S. Karato et Vr. Murthy, CORE FORMATION AND CHEMICAL-EQUILIBRIUM IN THE EARTH .1. PHYSICAL CONSIDERATIONS, Physics of the earth and planetary interiors, 100(1-4), 1997, pp. 61-79
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.