A dynamic origin for the global asymmetry of lunar mare basalts

We propose that the hemispheric asymmetry (Fig. 1) of mare basalts may be e xplained as a result of hydrodynamic instabilities associated with a layer of mixed ilmenite-rich cumulates (IC) and olivine-orthopyroxene (OPx) overl ying a metallic core. This mixed layer (MIC) should form shortly after the solidification of the magma ocean (P.C. Hess and E.M. Parmentier, Earth Pla net. Sci. Lett. 134 (1995) 501-514) because of gravitational differentiatio n of chemically dense IC material that is expected to form below the anorth ositic crust in the final stages of magma ocean crystallization (A.E. Ringw ood and S.E. Kesson, Proc. Lunar Planet. Sci. Conf. 7 (1976) 1697-1722). IC material is rich in heat producing elements, and thermal expansion due to radiogenic heating causes the MIC layer to become less dense than overlying mantle. The time required for the MIC layer to become thermally buoyant ma y explain a delay of mare volcanism until about 500 Ma after solidification of the magma ocean. Our analyses of the resulting Rayleigh-Taylor instabil ity and numerical modeling of thermo-chemical convection show that the inst abilities produce spherical harmonic degree I thermal and compositional str ucture if a lunar metallic core is sufficiently small, less than 250 km in radius. (C) 2000 Elsevier Science B.V. All rights reserved.