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

Abstract

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.

Spectral Analysis of Mare Ingenii Basin (Lunar Farside)

“…The SPA basin is heterogeneous in composition, showing anorthositic to noritic and gabbro‐noritic compositions (Lemelin et al., 2019; Moriarty & Pieters, 2018; Tompkins & Pieters, 1999; X. Zhang et al., 2019). Indeed, SELENE/Kaguya Multiband Imager (MI) derived maps of mineral abundances revealing that the SPA spectral signature is clearly characterized by a mafic component (Jolliff et al., 2000; Lemelin et al., 2019; Yamamoto et al., 2023), chiefly ascribed to orthopyroxene occurrence compared to the neighboring areas (Moriarty & Pieters, 2018). Actually, four distinct compositional zones have been broadly identified by Moriarty and Pieters (2018) across the SPA basin.…”

“…It might have rather revealed portions of the deeper lunar crust, since pure anorthositic compositions occur near the surface whereas more mafic compositions (e.g., norites, gabbros and troctolites) likely dominate the lower crust (i.e., around 5–10 km depth; Pieters, 1986; Pieters et al., 1997; Tompkins & Pieters, 1999). Indeed, they are not found in the central area of the SPA basin but rather as scattered points located on the outer rim, nor in the crater floors but along the peak rings (Yamamoto et al., 2012, 2023). This could validate the hypothesis for a certain local differentiation occurring in the melt layers during the SPA basin impact event, as suggested by Yamamoto et al.…”

“…Indeed, the Ingenii basin is located within the outer border of the SPA basin, the deepest and the oldest impact basin in our Solar System, which likely exposed the lower lunar crust. M 3 data suggest a strong contribution of the mafic component in the basin, with dominant pyroxene minerals (and local olivine‐rich exposures; Yamamoto et al., 2012, 2023), likely representing the deeper section of the lunar crust dominated by gabbros, norites, and troctolites (Anbazhagan et al., 2021; Jolliff et al., 2000). Several hypotheses have been proposed to account for this mafic exposures, such as a huge collision that penetrated through the Moon's crust (Pieters et al., 1997; Yamamoto et al., 2012) or excavation of the mantle (Lemelin et al., 2019; Lucey et al., 1998; Melosh et al., 2014; Miljkovic et al., 2013; Nakamura et al., 2012; Petro & Jolliff, 2013; Potter et al., 2012; Yamamoto et al., 2010).…”

“…However, such exposures of mafic minerals, especially of low-Ca and Ca-free pyroxenes, have been often interpreted as excavation of the lunar mantle (Lemelin et al, 2019;Lucey et al, 1998;Melosh et al, 2014;Miljkovic et al, 2013;Nakamura et al, 2012;Petro & Jolliff, 2013;Potter et al, 2012;Yamamoto et al, 2010). This hypothesis has been also suggested in order to account for olivine-rich exposures found in the central peaks of some regions (e.g., Schrödinger basin and Zeeman crater) or pyroxene-rich materials in some fresh craters (Yamamoto et al, 2012(Yamamoto et al, , 2023). Yet notably, by observing the Kaguya MI maps, the SPA basin is still dominated by a plagioclase-rich mineralogy (∼55-65 wt%), even if at a lesser extent compared to the overall highlands distributions (∼75-85 wt%).…”