Zinc isotopic evidence for recycled carbonate in the deep mantle

Carbonate, the major carbon reservoir on Earth's surface, can enter into the mantle by subduction. However, evidence for recycled surficial carbonates in the deep mantle is still scarce. Ocean island basalts from Cook-Austral islands and St. Helena Island, widely called HIMU basalts because of their high μ = ²³⁸U/²⁰⁴Pb sources, are thought to be fed by mantle plumes originating in the lower mantle. Here we report exceptionally high δ⁶⁶Zn values (δ⁶⁶Zn = 0.38 ± 0.03‰) of these HIMU lavas relative to most published data for oceanic basalts (δ⁶⁶Zn = 0.31 ± 0.10‰), which requires a source contributed by isotopically heavy recycled surficial carbonates. During subduction of the oceanic lithosphere, melting of mixed surficial carbonates and basaltic crust in the deep mantle generates carbonatite melts, which metasomatizes the nearby mantle and the resultant carbonated mantle ultimately evolves into a high-δ⁶⁶Zn HIMU source. High-δ⁶⁶Zn signatures of HIMU basalts, therefore, demonstrate that carbonates can be transported into Earth's deep mantle.