Towards geochemical alternatives to geophysical models of the internal structure of the lunar mantle and core

Abstract

Based on the Markov Chain Monte Carlo method, we present a reconciliation analysis of the three most important but not directly related sets of seismic, selenodetic, and geochemical data (Apollo seismic travel times, mean mass (M) and moment of inertia (MOI), tidal Love number (k2), quality factors, bulk silicate Moon (BSM) models) to determine the composition and internal structure of the four-layer mantle, deep-seated transition layer (LVZ), liquid outer and solid inner Fe-based core. A distinctive feature of this approach is the inclusion of geochemical parameters as “observed” values when calculating the likelihood function in combination with phase-equilibrium computations. We consider the effect of two bulk composition models with different refractory oxide contents (models E with terrestrial values of Al2O3 and models M with a higher Al2O3 content) and variations in temperature profiles on the geochemical and geophysical parameters of the mantle, LVZ, and core. The inversion results show that all successful E and M models are grouped around bulk FeO 11–13 wt% and Mg# 79–81, indicating a significant difference in the composition of the silicate Earth (BSE) and its satellite. A slight SiO2 enrichment is necessary for the pyroxenite upper mantle, where low-Ca orthopyroxene, not olivine, is the dominant mineral. The primordial lower mantle (=BSM) is enriched in Al compared to the overlying differentiated layers: ∼4.5% Al2O3 (1 × BSE) for models E and ∼6 wt% Al2O3 (1.5 × BSE) for models M. Comparison of measured high-pressure density and sound velocity data for liquid Fe(Ni)-C-S-Si alloys with inverted models constrains the relatively narrow range of physical parameters for the lunar core and favors the presence of Fe(Ni)-S liquid outer core with 3–10 wt% S. The results correspond to inverted P-wave velocities in the range of 3600–4100 m/s and densities in the range of 6200–7000 kg/m3 for the outer core with a radius of 300–350 km and are consistent with the speed of sound (but not density) and the radius of the lunar outer core by Weber et al. (2011).