Experimental evidence shows that yttrium does not behave like a heavy rare earth element in chloride-bearing hydrothermal fluids

Previous investigations of yttrium complexing in chloride-bearing hydrothermal fluids have produced contradictory results. To investigate these discrepancies, EXAFS and XANES analyses were carried out on synchrotron X-ray absorption spectroscopic data collected from Y(III)-chloride aqueous solutions at temperatures up to 500 °C and pressures up to 260 MPa in a hydrothermal diamond anvil cell. Fitting of the EXAFS data indicates that the Y(H2O)73+ hydrated complex is predominant under ambient conditions, which is fully supported with the XANES modeling using a YO7 cluster having C2v point group symmetry. The trend in the angular momentum projected density of states (l-DOS), obtained from modeling of the XANES spectra, indicates a shift in the distribution of the d-DOS toward lower energies with increasing temperature in the antibonding states associated with the white-line feature. This trend is attributed to increasing Cl association and decreasing H2O association with Y as the temperature of the aqueous sample is increased. Our results indicate that the onset of Cl association occurs at relatively low PT conditions and the number of Cl ligands of the Y(H2O)mCln+3-n step-wise complexes increases with increasing temperature, with a trend toward neutral Y(III) – Cl species at 500 °C. Comparison of the temperature-dependent trends in number of Cl ligands of Y(H2O)mCln+3-n step-wise complexes to that of REE(H2O)mCln+3-n complexes indicates that Y geochemical behavior mirrors that of LREE and thereby has greater mobility than HREE in chloride-bearing hydrothermal fluids. The Y – Cl complexing trends revealed in this study are entirely consistent with Y-Ho fractionation in hydrothermal environments where Y – ligand interactions are dominant factors over charge and ionic radius. X-ray absorption and X-ray fluorescence data were also collected from silica glass capillary tubes in which Y chloride solutions were held at 335 °C for 2, 5, and 20 hours. Analysis of the opened glass tubes at ambient conditions after hydrothermal treatment shows that a significant amount of Y diffused into the surrounding glass during heating, which may account for the lack of Cl coordination reported in a previous study.