Ion-mass transport in anhydrous basaltic melts in an electric field

Abstract

The transport processes in anhydrous basalt melt were investigated experimentally in electrochemical cell apparatus, imposing a constant electric field with intensity E ranging from 0.11 to 1.26 V/cm at temperatures comprising between 1500 and 1600 K. Within T limits the melt viscosity was calculated to decrease from 3200 to 1170 P, the electric conductivity changed from 3×10 −5 to 20×10 −5 (Ω cm) −1, and the ratio of the activation energies changes from 1.02 to 1.56. Frozen diffusional profiles of annealed and quenched samples were determined at 400 mV external voltage. Mass transport processes were analyzed by a system of differential equations describing unsteady-state multicomponent chemical diffusion in the approximation of effective diffusion coefficients. Equations were solved with semi-infinite boundary conditions. Expressions for the concentration dependence of melt constituents and the corresponding partial current densities were derived and fitted to obtain the effective diffusion coefficients for the Fe and Mg oxide components. The values of the diffusion coefficients are of the order of 10 −7 cm 2/s and the corresponding values of the ion-mobility are of the order of 10 −6 cm 2/s V. In the approximation of a limiting diffusion current, a distance of about 5 m is estimated to be traversed by the Fe and Mg melt constituents in the vicinity of the chamber's wall in about 100 years. Hence, during time periods, which are short compared to the geological time scale, electrochemically induced macro-inhomogeneities might arise in magmatic melts which would possibly influence to a certain extent the crystallization processes.