Numerical Examination of the Dynamics of Subducted Crustal Materials with Different Densities

Crustal materials subducte d into the deep Earth are the major source of the mantle chemical heterogeneity, and their dynamics are expected to strongly depend on the density contrasts between the subducted crustal materials and surrounding mantle materials. Here, we estimate the bulk density variations of some different lithologies, pyrolite, mid-ocean ridge basalt (MORB), tonalite-trondhjemite-granodiorite (TTG), and also more hypothetical anorthosite and KREEP (basalt enriched in potassium, rare Earth elements, and phosphorus), in the lower mantle pressure range at static temperature, and test numerically the evolution of these subducted crustal materials within the mantle convection. We identify the following four features. (1) Since the TTG crust gains buoyancy in the lower mantle, it eventually ascends through time. (2) Although the cold anorthositic crust could sink to the core-mantle boundary (CMB), it would be heated by the heat flux from the core and finally assimilated into the lower mantle materials. (3) In contrast, dense KREEP basalt could be expected to sink to the CMB and gravitationally stabilized through geological time. (4) MORB shows an intermediate behavior between anorthosite and KREEP. The results suggest that dense KREEP could be primordial chemical reservoirs if it was generated in the early Earth as well as in the Moon.