(received february 1, 1986; accepted september 24, 1986)

Geochemical Journal, Vol. 20, pp. 261 to 272,1986

Extraction and isoto

inclusionspic analysis of fluid

in halites

JUSKE HORITA and SADAO MATSUO

Department of Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152, Japan

(Received February 1, 1986; Accepted September 24, 1986)

Basic technique for the extraction and isotopic analysis of fluid inclusions in halite was investigated using

synthetic single crystals of halite and three natural halite samples from China.

Vacuum ball-mill, vacuum decrepitation and vacuum melting methods were examined for the extraction

of fluid inclusions. Results of analyses on SD and 6'x0 of the water in fluid inclusions extracted by the

ball-mill method from synthetic single crystals agreed with those of the mother solution from which the single

crystals were formed. Although the 8'80 value of the water extracted by the melting method agrees well with

that of the mother solution, the bD value was about 7 % more negative than that of the mother solution . There was a great difference in both SD and S'50 of the water extracted by the two methods applied to

the identical natural halite samples; the melting method gave consistently more negative values for both SD

and b'x0 compared with those by the ball-mill method. The difference was interpreted as a result of the

hydrolysis of NaCI with H20 combined with the formation of oxides of alkaline-earth elements in brine

inclusions in natural halite samples in the process of the melting method . Although the melting method has an advantage of complete recovery of volatiles in halite samples

, the chemical and isotopic compositions of volatiles can not be retained owing to a variety of thermal reactions

which occur at high temperatures. It was concluded that the ball-mill method is much superior to the melting

method in order to obtain isotopic and chemical information on fluid inclusions in halite , though the recovery of fluid inclusions by the ball-mill method is not 100 %, and high concentrations of Mg2+ and Ca 21 in fluid

inclusions require correction for the 8D and 61x0 values of the water.

INTRODUCTION

Since the pioneering work by Roedder (1958) on the extraction and analysis of fluid inclusions in minerals, a number of studies have been made to analyze fluid inclusions in evaporites in an attempt to obtain information on the mother solution from which the evaporites were formed. Lately, Roedder (1984) reviewed the studies

on fluid inclusions in salt. Holser (1963) analyzed brine inclusions in Permian halite, and acquired potential information about the condition of the salt deposition. Kramer (1965) made chemical analyses of brine inclusions of salts from a Silurian saline group of Ontario and Ohio to compare the chemical composition of the brine with that of contemporary oceans. Petrichenko and Shaydetskaya (1968) measured

pH, Eh and chemical composition of the brine in a large inclusion of recrystallized halite from the Artemovsk rock salt deposit in Donnbass and discussed the possible source of the fluid. Freyer and Wagener (1975) investigated atmospheric gases (N2, 02, C02 and Ar) and decay products of organic matters (CH4, NH3 and H2S) contained in Permian halite in order to deduce the change in the composition of the atmosphere in the past. Precise information on the nature and occurrence of halite formation including the water content has been investigated by Knauth and Kumar (1981) and Roedder and Bassett (1981) etc. in order to check the utility and adequacy of salt domes as nuclear waste storage sites.

Water is the major and ubiquitous component of fluid inclusions in halite. Hence, the isotopic composition of water in fluid inclusions

261

262J. Horita and S. Matsuo

provides us with information on the paleowater which was entrapped during the formation of halite. The D/H ratio of water in fluid inclusions in halite was first reported by Holser et al. (1963). Knauth and Beeunas (1986) carried out an extensive isotopic study on brine inclusions in Permian halites. However, their result has serious problems. The major purpose of this study is to establish the technique for extraction and isotopic analysis of water in fluid inclusions in halite. In this study, the ball-mill, melting and decrepitation methods have been adopted for the extraction of fluid inclusions from halite.

EXPERIMENTAL

Purification of natural halites: ultrasonic washing All halite samples are in general impure to

some extent. They contain fine-grained mate

rials such as mud, clay and other evaporite

minerals. Impurities in halite samples must be

separated as much as possible before fluid

inclusion analyses. In our case, the most impor

tant information to obtain is the D/H and 180/160 values of the water in fluid inclusions .

In this connection, some hydrous minerals such

as gypsum as well as clay minerals should be

carefully removed.

