Thermal history of the Erg Chech 002 parent body: Early accretion and early differentiation of a small asteroid

TL;DR

This study models the thermal and accretion history of the Erg Chech 002 parent body, revealing early differentiation of a small asteroid with a magma ocean, and analyzes its ejection and surface conditions.

Contribution

It provides the first thermo-chronological model of EC 002's parent body, estimating its size, formation time, and differentiation process based on multiple isotopic ages.

Findings

Parent body radius of 20-30 km formed 0.1 Ma after CAIs.

Early interior melting and differentiation by 0.5 Ma.

EC 002 originated from a partially differentiated region, not the magma ocean zone.

Abstract

The history of accretion and differentiation processes in the planetesimals is provided by various groups of meteorites. Sampling different parent body layers, they reveal the circumstances of the metal-silicate segregation and the internal structures of the protoplanets. The ungrouped achondrite Erg Chech 002 (EC 002) added to the suite of samples from primitive igneous crusts. Here we present models that utilize thermo-chronological data for EC 002 and fit the accretion time and size of its parent body to these data. The U-corrected Pb-Pb, Al-Mg, and Ar-Ar ages used imply a best-fit planetesimal with a radius of 20-30 km that formed at 0.1 Ma after CAIs. Its interior melted early and differentiated by 0.5 Ma, allowing core and mantle formation with a transient lower mantle magma ocean, and a melt fraction of <25 % at the meteorite layering depth. EC 002 formed from this melt at a depth of 0.8 km in a partially differentiated region covered by an undifferentiated crust. By simulating collisions with impactors of different sizes and velocities, we analyzed the minimum ejection conditions of EC 002 from its original parent body and the surface composition of the impact site. The magma ocean region distinct from the layering depth of EC 002 implyes that it was not involved in the EC 002 genesis. Our models estimate closure temperatures for the Al-Mg ages as 1060 K to 1200 K. A fast parent body cooling attributes the late Ar-Ar age to a local reheating by another, late impact.