Tracing decarbonated eclogite in the mantle sources of Tarim continental flood basalts using Zn isotopes

Abstract

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/86Sri between 0.70720 and 0.70779; εNd 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/86Sri (0.70459‒0.70518) and εNd(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.