Possible density segregation of subducted oceanic lithosphere along a weak serpentinite layer and implications for compositional stratification of the Earthʹs mantle

There is growing evidence that the top part of the oceanic mantle is pervasively serpentinized prior to subduction. Because the interior of a subducting slab heats up slowly, the serpentinized layer can be preserved for tens of Myr, thereby forming a weak zone that allows for mechanical decoupling between the oceanic crust and underlying lithospheric mantle. Once the crust is eclogitized, a shear stress would be induced by a compositionally-driven buoyancy difference between the crust and the lithospheric mantle. By simple force balance, we show that the downward slip velocity of the crust relative to the lithospheric mantle is similar to subduction velocities themselves; hence, conditions necessary for segregation of eclogitized crust from lithospheric mantle are generally met well before the slab approaches the lower mantle. The segregated components are predicted to journey to different resting grounds. Depleted lithospheric mantle, being slightly less dense than the ambient mantle, would eventually rise upward and congregate in the upper mantle while eclogitic crust would settle in a neutrally buoyant state near the bottom of the transition zone or at the base of the lower mantle. We speculate that this process gradually leads to the irreversible compositional stratification of the Earthʹs mantle.