Heavy Molybdenum Isotopes Observed in Ultramafic Igneous Rocks: Anoxic Sediments Fingerprint in Paleo-Oceanic Subduction Zone Magmatism

Recycling of crustal materials in subduction zones leads to changes in the nature of subarc mantle and composition of arc magmas. However， this recycling mechanism remains unclear， especially the contribution of specific subduction sediment types to the mantle is difficult to observe directly. Here， we studied the Mo-Sr-Nd-Hf isotopes of Middle Permian arc ultramafic igneous rocks in Northeast China， which provide important insights into the sediment recycling of arc magmas. The studied rocks have high δ^98/95Mo values （-0.10‰ to +0.05‰） and Ce/Mo ratios （32.78 to 57.07）， as well as depleted to enriched Sr-Nd-Hf isotopic compositions. According to the Mo-Sr-Nd isotopic mixing model， their mantle source incorporated 0.1-0.5 % of anoxic terrigenous sediment-derived hydrous melts and altered oceanic crust-derived aqueous fluids， which leads to its mantle heterogeneity and variation in the heavy Mo isotopes within the ultramafic cumulates. As a result， we propose that the metasomatism of anoxic sediment-derived components in subduction channels may impose a characteristic Mo isotope fingerprint on the mantle wedge， ultimately endowing the arc magmas with heavy Mo isotope signature in paleo-oceanic subduction zone.