Carbon in igneous minerals of granulites from Rogaland (Norway); nuclear microanalysis and isotope geochemistry

The occurrence of carbon is reported in zircons, olivines and spinels from three acidic igneous rocks from the Bjerkreim-Sokndal lopolith which crystallized at function signO (sub 2) close to FMQ buffer conditions. Up to 6600 +- 990 ppm of C were measured from the zircon sub-surface down to 5 mu m deep through the reaction (super 12) C(d,rho (sub 0) ) (super 13) C. In olivine and magnetite C contents of several hundred ppm were detected. To study the localization of C, X-ray photo-electron spectroscopy, SEM, TEM and isotopic analysis methods were used. In zircon, the C may be present in three forms: 1) as thin coatings of carbonates on crystal surfaces and microfracture walls; 2) as minute graphite inclusions, and 3) as carbonaceous films. This last species accounts for 65-75% of the total C extracted from zircon; it was probably formed by the reduction of fluids on microcracks produced during cooling.