No. 9616

1
No. 96-16
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 331.46.6ºC 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.
ltMicroscopic observation at room temperaturegt
0molCO2 (Sterner et al., 1988) 0molCO2
(Potter et al. 1977) 0molCO2 (Bodnar.,
1994) 5molCO2 (Schmidt et al., 1995)
10molCO2 (Schmidt and Bodnar, 2000)
5molCO2 (this study)
ltFeature of synthetic fluid inclusions gt
???
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
ltSetting of P-T conditionsgt
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
ltSynthesis of Fluid Inclusionsgt
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
ltMeasurement of halite solubilitiesgt
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-5CO2 solution in this study.
ltCO2 source use of silver oxalategt
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). s 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?2Ag2CO2)
Oxalic acid(H2C2O4)
ltspecificationgt
A schematic diagram of an experimental apparatus
equipped in this study.
Maximum Temperature 650ºC Maximum Pressure
145MPa
(Krüger and Diamond, 2001)
References
ltDetermination of halite solubilitygt
100
Bakker, R. J. (1999) Chem. Geol. 154,
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Geochim. Cosmochim. Acta 58, 1053-1063. Bowers,
T. S. and Helgeson H. C. (1983) Geochim.
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(1987) Geochim. Cosmochim. Acta 51, 1965-1975.
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J. L. R. and Zheng, Y. F. (2001) J. Metamolphic
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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 T500 ºC, P110.55.8MPa, and 5
mol CO2 graphically.
60
NaCl solubility (wt. )
40
Cooling
Heating
experimental temperature
Ambient temperature
20
Temperature (ºC)
0
200
400
600
800