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Boron concentration and isotopic composition of halite from experiments and salt lakes in the Qaidam Basin
Geochimica et Cosmochimica Acta, Vol. 64, No. 13, pp. 2177–2183, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/00 $20.00 ⫹ .00
Pergamon
PII S0016-7037(00)00363-X
Boron concentration and isotopic composition of halite from experiments and salt lakes in the Qaidam Basin W. G. LIU,1,2,3,* Y. K. XIAO,2 Z. C. PENG,3 Z. S. AN,1 and X. X. HE3 1
Key Laboratory of Loess and Quaternary Geology, Chinese Academy of Sciences, Xi’an, 710054, China 2 Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, 810008, China 3 Department of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, China (Received September 2, 1999; accepted in revised form March 1, 2000)
Abstract—The concentration and isotopic abundance of boron in salt can be used to trace paleosalinities and depositional environments for marine and non marine evaporites. However, the mechanism of incorporating boron into halite during evaporation of salt lake brines is subject to dispute, and there have been few studies of boron concentrations and isotopic compositions during this process due to the low boron concentration in halite. A group of evaporation experiments from artificial solutions and salt lake brines have been analyzed in this study. The results of boron concentration and isotopic analyses demonstrate that the boron in halite comes mainly from fluid inclusions, with a lesser amount from coprecipitation. The isotopic fractionation factors between the brine and halite are from 0.9857 to 1.0000 for the evaporation experiments, and 0.9945 to 1.0009 for natural samples from the salt lake. The ␦11B values of halite from the Qaidam Basin salt lakes vary from ⫺4.7 to 25.8‰, compared to ⫺4.7 to 31.4‰ in the salt lake brines. These values are controlled by the boron isotopic composition of the boron sources, pH values and Na/Ca ratios in the salt lake brines. The variation of boron isotopes in halite may be used to trace the hydrochemical evolution and paleoevaporation environment in salt lakes. Copyright © 2000 Elsevier Science Ltd
␦11B value of halite is lower than that in the parent seawaters, suggesting that the boron is incorporated into the mineral lattice (Vengosh et al., 1992). In this work, we carried out evaporation experiments on artificial and natural brines and investigated the boron isotopic composition of the crystallized halite. In addition, the boron concentration and isotopic composition of halite from salt lakes in Qaidam Basin were determined. The aim of this work is to understand the mechanism of boron removal into halite, and to study the relationship between the ␦11B value of halite and the evaporation process in the parent brine.
1. INTRODUCTION
The boron isotopic composition of evaporite halite and brine has been used to distinguish between marine and non-marine sediment environments. The ␦11B values from a number of marine and non-marine borate minerals of different ages, range from 18 to 31‰ for nine marine minerals, to ⫺22 to 0‰ for twenty-five non-marine minerals (Swihart et al., 1986). The boron isotopic composition of brine, groundwater and sediments from modern Australian salt lakes indicate that the salt lake brines with high ␦11B values have a marine origin (Vengosh et al., 1991). The wide variations of boron isotopic compositions in the Qaidam Basin demonstrate that variations in the ␦11B of the source water are important in these salt lakes, and the lower ␦11B values indicate that the boron within salt lakes is largely of non-marine origin (Xiao et al., 1992). These studies suggest that the variations in boron isotopic compositions are caused by boron isotopic fractionation between brine, clay, boron minerals and/or carbonates. The isotopic fractionation factors are consistent with the theoretical calculations by Kakihana et al. (1977). Most studies have focussed on high boron concentration minerals, hence the boron isotopic geochemistry of evaporite minerals has not been widely applied to studies of paleosedimentary environments. Halite is the main mineral deposited during brine evaporation, however, there has been little work on the boron isotopic composition of halite due to the low boron concentration in halite. In addition, the mechanism of boron removal from brine into halite is not clear. Dong (1984) investigated the composition of trace elements in evaporite halite and indicated that boron may be present in the form of parent fluid inclusions. However, evaporation experiments on seawater showed that the
2. EVAPORATION EXPERIMENTS AND SALT LAKE SAMPLES Three sets of evaporation experiments were carried out in this study as following: 1. A primary consideration of experiment (1) was to examine the variation of boron concentrations and isotopic compositions during pure NaCl precipitation, without the interference of Ca and Mg ions in the brine. 1.2 g of H3BO3 was added to 500 mL of 4.75 M NaCl solution. 2. In experiment (2), 1.5 g CaCl2, 1.5 g Na2SO4 and 1.2 g H3BO3 were added to 500 mL of 4.07 M NaCl solution and dissolved completely. The purpose of this procedure was to study the variation of boron isotopes during precipitation of halite from the synthetic brine, and to compare with experiment (1) in order to examine the affects of Ca and SO2⫺ in halite. 4 3. Experiment (3) included two procedures; First (E2-0), the halite and associated salt lake brines were kept in a tight polyvinyl bottle at 10 –20°C for a period of six years, during which time about 10 g of halite had crystallized from the 500 mL of solution. Second (E2-1), the brine that was from the E2-0 was used as parent solution in experiment (E2-1). The aim of this experiment was to study variations of boron concentrations and isotopic compositions as a function of varying evaporation speeds and amounts of fluid inclusions in halite.
* Address to whom correspondence should be addressed. 2177
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Liu et al. Table 1. Boron concentrations and isotopic compositions of evaporation experiments. Brine
Halite
␦11B Value (‰) Sample B (g/g) pH Exp. 1 Z4-1 Z4-2 Z4-3 Z4-4 Z4-5 Exp. 2 Z1-0 Z1-1 Z1-2 Z1-3 Z1-4 Z1-5 Exp. 3 E2-0 E2-1
512 677 710 1120 2871 354 460 585 712 1038 2303 80 230
Run 1
Run 2
␦11B Value (‰) Run 3
Average
B (g/g)
Run 1
Run 2
Run 3
Average
␣
10.8 ⫾ 0.2 10.7 ⫾ 0.2 10.8 ⫾ 0.4 10.9 ⫾ 0.3
0.9999 0.9995 0.9999 1.0000
5.5 5.5 5.3 4.8
11.4 ⫾ 0.1 10.7 ⫾ 0.1 11.1 ⫾ 0.6 10.8 ⫾ 0.1 10.6 ⫾ 0.1
11.3 ⫾ 0.2 11.1 ⫾ 0.6 11.2 ⫾ 0.1 11.1 ⫾ 0.1 10.8 ⫾ 0.1 11.1 ⫾ 0.1
11.4 ⫾ 0.1 10.9 ⫾ 0.3 11.2 ⫾ 0.1 11.0 ⫾ 0.2 10.8 ⫾ 0.3
4.4 8.0 12.2 39.7
10.9 ⫾ 0.1 10.8 ⫾ 0.4 10.6 ⫾ 0.2 10.6 ⫾ 0.1
6.1 6.0 5.8 5.3
11.5 ⫾ 0.1 11.3 ⫾ 0.1 11.1 ⫾ 0.1 10.7 ⫾ 0.1 10.6 ⫾ 0.1 10.7 ⫾ 0.1
11.4 ⫾ 0.1 11.1 ⫾ 0.3 11.0 ⫾ 0.4 10.5 ⫾ 0.1 10.5 ⫾ 0.1 10.4 ⫾ 0.1 11.3 ⫾ 0.1 11.1 ⫾ 0.1
11.5 ⫾ 0.1 11.2 ⫾ 0.1 11.1 ⫾ 0.1 10.6 ⫾ 0.1 10.5 ⫾ 0.1 11.0 ⫾ 0.3
6.9 12.4 17.7 48.0
10.7 ⫾ 0.1 10.8 ⫾ 0.1 10.9 ⫾ 0.1 10.8 ⫾ 0.1 0.9996 9.7 ⫾ 0.1 8.7 ⫾ 0.3 9.2 ⫾ 0.7 0.9987 7.0 ⫾ 0.1 7.0 ⫾ 0.1 7.2 ⫾ 0.3 7.1 ⫾ 0.1 0.9966 7.5 ⫾ 0.1 7.8 ⫾ 0.1 8.1 ⫾ 0.1 7.8 ⫾ 0.3 0.9965
7.1 21.1 ⫾ 0.1 20.8 ⫾ 0.1 20.9 ⫾ 0.1 20.9 ⫾ 0.1 7.6 20.9 ⫾ 0.1 20.6 ⫾ 0.1 20.8 ⫾ 0.2
4.9 8.7
7.1 ⫾ 0.1 6.3 ⫾ 0.1 6.8 ⫾ 0.1 6.7 ⫾ 0.4 0.9857 17.7 ⫾ 0.1 17.7 ⫾ 0.1 17.9 ⫾ 0.1 17.8 ⫾ 0.2 0.9971
10.6 ⫾ 0.1 10.4 ⫾ 0.1 10.8 ⫾ 0.1 11.2 ⫾ 0.1 10.9 ⫾ 0.1 11.1 ⫾ 0.1
Note. Run 1–3 are total sample repeated, precision are expressed within 2 confidence limits (95%).
