Suppression and stimulation of seafloor hydrothermal convection by exothermic mineral hydration

The effect of serpentinization on hydrothermal convection is explored using a dynamic 2D numerical model. Serpentinization is a highly exothermic mineral hydration process that consumes large quantities of water. The reaction is ubiquitous in the oceanic lithosphere and is generally associated with hydrothermal activity. Here, the thermal and hydration effects are incorporated into conservation equations describing fluid flow and heat transfer in hydrothermal systems. Models representing two different geological scenarios are explored. The "permeability-initiated" case simulates rapid uplift of ultramafic basement rock accompanied by rock fracturing, while the "temperature-initiated" scenario simulates the uplift of an ultramafic complex followed by a magmatic event at depth. In both models, simulations of convection with and without serpentinization demonstrate that mineral alteration can have an important effect on hydrothermal flow patterns and vent temperatures. Two parameters determine the impact of serpentinization on the system: (1) the basal temperature (T-b), and (2) the dimensionless Rayleigh number (Ra). At Ra < 50 and T-b < 170, peak temperatures were only slightly affected by serpentinization and convection was initially suppressed due to the downward flow of water induced by the hydration reaction. At higher Rayleigh numbers and basal temperatures, serpentinization effectively stimulated the onset of convection and significantly affected transient vent temperatures; differences in peak vent temperatures between the standard convective case and that with serpentinization were found to be as high as 170 degrees C. Thus, together with the geothermal heat flux, serpentinization could act to control temperatures at seafloor hydrothermal sites. (c) 2006 Elsevier B.V. All rights reserved.