Geochemical Journal, Vol. 25, pp. 377 to 385, 1991 Boron isotope fractionation accompanying boron mineral formation from aqueous boric acid-sodium hydroxide solutions at 250C TAKAO OI, JUNPEI KATO, TOMOKO OSSAKA and HIDETAKE KAKIHANA Department of Chemistry, Sophia University, 7-1 Kioicho, Chiyodaku, Tokyo 102, Japan (Received November 22, 1990; Accepted October 31, 1991) A series of experiments was carried out in which boron minerals were precipitated from pH and chemical composition-controlled solutions at 25°C and boron isotope fractionation accompanying the mineral deposition was measured. Borax, sassolite and sborgite were synthesized from aqueous boric acid-sodium hydroxide solutions. Borax was obtained from solutions with higher pH and sassolite from solutions with lower pH, irrespective of the mole ratio of B and Na in the solution. The fractionation factor, S, defined as S = ("B / 10B)minerai/ ("B /'OB)s°luti°nincreases with increasing the pH of the solution for both borax and sassolite. A cross-over point at which S=1 was found at about pH=9.5 for borax, while S was always larger than unity for sassolite. The present results of boron isotope fractionation are consistent with theoretical prediction of equilibrium isotope effects, although the experimental equilibrium constant of the boron isotope exchange reaction between B(OH)3 and B(OH)4 at 25'C is larger than that theoretically predicted (1.03-1.05 compared to 1.0194). In a previous paper (Oi et al., 1989), we show INTRODUCTION ed that evaporite boron minerals with the same Variations in boron isotopic compositions in geologic origin but with different structural for nature are of geochemical and cosmochemical mulae have different boron isotopic composi importance. They are utilizable for studying tions. That is, minerals with higher B03/1104 nucleosynthetic mechanism at the early stage of ratios (the ratio of the number of B03 triangle the solar system and the galaxy (Ishikawa and units to the number of B04 tetrahedron units in Nakamura, 1989), and are also used for studying the structural formula of a mineral) have higher the sedimentary cycle of boron (Spivack et al., "B/10B ratios . This observation is consistent 1987), hydrothermal and geothermal processes with prediction from the theory of isotope effects (Spivack and Edmond, 1987), the interaction of based on the isotopic reduced partition function magmas with sea water (Kanzaki et al., 1979; ratios (RPFRs) (Bigeleisen and Mayer, 1947). Nomura et al., 1982; Oi et al., 1991) and the However, a quantitative approach is still re genesis of ore deposits (Palmer and Slack, 1989; quired. The previous study showed that the type Slack et al., 1989). To make full use of boron of mineral formed and its boron isotopic ratio isotopes as a tracer in natural systems, it is are strongly dependent on the chemical composi necessary to have a good understanding of tion and pH of original solution from which the boron isotope effects which occur at each step of mineral was deposited. Therefore, to elucidate boron reaction in nature. quantitatively boron isotope fractionation in the 377 378 T. Oi et al. boron mineral formation from boron-bearing Table 1. The initial conditions of the solution phase solutions, we carried out a series of synthesis ex B:Na B concn. Volume periments with chemical composition and pH Run No. pH mole ratio (moldm-3) (Cm') controlled boric acid-sodium hydroxide solu B17 11.52 1 1 0.885 199.3 tions. The boron isotopic ratios of solutions and B08 11.01 1 1 0.812 203.6 minerals were then measured. In this paper, we B18 11.01 1 1 0.880 200.7 report the results of these experiments and B09 10.01 1 1 0.785 211.1 B10 9.01 1 1 0.761 217.6 discuss the boron isotope fractionation accompa B23 8.01 1 1 0.851 223.3 nying boron mineral formation from aqueous B25 8.00 1 2 0.680 252.0 solution. B24 7.01 1 2 0.775 226.0 B27 8.99 2 1 0.851 202.3 B29 7.00 2 1 0.809 212.5 B30 EXPERIMENTAL 6.00 2 1 0.797 215.8 The experimental procedures are briefly as follows. A saturated boric acid solution was first (Nomura et al., 1973; Oi et al., 1989). In brief, prepared at 23'C. To this solution was added boron was extracted from liquid samples and sodium hydroxide so that the mole ratio