Imbrium Age for Zircons in Apollo 17 South Massif Impact Melt Breccia 73155

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 Pb-207/Pb-206 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.