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Partial Melting in Mantle Eclogite Xenoliths: Connections with Diamond Paragenesis

Worldwide, most eclogite xenoliths from kimberlites display evidence of partial melting, brought about by the introduction of K-Na-rich fluids. This is particularly well developed in eclogites from Udachnaya, Mir, and other Yakutian kimberlites in Russia. The crystallization products of these incomplete reactions, typically represented as a "spongy" texture around primary omphacite, include secondary clinopyroxene (with lesser Na2O), spinel, feldspar, and glass. In intragranular partial-melt veins, orthopyroxene, plagioclase, amphibole, and phlogopite also can be present. With partial melting of kyanite eclogites, corundum and mullite are formed. In all cases, primary garnet shows evidence for partial melting, with the formation of orthopyroxene, spinel, olivine, and glass, with typical development of kelyphitic rims. The chemistry of the systems involved indicates that melting was definitely not isochemical, but involved the introduction of metasomatic fluids rich in alkalis, mainly K, and probably volatiles. There are indications from the secondary assemblages that different reactions occurred as a function of the chemistry of the primary minerals and that of the metasomatic fluids. The source of the fluids that induced most of the partial melting of the eclogites was from the kimberlitic melt. The alteration of the primary eclogitic minerals by these fluids may have begun upon entrapment of the xenoliths by the kimberlite, but continued even during upward movement of the kimberlite to depths of less than ~30 km. Evidence for the various reactions observed is evaluated in detail, and scenarios are depicted that might account for the observed reaction products. In addition, spatial correlations exist between the presence of some diamonds and the partial-melt products, as well as zones of alteration. We postulate that the partial melting process was connected with the resorption of some diamonds and the formation of last-stage fibrous diamond coatings and possibly some microdiamonds.