For the purpose of establishing a method for the purification of halite from rock salt samples, a series of experiments were attempted to examine the influence of washing by water on fluid inclusions in halite. After removal of the impurities in a rock salt sample by hand picking, 6 g each of samples coarser than 6 mesh size was washed with the Antarctic ice-melt water (SD -300%o) in an ultrasonic bath for 3 60 min

and dried in air in the hope that the solvent water would penetrate into micro cracks to help the separation of insoluble impurities. A halite sample (Detroit, Michigan, U.S.A.) was separated into two groups: one was subjected to ultrasonic washing prior to the extraction procedure of water by the ball-mill method mentioned later, and the other without ultrasonic wash

ing. D/H ratios of the extracted water and water contents were compared for these two groups

(Table 1). Isotope ratios are presented in the delta

expression defined as:

Rsample b (%0) (R standard 1) x 1000,

where R represents D/H or 1110/160 . The standard is Standard Mean Ocean Water

(SMOW) and the overall experimental errors of S values are ± 2 %o for SD and ± 0.2 %o for S 180. As seen in Table 1, it is obvious that the

sample treated with the Antarctic ice-melt water

gives a higher water content and lower SD value than those of the sample without treatment. The

lower bD values are due to the incorporation of

the Antarctic water into the crystal during the

pretreatment, which can not be removed during drying. From this result, it is concluded that the

pretreatment with pure water is not adequate, and only hand picking was adopted. It is to be

noted that the scattering of the H20 content and

the bD value for samples without pretreatment

gives us an idea on the heterogeneous distribution in size and number of fluid inclusions in

halite.

Table 1. Effect of ultrasonic washing of halite (Detroit, Michigan, U.S.A.) with Antarctic icemelt water (aD -300%o) on the content and dD value of the water extracted by the ball-mill method

Washing

(min) Content (µmol/g)H2O

8D (%o)

3

30

60

No washing

No washing

No washing

No washing No washing

156.0

116.9

129.4

78.9

94.3

97.1

105.5 72.1

134.1±20.0

89.6 ± 13.7

-117

-104

-123

-85 -81

-89

-85

-86

-115 ± 10

-85±3

Identification of foreign minerals in natural halites: mineral separation by density difference

Extraction and isotopic analysis of fluid inclusions in halites 263

Powder X-ray diffraction analysis of natural halite samples was applied to identify foreign minerals. The bulk halite samples were crushed and the grains with size between 100 mesh (149 µm) and 200 mesh (74 µm) were collected. Using liquids with different mixing ratio of methylene iodide; CH212 (d = 3.325 g/cm3) and acetone (d = 0.792 g/cm3), crushed halite samples were separated into three fractions, based on whether the specific gravity is higher, lower or almost the same as that of pure NaCI (d = 2.17 g/cm3). The fractions of which specific gravity is higher or lower than that of pure NaCl, were subjected to powder X-ray diffraction analysis after the heavy liquid was completely removed by evaporation. The only mineral phases identified for the three Chinese halite samples used in this study were quartz, albite, anhydrite and sylvite, and no hydrous minerals were identified.

Thus, each method has both advantage and

disadvantage.

a) Ball-mill method A ball-mill made of Pyrex glass described by

Kita (1981) was used in this study (Fig. 1). Samples can be pulverized in a vacuum by repeated up-and-down motion of an alumina ball (with a diameter of about 30 mm) in the mill by manual shake. The size distribution of powdered samples was tested by Kita (1981) for quartz and granite. In the case of halite, about 60 wt% of samples, whose initial size is coarser than 6 mesh (> 3.36 mm in diameter), can be pulverized to finer than 100 mesh (< 0.149 mm in diameter) after 30 minutes of shake, which is

Extraction procedure for fluid inclusions in halite There are two major methods for the extrac

tion of fluid inclusions in minerals, i.e., mechanical and thermal. The mechanical method was developed by

Roedder (1958); it is based on grinding in a conventional metal ball-mill or crushing within a sealed metal tube. In the coventional ballmill method, the adsorption of released gases on the newly formed surface of the crushed sample and/or the reaction of the gases with the metal wall of a ball-mill are serious. In the crushing method, the extraction yield of fluid inclusions is poor and the metal tube can not be re-used. The thermal method is based on decrepita

tion (Roedder et al., 1963) or melting at an elevated temperature, both of which were ex

pected to expel all the volatiles contained in a sample. In thermal methods, some unremovable

coexisting minerals, such as carbonate, sulfate

and hydrous minerals contained in the halite

sample decompose and liberate gases on heat

ing. In addition, thermal reactions occur easily

among gases and/or between gases and solid.

