CB chondrite
CB chondrites are extremely metal-rich carbonaceous chondrites whose chondrules and metal probably condensed from an impact-generated vapour plume.
Few meteorites blur the line between stone and iron like the CB chondrites, or bencubbinites, after Bencubbin, found in Western Australia in 1930. They are mostly metal: iron-nickel typically makes up well over half the rock by volume, with silicates mainly as cryptocrystalline or skeletal olivine–pyroxene chondrules. They are split into CBa, with centimetre-sized metal and chondrules, and CBb, finer-grained with chemically zoned metal grains. Gujba, which fell in Nigeria on 3 April 1984, is a CBa and the only observed CB fall.
CB components look very different from normal chondrules. Their metal grains show zoning in nickel, cobalt and chromium consistent with condensation from cooling gas, and the chondrules lack relict grains. Lead–lead dating of Gujba chondrules gave about 4,562 million years, roughly five million years after the first CAIs, much later than most nebular chondrules. This led to the widely discussed model that CB chondrules and metal condensed in a vapour–melt plume produced by a collision between planetary embryos.
CB chondrites belong to the CR clan with the CR and CH groups. They are recognised by their exceptional metal content, distinctive textures, and bulk nitrogen strongly enriched in nitrogen-15.
Related terms
Level: Expert Advanced