5:48 PM - 5:51 PM
[SMP10-P02] Diamond formation through intermediate sp2 carbon from fluid in dolomite marble during the Kokchetav UHP metamorphism
3-min talk in an oral session
We examined over 5,000 diamond grains in 40 thin sections of dolomite marbles under a transmission optical microscope. Five sp2 carbon inclusions have been discovered in five diamond grains. These host diamond grains are 4-15 μm in diameter. These graphitic carbon inclusions are black under a microscope and their sizes are 1-5 μm across.
The microdiamond in dolomite marble has been classified into S-type, T-type, and R-type grains on the basis of the morphologies , Raman spectra , cathodoluminescence spectra , and carbon isotopic compositions . R-type and the core of S-type formed at 1st stage, and T-type and the rim of S-type crystallized at 2nd stage from H2O-rich fluid. All sp2 carbon inclusions were found only in the rim of S-type (one grain) and T-type (four grains).
Using multilayered 2D Raman mappings at different focal depths with solid-state laser (487.9 nm), Ar+ laser (514.5 nm), and He-Ne laser (632.8 nm), the Raman spectra of the examined graphitic carbon inclusions show a peak at ca. 1580 cm-1 (assigned to G-band caused by sp2 bond of carbon), and these sp2 carbon inclusions are completely included inside the host diamond grains. The G-bands of peak position with FWHM for the sp2 carbon inclusions are as follows: (the rim of S-type) 1572.0 cm-1 with 17.8 cm-1, 1581.3 cm-1 with 17.7 cm-1, and 1576.5 cm-1 with 16.5 cm-1; (T-type) 1574.9-1584.0 cm-1 with 18.0-28.3 cm-1, 1580.3-1587.1 cm-1 with 17.3-41.9 cm-1, and 1581.5-1584.2 cm-1 with 17.7-31.0 cm-1. The relative peak intensities of G-band to the host diamond band (ca. 1332 cm-1) are less than 10 %, and the strongest G-band peaks were detected at the center of the host diamond grains. The spectra of the inclusions often show disordered graphite bands; D1-band (ca. 1360 cm-1) and D2-band (ca. 1620 cm-1), but these bands are usually weak rather than G-band.
The discovered sp2 carbon inclusions were formed at the 2nd stage of the diamond formation, and could be relics of an intermediate metastable phase precipitated from H2O-rich fluid and followed by the transformation to diamond. This interpretation is consistent with the previous studies of diamond synthesis using C-O-H fluid at diamond stability fields (e.g. ).
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