Archean mantle heterogeneity and the origin of diamondiferous eclogites, Siberia: Evidence from stable isotopes and hydroxyl in garnet

Abstract

Data are presented for the O isotopic composition of clinopyroxene and garnet, the C isotopic composition of diamond, and the OH- content of garnet from eclogite xenoliths brought to the surface by the Udachnaya kimberlite pipe, Yakutia, Siberia. Radiogenic isotopic data suggest that the eclogites could have been derived from an ultradepleted mantle at approximately 2.9 Ga (Pearson et al., 1995; Snyder et al., in preparation). O isotopic compositions generally show equilibration between the eclogitic minerals (Δ_(cpx-Grt) = 0.11-0.41‰) and have δ^(18)O_(SMOW) for both garnet and clinopyroxene that lie near the range of accepted mantle values of 5.7±0.7‰. However, several eclogites indicate significant deviations from this range, at higher values of 6.8-7.0‰. Also, two eclogites lie at the lower end of the mantle range, at values of 4.8 and 5.0‰ (all in clinopyroxene). C isotopic compositions of diamonds all have δ^(13)C_(PDB) in the range of -1 to -7‰ and are centered at approximately -5‰, also within the range of accepted mantle values. The OH- contents of the garnet are generally between 0 and 22 ppm (as H_(2)0), although two samples exceed 70 ppm. This range of OH- is similar to eclogitic garnet from the Kaapvaal craton of southern Africa. The mantle C isotopic values of associated diamonds, the majority of O isotopic data, and the low OH- content of the minerals, although suggesting a general lack of crustal participation in the formation of the Udachnaya eclogites, do not rule out the participation of some ancient crustal material. That these eclogites include both ^(18)O-enriched and ^(18)O-depleted types suggests that the protoliths may have been overprinted by both low- and high-temperature hydrothermal events (cf. Jacob et aI., 1994). A positive correlation between δ^(18)O and ^(87)Sr/^(86)Sr allows the interpretation of these eclogites as representing a cross section of an Archean ophiolite. However, the lack of a single coherent grouping on a plot of δ^(18)O vs. ^(87)Sr/^(86)Sr suggests that, if the Udachnaya eclogites were derived from oceanic crust, they cannot be cogenetic and must represent at least two separate ophiolite sequences. Conversely, if the eclogites are found to be cogenetic, then a totally different process affected their formation and a probable metasomatic mechanism was operative. Because of the total lack of correlation of δ^(18)O with other geochemical parameters, we find no compelling evidence that all eclogites are derived ultimately from oceanic crust.

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