Experiments (1), (2) and (E2-1) were kept at 30 ⫾ 3°C in a clean air hood. At each step of the evaporation process, the halite was separated from solution, and successively washed with alcohol, high purity water and alcohol in order to remove the surface brine. The halite was then dried at about 50°C. Qaidam Basin is located in the east of the Qinghai-Tibet plateau where there are many non-marine evaporated salt lakes and playas. The boron concentrations of the brines in these salt lakes are higher than that in the ocean. Seven halite samples were collected from these salt lakes, and they were cleaned using the method described in the evaporation experiments.
were first outgassed at 3.0 A for one hour. 3 l of a graphite slurry was spread across the whole of the top of the filament, then approximately 2 g of boron was loaded with equimolar of mannitol and cesium (Cs2CO3) and evaporated to dryness at ⬍50°C. A VG354 mass spectrometer was used for this work. The isotopic analysis was started when the pressure in the source section reached (6 –3) ⫻ 10⫺7 Tor. The Cs2BO⫹ 2 ion beam intensity was typically about 3 ⫻ 10⫺12 A. The data were collected on a faraday cup by switching magnetically between masses 308 and 309. The isotopic ratios were corrected for the contribution of 17O to the mass 309 by following: 11
3. SAMPLE TREATMENT AND ANALYSIS The boron concentrations of the samples were measured using the azomethin-H spectrophometric method described by Kiss (1988). The external precision (2) of the measurements was ⫾2%. Boron for isotopic determination was separated from the sample solution by a two step ion exchange procedure using Amberlite IRA743 boron-specific anion-exchange resin (Kiss, 1988). The advantages of boron-specific resin are the high recovery for boron extraction (100%) without boron isotopic fractionation during the exchange process, and the high capacity for boron retention levers allowing the use of less exchange resin and chemical reagents to reduce blank (Hemming et al., 1992; Gaillardet et al., 1995; Xiao et al., 1997). The halite was dissolved in low-boron water and, in the case of samples from salt lakes, centrifuged for about 20 min to remove the small amount of detrital minerals. The brine was then diluted with low-boron water. These solutions, containing about 10 g B, (pH at 6 to 7) were passed through a 0.8 mL Amberlite (120 to 200 mesh) resin column using low-boron water, 2 M NH4OH and low-boron water to remove the cations. 12 mL of 0.1 M HCl was then used to elute the boron from the resin. The eluant was then evaporated to dryness at 65°C by adding an equal amount of mannitol to the boron to suppress volatilization of boron. The residue was dissolved in low-boron water and passed through a 0.15 ml Amberlite resin column (120 to 200 mesh) to further purify the sample. After adding more mannitiol, the eluate was again evaporated to dryness at 60°C for mass spectrometry analysis. Low boron water for the experiments was obtained by passing pure water through a column loaded with Amberlite IRA 743 boronspecific resin. The boron blank, as determined by isotope dilution mass spectrometry, was less than 40 ng. The process of boron isotopic measurement is essentially the same as that described elsewhere (Xiao et al., 1988). The single Ta filaments
B/10B ⫽ R (309/308) ⫺ 2 (17O/16O) ⫽ R (309/308) ⫺ 0.00075
The 11B/10B ratio of NBS SRM 951 was repeatedly determined to be 4.0493 ⫾ 0.0006 (2) with a relative external reproducibility of 0.15‰ (n ⫽ 10). The isotopic composition of boron in the samples is expressed as a permil deviation relative to the NBS SRM 951 boric acid standard. 4. RESULTS AND DISCUSSION
4.1. Boron Distribution in the Halite During the Evaporation Experiment The boron concentrations of the minerals and residual brines in the partial evaporation experiments are given in Table 1. The distribution coefficient is expressed as: Kd ⫽ ([B]mineral/[B]solution) The Kd values of halite are lower (0.0066 to 0.031) than those of absorbed clay (0.75 to 2.85) (Palmer et al., 1987), coprecipitated carbonate (2.2 to 10.4) (Hemming et al., 1992), and sediments (clay and boron minerals) (1.87 to 30.34) from salt lakes in the Qaidam Basin (Xiao et al., 1992). Halite is the main evaporite mineral in salt lakes, especially in the late stage of salt lakes and playas. The low Kd value of halite indicates that dissolved boron in the salt lake brines is largely concentrated during the stage of halite crystallization. Figure 1 shows that the boron concentration of the halite
Boron concentration and isotopic
Fig. 1. Relationship between boron concentration of the halite and brines.
increases with increasing boron contents in the saline solutions, according to the degree of evaporation. Dong (1984), suggested that boron may be present in the fluid inclusions in the halite. In contrast, others have indicated that the mechanism of boron removal from highly evaporated sea water may be by direct precipitation of Mg-borate minerals and/or formation of a Mgborate ion pair (MgB(OH)⫹ 4 ) and coprecipitation with other salts (Vengosh et al., 1992). The crystallized halite was examined under a binocular microscope, and a large number of fluid inclusions were found at a magnification of ⫻180 (Fig. 2). The relationship between the boron concentrations and distribution coefficients indicates that the Kd value in the NaCl solutions containing Ca and SO4 are slightly higher than those in the pure NaCl solutions (Fig. 3). This suggests that boron may be coprecipitated with CaSO4. 4.2. Boron Isotopic Compositions in Halite from the Evaporation Experiments The isotopic composition of the crystallized halite and saline solutions from experiments 1 to 3 are given in Table 1, together with the isotopic fractionation factor (␣) between solid and liquid, where ␣ is defined as:
Fig. 2. Photomicrograph of primary fluid inclusion in crystallized halite (at a magnification of ⫻180).
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Fig. 3. Relationship between halite kd and the boron concentration of the brines.
