Geochemical constraints on the seismic properties of the lunar mantle

Thermodynamic modeling of phase relations and physical properties in the and velocity distribution in the mantle, and core radii of the Moon from the mass and moment of inertia constraints from recent Lunar Prospector gravity measurements. For the computation of the phase diagram for a given chemical composition, we have used a method of minimization of the total Gibbs free energy and self-consistent thermodynamic data for minerals and solid solutions. For the crustal thickness and density of 60 km and 3.0 g cm^-3, the largest core radii are estimated to be 530 km for an FeS (troilite)-core (4.1% of total mass), 490 km for a eutectic Fe-FeS-core (3.4%), 445 km for an Fe-10 wt.% S-core (2.9%), and 350 km for an Fe-core (2%). The existence of an Fe-FeS-core of intermediate radius of 25-400 km is compatible with recent lunar core information and lend support to a chemically stratified lunar mantle. The sensitivity of density and seismic velocities to temperature, pressure and composition variations is examined. The modeling results indicate that we can determine the permissible density and velocity ranges in the zoned lunar mantle with confidence. The maximum and minimum density changes at depths of the lunar mantle, consistent with geochemical constraints and mass and moment of inertia requirements, are found to be: 3.22 g cm^-3 ≤ p ≤ 3.44 g cm ^-3 at depths of 60-500 km and 3.34 g cm^-3 ≤ p ≤ 3.52 g cn ^-3 at depths between 500 km and the core mantle boundary. Core-free models of the Moon are possible only if the lower mantle densities are between 3.49 and 3.52 g cm^-3 (20 wt.% < FeO << 30 wt.%). The calculated field seismic velocities in the zoned lunar mantle shows that over a broad range of mantle compositions, temperatures, and densities the maximum variations are as follows: 4.23 km s^-1 ≤ Vs ≤ 4.69 km s^-1 and 7.41 km s^-1 ≤ Vp ≤ 8.24 km s^-1 at depths of 60-500 km, and 4.24 km s^-1 ≤ Vs ≤ 4.64 km s^-1 and 7.63 km s^-1 ≤ Vp ≤ 8.39 km s^-1 at depths between 500 km and the core mantle boundary. These petrologically and geophysically admissible ranges of velocities and densities may constitute a significant constraint for the analysis of internal structure of the Moon using the Apollo seismic data.