Insights from X-ray absorption/fluorescence spectroscopy and ab-initio molecular dynamics on concentration and complexation of Zr and Hf in aqueous fluids at high pressure and temperature

International audience ; High-field-strength elements such as Zr and Hf are important geochemical indicators for processes in the deep Earth. Aqueous fluids play a significant role in the transport of heat and matter in these systems. However, concentrations and complexation of Zr and Hf in these fluids at high pressure (P) and temperature (T) conditions are largely unknown. Zr/Hf contents were determined in-situ at P and T in aqueous fluids containing NaOH, HCl or Na-Al silicate components in equilibrium with zircon or baddeleyite. Concentrations are strongly enhanced in comparison to pure H2O. The variation of Zr/Hf contents with fluid composition and their respective variation with P and T point to differences in the Zr/Hf complexation. High-resolution Zr K-edge XANES spectra were collected in-situ at P and T as well as Hf L-3-edge XANES spectra. Analysis of spectra evidences considerable differences in the coordination of Zr and Hf as a function of fluid composition. Zr-[7] is found in NaOH solution. Zr-[7] is also found in HCl solution, however, coordinated by both chlorine and oxygen. In fluids with Na-silicate components, Zr and Hf are 6-fold coordinated in zircono-silicate complexes, likely complexed with Na and Si. Further insight into the speciation was obtained by ab-initio molecular dynamics simulations of Zr monomers in H2O-NaOH and H2O-HCl. In basic solutions, Zr is five-fold coordinated by oxygen, whereas in acidic solutions, mixed oxychloride complexes are formed. The oxychloride complexes also seem to be present in experimental fluids, whereas the Zr coordination in basic fluids is underestimated by the simulations. The most likely explanation for this discrepancy is the formation of Zr oligomers in the fluid