2D mapping of Iron content, oxidation state and local structure by XANES: an application to natural volcanic glasses

XANES imaging by collecting several x-ray fluorescence maps at various excitation ener-gies across the absorption edge of a given element is a powerful tool for the characterization of heterogeneous samples such as rocks or cultural heritage artefacts. Examining the fluorescence yield at different energies can differentiate the oxidation state and/or the speciation at every pixel in the image. In fluorescence detection, even the chemical state of trace elements with concentration in the ppm range can be studied. We will report on XANES imaging experiments performed at the Italian beamline BM08 of the European Synchrotron Radiation Facility on thin sections of natural volcanic glasses from Pantelleria Island (Italy). By using a beam size of 200μmx200μm we collected x-ray fluorescence maps at different energies across the Fe K-edge. From the analysis of maps collected above the absorption edge and on selected features of the XANES such as the edge and pre-edge peak, we obtained qualitative information on the spatial distribution of iron in the sample as well as on variations of the Fe oxidation state and local symmetry within the sampled area. After the two-dimensional elemental and chemical mapping, full XANES spectra were collected in selected points in order to extract quantitative information on the Fe oxidation state and coordination from the pre-edge peak fitting (Giuli et al., 2011). Experiments with a probe size such as that currently available at BM08 beamline find application for the characterization of cm-sized zones of samples for which the spatial resolution achieved with extreme micro-beams is not needed.