Boron isotope variations during fractional evaporation of sea water: New constraints on the marine vs. nonmarine debate

Examination of boron isotopes, elemental B, Br, and Li in brines, and coprecipitated salts during fractional evaporation of sea water shows that Br, Li, and B in the evaporated sea water have lower concentrations than expected, as determined from mass-balance calculations. The deficiency is found beyond a degree of evaporation of ~30 and is associated with a gradual increase in the ô n B values of the evaporated sea water, from 39°/oo to 54.7°/oo (relative to standard NBS 951). The high ô n B values of the brines and the relatively lower ô n B values of the coexisting precipitates (MgS0 4 and K-MgS0 4 salts; S n B = 11.4/00 to 36.0°/00) suggest selective uptake of B by the salts. Applying Rayleigh distillation equations, the empirical fractionation factors for the depletion of the salts in n B are estimated as 30°/oo (a. = 0.969) for the early stages of precipitation (gypsum and halite range) and 20°/oo (a = 0.981) for the late stages (K-MgS04 minerals). Coprecipitation of B(OH)4~ species with the salts, and/or precipitation of Mg-borate minerals with a coordination number of 4 are the proposed mechanisms for boron isotope fractionation during fractional evaporation of sea water. The boron isotope composition of sea water ( ô l l B = 39%0) is significantly higher than that of continental water (i5B = -3°/oo ±5%o). Our study shows that salt deposits may be depleted in U B by 20°/oo to 30°/oo relative to their parent brines. These variations suggest that boron isotopes can be used to determine the origin (marine vs. nonmarine) of brines and ancient evaporitic environments. INTRODUCTION The identification of the origin of evaporites (i.e., marine vs. continental) is important for the interpretation of ancient saline and hypersaline environments. Conventional geochemical methods, such as analysis of bromide content, cannot exclusively distinguish marine from nonmarine sources (Hardie, 1984, 1990; Sonnenfeld, 1985; Holser, 1979; Lowenstein and Spencer, 1990). For example, Hardie (1984, 1990) demonstrated that the compositions of some modern nonmarine saline lakes are similar to that of evaporated sea water. Boron isotope geochemistry can determine the origin of brines and evaporites. Sea water is enriched in n B relative to both oceanic crust and the continental crust (Spivack and Edmond, 1987; Spivack et al., 1987). The difference is due to isotopic fractionations that occur when boron is removed from sea water onto detrital clays, weathered basalts, and carbonate minerals (Schwarcz et al., 1969; Spivack and Edmond, 1987; Vengosh et al., 1991a). Previous studies showed that ancient marine evaporite borates are enriched in U B relative to their nonmarine counterparts (Swihart et al., 1986; Oi et al., 1989). Nevertheless, the 'Present address: Hydrological Service, Water Commission, Ministry of Agriculture, P.O. Box 6381, Jerusalem 91063, Israel. behavior of boron isotopes during evaporation and crystallization processes has not been studied. The purpose of this study is to examine the variations of boron isotopes, elemental B, Br, and Li in evaporated sea water and coprecipitated salts during artificial evaporation of sea water accompanied by fractional mineral crystallization. EXPERIMENTAL PROCEDURES Two sets (Y and R series) of 201 of Mediterranean sea water were evaporated for 73 days in a clean, air-filtered fume hood. At different stages of evaporation, measured fractions of residual brines and precipitated salts were separated from the main brine reservoir for chemical and isotopic analyses. The residual brine was not allowed to interact with brines and salts of previous stages. The brines were filtered several times after separation, and the precipitates were rinsed with ethanol, dried at 60 °C, and crushed. Further details of the technical procedures were reported in Raab and Spiro (1991). Boron isotope composition was determined by negative thermal-ionization mass spectrometry (Vengosh et al., 1989, and references therein). Brines and F^O-dissolved salts were deposited directly onto Re filaments and evaporated to dryness before loading into a reverse-polarity, NUCLIDE-type, solid-source mass spectrometer at the Australian National University (Vengosh etal., 1989,1991a). The U B / 1 0 B ratios are reported as per mil deviation (<5"B) relative to the standard NBS SRM 951: <5B = ( [ n B / ' ^ s a m p l e / 1 • B / ^ B N B S 95ll D X 1 0 0 0 . The mean of the absolute 1 *B/ B ratios of the NBS SRM 951 replicates, analyzed along with the samples, is 3.987 ± 0 . 0 0 6 . The average <5B value of 14 sea-water replicates was 37.7°/oo, with a standard deviation of 1.5°/Oo. Boron concentrations were determined by isotope dilution mass spectrometry (Vengosh et al., 1989), lithium by inductively coupled plasma spectrometry, and bromine by X-ray fluorescence spectroscopy. RESULTS AND DISCUSSION The experimental evaporation of Mediterranean sea water yielded progressively concentrated brines and the expected soluble salts for such evaporation, i.e., carbonate, gypsum, halite, MgS04> and K-MgS04 minerals (Raab and Spiro, 1991). The concentrations of Br, Li, and B, as well as the S 'B values of both the evaporated sea water and salts, increased with evaporation, whereas the salts yielded relatively lower B contents and S U B values (Table 1). We used the Br, Li, and B contents of the evaporated sea water to detect the degree of evaporation. Assuming that Br, Li, and B accumulate in conservative amounts in the solution, and considering the volume of the brines and the aliquots of the separate fractions that were taken during the experiment for chemical analyses, we calculated the expected concentrations of these elements in each of the evaporation stages. We defined «(volume), which is the absolute degree of evaporation, as the ratio between the expected concentrations of the conservative elements and their concentrations in ocean water. Although the measured Br, Li, and B contents of the brine increased with evaporation, at late stages of evaporation the measured concentrations were lower than expected. The degree of evaporation calculated for boron (e [B]), lithium (e [Li]), and bromide (e [Br]) were lower than that of the f (volume) values (Table 1, Fig. 1). Previous investigations, both in artificial and natural conditions, used these elements as indicators for the degree of evaporation of sea water (McCaffrey et al., 1987, and references therein). Our experGEOLOGY, v. 20, p. 799-802, September 1992 799 iment shows, however, that the three elements are not absolutely conservative—they do not accumulate solely in the liquid phase. In the late stages of evaporation the measured apparent degree of evaporation of lithium was higher than that of boron, which was, in turn, higher than that of bromide (Fig. 1). Thus, considerable amounts of all these elements contribute to the