KREEPy Bullialdus Crater: Probing the Links Between Mineralogy, Thorium, and Lunar Volatiles

The central peak of Bullialdus Crater has long been recognized as being dominated by a strong noritic signature (e.g., 1,2,3). Results of spectral fits to the central peak of Bullialdus suggest a relatively high Mg# in the low-Ca pyroxenes (4), within the range of values observed for Mg-suite lunar samples (e.g., 5). Centered at -20.7°, 337.5° in Mare Nubium, Bullialdus Crater lies within the high-thorium Procellarum KREEP Terrane (e.g., 6). In fact, based on orbital gamma-ray data, Bullialdus is the location of a clear Thorium enhancement, which is important because Th commonly serves as a proxy for detecting KREEP-rich materials on the lunar surface (7). In addition to providing a window into the complex petrology of the lunar crust, Bullialdus Crater may also provide insight into the distribution of native lunar volatiles. Multiple lunar data sets have demonstrated that some lunar surface materials exhibit a 2.8 - 3.0 μm absorption band, indicative of a hydroxyl or water component (e.g., 8, 9,10). An increasing number of studies are also revealing that the lunar mantle may have contained more water than originally assumed (11, 12, 13) and some of this increased water may be related to KREEP materials (14). Observations of the central peak of Bullialdus Crater indicate that the pyroxenes exhibit a distinctive 2.8 μm band that is significantly stronger than the immediate surroundings, possibly indicating the presence of a hydroxyl component. We will present the association of hydroxyl absorptions at and around Bullialdus Crater with the thorium anomalies and explore whether the hydroxyl observed in the Bullialdus central peak is likely to be solar-wind implanted hydrogen or native, bound hydroxyl within the norites.