First-principles calculations of equilibrium silicon isotope fractionation in metamorphic silicate minerals

Abstract

In this study, silicon isotope fractionation factors for metamorphic silicate including muscovite, epidote, and kyanite are calculated using first-principles methods based on density-functional theory. The reduced partition function ratio (103lnβ) of 30Si/28Si decreases in the order of muscovite > epidote ≈ kyanite. Combining with previous calculations, our results show that the equilibrium silicon isotope fractionation factors are correlated with average Si–O bond length. These results predict that measurable Si isotope fractionation can occur during metamorphic fluid activities. At 1000 K, the calculated 103lnα values for quartz–epidote, quartz–kyanite, and quartz–muscovite are 0.51‰, 0.49‰, and 0.08‰, respectively. The calculation results are generally consistent with the observed Si isotopic data in two metamorphic eclogite-vein systems, showing that our work can provide useful information in investigating the Si isotope fractionation in metamorphic fluid evolution. Keywords: Si isotopes, First-principles calculation, Metamorphic fluid