A 4,565-My-old andesite from an extinct chondritic protoplanet

1Jean-Alix Barrata,2Marc Chaussidon,3Akira Yamaguchi,4Pierre Beck,5Johan Villeneuve,5David J. Byrne,5Michael W. Broadley,5Bernard Marty
Proceedings of the National Academy of Sciences of the United States of America (PNAS) (in Press) Link to Article [https://doi.org/10.1073/pnas.2026129118]
1Univ Brest, CNRS, IRD, Ifremer, LEMAR, F-29280 Plouzané, France;
2Institut de physique du globe de Paris, CNRS, Université de Paris, F-75005 Paris, France;
3National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan;
4CNRS, Institut de Planetologie et d’Astrophysique de Grenoble, F-38400 Saint Martin d’Hères, France;
5Université de Lorraine, CNRS, CRPG, F-54000 Nancy, France

The age of iron meteorites implies that accretion of protoplanets began during the first millions of years of the solar system. Due to the heat generated by 26Al decay, many early protoplanets were fully differentiated with an igneous crust produced during the cooling of a magma ocean and the segregation at depth of a metallic core. The formation and nature of the primordial crust generated during the early stages of melting is poorly understood, due in part to the scarcity of available samples. The newly discovered meteorite Erg Chech 002 (EC 002) originates from one such primitive igneous crust and has an andesite bulk composition. It derives from the partial melting of a noncarbonaceous chondritic reservoir, with no depletion in alkalis relative to the Sun’s photosphere and at a high degree of melting of around 25%. Moreover, EC 002 is, to date, the oldest known piece of an igneous crust with a 26Al-26Mg crystallization age of 4,565.0 million years (My). Partial melting took place at 1,220 °C up to several hundred kyr before, implying an accretion of the EC 002 parent body ca. 4,566 My ago. Protoplanets covered by andesitic crusts were probably frequent. However, no asteroid shares the spectral features of EC 002, indicating that almost all of these bodies have disappeared, either because they went on to form the building blocks of larger bodies or planets or were simply destroyed.

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