An Examination of the Carbonaceous Materials in the S3 Bed of the Barberton Greenstone Belt, South Africa

Abstract

Carbonaceous materials found in Archean rocks have been the source of study and controversy for the last two decades due to questions of the biogenecity of these particles. One of the key locations for these studies is the Barberton Greenstone Belt (BGB), in South Africa which contains some of the oldest known rocks on Earth, ranging in age from 3.5 to 3.2 billion years old. Preserved within the Onverwacht and Fig Tree Groups of the BGB are spherules that formed by the condensation of an impact-produced global vapor plume. The spherules are distal deposits that would have been deposited globally, but are only preserved at this location and in western parts of Australia. Like several other sediments in the BGB, there is evidence of minor amounts of carbonaceous particles contained within the spherule beds. Four individual impact events are preserved in distinct beds designated as S1, S2, S3, and S4. Due to the wide distribution of this bed in a variety of depositional settings, including both protected shallow and deep water depositional settings where there is little evidence of reworking, the S3 bed is an ideal choice for mineralogical, geochemical, and petrographic studies of impact spherules. This research examines samples from four different locations of the S3 spherule bed layer, the Barite Syncline, Maid-of-the-Mist, Sheba Mine, and Loop Road locations, in order to determine the origin of carbonaceous particles contained within the bed. Several geologic processes could account for the presence of the carbonaceous materials within the S3 spherule bed layer. These processes include: (1) Diagenetic processes, (2) Fisher-Tropsch Synthesis, (3) Microbial activity, and (4) Primary condensates from the impact plume. In order to distinguish between these processes, the spatial distribution of the carbonaceous matter was mapped, noting the mineral associations with these grains. Petrographic and electron microanalytical studies of the S3 samples reveal the presence of carbonaceous material in the sections with highly concentrated spherules, Barite Syncline, Loop Road, and Sheba Mine locations, but not in the samples from the Maid-of-the-Mist location, where there is a low concentration of spherules and abundant admixed volcanic detritus. Only Fischer-Tropsch Synthesis can be excluded as a process responsible for the origin of carbonaceous materials in the S3 beds. Though there is no direct evidence of the biogenecity of the observed carbonaceous materials, other textual observations within the S3 spherule bed are consistent with microbial activity, including Ambient Inclusion Trails and an unusual feature with a cyanobacteria-like morphology. While microbial activity cannot be ruled out as a process responsible for the origins of the carbonaceous materials, the findings of this study indicate that the carbon was mobilized from within the spherules during diagenesis. The location of carbon along spherule rims and microfractures within the spherules can also be attributed to diagenetic processes, such as fracture flow, dissolution, and replacement. A plausible explanation is that the carbon was a primary condensate from the impact plume, but has been diagenetically remobilized locally into microfractures and along the rims of the spherules.