ABSTRACT

The distribution pattern of the chemical elements of the Earth's surface indicates that the bulk composition of the Earth as a whole is similar to that of volatile-depleted CI chondrites. On the basis of the above bulk composition of the Earth, the known chemical composition of the Earth's crust, and the well accepted compositional model for the upper mantle, the chemical compositions of the lower mantle and the core have been calculated. It has been found that the silica content (the most abundant chemical component of the mantle) of the lower mantle is about 20wt% more enriched than the upper mantle. Furthermore, the iron content of the lower mantle is likely to be depleted relative to that of the upper mantle, since iron is chemically incompatible with the major mineral phase in the lower mantle, which is probably composed of 95 wt% of silicates with perovskite modifications. The possible stable mineral assemblages of the various parts of the mantle are given. It has also been calculated that the outer core contains about 15wt% of a light element, which is in line with, but independent of, previous estimates based primarily on geophysical constraints. Various reasons suggest that oxygen is the major light element in the core. The simple and direct correlations in the abundances of the major and minor elements, and in the general distribution of the chemical elements between the Earth and CI chondrites suggest a comparatively simple model for the origin of the Earth. Only some volatile and light elements in the CI composition were volatilized and escaped from the accreting Earth. The core of the Earth was developed by high-pressure disproportionation reactions in iron-rich silicates simultaneously with accretion.