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 delta-B-11, 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 x 10(-4) to 4.9 x 10(-4), and delta-B-11 values of 54 parts per thousand to 59 parts per thousand (relative to NBS 951). The depletion of total B and the high positive delta-B-11 values relative to seawater (B/Cl ratio = 7.9 x 10(-4); delta-B-11 = 39 parts per thousand) 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 x 10(-6) to 4 x 10(-4), and delta-B-11 values of 25 parts per thousand to 48 parts per thousand. The delta-B-11 values of these brines are different from those of groundwaters from the Great Artesian Basin (delta-B-11 = -15.9 parts per thousand to 2.2 parts per thousand; with high molar B/Cl ratios of 1 x 10(-3) to 3.8 x 10(-2)), country rocks (delta-B-11 = -8.7 parts per thousand to 6.8 parts per thousand), and modern detrital sediments present in these salt lakes (delta-B-11 = -3.2 parts per thousand to 12.3 parts per thousand). The delta-B-11 values of these salt lakes overlap with those of surface and brackish waters (delta-B-11 = 28 parts per thousand to 35 parts per thousand) and with the B isotopic composition of seawater (delta-B-11 = 39 parts per thousand). Both low molar Na/Cl ratios (< 1) and high delta-B-11 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 delta-B-11 values (> 39 parts per thousand) of some brines indicate interaction of the brines with detrital sediments within the salt lake systems, delta-B-11 values < 39 parts per thousand suggest mixing of brines of marine origin from which B was partly removed by adsorption, with waters of terrestrial origin with low delta-B-11 values. Na/Cl ratios are used as indicators of the origin of the salts as well as of halite dissolution-precipitation. The delta-B-11 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 B-10 from the solution affects the bulk solution, whereas at high W/R ratios, the delta-B-11 value of a solution is not affected by adsorption. Although the delta-B-11 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.