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2 Ca and K Isotope Fractionation by Diffusion in Molten Silicates: Large Concentration Gradients Are Not Required to Induce Large Diffusive Isotope Effects
James M. Watkins1, John N. Christensen2, Donald J. DePaolo2,3, and Frederick J. Ryerson4
1 Department of Earth Sciences, University of Oregon, Eugene, Oregon, USA
2 Earth and Environmental Science Area, Energy Geosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
3 Department of Earth and Planetary Science, University of California, Berkeley, California, USA
4 Atmospheric, Earth, and Energy Division, Lawrence Livermore National Laboratory, Livermore, California, USA
ABSTRACT
Laboratory experiments were used to investigate diffusive isotopic fractionation of calcium and potassium in phonolite‐rhyolite diffusion couples. The starting compositions have very different SiO2 and K2O, but similar CaO. These were juxtaposed and held in a completely molten state at 1450°C and 1.0 GPa for durations of 2.5 or 6 hours in a piston cylinder apparatus. The resulting major‐element diffusion profiles exhibit many complexities, including uphill diffusion of all major oxide components. The diffusive fluxes for SiO2, K2O, and CaO were modeled using a published modified effective binary diffusion model, whereby diffusion is driven by activity gradients that are solely a function of the time‐evolving SiO2 concentration. Both Ca and K exhibit large diffusive isotope effects that can be explained by imposing a mass dependence on the diffusion coefficients used to model the major‐element profiles. The mass dependence is parameterized in terms of the inverse ratio of the isotope masses raised to an empirically determined exponent β (i.e., Di/Dj = [mj/mi]