Early accretion of planetesimals unraveled by the thermal evolution of the parent bodies of magmatic iron meteorites

Edouard Kaminski, Angela Limare, Balthasar Kenda, Marc Chaussidon

Earth and Planetary Science Letters
Volume 548, 15 October 2020, 116469



• Hf/W ages of magmatic iron meteorites define a common thermal evolution.
• We propose a convective model based on lab-scale experiments with internal sources.
• Inversion of the Hf/W record defines a universal accretion scenario.
• Accretion starts by the ≈ instantaneous formation of 60 ±30 km nuclei.
• Accretion continues by ≈3 Myr of continuous pebbles accretion.”

“The timing of formation of 100-300 km size planetesimals in the protoplanetary disk remains largely unconstrained. Recent models show that gravitational collapse of boulders in overdense regions of a dusty accretion disk can overcome the meter-sized barrier and lead to rapid formation of planetesimals with size of several km that further grow by pebble accretion. Hf/W ages indicate that the first large planetesimals to form could be the parent bodies of magmatic iron meteorites. These ages have been so far used to constrain timing of accretion considering (i) instantaneous accretion, and (ii) purely conductive heat transfer in the planetesimal. To relax these hypotheses we model the thermal evolution of a planetesimal in course of accretion and we take into account the possibility of convection onset. Our model is further based on considering the possibility of a common thermal evolution for all the parent bodies of iron meteorites. Within that framework we show that the planetesimals could have grown following a universal accretion law starting at the very beginning of the history of the disk by a nearly instantaneous formation of 60 ± 30 km size nuclei, followed by a growth via pebble accretion at a much slower pace to reach final sizes of 150–300 km in about 3 Myr. In this universal scenario, complete melting and total differentiation are not bound to happen in the parent body due to the continuous accretion of cold pebbles. The model, though calibrated here on iron meteorites, is general and can in principle be applied to other types of planetesimals such as for instance the parent bodies of CV chondrites.”