Isotope mass fractionation during evaporation of Mg2SiO4

Calcium- and aluminium-rich refractory inclusions (CAIs) in carbonaceous chondrites can contain Mg, Si and Cr with isotopic compositions that are significantly mass-fractionated, up to a few per cent per a.m.u., relative to normal (terrestrial) values. The oxygen isotopic composition of so-called FUN (fractionated and unknown nuclear effect) CAIs is also interpreted to be highly mass-fractionated from a postulated 16O-rich reservoir. Isotope fractionation between vapour and solids or liquids is likely to be responsible for these effects. Because of the large mass fractionations observed, it has been proposed that they are kinetic isotope effects produced by non-equilibrium evaporation and/or condensation processes that occurred in the primordial solar nebula. To test this idea, we have partially evaporated synthetic forsterite (Mg2SiO4) in vacuum for various durations and at different temperatures. The residual charges obtained when molten Mg2SiO4 is evaporated to 12% of its initial mass are enriched in heavy isotopes by ∼20, ∼30 and ∼15 per mille a.m.u.-1 for O, Mg and Si, respectively, whereas solid forsterite evaporated to a similar residual mass fraction shows negligible fractionations (<1 per mille a.m.u.-1). These results imply that CAIs must have been at least partially molten if the observed large mass fractionation effects were caused by evaporation. (author)