45
Chemical Geology (Isotope GeoscienceSection), 94 ( 199 1) 45-54
Elsevier Science Publishers
B.V., Amsterdam
Measurement of the hydrogen and oxygen isotopic
compositions of concentrated chloride brines and brines from
fluid inclusions in halite
G.D. Koehler, D. Chipley and T.K. Kyser
Department of GeologicalSciences.Universityof Saskatchewan,Saskatoon,Sask.S7N 0 WO,Canada
(Received January 9, I99 1; revised and accepted June 5, 199 I )
ABSTRACT
Koehler, G.D., Chipley, D. and Kyser, T.K., 1991. Measurement of the hydrogen and oxygen isotopic compositions of
concentrated chloride brines and brines from fluid inclusions in halite. Chem. Geol. (Isot. Geosci. Sect.), 94: 45-54.
Solutions of l-5 m NaCI, KCl, CaC12 and MgC12, and synthetic brines were analyzed for hydrogen and oxygen isotopic
compositions by vacuum distillation-microequilibration
techniques. No effect on the hydrogen isotopic composition was
observed for any of the solutions within the precision of the technique of It Sob determined from analyses of distilled
water. Oxygen isotopic compositions of solutions measured by vacuum distillation-microequilibration
indicate that oxygen isotopes are not fractionated during analysis in NaCl, KC1 and CaCl, solutions. Vacuum distillation-microequilibration analyses of MgC12 solutions show depletions in “0 relative to pure water that can be related to the concentration of
Mg’+ in the solution. The greatest depletion in IgO occurs in the 4 m MgC12 solutions which have 6”O-values 6Oh lower
than that measured for pure water. Corrections based on the measurement of the 6’80-values of the pure MgC12 solutions
can also be applied to mixed chloride brines. Both melting and crushing of halite grown in solutions of known isotopic
composition followed by microequilibration
to determine the stable isotopic composition of fluid inclusions give results
within the precision determined from analyses of distilled water.
1. Introduction
Many of the techniques used for analysis of
the isotopic composition of brines are slight
modifications of the techniques used to analyze dilute waters. The validity of these techniques commonly used to determine the hydrogen and oxygen isotopic compositions of
brines have been questioned becauseof the effect of cations on the structure of water (Sofer
and Gat, 1972, 1975; Stewart and Friedman,
1975; Horita and Matsuo, 1986; Horita and
Gat, 1988; Ho&a, 1989). Brines of a few millilitres are usually analyzed for oxygen isotope
ratios by Cot-brine equilibration (Epstein and
Mayeda, 1953) and corrections are made for
the effect of various ions on the activities of
0168-9622/91/$03.50
0 1991 Elsevier Science Publishers
the isotopic species (Sofer and Cat, 1972).
Hydrogen isotope ratios are usually determined by vacuum distillation of the water from
l-5 ~1of brine and decomposition of the water
on hot uranium (Bigeleisen et al., 1952).
Smaller brine samples, of < 1 ml, can be analyzed by vacuum distillation of the pure water
from the brine, followed by C02-HZ0 microequilibration and decomposition of the water
for simultaneous hydrogen and oxygen isotope
ratio determinations although the chemical
compositions of the brines may affect the results (Kishima and Sakai, 1980).
Since the pioneering work of Taube ( 1954)
on the effect of ions in solution on the 18O/16O
ratio of COZ in equilibrium with chloride and
other electrolyte solutions, many studies have
B.V. All rights reserved.
