Anisotropy of Mg isotopic fractionation during evaporation and Mg self-diffusion of forsterite in vacuum

Evaporation of solid materials under low-pressure conditions could play important roles in chemical and isotopic fractionations in the early solar system. We have studied anisotropy of isotopic fractionation of 26Mg and 25Mg during kinetic evaporation of forsterite (Mg2SiO4), which is potentially a powerful tool to understand thermal histories of crystals in the early solar system. Ion-microprobe depth profiling revealed that the Mg isotopic zoning profiles of forsterite evaporated at 1500–1700 °C are notably differing along the a-, b-, and c-axes, which can be attributed to anisotropy in self-diffusion coefficient of Mg (D) and an isotopic fractionation factor for evaporation of Mg (α). The D and α were obtained from zoning profiles by applying the diffusion-controlled isotopic fractionation model of Wang et al. [1999. Evaporation of single crystal forsterite: Evaporation kinetics, magnesium isotope fractionation, and implications of mass-dependent isotopic fractionation of a diffusion-controlled reservoir. Geochim. Cosmochim. Acta 63(6), 953–966.].The D is largest and smallest along the a- and c-axes, respectively. The activation energy of 560–670 kJ/mol indicates that Mg diffusion at 1500–1700 °C occurred in the intrinsic diffusion regime.The α seems to be larger along the a- or c-axes than along the b-axis. The α along the a- or c-axes show weak temperature dependence. The α along all the crystallographic orientations is closer to unity than that expected from the kinetic theory of gases. These lines of evidence suggest that surface processes such as breaking of bonds and surface diffusion are responsible for the isotopic fractionation.