DIAMONDIFEROUS ECLOGITES FROM YAKUTIA, SIBERIA - EVIDENCE FOR A DIVERSITY OF PROTOLITHS

Major-element and REE compositions of 14 diamondiferous eclogites from the Udachnaya kimberlite in Yakutia, Siberia have been determined by electron microprobe and secondary ion mass spectrometer (SIMS). Based on previous clinopyroxene classification schemes (e.g., Taylor and Nea l 1989), all of these eclogite xenoliths belong to Group B/C, although some of the garnet compositions and mineral REE abundances are incons istent with the indicated groups. This demonstrates the inadequacy of the classification scheme based on African eclogites for application t o Siberian samples. Because of the coarse grain size of the Udachnaya nodules, meaningful modal abundances could not be obtained. However. r econstructed REE compositions using various garnet: clinopyroxene rati os demonstrate relative insensitivity to changes in mode for common ec logitic assemblages. Many of these reconstructed REE compositions show LREE depletions. Some depletions are consistent with an origin (eithe r directly or through partial melting) as ''normal'' or Type-I ocean f loor basalt. Others, however, require material of eclogitic or pyroxen itic affinities to undergo partial melting; this facilitates the deple tion of LREE while leaving the HREE at nearly original levels. Many of the eclogites of South Africa are consistent with a protolith of ''an omalous'' or Type II ocean floor basalt. This fundamental difference b etween the two regions is the likely cause of the inconsistencies with the chemically-based classification.