ºC, P = 0.001 bar, 23.2 wt% NaCl); P = peritectic point (L + V + HH + H, T = 0.1ºC, P = 0.004 bar, 26.2 wt.% NaCl). Within the shaded region, fluid immiscibility to produce a high salinity liquid in equilibrium with a lower salinity vapor is possible (modified from Bodnar et al. 1985a). The shaded area of the inset is a schematic representation of the liquid-vapor two -phase region for a composition of 20 wt.% NaCl. • the liquid-vapor curve for pure H2O, that extends from the triple point of pure water (TPH 2O : T = 0.01ºC, P = 0.006 bar) to the critical
the NaCl triple point (TPNaCl: T = 801ºC, P ≈ 1 bar); • the NaCl liquid -vapor curve that extends from the NaCl triple point to the NaCl critical point (CPNaCl: T ≈ 3327ºC, P ≈ 235 bars); • the locus of critical points that extends from the critical point of H2O to the critical point of NaCl. Within the region of P-T space bounded by these various phase surfaces (labeled "L + V" on Fig. 4-1), a fluid with a given bulk composition may exist as either a single-phase liquid or singlephase vapor, or may split into two coexisting phases (Sourirajan & Kennedy 1962, Bodnar et al. 1985a). The shaded region of Figure 1 (labeled "L + V) represents the complete P-T range over which liquid-vapor immiscibility is possible. For any
point of H 2O (CPH 2O : T = 374.1ºC, P = 220 bars);
• the locus of liquid-vapor-ice triple points (L+V+I) or ice-melting curve that extends from the triple point of H2O to the eutectic point (E, I+L+V+Hydrohalite (HH); T = -21.2°C, P ˜ 0.001 bars); • the locus of liquid-vapor-hydrohalite triple points (L+V+HH) that extends from the eutectic to the peritectic, (P, L+V+Halite (H) +HH; T = 0.1°C, P ˜ 0.004 bars); • the locus of liquid-vapor-halite triple points (L+V+H) that extends from the peritectic (P) to
particular composition, the region of immiscibility occupies a somewhat smaller region of P-T space, as described by Bodnar et al. (1985a). For example, an H2O-NaCl fluid with a bulk composition of 20 wt.% NaCl would become immiscible within the shaded region labeled "L+V 20 wt.% NaCl" shown in the inset on Figure 4-1, but would exist as a single -phase fluid (either liquid or vapor, depending on the P-T conditions) at temperatures and pressures outside of the shaded region. How one determines the compositions of the coexisting liquid and vapor phases within the two-phase region is described below.
For salinities 50 wt.% NaCl, and the actual salinity in the inclusion will be lower than that predicted by equation (2). The magnitude of the error in both cases depends upon the pressure in the inclusion at halite dissolution, as well as on the salinity. However, if it is assumed that the pressure will generally not be higher than about 2 kbars (otherwise the inclusion would decrepitate; see Bodnar et al., 1989), possible errors range from an underestimate of the salinity by ~1.3 wt.% for a halite dissolution temperature of 200°C, to an overestimate of the salinity by ~2.6 wt.% for a halite dissolution temperature of 600°C.
