Reequilibrationof fluid inclusions - Geochemistry - Virginia Tech

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
rjb@vt.edu
INTRODUCTION
Aqueous fluids containing various
amounts of salts are common in many geologic
environments. In most environments, NaCl, KCl or
CaCl 2 is the dominant salt, but MgCl 2 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 H 2 O (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 H 2 O-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 H 2 O-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 H 2 O-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 H 2 O-NaCl, owing to its
importance not only in geologic studies, but also
in many industrial and engineering applications.
PVTX data for H 2 O-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
H 2 O-NaCl eutectic temperature (-21.2°C).
Properties of H 2 O-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 H 2 O-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.
H 2O-NaCl SYSTEM
PTX Topology of the H 2 O-NaCl System
The H 2 O-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 H 2 O-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 H 2 O-NaCl liquidvapor
two-phase region is bounded by:
81

FIG.4-1. Distorted, schematic PT projection of the H 2 O-NaCl liquid-vapor envelope. TP H2 O = H 2 O triple point (T
= 0.01ºC, P = 0.006 bar); CP H2 O = H 2 O 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 H 2 O, that
extends from the triple point of pure water
(TP H2 O: T = 0.01ºC, P = 0.006 bar) to the critical
point of H 2O (CP H2 O: 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 H 2 O 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
the NaCl triple point (TP NaCl : T = 801ºC, P ≈ 1
bar);
• the NaCl liquid -vapor curve that extends from
the NaCl triple point to the NaCl critical point
(CP NaCl : T ≈ 3327ºC, P ≈ 235 bars);
• the locus of critical points that extends from the
critical point of H 2 O 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
82

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 H 2 O-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.
Determination of Inclusion Compos ition
Before one can interpret microthermo -
metric data obtained from fluid inclusions, the
composition of the inclusions must be known so
that PVTX data for the appropriate chemical
system may be used. Compositions of fluid
inclusions represented by the H 2 O-NaCl system
are easily determined from temperatures of phase
changes during microthermometric analysis.
Figure 4-2 is a T-X projection of the H 2 O-NaCl
system showing the phases that are stable at
various temperature-composition conditions. Note
that vapor is present and in equilibrium everywhere
on the diagram.
For salinities

FIG.4-3. Phase behavior of an H 2 O-NaCl fluid inclusion with a salinity of 10 wt.% during heating from low
temperature. At -46°C the inclusion contains a glassy solid that devitrifies at the eutectic temperature
(-21.2°C) to produce a fine-grained mixture of hydrohalite and ice. With slight heating above the eutectic
temperature the hydrohalite disappears and the ice re-crystallizes to form several large crystals. With
continued heating, the ice crystals gradually dissolve, leaving a single small crystal at -7°C, which dissolves
completely at -6.6°C.
TABLE 4-1.
Salinities (wt.%) corresponding to measured freezing point depressions (degrees
Celsius) calculated according to Bodnar (1993)
FPD .0 .1 .2 .3 .4 .5 .6 .7 .8 .9
0. 0.0 0.2 0.4 0.5 0.7 0.9 1.1 1.2 1.4 1.6
1. 1.7 1.9 2.1 2.2 2.4 2.6 2.7 2.9 3.1 3.2
2. 3.4 3.6 3.7 3.9 4.0 4.2 4.3 4.5 4.7 4.8
3. 5.0 5.1 5.3 5.4 5.6 5.7 5.9 6.0 6.2 6.3
4. 6.5 6.6 6.7 6.9 7.0 7.2 7.3 7.5 7.6 7.7
5. 7.9 8.0 8.1 8.3 8.4 8.6 8.7 8.8 9.0 9.1
6. 9.2 9.3 9.5 9.6 9.7 9.9 10.0 10.1 10.2 10.4
7. 10.5 10 6 10.7 10.9 11.0 11.1 11.2 11.3 11.5 11.6
8. 11.7 11.8 11.9 12.0 12.2 12.3 12.4 12.5 12.6 12.7
9. 12.9 13.0 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8
10. 13.9 14.0 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9
11. 15.0 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9
12. 16.0 16.1 16.2 16.2 16.3 16.4 16.5 16.6 16.7 16.8
13. 16.9 17.0 17.1 17.2 17.3 17.3 17.4 17.5 17.6 17.7
14. 17.8 17.9 18.0 18.0 18.1 18.2 18.3 18.4 18.5 18.6
15. 18.6 18.7 18.8 18.9 19.0 19.1 19.1 19.2 19.3 19.4
16. 19.5 19.5 19.6 19.7 19.8 19.8 19.9 20.0 20.1 20.2
17. 20.2 20.3 20.4 20.5 20.5 20.6 20.7 20.8 20.8 20.9
18. 21.0 21.0 21.1 21.2 21.3 21.3 21.4 21.5 21.5 21.6
19. 21.7 21.8 21.8 21.9 22.0 22.0 22.1 22.2 22.2 22.3
20. 22.4 22.4 22.5 22.6 22.7 22.7 22.8 22.9 22.9 23.0
21. 23.1 23.1 23.2
84

