CONSTITUTION OF THE MOON 5 - CONSTRAINTS ON COMPOSITION, DENSITY, TEMPERATURE, AND RADIUS OF A CORE

Thermodynamic modeling of phase relations and physical properties of m ulticomponent mineral systems was used to develop a method for solving the inverse problem of determining the bulk composition, density and temperature distribution in the lunar mantle, and radius of a core fro m the totality of geophysical evidence (seismic velocities, moment of inertia, and mass of the Moon). A derived self-consistent model of the chemical composition and the internal structure of the Moon may be co nsidered as a preliminary geochemical-geophysical reference model and may be used as a significant constraint for the constitution of the Mo on. The geophysically admissible ranges of the chemical compositions i n various zones of the lunar mantle were determined within the system CaO-FeO-MgO-Al2O3-SiO2. It has been found that the isochemical models fail to explain the topology of the seismic structure; the lunar mantl e is chemically stratified. Compositional models of the zoned Moon are used to estimate the bulk composition of the silicate portion of the Moon (mantle + crust), wt.%: 4 < CaO < 5%. 5.5 < Al2O3 < 7%, 10 < FeO < 12%, 28 < MgO < 30%, 48 < SiO2 < 50%. The chemical composition of th e Moon bears no genetic relationship to the terrestrial material as we ll as to any of the known chondrites. It is suggested that the mean se ismic velocities by Nakamura [Nakamura, Y., 1983. Seismic velocity str ucture of the lunar mantle. J. Geophys, Res., 93: 6338-6352.] are too small in the middle mantle (by about 1-2%) and almost certainly too la rge in the lower mantle (by about 2-3%). A probable law of temperature distribution in the lunar mantle is obtained. Models of the internal structure of the Moon without density inversion are constructed, and t he sizes of a core are determined: 500-590 km in radius for a FeS-core and 320-390 km for a Fe-core. A comparison of an allowed ranges of Fe -core radii for the Moon and Jupiter's satellites has been carried out . (C) 1998 Elsevier Science B.V. All rights reserved.