FIG.4-7. Pressure-temperature diagram showing the three different modes of homogenization possible for a 40 wt.% NaCl fluid inclusion and the P-T fields in which the inclusions were trapped. The halite in inclusion "A" will dissolve at about 323°C, followed by bubble disappearance at 500°C. Inclusion "C" will display vapor bubble disappearance at 200°C followed by halite dissolution at ~300°C. Both the vapor bubble and halite in inclusion "B" will disappear at 323°C. (modified from Bodnar, 1994)
reequilibrated following entrapment (see Bodnar, 2003), and if there is evidence that the inclusions were trapped in a boiling or immiscible fluid system, then the homogenization temperature is equal to the trapping temp erature. In this case, the trapping pressure is equal to the vapor pressure in the inclusion at the temperature of homogenization. Atkinson (2002) recently developed an empirical equation describing the vapor pressure of H2O-NaCl solutions as a function of salinity and temperature. Figure 4-9 shows vapor pressure curves calculated from the equations presented by Atkinson (2002). Isopleths (lines of constant composition) shown on Figure 4-9 and labeled in wt.% NaCl represent the liquid-limb or bubble-curve (see Diamond, 2003, his Fig. 3-4) for an H2O-NaCl solution having the composition indicated on each curve. Thus, the isopleth labeled "60" represents the bubble curve for a 60 wt.% NaCl composition. Along this line a liquid with a salinity of 60 wt.% NaCl coexists with a vapor phase of lower salinity. The salinity of the vapor phase varies along the line according to PTX relations in the H2O-NaCl system (Sourirajan & Kennedy 1962, Bodnar et al. 1985a). [At the critical point the salinities of the liquid phase and the vapor phase would be equal.] The homogenization conditions (the temperature and the pressure in the inclusion at the moment of homogenization) of any H2O-NaCl fluid inclusion
FIG.4-8. Halite liquidi in the NaCl-H2O system. Along each liquidus line a liquid with the salinity indicated (in wt.% NaCl) is in equilibrium with halite. L+V+H refers to the vapor-saturated halite solubility curve. (modified after Bodnar, 1994). Vapor Pressures of H2O-NaCl Solutions One of the goals of most fluid inclusion studies is to determine the temperature and pressure of formation of the inclusions and, by inference, the host phase formation conditions. The first step in this process is to measure the homogenization temperature of the inclusions. If there is no evidence that the fluid inclusions have
FIG.4-9. Vapor pressure curves (labeled in wt.% NaCl) for H2O-NaCl calculated using equations in Atkinson (2002). Numbers (0, 10, 20, 30 40) along the locus of critical points represent the critical points for solutions having the salinity listed (wt.% NaCl).
with a composition of 60 wt.% NaCl that homogenizes to the liquid phase must lie along the 60 wt.% isopleth. If fluid inclusions are trapped in the one phase fluid field, then the homogenization temperature (along the isopleth corresponding to the inclusion composition) represents the minimum temperature of formation. In this case, a pressure correction must be added to the measured homogenization temperature to obtain the trapping temperature. The trapping temperature must lie along the line of constant density (or volume) that originates on the liquid-vapor curve and extends into the one-phase field (Figs. 4-10, 4-11). The first step in estimating the trapping temperature is to determine the starting point for the constant density line, or isochore, on the bubble curve. This point corresponds to the measured temperature of homogenization and the bubble curve pressure at the homogenization temperature.
Equation (4) predicts the slope of iso-Th lines for H2O-NaCl solutions having salinities from 0-40 wt.% NaCl, and homogenization temperatures from 50 to 700°C or the critical temperature, whichever is lower. Slopes of iso-Th lines predicted by equation (4) are valid to the upper limits of the experimental data, which is 6 kbars. Iso-Th lines calculated from equation (4) have been used to construct "isochore" diagrams for H2O-NaCl inclusions and are shown on Figures 4-10 and 411. Densities along these iso-Th lines can be approximated using data for PVT properties of H2O-NaCl solutions along the liquid-vapor curve (cf. Bodnar 1983). PVT data required to extend isochores beyond the range indicated in Figs. 4-10 and 4-11 do not presently exist. Some workers (cf. Anderko & Pitzer 1993) have developed theoretical equations of state to predict PVT properties of H2O-NaCl to P-T conditions beyond those shown here. Zhang & Frantz (1987) determined slopes of isochores for a range of compositions in the NaClKCl-CaCl2-H2O from 300° to 700°C and 1-3 kbars. Bakker & Brown (2003) summarize the various numerical models that are available to determine PVTX properties of inclusion fluids.
