A microbeam XAFS study of aqueous chlorozinc complexing to 430 degrees C in fluid inclusions from the Knaumuehle granitic pegmatite, Saxonian granulite massif, Germany

The synchrotron X-ray microprobe (X26A) at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory was used to collect zinc K-edge absorption spectra from saline (ca. 36 wt% NaCl equiv.) fluid inclusions in quartz from the Knaumuhle granitic pegmatite, in the Saxonian Granulite Massif, Germany, at temperatures ranging between 30 degrees and 430 degrees C. XAFS spectra were also obtained from one fluid inclusion that was experimentally re-equilibrated at a high pressure of hydrogen. The temperature of the fluid inclusions was controlled during analysis with a programmable heating stage. Analysis of the XAFS data shows that ZnCl ₄ (super 2-) is the dominant aqueous zinc species in the fluid inclusions up to the average trapping temperature (430 degrees C). Furthermore, the mean Zn-Cl bond length decreases uniformly from 2.31+ or -0.01 Aa at room temperature to 2.26+ or -0.02 Aa at 430 degrees C. The predominance of the tetrahedral chlorocomplex, rather than ZnCl ₂₀ , at high temperatures is most probably due to the high chloride concentrations of the inclusion brine.