Boron isotope geochemistry of Australian salt lakes
Boron isotope geochemistry has been investigated in brines, groundwaters, and sediments from the modern Australian salt lakes of Victoria, South Australia, and Western Australia by applying negative thermal-ionization mass spectrometry techniques. The geochemical history of the brines has been reconstructed by using δ11B, B Cl, and Na Cl ratios. The Victorian volcanic-crater lakes of southeastern Australia have water salinities of up to 60 g/L, molar Na Cl ratios (0.87) similar to the marine ratio, molar B Cl ratios of 2.9 × 10-4 to 4.9 × 10-4, and δ11B values of 54%. to 59%. (relative to NBS 951). The depletion of total B and the high positive δ11B values relative to seawater ( B Clratio = 7.9 × 10-4; δ11B = 39%.) are attributed to a marine (cyclic) salt origin together with adsorption processes in closed systems with low water/sediment ( W R) ratios. In contrast, salt lakes from South Australia and Western Australia which are large shallow playas associated with halite, gypsum, and detrital clay minerals have interstitial and surface brines characterized by salinities of 80 to 280 g/L, molar Na Cl ratios of 0.85 to 1, molar B Cl ratios of 4 × 10-6 to 4 × 10-4, and δ11 values of 25%. to 48%.. The δ11 values of these brines are different from those of groundwaters from the Great Artesian Basin (δ11 = -15.9%. to 2.2%.; with high molar B Cl ratios of 1 × 10-3 to 3.8 × 10-2), country rocks (δ11B = -8.7%. to 6.8%.), and modern detrital sediments present in these salt lakes (δ11B = -3.2%. to 12.3%.). The δ11B values of these salt lakes overlap with those of surface and brackish waters (δ11B = 28%. to 35%.) and with the B isotopic composition of seawater (δ11B = 39%.). Both low molar Na Cl ratios (<1) and high δ11B values suggest that the source of the bulk of the dissolved solids in the Australian brines is dominated by cyclic salts, derived from seawater, rather than from local rock weathering. While the low B Cl ratios and high δ11 values (>39%) of some brines indicate interaction of the brines with detrital sediments within the salt lake systems, δ11 values < 39% suggest mixing of brines of marine origin from which B was partly removed by adsorption, with waters of terrestrial origin with low δ11 values. Na Cl ratios are used as indicators of the origin of the salts as well as of halite dissolution-precipitation. The δ11 values and B Cl ratios are sensitive to a marine or non-marine origin, adsorption of boron onto clays, and the effective water/sediment ratio. At low W R ratios, the preferential removal of 10B from the solution affects the bulk solution, whereas at high W R ratios, the δ11 value of a solution is not affected by adsorption. Although the δ11 value of borate minerals may be a discriminant of marine or non-marine origin, boron isotopes are less distinctive in evaporative environments where boron is not an abundant component and where water/sediment interaction occurs. © 1991.
Vengosh, A; Chivas, AR; McCulloch, MT; Starinsky, A; Kolodny, Y
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