Recycling of carbonated sediments into the mantle: Insights from Li-Mg-Zn isotopes of mafic rocks in the eastern North China Craton

Subduction-driven recycling of volatiles plays a crucial role in shaping our planet. Nevertheless, the mechanisms for the transfer of carbon from a subducting slab into the overlying arc lithosphere remain enigmatic. In this study, we present novel findings using Li-Mg-Zn isotopic systematics along with radiogenic Sr-Nd-Hf isotopes from four Late Jurassic (emplaced at 157−150 Ma) subduction-related mafic igneous rocks in the eastern North China Craton. We aim to investigate the role of carbonated oceanic slabs in generating mafic arc magmatism. These mafic rocks show lighter Li (δ7Li = −3.67‰ to −0.67‰) isotopic compositions relative to the mantle. They exhibit light Mg (δ26Mg = −0.38‰ to −0.32‰) and normal mantle-like Zn (δ66Zn = 0.26‰−0.30‰) isotopic compositions, indicating “Mg-Zn isotopic decoupling.” Modeling requires a complex mantle source region encompassing 4.5%−14.0% dehydrated altered oceanic crust and 0.3%−6.0% carbonate-bearing silicate sediments. Abundant amphibole phenocrysts, arc-like trace element patterns, and enriched Sr-Nd-Hf isotopic compositions suggest a hybridized subcontinental lithospheric mantle source with significant slab-derived hydrous carbonatitic liquid contributions. We propose that water significantly depresses the thermal stability of Mg-rich carbonates and facilitates the generation of hydrous carbonatitic liquids at sub-arc depths. Consequently, our findings elucidate that arc magmatism originates from a mantle source infiltrated by hydrous carbonatitic liquids from subducting slabs. The combined analysis of Li-Mg-Zn isotopes provides a case study for identifying the recycling of carbon in subduction zones.