The Influence of Mineralogical Compositon on Viscosity and Dynamics of the Subducting Lithosphere

The viscosity of mantle rocks depends sensitively on mineralogy as well as on temperature, pressure, volatile content, and deviatoric stress. The viscosity of major mantle minerals -olivine, pyroxene, garnet, and their high-pressure phases, can differ up to three orders of magnitude depending on pressure, temperature and deviatoric stress. The viscosity of a rock will also vary according to the volume fraction of the individual minerals and their high-pressure phases, because different minerals undergo phase transitions at different depths. We present a numerical model of stress and deformation in subducting slabs for different mineralogical compositions of the slab (we distinguish the viscosity of basalt, harzburgite and lherzolite layers). First, the temperature distribution and phase transitions are calculated. These two fields are then used to compute the stress and deformation by including the effects of buoyancy forces, inhomogeneous thermal strain distribution, and volume reduction due to phase transitions. A viscoelastic rheology is considered with diffusion creep, dislocation creep and Pierls creep. The computations are restricted only to the subducting plate; the mantle interaction can be simulated by boundary conditions on the surfaces of the descending slab. The viscosity variation in the subducting slab will be discussed as well as its implications for the dynamics of the subducting lithosphere.