Roles of convection in the evolution of planetary interiors and terrestrial lithospheres.

摘要

Planetary mantle interiors deform by solid-state creep and behave like viscous fluids on long timescales. By studying the behavior of fluids, it is possible to study mantle flow and convection. Suites of numerical experiments can aid in the understanding of how different fluid characteristics affect the behaviors of connective motions. Scaling laws and parameterizations can be created and applied to planetary evolution questions. Two important influences on connective behavior are viscosity and the driving density differences. Solid-state creep predicted for mantle materials is strongly dependent on temperature and pressure. This, then, suggests the importance of examining the behaviors of fluids with temperature and pressure-dependent viscosities. In order for some of our numerical experiments to be properly resolved, we must use viscosities that are not as strongly dependent on temperature as expected in mantle minerals. We can then use our experiments as a basis for the scaling laws needed to extrapolate to the more strongly temperature-dependent viscosity thought to characterize the mantle of the Earth and other planets. The density of silicate minerals varies with both temperature and composition. Therefore, the density in our numerical models is considered to be a function of both temperature and inherent composition. Compositional stratification can result from magma ocean cumulate overturn or from large-scale mantle melting. Convection is studied in the presence of both stable and unstable stratifications. In the presence of stable compositional stratification, the onset time of thermal convection can be significantly delayed, and the vigor and depth extent of this convection can be greatly reduced. Strongly temperature-dependent viscosity reduces the available buoyancy to drive convection and therefore increases the impact of stable compositional stratification. In the early history a planet, gravitational overturn in unstable stratified mantles can be fast and fairly complete. The resulting inverted temperature structure could aid in the creation of an early crust and magnetic field. We also explore the effect that temperature and pressure-dependent viscosities have on the evolution of terrestrial lithospheres by using flux balance calculations and numerical evolution models. We focus on the influence of small-scale convection, crustal radioactive heating, and basal erosion.