Recycled oceanic crust is believed to have played an important role in the formation of continental flood basalts, whereas the involvement of large amounts of CO₂ derived from recycled marine carbonate in their mantle sources is highly debated. Zn isotopes have great potential to trace recycled carbonate due to the distinctly different δ⁶⁶Zn values between marine carbonates and the mantle. Representative continental flood basalt samples from Keping (Group1) and Yingmai and Shengli (Group 2) in Tarim, NW China, were collected to investigate their mantle sources, and their Zn isotopes were studied systematically for the first time. The Zn isotope values of Keping basalts (between 0.29‰ ± 0.03‰ and 0.32‰ ± 0.05‰) are higher than values in the primitive mantle (δ⁶⁶Zn = 0.16‰ ± 0.06‰) but similar to those of mid-ocean-ridge basalt (MORB; δ⁶⁶Zn = 0.24‰？0.31‰). Considering their high fractionation (e.g., Mg# = 0.28？0.37) and potential involvement of crustal contamination (⁸⁷Sr/⁸⁶Sr_i between 0.70720 and 0.70779; ε_Nd between ？3.2 and ？1.9), their Zn isotopes may not conclusively point to a carbonated mantle source. In contrast, Yingmai and Shengli basalts show heavy Zn isotope values (between 0.32‰ ± 0.03‰ and 0.39‰ ± 0.03‰), nonradiogenic ⁸⁷Sr/⁸⁶Sr_i (0.70459？0.70518) and ε_Nd(t) (between ？1.3 and 0.1) signatures, and less fractionation (Mg# = 0.46？0.71), implying the involvement of recycled carbonate components in their mantle source. Nonetheless, the lack of negative Zr, Hf, and Ti anomalies, low CaO/Al₂O₃ ratios, and high SiO₂ contents preclude direct melting of carbonate-bearing mantle. Alternatively, these features may suggest the melting of decarbonated subducted eclogite and variable interaction with subsolidus peridotite. This assumption is consistent with the positive correlations of δ⁶⁶Zn with Gd/Yb and Sm/Yb ratios and Zn content as a result of mixing between eclogite-derived high-δ⁶⁶Zn melt and peridotite-derived mantle-like δ⁶⁶Zn melt. Our study provides a new model to reconcile the geochemical features of the Tarim continental flood basalts and highlights the potential role of decarbonated eclogite in the formation of continental flood basalts.

Article link: https://doi.org/10.1130/B36502.1