Forbidden mineral assemblage coesite-disordered graphite in diamond-bearing kyanite gneisses (Kokchetav Massif)

Confocal 2D and 3D Raman imaging was applied to SiO₂ inclusions in kyanite porphyroblasts from diamond-bearing kyanite gneisses of the Kokchetav Massif (Northern Kazakhstan). The study reveals a mineral assemblage of coesite-disordered graphite which is forbidden in ultrahigh pressure-high temperature (UHP-HT) metamorphic complexes. Disordered graphite in coesite inclusions is optically undetectable and has been identified exclusively by Raman spectroscopy. Raman imaging of the forbidden coesite-disordered graphite assemblage fails to detect bands assigned to fluid components (e.g. CO₂, N₂, CH₄, H₂Oliq), but the Raman spectra of fluid inclusions in kyanite, garnet and quartz demonstrate the presence of CO₂, N₂, CH₄, H₂Oliq and disordered graphite. The disordered graphite most likely formed by precipitation from a C―O―H fluid during the retrograde stage. The fluid was entrapped by kyanite with coesite inclusions near the peak metamorphic conditions. The lack of disordered graphite around diamond crystals in our samples does not support the generally accepted origin of disordered graphite in ultra high pressure metamorphic rocks by partial diamond graphitization during exhumation.