Dual slab stagnation depths controlled by grain-size-induced sporadic low-viscosity zones at around 1000 km depth

Seismic tomography reveals widespread stagnant slabs, mainly at depths of ~ 660 and ~ 1000 km, yet a unified explanation for both depths remains lacking. Here we use numerical models that self-consistently track grain-size evolution to investigate controls on slab stagnation depth. Our models show that fossil slabs can induce mantle downwelling that crosses the 660-km discontinuity, where phase-transition-induced grain-size reduction generates a localized low-viscosity zone (LVZ) between 660 and 1000 km depths. Under slow trench retreat and in the presence of an LVZ, slabs penetrate the 660 km discontinuity and stagnate near ~ 1000 km depth. Conversely, under fast trench retreat or without an LVZ, slabs stagnate near ~ 660 km depth or penetrate into the deep mantle. These results highlight sporadic LVZs as a mechanism for the two stagnation depths and imply laterally inhomogeneous mid-mantle viscosity structure driven by sinking fossil slabs from past subduction.