Paragenetic relations of diamond with silicate and carbonate minerals in the carbonatite-diamond system: Experiments at 8.5 GPa |
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Authors: | A V Spivak Yu A Litvin |
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Institution: | 1.Institute of Experimental Mineralogy,Russian Academy of Sciences,Chernogolovka, Moscow oblast,Russia |
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Abstract: | Phase relations of diamond and syngenetic minerals were experimentally investigated in the multicomponent system natural carbonatite-diamond
at a pressure of 8.5 GPa and temperatures of 1300–1800°C (within the thermodynamic stability field of diamond). Under such
conditions, the natural carbonatite of the Chagatai complex (Uzbekistan) acquires the mineralogy of Ca-rich eclogites (grospydites).
The melting phase diagram of this system (syngenesis diagram) was constructed; an important element of this diagram is the
diamond solubility curve in completely miscible carbonate-silicate melts (solubility values are 15–18 wt % C). The diamond
solubility curve divides the phase diagram into two fields corresponding to (1) phase relations involving diamond-undersaturated
melts-solutions of carbon with garnet as a liquidus phase (region of diamond dissolution) and (2) phase relations with diamond-saturated
melts-solutions with diamond as a liquidus phase (region of diamond crystallization). During a temperature decrease in the
region of diamond crystallization from carbonate-silicate melts, the crystallization of diamond is accompanied by the sequential
formation of the following phase assemblages: diamond + garnet + melt, diamond + garnet + clinopyroxene + melt, and diamond
+ garnet + clinopyroxene + carbonate + melt, and the subsolidus assemblage diamond + garnet + clinopyroxene + carbonate is
eventually formed. This is indicative of the paragenetic nature of silicate and carbonate minerals co-crystallizing with diamond
and corresponding primary inclusions trapped by the growing diamond. A physicochemical mechanism was proposed for the formation
of diamond in carbonate-silicate melts. The obtained results were used to analyze the physicochemical behavior of a natural
diamond-forming magma chamber. |
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