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

Monday, 15 December 2014
Sho Miura and Yoshihide Ogasawara, Waseda University, Tokyo, Japan
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 1st stage and the rim of S-type and T-type at 2nd 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 2nd 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 1st stage.