PHASE-TRANSITIONS IN THE MARTIAN MANTLE AND THE GENERATION OF MEGAPLUMES

The influences of two exothermic phase transitions, the olivine to bet a-spinel and the beta- to gamma-spinel transition, on Martian mantle c onvection have been studied with an axisymmetric spherical-shell model . There are depth dependences in the thermal expansivity and gravity, which, taken together, decrease by a factor of between 2 to 3 across t he mantle. We have discovered an unique mechanism for the amplificatio n and superheating of megaplumes in Mars, which works particularly wel l because of the optimal heights of the two destabilizing exothermic p hase transitions above the Martian core-mantle boundary (CMB): The col d descending mantle flow is accelerated by its interactions with the p hase transitions and the nearby CMB forces the flow to bend over sharp ly. Together these effects result in strong localized viscous heating, which will generate and accelerate a rising plume and may even cause melting in its deep interior and produce komatiites.