Overview

Attempts to estimate mantle composition fall into two broad categories.

Cosmochemical approaches take meteorites as the basic building blocks; these materials are processed or mixed in order to satisfy such constraints as core size, heat flow and crustal ratios of certain elements. These models constrain the bulk chemistry of the Earth rather than that of the mantle alone. The only primitive meteorites that match satisfactorily both the stable isotope and redox characteristics of the Earth are the enstatite chondrites. The actual material forming the Earth is unlikely to be represented by a single meteorite class. It may be a mixture of various kinds of meteorites and the composition may have changed with time. The oxidation state of accreting material may also have changed with time.

Petrological models take the present continental crust, basalts and peridotites as the basic building blocks. Since basalts represent melts, and peridotites are thought to be residues, some mixture of these should approximate the composition of the upper mantle. Peridotites are the main mantle reservoirs for elements such as magnesium, chromium, cobalt, nickel, osmium and iridium. The continental crust is an important reservoir of the crustal elements; potassium, rubidium, barium, lanthanum, uranium and thorium. Enriched magmas, such as kimberlites, are rare but they are so enriched in the crustal elements that they cannot be ignored.