The luminescent carbon-bearing microinclusion enigma in the Kimi Unit, Rhodope, Greece: Raman microscopic point analyses and mapping with different lasers

The Kimi Unit of the Rhodope Metamorphic Province (RMP), NE Greece, experienced ultrahigh-pressure metamorphism (UHPM), as documented by the unequivocal presence of diamond microinclusions in metapelitic garnet porphyroblasts. Certain peculiar lozenge-shaped 2-8 μm sized inclusions in diamond-bearing garnets reveal a broad composite and asymmetric triplet band (phase XXX) at ∼1331 cm^-1 in their Raman spectra acquired with a 632.8 nm He-Ne laser, initially attributed to an sp³-hybridized C-polymorph. These have been meticulously re-investigated by means of combined 2-wavelength (514.5 nm/632.8 nm laser) Raman microscopy. Raman mapping has been extensively employed in order to examine the spatial distribution of phase XXX and of other phases in these polyphase inclusions and to explore for additional Raman bands. The triplet band at ∼1331 cm^-1 measured with the 632.8 nm laser shifts to much higher wavenumbers (∼4966 cm^-1) when excited with a 514.5 nm Ar⁺ laser, proving that the XXX triplet is not a real Raman band but a luminescence one at ∼691.1 nm. Numerous hypotheses on the nature of the mysterious phase XXX (e.g. Cr³⁺-bearing mineral, carbonate, C polymorph, gas, organic phase) are explored and discussed but all are shown to be unsatisfactory. It is suggested that XXX occurs as nanocrystals that luminesce strongly giving the appearance (in Raman maps) of being larger.