固物研究生论坛 -First-principles investigation of the concentration effect

The K concentration effects on K-O bonds and the reduced partition function ratios of ⁴¹K/³⁹K in alkali feldspars have been calculated using the density function theory (DFT) method. The average K-O bond length increases gradually with increasing K content (there after presented as K / (K + Na) molar ratio) in alkali feldspars from 2.72 Å in alkali feldspar with K / (K + Na) of 1/16 to 2.88 Å in microcline (K / (K + Na) = 1). The equilibrium isotope fractionation factors among alkali feldspars, 10³ln^41K-39Kα_{feldspar-microcline}, are negatively and linearly correlated with their average K-O bond lengths. 10³ln^41K-39Kα_{feldspar-microcline} between alkali feldspar with K / (K + Na) = 1/16 and microcline are 2.2‰ at 300K and 0.4‰ at 700K, which are comparable to the ⁴¹K/³⁹K variation observed in many terrestrial pegmatites (d⁴¹K_{NIST SRM 999b} varies from -0.11 ± 0.12‰ to -1.36 ± 0.34‰) and extraterrestrial samples such as martian meteorites (δ⁴¹K_BSE = +0.12 ± 0.12‰) and lunar mare basalts (δ⁴¹K_BSE = +0.44 ± 0.16‰) (Wang and Jacobsen, 2016a, 2016b; Chen et al., 2018; Morgan et al., 2018). Therefore, the concentration effect on K isotope fractionations needs to be considered in applying K isotopes to understand geochemical (Parendo et al., 2017; Santiago Ramos et al., 2018) and cosmochemical (Wang and Jacobsen, 2016b) processes, such as the formation of the Moon and the evolution of the Earth’s crust.