International Geologiical Congress - Oslo 2008

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MPM-10 Fluids and melts in the Earth?s mantle: From natural observation to HT-HP experiment

 

Physico-chemical conditions of formation of mantle-derived diamond and its syngenetic inclusions by HPHT experimental evidence

 

Yuriy Litvin, Institute of Experimental Mineralogy (Russian Federation)
 

 

Significance of multi-component system MgO - CaO - FeO (Fe, Fe2O3) - MnO - Na2O - K2O - Al2O3 - Cr2O3 - TiO2 - ZrO2 - SiO2 - P2 O5 - CuS - (Cu2S) - FeS (FeS2) - NiS - KCl - NaCl - SiC - CO2 (CO, CH4) - H2O - C in composition of parental medium for mantle-derived diamonds and their syngenetic inclusions is evident from mineralogy of the inclusions. The parental medium is heterogeneous and composed of Mg, Ca, Fe, Ti, Zr, Al, Si oxides, Mg, Fe, Ca silicates, Na, K, Mg, Ca, Fe aluminosilicates, Fe, Ni, Cu sulfides, Ca phosphate, Mg, Ca, Fe, K, Na carbonates, K, Na chlorides, Si carbide, water H2O, carbon dioxide CO2, sometime - native iron and its alloys, methane, etc. But, there is no mineralogical criterion for diamond-forming efficiency of them and, hence, for determination which material forms a growth medium for dominating mass of diamonds. Efficiency of natural materials for diamond formation is successfully checked with experimental criterion of nucleation of diamond phase. It turned out, that not alone but a majority of relevant chemically different materials, if melted, are effective for diamond nucleation. Multi-component melts of carbonate, silicate, carbonate-silicate, sulfide, chloride, fluid-carbonate, fluid-silicate materials and strongly compressed and overheated phases of water, carbon dioxide and their mixtures are among them. Hence, chemical way of formation of mantle-derived diamond is uncertain, if even the experimental criterion data are combined with the mineralogical database, and genetic solution is unattainable. All this led to another more rigid experimental criterion of syngenesis of diamonds and their primary inclusions that requires that growth medium should be equally efficient for diamond nucleation and formation of the inclusions. By the syngenesis criterion, a capability to form simultaneously both diamonds and suits of trapped minerals was demonstrated for multicomponent carbonate-silicate melts derived from peridotite- and eclogite-bearing materials. The criterion disclosed that homogeneous carbonate-silicate and sulfide melts, both diamond-forming, are completely immiscible, but the sulfide melts, in contradistinction to the carbonate-silicate ones, are not capable to form silicate and carbonate minerals in combination with diamond. The observations obtained with HPHT experimental criteria correlate well with the model of carbonate-silicate (carbonatitic) growth medium for the major mass of mantle-derived diamonds. The growth melt with dissolved carbon consists of major carbonate and silicate components as well as minor components as soluble in the carbonate silicate-melts (oxides, phosphates, chlorides, carbon dioxide, water, some other) so insoluble due to their liquid immiscibility with the major carbonate-silicate ones (sulfides, native metals). Diamonds were formed under the mechanism of labily oversaturated solution in respect to diamond in carbonate-silicate melt. Support: INTAS project 05-1000008-7927.

 

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