Thermo-chemical constraints on the interior structure and composition of the lunar mantle

Abstract

Based on a self-consistent thermodynamic-geophysical approach, we convert the recent seismic models of the lunar mantle to the temperature-depth profiles using Gibbs free energy minimization and petrological constraints in the Na2O–TiO2–CaO–FeO–MgO–Al2O3–SiO2 system. Our calculations are unable to explain the reasonable distribution of temperature for a single homogeneous composition throughout the entire lunar mantle with concentrations of CaO and Al2O3 in the range of 2–6.5%, and FeO content between 8.5% and 13%. The results lend support to the chemically stratified lunar mantle with a change in composition from predominantly pyroxenite upper mantle depleted in Ca and Al to predominantly fertile lower mantle enriched in Ca and Al with larger amounts of garnet. Such a zoned structure places significant constraints on any theory of lunar origin. Unlike the Earth’s mantle, compositional effects play a dominant role in determining the lunar mantle temperatures of the same observational model. Seismically derived temperatures allow us to constrain thermal structure of the lunar mantle and estimate the upper mantle heat flow (3.8–4.7mWm−2), which is not consistent with that found from the Apollo heat flow and thorium abundance measurements. Lower mantle temperatures are well below the probable solidus condition and can be evaluated at the level of 1420–1550°C at the core-mantle boundary without requiring a melt layer. We find that regardless of the composition, the positive S-wave velocity gradient in the lunar mantle leads to a negative temperature gradient, which has no physical basis. The resulting temperature profiles provide an effective independent tool that allows us to discriminate between the available seismic and petrological models.