Moissanite (Sic) from Kimberlites: Polytypes, Trace Elements, Inclusions and Speculations on Origin

Lithos 122 (2011) 152–164 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Moissanite (SiC) from kimberlites: Polytypes, trace elements, inclusions and speculations on origin A.A. Shiryaev a,⁎, W.L. Griffin b, E. Stoyanov c a Institute of Physical Chemistry and Electrochemistry RAS, Leninsky pr. 31, Moscow 119991, Russia b GEMOC ARC Key Centre, Macquarie University, NSW 2109, Australia c Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California at Davis, Davis, CA 95616, USA article info abstract Article history: An extensive collection of moissanite (SiC) grains from the Mir, Aikhal and Udachnaya kimberlite pipes of Received 26 July 2010 Yakutia has been characterized in terms of structural perfection, defects and the major- and trace-element Accepted 24 December 2010 chemistry of SiC and its included phases. The natural grains are clearly distinct from synthetic SiC produced by Available online 1 January 2011 various methods. Most of the natural SiC grains are 6H and 15R polytypes. Some of the grains (b10%) show extremely complex Raman spectra indicating strongly disordered structures. Some grains also show zoning in Keywords: impurities, C-isotope composition and cathodoluminescence brightness. Moissanite (SiC) Iron silicide Inclusions are heterogeneously distributed within the natural SiC; their size varies from a few nanometers to – – Oxycarbide hundreds of microns. The most abundant inclusions in SiC are Si metal and iron silicide (FeSi2); a Si C O Raman spectroscopy phase with stoichiometry close to Si4(C,O)7 probably is related to the silicon oxycarbides. FeSi2 commonly LAM-ICP-MS appears to have exsolved from Si metal; in some cases Ti metal then has exsolved from FeSi2 to form Electrochemical deposition symplectites. Trace elements are strongly concentrated in the inclusions of FeSi2 and Si4(C,O)7. The trace- element patterns of these phases are generally similar in the different kimberlites, but there are some consistent minor differences between localities. The trace-element patterns of FeSi2 and Si4(C,O)7 are strongly enriched in LREE/HREE and are broadly similar to the patterns of kimberlites, carbonatites and some diamond-forming fluids. However, extreme negative anomalies in Eu (and Sm) suggest highly reducing conditions. Yb also shows strong negative anomalies in FeSi2 from all three localities, and in Si4(C,O)7 from Aikhal and Mir, but not in those from Udachnaya. Trace-element chemistry and the nature of the inclusions provide a reliable basis for distinguishing natural and synthetic SiC. Textural and chemical features and the presence of oxidation products (Si4(C,O)7 and SiO2) suggest that moissanite grew at high temperatures and elevated pressures and was subsequently partly oxidised, also at high T. Several important features of moissanite grains from kimberlites are consistent with the formation of natural SiC by electrochemical processes in carbonate-silicate melts. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Diamonds have played an important role in defining the types of fluids that circulate in some parts of the SCLM, because some types of Recent research has brought an emerging recognition that the diamonds commonly contain visible fluid inclusions whose major- subcontinental lithospheric mantle (SCLM), especially in the ancient element, trace-element and isotopic compositions can be determined cratonic roots, has undergone major compositional modification by in-situ microanalytical techniques (e.g. Rege et al., 2010; Weiss through time. These studies suggest that the primary rocks of the et al., 2009) or by solution techniques (e.g. McNeill et al., 2009). The Archean SCLM were magnesian dunites and harzburgites, highly precipitation of diamonds also underlines the importance of redox depleted by the removal of melts, and that these rocks have been reactions between fluids and their mantle wall-rocks, and the need to repeatedly affected by metasomatic processes (Griffinetal.,2009and develop better tools for studying redox processes. In this paper we references therein). These fluid-mediated processes have refertilised present new data on natural silicon carbide (SiC), another highly the barren SCLM, adding some of the components originally extracted reduced phase from the deep lithosphere, and explore its implications during large-scale partial melting. These studies illustrate the impor- for redox processes in the SCLM. tance of understanding fluid-related processes in the deep lithosphere. SiC occurs in nature as the mineral moissanite, and there are several reviews of the extensive literature on its occurrence (e.g. Derkachenko et al., 1972; Kaminsky et al., 1968; Lyakhovich, 1979; ⁎ Corresponding