COMPOSITION OF THE UPPER-MANTLE OF THE MO ON

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 and geophysical observations including the seismic data, moment of inertia and mass of the Moon. Calculations of seismic velocity and density pr ofiles as well as phase equilibrium calculations in the CaO-FeO-MgO-Al 2O3-SiO2 system have been undertaken to determine whether or not some of the bulk compositions and equilibrium assemblages match geophysical observations. It has been shown that only a relatively small number o f compositional models satisfy the seismic velocity values at depths o f the upper mantle as well as the moment of inertia constraints. These models show that phase changes from olivine-spinel-pyroxenite field t o olivine-garnet pyroxenite field or from plagioclase-spinel-pyroxenit e field to quartz-pyroxenite (+-garnet) field are not able to explain the nature of the 270 km discontinuity. The calculated velocity profil es at depths of the upper mantle agree with seismic profiles within th e limits of uncertainties in both thermodynamic and seismic data. This is a reason to believe that the entire upper mantle depth interval 58 -270 km is chemically uniform and may be composed of pyroxenite contai ning 0,5-5 mol.% of free silica. Compositional models of the zoned Moo n were used to estimate bulk composition of the silicate portion of th e Moon. The calculated lunar bulk silicate compositon (mantle + crust) as well as the bulk compositon of the entire lunar mantle generated b y the geophysical data show that the concentrations of FeO, SiO2 and r efractory elements (Ca, Al) are significantly higher than those in the Earth's upper mantle; the-bulk Moon is enriched in refractories by a factor of 1,5-2,5 compared with the terrestrial mantle and CI meteorit es. The Fe/Si atomic ratio is equal to 0,18 for the silicate portion o f the Moon (crust + mantle) and 0,22 for the Moon as a whole (crust mantle + core). Without specifying a mechanism of the origin of the Mo on, we may conclude that the Earth and its satellite formed from compo sitionally different materials. Composition of the Moon remains unusua l in comparison with the Earth, chondrites and achondrites.