Composition of the lunar highland crust from near‐infrared spectroscopy

Near‐infrared reflectance spectra of the lunar highlands are synthesized to obtain global information about the composition of the lunar near‐side crust. Immature exposed surfaces from small fresh craters, mountains and massifs, and large craters with central peaks are included in a survey of rock types across the lunar near side. Compositions recognized in these data include anorthosite, noritic, gabbric, and troctolitic mineral assemblages. Three quarters of the areas studied that represent samples from the upper 1–2 km of lunar crust exhibit noritic compositions with different amounts of pyroxene and/or brecciation alteration. Norites are ubiquitous across the near side and show no spatial clustering associated with any of the major basins. In contrast to this noritic megaregolith of the upper crust, material representing stratigraphically deeper zones (5–15 km) of the lunar crust is dominated by gabbros, anorthosites, and troctolites, with less than a quarter of the areas studied exhibiting a noritic composition. The origin of the noritic megaregolith and its relation to deeper crustal material is thus somewhat of an enigma. The current limited statistics for areas identified as anorthosites and troctolites are insufficient to derive global spatial distributions. Gabbroic areas, however, are notably concentrated in the western hemisphere. Although incomplete for a global assessment of crustal materials, these data demonstrate that the lunar near‐side crust is clearly mineralogically heterogeneous, both laterally and vertically, and not well mixed below 1–2 km.