Basalt Accumulation Fuels Warm Slab’s Subduction into Lower Mantle

How and why oceanic slabs subduct into the lower mantle is crucial in understanding the mantle dynamics and evolution, but remains unclear. Here we present seismic images of the mantle transition zone (MTZ) beneath the Sumatra subduction zone that reveal phase transformations of both the olivine and garnet systems, exhibiting a relatively warm slab in a ~250-km-wide passage from ~350 to ~750 km, then broadening to ~600 km wide at ~780 km depths. The amplitudes of converted waves at the interfaces of the MTZ base and the topmost of the lower mantle suggest up to ~75±25% of basalts in a basalt-harzburgite assemblage and thus intensive enrichment of oceanic crust near the upper-lower mantle boundary, and a gradual reduction of basalt content toward the depthsof ~780 km. Mineral physics modeling shows that such basaltic enrichment provides sufficient positive density contrasts and driving forces for downwelling of the slab assemblage into the lower mantle, and negative density contrasts and thereby buoyancy for the upward concentration of basalts in the uppermost lower mantle. The residual part of the slab assemblage subjected to the basalt reduction is rheologically more weak and conducive to itsflattening and broadening between 750 to 1200 km depths.