Howardites
Group · Stony meteorites › Chondrites › Carbonaceous chondrites

CB chondrites (bencubbinites)

More than half metal, with chondrules made in a planet-sized collision.

CB chondrite
Bencubbin, CBa, Australia
Classification
Stony meteorites › Chondrites › Carbonaceous chondrites › CB chondrites (bencubbinites)
Share of witnessed falls
about 0.1%
In our catalogue
18 meteorites, 1 seen to fall
Heaviest
Bencubbin, 118 kg
Earliest recorded fall
Gujba, 1984
Type specimen
Bencubbin, found in Western Australia in 1930

CB chondrites, named after Bencubbin in Australia, are more than 50% iron-nickel metal, yet they are classed as chondrites because their chemistry ties them to the CR clan. They are split into CBa and CBb subgroups.

Their chondrules formed about 5 million years after the first solids, later than in other chondrites. The leading explanation is that they condensed in a hot cloud of vapour from a high-speed collision between two planetesimals.

Mineralogy and chemistry

CB chondrites are the most metal-rich chondrites: iron-nickel metal makes up about 60% to 70% of the rock. The rest is chondrules and fragments of iron-poor silicates, with almost no fine matrix and very few CAIs. Their chondrules are unusual, with barred olivine and cryptocrystalline textures and no rims of fine dust, suggesting they formed and were gathered up quickly in a single event.

The CBa subgroup, which includes Bencubbin, Gujba and Weatherford, has centimetre-sized chondrules and metal nodules. The CBb subgroup, including Hammadah al Hamra 237, has smaller chondrules and metal grains, many chemically zoned in nickel and cobalt from core to rim. Oxygen isotopes link both to the CR clan, and they are rich in nitrogen-15.

Parent body and formation

Lead isotopes date CB chondrules to about 4,562 to 4,563 million years ago, nearly 5 million years after the first CAIs and after most planetesimals had formed. The leading explanation is a hypervelocity collision between two planetesimals, at least one of them already melted, that produced an incandescent plume of vapour and melt. Chondrules and metal condensed from the plume within hours to days and were swept together. The CH chondrites probably formed in the same event, at a different distance from its centre.

Abundance and famous examples

Only a few dozen CBs are known. Bencubbin, the type specimen, was ploughed up in Western Australia in 1930; it is a breccia of CB material with fragments of other meteorite types. Gujba, which fell in Nigeria on 3 April 1984, is the only CB fall, with about 100 kg recovered. Weatherford (Oklahoma) and several Saharan finds make up most of the rest.

Why they matter

The young, uniform ages of CB chondrules make them a precise clock for a single event. Their metal grains, some zoned in nickel from core to rim over a few tens of micrometres, cooled within hours, which is only possible in a dense plume, not in the thin gas of the solar nebula. CBs are therefore a direct record of a planetesimal collision during the time when the planets were being assembled.

The same nitrogen-15 excess seen in CR and CH chondrites is found in CB metal and silicates, which supports the view that all three groups drew on a common reservoir in the outer Solar System before the CB-making collision took place.

How to recognise one

A CB looks at first like a stony-iron or an iron with stony bits. A cut face shows metal nodules and rounded silicate chondrules, sometimes larger than a pea. CBs are very heavy, strongly magnetic and rust readily. Without laboratory work they can be confused with mesosiderites or with irons carrying silicate inclusions.

Famous examples

Related groups

See all 18 CB chondrites (bencubbinites)

Photos shared under Creative Commons licences (Wikimedia Commons).