Viton O-ring

Pyrex glass

alumina ball

mineral samplea

300mm

-1 45mm~

Fig. 1. Ball-mill made of Pyrex glass used in this study

264 J. Horita and S. Matsuo

much more effective than in the case of quartz.

Longer shake does not much help so far as the

powder size is concerned, because of the absorption of the mechanical energy by fine powders.

After impurities in halite had been removed as much as possible by hand picking, about 4 6 g of a sample with the grain size of around 6 mesh was introduced into the ball-mill with an alumina ball. The ball-mill was externally heated at 130 150°C (heating rate was ca. 4°C/min.) in a vacuum for more than 12 h. According to Knauth and Kumar (1981) and Roedder and Bassett (1981), the adsorbed water on the surface of halite samples can be removed by submitting the sample to a high vacuum at 23

35°C for 30 minutes or longer. However, clay and some other hydrous minerals which are considered to exist in natural halite samples will dehydrate slowly above 100°C. Roedder and Belkin (1979) found that many inclusions (< 100 µm) did not decrepitate even at 250°C when the heating rate was sufficiently low. Knauth and Kumar (1981) reported that the water, which they considered to have been derived from fluid inclusions, started to outgas at 280°C in a vacuum. Based on the above information, the preheating temperature was set at 130 -- 150°C to expel adsorbed atmospheric moisture and gases as well as trapped water and gases in fine cracks. The ball-mill was then cooled to room temperature and the sample was pulverized by manual shake for 30 minutes. The ball-mill was reconnected to the extraction line and heated to 180 200°C to ensure the release of gases adsorbed on the surface of powdered halite. The released gases (mainly H20, COD were collected at the liq. N2 temperature for 4 10 h. Non-condensable gases were collected by a Toepler pump for volumetry. H20 was separated from C02 by repeated trap works and subjected to hydrogen and oxygen isotopic analyses by the C02-H20 equilibration technique adapted for milligram

quantities of H20 (Kishima and Sakai, 1980). By this technique, simultaneous analyses of

hydrogen and oxygen isotopic ratios of the same

aliquot of a few milligram water sample can be

made. The water was finally converted to H2 for

D/H ratio analyses. Based on the amount of H2

measured, we can calculate the amount of H20

released by the extraction procedure and the

apparent content in turn.

The amount of contaminants derived from the ball-mill and associated system during the extraction procedure was evaluated using an empty ball-mill containing an alumina ball. The contaminants were water (2.3 µmol), CO2 (0.03 tmol) and non-condensable gases (0.20 µmol). The amounts of blank H20 and C02, considered to have been derived mostly from the alumina ball, were negligible compared with those of H2O and CO2 recovered from the sample. Non-condensable gases may be mainly due to the slow leakage of air.

In order to check whether or not adsorption of the released gases on the surface of the powdered sample affects the analytical results, we carried out the following experiments. About 5 mg of the laboratory water standard was weighed and sealed in a Pyrex glass capillary. The capillary and about 4 g of the halite powder finer than 400 mesh (< 37 µm) were introduced into the ball-mill together with an alumina ball. The reason for the use of fine powder of halite is to minimize the influence of the water from fluid inclusions. Then, the standard procedure of the ball-mill method was performed. The yields of water were almost 100 %. SD values of the recovered water are slightly more negative (M8D = -2.6%o) than that of the loaded water. 8180 values agree well with that of the loaded water within the experimental error. From this experiment, it was shown that the ball-mill method has no serious problems for adsorption and reaction of water with the glass wall.

b) Melting method After impurities in the Chinese halite sam

ples were removed by hand picking, about 5 g of the sample was introduced into a quartz tube.

The tube was connected to the extraction line


and preheated in a vacuum as described in the previous section. The temperature was then raised to 850°C at the rate of about 10°C/min.

