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Meteorite reveals oldest known planetary crust

A team of researchers from the Institut Universitaire Européen de la Mer in Brest (CNRS, Université de Bretagne-Occidentale), the Institut de Physique du Globe in Paris (Université de Paris/IPGP/CNRS), the National Institute of Polar Research in Tokyo, the Institut de Planétologie et d'Astrophysique in Grenoble (CNRS, Université Grenoble-Alpes) and the Centre de Recherches Pétrographiques et Géochimiques de Vandoeuvre-lès-Nancy (CNRS, Université de Lorraine), have just discovered that a meteorite (Erg Chech 002), found in the Algerian Sahara in May 2020, is a fragment of the oldest crust known for a small planet in the solar system. This lava was emitted 4565 million years ago (Ma), i.e. less than 2.25 Ma after the start of the formation of the solar system, or almost 50 Ma before the formation of our Earth.

Meteorite reveals oldest known planetary crust

Publication date: 16/03/2021

Press, Research

Related themes : Origins

Meteorites are mainly fragments of small bodies1 from the asteroid belt between Mars and Jupiter. The nature of asteroids gives us a picture of the geological diversity of the first objects formed in the accretion disc around the Sun, which then served as the basic materials for the formation of planets such as the Earth and Mars. To understand the formation of planets, it is essential to discover and analyse the fragments of the oldest bodies that give access to the first stages of planet formation. Such rocks are dramatically rare, just a handful among the 65,000 meteorites recorded to date, making the discovery of the Erg Chech 002 meteorite exceptional.

Erg Chech 002 is an andesite, a lava rich in silica, sodium and potassium, similar in composition to the lava of Mont Pelée (Martinique) or the Volvic stone used to build Clermont Ferrand. Here, the lava is the result of the partial fusion of a body of chondritic composition (formed by the accumulation of the first solids formed around the sun).

By studying its mineralogy, chemical composition and magnesium isotopic composition, we can reconstruct the history of its parent body. The oldest age known to date for the first grains condensed at very high temperatures around the Sun is 4567.3 Ma. This age is often referred to as the “age of the solar system”.

The parent body of Erg Chech 002 would have formed around 1 Ma later, growing to a size of the order of a few tens or even a hundred kilometres in diameter, and beginning to melt around 1.5 Ma. The magma produced, which was fairly viscous and less dense than the unmelted rock, took a few hundred thousand years to migrate to the surface under the effect of gravity.

A sample of the Erg Chech 002 meteorite. This rock contains large green crystals of pyroxene (silicate of iron and magnesium) bathed in a clear paste containing feldspars (silicates of aluminum, calcium and alkaline elements) and silica. (© A. Irving)

The parent body of Erg Chech 002 is thought to have formed around 1 Ma later, reaching a size of several tens or even a hundred kilometres in diameter, and to have begun to melt around 1.5 Ma. The magma produced, which was fairly viscous and less dense than the unmelted rock, took a few hundred thousand years to migrate to the surface under the effect of gravity.

Erg Chech 002 then crystallised, precisely 4,565 million years ago. Over a period of a few decades, the lava cooled to a temperature of 900°C, before being torn from its parent body by an impact while still hot. The meteorite then began a very long interplanetary journey before being found in the Sahara in 2020.

The Erg Chech 002 meteorite demonstrates that crusts of andesitic composition must have been common on the surfaces of the Solar System’s first small planets. However, spectroscopic analysis of the asteroid belt shows that none of the known asteroids have a composition similar to Erg Chech 002. The preservation of Erg Chech 002 since 4565 Ma remains mysterious and much remains to be done on this precious meteorite. While its study has already provided a great deal of information about the formation of the first crusts in the solar system, it is still not known whether its parent body harboured a small core comparable to certain very ancient iron meteorites. The data now available does not rule out the possibility that it did.

1 – A small solar system body is a natural object orbiting the Sun that is neither a planet nor a dwarf planet.

Ref : Barrat J.A., Chaussidon M., Yamaguchi A., Beck P., Villeneuve J., Byrne D.J., Broadley M.W., Marty B. (2021) A 4565 Myr old andesite from an extinct chondritic protoplanet. Proc. Natl. Acad. Sci. U.S.A. – https://doi.org/10.1073/pnas.2026129118

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