␣ ⫽ R (11/10)solid/R (11/10)liquid
(1)
In the experiment containing pure NaCl solution, the ␦11B value of the halite does not vary (total range 10.7 to 10.8‰), while a small variation is seen in the halite crystallized from the Na-Ca-Cl-SO4 solution (7.1 to 10.7‰). One possible interpretation of these results is that different mechanisms of boron isotopic fractionation existed in the two different solutions. The relative enrichment and depletion of 11B in brines and precipitates during evaporation and fractional crystallization of sea water and salt lakes brine have been used to explain the boron isotopic variations observed in evaporite minerals (Swihart et al., 1986; Qi et al., 1989; Vengosh et al., 1991). These previous studies were based on the distribution of trigonal BO3 and tetrahedral BO4 units in each mineral, but they neglected the possible presence of fluid inclusions in the minerals. Table 1 indicates that the isotopic fractionation factors vary from 0.9995 to 1.0000 during crystallization of pure NaCl from solution. The ␦11B values of the halite are the same as those in the coexisting saline solution, and do not vary with changing pH values, or with the B and Ca concentrations in solution. This suggests that boron does not enter the lattice of NaCl, so there is no boron isotopic fractionation when NaCl crystallized from solution; i.e., the boron in the halite is entirely contained within the fluid inclusions, and the ␦11B values of the evaporates remain the same as those of the parent solution. The ␦11B values of halite that crystallized from solutions containing Ca, SO4, Na and Cl are lower than that which crystallized from solutions containing only Na and Cl, and they have a lower ␦11B than the parent solution, (Fig. 4). The boron isotopic fractionation factor between solid and liquid ranges from 0.9965 to 0.9996, and increases with increasing Ca content, Na/Ca and the degree of evaporation. During evaporation of the Na-Ca-Cl-SO4 solution, boron may be partly removed from solution by coprecipitation with CaSO4. Because there may be a relatively large degree of boron isotopic fractionation during boron precipitation, the ␦11B value of halite containing precipitated minerals during crystallization is lower than that in pure halite. Hence, the small degree of boron isotopic fractionation found in the evaporation experiments of Na-Ca-Cl-SO4 solutions. This interpretation can explain why the ␦11B value in
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Fig. 5. ␦11B values in brines vs. boron concentration of brines from evaporation experiments. The ␦11B values of solutions have not apparent variation with increasing degree of evaporation during the halite crystallization.
Fig. 4. Results of evaporation experiments. A) ␦11B of crystallized minerals vs. Broon content. B) Isotopic fractionation vs. Ca content of solution.
the high Ca concentration brines is higher than that in those with lower Ca levels. The ␦11B value of the crystallized halite also shows a progress decrease with decreasing pH values of the saline solution. Table 1 presents the results of evaporation experiment (3). Sample E2-0 was from the Gashkule lake brine that was preserved in a sealed bottle, at 10 to 20°C, for six years. During this period 10 g of halite precipitated from 500 ml of brine. The crystallized halite was examined by a binocular microscope and a small number of fluid inclusions were found. Zhang (1993) suggested that a large number of fluid inclusions in halite is usually associated with fast evaporation and large temperature variations in salt lakes. The ␦11B value of the halite in experiment E2-0 is lower than that in the experiment E2-1. The boron isotopic fractionation factor between the halite and brine in E2-0 is 0.986. This value is in good agreement with the average of the empirical ␣ value between sediments and brines from salt lakes (Xiao et al., 1992). Experiment (E2-0) suggests that fluid inclusions are not the only source of boron in the halite, and that boron isotopic fractionation is controlled by coprecipitation with halite when fewer fluid inclusions are formed in halite during slow crystallization. The isotopic fractionation factor of E2-1 (0.997) is similar to that in the evaporation experiments (1) or (2). The results support the hypothesis that boron in halite is mainly present within fluid inclusions and that the boron isotopic fractionation is controlled by the
evaporation temperature which determines the number of fluid inclusions in the halite. The small degree of boron isotopic fractionation and the low boron content (4.4 – 48 g) in halite from evaporation experiments suggest that the precipitated minerals in the halite are not borate minerals. The results of X-ray diffraction studies indicate that there is a small amount of Ca-sulphate in the halite from experiments (2) and (3), including CaSO4 䡠 2H2O and CaSO4 䡠 6H2O. In experiment (2), the highest concentration of CaSO4 䡠 2H2O found in the halite was less than 2% in the Z1-5. In experiment E2-1, the concentration of CaSO4 䡠 6H2O in the halite was less than 1%. The ␦11B values of the saline solutions do not show any obvious change during the evaporation and crystallization stages (Fig. 5), during which the boron concentration of these solutions ranged from 354 to 2871 g/g. This reflects the fact that relatively little boron is partitioned into the halite from the brine, and that this process is not associated with a large degree of isotopic fractionation. This result is similar to that obtained from evaporation of seawater, which also showed that the ␦11B value in the residual brine did not vary during precipitation of halite (Vengosh et al., 1992). 5. HALITE AND BRINE IN THE SALT LAKES OF QAIDAM BASIN
The boron isotopic compositions of seven samples of halite and coexisting brine are given in the Table 2, together with their boron concentrations and the calculated isotopic fractionation factors. The pH values and Na/Ca ratio of the brines are also presented in Table 2. The ␦11B value of the halite has a wide range from, ⫺4.7 to 25.8‰, similar to the range deserved in the coexisting brine, ⫺4.0 to 31.4‰. The ␦11B values of the halite and coexisting brine have similar characteristics in that the ␦11B values decrease with increasing boron concentrations and pH values in the salt lake brines, and decreasing Ca contents in the brine (Fig. 6). The results show that the ␦11B values of the halite are mainly controlled by the boron isotopic composition of the salt lake brines. The small isotopic fractionation factor (0.9945 to
Boron concentration and isotopic
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Table 2. Boron concentrations and isotopic compositions of brine and halite from salt lakes, Qaidam Basin. Brinea
Halite
␦11B Value (‰) Sample
pH
B (g/ml)
␦11B (‰)
Na/Ca
B (g/g)
Run 1
Run 2
Gasikule Mahai Balun Xiaochaidam Dachaidam Chaikai Niulang
7.11 7.98 6.50 8.14 7.78 7.68 5.34
101.6 162.9 10.1 285.6 404.3 15.4 53.6
17.9 ⫺4.0 31.4 ⫺0.5 5.3 9.9 24.5
193.8b 208.3 12.1 167.0 255.7 130.6 0.7
8.0 11.6 0.4 13.0 16.0 7.4 3.8
12.1 ⫾ 0.3 ⫺4.7 ⫾ 0.2 25.7 ⫾ 0.3 0.4 ⫾ 0.1 3.8 ⫾ 0.1 6.0 ⫾ 0.1 20.3 ⫾ 0.3
12.5 ⫾ 0.1 ⫺5.1 ⫾ 0.1 25.9 ⫾ 0.1 0.4 ⫾ 0.1 3.7 ⫾ 0.1 5.6 ⫾ 0.1
Run 3 ⫺5.3 ⫾ 0.1
6.1 ⫾ 0.1
Average
Kd
␣
12.3 ⫺5.0 25.5 0.4 3.8 5.9 20.0
0.079 0.071 0.041 0.049 0.040 0.480 0.071
0.9945 0.9993 0.9945 1.0009 0.9984 0.9960 0.9956
Note. Run 1–3 are total sample repeated, precision are expressed within 2 (95%) confidence limits. The results of the brines are from Xiao et al. (1999). b This result is from Zhang (1987). a
1.0009) is consistent with the results of the evaporation experiments discussed above, and suggests that fluid inclusions dominate the boron isotopic composition of the halite. However, the isotopic fractionation factors between the halite and brines from the salt lakes differ from the experiments (1) in that it does not appear to be dependent on the Ca content of the brines. The relationship between the ␦11B values and pH is different in the evaporation experiment and the salt lakes because of the large variations in pH in the latter.
Based on the results from the experiments and the salt lakes, boron isotopic compositions of the halite can be described thus,
␦11Bhalite ⫽ ␦11Bb F1 ⫹ ␦11 Bc F2
(2)
where the ␦11Bb and ␦11Bc are the boron isotopic compositions of the fluid inclusions and the boron coprecipitated with the halite. F1 and F2 are the fractions of boron in the fluid inclusions and coprecipitated with the halite. Equation (2) can also be described as;
␦11Bhalite ⫽ ␦11Bb F1 ⫹ ␦11Bb F2 ⫺ 103(1 ⫺ ␣) F2
(3)
The ␣ in equation (1) is given as:
␣ ⫽ (␦11Bc ⫹ 103)/(␦11Bb ⫹ 103)
(4)
The mechanism of boron isotopic fractionation during coprecipitation and absorption has been studied in many fields in the last few years. The ␦11Bc value depends on the pH and boron concentration of the solution (Kakihana et al., 1977; Palmer et al., 1987; Hemming et al., 1995). The B(OH)⫺ 4 species in solution tends to be enriched in 10B and is preferentially removed from solution and incorporated into the solid phase. The mechanism controlling boron isotope variations in halite is different from those identified in previous investigation of sediments. A preliminary explanation for the variation of the boron isotopic composition of halite for different evaporative condition is given as follows.