(m.p. of halite is 800°C). The released gases were collected in a trap cooled by liq. nitrogen. Non-condensable gases were collected by a Toepler pump for volumetry. The trapped gases were composed of H20, C02, S02, HCl and hydrocarbons. Carbon dioxide was separated from the other gases at the melting point of n-pentane (-131°C), and subjected to isotope ratio measurements after measuring the amount. Hydrocarbons (mainly n-butane) and SO2 were separated from H20 at dry ice-alcohol temperature (-72°C). Most of the HCl behaves similarly to H20 in the above procedure. The recovered water including a small amount of HCl was subjected to hydrogen and oxygen isotopic analyses by the technique mentioned previously. The composition and amount of the blank run during the melting procedure are water (1.3 µmol), CO2 (0.40 i mol) and non-condensable gases (0.30 µmol). The amount of the blank is also negligible except for C02.

c) Decrepitation method The apparatus used is the same as that for

the melting method. About 7 g of No.1 Chinese halite sample to be described in the later section was preheated in a vacuum as mentioned previously. The temperature was then raised to 500°C and kept for 118 h. Another run was made at 600°C for 93 h. All the extracted water was separated from other gases and subjected to hydrogen and oxygen isotopic analyses.

Synthesis of NaCI single crystals In order to check whether or not the isotopic

ratios of the water-in fluid inclusions in halite are

the same as those of the mother solution from

which halite is crystallized, NaCl single crystals

were synthesized in the laboratory and the

isotopic ratios of the water extracted from fluid

inclusions and that of the mother solution were

compared.

A saturated NaCI solution was prepared at 60.3°C using a reagent grade NaCI of 99.9 % chemical purity. The solution was contained in a

glass vessel with a volume of 1,000 ml. Small seed crystals were attached to a glass rod and the

glass rod was held horizontally with a central Teflon rod which was directly connected to a variable motor. The rotaiton speed of the glass rod was about 50 60 rpm. Turbulent conditions enhance the growth rate of halite crystals. The surface of the solution was covered with benzene to prevent evaporation of the solution. The glass vessel was kept in a bath. The cooling rate of the solution was kept low, about 1.2°C

per day, to grow large single crystals of NaCl. The final temperature of the solution was 18°C. It took about 34 days for the growth of NaCl single crystals totally heavier than 6g. These single crystals were subjected to fluid inclusion extraction and analyses by both ball-mill and melting methods. The mother solution was also sampled for isotopic analyses before and after the growth of the crystals.

Trace-element analyses of natural halites Trace elements in natural halite samples were analyzed. Three Chinese samples were dissolved in dil. HCl solution and the insoluble components were filtered. Concentrations of calcium, magnesium and potassium were determined by atomic absorption spectrophotometry using an air-acetylene flame. Iron and sulfate were determined by the gravimetric analysis. Bromine and iodine were analyzed by the sodium hypochlorite method (Utsumi et al., 1963). Nitrate could not be detected and carbonate could not be analyzed.

RESULTS AND DISCUSSION

Synthetic NaCI single crystals (an example of NaCl-type brine inclusions) Synthetic NaCI single crystals are shown in

Fig. 2a. The inclusions consist mainly of two types: (1) sheets and zones of relatively small size vesicles (mostly < 10 µm) which are particu

3

266

a)

ht

ge

e.f

J. Horita and S.

b)

c)

0

Matsuo

larly dense in population with only liquid phase

(Fig. 2b): (2) randomly arrayed large inclusions (> 100 µm) containing also a gas phase (Fig. 2c). The gas phase is considered to be composed of water vapor produced owing to the contraction of the fluid by the drop in the growth temperature. The negligible amounts of volatiles other than water supports this interpretation. Fluid inclusions in synthetic single crystals

were also extracted and analyzed by the two methods. The results of these analyses are given

in Table 2. The water content of synthetic

crystals is by an order of magnitude larger than

that of natural halite samples, probably because

the growth rate of the halite crystals is much

higher than that of natural halite. The water

content varies for each hand specimen of the

same sample, indicating a heterogeneous dis

tribution of fluid inclusions within a single

crystal. As seen in Table 2, the isotopic composi

tions of the original mother solution did not

change after the growth of NaCl single crystals.

