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

Cyril Sturtz, Angela Limare, Marc Chaussidon, Édouard Kaminski

In Press, Journal Pre-proof, Available online 28 May 2022



• We study the formation and the thermal evolution of planetesimals that accreted undifferentiated material during the first few Myr following the CAI formation.
• The protracted accretion of cold chondritic material results in the preservation of a few to tens of km thick undifferentiated crust on top of a magma ocean.
• Basal crustal melts rise in the crust where they stall and crystallize to produce andesitic rocks such as Erg Chech 002.”

“Meteorites are interpreted as relics of early formed planetary bodies, and they provide information about the processes that occurred in the first few Myers of our solar system. The ages measured for some differentiated meteorites (achondrites), indicate that planetesimals formed a differentiated silicate crust as early as about 3 Myr after the beginning of the solar system. The composition of the recently discovered achondrite Erg Chech 002 (EC002), the oldest andesitic rock known so far, betokens partial melting of a chondritic source taking place as early as about 1 Myr before all other known achondrites. However, thermal models of early accreted planetesimals predict massive melting of the planetesimal during core/mantle differentiation and cannot account for the preservation of a substantial amount of chondritic material. In this paper, we propose a way to interpret petrological and geochemical constraints provided by differentiated meteorites by introducing a refined thermal model of planetesimals formation and evolution. We demonstrate that continuous, protracted accretion of cold undifferentiated material upon a magma ocean over a timescale 2 times larger than the lifetime of the 26Al heat source leads to the preservation of a few km thick chondritic crust. During accretion, the heat released by radioactive decay further induces episodes of partial melting at the base of the crust, which can led to the formation of andesitic rocks such as EC002. Using the available constraints on the age of EC002 and its cooling rate, the application of our model constraints the terminal radius of its parent body between 70 and 130 km.”