46
demonstrated that water molecules are selectively incorporated into the hydration spheres
of ions in solution resulting in a change in the
activity of ‘*O and D relative to 160 and H in
the remaining water (e.g., Sofer and Gat, 1972,
1975; Stewart and Friedman, 1975). Therefore, the stable isotope ratios of hydrogen and
oxygen in brines by gas-liquid equilibration
usually reflect the isotope activity ratios, and
theseratios are difficult to compare to the more
informative isotope concentration ratios measured by water decomposition. Various correction factors that depend on the type and concentration of ions in solution have beenderived
to adjust hydrogen and oxygen isotope activity
ratios to isotope concentrations ratios for chloride brines (Sofer and Gat, 1972, 1975; Stewart and Friedman, 1975). However, for very
small samples of brine such as those obtained
from fluid inclusions in minerals, it is difficult
to measurethe concentrations of ions required
to correct isotope values from activity ratios to
concentration ratios. Also, these correction
factors, particularly in concentrated Mg- or
Ca-chloride solutions, are not well defined as
demonstrated by conflicting hydrogen correction values obtained by different studies (e.g.,
Sofer and Gat, 1975; Stewart and Friedman,
1975). To avoid corrections for ions in solution and allow analysis of hydrogen and oxygen isotope ratios on the same small sample of
brine, the water can be removed from the brine
by vacuum distillation and the pure water analyzed by C02-HZ0 equilibration for oxygen
isotope concentration ratios and decomposition on hot uranium (750°C) for hydrogen
isotope concentration ratios (Kishima and
Sakai, 1980). However, based on limited data,
Horita and Matsuo ( 1986) and Horita ( 1989)
suggestthat significant fractionations of hydrogen and oxygen isotopes occur during vacuum distillation of brines due to precipitation
of hydrated phasesor hydroxides. They have
proposedalternative analytical proceduresthat
require large volumes of brine and complex
distillation procedures that are not only labo-
G.D. KOEHLER
ET AL.
rious, but cannot be applied to small samples.
The purpose of this study is to present a
technique that allows simple and accurate
measurement of the stable isotopic compositions of water from chloride brines of even the
limited volume in fluid inclusions. Fractionation corrections are not necessaryusing this
technique except for Mg-rich brines which require a minimal correction for oxygen isotope
ratios only. This lack of corrections neededto
determine directly the concentration ratios of
hydrogen and oxygen isotopes from chloride
brines using this technique suggeststhat extraction and vacuum distillation of water from
fluid inclusions in chloride minerals will not
involve significant isotopic fractionations.
2. Methods, techniques,and material studied
2.1. Hydrogen isotopeanalyses
The hydrogen isotopic composition of small
samples of water and brine are routinely determined using vacuum distillation followed by
quantitative decomposition of the water over
hot uranium (Fig. 1) as described by Bigeleisen et al. ( 1952). Small samples ( l-5 ~1) of
water or brine are introduced into the vacuum
line by injection with a micro-syringe through
a rubber septum in the injection port. All the
water is distilled from the brine into a cold trap
at - 196°C by heating the injection port to
250°C for 3-5 min. with a heat gun. Small
amounts of non-condensible gassesremaining
after vacuum distillation, which may represent
dissolved atmospheric gasses or leakage
through the injection port, are evacuatedfrom
the system. The sample is then exposedto uranium at 750°C and the isotopic composition
of the resultant Hz measured on the mass spectrometer. Replicate analyses of pure water
standards indicate a reproducibility of &I-values of + l%o using this vacuum distillation
method.
MEASUREMENT
OFTHE
HYDROGEN
AND OXYGEN
ISOTOPIC
COMPOSITIONS
To
Vacuum
TO
Vacuum
Equilibralifm
vessel
volume = l-2 ml.
chalwalm
sample vessel
Fig. 1. Apparatus used for hydrogen isotope analysis and loading microequilibration vesselsfor hydrogen and oxygen
isotope analysis of waters, synthetic brines, and fluid inclusion brines in halite.
2.2. Hydrogen and oxygen isotopeanalysesby
microequilibration
Determination of the hydrogen and oxygen
isotopic composition of small samples
( < 1 ml) of water or brine is accomplished using the vacuum distillation-microequilibration technique of Kishima and Sakai ( 1980).
A brine or water sample of l-5 ~1is distilled in
vacuum using the same procedure as for hydrogenisotope analysisdescribedin Section 2.1
and the resulting pure water is frozen into a
small volume ( 1-2 ml) equilibration vessel
(Fig. 1) along with a known quantity (60-70
pmol) of pure CO2 of known isotopic composition. The equilibration vesselsare then immersed in a water bath regulated to 25 t 0.1 ‘C
for 48 hr.
