Salt Lake System Research Articles

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, BCl, and NaCl ratios.The Victorian volcanic-crater lakes of southeastern Australia have water salinities of up to 60 g/L, molar NaCl ratios (0.87) similar to the marine ratio, molar BCl 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 (BClratio = 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 (WR) 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 NaCl ratios of 0.85 to 1, molar BCl 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 BCl 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 NaCl 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 BCl 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.NaCl ratios are used as indicators of the origin of the salts as well as of halite dissolution-precipitation. The δ11 values and BCl ratios are sensitive to a marine or non-marine origin, adsorption of boron onto clays, and the effective water/sediment ratio. At low WR ratios, the preferential removal of 10B from the solution affects the bulk solution, whereas at high WR 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.

Classification and Genesis of Calcrete and Gypsite Lithofacies in Paleodrainage Systems of Inland Australia and their Relationship to Carnotite Mineralization

Calcrete and gypsite lithofacies are chemical sedimentary rocks irre spective of age and/or stratigraphic position. The depositional environments of these deposits indicate that, since early Tertiary silting up of inland drainage developed internal salt lake systems with gypsite as the dominant evaporitic lithotype. Elongate bodies of phreatic calcrete formed in paleochannels. Almost all primary calcretes and gypsites have been modified by phreatic and vadose diagenetic processes. Because the diagenetic processes are complex, stratigraphic analysis is difficult. Carnotite mineralization is probably not contemporaneous with calcretization. Assuming that waters from fertile (uraniferous) sources are flowing through the sediments, several controls will govern carnotite mineralization; ponding by physical or chemical barriers, solution chemistry (P CO2 ; CO 3 /HCO 3 , V, U, and K concentrations; Eh, pH), and postdepositional physical or chemical changes. For significant carnotite bodies to remain, primary deposition must have been followed by a freezing mechanism to preserve the deposit. Diagenetic conditions which may cause remobilization vary between deposits; therefore it may be unwise to use a single deposit as the basis for a widely applicable exploration model.--Modified journal abstract.

Fulbright Awards in Earth sciences

Several of the 500 Fulbright awards for 1981–1982 are available for work in earth sciences: Argentina—edaphology and volacanic soils; Australia—Metamorphic rocks, the Australian salt lake system, and soil taxonomy; Austria—physical geography, Fiji—environmental studies; Ivory Coast—tectonic petrology and paleontology; Nepal—meteorology; Norway—paleontology; Pakistan—geology; Tanzania—Physical oceanography or geophysics; United Kingdom—oceanography; U.S.S.R.—several specialities; and unspecified fields in Honduras, Korea, and Liberia.