Chemical Composition, Mineralogy, and Physical Properties of the Moon’s Mantle: A Review

Abstract

The problem of the internal structure of the Moon plays a special role in understanding its geochemistry and geophysics. The principal 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, and geochemical and isotopic studies of lunar samples. Despite the high degree of similarity of terrestrial and lunar matter in the isotopic composition of several elements, the problem of the similarity and/or difference in the major-component composition of the silicate shells of the Earth and its satellite remains unresolved. This review paper summarizes and critically analyzes information on the composition and structure of the Moon, examines the main contradictions between geochemical and geophysical classes models for the mantle structure, both within each class and between the classes, related to the estimation of the abundance of Fe, Mg, Si, Al, and Ca oxides, and analyzes bulk silicate Moon (BSM) models. The paper describes 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. Two new classes of the chemical composition of the Moon enriched in silica (∼50% SiO₂) and ferrous iron (11–13% FeO, Mg# 79–81) relative to the bulk composition of the silicate component of the Earth (BSE) are discussed: (i) models E with terrestrial concentrations of CaO and Al₂O₃ (Earth-like models) and (ii) models M with higher contents of refractory oxides (Moon-like models), which determine the features of the mineralogical and seismic structure of the lunar interior. A 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 are revealed between contents of major oxides in the silicate shells of the Earth and the Moon. Calculations were carried out for 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. Arguments are presented in support of the SiO₂- and FeO-enriched (olivine pyroxenite) lunar mantle, which has no genetic similarity with Earth’s pyrolitic mantle, as a geochemical consequence of the inversion of geophysical parameters and determined by cosmochemical conditions and the mechanism that formed the Moon. The dominant mineral of the lunar upper mantle is high-magnesium orthopyroxene with a 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 dominant mineral of the lunar mantle, do not match Apollo seismic data. The geochemical constraints in the scenarios for the formation of the Moon are considered. The simultaneous enrichment of the Moon in both SiO₂ and FeO relative to the 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 is in conflict with modern scenarios of the formation of the Moon and with similarities 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.