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.