Trace element and U-Pb-Hf isotopic systematics of kimberlitic zircons: Classification, textures and implications for the origin of zircon “megacrysts” as antecrysts of variable antiquity

Zircon megacrysts in kimberlites comprise part of the low-Cr megacryst suite and are thought to form in the mantle during the final stages of evolution of melts that are related spatially and temporally to the host kimberlites. Kimberlitic zircon megacrysts provide crucial insights into their deeply derived parent magmas. Here, we present new U-Pb age, trace element and Hf isotopic data from 135 zircons from 29 kimberlite and four lamproite localities, along with detailed textural analyses. Cathodoluminescence (CL) imaging allows classification of the zircon textures into four broad groups: homogeneous zircons, laminar-oscillatory zoned zircons, zircons with thick core bands and overgrowths, and patchy homogeneously zoned zircons with thin irregular boundaries. A detailed examination of the different zircon textural groups reveals correlations between zircon zoning and trace element, Hf isotope geochemistry, and/or U-Pb ages, offering insights into zircon formation. We propose a genetic model for zircon megacryst formation, involving the immiscibility of silicate-rich and silica-poor CO 2 -rich liquids within a carbonate-silicate parental melt, derived from a carbonate-rich asthenospheric mantle, similar to kimberlites. In this model, kimberlitic zircon megacrysts crystallise from the silico‑carbonatitic CO 2 -rich melt fraction and are subsequently erupted, either immediately or at a later stage, by the host kimberlite or related melts. The complex evolution of the spectrum of kimberlitic zircon megacrysts, including variable mantle residence times and temperatures prior to kimberlite entrainment, would result in different types of compositional zoning. Overall, our observations support a model in which zircon megacrysts in kimberlites represent antecrysts of varying antiquity. • Kimberlitic zircon megacrysts are analysed for trace elements and U-Pb-Hf isotopes. • CL images reveal four different groups of zoning in zircon megacrysts. • Differently zoned zircons are correlated with trace element and Hf composition. • Zircon formation involves the immiscibility of carbonate-silicate parental melt. • Zircon megacrysts in kimberlites are best viewed as antecrysts of varying history.