Formation of diamond and syngenetic minerals in the mantle multi-component silicate-carbonate-sulfide-carbon substance: the key role of carbonatites

Litvin Yu.A.*, Kuzyura (Shushkanova) šA.V.*, šBobrov A.V.**,

Vasil’yev P.G.**, Okoyomova V.Yu.**

*Institute of Experimental Mineralogy, Chernogolovka, Russia; **Moscow State University, Moscow, Russia.


ššššššššššššššš In this work, melting phase relations of multi-component multi-phase system composed of eclogitic omphacite (Omph-Cpx) + garnet (Grt)š assemblage, K-Mg-Ca-carbonatite, sulfide pyrrhotite (Po) + pentlandite (Pn) + chalcopyrite (Ccp) material and carbon (graphite, diamond) šis first physico-chemically studied in high-pressure high-temperature experiment in the context of diamond genesis. In general [1], experimental physico-chemical research of diamond origin in the Earth’s mantle calls for compositions of starting materials that are chemically close to or reproduce important constituent parts of the multi-component diamond-parent medium of essential chemical variability.š

ššššššššššššššš At present, the criterion of syngenesis of diamonds and their primary inclusions [2] is crucial for physico-chemical experimental untangling petrological and mineralogical details of diamond genesis. In connection with this, ซsyngenesisป phase diagram is solely suited and most informative. šThe syngenesis diagram is taken its name from the fact that it unravels the physico-chemical ซsecretsป of syngenetic relations between diamond and all the other solid, liquid and volatile components and phases involved into diamond-forming processes under specific chemical and PT physical conditions. The syngenesis diagram of the multi-component system under study at 7.0 GPa is represented by its two-measure pseudo binary section ซeclogite28carbonatite42sulfide30 – diamondป (wt. %) where {eclogite = [(Grt: SiO2 40.00, TiO2 0.46, Al2O3 22.00, Cr2O3 0.04, FeO 20.90, MnO 0.52, MgO 9.02, CaO 8.18, Na2O 0.17)40(Omph-Cpx: SiO2 54.80, TiO2 0.48, Al2O3 9.79, Cr2O3 0.05, FeO 6.40, MnO 0.07, MgO 8.97, CaO 13.10, Na2O 6.70, K2O 0.30)60]}28{carbonatite = (K2CO3)50(MgCO3)25(CaCO3)25}42{sulfide = [Po: FeS]40[Pn: (Fe, Ni)9S8]40[Ccp: CuFeS.2)20]}30; graphite is used as starting carbon material. Major constituent parts of the whole system under study are ซeclogite40carbonatite60 – diamondป and ซsulfide – diamondป systems which were taken separately for experimental study. Two separate silicate40carbonate60 (carbonatite) – diamond and sulfide - diamond syngenesis diagrams were constructed at 7.0 GPa. The curves of diamond solubility in carbonatite and sulfide melts under partial and complete melting of the systems are determined and plotted as the key details of the proper diagrams. The curves (together with solidus lines) are boundary for P-T-composition conditions of diamond-forming phase fields and important for understanding the physico-chemical mechanism of diamond formation.

Earlier experimental study of the simplified silicate-carbonate-sulfide system [3, 4] demonstrates the complete carbonate-silicate liquid miscibility as well as complete immiscibility of homogeneous carbonate-silicate and sulfide melts which both are effective diamond-forming media. It was also found that syngenetic formation of diamond with silicate and carbonate minerals in diamond-forming sulfide melt is impossible because solubility of silicate and carbonate components in sulfide melts is critically low.ššššššššššššššš

The effect of complete immiscibility of carbonatite and sulfide melts makes possible to superpose the both sub-system diagrams experimentally studied here by projecting the image of one of them onto another one. As a result the combined syngenesis diagram for the whole eclogite-carbonatite-sulfide-diamond system is constructed. Special controlling experiments testify the validity of the combined syngenesis diagram. The pseudo binary diagram demonstrates clearly the interrelation between syngenesis conditions of diamond and trapped phases in dependence of PT parameters. In accordance with temperature and composition (pressure is constant in this case), conditions of syngenetic formation of diamond with silicate-carbonate (carbonatite) and sulfide melts, garnet, sulfide solid solution, clinopyroxene and carbonate minerals as well as their assemblages go evident. Hence the syngenesis diagram reveals potential scenarios of capturing each of the coexisting phases by growing diamond and PT conditions for these events.šš

ššššššššššššššš Present experimental study is motivated by (1) a close natural association of diamond with silicate, carbonate and sulfide minerals and melts which is identified by thorough mineralogical study of the growth inclusions in diamond [5-9], (2) still disputable estimation of chemical composition of diamond-parent medium between mineralogists that is contradictory favored as predominant silicate [10, 11], metal [12, 13], carbonatite [14, 15], kimberlite [16, 17],š sulfide [18, 19], C-O-H volatile [20, 21] one and (3) the first high-pressure experimental results for melting phase relations of extremely simplified silicate (pyrope) – carbonate (aragonite) – sulfide (pyrrhotite) system under PT conditions of diamond stability [3, 4].

The resulting data of this study as well as relevant experimental and mineralogical data support the view [2] that multi-component carbonatite (carbonate-silicate) melt of variable composition is of decisive importance in origin of the most mantle-derived diamonds and their syngenetic inclusions. The heart of the diamond-parent carbonatite medium is argued as completely miscible carbonate-silicate melt over-saturated with dissolved elemental carbon. The carbonatite melt hosts minor chemically variable components and phases as miscible and soluble (phosphate, chloride, C-O-H volatile, etc) so immiscible and insoluble (sulfides, native metals, etc). Influence of the immiscible components on PT conditions and kinetics of diamond nucleation and crystallization in parent carbonatite melts seems not to be essential. Influence of the soluble components may be more perceptible but not critical because of their comparatively lowered content in parent carbonatite melt (a steady-state of C-O-H volatile phases under mantle conditions is of low probability). šš

Reliable approximation of the parent composition is multi-component system MgO – CaO - FeO (Fe, Fe2O3) – MnO – Na2O – K2O – Al2O3 – Cr2O3 – TiO2 – ZrO2 - SiO2 – P2O5 – CuS (Cu2S) – FeS (FeS2) – NiS – KCl – NaCl – SiC – CO2 (CO, CH4) – H2O – C. Congruent carbonate melting, complete carbonate-silicate liquid miscibility and high elemental carbon solubility in carbonatite melts under conditions of diamond PT stability are key factors in formation of diamond-parent carbonatite melt.

Process of formation of parent medium for diamonds relates to conditions of origin of carbonatite magma in Earth’s mantle. The process may begin with metasomatic mantle peridotite carbonatization by chemically active volatile C-O-H agents of plume origin. Ensuing formation and evolution of carbonate-silicate magma with hosting miscible components (including carbon) and immiscible phases lead to multi-component diamond-forming carbonatite.

Experimental study of physico-chemical role of the components highly soluble in multi-component diamond-parent carbonatite melts (C-O-H volatile is among them) becomes urgent.š

ššššššššššššššš This study is supported by the INTAS project 05-1000008-793,RFBR grants 08-05-00110-aš and 09-05-00751-a..



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