Magmatic differentiation and convective stirring of the mantle in early planets 2: Effects of the properties of mantle materials

Abstract

Summary Magmatism and mantle convection closely couple with each other, when the mantle is hot and magmatism is extensive. Here, I explore dynamics of the coupled system for a planet of the Earth's size based on a two-dimensional numerical model. Magmatism occurs as a permeable flow of basaltic magma generated by decompression melting through matrix, while mantle convection is driven by thermal, compositional, and melt-buoyancy as well as the volume change of matrix caused by magma-migration. The viscosity of matrix strongly depends on temperature. Basaltic solid materials are denser than the average mantle materials except at the top of the lower mantle and in the crust. A positive feedback between magmatism and mantle convection caused by melt-buoyancy (the MMUb feedback) dominates mantle dynamics, when the Rayleigh number Ra is higher than a threshold RMMUb, around 107: buoyancy of magma generated by mantle upwelling flow drives the upwelling flow itself. The feedback reinforces convective stirring of the mantle and also allows the buoyancy of basaltic materials at the top of the lower mantle to strongly impede mass exchange between the upper and lower mantles. As a consequence, an extensive magmatism can only moderately differentiate the mantle at Ra > RMMUb. At Ra < RMMUb, another type of positive feedback is important (the MMUc feedback): volume change of matrix caused by magma-migration drives the mantle upwelling flow that generates the magma. This feedback does not noticeably enhance convective stirring and hence enhances differentiation of the mantle by magmatism. I also calculated several cases where melt is denser than matrix at the base of the upper mantle and found that a compositional discontinuity develops along the top of the lower mantle to make the mantle layered, when Ra < RMMUb. In the early Earth where magmatism was probably extensive, the MMUb feedback is most likely to have operated to keep the mantle rather homogeneous despite differentiation by the magmatism.