Abstract Subducted slabs transport volatiles into the deep mantle, greatly influencing material recycling, earthquakes, and magmatism in subduction zones. However, identifying the nature and amounts of fluids from different sources, including basaltic oceanic crust, sediment, and serpentinite, is difficult. Here, we report major and trace elements, Mo–Mg–Sr–Nd isotopes, and zircon Hf–O isotopes of early Paleozoic mafic rocks from an orogenic system. These rocks have arc trace element characteristics, enriched radiogenic isotopic compositions, and heavy Mo–Mg–O isotopic compositions, suggesting the existence of different subducted components in the mantle source. Notably, the heavy Mo and Mg isotopic signatures of these rocks mainly originated from serpentinite‐derived fluids. Our study indicates that 90% of the fluid component incorporated into the mantle sources was derived from serpentinite. This suggests that serpentinite probably dominates the fluid cycle in paleo‐oceanic subduction zone.

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