A corundum-rich inclusion in the Murchison carbonaceous chondrite

A corundum-hibonite inclusion, BB-5, has been found in the Murchison carbonaceous chondrite. This is the first reported occurrence of corundum as a major phase in any refractory inclusion, even though this mineral is predicted by thermodynamic calculations to be the first condensate from a cooling gas of solar composition. Ion microprobe measurements of Mg isotopic compositions yield the unexpected result for such an early condensate that 26Mg excesses are small: δN26Mg = 7.0 ± 1.6%. for hibonite and 5.0 ± 4.8%. for corundum, despite very large 27Al24Mg ratios, 130 and 2.74 × 104, respectively. Within the errors, δN26Mg does not vary over this exceedingly large range of 27Al24Mg ratios. The extreme temperature required to melt this inclusion makes a liquid origin unlikely, except possibly by hypervelocity impact involving refractory bodies. If, instead, BB-5 is a direct gas-solid condensate, textural evidence implies that corundum formed first and later reacted to produce hibonite. In this model, BB-5's uniform enrichment in 26Mg must be a characteristic of the reservoir from which it condensed. Because severe difficulties are encountered in making such a reservoir by prior decay of 26Al, nebular heterogeneity in magnesium isotopic composition is a preferred explanation.