INTERCONNECTIVITY OF CARBONATE MELT AT LOW MELT FRACTION

Experiments were performed to investigate the interconnectivity of sod ium carbonate melt in a synthetic dunite at low melt fractions at 1 GP a and 1300 degrees C. Carbonate melt was found to remain interconnecte d to 0.05 wt% melt in fine-grained olivine (all grains initially less than or equal to 63 mu m). The presence of this interconnected melt en hances the bulk transport of iron through the polycrystalline sample b y two to almost three orders of magnitude, depending on melt fraction. This effectively reduces the diffusive length scale of the bulk sampl e to that of the individual grains, with transport limited by the rate of lattice diffusion for those species soluble in the carbonate melt. These experiments also show that 0.03-0.07 wt% melt demarcates a lowe r limit to melt interconnectivity, despite a melt-olivine dihedral ang le of 25-30 degrees, less than the critical dihedral angle of 60 degre es. In this range of melt abundance grain-edge tubules pinch off and d iffusion is short-circuited only for the length of the melt pores. Int erconnectivity is likely present to melt fractions lower than 0.05 wt% in carbonated olivine-rich mantle with grain size on the order of 1 m m. Combining bulk diffusivity measurements with models of permeability suggests that carbonate melts may be more mobile at low melt fraction that previously estimated, increasing the potential of carbonate melt s to transport soluble elements and carbon-rich volatiles through the upper mantle.