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Aqueous fluids play a significant role in the transport of heat and matter in the Earth's crust and the upper mantle, and high-field-strength elements such as Zr are important geochemical tracers for these processes. However, the dissolution mechanism and complexation of Zr in the fluids at high pressure and temperature are unknown, in part because very low concentrations present severe experimental challenges. Here, we present an experimental setup for in-situ investigation of the coordination environment of elements at low concentrations in aqueous fluids up to 800 °C and 1.5 GPa using XAFS. Experiments were carried out in a modified hydrothermal diamond-anvil cell optimised for the detection of the fluorescence signal. We have investigated the effects of silicate components dissolved in aqueous fluids on the Zr solubility and complexation at high pressure and temperature. The observed Zr concentrations in fluids containing 7–33 wt% Na2Si2O5 and variable Al contents were between 75 and 720 ppm at 500 to 750°C and ~300 MPa to ~700 MPa. Initial XAFS results show clear differences between spectra of Zr in an HCl solution and in an H2O-Na2Si2O5 aqueous fluid, implying considerable differences in the Zr complexation.

XAFS measurements on Zr in aqueous fluids at high pressures and temperatures using a modified hydrothermal diamond-anvil cell