Petrography and geochemistry of five new Apollo 16 mare basalts and evidence for post-basin deposition of basaltic material at the site

Abstract

We present the petrography and geochemistry of five 2-4 mm basalt fragments from the Apollo 16 regolith. These fragments are 1) a high-Ti vitrophyric basalt compositionally similar to Apollo 17 high-Ti mare basalts, 2) a very high-Ti vitrophyric basalt compositionally similar to Apollos 12 and 14 red-black pyroclastic glass, 3) a coarsely crystalline high-Al basalt compositionally similar to group 5 Apollo 14 high-Al mare basalts, 4) a very low-Ti (VLT) crystalline basalt compositionally similar to Luna 24 VLT basalts, and 5) a VLT basaltic glass fragment compositionally similar to Apollo 17 VLT basalts. High-Ti basalt has been reported previously at the Apollo 16 site; the other basalt types have not been reported previously. As there are no known cryptomaria or pyroclastic deposits in the highlands near the Apollo 16 site (ruling out a local origin), and scant evidence for basaltic material in the Apollo 16 ancient regolith breccias or Apollo 16 soils collected near North Ray Crater (ruling out a basin ejecta origin), we infer that the basaltic material in the Apollo 16 regolith originated in maria near the Apollo 16 site and was transported laterally to the site by small- to medium-sized post-basin impacts. On the basis of TiO2 concentrations derived from the Clementine UVVIS data, Mare Tranquillitatis (~300 km north) is the most likely source for the high-Ti basaltic material at the Apollo 16 site (craters Ross, Arago, Dionysius, Maskelyne, Moltke, Sosigenes, Schmidt), Mare Nectaris/Sinus Asperitatis (~220 km east) is the most likely source for the low-Ti and VLT basaltic material (craters Theophilus, Madler, Torricelli), and a large regional pyroclastic deposit near Mare Vaporum (600 km northwest) is the most likely source region for pyroclastic material (although no source craters are apparent in the region).