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

Thorium-rich, mafic impact-melt breccias from the Apollo 14-17 missions, th at is, those breccias identified with the composition known as "LKFM," are regarded largely as products of basin-forming impacts that penetrated the f eldspathic crust and sampled underlying mafic material and magma-ocean resi duum carrying the compositional signature of KREEP (potassium, rare earth e lements, phosphorous). Despite considerable compositional variation among s uch breccias, compositions of all of them correspond to mixtures of only fo ur components: (1) a norite with composition generally similar to that of A pollo 15 basalt (mean abundance: 58%; range: similar to 30-95%), (2) Fo(sim ilar to 90) dunite(mean: 13%, range: 1-27%), (3) feldspathic upper crust(me an: 29%, range: 4-50%), and FeNi metal (0.1-1.7%). Petrographic evidence ha s shown that much of the feldspathic component, but none of the KREEP compo nent, is elastic. This observation and the high proportion of KREEP norite component in the breccias suggest that the melt zone of the impact or impac ts forming the breccias contained little feldspathic material but consisted predominantly of material with the average composition of KREEP norite. Th e dunite component probably derives ultimately from the upper mantle. These conclusions support the hypothesis that the breccias were not formed in ty pical feldspathic crust but instead by one or more impacts into what is des ignated here "the great lunar hot spot," that is, the anomalous Th-rich ter rane in the Imbrium-Procellarum area identified by the Apollo and Lunar Pro spector 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-sui te plutonic rocks of the Apollo collection are likely all differentiation p roducts of the hot spot, not of plutons that might occur in typical feldspa thic crust.