Molybdenum isotopes record recycling of subducting sediment in active continental margin, Northeast China

Abstract

Highly variable molybdenum (Mo) isotope compositions have been reported for mafic arc volcanics above oceanic subduction zones. The heavy Mo isotope signatures of mafic arc magmas are generally attributed to addition of high δ98Mo oceanic crust-derived fluids to the subarc mantle sources. However, subduction zone fluids at subarc depths may occur in the forms of aqueous solution and hydrous melts, and seafloor sediments would also be subducted to subarc depths to produce hydrous melts. If such sediment-derived melts were incorporated into the mantle wedge, it is intriguing what is their effect on the Mo isotope composition of mafic arc magmas. Here we report Mo isotope data for Early Jurassic mafic igneous rocks from Northeast China, which was an active continental margin during subduction of the Paleo-Pacific Plate in the Mesozoic. These rocks have δ98Mo values of −0.26‰ to 0.17‰, which are mostly higher than the normal mantle value of −0.20‰ ± 0.01‰. They also exhibit arc-like trace element compositions, moderate (87Sr/86Sr)i ratios of 0.7042 to 0.7047, and neutral εNd(t) values of −2.8 to 2.2. The high δ98Mo values are associated with low Mo/Ce ratios, enriched SrNd isotopes, and high melt-mobile element contents in the mafic igneous rocks. These geochemical features indicate that the heavy Mo isotope signatures in these rocks were mainly inherited from subducting sediment-derived melts. Subducting Paleo-Pacific oceanic crust-derived aqueous solutions and seafloor sediment-derived hydrous melts would metasomatize the overlying mantle wedge beneath the northeastern China continent, and the sediment-derived melts would have contributed the heavy Mo isotope signatures to the mantle sources of mafic arc magmas in the active continental margin. Therefore, the Mo isotope study of mafic igneous rocks is a powerful means to decipher the recycling of crustal components at convergent plate margins.