Mantle metasomatism and melting in deep-seated xenoliths from the Udachnaya pipe, their possible relationship with diamond and kimberlite formation

Different varieties of modal metasomatism in mantle-derived xenoliths from the Udachnaya pipe and also the phenomena of partial melting in xenoliths and low-Cr garnet megacrysts from the Udachnaya and Mir kimberlites are discussed. Two groups of rocks with primary or texturally equilibrated mica are distinguished (A and B). Group A includes common coarse granuloblastic Iherzolites, harzburgites, pyroxenites, and websterites of the spinel and garnet facies, which demonstrate mantle metasomatism and veining. The introduction of metasomatising fluids (or melts) at this stage appears to occur during general metamorphism of the rocks. Group B, containing texturally equilibrated mica is believed to be of possible magmatic origin. Metasomatised garnet peridotites and pyroxenites Group C contain Ph1 + Cr-Di + Cr-Sp aggregates replacing garnet. Phlogopite from this reaction association is an intermediate textural type between primary and secondary micas and correlates in composition with primary mica from the Kimberley area xenoliths. Group C Cr-spinels are compositionally heterogeneous ranging from Al-Cr-spinels to chromites which are similar as a whole to moderate Cr-spinels of the diamond paragenesis. Group C metasomatism is followed by late-stage metasomatism and partial melting. Oxygen fugacities for spinel-ilmenite pairs from late-stage products and from melt patches in xenoliths and megacrysts indicate a high oxidized state (FMQ) both for the late-stage metasomatism and melting. This complicated process appears to start under reduced conditions that involve fO(2) inversion of asthenospheric fluid within the diamond and kimberlite-forming cycle.