Mg and Zn isotope evidence for two types of mantle metasomatism and deep recycling of magnesian carbonates

To test the ability of Mg and Zn isotopes in discriminating between different types of mantle metasomatism and identifying deep carbon cycling, here we present a comparative study on two types of Cenozoic lavas in SE Tibet, i.e., potassic-ultrapotassic lavas and alkali basalts. The contrasting chemical compositions, Sr-Nd isotopic ratios and olivine chemistry between them suggest distinct sources in the enriched lithospheric mantle and asthenosphere, respectively. 26Mg and 66Zn values of the K-rich lavas are shifted toward slightly heavier and lighter values relative to global oceanic basalts, respectively, indicating source metasomatism by recycled siliciclastic sediments. By contrast, the alkali basalts possess remarkably light 26Mg and heavy 66Zn values that are typically characterized by marine carbonates. The coupling of high 66Zn with high Zn contents and Zn/Fe ratios further suggests a pyroxenite source containing recycled Zn-rich magnesian carbonates. This is corroborated by the similarity in major elements between the alkali basalts and experimental partial melts of CO2 + pyroxenite. Thus, Mg and Zn isotopes could be an effective tool of discriminating between silicate and carbonate metasomatism in the mantle. Notably, the occurrence of the alkali basalts is spatially consistent with a stagnant slab in the mantle transition zone (410–660 km), the latter of which is interpreted to represent the deeply subducted oceanic slab. These findings thus provide evidence for recycling of carbonates into the deep mantle, which represents a long-term circulation of subducted carbon compared with that in arc systems and has crucial significances for global deep carbon cycling.