USE OF THE HYDROTHERMAL DIAMOND-ANVIL CELL FOR X-RAY SPECTROSCOPIC STUDIES OF METAL ION COMPLEXES, HYDROTHERMAL LEACHING AND ELEMENT PARTITIONING AT ELEVATED TEMPERATURES AND PRESSURES

Knowledge of the structure and speciation of REE and other metal ion complexes in crustal fluids is necessary for a better understanding of the transport, fractionation, and mineralization of such elements in deposits of hydrothermal or metasomatic origin. Results from x-ray absorption fine structure (XAFS) measurements of zinc(II) bromide aqueous solutions when heated up to 660 °C and 800 MPa show progressive shedding of outer-hydration shell water molecules of the ZnBr₄^2- complex. Such a loss of outer shell water molecules of the ZnBr₄^2- and other metal halide complexes is due to progressive hydrogen-bond breaking with increased temperature (T) in hydrothermal fluids. XAFS measurements at up to 500 °C and 480 MPa of REE ions (La³⁺, Nd³⁺, Gd³⁺, and Yb³⁺) in chloride solutions show formation of step wise REE(H₂O)_mCl_n^+3-n complexes, where m steadily decreases from ~6 or 7 and n steadily increases (to ~2) with T. Our results show that the light REE chloro complexes are potentially slightly more stable than those of the heavy REE trivalent ions under hydrothermal conditions. Determination of leaching rates of trace elements is useful for successful modeling of dissolution kinetics of minerals in hydrothermal fluids. Results from x-ray fluorescence (XRF) microprobe analysis of metamict zircon in aqueous fluids show variable leaching rates of Pb indicative of dissolution of nanometer size galena inclusions within nearly amorphous zircon upon heating between 300 to 700 °C and up to 200 MPa of pressure. Analysis of the local structure and partial composition of each phase is necessary for an understanding of transport and fractionation of trace elements or minor constituents of a melt + aqueous fluid system. High spatial resolution of the synchrotron x-ray microprobe, combined with innovations made to the hydrothermal diamond-anvil cell, allow for in-situ spectroscopic analysis of individual phases of the sample. Microprobe XRF and Nb K-edge XAFS measurements have been made on an Nb-bearing granitic glass in H₂O and in a 1 M Na₂CO₃ aqueous solution at up to 880 °C and 700 MPa. XRF measurements indicate the onset of significant transfer of Nb from the silicate melt into the aqueous phase at ~ 500 °C in the system whereas XAFS spectra show reorganization of the silicate structure surrounding Nb in the melt when compared to that of the glass.