Discovery of graphitic carbon inclusions (metastable phase?) in the metamorphic microdiamonds in the Kokchetav UHP dolomite marbles

Abstract:

The microdiamonds in the Kokchetav dolomite marble have been classified into S-, T- and R-type based on the morphologies, C isotope compositions, and other characteristics, and formed at two stages in the diamond stability field; the core of S-type and R-type at 1^st stage and the rim of S-type and T-type at 2^nd stage (Ishida et al., 2003; Yoshioka and Ogasawara, 2005; Imamura et al., 2013). Graphitic carbon inclusions were discovered in diamond grains in garnets in dolomite marble from the Kumdy-Kol area. We examined over 5000 diamonds in 40 thin sections and recognized 15 inclusions in 15 diamonds under transmitted light microscope. These host diamonds measure 4-15 µm. The inclusions are black-colored and measure 1-5 µm, and were found in the rim of S-type, T-type, and probably R-type. The inclusions were identified by laser Raman spectroscopy; one inclusion in the rim of S-type, four inclusions in T-type, and four inclusions in R-type. All examined inclusions are completely included in diamonds confirmed by 2D Raman mappings at different focal depths.

All Raman spectra of the inclusions show a peak at ca. 1580 cm^-1 with FWHM of 15-23 cm^-1. This peak is assigned to the G-band by sp² bond of carbon. All intensity of G-band is <10 % of diamond-band at peak. Most of the peaks of G-band are located higher wavenumber. The peak positions with FWHM of G-band are (in the rim of S-type): 1579.7 cm^-1 with 19.8 cm^-1, (in T-type): 1581.5-1585.8 cm^-1 with 15.6-22.2 cm^-1, and (in R-type): 1580.3-1595.7 cm^-1 with 15.0-17.2 cm^-1. The variation of these peak positions may be caused by the internal (residual) pressure. Raman spectra of some inclusions in the rim of S-type, T-type, and R-type show peaks at ca. 1350 cm^-1 (D1-band) and ca. 1620 cm^-1 (D2-band), which are assigned to structure defect of sp² bond.

The graphitic carbon inclusions discovered in the rim of S-type and T-type could be the relics of metastable intermediate graphitic carbon to form diamonds from aqueous fluids (see Akaishi et al., 2000; Yamaoka et al., 2000) in dolomite marble at 2^nd stage. However, the graphitic carbon inclusions in R-type are still unclear; possible explanation is that the inclusions are the relics of graphite trapped in diamond grains when diamonds formed or the relics of metastable intermediate graphitic carbon for the R-type and the core of S-type formations at 1^st stage.