Abstract:
We report new measurements of self diffusion coefficients (D) for Mg, Nd, Eu, Th, and U in a haplobasaltic (Fo15Di40An45) melt at 1 atm, 1400-1500°C, and oxygen fugacities corresponding to air and the Fe-FeO buffer. Diffusion couples consisted of isotopically distinct melts of the same chemical composition, and isotopic concentration profiles in quenched couples were measured with an ion probe. The valence state distributions of Eu and U were determined from absorption spectroscopy and model calculations, which demonstrate a shift from Eu3+ and U5.5+ in air to Eu2.5+ and U4+ at Fe-FeO. DMg,DNd , and DTh are independent of oxygen fugacity and agree well with our previous measurements. DEu = DNd in air and increases by 42% at Fe-FeO, while DU = DTh in air and shows a possible small increase of ~20% at Fe-FeO. The change in DEu with oxygen fugacity matches the established ionic radius and charge dependence for Mg, Ca, Ba, Nd, Yb, Ti, and Zr, while diffusion coefficients for Zr, Th, U4+, and U5.5+ are independent of ionic radius and charge. Activation energies for all cations are approximately equal, independent of oxygen fugacity, and approximately match the activation energy for viscous flow. In addition, activation energies and diffusion coefficients recently measured for O and Si in basalt agree well with the present values. The good agreement between the various activation energies and between network modifier and network former diffusivities is consistent with a model in which diffusion of network modifying cations in low viscosity melts is controlled largely by the extrinsic influence of the melt network reorganization, with an additional influence from the intrinsic mobilities of the individual cations. The constant diffusion coefficient defined by the high ionic radius and charge elements is interpreted to represent the characteristic network diffusivity for this composition, which dominates over the intrinsic diffusivities for these elements. Elements with faster intrinsic diffusivities still display a small ionic radius and charge dependence. Diffusion coefficients in high viscosity melts are expected to be decoupled from the network, and thus may display a much greater dependence on ionic radius and charge.