Fig. 6. A) ␦11B value in the halite and brine vs. pH value of brine. B) ␦11B value in the halite and brine vs. Ca content of brine. The Ca content of brine in Gashikule Lake is from early result (Zhang 1987) and is not presented in Figure 6B.
1. In pure halite (F2 ⫽ 0), the boron is only present within fluid inclusions in the halite. In this case, ␦11B values of the halite correspond to that in brine (␦11Bhalite ⫽ ␦11Bbrine). 2. When the halite is formed at a low temperature by slow crystallization, for example as in the evaporation experiment (E2-0), there are very few fluid inclusions in the halite (F1 ⫽ 0). In this case, boron is removed from the solution by coprecipitation and the ␦11B in halite is controlled by that of the coprecipitated phase ␦11Bc. 3. In most halite, the boron composition consists of a mixture of that which has been coprecipitated and of that in the fluid inclusions. The ␦11B value in halite is related to the pH value and Ca content of the brine (Fig. 6). In high pH (⬎7.5) and low Ca (⬍1 g/L) brines, the boron isotopic fractionation between the sediments and the brine is small relative to that
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Fig. 7. Relationship between the ␦11B value of the halite and salt lake brines.
at low pH (Palmer et al., 1987). In addition, a low Ca concentration in the brine reduces the contribution of boron from coprecipitation in the halite. Where boron is mainly located within the fluid inclusions, the ␦11B of the halite is in close agreement with the isotopic composition of the brine. In low pH (⬍7.5) and high Ca (⬎1 g/L) brines, relatively large isotopic fractionation (0.9945 to 0.9960) is produced by decreasing the pH value. The more Ca is present in brine, the more boron coprecipitated with the halite, hence the ␦11B value of the halite should be lower than that in the corresponding brine. 6. GEOLOGICAL IMPLICATIONS
The trace element boron is relatively enriched in salt lakes, especially in Qaidam Basin of the Qinghai-Tibet plateau. The small boron isotopic fractionation and similar chemical composition of the halite and brine mean that the boron isotopic composition of the halite is approximately equal to that in the salt lake brines. Figure 7 shows that there a positive relationship between the boron isotopic composition of the halite and the salt lake brines. Therefore, ␦11B values in halite provide a record of the boron isotopic composition of salt lake brine during crystallization of halite. The ␦11B value of the halite is related to the chemical composition and evaporation environment of the salt lakes. High boron concentrations and low ␦11B values in the halite are associated with high boron concentrations in the salt lake brines. ␦11B values in halite may be a indicator that boron is enriched or depleted during evolution of salt lake brines. Figure 8 shows that ␦11B values in halite from salt lakes of boron mine area, together with high boron content, for example, in Dachaidan, Xiaocaidan and Mahai salt lakes, are lower than that in halite from other salt lakes. Zhang et al. (1993) indicated that primary fluid inclusions in clear halite are the remains of brine from when the halite crystallized and are a useful tool with which to trace the brine evolution of salt lakes. Boron isotopic fractionation between halite and brine is small in sulphate type salt lakes when that brine has a high pH value (6.5 to 8.1) and low Ca content (⬍0.6 g/L). The ␦11B value of the halite then directly records the boron isotopic composition of the primary brine. The pH value of brine
Fig. 8. Boron isotopic composition of halite in salt lakes as a function of B concentration.
usually decreases with progressive evolution of salt lake. The chloride type brine is the final stage of a salt lake, and has a lower pH value and high Ca and Mg content compared with sulphate type brines (Zhang et al., 1987). The ␦11B value of brine is increased by the larger isotope fractionation at low pH value, a large amount of coprecipitated boron and high Ca contents. However, the distribution coefficient between absorbed boron and brine in solution is low at low pH (Palmer et al., 1987), so the ␦11B value of halite precipitated from chloride type brines is higher than that formed from in the sulphate type brines. Hence, the boron isotopic composition of halite in salt lake is directly related to the evolution of boron isotopes in that salt lake. In addition, fluid inclusions are present in other evaporite minerals, and not just halite. Hence, boron within these evaporite minerals may also have a significant contribution from fluid inclusions, therefore, it is important to consider the affect of inclusions in minerals in the study of the boron isotopic compositions of evaporite minerals. 7. CONCLUSION
The results of the evaporation experiments and studies of samples from salt lakes show that similar processes control the variation of boron concentrations and isotopes. The evidence that boron in halite is mainly contained within fluid inclusions and not in coprecipitated minerals has been provided by considering the boron concentration and isotopic composition of halite and brine from evaporation experiments and salt lakes in Qaidam Basin. The ␦11B of halite varies from ⫺5.0 to 25.8‰, and shows a range of ⫺4.7 to 31.4‰ for the salt lake brines. The characteristics of boron concentrations and isotopic compositions in halite are as following: 1. The distribution coefficient between halite and associated brine is lower than that in other evaporite minerals. This results in the concentration of boron in the brine during halite crystallization. 2. The boron isotopic composition of halite is controlled by a mixture of boron within the fluid inclusions and boron that has been coprecipitated with the halite. 3. There is only a small boron isotopic fractionation produced during crystallization of halite from brine. This isotopic
Boron concentration and isotopic
fractionation has a narrow range of 0.9945 to 1.0009 for salt lakes in Qaidam basin, and increases slightly with increasing Ca content and decreasing pH value in the brine. 4. The boron isotopic composition of the brine is not affected by crystallization of halite within the salt lakes. The ␦11B value of halite may directly record the boron isotopic composition of brine in salt lakes. This may be used to reveal the boron isotopic composition of ancient brines and trace the evolution of brines and the paleosedimentary environment of salt lakes. Acknowledgments—We are very grateful to M. R. Palmer, N. G. Hemming and two anonymous reviewer for their through reviews, especially in correcting English by M. R. Palmer. This work was supported by Key Project of CAS (KZ951-A1-402), Knowledge Innovation Program of CAS (KZCX1-Y-05) and Natural Science Foundation of Qinghai Province (97-07-09). REFERENCES Dong J. H., (1984) Mineralogical chemistry of halite in modern salt lakes of China. Acta Mineral. Sinica 1, 29 –34 (in Chinese). Gaillarelet J. and Allegre C. J. (1995) Boron isotopic compositions of corals: Seawater or diagenesis record? Earth Planet. Sci. Lett. 136, 665– 676. Hemming N. G. and Hanson G. N. (1992) Boron isotopic composition and concentration in modern marine carbonate. Geochim. Cosmochim. Acta 56, 537–543. Hemming N. G., Reeder R. J., and Hanson G. N. (1995) Mineral-fluid partitioning and isotopic fractionation of boron in synthetic calcium carbonate. Geochim. Cosmochim. Acta 59, 371–379. Kakihana H., Kotaka M., and Satoh S. (1977) Fundamental studies on the ion-exchange separation of boron isotopes. Chem. Soc. Jap. Bull. 50, 158 –163. Kiss E. (1988) Ion-exchange separation and spectrophotometric deter-
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mination of boron in geological materials. Anal. Chim. Acta 211, 243–256. Palmer M. R., Spivack A. J., and Edmond J. M. (1987) Temperature and pH controls over isotopic fractionation during adsorption of boron on marine clay. Geochim. Cosmochim. Acta 51, 2319 –2323. Qi T., Nomura M., Ossaka T., Okamoto M., and Kakihana H. (1989) Boron isotopic composition of some boron minerals. Geochim. Cosmochim. Acta 53, 3189 –3195. Swihart G. H., Moore P. B., and Callis E. L. (1986) Boron isotopic composition of marine and nonmarine evaporite borates. Geochim. Cosmochim. Acta 50, 1297–1301. Vengosh A., Chivas A. R., McCulloch M. T., Starinsky A., and Kolodny Y. (1991) Boron isotopic geochemistry of Australian salt lakes. Geochim. Cosmochim. Acta 55, 2591–2606. Vengosh A., Starinsky A., Kolodny Y., Chivas A. R., and Raab M. (1992) Boron isotope variations during fractional evaporation of sea water: New constraints on the marine vs. Nonmarine debate. Geology 20, 799 – 802. Xiao Y. K., Beary E. S., and Fasset J. D. (1988) An improved method for the high precision isotopic measurement of boron by thermal ionization mass spectrometry. Int. J. Mass Spectrom. Ion Proc. 85, 203–313. Xiao Y. K., Sun D. P., Wang Y. H., Qi H. P., and Jin L. (1992) Boron isotopic compositions of brine, sediments, and source water in Da Qaidam Lake, Qinghai, China. Geochim. Cosmochim. Acta 56, 11561–1568. Xiao Y. K., Xiao Y., Swihart G. H., and Liu W. G. (1997) The investigation of ion exchange technique for extracting boron from aqueous fluids by boron specific ion exchange resin. Acta Geoscientia Sinica. 18, 286 –289. Xiao Y. K., Shirodkar P. V., Liu W. G., Sun D. P., Wang Y. H., and Jin L. (1999) Boron isotopic geochemistry of salt lakes, Qaidam Basin, China. Advance of Nature Science, 917, 612– 618. Zhang P. X. (1987) The Saline lakes in Qaidam Basin (in Chinese) Science Press. Zhang P. X. (1993) Origin of ancient anomaly potash evaporates. (in Chinese). Science Press.
