The mean global heat flow of a planet depends mainly on the initial mantle temperature, the distribution of the radiogenic heat sources, and the mechanism of the heat transport within the mantle. It is well known that subsolidus convection is the dominant mechanism of the heat transport within the mantle. With the help of parametrized convection models (Franck and Bounama, 1995; and references given therein) temporal variations of the average mantle temperature and the surface heat flow in terms of the Rayleigh number can be calculated under certain initial conditions. In our numerical experiments we calculate the thermal and the degassing history for two different initial average mantle temperatures: 2400 K and 3000 K. Looking at the two different starting temperatures, we find qualitatively different behaviour in the first 1-1.5 Gyr of thermal evolution. After this stage the two curves converge rapidly showing the so-called readjustment-effect. After readjustment the quantities decrease monotonically. Furthermore, we investigate the influence of continental growth and the thermal and degassing history by using simple analytical continental growth models. We compare our results with that of laboratory melting experiments of komatiites providing estimates for the secular mantle cooling and discuss relations between global mean heat flow and the areal spreading rate.