What Do Nectaris Basin Impact Melt Rocks Look like and Where Can We Find Them?

Introduction: The formation of the Nectaris basin is a key event defining the stratigraphy of the Moon. Its absolute age, therefore, is a linchpin for lunar bombardment history. Fernandes et al. [1] gave a thorough account of the history of different samples thought to originate in Nectaris, with the upshot being there is little agreement on what samples represent Nectaris, if any. We are revisiting the effort to identify Nectaris basin impact-melt rocks at the Apollo 16 site, to model their emplacement, and to use these parameters to examine other sites where Nectaris impact melt is more abundant and/or more recognizable for potential further study. Nectaris melt in Apollo 16 soil? A compendium of all the rocks so far dated (only a fraction of all possible samples) from the Apollo 16 collection is shown in Fig. 1. Though it reflects our known bias as a community toward dating radiogenic-rich, mafic impact-melt rocks, it does show several important features: a clear time of crystallization of lunar crustal rocks, represented by the abundant (though undersampled) ferroan anorthosites; a small group of ~4.1-4.2 Ga samples as noted by [1]; two distinct groups of impact-melt samples clustered ~3.95 and ~3.85 Ga; a tail off of assorted impact-melt compositions similar to the lunar meteorite ages [2]; and a handful of glassy materials spanning time until present. At the Apollo 16 site, the formation of the Imbrium basin was undoubtedly the last major modification to the surface, forming the Cayley plains and possibly