Potassium isotopes reveal lithospheric control on the evolution of deep-sourced carbonated silicate magmas

Abstract

Kimberlites and related CO2-rich ultramafic rocks such as aillikites commonly show Nd-Hf isotope decoupling (unradiogenic εHf at a given εNd), attributed to either recycled oceanic slabs or interaction with metasomatized continental lithospheric mantle (CLM). We present potassium isotope data (δ41K) for Neoproterozoic aillikites from West Greenland and use δ41K-ε176Hf covariations to test these models. Aillikite δ41K ranges from −0.62‰ to −0.31‰ and correlates positively with εHf, indicating mixing between distinct mantle reservoirs rather than magmatic differentiation. The data can be reproduced by mixing a K-poor, high-δ41K carbonatitic melt with a K-rich, low-δ41K CLM metasome characterized by unradiogenic Hf. We infer a two-stage evolution in which CO2-rich melts formed in the asthenosphere from domains containing recycled carbonate-rich oceanic material and then acquired their potassic compositions and decoupled Nd-Hf isotope signatures during ascent through heterogeneous CLM. The Nd-Hf isotope decoupling in aillikites and kimberlites therefore records interaction between deep CO2-rich melts and ancient CLM domains. Primitive CO2-rich intraplate magmatism, especially in cratonic settings, offers unique insights into deep carbon cycling and the fate of CO2-rich melts on their way to Earth’s surface.