Grain size-dependent viscosity convection and the thermal evolution of theEarth

Scaling relationships for grain size-dependent viscosity convection are der ived assuming that eutectoid-like transformations at 660 km depth produce v ery small grains and that the grain size is mainly controlled by a coupled Ostwald ripening of Mg-perovskite, Ca-perovskite and magnesiowustite. Param eterized convection calculations based on these relationships show that the thermal evolution of the Earth strongly depends on the grain growth parame ters. The Earth can either quickly forget the initial conditions with tempe rature and heat loss following the decaying radiogenic heat production (Toz er-type evolution) or the initial conditions can essentially determine the temperature and heat loss (Christensen-type evolution). Christensen-type mo dels can easily satisfy both the present-day heat flux and geochemical cons traints on abundances of radiogenic isotopes. For example, this happens if the rate-con trolling process for both grain growth and viscous creep is vo lume diffusion and the activation enthalpy for grain growth is about 50% hi gher than that for viscous creep. (C) 2001 Elsevier Science BN. All rights reserved.