After equilibration, the CO2 and the water
are separated by differential freezing using a
dry-ice slush (-78°C). Both the quantity and
stable isotopic composition of the CO2 are
measured. The remaining water is quantitatively decomposed to H2 by reaction with hot
uranium to obtain its hydrogen isotopic composition. The amount of hydrogen gas produced, which representsthe quantity of water
in the equilibration vessels,is measured manometrically (Fig. 1) .
GL80-valuesof the water before equilibration are calculated by mass balance:
(&I*o) bH*O I+ (&0d (2k0,)
= G%ko) (nHzO
)+(&0,Wk0,)
(1)
where 6’ and af represent the isotopic compositions of the H20 and CO2 before and after
equilibration, respectively;and n representsthe
number of moles of H20 and C02. SfiZo is calculated from SLo, assuming equilibrium fractionation at 25 ‘C, and a C02-Hz0 fractionation factor of 1.0412 ( O’Neil et al., 1975) . The
effect of both water vapour and liquid water,
which are present in the equilibration vessels,
on the calculated aI80 of the water was found
4x
to be small ( < 0. l%o). Because this effect is
small compared to the error in the method, it
was omitted from the calculation of the
6’80-value of the water.
Samples that differ by > 1 pmol in the
amount of, or by OS%0 in the G”C-value of,
the CO1 introduced and extracted from the
sample vesselsarediscarded. Differences in the
amount or carbon isotopic composition of the
CO2 before and after equilibration result from
leakageof CO* during equilibration or loss and
fractionation of COZ during the loading or extraction process. Samples that have a large
amount of non-condensible gassesat any stage
during the loading or extraction procedure are
also discarded, as again leakageis suspected.
The reproducibility of values using this
method was determined from repeat analyses
of NBS standards and an in-house distilled
water standard, designatedUofs (for University of Saskatchewan) Standard, which has
6D- and G”O-values of - 136 and - 16.7%0,
respectively. Repeat analyses of UoIS Standard by vacuum distillation-microequilibration indicate reproducabilities for the method
of + 5%0for hydrogen and _’ 1%o for oxygen
(Fig. 2).
A modification of the microequilibration
technique of Kishima and Sakai ( 1980) resulted in a reduction in the number of samples
lost due to leakage of the microequilibration
vessels,Water produced from vacuum distillation of brines or extracted from fluid inclusions in halite was frozen into a length of
4-mm-ID Pyrexm tubing which servesas a microequilibration vessel. A measured quantity
of CO2 of known isotopic composition is frozen into the tubing, and the tube sealed at an
appropriate length ( - 8 cm) with a torch. Extraction consists of breaking the sealedtube in
vacuum and separation of the COZ and water
for analysis. This modification produces similar precision of SD- and 6180-values (Fig. 2),
but is simpler, faster, and more reliable than
microequilibration using the reusable microequilibration vessels.
GD.
KOEHLEK
ET AL.
Fig. 2. Differences between the hydrogen and oxygen isotopic compositions of UofS (University of Saskatchewan) Standard water analyzed by standard vacuum distillation for hydrogen and CO*-HZ0 equilibration for
oxygen and those obtained by the vacuum distillationmicroequilibration technique. d6D and dS’*O are the
b-Dknown -cm ITllWX~“lllb~ZWO” and
6’80 known d’80 m,croequiiibratlon,
respectively, and the cross represents
the values determined from repeat analysesusing vacuum
distillation for hydrogen isotope analysis and C02-HI0
equilibration for oxygen isotope analysis. Errors for the
vacuum distillation-microequilibration technique as indicated by Uofs Standard are + 5O%a
for m-values and
-+ I%0 mil for 6i80-values. Circlps represent analyses using reusable valved microequilibration vesselsand triangles represent analysesusing sealed glasstubes.
