Abstract
Experimental study of changes in the composition of Ca-Al-inclusions of chondrites during evaporation indicates a close relationship of chemical and isotopic fractionation of this substance. The theoretical description of the coupling is carried out using the equations of the evaporation rate of the melt component (the Hertz-Knudsen equation) and isotope Rayleigh fractionation. The form of Rayleigh equation taken in foreign literature, derived from the Hertz-Knudsen equation, faces difficulties in interpreting experimental data. The discrepancy between the experimental and model data is explained by the fact that the «ideal» isotope fractionation factor used in the Rayleigh equation does not take into account its dependence on the temperature and composition of the evaporating melt. The article presents an alternative expression of the Rayleigh equation and a new expression of the rate of evaporation of Hertz-Knudsen, taking into account the activity of the melt component. The activity of the component is determined by the acidity-basicity index of the melt Ca-Al-inclusion, which, in turn, affects the evaporative fractionation of magnesium and silicon isotopes.
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