Numerical models of magmatism in convecting mantle with temperature‐dependent viscosity and their implications for Venus and Earth

Abstract

Numerical models are presented for magmatism in a convecting mantle that contains internal heat source. Mantle convection is modeled by a convection of binary eutectic material with Newtonian temperature‐dependent rheology driven by thermal, compositional, and melt buoyancy as well as the buoyancy from the “660‐km” phase transitions. Mantle magmatism is modeled by a gravitationally induced permeable flow of magma generated by pressure release melting through matrix. Magma mostly has the eutectic composition, that is, basaltic composition. The numerical models suggest that magmatism and mantle convection strongly influence each other to form a coupled system when the internal heating is sufficiently strong. Episodic magmatism takes place to chemically differentiate the mantle; basaltic materials occupy deeper part of the lower mantle, while magma residue occupies the uppermost mantle. Compositional buoyancy of the differentiated materials makes mantle convection sluggish except when magmatic activities take place. The style of magmatism and mantle convection depends on the viscosity contrast across the cold thermal boundary layer (TBL) along the top surface boundary. When the viscosity contrast is large, the fluid in the coldest part of top TBL becomes a stagnant lid and a conspicuous lateral heterogeneity arises in the upper mantle. Magmatism is induced by hot uprising diapirs originating in the top of the lower mantle. The diapirs induce convective circulation within the upper mantle when the lower mantle is rather cold but induce flushing event, that is, a massive flow across the 660‐km phase boundary and the resulting vigorous magmatism, when the lower mantle becomes sufficiently hot owing to the internal heating. When the viscosity contrast is moderate, the fluid in the coldest part of top TBL behaves as a moving lid. Both the moving lid and hot uprising diapirs induce magmatic activity; the magmatic activity due to a moving lid resembles ridge volcanism. The mantle of Venus and that of the early Earth are suggested to have been on the regime modeled here, and the magmatic activity by flushing event is compared to the magmatism of volcanic plain formation on Venus.