Influence of initial CMB temperature and other parameters on the thermal evolution of Earth's core resulting from thermochemical spherical mantle convection

Abstract

Here we present a wide-ranging parameter study of the effects of initial core-mantle boundary (CMB) temperature, concentration of radioactive potassium in the core, and density difference between harzburgite and mid-ocean ridge basalt (MORB) in a coupled spherical model of thermochemical mantle convection and parameterized core heat balance. The initial CMB temperature is expected to be much higher than the solidus temperature of silicates at the base of the mantle. The results indicate that as with previous, purely thermal convection models, the final state of the system is only weakly dependent on initial CMB temperature unless the CMB becomes blanketed by a global layer of dense material. Fully 3-D spherical cases have a very similar core evolution to cases in a 2-D spherical annulus, giving confidence in the applicability of 2-D spherical annulus geometry for modeling Earth's evolution. Obtaining a successful thermal evolution, in the sense of obtaining the correct present-day inner core size and maintaining a geodynamo over geological time, is helped by the accumulation of piles of dense material at the CMB (subducted MORB in the present calculations) and a concentration of radiogenic K in the core in the range 400-800 ppm. The present-day CMB heat flow is predicted to be around 9 TW. While this is lower than estimates based on calculating temperature gradients in regions where the postperovskite transition is seismically imaged, these tend to be areas of higher than average heat flux and thus likely overestimate the global heat flow. Copyright 2010 by the American Geophysical Union.