FIG.4-4. Vapor-saturated phase relations in the NaCl- H 2 O system at low temperatures constructed from data
presented in Hall et al. (1988), Sterner et al. (1988) and Bodnar et al. (1989). I = ice; L = liquid; HH =
hydrohalite; H = halite; P = peritectic (0.1°C, 26.3 wt.% NaCl); E = eutectic (-21.2°C, 23.2 wt.% NaCl).
relating the freezing-point depression to salinity
according to:
Salinity (wt.%) = 0.00 + 1.78 - 0.0442 2
+ 0.000557 3 (1)
where is the freezing point depression (FPD) in
degrees Celsius [Note that FPD is simply the
negative of the freezing temperature. Thus, a
freezing temperature of -10°C corresponds to an
FPD of 10 degrees Celsius]. Equation (1)
reproduces the original experimental data of Hall et
al. (1988) to better than ±0.05 wt.% NaCl at all
temperatures from 0.0°C to -21.2°C, the eutectic
temperature for H 2 O-NaCl. Table 4-1 lists the
salinity as a function of freezing-point depression
(FPD) in 0.1 degree Celsius increments for compositions
ranging from pure water to the eutectic
composition, calculated using equation (1).
While this chapter does not consider the
practical aspects of fluid inclusion microthermo -
metry, it is worth noting for the beginning
inclusionist that it is not always easy (or possible)
to determine the eutectic temperature or the final
ice-melting temperature accurately. Often, the
formation of ice during cooling cannot be
observed, and little or no difference in appearance
is seen during heating to determine eutectic and
final ice melting, especially for small or thin
(tabular) inclusions. For a detailed discussion of
techniques for studying such inclusions, the
reader is referred to Goldstein & Reynolds (1994;
their Chapter 7) and Wilson et al. (2003).
Inclusions having salinities between the
eutectic composition (23.2 wt.%) and the peritectic
composition (26.3 wt.%) also freeze to produce a
mixture of ice and hydrohalite when cooled to low
temperatures. However, when these inclusions
are heated to the eutectic temperature the phase
that is completely consumed (melts) is the ice,
leaving hydrohalite and liquid in the inclusion.
With further heating the hydrohalite dissociates
and disappears at some temperature between the
eutectic (-21.2°C) and the peritectic (0.1°C)
temperatures (Fig. 4-4). Hydrohalite does not
easily recrystallize during heating from the
eutectic and remains as many small crystals, unlike
ice, which normally recrystallizes to form a few
large crystals (see Fig. 4-3). The salinity of an
inclusion in which hydrohalite is the last solid to
dissolve is determined from the known relationship
between salinity and hydrohalite dissociation
temperature as presented by Sterner et al. (1988)
and Bodnar et al. (1989). It should be noted that
hydrohalite dissociation can be very sluggish
compared to ice melting and hydrohalite has been
known to persist metastably for several minutes to
hours at temperatures above 0.1°C (Roedder,
1984). Inclusions with salinities between the
85