2.3. Fluid inclusions in halite
Brines trapped as fluid inclusions in halite
are extracted by melting or crushing the halite
in vacuum. In the melting method, -0.5 g of
clean halite is placed into a molybdenum boat
and preheatedat 110’ C for 1 hr. in vacuum to
drive off any adherent water. The temperature
of the molybdenum boat is then raised slowly
to - 800” C using an induction furnace to melt
the halite and releasethe water from fluid inclusions (Knauth and Beeunas, 1986). In the
crushing method, -0.5 g of clean halite is
placed in a stainless-steelpipe, which is closed
at one end and connected to the vacuum system at the other. After preheating for 1 hr. at
110’ C to remove any adherent water, the halite is crushed by squeezingthe pipe in a vice.
The pipe is reconnectedto the vacuum line and
heated to 350°C for w-30 min. to collect the
water liberated by crushing (O’Neil et al.,
1986). The water collected using both these
methods is analyzed for its stable isotopic
MEASUREMENT
OF THE HYDROGEN
AND OXYGEN
ISOTOPIC
composition using the microequilibration
technique described in Section 2.2.
2.4. Preparation and analysis of materials
TABLE I
Chemical compositions of solutions for isotopic analyses
Molality
pH
MgCI,:
Im
2m
3m
4m
5m
Molality
pH
1.oo
1.89
2.53
3.90
4.94
7
KCl:
1.01
1.99
3.05
4.00
4.91
7
7
5
CaC&:
Im
2m
3m
4m
Sm
Sample
lm
2m
3m
4m
5m
I
7
NaCI:
1.oo
2.04
2.86
3.96
5.01
7
7
6
lm
2m
3m
4m
5m
1.00
2.00
3.03
4.06
5.06
TABLE II
Chemical compositions of prepared brines for stable isotope
analysis and a natural brine actively precipitating halite
Sample
Solutions of l-5 m NaCl, KCl, CaCl, and
MgCl* were prepared by dissolving appropriate quantities of anhydrous reagent grade
MgC& ( l-3 wt.% H,O), and anhydrous analytical grade NaCl, KC1 and CaC& in water of
known isotopic composition (Table I). Anhydrous MgCl* and CaClz react exothermically
when mixed with water. To minimize changes
in the isotopic composition of the solutions due
to evaporation caused by this reaction, the
mixing flasks were immersed in an ice-saltwater bath.
In addition to these solutions, artificial
brines were prepared. Thesebrines have chemical compositions similar to brines derived
from the evaporation of seawaterto halite precipitation, sylvite precipitation (McCaffery et
al., 1987), and carnallite precipitation
(Braitsch, 1962). A brine chemically similar
to brines encountered in potash mines in Saskatchewan was also prepared (Wittrup and
Kyser, 1990) (Table II).
Halite was precipitated from a pure NaCl
supersaturated solution by lowering the tem-
Sample
49
COMPOSITIONS
7
7
7
SHB
SFB
CFB
SPLB
405-00-23
Molality of solute
PH
KCI
MgCb
CaCI,
NaCl
0.502
0.31 I
0.046
0.060
1.5
2.771
4.120
4.643
0.029
0.1
0.000
0.000
0.000
0.049
0.2
3.536
0.500
0.066
2,315
4.6
7
6
5
I
6
SHB=Synthetic
Halite Facies Brine; SFB=Sylvite
Facies
Brine; CFB= Carnallite Facies Brine; SPLB = Synthetic Potash Leak Brine; 405-00-23= natural halite pool from potash
mine at Rocanville, Saskatchewan.
perature of the solution from 80’ to 25 ‘C over
a period of 3 days. In addition, samples of halite and brine were collected from a natural
brine pool at the PCS Rocanville potash mine
in Saskatchewan (Table II). Microscopic examination of natural and synthetic halite crystals revealed many small fluid inclusions.
Samples of natural halite to be used for isotope
analysis are hand-picked and cleaned of foreign minerals and then washed with carbon
tetrachloride to remove any organic material
and surface oils from the mineral.
The D/H ratios of hydrogen and the
‘80/‘60 ratios of CO* were determined with a
Finnigan @Mat Delta and a Finnigan @Mat 25I
stable isotope ratio mass spectrometer, respectively. Stable isotopic compositions are reported in the familiar delta notation defined
as:
8 (%o) = [ uLnpIe/Rstandard
I- 11x 1000
(2)
where R is the ‘sO/‘6O or D/H of the sample
or the standard, SMOW. All results were normalized to the stable isotopic composition of
the pure water used to mix the solutions, da=
6solution
- 4vater.The m>- and 6’*0-value of the
pure water is 3 141 and - 17.0%0,respectively.
