Mapping trace-metal (Cu, Zn, As) distribution in a single fluid inclusion using a third generation synchrotron light source
Résumé
The concentration and spatial distribution of relatively dilute (10 to 100 ppm) trace metals (Cu, Zn and As) were determined for individual fluid inclusions from a gold-quartz vein at Brusson, western Italian Alps, using synchrotron radiation X-ray fluorescence (SXRF). The analyses were performed on beamline ID-22 Micro-FID (Fluorescence, Imaging, Diffraction) of the European Synchrotron Research Facility (ESRF) Grenoble, France. The experimental set-up ensured a focal spot at the sample position of 2×7 μm and a flux of 1010 photons/s. Fluorescence X-ray maps were collected for several fluid inclusions. An example of the distribution of As in a fluid inclusion oriented at high angle to the wafer surface is presented. For each X-ray fluorescence spectrum, the (Kα/Kβ)As ratio of As was used as a means of estimating the thickness of quartz traversed by the photon beam. This value was used in turn for correcting As count rates. The correction procedure resulted in uniformising the count rates of As in the liquid portion of the inclusion and allowed visualizing the vapour bubble which contains only trace amounts of As. The strategy for computing the composition of the inclusion fluid was to use one element from crush leach analysis (As) as an internal standard to calculate the concentrations of the other cations (Cu and Zn) present in the solution. Results of this calculation show that element concentration estimates calculated using SXRF spectra are in relatively good agreement with those determined by crush-leach analysis. This indicates that the fluid trapped in the sample studied was homogeneous with respect to electrolyte composition. These results indicate that third generation synchrotron light source is a promising technique for tracking trace metals in individual inclusions (down to the tenths of ppm level) and, hence, as an invaluable tool for reconstructing the relative chronology of paleo-hydrothermal events responsible for the concentration of economic resources.