Geochemistry of lunar crustal rocks from breccia 67016 and the composition of the Moon

Abstract

Global differentiation of the Moon has produced a plagioclase-rich crust overlying an ultramafic mantle. We have conducted a major and trace element study of anorthositic clasts from an Apollo 16 breccia to investigate the geochemical features of these highlands lithologies and their role in lunar crustal evolution. Samples analyzed for this study have aluminous, alkali-poor compositions and varied FeO and MgO contents (Al 2O 3 25–35%, Na 2O 0.3–1%, K 2O < 0.1%, FeO and MgO 0.5–8%). Three compositional groups are recognized. One group is poor in mafic constituents (Al 2O 3 ≥ 30%, FeO and MgO ≤ 4%), with the low abundances of lithophile trace elements typical of lunar anorthosites. The other two groups are more mafic (Al 2O 3 ≤ 28%, MgO and FeO ≥ 5%) and are distinguished from each other by FeO MgO ratios > 1 (ferroan noritic) and <1 (magnesian troctolitic), respectively. These mafic-enriched varieties have considerably higher lithophile element concentrations, at levels similar to that of the bulk lunar crust. The ferroan noritic clasts may represent a fundamental type of igneous rock in the lunar crust which has not been widely recognized. Li Yb ratios of highlands and mare rocks demonstrate the complementary nature of the lunar crust and mantle, which is best explained by a global differentiation event such as crystallization of a magma ocean. The most primitive mantle-derived lunar material for which data are available, the Apollo 15 green glass, has Li Yb virtually identical to those of the mafic-enriched anorthosites, providing an estimate of the magma ocean composition. The Moon and the Earth have Li Yb ratios that are similar to each other, but the absolute abundances of both Li and Yb appear to be higher in the Moon. The Li Yb ratios of the Earth and Moon are distinct from those of CI and ordinary chondrites and the eucrite parent body but are similar to those of CV, CO and EL chondrites, precluding a unique match between the composition of the Earth and Moon.