Paired megacrystic zircon (U-Th)/He and U/Pb dating reveals kimberlite eruption ages, protracted emplacement histories, and insights into megacryst genesis

Abstract

The timing of the processes occurring in the lithosphere during the ascent and eruption of kimberlites has been challenging to ascertain. Here, we present a novel application of paired (U-Th)/He and U/Pb geochronology on megacrystic zircon to date both the crystallization of the megacryst suite and the host kimberlite eruption. Unlike Pb, He is not retained in the crystal at temperatures >250 °C. For megacrystic zircon that crystallizes in the mantle, He retention begins upon kimberlite eruption such that the zircon (U-Th)/He (ZHe) dates should record kimberlite emplacement. We collected cathodoluminescence, trace element, U/Pb, and (U-Th)/He data for nine megacrystic zircon from five kimberlites: Diavik and Panda (Canada), Juína and Batovi-9 (Brazil), and Monastery (South Africa). For all zircon except from Diavik, ZHe and U/Pb dates overlap within uncertainty, indicating megacryst crystallization during or shortly before kimberlite eruption, and suggesting a genetic link between the kimberlite magma and megacryst suite. For Diavik zircon, the ZHe dates are ∼2.5 Gyr younger than the U/Pb dates, documenting that the megacrysts and the host kimberlite magma are genetically unrelated. ZHe dates from three of the kimberlites overlap with eruption ages interpreted from other methods. However, for Panda and Monastery, our paired ZHe-U/Pb dates differ from previous results, consistent with multiple eruptions over ∼5 Myr and implying that protracted eruptive phases may be more common than previously thought. Our work shows that (U-Th)/He dating of megacrystic zircon accurately dates kimberlite emplacement, and when coupled with U/Pb dating can yield insights into megacryst-kimberlite relationships.