Mineral physics observations suggest that distinct density and rheolog
ical differences exist between the crustal component of oceanic lithos
phere and the underlying mantle. We have conducted numerical experimen
ts to investigate the influence of both density and viscosity on the e
ffectiveness of recycling of oceanic crust into the lower mantle. Conf
irming previous results, the density inversion at 670 km depth alone i
s not sufficient to prevent crustal recycling. However, a soft layer m
ay exist between the strong garnet crust and cold slab interior. Model
s employing a simplified Newtonian sandwich model show that this thin,
weak layer can effectively decouple the crust and slab. Once entraine
d into the lower mantle, the then lighter crust can rise sufficiently
fast as a Rayleigh-Taylor instability to avoid further entrainment. Th
ese results suggest that the crustal component of slabs may be trapped
at 670 km depth, leading to a garnet enriched transition zone.