A MODEL FOR THE LAYERED UPPER-MANTLE

Numerical modeling of mantle convection by Liu (1994, Science, 264: 19 04-1907) favors a two-layer convection, if the results are reinterpret ed for the correct phase relations in (Mg,Fe)(2)SiO4. The resulting ch emical isolation of the upper and lower mantle suggests a highly diffe rentiated and layered upper mantle to account for the discrepancy betw een the observed compositions of mantle xenoliths and the cosmic abund ances of elements. It is shown that a layered upper mantle with a hidd en reservoir can have a structure consistent with the observed seismic velocity profiles and an average bulk composition corresponding to th e cosmic abundances. The evolution of the upper mantle and the origin of komatiites are discussed in the context of the proposed model. (C) 1997 Elsevier Science B.V.