RARE-EARTH ELEMENT DISTRIBUTION IN LAKE TYRRELL GROUNDWATERS, VICTORIA, AUSTRALIA

The rare-earth element (REE) geochemistry of the saline to hypersaline, acidic brines of Lake Tyrrell, Victoria, Australia, were analyzed in order to determine the relative importance of concentration and modification processes in the lake system. Several types of brines were identified on the basis of different REE patterns and concentrations. Regional groundwater flowing into Lake Tyrrell yields the highest concentrations of REE, while a transitional water group is identified on the basis of lower REE concentrations. A third water group, the reflux brine, is characterized by extremely low REE concentrations. The REE patterns suggest that REE are introduced to the Lake Tyrrell system by the regional groundwater which has concentrated REE from the aquifer material in its acidic waters. Eventually, REE are removed from the evolving water mass as higher levels of pH are reached. The data of the study strongly suggest that REE are scavenged from solution by Fe-oxyhydroxide phases which precipitate as the pH of the system rises. Water-rock interactions probably control pH in the lake system and, therefore, ultimately control the concentration of REE in solution in this system. Our results are directly correlated with studies of hydrothermal vents and geothermal solutions which have shown the concentration of REE in solution to be dominantly controlled by pH.