Model evaporation of FeO-bearing liquids: Application to chondrules

Models for thermodynamic behavior of FeO-bearing liquids are required for understanding the separate roles of evaporation, condensation and crystallization in the formation of free-floating silicate liquid droplets in the early solar nebula. These droplets, frozen as chondrules, are common in chondritic meteorites. Evaporation coefficients for Fe and FeO of ∼0.2 are calculated here from existing data using silicate liquid activity models. These models, used to describe gas-liquid-solid equilibria and to constrain kinetic processes, are compared and found similar, and the effects of liquid non-ideality are assessed. A general approach is presented for predicting the evaporation behavior of FeO-bearing Al₂O₃-CaO-SiO₂-MgO liquids in H₂-rich gas above 1400 K at low total pressure. Results are vapor pressure curves for Fe, FeO and other gas species above typical chondrule liquids, suitable for predicting compositional trajectories of residual liquids evaporating in a hydrogen-dominated vapor. These predictions are consistent with chondrule formation in the protoplanetary disk in heating events of short duration, such as those expected from shock wave or current sheet models.