CORE FORMATION AND CHEMICAL-EQUILIBRIUM IN THE EARTH .2. CHEMICAL CONSEQUENCES FOR THE MANTLE AND CORE

We present here a new model of core formation which is based on the cu rrent understanding of planetary accretion and discuss its implication s for the chemistry of the Earth's mantle and core. Formation of the E arth by hierarchical accretion of progressively larger bodies on a tim e scale much longer than that of solid body differentiation in the neb ula indicates that a significant fraction of metal in the core could b e inherited from preterrestrially differentiated planetesimals. An ana lysis of the segregation of this iron to form the core suggests that m ost of the metal settles to the core without interaction with silicate s; only a small fraction of the metal chemically equilibrates at high temperatures and pressures with the silicates. The siderophile element abundances in the mantle are considered to be a consequence of a two- step equilibration with iron, once preterrestrially in the planetesima ls at low temperatures and pressures, and later in the Earth at high t emperatures and pressures. The highly siderophile elements such as Re, Au and the platinum group elements in the mantle are essentially excl uded from silicates from the preterrestrial equilibration. We attribut e the abundances of these elements in the mantle to the later equilibr ation in the Earth at substantially reduced metal-silicate partition c oefficients (D-met/sil), for which there is a considerable experimenta l evidence now. Mass balance considerations constrain the fraction of core metal involved in such an equilibration at approximately 0.3-0.5% . The model accounts for the levels and the near-chondritic ratios of the highly siderophile elements in the mantle. The mantle abundances o f the less siderophile elements are largely determined by preterrestri al metal-silicate equilibrium and are not significantly affected by th e second equilibration. The extreme depletion of sulfur and the lack o f silicate melt-sulfide signature in the noble metal abundances in the mantle are natural consequences of this mode of core formation. Sulfu r was added to the magma ocean during the high-T, high-P equilibration in the Earth, not extracted from it by sulfide segregation to the cor e. Except for Ni and Co, the overall siderophile abundances of the man tle can be well matched in this two-step equilibration model. The mant le characteristics of Ni and Co are unique to the Earth and hence sugg est a terrestrial process as the likely cause. One such process is the flotation and addition of olivine to the primitive upper mantle. In o ur model of core formation, neither the elemental and isotopic data of Re-Os, nor the low sulfur content of the mantle remains as an objecti on to the existence of a magma ocean and olivine flotation. The small fraction of core metal that equilibrates with silicates at high T and P suggests that the light elements O, Si or H are unimportant in the c ore, leaving S (and possibly C) as prime candidates. Sulfur, as FeS as sociated with incoming iron metal, is directly sequestered to the core along with the bulk of the iron metal. It appears unlikely that other light elements can be added to the core after its formation. U and Th are excluded from the core but the model allows for entry of some K; however, the extent to which K sen es as a heat source in the core rem ains uncertain. The model is testable in two ways. One is by investiga tion of the metal-silicate partitioning at high temperatures and press ures under magma ocean conditions to determine if the (D-met/sil) valu es are lowered to the levels required in the model. The other is by ex periments to determine if a solvus closure between metal and silicate liquids occurs at high temperatures relevant to a magma ocean.