Positive correlation between shock stage and petrologic type in ordinary chondrites: Implications for the internal structures and thermal histories of ordinary-chondrite parent asteroids

Abstract

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.