A chemical and petrological model of the lunar crust and implications for lunar crustal origin

Abstract

We utilize a variety of lunar sample and orbital geochemical data in conjunction with current knowledge of impact‐cratering processes to develop a chemical and petrological model of the lunar crust. Orbital chemical data indicate that the upper highlands surface on the moon has the bulk composition of “anorthositic gabbro” (Al2O3 26–28 wt %); greater than 90% of the area covered is dominated by material having compositional affinities with the ferroan anorthosites, rather than with Apollo‐type Mg‐suite rocks. Considerations of the cumulative bombardment history of the moon indicate that the outer zone of impact brecciation extends tens of kilometers into the crust; given the resolution of the orbital gamma ray data, the ferroan‐anorthositic composition derived for the highlands surface may represent the bulk composition of the upper half of the lunar crust. The observed enrichment in “noritic” components in basin ejecta with increasing basin size, together with considerations of impact melt petrogenesis in lunar basins, suggest that the bulk composition of the lower lunar crust is “noritic” (Al2O3 ∼ 20 wt %); samples of lower crustal material are probably present in the Apollo collections in the form of low‐K Fra Mauro (LKFM) and very high alumina (VHA) basaltic impact melts. Our estimated value of total crustal Al2O3 content (24–25 wt %) suggests that the lunar crust contains too much aluminum to have originated by a purely “serial magmatism” mechanism. The large abundance of plagioclase within the lunar crust is probably a result of global‐scale fractionation of plagioclase in early lunar history. The “magma ocean” hypothesis of crustal origin more readily explains the bulk composition of the lunar crust.