Multiple shock events recorded in the Northwest Africa 2139 LL6 chondrite: Implications for collisional histories of the LL chondrite parent body

Abstract

LL chondrites have experienced multiple shock events; however, the relations of each shock event and their timing have rarely been investigated. To demonstrate the relations between each shock texture and shock chronological ages, we conducted both petrological and chronological (40Ar/39Ar and I‐Xe ages) studies using aliquots subsampled from the same chip of the Northwest Africa (NWA) 2139 LL6 chondrite. Our 40Ar/39Ar studies and petrological observation reveal that NWA 2139 recorded at least three impact events before 4.17 ± 0.10 Ga, thus resulting in a complex brecciated texture, silicate darkening, and thick shock veins. An intense heating event occurred at 4.17 ± 0.10 Ga, which recrystallized the thick veins and healing cracks. Then, a weak shock event occurred at <3.9 Ga. Combined with 40Ar/39Ar data of other LL chondritic materials, this study supports that the LL chondrite parent body was possibly broken up by 1.7 Ga, and that most of the breakup likely occurred within 3.8–4.2 Ga.