Geochemical Constraints on the Cold and Hot Models of the Moon’s Interior: 1–Bulk Composition

Resumo

The variations of the bulk composition of the silicate Moon (crust + mantle = Bulk Silicate Moon, BSM) depending on the thermal state are explored based on the joint inversion of gravitational, seismic, and petrologic data within the Na₂O–TiO₂–CaO–FeO–MgO–Al₂O₃–SiO₂ system. The mantle bulk temperature T_mean determining the mineral composition and physical properties of the Moon is adopted as the integral characteristic of thermal state. By parameter T_mean, all thermal models of the Moon can be conventionally broken down into the “cold” with T_mean ~ 690–860°C and the “hot” with T_mean ~ 925–1075°C. The estimations of refractory oxide abundance in lunar rocks depending on the thermal state are included in two different groups. Cold models of BSM are comparable by the bulk content of Al₂O₃ ~ 3.0–4.6 wt % to those for the silicate Earth (Bulk Silicate Earth, BSE), while hot models of BSM are significantly enriched with Al₂O₃ ~ 5.1–7.3 wt % (Al₂O₃ ~ 1.2–1.7 × BSE) as compared with BSE. On the contrary, independent of the temperature distribution, both types of BSM models are characterized by nearly constant values of bulk concentrations of FeO ~ 12–13 wt % and magnesian number MG# 80–81.5 (MG# = [MgO/(MgO + FeO) × 100]), which differ markedly from those for BSE (FeO ~ 8% and MG# 89). It means that for all possible temperature distributions, the silicate fraction of the Moon is FeO-enriched and MgO-depleted in relation to BSE. These arguments discard the possibility of the Moon’s formation out of the material of the Earth’s primitive mantle. In spite of the almost complete coincidence of the isotopic systems, this apparently undeniable fact has no adequate explanation in the existing canonical models of the Moon’s origin and should result in additional constraints on the dynamic processes in models of the formation of the Earth–Moon system. However, the problem of the similarity of and/or difference between compositions of the Moon and the Earth regarding the abundance of refractory elements, which is very important for the geochemistry of the Moon and the Earth’s mantle, remains unresolved and requires further study.