Deep mantle anomalies block early Earth melting, challenging a primordial origin.

Earth’s deep mantle hosts enigmatic structures known as Large Low Velocity Provinces (LLVPs), which sit atop the core–mantle boundary and influence mantle dynamics and plume generation. Understanding their origin is central to reconstructing Earth’s early thermal and compositional evolution. Several hypotheses suggest that present-day LLVPs are the disrupted remnants of a globally continuous dense layer that formed early in Earth’s history. In this study, we show that a laterally continuous, non-convecting proto-LLVP layer would have severely inhibited strong plume formation and suppressed upper mantle melting throughout the Archean contradicting extensive geological evidence for widespread volcanism and early crust formation. By incorporating melting in global mantle convection models, we find that even high mantle potential temperatures, increased radiogenic heating in the basal layer, and elevated core-mantle boundary temperatures cannot overcome the thermal and physical barrier imposed by a continuous non-convecting basal layer. These findings rule out the scenario of a globally continuous basal layer in early Earth, prior to modern-style plate tectonics, and instead support hypotheses of LLVP formation as spatially separate or later-evolving structures. By reconciling mantle dynamics with observed Archean melting signatures, this study places strong constraints on the timing and geometry of LLVP formation, advancing our understanding of Earth’s early thermal evolution and the origin of deep mantle anomalies.