Identification of Potential Mantle Rocks Around the Lunar Imbrium Basin

Abstract

Basin-forming impacts expose material from deep within the interior of the Moon. Given the number of lunar basins, one would expect to find samples of the lunar mantle among those returned by the Apollo or Luna missions or within the lunar meteorite collection. However, only a few candidate mantle samples have been identified. Some remotely detected locations have been postulated to contain mantle-derived material, but none are mineralogically consistent upon study with multiple techniques. To locate potential remnants of the lunar mantle, we searched for early-crystallizing minerals using data from the Moon Mineralogy Mapper (M 3) and the Diviner Lunar Radiometer (Diviner). While the lunar crust is largely composed of plagioclase, the mantle should contain almost none. M 3 spectra were used to identify massifs bearing mafic minerals and Diviner was used to constrain the relative abundance of plagioclase. Of the sites analyzed, only Mons Wolff was found to potentially contain mantle material. Plain Language Summary During the Moon's early development, minerals such as olivine and pyroxene would have crystallized first, sinking toward the lunar interior and becoming the primary components of the mantle. After approximately 70-80% of the magma ocean solidified, plagioclase began to crystallize and floated on the iron-rich residual melt. The lunar highland crust is characterized by an abundance of plagioclase, whereas samples of the mantle should contain very little plagioclase. Considering the size and number of large impact basins on the Moon, one would expect that some of these dug through the lunar crust, exposing lunar mantle. However, very few candidates for mantle material have been identified among the lunar samples on Earth. This study uses near-infrared data from the Moon Mineralogy Mapper to identify sites on the surface that contain early-crystallizing minerals (olivine and pyroxene), which are indicative of mantle material. These sites were then analyzed using data from the Diviner Lunar Radiometer, which is able to constrain the abundance of these minerals relative to the amount of plagioclase present. Based on our analysis, the Imbrium Basin contains only one instance of rocks that are mineralogically consistent with being sourced from the mantle.

Global Distribution and Geological Context of Co‐Existing Occurrences of Olivine‐Rich and Plagioclase‐Rich Materials on the Lunar Surface

“…In any case, it is difficult to determine how the co‐existence and isolated olivine‐rich sites are formed on the lunar surface using only remote‐sensing data. Sample return missions and in‐situ analysis might provide important information for this issue (e.g., Bretzfelder et al., 2020; Li et al., 2019; Tartèse et al., 2019). In this case, the compositional information of bulk rocks, the Mg number of olivine, the mineral phases coexisting with olivine on a microscale and their textures, and the crystallization age may be important.…”

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

“…The role of exposure events has important implications for the interpretation of mineral abundance using remote sensing data. Bretzfelder et al (2020) have reported that many parts of the SI and MA do not possess mantle-derived rocks, as evidenced by their excessively high concentrations of plagioclase mixed with mafic minerals. Nevertheless, we note that their argument is based on an overview of large-scale images spanning tens of kilometers, not on a detailed description of areas where LCP-rich or olivine-rich rocks are exposed, such as the outcrops shown in the present work.…”

“…Bretzfelder et al. (2020) presented average spectral coverage of tens to hundreds of kilometers in the Imbrium basin; however, they did not include a detailed description of the distribution and geological context of each LCP‐rich outcrop.…”

Diverse Geological Evolution of Impact Basins on the Moon

“…For example, Spudis [6] studied the geology and composition of important basin deposits, including the Orientale and Nectaris basins, and systematically summarized the geological characteristics of another 45 lunar impact basins. Thus far, studies of the lunar basins have mostly focused on the structure of individual basins or considered the compositional characteristics of the basins [10][11][12]. In our research, we conducted a comparative analysis of three lunar basins, Dirichlet-Jackson, Nectaris, and Orientale, which were formed at different moments of early lunar history.…”