H2O-NaCl Isochores Once the composition, homogenization temperature and vapor pressure in the inclusion at homogenization have been determined, it is necessary to determine the slope of the isochore along which the inclusion was trapped in order to estimate a pressure correction. The relationship between trapping temperature and pressure, salinity, and homogenization temperature for H2ONaCl inclusions has been determined using the synthetic fluid inclusion technique (Bodnar & Vityk 1994). The results are represented by an equation of the form: dP/dT (bar/°C) = a S + b S * Th + c S * Th 2
Interpretation of Inclusions Trapped in a TwoPhase (Immiscibility) Field Fluid inclusions having compositions approximated by H2O-NaCl and trapped in a boiling or immiscible fluid system are common in many geologic environments, including terrestrial geothermal systems and their fossil equivalents, the epithermal precious-metals deposits (Bodnar et al. 1985b), and magmatic-hydrothermal ore deposits associated with silicic magmas (Bodnar 1992, 1995, Beane & Bodnar 1995, Roedder & Bodnar 1997). Inclusions trapped under conditions of immiscibility are valuable P-T indicators because the homogenization temperature equals the formation temperature (Roedder & Bodnar 1980), eliminating the need for a pressure correction to obtain the trapping temperature. The complete range of P-T conditions over which immiscibility may occur in the H2O-NaCl system is unknown, although experimental (Bodnar et al. 1985a) and theoretical (Pitzer 1984) studies indicate that the two-phase region extends to at least 2 kbars and temperatures in excess of 3,000°C (Fig 4-1). For any composition, the two phase region extends to temperatures at least as high as the critical temperature. Knight &
where dP/dT is the slope of the iso-Th line (˜ isochore), Th is the homogenization temperature in degrees Celsius, and "aS", "bS", and "c S" are salinity-dependent fitting parameters defined by: a S = 18.28 + 1.4413 S + 0.0047241 S2 – 0.0024213 S3 + 0.000038064 S4
b S = 0.019041 – 1.5268 x 10-2 S + 5.6012 x 10-4 S2 – 4.2329 x 10-6 S3 – 3.0354 x 10-8 S4
c S = –1.5988 x 10-4 + 3.6892 x 10-5 S – 1.9473 x 10-6 S2 + 4.1674 x 10-8 S3 – 3.3008 x 10-10 S4
FIG.4-10. Iso-Th lines for NaCl-H2O inclusions having salinities of 0, 5, 10, 15, 20 and 25 wt.% NaCl calculated using data from Bodnar & Vityk (1994).
In the H2O-NaCl system, as in any twocomponent system, the comp ositions of the coexisting phases at any P-T condition are defined by the isopleths that intersect at that point on a PT diagram. Ideally, the technique that would be used to define the P-T formation conditions for H2O-NaCl inclusions trapped in the two-phase field would be to determine the salinities of the coexisting vapor-rich and liquid-rich inclusions. Then, these data would be referred to the appropriate phase diagram for H2O-NaCl to determine the unique P-T condition at which these two compositions may coexist. For example, consider a 20 wt.% NaCl composition at some P-T condition within the two-phase, liquid + vapor field, as shown by the star in the inset in Figure 4-1. Assuming that the P-T conditions are 700°C and 1 kbar, the two phases that are in equilibrium are a 4 wt.% NaCl vapor and a 49 wt.% NaCl liquid (Bodnar et al. 1985a). At room temperature, inclusions that trapped the vapor phase will be vapor-rich with a small rim of low-salinity (4 wt.%) liquid, and inclusions that trapped the liquid phase will contain a halite crystal and a smaller vapor bubble (Fig. 4-12). Assuming that inclusions trapped only the vapor or only the liquid phase, both the vapor-rich and the halite-bearing inclusions would homogenize at 700°C, which is equal to the trapping temperature. Unfortunately, this "ideal" approach generally cannot be used to determine the P-T formation conditions for inclusions trapped in the two-phase field, for several reasons. First, it is well known that the vapor-rich inclusions almost always trap some small amount of liquid along with the vapor phase (Bodnar et al. 1985a,b). Therefore, the