On the other hand, the SD value of the water

extracted by the ball-mill method is about 3%c

higher than that of the original mother liquid,

and about 10%o higher than that of the water

extracted by the melting method. The difference

in SD values by the two methods certainly

exceeds the experimental error. The 5180 values

Table 2. Isotopic analyses of water extracted fromsynthetic halite and mother solution used for its synthesis

Samples H2O (µmol/g)

SD (%o)

8180

MI)

Fig. 2. a) Single crystals of synthetic halite, b) and c)

photomicrographs of fluid inclusions in a synthetic halite

Water extracted

by ball-milling

by melting

Mother solution

before crystallization

after crystallization

av.

693

403 548

200*

-58

-60

-59 -69

-62

-61

av. -62

-8 .1 -8 .6 -8 .4 -8 .4

-8 .4 -8 .7 -8 .6

* The lower water content obtained by the melting

method seems to be due to the heterogeneous distribution of fluid inclusions in the sample.


of the water extracted both by ball-mill and by

melting methods, however, agree with that of

the mother solution within the experimental

error. As will be mentioned later, 5180 values of

the water extracted by the two methods were

significantly different for natural samples.

Several investigations have been made about the effect on changes in ductility and surface electric conductivity of alkali halide crystals, principally NaCI, when they are exposed to a variety of atmospheres (H20, C02, N2, 02, 03 etc.). Otterson (1960) has reported that NaOH is formed in variable amounts in a wide variety of NaCI crystals. The highest concentration of NaOH occurred in NaCI fused in moist air and the lowest concentration was found in naturally occurring rock salt. The presence of NaOH in NaCI crystals is caused by the hydrolysis of NaCl on melting in moist air,

NaC1 + H2O (g) NaOH + HCl (g) (1)

Barr et al. (1962) have demonstrated that the hydrolysis can occur at a temperature as low as 250°C. According to Johnson (1935) and Barr et al. (1962), the extent of the reaction between NaCI crystals and H20 is much greater than that predicted on the basis of the thermodynamic properties of the bulk phases. The discrepancy can be attributed to the fact that the experiments were conducted under gas-flowing conditions. Based on the above investigations and our

finding that HCl was present in the volatiles from natural Chinese halite samples extracted by the melting method, it is certain that a

portion of the H20 included in fluid inclusions has been converted into HCl after reaction with NaCI at elevated temperatures up to 850°C. On the other hand, HCl was not found in the volatiles extracted by the ball-mill method from natural Chinese halite samples. Thermodynamic calculations show that high temperature enhances the conversion of H20 into HCI. There is no direct information on both hydrogen and oxygen isotopic fractionations relevant to reaction (1). According to the equilibrium partition

function ratio of hydrogen isotopes between H20 and HCI, deuterium is much depleted in

HCl relative to H20 (Urey, 1947). The isotopic exchange equilibrium between H2O and HC1 may not be attained in the process of gas

extraction by the melting method. As mentioned before, in the experimental

procedure for the separation of H20 from the extracted volatiles, the complete separation of

H20 from HCl is difficult. Hydrogen chloride is

also reduced to H2 in a uranium furnace in which

H20 is reduced to H2 for isotopic ratio measure

ments. Therefore, SD value for the H2 recov

ered by the melting method from the synthetic

NaCl in Table 2 is considered to be the

weighted mean value of those of H20 and HCI.

The lower SD value of the H20 extracted by

the melting method compared with those in the

ball-mill method can be attributed to the con

tribution of HCl with SD values lower than that

of H20 in the case of the melting method. This

would mean that the sodium hydroxide enriched

in deuterium seems to remain undecomposed at

850°C.

For the 5180 values of the H20 extracted, no difference can be found between the two extraction methods (Table 2). We have to assume that there is no oxygen isotopic fractionation at 850°C between H20 and NaOH (reaction (1)) in the process of the melting method.

Natural halite samples (Chinese halites) In this study, Chinese halite samples were

analyzed as examples of natural halite. The

descriptions of the three halite samples are given

below.

No.1 : The sample was taken from the Si Mao

depression in the southern part of Yun

nan Province. Its geologic age is Cre

taceous.

