2001 Volume 77 Issue 3 Pages 37-42
Magnesium and Silicon are the most abundant rock forming elements in the terrestrial planets, and Mg/Si fractionation among planets and meteorites is believed to have occurred in the early solar nebula. In order to evaluate Mg/Si fractionation in the solar nebula, evaporation and condensation of magnesium silicates moving vertically in the protoplanetary disk are simulated based on experimental kinetics and an astrophysical model. Evaporation of forsterite dust particles is shown to be a rapid process and be regarded as an equilibrium reaction during the vertical drift. Evaporation of enstatite is controlled by diffusion through the forsterite layer that is formed as an evaporation residue. Since the diffusion distance under kinetic conditions during the vertical drift is about 1μm, evaporation of enstatite grains would be nearly equilibrium for grains of 1μm and non-equilibrium for grains of 10μm. Condensation of forsterite is simulated as grain growth after heterogeneous nucleation on pre-existed refractory grains. It is found that condensation of forsterite proceeds with keeping equilibrium. Enstatite is formed in the nebula via a reaction between forsterite and gas, and the formation reaction is controlled by diffusion through the enstatite layer. Formation of enstatite would be kinetically inhibited due to its slow diffusion kinetics. The present result implies that the temperature range, in which forsterite is a dominant solid component, would be widened to lower temperature than at equilibrium, and Mg/Si fractionation, which has been explained by extraction of materials with a high Mg/Si ratio such as forsterite from the system, would occur more easily than that in the equilibrium model.