Delayed Growth of Ferropericlase and Bridgmanite Controls Slab Residence at the 660‐km Discontinuity

Abstract

Cold subducted oceanic lithosphere may stall in the uppermost portions of the lower mantle (660-700 km depth) as a consequence of its slow thermal equilibration and the negative dP/dT of reactions forming ferropericlase and bridgmanite. Evaluations of slab buoyancy in the lower parts of the transition zone have mostly neglected the effects of dynamic P-T paths and the temperature-dependent expansion or contraction of stalled slabs. Forward predictions from mineral equilibria and thermal modeling posit slab residence at the base of the mantle transition zone for c. 150-160 Myr, dependent on geotherm and the depth of stagnation (between 660-700 km). Slab components can attain density thresholds following heating and phase transformations over distinct periods: Basaltic crust will become positively buoyant after c. 4 Myr whereas ultramafic components will become negatively buoyant after c. 150-160 Myr. Slab components may thus separate and independently transit or escape the transition zone under circumstances of equilibrated mineral assemblages. The 660 km seismic discontinuity presents a filter for the partial recycling of the upper oceanic lithosphere into the upper mantle.