Diffusion in silicate melts: I. Self diffusion in CaO[sbnd]Al 20 3[sbnd]SiO 2 at 1500°C and 1 GPa

Abstract

Self diffusion coefficients of calcium (DCa), aluminum (DAl), silicon (DSi), and oxygen (DO) were measured in molten CaOAl 20 3SiO 2 at 1500°C and 1 GPa over a range of melt compositions, using the isotope tracer method. For all but one composition, the measured self diffusion coefficients decrease in the order DCa > D Al > D O > D Si, with D Ca ∼ 4–19 D Si , D Al ∼ 2D Si, and D O ∼ 1–2 D Si. The relative uncertainties, based on replicated experiments, are 8% for DCa, 27% for DAl, 28% for DSi, and 18% for DO. Although the self diffusion coefficients of calcium, aluminum, silicon, and oxygen increase with the decrease of melt viscosity, they do not obey the Stokes-Einstein equation or the Eyring equation. The compositional dependence of the self diffusion coefficients of calcium, aluminum, and oxygen were parameterized in terms of melt viscosity (Pa · s) via the power law relation: DCa = (115.141/viscosity 0.389) × 10 −12m 2s −1DA1 = (47.302/viscosity 0.800) × 10 −12m 2s −1DO = (33.080/viscosity 0.722) × 10 −12m 2s −1The compositional dependence of the self diffusion coefficients of Si was parameterized in terms of the degree of melt polymerization (NBO/T) DSi = [0.669 exp(4.195 NBO/T)] × 10 −12m 2s −1There is an abrupt change in the diffusion behavior of Al and Si between melts of more and less polymerized compositions, occurring at a NBO/T value of approximately 0.6, which may be indicative of a major change in melt structure. The hypothesized structural transition, the nature of which remains to be identified, may control other aspects of diffusion such as the sign of the activation volume for self diffusion of network-forming cations and the magnitude of the activation energy.The self diffusion coefficients of calcium, aluminum, silicon, and oxygen were used to calculate electrical conductivities of molten CaOAl 2O 3SiO 2 at 1500°C and 1 GPa using the generalized NernstEinstein relation. The estimated electrical conductivity implies that calcium, silicon, and oxygen contribute nearly equally to the total conductance: 31 % from calcium, 29% from silicon, and 30% from oxygen, which is quite different from what one would expect for alkali-bearing melts in which Na and K are believed to be the dominant conducting species.