Home – Home – Jornals – Russian Geology and Geophysics 2018 number Неопубликованное

Experimental studies aimed at assessing the possibility of diamond crystallization during interactions of alkaline earth and transition metal carbonates with sulfur, as well as at characterizing the obtained diamond crystals and inclusions in them, were carried out on a multi-anvil high-pressure "split sphere" apparatus (BARS). The experiments were performed at constant P,T,t-parameters: 6.3 GPa, 1550 ºС and a duration of 20 hours, in the Mg,Ca-carbonate-sulfur system with different CaO/MgO ratios and Fe,Mg,Ca-carbonate-sulfur system with variable FeO/CaO/MgO proportions. It was experimentally established that in the alkaline earth carbonate-sulfur systems, a Ca,Mg-carbonate melt with dissolved sulfur (1.2-6.0 wt.%) is formed, which is a diamond growth medium. The stable growth form of diamond crystals is the octahedron, and an increase in the Ca# of the system is accompanied by an increase in the growth rate of the {111} faces. It was demonstrated that in the Fe,Mg,Ca-carbonate–sulfur system, the crystallization medium and the source of diamond carbon is the Fe,Mg,Ca-carbonate melt with dissolved sulfur, and diamond growth occurs as a result of the redox interaction of the sulfide and carbonate melts. With an increase in the ferruginousity of the system from 0.41 to 0.78, there occurs (1) an increase in the growth rate of the {100} faces (from 1.0 to 5.4 μm/h); (2) a change in the predominant elements of the octahedron face microrelief from triangular growth layers to hexagonal vicinals; and (3) an increase in the number and size of inclusions, as well as the evolution of their phase composition. Diamond inclusions are formed by melt preservation on the {100} faces during the transformation of a cuboctahedron into an octahedron. The inclusions contain quenched carbonate melt (quenching phases — carbonate, magnetite, and graphite), sulfur melt, and Fe-S-O melt. The discovery of a carbonate+magnetite+graphite association in the inclusions confirms the occurrence of a siderite redox dissociation reaction (3FeCO₃ = Fe₃O₄ + 3C⁰ + 2.5O₂ fluid) at the P,T-parameters of the lithospheric mantle. These results shed light on the possible origin of magnetite inclusions in upper-mantle diamonds and suggest that the presence of magnetite in syngenetic inclusions may be an indicator of the participation of iron-bearing carbonates in natural diamond formation processes. The results of the conducted studies indicate that Mg,Ca- or Fe,Mg,Ca-carbonate melts with dissolved sulfur can be classified as natural diamond-forming environments, and the identified features of the morphology of diamond crystals, the specifics of its growth and the composition of inclusions depending on the cationic composition of the carbonate melt with dissolved sulfur can be used as indicators of diamond crystallization in similar melts in nature.