Theoretical Analysis of Mg and Si Chemical and Isotopic Fractionation at Vaporization of Ca–Al Inclusions of Chondrites

Abstract

Experimental study of changes in the composition of Ca–Al inclusions of chondrites during evaporation indicates the chemical and isotopic fractionation of this material are closely interrelated. The coupling is theoretically described using an equations for the evaporation rate of a component of melt (the Hertz–Knudsen equation) and Rayleigh isotope fractionation. The form of the Rayleigh equation (which was derived from the Hertz–Knudsen equation) conventionally used in the foreign literature 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 paper presents an alternative expression for the Rayleigh equation and a new expression of the Hertz–Knudsen evaporation rate with regard for the activity of melt components. The activity of the components is determined by the acidity–basicity index of the melt Ca–Al inclusion, which, in turn, affects the evaporative fractionation of Mg and Si isotopes.