STUDY OF CARBONACEOUS MATTER WITHIN THE 3,240 MA S3 SPHERULE BED OF THE BARBERTON GREENSTONE BELT, SOUTH AFRICA

The origin of carbonaceous matter found in Archean rocks of the Barberton Greenstone Belt (BGB) in South Africa has been the source of study and controversy for the last two decades [e.g. 1, 2]. Preserved within the Onverwacht and Fig Tree Groups of the BGB are four spherule bed layers, ranging in age from 3,500Ma to 3,240Ma, that formed by the condensation of vaporized rock after four separate meteor impact events [3]. The focus of this study is the S3 spherule bed layer, which is located at or near the base of the Fig Tree Group and is dated to 3,243 ± 4 Ma from an underlying tuff [4]. This layer outcrops in a number of locations throughout the belt, and represents deposits from a variety of depositional environments. Carbonaceous matter was identified in the spherule bed by laser Raman and SEM analysis [5]. Several processes could account for the presence of the carbonaceous materials within the S3 spherule bed layer. These processes include, but are not limited to, (1) hydrothermal processes, (2) Fisher-Tropsh synthesis, (3) microbial activity, and/or (4) primary impact related origins. We report the results of petrographic and microprobe analyses of spherule beds in four sections of S3 in an attempt to identify the origin(s) of the carbonaceous material. Preliminary petrographic analysis suggests that the carbonaceous material is predominately located along grains boundaries, in microfractures, and around spherule rims. These findings suggest that the carbonaceous material is not primary, but is a result of post-depositional diagenetic processes and/or microbial processes.

References: [1] van Zuilen, M. A., et al., 2007, [2] Walsh, 1992, [3] Lowe, D, et al., 2003, [4] Kröner, A., et al., 1991, [5] Davatzes, A.E.K., 2007.