COMPARATIVE DEGASSING HISTORY OF EARTH AND VENUS. S. Franck and C. Bounama,

Introduction: Parameterized convection models have provided important results for the thermal history of the Earth and of other planets [1,2,3]. Schubert et al. [4] calculated a secular cooling of about 150°C-300°C since the Archaean and showed that a substantial portion of the present terrestrial heat flux is primordial with a Urey ratio of 0.65 to 0.85. For the first time Jackson and Pollack [5] investigated the effects of volatile dependent rheology on the thermal evolution of the Earth. The first selfconsistent model to calculate the loss of volatiles during the Earth’s evolution and to study the feedback between heat transport and both temperature and volatile dependent viscosity was presented by McGovern and Schubert [6]. Our calculations of whole mantle convection and a dependence of the viscosity on the temperature and the volatile content are based on a functional relationship between creep rate and water fugacity [7] (hereafter referred to as FB-model). Model Description: Our coupled thermal and degassing model is shown in Figure 1., where we have sketched how changes in the mantle heat flow are related to changes in average mantle temperature and radiogenic heat production. It contains the mechanisms for degassing of volatiles at mid-ocean ridges and regassing via subduction, respectively.