ABSTRACT

Evaporation experiments were performed in vacuo on the multicomponent melt, FeO-MgO-SiO₂-CaO-Al₂O₃ with the solar elemental abundances. The analysis of experimental results shows that the rate-determining step of evaporation is the vaporization reactions occurring on the melt surface, amongst other possible rate processes. The reaction modes are determined for the components, FeO, MgO and SiO₂. FeO, unlike other components, vaporizes through the disproportionation reaction into metallic and ferric irons in the melt phase. ‘Volatility’ is defined as a physical quantity so as to describe the evaporational fractionations of elements. On the basis of this concept, the experimental results of the present and previous works together with thermodynamic data are organized to characterize the evaporation sequence of primitive condensed materials. Its gross feature is the sequential evaporation of the components in the order of Fe, Mg, Si, Ca and Al. The details of the evaporation sequence depend on temperature and initial valences of Fe. The evaporation sequence is comparatively discussed with the condensation sequence of the nebular gas, which has been well examined by previous workers. Both sequences, combined together, provide a basic framework to clarify the chemical fractionation processes which have differentiated primitive materials into the planetary and meteoritic materials. The chemical diversity of chondrules and inclusions in meteorites is interpreted as mainly due to the evaporational or condensational fractionation, but it also invokes other processes such as metal/silicate separation and non-equilibrium reaction with a surrounding gas.