Slab stagnation sustained by wet harzburgite and dry basaltic slab assemblages

Slab stagnation in the mantle transition zone is critical for the mantle dynamics and evolution of Earth's interior but remains poorly understood. Here we present seismic images that show two key features near Eurasia's northeastern margin: 1) a 700-km-deep, 400-km-wide discontinuity within the slab stagnation zone, matching the akimotoite-bridgmanite transition in an anhydrous basaltic assemblage at adiabatic temperature of ~ 1000 K, and 2) the 660-km discontinuity depressed below the Clapeyron slope of the post-spinel phase transition by ~ 15 and ~ 10 km, likely induced by water, respectively in front of the Pacific plate and at the center of the slab stagnation zone. Our mineral physics modeling indicates that slab stagnation is initiated and sustained predominantly by buoyancy from the excess depressions in the pyrolite and harzburgite mantle and aided by buoyancy of the dry basaltic assemblage in the uppermost lower mantle.