Coupled fluid flow and reaction in mid-ocean ridge hydrothermal systems: The behavior of silica

Abstract

We apply a kinetic model of water-rock interaction to problems of silica mass transfer in mid-ocean ridge hydrothermal systems. A one-component steady state formulation is used to illustrate that the quartz geobarometer must be used with caution in mid-ocean ridge systems because the assumptions required by the technique may not be met. The results of this model suggest that there can be measurable mass transfer between wall rock and fluid in the upwelling zone of even vigorously flowing mid-ocean ridge hydrothermal systems. In addition, we attempt to quantify the effect of conductive cooling in the upwelling zone on the accuracy of the quartz geobarometer.The porosity and permeability fields constitute a critical link between the fluid dynamic equations and the equations describing chemical reactions in water-rock systems. We utilize a quasi-steady state approximation to evaluate the changes in porosity over time due to dissolution and precipitation of quartz along the flow path of a mid-ocean ridge hydrothermal system. There is little doubt that many mid-ocean ridge fluids encounter pressure and temperature conditions in which the solubility of quartz reaches a maximum. Our calculations suggest that precipitation of quartz from fluids passing through the solubility maximum within the oceanic crust may decrease the local porosity on a time scale of decades.