Allan Hills 85085: A subchondritic meteorite of mixed nebular and regolithic heritage

Allan Hills 85085 is a tiny (12-g) meteorite whose closest relatives are the Renazzo and Al Rais chondrites and the subchondritic host material in the Bencubbin meteorite. Its bulk composition is generally chondritic but siderophile abundances are tens of per cent higher and abundances of moderately volatile elements factors of 2–4 × lower than in known authentic chondrites. In addition to this unusual composition ALH85085 has an extremely small particle size, metal that shows no evidence of metamorphic equilibration and a very high abundance of pyroxenitic lithic particles. Chondrules are rare and small; most are pyroxenitic and chemically similar to the lithic particles. Previous workers have inferred that most or all of these strange features of ALH85085 resulted from nebular processes, but we suggest that melting, vaporization, outgassing, condensation and size-sorting in a cloud of impact ejecta offer a more viable alternative. Until additional similar materials have been discovered and characterized, it seems safest to infer nebular properties and processes on the basis of the record in chondrites regarding which there is little doubt that nebular processes were dominant.