Organic matter in the Kavirondian sedimentary rocks of Archaean period in Kenya

抄録

Concentrations and chemical forms of carbon and nitrogen are examined on the Kavirondian sedimentary rocks of Archaean period in Kenya (2, 500–2, 800Ma). Elemental composition and graphitization degree of kerogen fraction (carbonaceous matter) in the Kavirondian shales are also examined. Among various types of the sedimentary rocks, i.e., shales, sandstones, matrices of conglomerates and pyroclastics, only shales contain significant amounts of organic carbon and insoluble nitrogen in KCl solution. But their concentrations are low in comparison with those in Phanerozoic muddy sediments. The insoluble nitrogen is composed mainly of ammonium-nitrogen and contains a small amount of organic nitrogen. The ammonium-nitrogen must have been inherited from organic matters of sedimentary rocks as it is presumed from geochemical studies of nitrogen in Phanerozoic muddy sediments of Japan. The kerogen fractions in two Kavirondian shales are graphite, but their graphitization degree has not yet reached the fully ordered state. The graphitization degree is conformable with metamorphic grade of the shales. The elemental composition of the kerogen fractions shows that these kerogens have high H/C ratio and low N/C ratio in comparison with those of other Archaean shales in other areas.