Surficial carbonates in mantle plume sources revealed by magnesium isotopes

Deep recycling of surficial carbonates into the mantle may play a critical role in shaping Earth's long-term habitability, yet its contribution to the deep mantle carbon budget remains uncertain. Magnesium isotopes provide a promising tracer for addressing this issue, because of the uniquely light isotopic signatures of sedimentary carbonates. Here, we present high-precision magnesium isotopic compositions (δ²⁶Mg) for 152 globally distributed plume-related lavas sampling diverse mantle end-members. We find that δ²⁶Mg values below the bulk silicate Earth range are commonly associated with increasingly pronounced radiogenic isotopic signatures indicative of recycled crustal components. These features are best explained by the incorporation of isotopically light magnesium derived from subducted carbonate-bearing crustal materials into their mantle sources. On the basis of these observations, the lower mantle is estimated to contain up to ∼130 or ∼230-parts per million recycled carbon, potentially constituting a substantial portion of its total carbon budget.