ИСТИНА

Origin of diamond-bearing peridotite and eclogite rocks in kimberlites is cleared up based on mantle-carbonatite model of diamond genesis (Litvin, 2007 2009, 2013). Within the context of this model, representative data of analytical mineralogy of primary inclusions in diamonds are co-ordinated with results of physicochemical experiments on syngenetic diamond and inclusion phase relations (Litvin et al., 2012). The agreed data provide an evidence that diamond-parental media are presented by changeable carbon-saturated peridotite-carbonatite and eclogite-carbonatite melts of carbonatite specialization. The multicomponent parental melts are capable of formation not diamonds only but the great variety of their major and minor primary inclusions. It is evaluated from mantle xenoliths in kimberlites that the upper mantle is mainly composed of diamond-free peridotites which dominate over eclogites as 9 to 5 % (Mathias et al., 1970). At the same time diamond-bearing peridotites and eclogites occur rarely as demonstrated for S. Africa and Yakutia (Sobolev N., 1977). Nevertheless, origin of diamond-bearing rocks belongs to the key problems of genetic mineralogy of diamond and the mantle petrology. This is conditioned by dissimilar physicochemical and environmental conditions of formation of diamond-bearing and diamond-free peridotites and eclogites as reported below. It is symptomatic that garnets included into diamond of diamond-bearing eclogite sample are compositionally in close agreement with garnets of the sample (Sobolev V. et al., 1972). As a whole with respect to diamond-free eclogitic garnets, a tolerant indication of garnets from diamond-bearing eclogite (similar to garnets of eclogite paragenesis from inclusions in diamonds as well as intimate eclogitic garnet intergrowths with diamonds) is marked by elevated Na2O content (0.10-0.22%) created by admixtured Na-majorite component Na2MgSi5O12 (Bobrov & Litvin, 2011). For peridotites, garnets of diamond-bearing samples (as for inclusions in diamonds and intergrowths with diamonds) are indicated by high Cr2O3 and low CaO content over diamond-free ones. This compositional dissimilarity is compatible with formation: (1) of diamond-bearing rocks together with phases as trapped by diamonds so intimatel intergrown with them in localized chambers of partially melted multicomponent peridotite-eclogite-carbonatite-sulphide-carbon system of changeable composition; (2) of diamond-free rocks as products of scale processes of the upper mantle magmatism based on carbonatite-free peridotite-eclogite-sulphide-carbon system; the upper mantle peridotite is major host-rock for the chambers of diamond-parental magma. The chambers of diamond-parental carbonatite magma may originate and evolve in a following way: (1) metasomatic-magmatic stage which is resulted in partial carbonatization of the mantle peridotite under attack of K-CO2- bearing metasomatic agents and generation of carbonate melts; (2) dissolving-magmatic stage when major and accessory minerals of the host-rock, volatiles, carbon dissolve in carbonate melt as well as insoluble sulphide phases penetrate into the melts; eventually, completely miscible peridotite-carbonatite-carbon magma parental for diamond and paragenetic minerals (with inclusions of xenogenetic sulphide minerals and melts) are formed; (3) fraction- crystallization stage (after consolidation of the chamber into a self-dependent body) over a period of natural cooling of primarily partially melted parental magma up to its solidus temperature; the cooling activates a physicochemical control that is created by PT-phase relations for the parental magma composition, i.e., syngenesis phase diagram on a representative polythermal section of peridotite-eclogite-carbonatite-diamond system at 7 GPa under conditions of fractional crystallization (Litvin, 2013). Parental carbonatite melts, while compositionally evolve under fractional crystallization, are physicochemically capable to crystallization of diamond and sequential formation of minerals of peridotitic and eclogitic paragenese (presented as primary inclusions in diamonds). The paragenetic peridotite-eclogite transition in the course of ultrabasic-basic fractional evolution of parental melts is revealed in physicochemical experiments as the effect of “peridotite-to-eclogite” tonnel (Litvin, 2013). Diamond-bearing peridotite and eclogite rocks and intimate mineral intergrowths with diamond are also formed in the chambers of diamond-parental carbonatite magmas under these physicochemical conditions. Period of formation of diamond, paragenetic phases, diamond-bearing rocks and intimate intergrowths is commensurable to lifetime of the mantle chamber of diamond-parental carbonatite magma. Diamond-free rocks among mantle xenoliths in kimberlites represent samples of the enclosing host-rocks for the chambers of diamond-parental carbonatite magma.