Graphite in Precambrian rocks of Southern Africa: implications on the carbon content of metamorphic rocks

Abstract

Data on the occurrence of graphite and other highly reconstituted carbonaceous matter as potential traces of carbon originally derived from organic matter are compiled from Precambrian rocks of Namibia, Botswana, South Africa, and Zimbabwe. While carbonaceous matter occurs in sediments throughout Precambrian sequences of the area, graphite occurrences appear to be concentrated in high-grade metamorphic rocks of the Early Archaean Beithridge Complex of the Limpopo metamorphic belt and the late Precambrian Damara Supergroup of Namibia. The original sediments were mostly carbonaceous shales and rarely bituminous carbonates. The data compiled show that metamorphic rocks can contain appreciable amounts of carbon that cannot be disregarded in any calculation of carbon cycles. Previous estimates, weighted in favour of gneisses, give an average content of 200 ppm C org for metamorphic rocks, i.e., the same as for igneous rocks. The inclusion of sediments of lower metamorphic grades in the calculation raises the mean to 0.054% C org in metamorphic rocks. This results in a total of 6.75 × 10 21 mg C org in the metamorphic portion of the crust, or ∼ 33% of the total C org contained in the crust. This is a proportion which suggests a need for a revision of the presently accepted balance calculations of the interconnected carbon and oxygen cycles.