Studying the mineralogy and chemical compositions of primitive meteorites can reveal the conditions under which materials formed in the early solar system and the processes that altered them on asteroidal parent bodies. Collisional processes were ubiquitous in the early solar system, and the records of those events can be preserved in asteroidal meteorites recovered on Earth. The interpretation of the collisional history of a meteorite may not, however, be straightforward because impact shock can cause thermal metamorphism and resetting of mineral ages.

The Chelyabinsk meteorite was recovered from the February 15, 2013, impact of a large meteor over Chelyabinsk Oblast, Russia. Detailed studies of this meteorite led to its classification as an LL ordinary chondrite having three distinct lithologies: a light-toned host rock, a dark-toned portion containing melt phases, and a shocked region of fully melted and recrystallized material. Attempts have been made to constrain the ages of these lithologies through radioactive dating of phosphate minerals, which record low-to-medium closure temperatures. Phosphate ages indicated that the melting and brecciation that led to the three lithologies occurred as a single event, which disagrees with textural observations suggesting two mixing events.

To refine the phosphate ages from mineral textures and better constrain the collisional history of Chelyabinsk, Dr. Craig Walton (University of Cambridge) and colleagues analyzed microtextures of minerals to age-date portions of Chelyabinsk that were associated with both pristine and fractured phosphates using scanning electron microscopy, electron backscattered diffraction, and other techniques. They found that the pristine phosphates recorded concordant U-Pb ages, implying that they preserve their crystallization ages. On the other hand, the fractured phosphates show discordant ages, implying loss of Pb. These results indicate that an ancient (4.473 billion years ago) high-energy collision affected all the phosphates, and then a milder collisional and re-heating event approximately 9 million years ago affected the fractured phosphates. READ MORE