Molybdenum isotopic constraints on the origin of EM1-type continental intraplate basalts

Abstract

The origin of enriched mantle 1 (EM1) component in the mantle has long been debated. EM1 source of basalts has been variously attributed to recycled sub-continental lithosphere, recycled lower continental crust, or recycled oceanic crust plus pelagic sediments. Molybdenum stable isotope systematics has the potential to resolve this debate, because of the significant isotopic differences between mantle and crustal reservoirs. Here we present molybdenum isotope data for EM1-type continental intraplate basalts from Nuominhe (NMH), Northeast China. The NMH basalts display significant variations in both δ98/95Mo (98Mo/95Mo ratio relative to NIST SRM 3134, −0.49 to −0.15‰) and Mo/Ce (0.009–0.037), and strong correlations between δ98/95Mo, Mo/Ce and Hf isotope and trace elements. EM1 end-member of the NMH basalts shows the lightest δ98/95Mo and lowest Mo/Ce values. The involvement of continental crust and sub-continental lithospheric mantle cannot account for the light Mo isotopic compositions of the basalts. By contrast, the recycled oceanic crust with pelagic sediments, characterized by low δ98/95Mo and Mo/Ce values, is the satisfactory candidate for the EM1 component in the NMH basalts. Our results also suggest a residual slab reservoir with light δ98/95Mo values (<−0.5‰) in the mantle beneath Northeast China, representing the complementary reservoir to heavy Mo isotope continental crust. Identification of this reservoir makes the Mo isotope system a powerful tracer of crust-mantle interactions.