Raman spectroscopic study of metamorphic diamonds in garnet-clinopyroxene rock from the Kokchetav Massif

Diamond-bearing garnet-clinopyroxene rock (sample nos. 24997 and 25018) contains large-grained “microdiamonds” (ca. 150 µm at average) which are described as 2 types, cuboids and “ball-shaped” morphologies. Recently, we found another type of 2 grains; small-grained type (ca. 10 µm, rounded-shaped with smooth surface), completely included in garnet in the same rock with large-grained diamond. The Raman and photoluminescence (PL) spectroscopy were conducted in order to characterize diamond in garnet-clinopyroxene rock among the various diamonds in the Kokchetav UHPM rocks.

12 cuboids and 2 small-grained diamonds were analyzed by Raman spectroscopy using Ar⁺ laser (514.5 nm). The positions with the full widths at half maximum (FWHM) of Raman band of diamond are (cuboids): 1331.1-1332.1 cm^-1 with 4.23-5.34 cm^-1, and (small-grained): 1334.0-1334.7 cm^-1 with 4.75-4.79 cm^-1. The higher peak positions of small-grained type compared to cuboids were caused by internal (residual) pressure from the host garnet. 2D Raman mappings at different focal depths were also conducted to examine inside the diamond grains. At this moment, no specific variation and no inclusion have been detected in the diamond grains.

PL spectra of 2 cuboids and 2 small-grained diamonds were measured using excitation lines of the Ar⁺ laser and the He-Ne laser (632.8 nm) to confirm PL band. All examined diamonds have a PL band at ca. 639 nm (NV^- center) throughout grains. Using Ar⁺ laser, these 4 diamond grains (both cuboid and small-grained) showed strange PL bands at 554.5-557.5 nm (ca. 1450 cm^-1), and their intensity distributions were strongly heterogeneous (sporadic within a diamond grain) within 1-3 µm scale. These PL bands have been already reported in other Kokchetav diamonds in dolomite marble and garnet-biotite gneiss.

Although a wide variation of diamond-bearing rocks occur in the Kokchetav Massif and the formation environments of diamonds are quite diverse, in Raman spectra no critical difference was recognized up to now. Our studies just started and need to become broadened also to other diamond-bearings lithologies.