Thermochemical constraints on the thermal state, composition, and mineralogy of the upper mantle of the Moon: Evidence from the seismic models

The thermal state, heat flow, and thermochemical evolution of the Moon are still debatable, and the temperature of lunar interiors is one the most uncertain physical parameters. Transformation of profiles of the velocities of the P and S seismic waves in the lunar mantle obtained by processing the Apollo lunar seismic data into the temperature-depth relationships was performed by the method of thermodynamic modeling in the Na 2 O-TiO 2 -CaO-FeO-MgO-Al 2 O 3 -SiO 2 system. This was the basis for testing of four seismic models in relation to the thermal regime and chemical composition of the mantle in a wide range of the concentrations of CaO (2–5%), Al 2 O 3 (2–6.5%), and FeO (8.5–13%). In contrast to the Earth’s mantle, the chemical composition is of key importance for conversion of the velocities in the same seismic model into the temperature effects. The most probable composition of the upper mantle corresponds to olivine-bearing pyroxenite depleted in refrectory oxides (∼2 wt % CaO and Al 2 O 3 ). Based on the seismic models, constraints on the temperature distribution in the mantle, heat flow, and uranium concentration in the Moon were established. Estimation of the upper limits of the total heat flow resulted in approximately half compared with the Apollo measurements. The results of conversion of the velocities of the P and S seismic waves into the temperature-depth relationships show that, independently on the composition, the positive gradient in the velocities of the P and S waves results in the negative temperature gradient in the mantle, which does not have a physical basis. The velocities of P and S waves should be almost constant or decrease slightly (especially V S ) as a result of the influence of the temperature increasing more rapidly than the pressure for an adequate distribution of temperature in the lunar mantle. The suggested approach to testing of the velocity structure of the lunar mantle based on the methods of thermodynamics and mineral physics provides an independent instrument for estimation of the reliability of the studied seismic model and its consistence with the petrological and thermal models. The main result of this study is self-consistent information on the distribution of the velocities of the P and S waves-temperature ( T P,S )-chemical and mineral composition-density-depth, which provides more reliable constraints on the internal structure of the Moon.