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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 0 / 00 to 36.0°/ 00) suggest selective uptake of 10 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-MgS0 4 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 ll B = 39% 0) is significantly higher than that of continental water (i5 n B =-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.

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