Pergamon
PII S0016-7037(00)00363-X
Boron concentration and isotopic composition of halite from experiments and salt lakes in the Qaidam Basin W. G. LIU,1,2,3,* Y. K. XIAO,2 Z. C. PENG,3 Z. S. AN,1 and X. X. HE3 1
Key Laboratory of Loess and Quaternary Geology, Chinese Academy of Sciences, Xi’an, 710054, China 2 Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, 810008, China 3 Department of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, China (Received September 2, 1999; accepted in revised form March 1, 2000)
Abstract—The concentration and isotopic abundance of boron in salt can be used to trace paleosalinities and depositional environments for marine and non marine evaporites. However, the mechanism of incorporating boron into halite during evaporation of salt lake brines is subject to dispute, and there have been few studies of boron concentrations and isotopic compositions during this process due to the low boron concentration in halite. A group of evaporation experiments from artificial solutions and salt lake brines have been analyzed in this study. The results of boron concentration and isotopic analyses demonstrate that the boron in halite comes mainly from fluid inclusions, with a lesser amount from coprecipitation. The isotopic fractionation factors between the brine and halite are from 0.9857 to 1.0000 for the evaporation experiments, and 0.9945 to 1.0009 for natural samples from the salt lake. The ␦11B values of halite from the Qaidam Basin salt lakes vary from ⫺4.7 to 25.8‰, compared to ⫺4.7 to 31.4‰ in the salt lake brines. These values are controlled by the boron isotopic composition of the boron sources, pH values and Na/Ca ratios in the salt lake brines. The variation of boron isotopes in halite may be used to trace the hydrochemical evolution and paleoevaporation environment in salt lakes. Copyright © 2000 Elsevier Science Ltd
␦11B value of halite is lower than that in the parent seawaters, suggesting that the boron is incorporated into the mineral lattice (Vengosh et al., 1992). In this work, we carried out evaporation experiments on artificial and natural brines and investigated the boron isotopic composition of the crystallized halite. In addition, the boron concentration and isotopic composition of halite from salt lakes in Qaidam Basin were determined. The aim of this work is to understand the mechanism of boron removal into halite, and to study the relationship between the ␦11B value of halite and the evaporation process in the parent brine.
1. INTRODUCTION
The boron isotopic composition of evaporite halite and brine has been used to distinguish between marine and non-marine sediment environments. The ␦11B values from a number of marine and non-marine borate minerals of different ages, range from 18 to 31‰ for nine marine minerals, to ⫺22 to 0‰ for twenty-five non-marine minerals (Swihart et al., 1986). The boron isotopic composition of brine, groundwater and sediments from modern Australian salt lakes indicate that the salt lake brines with high ␦11B values have a marine origin (Vengosh et al., 1991). The wide variations of boron isotopic compositions in the Qaidam Basin demonstrate that variations in the ␦11B of the source water are important in these salt lakes, and the lower ␦11B values indicate that the boron within salt lakes is largely of non-marine origin (Xiao et al., 1992). These studies suggest that the variations in boron isotopic compositions are caused by boron isotopic fractionation between brine, clay, boron minerals and/or carbonates. The isotopic fractionation factors are consistent with the theoretical calculations by Kakihana et al. (1977). Most studies have focussed on high boron concentration minerals, hence the boron isotopic geochemistry of evaporite minerals has not been widely applied to studies of paleosedimentary environments. Halite is the main mineral deposited during brine evaporation, however, there has been little work on the boron isotopic composition of halite due to the low boron concentration in halite. In addition, the mechanism of boron removal from brine into halite is not clear. Dong (1984) investigated the composition of trace elements in evaporite halite and indicated that boron may be present in the form of parent fluid inclusions. However, evaporation experiments on seawater showed that the
2. EVAPORATION EXPERIMENTS AND SALT LAKE SAMPLES Three sets of evaporation experiments were carried out in this study as following: 1. A primary consideration of experiment (1) was to examine the variation of boron concentrations and isotopic compositions during pure NaCl precipitation, without the interference of Ca and Mg ions in the brine. 1.2 g of H3BO3 was added to 500 mL of 4.75 M NaCl solution. 2. In experiment (2), 1.5 g CaCl2, 1.5 g Na2SO4 and 1.2 g H3BO3 were added to 500 mL of 4.07 M NaCl solution and dissolved completely. The purpose of this procedure was to study the variation of boron isotopes during precipitation of halite from the synthetic brine, and to compare with experiment (1) in order to examine the affects of Ca and SO2⫺ in halite. 4 3. Experiment (3) included two procedures; First (E2-0), the halite and associated salt lake brines were kept in a tight polyvinyl bottle at 10 –20°C for a period of six years, during which time about 10 g of halite had crystallized from the 500 mL of solution. Second (E2-1), the brine that was from the E2-0 was used as parent solution in experiment (E2-1). The aim of this experiment was to study variations of boron concentrations and isotopic compositions as a function of varying evaporation speeds and amounts of fluid inclusions in halite.
