Terrestrial planetary dynamics: a view from U, Th geochemistry

Abstract

The migration of U and Th inside a planet is controlled by its oxidation state imposed by the volatile composition. In the deep interior of a planet, an absence of oxidative volatiles will cause U and Th to stay in a state of metal or low valance compounds with a big density. Consequently, they migrate to the bottom of its mantle first, and then are gradually sequestered to its liquid metal core. Earth is rich in oxidative volatiles including water, therefore, U and Th in the core can be moved up by an internal circulation system consisting of the outer core, hot super plumes, asthenosphere and subduction zone (or cold super plumes). This internal circulation system is the key for the formation of plate tectonics, the geodynamo and the consequent geomagnetic field. Moreover, plentiful oxidative volatiles and water within Earth is the precondition to form such a circulation system. In the early stage (> 4 Ga), Mars developed an Earth-like internal circulation system due to relatively large amount of oxidative volatile compositions coming from its building material. This would have produced a dynamo and correspondingly an Earth-like magnetic field. However, this internal circulation system was destroyed by one or several giant impact events in the early stage, which drove off these volatile compositions. These events also shaped the striking hemispheric dichotomy structure on the Martian surface. The other result is that its dynamo and geomagnetic field have also disappeared. Since then, Mars has been the same as Mercury and Venus in that the heat release from the U and Th in their cores can not be moved by an internal circulation system gently, but by sporadically catastrophic resurfacing events (Venus), or super plumes (Mars) or gradual heat conduction (Mercury).