Hydrothermal calcite-fluid REE partitioning experiments at 200 °C and saturated water vapor pressure

Abstract

Calcite is a common vein mineral associated to ore deposits and alteration zones geothermal systems. The concentration of rare earth elements (REE) in calcite can potentially be used to fingerprint the changing physicochemical conditions during mineralization from hydrothermal fluids. Previous calcite-fluid partitioning experiments have been carried out at ambient temperature but models aiming at predicting the behavior of REE at elevated temperature have not yet been developed. This study aims at determining the controls on REE incorporation into calcite mineralized from hydrothermal aqueous fluids above 100 °C. Here, we present a series of hydrothermal batch-type REE partitioning experiments at 200 °C and saturated water vapor pressure (15.5 bar). The experiments were designed to synthesize REE-doped calcite from fluid mixing with varying initial REE concentrations (250 ppb to 1000 ppb) and permit in situ sampling of the aqueous fluids. Calcite-fluid partition coefficients (KD) were observed to depend on the ionic radius of the REE and vary as a function of initial REE concentrations: logKD of 0.86–1.67 at 250 ppb REE; logKD of 1.05–1.70 at 500 ppb REE; logKD of 0.47–1.07 at 1000 ppb REE. These data fit a parabola according to the lattice strain model from which calculated Young’s modulus (ES) values range between 21.6 and 63.2 GPa. The fits indicate that the partitioning of the light REE is controlled by the strain-induced Ca2+ substitution in the calcite structure, whereas the heavy REE deviate from these trends. The development of a binary REE(OH)3 - CaCO3 solid solution model based on the dual-thermodynamic (DualTh) approach suggests that the REE partitioning is controlled by the following possible coupled substitutions at pH of 6:Eu3++3OH-⇔Ca2++CO32-Eu3++O2-+OH-⇔Ca2++CO32-The lattice strain fits are useful to interpret deviations of the experimental data from the solid solution model and possible non-ideal mixing behavior. Application of this model to natural systems yields some limitations because of the variability of experimental mineral-fluid KD values with REE concentrations in the aqueous fluids. The DualTh approach considers the aqueous speciation and activity-concentration relationships of the aqueous fluids, and therefore, provides an efficient method to evaluate the partitioning of REE between mineral and aqueous fluids. This research is a first step in building an experimental database of calcite-fluid partitioning data at hydrothermal conditions and aims at developing more accurate models for predicting the behavior of the REE in natural mineral-fluid systems.