Diamondiferous xenoliths from crustal subduction: garnet oxygen isotopes from the Nyurbinskaya pipe, Yakutia

Oxygen isotopic ratios suggest that many and possibly all eclogite xenoliths from kimberlites are representatives of subducted crust. Such oxygen data from all diamond pipes studied to date form an approximate bell-shaped distribution curve centered on that of the mantle, with a significant number of values both below and above mantle, indicative of high- and low-temperature (resp.) hydrothermal seawater processing of the protoliths prior to subduction. The Nyurbinskaya pipe of Yakutia, a newly developed diamondiferous kimberlite, has yielded an unprecedented array of xenoliths, each containing diamonds. Some 121 of these diamondiferous samples were selected for this study. Garnets were separated from the different types of diamondiferous xenoliths, including mostly eclogites, but also some garnet websterites (pyroxenites), and peridotites. The delta O-18 ratios of the majority of the peridotitic garnet samples lie within the range of the average mantle, except for one with a delta O-18 value of 6.57 parts per thousand. Garnets from xenoliths of websterite. paragenesis, in general, have delta O-18 values above 6.0 parts per thousand, with two samples as high as 7.3 and 8.59, and only two samples as low as 5.9 and 6.0 parts per thousand. Eclogitic garnets have a range of delta O-18 from 4.7 to 9.7 parts per thousand, with > 80 % above 6 parts per thousand, but still within the general range reported for garnets from eclogite xenoliths from the Siberian platform (2.5 and 8.0 parts per thousand). There does not appear to be any correlation between the major-element composition of the garnets and their delta O-18 values. These eclogitic garnet delta O-18 values form a rather steep-sided, near Gaussian distribution, centered about 6.6 parts per thousand. These new garnet oxygen-isotope ratios are not only additional evidence for subduction of oceanic crust but also evidence for a major involvement of the upper, low-temperature metasomatized portion of the crustal section. These eclogites, garnet websterites, and peridotites have also experienced extensive late-stage mantle metasomatism, with inhomogeneities in some garnet, typical of many diamondiferous xenoliths.