Molybdenum isotopes record slab inputs to the mantle source of late Triassic Arc Lavas, Northeast China

The geochemical study of arc lavas provides an important window for the exploration of slab input into deep mantle in subduction zones. However, the classification of subduction compositions and the contribution degree that reaches the mantle are difficult to observe directly, and this recycling mechanism is still poorly understood. Here we report Mo-Sr-Nd-Hf isotope data for Late Triassic mafic rocks from Northeast China, which were products of Mongol–Okhotsk Ocean slab subduction in the Early Mesozoic. They have δ^98/95Mo values of −0.24 ‰ to 0.36 ‰, which are mostly higher than the normal mantle value of −0.20 ‰ ± 0.01 ‰. These rocks also exhibit arc-like trace element compositions and depleted whole-rock Sr-Nd-Hf isotopes, with (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7039 to 0.7043, ε_Nd(t) values of 4.0 to 8.5 and ε_Hf(t) values of 6.9 to 11.1. In addition, they also have elevated zircon ε_Hf(t) values of 7.7 to 12.7. These features, combined with high La/Sm, Th/La, Ba/La, and Ba/Th ratios, suggest that the heavy Mo isotope signatures in the arc rocks were mainly inherited from altered oceanic crust-derived aqueous solutions with minor anoxic sediment-derived melts. Therefore, the combined Mo-Sr-Nd-Hf isotopic studies in mafic igneous rocks are a powerful means to trace the geochemical fluxes in subduction zones, especially with the studies of molybdenum isotope system providing new insights into the recycling of crustal materials at the convergence plate margin.