On the relative importance of mineral phase transitions and viscosity stratification in controlling the sinking rates of detached slab remnants

Abstract

We employ a two-dimensional control-volume model of convection in a cylindrical shell to study sinking rates of cold parcels of material in the Earth's mantle. Our model uses periodic boundary conditions, at 0 and 360 degrees, to eliminate side-wall effects and includes mineral phase transitions at 410 km and 660 km depths and a depth-dependent viscosity. We compare the rates of sinking of cold material with and without phase boundaries and with and without viscosity increases with depth. When the viscosity increase is sufficiently large we find the sinking rate is relatively insensitive to the presence or absence of phase boundaries; the effects of viscosity dominate. If recent estimates of large viscosity increases in the lower mantle are correct, our model could account for mid-mantle slab remnants of Jurassic age independent of the existence of phase transition boundaries in the mantle. Copyright 2007 by the American Geophysical Union.