Info

[ | Komatiite Dacite-rhyolite

Figure 3. Putative biogenic structures from the Barberton greenstone belt (BGB), South Africa. (a) Location of the BGB on the South Africa-Swaziland border; (b) stratigraphy of the -3,500-3,200 Ma Onverwacht Group; (c) etched fracture surface of chert from the -3,400 Ma Kromberg Formation showing short rod-shaped bateriomorph structures embedded in quartz (Westall et al., 2001); (d) petrographic thin section of spindle structure in chert from the base of the Kromberg Formation (Walsh, 1992); (e) petrographic thin section of putative microbial mat layer from the 3,416 Ma Buck Reef Chert; (f) petrographic thin section of solid filaments in chert from the -3,450 Ma upper Hooggenoeg Formation (Walsh, 1992); (g) pet-rographic thin section of mineralized microtubular structures within the formerly glassy margin of pillow basalt from the -3,450 Ma upper Hooggenoeg Formation. Figure 3c is reprinted from Precambrian hyaloclastites (Fig. 3g). These have been described from within the Komati, Hooggenoeg and Kromberg Formations, with the best developed structures coming from the 3,472-3,456 Ma upper Hooggenoeg Formation (Furnes et al., 2004; Banerjee et al., 2006). The mineralized tubular structures are 1-10 |im in width and up to 200 |im in length and extend away from "root zones" of fine grained titanite associated with re-healed fractures. The tubular structures are segmented in some areas (Fig. 3g) by cross-cutting chlorite formed during ocean floor alteration very soon after initial eruption of the basalts (Furnes et al., 2004). The replacement of titanite by low grade metamorphic chlorite within the tubes suggests the tubes were formed prior to this alteration and are indeed ancient structures. X-ray element mapping reveals the presence of linings enriched in carbon along the margins of the tubes, and this carbon is argued to represent decayed organic material from the trace maker (Banerjee et al., 2006). Moreover, carbon isotope analysis of disseminated carbonates in the pillow rims gives 513C values of +3.9%o to -16.4%o compared to +0.7%o to -6.9%o for the crystalline pillow interiors, and these lower values from the pillow rims are consistent with microbial fractionation (Furnes et al., 2004). Higher resolution in situ chemical and isotopic analyses are now needed to substantiate this claim. These findings highlight the preservation potential of volcanic rocks that have until recently been somewhat neglected in studies of Archean life. (Extensive reviews of microbial ichnofossils found in volcanic glass from the modern seafloor, Phanerozoic ophiolites and Precambrian greenstone belts are given in McLoughlin et al., 2008b and Furnes et al., 2007.)

0 0

Post a comment