Earth’s Oldest Fossils
using SPIERSalign [ 17 ]. T e resultant stacks were imported into AVIZO 8.0 where carbonaceous material was segmented allowing 3D models of the carbon associated with the fi laments to be produced. T e models were visualized and rendered in AVIZO 8.0, and images were captured from multiple orientations in 3D space.
Results TEM analyses of ultrathin speci-
Figure 2 : (a) HAADF-STEM image of a fi lament from the 3.46 billion-year-old Apex chert. b) False color ChemiSTEM three-element overlay X-ray emission map of area boxed in (a). The fi lament comprises stacks of sheet-like phyllo- silicate grains (green) with carbon (yellow) and iron (red) interleaved between some of the sheets and around some of the fi lament margins. This distribution of phases is incompatible with a biological origin for the fi lament. In places, carbon completely coats the iron phase (arrow) suggesting carbon is the last phase to become associated with the fi laments, again incompatible with a biological origin.
Figure 3 : (a) HAADF-STEM image of a second fi lament from the Apex chert. (b) ChemiSTEM elemental X-ray emission map of the area indicated by the green box in (a) where green is aluminium from phyllosilicate, red is iron from iron oxide, and yellow is carbon. A number of quartz grains (q) are also intermixed with the phyllosilicate sheets within the fi lament. (c) Higher-magnifi cation ChemiSTEM carbon map of the region indicated by the yellow box in (a), emphasizing how narrow strings or sheets of carbon (yellow) are interleaved with phyllosilicate and quartz grains (black). (d–e) Selected area electron diffraction patterns from the regions of the TEM wafer indicated in (a). DP1 is consistent with the [001] zone axis pattern from a 2:1 phyllosilicate. DP2 shows a pattern of ring arcs, representative of a set of closely aligned grains of a 2:1 phyllosilicate with the incident beam parallel to the {00l} plane (adapted with permission from [ 14 ]).
and maximize the contrast between mineral phases of diff ering masses. Sequential slices were milled with a 2 nA Ga + beam current, and the slice spacing was 200 nm. Each newly milled face was imaged (2048 × 1536 pixels) with an image capture time of ~80 seconds to obtain images of high enough quality for subsequent 3D reconstruction. Qualitative elemental mapping of selected FIB-milled faces was performed using an Oxford X-max SDD energy dispersive X-ray spectrometry (EDS) system, with detection limits of about 1 atomic % to confi rm that the chemistry correlated with the morphological features observed.
3D reconstruction and visualization . Sequential FIB-SEM nano-tomography images were stacked, aligned, and cropped
14
mens through Apex fi laments reveal fi lament morphologies at the sub- micrometer scale that are characteristic of a mineralic origin, with complex in- tergrowths of mineral phases ( Figures 2 and 3 ). Each fi lament is made up of multiple plate- or sheet-like grains of phyllosilicate ( Figures 2 b and 3 b, green), located within a matrix of microcrystalline quartz. Occasionally quartz is also seen inter-grown with the phyllosilicate within a fi lament ( Figure 3b ). ChemiSTEM mapping shows that the phyllosilicate mineral(s) contain the elements K, Al, Si, and O, plus variable amounts of Ba and minor Mg. Electron diff raction patterns of this mineral obtained in the TEM possess d-spacings consistent with a 2:1 layered phyllosilicate crystal lat- tice structure ( Figures 3 d– 3 e). T is structure is found in micas such as muscovite and in some clay minerals [ 18 ]. T e nano-morphology of the phyllosilicate, appearing as a worm- like stack of crystals, closely resembles vermiculite, a common alteration product of mica. However, the chemi- cal composition is spatially heteroge- nous on the nano- to micro-scale. T is, together with the presence of barium, suggests that the phyllosili- cate is likely a complex hydrothermal
association of mica alteration products that are best termed vermiculite-like.
Further ChemiSTEM mapping shows that carbon
( Figures 2 b, and 3 b– 3 c, yellow) and iron ( Figures 2 b and 3 b, red) are closely associated with the phyllosilicate filaments. Both carbon and iron are seen interleaved between sheets of phyllosilicates within the body of the filaments; these elements also coat the outer margins of some parts of the filaments. In addition, carbon occurs away from the filaments within the quartz matrix where it forms a boundary phase between quartz grains ( Figure 2a ). These data indicate significant redistribution of carbon both within and around the Apex filaments, in marked contrast to patterns found by
www.microscopy-today.com • 2016 January
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68