Abstract:
We have measured self diffusion coefficients (D) for a wide variety of elements in a Fo15Di40An45 melt at 1 bar from 1350-1500°C. These measurements, including the first reported data for Yb, Ti, Zr, U, and preliminary values for Th and Pb, provide an internally consistent set of self diffusivities for elements covering a wide range in ionic radius and charge. Diffusion couples were formed by juxtaposing isotopically enriched and isotopically normal melts of the same chemical composition, and isotopic concentration profiles were measured with an ion probe. Well-defined diffusion coefficients were determined to within a reproducibility of +/-15%. The data show good Arrhenian behavior with activation energies ranging from 170 kJ/m for Mg2+ to 216 kJ/m for Zr4+. The diffusion coefficient for U is interpreted as the weighted sum of variable proportions of U5+ and U6+, withDU5+ > DU6+. Activation energies for most elements are similar and agree well with the activation energy for viscous flow, suggesting a structural relationship between diffusion and viscosity. In contrast, diffusion coefficients display a systematic decrease with increasing ionic radius and increasing charge at any given temperature, and at 1400°C range from 8.1 x 10-7 cm2/s for Mg2+ to 0.9 x 10-7 cm2/s for U5-6+. These observations suggest that diffusion of network modifying cations in low viscosity melts reflects a combination of the intrinsic mobilities of the individual cations and viscous flow of the melt network. While there is some indication that self diffusion coefficients are influenced by a combination of mechanical and electrostatic effects, a complete physical explanation for the variation of diffusivity with ionic radius and charge does not exist. The width of a boundary layer around a growing crystal is proportional to D, and hence the small range in diffusion coefficients measured in this study implies that trace element fractionation during crystal growth in basaltic compositions will be minimal.