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

The subduction zones are vital places for material cycling and energy exchange between the Earth's surface and interior. Previous researches mainly focuses on the effects of deep magma activities in controlling surface processes. However, the influence of surface processes on the nature of arc magmas in subduction zones remains poorly understood. Nevertheless, subducted sediments may preserve records of surface climatic fluctuations, leading to distinct chemical heterogeneities in Earth's interior. Northeast (NE) Asia, as a typical region of sequential tectonic regimes, provides potential in studying various influences of surface processes on the nature of arc magmas in subduction zones owing to occurrence of Permian and early Mesozoic mafic arc rocks with different geochemical features.

Previous studies suggest that the early Permian calc-alkaline volcanic rocks in the eastern margin of the Jiamusi Massif, together with the Yuejinshan accretionary complex, reveal that westward subduction of the Paleo-Asian oceanic plate occurred beneath the Jiamusi Massif, whereas the Late Triassic and Early Jurassic calc-alkaline igneous rocks, along with the coeval porphyry-type Cu-Mo deposits and Jurassic accretionary complexes in eastern Jilin and Heilongjiang provinces (NE China), indicate that the initial subduction of the Paleo-Pacific plate beneath Eurasia took place during the Late Triassic-Early Jurassic.

New whole-rock Mo-Zn-Sr-Nd-Pb isotopic data for these early Permian (293 Ma) and the Late Triassic (202–213 Ma)-Early Jurassic (183–185 Ma) mafic igneous rocks indicate: 1) that the synergistic changes in Sr-Nd-Pb isotope compositions have revealed the contribution of global subducting sediments (GLOSS); 2) that the consistent Zn isotopic compositions (δ⁶⁶Zn = 0.20‰ to 0.30‰), similar to those of mid-ocean ridge basalts (MORB, δ⁶⁶Zn = 0.28‰ ± 0.06‰; Wang et al., 2017), excluded the potential contribution of carbonates (generally low δ⁶⁶Zn) and the mantle partial melting (no correlations with MgO); 3) that the early Permian basaltic rocks exhibit generally lighter Mo isotopic signatures (δ⁹⁸Mo = -0.99‰ to -0.07‰) compared to the depleted MORB mantle (DMM, δ⁹⁸Mo = -0.204‰ ± 0.008‰; McCoy-West et al., 2019), suggesting that the early Permian mafic arc magmas were sourced from a lithospheric mantle modified by oxidized sediment; and 4) that the Late Triassic-Early Jurassic gabbros display generally heavier Mo isotopic compositions (δ⁹⁸Mo = -0.18‰ to 0.54‰) than DMM, suggesting the Late Triassic-Early Jurassic gabbros were sourced from a lithospheric mantle modified by reduced sediment. Taken together, we conclude that the lithospheric mantle in NE Asia experienced the transformation from oxidized to reduced sediment modifications during early Permian to early Mesozoic and that different surface processes control nature of arc magmas in subduction zones. These conclusions are also supported by the late Paleozoic-early Mesozoic stratigraphic records. In summary, our investigation demonstrates that arc magmas exhibit limited geochemical variability in non-redox-sensitive elemental signature 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.

This work was financially supported by the National Natural Science Foundation of China (Grant: U2244201).

• Wang et al. (2017). Geochimica et Cosmochimica Acta, 198, 151–167.
• McCoy-West et al. (2019). Nature Geoscience, 12, 946–951.