50
G.D. KOEHLER
ET AL.
3. Results and discussion
Measurements of the D-values of the NaCl,
KCl, CaCl, and MgCl* solutions by the vacuum distillation method (Bigeleisen et al.,
1952) differ from the m-value of pure water
by up to 5O/oo,
but there is no consistent relationship between AD-values and the molality
of any of the solutions (Fig. 3 ) . The decreased
precision of the GD-valuesfrom 2 1O/o0
for pure
water to + %o for the brine samples (Fig. 3)
may be attributed to trapping of water as inclusions or adsorption of water on residual salts
during evaporation. Reaction of the water with
the salt during distillation to produce stable
hydroxides which result in significant fractionations of hydrogen isotopes as proposed by
Horita ( 1989) is not evident from our data.
m-values measured by vacuum distillation-microequilibration
for each solution
(Kishima and Sakai, 1980) are also usually
within the 2 5%0error indicated by measurement of Uofs Standard water (Fig. 4a). Those
valuesthat deviate by amounts greaterthan the
aI5
3
2’
a.1
-3
-5
-'OlZ
3
W’,
T
5
6
4
5
6
d) 5
2
“0
molal
4
molal
molal
12
3
molal
Fig. 3. Effect of the molality of: (a) CaCl*; (b) M&I,;
(c) KCl; and (d) NaCl, on the hydrogen isotopic composition of solutions analyzed using vacuum distillation.
The dashed line corresponds to no effect on the m)-value
of the pure water from which the solutions were prepared.
The symbols represent average values obtained by at least
4 analysesof each solution and the range in values is indicated by the bars for each solution.
3
4
5
6
molal
d
0
12
3
4
4
. .
5
6
molal
Fig. 4. Differences between the: (a) relative &B-values;
and (b) relative 6”O-values, of various solutions of CaC12
(+), MgC12 (A), KC1 (0) and NaCl (m) obtained by
vacuum distillation-microequilibration.
Dashed lines
represent the error obtained from repeat analysesof Uofs
Standard (Fig. 2 ) . Only M&l2 solutions show a decrease
in the 6’80-values that correspond to MgC12 concentrations relative to the 6’80-values of the water from which
the solutions were prepared.
+- 5%0 error show no consistent correlation
with molality.
Except for a few values, the 6’*0-values of
the NaCl, KC1 and CaClz solutions measured
by vacuum distillation-microequilibration
agree to within l%o of the cT”O-value of the
water from which the solutions were prepared,
suggestingthat no fractionation of oxygen isotopes occurs (Fig. 4b). Similar results were
found previously by Zuber et al. ( 1978) for solutions of NaCl, KC1 and CaC12up to 5 m and
MgCl* solutions up to 1.25 m. MgCl* solutions
show a depletion of “0 that can be related to
the concentration of Mg2+ in solution (Fig.
4b). This trend reachesa maximum depletion
of 6%0at 4 m MgC12,but then reversesso that
5 m solutions are only depleted by 3-4%0. It is
unlikely that these depletions are due to processesthat may have occurred during preparation of the 4 or 5 m MgC12solutions because
this would be reflected by a similar trend in the
hydrogen isotopic compositions. CO2 equilibrated with the MgCl, solutions using the
MEASUREMENT
OF THE HYDROGEN
AND OXYGEN
ISOTOPIC
a)505----a2m
COMPOSITIONS
method of Epstein and Mayeda ( 1953) gives
oxygen isotope activities that can be related to
the known oxygen isotope concentration ratios
of these solutions by the corrections of Sofer
and Gat ( 1972). This further suggeststhat
fractionation of oxygen isotopes were minimal
during preparation of the solutions.
During preparation of the MgClz solutions a
black precipitate was noticed and subsequent
pH measurementsrevealed that pH-values decrease with increasing molality (Table I).
