Di¡usive fractionation of trace elements during production and transport of melt in Earth’s upper mantle

We have developed a numerical model to investigate the importance of diffusive chemical fractionation during production and transport of melt in Earth’s upper mantle. The model incorporates new experimental data on the diffusion rates of rare earth elements (REE) in high-Ca pyroxene [Van Orman et al., Contrib. Mineral. Petrol. 141 (2001) 687^703] and pyrope garnet [Van Orman et al., Contrib. Mineral. Petrol., in press], including the dependence of diffusivity on temperature, pressure and ionic radius. We find that diffusion exerts an important control on REE fractionation under conditions typical of melting beneath slow spreading centers, provided that grain radii are V0.5 mm or greater. When partitioning is diffusion-limited, REE are fractionated less efficiently than under equilibrium conditions, and this effect becomes more pronounced as the melting rate, grain size, and efficiency of melt segregation increase. Data for the light REE in abyssal peridotite clinopyroxene (cpx) grains from the slow spreading America^ Antarctic and Southwest Indian ridge systems are better explained by melting models that allow for diffusive exchange than by models that assume complete solid^melt equilibration. The data are best fit by models in which the initial cpx grain radii are V2^3 mm and melt extraction is very efficient (near fractional). Diffusion is likely to play a strong role in REE fractionation during intergranular melt transport in the upper mantle. Complete equilibrium between solid and melt requires very sluggish melt transport, with ascent rates on the order of a few centimeters pe