Hydrothermal cooling of the ocean crust: Insights from ODP Hole 1256D

The hydrothermal heat flux removed by fluid flow through the SDCs, is sufficient to remove only ∼20 to 60% of the available latent and sensible heat from the lower crust. Consequently, there must be additional thermal and chemical fluid-rock exchange deeper in the crust, at least of comparable size to the upper crustal hydrothermal system. Two scenarios are proposed for the potential geometry of this deeper hydrothermal system. The first requires the downward expansion of the upper crustal hydrothermal system ∼800 m into the lower crust in response to a downward migrating conductive boundary layer. The second scenario invokes a separate hydrothermal system in the lower crust for which fluid recharge bypasses reaction with the sheeted dikes, perhaps via flow down faults.