In situ Pb‐Pb dating of silica‐rich Northwest Africa (NWA) 6594 basaltic eucrite and its constraint on thermal history of the Vestan crust

1,2Shiyong Liao,1Weibiao Hsu,1Ying Wang,1Ye Li,2Chipui Tang,2Bao Mei
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13408]
1CAS Center for Excellence in Comparative Planetology, Purple Mountain Observatory, Nanjing, 210034 China
2State Key Laboratory for Lunar and Planetary Sciences, Macau University of Science and Technology, Taipa, Macau
Published by arrangement with John Wiley & Sons

Eucrites represent one of the major lithologies of the Vestan upper crust, which had experienced pervasive and intense thermal metamorphism. To better constrain the timing and mechanism of thermal metamorphism, we carried out in situ Pb‐isotope analysis of an unbrecciated basaltic eucrite NWA 6594 on the basis of detailed mineralogical and petrographic investigations. Zircon Pb‐Pb dating reveals that NWA 6594 emplaced before or at 4547 ± 11 Ma (95% confidence, MSWD = 1.3). Studies of silica minerals indicate that NWA 6594 had experienced intense thermal metamorphism after emplacement, followed by a late impact reheating and rapid cooling. Apatite grains yield a weighted mean Pb‐Pb age of 4523 ± 2 Ma (95% confidence, MSWD = 0.76). This age could not be attributed to slow cooling after the initial crystallization, but most likely related to an independent thermal event that induced thermal metamorphism. The protracted time lag (~24 ± 13 Myr) between zircon and apatite closure ages indicates that this thermal event is most probably induced by an intense impact event that was synchronous with the metal–silicate mixing event recorded by mesosiderites. HEDs may have experienced multiple stages of thermal metamorphism after emplacement. The late impact reheating occurred after thermal metamorphism, which caused crystallization of tridymite.

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