Mantle evolution in Venus due to magmatism and phase transitions: From punctuated layered convection to whole‐mantle convection

A series of numerical models of magmatism and mantle convection with a stagnant lithosphere are developed to understand the mantle evolution in Venus. Magmatism is modeled as a permeable flow of basaltic magma generated by decompression melting, and the solid‐state convection of mantle materials with temperature‐dependent Newtonian rheology is affected by the garnet‐perovskite transition and the postspinel transition. In our preferred models, the mantle evolves in two stages: The earlier stage is characterized by layered mantle convection punctuated by repeated bursts of hot material from the deep mantle to the surface. Mantle bursts induce vigorous magmatism and also cause the basaltic crust, enriched in heat‐producing elements (HPEs), to recycle into the mantle. A part of the recycled basaltic crusts accumulates along the postspinel boundary to form a barrier, and this basalt barrier causes mantle convection to become layered. At a later stage, when the HPEs have already decayed, in contrast, the basalt barrier disappears and whole‐mantle convection occurs more steadily. Mild magmatism is induced by small‐scale partial melting at the base of the crust and hot plumes from the deep mantle. The internal heating by the HPEs that recycled into the mantle in the earlier stage allows the magmatism of the later stage to continue throughout the calculated history of mantle evolution. The two stages arise when the barrier effect of the postspinel transition is weak and the lithosphere is mechanically strong enough. The two‐stage evolution model meshes with the observed history of magmatism and the lithosphere on Venus.