Mineral equilibria of diamond-forming carbonate-silicate systems |
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Authors: | A V Bobrov Yu A Litvin |
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Institution: | 1.Faculty of Geology,Moscow State University,Moscow,Russia;2.Institute of Experimental Mineralogy,Russian Academy of Sciences,Chernogolovka,Russia |
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Abstract: | Based on experimental and mineralogical data, the model of mantle carbonate-silicate (carbonatite) melts as dominating parental
media for natural diamonds was substantiated. It was demonstrated that the compositions of silicate constituents of parental
melts were variable and saturated with respect to mantle rocks, namely pyrope peridotite, garnet pyroxenite, and eclogite.
Based on concentration contributions and role in diamond genesis, major (carbonate and silicate) and minor (admixture) components
were distinguished. The latter components may be both soluble (oxides, phosphates, chlorides, carbon dioxide, and water) and
insoluble (sulfides, metals, and carbides) in silicate-carbonate melts. This paper presents the results of a study of diamond
crystallization in multicomponent melts of variable composition with carbonate components (K2CO3, CaCO3 · MgCO3, and K-Na-Ca-Mg-Fe carbonatite) and silicate components represented by model peridotite (60 wt % olivine, 16 wt % orthopyroxene,
12 wt % clinopyroxene, and 12 wt % garnet) and eclogite (50 wt % garnet and 50 wt % clinopyroxene). Carbonate-silicate melts
behave like completely miscible liquid phases in experiments performed under the P-T conditions of diamond stability. The concentration barriers of diamond nucleation (CBDN) in melts with variable proportions
of silicates and carbonates were determined at 8.5 GPa. In the peridotite system with K2CO3, CaCO3 · MgCO3, and carbonatite, they correspond to 30, 25, and 30 wt % silicates, respectively, and in the eclogite system, the CBDN is
shifted to 45, 30, and 35 wt % silicates. In the silicate-carbonate melts with higher silicate contents, diamond grows on
seeds, which is accompanied by the crystallization of thermodynamically unstable graphite. At P = 7.0 GPa and T = 1200−1800°C, we studied and constructed phase diagrams for the multicomponent peridotite-carbonate and eclogite-carbonate
systems as a physicochemical basis for revealing the syngenetic relationships between diamond and its silicate (olivine, ortho-
and clinopyroxene, and garnet) and carbonate (aragonite and magnesite) inclusions depending on the physicochemical conditions
of growth media. The results obtained allowed us to reconstruct the evolution of diamond-forming systems. The experiments
revealed similarity between the compositions of synthetic silicate minerals and inclusions in natural diamonds (high concentrations
of Na in garnets and K in clinopyroxenes). It was experimentally demonstrated that the formation of Na-bearing majoritic garnets
is controlled by the P-T parameters and melt alkalinity. Diamonds with inclusions of such garnets can be formed in alkalic carbonate-silicate (aluminosilicate)
melts. A mechanism was suggested for sodic end-member dissolution in majoritic garnets, and garnet with the composition Na2MgSi5O12 and tetragonal symmetry was synthesized for the first time. |
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