Is a Consensus Possible between Geochemical and Geophysical Models of the Internal Structure of the Lunar Mantle?

Significant inconsistencies persist between geophysical and geochemical groups of lunar composition models, as well as within each group. The primary issues are related to the assessment of the content of main petrogenic elements (Fe, Mg, Si), including refractory elements (Al, Ca). In this study, we examined the impact of the chemical composition and mineralogy of different compositional models on the seismic and density structure of the lunar interior in combination with phase equilibrium calculations. Two groups of new geophysical models containing 51–52 wt % SiO2 are considered—conventionally Earth-like models with terrestrial Al2O3 content and Moon-like models with increased Al2O3 content, obtained on the basis of seismic and selenodesic constraints using a Markov chain Monte Carlo method, as well as the most popular geochemical models TWM (Taylor, 1982) and LPUM (Longhi, 2006) with ∼45–46 wt % SiO2. For these geophysical and geochemical models, we explicitly calculated sound velocities and densities of mantle-phase assemblages for a range of proposed selenotherms and compared the results with high-quality Apollo seismic data (Garcia et al., 2019). Regardless of the abundance of refractory elements, we find very satisfactory agreement between the physical properties of our new geophysical models and the Apollo seismic data, confirming the enrichment of the lunar mantle in FeO (∼12 wt %) and SiO2 (∼50 wt %) at depths from 50 to 500 km. This enrichment of SiO2 corresponds to the predominance of pyroxenes, especially low-Ca orthopyroxene, over olivine. In contrast, sound velocities in silica-unsaturated, olivine-dominated compositions enriched (TWM) or depleted (LPUM) in FeO and Al2O3 are inconsistent with the Apollo lunar seismic data. The inferred compositions contain significantly more FeO and SiO2 than the bulk silicate Earth, which poses problems associated with the formation of the Moon from a pyrolite mantle during a giant impact scenario.