Mg and Zn Isotope Evidence for Two Types of Mantle Metasomatism and Deep Recycling of Magnesium 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, 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 delta Mg-26, slightly lighter delta Zn-66 relative to global oceanic basalts, high Sr-87/Sr-86, and low Nd-143/Nd-144, indicating source metasomatism by recycled siliciclastic sediments. By contrast, the alkali basalts possess remarkably lighter delta Mg-26 and heavier delta Zn-66 values relative to the mantle that are typically characterized by carbonates. The coupling of high delta Zn-66 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 + CO2. 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.