MANTLE METASOMATISM AND MELTING IN DEEP-S EATED 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 x enoliths and low-Cr garnet megacrysts from the Udachnaya and Mir kimbe rlites are discussed. Two groups of rocks with primary or texturally e quilibrated mica are distinguished (A and B). Group A includes common coarse granuloblastic Iherzolites, harzburgites, pyroxenites, and webs terites of the spinel and garnet facies, which demonstrate mantle meta somatism and veining. The introduction of metasomatising fluids (or me lts) at this stage appears to occur during general metamorphism of the rocks. Group B, containing texturally equilibrated mica is believed t o 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 t extural type between primary and secondary micas and correlates in com position with primary mica from the Kimberley area xenoliths. Group C Cr-spinels are compositionally heterogeneous ranging from Al-Cr-spinel s to chromites which are similar as a whole to moderate Cr-spinels of the diamond paragenesis. Group C metasomatism is followed by late-stag e metasomatism and partial melting. Oxygen fugacities for spinel-ilmen ite 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 st art under reduced conditions that involve fO(2) inversion of asthenosp heric fluid within the diamond and kimberlite-forming cycle.