Evaporation kinetic and isotopic fractionation of minerals in the early solar nebula

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In order to describe the chemical and isotopic fractionation due to eveporation of minerals in the early solar nebula as a function of temperature, pressure, and time, evaporation experiments were carried out by using forsteirte and olivine as starting materials.Crystals show strong evaporation anisotropy for the evaporation rate and microstructures. The microstructures are controlled by density and nature of dislocation, which results in different microstructures developing on the surface of forsterite and natural olivine. Evaporation rate of forsterite was obtained as a function of temperature, and thus the results can be used to investigate the possilility of isotopic fractionation of Mg, Si, and O.The degree of chemical and isotopic fractionantion from a condensed phases is controlled by elemental diffusion, which is successfully shown by an equation including evaporation rate, diffusion rate, and fractionation factor as independent parameters.The diffusion coefficient of an element can be obtained in the evaporation experiments if a steady state for evaporation is achieved. This technique is powerful at high temperatures when other conventional experiments have difficulties in making c desirable conditions. The Mg-Fe inter-diffusion coefficient was obtained in the present study, which is one of the most important parameter for geochemical processes. The value is close to that extrapolated from lower temperature experiments by Meisner (1974) , but is a little higher than that. The reason for a higher value is either (1) the present value is correct becaouse diffusion mechanism changes at high temperatures, (2) the present value is correct due to generation of vacancies at the evaporating surface, or (3) the value is incorrect due to overestimation of evaporation coefficient. In the present study, the chemical and isotopic fractionation was modeled as a function of temperature and time, which enables us to estimate the chemical fractionation in the solar nebula.