Meteorites demystified: A beginner’s guide (Part 2)

Helen Gould (UK)

Chemistry is the key to identifying the source of a meteorite. The commonest rock in the Solar System – and on Earth – is basalt. Erupted at mid-ocean ridges and many hotspot volcanoes, it also floors the oceans. However, each of these situations can be identified as geochemically different from one another.

Some meteorites have geochemical signatures associated with individual asteroids, being either enriched or poor in specific minerals. The ratios of their minerals are plotted against one another, then the shape and co-ordinates of the plots are cross-referenced to a database. This process has allowed distinct groups of meteorites with similar geochemistry to be identified, suggesting that the meteorites in each cluster plotted came from the same source.

There are five sub-groups of achondrites of various chemical composition, including eucrites, diogenites, SNC, lunar achondrites and ureilites. The name means they don’t contain chondrules.

Most are of igneous origin, but lunar achondrites resemble fragmental sedimentary rocks. The only “weathering” on the Moon comes from impacting meteorites, but this breaks up rocks and reforms them into breccias – jumbles of jagged fragments fused together.

Eucrites

Eucrites are basaltic meteorites containing low-calcium proxenite and plagioclase feldspar with metallic iron, troilite (iron sulphide) and silicates. They probably all crystallised at or just below the surface of their source bodies.

Diogenites

Diogenites consist of calciumpoor pyroxenite, which is an igneous rock resembling the ocean crust.

SNC

SNC meteorites have been identified as coming from Mars. SNC stands for the ‘Shergotty, Nakhla and Chassigny’ meteorites. They are layered rocks whose crystals probably accumulated by gravity settling. Similar to terrestrial basalts, they likely represent volcanic or shallow intrusive rocks there and suggest relatively recent volcanism, 1,300 to 200 million years (Ma) ago.

Martian volcanism continued until geologically recent times. Nitrogen and oxygen bubbles trapped in these rocks match Viking atmospheric measurements taken in 1976. The rocks are volatile-rich and younger than about. 1,300Ma (the Shergotty meteorite has been dated at 360Ma).

Lunar achondrites

Lunar achondrites, mostly found in the Allan Hills area of Antarctica, contain breccias and anorthosites. Terrestrial impacts form breccias from crystal fragments and shattered rock, such as suevite, a terrestrial breccia from an impact.

Images courtesy of Meteorite Australia: www.meteroites.com.au

NASA website: http://www.fotoimagelibrary.com

Further reading

Meteorites, HMSO/Natural History Museum publication, 1994. R. Hutchinson & A Graham (P 4, 20-25, 34-35, 37-38).

Meteorites and their parent planets, by Harry Y. McSween, Cambridge University Press, Cambridge (1999), 324 pages (Hardcover), ISBN-13: 978-0521583039.

Spacerocks: A Collectors’ Guide to Meteorites, Tektites and Impactites, by David Bryant, Heathland Book (2018), 156 pages (Paperback), ISBN: 9781999741723.

The Nine Planets: Meteors, Meteorites and Impacts

The Robert Haig collection of meteorites, by Robert Haag, 126 pages, Robert Haag Meteorites, Tucson, Arizona (2003).

The Tunguska Mystery, by Vladimir Rubtsov and Edward Ashpol, Springer+Business Media, LLC (2009), 318 pages (Hard back), ISBN: 978-03-87765-75-7.

The two articles in this series comprise:
Meteorites demystified: A beginner’s guide (Part 1)
Meteorites demystified: A beginner’s guide (Part 2)