Ultrafast mantle impregnation by carbonatite melts

Carbonatitic melts are often invoked as responsible for metasomatism in the mantle because of their unique chemical and physical properties. Here we report on infiltration experiments demonstrating that such melts can percolate very quickly in polycrystalline olivine. Carbonatites can travel over several millimeters in one hour and the infiltration rate is kinetically controlled by cation diffusion in the melt. The observed rates are several orders of magnitude higher than those previously found for basalt infiltration in mantle lithologies. Infiltration proceeds by a dissolution-precipitation mechanism wherein porosity is created in the dunite by dissolution of olivine at grain edges. This reaction is accompanied by forsterite reprecipitation in the carbonatite reservoir. Such a mechanism would likely favor chemical exchange between melt and matrix during percolation. We propose a migration model combining infiltration and compaction by which carbonatite melts can travel upward in the mantle over hundreds to thousands of meters on time scales of 0.1-1 m.y.