THE ORIGINS OF YAKUTIAN ECLOGITE XENOLITHS

Abstract

Owing to the association with diamonds, eclogite xenoliths have received disproportionate attention given their low abundance in kimberlites. Several hypotheses have been advanced for the origin of eclogite xenoliths, from the subduction and high-pressure melting of oceanic crust, to cumulates and liquids derived from the upper mantle. We have amassed a comprehensive data set, including major- and trace-element mineral chemistry, carbon isotopes in diamonds, and Rb–Sr, Sm–Nd, Re–Os, and oxygen isotopes in ultrapure mineral and whole-rock splits from eclogites of the Udachnaya kimberlite pipe, Yakutia, Russia. Furthermore, eclogites from two other Yakutian kimberlite pipes, Mir and Obnazhennaya, have been studied in detail and offer contrasting images of eclogite protoliths. Relative to eclogites from southern Africa and other Yakutian 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 significant oxygen, sulfur, and carbon isotopic variation relative to the mantle; and intermineral radiogenic isotopic equilibration. Several of these eclogites could be derived from ancient, recycled, oceanic crust, but many others exhibit no evidence for an oceanic crustal protolith. The apparent lack of stable-isotope variation in the Udachnaya eclogites could be due to the antiquity of the samples and consequent lack of deep oceanic and biogenically diverse environments at that time. Those eclogites that are interpreted to be non-recycled have compositions characteristic of Group A eclogites from other localities that also have been interpreted as being directly from the mantle. At least two separate and diverse isotopic reservoirs are suggested by Nd isotopic whole-rock reconstructions. Most samples were derived from typical depleted mantle. However, two groups of three samples each indicate both enriched mantle and possible ultra-depleted mantle present beneath Yakutia during the late Archean and early Proterozoic. The vast majority of eclogites studied from the Obnazhennaya pipe also exhibit characteristics of Group A eclogites and are probably derived directly from the mantle. However, the eclogites from the Mir kimberlite are more typical of other eclogites world-wide and show convincing evidence of a recycled, 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.