Ion and electron microprobe study of troctolites, norite, and anorthositesfrom Apollo 14: Evidence for urKREEP assimilation during petrogenesis of Apollo 14 Mg-suite rocks

Most of the Moon's highland crust formed during the period 4.65-4.45 Ga ago from a vast magma ocean up to 800 km deep (Hess and Parmentier, 1995). Thi s early lunar crust comprises Fe-rich anorthosites with calcic plagioclase compositions. Subsequent evolution of the highland crust was dominated by t roctolites, anorthosites, and norites of the Mg-suite. This plutonic series is characterized by calcic plagioclase, and mafic minerals with high mg# ( =100*Mg/[Mg + Fe]). These rocks evidently formed by partial melting of ultr amafic rocks of the lunar mantle, but their bulk rock incompatible element characteristics are too enriched to represent such a primitive source. Prev ious studies have suggested that this enrichment in incompatible trace elem ents is the result of metasomatism of the crust by fluids rich in REE and P . The products of this suggested metasomatic event are PEE-rich phosphates (typically whitlockite) deposited interstitially. Alternatively, the incomp atible element-rich nature of these plutonic rocks may represent a characte ristic of their parent magma, acquired prior to crystallization of the plut ons. In an effort to distinguish the origin of this important lunar rock series, we have analyzed the REE content of primary cumulus phases in ten Mg-suite cumulates using SIMS, along with their major and minor element composition s by electron microprobe analysis. Nine of these samples have high mg#s, co nsistent with their formation from the most primitive parent melts of the M g-suite. The data presented here show that Mg-suite troctolites and anortho sites preserve major and trace element characteristics acquired during thei r formation as igneous cumulate rocks and that these characteristics can be used to reconstruct related aspects of the parent magma composition. Our d ata show that primitive cumulates of the Mg-suite crystallized from magmas with REE contents similar to high-K KREEP in both concentration and relativ e abundance. The highly enriched nature of this parent magma contrasts with its primitive major element characteristics, as pointed out by previous wo rkers. This enigma is best explained by the mixing of residual magma ocean urKREEP melts with ultramagnesian komatiitic partial melts from the deep lu nar interior. The data do not support earlier models that invoke crustal me tasomatism to enrich the Mg-suite cumulates after formation, or models whic h call for a superKREEP parent for the troctolites and anorthosites. Copyri ght (C) 1998 Elsevier Science Ltd.