Effect of thermal state on the chemical composition of the mantle and the sizes of the moon’s core

Abstract

Based on the joint inversion of seismic and gravity data in combination with the Gibbs free energy minimization method for calculating phase equilibria in the framework of the Na2O-TiO2-CaO-FeO-MgO-Al2O3-SiO2 system, the influence of the thermal state on the chemical composition models of the mantle and the sizes of the Fe-S core of the Moon has been studied. The boundary conditions used are seismic models from Apollo experiments, mass and moment of inertia from the GRAIL mission. As a result of solving the inverse problem, constraints on the chemical composition (concentration of rock-forming oxides) and the mineralogy of a three-layer mantle are obtained. It is shown that regardless of the temperature distribution, the FeO content of 11–14 wt.% and magnesian number MG# 80–83 are approximately the same in the upper, middle and lower mantle of the Moon, but differ sharply from that for the bulk composition of the silicate Earth (Bulk Silicate Earth = BSE, FeO ~8 wt% and MG# 89). On the contrary, estimates of the Al2O3 content in the mantle rather noticeably depend on the temperature distribution. For the considered scenarios of the thermal state with a difference in temperature of 100–200°C at different depths, Al2O3 concentrations increase from 1–5% in the upper and middle mantles to 4–7 wt.% in the lower mantle with garnet amounts up to 20 wt.%. For the “cold” models, the bulk abundance of aluminum oxide in the Moonis Al2O3 ~1–1.2 × BSE, and for the “hot” models it can be in the range of 1.3–1.7 × BSE. Concentrations of SiO2 to a lesser extent depend on the temperature distribution and constitute 50–55% in the upper and 45–50 wt.% in the lower mantle; orthopyroxene, rather than olivine, is the dominant mineral of the upper mantle. Based on the modeling of the density of Fe-S melts at high Р-Т parameters, the sizes of the lunar core are estimated. The Fe-S core radii with an average density of 7.1 g/cm3 and a sulfur content of 3.5–6 wt.% are in the range of 50–350 km with a most likely value of about 300 km and rather weakly depend on the thermal regime of the Moon. The simulation results suggest that a lunar mantle is stratified by chemical composition and indicate significant differences in the compositions of the Earth and its satellite.