Fitting Thermal Evolution Models to the Chronological Record of Erg Chech 002 and Modeling the Ejection Conditions of the Meteorite

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 thermochronological data for EC 002 and fit the accretion time and size of its parent body to these data. The U-corrected Pb–Pb pyroxene, Pb–Pb phosphate, and Ar–Ar ages used imply a best-fit planetesimal with a radius of 20–30 km that formed at 0.1 Ma after calcium-aluminum-rich inclusions. 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 implies that it was not involved in the EC 002 genesis. Our models estimate closure temperatures for the Al–Mg ages as 1030–1200 K. A fast parent body cooling attributes the late Ar–Ar age to a local reheating by another, late impact.

Petrogenesis of Erg Chech 002 Achondrite and Implications for an Altered Magma Ocean

“…The concept of a plumbing system typically entails the partial melting of the mantle within a differentiated parent body, necessitating significant heat and magma convection in a large-scale magma chamber. However, this seems impractical in a parent body of a size range around ∼20-30 km (Neumann et al 2023). Furthermore, the low REE content of the xenocrysts dismisses the possibility that they derive from partial melts of a differentiated planetary mantle.…”

Recurrent planetesimal formation in an outer part of the early solar system

“…These models were also applied to primitive achondrites, carbonaceous chondrites, and the NEA Ryugu 12 , 13 . For differentiated parent bodies, such models consider metal-silicate separation, including formation of differentiated layers, such as Fe-Ni core and silicate mantle, magma ocean evolution, and liquid-state convection 5 , 12 , 14 – 16 .…”

“…Heavily metamorphosed or partially molten meteoritic materials lack carbonates. Thus, other components, such as whole-rock, olivine, pyroxene, plagioclase, and phosphates can be dated using the U/Pb-Pb, Al–Mg, and Mn–Cr chronometers 15 , 16 . Strictly speaking, age data yield the cooling time below a critical closure temperature, below which daughter radionuclides cannot be diffusively exchanged between the dated mineral and the surrounding mineral matrix.…”