The relation of mantle heterogeneity to the bulk composition and origin of the Earth

Abstract

The heterogeneity of the mantle can be viewed in the context of models for accretion of the terrestrial planets from the solar nebula. Oxygen isotope ratios and mineralogy indicate the existence of hot planetesimals of diverse compositions. Assuming that nebular condensates range from a reduced state near the Sun to an oxidized state near Jupiter, a new model is proposed for heterogeneous accretion of the Earth beginning with hot, reduced condensates and ending with cool, oxidized condensates. The Ganapathy—Anders cosmochemical model for the bulk composition of the Earth was tested by summing measured compositions for the three outer zones and likely compositions for the inner zones. Revisions are suggested, including reduction of the content of the early condensate from that suggested by taking [U] « 30 ng/g, as suggested by naive interpretation of the heat flow. Elements that enter magma in preference to pyroxene or olivine are mainly confined to the outer 200 km. Elements that are chalcophilic under reduced conditions may be partly in the outer core. Hydrogen loss should result in an inward wave of oxidation, and may result in conversion of carbon to CO 2 .