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, that is, K-rich (potassic-ultrapotassic) lavas and Na-rich alkali basalts. The contrasting bulk rock chemical compositions, Sr-Nd isotopic ratios, and olivine chemistry between them suggest distinct sources in the lithospheric mantle and asthenosphere, respectively. The K-rich lavas have mantle-like δ 26 Mg, slightly lighter δ 66 Zn relative to global oceanic basalts, high 87 Sr/ 86 Sr, and low 143 Nd/ 144 Nd, indicating source metasomatism by recycled siliciclastic sediments. By contrast, the alkali basalts possess remarkably lighter δ 26 Mg and heavier δ 66 Zn values relative to the mantle that are typically characterized by carbonates. The coupling of high δ 66 Zn with high-Zn contents and Zn/Fe ratios further suggests a pyroxenite source containing recycled Zn-rich magnesium carbonates. This is strongly corroborated by the similarity in major elements between the alkali basalts and experimental partial melts of pyroxenite + CO 2. Thus, mantle silicate and carbonate metasomatism contributed to the origin of K-rich and Na-rich lavas in SE Tibet, respectively. 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 observations provide evidence for recycling of carbonates into the deep mantle, which represents a long-term circulation of subducted carbon compared with that of arc-trench systems and has crucial significance for global deep carbon cycling.
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