Thermal evolution of the Earth: Secular changes and fluctuations of plate characteristics S. Labrosse a, ⁎ , C. Jaupart b a Laboratoire des sciences de la Terre, École Normale Supérieure de Lyon, Université de Lyon, CNRS UMR 5570, 46 Allée d'Italie 69364 Lyon Cedex 07, France b Equipe de dynamique des fluides géologiques, Institut de Physique du Globe de Paris, Université Paris Diderot CNRS UMR 7154, 4, place Jussieu, 75252, Paris Cedex 05, France Received 26 July 2006; received in revised form 24 May 2007; accepted 24 May 2007 Editor: R.D. van der Hilst Available online 2 June 2007 Abstract The average secular cooling rate of the Earth can be deduced from compositional variations of mantle melts through time and from rheological conditions at the onset of sub-solidus convection at the end of the initial magma ocean phase. The constraint that this places on the characteristics of mantle convection in the past are investigated using the global heat balance equation and a simple parameterization for the heat loss of the Earth. All heat loss parameterization schemes depend on a closure equation for the maximum age of oceanic plates. We use a scheme that accounts for the present-day distribution of heat flux at Earth's surface and that does not depend on any assumption about the dynamics of convection with rigid plates, which remain poorly understood. We show that heat supply to the base of continents and transient continental thermal regimes cannot be ignored. We find that the maximum sea floor age has not changed by large amounts over the last 3 Ga. Calculations lead to a maximum temperature at an age of about 3 Ga and cannot be extrapolated further back in time. By construction, these calculations are based on the present-day tectonic regime characterized by the subduction of large oceanic plates and hence indicate that this regime did not prevail until an age of about 3 Ga. According to this interpretation, the onset of rapid continental growth occurred when the current plate regime became stable. © 2007 Published by Elsevier B.V. Keywords: cooling of the Earth; plate tectonics; early Earth; continental growth 1. Introduction The thermal evolution of the Earth has usually been addressed using the so-called “parameterized” approach such that the surface heat flux is related to the temperature difference across the convecting layer. This scaling law was derived from theoretical argu- ments, numerical calculations and laboratory experi- ments on convecting systems that are much simpler than the Earth. It is written as q ¼ B k DT D Ra b ; ð1Þ where k is thermal conductivity, D the vertical extent of the convecting layer and ΔT is the super-isentropic Earth and Planetary Science Letters 260 (2007) 465 – 481 www.elsevier.com/locate/epsl ⁎ Corresponding author. E-mail address: stephane.labrosse@ens-lyon.fr (S. Labrosse). 0012-821X/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.epsl.2007.05.046