RHEOLOGY AND VOLATILE EXCHANGE IN THE FRAMEWORK OF PLANETARY EVOLUTION

The thermal history of an Earth-like planet is studied with the help of a parameterized mantle convection model including the volatile exchange between mantle and surface reservoirs. The weakening of mantle silicates by dissolved volatiles is described by a functional relationship between creep rate and water fugacity. We use flow law parameters of diffusion creep in olivine under dry and wet conditions to investigate the negative feedback between dissolved volatiles and outgassing by vigorous mantle convection. The mantle degassing rate is considered as directly proportional to the seafloor spreading rate which in turn is dependent on the mantle heat flow. The rate of regassing also depends on the seafloor spreading rate as well as on other factors like the efficiency of volatile recycling through island are volcanism. We calculate time series over 4.6 Gyr for the average mantle temperature, the mantle viscosity, the mantle heat flow, the volatile loss, the Rayleigh number, and the Urey ratio. Mantle water is outgassed rapidly within a time scale of less than 200 Myr for all numerical simulations. The present values of the calculated parameters for the Earth are in the generally accepted range. In the case of Venus we find a less efficient outgassing in accordance with data about occurrence of the radiogenic noble gasses.