New lunar meteorite Northwest Africa 2996: A window into farside lithologies and petrogenesis

The Northwest Africa (NWA) 2996 meteorite is a lunar regolith breccia with a “mingled” bulk composition and slightly elevated incompatible element content. NWA 2996 is dominated by clasts of coarse-grained noritic and troctolitic anorthosite containing calcic plagioclase (An#~98) and magnesian mafic minerals (Mg#~75), distinguishing it from Apollo ferroan anorthosites and magnesian-suite rocks. This meteorite lacks basalt, and owes its mingled composition to a significant proportion of coarse-grained mafic clasts. One group of mafic clasts has pyroxenes similar to anorthosites, but contains more sodic plagioclase (An#~94) distinguishing it as a separate lithology. Another group contains Mg-rich, very low-titanium pyroxenes, and could represent an intrusion parental to regional basalts. Other clasts include granophyric K-feldspar, disaggregated phosphate-bearing quartz monzodiorites, and alkali-suite fragments (An#~65). These evolved lithics are a minor component, but contain minerals rich in incompatible elements. Several anorthosite clasts contain clusters of apatite, suggesting that the anorthosites either assimilated evolved rocks or were metasomatized by a liquid rich in incompatible elements. We used Lunar Prospector gamma-ray spectrometer remote sensing data to show that NWA 2996 is most similar to regoliths in and around the South Pole Aitken (SPA) basin, peripheral regions of eastern mare, Nectaris, Crisium, and southern areas of Mare Humorum. However, the mineralogy of NWA 2996 is distinctive compared with Apollo and Luna mission samples, and is likely consistent with an origin near the SPA basin: anorthosite clasts could represent local crustal material, mafic clasts could represent intrusions beneath basalt flows, and apatite-bearing rocks could carry the SPA KREEP signature.