Interconnectivity of carbonate melt at low melt fraction

Experiments were performed to investigate the interconnectivity of sodium carbonate melt in a synthetic dunite at low melt fractions at 1 GPa and 1300° C. Carbonate melt was found to remain interconnected to 0.05 wt% melt in fine-grained olivine (all grains initially ≤ 63 μm). The presence of this interconnected melt enhances the bulk transport of iron through the polycrystalline sample by two to almost three orders of magnitude, depending on melt fraction. This effectively reduces the diffusive length scale of the bulk sample 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 lower limit to melt interconnectivity, despite a melt-olivine dihedral angle of 25–30°, less than the critical dihedral angle of 60°. In this range of melt abundance grain-edge tubules pinch off and diffusion is short-circuited only for the length of the melt pores. Interconnectivity is likely present to melt fractions lower than 0.05 wt% in carbonated olivine-rich mantle with grain size on the order of 1 mm. 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 melts to transport soluble elements and carbon-rich volatiles through the upper mantle.