Metamorphic Diamond Formation under H2O-Fluid Conditions in Diamond-bearing Garnet-Clinopyroxene Rock from the Kokchetav Massif

Abstract:

H₂O-fluid inclusions and carbonate inclusions were identified in metamorphic diamond in garnet-clinopyroxene rock from the Kokchetav massif by micro-FTIR spectroscopy.

Metamorphic diamond was first reported in the Kumdy-Kol area of the Kokchetav Massif (Sobolev & Shatsky 1990). Kokchetav metamorphic diamonds occur in dolomite marbles, gneisses, and garnet-clinopyroxene rock, and show various features of morphology and occurrence. Among these diamond-bearing rocks, dolomite marble contains the highest concentrations of microdiamond (10-20 μm across, 2700 carat/t; Yoshioka et al. 2001). The largest "microdiamonds" (> 100 μm across) occur in garnet-clinopyroxene (Schertl et al. 2004). Recently, the same rock type but diamond-free one was studied; this diamond-free garnet-clinopyroxene rock contains exsolved coesite-bearing titanite suggesting precursor supersilicic compositions at UHP conditions (Sakamaki & Ogasawara 2014). Garnet and clinopyroxene in both diamond-bearing and diamond-free garnet-clinopyroxene rocks contain significant amounts of water (3000 ppm wt. H₂O at average) as structural OH and submicron-sized H₂O-fluid inclusions. The host garnet and clinopyroxene of diamond were grown under H₂O-rich environments (AGU Fall Meeting 2014 abstract, #V13B-4775).

Cubic diamond grains (approximately 100 μm across) chemically separated from diamond-bearing garnet-clinopyroxene rock was used in this study. Micro-FTIR analyses were conducted using a KBr pellet as an IR transparent window in N₂ gas atmosphere. The IR spectra shows CO₃^2- bands at 1455 cm^-1 (weak), broad H₂O bands at 3428 cm^-1 (strong), and sharp OH bands at 3555 cm^-1 (strong) were identified. These bands are assigned to H₂O-fluid inclusions, aragonite, and a hydrous silicate mineral (probably phengite), respectively. These bands are similar to those in De Corte et al. (1998). Strong IR absorption bands by C-N bonds at 1282 cm^-1 (A center, very strong), 1180 cm^-1 (B center, very weak), and 1133 cm^-1 (C center, weak) are also detected.

H₂O-fluid inclusions of the diamond examined in this study indicate that the diamond was crystallized in H₂O-rich environments while its host minerals were formed.