Diffusion of major and trace elements in compositionally complex Cl- and F-bearing silicate melts

Abstract

Chemical diffusion experiments on halogenated, multicomponent silicate melts at 10 MPa, 200 MPa and 1.0 GPa have yielded diffusivities for both major (Al, Fe, Si) and trace elements (Mn, Zn, Y, Zr, Nb) over the temperature range 900 to 1400 ° C. At any given temperature and pressure, diffusivities of all elements are the same within an order of magnitude; they display no apparent dependence upon cation size, but do correlate crudely with cation field strength ( Z/ r). Comparison with diffusion data on unhalogenated melts revealed that addition of Cl and F tends to increase cation diffusivities and lower activation energies for diffusion. The effect of increasing pressure is also to enhance diffusion for all elements studied except the high field-strength network-modifiers Zr and Nb. As pressure increases, low Z/ r elements experience a decrease in activation energy for diffusion, whereas high Z/ r elements display increasing activation energies. It is proposed that the change in activation energy is related to the existence of two distinct network structures characterized by different TOT angles. The network structures with lower TOT angles apparently can provide charge-balanced sites for low Z/ r cations but not for high Z/ r cations. The effect of elevated pressure on the melt structure may be to increase the proportion of network structures having low TOT angles. The resulting increase in available sites for low Z/ r cations is believed responsible for the observed reduction in activation energy for diffusion of the cations.