70 Physics of the Earth and Planetary Interiors, 38 (1985) 70-80 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands The primodial terrestrial magma ocean and its implication for stratification of the mantle Eiji Ohtani Department of Earth Sciences, Faculty of Science, Ehime University, Matsuyama 790 (Japan) (Received May 14, 1984; revision accepted October 1, 1984) Ohtani, E., 1985. The primodial terrestrial magma ocean and its implication for stratification of the mantle. Phys. Earth Planet. Inter., 38: 70-80. Fractional crystallization behaviour of a magma ocean extending to lower mantle depths was deduced from estimations of melting relations for the deep mantle and the density relationships between ultrabasic liquid and mantle minerals. The accretional growth of the Earth necessarily involves a molten zone (magma ocean) in the outer layer of the growing Earth. The fractionation by melting during accretion results in primary stratification composed of a molten ultrabasic upper mantle (magma ocean), a perovskite-rich lower mantle, and an iron core. A certain amount of A1203 and CaO was removed from the magma ocean and retained in the lower mantle due to eclogite fractionation in the early stage of accretion and the perovskite fractionation in the later stage of accretion. Models of the stratification of the upper mantle arising from fractional crystallization of the magma ocean and subsequent convective disturbance were deduced on the basis of estimations of melting relations for the deep mantle and the density relationships between the ultrabasic liquid and mantle minerals. The stratification of the mantle, which is consistent with geophysical constraints is as follows; the upper mantle is composed of two layers, the upper olivine-rich layer and the lower garnet-rich layer with a thickness around 200 km, and the lower mantle with a perovskite-rich composition. In this model, both the 400 and 650 km discontinuities are the chemical boundaries. 1. Introduction Various models of the accretion and core for- mation of the Earth have proposed that the outer layer of the Earth was much hotter in the early stages of terrestrial evolution (e.g., Hanks and Anderson, 1969: Ringwood, 1975). The possible existence of a primodial terrestrial magma ocean has been discussed by many authors. Ringwood (1975) suggested-that a partially molten zone ex- tended to depths of 400 km and that there was complete melting of the uppermost mantle im- mediately after core formation. Recent calcula- tions of the temperature regime of the accreting Earth suggested that the outer layer of the proto- Earth might have been molten during accretion. Wetherill (1976) suggested that the depth of the magma ocean was < 200 km, whereas Kaula (1979) and Hayashi et al. (1979) inferred melting up to a depth of > 1000 kin. The upper mantle stratigra- phy resulting from fractional crystallization of a terrestrial magma ocean extending to depths of - 120 km was discussed in detail by Hofmeisner (1983). Efforts to extend melting curves of mantle minerals to higher pressures have been carried out and melting curves of major minerals have been obtained to pressures exceeding 10 GPa (e.g., Ohtani, 1979). Herzberg (1983) and Ohtani (1983b, 1984) independently estimated the density of sili- cate liquids at high pressures by using the melting curves of various mantle minerals, to obtain a more detailed understanding of the fractional crystallization and gravitational differentiation, which occurred in the deep mantle during the early stage of terrestrial evolution. This paper discusses 0031-9201/85/$03.30 © 1985 Elsevier Science Publishers B.V.