Exploring the Complex Impact History of the Apollo 16 Landing Site Through High Spatial Resolution 40Ar39Ar Geochronology

Exploring the Complex Impact History of the Apollo 16 Landing Site Through High Spatial Resolution 40Ar39Ar Geochronology Exploring the Complex Impact History of the Apollo 16 Landing Site Through High Spatial Resolution 40Ar/39Ar Geochronology One of the top priorities in lunar science for NASA is to establish an absolute chronology of lunar impact events, with significant implications for understanding the bombardment history and evolution of the Earth and the other planets of the inner solar system. Realizing this goal requires the precise and accurate dating of impact events recorded in lunar impact rocks (impactites). Samples returned from the Descartes Mountains and Cayley Plains formations at the Apollo 16 landing site contain abundant impact melts that represent four major compositional groups, and exhibit conventional 40Ar/39Ar ages that span nearly 400 million years and likely reflect multiple impact events. Previous researchers have attempted to constrain the formation ages for the Nectaris and Imbrium basins by analyzing Apollo 16 impactites, but the genetic relationships between dated samples and a particular basin are uncertain at best. This is largely due to the unknown and undoubtedly mixed provenance of impact breccia materials in the Descartes Mountains and Cayley Plains formations, which were presumably emplaced as ejecta from the Nectaris and Imbrium basins, respectively. Since the origin of any particular dated specimen is unclear, it seems advantageous to acquire precise and accurate dates for all events recorded in Apollo 16 impactites to better understand the complex impact history preserved in the Descartes-Cayley Plains region. This requires numerous analyses of a variety of impact melts from the Apollo 16 sample suite, as well as a statistical treatment of the resultant dataset to determine the number and timing of impact events. We will apply high spatial-resolution laser microprobe techniques to acquire multiple in situ 40Ar/39Ar dates from thin sections of each of six impactites from the Apollo 16 sample suite that are known to represent all four major impact melt compositional groups. An important advantage of this approach is that the petrographic context of the dated material is preserved, helping to inform age interpretations. In this way, we hope to clarify or expand upon previous impact chronologies of the Descartes-Cayley Plains region and to address some of NASAs primary objectives in lunar science.