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 mu = U-238/Pb-204 sources, are thought to be fed by mantle plumes originating in the lower mantle. Here we report exceptionally high delta Zn-66 values (delta Zn-66 = 0.38 +/- 0.03 parts per thousand) of these HIMU lavas relative to most published data for oceanic basalts (delta Zn-66 = 0.31 +/- 0.10 parts per thousand), 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-delta Zn-66 HIMU source. High-delta Zn-66 signatures of HIMU basalts, therefore, demonstrate that carbonates can be transported into Earth's deep mantle. Zhang et al. perform high-precision zinc (Zn) isotopic analysis on lavas from St. Helena Island in the Atlantic, and Cook-Austral Islands in the Pacific, and confirm that ancient superficial carbonates were transported into the deep mantle billions of years ago.

Automated summary

Researchers have confirmed that ancient superficial carbonates were transported into the deep mantle billions of years ago by analyzing lavas from St. Helena Island in the Atlantic and Cook-Austral Islands in the Pacific.