eutectic and peritectic compositions are rarely
reported in the literature. This may reflect the
failure to recognize hydrohalite as the last phase
to melt in the inclusion; many reports of "icemelting"
may in fact be hydrohalite dissociation.
A slow response in the rate of melting of the solid
during heating of a frozen inclusion is an
indication that the phase being monitored is
hydrohalite rather than ice.
Inclusions with salinities greater than the
peritectic composition (26.3 wt.% NaCl) should
show halite as the last solid phase to disappear
during heating from low temperatures (Fig. 4-2). In
practice, inclusions with salinities less than 30-35
wt.% NaCl often fail to nucleate a halite crystal,
even during repeated thermal cycling at
temperatures below 0°C. For this reason, there are
relatively few reported salinities in the range 25-35
wt.% in the literature.
While freezing data for pure H 2 O-NaCl
inclusions that contain halite cannot be used to
determine the salinity, most (all) natural
inclusions do not contain pure H 2 O-NaCl. As
such, freezing of natural halite-bearing inclusions
can provide valuable information concerning the
chemical system that should be used to interpret
the microthermometric data obtained from the
inclusions. When halite-bearing inclusions are
cooled for the first time, generally all of the halite
does not react with the solution to produce
hydrohalite, and it is necessary to warm the
inclusion to a temperature where the reaction
proceeds. After all the halite disappears, the
inclusion can be cooled a second time to produce
the equilibrium assemblage. A more detailed
discussion of the behavior of halite-bearing
inclusions during freezing, and application of
these data to determine fluid compositions, is
given in Samson & Sinclair (1992).
The solubility of halite under vaporsaturated
conditions has been determined by
Sterner et al. (1988), and the data have been fitted
to an equation describing salinity as a function of
halite dissolution temperature according to:
Salinity (wt.%) = 26.242 + 0.4928 + 1.42 2
- 0.223 3 + 0.04129
4 + 6.295 x 10
-3
5
- 1.967 x 10 -3 6 + 1.1112 x 10 -4
7
(2)
where = T(°C)/100. Equation (2) accurately
represents the solubility of NaCl in water from the
peritectic temperature (0.1°C) to the NaCl triple
point (801°C). Salinities calculated using Equation
(2) are shown on Figure 4-5 and listed in Table 4-2.
During heating of a halite-bearing fluid
inclusion, it is possible for the halite crystal to
disappear at a temperature higher than, lower than,
or at the same temperature as the vapor
bubble (Fig. 4-6). Equation (2) is theoretically
FIG.4-5. Solubility of NaCl in water under vaporsaturated
conditions calculated using the FORT -
RAN program SALTY (Bodnar et al., 1989)
TABLE 4-2. Halite solubility (in wt.%) as a function of temperature [Tm(halite)] calculated using equation (2).
Tm (halite) 0 10 20 30 40 50 60 70 80 90
0 26.2 26.3 26.4 26.5 26.7 26.8 27.0 27.2 27.4 27.7
100 28.0 28.3 28.6 28.9 29.3 29.7 30.1 30.5 30.9 31.4
200 31.9 32.4 32.9 33.5 34.1 34.7 35.3 36.0 36.7 37.4
300 38.2 38.9 39.8 40.6 41.5 42.4 43.3 44.3 45.3 46.4
400 47.4 48.5 49.7 50.8 52.0 53.3 54.5 55.8 57.1 58.4
500 59.8 61.1 62.5 63.9 65.3 66.8 68.2 69.6 71.1 72.5
600 74.0 75.4 76.9 78.3 79.7 81.1 82.5 83.9 85.3 86.6
700 87.9 89.2 90.5 91.8 93.0 94.2 95.4 96.6 97.7 98.9
800 100.
86

FIG.4-6. Series of photomicrographs depicting the
three different modes of homogenization possible
for halite-bearing fluid inclusions. All inclusions
have a salinity of 40 wt.% NaCl, and the scale bar
represents 25 micrometers for each inclusion.
(from Bodnar, 1994).
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)
valid only for fluid inclusions in which the halite
and vapor bubble disappear at the same
temperature, i.e., homogenization occurs along the
three phase liquid + vapor + halite curve (Fig. 4-1).
However, equation (2) can be used to approximate
the salinity within a few percent for inclusions in
which liquid -vapor homogenization is higher than
the halite dissolution temperature by several tens
of degrees (see Chou, 1987). If the liquid-vapor
homogenization temperature is less than the halite
dissolution temperature, then trapping must have
occurred at a P-T condition such that the
inclusion isochore intersects the halite liquidus
before intersecting the liquid-vapor curve during
cooling (Isochore "C", Fig. 4-7).
During heating, halite dissolution, and
therefore total homogenization, occurs along the
halite liquidus (Bodnar, 1994). For inclusions that
homogenize along the liquidus by halite
dissolution, equation (2) ma y over or
underestimate the salinity in the inclusion,
depending on the slope of the liquidus in P-T
space, which varies as a function of salinity
according to (Bodnar, 1994):
dT/dP (°C/kbar) =
–38.38 + 0.90 S – 0.0029 S 2 (3)
where "S" is the inclusion salinity in weight
percent NaCl.
The position of the 50 wt.% NaCl
liquidus is independent of pressure (i.e., dT/dP =
0), and an inclusion containing ~50 wt.% NaCl will
show the same halite dissolution temperature
(423ºC) regardless of whether dissolution occurs
along the 3-phase curve or along the liquidus at
higher pressures (Fig. 4-8). For lower salinities,
the slope of the liquidus (dT/dP) is negative,
meaning that the actual salinity in the inclusion is
higher than that predicted by equation (2). Conversely,
the slope of the liquidus is positive 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.
87

FIG.4-8. Halite liquidi in the NaCl-H 2 O 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 H 2 O-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
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 H 2 O-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 H 2 O-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 H 2 O-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 H 2 O-NaCl fluid inclusion
FIG.4-9. Vapor pressure curves (labeled in wt.% NaCl) for H 2 O-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).
88

