The speciation of copper in natural fluid inclusions at temperatures up to 700 °C

Copper K-edge X-ray absorption spectra were recorded from natural polyphase brine fluid inclusions in miarolitic quartz from the Omsukchan Granite, Russia, as a function of temperature up to 700 °C. The inclusions studied represent two distinct fluids, which contain either 0.02 or 0.94 wt.% Cu homogeneously distributed in solution at temperatures above salt dissolution (350-550 °C). X-ray absorption near edge structure (XANES) spectra of the solution phase in the Cu-poor inclusions exhibit an intense edge feature near 8983 eV that is characteristic of the linear [CuCl₂]^- complex. This spectrum was obtained at all temperatures between 200 and 700 °C and from around 15 inclusions. Extended X-ray absorption fine structure (EXAFS) spectra recorded at 550 °C were modelled to give a Cu-Cl bond length of 2.11(2) Å, also consistent with [CuCl₂]^-. XANES spectra of the Cu-rich inclusions appear similar to that of [CuCl₂]^- but are shifted to higher energy. This spectrum was obtained between 350 and 700 °C and from all three inclusions studied. EXAFS recorded at 700 °C gave a Cu-ligand bond length of 2.19(1) Å. This complex is yet to be identified. The results for the Cu-poor inclusions indicate that [CuCl₂]^- is stable at magmatic temperatures, extending the known range of this complex by 200 °C. This is the first time that Cu speciation has been determined at these temperatures and provides an example of how inclusions can be used as sample cells for the spectroscopic study of fluids at extreme conditions.