Cosmochemical Estimates of Mantle Composition

As regards the rock-forming elements, the bulk composition of the Earth is basically chondritic (i.e., solar) with approximately equal abundances of magnesium, silicon, and iron atoms. In detail, however, there are some variations in chemistry among chondritic meteorites, and from a detailed comparison with meteorites it is concluded that the bulk Earth composition has similarities with the chemical composition group of carbonaceous chondrites. 1. The Earth and most groups of carbonaceous chondrites are enriched in refractory elements, other types of chondrites are depleted. 2. If the Earth’s metal core contains 5% Si, then the Earth and carbonaceous chondrites have the same CI Mg/Si ratios. Ordinary and enstatite chondrites have significantly lower ratios. 3. Although the depletions of moderately volatile elements in the Earth are larger than in any group of carbonaceous chondrites, the Earth and carbonaceous chondrites show similar patterns of depletion of the moderately volatile elements: in particular, both are depleted in the alkali elements and in manganese. Enstatite and OCs are also depleted in volatile elements, but their depletion patterns are different and sodium and manganese are not depleted relative to silicon. 4. The Earth and carbonaceous chondrites lie on the same ⁵³Cr/⁵²Cr versus ⁵³Cr/⁵⁵Mn isochron, indicating that the depletion of manganese and probably of all other moderately volatile elements in the Earth and in carbonaceous chondrites occurred shortly after the first solids had formed in the solar nebula. There are also some differences between the chemistry of carbonaceous chondrites and the Earth. 1. The bulk Earth has.