The rare earth element geochemistry of Mono Lake water and the importance of carbonate complexing

Abstract

Rare earth element (REE) concentrations for the alkaline, saline waters of Mono Lake in eastern California are reported. The total REE concentrations of the lake water ranged from 4,681 to 7,979 pmol kg−1 . Shale‐normalized REE profiles for the lake water indicate that the heavy REEs (HREE) are enriched 20–200 times over the light REEs (LREE) compared to shale.The speciation of the REEs in Mono Lake was modeled with a combined specific ion interaction and ion‐pairing model which allowed activity coefficients for the major solutes (γM, γX), the REEs (γM), and the REE ion pairs (γMX) to be determined in the high ionic strength lake waters (I = 1.84 m). The speciation model suggests that essentially all the REEs in solution are complexed with 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 enrichments. Activity product calculations for REE phosphate coprecipitates in the high phosphate waters of Mono Lake (i.e. 800–1,000 µmol kg−1) indicate 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.