THE ORIGINS OF YAKUTIAN ECLOGITE XENOLITHS

Owing to the association with diamonds, eclogite xenoliths have receiv ed disproportionate attention given their low abundance in kimberlites . Several hypotheses have been advnnced for the origin of eclogite xen oliths, from the subduction and high pressure melting of oceanic crust , to cumulates and liquids derived from the upper mantle. We have amas sed a comprehensive data set, including major- and trace-element miner al chemistry, carbon isotopes in diamonds, and Rb-Sr, Sm-Nd, Re-Os, an d oxygen isotopes in ultrapure mineral and whole-rock splits from eclo gites of the Ud-achnaya kimberlite pipe, Yakutia, Russia. Furthermore, eclogites from two other Yakutian kimberlite pipes, Mir and Obnazhenn aya, have been studies in detail and offer contrasting images of eclog ite protoliths. Relative to eclogites from southern Africa and other Y akutian localities. Udachnaya eclogites are notable in the absence of chemical zoning in mineral grains, as well as the degree of light rare earth element (LREE) depletion and unradiogenic Sr; lack of significa nt oxygen, sulfur, and carbon isotopic variation relative to the mantl e; and intermineral radiogenic isotopic equilibration. Several of thes e eclogites could be derived from ancient, recycled, oceanic crust, bu t many others exhibit no evidence for an oceanic crustal protolith. Th e apparent lack of stable-isotope variation in the Udachnaya eclogites could be due to the antiquity of the samples and consequent lack of d eep oceanic and biogenically diverse environments at that time. Those eclogites that are interpreted to be non-recycled have compositions ch aracteristic of Group A eclogites from other localities that also have been interpreted as being directly from the mantle. At least two sepa rate and diverse isotopic reservoirs are suggested by Nd isotopic whol e-rock reconstructions. Most samples were derived from typical deplete d mantle. However, two groups of three samples each indicate both enri ched mantle and possible ultra-depleted mantle present beneath Yakutia during the late Archean and early Proterozoic. The vast majority of e clogites studied from the Obnazhennaya pipe also exhibit characteristi cs of Group A eclogites and are probably derived directly from the man tle. However, the eclogites from the Mir kimberlite are more typical o f other eclogites world-wide and show convincing evidence of a recycle d, oceanic crustal affinity. We concur with the late Ted Ringwood that eclogites can be formed in a variety of ways, both within the mantle and from oceanic crustal residues.