Microscale distribution and behavior of Th, REE, and K during regolith formation processes on the Moon: Implications for remote sensing of the surfaces of airless planetary bodies

Introduction: Remotely sensed geochemical data provide extremely valuable information concerning the global distribution of elements on a planetary surface, and are valuable in the identification of major crustal terranes. Planetary geochemical maps of the Moon and Mars for several elements such as Th, rare earth elements (REE), K, and Fe have been or are currently being prepared and have yielded significant insights into large-scale planetary processes [e.g., 1,2,3,4]. With respect to the lunar surface, the most precise chemical information obtained by remote sensing thus far has been collected by the Lunar Prospector and Clementine missions, which characterized the Fe, Th and REE concentrations of the lunar regolith across the entire surface of the planet. These measurements represent compositional variation in planetary regolith that reflect bedrock geology/geochemistry to varying degrees. It is important to understand how regolith processes affect the behavior of Th, REE, and K when interpreting these data. For example, Th, REE and often K, are enriched in finer fractions of the lunar regolith [5,6]. Enrichments may represent higher KREEP or exotic component(s) in the finer fraction, or simple comminution of friable mesostasis and feldspar into the finest soil fraction. For example, Papike et al. [7] observed that impact-induced volatilization will fractionate K from the REE. Delano [8] and Vaniman [9] demonstrated that glasses in lunar regoliths and regolith breccias can provide abundant information about impact melting and the igneous lithologies from which glasses are derived. In order to achieve this goal, the affects of impact melting on primary crustal chemical signatures were examined in regolith glasses. In this study, we combine electron and ion microprobe techniques in the analysis of Sm, Th, K, and major elements. In doing so, we demonstrate the effect of impact volatilization on K/(Sm,Th) ratios in regolith glasses and examine concentrations of these constituents in potential lunar crustal lithologies. These data sets will contribute to a more accurate interpretation of planetary information obtained through remote sensing. are thin sections of lunar regolith containing both impact and volcanic glasses. The glasses were first identified by plane light microscopy and backscattered imaging us