High-pressure melting relations of diamond-forming carbonatites: formation of syngenetic peridotitic and eclogitic minerals (experiments at 7.0 and 8.5 GPa)

Bobrov A.V.*, Spivak A.V.**, Divaev F.K.***, Dymshits A.M.*, Litvin Yu.A.**

*Moscow State University, Moscow, Russia; **Institute of Experimental Mineralogy, Chernogolovka, Russia; ***Institute of Mineral Resources, Tashkent, Uzbekistan

 

A substantial body of experimental results on diamond synthesis in carbonate media has been gained in recent years. There are primary inclusions in diamond crystals containing fluid-carbonate material [1], which can be modeled by the system K2ONa2OCaOMgOFeOCO2 and was considered as a medium of diamond formation under high pressure [2]. Diamond was successfully synthesized in multicomponent carbonate-silicate melts at 57 GPa and 12001500C [3]. The discovery of diamond in carbonate-silicate rocks of the Chagatai Complex [4] motivated our interest in the experimental study of phase relations in these rocks [5]. Our experiments allowed us to simulate the initial (mantle) silicate assemblage of the carbonate-silicate rocks of the Chagatai Complex. This assemblage was similar to that of high-Ca eclogite and grospidite from kimberlite pipes, which provided an explanation for diamond discoveries in these rocks. The association of carbonatite and kimberlite that was described in the Slave Craton (Canada) emphasized the role of carbonatite as a possible new source of diamond [6]. Furthermore, diamonds were found in carbonatite matter of Hawaiian magmas [7].

The discovery of diamondiferous carbonatites provides additional support to the theory of diamond genesis in the Earth's mantle, according to which diamonds were formed in strongly compressed carbonatite melts [8]. The carbonatite nature of the parental media in which spontaneous nucleation and growth of diamond occurred under PT-conditions of its thermodynamic stability obtained convincing experimental support through the simulation of diamond crystallization in melts chemically analogous to natural parent media both in simplified model systems and real multicomponent compositions [8]. Under such conditions the matter of primary fluid-mineral inclusions transforms to the state of carbonatite melts with an essential content of alkali silicate components.

The aim of this study was to study experimentally diamond nucleation and simulate the high-pressure mineral assemblages in peridotite-carbonate and eclogite-carbonate systems with dissolved carbon. We simulated crystallization of diamonds in melts with variable compositions of model peridotite [60 wt% olivine (Ol), 16 wt% orthopyroxene (Opx), 12 wt% clinopyroxene (Cpx), 12 wt% garnet (Grt)] and eclogite [50 wt% Grt, 50 wt% Cpx]. with carbonate of dolomite composition (CaCO3MgCO3), K2CO3, and multi-component K-Na-Ca-Mg-Fe-carbonatites. Carbonate-silicate melts in all experiments performed at PT-conditions of diamond stability are completely soluble. Concentration barriers of nucleation (CBN) [9, 10] were estimated at a pressure of 8.5 GPa for variable concentrations of silicate and carbonate components in parental melts: 25, 30, and 30 wt.% of peridotite components and 35, 45, and 35 wt.% of eclogite components, for CaCO3MgCO3, K2CO3, and model carbonatite, respectively. At higher silicate concentrations in carbonate-silicate melts, diamond grows only on seed being accompanied by thermodynamically unstable graphite phase.

The appearance of peridotite minerals syngenetic to diamond in the studied diamond-forming melts was established in special run series at P = 7.0 GPa and T = 12001800 for the composition of peridotite30carbonate70 (wt.%). Ol is a liquidus phase in the system with (CaCO3MgCO3); at T < 1700, an association of Cpx + Ol + carbonate-silicate melt (L) is stable; at 1600, Grt is added. The prevalence of Cpx over other silicates was established for this system; none of the runs demonstrated the presence of Opx. It is assumed that in CaCO3-rich systems Opx enters into reaction like 2MgSiO3 + CaCO3 → CaMgSi2O6 + MgCO3 and practically is not presented as a proper phase. In the system of peridotitealkali carbonate (K2CO3) the following assemblages are established: Opx(Ol) + L (1800C); Opx + X phase [11] + L (1500C); Opx + Ol + carbonate + L (1300C), Opx + Ol + Si-wadeite + carbonate (1200). Crystallization of melts with model multi-component (K-Na-Ca-Mg-Fe) carbonatite proceeds with the following change of mineral parageneses as the temperature decreases: Ol + L → Ol + Cpx (Grt) + L → Ol + Cpx (Grt) + carbonate. In principle, the appearance of Opx is possible in this system, but only if initial peridotite is enriched in this component, and alkali carbonate (K2CO3) essentially prevails over CaCO3 among carbonate phases. In the eclogite-carbonate system, for the composition of [Grt50Cpx50]35Carb65 (wt%), Grt and Cpx were obtained as liquidus phases depending on the starting composition. Melt crystallization proceeds by the following scheme with a temperature decrease: L Cpx+L (Grt+L) Grt+Cpx+L (Grt+Carb+L) Grt+Cpx+Carb+L Grt+CPx+Carb. It should be specially emphasized that the compositions of the minerals synthesized in experiments were quite similar to the compositions of starting phases. Some features which are typical for inclusions in diamonds, such as Na2O admixture in garnet (up to 0.5 wt.%) and K2O admixture in clinopyroxene (up to 1 wt.%) were established in run products. Incorporation of Na in Grt and K in CPx provides evidence not only for high pressure, but also demonstrates and important role of alkaline components in silicate-carbonante (carbonatite) melts during the diamond formation [12, 13].

The results obtained correspond to the carbonatite (carbonate-silicate) model of diamond genesis and point on theoretical possibility of crystallization of peridotitic and eclogitic silicate minerals syngenetic to diamond in silicate-carbonate melts under PT-conditions of diamond stability. The definite set of mineral inclusions in diamond is determined by the chemistry of carbonate-silicate systems including carbonate composition.

This study is supported by the INTAS project 05-1000008-7938, RFBR grant 08-05-00110-a and the RF President grant 4122007.5.

 

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