salinity determined from the freezing-point depression of the liquid in the vapor-rich inclusion does not represent the composition of the vapor phas e present at trapping but, rather, some salinity intermediate between the vapor and liquid compositions. However, even if inclusions which trapped only vapor could be identified and their salinities determined, data for the P-T locations of low salinity is opleths beyond the critical point are scarce and not of sufficient accuracy to adequately constrain the P-T formation conditions (Sourirajan & Kennedy 1962, Bodnar et al. 1985a). Finally, the homogenization temperatures of the vapor-rich inclusions generally cannot be determined with
FIG.4-11. Liquidi and iso-Th lines for NaCl-H2O inclusions having salinities of 30 and 40 wt.% NaCl. Constructed from data in Bodnar (1994), Cline & Bodnar (1994) and Bodnar & Vityk (1994). Bodnar (1989) determined the critical properties for H2O-NaCl solutions having salinities =30 wt.% NaCl, and described the relationship between salinity and the critical temperature (Tc ) as: TC (ºC) = 374.1 + 8.800 f + 0.1771 f 2 – 0.0211 f 3 + 7.334 x 104 f 4
where f is the salinity in weight percent NaCl. These data indicate two -phase behavior up to at least 800°C and 1.5 kbars for a 30 wt.% NaCl solution. This range includes the P-T conditions of many crustal magmatic-hydrothermal systems.
FIG.4-12. P-X diagram for the system H2O-NaCl showing compositions of coexisting phases in the liquid + vapor region as a function of pressure at 700°C. Any pressure-composition combination under the 700°C solvus is in the two-phase liquid + vapor field where a higher salinity liquid is in equilibrium with a lower salinity vapor phase. For example, a fluid with a bulk composition of 20 wt.% NaCl at 700°C and 1 kbar is in the two -phase field, and would split into a liquid with a salinity of 49 wt.% NaCl and a vapor with a salinity of 4 wt.% NaCl. At room temperature the inclusions would appear as shown schematically and by the photographs of fluid inclusions trapped in the two-phase field. sufficient accuracy to confirm that they were trapped at the same P-T condition as the coexisting halite-bearing inclusions owing to the inability to visually estimate when the vapor phase fills the inclusion (Bodnar et al. 1985a,b, Sterner 1992). The procedure that is recommended to define formation conditions for fluid inclusions trapped in the two-phase (liquid + vapor) field includes a combination of petrographic and PVTX techniques. If immiscibility is suggested, based on careful observation of a Fluid Inclusion Assemblage (FIA) (Goldstein & Reynolds 1994) along growth zones and/or healed fractures, the salinities and homogenization temperatures of the liquid-rich (halite-bearing) inclusions are determined. It should be noted here that if there is petrographic evidence to suggest immiscibility, and the halite-bearing, liquid-rich inclusions homogenize by halite dissolution (at a temperature higher than the vapor-bubble disappearance temperature), then the halite-bearing and vaporrich inclusions can not represent an immis cible pair. Phase equilibrium constraints do not permit
inclusions that homogenize by halite dissolution (i.e., those trapped in Field "C", Fig. 4-7) to be trapped in equilibrium with a vapor phase (see Roedder & Bodnar 1980, Bodnar 1994), except along the three-phase (liquid + vapor + halite) curve (L+V+H, Fig. 4-8). However, even in this case, halite dissolution can only occur at a temperature higher than vapor disappearance if the inclusion traps halite along with the liquid phase. Assuming that the halite-bearing inclusion trapped the liquid phase (and only the liquid phase) in an immiscible fluid system, the composition of the inclusion is determined from the halite dissolution temperature, and the trapping temperature is equal to the homogenization temperature. Once trapping in the two-phase field is confirmed from petrographic observations and the salinity and homogenization temperature of the liquid-rich (usually halite-bearing) inclusions have been determined, these data are referred to bubble-point or vapor-pressure curves for H2ONaCl solutions (Fig. 4-9). The intersection of the vapor pressure isopleth corresponding to the
