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

The transport processes in anhydrous basalt melt were investigated experime ntally in electrochemical cell apparatus, imposing a constant electric fiel d with intensity E ranging from 0.11 to 1.26 V/cm at temperatures comprisin g between 1500 and 1600 K. Within T limits the melt viscosity was calculate d to decrease from 3200 to 1170 P, the electric conductivity changed from 3 X 10(-5) to 20 X 10(-5) (Omega cm)(-1), and the ratio of the activation en ergies changes from 1.02 to 1.56. Frozen diffusional profiles of annealed a cid quenched samples were determined at 400 mV external voltage. Mass trans port processes were analyzed by a system of differential equations describi ng 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 a nd 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) cm2/s and the corresponding values of the ion-mobility are of th e order of 10(-6) cm2/s V. In the approximation of a limiting diffusion cur rent, 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 ti me scale, electrochemically induced macro-inhomogeneities might arise in ma gmatic melts which would possibly influence to a certain extent the crystal lization processes. (C) 2001 Published by Elsevier Science B.V.