Hydroxyl contents, and carbon and oxygen isotopic compositions of diamondiferous eclogites from the Udachnaya Kimberite pipe, Yakutia, Siberia

Planetary Geosciences Institute
Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996, U.S.A

Enrico Fermi Institute
University of Chicago, Chicago, Illinois 60637, U.S.A.

Department of Geosciences
Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A.

Division ofGeological and Planetary Sciences
California Institute of Technology, Pasadena, California 91125, U.S.A.

Institute of Mineralogy and Petrography
Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia


Data are presented for theO isotopic composition of clinopyroxene and garnet, the C isotopic composition of diamond, and the OH content of gamet 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 (elta Cpx-Grt = 0.11-0.41 per mil) and have delta 18OSMOW for both garnet and clinopyroxene that lie near the range of accepted mantle values of 5.7 ± 0.7 per mil. However, several eclogites indicate significant deviations from this range, at higher values of 6.8-7.0 per mil. Also, two eclogites lie at the lower end of the mantle range, at values of 4.8 and 5.0 per mil (all in clinopyroxene). C isotopic compositions of diamonds all have delta 13CPDB in the range of -1 to -7 per mil and are centered at approximately -5 per mil, also within the range of accepted mantle values. The OH contents of the garnet are generally between 0 and 22 ppm (as H2O), 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 18O-enriched and 18O-depleted types suggests that the protoliths may have been overprinted by both low- and high-temperature hydrothermal events (cf. Jacob et al., 1994). A positive correlation between delta 18O and 87Sr/86Sr 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 delta 18O vs 87Sr/86Sr 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 delta 18O with other geochemical parameters, we find no compelling evidence that all eclogites are derived ultimately from oceanic crust.