Hydrothermal convection at slow-spreading mid-ocean ridges

Abstract

The discrepancy observed between measured heat flow data and the heat flow predicted by thermal models of mid-ocean ridges is commonly explained by the presence of hydrothermal convection in young oceanic crust. Numerical modelling of fluid flow through porous media has been used to investigate what permeability, depth of penetration and mass fluxes are necessary to produce conductive heat flow distributions compatible with observed heat flow data at spreading centers. The results presented here were calculated for oceanic crust between 0 and 2 m.y. old at a mid-ocean ridge with a half-spreading rate of 1 cm/yr. The calculations show that theoretical and observed heat flow near mid-ocean ridges can be brought into better agreement if non-uniform rather than uniform permeability is assumed in the oceanic crust. If the crustal permeability is uniform, the percentage of heat flow values (⩾25%) which are increased by upwelling flow above the predicted values is significantly higher than that observed at mid-ocean ridges (<10%). If — due to faulting, for example — zones of high permeability exist in crust of low permeability, upwelling flow can be concentrated and the area of increased heat flow can be greatly reduced. In the latter case, the percentage of sea floor near mid-ocean ridges (⩾90%) where heat flow is depressed below the predicted values corresponds well with observed heat flow distributions near active spreading centers. Average temperatures in crust with hydrothermal convection are considerably lower than those in purely conductive crust. This difference in average temperatures should result in crestal offsets at mid-ocean ridges. The observation that crestal offsets attributable to convective cooling are not larger than 50 m suggests that the depth of penetration of convection strong enough to produce conductive heat flow compatible with observed heat flow distributions is less than 5 km in crust younger than 2 m.y. The downward flow of cold ocean water into the sea floor greatly reduces conductive heat flow. The magnitude of this inflow and hence the degree of heat flow reduction depends on the average permeability of the oceanic crust. Comparison of heat flow measurements from the FAMOUS area and from the Galapagos Spreading Center to heat flow over downwelling areas in our models indicates that the average permeability (including faults or high permeability zones) in young oceanic crust is not larger than 2.5 millidarcy. Finally, the integrated mass flux through sea floor between 0 and 2 m.y. old was found to be approximately 4 · 10 6 g yr −1 (cm of ridge). If this mass flux is considered representative for spreading centers and if older crust is included into the calculations, a total mass of ~1 · 10 17 g yr −1 is convected through the worldwide system of mid-ocean ridges.