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

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’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.

Structure of differentiated planetesimals: A chondritic fridge on top of a magma ocean

“…As inferred from petrological data, EC002 was formed by ≈ 25% partial melting of a chondritic source (Barrat et al, 2021). In the following, we establish in which conditions chondritic material can be preserved in a differentiated planetesimal.…”

“…Achondrites with andesitic or trachyandesitic compositions are very rare, e.g., GRA 06128 and 06129 (Day, Ash, Liu, Bellucci, III, McDonough, Walker and Taylor, 2009;Shearer, Burger, Neal, Sharp, Spivak-Birndorf, Borg, Fernandes, Papike, Karner, Wadhwa, Gaffney, Shafer, Geissman, Atudorei, Herd, Weiss, King, Crowther and Gilmour, 2010), Almahata Sitta ALM-A (Bischoff, Horstmann, Barrat, Chaussidon, Pack, Herwartz, Ward, Vollmer and Decker, 2014), NWA 11119 (Srinivasan, Dunlap, Agee, Wadhwa, Coleff, Ziegler, Zeigler and McCubbin, 2018) and NWA 11575 (Agee, Habermann and Ziegler, 2018). Recently, a new achondrite, Erg Chech 002 (EC002), has been recognized as the oldest andesitic meteorite known so far (Barrat, Chaussidon, Yamaguchi, Beck, Villeneuve, Byrne, Broadley and Marty, 2021).…”

“…The formation of CAIs is generally considered to date the episode of high-temperature condensation, or reprocessing of these condensates, which produced the first Solar system dust in the accretion disk. A crystallization age of 2.255 ± 0.015 Myr after CAIs have been inferred for EC002 from 26 Al − 26 Mg chronology (Barrat et al, 2021), i. e. ≈ 1.2 Myr older than all other known achondrites, and contemporaneous with the formation of the metallic cores of the parent bodies of iron ⋆ Tanton, 2013). However, the petrology and geochemistry of some differentiated meteorites, such as EC002, indicate that their parent liquids formed directly from chondritic material at shallow depth in planetesimals.…”

The Meteoritical Bulletin, No. 109

“…Meteorite finds that are notable for their mass, rarity, and/or scientific interest include Erg Chech 002 (32 kg ungrouped achondrite, described in detail in Barrat et al. 2021), NWA 13133 (90 kg EL6), Oiuru 001 (450 kg IVB iron), Orlov Dol (151 kg IID iron), NWA 13583 (78 kg eucrite), NWA 11333 (56 kg ureilite), Balambala (the 7th IIF iron), and NWA 13368 (nakhlite).…”

Evolved components in the eucrite parent body: Records in the layered eucrite Northwest Africa 8021

“…Most achondritic meteorites are ultramafic or basaltic in chemistry. However, a few andesitic to dacitic meteorites derived from asteroids were also reported in the last decade (e.g., Barrat et al., 2021; Bischoff et al., 2014; Day et al., 2009; Srinivasan et al., 2018). Their formation may be related to either high‐degree evolution or partial melting of chondritic parent bodies (Collinet & Grove, 2020; Lunning et al., 2017; Usui et al., 2015).…”