THE RARE-EARTH ELEMENT GEOCHEMISTRY OF MONO-LAKE WATER AND THE IMPORTANCE OF CARBONATE COMPLEXING

Rare earth element (REE) concentrations for the alkaline, saline water s of Mono Lake in eastern California are reported. The total REE conce ntrations of the lake water ranged from 4,681 to 7,979 pmol kg-1. Shal e-normalized REE profiles for the lake water indicate that the heavy R EEs (HREE) are enriched 20-200 times over the light REEs (LREE) compar ed to shale. The speciation of the REEs in Mono Lake was modeled with a combined specific ion interaction and ion-pairing model which allowe d activity coefficients for the major solutes (gamma(M), gamma(X)), th e REEs (gamma(M)), and the REE ion pairs (gamma(MX)) to be determined in the high ionic strength lake waters (I = 1.84 m). The speciation mo del suggests that essentially all the REEs in solution are complexed w ith carbonate ions and that >99% of each REE is complexed as Ln(CO3)2- species. The carbonate ion concentration of Mono Lake water is 0.27 m . Stability constants for these complexes increase with atomic number; consequently, these complexes are responsible for the HREE enrichment s. Activity product calculations for REE phosphate coprecipitates in t he high phosphate waters of Mono Lake (i.e. 800-1,000 mumol kg-1) indi cate that the lake water is close to saturation with respect to these precipitates and suggest that the coprecipitates may effectively limit the maximum dissolved REE concentrations in the lake.