3-DIMENSIONAL SIMULATIONS OF PLUME-LITHOSPHERE INTERACTION AT THE HAWAIIAN SWELL

Rapid lithospheric thinning by mantle plumes has not been achieved in numerical experiments performed to date. Efficient thinning depends on small-scale instabilities that convectively remove lithospheric mater ial. These instabilities are favored by hotter plumes or stronger temp erature dependence of viscosity, and a simple scaling independent of r heology controls their onset. This scaling allows extrapolation of the results of numerical experiments to the Earth's mantle. Mantle plumes between 100 and 150 kelvins hotter than the background mantle should exhibit small-scale convective rolls aligned with the plate motion. Th e unusual variation in heat flow across the Hawaiian swell may be due to such instabilities. It was found that the spreading of the plume cr eates a downwelling curtain of material that isolates it from the rest of the mantle for distances of at least 1000 kilometers from the plum e origin. This isolation has important consequences for the geochemica l heterogeneity of the lithosphere and upper mantle.