Widespread two-layered melt structure in the asthenosphere

Partial melts in the Earth’s convecting mantle influence its physical and chemical state, particularly the plasticity of the asthenosphere and the dynamics of plate tectonics. Melt compositions change systematically with the depth of mantle melting, but there are currently few quantitative constraints. Here we measure major and trace elements, combined with Hf–Nd isotope measurements, for basalts from North China. In addition, we compile a dataset of basalts from various oceanic and continental settings and find a quantitative link between the depth of basaltic melt extraction and its mean Y/Yb ratio. We show that a bimodal Y/Yb distribution is widespread in oceanic and continental basalts, consistent with two distinct depths of melt accumulation in the asthenosphere. Silica-rich basaltic melt accumulates at depths of 80–110 km and silica-poor, iron-rich melt at depths of 140–165 km, with a melt-free gap at a depth of 110–140 km. Our findings suggest that a two-layered melt structure may be more widespread in the asthenosphere than previously thought, particularly in areas of active or passive mantle upflow. The presence of two melt layers beneath fast-spreading plates and rifted continental margins may reduce the basal drag force on cratonic roots and aid with the breakup of cratons and supercontinents.