No.2 : The sample was taken from the Qian

Jiang depression, Hubei Province. Its

geologic age is Tertiary.No.3 : The sample was collected from the west

part of the Qaidam basin, Qinghai Pro vince. Its geologic age is Quaternary,


but the sample has been considered to be

redeposited halite of Tertiary origin.

Generally, natural halites contain some impurities as foreign minerals and organic matter and the separation of the impurities is difficult as discussed before. Another difference between synthetic NaCI crystals and natural halites is that most of brine inclusions in natural halites contain some components other than NaCl (Roedder, 1972; Roedder and Belkin, 1979). Although some foreign minerals such as quartz, anhydrite, sylvite and albite (Table 3) were identified in Chinese halites, chemical analysis of brine inclusions was not yet successful. Bulk chemical analyses of Chinese halites (including both of acid-soluble foreign minerals and brine inclusions) revealed the existence of K+, Mg 2+, Ca 2+ and S02 (Table 3).

A variety of thermal reactions would occur

on heating the "impure" natural halites,

decomposition H2O, CO2, SO2 and impurities hydrocarbons (2)

In addition, the precipitation due to the eva

poration of water, hydrolysis and dehydration of MgCl2 dissolved in brine inclusions would occur during the progressive heating,

dissolved salts precipitation MgC12 nH2O

dehydration

hydrolysis Mg (OH) Cl _HCl > MgO (3)

-HCl

Calcium chloride may react in the same way as the reaction (3). As a result of the above thermal

reactions including reaction (1), not contents, but also isotopic ratios of would be significantly altered.

only the

volatiles

a) Volatile components except for water The volatiles incorporated in these samples

were extracted by both the ball-mill and melting

methods. The decrepitation method was applied

only to No.1 sample. By the ball-mill method,

H20, a trace amount of C02, hydrocarbons, and

atmospheric components were detected. On the

other hand, H20, a considerable amount of

C02, small amounts of SO2, hydrocarbons,

hydrogen chloride and atmospheric components

were detected by the melting method. The

excess C02, SO2 and hydrocarbons by the

melting method are considered to have been

evolved by the pyrolysis of carbonate, sulfate

and organic matters incorporated in fluid inclu

sions and/or crystal lattice of halite, and derived

from impurities mixed in halite crystals.

The contents and isotopic compositions of H20, CO2 and SO2 (contents only) extracted by the two methods are given in Table 4 and 5. No measurable amount of CO2 was recovered by the ball-mill method (Table 4), which indicates that only a minute amount of gaseous CO2 is present in fluid inclusions in Chinese halite samples. In contrast, a large amount of CO2 was recovered by the melting method. Especially, No.2 sample revealed that the apparent CO2 content was much more than H20 content (Table 5). There is a carbonate layer on the surface of the lump of No.2 halite sample, which could not be removed by hand picking.

Table 3. Analytical results of trace elements and identified foreign minerals in Chinese halite samples after mineral separation

SampleCa

(ppm)

Mg

(ppm)

K

(ppm) Fe

(ppm)

so,

(ppm)

Br

(ppm)

I (ppm)

identified foreign minerals

No.1 (Cretaceous)

No.2 (Tertiary)

No.3 (Quaternary)

400

740

1000

49

520

580

1800

120

250

n.d.*

n.d.*

1500

344

3000

89

85

-0

3

130

97

-0

Quarts, Anhydrite, Sylvite

Albite

Quartz

*n .d.: not determined


Table 4. Contents and isotopic compositions of water extracted from Chinese halite samples by the ball-mill method (The range corresponds to 1 a)

Sample Content (µmol/g)

H20

bD 6180 (%o)

Number of runs

No.1 (Cretaceous)

No.2 (Tertiary)

No.3 (Quaternary)

7.9 ± 1.1

9.2 ± 1.6

41.2 ± 3.0

-68 ± 4

-38 ± 5

-29 ± 1

-2 .2 ± 0.5

3.3 ± 0.7

6.0 ± 0.3

7

5

2

Table 5. Contents and isotopic compositions of volatiles extracted from Chinese samples at 850°C by the melting method (The range corresponds to 1 Q)

halite

Sample

H20

Content 6D (µmol/g) (%o)

6180 (%o)

Content (µmol/g)

C02

6'3C 6180 (%o) (%o)

