Redox and speciation micromapping using dispersive X-ray absorption spectroscopy:: Application to iron chlorite mineral of a metamorphic rock thin section

The first dispersive mu-XANES mapping is presented here. The experiments have been conducted at the iron K-edge, on the Dispersive-EXAFS'' beamline of the European Synchrotron Radiation Facility (France). The mapping has been performed on a polished thin section of 30 mm of a natural metamorphic rock including different types of minerals. Because of the high X-ray absorption due to the thickness of the glass sample holder (similar to 1 mm), the data have been collected in the fluorescence mode using the so-called Turbo-XAFS'' design. The effective spot size was approximately 10 x 10 microns, and an area 390 x 180 microns in size was mapped. Improvements of the acquisition process allowed collection of each XANES spectrum in 1.5 s and with a step size of 5 microns so that 2808 spectra with full XANES information were collected. Then, automatic procedures for data reduction and mapping reconstruction were developed using Matlab(R). The results are maps of iron content, oxidation state, and speciation, with a 5 mm spatial resolution after two-dimensional deconvolution. Subsequent analyses of the reconstructed images provide some quantitative calibrations.