Trace Elemental Imaging
Results T is approach was applied to a
Figure 4 : Synchrotron XRF mapping of trace elements in Palaeospondylus gunni (MNHN-GBP 92), an early vertebrate from the Middle Devonian (~390 Myr) of Scotland. (a) Light optical micrograph and distributions of yttrium; the REEs lanthanum, cerium, neodymium and samarium; and uranium that can be clearly mapped in this fossil (scan step: 30 × 30 µm 2 , 28,231 pixels; distributions reconstructed from a full spectral decomposition of the data). (b) Reconstruction of the entire organism, with the box area corresponding to what is shown in (a). (c–d) Mean XRF spectra from the red (c) and white (d) box areas in the Y map (168 pixels), respectively characteristic of the fossil and the sedimentary matrix, showing contributions from transition metals and the elements mapped in (a).
beamline (SOLEIL synchrotron, France), at an excitation energy of 17.2 keV, selected for excitation of K-lines from phosphorous to yttrium and L-lines from cadmium to lead. T e X-ray beam was collimated by 2 bendable mirrors, monochromatized using a Si(111) double-crystal monochromator, and focused using Kirkpatrick-Baez mirrors down to a spot size of 11 × 7 µm 2 . T e sample was mounted on a xyz scanner stage, allowing ±12 mm movements with better than 500 nm accuracy. T e sample was oriented at 45° to the incident beam and at 45° to the XRF detector, a silicon driſt detector (SDD), placed in the horizontal plane ( Figure 1c ). Counting time per pixel was set to 500 ms to attain good statistics on trace elements at this energy. All the elemental distributions presented herein have been reconstructed from a full spectral decomposition performed with the PyMCA data-analysis soſt ware [ 15 ] using batch-fi tting procedure, Pseudo-Voigt peak shape, and polynomial baseline subtraction.
2015 May •
www.microscopy-today.com Discussion
Visualization of as-yet unobserved anatomical details . Such an approach is particularly suited to fl attened fossils given that X rays will penetrate the fossil to a depth of a several tenths of a millimeter (about 250 µm and 300 µm for strontium and yttrium, respectively). Comparing fl uorescent lines from several elements allows tuning the probed depth and distinguishes, to some extent, surface from subsurface. T us, synchrotron XRF mapping makes it possible to obtain a detailed, accurate view of the anatomy of a fossil without the need for prior delicate sample preparation and therefore appears to be a useful complement to light microscopy and SEM in visualizing anatomical features of fossils that are hidden under well-preserved decay-prone tissues or a sedimentary matrix that is not able to be prepared. T e great advantages of the method are its non-destructiveness and its
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series of fl attened fossils as old as the middle Devonian (~390 Myr [million years ago]). In particular, the speci- mens studied included exquisitely pre- served fi sh and shrimp from the late Cretaceous (~100 Myr) of Morocco, which display fi nely mineralized muscles observed by SEM imaging ( Figure 2 ). T e elemental distributions that were obtained, greatly improved the dis- crimination of hard tissues (bones, carapaces, or cuticles) from both the sedimentary matrix and the fossilized soſt tissues (muscles) on the basis of variations in relative elemental concentrations ( Figures 3 and 4 ). For instance, in a newly discovered teleost fi sh of unknown affi nities bearing a large, notched elongated bone unique to the fossil record, the technique revealed certain bones, such as the entire skull together with vertebrae and rib insertions, concealed under a fi ne layer of unpreparable clay ( Figure 3 ). Although this fossil is the only teleost ever reported to display such a peculiar bone, and therefore a yet undescribed taxon, most critical features of the skull, fi ns, vertebrae, and rib insertions remained hidden under the sediment, precluding an accurate description of this new genus and species. In Palaeospondylus , a mysterious, fi sh-like fossil vertebrate of highly debated affi nities from the middle Devonian of Scotland (~390 Myr), the overall fossil morphology is easily distinguishable by REEs mapping, but also by mapping traces of actinides such as uranium ( Figure 4 ).
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