Electron microprobe analysis of the solid revealed a Mg-O-Cl phase and Zr, the latter of
which is probably a contaminant in the MgCl*
reagentand possibly served asa nucleation site
for precipitation of the Mg-O-Cl phase. Precipitation of the Mg-O-Cl phase, which has a
6’80-value of +8%o, caused some fractionation of the oxygen isotopes in the brine and released H+, which resulted in low pH-values
(Table I ) . However, mass-balance calculations indicate that incorporation of “0 into the
solid phase cannot account for the observed
depletion of I80 in the water becausethe quantity of Mg-O-Cl that precipitated would only
result in a depletion of 0.1%oin the most concentrated MgC12solution.
Results from vacuum distillation and hydrogen isotope analyses of synthetic brines are
similar to those found for the single-salt solutions (Fig. 5a). No systematic hydrogen isotope fractionations occur during vacuum distillation followed by hydrogen isotope analysis;
however, the m-values determined for these
brines are more variable than those obtained
from pure water. This variability is probably
attributable to the same processesthat occur in
the single-salt solutions. Similarly, m-values
obtained from vacuum distillation-microequilibration of synthetic brines are within the
+- 5%o precision of the method, and indicate
that no substantial hydrogen isotope fractionations occur for any of the synthetic brines
(Fig. 5b). Depletions of I80 in the synthetic
brines obtained using vacuum distillationmicroequilibration (Fig. 5b) correlate with the
-5
51
i
.15-
SPLB SHB
SFB
Cl-3
b) 5
-10
-5
0
5
As”0
Fig. 5. a. Relativem-valuesobtained
by vacuumdistillation and hydrogen isotope analysis of Synthetic Potash
Leak Brine (SPLB), Synthetic Halite FaciesBrine (SHB),
Sylvite Facies Brine (SFB) and Camallite Facies Brine
( CFB). The symbols represent the mean values of at least
5 analysesand bars represent the spread of values for each
sample.
b. Relative SD- and 6’80-values of SPLB (0 ), SHB (A ),
SFB (0) and CFB ( 0 ), obtained by vacuum distillation-microequilibration. The cross represents the relative
hydrogen and oxygen stable isotopic compositions of the
water from which the solutions were prepared and the
dashed box represents the error in the vacuum distillation-microequilibration method as determined from repeat analyses of standards (Fig. 2 ). None of the values
have been corrected for the effect of Mg.
molality of Mg2+ in the brines and tend to follow the trend observed in the MgC12solutions
(Fig. 6). It is likely that of all the cations in
solution, only Mg causesfractionation of oxygen isotopes during vacuum distillation, as
would be predicted from the results of the single-salt solutions (Fig. 4).
The stable isotopic compositions of water
from fluid inclusions in halite, analyzed by
both crushing and melting methods, agreewell
with those of the respective brines from which
the halites were precipitated (Fig. 7). Some
scatter is expected for natural halites because
these sampleswere collected from a brine pool
in a potash mine which is fed by leaks into the
G D.KOEHLERET&L.
Molal
MgCl 2
Fig. 6. Relationship between the oxygen isotoptc compositions of; Synthetic Potash Leak Brine (0); Synthetic
Halite Brine ( A ); Sylvite FaciesBrine (0); and Carnallite Facies Brine (0 ), and the concentration of MgCIZ in
the solution. The dashed line represents the trend observed in the pure MgC12 solutions (Fig. 4b) and the bar
represents the uncertainty in each measurement.
-150 -15
-10
-5
0
8%
Fig. 7. Stable isotopic compositions of fluid inclusions in
halite analyzed by the crushing method (A ) and melting
method (0 ) and the corresponding brines from a brine
pool at PCS Rocanville 405-00-23 ( 0 ).
mine. The stable isotopic composition of the
pool varies either through evaporation or
changesin the stable isotopic composition of
the inflowing brine as is demonstrated by different isotopic compositions of the pool sampled two years apart (Fig. 7 ). Synthetic halites
were precipitated from a pure NaCl brine with
m>- and 6 “O-values of - 140+ 5 and
- 16.92 l%o, respectively, and the isotopic
composition of water released from fluid inclusions by melting the synthetic halite agree
with those of the parent brine (SD=-1 35%0,
6’80=-15.9YW).
