Slow Si-Al interdiffusion in garnet and stagnation of subducting slabs

► Si-Al interdiffusion rates in majoritic garnet have been determined. ► The diffusion rate is low compared to the Si diffusion rate in other silicate minerals in the deep mantle. ► Low-density metastable regions should exist in the slab due to the low diffusion rate. ► The subducted slabs may be buoyantly trapped in the mantle transition regions.