春山 純一

We 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.