Subducted Crustal Signatures in Diamondiferous Xenoliths from the Nyurbinskaya Pipe, Yakutia, Russia: Trace-Element and Oxygen Isotope Characteristics of Garnets

The recently discovered Nyurbinskaya kimberlite has yielded an exceptional number of diamondiferous xenoliths during early production stages. Xenolith studies from this highly-diamondiferous kimberlite provide an unparalleled opportunity to constrain the characteristics and distribution of diamond-bearing source materials in the mantle beneath the Archaean Anabar crustal province. A large suite of xenolith-hosted garnets (n=121) studied by Spetsius et al. (2008) have an unusual range of oxygen isotope compositions defining two populations. One population is dominated by eclogite (n=96), whereas the other is dominated by peridotite (n=25). The large eclogite dominated population defines a narrow distribution of δ18O (6.6 ± 0.5‰; 2SD), above the mantle garnet value (5.3 ± 0.6‰; Valley et al., 1998). Uncommon eclogitic garnets lie outside this range and reach a maximum δ18O value of ~9.7‰. These δ18O characteristics are unlike garnets from any other diamondiferous eclogite populations found in kimberlites worldwide (e.g., Mir, Udachnaya, Roberts Victor, Sloan, and Kirkland Lake). These populations have δ18O distributions centered about mantle values. The fact that garnets from nearly 100 mantle xenoliths from this pipe consistently contain high δ18O warrants close-examination of the trace-element chemistry of these garnets.Here we report the major- and trace-element compositions of garnet fragments separated from 84 xenoliths from the Nyurbinskaya kimberlite pipe that represent a subset of those studied by Spetsius et al. (2008). Trace-element contents in garnets of peridotitic, pyroxenitic, and eclogitic associations define several groups with both sinusoidal and LREE-depleted REE patterns. The trace-element characteristics and high δ18O values of eclogitic garnets suggest derivation from an oceanic crust that experienced extensive low-temperature hydrothermal alteration prior to subduction. Additionally, several pyroxenitic and peridotitic samples have δ18O and trace-element characteristics suggesting interaction with subducted crust, whereas harzburgitic garnets have low δ18O within the range of upper mantle estimates.The crustal signature observed in the diamondiferous xenoliths entrained by the Nyurbinskaya kimberlite may be linked to modification of mantle peridotites and mantle melts by subduction zone fluids (Taylor et al., 2003), or the incorporation of subducted crust into lithospheric or asthenospheric mantle by solid-state processes (e.g., Allegre and Turcotte, 1986; Pearson and Wittig, 2008). This study provides O-isotope and trace-element information to assess the origin of the crustal signature in a large number of mantle xenoliths; these results have important implications for the evolution of Siberian lithosphere. Additionally, these findings place constraints on the origins of highly-diamondiferous mantle reservoirs and improve understanding of Yakutian diamond genesis.