CONSTITUTION OF THE MOON .3. COMPOSITION OF MIDDLE MANTLE FROM SEISMIC DATA

The constitution of the lunar middle mantle, which is located between two prominent discontinuities at depths of 270 and 500 km, has been in vestigated by a method of thermodynamic modeling. On the basis of the THERMO-SEISM data base, self-consistent phase equilibrium assemblages and synthetic seismic profiles have been calculated for various geoche mical models with different contents of the major petrogenic oxides. I t is shown that none of the analyzed compositional models satisfies th e seismic velocity profiles simultaneously in the middle and lower man tle. Phase changes in any petrological model considered do not explain the nature of the 500 km discontinuity. Pyroxenite models having 50-5 2% SiO2 and 15-16% FeO (mg-number 74-76) have the best fit to the midd le-mantle mean velocities of Nakamura (J. Geophys. Res., 88: 677-686, 1983) and satisfy the mean density and lower-mantle density constraint s. These results and the calculations of Kuskov and Fabrichnaya (Phys. Earth Planet. Inter., 83: 197-216, 1994) strongly reinforce the case for a chemically stratified Moon interior.