EFFECTS OF WATER-DEPENDENT CREEP RATE ON THE VOLATILE EXCHANGE BETWEEN MANTLE AND SURFACE RESERVOIRS

A parameterized model of mantle convection, including the volatile exchange between mantle and surface reservoirs, is used to study the thermal history of the Earth. The influence of dissolved volatiles on mantle rheology is reformulated. The weakening of mantle materials 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. The mantle degassing rate is considered as directly proportional to the seafloor spreading rate, which, in turn, is dependent on the mantle heat flow. To calculate the spreading rate, we assume that the heat flow under the mid-ocean ridges is double the average mantle heat flow. The rate of regassing also depends on the seafloor spreading rate, as well as on other factors such as the efficiency of volatile recycling through island are volcanism. Both mechanisms (degassing and regassing) are coupled self-consistently with the help of a parameterized convection model under implementation of a different formulation of mantle viscosity. The model is run for 4.6 Gyr. Time series of average mantle temperature, volatile loss, mantle viscosity, mantle heat flow, Rayleigh number and Urey ratio are calculated. The effects of changing initial parameters have been tested and found negligible for the final results. Mantle water is outgassed rapidly within a time scale of less than 200 Myr for all numerical simulations. The present-day values of calculated parameters are in the generally accepted range.