Moissanite (natural SiC) has been recovered from podiform chromitites of
several ophiolite complexes, including the Luobusa and Donqiao
ophiolites in Tibet, the Semail ophiolite in Oman and the United Arab
Emirates, and the Ray-Iz ophiolite of the Polar Urals, Russia. Taking
these new occurrences with the numerous earlier reports of moissanite in
diamondiferous kimberlites leads to the conclusion that natural SiC is a
widespread mineral in the Earth's mantle, which implies at least locally
extremely low redox conditions. The ophiolite moissanite grains are
mostly fragments (20 to 150 mu m) with one or more crystal faces, but
some euhedral hexagonal grains have also been recovered. Twinned
crystals are common in chromitites from the Luobusa ophiolite. The
moissanite is rarely colorless, more commonly light bluish-gray to blue
or green. Many grains contain inclusions of native Si and Fe-Si alloys
(FeSi(2), Fe(3)Si(7)).
Secondary ion mass spectrometric (SIMS) analysis shows that the
ophiolite-hosted moissanite has a distinctive (13)C-depleted isotopic
composition (delta(13)C from -18 to -35 parts per thousand, n=36), much
lighter than the main carbon reservoir in the upper mantle (delta(13)C
near -5 parts per thousand). The compiled data from moissanite from
kimberlites and other mantle settings share the characteristic of
strongly (13)C-depleted isotopic composition. This suggests that
moissanite originates from a separate carbon reservoir in the mantle or
that its formation involved strong isotopic fractionation. The degree of
fractionation needed to produce the observed moissanite compositions
from the main C-reservoir would be unrealistically large at the high
temperatures required for moissanite formation. Subduction of biogenic
carbonaceous material could potentially satisfy both the unusual
isotopic and redox constraints on moissanite formation, but this
material would need to stay chemically isolated from the upper mantle
until it reached the high-T stability field of moissanite. The origin of
moissanite in the mantle is still unsolved, but all evidence from the
upper mantle indicates that it cannot have formed there, barring special
and local redox conditions. We suggest, alternatively, that moissanite
may have formed in the lower mantle, where the existence of
(13)C-depleted carbon is strongly supported by studies of
extraterrestrial carbon (Mars, Moon, meteorites). (C) 2009 Elsevier B.
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