Silicate diffusion in alkali-carbonatite and hydrous melts at 16.5 and 24Â GPa: Implication for the melt transport by dissolution-precipitation in the transition zone and uppermost lower mantle

Using obtained diffusivities we estimated possible migration rates of dispersed melt inclusion in the deep mantle by means of dissolution-precipitation considering different driving forces. The rates of melt migration driven by the lateral thermal gradient of 1 °C/km in the mantle plume range from 4 × 10−8 to 8 × 10−7 m/year. This means that during plume ascent time of about 50 Ma, the melt can be moved by 2-40 m. These values clearly demonstrate that the thermal gradient is very weak driving force in terms of melt segregation in the deep mantle. On the other hand, at typical mantle stress of 1 MPa and droplet size of 100 μm the migration rates of the HM, KMCH and KMC melts are estimated to be 22.5, 0.9 and 0.2 m/year, respectively, which are 2-3 orders of magnitude faster than ascent rate of the mantle plume. This implies that all melt droplets on the way of ascending plume would be entrapped by the stressed zone in front of plume and accumulated in the plume head. This mechanism may explain segregation of mantle magmas with the source regions deeper than 150-250 km, such as kimberlites.