The precision in the measurement of the isotopic composition of water in inclusions released by crushing halite followed by heating
to moderate temperature ( 350°C) to distill the
water from the salt is similar to that of vacuum
distillation-microequilibration
of concentrated chloride brines. Further, water released
by melting of synthetic halite at high temperature ( 800oC ) followed by microequilibration
also yields m>- and 6”O-values to within 5 and
lo/~, respectively, of the parent brine. These
results support the findings of Knauth and
Beeunas ( 1986) that both the melting and
crushing methods can be usedto determine the
stable isotopic composition of water from fluid
inclusions in halite.
Chemical compositions of fluid inclusions in
halite can be measured directly (Lazar and
Holland, 1988) or estimated from freezingpoint depression measurements (Roedder,
1984). Because Mg*+ in solution results in
fractionation of “0 of solutions during distillation, a good estimate of the Mg*+ concentration in fluid inclusion brines is a necessaryprerequisite to stable isotope determinations of
fluids in inclusions in halite. Becausethe halite
pool has a low Mg2+ concentration (Table II),
no correction of the oxygen isotopic compositions of water from fluid inclusions are necessary.Synthetic halites, precipitated from a pure
NaCl solution, also do not require any corrections. However, natural samples of halite may
contain varying concentrations of Mg*+ in
their fluid inclusions and therefore will require
small corrections for oxygen isotopic compositions only, providing that the MgC12concentration can be estimated.
Significant fractionation of hydrogen and
oxygen isotopes by the formation of NaOH
during melting of halite, as proposedby Horita
and Matsuo ( 1986), is not indicated by our
data. Melting of NaCl in moist air will only
produce l-2.5 pmol NaOH/g NaCl (Johnson,
1934;Otterson, 1960) and yields of water from
fluid inclusions in halite are usually 600-800
pmol H,O/g NaCl. From mass-balanceconsiderations, the small amount of NaOH produced by reaction of fluid inclusion waterswith
halite during melting in vacuum could not pro-
MEASUREMENTOFTHEHYDROGENANDOXYGENlSOTOPlCCOMPOSlTlONS
duce any significant variations in the stable
isotopic composition of fluid inclusion brines.
4. Conclusions
The vacuum distillation-microequilibration technique allows hydrogen and oxygen
isotopic analysis of up to 5 m Na-K-Ca chloride solutions of with a precision of It 5%0for
hydrogen and 2 lo/o0for oxygen. Pure MgC12
solutions and chloride brines containing Mg2+
also yield SD-values within + 5%0of the true
value of the water but require a correction to
the 6180-values measured to a maximum of
+6%0 for 4 m MgClz and smaller corrections
for less concentrated and more concentrated
Mg-brines. This behaviour suggeststhat fractionation of oxygen isotopes by reaction with
MgC12during vacuum distillation is a result of
complex processes.
No fractionation of hydrogen or oxygen isotopes occur during melting or crushing of halite for stable isotope analysis of fluid inclusion brines. Corrections of the oxygen isotopic
composition of fluid inclusion brines in halite
can be estimated if the concentration of Mg in
fluid inclusion brines can be measureddirectly
or estimated by typical fluid inclusion methods.
Acknowledgements
The authors would like to thank the member
companies of the Saskatchewan Potash ProducersAssociation for their continued funding
of evaporite research and for accessto their
mines. Also, thanks to Randy George for his
guidancein electron microprobe analyses.Two
anonymous reviewers are thanked for their
constructive comments and suggestions.
References
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Epstein, S. and Mayeda, T., 1953. Variation of the “0
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Horita, J., 1989. Analytical aspectsof stable isotopes in
brines. Chem. Geol. (Isot. Geosci.Sect.), 79: 107-l 12.
Horita, J. and Gat, J.R., 1988. Procedure for the hydrogen isotope analysisof water from concentrated brines.
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