First-principles investigation of equilibrium isotopic fractionation of O- and Si-isotopes between refractory solids and gases in the solar nebula
Résumé
First-principles methods based on density functional theory (DFT) allow the calculation of isotopic partition functions of small gaseous molecules and crystalline compounds with comparable levels of accuracy. We applied these methods to a few species of interest in cosmochemical problems, with a special emphasis on molecules up to now rather neglected in isotopic cosmochemistry: silicon monoxide and silicon monosulfide. These species are likely to play an important role in future cosmochemical studies and already appear in studies such as those related to mass independent fractionation (MIF) generation. We explore the equilibrium mass dependent fractionation (MDF) of these molecules with major gaseous solar species (CO, H2O) and some key meteorite minerals: examples of results are given for the role of SiO on silica enrichment of magnesian chondrules and for the isotopic composition of SiO and SiS during the genesis of silicon solid-solutions in chondritic Fe-Ni metal, a possible starting product for planetary cores. It is shown that the oxygen isotopic composition of early solar silicon monoxide likely lays at the intersection (δ18O=2.12±0.4‰, δ17O=1.1± 0.2‰) of the Young Russell line with the MDF lines corresponding to the main inner solar system bodies. This can be related to solar water abundance in the frame of self-shielding models. At the time of E chondrites' formation the δ30Si of the nebular gas, dominated by SiS, was ~−6‰.