HYDROTHERMAL CIRCULATION IN THE OCEANIC CRUST: INTEGRATED FIELD MEASUREMENTS AND NUMERICAL MODELING, CRAIG FORSTER STYLE

Hydrothermal circulation in young oceanic crust constitutes one of the most significant fluid fluxes within the solid Earth. Craig Forster’s work with Earl Davis and David Chapman on the eastern flank of the Juan de Fuca Ridge combined detailed seafloor heat-flow measurements, physical-property measurements on core samples, and numerical modeling to set important quantitative constraints on the circulation. More importantly, the work provides an example of the power of integrated observation and modeling studies that characterized much of Craig’s work. In very young and thinly sedimented sea floor, heat-flow observations are below that expected for sea floor spreading models because much of the heat is removed by water ventilating through topographic high points or outcrops. The quantitative heat flow deficit is consistent with a water flux through the crust of ~10 ¹⁵ kg per year, equivalent to the entire ocean passing though its crust in about a million years. The hydrologic regime in these young areas is generally dominated by high Rayleigh- and Nusselt-number pore-water convection, with high permeabilities in the uppermost few hundred meters of young igneous crust. In one area of 1 My old seafloor, characterized by unusually smooth basement topography and uniform sediment cover, we observed a coherent heat flow pattern of four maxima and minima with an average half-wavelength of 600 m and an amplitude variation of 35 mW m^-2. This pattern could be caused by cellular convection. Numerical models simulating hydrothermal circulation in a confined, permeable upper crust are able to reproduce this pattern if the permeability is 2 x 10^-12 m², not very different from values determined in deep sea boreholes. Ultimately, a decreasing thermal forcing in older sea floor, increasing sediment thickness, and hydrothermal alteration of the permeable crust all combine to shut off this hydrothermal circulation and return the crust to a more predictable, conductive thermal regime.