DIFFUSIVE FRACTIONATION OF TRACE ELEMENTS DURING PRODUCTION AND TRANSPORT OF MELT IN EARTH'S UPPER MANTLE

Abstract

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 ∼0.5 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 ∼2–3 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 per year. Complete disequilibrium, on the other hand, requires rapid transport (>30 m yr−1) through a permeable network with channel spacing considerably larger than the grain size. Diffusive fractionation during percolation through depleted spinel peridotite can lead to a wide variety of REE patterns in the melt, depending on the porosity and channel spacing of the melt network. These include ultra-depleted patterns when the porosity is very small (<0.005 for an intergranular network with 2 mm grain radii) and patterns of relative LREE enrichment at moderate porosity (0.02–0.03).