of purified by methyl borate distillation and ion-ex boron to sodium becomes 1:1, 1:2 or 2:1. The change. Sodium hydroxide was then added so pH of the solution was then adjusted by 5 M that the mole ratio of B to Na became about (1 M =1 mol / dm') HCl and then used as the 1:1.5. The resultant solution was loaded on a initial solution of each experiment (Table 1). boat-shaped rhenium filament. The boron About 200 cm3 of this initial solution contained isotopic ratio was determined by measuring the in a beaker was placed in a water bath tempera height ratio of Na210BO2and Na211BOi peaks . ture-controlled at 25±0.2°C. While the beaker In the case of solid samples, the samples were was in the bath, it was not shaken nor was the first dissolved in pure water and the subsequent solution stirred, but the pH of the solution was procedure was the same as that for the liquid monitored frequently. Minerals were samples. The 95% confidence limit of measure precipitated from the solution by concentration ment was typically about ±0.2%. Each sample of the solution due to water evaporation, was measured 2 or 3 times and the average was without any artificial manipulation such as inser taken as the isotopic ratio of the sample tion of a seed crystal. Upon the mineral deposi (Nomura et al., 1973). tion, the pH of the solution phase was measured and the solid and the liquid phases were separated by suction filtration using a chilled RESULTS glass filter. The precipitate was air-dried and the Experimental results are summarized in mineral phase was identified by X-ray powder Tables 2 and 3. It took about 2 days to 3 weeks diffraction using a Rigaku Denki X-ray spec to precipitate minerals. Any correlation was not trometer. The amounts of boron precipitated found between the conditions of initial solutions and remaining in solution were determined by and the time that elapsed before mineral deposi conventional acid-base titration or by induc tion started (deposition time). For each experi tively coupled plasma atomic emission spec ment, the pH of the solution was fairly constant trometry. until the initiation of mineral precipitation . The The boron isotopic ratios of the solution and pH of the solution after precipitation became minerals were measured by the surface ioniza slightly higher than that of the initial solution tion method with a Varian MAT CH-5 mass spec when the latter was above 10 but became slightly trometer at Tokyo Institute of Technology . The lower when the latter was below 9. The mole frac details of mass spectrometry are given elsewhere tion of boron transferred from the solution B isotope fractionation 379 Table 2. Experimental results other than isotopic data Deposition Approximate Solution phase Solid phase Run time (h) Volume of water No. evaporated B conc. Deposited Mole (cm3) pH (moldm-3) mineral fractions B17 280.3 109 11.54 1.969 borax 0.173 B08 280.5 110 11.48 1.766 borax 0.189 B18 174.0 78 11.02 1.156 borax 0.272 B09 40.0 27 10.32 0.459 borax 0.499 B10 40.0 27 8.75 0.456 borax 0.425 B27 503.1 135 8.22 1.115 borax 0.611 B25 195.3 83 7.58 0.942 borax 0.183 B23 259.5 104 7.35 1.637 borax 0.138 B24 476.2 168 5.44 1.625 sassolite +sborgite B29 140.0 39 5.91 2.190 sassolite 0.063 B30 331.9 88 5.48 1.254 sassolite 0.115 a=the amount of B in the mineral divided by the amount of B in the initial solution , calculated using the B contents in the solid phase and in the initial solution. Table 3. Isotopic data Solution phase Solid phase Run S Mineral 11B/10B No. 11B/ 10B a"B a"B B17 borax 4.031±0.009 -3 .0 4.087 ±0.009 +10.8 1.0138 B08 borax 4.028±0.009 -4 .0 4.067 ±0.006 +5.7 1.0097 B18 borax 4.040±0.008 -0 .9 4.087 ±0.008 +10.8 1.0118 B09 borax 4.046±0.003 +0.5 4.065 ±0.009 +5.3 1.0048 -3 B10 borax 4.072±0.003 +7.1 4.028 ±0.004 .9 0.9890 -2 B27 borax 4.056±0.001 +3.0 4.033±0.006 .7 0.9943 -6 B25 borax 4.069±0.006 +6.4 4.019±0.004 .2 0.9875 -7 B23 borax 4.053 ±0.001 +2.3 4.015 ±0.005 .0 0.9907 B24 sassolite 4.038 ±0.006 -1 .4 4.052±0.006 +2.0 1.0034 -3 sborgite 4.029±0.002 .6 0.9978 B29 sassolite 4.043±0.002 -0 .1 4.061±0.003 +4.2 1.0044 B30' sassolite 4.045 ±0.003 +0.3 4.047 ±0.004 +0.9 1.0006 phase to the solid phase ranges from 0.063 to Chloride ions were not found in the minerals.