The Mechanism of Disordered Graphite Formation in UHP Diamond-Bearing Complexes

Abstract

Kyanite gneiss from the “New Barchinsky” locality (Kokchetav Massif) was studied in detail. This rock is characterized by zonal distribution of the C and SiO₂ polymorphs in kyanite porphyroblasts: (1) cores with graphite and quartz inclusions; (2) clean overgrowth zone with inclusions of cuboctahedral diamond crystals. The Raman mapping of SiO₂ polymorphs originally showed the presence of an association of disordered graphite + coesite “prohibited” in HT diamond-bearing rocks. Graphitization of diamond is the only likely mechanism of the disordered graphite formation in HT diamond-bearing rocks. However, the absence of disordered graphite in association with diamond in kyanite porphyroblasts from kyanite gneiss from the “New Barchinsky” locality eliminates the process of diamond graphitization at the retrograde stage. Most likely, crystallization of disordered graphite occurred at the retrograde stage from the UHP C–O–H fluid.