CARBON ISOTOPIC COMPOSITION AND ORIGIN OF SIC FROM KIMBERLITES OF YAKUTIA, RUSSIA

The stability of moissanite (SIC) has been computed for upper mantle conditions using the internally optimized thermodynamic dataset for the Mg-Si-O compounds of Fei et al. (1990). The computations consider the effects of pressure and temperature on the elastic properties of phases involved in the reactions. The maximum stability of moissanite throughout the upper mantle is typically five to six orders of magnitude lower in oxygen fugacity (f(o2)) than the Fe metal-wustite oxygen buffer at equivalent temperature and pressure, in agreement with previous calculations. Under conditions of SiC stability, silicates will be Fe-free, Fe metal will contain substantial amounts of Si but little C in solution, and Mg-rich sulfides will be stable. Moissanite from the heavy mineral concentrate of the Mir and Aikhal kimberlite pipes, Yakutia, has been studied. Moissanite crystals are gemmy and vary in color from a characteristic blue-green to pale green to nearly colorless to blue-black. Most exhibit crystallographic faces and are in the size range 0.5 to 1 mm in long dimension. Their compositions include small quantities of Fe, which is ubiquitous, Al, Ca, V, Cr, and Mn, all of which may be present in concentrations > 100 ppmwt. Mineral inclusions are present in some crystals. Silicon metal is the most common; inclusions of ferrosilicite (Fe3Si7), Fe-Ti silicides, REE silicate, and sinoite (Si2N2O) have also been observed. The carbon isotopic compositions of individual moissanite grains have been determined by ion microprobe. The nine analyzed crystals from Aikhal and fourteen from Mir are characterized by a narrow range in delta(13)C values of -22 to -29 parts per thousand; the majority of crystals fall within a more restricted range of -24 to -27 parts per thousand. Two grains were analyzed for N and found to have a delta(15)N of +9.7 +/- 4.0 and + 5.6 +/- 2.0 parts per thousand. Five mechanisms for the formation of moissanite are considered. Moissanite may be a relict of a reduced, primordial Earth and now present only as a trace phase in an otherwise oxidized mantle. Alternatively, there may be present-day global regions of the Earth that are both highly reduced and characterized by light carbon isotopic compositions. Although these possibilities cannot be disproved, they are not supported by observations. Two other possibilities, namely that moissanite stability extends to more oxidized conditions at pressures of the lower mantle or that it may form metastably, cannot be evaluated with present knowledge. The possibility most consistent with, although not proven by, the isotopic data is that moissanite formed by metamorphism of reduced, carbonaceous sediments during subduction.