CO chondrites
Small-chondrule carbonaceous chondrites, close relatives of the CMs.
- Classification
- Stony meteorites › Chondrites › Carbonaceous chondrites › CO chondrites
- Share of witnessed falls
- about 0.6%
- In our catalogue
- 391 meteorites, 6 seen to fall
- Heaviest
- Kainsaz, 200 kg
- Earliest recorded fall
- Ornans, 1868
- Most recent fall
- Moss, 2006
- Type specimen
- Ornans, fell in France in 1868
CO chondrites, named after Ornans in France, have very small chondrules, averaging about 0.15 mm, packed tightly with little matrix. All are petrologic type 3 and fairly poor in water and other volatiles.
Their oxygen isotopes and their small refractory inclusions link them closely to the CM chondrites, and the two are thought to come from related parent bodies.
Mineralogy and chemistry
CO chondrites are crowded with very small chondrules, mostly 0.1 to 0.2 mm across, which make up about 40% to 50% of the rock. Small CAIs and amoeboid olivine aggregates are common, and fine-grained matrix fills about a third of the volume. They contain a few per cent of iron-nickel metal and troilite, and much less water than CMs.
Every CO is petrologic type 3, but they show a clear sequence of gentle heating, recorded as subtypes from CO3.0 to about CO3.8. With rising temperature, iron spreads from the matrix into the chondrules, and presolar grains and fragile organic matter are destroyed. Antarctic finds such as Allan Hills A77307 and Dominion Range 08006 are among the most primitive COs known, preserving abundant presolar silicate grains. Their oxygen isotopes lie well below the terrestrial line, close to those of the CM and CV chondrites.
Parent body and formation
The smooth heating sequence fits an asteroid that warmed from inside, like a small version of the ordinary chondrite parent bodies, but kept its temperatures relatively low. The similarity of their chondrules, inclusions and oxygen isotopes to CMs has led many scientists to group CM and CO together as a clan, possibly formed from the same reservoir of material, with CMs accreting more ice.
Abundance and famous examples
COs are about 0.6% of witnessed falls. Ornans, which fell in the French Jura in 1868, is the type specimen. Kainsaz, which fell in Tatarstan, Russia, in 1937, is the largest CO fall at about 200 kg. Lancé (France, 1872), Warrenton (Missouri, 1877) and Felix (Alabama, 1900) are other falls. Many more have been found in hot deserts and in Antarctica.
Why they matter
Because COs record a gentle heating sequence without the complications of water, they act as a natural experiment in how heat changes primitive material. Researchers use them to watch, step by step, how presolar grains disappear, how glass in chondrules recrystallises and how noble gases are lost. Their small CAIs carry the same early oxygen isotope signature as the giant inclusions of CV chondrites, linking the two groups to the same population of the first solids.
CO chondrites are also prized by researchers studying the matrix of chondrites, because the least heated examples keep fine-grained dust almost as it was when it was swept up, including grains of glassy silicate that formed in the cold outer disc.
How to recognise one
A CO is dark grey and looks almost sugary on a broken face, because the chondrules are so tiny and tightly packed. It is harder and denser than a CM, and contains a little metal, so it may respond weakly to a magnet. Telling a CO from a CM or CV needs microscopic and isotopic study.
Famous examples
- Ornans CO3.4, France, 1868 · 6 kg
- Kainsaz CO3.2, Russia, 1937 · 200 kg
- Lancé CO3.5, France, 1872 · 51.7 kg
- Warrenton CO3.7, United States, 1877 · 1.6 kg
- Felix CO3.3, United States, 1900 · 3.2 kg
- Allan Hills A77307 CO3.0, Antarctica, 1977 · 181 g
Related groups
- CM chondrites Water-rich carbonaceous chondrites full of organic molecules, Murchison and Winchcombe among them.
- CV chondrites Large chondrules and big white inclusions, the oldest solids in the Solar System. Allende is the classic.
- Carbonaceous chondrites Rare, often dark chondrites rich in carbon, water-bearing minerals and organic compounds, including the building blocks of life.
Photos shared under Creative Commons licences (Wikimedia Commons).