Alan S. Kornacki

Microdistributions of Mg isotopes and REE abundances in a Type A calcium-aluminum-rich inclusion from Efremovka☆

Abstract Ion probe measurements were performed in the core and rim of a compact Type A inclusion, E2, from the CV3 chondrite Efremovka. Rare earth element (REE) and selected trace element (Sc, V, Sr, Y, Zr, Nb, Ba, Hf) abundances as well as Mg, Si, and Ti isotopes were measured. The REE are concentrated in perovskite except for Eu which mainly resides in melilite. REE patterns in E2 perovskite are unfractionated and have a Eu depletion. No systematic differences in REE concentrations or patterns between core and rim perovskites were found. In melilite, the concentrations of Ba, La, Ce, and Dy are independent of the distance from the rim. The Mg isotopic composition of melilite and spinel is uniformly heavy (by ~ 10%./ amu) in the interior and smoothly grades to normal in and near the rim. This indicates that the rim must have formed by condensation of minerals onto a solid core at high temperatures. Excesses of 26 Mg are present and correlate with the 27 Al 24 Mg ratio, both in the core and rim regions of the inclusion. The slope of the apparent isochron in the rim region is higher than the slope measured in the inner core, providing evidence that 26 Al was inhomogeneously distributed in the early solar nebula. In addition, the intercepts differ, pointing to a heterogeneous 26 Mg 24 Mg ratio. No Si isotopic mass fractionation was found. The Ti isotopic composition of E2 perovskite is normal.

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 (CAIs) 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 CAIs 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 CAIs 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 CAIs 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 CAIs can be partly explained if Anders and Ebihara [38] overestimated the cosmic abundance of Hf by approximately 10%.