Thermo-chemical constraints on the lunar bulk composition and the structure of a three-layer mantle

The main purpose of the present study is to evaluate the lunar bulk composition (BSM) and the composition of a three-layer mantle based on a joint inversion of lunar mass and moment of inertia, and the mantle seismic velocity profiles in combination with Gibbs free energy minimization. As an integral characteristic of the thermal state, the mean volume temperature T-mean of the lunar mantle controlling the mineral composition of the Moon was chosen. In terms of T-mean, all thermal models can be conditionally divided into cold ones with T-mean similar to 690-860 degrees C and hot ones with T-mean similar to 925-1075 degrees C. We find that irrespective of the thermal state, BSM is characterized by practically constant values of bulk FeO similar to 12-13 wt% and Mg# 80-81.5, which indicates a significant difference in the composition of the silicate Earth (BSE) and its satellite. The FeO content of 11-14 wt% and Mg# 80-83 are approximately identical in the upper and lower mantle. The bulk SiO2 abundance depends little on the thermal state and ranges from 45 to 52 wt%, with probable concentrations of SiO2 being 50-55% in the upper mantle and 45-50 wt% in the lower mantle; orthopyroxene, and not olivine, is the predominant mineral of the upper mantle. On the contrary, the lunar abundance of alumina, depending on the thermal state, falls into two different groups. Cold models of BSM with a content of 3-4.6 wt% Al2O3 are comparable to those of BSE, whereas for hot models the bulk-alumina content can be in the range of 1.2-1.7 x BSE. The results indicate a tendency for a gradual increase in the alumina content with depth: from 1 to 2% in the upper mantle to 4-7 wt% Al2O3 in the lower mantle with an increasing amount of garnet. The composition of the middle mantle remains controversial, since it may partially overlap with the compositions of the overlying and underlying shells. Our simulation results show that a chemically stratified lunar interior is consistent with the observations and has similar concentrations of FeO and MgO but different concentrations of Al2O3 and SiO2 in different zones of the mantle. However, the question of the similarity and/or difference in the composition of the silicate Earth and its satellite with respect to the abundance of refractory elements, which is of fundamental importance for lunar geochemistry, remains unresolved and requires further research.