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.
H 2 O-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 H 2 O-
NaCl 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 (4)
where dP/dT is the slope of the iso-Th line
(˜ isochore), Th is the homogenization
temperature in degrees Celsius, and "a S ", "b S ", and
"c S " are salinity-dependent fitting parameters
defined by:
a S = 18.28 + 1.4413 S + 0.0047241 S 2
– 0.0024213 S 3 + 0.000038064 S 4 (5)
b S = 0.019041 – 1.5268 x 10 -2 S
+ 5.6012 x 10 -4 S 2 – 4.2329 x 10 -6 S
3
– 3.0354 x 10 -8 S
4
(6)
c S = –1.5988 x 10 -4 + 3.6892 x 10 -5 S
– 1.9473 x 10 -6 S 2 + 4.1674 x 10 -8 S 3
– 3.3008 x 10 -10 S
4
(7)
Equation (4) predicts the slope of iso-Th lines for
H 2 O-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 H 2O-NaCl
inclusions and are shown on Figures 4-10 and 4-
11. Densities along these iso-Th lines can be
approximated using data for PVT properties of
H 2 O-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
H 2 O-NaCl to P-T conditions beyond those shown
here. Zhang & Frantz (1987) determined slopes of
isochores for a range of compositions in the NaCl-
KCl-CaCl 2 -H 2 O 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.
Interpretation of Inclusions Trapped in a Two-
Phase (Immiscibility) Field
Fluid inclusions having compositions
approximated by H 2 O-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 H 2 O-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 &
89

FIG.4-10. Iso-Th lines for NaCl-H 2O inclusions having salinities of 0, 5, 10, 15, 20 and 25 wt.% NaCl calculated
using data from Bodnar & Vityk (1994).
90

FIG.4-11. Liquidi and iso-Th lines for NaCl-H 2 O
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
H 2 O-NaCl solutions having salinities =30 wt.%
NaCl, and described the relationship between
salinity and the critical temperature (Tc) as:
T C (ºC) = 374.1 + 8.800 f + 0.1771 f 2
– 0.0211 f 3 + 7.334 x 10 4 f 4 (8)
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.
In the H 2 O-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 P-
T diagram. Ideally, the technique that would be
used to define the P-T formation conditions for
H 2 O-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 H 2 O-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
91

FIG.4-12. P-X diagram for the system H 2 O-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 H 2 O-
NaCl solutions (Fig. 4-9). The intersection of the
vapor pressure isopleth corresponding to the
92

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 (P f ) 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 H 2 O-NaCl.
OTHER AQUEOUS SYSTEMS
H 2 O-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 H 2 O-
NaCl-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 H 2 O-NaCl is -21.2°C,
whereas the eutectic for the ternary H 2 O-NaCl-KCl
is -22.9°C (Fig. 4-14). Owing to the difficulty in
recognizing first
93

FIG.4-14. Isotherms in the vapor-saturated ice field in the H 2 O-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 H 2 O-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).
94

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).
H 2 O-NaCl-CaCl 2
Fluid inclusions approximated by the
H 2 O-NaCl-CaCl 2 system are common in many
environments, including sedimentary basins and
medium to high-grade metamorphic rocks. Fluid
inclusions containing H 2 O-NaCl-CaCl 2 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 H 2O-NaCl (-21.2°C) and H 2O-NaCl-
KCl (-22.9°C) eutectics, and these are usually
interpreted to indicate the presence of calcium or
other divalent cations in solution. In some cases,
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 H 2O-NaCl-CaCl 2 system freeze to
form a mixture of ice, hydrohalite and antarcticite
(CaCl 2 •6H 2 O). Eutectic melting is first observed at -
52°C during heating (Fig. 4-17). Except for
extremely CaCl 2 -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. 4-
18). 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
FIG.4-16. Vapor-saturated solubility relations in the H 2 O-NaCl-KCl system calculated using equations in
Bodnar et al. (1989).
95

FIG.4-17. Vapor-saturated phase equilibria in the H 2 O-NaCl-CaCl 2 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 H 2 O-NaCl-CaCl 2 inclusions to coarsen
the phases, making it easier to identify the phases
FIG.4-18. Isotherms (in degrees Celsius) of the ice
liquidus at 1 atmosphere pressure in the H 2O-
NaCl-CaCl 2 system (modified after Oakes et al.
1990).
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
H 2 O-NaCl-CaCl 2 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 &
96

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 H 2 O-NaCl-
CaCl 2 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.
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 H 2 O-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.
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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