Trace Elemental Imaging
in fossil bones and teeth, as well as sedimentary apatites [ 11 ]. REEs are also known to be critical proxies in paleoenvironment and taphonomic reconstructions because their frac- tionation depends on fossilization and diagenetic conditions [ 11 – 12 ].
Figure 3 : Synchrotron XRF mapping of major-to-trace elements of the anterior part (skull on the right) of a new fossil fi sh (MHNM-KK-OT 03a) from the Djebel Oum Tkout Lagerstätte (Upper Cretaceous, ~100 Myr, Morocco). Optical photograph (a) and false color overlays of iron (blue) and two REEs, neodymium (red) and yttrium (green), show distributions reconstructed from a full spectral decomposition of the data (b; modifi ed from [ 13 ]). Acquisition parameters: 100 × 100 µm 2 scan step, 50,851 pixels. (c–d) Mean XRF spectra and main elemental contributions from the red (c) and white (d) box areas in (b) (100 pixels), respectively characteristic of fossil bone and the sedimentary matrix. Red, white, and yellow arrows in (b) respectively highlight the notched elongated bone, hidden vertebrae, and rib insertions.
from the increase in X-ray fl ux at synchrotron facilities and from the development of stable X-ray focusing optics, novel detectors, and new data collection strategies that minimize experimental dead times. Fast XRF mapping, based on the collection of integrated intensity in preselected spectral regions of interest, was used to map major and minor element concentrations, such as copper and zinc, in fossil bird feathers [ 7 , 8 ]. T e same group also studied lizard skins [ 9 ] and plants [ 10 ]. T is article presents new methodological developments regarding trace elemental imaging in fossils, particularly focusing on strontium, yttrium, and the rare earth element (REE) series, known to be present in signifi cant quantities
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Materials and Methods Synchrotron XRF mapping . For the past twenty years, paleontolo- gists have used point spectroscopy and mapping methods to analyze the elemental composition of fl attened fossils. Scanning electron microscopy (SEM) can reveal surface anatomical details that are not evident under light microscopy. But while the spatial resolution of SEM X-ray emission spectroscopy is about 1 µm, this method is of limited interest for trace elemental imaging because the minimum ele- mental detection is in the range of 0.1 to 5 wt%, depending on the element analyzed. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) allows microscale mapp- ing of trace elements such as REEs in fossil bones, but local ablation of the area investigated precludes com- plementary characterization and use of this technique on rare specimens. Synchrotron-based XRF appears as a very promising tool to map trace elemental distributions in ancient fossils [ 7 – 10 , 13 ]. XRF spectroscopy detects the secondary X-ray emission from atoms bombarded with X rays generated by an X-ray source, here a synchrotron ( Figure 1 a– 1 b). Radiative de-excitation results in lines in the emission spectrum that are characteristic of the chemical elements making up the sample. Used in raster-scanning mode, XRF allows microscale mapping of elements. In paleontology, synchrotron XRF mapp- ing has previously been used in collect-
ing integrated intensity in preselected spectral regions of interest, thereby allowing great analytical speed, but hampering trace REE mapping because most corresponding L-lines fall in an energy domain where K-lines from transition metals dominate the signal [ 13 ].
Data acquisition . Collection of full-range XRF spectra coupled with spectral decomposition for all constitutive elements, or a much faster statistical analysis based on the Kullback-Leibler divergence, which is a natural dissimilar- ities measure for probability distributions [ 14 ], was recently demonstrated to allow mapping a large set of trace elements, including REEs [ 13 ]. XRF maps were collected at the DIFFABS
www.microscopy-today.com • 2015 May
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