The search for rock : hunting rare lithologies with high-resolution global lunar mineral maps

Introduction: Efforts have been made to identify meteorite source regions using Lunar Prospector Gamma-Ray Spectrometer (LPGRS) data and Clementine reflectance data (e.g., [1-4]), but the low spatial resolution (≥0.5 degree per pixel) of LPGRS measurements [5] and relative instrumental insensitivity to Fepoor lithologies means potential source regions can often only be identified at a broad scale. The resolution of compositional remote sensing data from recent missions such as Lunar Reconnaissance Orbiter and Kaguya now approaches the spatial scale of individual outcrops, offering a new opportunity to both potentially identify the source regions of lunar meteorites and place Apollo samples in more precise geologic context. Here, we present a new method for identifying the source regions of rare lunar lithologies using Kaguya Multiband Imager (MI) data. Methods: To locate source regions of lunar samples, we first obtained the mineralogical composition of samples of lithologies rare among the Apollo collection but with known provenances. These lithologies, such as Mg-suite rocks, are poorly sampled in the Apollo collection, yet are critical to understanding the structure and evolution of the lunar crust [6]. We then used near global mineralogical maps derived from the Kaguya MI data [7], and polar mineral maps derived from the Kaguya Spectral Profiler data [8] to locate areas that match the mineralogical composition of each sample +/~8 wt.% (i.e., the error on the mineral abundance maps) [7]. After “ground-truthing” our method with Apollo samples, we used the same technique to locate areas that match the mineral composition of a meteorite with unknown source. We identified three samples of rare Apollo lithologies with known provenance: norite 77215, norite 78235 and troctolite 76535. Sample 77215 is a single lithology subsampled from the Station 7 boulder [9]. This boulder is a breccia [10] and has a wellconstrained source on the North Massif [11,12]. Sample 78235 was sampled from the Station 8 boulder (which was likely ejected from a small crater near Taurus-Littrow [13]). Rake sample 76535 was collected at Station 6 [9]. For a sample with unknown source, we selected lunar meteorite Northwest Africa 5744, a granulitic anorthositic troctolite (e.g., [14,15]). The NWA 5744 group consists of several paired stones, including NWA 8687 [14,15]. We obtained two 27 × 46 mm thin sections of these meteorites for study. Sample analysis. Bulk mineral and Th compositions of 76535, 77215, and 78235 were obtained from the Lunar Sample Compendium. Modal mineralogy of NWA 5744 was determined with point counting on false color X-ray element maps of sections of NWA 5744 and its pair NWA 8687 [15]. Mineral proportions of plagioclase, pyroxene, and olivine were normalized to 100 %. Bulk mineral proportions among the NWA 5744 group pairs are variable [14,15], so we searched for two compositions. Literature bulk Th was used for NWA 5744 (~0.1 ppm [14]).