The abundance and relative volatility of refractory trace elements in Allende Ca,Al-rich inclusions: implications for chemical and physical processes in the solar nebula

Abstract

We have determined the relative volatility of lithophile refractory trace elements (LRTE) in the solar nebula by first calculating 50% condensation temperatures for 26 LRTE oxides (assuming ideal solid solution in perovskite and, where appropriate, in hibonite and melilite). The measured abundances of 25 LRTE and six siderophile refractory trace elements in 97 Group I, 11, 111, and V Allende Ca,Al-rich inclusions (CAI's) and in ultra-refractory inclusions then are used to empirically modify LRTE and refractory siderophile volatility sequences that are based on condensation temperatures alone. Sc is significantly less depleted in Group II Allende CAI's than are other LRTE (e.g., Zr; Hf; Y) that also have very high oxide condensation temperatures; Ba, Sr, and Eu (three moderately volatile LRTE) are significantly more enriched in Ca-rich Group II and Group III Allende CAI's than they are in spinel-rich varieties of these inclusions. We explain these observations and several others by invoking dust fractionation s, and crystal-chemical and diffusion effects during the formation of Allende CAI's that overprinted volatility based on solid/gas reactions alone. For example, we show that hibonite probably was not the carrier of the super-refractory lithophile component that is missing from Group II Allende inclusions. Anomalous Hf depletions (relative to Zr and Y) that are common in Group I, III, and V Allende CAI's can be partly explained if Anders and Ebihara [38] overestimated the cosmic abundance of Hf by approximately 10%.