Tracing Sediment Melt Activity in the Sub‐Continental Lithosphere: Insights From Zn‐Fe Stable Isotopes.

Recycling of upper crustal sediments through slab subduction contributes to sub‐continental lithospheric refertilization and heterogeneity. However, the nature of recycled upper crustal components is unclear and direct evidence for sediment melt activity in the sub‐continental lithosphere is lacking. Here, we integrate major and trace elements, zircon U‐Pb dating, Sr‐Nd‐Zn‐Fe isotopic compositions of clinopyroxenites (crust‐mantle boundary) and a "glassy" xenolith from the North China Craton to relate their petrogenesis to the potential recycling of upper continental crust and provide direct insight into the sediment melt‐rock interaction. The clinopyroxenites have relatively uniform δ56Fe values (the permil deviation of the 56Fe/54Fe ratio from the IRMM014; −0.05‰–0.07‰, except for one outlier) and are not affected by melt metasomatism. The clinopyroxenites have highly variable whole‐rock δ66Zn values (the permil deviation of the 66Zn/64Zn ratio from the JMC‐Lyon standard) between 0.04‰ and 0.46‰, that closely correlate with Rb/La, K/U, Ba/Th, and Th/Nb ratios, and generate arrays that trend toward a composition similar to the "glassy" xenolith. The "glassy" xenolith has a high δ66Zn value (0.43‰ ± 0.05‰, 2SD) and a significantly low 143Nd/144Nd ratio (0.510991). This evidence implies that the "glassy" xenolith may represent a quenched sediment melt formed by the melting of carbonate‐bearing terrigenous sediments that may also be responsible for the metasomatism of clinopyroxenite xenoliths. The geochemical evidence from the "glassy" and clinopyroxenite xenoliths provides a direct evidence for the activity of sediment melt with upper continental crust components in the sub‐continental lithosphere. Plain Language Summary: Slab‐derived sediment melt activity in the overlying lithosphere is crucial for element cycling and lithospheric evolution. To understand the nature of recycled sediment and its impact on the overlying lithosphere, we analyzed the Zn‐Fe‐Sr‐Nd isotopic compositions of clinopyroxenites and a "glassy" xenolith from the North China Craton. The δ66Zn values of clinopyroxenites show correlations with proxies for hydrous melt metasomatism (Rb/La, K/U, Ba/Th, and Th/Nb ratios) and generate arrays that trend toward a composition similar to the "glassy" xenolith. The identification of "glassy" xenolith provides direct insight into the sediment melt‐rock interaction. Furthermore, this study proves the validity of using Zn isotopes as a powerful tool for tracing sediment melt activity in the sub‐continental lithosphere. Key Points: The Zn isotopic variations (0.04‰–0.46‰) of the clinopyroxenites are produced by melt metasomatismThe "glassy" xenolith may represent the preserved sediment melt with recycled carbonate‐bearing terrigenous componentsZn isotopes can be used to trace the activity of slab‐derived sediment melts in the sub‐continental lithosphere [ABSTRACT FROM AUTHOR]