Implications of metallic iron for diamonds and nitrogen in the sublithospheric mantle

Recently published experimental data suggest that the Earth's mantle below similar to 250 km is saturated in metallic Fe, reaching a concentration of similar to 1 wt.% Fe-o throughout the lower mantle and buffering oxygen fugacity by Fe-o <-> Fe2+ reactions. Metallic Fe in the mantle bears important implications for N, which behaves as a moderately siderophile element. Here, we propose that the trapping of N in (Fe, Ni) metal and Fe-carbides during diamond growth may account for the characteristically low N content of diamonds from the sublithospheric mantle. This model may also explain the origin of especially valuable, large, anhedral, flawless Type II (N-free) diamonds, like the Cullinan, that comprise a minor part of world diamond production. Partition coefficients of N (D-N) between diamond and metallic Fe within D-N = 0.0005-0.013 have been demonstrated in high-pressure experiments for diamond synthesis and in a natural sample of a N-poor lower mantle diamond with inclusions of Fe-nitrocarbide. More N is incorporated into diamond if it grows in the lithosphere, where there is no ambient Fe-o to trap N. As a broader implication, the recognition of Fe-o in the mantle and its affinity for N suggests metallic Fe should be a major host of mantle N. Retention of primordial mantle N in metallic Fe could explain the high N/Ar-36 and low N-15/N-14 ratios of the mantle compared with the atmosphere.