RHEOLOGICAL CONTROL OF OCEANIC-CRUST SEPARATION IN THE TRANSITION ZONE

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.