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

Abstract : The first dispersive m-XANES mapping is presented here. The xperiments 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 ( 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 10 microns, and an area 390 180 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 Matlab1. 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.