Thermochemical convection in porous media : an application to hydrothermal systems and magmetic intrusions

Abstract

The motion of aqueous and magmatic liquids has a large influence on the evolution of the cooling and chemically differentiating Earth. In many geological environments, these fluids percolate through the pores and fractures of a solid matrix, which is composed of (un)conso-lidated rocks or solidified magma. The major driving forces within the liquid arise from the pressure, temperature and compositional variations present within the liquid, or fromthe den-sity difference between liquid and solid states. Buoyancy driven flow (also called convection) in these porous or fractured media is capable to efficiently transport heat and chemical ele-ments within and between geological environments. As a consequence, convection in porous or fractured media plays a significant role in, or even dominates, a wide variety of geological processes [Jaupart and Tait, 1995; Ingebritsen and Sanford, 1998]. In the oceanic crust, for instance, hydrothermal circulation accounts for about one third of the heat flux through the ocean floor or 25 percent of the global heat loss [Lowell et al, 1995]. To dissipate this heat, estimated at 10 13 W, the mass of the oceans is circulated through the crust approximately every one million years [Elderfield and Schultz, 1997]. These sub-seafloor hydrothermal systems are driven by heat loss from magma lenses beneath spreading centers and from already solidified basaltic crust. Advective fluxes are important for all the major ocean basins to seafloor ages of 65 10 Myr [Stein and Stein, 1994]. Ninety percent of the oceanic heat flux occurs on the sedimented flanks of the spreading centers [Morton and Sleep, 1985]. It was also at these flanks that convective circulation was first invoked as an explanation for the missing heat in the measured heat flow values, as compared with those predicted by theoretical models of conductively cooling oceanic crust.