S02

Content (µmol/g)

Nontrapped

gas (µmol/g)

Number of runs

No.1 (Cretaceous)

No.2 (Tertiary)

No.3 (Quaternary)

11.2

±1.2

8.9

±1.9

51.0

±14.3

-91

±4

-82

±14

-45

±5

-21 .9

±2.9

-14 .2

±12.8

-16 .8

±4.1

0.5

±0.2

13.1

±2.8

10.4

±3.5

-8 .2 +20.1

±2.9 ±1.6

-10 .0 +32.4

±0.5 ±0.8

-0 .9 +25.4

±0.2 ±1.3

0.40

±0.17

3.4 ±0.4

1.3

±0.7

0.19

±0.03

0.58 ±0.10

1.0

±0.4

4

4

4

Table 6. Analytical results of volatiles extracted fromNo.1 sample (Cretaceous) by the decrepitation method with independent heating at 500 and 600°C

Temp. Duration

(°C) (h) Content 6D(µmol/g) (%o)

H,O CO, SO,

6'x0 Content Content (%o) (tmol/g) (µmol/g)

500

600

118

93

9.2 -86 -9.4

10.9 -78 -10.4

0.26

0.21

0.25

0.25

6D and 6180 values of the H2O extracted by the ball-mill and melting methods are shown in Tables 4 and 5, respectively.

A large amount of C02 could have been derived

from the carbonate by pyrolysis.

As seen in Table 5, S02 was recovered by the

melting method. Since no SO2 was detected in

the ball-mill method, SO2 could have been

derived from sulfates on heating. As seen in

Table 3, anhydrite was identified as an inde

pendent mineral in No.1 sample. Pyrolysis reactions of carbonate and sulfate to produce

CO2 and SO2 are supported by the fact that the concentrations of CO2 and SO2 recovered by the decrepitation method conducted at lower temperature are less than those obtained by the melting method for No.1 sample (Table 6).

Minute to sizable amounts of hydrocarbons

were detected though a detailed analysis was not

made. Non-condensable gases were always

found in measurable amounts. Some portion of

these gases, however, is suspected to be due to

the slow leakage of air during manipulations in a

vacuum.

613C and 8180 values of CO2 extracted by the melting method (Table 5) are not easy to interpret because most of the C02 is derived from carbonate minerals contained in the halite samples; the origin of these carbonate minerals is not known.

b) Water The most interesting and complicated prob

lem that arose in this study is the large difference in both 8D and 8180 values of the H20


extracted from the same samples by the ball-mill and melting methods, though a difference in the apparent water content is not much as seen in Table 7. The apparent concentration of the H20 extracted by both methods are almost the same in the case of No.2 sample; this may show the heterogeneous distribution of fluid inclusions in both size and population. The values of SD and 61x0 of the H20 extracted by the melting method are unbelievably lower than those of the H20 extracted by the ball-mill method. In addition, the 8D and 8180 values of the H20 extracted by the decrepitation method (500 and 600°C) are between those of the H20 by the ball-mill and melting methods for No.1 sample (Table 6). From these facts, thermal reactions at an elevated temperature are inferred to be responsible for the difference in both the SD and 8180 values of the H20 extracted by the ball-mill and melting methods.

In the ball-mill method, reactions (1) and (2) are considered to be negligible. In order to estimate the isotopic fractionation involved in reaction (3), vacuum distillation experiments to heat some synthetic brines containing MgCl2 and CaC12 up to 200°C (almost the same as post-heating temperature in the ball-mill method) were carried out. The distillate was definitely lower in 8D and 8180 values than those of original brines. This supports the contention that the isotopic fractionation of water takes place as a result of reaction (3) during the post-heating in the ball-mill method when the brine inclusions contain Mg2+

Table 7. Comparison of 6D and 6180 values of H20 extracted by the two methods

SampleRatio of H,O content *ASD *A6'"O

(melting/ball-mill)

No.1 (Cretaceous)

No.2 (Tertiary)

No.3 (Quaternary)

1.42

0.97

1.24

-23 -19 .7

-44 -17 .5

-16 -22 .8

*dd = dmelting 6ball-mill

and Ca 2+. Hence, the isotopic ratios of water by

the ball-mill method could be incorrect when

Mg 2+ and Ca 2+ are dissolved in brine inclusions

in Chinese halites.

