Diamond-forming efficiency of chloride-silicate-carbonate melts

Abstract

According to the carbonatite model of the diamond formation [Litvin, 2007] based on the large volume of mineralogical and physicochemical experimental information, carbon-silicate-carbonate (carbonatitic) melts with widely variable compositions are the growth medium for most mantle diamonds and inclusions in them. In addition to the major completely miscible carbonate and silicate components (minerals of peridotite and eclogite assemblages), such melts contain minor soluble components (oxides, phosphates, chlorides, C-O-H-N fluids, and others), as well as minor completely immiscible and insoluble solid and melt phases (sulfides, metals). The diamond-forming efficiency of silicate-carbonate melts clearly corresponds to the important criterion of syngenesis of diamond with its silicate and carbonate inclusions. High-pressure experimental associations not only comprise the whole set of minerals typical for inclusions in diamonds of peridotitic (olivine, garnet, clino-, and orthopyroxenes) and eclogitic (garnet and clinopyroxene) type, but demonstrate the characteristic features of minerals of diamond paragenesis. These comprise significant admixtures of Na in garnets and K in clinopyroxenes, which are the reliable indicators of crystallization of these minerals from alkaline silicate-carbonate melts. Experimental investigations of multicomponent peridotite-carbonatite and eclogite-carbonatite systems at a standardized pressure of 8.5 GPa within the narrow pressure range of 1760–1820°C demonstrated that the diamond-forming efficiency of their carbon-bearing melts had concentration limitations [Bobrov and Litvin, 2009]. The compositions effective for the diamond nucleation comprise only significantly carbonatite parts of the systems and are limited by the concentration barriers of diamond nucleation (CBDN) in the cases of K-Na-Ca-Mg-Fe-carbonatite, Ca-Mg-, and Kcarbonate compositions (at the concentrations of 30, 25, and 30 wt % of peridotite components and 35, 30, and 45 wt % of eclogite component, respectively). This means that the inhibitory influence of peridotite and eclogite components dissolved in carbon-bearing carbonatite melts on diamond nucleation is observed only at their relatively low concentrations. We established the diamond-forming efficiency of chloride [Litvin, 2003] and chloride