Northwest Africa 773: Lunar origin and iron-enrichment trend

Abstract

The meteorite Northwest Africa 773 (NWA 773) is a lunar sample with implications for the evolution of mafic magmas on the moon. A combination of key parameters including whole-rock oxygen isotopic composition, Fe/Mn ratios in mafic silicates, noble gas concentrations, a KREEP-like rare earth element pattern, and the presence of regolith agglutinate fragments indicate a lunar origin for NWA 773. Partial maskelynitization of feldspar and occasional twinning of pyroxene are attributed to shock deformation. Terrestrial weathering has caused fracturing and precipitation of Carich carbonates and sulfates in the fractures, but lunar minerals appear fresh and unoxidized. The meteorite is composed of two distinct lithologics: a two-pyroxene olivine gabbro with cumulate texture, and a polymict, fragmental regolith breccia. The olivine gabbro is dominated by cumulate olivine with pigeonite, augite, and interstitial plagioclase feldspar. The breccia consists of several types of clasts but is dominated by clasts from the gabbro and more FeO-rich derivatives. Variations in clast mineral assemblage and pyroxene Mg/(Mg + Fe) and Ti/(Ti + Cr) record an igneous Fe-enrichment trend that culminated in crystallization of fayalite + silica + hedenbergite-bearing symplectites. The Fe-enrichment trend and cumulate textures observed in NWA 773 are similar to features of terrestrial ponded lava flows and shallow-level mafic intrusives, indicating that NWA 773 may be from a layered mafic intrusion or a thick, differentiated lava flow. NWA 773 and several other mafic lunar meteorites have LREE-enriched patters distinct from Apollo and Luna mare basalts, which tend to be LREE-depleted. This is somewhat surprising in light of remote sensing data that indicates that the Apollo and Luna missions sampled a portion of the moon that was enriched in incompatible heat-producing elements.