抄録
The phase transitions in the peridotite composition can account for the observed density and seismic wave velocity profiles of the upper mantle and the transiton zone. On the other hand, the lower mantle is not well constrained by the present experimental data, because of large uncertainties in the physical properties of the lower mantle minerals, and the temperature distribution in the lower mantle. Both a homogeneous mantle with a peridotite composition, and a stratified mantle with a peridotite upper mantle and a silica rich lower mantle can be consistent with the observed seismic data. The source upper mantle of the Al-depleted komatiite observed in the late Archean (2.5 Ga old) shows chondritic abundances of some key refractory lithophile elements compatible with the ultrahigh pressure minerals such as majorite and Mg-perovskite. The Al-depleted and Al-enriched komatiites generally observed in the early Archean (3.5-3.8 Ga old) show evidence of the majorite fractionation, melting at the depths of the transition zone. Recent Hf isotopic data of Al-depleted komatiites imply that the source mantle of the komatiites shows no evience of chemical layering in the early Archean; the source mantle of such komatiites was chondritic in terms of some refractory lithophile elements, such as Al, Ti, Sc, Hf, HREE. Solid state convection might have homogenized the stratification formed by global melting during the accretional stage of the Earth.
