Lloyd R. Snowdon

Thermal metamorphism of bitumen in Archean rocks by ultramafic volcanic flows

Abstract Thermally metamorphosed bitumen samples associated with an Archean ultramafic volcanic sequence near Timmins, Ontario, Canada were examined using reflected and transmitted light microscopy and X-ray diffraction. The bitumen occurs in aphanitic to fine-grained chemical and clastic meta-sediments intercalated with the volcanic flows and in derived metasedimentary xenoliths engulfed in the flows. This thermally altered bitumen was apparently derived from biogenic organic carbon in the original Archean sediments. The organic components of the samples consist of epi-impsonite to cata-impsonite and pyrolytic carbons. The latter occur in conical, layered, radial and spherulitic forms. The maximum temperature of formation of the pyrolytic carbon has been estimated at 700°C, which is consistent with its occurrence in xenoliths in these high temperature (1300–1500°C) volcanic flows.