Imbrium Age for Zircons in Apollo 17 South Massif Impact Melt Breccia 73155. Issue 12 (3rd December 2019)

• Record Type: Journal Article
• Authors: Zhang, Bidong; Lin, Yangting; Moser, Desmond E.; Hao, Jialong; Shieh, Sean R.; Bouvier, Audrey
• Is Part Of: Journal of geophysical research. Volume 124:Issue 12(2019)
• Page Start: 3205
• Page End: 3218
• Publication Date: 2019-12-03
• Subjects: Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2019JE005992
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: The Apollo 17 poikilitic impact melt breccias were initially interpreted to be directly related to the Serenitatis basin formation. Here we present petrological, geochemical, and U‐Pb geochronological investigations of the poikilitic impact melt of Apollo 73155, 69 which was sampled from the South Massif of the Taurus‐Littrow valley. Nanoscale secondary ion mass spectrometry analyses of well‐preserved poikilitic zircons from sample 73155, 69 yield a uniform 207 Pb/ 206 Pb age population with a weighted mean age of 3921 ± 14 Ma. The zircons have distinctly high concentrations of Y (3279−6347 ppm) and Th (254−302 ppm) compared with other Apollo 17 zircons. These characteristics, together with their textures, are remarkably similar to those of zircons from high‐Th melt breccias from Apollo 12 samples and lunar meteorite Sayh al Uhaymir 169 attributed to the formation of the high‐Th deposits of the Imbrium basin. We therefore propose an Imbrium provenance for melt component in 73155, 69. Our results provide new evidence for components of Imbrium‐type ejecta occurring among the South Massif breccias. Consequently, the Apollo 17 poikilitic impact melt breccias may not represent Serenitatis ejecta. Plain Language Summary: Mare Serenitatis is one of the largest basins on the near side of the Moon, and its age is important for calculating lunar impact intensity over the ~600 million years after the solar system formation. The poikilitic impact melt breccias brought back to Earth by the Apollo 17 mission have been widely seen as ejecta from the Serenitatis impact basin and have been used to date its formation. We observe that zircons in Apollo 17 poikilitic impact melt breccia 73155, 69 have age, textural, and geochemical characteristics linked to the younger, 3.9 billion‐year‐old Imbrium basin and conclude that at least some components of the Apollo 17 site are Imbrium ejecta. Our findings further highlight the necessity to revisit the Serenitatis origin for the Apollo 17 poikilitic breccias. Key Points: Impact‐grown poikilitic zircons are found in Apollo 17 breccia 73155 from the South Massif The 73155 zircons have age, texture, and trace element concentrations identical to the zircons from representative Imbrium breccias These zircons are evidence for components of Imbrium ejecta emplaced into the Apollo 17 breccias