Standard partial molal thermodynamic properties including association constants for 246 inorganic aqueous rare earth element (REE) complexes with chloride, fluoride, hydroxide, carbonate, sulfate, bicarbonate, nitrate, and orthophosphate can be calculated at pressures from 1 to 5000 bars and temperatures from 0 to 1000 degrees C, using experimental data from the literature and correlation algorithms. Predicted association constants for REE complexes are used to calculate the speciation of the REEs in simulated and natural fluid compositions over ranges of pH, temperature, and pressure. Our results demonstrate that in a generalized chloride-rich hydrothermal fluid, REE transport may be facilitated by formation of chloride, fluoride, and hydroxide complexes at acidic, neutral, and basic pH conditions, respectively. The HREEs (Gd-Lu) are complexed more strongly by fluoride, and less strongly by chloride, than the LREEs (La-Sm), whereas at basic pH conditions HREEs and LREEs strongly associate with hydroxide to an equivalent degree. Estimates of REE speciation in natural hydrothermal solutions that differ in composition and geologic setting demonstrate that different REE-complexes predominate in different environments. It follows that ad hoc assumptions about the identity of complexes which are responsible for REE mobility in a given geologic setting are not necessary.