* Address to whom correspondence should be addressed. 2177
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Liu et al. Table 1. Boron concentrations and isotopic compositions of evaporation experiments. Brine
Halite
␦11B Value (‰) Sample B (g/g) pH Exp. 1 Z4-1 Z4-2 Z4-3 Z4-4 Z4-5 Exp. 2 Z1-0 Z1-1 Z1-2 Z1-3 Z1-4 Z1-5 Exp. 3 E2-0 E2-1
512 677 710 1120 2871 354 460 585 712 1038 2303 80 230
Run 1
Run 2
␦11B Value (‰) Run 3
Average
B (g/g)
Run 1
Run 2
Run 3
Average
␣
10.8 ⫾ 0.2 10.7 ⫾ 0.2 10.8 ⫾ 0.4 10.9 ⫾ 0.3
0.9999 0.9995 0.9999 1.0000
5.5 5.5 5.3 4.8
11.4 ⫾ 0.1 10.7 ⫾ 0.1 11.1 ⫾ 0.6 10.8 ⫾ 0.1 10.6 ⫾ 0.1
11.3 ⫾ 0.2 11.1 ⫾ 0.6 11.2 ⫾ 0.1 11.1 ⫾ 0.1 10.8 ⫾ 0.1 11.1 ⫾ 0.1
11.4 ⫾ 0.1 10.9 ⫾ 0.3 11.2 ⫾ 0.1 11.0 ⫾ 0.2 10.8 ⫾ 0.3
4.4 8.0 12.2 39.7
10.9 ⫾ 0.1 10.8 ⫾ 0.4 10.6 ⫾ 0.2 10.6 ⫾ 0.1
6.1 6.0 5.8 5.3
11.5 ⫾ 0.1 11.3 ⫾ 0.1 11.1 ⫾ 0.1 10.7 ⫾ 0.1 10.6 ⫾ 0.1 10.7 ⫾ 0.1
11.4 ⫾ 0.1 11.1 ⫾ 0.3 11.0 ⫾ 0.4 10.5 ⫾ 0.1 10.5 ⫾ 0.1 10.4 ⫾ 0.1 11.3 ⫾ 0.1 11.1 ⫾ 0.1
11.5 ⫾ 0.1 11.2 ⫾ 0.1 11.1 ⫾ 0.1 10.6 ⫾ 0.1 10.5 ⫾ 0.1 11.0 ⫾ 0.3
6.9 12.4 17.7 48.0
10.7 ⫾ 0.1 10.8 ⫾ 0.1 10.9 ⫾ 0.1 10.8 ⫾ 0.1 0.9996 9.7 ⫾ 0.1 8.7 ⫾ 0.3 9.2 ⫾ 0.7 0.9987 7.0 ⫾ 0.1 7.0 ⫾ 0.1 7.2 ⫾ 0.3 7.1 ⫾ 0.1 0.9966 7.5 ⫾ 0.1 7.8 ⫾ 0.1 8.1 ⫾ 0.1 7.8 ⫾ 0.3 0.9965
7.1 21.1 ⫾ 0.1 20.8 ⫾ 0.1 20.9 ⫾ 0.1 20.9 ⫾ 0.1 7.6 20.9 ⫾ 0.1 20.6 ⫾ 0.1 20.8 ⫾ 0.2
4.9 8.7
7.1 ⫾ 0.1 6.3 ⫾ 0.1 6.8 ⫾ 0.1 6.7 ⫾ 0.4 0.9857 17.7 ⫾ 0.1 17.7 ⫾ 0.1 17.9 ⫾ 0.1 17.8 ⫾ 0.2 0.9971
10.6 ⫾ 0.1 10.4 ⫾ 0.1 10.8 ⫾ 0.1 11.2 ⫾ 0.1 10.9 ⫾ 0.1 11.1 ⫾ 0.1
Note. Run 1–3 are total sample repeated, precision are expressed within 2 confidence limits (95%).
Experiments (1), (2) and (E2-1) were kept at 30 ⫾ 3°C in a clean air hood. At each step of the evaporation process, the halite was separated from solution, and successively washed with alcohol, high purity water and alcohol in order to remove the surface brine. The halite was then dried at about 50°C. Qaidam Basin is located in the east of the Qinghai-Tibet plateau where there are many non-marine evaporated salt lakes and playas. The boron concentrations of the brines in these salt lakes are higher than that in the ocean. Seven halite samples were collected from these salt lakes, and they were cleaned using the method described in the evaporation experiments.
were first outgassed at 3.0 A for one hour. 3 l of a graphite slurry was spread across the whole of the top of the filament, then approximately 2 g of boron was loaded with equimolar of mannitol and cesium (Cs2CO3) and evaporated to dryness at ⬍50°C. A VG354 mass spectrometer was used for this work. The isotopic analysis was started when the pressure in the source section reached (6 –3) ⫻ 10⫺7 Tor. The Cs2BO⫹ 2 ion beam intensity was typically about 3 ⫻ 10⫺12 A. The data were collected on a faraday cup by switching magnetically between masses 308 and 309. The isotopic ratios were corrected for the contribution of 17O to the mass 309 by following: 11
3. SAMPLE TREATMENT AND ANALYSIS The boron concentrations of the samples were measured using the azomethin-H spectrophometric method described by Kiss (1988). The external precision (2) of the measurements was ⫾2%. Boron for isotopic determination was separated from the sample solution by a two step ion exchange procedure using Amberlite IRA743 boron-specific anion-exchange resin (Kiss, 1988). The advantages of boron-specific resin are the high recovery for boron extraction (100%) without boron isotopic fractionation during the exchange process, and the high capacity for boron retention levers allowing the use of less exchange resin and chemical reagents to reduce blank (Hemming et al., 1992; Gaillardet et al., 1995; Xiao et al., 1997). The halite was dissolved in low-boron water and, in the case of samples from salt lakes, centrifuged for about 20 min to remove the small amount of detrital minerals. The brine was then diluted with low-boron water. These solutions, containing about 10 g B, (pH at 6 to 7) were passed through a 0.8 mL Amberlite (120 to 200 mesh) resin column using low-boron water, 2 M NH4OH and low-boron water to remove the cations. 12 mL of 0.1 M HCl was then used to elute the boron from the resin. The eluant was then evaporated to dryness at 65°C by adding an equal amount of mannitol to the boron to suppress volatilization of boron. The residue was dissolved in low-boron water and passed through a 0.15 ml Amberlite resin column (120 to 200 mesh) to further purify the sample. After adding more mannitiol, the eluate was again evaporated to dryness at 60°C for mass spectrometry analysis. Low boron water for the experiments was obtained by passing pure water through a column loaded with Amberlite IRA 743 boronspecific resin. The boron blank, as determined by isotope dilution mass spectrometry, was less than 40 ng. The process of boron isotopic measurement is essentially the same as that described elsewhere (Xiao et al., 1988). The single Ta filaments
B/10B ⫽ R (309/308) ⫺ 2 (17O/16O) ⫽ R (309/308) ⫺ 0.00075
The 11B/10B ratio of NBS SRM 951 was repeatedly determined to be 4.0493 ⫾ 0.0006 (2) with a relative external reproducibility of 0.15‰ (n ⫽ 10). The isotopic composition of boron in the samples is expressed as a permil deviation relative to the NBS SRM 951 boric acid standard. 4. RESULTS AND DISCUSSION
4.1. Boron Distribution in the Halite During the Evaporation Experiment The boron concentrations of the minerals and residual brines in the partial evaporation experiments are given in Table 1. The distribution coefficient is expressed as: Kd ⫽ ([B]mineral/[B]solution) The Kd values of halite are lower (0.0066 to 0.031) than those of absorbed clay (0.75 to 2.85) (Palmer et al., 1987), coprecipitated carbonate (2.2 to 10.4) (Hemming et al., 1992), and sediments (clay and boron minerals) (1.87 to 30.34) from salt lakes in the Qaidam Basin (Xiao et al., 1992). Halite is the main evaporite mineral in salt lakes, especially in the late stage of salt lakes and playas. The low Kd value of halite indicates that dissolved boron in the salt lake brines is largely concentrated during the stage of halite crystallization. Figure 1 shows that the boron concentration of the halite
Boron concentration and isotopic
Fig. 1. Relationship between boron concentration of the halite and brines.
increases with increasing boron contents in the saline solutions, according to the degree of evaporation. Dong (1984), suggested that boron may be present in the fluid inclusions in the halite. In contrast, others have indicated that the mechanism of boron removal from highly evaporated sea water may be by direct precipitation of Mg-borate minerals and/or formation of a Mgborate ion pair (MgB(OH)⫹ 4 ) and coprecipitation with other salts (Vengosh et al., 1992). The crystallized halite was examined under a binocular microscope, and a large number of fluid inclusions were found at a magnification of ⫻180 (Fig. 2). The relationship between the boron concentrations and distribution coefficients indicates that the Kd value in the NaCl solutions containing Ca and SO4 are slightly higher than those in the pure NaCl solutions (Fig. 3). This suggests that boron may be coprecipitated with CaSO4. 4.2. Boron Isotopic Compositions in Halite from the Evaporation Experiments The isotopic composition of the crystallized halite and saline solutions from experiments 1 to 3 are given in Table 1, together with the isotopic fractionation factor (␣) between solid and liquid, where ␣ is defined as:
Fig. 2. Photomicrograph of primary fluid inclusion in crystallized halite (at a magnification of ⫻180).
