Vol 69, No 12 (2024)

The problem of the internal structure plays a special role in the geochemistry and geophysics of the Moon. The main sources of information about the chemical composition and physical state of the deep interior are seismic experiments of the Apollo expeditions, gravity data from the GRAIL mission, geochemical and isotopic studies of lunar samples. Despite the high degree of similarity of terrestrial and lunar matter in the isotopic composition of a number of elements, the question of the similarity and/or difference in the composition of the silicate shells of the Earth and its satellite in relation to the main elements remains unresolved. The review article summarizes and critically analyzes information on the composition and structure of the Moon, examines the main contradictions between geochemical and geophysical classes of mantle structure models both within each class and between the classes, related to the estimation of the abundance of the major element oxides Fe, Mg, Si, Al, Ca, and analyzes bulk silicate Moon (BSM) models. The principles of the approach to modeling the internal structure of a planetary body, based on the joint inversion of an integrated set of selenophysical, seismic, and geochemical parameters combined with calculations of phase equilibria and physical properties, are presented. Two new classes of the chemical composition of the Moon, enriched in silica (~50% SiO₂) and ferrous iron (11-13% FeO, Mg# 79–81) in relation to the bulk composition of the silicate component of the Earth (BSE) are discussed — models E with terrestrial concentrations of CaO and Al₂O₃ (Earth-like models) and models M with higher refractory oxide content (Moon-like models), which determine the features of the mineralogical and seismic structure of the lunar interior. The probabilistic distribution of geochemical (oxide concentrations) and geophysical (P-, S-wave velocities and density) parameters in the four-layer lunar mantle within the range of permissible selenotherms was obtained. Systematic differences in the content of rock-forming oxides in the silicate shells of the Earth and the Moon have been revealed. Calculations of the mineral composition, P-, S-wave velocities, and density of the E/M models and two classes of conceptual geochemical models LPUM (Lunar Primitive Upper Mantle) and TWM (Taylor Whole Moon) with Earth’s silica content (~45 wt.% SiO₂) and different FeO and Al₂O₃ contents were carried out. The justification of the SiO₂-FeO-enriched (olivine-pyroxenite) lunar mantle, which has no genetic similarity with Earth’s pyrolitic mantle, is provided as a geochemical consequence of the inversion of geophysical parameters and determined by cosmochemical conditions and the Moon’s formation mechanism. The major mineral of the lunar upper mantle is high-magnesium orthopyroxene with low calcium content rather than olivine, as confirmed by Apollo seismic data and supported by spacecraft analysis of spectral data from a number of impact basin rocks. In contrast, the P- and S-wave velocities of the TWM and LPUM geochemical models, in which olivine is the major mineral of the lunar mantle, do not match the Apollo seismic data. The geochemical constraints in the scenarios for the formation of the Moon are considered. The simultaneous enrichment of the Moon in SiO₂ and FeO relative to pyrolitic mantle of the Earth is incompatible with the formation of the Moon as a result of a giant impact from terrestrial matter or an impact body (bodies) of chondritic composition and becomes the same obstacle in modern scenarios of the formation of the Moon as the similarity in the isotopic compositions of lunar and terrestrial samples. The problem of how to fit these different geochemical factors into the Procrustean bed of cosmogonic models for the Earth-Moon system formation is discussed.

As a result of a study of igneous rocks of the basalt — andesite series, dredged on the Shaka Ridge in the South Atlantic, it was found that they differ from the basalts of mid-ocean ridges and ocean islands, and have an age of 183–186 Ma, corresponding to the time of manifestation of the Karoo-Mod mantle plume in central Gondwana. The input of ice-rafted debris into the study area due to ice transportion is considered unlikely. Geochemical and isotopic features of the studied igneous rocks show their similarity with the Jurassic mafic complexes of the Ferrar province in Antarctica and the Falkland Islands, formed during the intrusion of the Karoo Maud plume and paleo-Pacific subduction. Based on the all data obtained, it was concluded that the Shaka Ridge is a continental block that was moved during the opening of the South Atlantic in the Early Cretaceous-Early Miocene from the continental margin of Africa along an extended transform fault into the present Bouvet triple junction area.