Abstract
Graphite associated with nickeliferous pyrrhotites occurs in the Oshirabetsu gabbroic body in southeast part of the Hidaka Belt, Hokkaido. Brecciated and spherulitic aggregates of graphite are mainly found in norite and diorite.
Stable carbon isotope ratio and degree of graphitization of the graphite and carbonaceous material in the Nakanogawa Group around the gabbroid were determined.
Crystallite thickness (Lc, (002)) of the graphite is more than 1000 Â and lattice strain along c-axis (εc) is very small, which means that the graphite is a perfect graphite crystal.
Carbon isotopic composition of the graphite ranges from -23.34 to -23.88‰ (δ13CPDB) and that of carbonaceous material in the Nakanogawa Group is about -25.7‰.
The dδ13C values of the graphite range within isotopic composition of carbon in coal and petroleum. The slight difference of δ13C value between the graphite and the carbonaceous material was due to the fractionation of 13C between the graphite and hydrothermal fluid.
These results suggest that the graphite in the Oshirabetsu gabbroic body is of sedimentary origin and carbonaceous sedimentary rocks were incorporated in ascending gabbroic magma.
