CHEMICAL AND ISOTOPIC FRACTIONATION DURING THE EVAPORATION OF THE FEO-MGO-SIO2-CAO-AL2O3-TIO2 RARE EARTH ELEMENT MELT SYSTEM

Abstract

A synthetic material with solar elemental proportions of iron, magnesium, silicon, titanium, calcium, and aluminum oxides and doped with rare earth elements was evaporated in a vacuum furnace at 1800 and 2000°C for different durations to study its chemical and isotopic evolution during the evaporation process. It is demonstrated that kinetic evaporation of solar composition 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 ~ magnesium > titanium. 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, but also of the bulk chemical composition of the condensed phase. During the evaporation process, cerium is almost as volatile as iron. The 2,000-fold cerium depletion found in some refractory inclusions in carbonaceous chondrites was reproduced in the evaporation experiment and can be readily explained as a result of evaporation of preexisting meteoritic material. Kinetic isotopic fractionation of magnesium, oxygen, and silicon follows the Rayleigh distillation law during the laboratory evaporation of synthetic solar composition material. This implies that the residue is well mixed during the evaporation process and that the evaporation kinetic processes (both chemical and isotopic) are surface reaction-controlled. The isotopic mass fractionation 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 gas species of each element.