¹Kim A.Cone,^1,2Richard M.Palin,¹Kamini Singh
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113787]
¹Geology and Geological Engineering Department, Colorado School of Mines, 1516 Illinois St., Golden, CO 80401, USA
²Department of Earth Sciences, University of Oxford, 3 South Parks Road, Oxford OX1 3AN, UK
Copyright Elsevier

Diversity of lunar basalt characteristics is partly a consequence of lunar mantle heterogeneity. Although the cumulate mantle overturn hypothesis is the current standard model invoked to explain mantle asymmetries of unknown length scale in both compositional and geometrical space, successful petrological modeling of this mixing event requires a specific set of parameters not currently agreed upon. In contrast, surface basalt patterns may yield clues to both localized and nearside lunar interior structure.

Using two multidimensional data analysis approaches – principal component analysis (PCA) and K-means cluster analysis (KCA) – we report the patterns produced from basalt characteristics over changing spatial scales, from intra-site to inter-site to nearside. The data are sourced from a newly developed, self-contained database of lunar basalt characteristics (ApolloBasaltDB), which includes major element oxides, mineral modes, ages, and textures for petrological and statistical modeling. Through the simultaneous considerations of multiple basalt characteristics contained in the database, we find that terrestrial-based basalt classifications cannot adequately describe the complex and overlapping distribution patterns of major element oxides and mineral modes that define multiple distinct basalt groupings over multidimensional space. These patterns provide opportunities for alternative lunar basalt classification schemes. Our analyses suggest that Al2O3 volumetric content is more diverse inside the Procellarum KREEP Terrane rift boundary versus content for older Apollo samples in close proximity to the eastern arm of the same rift boundary. Northernmost basalt samples show increased pyroxene diversity. Easternmost sites suggest anti-correlations in modal ilmenite and plagioclase, based on major element oxide PCA biplots, while nearside analyses of either major element oxides or mineral modes similarly suggest plagioclase (and Al2O3) diversity comes at the expense of ilmenite (and TiO2) diversity. There is evidence to suggest that approximate mineral content can be extracted from major element oxide data based on correlative patterns between major element oxide PCA biplots and mineral mode PCA biplots. These patterns have implications for remote sensing missions in that onboard data manipulation may provide lithologic basalt vectors of interest.

¹Paul H.Warren,¹Alan E.Rubin
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113771]
¹Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA 90095-1567, USA
Copyright Elsevier

The 14321 lunar granite (14321g) has recently been reinterpreted (Bellucci et al., 2019) as a piece of the Hadean Earth, impact-transported to the Moon. In principle, samples of such derivation may afford important insights into the nature of Earth’s Hadean crust. We have tested the terrestrial provenance hypothesis by comparing trace-element data from 14321g versus other lunar evolved rocks and a large data base for terrestrial granites. Volatile trace metals Zn, Ga and Ge are depleted in 14321g below the terrestrial granite ranges; and relative to the terrestrial granite averages by factors of 27, 2.0 and 18, respectively. Evidence from shocked chondrites, Martian meteorites, and impact-shock studies in general, indicates that such major depletions are unlikely to develop without near-complete shock-melting, which clearly did not occur in the mostly still-crystalline 14321g. Moreover, other aspects of compositional disparity between 14321g and terrestrial granites involve exclusively refractory trace elements. Compared to terrestrial granites of similarly high Ba content, 14321 is enriched in Ta by a factor of 10; and the few terrestrial granites that are as Ta-rich as 14321 have 10 times lower Ba. The refractory-element ratio Lu/Sm is also close to 10 times higher in 14321g than in terrestrial granites of similarly high Ba content. Other highly evolved lunar rocks, “felsites”, strongly resemble 14321g in all these respects. We conclude that 14321g is probably of wholly lunar derivation. This finding stands in contradiction to a recent inference from Ti-in-quartz modeling (Bellucci et al., 2019) that 14321g crystallized at a pressure of 0.69 GPa. The geodynamically limited Moon was presumably never capable of forming, or burying, such a highly granitic material ~100 km below the base of its crust, nor of excavating material from such a depth to the surface.