First-principles investigation of the concentration effect on equilibrium fractionation of K isotopes in feldspars

摘要

The K concentration effects on K-O bond length and the reduced partition function ratios of K-41/K-39 in alkali feldspars have been explored using the density functional theory (DFT) method. In alkali feldspars, the average K-O bond length increases with increasing K content, measured as K/(K + Na) molar ratio, ranging from 2.724 angstrom in alkali feldspar with a K/(K + Na) of 1/16 to 2.880 angstrom in microcline (K/(K + Na) = 1). Our results show large K concentration effect on the calculated reduced partition function ratio. For example, 10(3)ln (41K/39K)alpha(feldspar-microchne) between alkali feldspar with a K/(K + Na) of 1/16 and microcline are 2.21 parts per thousand at 300 K and 0.42 parts per thousand at 700 K, which are comparable to the K-41/K-39 variation observed in natural samples. Furthermore, isotope fractionation, 10(3)ln (41K/39K)alpha(feldspar-microchne), is negatively linearly correlated with the average K-O bond length in alkali feldspars. Therefore, the concentration effect on K isotope fractionations needs to be considered in the applications of K isotopes in the fields of geochemistry and cosmochemistry, such as the formation of the lunar anorthositic crust and the evolution of the Earth's crust. (C) 2018 Elsevier Ltd. All rights reserved.