Condensation and fractionation of rare earths in the solar nebula

Using the most recent thermodynamic data, we calculated the condensation behavior of REE and investigated several models to explain ‘group II’ REE patterns in Allende inclusions. All models involve removal of large fractions of the more refractory heavy REE in an early condensate, probably perovskite, followed by condensation of the remainder at lower temperature. Boynton (1975 Geochim. Cosmochim. Acta39, 569–584), found that the pattern of one such inclusion could not be fit by that of the gas remaining after ideal solution of REE in perovskite and, assuming the presence of only one REE component, calculated relative activity coefficients for REE in perovskite that would be needed to produce a match. In attempting to fit 20 group II patterns with this type of model, we found that these activity coefficients could not be used for most inclusions and that the relationship between ionic radius and required activity coefficients had to change rapidly and irregularly over a narrow range of perovskite removal temperature. Because this feature and the high degree of nonideality needed are most unreasonable, we propose a different model in which two REE components control the patterns: (1) the gas remaining after removal of perovskite in which REE dissolve in ideal solution; (2) a material uniformly enriched in all REE. Two-component models in which solid solution of REE in perovskite is slightly non-ideal and activity coefficients vary negligibly over a narrow temperature range cannot be ruled out. By varying perovskite removal temperatures and the relative proportions of the two components, all 20 REE patterns can be satisfactorily explained.By using a thermodynamically reasonable model, we conclude that perovskite removal occurred over a very narrow temperature range, that multiple refractory element-bearing components are present, indicating a complex history for these inclusions, and that the undeniable gas-solid fractionations that produced the REE patterns may have taken place under somewhat more reducing conditions than those of a normal solar gas.