抄録
Vapor growth experiments of Mg-silicates (mainly forsterite) and its isotopic mass fractionation were reported. Vapor growth sequences at high ambient gas pressures (70 Pa He and 1.4 Pa H_2) can be explained by maximum fractionation sequences based on thermo-chemical calculations for solid-gas equilibria, while those at low pressures (1.4 Pa He and vacuum) deviate from the maximum fractionation sequences. Change in crystal morphology (euhedral to dendritic) in the experiments can be explained by nucleation delay and accompanied change in supercooling in vapor. Mg and Si isotopic mass fractionation takes place as a function of temperature, and this can be explained by Rayleigh fractionation model where solids fractionate isotopically toward the heavy mass with respect to the vapor phase. The mass fractionation is small at large ambient gas pressures (70 Pa He), while it is large at low ambient pressures (1.4 Pa He and H_2 and vacuum). The small fractionations took place near isotopic solid-gas equilibrium, while the large fractionations took place due to kinetic effect, and this is almost consistent with the vapor growth sequence-pressure relation (major elemental fractionation). Isotopic effect of crystal growth was discussed, and it is concluded that the kinetic effect on the isotopic mass fractionation in the experiments is considered to be caused by mass transportation (diffusion) processes in the vapor phase. In order to understand interface kinetics effect, isotopic sector zoning should be examined.
