Stability of a chemically layered mantle

Abstract

A system composed of two immiscible fluid layers and heated from below is analyzed to determine its stability to convective motions. The model is applicable to the mantle if some fraction of the density increase with depth results from changes in chemical composition. For different parameter ranges, three modes of instability are possible: (1) convection of the entire depth of the fluid, (2) separate convection within each layer, and (3) overstability in the form of standing waves on the interface between layers. Earth models that include an increase in density due to iron enrichment below 650 km suggest that mantle convection can exist above and below but not through the 650-km level. The model results are combined with a prior understanding of mineralogical phase changes to suggest a possible mechanism for the deepest earthquakes.