Effects of surface processes on nature of arc magmas in subduction zones revealed by Mo-Zn isotopes

The features of arc magmas are inherited from the modified lithospheric mantle sources. Within oceanic subduction zones, the subducted sediment-derived fluids play a critical role in governing the geochemical variations of arc magmas. Nevertheless, it remains unclear whether the chemical signatures of arc magmas preserved from these sediments are indicative of extreme paleoenvironmental conditions on Earth’s surface. Here, we present whole-rock Mo-Zn isotopes for sequential arc magmas in Northeast Asia that chronologically document the subductions of the Paleo-Asian Ocean and Paleo-Pacific plates during the Early Permian to Early Jurassic. The Early Permian (293 Ma) and the Late Triassic (213−202 Ma) to Early Jurassic (185−183 Ma) mafic igneous rocks have Zn isotopic compositions (δ66Zn = 0.20‰−0.30‰) similar to those of mid-ocean-ridge basalts (MORBs; δ66Zn = 0.28‰ ± 0.06‰). Notably, the Early Permian basaltic rocks exhibit significantly lighter Mo isotopic signatures (δ98Mo = −0.99‰ to −0.07‰) compared to the depleted MORB mantle (DMM; δ98Mo = −0.204‰ ± 0.008‰). In contrast, the Late Triassic−Early Jurassic gabbros display heavier Mo isotopic compositions (δ98Mo = −0.18‰ to 0.54‰). These results, along with average oxygen fugacity values of 3.14 (n = 6, the Early Permian basalts) and 2.52 (n = 7, the Late Triassic and Early Jurassic gabbros), suggest that the Early Permian mafic arc magmas were sourced from a lithospheric mantle modified by oxidized sediment-derived fluids, whereas the Late Triassic−Early Jurassic gabbros were sourced from a lithospheric mantle modified by reduced sediment-derived fluids. Our investigation demonstrates that arc magmas exhibit limited geochemical variability in their Zn isotope signatures despite extreme environmental perturbations, but redox-sensitive isotopes (such as Mo) could serve as sensitive tracers of recording climatic fluctuations, especially in paleo-surface redox events.