Ages of the Obnazhennaya and Olivinovaya Pipes from the Kuoika Kimberlite Field, Siberian Craton: Constraints from Kimberlitic Zircons

Kimberlitic zircons represent a common type of mantle-derived zircon megacryst entrained in kimberlites, and they have been shown to reliably record ages of kimberlite emplacement. Here, we present results of U–Pb geochronology, as well as oxygen-isotope, trace-element, and mineral-inclusion analyses of zircon megacrysts from the Obnazhennaya and Olivinovaya kimberlite pipes of the Kuoika field, northern Siberian craton. These zircons exhibit mantle-like oxygen-isotope compositions (δ18O = 5.11–5.86‰), low U concentrations (5–90 ppm), low total rare-earth-element (REE) concentrations (8–186 ppm), relatively flat heavy-REE patterns, and weak or absent Eu anomalies. Together, these characteristics indicate that the analyzed megacrysts are mantle-derived kimberlitic zircons, an interpretation further strengthened by the identification of olivine inclusions in one of the zircons. As such, these are the first kimberlitic zircons reported from the Obnazhennaya and Olivinovaya kimberlites. U–Pb dating of these zircons yielded Late Jurassic concordia ages of 152.5 ± 2.7 Ma (2σ) for the Obnazhennaya pipe and 148.5 ± 3.4 Ma (2σ) for the Olivinovaya pipe, which are consistent with the Middle Jurassic-to-Early Cretaceous age range previously reported for kimberlite magmatism within the Kuoika field. Our U–Pb age for the Obnazhennaya pipe is consistent with previous age estimates obtained using other geochronological methods and therefore provides a new, independent constraint on its emplacement age. The U–Pb age obtained for kimberlitic zircons from the Olivinovaya pipe provides the first reliable age constraint for this kimberlite.