Geochemical Constraints on the Cold and Hot Models of the Moon’s Interior: 2—Three-Layer Mantle

Abstract

Internally consistent models of the thermal state, chemical composition and mineralogy of the three-layer mantle of the Moon are constructed based on the joint inversion of gravity, seismic and petrological-geochemical data within the Na2O-TiO2-CaO-FeO-MgO-Al2O3-SiO2 system. Geochemical constraints on the chemical composition and physical properties in three zones of the mantle are obtained in terms of the cold and hot models. Velocities of P-waves in the lower mantle (∼8 km/s) are higher than in the upper mantle (∼7.7 km/s). The behavior of velocities of S-waves is conservative, they are observed in the interval 4.40–4.45 km/s in all zones of the mantle. It was found that, independently of the temperature distribution, the most probable concentrations of FeO, ∼11–14 wt % and MgO, 28–31 wt % and the values of the magnesian number MG# 80–83 are approximately the same in the upper and the lower mantles of the Moon, but drastically differ from those in the bulk composition of the silicate Earth (Bulk Silicate Earth, BSE, FeO 8%, MG# 89). On the contrary, the estimates of Al2O3 concentration in the three-layer mantle noticeably depend on the thermal state. The results of solution of the inverse problem indicate the trend towards the gradual increase in the Al2O3 content with depth, from the upper to the lower mantle to 4–7% with the higher content of garnet. For the cold models of the lower mantle of the Moon, the bulk content of Al2O3 is ∼1 × BSE, and for the hot models it can be in the interval of 1.3 × BSE-1.7 × BSE. The abundance of SiO2 depends, to a lesser degree, on the thermal state and is 50–55% in the upper and 45–50% in the lower mantle. The high pyroxene content of the upper mantle of the Moon is the geochemical consequence of the geophysical models used with the inversion into composition and temperature relations; orthopyroxene, instead of olivine, is the dominant mineral of the upper mantle. Concentrations of SiO2 in the lower (undifferentiated) mantle showing the bulk composition of the silicate Moon (Bulk Silicate Moon, BSM), are consistent with the geochemical estimates of 45–48% of SiO2 for the BSM and close to those for the Earth’s mantle (45–47%). The composition of the mantle middle zone remains discussible, since it might be partially overlapped with compositions of the over- and underlying envelopes. The results of the model suggest that the mantle of the Moon is stratified in chemical composition. For the considered thermal state models, the mantle of the Moon is enriched in FeO and depleted in MgO in relation to the primitive Earth mantle, which indicates considerable differences between compositions of the Earth and its satellite.