¹Lang Zhang, ¹Ai-Cheng Zhang, ¹Xiao-Wen Liu, ²Yan-Jun Guo, ¹Jia-Ni Chen, ³Yuan-Yun Wen, ⁴Qiu-Li Li, ⁴Yu Liu, ⁴Xiao-Xiao Ling, ⁵Jin S. Zhang
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.06.032]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
²CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing, China
³Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
⁴State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
⁵Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843, USA
Copyright Elsevier

Mineralogical records of strong shock metamorphism (around or above 20 GPa) are common in L-group chondrites, Martian meteorites, and lunar meteorites, but rarely reported in Howardite-Eucrite-Diogenite (HED) meteorites. Here, we report detailed mineralogical observations of shock-induced features and ion-microprobe merrillite U-Pb ages from the pigeonite cumulate eucrite Northwest Africa (NWA) 8326. Shock-induced mineralogical features in NWA 8326 contain: (i) planar fractures in pyroxene and partial maskelynitization of plagioclase; (ii) presence of high-pressure minerals such as tissintite, stishovite, vacancy-rich augite, super-silicic garnets within melt veins, and xieite, tuite, and reidite in the host rock outside melt veins. We also observed fine-grained clinoenstatite and pigeonite at the edges of shock melt and propose they formed through metastable crystallization. Our study indicates that NWA 8326 experienced shock metamorphism of at least 20 GPa, comparable to those observed in L-group chondrites, Martian meteorites, and lunar meteorites. We propose that the relatively low shock pressures inferred for shocked eucrites in previous investigations could be due to the absence of suitable high-pressure mineralogical indicators. The ion-microprobe 207Pb/206Pb age of merrillite in NWA 8326 is 4238 ± 32 Ma (95 % confidence) and represents the timing of the shock metamorphism. The similarity of the impact ages across NWA 8326, some eucrites, lunar samples/meteorites, and chondrites suggests that there were probably widespread impact events at ∼4.2 Ga in the Solar System.