Sluggish post-garnet transformation controls slab stagnation at the uppermost lower mantle

The behavior of subducted slabs in the deep mantle is crucial for understanding Earth's evolution and mantle dynamics. Seismic observations reveal widespread slab stagnation at around 1000 km, yet no major phase transitions have been found at that depth, leaving the underlying mechanisms enigmatic. Here we show that sluggish kinetics of the post-garnet transformation could induce a critical density deficit within subducting slabs. This deficit sustains metastable garnet over tens of millions of years, effectively stalling slabs at the uppermost lower mantle. Our results reveal that slab stagnation at this depth is transient and inherently tied to delayed phase transformation kinetics, consistent with geophysical inferences. This offers an alternative explanation for slab stagnation in a pyrolitic mantle without requiring long-standing rheological or chemical explanations alone. These findings highlight a critical, yet underexplored, role of phase transformation kinetics in slab behavior and deep mantle dynamics.