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 delta Fe-56 values (the permil deviation of the Fe-56/Fe-54 ratio from the IRMM014; -0.05 parts per thousand-0.07 parts per thousand, except for one outlier) and are not affected by melt metasomatism. The clinopyroxenites have highly variable whole-rock delta Zn-66 values (the permil deviation of the Zn-66/Zn-64 ratio from the JMC-Lyon standard) between 0.04 parts per thousand and 0.46 parts per thousand, 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 delta Zn-66 value (0.43 parts per thousand +/- 0.05 parts per thousand, 2SD) and a significantly low Nd-143/Nd-144 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.