Isotope fractionation and mass fractionation laws of Fe and Ni during evaporation

High-temperature evaporation is a fundamental process shaping the chemical and isotopic signatures of planetary materials, yet the specific physical laws governing isotopic fractionation during evaporation remain debated. We investigated the mass-dependent isotope fractionation of Fe and Ni during vacuum evaporation of metallic melts. The residues exhibit large isotope fractionation, reaching up to 11.4 ‰ for δ 56/54 Fe and 2.9 ‰ for δ 60/58 Ni. Determining the isotope fractionation factors for Fe and Ni [ 𝛼 = ( 𝑚 𝑖 ⁢ 1 / 𝑚 𝑖 ⁢ 2 ) 𝛽 ] using the Rayleigh distillation model yielded a 𝛽 value of 0.5 for both Fe and Ni. Further, the evaporation coefficients of Fe and Ni were found to be similar ( 𝛾 F e ≃ 𝛾 N i ). These results suggest that Fe and Ni evaporation from metallic melts is ideal and uninhibited by kinetic barriers. We used high-precision three-isotope plots to constrain the mass fractionation law governing the evaporation process. Both Fe and Ni in the evaporation residue strictly follow the law of 𝑛 = − 0 . 5 , distinct from the exponential law ( 𝑛 = 0 ) typically assumed for kinetic processes. We propose a theoretical framework to reconcile this observation with kinetic gas theory: while the intrinsic instantaneous isotope fractionation between vapor and melt follows the exponential law, the apparent law observed in the residue is shifted by the Rayleigh distillation process. We derive a general relationship showing that the divergence between the apparent law and the intrinsic law during a Rayleigh distillation process is controlled by the magnitude of isotope fractionation ( 𝛽 ), 𝑛 a p p r ≃ [ 1 + 𝛽 𝑖 ⁢ 2 / 𝑖 ⁢ 1 2 ⁢ l n ⁡ ( 𝑚 𝑖 ⁢ 1 𝑚 𝑖 ⁢ 2 ) ] ⁢ 𝑛 i n s t − 𝛽 𝑖 ⁢ 2 / 𝑖 ⁢ 1 . This framework unifies conflicting observations regarding mass fractionation laws in previous evaporation experiments. The implications of these results for the evaporation history of Type-I cosmic spherules and for identifying nucleosynthetic isotopic anomalies are discussed.