A numerical study of magmatism and mantle convection in Mars: Implications for its crust, surface environment, and magnetic field

Abstract:

I present a two-dimensional numerical model of magmatism in the convecting mantle in Mars. The mantle is initially wet and internally heated by heat-producing elements (HPEs). Mantle convection occurs as a thermo-chemical convection. Magmatism is modeled by decompression melting of upwelling mantle materials and upward permeable flow of the generated basaltic magma through the convecting mantle. Water is transported by mantle convection and migrating magma; water is extracted from the mantle, when it is transported to the surface by magma. Initially, the shallow mantle is hot, while the deep mantle is rather cold, as predicted from a classical model of planetary formation: the initial temperature at the core is the same as that of the shallow mantle. At the beginning of the calculation, an extensive magmatism forms the crust, as the magma ocean (MO) probably did in Mars. When the deep mantle is not initially so cold and is rather soft, however, a vigorous magmatism generates a new crust that replaces the older MO crust within 10-20 Myr (see the frame of 0.006 Gyr in Figure b). Magmatism also makes the mantle compositionally stratified. The stratification suppresses magmatism for the next few hundred million years (Figure f). The HPEs in the deep mantle and the recycled MO crust, however, eventually induce hot upwelling plumes to resume magmatism (Figure a for 0.649 Gyr). The HPEs also reduce the heat flux on the core-mantle boundary within a short period of time (Figure e). The magmatism further extracts HPEs and water from the mantle to let the magmatism itself decline (Figures f and g). When the deep mantle is initially colder and stiffer, however, magmatism becomes milder, and a large portion of the MO crust remains at the top of the mantle for the calculated 4.5 Gyr history. Because of the limited recycling of the MO crust, the deep mantle is only mildly heated, and the heat flux on the core-mantle boundary remains high enough to drive the core dynamo for 100-200 Myrs. The models suggest that (a) there was a period when magmatism is quiescent and the surface is arid in the earliest Mars, and that (b) the surface became more temperate due to degassing by plume magmatism after the arid period. The models also suggest that how much of the MO crust remained on the surface controlled how long magnetic field continued in early Mars.