EVIDENCE FOR EARLY FRAGMENTATION-REASSEMBLY OF ORDINARY CHONDRITE (H, L, AND LL) PARENT BODIES FROM REE-IN-TWO-PYROXENE THERMOMETRY

Ordinary chondrites (OCs) are variably thermally metamorphosed meteorites thought to originate from three different asteroid parent bodies (H, L, and LL). Their thermal evolutions are frequently explained by the onion shell model; however, a competing hypothesis is the fragmentation-reassembly model. The onion shell model proposes an internally heated, concentrically stratified, and undisrupted thermal structure, with cooling rates inversely correlated with petrologic type. The alternative fragmentation-reassembly model invokes catastrophic collisional disruption, followed by rapid reaccretion of hot fragments, forming rubble pile bodies. Fragmentation would result in fast cooling of collisional fragments at the time of disruption. Discrimination between these scenarios may be possible through the application of geothermometry and geospeedometry, which are used to constrain the temperatures and rates through which igneous and metamorphic rock samples cool.

Most published cooling rate data for OC parent bodies are based on methods that record rates through low closure temperatures (~500-200 °C; e.g., metallography, ⁴⁰Ar–³⁹Ar, ²⁴⁴Pu fission track) rather than from peak metamorphic temperatures. We applied a relatively new REE-in-two-pyroxene method, in conjunction with conventional two-pyroxene thermometry, to estimate cooling rates for 18 equilibrated OC samples at peak temperatures. We obtain fast cooling at rates ≳0.5 °C/y from peak temperatures of ~900 °C, inconsistent with slow cooling rates expected for an onion shell. Ca-in-olivine thermometry corroborates the REE-in-two pyroxene results, suggesting that the OCs cooled through closure temperatures of ~700 to 800 °C at ~10^-2 to 10^-1 °C/y. These rates are three to six orders of magnitude faster than rates determined using methods sensitive to low temperature cooling. We developed a novel thermal model that incorporates fragmentation of an initial onion shell body and reassembly into a rubble pile that reproduces both fast cooling from high temperature intervals and slow cooling at low temperatures. We hypothesize that OC parent bodies initially possessed onion shell thermal structures, but experienced collisional breakup at peak or near-peak temperatures, then reaccreted rapidly to form thermally stable rubble-pile asteroids.