CORE FORMATION AND CHEMICAL-EQUILIBRIUM IN THE EARTH .1. PHYSICAL CONSIDERATIONS

Current models of planetary formation suggest a hierarchy in the size of planetesimals from which planets were formed, causing formation of a hot magma ocean through which metal-silicate separation (core format ion) may have occurred, We analyze chemical equilibrium during metal-s ilicate separation and show that the size of iron as well as the therm odynamic conditions of equilibrium plays a key role in determining the chemistry of the mantle (silicates) and core (iron) after core format ion. A fluid dynamical analysis shows that the hydrodynamically stable size of iron droplets is less than similar to 10(-2) m for which both chemical and thermal equilibrium should have been established during the separation from the surrounding silicate magma. However, iron may have been separated from silicates as larger bodies when accumulation of iron on rheological boundaries and resultant large scale gravitatio nal instability occurred or when the core of colliding planetesimals d irectly plunged into the pre-existing core. In these cases, iron to fo rm the core will be chemically in dis-equilibrium with surrounding sil icates during separation, The relative role of equilibrium and dis-equ ilibrium separation has been examined taking into account of the effec ts of rheological structure of a growing earth that contains a complet ely molten near surface layer followed by a partially molten deep magm a ocean and finally a solid innermost proto-nucleus, We show that the separation of iron through a completely molten magma ocean likely occu rred with iron droplets assuming a hydrodynamically stable size (simil ar to 10(-2) m) at chemical equilibrium, but the sinking iron droplets are likely to have been accumulated on top of the partially molten la yer to form a layer (or a lake) of molten iron which sank to deeper po rtions as a larger droplet. The degree of chemical equilibrium during this process is determined by the size of droplets which is in turn co ntrolled by the size and frequency of accreting planetesimals and the rheological properties of silicate matrix, For a plausible range of pa rameters, most of the iron that formed the core is likely to have been separated as large droplets or bodies and chemical equilibrium with s ilicate occurred only at relatively low temperatures and pressures in a shallow magma ocean or in their parental bodies. However, a small po rtion of iron that separated as small droplets was in chemical equilib rium with silicate at high temperatures and pressures in a deep magma ocean during the later stage of core formation. Therefore the chemistr y of the core is mostly controlled by the chemical equilibrium with si licates at relatively low temperatures and pressures, whereas the chem istry of the mantle controlled by the interaction with iron during cor e formation is likely to have been determined mostly by the chemical e quilibrium with a small amount of iron at high temperatures and pressu res. (C) 1997 Elsevier Science B.V.