ERG CHECH 002, the oldest andesite in the solar system

Summary : 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 non-carbonaceous chondritic reservoir, with no depletion in alkalis relative to the sun’s photosphere, and at a high melting rate of around 25%. Moreover, EC 002 is to date, the oldest known piece of an igneous crust with a 26Al-26Mg crystallization age of 4565.0 Myr. Partial melting took place at 1220 °C, up to several hundred kyr before, implying an accretion of the EC 002 parent body circa 4566 Myr 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.