Rapid cooling of H, L, and LL chondrites and lodranite meteorites suggests collisional fragmentation of their parent bodies at peak or near-peak temperatures and long (10s-10,000s y) reassembly timescales

Application of REE-in-two-pyroxene thermometry, two-pyroxene solvus thermometry and Ca-in-olivine thermometry to meteorites from H, L, LL chondrite and acapulcoite-lodranite parent bodies reveals fast (~1 °C/y) cooling from peak or near-peak temperatures [1,2]. For these parent bodies, rapid cooling persisted through ~700 °C, and then declined dramatically to rates two-six orders of magnitude slower, according to speedometric methods sensitive to lower temperature cooling [e.g., 3,4]. These disparate rates can be reconciled by fragmentation-reassembly events, where the parent asteroids were collisionally disrupted at peak or near-peak temperature, producing smaller fragments that radiated heat away into space before gravitational reassembly into second-generation parent asteroids that cooled slowly [e.g., 5]. Assuming conductive cooling, we modeled the thermal evolution of fragments after collisional disruption but before reassembly [6]. We show that the cooling rate at any fixed temperature decays as 1/t so that cooling rates measured at a given closure temperature provide a constraint on both reassembly time and fragment radius. The cooling rates between 10-2 and 10 °C/y observed in chondrites and lodranites require reassembly times of at least 10 to 104 y and fragment radii of at least 100s to 1000s of m. These durations are much longer than expected compared to freefall times (<1 y) [7], suggesting physical processes such as degassing or escape from the initial mass well of the parent body prolonged reassembly, or the fragments cooled more rapidly by a process other than conduction. The slower cooling rates through temperatures <700 °C were successfully modeled assuming reassembly into rubble-pile meteorite parent asteroids [1]. Our work suggests that fragmentation-reassembly events were ubiquitous within 10 Myr of Solar System formation, and that they played an important role in physically and chemically modifying planetary precursor materials. [1] Lucas et al., 2020, GCA 290, 366-390. [2] Lucas et al., in prep. [3] Scott E.R.D, 2002, Asteroids III, 697-709. [4] Pellas P. & Storzer D., 1981, PRSL 374, 253-270. [5] Grimm, R.E., 1985, JGR 90, 2022-2028. [6] Ren et al., in prep. [7] Love, S.G. & Ahrens, T.J., 1996, Icarus 124, 141-155.