Zircon solubility and zirconium complexation in H2O+Na2O+SiO2±Al2O3 fluids at high pressure and temperature

Abstract

Zircon is an important host mineral for many high-field strength elements (HFSE), particularly Zr and Hf. Thus, its solubility in geologic fluids at high pressure and temperature plays an important role in terrestrial cycling of these elements during processes in the Earth's crust and mantle. We performed in-situ high-pressure, high-temperature measurements of zircon solubility in H₂O–Na₂Si₃O₇, H₂O–Na₂Si₃O₇+Al₂O₃, H₂O–Na₂Si₂O₅, H₂O–NaAlSi₃O₈ fluids, as well as of baddeleyite solubility in H₂O–NaOH fluids, by in-situ synchrotron radiation X-ray fluorescence analysis using hydrothermal diamond-anvil cells. Zirconium complexation in fluids in equilibrium with zircon was constrained by in-situ X-ray absorption near-edge structure (XANES) spectroscopy. Zircon solubility is strongly enhanced by addition of Na₂Si₃O₇ to H₂O. The degree of enhancement increases with Na₂Si₃O₇ concentration. The Zr content of fluids containing 10 wt% Na₂Si₃O₇ reached up to 86±2 ppm Zr at 550 °C and 400 MPa. At 30 wt% Na₂Si₃O₇, the maximum Zr concentration was 997±6 ppm at 600 °C and 440 MPa. Zircon solubility in these fluids decreases considerably with pressure and increases slightly with temperature. Addition of Al₂O₃ decreases the zircon solubility. In H₂O–NaAlSi₃O₈ fluids, the Zr concentrations are in the sub-ppm to ppm range. Zr concentrations in NaOH solutions in equilibrium with baddeleyite reached up to 390±2 ppm at 600 °C and 930 MPa and increase with pressure and temperature. In-situ XANES spectra collected on Zr in H₂O–Na₂Si₃O₇, H₂O–Na₂Si₃O₇+Al₂O₃, H₂O–NaOH, and H₂O–HCl fluids in equilibrium with zircon provide evidence for strong differences in the Zr complexation between these fluids. Comparison of XANES spectra to those of model compounds and ab-initio simulation of XANES spectra revealed ^[8]Zr for the HCl solution, ^[7]Zr for the NaOH solution, and ^[6]Zr for the Na–Al-silicate-bearing solutions. For the latter solutions, formation of alkali zircono-silicate complexes is indicated by the strong dependence of zircon solubility on Na/Al and the similarity of the XANES spectra to spectra simulated based on the local structure around Zr in the alkali zircono-silicates vlasovite and catapleite. Alkali zircono-silicate complexes are responsible for the enhancement of Zr concentrations in Na–Al-silicate-bearing solutions and very likely play an important role for mobilization of HFSE during fluid–rock interaction. Because high alkali/Al can be expected in aqueous fluids at high pressure and temperature due to incongruent dissolution of feldspar and mica, the increase of zircon solubility along the NaAlSi₃O₈–Na₂Si₃O₇ join points to potentially considerable Zr or HFSE transport by silicate-bearing aqueous fluids in the lower crust and upper mantle.