A 4565 Myr old andesite from an extinct chondritic protoplanet

TL;DR

This paper reports the discovery of the oldest known igneous crust sample, EC 002, from a protoplanet, revealing early planetary differentiation processes and suggesting such bodies were common but mostly destroyed or incorporated into larger planets.

Contribution

It provides the first detailed analysis of a primitive andesite crust from an early protoplanet, dating back 4.565 billion years, offering new insights into early planetary formation.

Findings

EC 002 is the oldest known igneous crust, dated at 4,565 million years.

The crust originated from partial melting of a chondritic reservoir at 1220°C.

Most early protoplanets with similar crusts have been destroyed or incorporated into larger bodies.

Abstract

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 photosphere and at a high degree of melting of around 25 percents. 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.