Evaporation kinetics of forsterite and implications for the early solar nebula

Abstract

THE planets and meteorites originated in a disk of gas and dust—the solar nebula—in which thermal processing fractionated the elements to varying degrees between gaseous and condensed phases^1,2. A record of these effects is preserved in the chemical, mineralogical and isotopic compositions of meteorites, which accreted in the nebula. Previous attempts to interpret these fractionation effects have been based largely on thermodynamic calculations^3,4, which assume that phases remained in equilibrium with one another. Vapour–solid equilibria in the system Mg–Si–O–H have also been studied experimentally⁵. But equilibrium is only a first approximation to the real situation: internal evidence in the chondritic meteorites⁶ shows that the thermal processing was rapid, and kinetic effects (non-equilibrium fractionation) must have played an important part in the evaporation and condensation events that created the chondrules, refractory inclusions and dust grains that are preserved in chondrites. Here I report the results of experiments to study the kinetics of evaporation of Mg₂SiO₄ (forsterite), the most abundant mineral in planets and meteorites. Solid and liquid Mg₂SiO₄ evaporate very slowly, only about one-tenth as fast as they would if the vapour species produced by evaporation were those predicted by equilibrium thermodynamics, and if no energy barrier in excess of the energy of reaction impeded the process. This large kinetic effect is due mainly to the transient production of gaseous SiO₂ on evaporation. As a result, forsterite may evaporate at the same rate as an intrinsically more refractory material.