Carbonatitic mineralogy of natural diamond-forming fluids

M. Kopylova*, O. Navon, L. Dubrovinsky, G. Khachatryan

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

42 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)126-137
Number of pages12
JournalEarth and Planetary Science Letters
Volume291
Issue number1-4
DOIs
Publication statusPublished - 1 Mar 2010
Externally publishedYes

Keywords

  • carbonatite
  • diamond
  • fluid inclusion
  • mineralogy
  • vibrational spectroscopy
  • X-ray diffraction

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