MAGNESIUM ISOTOPIC FRACTIONATION OF SILICATES PRODUCED IN CONDENSATION EXPERIMENTS

Isotopic mass fractionations of magnesium have been studied for silicates produced in condensation experiments for the purpose of studying the evaporation-condensation process in the early solar nebula in connection with the isotopic data observed in various primitive materials. The silicate gas, which is evaporated from forsterite, was fractionated to produce condensates at various temperatures below 1350-degrees-C down to room temperature, and the amount of isotopic mass fractionation varied considerably with the condensation temperature, T(c). The variation in the Mg-26/Mg-24 ratio with respect to the initial evaporation source, DELTA-26, varies from +8 parts-per-thousand to 30 parts-per-thousand as T(c) changes from 1350-degrees-C to room temperature. The obtained (DELTA-26) - T(c) relationship is explained in terms of the Rayleigh fractionation model, and the relationship indicates that the condensates fractionate isotopically toward the heavy mass with respect to the gas phase at temperatures from 700-degrees-C to 1350-degrees-C. The obtained data show that the condensates can be fractionated both toward the heavy and light isotopes with respect to the initial source according to the fraction of the residual gas phase, f(T(c)). The evaluation of mass fractionation between the condensed phase and the residual gas phase is essential for the analysis of various isotopic data on primitive materials.