FIG.4-13. Recommended technique for determining the trapping conditions for a 40 wt.% NaCl fluid inclusion trapped in the two -phase (liquid + vapor) field. See text for explanation. inclusion composition (i.e., the line labeled "40" on Fig. 4-13) with the measured homogenization temperature [Th(L-V)] in P-T space defines the pressure (Pf ) at the time of trapping. Thus, a halitebearing inclusion with a halite dissolution temperature of ~323°C, corresponding to a salinity of 40 wt.% NaCl, and a homogenization temperature of 500°C would have been trapped at about 480 bars according to Figure 13. The vapor phase that is in equilibrium with a 40 wt.% NaCl liquid at 500°C and 500 bars has a salinity of about 1 wt.% NaCl (Bodnar et al. 1985a). Thus, the icemelting temperature of the vapor-rich inclusions that coexist with the halite-bearing inclusions should be -1.7°C, or lower if the inclusions trapped some liquid along with the vapor. Consistent ice-melting temperatures higher than 1.7°C might indicate that the vapor-rich and halitebearing inclusions are not coeval, assuming that the experimental data for compositions of coexisting phases at this temperature and pressure are correct and that the inclusion compositions are adequately described using PVTX data for the system H2O-NaCl.
OTHER AQUEOUS SYSTEMS H2O-NaCl-KCl In magmatic -hydrothermal systems associated with granitic magmas, the dominant cations in solution are usually Na and K (Burnham 1979, 1997). In this case, PVTX data for the H2ONaCl-KCl system are most appropriate for interpreting fluid inclusion microthermometric data. Phase relations in the low temperature (icestable) region of the ternary have been determined by Hall et al. (1988), and those in the high temperature (sylvite ± halite stable) region have been determined by Sterner et al. (1988). A Fortran model describing phase equilibria in the entire ternary system was developed by Bodnar et al. (1989). If fluid inclusions contain two phases (liquid and vapor) at room temperature, one would generally not be able to determine if the inclusions contain both NaCl and KCl based on microthermo metric analysis. The eutectic temperature for the system H2O-NaCl is -21.2°C, whereas the eutectic for the ternary H2O-NaCl-KCl is -22.9°C (Fig. 4-14). Owing to the difficulty in recognizing first
FIG.4-14. Isotherms in the vapor-saturated ice field in the H2O-NaCl-KCl system (modified after Hall et al. 1988). melting during heating of frozen inclusions, it is unlikely that one would be able to distinguish between inclusions that begin to melt at -21.2° and those that start to melt at -22.9°C. The system H2O-NaCl-KCl is most often used to interpret microthermometric results from fluid inclusions that contain both halite and sylvite daughter minerals. Such inclusions are common in many granitic rocks, and are nearly ubiquitous in porphyry copper deposits (Bodnar 1992, 1995, Bodnar & Beane 1980, Roedder & Bodnar 1997). In most cases, the composition of inclusions containing both halite and sylvite is such that the sylvite daughter mineral dissolves first, followed by the halite. In any case, halite and sylvite are easily distinguished based on the
behavior during heating from room temperature to 150°C. At temperatures between the ternary eutectic (-22.9°C) and approximately 150°C, NaCl shows retrograde solubility in the presence of a KCl-saturated solution, whereas KCl solubility in an NaCl-saturated solution increases with temperature over this same temperature range. Thus, asan inclusion containing halite and sylvite is heated from room temperature, the sylvite phase dissolves noticeably while the halite phase grows as NaCl precipitates. Halite precipitation during heating to 150°C is most often manifest as a noticeable sharpening of the corners of the halite crystal (compare the appearance of halite at 25°C and 100°C; Fig. 4-15).
