Elemental mapping of exceptionally preserved ‘carbonaceous compression’ fossils

Elemental mapping offers rapid qualitative analysis of the mineralogy of exceptionally preserved fossils and, importantly, can be undertaken on unprepared samples. The results of such analyses are sensitive to several variables including the operating conditions (particularly accelerating voltage) and the composition of the material analysed: the higher the accelerating voltage employed and the lower the atomic number of the material impacted by the electron beam the larger the excitation volume (in effect a greater thickness of the specimen) that contributes to the resultant elemental map. Fossil non-biomineralised arthropods are commonly preserved as ‘carbonaceous compresssions’: the cuticle is organically preserved and, in hand specimen, essentially two dimensional. The external and internal surfaces of this cuticle may be coated by authigenic minerals. Examples include arthropods from the Middle Cambrian Burgess Shale exceptional fauna. It is theoretically possible that the proportion of the total counts generated by a thin surficial film of low atomic number (e.g., a carbonaceous cuticle) will be insufficient to distinguish it as a peak above the background spectrum; i.e., it will be effectively transparent in elemental maps. Simulated energy dispersive X-ray spectra for a windowless ED detector suggest that at an accelerating voltage of 15 keV a carbon layer less than about 0.2 μm thick will be undetectable, and at 5 keV a layer less than a few hundred angstroms thick. These values will increase if the energy dispersive spectrometer is fitted with a Super Ultra Thin Window.