Heavy potassium isotopes in carbonatites reveal oceanic crust subduction as the driver of deep carbon cycling

Carbon cycling between surface and mantle reservoirs is pivotal in fostering habitability of Earth. A critical yet poorly constrained parameter is whether crustal carbon can “survive” devolatilization processes that accompany slab subduction and therefore influence deep carbon budgets. Carbonatites provide a key record to address this important topic. Here, we present high-precision potassium isotope data for a large set of carbonatite samples from both continental and oceanic settings, spanning from 2 billion years ago to the present. Modeling suggests that the heavy potassium isotopic compositions of carbonatites are inherited from their mantle sources, rather than resulting from magmatic and postmagmatic processes. Our results demonstrate a strong link between the subduction of oceanic crust and the recycling of carbonates into the mantle sources of carbonatites. These findings support the hypothesis that subduction of carbonate-bearing altered oceanic crust has been a critical mechanism for transferring carbon into the deep Earth through time.