Synchrotron XRFD mapping published in Roy. Soc. Interface

Even the most spectacularly preserved fossils, with soft tissues ‘petrified’ as the product of a complex interplay of biological and geological processes, have experienced alterations that distort palaeontological reconstructions. Many works have now emphasized the need to precisely constrain these processes prevailing during fossilization, and circumvent potential biases. Nonetheless our knowledge is essentially limited to the biological controls occurring in the first days to months after the death of an organism, as revealed by decay experiments in the laboratory, but the geological processes remain largely unknown. Particularly, the mineralogical nature of fossils is rarely investigated, or mostly through their elemental composition.

In a manuscript published today to the Journal of the Royal Society Interface, my collaborators and myself describe a novel synchrotron imaging approach to generating megapixel mineralogical maps across millimeter-thick cross-sections through fossils and pluri-centimeter-large flat fossils. Referred to as “synchronous synchrotron rapid scanning X-ray fluorescence and diffraction mapping (SRS-XRFD)”, it allows for mineral phases identification and localization at the microscale over centimetric lateral size objects, and further gives information on texture (preferential orientation), crystallites size and local strain, showing great potential for taphonomic studies, as well as other poorly understood (bio)mineralization processes in life and environmental sciences. We also illustrate that mineralogical contrasts between fossil tissues and/or the encasing sedimentary matrix can be used to visualize hidden anatomies in fossils.

SRS-XRFD of a millimeter-thick transversal section of the thylacocephalan arthropod Dollocaris ingens (specimen MNHN.F.A66910) from the La-Voulte-sur-Rhône Lagerstätte (Jurassic, France). (a) Schematic view of the setup; while laterally scanning the sample with a high energy X-ray beam, an XRD image (green square) is collected at each pixel using a 2D area detector that intercepts portions of the diffraction rings (red). Simultaneously, an XRF dataset is also acquired in reflection. (b) Mean XRF spectrum from a 25-pixel area around the beam location (top), and false color overlay of zinc (red), arsenic and lead (green) and manganese (blue) distributions (bottom). (c) XRD images from pixels in the gills and heart, showing contrasting peaks. (d) Mean diffractograms extracted from the XRD map for three different 25-pixel areas in the gills, heart and muscles (colors; acquired at 18 keV), sum diffractogram from the map (black; acquired at 18 keV), and powder XRD diffractograms obtained from the gills and heart of a more anterior section of the same specimen (grey; acquired at Cu Kα). C, D and A phases are calcite, dolomite and arsenopyrite, respectively. (e) XRD intensity maps for 2θ (18 keV) = 10.68° (blue), 13.07° (green) and 13.68° (red), showing different mineralogical contrasts associated to the diffraction peaks highlighted by the corresponding colors in (d). Acquisition parameters: 9×11 µm2 (H×V) beam spot size, 75×75 µm2 scan step, 108,900 pixels, 150 (XRF) and 180 (XRD) ms couting times per pixel. Scale bar = 5 mm.

Reference: Gueriau P., Réguer S., Leclercq N., Cupello C., Brito P.M., Jauvion C., Morel S., Charbonnier S., Thiaudière D. & Mocuta C. 2020. Visualizing mineralization processes and fossil anatomy using synchronous synchrotron X-ray fluorescence and X-ray diffraction mapping. Journal of the Royal Society Interface 17: 20200216. Find the article (Open Access) here