ALBERT

Description: Water content in the mantle transition zone (MTZ) has been broadly debated in the Earth science community as a key issue for plate dynamics. In this study, a systematic series of three-dimensional (3D) numerical simulation are performed in an attempt to verify a hypothesis regarding the behavior of subducted oceanic plate and the role of crust in the MTZ under wet conditions. This hypothesis is based on the seismic observations of structural variations associated with large-scale flattened high velocity anomalies (i.e., stagnant slabs) in the MTZ. In our model, weak (low-viscosity) fault zones (WFZs), which presumably correspond to the fault boundaries of large subduction earthquakes, are imposed on the top part of subducting plates. The phase transitions of olivine to wadsleyite and ringwoodite to perovskite+magnesiowüstite under both “dry” and “wet” conditions are considered based on recent high pressure experiments. The effect of viscosity reduction of wet garnet is also considered for oceanic crust in the MTZ. Results show that there is a substantial difference in the behaviors of subducting plate and the trace of crustal materials between the models under dry and wet conditions. Under wet conditions the subducting plate tends to stagnate with a maximum lateral extent of over 1000 km. The weaker and denser crustal material (garnet rich) is fed into the WFZs and is advected faster than the plate main body of peridotite (olivine rich). Thus, the viscosity and density contrast between the crustal materials and the peridotite permits the plate segmentation under a wet MTZ condition.