CARBON IN IGNEOUS MINERALS OF GRANULITES FROM ROGALAND (NORWAY) - NUCLEAR MICROANALYSIS AND ISOTOPE GEOCHEMISTRY

The occurrence of carbon is reported in zircons (first occurrence), ol ivines and spinels from Rogaland (Norway). Minerals were separated fro m hypersthene granites (charnockites, granulite facies) which crystall ized at fO2 close to FMQ buffer conditions. Up to 6600 +/- 990 ppm of carbon were measured from the zircon sub-surface down to 5 mum deep (d eep carbon) through the nuclear reaction C-12(d, p(o))C-13. No carbon diffusion at depth or towards the surface was observed. In olivine and magnetite, deep carbon contents of several hundreds of ppm were detec ted. In an attempt to determine the carbon localization, speciation an d origin we used several techniques: electron spectroscopy, scanning e lectron microscopy, transmission electron microscopy as well as isotop ic analysis of carbon released by step-heating of zircon. The results obtained by these various techniques suggest that carbon in zircon is chiefly present in three forms: (1) as thin coating of carbonates on c rystal surfaces and on late microfracture walls, (2) as minute graphit e inclusions, and (3) as carbonaceous films. This last species account s for 65 to 75% of the total amount of carbon extracted from zircon. I t was probably formed by the reduction of pervasive fluids (magmatic?) on microcracks produced during cooling of the rocks. No evidence for dissolved carbon was found.