The state of Zr and Hf in chloride hydrothermal fluids from in situ X-ray absorption spectroscopy

In order to quantify the hydrothermal mobility of Zr and Hf in chloride fluids we performed an investigation of the speciation of these metals by means of in situ X-ray absorption spectroscopy (XAS). The experiments included registration of Zr K-edge X-ray absorption near edge structure/extended X-ray absorption fine structure (XANES/EXAFS), and Hf L1-edge (XANES) and Hf L3-edge (EXAFS) spectra. The capillary method, when the experimental solution together with baddeleyite ZrO2(cr) or HfO2(cr) is sealed inside a silica glass capillary, was used. The spectra were recorded between 320 °C/600 bar and 480 °C/2200 bar (Zr) and at 370 °C/2100 bar (Hf) after the stationary solubility of the solid phase was attained. The aqueous fluid composition varied from 3.8 m HCl to 3.5 m HCl/3.2 m CsCl and 3.3 m HCl/2.2 m NaCl (Zr), and was set to 7.7 m HCl in the case of Hf. The composition of the dominant complexes was determined by means of EXAFS fitting and theoretical XANES modeling as ZrCl3(OH)2−/ ZrCl5(H2O)− and HfCl4OH−/ HfCl5(H2O)−. In Zr complexes the metal-to-ligand distances varied between 2.10 ± 0.06 Å - 2.16 ± 0.06 Å (OH−), 2.22 ± 0.03 Å - 2.32 ± 0.07 Å (OH2), and 2.46 ± 0.02 Å - 2.49 ± 0.02 Å (Cl−) depending on the fluid PT-compositional parameters. In Hf-bearing species the average HfO distance is 2.07 ± 0.12 Å, and the HfCl distance is 2.43 ± 0.03 Å. Both the distances are slightly shorter compared to the interatomic distances in Zr complexes. Higher number of Cl ligands and shorter metal-to-ligand distance determined for Hf complexes mean higher affinity of Hf to oxygen-bearing ligands and chloride ion, which can result in separation of Zr and Hf in hydrothermal processes. Reactions of the dissolution of Zr- and Hf-bearing minerals imply that both ionic associate HCl(aq) and ion Cl− are necessary for the formation of the negatively charged complexes. The region, that best satisfy these conditions, is located between ca. 300 and 450 °C. At higher temperature the neutral Zr/Hf-OH-Cl complexes can support the hydrothermal mobility of these metals. The complexation mechanisms determined for Zr and Hf demonstrate that deep chloride fluids can play an important role in mobilization and separation of these metals in subduction zones and in the fluid-rock interaction processes.