Mixing of Two Carbon Sources Controls the Spread in
d13C (-20” to -60”) in Sediments and TufÞtes of the Sukumaland Greenstone Belt, NW-Tanzania

D. Rammlmair BGR, Hannover, Germany

A. M. van den Kerkhof IGDL-Univ. Göttingen, Germany

E. Faber BGR, Hannover, Germany

Ch. Reutel IGDL-Univ. Göttingen, Germany

The 'organic carbon' within some sedimentary and tuffitic horizons of the Sukumaland Greenstone Belt can be attributed to carbon from a synsedimentary and a thermally generated methane source.

The synsedimentary source, with total organic carbon contents between 0.1 and 0.25%, is documented by relics of microbial mats and globular, biogenic objects (diameter approx. 10 µm) of Acritarcheaen affinities. The kerogen suffered very low grade (VLG) to medium grade (MG) metamorphic overprint. The d13C values (relative to PDB) range from -28” (VLG) to
-23” (MG), H/C ratios [mol%] are 1.30 (VLG) to 0.60 (MG), N [mol%] is around 0.045 (VLG) showing a relative enrichment towards medium grade. Vitrinite reflectivity is low (VLG: Rmax ª 3%) to high (MG > 10 % Rmax).Raman analyses of graphite in the low-grade rocks show lower degrees of structural order with "disordered-to-ordered" peak intensity ratios D/(O+D) of 0.18-0.74 (mostly 0.30 to 0.55), compared to higher-grade rocks (0.0-0.24).

The methane derived carbon is physically expressed by a very fine grained size fraction (<0.6 µm split from carbon concentrates; 0.1µm from scanning electron microscopy) . This carbon is almost H and N free, reaches up to 3.5% TOC and shows the extremely low d13C of -60”. The occurrence of both highly organised medium-grade metamorphosed graphite and lower organised methane-derived carbon documents that addition of carbon via oxidation of methane took place after the peak of metamorphism. This excludes a bacterial source of light isotope carbon. The source of methane is provided by accumulated
C-enriched sediments, but complex open system isotope fractionation has to be assumed to provide the extreme 13C depletion of the methane.