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 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.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2014
- Bibcode:
- 2014AGUFM.V13B4771M
- Keywords:
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- 3621 Mantle processes;
- MINERALOGY AND PETROLOGY;
- 3652 Pressure-temperature-time paths;
- MINERALOGY AND PETROLOGY;
- 3653 Fluid flow;
- MINERALOGY AND PETROLOGY;
- 3654 Ultra-high pressure metamorphism;
- MINERALOGY AND PETROLOGY