Geochemical characteristics of naturally acid and alkaline saline lakes in southern Western Australia

摘要

Abundant shallow saline lakes on the Archean Yilgarn Craton in southern Western Australia exhibit a rare spectrum of geochemical conditions. Here the field geochemistry over three seasons (pH, salinity, and temperature), as well as major ions, trace elements, and H, O, and S stable isotopes of surface waters and shallow groundwaters from 59 ephemeral takes in southern Western Australia (WA) are reported. Approximately 40% of the lakes and 84% of the measured groundwaters in WA are extremely acidic (pH < 4) and pHs are observed as low as 1.7. The salinity of lake waters and groundwaters ranges from rare freshwaters to common saline waters and brines with total dissolved solids >28%. The fluids are typically Na-Cl to Na-Mg-Cl-SO4 brines with variable yet locally high amounts of Ca, K, Al, Fe, Si, and Br. The acid brine fluid compositions are unusual. For example, in some fluids the amount of Al Ca, the amount of Br > K, and comparison of total S to SO42- values suggest the presence of other uncommon S-bearing species. Trends in delta O-18 and delta(2) H illustrate the separation between surface lake water and shallow groundwaters, and indicate the contribution of meteoric waters to the lakes. The chemical and isotopic compositions of these fluids indicate a spatially and temporally dynamic, yet regionally consistent, history of brine evolution that is fundamentally different from most other terrestrial closed basin brines. The WA lake brines do not evolve from surface evaporation of dilute inflow waters, but rather are fed by highly evolved regionally acid saline groundwaters. The lake waters then diversify with locally varying surface and near-surface processes such as meteoric dilution by flooding, evapoconcentration, mineral precipitation and dissolution, and fluid mixing. The WA lake waters and groundwaters are somewhat similar to those in an entirely different geologic setting in northeastern Victoria, illustrating the potential for different geochemical pathways to lead to the formation of similar lacustrine acid brines. Although these types of environments are rare in modern settings, ancient ephemeral acid saline lake deposits have been recognized in the geologic record on Earth and on Mars, indicating that natural evolution of acid saline waters may be more ubiquitous than previously recognized. (c) 2008 Elsevier Ltd. All rights reserved.