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

Abstract

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 TiO 2 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).

The geochemistry and provenance of Apollo 16 mafic glasses

“…6), but it does not match any of the Apollo 11 or 17 high-Ti basalt suites. Glasses of this composition have been described in almost all of the previous surveys of Apollo 16 glass and several lithic basalts with a similar major element composition have been identified in the Apollo 16 regolith (Delano, 1975;Vaniman et al, 1978;Zeigler et al, 2005a).…”

“…A third type of mafic material consists of the products of mare volcanism. Despite the Apollo 16 site being 220 km from the nearest mare, fragments of mare basalt and pyroclastic glass occur in the Apollo 16 regolith (Dowty et al, 1974a,b;Delano, 1975;Murali et al, 1976;Vaniman et al, 1978;Simon and Papike, 1987;Takeda et al, 1987;Zeigler et al, 2005a). Again, all known samples are small.…”

“…The concept that basaltic material at the Apollo 16 site was deposited by post-basin impacts was first presented in Zeigler et al (2005a), so only a brief summary is provided here for convenience. The Apollo 16 ancient regolith breccias (McKay et al, 1986) and the soils collected near North Ray Crater contain material from Imbrium, the last basin ejecta deposit at the Apollo 16 site, but practically no material deposited by post-Imbrium impacts.…”

“…The >1-mm grain-size fraction of the Apollo 16 regolith contains much less mare volcanic material (1% or less; Delano et al, 1973;Zeigler et al, 2005a) than the 6% predicted by the mass balance model for the <1-mm fraction. This discrepancy led us to suspect that most of the mare component of the Apollo 16 regolith occurs as pyroclastic ''ash,'' which typically occurs as glassy spherules of 40-50 lm diameter (Korotev, 1997).…”

Apollo 17 regolith, 71501,262: A record of impact events and mare volcanism in lunar glasses

“…Finally, several episodes of VLT volcanism have been reported at the Apollo 14 (e.g., Delano 1986Delano , 1988 and Apollo 16 landing sites (e.g., Zeigler et al 2006b), as well as in lunar meteorites (e.g., EET 87521; Asuka-881757; Y-793274; Y-793169; Asuka-31; QUE 94281; and NWA 773) with one reported U-Pb age of ~3840 Ma (Misawa et al 1993) and another reported U-Pb age of ~4350 Ma (Terada et al 2007). These occurrences suggest that VLT mare volcanics are not only widely distributed on the Moon but also have a large range of isotopic ages (~2700 Ma to ~4350 Ma).…”