ACID SALINE WEATHERING EXPERIMENTS ON AUSTRALIAN ACID LAKE HOST ROCKS

Acid saline waters have been documented in southern Western Australia and have been suggested as possible past surface and groundwaters on Mars. The origin of the acidity remains unsolved, but role of weathering of host rocks may be significant. We have been conducting weathering experiments in the lab to test how water chemistry is affected by specific host rocks. Samples of various rocks, including granite, gneiss, quartzite, amphibolite, and sandstone, were collected from the vicinity of acid saline lakes in Western Australia for use in these experiments. Natural weathering rinds were removed in the lab. A representative fresh sample of each rock type was immersed in pH 2 and pH 4 sulfuric acid solutions, NaCl-saturated pH 2 sulfuric acid solution, and pH 7 distilled water. Samples of each fluid without a rock were used as controls. Each fluid-rock sample was allowed to sit in a fume hood for a month. Measurements of water depth, salinity, pH, and temperature were made on a daily - weekly basis for one month. In addition, observations of water and rock appearance were recorded. The control samples all decreased in pH ovcer time, but had no changes in salinity or fluid appearance. The solution set containing distilled water decreased 1-2 pH units, while the other solution sets decreased 0.3 to 2.1 pH units over the month trial. Only one sample, the sandstone in pH 4 solution, had an increase in pH (1 pH unit). The most acidic waters also turned yellow, regardless of the rock type. All NaCl-saturated acid solutions produced salts, including halite, gypsum and/or unidentified crystals. Rock type played a minor role in change of pH and salinity over time. However, there were few noticeable changes in the rocks in terms of weathering features. Our preliminary conclusions are that evapoconcentration plays a greater role in lowering the acidity than does specific rocks types.