The great lunar hot spot and the composition and origin of the Apollo mafic (“LKFM”) impact-melt breccias


  • Randy L. Korotev


Thorium-rich, mafic impact-melt breccias from the Apollo 14–17 missions, that is, those breccias identified with the composition known as “LKFM,” are regarded largely as products of basin-forming impacts that penetrated the feldspathic crust and sampled underlying mafic material and magma-ocean residuum carrying the compositional signature of KREEP (potassium, rare earth elements, phosphorous). Despite considerable compositional variation among such breccias, compositions of all of them correspond to mixtures of only four components: (1) a norite with composition generally similar to that of Apollo 15 basalt (mean abundance: 58%; range: ∼30–95%), (2) Fo∼90 dunite (mean: 13%, range: 1–27%), (3) feldspathic upper crust (mean: 29%, range: 4–50%), and FeNi metal (0.1–1.7%). Petrographic evidence has shown that much of the feldspathic component, but none of the KREEP component, is clastic. This observation and the high proportion of KREEP norite component in the breccias suggest that the melt zone of the impact or impacts forming the breccias contained little feldspathic material but consisted predominantly of material with the average composition of KREEP norite. The dunite component probably derives ultimately from the upper mantle. These conclusions support the hypothesis that the breccias were not formed in typical feldspathic crust but instead by one or more impacts into what is designated here “the great lunar hot spot,” that is, the anomalous Th-rich terrane in the Imbrium-Procellarum area identified by the Apollo and Lunar Prospector gamma-ray spectrometers. The LKFM composition is a special product of the great lunar hot spot and is not the average composition of the lower crust in typical feldspathic highlands. Similarly, Mg-suite and alkali-suite plutonic rocks of the Apollo collection are likely all differentiation products of the hot spot, not of plutons that might occur in typical feldspathic crust.