Ultramafic outcrop in the centre of Vredefort: Possible exposure of upper mantle?

Abstract

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). This early lunar crust comprises Fe-rich anorthosites with calcic plagioclase compositions. Subsequent evolution of the highland crust was dominated by troctolites, 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 ultramafic rocks of the lunar mantle, but their bulk rock incompatible element characteristics are too enriched to represent such a primitive source. Previous studies have suggested that this enrichment in incompatible trace elements is the result of metasomatism of the crust by fluids rich in REE and P. The products of this suggested metasomatic event are REE-rich phosphates (typically whitlockite) deposited interstitially. Alternatively, the incompatible element-rich nature of these plutonic rocks may represent a characteristic of their parent magma, acquired prior to crystallization of the plutons.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 compositions by electron microprobe analysis. Nine of these samples have high mg#s, consistent with their formation from the most primitive parent melts of the Mg-suite. The data presented here show that Mg-suite troctolites and anorthosites preserve major and trace element characteristics acquired during their formation as igneous cumulate rocks and that these characteristics can be used to reconstruct related aspects of the parent magma composition. Our data show that primitive cumulates of the Mg-suite crystallized from magmas with REE contents similar to high-K KREEP in both concentration and relative abundance. The highly enriched nature of this parent magma contrasts with its primitive major element characteristics, as pointed out by previous workers. This enigma is best explained by the mixing of residual magma ocean urKREEP melts with ultramagnesian komatiitic partial melts from the deep lunar interior. The data do not support earlier models that invoke crustal metasomatism to enrich the Mg-suite cumulates after formation, or models which call for a superKREEP parent for the troctolites and anorthosites.