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Fig. 3. Relationship between halite kd and the boron concentration of the brines.
␣ ⫽ R (11/10)solid/R (11/10)liquid
(1)
In the experiment containing pure NaCl solution, the ␦11B value of the halite does not vary (total range 10.7 to 10.8‰), while a small variation is seen in the halite crystallized from the Na-Ca-Cl-SO4 solution (7.1 to 10.7‰). One possible interpretation of these results is that different mechanisms of boron isotopic fractionation existed in the two different solutions. The relative enrichment and depletion of 11B in brines and precipitates during evaporation and fractional crystallization of sea water and salt lakes brine have been used to explain the boron isotopic variations observed in evaporite minerals (Swihart et al., 1986; Qi et al., 1989; Vengosh et al., 1991). These previous studies were based on the distribution of trigonal BO3 and tetrahedral BO4 units in each mineral, but they neglected the possible presence of fluid inclusions in the minerals. Table 1 indicates that the isotopic fractionation factors vary from 0.9995 to 1.0000 during crystallization of pure NaCl from solution. The ␦11B values of the halite are the same as those in the coexisting saline solution, and do not vary with changing pH values, or with the B and Ca concentrations in solution. This suggests that boron does not enter the lattice of NaCl, so there is no boron isotopic fractionation when NaCl crystallized from solution; i.e., the boron in the halite is entirely contained within the fluid inclusions, and the ␦11B values of the evaporates remain the same as those of the parent solution. The ␦11B values of halite that crystallized from solutions containing Ca, SO4, Na and Cl are lower than that which crystallized from solutions containing only Na and Cl, and they have a lower ␦11B than the parent solution, (Fig. 4). The boron isotopic fractionation factor between solid and liquid ranges from 0.9965 to 0.9996, and increases with increasing Ca content, Na/Ca and the degree of evaporation. During evaporation of the Na-Ca-Cl-SO4 solution, boron may be partly removed from solution by coprecipitation with CaSO4. Because there may be a relatively large degree of boron isotopic fractionation during boron precipitation, the ␦11B value of halite containing precipitated minerals during crystallization is lower than that in pure halite. Hence, the small degree of boron isotopic fractionation found in the evaporation experiments of Na-Ca-Cl-SO4 solutions. This interpretation can explain why the ␦11B value in
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Liu et al.
Fig. 5. ␦11B values in brines vs. boron concentration of brines from evaporation experiments. The ␦11B values of solutions have not apparent variation with increasing degree of evaporation during the halite crystallization.
Fig. 4. Results of evaporation experiments. A) ␦11B of crystallized minerals vs. Broon content. B) Isotopic fractionation vs. Ca content of solution.
the high Ca concentration brines is higher than that in those with lower Ca levels. The ␦11B value of the crystallized halite also shows a progress decrease with decreasing pH values of the saline solution. Table 1 presents the results of evaporation experiment (3). Sample E2-0 was from the Gashkule lake brine that was preserved in a sealed bottle, at 10 to 20°C, for six years. During this period 10 g of halite precipitated from 500 ml of brine. The crystallized halite was examined by a binocular microscope and a small number of fluid inclusions were found. Zhang (1993) suggested that a large number of fluid inclusions in halite is usually associated with fast evaporation and large temperature variations in salt lakes. The ␦11B value of the halite in experiment E2-0 is lower than that in the experiment E2-1. The boron isotopic fractionation factor between the halite and brine in E2-0 is 0.986. This value is in good agreement with the average of the empirical ␣ value between sediments and brines from salt lakes (Xiao et al., 1992). Experiment (E2-0) suggests that fluid inclusions are not the only source of boron in the halite, and that boron isotopic fractionation is controlled by coprecipitation with halite when fewer fluid inclusions are formed in halite during slow crystallization. The isotopic fractionation factor of E2-1 (0.997) is similar to that in the evaporation experiments (1) or (2). The results support the hypothesis that boron in halite is mainly present within fluid inclusions and that the boron isotopic fractionation is controlled by the
evaporation temperature which determines the number of fluid inclusions in the halite. The small degree of boron isotopic fractionation and the low boron content (4.4 – 48 g) in halite from evaporation experiments suggest that the precipitated minerals in the halite are not borate minerals. The results of X-ray diffraction studies indicate that there is a small amount of Ca-sulphate in the halite from experiments (2) and (3), including CaSO4 䡠 2H2O and CaSO4 䡠 6H2O. In experiment (2), the highest concentration of CaSO4 䡠 2H2O found in the halite was less than 2% in the Z1-5. In experiment E2-1, the concentration of CaSO4 䡠 6H2O in the halite was less than 1%. The ␦11B values of the saline solutions do not show any obvious change during the evaporation and crystallization stages (Fig. 5), during which the boron concentration of these solutions ranged from 354 to 2871 g/g. This reflects the fact that relatively little boron is partitioned into the halite from the brine, and that this process is not associated with a large degree of isotopic fractionation. This result is similar to that obtained from evaporation of seawater, which also showed that the ␦11B value in the residual brine did not vary during precipitation of halite (Vengosh et al., 1992). 5. HALITE AND BRINE IN THE SALT LAKES OF QAIDAM BASIN
The boron isotopic compositions of seven samples of halite and coexisting brine are given in the Table 2, together with their boron concentrations and the calculated isotopic fractionation factors. The pH values and Na/Ca ratio of the brines are also presented in Table 2. The ␦11B value of the halite has a wide range from, ⫺4.7 to 25.8‰, similar to the range deserved in the coexisting brine, ⫺4.0 to 31.4‰. The ␦11B values of the halite and coexisting brine have similar characteristics in that the ␦11B values decrease with increasing boron concentrations and pH values in the salt lake brines, and decreasing Ca contents in the brine (Fig. 6). The results show that the ␦11B values of the halite are mainly controlled by the boron isotopic composition of the salt lake brines. The small isotopic fractionation factor (0.9945 to
Boron concentration and isotopic
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Table 2. Boron concentrations and isotopic compositions of brine and halite from salt lakes, Qaidam Basin. Brinea
Halite
␦11B Value (‰) Sample
pH
B (g/ml)
␦11B (‰)
Na/Ca
B (g/g)
Run 1
Run 2
Gasikule Mahai Balun Xiaochaidam Dachaidam Chaikai Niulang
7.11 7.98 6.50 8.14 7.78 7.68 5.34
101.6 162.9 10.1 285.6 404.3 15.4 53.6
17.9 ⫺4.0 31.4 ⫺0.5 5.3 9.9 24.5
193.8b 208.3 12.1 167.0 255.7 130.6 0.7
8.0 11.6 0.4 13.0 16.0 7.4 3.8
12.1 ⫾ 0.3 ⫺4.7 ⫾ 0.2 25.7 ⫾ 0.3 0.4 ⫾ 0.1 3.8 ⫾ 0.1 6.0 ⫾ 0.1 20.3 ⫾ 0.3
12.5 ⫾ 0.1 ⫺5.1 ⫾ 0.1 25.9 ⫾ 0.1 0.4 ⫾ 0.1 3.7 ⫾ 0.1 5.6 ⫾ 0.1
Run 3 ⫺5.3 ⫾ 0.1
6.1 ⫾ 0.1
Average
Kd
␣
12.3 ⫺5.0 25.5 0.4 3.8 5.9 20.0
0.079 0.071 0.041 0.049 0.040 0.480 0.071
0.9945 0.9993 0.9945 1.0009 0.9984 0.9960 0.9956
Note. Run 1–3 are total sample repeated, precision are expressed within 2 (95%) confidence limits. The results of the brines are from Xiao et al. (1999). b This result is from Zhang (1987). a
1.0009) is consistent with the results of the evaporation experiments discussed above, and suggests that fluid inclusions dominate the boron isotopic composition of the halite. However, the isotopic fractionation factors between the halite and brines from the salt lakes differ from the experiments (1) in that it does not appear to be dependent on the Ca content of the brines. The relationship between the ␦11B values and pH is different in the evaporation experiment and the salt lakes because of the large variations in pH in the latter.
