The effect of the transformation of basalt to eclogite on the internal dynamics of Venus

Differentiation that occurs in zones of partial melting, forms less dense depleted mantle and basaltic crust. While the basaltic crust thickens, it transforms into denser eclogite. Typical values of Earth parameters are assumed to characterize Venus mantle properties except for the gravity acceleration equal to 8.8 m s−2, the surface temperature equal to 700 K and an unmoving surface. Such values provide a thermal Rayleigh number equal to 3 × 105. It can be applied to the formation of the Earthʹs surface during its primitive evolution and to the evolution of Venus considered as a one plate planet. In a 2D rectangular ☐, a steady pattern of thermal convection is achieved before the compositional effects are taken into account. Rocks are modeled with 130,000 particles characterized by their type (peridotite, basalt, eclogite) and their density. This model provides chemical buoyancy related to the distribution of these heterogeneities and the magmatism and its origin. A first study takes into account the chemical buoyancy created by mantle heterogeneities and basalt. In a second study, chemical buoyancy induced by dense eclogite is included. The comparison of the results of these two models allows us to quantify separately the dynamics induced by mantle heterogeneities and that induced by denser eclogite. The less dense depleted mantle accumulates in a surface-layer and tends to make the downwelling zones disappear, favoring more elongate cells. When the depleted mantle reaches the base of the upper mantle, compositional buoyancy enhances the development of new upwellings and new partial melting zones. The amount of magma becomes strongly time dependent and new magma centers are created. The competition between thermal and chemical buoyancy, in addition to the stabilization of the upper depleted layer decreases the efficiency of heat and mass transfer. Once the basalt/eclogite transformation is taken into account, gravitational instabilities form, yielding a recycling of crust into the mantle and lamination of the depleted lithospheric layer. These instabilities carry basaltic crust in the domain of stability of eclogite and limit the crustal thickness to a value lower than 40 km. The formation of compositional upwelling becomes less efficient while thermal downwellings are enhanced by the buoyancy of eclogite. This moderation of chemical perturbations induced by mantle heterogeneities favor both heat transfer and mass transfer. Therefore, both the cooling of the planet and the recycling of eclogite to the partial melting zones are more rapid. The buoyancy of eclogite enhances the recycling of crust in partial melting zones and may provide some “instantaneous” volcanic events at high rates which result from the melting of recycled eclogite. Finally, chemical and thermal convection stabilizes larger cells than a purely thermal convection would. While the mean aspect ratio of the cells is close to 1.3, in a purely thermal convection, it tends to 4 when chemical convection of mantle is included and it tends to 2 when chemical convection of both mantle and eclogite are taken into account.