author. Marshintsev, 1990). Moissanite has been found as inclusions in E-mail address: [email protected] (A.A. Shiryaev). diamonds (Jaques et al., 1989; Klein-BenDavid et al., 2007; Leung 0024-4937/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2010.12.011 A.A. Shiryaev et al. / Lithos 122 (2011) 152–164 153 et al., 1990; Moore and Gurney, 1989), and in mantle-derived addressed by avoiding the use of SiC-containing machinery and by magmatic rocks such as kimberlites (Bobrievich et al., 1957) and working in clean rooms. The kimberlitic moissanite grains are up to volcanic breccias (Bauer et al., 1963; Di Pierro et al., 2003; Gorshkov et 1 mm across. Most of the grains are transparent and they show al., 1995). These finds suggest that SiC may be rare but ubiquitous in at various colors, of which bluish-green is most common. As earlier least the deeper parts of the subcontinental lithospheric mantle. More observed (Derkachenko et al., 1972; Marshintsev, 1990) most grains enigmatic occurrences include high- and low-grade metamorphic are fractured, but some preserve well-formed crystallographic faces. rocks, limestones, pegmatites and chromitite pods within ophiolites Raman microspectroscopy was employed to determine the (Gnoevaja and Grozdanov, 1965; Lyakhovich, 1967, 1979; Marshint- polytypes, to assess the degree of crystalline perfection, and to sev, 1990 and references therein; Shiryaev et al., 2008a; Trumbull et identify some inclusions. Most analyses were performed using a al., 2009). These occurrences will not be dealt with here, but will be 514.4 nm laser in nearly back-scattering geometry either on polished the subject of future studies aimed at a general understanding of grains placed in epoxy for chemical analysis or on loose grains. The redox processes in the lithosphere. laser spot was 5–20 μm in diameter and the explored wavelength Grains of natural SiC often contain inclusions of highly reduced range was between 90 and 3750 cm− 1. This geometry allows rapid phases such as native Si (Marshintsev, 1990) and Fe-, Mg-, Ti-, identification of polytypes, but the relative intensities of various peaks Cr-silicides (Di Pierro et al., 2003; Marshintsev, 1990; Marshintsev cannot be compared for the polished samples since the exact et al., 1967; Mathez et al., 1995), which imply formation under orientation of the polished grains is sample-dependent. A silicon extremely reducing conditions, well below the iron-wustite buffer chip was employed for wavelength calibration. The influence of the that is commonly regarded as a lower limit on the fO2 of the mantle. If laser power on line positions is not pronounced for SiC; nevertheless, this implication is accepted, it requires a mechanism for drastically the power was kept low to reduce possible artefacts. Photolumines- lowering the fO2 of the subcontinental mantle, at least locally. cence was generally weak and posed no problems in data analysis. Alternatively, other mechanisms must be proposed and tested. In The SiC grains were cast in epoxy and polished prior to analysis. either case, moissanite can potentially provide significant insights into Major elements were analysed at GEMOC, Macquarie University, redox processes in the deep continental lithosphere. using a CAMECA SX100 electron microprobe, fitted with 5 wave- In the past, most finds of moissanite in nature have been ascribed length dispersive spectrometers, using an accelerating voltage of to industrial contamination during sample preparation. Discrimina- 15 kV and a sample current of 20 nA. The diameter of the electron tion of natural vs synthetic SiC is indeed a problem; Bauer et al. (1963) beam was b5 μm. Standards included natural minerals and synthetic noted that “the morphological, physical and chemical properties of oxides; matrix corrections were done as described by Pouchou and natural moissanite and synthetic silicon carbide agree very closely”. Pichoir (1984). Counting times were 10 s for peaks and 5 s for Several criteria have been proposed for distinguishing natural background on either side of the peak. moissanites from their synthetic analogues and the most commonly Trace-element contents of minerals were analysed at GEMOC mentioned are the polytype abundance and inferred narrow compo- using an Agilent 7500cs ICPMS attached to a 266 nm Nd-YAG laser sitional range of “synthetic” inclusions. However, many of these operating at 10 Hz; the beam diameter was varied between 40 and criteria are unsuitable for such discrimination. The composition of 150 μm depending on the type of material being analysed. The NIST inclusions in synthetic SiC varies considerably between the samples 610 glass was used as the external standard and EMP data for Si were grown by different methods.

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