Mo isotopes record recycling of anoxic sediment in a Paleo-oceanic subduction zone

Abstract

Mafic arc igneous rocks with oceanic crust-derived fluids in their mantle sources generally show heavy Mo isotope compositions. However, these rocks may exhibit variable Mo isotope compositions due to involvement of seafloor sediments under different redox conditions. Thus, it is intriguing how Mo isotope recycles with sediment subduction and how Mo isotope fractionation occurs during this process. Here we report Mo isotope data for Paleozoic mafic arc igneous rocks from the Qinling-Tongbai orogenic belt in Central China. The Qinling mafic rocks have δ98Mo values of −0.03 to 0.40‰, and the Tongbai mafic rocks have δ98Mo values of −0.26 to 0.17‰. These values are considerably higher than the normal mantle values of −0.20 ± 0.01‰. In combination with low Mo/Ce ratios, enriched SrNd isotope compositions, and high melt-mobile incompatible trace element contents for these rocks, the high δ98Mo values suggest that anoxic sediment-derived melts with heavy Mo isotopes were incorporated by oceanic subduction into the mantle sources. The high δ98Mo values of the mafic rocks indicate the limited Mo isotope fractionation during subduction of the anoxic sediment. This inference is further verified by modeling of Mo isotope fractionation under reducing conditions in the subduction zone. In this regard, Mo isotopes can be used to identify the recycling of sediments in different types and thus provide geochemical constraints on the redox conditions of oceanic subduction zones.