PHASE-TRANSITIONS IN THE MARTIAN MANTLE - IMPLICATIONS FOR THE PLANETS VOLCANIC HISTORY

The influences of phase transitions on Martian mantle convection and m elting in the Martian mantle have been studied with an axisymmetric sp herical-shell model, An extended Boussinesq model in which viscous and a,diabatic beatings are included has been used, There are depth depen dences in the thermal expansivity and gravity, which taken together, d ecrease by a factor of between 2 and 3 across the Martian mantle. The two destabilizing exothermic phase transitions, olivine to beta-spinel and beta- and gamma-spinel transitions, above the Martian core-mantle boundary (CMB) accelerate the mantle flow and result in an amplificat ion and superheatiag of plumes in Mars, The additional consideration o f the endothermic phase transition, spinel to perovskite transition, w hich was only likely present in the early evolution of the planet, has little influence on Martian mantle convection in this model. Strong l ocalized viscous heating is generated near the CMB and underneath the lithosphere because here the flow bends over sharply, A possible volca nic evolution of Mars can be derived from a comparison of the temperat ure fields with the solidus of anhydrous peridotite if the cooling of the Martian mantle is taken into account. In the early evolution the m antle was molten to a high degree along the plume asis, which possibly resulted in a strong differentiation of the mantle, As the placet coo led down, the region of melt generation receded to where the maximum o f viscous heating occurred: at the CMB and immediately underneath the lithosphere. Upon further cooling, the deep-mantle melt source region became subsolidus. The most recent volcanism on Mars was most likely g enerated at a shallow depth below the lithosphere.