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

Presentation #321.03 in the session Asteroids: Origins (Oral Presentation)

Published onOct 23, 2023
Thermal history of the Erg Chech 002 parent body: Early accretion and early differentiation of a small-size planetesimal

The history of accretion and differentiation processes in the precursors of planets is provided by various groups of primitive chondritic and achondritic meteorites. Sampling different layers of their parent bodies, they can be used to trace the circumstances of the metal-silicate segregation and the internal structures of the protoplanets. The ungrouped achondrite meteorite Erg Chech 002 added to the suite of samples from primitive igneous crusts. Here we present models that utilize thermo-chronological data for Erg Chech 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 for the models imply a best-fit planetesimal with a radius of 20 − 30 km that accreted at 0.1 Ma after the beginning of the solar system. Its interior melted early and differentiated by 0.5 Ma after CAIs, producing a high melt fraction at depth, allowing core and mantle formation with a transient mantle magma ocean, and a melt fraction of <25 % at the meteorite layering depth. The meteorite material formed in situ from such a partial melt at a depth of 0.8 km in a partially differentiated source region that was covered by a thin undifferentiated chondritic crust. By simulating collisions with impactors of different sizes and velocities, we analyzed, further, the minimum ejection conditions of Erg Chech 002 from its original main body. A likely coverage of the source region with the rubble from the undifferentiated crust during the meteorite producing impact and subsequent adjacent impacts on the parent body implies that this object may still be present in the asteroid belt, but its spectrum would not match the spectral features of Erg Chech 002. The timescale of thermal convection in the metallic core of <4 Ma is too short for magnetization of the chondritic crust, attributing any remanent magnetization of Erg Chech 002 to the influence of the early solar nebula field. The mantle magma ocean crystallizes on a timescale of 3 Ma and its spatial extension is distinct from the burial depth of Erg Chech 002 derived, implying that its melt was not involved in the Erg Chech 002 genesis. Furthermore, our best-fit 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 late impact that reheated the parent body locally. Despite a low degree of melting at the meteorite burial depth, large melt fractions produced in deeper interior fit well into the general accretion time and melting chronology of achondritic iron and rocky meteorite parent bodies.

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