Melting of eclogitic oceanic crust for the low-velocity zone within Earth’s upper asthenosphere

The low-velocity zone (LVZ) in shallow asthenosphere is crucial for Earth’s geodynamics and is widely linked to the presence of partial melts. Incipient melts from mantle peridotite melting are highly mobile and tend to quickly escape from their sources; however, it remains enigmatic how the LVZ can extend to the depths where partial melting initiates. Here, we identify a suite of primitive low-MgO basalts derived from an eclogitized oceanic crust at ∼200 km within the LVZ, providing a compelling scenario for the LVZ formation. These primitive basalts exhibit significantly heavier Fe isotopes and lighter Ca-Mo-O isotopes compared to mid-ocean-ridge basalts, indicating that they originated from eclogitized oceanic crust rather than peridotite. This highlights the fact that oceanic crust recycled into the mantle can melt to form primitive Mg-poor and Si-rich melts. Given their relatively high density and viscosity, these melts show low-mobility and accumulate at depths of ∼150−200 km, forming a low-velocity layer within the asthenosphere.