Introduction: The compositional variations [1 and references therein] in the zoned metal grains from the CH and CB chondrites are generally interpreted as unequivocal evidence for their condensation from a parcel of H 2-rich gas with solar or somewhat increased dust/gas (D/G) ratios [2,3]. Although different origins of such gas are discussed [e.g., 4-7], its high enrichment in H 2 [8] and low a...