Lunar Anorthosites: Rare-Earth and Other Elemental Abundances

Abstract

Elemental abundances of major (Ti, Al, Fe, and Ca), minor (Na, Mn, and Cr), and trace elements [14 rare-earth elements (REE), Y, In, Cd, Rb, Cs, Ba, Co, and Sc] in lunar anorthosites separated from Apollo 11 sample 10085 coarse fines have been determined by means of instrumental and radiochemical neutron activation analysis. The REE distribution pattern of lunar anorthosites, relative to ordinary chondrites, has a positive Eu anomaly. On the assumption that (i) the lunar composition is similar to that of ordinary chondritic meteorites low in total Fe (∼ 13 percent); (ii) lunar anorthosites are derived from highland cratering events and are representative of the highlands; and (iii) the moon differentiated into olivine, hypersthene, and basaltic and anorthositic phases, and plagioclase crysstallization began after ∼ 93 percent solidification, then mass balance calculations yield ∼ 30-kilometer and ∼ 10-kilometer thicknesses for the lunar highlands for the melting and chemical differentiation of the entire moon and of the upper 200 kilometers, respectively. Corresponding thicknesses of the basaltic basement rocks were ∼ 5 kilometers and ∼ 2 kilometers, respectively. Alternatively, if the anorthosites of this study are representative of the highlands and the onset of plagioclase crystallization occurred after ∼ 50 percent solidification of the initially melted moon, calculations with REE and Ba partition coefficients suggest that the REE and Ba abundances in the primeval moon were similar to those observed in basaltic achondrites.