Iron and titanium distribution on the moon from orbital gamma ray spectrometry with implications for crustal evolutionary models

A set of Fe and Ti maps and regional values are obtained from the Apollo 15 and 16 orbital gamma ray data by energy band analysis. High‐Ti basalts predominate the early and late stages of mare volcanism with high‐Fe basaltic volcanism in the interim. The first evidence of a high‐Ti‐KREEP basalt association is found in the Aristarchus region. A N‐S asymmetry for Fe and Ti in the east limb and farside highlands complicates the E‐W asymmetry for Th but substantiates crustal inhomogeneity. The observed crustal inhomogeneity adds an additional objection to the primitive source model for crustal evolution. The high‐Ti‐KREEP basalt association and the general trend of decreasing mare basalt Ti with time lend support to the cumulate source model; however, this model cannot account for young, high‐Ti maria and the absence of KREEP in high‐Ti maria. The dynamic assimilation model better accounts for chemical variations observed on the moon.