Chemical and isotopic fractionation during the evaporation of the FeO-MgO-SiO2-CaO-Al2O3-TiO2 rare earth element melt system

A synthetic material with solar elemental proportions of iron, magnesium, s ilicon. titanium, calcium, and aluminum oxides and doped with rare earth el ements was evaporated in a vacuum furnace at 1800 and 2000 degreesC for dif ferent durations to study its chemical and isotopic evolution during the ev aporation process. It is demonstrated that kinetic evaporation of solar com position material can produce residues of calcium-, aluminum-rich inclusion bulk chemistry. The evaporation sequence of the main constituents in this solar composition material is iron > silicon approximate to magnesium > tit anium, Calcium and aluminum remain unevaporated after evaporation of 95% of the solar composition material. The chemical fractionation between the gas and condensed phase is a function not only of temperature and pressure, bu t also of the hulk chemical composition of the condensed phase. During the evaporation process, cerium is almost as volatile as iron. The 2,000-fold c erium depletion found in some refractory inclusions in carbonaceous chondri tes was reproduced in the evaporation experiment and can be readily explain ed as a result of evaporation of preexisting meteoritic material. Kinetic i sotopic fractionation of magnesium, oxygen, and silicon follows the Rayleig h distillation law during the laboratory evaporation of synthetic solar com position material. This implies that the residue is well mixed during the e vaporation process and that the evaporation kinetic processes (both chemica l and isotopic) are surface reaction-controlled. The isotopic mass fraction ation factors are lower than those predicted from theoretical calculations by using the square root of mass ratios of likely evaporating species. Thus , the surface reaction is more complicated than decomposition into single g as species of each element. Copyright (C) 2001 Elsevier Science Ltd.