In the melting method, reaction (1) fractionates the hydrogen isotope ratio, but not the oxygen isotope ratio. On the other hand, at such a high temperature as 850°C, all the Mg 2+ (and Ca 21) ions would be converted to oxides (reaction (3)) and 180 would be significantly enriched in oxides relative to the original H2O, leaving the H20 depleted in 180. The contents and isotopic ratios of water owing to reaction (2) are difficult to be evaluated. Consequently, SD and 8180 values obtained by the melting method are by all means altered and impossible to be corrected. As an explanation of the large o8 = Smelting Sball-mill found in natural halites, the formation

of NaOH (reaction (1)) and MgO (and CaO) (reaction (3)) in the melting method can be considered. As a result of the reaction (1) and

(3), deuterium and 180 will be significantly depleted in the extracted water as discussed before. Although there is an alternative idea to explain the large M8D and 08180 values; the contribution of H20 other than brine inclusions such as water from clay and hydrated saline minerals by the melting method (reaction (2)), we have no information on the content and isotopic ratios of the non-inclusion water.

Necessary correction for isotopic ratios of the extracted water by the ball-mill method Not only the melting method, but also the

ball-mill method have problems on the isotopic

ratios of the extracted water, if brine inclusions

in natural halites contain Mg 2+ and Ca2+ as

discussed in the previous section.

The procedure for the correction of 8D and 8180 values of the extracted water by the ball-mill method is as follows: 1) chemical analysis of brine inclusions. If the brine is a pure Na(K)-Cl solution, no correction is required. If the brine contains significant amounts of Mg2+ and Ca 21, then,


2) the vacuum distillation of a synthetic solution with the same chemical composition as that of the brine inclusions is carried out at the same temperature as the post-heating temperature and 8D and 8180 values of the distillate are compared with those of the water which makes up the synthetic solution, 3) the observed 8D and 8180 values of the extracted water are corrected on the basis of the isotopic fractionation factor obtained by step 2).

of natural brine samples. It should be noticed that all of the hydrogen isotopic data of brine samples published up to the present except those obtained by the vapor-liquid equilibration method (Sofer and Gat, 1975 and Stewart and Friedman, 1975) might be erroneous to some extent because the conventional hydrogen isotopic analysis includes vacuum distillation of brine samples and fractionation of hydrogen isotopes might be inevitable.

CONCLUDING REMARKS

Important technical information obtained by the present study and problems to be solved in the future study are summarized below. 1) Pure halite cannot be sampled and could not

be purified without contamination by the

pretreatments through ultrasonic washing.2) The melting method is proven to be inade

quate for the extraction and analysis of fluid inclusions in both synthetic and natural halite

samples because of a variety of thermal reactions taking place at an elevated temper ature up to 850°C.

3) The ball-mill method is the best method so far tried to obtain isotopic and chemical information of volatiles in fluid inclusions in

halite and the extraction efficiency of fluid inclusions in natural halites is more than

70 % (from Table 7).4) Isotopic data on H20 from brine inclusions obtained by the ball-mill method are some times not accurate depending on the chemi

cal composition of brine inclusions, especial ly the presence of Mg2+ and Ca t+, and should be corrected.

5) Chemical analyses of brine inclusions in halite are necessary to correct the isotope ratios of the extracted water.

The nature and concentration of the solute in

fluid inclusions of halite make it difficult to

obtain the accurate isotopic composition of

brine inclusions. This problem is also crucial to

measure the hydrogen isotopic composition

Acknowledgements-We are indebted to Prof. Chen Ke-zao of Qinghai Institute of Salt Lake, Academia Sinica, who provided us with a variety of Chinese halite samples with geological information. Our thanks are due to Prof. I. Zak of the Hebrew University of Jerusalem, who gave us useful information on evaporites, suggestions and discussions pertinent to this study. Our

gratitude is extended to Dr. K. Tsukamoto of Tohoku University, who instructed us how to synthesize halite single crystals. We are thankful to Prof. W.T. Holser of Oregon University for giving us the basic knowledge of the chemistry of fluid inclusions in evaporites.

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(received february 1, 1986; accepted september 24, 1986)

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