Core-Mantle equilibrium: Comments on a paper by R. Brett

Brett's arguments in the preceding paper are critically reviewed. It is concluded that the abundances of Ni, Co, Cu, Au and Pt in the upper mantle, the oxidation state of this region and the nature of the volatiles inferred to have been degassed from the upper mantle are not readily explained if the earth has accreted from a well mixed reservoir of preexisting metal and silicate particles in the solar nebula which were equilibrated within the earth prior to separation of the core. The data can be readily explained if it is assumed that the earth accreted inhomogeneously in a state which was initially out of overall chemical equilibrium and that equilibrium, although approached, was not finally achieved during core-segregation.