FIG. 12.

Morphology and geochemistry of network of laminae in sediment of the lower supratidal zone, 3.48 Ga Dresser Formation, Pilbara, Western Australia. (A) Thin-section photomicrograph of dark brown laminae forming a carpetlike network (compare to filamentlike textures in Fig. 11); box in (A) is enlarged in (B); boxed area in (B) indicates area analyzed by Raman in (C) to (E). (C) Raman chemical maps of part of filamentous texture showing that it is composed of hematite and organic carbonaceous matter enclosed within a matrix of quartz. The hematite map was produced using the ∼415 cm⁻¹ hematite Raman peak, the quartz map using the ∼465 cm⁻¹ Raman quartz peak, and the carbon map using the ∼1600 cm⁻¹ Raman carbon peak. (D) Raman spectrum from an area of the filamentous texture rich in hematite (H) and carbon (C) showing the typical peaks for each mineral. Note that carbon and hematite both have major peaks in the 1320–1350 cm⁻¹ region, so the presence of organic carbon must be confirmed and mapped using the 1600 cm⁻¹ peak (Rividi et al., 2010; Marshall et al., 2011). Carbonate (labeled “Carb”) is occasionally found in the vicinity of these filamentous textures. (E) Raman spectrum from hematite-rich area lacking organic carbon. Note absence of carbon 1600 cm⁻¹ peak. Unlabeled peaks in (D–E) are from the quartz matrix.