¹A. C. Stadermann,²T. M. Erickson,¹L. B. Seifert,³Y. Chang,³Z. Zeszut,³T. J. Zega,⁴Z. D. Michels,³J. J. Barnes
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14282]
¹NASA Johnson Space Center, Houston, Texas, USA
²Jacobs JETS Contract at NASA Johnson Space Center, Houston, Texas, USA
³Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
⁴Department of Geosciences, University of Arizona, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

The temperature and pressure conditions experienced by rocks during an impact event can be constrained using petrologic and microstructural analysis and is crucial to providing ground truth to the impact cratering process. Suevite is a polymict, impact melt-bearing breccia, specific to Ries crater in Germany. There are competing models for suevite formation and emplacement, such as clastic flows pushed out of the crater rim or ejecta plume fallback. Knowledge of the temperature and pressure pathways recorded by grains within the suevite can help distinguish between these and other models. The accessory phase zircon (ZrSiO4) and its high-pressure polymorph reidite are particularly useful in such circumstances as they are highly refractory minerals that can record the high-temperature and/or high-pressure conditions of an impact event. Here, we present evidence for a wide array of temperature and pressure conditions recorded in zircon grains within a single thin section of suevite. Zircons in this study range from unshocked to highly shocked (>53 GPa), and record temperatures more than 1673°C. These findings confirm previous studies concluding that suevites contain material exposed to very diverse pressure and temperature conditions during initial shock compression and excavation but do not, as a whole, experience extreme temperatures (>1673°C) or pressures (>30 GPa).

^1,2Alan E. Rubin
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14284]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, USA
²Maine Mineral & Gem Museum, Bethel, Maine, USA
Published by arrangement with John Wiley & Sons

H, L, and LL chondrites all exhibit positive correlations between mean shock stage and petrologic type. At a given shock energy, hot samples exhibit more intense shock features than cold samples. After the ordinary-chondrite (OC) parent asteroids were collisionally disrupted, jumbled, and gravitationally reassembled, the correlations between mean shock stage and petrologic type may have resulted from stochastic collisions into material of different temperatures that were randomly distributed in the near-surface regions of the rubble-pile asteroids. Late-stage processes including shock events and post-shock annealing affected the preexisting correlations to only minor degrees. This model, combined with literature data, permits the following scenario: Each principal OC asteroid initially had an onion-shell structure with deeply buried type 6 materials cooling slowly, yielding young closure ages in Pb-phosphate data. The OC bodies were disrupted at ~60 Ma, locking in the Pb-phosphate record of the onion-shell structure. The H-chondrite parent body was collisionally disrupted somewhat later than the L or LL bodies and was thus somewhat cooler at the time of disruption. In the OC asteroidal rubble piles, materials of different petrologic types cooled at similar rates through ~500°C, precluding a correlation between petrologic type and metallographic cooling rate. Shortly after rubble-pile formation, materials of higher petrologic types remained hotter than materials of lower petrologic types. The hotter materials recorded more intense shock features from the common meteoroid flux, leading to positive correlations in each OC asteroid between petrologic type and mean shock stage. The cooler H-chondrite materials manifested a lower range in mean shock stage.