CONSTITUTION OF THE MOON .4. COMPOSITION OF THE MANTLE FROM SEISMIC DATA

The main goal of this paper is to estimate the chemical composition of the lunar upper mantle as well as the bulk composition of the silicat e portion of the Moon based on a method of thermodynamic modeling (pha se equilibrium calculations in the CaO-FeO-MgO-Al2O3-SiO2 system) and geophysical observations including the seismic data, moment of inertia and mass of the Moon. It has been found that the upper mantle is chem ically uniform and may be composed of pyroxenite containing 0.5-2 mol. % of free silica. The calculated lunar bulk silicate composition (mant le + crust) as well as the bulk composition of the entire lunar mantle generated by the geophysical data suggest that the concentrations of FeO, SiO2 and refractory elements (Ca, Al) are significantly higher th an those in the Earth's upper mantle. The Fe/Si atomic ratio is equal to 0.18 for the silicate portion of the Moon and 0.22 for the Moon as a whole (crust + mantle + core); the latter value is the lowest known Fe/Si ratio of any object in the solar system. Phase changes in any mo del considered in the dry CFMAS system are not able to explain the nat ure of the 270km and 500km discontinuities; it is concluded that the l unar mantle is chemically stratified. Thermodynamic modeling and geoph ysical observations allow some constraints on the internal density dis tribution and suggest the presence of a lunar core: 480 km in radius f or the FeS-core and 310km for the Fe-core. As compared with the Moon, an allowed range of the Fe-core and FeS-core radius for Io is estimate d to be 300-680 km and 450-1050 km, respectively. Without specifying a mechanism for the origin of the Moon, we may conclude that the Earth and its satellite formed from compositionally different materials. Com position of the Moon remains unusual in comparison with the Earth and chondrites. (C) 1997 Elsevier Science B.V.