Stability of Chemically Distinct Mantle Reservoirs

The geochemic observations of isotopic ratios are often interpreted as an evidence of the existence of the stable reservoirs of primitive material in the deep mantle. Until recently, lower mantle was often considered to be such reservoir and 660-km discontinuity was supposed to be its upper boundary. From the late nineties when new global seismic tomographic models resolved the slab like structures continuously passing through the 660-km boundary, the upper boundary of the reservoir is usually supposed to be located somewhat deeper in the mantle - at a depth of about 1000 km or even at 1700 km. To keep the upper and the lower (primitive) layer convecting separately, a density contrast associated with a difference in chemical composition is necessary. Numerical simulations of double-diffusive convection in the mantle have shown that the contrast of 2 - 5 % is sufficient to preserve the mixing of the two layers. Another factor influencing the efficiency of mixing is the viscosity contrast between the two layers. Inversions of the geoid and postglacial rebound data predict an increase of viscosity by 1 - 2 orders of magnitude somewhere in the depth range 660 - 1500 km. In the present study we investigate the stability of the lower layer as a function of the viscosity contrast and the density difference between the layers. The stability is studied by numerical simulations of double-diffusive convection in the 2-D axisymmetric model with a depth dependent viscosity. The convection simulations are started from the temperature distribution that is taken from the fully layered flow situation. The viscosity contrast between the layers is considered to be 0 - 2 orders of magnitude. The amplitude of dynamic topography of the boundary between the two layers is used as a measure of the stability. Our preliminary results indicate that the density contrast of at least 2-3 % is necessary to keep the layers convecting separately. The increasing viscosity contrast seems to suppress the tendency to layering. While in the isoviscous case the 2 % density contrast is sufficient to preserve the existence of a relatively stable reservoir, the viscosity jump by a factor of 100 causes relatively fast increase of the boundary topography.