Source rejuvenation vs. re-heating: Constraints on Siberian kimberlite origin from U[sbnd]Pb and Lu[sbnd]Hf isotope compositions and geochemistry of mantle zircons

Abstract

We have studied a suite of mantle zircons from several differently aged pipes of the Siberian kimberlite province via UPb and LuHf isotope analyses and trace element compositions. The UPb ages we obtained confirmed four main episodes (Silurian, Devonian, Triassic and Jurassic) of kimberlite activity on the Siberian craton. The Druzhba pipe had two populations of zircons dating from the Silurian and Devonian, respectively. The geochemical features of our suite of mantle zircons show low concentrations of U, Th and heavy rare earth elements (REEs), positive Ce anomalies, and weak or absent Eu anomalies, which is in accord with the mantle-derived nature of the zircon. Despite having broadly similar geochemistry, zircons from differently aged kimberlites had some clear differences arising from variations in the composition of the protokimberlite metasomatic melt and from peculiarities of fractional crystallization. The Th/U ratios were highest in the Silurian zircons and sharply decreased toward the Devonian. The Triassic zircons had elevated and highly variable Ce/Nb ratios with low and nearly constant Th/U ratios.Zircons from Siberian kimberlites with different UPb ages showed systematic variations in their initial Hf isotope compositions. The oldest Silurian kimberlite field, Chomurdakh, had two zircon populations: Silurian zircons, with ɛHft values in the range of +2.8 to +5.9 units, and Devonian zircons, with ɛHft values in the range of +1.6 to +2.0 units. Zircons from the Devonian field kimberlites were in the range of +5.6 to +9.6 ɛHft units. The Triassic kimberlitic zircons had the most juvenile Hf isotope composition, at +9.3 to +11.2 ɛHft units, while the Jurassic zircons had +6.9 ɛHft units. The combination of the UPb and LuHf isotope data suggests a periodic rejuvenation of the lithospheric mantle roots by low-volume melts from the asthenospheric mantle, resulting shortly after in kimberlite emplacements. Some Devonian and Jurassic kimberlites may have been melted by re-heating the Silurian and Triassic age sources, respectively, about 60 Myr after they were formed.