FIG.4-15. Behavior during heating of fluid inclusions containing sylvite (S) and halite (H) daughter minerals. (modified from Sterner & Bodnar 1984).
The composition of halite + sylvitebearing inclusions is determined from the temperatures of dissolution of the two phases. As long as both phases are present, the composition of the liquid phase is defined by the halite-sylvite cotectic (Fig. 4-16). After dissolution of one of the phases, the liquid composition moves toward either the NaCl corner (sylvite dissolves first) or the KCl corner (halite dissolves first). The bulk composition of the inclusion is defined by the temperature of dissolution of the last phase, using PTX data for the ternary system (Fig. 4-16).
these low temperature events do not represent eutectic melting but, rather, represent metastable (or stable) crystallization of the inclusion contents. For a more detailed discussion of low temperature behavior in complex aqueous inclusions, the reader is referred to Davis et al. (1990) and Samson & Walker (2000). Most two -phase (liquid + vapor) inclusions in the H2O-NaCl-CaCl2 system freeze to form a mixture of ice, hydrohalite and antarcticite (CaCl 2•6H2O). Eutectic melting is first observed at 52°C during heating (Fig. 4-17). Except for extremely CaCl2-rich compositions, antarcticite will disappear at the eutectic, leaving a fine-grained mixture of ice and hydrohalite in the liquid phase. With continued heating the liquid composition follows the hydrohalite-ice cotectic (Fig. 4-18) until the hydrohalite phase completely disappears. The path then proceeds into the ice field and moves towards the ice corner with continued heating. The bulk composition is defined by the intersection of the melting path with the appropriate isotherm in the ice-stable field (Fig. 418). For example, if hydrohalite disappears at -25°C and ice melts at -10°C, the inclusion would have a composition indicated by the open circle on the 10°C isotherm on Figure 18. In practice, it is very difficult to distinguish between ice and hydrohalite, and to determine the
H2O-NaCl-CaCl2 Fluid inclusions approximated by the H2O-NaCl-CaCl2 system are common in many environments, including sedimentary basins and medium to high-grade metamorphic rocks. Fluid inclusions containing H2O-NaCl-CaCl2 are most often identified based on low first melting temperatures observed during freezing studies. The eutectic in this ternary system is ˜ -52°C (Fig. 4-17), usually resulting in recognizable melting at temperatures in the range -40° to -50°C. Many workers report "eutectic events" at temperatures well below the H2O-NaCl (-21.2°C) and H2O-NaClKCl (-22.9°C) eutectics, and these are usually interpreted to indicate the presence of calcium or other divalent cations in solution. In some cases,
FIG.4-16. Vapor-saturated solubility relations in the H2O-NaCl-KCl system calculated using equations in Bodnar et al. (1989).
FIG.4-17. Vapor-saturated phase equilibria in the H2O-NaCl-CaCl2 system showing isotherms (in degrees Celsius) of halite solubility and ice-melting. (modified after Vanko et al. 1988). temperature at which the hydrohalite disappears, during the initial heating sequence owing to the fine-grained nature of these phases. Haynes (1985) described a technique involving sequential freezing of H2O-NaCl-CaCl2 inclusions to coarsen the phases, making it easier to identify the phases
and determine the melting temperatures. Samson & Walker (2000) described a cryogenic Raman technique that can be used to detect the presence or absence) of hydrohalite in fluid inclusions during low-temperature microthermometry. Fluid inclusions approximated by the H2O-NaCl-CaCl2 system and containing halite daughter minerals at room temperature have been reported from many different geologic environments, including submarine hydrothermal systems. Ideally, the composition can be determined by measuring the temperature of hydrohalite dissolution along the hydrohalitehalite cotectic, followed by measurement of the halite dissolution temperature at higher temperature. However, the temperature at which the last hydrohalite crystal dissolves is difficult to determine accurately because the crystals are often small and melting is extremely sluggish compared to melting of ice or halite. And, because the hydrohalite isotherms intersect the hydrohalite-halite cotectic at a low angle, a small error in the temperature of hydrohalite dissolution represents a relatively large error in the Na/Ca ratio and in the "take off" point into the halite field. Moreover, as the Na/Ca ratio changes, the point of intersection with the halite dissolution isotherm changes, resulting also in an error in the total salinity of the inclusion (Williams-Jones &
FIG.4-18. Isotherms (in degrees Celsius) of the ice liquidus at 1 atmosphere pressure in the H2ONaCl-CaCl2 system (modified after Oakes et al. 1990).
