The Evolution of Venusian Mantle Dominated by the Magmatism-Mantle Upwelling Feedback and Basalt-Barrier: a numerical study

Mantle upwelling flow may generate magma by decompression melting while the buoyancy of generated magma may reinforce the upwelling flow itself. This positive feedback between magmatism and mantle upwelling (the MMU feedback) operates to episodically cause plume magmatism in a compositionally stratified hot mantle, when the mantle is sufficiently soft. I numerically explored how the MMU feedback can exert control over the evolution of Venusian mantle. I assumed that the modeled mantle is initially hot enough to cause an extensive magmatism. At typical values of model parameters expected for Venus, this magmatism compositionally differentiates the mantle; a fraction of the basaltic crust generated by the magmatism recycles into the mantle and accumulates along the top of the lower mantle to form the basalt-barrier, that is, a layer of basaltic composition that impedes convective heat and mass exchange across the layer. The basalt-barrier allows heat to build up in the lower mantle owing to internal heating. Eventually, the MMU feedback begins to induce a series of partially molten plumes that episodically pump-up hot materials in the lower mantle through the basalt barrier to cause an extensive magmatism, or mantle burst, in the upper mantle. The basalt-barrier, however, fades away and magmatism becomes milder and more continuous on the later stage as the heat-producing elements decline. In the parameter range I searched, the mantle evolves in this way if the mantle is not too stiff for the MMU feedback and magma-segregation is efficient enough to differentiate the mantle: the mantle remains compositionally homogeneous and is monotonously cooled, and magmatism monotonously declines with time, when magma-segregation is less efficient. Strong deformation of the crust caused by mantle burst in the early stage of mantle evolution can account for the tessera terrain while the continuous and mild magmatism on the later stage can account for the formation of volcanic plains, both observed on Venus.