CONSTITUTION OF THE MOON .2. COMPOSITION AND SEISMIC PROPERTIES OF THE LOWER MANTLE

To constrain the composition of the Moon we have calculated the stable phase assemblages and their seismic properties by the method of minim ization of Gibbs free energy, to determine whether various composition al models are consistent with the seismic observations for the lunar l ower mantle at depths between 500 and 1000 km. P-Wave and S-wave veloc ities have been calculated for a series of proposed bulk composition m odels. Compositional models for the Moon (impactor composition, Orguei l composition, etc.) and Earth-like compositions (lherzolite and pyrol ite) have been considered. It is found that impactor and Orgueil model s are unlikely for the lower mantle. Within reasonable temperature adj ustment in the temperature profile, all of the terrestrial composition al models come close to the seismic data below the 500 km discontinuit y. It is found that a composition enriched in Al2O3 (up to about 6 wt. %) and having pyrolite content of the other major oxides ('lunar pyrol ite') is adequate to describe the seismic properties of the lunar lowe r mantle. Within the uncertainty of calculations and resolution of, se ismic data, an Al-rich model with a terrestrial Ol/Px ratio and unusua lly enriched in garnet gives the best fit to the lunar lower-mantle se ismic properties. Despite the uncertainty of the seismic data it is sh own that none of the analysed compositional models satisfies the seism ic constraints simultaneously in the middle and lower mantle. Phase ch anges in any model considered are not able to explain the nature of th e 500 km discontinuity. It is concluded that the lunar mantle is chemi cally stratified.