Bulk Chemical and Isotopic Compositions of Spinel-, Hibonite-rich Spherules: Clues to Their Origin

Introduction: Melilite-free, spinel-, hibonitebearing spherules, a major type of refractory inclusion in CM2 chondrites, consist of phases that are predicted by thermodynamic calculations to condense from a cooling gas of solar composition but are not predicted to coexist with each other. Throughout updates and additions to the data base over the years, calculations [e.g. 1-3] consistently show that melilite should condense after hibonite and before spinel, and that hibonite should react completely with the gas to form spinel. Despite this, hibonite-, spinel-bearing inclusions that are melilitefree or very melilite-poor are much more abundant in CM chondrites than hibonite-melilite or melilitespinel inclusions. The origin of the melilite-free inclusions has puzzled researchers for years. One way to reconcile the observed assemblages with the condensation calculations is if melilite originally present was lost due to evaporation of Ca, Si and Mg during heating of the precursors [4]. The loss of SiO2 and CaO from an Al2O3-rich CMAS liquid would stabilize spinel and prevent extensive Mg evaporation. If evaporation occurred while the inclusions were partially molten, then they should be measurably enriched in the heavier isotopes of the affected elements (i.e., FMg, FCa >0). Previous studies of Ca [5] and Mg [6] isotopes in hibonite-spinel inclusions showed –1030 wt% TiO2. Five inclusions have compositions that, on a plot of CaO vs. Al2O3, appear to project to sub-solar CaO/Al2O3 ratios along the condensate trajectory with the removal of spinel. This might reflect addition of spinel to inclusions formed prior to complete condensation of Ca, but even for these inclusions, correction to the lower CaO/Al2O3 ratios also results in negative MgO contents. To see how much melilite must be added to the