DIFFUSION IN SILICATE MELTS: II. MULTICOMPONENT DIFFUSION IN CAO-AL2O3-SIO2 AT 1500°C AND 1 GPA

Abstract

Chemical diffusion profiles in molten CaO-Al2O3-SiO2 have been measured over a large range of compositions at 1500°C and 1 GPa. The diffusion profiles have been inverted for effective binary diffusion coefficients (EBDCs) and for the chemical diffusion matrix. The EBDCs are shown to depend strongly on both composition and direction of diffusion in composition space. The dependence of EBDCs on direction in composition space, which for the system studied here can be as large as a factor of seven, severely limits the applicability of EBDCs to interdiffusion in any direction other than the one used to derive the EBDCs.The chemical diffusion matrix for molten CaO-Al2O3-SiO2 was determined using diffusion profiles from two or three mutually orthogonal diffusion couples in the ternary composition space. All features of the diffusion profiles shown in this work can be reoduced by representing the chemical fluxes in the three-component system as a linear combination of concentration gradients via a 2 x 2 diffusion matrix. Chemical diffusion in molten CaO-Al2O3-SiO2 shows clear evidence of strong diffusive coupling among the components. This can be seen in the uphill diffusion profiles of components that were initially uniform, in the fact that the apparent rate of diffusion of some components is a strong function of direction in composition space, and most quantitatively in the magnitude of off-diagonal elements of the diffusion matrix relative to the magnitude of the diagonal elements. SiO2, for example, is found to be strongly coupled with CaO in relatively silicic melts, whereas Al2O3 is strongly coupled with CaO in less silicic melts. Furthermore, the coupling of CaO with either Al2O3 or SiO2 reverses sign between more and less polymerized compositions. Interdiffusion profiles in natural melts have numerous features that suggest similar coupling between Al2O3 and CaO and between SiO2 and CaO.