Speaker: Pierre Gueriau, Synchrotron SOLEIL
Program Description
Synchrotron-based X-ray fluorescence (SR-XRF) has mainly been used in fossils to map organics and metal distributions at minor-to-trace levels based on the collection of integrated intensity in preselected spectral regions of interest. I will present recent developments performed at the SOLEIL synchrotron (south of Paris, France) regarding mapping and speciation of yttrium and rare earth elements (REEs), a series of key elements present at trace levels in fossils (100–1000 ppm range) and used in paleoenvironmental and taphonomic research as their distribution and fractionation depend on fossilization and diagenesis processes. They provided new information regarding:
(1) Fossil anatomy. Comparing fluorescent lines from light elements (information depth of a few ten µm) and heavier elements such as yttrium (information depth ca. 300 µm) allows tuning the probed depth and distinguishes fossil parts hidden under a thin layer of unpreparable sediment (Fig).
(2) Burial conditions. The collection of full-range XRF spectra coupled to spectral decomposition for all constitutive elements provide a unique non-destructive way to measure REE concentrations, used by geochemists to reconstruct environmental conditions.
(3) Signal homogeneity. Speciation maps can be produced through the collection of successive XRF maps at excitation energies across the Ce L3-edge, allowing unprecedented access to the distribution of the oxidation state of Ce within fossils at microscale.
I will finish with a few words about the development of a fast, simultaneous and multi-technique scanning environment (so-called ‘fly-scan’) at the SOLEIL synchrotron, which allows recording full-range XRF spectra with millisecond-range counting time per pixel, while synchronously collecting complementary structural and spectroscopic data (XAS, XRD).
