Magma convection and mixing, and periodic refilling, commonly occur in magma chambers. We show here that the presence of CO2 in the refilling magma is a very efficient mean of inducing buoyant-driven plume rise and large scale convection. Numerical simulations performed with an appositely developed code for the transient 2D dynamics of multicomponent compressible to incompressible fluids reveal several features of the processes of plume rise, convection and mixing in magma chambers associated with chamber refilling. A parametric study on CO2 abundance in the refilling magma shows that progressively larger amounts of this volatile produce a shift from simple plume rise and spreading near the chamber top, to complex patterns of flow circulation and large scale vorticity and mixing. Lower chamber depth and lower magma viscosity largely enhance the efficiency of mixing and convection, favoring the formation of multiple vortexes migrating with time.