Lunar Mantle Composition Based on Spectral and Geologic Analysis of Low‐Ca Pyroxene‐ and Olivine‐Rich Rocks Exposed on the Lunar Surface

Abstract

AbstractWe studied the lunar mantle composition based on the characteristics of low‐Ca pyroxene (LCP)‐rich and olivine‐rich rocks exposed on the lunar surface. Using data mining with SELENE (Kaguya) hyperspectral data, we found 531 sites with spectra dominated by an ultramafic LCP end‐member mineral, most of which are located in the Imbrium basin and the South Pole‐Aitken (SPA) basin. Stratigraphic analysis of each site revealed that LCP‐rich rock bodies are exposed on fresh geological features that are less affected by space‐weathering, such as steep slopes at peaks, and crater walls and ejecta deposits at smaller craters. We also found that, in the SPA and Imbrium basins, LCP‐rich bodies are more numerous and more widely distributed than olivine‐rich rocks, suggesting that LCP‐rich materials deep in the mantle were excavated during the formation of these huge basins. However, olivine‐rich rocks were abundant, and no LCP‐rich rocks were found in the Moscoviense, Crisium, and Humboldtianum basins, which are known to have almost‐zero crustal thicknesses, indicative of mantle excavation. Thus, our results suggest that the composition of rocks derived from the lunar mantle varies with the impact basin. Such a difference might indicate a layered structure of mantle composition, with the olivine‐rich upper mantle overlying the LCP‐rich mantle, a horizontal heterogeneity in the mantle composition, a regional heterogeneity in early lunar basaltic magmatism, or an impact melt origin. The layered structure or horizontal heterogeneity might have resulted from a mantle overturn caused by gravitational instability in the early stages of the lunar magma ocean.