Synthesis of diamonds with mineral, fluid and melt inclusions

Abstract

Experiments on the synthesis of inclusions-bearing diamond were performed in the SiO2–((Mg,Ca)CO3–(Fe,Ni)S system at 6.3GPa and 1650–1750°C, using a multi-anvil high pressure apparatus of the “split-sphere” type. Diamond synthesis was realized in the “sandwich-type” experiments, where the carbonate–oxide mixture acted as a source of both CO2-dominated fluid and carbonate–silicate melt, and Fe,Ni-sulfide played a role of reducing agent. As a result of redox reactions in the carbonate–oxide–sulfide system, diamond was formed in association with graphite and Mg,Fe-silicates, coexisting with CO2-rich fluid, carbonate–silicate and sulfide melts. The synthesized diamonds are predominantly colorless or light-yellow monocrystals with octahedral habit (20–200μm), and polycrystalline aggregates (300–400μm). Photoluminescence spectroscopy revealed defects related to nickel impurity (S3 optical centers), which are characteristic of many diamonds in nature. The density of diamond crystallization centers over the entire reaction volume was ~3×102–103cm−3. The overwhelming majority of diamonds synthesized were inclusions-bearing. According to Raman spectroscopy data, diamond trapped a wide variety of inclusions (both mono- and polyphase), including orthopyroxene, olivine, carbonate–silicate melt, sulfide melt, CO2-fluid, graphite, and diamond. The Raman spectral pattern of carbonate–silicate melt inclusions have bands characteristic of magnesite and orthopyroxene (±SiO2). The spectra of sulfide melt displayed marcasite and pyrrhotite peaks. We found that compositions of sulfide, silicate and carbonate phases are in good agreement not only with diamond crystallization media in experiments, but with data on natural diamond inclusions of peridotitic and eclogitic parageneses. The proposed methodological approach of diamond synthesis can be used for experimental simulation of the formation of several types of mineral, fluid and melt inclusions, observed in natural diamonds.