MYSTERIOUS RAMAN BANDS FROM UHP METAMORPHIC MICRODIAMONDS: A RELICT OF INTERMEDIATE PHASE FOR DIAMOND FORMATION

The Raman bands at about 1430-1480 cm^-1 suggesting carbon (or carbon-bearing) species were found in microdiamond entirely included in the host garnet of diamond-bearing dolomite marble, the Kokchetav UHPM terrane. Any extra phases relating to such Raman bands were not observed near laser spots in microdiamond. The Raman bands are relatively strong and broad: the relative intensities to the host diamond are 10-40 % (max. 90-110 %), and FWHMs are 25-45 cm^-1. Such unknown bands were first discovered in several domains of a very fine-grained (several micrometer) transparent T-type microdiamond. Later, similar bands were found in the core and rim of S-type and R-type. Recently, we confirmed the same bands in the microdiamonds in garnet in Grt-Bt gneiss from the Kokchetav.

As we used ordinary polished thin sections and the Raman bands at around 1450 cm^-1 may be attributed to contaminated organic materials used in thin section making, we carefully examined that these bands were not caused by the contamination but were attributed to the inside of microdiamond. We conducted 2D Raman mappings at different depths. The results showed that the domains (<1-3 µm) having the strongest bands were located in a diamond grain. This proved that the Raman bands at 1430-1480 cm^-1 were attributed to some materials in microdiamond entirely included in the host garnet. The possibility of the contamination during thin section making was excluded.

In spite of the difficulty to identify unknown phases only by Raman spectroscopy, we considered that the Raman bands could be attributed to some carbon or carbon-bearing species which were relicts as intermediate phases for microdiamond formation under UHP. Fullerenes are speculated as possible carbon materials for those mysterious Raman bands. The band at ca. 1470 cm^-1 is similar to A_g (2) band of C₆₀ fullerene. The large FWHM and the variation of peak positions at 1430-1480 cm^-1 may be caused by aggregations of several species of fullerenes. If our speculation is correct, metamorphic diamond could have high possibility to preserve metastable intermediate carbon phases in their crystal because the duration for diamond growth was much shorter than kimberlitic diamonds. We should pay attentions to such extra phase inside metamorphic diamonds in order to explain the formation mechanism.