LARGE-SCALE SEPARATION OF K-FRAC AND REEP-FRAC IN THE SOURCE REGIONS OF APOLLO IMPACT-MELT BRECCIAS, AND A REVISED ESTIMATE OF THE KREEP COMPOSITION

Mafic impact-melt breccias (IMB) from the Apollo landing sites-particu larly Apollo 14, Apollo 15, Apollo 16, and Apollo 17-are abundant and form compositionally distinct groups. These groups exhibit a range of major-element compositions and incompatible-element enrichments. Altho ugh concentrations of incompatible elements span a significant range, inter-element ratios vary little and have been used in the past to inf er a common KREEP component (KREEP = rich in potassium, rare-earth ele ments, phosphorus, and other alkali and high-field-strength elements). On the basis of an extensive, high-precision data set for melt-brecci a groups from different Apollo landing sites, variations in trace-elem ent signatures of the mafic impact-melt breccias ref;ect significant d ifferences in KREEP components of source regions. These differences ar e consistent with variable enrichment or depletion of source regions i n those trace elements that fractionated during the latest stages of r esidual-melt evolution and are more or less related to ''lunar granite .'' Compared to other sites, the source region of Apollo 14 impact mel ts had an excess of the elements that are concentrated in lunar granit e, suggesting either than this source region was enriched in such a co mponent (K-frac) or that it lost a corresponding mafic component (REEP -frac). Because these are impact-melt breccias formed in large (probab ly basin) impacts, the indicated geochemical separations must have occ urred on a broad scale. Variations in the incompatible-element concent rations of the IMB groups reported in this paper are used to calculate a revised KREEP incompatible-element composition. On the basis of sev eral extremely enriched lunar samples that retain the incompatible ele ments in KREEP-like ratios, the KREEP composition is extended to a lev el of 300 ppm La, or about three times the concentration of high-potas sium KREEP as estimated by Warren (1989).