The ordinary chondrite (OC) meteorites originate from at least three different original parent asteroids with slightly different chemical compositions, and are designated H, L, and LL for their high, low, and very low iron contents. These asteroids were thermally metamorphosed, but not melted, early in the history of the solar system, resulting in OC samples that show a sequence of petrologic types (referred to as types 3 through 6) corresponding to increasing degrees of heating and metamorphism. One model to explain this range of metamorphic types, commonly referred to as the “onion shell model,” posits that the asteroids cooled slowly from the outside inward while still intact, resulting in concentric layers. This model is not, however, consistent with all observed features of OCs. In particular, cooling rates recorded by metallic phases in OC are not correlated with petrologic type as the onion shell model would require. This has led to the alternative hypothesis that the original OC parent bodies were catastrophically disrupted and then reassembled into bodies with “rubble pile” structure, possibly in multiple stages.
New research led by Michael Lucas, a postdoctoral scholar at the University of Tennessee, examined 18 OCs with the aim of distinguishing between these two modes of origin. Lucas and colleagues applied mineral geothermometry and geospeedometry, techniques that are used to constrain the temperatures and rates through which rocks cool. In addition to conventional geothermometers (e.g., calcium-in-olivine), the authors applied a more recently developed geothermometer based on the temperature-dependent distribution of rare earth elements (REEs) between two different types of pyroxene minerals. This new technique measures the peak metamorphic temperature (up to 1400oC), while conventional geothermometers can help determine the temperature at which elements stopped being re-distributed (i.e., closure temperatures of around 500 to 1000oC). Using this methodology, the authors estimate cooling rates of about 0.3oC per year at peak temperatures of around 900oC, which is one thousand to one million times faster than previous methods calibrated for low-temperature (500oC) cooling (e.g., metallography). The authors then modeled a two-stage thermal history of rapid cooling, followed by slow cooling. They concluded that the rapid cooling reflects catastrophic disruption, while the slower cooling occurred in reassembled rubble piles. Thus, OC parent bodies could have originally had onion shell structures but were catastrophically disrupted and reassembled into rubble piles before becoming completely cold, a conclusion that had previously been reached by other researchers based on evidence from 207Pb-206Pb lead closure temperatures combined with metallographic cooling rates. Different mineral thermometers record different stages of this two-stage history, and thus do not necessarily contradict one another. Since OCs are among the most ancient planetary samples, this result confirms that the geological histories of planetesimals early in the history of the solar system were strongly influenced by collisional processes. READ MORE