A dynamic origin for the global asymmetry of lunar mare basalts

We propose that the hemispheric asymmetry (Fig. 1) of mare basalts may be explained as a result of hydrodynamic instabilities associated with a layer of mixed ilmenite-rich cumulates (IC) and olivine¯orthopyroxene (OPx) overlying 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 Planet. Sci. Lett. 134 (1995) 501¯514) because of gravitational differentiation of chemically dense IC material that is expected to form below the anorthositic crust in the final stages of magma ocean crystallization (A.E. Ringwood 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 may explain a delay of mare volcanism until about 500 Ma after solidification of the magma ocean. Our analyses of the resulting Rayleigh¯Taylor instability and numerical modeling of thermo-chemical convection show that the instabilities produce spherical harmonic degree 1 thermal and compositional structure if a lunar metallic core is sufficiently small, less than 250 km in radius.