Based on the results from the experiments and the salt lakes, boron isotopic compositions of the halite can be described thus,
␦11Bhalite ⫽ ␦11Bb F1 ⫹ ␦11 Bc F2
(2)
where the ␦11Bb and ␦11Bc are the boron isotopic compositions of the fluid inclusions and the boron coprecipitated with the halite. F1 and F2 are the fractions of boron in the fluid inclusions and coprecipitated with the halite. Equation (2) can also be described as;
␦11Bhalite ⫽ ␦11Bb F1 ⫹ ␦11Bb F2 ⫺ 103(1 ⫺ ␣) F2
(3)
The ␣ in equation (1) is given as:
␣ ⫽ (␦11Bc ⫹ 103)/(␦11Bb ⫹ 103)
(4)
The mechanism of boron isotopic fractionation during coprecipitation and absorption has been studied in many fields in the last few years. The ␦11Bc value depends on the pH and boron concentration of the solution (Kakihana et al., 1977; Palmer et al., 1987; Hemming et al., 1995). The B(OH)⫺ 4 species in solution tends to be enriched in 10B and is preferentially removed from solution and incorporated into the solid phase. The mechanism controlling boron isotope variations in halite is different from those identified in previous investigation of sediments. A preliminary explanation for the variation of the boron isotopic composition of halite for different evaporative condition is given as follows.
Fig. 6. A) ␦11B value in the halite and brine vs. pH value of brine. B) ␦11B value in the halite and brine vs. Ca content of brine. The Ca content of brine in Gashikule Lake is from early result (Zhang 1987) and is not presented in Figure 6B.
1. In pure halite (F2 ⫽ 0), the boron is only present within fluid inclusions in the halite. In this case, ␦11B values of the halite correspond to that in brine (␦11Bhalite ⫽ ␦11Bbrine). 2. When the halite is formed at a low temperature by slow crystallization, for example as in the evaporation experiment (E2-0), there are very few fluid inclusions in the halite (F1 ⫽ 0). In this case, boron is removed from the solution by coprecipitation and the ␦11B in halite is controlled by that of the coprecipitated phase ␦11Bc. 3. In most halite, the boron composition consists of a mixture of that which has been coprecipitated and of that in the fluid inclusions. The ␦11B value in halite is related to the pH value and Ca content of the brine (Fig. 6). In high pH (⬎7.5) and low Ca (⬍1 g/L) brines, the boron isotopic fractionation between the sediments and the brine is small relative to that
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Fig. 7. Relationship between the ␦11B value of the halite and salt lake brines.
at low pH (Palmer et al., 1987). In addition, a low Ca concentration in the brine reduces the contribution of boron from coprecipitation in the halite. Where boron is mainly located within the fluid inclusions, the ␦11B of the halite is in close agreement with the isotopic composition of the brine. In low pH (⬍7.5) and high Ca (⬎1 g/L) brines, relatively large isotopic fractionation (0.9945 to 0.9960) is produced by decreasing the pH value. The more Ca is present in brine, the more boron coprecipitated with the halite, hence the ␦11B value of the halite should be lower than that in the corresponding brine. 6. GEOLOGICAL IMPLICATIONS
The trace element boron is relatively enriched in salt lakes, especially in Qaidam Basin of the Qinghai-Tibet plateau. The small boron isotopic fractionation and similar chemical composition of the halite and brine mean that the boron isotopic composition of the halite is approximately equal to that in the salt lake brines. Figure 7 shows that there a positive relationship between the boron isotopic composition of the halite and the salt lake brines. Therefore, ␦11B values in halite provide a record of the boron isotopic composition of salt lake brine during crystallization of halite. The ␦11B value of the halite is related to the chemical composition and evaporation environment of the salt lakes. High boron concentrations and low ␦11B values in the halite are associated with high boron concentrations in the salt lake brines. ␦11B values in halite may be a indicator that boron is enriched or depleted during evolution of salt lake brines. Figure 8 shows that ␦11B values in halite from salt lakes of boron mine area, together with high boron content, for example, in Dachaidan, Xiaocaidan and Mahai salt lakes, are lower than that in halite from other salt lakes. Zhang et al. (1993) indicated that primary fluid inclusions in clear halite are the remains of brine from when the halite crystallized and are a useful tool with which to trace the brine evolution of salt lakes. Boron isotopic fractionation between halite and brine is small in sulphate type salt lakes when that brine has a high pH value (6.5 to 8.1) and low Ca content (⬍0.6 g/L). The ␦11B value of the halite then directly records the boron isotopic composition of the primary brine. The pH value of brine
Fig. 8. Boron isotopic composition of halite in salt lakes as a function of B concentration.
usually decreases with progressive evolution of salt lake. The chloride type brine is the final stage of a salt lake, and has a lower pH value and high Ca and Mg content compared with sulphate type brines (Zhang et al., 1987). The ␦11B value of brine is increased by the larger isotope fractionation at low pH value, a large amount of coprecipitated boron and high Ca contents. However, the distribution coefficient between absorbed boron and brine in solution is low at low pH (Palmer et al., 1987), so the ␦11B value of halite precipitated from chloride type brines is higher than that formed from in the sulphate type brines. Hence, the boron isotopic composition of halite in salt lake is directly related to the evolution of boron isotopes in that salt lake. In addition, fluid inclusions are present in other evaporite minerals, and not just halite. Hence, boron within these evaporite minerals may also have a significant contribution from fluid inclusions, therefore, it is important to consider the affect of inclusions in minerals in the study of the boron isotopic compositions of evaporite minerals. 7. CONCLUSION
The results of the evaporation experiments and studies of samples from salt lakes show that similar processes control the variation of boron concentrations and isotopes. The evidence that boron in halite is mainly contained within fluid inclusions and not in coprecipitated minerals has been provided by considering the boron concentration and isotopic composition of halite and brine from evaporation experiments and salt lakes in Qaidam Basin. The ␦11B of halite varies from ⫺5.0 to 25.8‰, and shows a range of ⫺4.7 to 31.4‰ for the salt lake brines. The characteristics of boron concentrations and isotopic compositions in halite are as following: 1. The distribution coefficient between halite and associated brine is lower than that in other evaporite minerals. This results in the concentration of boron in the brine during halite crystallization. 2. The boron isotopic composition of halite is controlled by a mixture of boron within the fluid inclusions and boron that has been coprecipitated with the halite. 3. There is only a small boron isotopic fractionation produced during crystallization of halite from brine. This isotopic
Boron concentration and isotopic
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