The origin of young mare basalts inferred from lunar meteorites Northwest Africa 4734, 032, and LaPaz Icefield 02205

Abstract

AbstractNorthwest Africa (NWA) 4734 is an unbrecciated basaltic lunar meteorite that is nearly identical in chemical composition to basaltic lunar meteoritesNWA032 and LaPaz Icefield (LAP) 02205. We have conducted a geochemical, petrologic, mineralogic, and Sm‐Nd, Rb‐Sr, and Ar‐Ar isotopic study of these meteorites to constrain their petrologic relationships and the origin of young mare basalts.NWA4734 is a low‐Ti mare basalt with a low Mg* (36.5) and elevated abundances of incompatible trace elements (e.g., 2.00 ppm Th). The Sm‐Nd isotope system datesNWA4734 with an isochron age of 3024 ± 27 Ma, an initial εNdof +0.88 ± 0.20, and a source region147Sm/144Nd of 0.201 ± 0.001. The crystallization age ofNWA4734 is concordant with those ofLAP02205 andNWA032.NWA4734 andLAP02205 have very similar bulk compositions, mineral compositions, textures, and ages. Their source region147Sm/144Nd values indicate that they are derived from similar, but distinct, source materials. They probably do not sample the same lava flow, but rather are similarly sourced, but isotopically distinct, lavas that probably originate from the same volcanic complex. They may have experienced slightly different assimilation histories in route to eruption, but can be source‐crater paired.NWA032 remains enigmatic, as its source region147Sm/144Nd definitively precludes a simple relationship withNWA4734 andLAP02205, despite a similar bulk composition. Their high Ti/Sm, low (La/Yb)N, and Cl‐poor apatite compositions rule out the direct involvement ofKREEP. Rather, they are consistent with low‐degree partial melting of late‐formedLMOcumulates, and indicate that the geochemical characteristics attributed to urKREEPare not unique to that reservoir. These and other basaltic meteorites indicate that the youngest mare basalts originate from multiple sources, and suggest thatKREEPis not a prerequisite for the most recent known melting in the Moon.