Title | Implications of metallic iron for diamonds and nitrogen in the sublithospheric mantle |
Publication Type | Journal Article |
Year of Publication | 2014 |
Authors | Smith EM, Kopylova MG |
Journal | CANADIAN JOURNAL OF EARTH SCIENCES |
Volume | 51 |
Pagination | 510–516 |
Date Published | may |
ISSN | 0008-4077 |
Abstract | 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 {\textless}-{\textgreater} 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. |
DOI | 10.1139/cjes-2013-0218 |