ILMENITE - MANTLE RESERVOIR FOR NITROGEN

Five ilmenite megacrysts from the Premier kimberlite pipe have been an alysed by Mossbauer spectroscopic and electron microprobe techniques f or their Fe3+/Fe2+ ratio; and by nuclear bombardment techniques for th eir N-2 content. Comparison of the Fe3+/Fe2+ ratio determined from ele ctron microprobe analysis for total Fe assuming stoichiometry with the direct determination of this ratio by Mossbauer techniques is suffici ently good that the former method which is very widely used may be con sidered reliable. The N-2 content of the ilmenite samples ranges from 0.5 to 2.0%, considerably in excess of that for most minerals. Heating experiments suggest that the N-2 is not located in liquid inclusions. Isotopic analysis of ilmenite is not available and so its origin is u ncertain, but these results suggest that ilmenite could be a major res ervoir for N-2 in the earth's mantle.