First-principles modeling of chlorine isotope fractionation between chloride-bearing molecules and minerals

Equilibrium Cl-37/Cl-35 fractionation factors in selected molecules, Cl-bearing crystalline solids, and silicates in which Cl occurs at trace or minor concentration level are determined from first-principles calculations, within the density functional theory (DFT) scheme. Results on benchmarking molecules and crystalline solids are consistent with the previous theoretical study of Schauble et al. (2003). The present study further documents the control of the isotopic fractionation properties of chlorine by its local bonding environment. Chloromagnesite and chlorapatite display similar isotopic fractionation properties due to relatively similar bonding environment. In contrast, trace Cl in Mg-serpentine (lizardite) and Mg-amphibole (anthophyllite) are enriched in Cl-37 with respect to chloromagnesite, due to the structural constraints exerted by the host structure on the substituted ion. This effect is even more pronounced when Cl is associated to hydroxylated cationic vacancies in forsterite. An effect of the local bonding environment on the Cl isotopic fractionation properties is also inferred for Cl- ions in saturated aqueous solutions. It explains the systematic departure between theoretical and empirical reduced partition function ratio observed for the alkaline chlorides, differing from the agreement observed for the hydrated Cl salts. The reduced partition function ratio of Cl- ions in concentrated solution of alkaline chlorides is smaller from that observed in dilute solutions by an amount potentially reaching 1% at 22 degrees C. Finally, the calculation of fractionation factors between gas (HCl(g), NaCl(g), KCl(g)) and solids (sodalite, chlorapatite, halite, HCl trihydrate) which likely prevailed in the solar nebula, sustains a model in which the Cl-37 enrichment of HCl(g) is produced by a Rayleigh type fractionation during chlorine condensation at temperatures between 400 and 500 K. This model could explain the heavier isotopic composition observed for bulk Earth and various chondrites compared to the nebular gas.