The Solar System's Earliest Chemistry: Systematics of Refractory Inclusions

Abstract

Refractory inclusions, or CAIs (calcium-aluminium-rich inclusions) are a unique ingredient in chondritic meteorites. As the name suggests, they are enriched in refractory elements, essentially reflecting a condensation sequence of phases from a cooling gas of solar composition. However, the widespread preservation of diverse isotopic anomalies is not compatible with the inclusions having been in a gaseous form. Rather, the CAIs appear to represent mixtures of condensate and refractory residue materials. The condensates formed from cooling solar gas and fractionation of that gas produced variations in the abundances of refractory elements according to volatility. Solar condensate has isotopically normal Ca and Ti isotopic compositions and has 26Al/27Al of the canonical value for the solar system at 5 × 10−5. Residues of material falling in toward the Sun are probably aluminous oxides such as corundum and hibonite, and preserve diverse Ca and Ti isotopic anomalies. Meteoritic inclusions from the Murchison meteorite show the best polarization of these components. Spinel-hibonite-perovskite inclusions (SHIBs) predominantly have normal Ca and Ti isotopes, 26Al/27Al at 5 × 10−5, and ultrarefractory fractionated REE patterns. Single hibonite crystal fragments (PLACs) have diverse Ca and Ti isotopic compositions and low 26Al/27Al because of the initially high proportion of 27Al in the residue. REE patterns in PLACs are variable in terms of the ultrarefractory fractionation of their REE patterns, as indicated by Tm/Tm∗, but are dominated by depletion in the less refractory REE Eu and Yb. Both PLACs and SHIBs homogenized with 16O-rich gas, enriched relative to terrestrial O by up to 7%, thus removing any isotopic heterogeneity from the PLAC precursors. CAIs formed close to the Sun where condensation and re-evaporation of REE was possible, and were then ejected back to planetary radii where they were eventually accreted onto planetesimals.