FIG. 6.

Tufted microstructures of the subtidal zone, 3.48 Ga Dresser Formation, Pilbara, Western Australia. (A) Thin section photomicrograph taken perpendicular to bedding showing a series of linked tufts. Arrow indicates the tuft analyzed in (B–C). (B) Optical image plus Raman chemical maps of a single tuft showing that the tuft is composed of quartz, pyrite, and significant amounts of carbonaceous matter. (C) Typical Raman spectra from two carbonaceous areas of the tuft show the presence of pyrite (P) and quartz (Q) plus two carbon peaks (C) at ∼1350 cm⁻¹ (the “D1” disordered peak) and ∼1600 cm⁻¹ (the “G” graphite peak). The D1 and G peak positions and widths, plus the D1/G peak heights and areas, are characteristic of thermally mature but disordered organic carbon that has experienced prehnite-pumpellyite to greenschist facies metamorphism (Beyssac et al., 2002). This degree of maturation is consistent with the known metamorphic grade of the Dresser Formation (Van Kranendonk et al., 2008) and indicates that the carbon is probably indigenous to these rocks.