Earth’s Oldest Fossils
Figure 1 : (a-e) Microfossil-type specimens from the 3.46 billion-year-old Apex chert. Note the wide variation in color and the presence of branching structures (arrows) that cast doubt on their interpretation as primitive bacterial microfossils. (f–k) Selection of fi laments closely comparable to the type specimens, from new material studied here using electron microscopy.
original “microfossil” type specimens ( Figures 1 a– 1 e) was compromised by the 193–380 μ m thick preparations [ 5 ] making optical petrography diffi cult. T ere is also an understandable prohibition (by the Natural History Museum, London) on destructive or intrusive techniques, meaning, for example, that high spatial resolution EM approaches cannot be performed on the original type specimens. T e materials studied here are therefore standard (~30 μ m thick) polished geological thin sections ( Figures 1 f– 1 k), making light optical characterization more straightforward and allowing the application of a range of high spatial resolution analytical techniques. Light microscopy shows that the fi lamentous microstructures found in our material are identical to the original type specimens, in terms of their morphology, abundance, and petrographic distribution.
Focussed ion beam (FIB) preparation of TEM thin
specimens . Prior to FIB milling the thin sections were examined by light microscopy, 2D and 3D laser Raman, and SEM to gain an understanding of the fi lament distributions and morphologies and to select the most appropriate targets for detailed study. T e TEM specimens were prepared using two dual-beam FIB systems (FEI Nova NanoLab and FEI Helios NanoLab G3) located respectively at the Electron Microscopy Unit (EMU), University of New South Wales, and the FEI factory, Brno, Czech Republic. T in specimens were, on average, 15 μ m × 8 μ m × 100 nm and were attached to Omniprobe ® copper
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TEM holders using platinum connector strips or deposited on continuous carbon TEM grids.
TEM analysis of FIB-milled thin specimens . TEM data were obtained using a FEI Titan G2 80-200 TEM/STEM with ChemiSTEM elemental X-ray mapping technology operating at 200 kV, located in the Centre for Microscopy, Characterisation and Analysis (CMCA) at T e University of Western Australia. Data included high-angle annular dark-fi eld (HAADF) scanning transmission electrom microscopy (STEM) images, EDS (ChemiSTEM) maps, and electron diff raction for mineral phase identifi cation.
FIB-SEM 3D nano-tomography . FIB milling and SEM imaging were performed on a Zeiss Auriga Crossbeam dual-beam instrument at EMU. T e protocol was a modifi ed version of that described in [ 16 ], with milling and imaging parameters optimized to suit the type of sample (that is, carbon, iron oxides, and phyllosilicates within a silica matrix). Regions of interest (ROI) were covered with a protective (about 1μ m thick) platinum layer, then front and side trenches were milled with a 10 nA Ga + ion beam. An additional gold coat was then applied, and the sample was heated in an oven at ~50 °C overnight to minimize any instability caused by degassing during the opening of the trenches. T e front face of the ROI was then cleaned and polished using a 2 nA beam current. Imaging was performed at 5 kV using the backscattered electron (BSE) detector in order to minimize charging eff ects
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