Ilmenite: mantle reservoir for nitrogen?

Five ilmenite megacrysts from the Premier kimberlite pipe have been analysed by Mössbauer spectroscopic and electron microprobe techniques for their Fe³⁺/Fe²⁺ ratio; and by nuclear bombardment techniques for their N₂ content. Comparison of the Fe³⁺/Fe²⁺ ratio determined from electron microprobe analysis for total Fe assuming stoichiometry with the direct determination of this ratio by Mössbauer techniques is sufficiently good that the former method which is very widely used may be considered reliable. The N₂ 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 N₂ is not located in liquid inclusions. Isotopic analysis of ilmenite is not available and so its origin is uncertain, but these results suggest that ilmenite could be a major reservoir for N₂ in the earth's mantle.