Carbonatitic mineralogy of natural diamond-forming fluids

A suite of 20 cuboid fibrous diamonds from the Democratic Republic of Congo was studied to determine mineral phases crystallized from diamond-forming fluids that were trapped as inclusions in diamonds. We identified minerals and non-crystalline components using their compositional trends in the electron microprobe analyses of inclusions, an innovative bulk X-ray diffraction analysis and characteristic FTIR and Raman peaks. The DRC diamonds contain fluid inclusions of the Ca–carbonatitic–silicic composition. Most common phases precipitated from the fluid are found to be high-Si micas (90–30% of the solids volume), complex non-crystalline Na–Ca–Mg–Fe carbonate matter and apatite (together 5–70%), leaving a residual aqueous solution of K, Cl and carbonate ions and gaseous CO2. A notable absence of carbonate minerals in bulk X-ray diffraction patterns combined with the vibrational spectroscopy observations on the C–O bonds indicates that C may be included in dissolved or amorphous carbonate matter. The modes of the most abundant phases are controlled by chemical trends of the bulk fluid compositions. Other relatively rare ∼ 30 minerals, including various minerals with structural and coordinated H2O, are detected by the vibrational spectroscopic and X-ray analyses. The fluid also contains some hydrocarbons associated with the carbonate material. The presence of some rare minerals and exotic compositions of solid-solution minerals in the fluid suggest crystallization from a closed system with high salinity–high aH2O–high aCO2 composition that persisted to relatively low temperatures and pressures. The bulk of the fluid crystallized in the diamond stability field (P < 7 GPa, T < 950 °C), but continued to form hydrocarbons, accessory and deuteric phases down to 200 °C and 0.2 GPa. Overall, the mineralogy of the fluid resembles carbonatite.