Remarkably preserved microtubular structures have recently been reported from a silicified sandstone unit at the base of the ~3,400 Ma Strelley Pool Chert (Wacey et al., 2008a; Brasier et al., 2006). Ongoing investigations of these structures show several lines of evidence consistent with a biogenic interpretation. The geological context of the sandstone unit appears conducive for life. This is evidenced by the presence of low angle cross bedding, channels and relatively high textural and compositional maturity, together indicating deposition in a shallow marine transgression, and providing the oldest such deposit in the rock record. That does not mean to say, of course, that any structures appearing in such a sandstone are necessarily biogenic. The microtubes are restricted to a small subset of early formed, well rounded clasts within the sandstone, and cross cutting relationships appear to exclude modern contamination, but as shown by Wacey et al. (2008a, Fig. 10), such contexts can be surprisingly complex and difficult to decode.
Pioneering work using nanoSIMS technology has detected biologically important elements such as carbon, nitrogen, phosphorus and sulfur, all occurring within or along the edges of the microtubes. Nano-scale in situ carbon isotope measurements from within the microtubes (average -26%o PDB) also fall within the range consistent with biological processing (Wacey et al., 2008a). The occurrence of pyrite grains associated with the microtubes, together with their cross-sectional profiles, led Wacey et al. to conclude that these structures are likely biologically mediated ambient inclusion trails (AITs; cf. Knoll and Barghoorn, 1974). This discovery is not without its problems though; such trails can form at several stages through the diagenetic and metamorphic history of the rock, so great care must be taken to constrain their age. There is also no consensus, as yet, on the degree to which AITs are biologically-mediated; laboratory experiments are urgently needed to better understand this process.
Further mineralized microtubular structures have recently been reported from inter-pillow hyaloclastite within the 3,350 Ma Euro Basalt Formation and are argued to represent bioerosion traces created by euendolithic organisms within this volcanic glass (Banerjee et al., 2007). These microtubes are 1-9 |im in width (averages of 2.4 |im) and up to 200 |im in length (average -50 |m).They are infilled with titanite (also known as sphene) which has been dated directly using U-Pb systematics to give an Archean age of 2.9 billion years old. This correlates with the youngest episode of regional metamorphism that affected the sample area and is interpreted to record metamorphic re-setting of the titanite which infills the microtubes and therefore a minimum age estimate for their formation. We await further results from these microtubular structures found in both meta-volcanic glass and silicified sediments with interest.
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