ON THE ROLE OF LARGE-SCALE MELTING, MELT EXTRACTION AND MANTLE OVERTURN IN THE EVOLUTION OF PLANETS

Large-scale melting and melt extraction appear to play a fundamental role in planetary evolution through compositional effects on buoyancy, distributions of heat producing elements, and solid-state creep rheology. Models based solely on solid-state convection driven by thermal buoyancy and with temperature-dependent thermally-activated creep rheologies are relatively well understood and are frequently proposed as a paradigm for planetary evolution. However, chemical fractionations that develop in magma oceans (MO) during the very rapid and earliest stages of planetary evolution can have fundamental consequences on geological time scales. Global and local MOs may be a consequence, respectively, of the conversion of accretionary energy to heat beneath a global insulating atmosphere and giant impacts that are believed to dominate final accretion. During the fractional solidification of a MO, residual liquids and the solids crystallizing from them become progressively richer in Fe relative to Mg; residual liquids retained interstitially as solids form become progressively richer in incompatible elements and water. The consequence is progressively denser silicate mantle enriched in heat producing elements. Rapid solidification results in unstable density stratification which can overturn on Myr-timescales to produce a stably stratified mantle with some fraction of heat producing elements concentrated at depth. Global scale overturn and the evolution of a stably stratified mantle may explain the inferred depth of melting and global asymmetry in surface distribution of mare basalts on the Moon, as well as the history of an internally generated magnetic field. The early, strong magnetic field on Mars may be explained as a consequence of overturn which places relatively cool mantle solidified near the surface the MO adjacent to the core. The stable stratification then present prevents solid-state thermal convection and continuing heatflux out of the core. Decompression melting during overturn may produce an early crust. Lateral heterogeneity that is a consequence of overturn may account for the long-lived Tharsis volcanic province, normally attributed to a single large thermally-driven deep mantle plume. Core cooling by such a plume may be problematic given the absence of a longterm magnetic field.