Compositions of Unzoned and Zoned Metal in the CBb

2Abstract–The CBb chondrites are rare, primitive, metal-rich meteorites that contain several features, including zoned metal, that has previously been interpreted as evidence for origins in the solar nebula. We have measured concentrations of Ni, Cu, Ga, Ru, Pd, Ir, and Au within both zoned and unzoned metal grains in the CBb chondrites QUE 94627 and HH 237, using laser ablation inductively coupled plasma mass spectrometry. Refractory elements Ni, Ru, and Ir are enriched in the grain cores, relative to the rims, in the zoned metal. All refractory elements are uniform across the unzoned metal grains, at concentrations that are highly variable between grains. Volatile elements Cu, Ga, and Au are usually depleted relative to chondritic abundances, and are most often uniform within the grains, but sometimes are slightly elevated at the outermost rim. The Pd abundances are nearly uniform, at close to chondritic abundances, in all of the metal grains. A condensation origin is inferred for both types of metal. The data support a model in which the zoned metal formed at high temperatures, in a relatively rapidly cooling nebular gas, and the unzoned metal formed at lower temperatures, and at a lower cooling rate. The CBb metal appears to have formed by a process very similar to that of the CH chondrites, but the CB