A corundum-rich inclusion in the Murchison carbonaceous chondrite

Although thermodynamic calculations predict corundum to be the first condensate from a cooling gas of solar composition, a corundum-hibonite inclusion, BB-5, has for the first time been found in the Murchison carbonaceous chondrite. Ion microprobe measurements of Mg isotopic compositions yield the result, unexpected in such an early condensate, that Mg-26 excesses are small despite large Al-27/Mg-24 ratios. The extreme temperature required to melt this inclusion makes a liquid origin unlikely, except by the hypervelocity impact of refractory bodies. Alternatively, B-5 is a direct gas-solid condensate, and its uniform Mg-26 enrichment must be a characteristic of the reservoir from which it condensed. Nebular heterogeneity in magnesium isotopic composition is the preferred explanation for the formation of such a reservoir.