Samson 1990). To avoid these problems, Vanko et al. (1988) and Williams -Jones & Samson (1990) used the ice-melting temperature and the halite dissolution temperatures to estimate compositions of halite-bearing fluid inclusions in the H2O-NaClCaCl2 system. This approach introduces relatively little error, as evidenced by comparing known and calculated compositions of synthetic fluid inclusions (Vanko et al. 1988). As an example, a halite-bearing inclusion in which ice melts at -25°C and halite dissolves at 350°C would have a composition indicated by the open circle on the 350°C isotherm shown on Figure 4-17.
above 573K. Geochim. Cosmochim. Acta 57, 1657-1680. A TKINSON, A.B. (2002): A model for the PTX properties of H2O-NaCl. Unpublished M.S. Thesis, Virginia Tech, Blacksburg VA 133pp. BAKKER, R.J. & BROWN , P.E. (2003): Computer modelling in fluid inclusion research. In I. Samson, A. Anderson, & D. Marshall, eds. Fluid Inclusions: Analysis and Interpretation. Mineral. Assoc. Can., Short Course Ser. 32, 175-204. BEANE, R.E. & BODNAR, R.J. (1995): Hydrothermal fluids and hydrothermal alteration in porphyry copper deposits. In Pierce, F.W. and Bohm, J.G., Porphyry Copper Deposits of the American Cordillera. Arizona Geological Society Digest 20, Tucson, AZ p. 83-93.
SUMMARY Fluid inclusions containing aqueous solutions with no detectable gases are arguably the most common type of fluid inclusion in most geologic environments. Interpretation of microthermo metric data from these inclusions requires PTX data to estimate the inclusion composition, and PVT data to determine trapping conditions. Many aqueous fluid inclusions are approximated by the H2O-NaCl system. While more complex aqueous fluid compositions are common in many environments, our ability to interpret these inclusions is hampered by (1) the difficulty of observing and identifying phase changes in complex aqueous solutions (especially in small, natural inclusions) and (2) the paucity of PVTX data to interpret these more complex compositions.
BODNAR, R.J. (1983): A method of calculating fluid inclusion volumes based on vapor bubble diameters and P-V-T-X properties of inclusion fluids. Econ. Geology 78, 535-542. BODNAR, R.J. (1985): Pressure-volumetemperature-composition (PVTX) properties of the system H2O-NaCl at elevated temperatures and pressures. Unpub. Ph. D. Dissertation, The Pennsylvania State University, University Park, PA, 183 pp. BODNAR, R. J. (1992): Can we recognize magmatic fluid inclusions in fossil hydrothermal systems based on room temperature phase relations and microthermo metric behavior?. Geological Survey of Japan, report No. 279, p. 26-30.
ACKNOWLEDGEMENTS Much of the information presented in this chapter represents studies by former students and post-doctoral researchers and visitors to the Fluids Research Laboratory, especially Don Hall, Charlie Oakes, Mike Sterner and Max Vityk. Phil Brown, Jean Cline and Iain Samson are thanked for their comments and suggestions on an earlier version of this manuscript. The National Science Foundation, Department of Energy, NASA, and the American Chemical Society have supported work in the Fluids Research Laboratory over the years. NSF Grants EAR-0001168 and EAR-0125918 provided support during preparation of this manuscript.
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